Reasons for using carnitine in managing dioxins-induced metabolic dysfunction-associated steatotic liver disease: A scoping review

Marwan Al-Nimer¹, Vian Wasta Esmail², Tavga Aziz³

1 College of Medicine, University of Diyala; 32001 Baqubah, Iraq

2 College of Pharmacy, University of Sulaimani; 46001-Sulaymaniyah, Kurdistan region, Iraq

3 College of Pharmacy, University of Sulaimani; 46001-Sulaymaniyah, Kurdistan region, Iraq

Corresponding author: Marwan Al-Nimer (alnimermarwan@ymail.com)

Abstract

Introduction: Preclinical studies and epidemiological observational surveys showed that dioxin is a toxic substance that causes hepatic steatosis, metabolic dysregulation, insulin resistance, and type 2 diabetes mellitus. Workers in industries who live near the contaminated areas or consume contaminated food are at risk of metabolic dysfunction-associated steatotic liver disease (MASLD).

Materials and Methods: This scoping review was conducted according to the Joanna Briggs Institute (JBI) methodology and summarised the published literature on the mechanisms of action of carnitine (s) that counteract the mechanisms of dioxin(s)-inducing MASLD/MDAFLD/NAFLD/NASH/ in the preclinical studies. We addressed this exploratory review as a broad question and did not intend to conduct a systematic review; for that reason, we did not follow PRISMA guidelines.

Results and Discussion: The mechanisms by which dioxin-induced MASLAD can be counteracted by L-carnitine or acetyl-L-carnitine at the same site of dioxin effects are discussed in this scoping review. Carnitine acts as a metabolic antagonist against dioxin, according to the findings of preclinical, clinical observational, controlled clinical trials, and meta-analysis studies. Therefore, using competitive or non-competitive aryl receptor-blocking drugs to stop dioxin's harmful effects is not necessary.

Conclusion: L-carnitine or acetyl-L-carnitine as a supplementary nutrient could prevent or improve the clinical picture of MASLD in people at risk of dioxin toxicity.

Graphical Abstract

Keywords: acetyl-L-carnitine; aryl hydrocarbon receptors; carnitine; dioxin; metabolic dysfunction associated steatotic liver disease; reactive oxygen species

Introduction

Recently, in June 2023, during the European Association for the Study of the Liver Congress in Vienna, a transformative revolution happened in hepatology generally and in fatty liver terminology specifically. Instead of the old term of non-alcoholic fatty liver disease (NAFLD), a more reasonable broader substitute, metabolic dysfunction-associated steatotic liver disease (MASLD), became the spotlight because it can cover huge and diverse causes of steatosis under its umbrella (Rinella et al. 2023). Among the five common and well-known cardiometabolic risk factors, at least one is certainly included in these updated definitions, while individuals without a known cause for steatosis or metabolic risk factors are categorized as cryptogenic steatotic liver disease (Rinella et al. 2023). Nowadays, the most common cause of systemic complications and cirrhosis of the liver is known to be induced by MASLD, which usually starts with lipid droplet accumulation within hepatocytes, causing lipotoxicity and inflammation, magnified according to the genetic susceptibility of affected individuals. Then, activation of various inflammatory cells such as macrophages and hepatic stellate cells augments the synthesis of intracellular matrix and subsequent hepatic fibrosis known as metabolic associated steatotic hepatitis (MASH) (Grgurevic et al. 2020). Advanced fibrosis may easily shift to an irreversible advanced stage of hepatic cirrhosis and related complications such as hepatocellular carcinoma (Grgurevic et al. 2020). The 10^th revision of the International Classification of Disease (ICD-10) created by WHO in 1983, is one of the widely used codes for diagnosis of disease or clinical presentations/complaints for many illnesses, including Non-Alcoholic Fatty Liver Disease (NAFLD). As a result of new terminologies and updated nomenclatures, many studies were carried out to assess the appropriateness of using the existing ICD NAFLD and NASH codes to code MASLD and MASH, respectively (Song et al. 2024). A study performed by Hagström et al. (2024) concluded that the ICD code for previous nomenclature could be updated based on the new terminologies and definitions and be used effectively without the efforts to make new codes for MASLD and MASH. The clinical course and features of MASLD are highly variable among different individuals and cannot be easily predicted. Most affected individuals remain asymptomatic until progression to cirrhosis is ensured. Others may complain of right upper quadrant pain, reduced health-related quality of life, psychological symptoms and fatigue related to sarcopenia which are nonspecific to liver disease at early stages of fat disposition in the hepatocytes (Spengler and Loomba 2015). Elevations in the levels of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) may also occur as a result of hepatocellular injury, and sometimes the level fluctuates over time especially at early stages of the disease (Torres et al. 2012). These symptoms are not easily recognized by patients and physicians as characteristic of MASLD, especially in patients with comorbidities (Younoss et al. 2020). Thus, the variable nature of the disease presentation, especially in those with generally good health conditions, may make it difficult for most patients to accept the fact of being diagnosed with MASLD even if they are at risk of early development of complications (Eskridge et al. 2021). With progression to more advanced stages, the patients may present with itching, ascites, hepatic encephalopathy, variceal bleeding and even thrombocytopenia (Eskridge et al. 2023).

The change in NAFLD terminology to MASLD was deemed necessary to include the coexistence of hepatic steatosis broadly and at least one of the following risk factors for cardiometabolic diseases such as obesity evidenced by high body weight or large waist circumference, impaired glucose metabolism, elevated blood pressure, high level of triglyceride (TG), and low level of high-density cholesterol (HDL-C) (Chalasani et al. 2018) (Fig. 1).

Figure 1. Overlapping features between hepatic steatosis and metabolic dysfunction associated steatosis liver disease with their progression.

Exceptions are underweight patients with MASLD (Younossi et al. 2018). A retrospective study by Yoneda et al. (2021) analyzed 2,452,949 MASLD patients’ registered data to examine cardiovascular disease (CVD) as an associated risk factor. The result showed that the incidence rates of CVD were 1.01 (95% CI, 0.98 to 1.03) and 2.69 (95%CI, 2.55 to 2.83) per 1000 person-years in the non-MASLD and MASLD groups, respectively. Furthermore, the overall prevalence of hypertriglyceridemia and diabetes was 13.6 and 4.3%, respectively, in the non-MASLD group versus 64.1 and 20.6% in the MASLD group, which concluded that the risk for CVD is increased with elevated triglyceride and blood glucose level. A meta-analysis of 20 studies investigated the association of central obesity with the development of MASLD after adjusting for general obesity and found that the pooled odds ratio in waist circumference and body mass index was of 2.34 (95%CI, 1.83 to 3.00) and 2.85 (95%CI, 1.60 to 5.08), respectively (Pang et al. 2015). On the other hand, other studies linked MASLD to hypertension, and a meta-analysis of 11 studies tried to discover the nature and strength of this association (Li et al. 2022). Eventually, it concluded that the presence of elevated blood pressure was significantly associated with an increased risk of incident MASLD (hazard ratio 1.63; 95%CI, 1.41 to 1.88) and a bidirectional association between the two conditions exists independently of cardiometabolic risk factors. Metabolic dysfunction-associated steatotic liver disease (MASLD) is a risk factor for liver cirrhosis, hepatocellular carcinoma, and systemic complications such as cardiovascular diseases, type 2 diabetes mellitus (T2DM), chronic kidney disease or systemic inflammation, etc. (Angulo et al. 2015; Adams et al. 2020; Targher et al. 2024; Thomas et al. 2024).

Dioxins are a group of compounds chemically related to polyhalogenated organic compounds, including polychlorinated dibenzo-p-dioxins (PCDDs) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyl, polychlorinated dibenzofurans, and polybrominated analogues, which are considered persistent environmental pollutants (Tuomisto 2019). These compounds are eliminated slowly from animals, humans, and the environment, making them persistent organic chemicals. The mechanism of toxicity occurs through the binding of dioxins to the aryl hydrocarbon receptor (AhR). Dioxins are principally produced as unintentional byproducts of various industrial processes, particularly those involving combustion, or it may occur from natural sources, e.g., food contamination (red meat, fish and dairy products); combustion processes (waste incineration and burning of fuels); and chemical manufacturing (chlorine, halogenated organic chemicals, and papers). Also, forest fires and volcanic eruptions can contribute to producing dioxins (Kulkarni et al. 2008). Therefore, it is expected to get dioxin toxicity in the population when the following factors are provided: contaminated food; workers in industries (e.g., paper or herbicides and pesticides industries), people who live near the industries, and certain people are more vulnerable to dioxin toxicity like children, pregnant women, breastfed infants, and those with a lower socioeconomic status because they lived in poor or highly industrialized contaminated areas (Weir 2005; Schecter et al. 2006; EFSA Panel on Contaminants in the Food Chain et al. 2018). Acute, subacute and chronic toxicities affect many organs and systems in the body (Fig. 2). The chronic toxicity of dioxins is associated with: cancer (soft-tissue sarcoma, non-Hodgkin lymphoma, and lung cancer); disrupts hormonal (thyroid, estrogen and progesterone) signaling pathways causing many reproductive and metabolic diseases; developmental impairment (intrauterine growth retardation, cognitive impairments, and developmental delays in children); cardiovascular events; and hepatic toxicity (NAFLD and cirrhosis) (Chen et al. 2010; Całkosiński et al. 2014; Hattori et al. 2014; Petriello et al. 2018; Vuong 2022). In addition to the determination of circulating levels of dioxins which are expressed per gram lipids, there are certain specific and non-specific biomarkers indicating exposure to the dioxins. These biomarkers are hepatocellular enzymes, inflammatory markers, hepatic accumulation of lipids, hormonal assay, oxidative stress markers (e.g. superoxide dismutase), lipid peroxidation marker (e.g. malondialdehyde (MDA), which is also used for assessment of ferroptosis), and DNA methylation markers (Yoshida and Ogawa 2000; Das et al. 2017; Wang et al. 2024).

Carnitine is a natural nutrient substance formed inside the body (endogenous), and it can be obtained from exogenous sources in the diet. It transports the long-chain free fatty acids into mitochondria for β-oxidation. In humans, it is available as free carnitine or acetylated derivatives. Methionine, lysine, niacin, pyridoxine, ascorbic acid, and iron are the precursor sources of endogenous L-carnitine, which makes up 25% of total body levels. In contrast, exogenous L-carnitine, which makes up 75% of total body levels, is found in dairy products, fish, and red meat; and as pharmaceutical supplements for individuals with nutritional deficiencies (Rebouche 1992). The circulating L-carnitine levels are significantly lower in women than men, and a plasma level of less than 20µmol/L indicates L-carnitine deficiency in all age groups (Valkner and Bieber 1982). Since acetyl-L-carnitine is easily absorbed from the gut, it has a higher bioavailability than L-carnitine.

Figure 2. Dioxins-induced toxicities and the proposed mechanisms of toxicities.

Acetyl-L-carnitine has potential therapeutic uses in neurodegenerative conditions, e.g., Alzheimer’s dementia because it crosses the blood-brain barrier. Despite carnitines having numerous pharmacological properties that mitigate the detrimental consequences of NAFLD, their use as therapeutic medicines has not been fully explored. Clinical studies showed that many medicines are useful in the management of NAFLD, and they are acting in different directions against the risk-associated factors, some of them improve the full picture of NAFLD, while others improve some clinical findings of NAFLD. Table 1 summarizes the pharmacological intervention reported in the clinical studies for treating fatty liver disease (Rong et al. 2023). We hypothesized that carnitine or its derivatives could prevent the mechanisms of action of dioxin-induced NAFLD because they both have a similar set of effects. This scoping review’s goal is to provide an answer to the question, “Do carnitines have a clinical application in the management of NAFLD?” Taking into account that dioxins are ecologically harmful chemicals that cause NAFLD with various effects.

Materials and Methods

This scoping review was conducted according to the Joanna Briggs Institute (JBI) methodology for scoping (https://synthesismanual.jbi.global, https://doi.org/10.46658/JBIMES-24-09). This scoping review summarised the published literature on the mechanisms of action of carnitine (s) that counteract the mechanisms of dioxin(s)-inducing MASLD/MDAFLD/NAFLD/NASH/ in the preclinical studies. We addressed this exploratory review as a broad question and did not intend to conduct a systematic review; for that reason, we did not follow PRISMA guidelines.

Research question and search strategy

The research question was “What are the mechanisms of action of carnitine (s) that attenuated or counteracted the dioxin(s)-inducing fatty liver, to be a reason for using it in the management of MASLD/MDAFLD/NAFLD/NASH?”. The relevant information on the mechanisms of action of dioxin (or its derivatives) in inducing MASLD/MAFLD/NAFLD/NASH as well as the mechanism of action of carnitine (or its derivatives) that is helpful in the management of these pathological conditions was obtained from a literature search conducted through MEDLINE (via PubMed), European PMC, Semantic Scholar, Dimensions, Scopus and Web of Science. Medical subheadings (MeSH) terms used for the search were “metabolic dysfunction associated steatotic liver disease”, “non-alcoholic fatty liver disease”, “non-alcoholic steatotic hepatitis”, “dioxin (s)” and “carnitine (s)”. The quality of studies using the JBI SUMARI Critical Appraisal Checklist was followed, and any disagreement was resolved by declining the study. Only full manuscripts written in English were considered.

Table 1.
Pharmacotherapy for non-alcoholic fatty liver disease

Results and Discussion

According to the mechanisms of the dioxin-inducing fatty liver and the actions of carnitine at these mechanisms, the following sections summarized the results that explained why carnitine could be a promising agent against dioxin toxicity.

Metabolic antagonism of carnitine against dioxin-induced fatty liver

Oxidative stress syndrome: the antioxidant properties of carnitine

The role of reactive oxygen species (ROS) in the development of MASLD is related to the upregulation of the lipid peroxidation process (García-Monzón et al. 2000; Sanyal et al. 2001; Videla et al. 2004), hepatic iron overload, and hyperinsulinemia (Muriel 2009). The sources of oxidative stress in this pathological condition are the oxidation of free fatty acids, forming hydrogen peroxides, superoxide anions, and lipid peroxides (Nehra et al. 2001). The metal iron is a direct pro-oxidant metal, and it is involved in the enzymatic catalytic process in the production of ROS. There is evidence that insulin under the state of hyperinsulinemia in MASLD can produce ROS (Goldstein et al. 2005) and stimulate certain growth factors, which are responsible for inducing the profibrogenic process in the liver (Wanless et al. 1989). One mechanism of dioxin-induced liver toxicity is the activation of the AhR in the hepatocytes, which results in a continuous production of higher levels of ROS and a significant increase in lipid droplets in the liver, producing a similar change observed in NAFLD (Furue et al. 2021; Ohashi et al. 2018). So, it is reasonable to suggest that L-carnitine could have promising beneficial effects on MASLD by inhibiting the generation of ROS and counteracting the accumulation of hepatic lipids and hypertriglyceridemia induced by dioxins (Choi et al. 2020). Synthesized carnitine in the liver showed hepatoprotection against toxins, including ammonia and inflammatory mediators in hepatic encephalopathy (Martí-Carvajal et al. 2019). This hepatoprotective effect is not related to the antioxidant property, but to its metabolic effect by reducing the ammonia output urea cycle. In addition, patients with chronic liver diseases are more likely to have low plasma levels of carnitine (Hanai et al. 2020). Moreover, the creatinine fractions detected by tandem mass spectrometry are significantly associated with stages of liver cirrhosis assessed by the Child-Pugh score (Miyaaki et al. 2020). The mechanism of L-carnitine action in the NAFLD is still unclear. The isomer D-carnitine lacks biological activity, but it inhibits the carnitine acetyltransferase and ultimately reduces the formation of acetyl-L-carnitine (Vashistha and Bhushan 2015). In contrast to L-carnitine, it has been found experimentally that D-carnitine supplementation induces hepatic inflammatory changes, production of ROS, and cellular apoptosis (Li et al. 2019). The antioxidant property of L-carnitine is achieved via the following mechanisms:

i.              Scavenging superoxide anion, hydrogen peroxide, and 2,2-diphenyl-1-picrylhydrazyl;

ii.             Inhibiting the free radical formation by chelating the metals (e.g., iron); inhibiting the enzymes that played a role in the production of ROS (e.g., xanthine oxidase);

iii.           Upregulatimg the antioxidant activity of catalase, glutathione peroxidase, glutathione reductase, and nitric oxide synthetase (endothelial) (Modanloo and Shokrzadeh 2019).

Metabolic derangements, including hepatic lipids and the development of Type 2 diabetes mellitus

An accumulation of fat at least ≥ 5% of the liver weight is considered the onset of MDAFL, and the grade of MASLD is grouped according to the accumulated percentages of fat (Chalasani et al. 2018). Hepatic lipids are regulated by four factors (Badmus et al. 2022):

i.              Lipids uptake. It is regulated by fatty acid binding-proteins (FABPs). FABP-1 plays an important role in this process, and significantly higher levels of FABP-1 are significantly correlated with hepatic steatosis in MASLD (Lu et al. 2023). A significant expression of the cluster of differentiation 36 (CD36) is associated with a significantly high hepatic fat content (Miquilena-Colina et al. 2011).

ii.             De novo lipogenesis: Dysregulation in the acetyl-CoA carboxylase, fatty acid synthase, and stearoyl-CoA desaturase-1 resulted in hepatic steatosis and hypertriglyceridemia (Hong et al. 2020). Increased activity of de novo lipogenesis led to the production of toxic lipid species that cause non-alcoholic steatotic hepatitis (Badmuset al. 2022).

iii.           Lipid β-oxidation: As a result of lipid accumulation, ω-oxidation of fatty acids by cytochrome P450 enzymes has occurred and generated ROS. Normally, the beta-oxidation of fatty acid occurs in mitochondria, and peroxisome proliferator-activated receptor-α (PPARα) plays a role in this process. It has been found that a decline in the expression of PPARα is associated with advanced MASLD and liver fibrosis (Francque et al. 2015).

iv.           Exportation of fatty acids: This process is achieved by apolipoprotein B100 and microsomal triglyceride transfer protein. A close relationship between a defect or down-regulation of microsomal triglyceride transfer protein is associated with the development of MASLD (Peng et al. 2014).

Dioxin-like polychlorinated biphenyls cause liver steatosis by inducing alteration in the gut microbiome, leading to a disruption in the lipid metabolism. At the same time, the non-dioxin derivatives act through different mechanisms by promoting diet-induced NAFLD (Wahlang et al. 2019). Livers of mice exposed to TCDD for 28 days showed a significant increase in free fatty acid, triglycerides, cholesterol esters, ceramides, and inhibiting secretion of very low-density lipoprotein, which is similar to that picture of hepatic steatosis (Nault et al. 2017). TCDD induced hepatic inflammation by accelerating arachidonic acid’s metabolism to up-regulate cyclooxygenase and lipoxygenase’s activity to produce leukotrienes, leading to steatohepatitis (Doskey et al. 2020). Some authors explained the progression of steatosis to steatohepatitis and fibrosis to the one-carbon metabolism induced by persistent AhR activation (Fling et al. 2020); others believed that incomplete β-oxidation of fatty acid is the reason for the progression (Cholico et al. 2021). TCDD targeted the aryl hydrocarbon receptor in the intermembrane space of organelles in mitochondria (miAhR), leading to degradation of miAhR and dysfunction of mitochondria, which clinically manifested as metabolic dysfunction, e.g. hepatic steatosis (Hwang et al. 2016). T2D is a contributing factor, and one of the diagnostic criteria of MASLD is also reported as a result of dioxin toxicity (Gang et al. 2022; Lee et al. 2022).

Dioxin is one of the six persistent organic pollutants that contribute to altering the metabolic and oxidative stress pathways, impairing glucose homeostasis and β-cell dysfunction (Hoyeck et al. 2022). There is much evidence that reports an association between dioxin exposure and the development of diabetes, as dioxin is considered an endocrine disruption pollutant. In one study carried out on 2898 participants who lived in an area with factory-released dioxin, 425 diabetes (14.7%) subjects were identified, and a higher serum dioxin level is a risk factor for developing diabetes mellitus (Huang et al. 2015). A systematic review and meta-analysis included 10 epidemiological studies that showed an inconsistency in the association between dioxin exposure and diabetes mellitus (Goodman and Narayan 2015). A recent preclinical study showed that the signalling of AhR in the beta-cell of the pancreas is a regulatory key for beta-cell function and glucose homeostasis (Hoyeck et al. 2024). Dioxin, like any endocrine-disrupting chemical, can induce insulin resistance by its direct effect on the β-cell of the pancreas, in addition to its indirect effect by inducing adipogenesis, stimulating lipid storage, and altering the production of adipocytokines via activation of peroxisome proliferator-activated receptor-alpha (PPARα) (Street et al. 2018; Papalou and Kandaraki 2019; Rotondo and Chiarelli 2020).

L-carnitine specifically antagonized the effect of dioxin on the lipid metabolism as it stimulates the lipolysis and inhibits the lipogenesis in the adipocyte, i.e. a metabolic antagonism at the molecular cell level (Lee et al. 2006). Carnitine derivatives can ameliorate steatosis by facilitating the β-oxidation of fatty acids and improving the metabolic dysfunction associated with a fatty liver, e.g. reducing the circulating glucose, enhancing insulin sensitivity, preserving the function of mitochondria, and restoring the oxidative phosphorylation activity (Hong and Lee 2021). Others suggested that L-carnitine reduces the hepatic lipids by reducing the production of ROS and increasing the activity of proteins that regulate the antioxidant properties, e.g. NrF2 and superoxide dismutase (Montesano et al. 2020). In addition, L-carnitine is useful in the management of T2DM. A meta-analysis that included 21 randomized clinical trials reported that a supplementation of one-gram daily carnitine could reduce body mass index and improve the glycemic and lipid profiles in T2DM patients (Mirrafiei et al. 2024). Preclinical studies demonstrated L-carnitine or its analogues could counteract the insulin resistance. A randomized controlled clinical trial showed supplementation of L-carnitine tartrate (500 mg twice daily for eight weeks) reduces the body mass index, fasting plasma glucose, insulin, and homeostatic model assessment of insulin resistance, when it is administered as monotherapy or in combination with synbiotics (Fallah and Mahdavi et al. 2023). In a systematic review and meta-analysis that included five studies, it was found that L-carnitine is a useful remedy to treat patients with insulin resistance (Xu et al. 2017). Therefore, L-carnitine does act as an AhR antagonist, but it minimizes the metabolic effects resulting from activation of hepatic AhR.

Anti-inflammatory effects

In an experimental animal model, TCDD-induced liver injury in the form of hepatic steatosis, significant deposition of lipid and neutrophil accumulation in the liver, elevated serum alanine aminotransferase enzyme, and interleukin 6 were observed (Olivero-Verbel et al. 2021). In another study, mice treated with TCDD for two weeks showed histopathological findings of liver fibrosis, which is associated with upregulation of the inflammatory biomarkers, including interleukin 1β and tumor necrosis factor-α (Pierre et al. 2014). Exposure to dioxin derivatives is associated with significant elevation of bilirubin and hepatic enzymes (ALT, AST, and alkaline phosphatase) (Kumar et al. 2014), accompanied by triggering to form and accumulate the lipid (i.e. de novo lipogenesis (Kawano and Cohen 2013; Geisler et al. 2017; Ipsen et al. 2018). Workers exposed to dioxins are at health risk for inflammation and liver diseases due to alterations in the lipid metabolism leading to high triglyceride deposits, ceramide, and sphingoid (Liang et al. 2021). In the preclinical studies, supplementation of L-carnitine reduces lipid accumulation, oxidative stress, hepatic fibrosis, and systemic inflammation, which is related to its effects on the expression of α-smooth muscle actin, peroxisome-activated receptor gamma, and nuclear factor kappa B (Mollica et al. 2020). Although L-carnitine did not antagonize the effect of dioxins at AhR, it specifically cancels the metabolic effects of dioxin at a molecular cell level (i.e. metabolic antagonist). It could be suggested that L-carnitine supplementation is useful to people exposed to dioxin and developed liver injury for the following reasons:

i.              Dioxin is a persistent and continuous toxin that induces hepatic lipotoxicity;

ii.             There is deprivation of the endogenous L-carnitine because it is synthesized in the liver;

iii.           Continuous L-carnitine supplementation will support the liver against inflammation, apoptosis, and oxidative stress that followed or accompanied the dioxin-induced hepatic lipotoxicity.

Anti-apoptotic effects

It has been documented that dioxins induce cellular apoptosis in a dose-dependent manner, and it is associated with a generation of ROS and nitrative reactive species (NRS; e.g. nitric oxide) (Yang and Lee 2010). The apoptotic effect that is mediated by ROS and NRS can be blocked by using ROS or NRS inhibitors. On the other side, acetyl-L-carnitine provides an antiapoptotic effect via inhibition of the lipid peroxidation process and restoring the antioxidant activity of glutathione (Zidan et al. 2018). Another study demonstrated that L-carnitine protects the platelet count and function (aggregation and viability) during storage by the evidence of reducing the expression of apoptotic biomarkers (e.g. miR-326) (Norouzi et al. 2024). This indicates that acetyl-L-carnitine can act as a non-specific inhibitor of the dioxin that is mediated by blocking ROS production and not through blocking the AhR.

Anti-fibrosis

Preclinical studies demonstrated that dioxin accelerates the development of liver fibrosis in mice fed high-fat (45%) by the evidence of increasing hepatic collagen staining and a significant elevation of serum ALT and AST levels (Duval et al. 2017). Other studies demonstrated the association between liver fibrosis and inflammation upon subacute-chronic exposure to the dioxin, suggesting that due to the overproduction of ROS and depletion of superoxide anion dismutase, in parallel increasing levels of interleukin 1β and Tumor necrosis factor-α, as a result of activation of AhR (He et al. 2013; Pierre et al. 2014). Patients with chronic hepatitis who live in dioxin-contaminated areas are more likely to have the liver fibrosis stage with a significantly high aspartate aminotransferase (Pham et al. 2022). Recently, a preclinical study demonstrated that acetyl-L-carnitine significantly prevents the development of liver fibrosis induced by propionic acid due to its anti-inflammatory and antioxidant properties (Alhusaini et al. 2023). Another clinical observational study of a small sample size demonstrated that a ten-week administration of L-carnitine significantly reduces the hepatic enzymes and improves the histopathological findings of NAFLD; it does not improve the scoring of hepatic steatosis or fibrosis (Lyu et al. 2024). These observations indicate that carnitine use is still in the preclinical studies for the management of MASLD.

Anti-ferroptosis

The main feature of ferroptosis is the accumulation of the byproducts of the lipid peroxidation process that is initiated by ROS. The precise link between ferroptosis and MASLD is still not well explored (Feng et al. 2022). Tong et al. (2023) reported that using liprostatin-1 (a ferroptosis inhibitor) alleviates most of the MASLD characteristics in mice, including reducing hepatic triglycerides, inhibiting lipid peroxidation products, e.g. MDA, and attenuating the metabolic derangements related to insulin resistance, mitochondrial dysfunction, and even liver fibrosis. An ex vivo experimental study showed a significant accumulation of MDA in the hearts of rats exposed to the TCDD, indicating that dioxin activates ferroptosis in the tissue (Mahdi et al. 2019). L-carnitine supplementation had no significant effect on malondialdehyde levels in patients with osteoarthritis (Baghban et al. 2021). It is interesting to mention herein that ROS play a major role in the activation of the lipid peroxidation process and ferroptosis, but neither dioxins nor carnitines showed specific effects on the ferroptosis process.

Clinical evidence of using carnitine as an effective antagonist against dioxin-induced hepatic toxicity

Table II shows that there is evidence that dioxins could cause fatty liver disease associated with metabolic derangement, e.g. insulin resistance and T2DM. On the other side, meta-analysis studies and clinical trials reported significant effects of using carnitine in the management of non-alcoholic fatty liver disease with or without insulin resistance. The effects of carnitine on metabolic dysregulation showed variable results, which could be related to the optimum dose, the derivative of carnitine, the duration of treatment, and associated comorbidities. It could be useful to use acetyl-L-carnitine instead of L-carnitine because it crosses the blood-brain barrier and improves the cognitive dysfunction associated with hepatic steatosis.

Table 2.
Evidence of metabolic dysfunction associated steatotic liver disease induced by dioxins and ameliorated by carnitine

Note: TCDD – 2,3,7,8 tetrachlorodibenzodioxin, TEQ – Toxic equivalency, PCDD/Fs – Polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs), ALT – alanine aminotransferase, AST – aspartate aminotransferase, NAFLD – non-alcoholic fatty liver disease. ↑ – increase, ↓ – decrease, ↔ – no effect.

Conclusions

Subclinical studies and epidemiological observational studies indicated that exposure to dioxins as persistent endocrine-disrupting chemicals is a cause of metabolic dysfunction associated with steatotic liver disease, as evidenced by hepatic steatosis and the presence of obesity, type 2 diabetes, and abnormal lipid profiles, which is due to the activation of hepatic aryl hydrocarbon receptor. Although carnitines have no antagonistic effects against aryl hydrocarbon receptors, they counteract most of the metabolic events resulting from the activation of aryl hydrocarbon receptors. Thus, carnitines’ metabolic antagonist actions may be used to treat MASLD induced by dioxin, and it can be recommended as a preventive supplementary nutrient for workers exposed to dioxin or to people who live near the dioxin-contaminated areas.

Additional Information

Conflict of interest

The authors declare the absence of a conflict of interests.

Acknowledgements

The authors appreciate the effort of the Faculty of Medicine at the University of Diyala and the Faculty of Pharmacy at Sulaimani University (Iraq) for encouraging us to write this review about a health problem concerning dioxin exposure and the increased number of case reports of fatty liver in our society.

Data availability

All of the data that support the findings of this study are available in the main text.

References

§  Abolfathi M, Mohd-Yusof BN, Hanipah ZN, Mohd Redzwan S, Yusof LM (2020) The effects of carnitine supplementation on clinical characteristics of patients with non-alcoholic fatty liver disease: A systematic review and meta-analysis of randomized controlled trials. Complementary Therapies in Medicine 48: 102273. https://doi.org/10.1016/j.ctim.2019.102273 [PubMed]

§  Adams LA, Roberts SK, Strasser SI, Mahady SE, Powell E, Estes C, Razavi H, George J (2020) Nonalcoholic fatty liver disease burden: Australia, 2019-2030. Journal of Gastroenterology and Hepatolology 35(9): 1628–1635. https://doi.org/10.1111/jgh.15009 [PubMed] [PMC]

§  Alhusaini AM, Alsoghayer R, Alhushan L, Alanazi AM, Hasan IH (2023) Acetyl-L-carnitine and liposomal co-enzyme q₁₀ attenuate hepatic inflammation, apoptosis, and fibrosis induced by propionic acid. Internationl Journal of Molecular Sciences 24(14): 11519. https://doi.org/10.3390/ijms241411519 [PubMed] [PMC]

§  Angulo P, Kleiner DE, Dam-Larsen S, Adams LA, Bjornsson ES, Charatcharoenwitthaya P, Mills PR, Keach JC, Lafferty HD, Stahler A, Haflidadottir S, Bendtsen F (2015) Liver fibrosis, but no other histologic features, is associated with long-term outcomes of patients with nonalcoholic fatty liver disease. Gastroenterology 149(2): 389–397. https://doi.org/10.1053/j.gastro.2015.04.043 [PubMed] [PMC]

§  Badmus OO, Hillhouse SA, Anderson CD, Hinds TD, Stec DE (2022) Molecular mechanisms of metabolic associated fatty liver disease (MAFLD): Functional analysis of lipid metabolism pathways. Clinical Science (London) 136(18): 1347–1366. https://doi.org/10.1042/CS20220572 [PubMed] [PMC]

§  Bae JC, Lee WY, Yoon KH, Park JY, Son HS, Han KA, Lee KW, Woo JT, Ju YC, Lee WJ, Cho YY, Lee MK (2015) Improvement of nonalcoholic fatty liver disease with carnitine-orotate complex in type 2 diabetes (CORONA): A randomized controlled trial. Diabetes Care 38(7): 1245–1252. https://doi.org/10.2337/dc14-2852 [PubMed]

§  Baghban F, Hosseinzadeh M, Mozaffari-Khosravi H, Dehghan A, Fallahzadeh H (2021) The effect of L-Carnitine supplementation on clinical symptoms, C-reactive protein and malondialdehyde in obese women with knee osteoarthritis: A double blind randomized controlled trial. BMC Musculoskeletal Disorders 22(1): 195. https://doi.org/10.1186/s12891-021-04059 [PubMed] [PMC]

§  Całkosiński I, Rosińczuk-Tonderys J, Bazan J, Dobrzyński M, Bronowicka-Szydełko A, Dzierzba K (2014) Influence of dioxin intoxication on the human system and possibilities of limiting its negative effects on the environment and living organisms. Annals of Agricultural and Environmental Medicine: AAEM 21(3): 518–524. https://doi/org/10.5604/12321966.1120594 [PubMed]

§  Chalasani N, Younossi Z, Lavine JE, Charlton M, Cusi K, Rinella M, Harrison SA, Brunt EM, Sanyal AJ (2018) The diagnosis and management of nonalcoholic fatty liver disease: Practice guidance from the American Association for the Study of Liver Diseases. Hepatology 67(1): 328–357. https://doi.org/10.1002/hep.29367 [PubMed]

§  Chen SC, Liao TL, Wei YH, Tzeng CR, Kao SH (2010) Endocrine disruptor, dioxin (TCDD)-induced mitochondrial dysfunction and apoptosis in human trophoblast-like JAR cells. Molecular Human Reproduction 16(5): 361–372. https://doi.org/10.1093/molehr/gaq004 [PubMed]

§  Choi M, Park S, Lee M (2020) L-Carnitine’s effect on the biomarkers of metabolic syndrome: A systematic review and meta-analysis of randomized controlled trials. Nutrients 12 (9): 2795. https://doi.org/10.3390/nu12092795 [PubMed] [PMC]

§  Cholico GN, Fling RR, Zacharewski NA, Fader KA, Nault R, Zacharewski TR (2021) Thioesterase induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin results in a futile cycle that inhibits hepatic β-oxidation. Scientific Reports 11(1): 15689. https://doi.org/10.1038/s41598-021-95214-0 [PubMed] [PMC]

§  Das DN, Panda PK, Sinha N, Mukhopadhyay S, Naik PP, Bhutia SK (2017) DNA damage by 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced p53-mediated apoptosis through activation of cytochrome P450/aryl hydrocarbon receptor. Environmental Toxicology and Pharmacology 55: 175–185. https://doi.org/10.1016/j.etap.2017.08.012 [PubMed]

§  Doskey CM, Fader KA, Nault R, Lydic T, Matthews J, Potter D, Sharratt B, Williams K, Zacharewski T (2020) 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters hepatic polyunsaturated fatty acid metabolism and eicosanoid biosynthesis in female Sprague-Dawley rats. Toxicology and Applied Pharmacology 398: 115034. https://doi.org/10.1016/j.taap.2020.115034 [PubMed] [PMC]

§  Duval C, Teixeira-Clerc F, Leblanc AF, Touch S, Emond C, Guerre-Millo M, Lotersztajn S, Barouki R, Aggerbeck M, Coumoul X (2017) Chronic exposure to low doses of dioxin promotes liver fibrosis development in the C57BL/6J diet-induced obesity mouse model. Environmental Health Perspectives 125(3): 428–436. https://doi.org/10.1289/EHP316 [PubMed] [PMC]

§  EFSA Panel on Contaminants in the Food Chain (CONTAM); Knutsen HK, Alexander J, Barregård L, Bignami M, Brüschweiler B, Ceccatelli S, Cottrill B, Dinovi M, Edler L, Grasl-Kraupp B, Hogstrand C, Nebbia CS, Oswald IP, Petersen A, Rose M, Roudot AC, Schwerdtle T, Vleminckx C, Vollmer G, Wallace H, Fürst P, Håkansson H, Halldorsson T, Lundebye AK, Pohjanvirta R, Rylander L, Smith A, van Loveren H, Waalkens-Berendsen I, Zeilmaker M, Binaglia M, Gómez Ruiz JÁ, Horváth Z, Christoph E, Ciccolallo L, Ramos Bordajandi L, Steinkellner H, Hoogenboom LR (2018) Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food. EFSA Journal 16(11): e05333 https://doi.org/10.2903/j.efsa.2018.5333 [PubMed] [PMC]

§  Eskridge W, Cryer DR, Schattenberg JM, Gastaldelli A, Malhi H, Allen AM, Noureddin M, Sanyal AJ (2023) Metabolic dysfunction-associated steatotic liver disease and metabolic dysfunction-associated steatohepatitis: The patient and physician perspective. Journal of Clinical Medicine 12(19): 6216. https://doi.org/10.3390/jcm12196216 [PubMed] [PMC]

§  Eskridge W, Vierling JM, Gosbee W, Wan GA, Hyunh ML, Chang HE (2021) Screening for undiagnosed non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH): A population-based risk factor assessment using vibration controlled transient elastography (VCTE). PLoS One 16(11): e0260320. https://doi.org/10.1371/journal.pone.0260320 [PubMed] [PMC]

§  Fallah F, Mahdavi R (2023) Ameliorating effects of L-carnitine and synbiotic co-supplementation on anthropometric measures and cardiometabolic traits in women with obesity: A randomized controlled clinical trial. Frontiers in Endocrinology 14: 1237882. https://doi.org/10.3389/fendo.2023.1237882 [PubMed] [PMC]

§  Feng G, Byrne CD, Targher G, Wang F, Zheng MH (2022) Ferroptosis and metabolic dysfunction-associated fatty liver disease: Is there a link? Liver International 42(7): 1496–1502. https://doi.org/10.1111/liv.15163 [PubMed]

§  Fling RR, Doskey CM, Fader KA, Nault R, Zacharewski TR (2020) 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) dysregulates hepatic one carbon metabolism during the progression of steatosis to steatohepatitis with fibrosis in mice. Scientific Reports 10(1): 14831 https://doi.org/10.1038/s41598-020-71795-0 [PubMed] [PMC]

§  Francque S, Verrijken A, Caron S, Prawitt J, Paumelle R, Derudas B (2015) PPARα gene expression correlates with severity and histological treatment response in patients with non-alcoholic steatohepatitis. Journal of Hepatology 63(1): 164–173. https://doi.org/10.1016/j.jhep.2015.02.019 [PubMed]

§  Furue M, Ishii Y, Tsukimori K, Tsuji G (2021) Aryl hydrocarbon receptor and dioxin-related health hazards-lessons from Yusho. International Journal of Molecular Sciences 22(2): 708. https://doi.org/10.3390/ijms22020708 [PubMed] [PMC]

§  Gang N, Van Allen K, Villeneuve PJ, MacDonald H, Bruin JE (2022) Sex-specific associations between type 2 diabetes incidence and exposure to dioxin and dioxin-like pollutants: A meta-analysis. Frontiers in Toxicology 3: 685840. https://doi.org/10.3389/ftox.2021.685840 [PubMed] [PMC]

§  García-Monzón C, Martín-Pérez E, Iacono OL, Fernández-Bermejo M, Majano PL, Apolinario A, Larrañaga E, Moreno-Otero R (2000) Characterization of pathogenic and prognostic factors of nonalcoholic steatohepatitis associated with obesity. Journal of Hepatology 33(5): 716–724. https://doi.org/10.1016/s0168-8278(00)80301-3 [PubMed]

§  Geisler CE, Renquist BJ (2017) Hepatic lipid accumulation: cause and consequence of dysregulated glucoregulatory hormones. Journal of Endocrinology 234(1): R1–R21. https://doi.org/10.1530/JOE-16-0513 [PubMed]

§  Goldstein BJ, Mahadev K, Wu X (2005) Redox paradox: insulin action is facilitated by insulin-stimulated reactive oxygen species with multiple potential signaling targets. Diabetes 54(2): 311–321. https://doi.org/10.2337/diabetes.54.2.311 [PubMed] [PMC]

§  Goodman M, Narayan KM, Flanders D, Chang ET, Adami HO, Boffetta P, Mandel JS (2015) Dose-response relationship between serum 2,3,7,8-tetrachlorodibenzo-p-dioxin and diabetes mellitus: a meta-analysis. American Journal of Epidemiology 181(6): 374–384. https://doi.org/10.1093/aje/kwu307 [PubMed] [PMC]

§  Grgurevic I, Podrug K, Mikolasevic I, Kukla M, Madir A, Tsochatzis EA (2020) Natural history of nonalcoholic fatty liver disease: implications for clinical practice and an individualized approach. Canadian Journal of Gastroenterology and Hepatology 2020: 9181368. https://doi.org/10.1155/2020/9181368 [PubMed] [PMC]

§  Hagström H, Adams LA, Allen AM, Byrne CD, Chang Y, Duseja A, Grønbæk H, Ismail MH, Jepsen P, Kanwal F, Kramer J, Loomba R, Mark HE, Newsome PN, Rinella ME, Rowe IA, Ryu S, Sanyal A, Schattenberg JM, Serper M, Sheron N, Simon TG, Spearman CW, Tapper EB, Villota-Rivas M, Wild SH, Wong VW, Yilmaz Y, Zelber-Sagi S, Åberg F, Lazarus JV (2024) The future of International Classification of Diseases coding in steatotic liver disease: An expert panel Delphi consensus statement. Hepatology Communications 8(2): e0386. https://doi.org/10.1097/HC9.0000000000000386 [PubMed]

§  Hanai T, Shiraki M, Imai K, Suetugu A, Takai K, Shimizu M (2020) Usefulness of carnitine supplementation for the complications of liver cirrhosis. Nutrients 12(7): 1915. https://doi.org/10.3390/nu12071915 [PubMed]

§  Hattori Y, Takeda T, Fujii M, Taura J, Ishii Y, Yamada H (2014) Dioxin-induced fetal growth retardation: the role of a preceding attenuation in the circulating level of glucocorticoid. Endocrine 47(2): 572–580. https://doi.org/10.1007/S12020-014-0257-3 [PubMed]

§  He J, Hu B, Shi X, Weidert ER, Lu P, Xu M, Huang M, Kelley EE, Xie W (2013) Activation of the aryl hydrocarbon receptor sensitizes mice to nonalcoholic steatohepatitis by deactivating mitochondrial sirtuin deacetylase Sirt3. Molecular and Cellular Biology 33(10): 2047–2055. https://doi.org/10.1128/MCB.01658-12 [PubMed]

§  Hong JH, Lee MK (2021) Carnitine orotate complex ameliorates insulin resistance and hepatic steatosis through carnitine acetyltransferase pathway. Diabetes and Metabolism Journal 45(6): 933–947. https://doi.org/10.4093/dmj.2020.0223 [PubMed] [PMC]

§  Hong S, Gordon D, Stec DE, Hinds TD (2020) Bilirubin: A Ligand of the PPARα Nuclear Receptor. In: Badr MZ (Ed.) Nuclear Receptors: The Art and Science of Modulator Design and Discovery. Springer International Publishing, 463–482. (Cham: 10.1007/978-3-030-78315-0_17)

§  Hoyeck MP, Matteo G, MacFarlane EM, Perera I, Bruin JE (2022) Persistent organic pollutants and β-cell toxicity: a comprehensive review. American Journal of Physiology. Endocrinology and Metabolism. 322(5): E383–E413. https://doi.org/10.1152/ajpendo.00358.2021 [PubMed] [PMC]

§  Hoyeck MP, Angela Ching ME, Basu L, van Allen K, Palaniyandi J, Perera I, Poleo-Giordani E, Hanson AA, Ghorbani P, Fullerton MD, Bruin JE (2024) The aryl hydrocarbon receptor in β-cells mediates the effects of TCDD on glucose homeostasis in mice. Molecular Metabolism 81: 101893. https://doi.org/10.1016/j.molmet.2024.101893 [PubMed] [PMC]

§  Huang CY, Wu CL, Yang YC, Chang JW, Kuo YC, Cheng YY, Wu JS, Lee CC, Guo HR (2015) Association between dioxin and diabetes mellitus in an endemic area of exposure in Taiwan: A population-based study. Medicine 94(42): e1730. https://doi.org/10.1097/MD.0000000000001730 [PubMed] [PMC]

§  Hwang HJ, Dornbos P, Steidemann M, Dunivin TK, Rizzo M, LaPres JJ (2016) Mitochondrial-targeted aryl hydrocarbon receptor and the impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin on cellular respiration and the mitochondrial proteome. Toxicology and Applied Pharmacology 304: 121–132. https://doi.org/10.1016/j.taap.2016.04.005 [PubMed] [PMC]

§  Kawano Y, Cohen DE (2013) Mechanisms of hepatic triglyceride accumulation in non-alcoholic fatty liver disease. Journal of Gastroenterology 48(4): 434–441. https://doi.org/10.1007/s00535-013-0758-5 [PubMed] [PMC]

§  Kulkarni PS, Crespo JG, Afonso CA (2008) Dioxins sources and current remediation technologies: A review. Environment International 34(1): 139–153. https://doi.org/10.1016/j.envint.2007.07.009 [PubMed]

§  Kumar J, Lind L, Salihovic S, van Bavel B, Ingelsson E, Lind PM (2014) Persistent organic pollutants and liver dysfunction biomarkers in a population-based human sample of men and women. Environmental Research 134: 25125–25126. https://doi.org/10.1016/j.envres.2014.07.023 [PubMed]

§  Ipsen DH, Lykkesfeldt J, Tveden-Nyborg P (2018) Molecular mechanisms of hepatic lipid accumulation in non-alcoholic fatty liver disease. Cellular and Molecular Life Sciences 75(18): 3313–3327. https://doi.org/10.1007/s00018-018-2860-6 [PubMed] [PMC]

§  Lee MS, Lee HJ, Lee HS, Kim Y (2006) L-carnitine stimulates lipolysis via induction of the lipolytic gene expression and suppression of the adipogenic gene expression in 3T3-L1 adipocytes. Journal of Medicinal Food 9(4): 468–473. https://doi.org/10.1089/jmf.2006.9.468 [PubMed]

§  Lee S, Lim Y, Kang Y, Jung K, Jee S (2022) The association between blood concentrations of PCDD/DFs, DL-PCBs and the risk of type 2 diabetes mellitus and thyroid cancer in South Korea. International Journal of Environmental Research and Public Health 19(14): 8745. https://doi.org/10.3390/ijerph19148745 [PubMed] [PMC]

§  Li JM, Li LY, Zhang YX, Jiang ZY, Limbu SM, Qiao F (2019) Functional differences between l- and d-carnitine in metabolic regulation evaluated using a low-carnitine Nile tilapia model. British Journal of Nutrition 122(6): 625–638. https://doi.org/10.1017/S000711451900148X [PubMed]

§  Li G, Peng Y, Chen Z, Li H, Liu D, Ye X (2022) Bidirectional association between hypertension and NAFLD: A systematic review and meta-analysis of observational studies. International Journal of Endocrinology 2022: 8463640. https://doi.org/10.1155/2022/8463640 [PubMed] [PMC]

§  Li M, Yang K, De Vivo I, Eliassen AH, Qureshi AA, Nan H, Han J (2023) Association between plasma L-carnitine levels and mitochondrial DNA copy number. BMC Molecular and Cell Biology 24(1): 35. https://doi.org/10.1186/s12860-023-00496-z [PubMed] [PMC]

§  Liang Y, Tang Z, Jiang Y, Ai C, Peng J, Liu Y, Chen J, Zhang J, Cai Z (2020) Serum metabolic changes associated with dioxin exposure in a Chinese male cohort. Environment International 143: 105984. https://doi.org/10.1016/j.envint.2020.105984  [PubMed]

§  Liang Y, Tang Z, Jiang Y, Ai C, Peng J, Liu Y, Chen J, Xin X, Lei B, Zhang J, Cai Z (2021) Lipid metabolism disorders associated with dioxin exposure in a cohort of Chinese male workers revealed by a comprehensive lipidomics study. Environmental International 155: 106665. https://doi.org/10.1016/j.envint.2021.106665 [PubMed]

§  Lu YC, Chang CC, Wang CP, Hung WC, Tsai IT, Tang WH, Wu CC, Wei CT, Chung FM, Lee YJ, Hsu CC (2020) Circulating fatty acid-binding protein 1 (FABP1) and nonalcoholic fatty liver disease in patients with type 2 diabetes mellitus. International Journal of Medical Sciences 17(2): 182–190. https://doi.org/10.7150/ijms.40417 [PubMed] [PMC]

§  Liu A, Cai Y, Yuan Y, Liu M, Zhang Z, Xu Y, Jiao P (2023) Efficacy and safety of carnitine supplementation on NAFLD: a systematic review and meta-analysis. Systematic Reviews 12(1): 74. https://doi.org/10.1186/s13643-023-02238-w [PubMed] [PMC]

§  Lyu J, Okada H, Sunagozaka H, Kawaguchi K, Shimakami T, Nio K, Murai K, Shirasaki T, Yoshida M, Arai K, Yamashita T, Tanaka T, Harada K, Takamura T, Kaneko S, Yamashita T, Honda M (2024) Potential utility of l-carnitine for preventing liver tumors derived from metabolic dysfunction-associated steatohepatitis. Hepatology Communications 8(5): e0425. Https://doi.org/10.1097/HC9.0000000000000425 [PubMed] [PMC]

§  Mahdi C, Ajeng M, Sari M (2019) The effects goat milk yoghurt casein on malondialdehyde (MDA) level of rats (Rattus norvegicus) exposured by 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCCD). IOP Conference Series: Materials Science and Engineering 509: 012026. https://doi.org/10.1088/1757-899X/509/1/012026

§  Martí-Carvajal AJ, Gluud C, Arevalo-Rodriguez I, Martí-Amarista CE (2019) Acetyl-L-carnitine for patients with hepatic encephalopathy. The Cochrane Database of Systematic Reviews 1(1): CD011451. https://doi.org/10.1002/14651858.CD011451.pub2 [PubMed] [PMC]

§  Miquilena-Colina ME, Lima-Cabello E, Sánchez-Campos S, García-Mediavilla MV, Fernández-Bermejo M, Lozano-Rodríguez T, Vargas-Castrillón J, Buqué X, Ochoa B, Aspichueta P, González-Gallego J, García-Monzón C (2011) Hepatic fatty acid translocase CD36 upregulation is associated with insulin resistance, hyperinsulinaemia and increased steatosis in non-alcoholic steatohepatitis and chronic hepatitis C. Gut 60(10): 1394–1402. https://doi.org/10.1136/gut.2010.222844 [PubMed]

§  Mirrafiei A, Jayedi A, Shab-Bidar S (2024) The effects of L-carnitine supplementation on weight loss, glycemic control, and cardiovascular risk factors in patients with Type 2 Diabetes: A systematic review and dose-response meta-analysis of randomized controlled trials. Clinical Therapeutics 46(5): 404–410. https://doi.org/10.1016/j.clinthera.2024.03.002 [PubMed]

§  Miyaaki H, Kobayashi H, Miuma S, Fukusima M, Sasaki R, Haraguchi M, Nakao K (2020) Blood carnitine profiling on tandem mass spectrometry in liver cirrhotic patients. BMC Gastroenterol 20(1): 41. https://doi.org/10.1186/s12876-020-01190-6 [PubMed] [PMC]

§  Modanloo M, Shokrzadeh M (2019) Analyzing mitochondrial dysfunction, oxidative stress, and apoptosis: Potential role of L-carnitine. Iranian Journal of Kidney Diseases 13(2): 74–86. [PubMed]

§  Mollica G, Senesi P, Codella R, Vacante F, Montesano A, Luzi L, Terruzzi I. (2020) L-carnitine supplementation attenuates NAFLD progression and cardiac dysfunction in a mouse model fed with methionine and choline-deficient diet. Digestive and Liver Disease 52(3): 314–323. https://doi.org/10.1016/j.dld.2019.09.002 [PubMed]

§  Montesano A, Senesi P, Vacante F, Mollica G, Benedini S, Mariotti M, Luzi L, Terruzzi I (2020) L-Carnitine counteracts in vitro fructose-induced hepatic steatosis through targeting oxidative stress markers. Journal of Endocrinological Investigation 43(4): 493–503. https://doi.org/10.1007/s40618-019-01134-2 [PubMed] [PMC]

§  Muriel P (2009) Role of free radicals in liver diseases. Hepatology International 3(4): 526–536. https://doi.org/10.1007/s12072-009-9158-6 [PubMed] [PMC]

§  Nault R, Fader KA, Lydic TA, Zacharewski TR (2017) Lipidomic evaluation of aryl hydrocarbon receptor-mediated hepatic steatosis in male and female mice elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin. Chemical Research in Toxicology 30(4): 1060–1075. https://doi.org/10.1021/acs.chemrestox.6b00430 [PubMed] [PMC]

§  Nehra V, Angulo P, Buchman AL, Lindor KD (2001) Nutritional and metabolic considerations in the etiology of nonalcoholic steatohepatitis. Digestive Diseases and Sciences 46(11): 2347–2352. https://doi.org/10.1023/a:1012338828418 [PubMed]

§  Norouzi M, Mesbah-Namin SA, Sharifi Z, Deyhim MR (2024) L-carnitine contributes to enhancement of viability and quality of platelet concentrates through changing the apoptotic and anti-apoptotic associated microRNAs. Transfusion Clinique et Biologique 31(2): 87–94. https://doi.org/10.1016/j.tracli.2024.01.007 [PubMed]

§  Ohashi H, Nishioka K, Nakajima S, Kim S, Suzuki R, Aizaki H, Fukasawa M, Kamisuki S, Sugawara F, Ohtani N, Muramatsu M, Wakita T, Watashi K (2018) The aryl hydrocarbon receptor-cytochrome P450 1A1 pathway controls lipid accumulation and enhances the permissiveness for hepatitis C virus assembly. The Journal of Biological Chemistry 293(51): 19559–19571. Https://doi.org/10.1074/jbc.RA118.005033 [PubMed] [PMC]

§  Olivero-Verbel J, Harkema JR, Roth RA, Ganey PE (2021) Fenofibrate, a peroxisome proliferator-activated receptor-alpha agonist, blocks steatosis and alters the inflammatory response in a mouse model of inflammation-dioxin interaction. Chemico-Biological Interactions 345: 109521. https://doi.org/10.1016/j.cbi.2021.109521 [PubMed]

§  Pang Q, Zhang JY, Song SD, Qu K, Xu XS, Liu SS, Liu C (2015) Central obesity and nonalcoholic fatty liver disease risk after adjusting for body mass index. World Journal of Gastroenterology 21(5): 1650–16622. https://doi.org/10.3748/wjg.v21.i5.1650 [PubMed] [PMC]

§  Papalou O, Kandaraki EA, Papadakis G, Diamanti-Kandarakis E (2019) Endocrine disrupting chemicals: An occult mediator of metabolic disease. Frontiers in Endocrinology 10: 112. https://doi.org/10.3389/fendo.2019.00112 [PubMed]

§  Peng XE, Wu YL, Lu QQ, Hu ZJ, Lin X (2014) MTTP polymorphisms and susceptibility to non-alcoholic fatty liver disease in a Han Chinese population. Liver International 34(1): 118–128. https://doi.org/10.1111/liv.12220 [PubMed]

§  Petriello MC, Brandon JA, Hoffman J, Wang C, Tripathi H, Abdel-Latif A, Ye X, Li X, Yang L, Lee E, Soman S, Barney J, Wahlang B, Hennig B, Morris AJ (2018) Dioxin-like PCB 126 increases systemic inflammation and accelerates atherosclerosis in lean LDL receptor-deficient mice. Toxicological Sciences 162(2): 548–558. https://doi.org/10.1093/toxsci/kfx275 [PubMed]

§  Pham PQ, Nguyen VB, Pham TT, Duong NX, Nguyen HT, Ha QV, Nguyen TD, Hoang TM, Dinh DT, Tran QTN, Bui LK, Vu TT, Phan MV, Luong TM, Nguyen K, Vu DA, Pham TN (2022) Histopathological alterations in the livers of chronic hepatitis patients exposed to agent orange/dioxin in Vietnam. Toxics 10(6): 315. https://doi.org/10.3390/toxics10060315 [PubMed]

§  Pierre S, Chevallier A, Teixeira-Clerc F, Ambolet-Camoit A, Bui LC, Bats AS, Fournet JC, Fernandez-Salguero P, Aggerbeck M, Lotersztajn S, Barouki R, Coumoul X (2014) Aryl hydrocarbon receptor-dependent induction of liver fibrosis by dioxin. Toxicological Sciences 137(1): 114–124. https://doi.org/10.1093/toxsci/kft236 [PubMed]

§  Rebouche CJ (1992) Carnitine function and requirements during the life cycle. FASEB Journal 6(15): 3379–3386. [PubMed]

§  Rinella ME, Lazarus JV, Ratziu V, Francque SM, Sanyal AJ, Kanwal F, Romero D, Abdelmalek MF, Anstee QM, Arab JP, Arrese M, Bataller R, Beuers U, Boursier J, Bugianesi E, Byrne CD, Castro Narro GE, Chowdhury A, Cortez-Pinto H, Cryer DR, Cusi K, El-Kassas M, Klein S, Eskridge W, Fan J, Gawrieh S, Guy CD, Harrison SA, Kim SU, Koot BG, Korenjak M, Kowdley KV, Lacaille F, Loomba R, Mitchell-Thain R, Morgan TR, Powell EE, Roden M, Romero-Gómez M, Silva M, Singh SP, Sookoian SC, Spearman CW, Tiniakos D, Valenti L, Vos MB, Wong VW, Xanthakos S, Yilmaz Y, Younossi Z, Hobbs A, Villota-Rivas M, Newsome PN; NAFLD Nomenclature consensus group (2023) A multisociety Delphi consensus statement on new fatty liver disease nomenclature. Hepatology (Baltimore) 78(6): 1966–1986. https://doi./org/10.1097/HEP.0000000000000520 [PubMed]

§  Romano M, Vacante M, Cristaldi E, Colonna V, Gargante MP, Cammalleri L, Malaguarnera M (2008) L-carnitine treatment reduces steatosis in patients with chronic hepatitis C treated with alpha-interferon and ribavirin. Digestive Diseases and Sciences 53(4): 1114–1121. https://doi.org/10.1007/s10620-007-9983-1 [PubMed]

§  Rong L, Zou J, Ran W, Qi X, Chen Y, Cui H, Guo J (2023) Advancements in the treatment of non-alcoholic fatty liver disease (NAFLD). Frontiers in Endocrinology 13: 1087260. https://doi.org/10.3389/fendo.2022.1087260 [PubMed] [PMC]

§  Rotondo E, Chiarelli F (2020) Endocrine-disrupting chemicals and insulin resistance in children. Biomedicines 8(6): 137. https://doi.org/10.3390/biomedicines8060137 [PubMed] [PMC]

§  Sanyal AJ, Campbell-Sargent C, Mirshahi F, Rizzo WB, Contos MJ, Sterling RK, Luketic VA, Shiffman ML, Clore JN (2001) Nonalcoholic steatohepatitis: association of insulin resistance and mitochondrial abnormalities. Gastroenterology 120(5): 1183–1192. https://doi.org/10.1053/gast.2001.23256 [PubMed]

§  Schecter A, Birnbaum L, Ryan JJ, Constable JD (2006) Dioxins: an overview. Environmental Research 101(3): 419–428. https://doi.org/10.1016/j.envres.2005.12.003 [PubMed]

§  Song SJ, Lai JC, Wong GL, Wong VW, Yip TC (2024) Can we use old NAFLD data under the new MASLD definition? Journal of Hepatology 80(2): e54–e56. https://doi.org/10.1016/j.jhep.2023.07.021 [PubMed]

§  Spengler EK, Loomba R (2015) Recommendations for diagnosis, referral for liver biopsy, and treatment of nonalcoholic fatty liver disease and nonalcoholic steatohepatitis. Mayo Clinic Proceedings 90(9): 1233–1246 https://doi.org/10.1016/j.mayocp.2015.06.013 [PubMed] [PMC]

§  Street ME, Angelini S, Bernasconi S, Burgio E, Cassio A, Catellani C, Cirillo F, Deodati A, Fabbrizi E, Fanos V, Gargano G, Grossi E, Iughetti L, Lazzeroni P, Mantovani A, Migliore L, Palanza P, Panzica G, Papini AM, Parmigiani S, Predieri B, Sartori C, Tridenti G, Amarri S (2018) Current knowledge on endocrine disrupting chemicals (EDCs) from animal biology to humans, from pregnancy to adulthood: highlights from a National Italian Meeting. International Journal of Molecular Sciences 19(6): 1647. https://doi.org/10.3390/ijms19061647 [PubMed] [PMC]

§  Targher G, Byrne CD, Tilg H (2024) MASLD: A systemic metabolic disorder with cardiovascular and malignant complications. Gut 73(4): 691–702. https://doi.org/10.1136/gutjnl-2023-330595 [PubMed]

§  Thomas JA, Kendall BJ, El-Serag HB, Thrift AP, Macdonald GA (2024) Hepatocellular and extrahepatic cancer risk in people with non-alcoholic fatty liver disease. The Lancet Gastroenterology & Hepatology 9(2): 159–169. https://doi.org/10.1016/S2468-1253(23)00275-3 [PubMed]

§  Tong J, Lan XT, Zhang Z, Liu Y, Sun DY, Wang XJ, Ou-Yang SX, Zhuang CL, Shen FM, Wang P, Li DJ (2023) Ferroptosis inhibitor liproxstatin-1 alleviates metabolic dysfunction-associated fatty liver disease in mice: potential involvement of PANoptosis. Acta Pharmacologica Sinica 44(5): 1014–1028. https://doi.org/10.1038/s41401-022-01010-5 [PubMed] [PMC]

§  Torres DM, Williams CD, Harrison SA (2012) Features, diagnosis, and treatment of nonalcoholic fatty liver disease. Clinical Gastroenterology and Hepatology 10(8): 837–858. https://doi.org/10.1016/j.cgh.2012.03.011 [PubMed]

§  Tuomisto J (2019) Dioxins and dioxin-like compounds: toxicity in humans and animals, sources, and behaviour in the environment. Wiki Journal of Medicine 6(1): 8. https://doi.org/10.15347/WJM/2019.008

§  Valkner KJ, Bieber LL (1982) Short-chain acylcarnitines of human blood and urine. Biochemical Medicine 28(2): 197–203. https://doi.org/10.1016/0006-2944(82)90070-9 [PubMed]

§  Vashistha VK, Bhushan R (2015) Bioanalysis and enantioseparation of dl-carnitine in human plasma by the derivatization approach. Bioanalysis 7(19): 2477–2488. https://doi.org/10.4155/bio.15.155 [PubMed]

§  Videla LA, Rodrigo R, Orellana M, Fernandez V, Tapia G, Quiñones L, Varela N, Contreras J, Lazarte R, Csendes A, Rojas J, Maluenda F, Burdiles P, Diaz JC, Smok G, Thielemann L, Poniachik J (2004) Oxidative stress-related parameters in the liver of non-alcoholic fatty liver disease patients. Clinical Science (London) 106(3): 261–268. https://doi.org/10.1042/CS20030285 [PubMed]

§  Vuong TP (2022) Research on the relationship between exposure to dioxins and cancer incidence in Vietnam. Toxics 10(7): 384. https://doi.org/10.3390/toxics10070384 [PMC]

§  Wahlang B, Hardesty JE, Jin J, Falkner KC, Cave MC (2019). Polychlorinated biphenyls and nonalcoholic fatty liver disease. Current Opinion in Toxicology 14: 21–28. https://doi.org/10.1016/j.cotox.2019.06.001 [PubMed] [PMC]

§  Wang C, Liu X, Zhai J, Zhong C, Zeng H, Feng L, Yang Y, Li X, Ma M, Luan T, Deng J (2024) Effect of oxidative stress induced by 2,3,7,8- tetrachlorodibenzo-p-dioxin on DNA damage. Journal of Hazardous Materials 472: 134485. https://doi.org/10.1016/j.jhazmat.2024.134485 [PubMed]

§  Wanless IR, Bargman JM, Oreopoulos DG, Vas SI (1989) Subcapsular steatonecrosis in response to peritoneal insulin delivery: A clue to the pathogenesis of steatonecrosis in obesity. Modern Pathology 2(2): 69–74. [PubMed]

§  Weir E (2005) Dioxin contamination and poisoning. Canadian Medical Association Journal 172(7): 873. https://doi.org/10.1503/cmaj.045283 [PubMed] [PMC]

§  Xu Y, Jiang W, Chen G, Zhu W, Ding W, Ge Z, Tan Y, Ma T, Cui G (2017) L-carnitine treatment of insulin resistance: A systematic review and meta-analysis. Advances in Clinical and Experimental Medicine 26(2): 333–338. https://doi.org/10.17219/acem/61609 [PubMed]

§  Yang JH, Lee HG (2010) 2,3,7,8-Tetrachlorodibenzo-p-dioxin induces apoptosis of articular chondrocytes in culture. Chemosphere 79(3): 278–284. Https://doi.org/10.1016/j.chemosphere.2010.01.040 [PubMed]

§  Yoneda M, Yamamoto T, Honda Y, Imajo K, Ogawa Y, Kessoku T, Kobayashi T, Nogami A, Higurashi T, Kato S, Hosono K, Saito S, Nakajima A (2021) Risk of cardiovascular disease in patients with fatty liver disease as defined from the metabolic dysfunction associated fatty liver disease or nonalcoholic fatty liver disease point of view: A retrospective nationwide claims database study in Japan. Journal of Gastroenterology 56(11): 1022–1032. https://doi.org/10.1007/s00535-021-01828-6 [PubMed] [PMC]

§  Yoshida R, Ogawa Y (2000) Oxidative stress induced by 2, 3, 7, 8-tetrachlorodibenzo-P-dioxin: An application of oxidative stress markers to cancer risk assessment of dioxins. Industrial Health 38(1): 5-14. https://doi.org/10.2486/indhealth.38.5 [PubMed]

§  Younossi Z, Anstee QM, Marietti M, Hardy T, Henry L, Eslam M, George J, Bugianesi E (2018) Global burden of NAFLD and NASH: Trends, predictions, risk factors and prevention. Nature Reviews. Gastroenterology & Hepatology 15(1): 11–20. https://doi.org/10.1038/nrgastro.2017.109 [PubMed]

§  Younossi ZM, Wong VW, Anstee QM, Romero-Gomez M, Trauner MH, Harrison SA, Lawitz EJ, Okanoue T, Camargo M, Kersey K, Myers RP, Goodman Z, Stepanova M (2020) Fatigue and pruritus in patients with advanced fibrosis due to nonalcoholic steatohepatitis: The impact on patient-reported outcomes. Hepatology Communications 4(11): 1637–1350. https://doi.org/10.1002/hep4.1581 [PubMed] [PMC]

§  Zakharova N, Luo C, Aringazina R, Samusenkov V (2023) The efficacy of L-carnitine in patients with nonalcoholic steatohepatitis and concomitant obesity. Lipids in Health and Disease 22(1): 101. https://doi.org/10.1186/s12944-023-01867-3 [PubMed] [PMC]

§  Zidan A, Hedya SE, Elfeky DM, Abdin AA (2018) The possible anti-apoptotic and antioxidant effects of acetyl l-carnitine as an add-on therapy on a relapsing-remitting model of experimental autoimmune encephalomyelitis in rats. Biomedicine & Pharmacotherapy 103: 1302–1311. https://doi.org/:10.1016/j.biopha.20 [PubMed]

Author Contribution

§  Marwan Al-Nimer, Professor of Therapeutics, Department of Clinical Pharmacology and Therapeutics, College of Medicine, University of Diyala, Baqubah, Iraq; e-mail: alnimermarwan@ymail.com; ORCID ID: https://orcid.org/0000-0002-5336-3353. The concept, design, literature review, interpretation, drafting, and revising and final approval of the version.

§  Vian Wasta Esmail, Assistant Professor in Clinical Pharmacy, Department of Clinical Pharmacy, University of Sulaimani, Kurdistan region, Iraq; e-mail: vian.wastaesmail@univsul.edu.iq; ORCID ID: https://orcid.org/0000-0002-5914-8136. Literature review, drafting, revising and final approval of the version.

§  Tavga Aziz, Professor of Toxicology, Department of Pharmacology and Toxicology, College of Pharmacy, University of Sulaimani, Kurdistan region, Iraq; e-mail: tavga.aziz@univsul.edu.iq; ORCID ID: https://orcid.org/0000-0003-2742-6127. Literature review, interpretation. drafting, revising and final approval of the version.

All authors critically revised the manuscript, approved the final version to be published, and agree to be accountable for all aspects of the work.