Therapeutic potential of dithiolethiones for hepatic diseases

Steatotic Liver Disease: Pathophysiology and Emerging Pharmacotherapies

Steatotic liver disease (SLD) displays a dynamic and complex disease phenotype. Consequently, the metabolic dysfunction-associated steatotic liver disease (MASLD)/metabolic dysfunction-associated steatohepatitis (MASH) therapeutic pipeline is expanding rapidly and in multiple directions. In parallel, noninvasive tools for diagnosing and monitoring responses to therapeutic interventions are being studied, and clinically feasible findings are being explored as primary outcomes in interventional trials. The realization that distinct subgroups exist under the umbrella of SLD should guide more precise and personalized treatment recommendations and facilitate advancements in pharmacotherapeutics. This review summarizes recent updates of pathophysiology-based nomenclature and outlines both effective pharmacotherapeutics and those in the pipeline for MASLD/MASH, detailing their mode of action and the current status of phase 2 and 3 clinical trials. Of the extensive arsenal of pharmacotherapeutics in the MASLD/MASH pipeline, several have been rejected, whereas other, mainly monotherapy options, have shown only marginal benefits and are now being tested as part of combination therapies, yet others are still in development as monotherapies. Although the Food and Drug Administration (FDA) has recently approved resmetirom, additional therapeutic approaches in development will ideally target MASH and fibrosis while improving cardiometabolic risk factors. Due to the urgent need for the development of novel therapeutic strategies and the potential availability of safety and tolerability data, repurposing existing and approved drugs is an appealing option. Finally, it is essential to highlight that SLD and, by extension, MASLD should be recognized and approached as a systemic disease affecting multiple organs, with the vigorous implementation of interdisciplinary and coordinated action plans. SIGNIFICANCE STATEMENT: Steatotic liver disease (SLD), including metabolic dysfunction-associated steatotic liver disease and metabolic dysfunction-associated steatohepatitis, is the most prevalent chronic liver condition, affecting more than one-fourth of the global population. This review aims to provide the most recent information regarding SLD pathophysiology, diagnosis, and management according to the latest advancements in the guidelines and clinical trials. Collectively, it is hoped that the information provided furthers the understanding of the current state of SLD with direct clinical implications and stimulates research initiatives.

Inflammatory and fibrotic mechanisms in NAFLD-Implications for new treatment strategies

Non-alcoholic fatty liver disease is comprised of either simple steatosis (non-alcoholic fatty liver) or a more advanced inflammatory and fibrogenic stage (non-alcoholic steatohepatitis [NASH]). NASH affects a growing proportion of the global adult and pediatric population, leading to rising rates of liver fibrosis and hepatocellular carcinoma. NASH is a multifactorial disease that is part of a systemic metabolic disorder. Here, we provide an overview of the metabolic underpinnings of NASH pathogenesis and established drivers of inflammation and fibrosis. Clarification of underlying fibrogenic and inflammatory mechanisms will advance the development of novel treatment strategies as there are no approved therapies at present. We discuss emerging experimental approaches and potential novel investigational strategies derived from animal models including the inflammasome, epigenetic reprogramming, Hippo signaling, Notch signaling, engineered T cells to remove fibrogenic HSCs, and HSC-specific targeting therapies. Recently completed and ongoing clinical trials and antifibrotics are discussed, illuminating the growing expectation that one or more therapies will yield clinical benefit in NASH in the coming years.

Nonalcoholic Fatty Liver Disease: A Drug Revolution Is Coming

The worldwide prevalence of non-alcoholic fatty liver disease is around 25%, and that of nonalcoholic steatohepatitis (NASH) ranges from 1.5% to 6.45%. Patients with NASH, especially those with fibrosis, are at higher risk for adverse outcomes such as cirrhosis and liver-related mortality. Although vitamin E, pioglitazone, and liraglutide improved liver histology in randomized trials, there are currently no Food and Drug Administration-approved drugs for NASH. Five pharmacologic agents-obeticholic acid, elafibranor, cenicriviroc, resmetirom, and aramchol-are being evaluated in large, histology-based phase 3 trials. Within 2 to 4 years, new and effective drugs for the treatment of NASH are expected. Additionally, many phase 2 trials are ongoing for various agents. Based on the results of phase 2 and 3 trials, combination treatments are also being investigated. Future treatment strategies will comprise drug combinations and precision medicine based on the different phenotypes of NASH and treatment response of the individual patient.

Adipokines in Liver Cirrhosis

Liver fibrosis can progress to cirrhosis, which is considered a serious disease. The Child-Pugh score and the model of end-stage liver disease score have been established to assess residual liver function in patients with liver cirrhosis. The development of portal hypertension contributes to ascites, variceal bleeding and further complications in these patients. A transjugular intrahepatic portosystemic shunt (TIPS) is used to lower portal pressure, which represents a major improvement in the treatment of patients. Adipokines are proteins released from adipose tissue and modulate hepatic fibrogenesis. These proteins affect various biological processes that are involved in liver function, including angiogenesis, vasodilation, inflammation and deposition of extracellular matrix proteins. The best studied adipokines are adiponectin and leptin. Adiponectin protects against hepatic inflammation and fibrogenesis, and leptin functions as a profibrogenic factor. These and other adipokines are supposed to modulate disease severity in patients with liver cirrhosis. Consequently, circulating levels of these proteins have been analyzed to identify associations with parameters of hepatic function, portal hypertension and its associated complications in patients with liver cirrhosis. This review article briefly addresses the role of adipokines in hepatitis and liver fibrosis. Here, studies having analyzed these proteins in systemic blood in cirrhotic patients are listed to identify adipokines that are comparably changed in the different cohorts of patients with liver cirrhosis. Some studies measured these proteins in systemic, hepatic and portal vein blood or after TIPS to specify the tissues contributing to circulating levels of these proteins and the effect of portal hypertension, respectively.

Randomised clinical trial: the efficacy and safety of oltipraz, a liver X receptor alpha-inhibitory dithiolethione in patients with non-alcoholic fatty liver disease

BACKGROUND

Oltipraz is a synthetic dithiolethione with an antisteatotic effect by inhibiting the activity of liver X receptor alpha (LXR-α). Recent studies demonstrated the disruptive role of oltipraz on LXR-α-dependent lipogenesis in hepatocytes and a high-fat diet mouse model.

AIM

To evaluate the efficacy and safety of oltipraz for reducing liver fat in subjects with non-alcoholic fatty liver disease (NAFLD).

METHODS

We performed a multicentre, double-blind, placebo-controlled, phase II study. Subjects with a liver fat >20% and hypertransaminasemia were randomised to the three groups: placebo (n = 22), 30 mg of oltipraz (n = 22) or 60 mg of oltipraz (n = 24) twice daily for 24 weeks. Changes in the liver fat from baseline to 24 weeks quantified using magnetic resonance spectroscopy were the primary outcome.

RESULTS

Compared with the placebo group (-3.2 ± 11.1%), absolute changes in the liver fat content increased in a dose-dependent manner: -7.7 ± 7.0% and -13.9 ± 10.7% for the low-dose and high-dose groups (P = 0.13 and P < 0.01). Per cent reduction in the liver fat content was also significantly greater in the high-dose group than in the placebo group (-34.6 ± 29.4% vs. -0.6 ± 62.9%, P = 0.046). Body mass indices (-1.0 ± 0.9% vs. -0.5 ± 1.4%, P = 0.04) significantly decreased in the high-dose group compared to the placebo group. However, absolute changes in insulin resistance, liver enzymes, lipids and cytokines were not significantly different among groups. The incidence of adverse events was comparable among groups.

CONCLUSIONS

Twenty-four-week oltipraz treatment significantly reduced the liver fat content in patients with NAFLD. Clinicaltrials.gov (NCT01373554).

Posttranslational Modifications of Lipid-Activated Nuclear Receptors: Focus on Metabolism

Posttranslational modifications (PTMs) occur to nearly all proteins, are catalyzed by specific enzymes, and are subjected to tight regulation. They have been shown to be a powerful means by which the function of proteins can be modified, resulting in diverse effects. Technological advances such as the increased sensitivity of mass spectrometry-based techniques and availability of mutant animal models have enhanced our understanding of the complexities of their regulation and the effect they have on protein function. However, the role that PTMs have in a pathological context still remains unknown for the most part. PTMs enable the modulation of nuclear receptor function in a rapid and reversible manner in response to varied stimuli, thereby dramatically altering their activity in some cases. This review focuses on acetylation, phosphorylation, SUMOylation, and O-GlcNAcylation, which are the 4 most studied PTMs affecting lipid-regulated nuclear receptor biology, as well as on the implications of such modifications on metabolic pathways under homeostatic and pathological situations. Moreover, we review recent studies on the modulation of PTMs as therapeutic targets for metabolic diseases.

Targeting Hepatic Fibrosis in Autoimmune Hepatitis

Hepatic fibrosis develops or progresses in 25 % of patients with autoimmune hepatitis despite corticosteroid therapy. Current management regimens lack reliable noninvasive methods to assess changes in hepatic fibrosis and interventions that disrupt fibrotic pathways. The goals of this review are to indicate promising noninvasive methods to monitor hepatic fibrosis in autoimmune hepatitis and identify anti-fibrotic interventions that warrant evaluation. Laboratory methods can differentiate cirrhosis from non-cirrhosis, but their accuracy in distinguishing changes in histological stage is uncertain. Radiological methods include transient elastography, acoustic radiation force impulse imaging, and magnetic resonance elastography. Methods based on ultrasonography are comparable in detecting advanced fibrosis and cirrhosis, but their performances may be compromised by hepatic inflammation and obesity. Magnetic resonance elastography has excellent performance parameters for all histological stages in diverse liver diseases, is uninfluenced by inflammatory activity or body habitus, has been superior to other radiological methods in nonalcoholic fatty liver disease, and may emerge as the preferred instrument to evaluate fibrosis in autoimmune hepatitis. Promising anti-fibrotic interventions are site- and organelle-specific agents, especially inhibitors of nicotinamide adenine dinucleotide phosphate oxidases, transforming growth factor beta, inducible nitric oxide synthase, lysyl oxidases, and C-C chemokine receptors types 2 and 5. Autoimmune hepatitis has a pro-fibrotic propensity, and noninvasive radiological methods, especially magnetic resonance elastography, and site- and organelle-specific interventions, especially selective antioxidants and inhibitors of collagen cross-linkage, may emerge to strengthen current management strategies.

Nature and Implications of Oxidative and Nitrosative Stresses in Autoimmune Hepatitis

Oxidative and nitrosative stresses can damage cellular membranes, disrupt mitochondrial function, alter gene expression, promote the apoptosis and necrosis of hepatocytes, and increase fibrosis in diverse acute and chronic liver diseases, including autoimmune hepatitis. The objectives of this review are to describe the mechanisms of oxidative and nitrosative stresses in inflammatory liver disease, indicate the pathogenic implications of these stresses in autoimmune hepatitis, and suggest investigational opportunities to develop interventions that counter them. The principal antioxidant defenses, including glutathione production, the activities of antioxidant enzymes, and the release of the nuclear factor erythroid 2-related factor 2, may be inadequate or suppressed by transforming growth factor beta. The generation of reactive oxygen species can intensify nitrosative stress, and this stress may not be adequately modulated by the thioredoxin-thioredoxin reductase system and induce post-translational modifications of proteins that further disrupt hepatocyte function. The unfolded protein response and autophagy may be unable to restore redox stability, meet metabolic demands, and maintain hepatocyte survival. Emerging interventions with highly selective site- and organelle-specific actions may improve outcomes, and they include inhibitors of nicotinamide adenine dinucleotide phosphate oxidase, nitric oxide synthase, and transforming growth factor beta. Pharmacological manipulation of nuclear transcription factors may favor expression of antioxidant genes, and stimulation of chaperone proteins within the endoplasmic reticulum and modulation of autophagy may prevent hepatic fibrosis and enhance cell survival. These interventions constitute investigational opportunities to improve the management of autoimmune hepatitis.

Dietary walnut reduces hepatic triglyceride content in high-fat-fed mice via modulation of hepatic fatty acid metabolism and adipose tissue inflammation

In this study, we evaluated the protective effects of dietary walnuts on high-fat diet (HFD)-induced fatty liver and studied the underlying mechanisms. Male C57BL/6J mice were fed either a regular rodent chow or HFD (45% energy-derived) with or without walnuts (21.5% energy-derived) for 20weeks. Walnut supplementation did not change HFD-induced increase in body weight or visceral fat mass. However, dietary walnuts significantly decreased the amounts of hepatic triglyceride (TG) observed in HFD-fed mice. The addition of walnuts significantly altered the levels of proteins, involved in the hepatic lipid homeostasis, including AMP-activated protein kinase, fatty acid synthase and peroxisome proliferator-activated receptor-α. Since adipocyte inflammation and apoptosis are reportedly important in regulating hepatic fat accumulation, we also evaluated the protective effects of walnuts on adipose tissue injury. Real-time polymerase chain reaction results revealed that adipose tissues isolated from mice fed the HFD+walnut diets showed significantly decreased levels of macrophage infiltration with suppressed expression of proinflammatory genes compared to those significantly elevated in mice fed HFD alone. These improvements also coincided with reduction of HFD-induced apoptosis of adipocytes by dietary walnuts. However, the supplemented walnuts did not significantly alter HFD-induced peripheral glucose intolerance or insulin resistance despite a trend of improvement. Collectively, these results demonstrate that the protective effects of walnuts against HFD-induced hepatic TG accumulation in mice are mediated, at least partially, by modulating the key proteins in hepatic lipid homeostasis and suppression of the genes related to adipose tissue inflammation and macrophage infiltration as well as prevention of adipocyte apoptosis.

LXR-α antagonist meso-dihydroguaiaretic acid attenuates high-fat diet-induced nonalcoholic fatty liver

Collaborative regulation of liver X receptor (LXR) and sterol regulatory element binding protein (SREBP)-1 are main determinants in hepatic steatosis, as shown in both animal models and human patients. Recent studies indicate that selective intervention of overly functional LXRα in the liver shows promise in treatment of fatty liver disease. In the present study, we evaluated the effects of meso-dihydroguaiaretic acid (MDGA) on LXRα activation and its ability to attenuate fatty liver in mice. MDGA inhibited activation of the LXRα ligand-binding domain by competitively binding to the pocket for agonist T0901317 and decreased the luciferase activity in LXRE-tk-Luc-transfected cells. MDGA significantly attenuated hepatic neutral lipid accumulation in T0901317- and high fat diet (HFD)-induced fatty liver. The effect of MDGA was so potent that treatment with 1mg/kg for 2 weeks completely reversed the lipid accumulation induced by HFD feeding. MDGA reduced the expression of LXRα co-activator protein RIP140 and LXRα target gene products associated with lipogenesis in HFD-fed mice. These results demonstrate that MDGA has the potential to attenuate nonalcoholic steatosis mediated by selective inhibition of LXRα in the liver in mice.

The polyphenol extract from Sechium edule shoots inhibits lipogenesis and stimulates lipolysis via activation of AMPK signals in HepG2 cells

Fatty liver may have implications for metabolic syndrome, such as obesity, hypertension, and diabetes. Therefore, the development of pharmacological or natural agents to reduce fat accumulation in the liver is an important effort. The Sechium edule shoots have already been verified to decrease serum lipids and cholesterol and prevent atherosclerosis. However, how Sechium edule shoots modulate hepatic lipid metabolism is unclear. This study was designed to investigate the effects and mechanisms of polyphenol extracts (SPE) of Sechium edule shoots in reducing lipid accumulation in oleic acid-treated HepG2 cells. We found that water extracts (SWE) of Sechium edule shoots could decrease serum and hepatic lipid contents (e.g., triacylglycerol and cholesterol). Furthermore, SWE and SPE through the AMPK (AMP-activating protein kinase) signaling pathway could decrease lipogenic relative enzymes, such as FAS (fatty acid synthase), HMGCoR (HMG-CoA reductase), and SREBPs (sterol regulatory element binding proteins), and increase the expression of CPT-I (carnitine palmitoyltransferase I) and PPARα (peroxisome proliferators activated receptor α), which are critical regulators of hepatic lipid metabolism. These observations suggested that Sechium edule shoots have potential for developing health foods for preventing and remedying fatty liver.

Emerging drugs for non-alcoholic steatohepatitis

INTRODUCTION

The prevalence of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) is increasing along with the worldwide epidemic of obesity and their strong association with metabolic syndrome. Currently existing pharmacological therapies include anti-oxidants, insulin-sensitizing agents, lipid-lowering drugs and cytoprotective agents, but there is a lack of consensus regarding the most effective and appropriate pharmacologic therapies for NASH. Clinical trials examining new therapeutic drugs for NASH that act via various mechanisms are being performed in several countries, and these drugs may strongly influence current NASH treatment.

AREAS COVERED

This article provides a review of recent data on the safety and efficacy of existing and emerging agents for the treatment of NASH.

EXPERT OPINION

Ideally, treatment for NASH should not only improve liver disease, but also reduce the risks of adverse cardiovascular outcomes and the development of diabetes and cancers. However, this goal is likely to be too high in the context of clinical trials designed to obtain approval for the treatment of liver disease. The only way to achieve the goal is to accumulate the results of these relatively short-term clinical trials.

Nrf2 activators attenuate the progression of nonalcoholic steatohepatitis-related fibrosis in a dietary rat model

Oxidative stress is considered to be a key mechanism of hepatocellular injury and disease progression in patients with nonalcoholic steatohepatitis (NASH). The transcription factor Nrf2 (nuclear factor-erythroid-2-related factor 2) plays a central role in stimulating expression of various antioxidant-associated genes in the cellular defense against oxidative stress. As the cytosolic repressor kelch-like ECH-associated protein 1 (Keap1) negatively regulates Nrf2, activation of Nrf2 facilitated by its release from Keap1 may represent a promising strategy in the treatment of NASH. To test this hypothesis, we used two chemically distinct types of Nrf2 activator. One is the thiol-reactive agent oltipraz (OPZ), a typical Nrf2 activator, and the other is a novel biaryl urea compound, termed NK-252 (1-(5-(furan-2-yl)-1,3,4-oxadiazol-2-yl)-3-(pyridin-2-ylmethyl)urea). NK-252 exhibits a greater Nrf2-activating potential than OPZ. Furthermore, in vitro binding studies revealed that NK-252 interacts with the domain containing the Nrf2-binding site of Keap1, whereas OPZ does not. This finding indicates that NK-252 is more potent than OPZ due to its unique mechanism of action. For in vivo animal model studies, we used rats on a choline-deficient L-amino acid-defined (CDAA) diet, which demonstrate pathologic findings similar to those seen in human NASH. The administration of OPZ or NK-252 significantly attenuated the progression of histologic abnormalities in rats on a CDAA diet, especially hepatic fibrosis. In conclusion, by using Nrf2 activators with independent mechanisms of action, we show that, in a rat model of NASH, the activation of Nrf2 is responsible for the antifibrotic effects of these drugs. This strategy of Nrf2 activation presents new opportunities for treatment of NASH patients with hepatic fibrosis.

AhR expression is increased in hepatocellular carcinoma

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor implicated in multiple cellular processes and its expression has been shown to play a critical role in tumorigenesis. However, the role of AhR in tumorigenesis of hepatocellular carcinoma remains unclear. In the current study, we investigated the role of AhR in hepatocellular carcinoma tumorigenesis and progression by (a) measuring the expression levels of AhR in liver lesions and (b) assessing the correlation between AhR expression and clinicopathologic parameters. The tissue microarray used in this study contained hepatocellular carcinoma tissues (n = 94), cancer adjacent normal hepatic tissues (n = 5) and normal hepatic tissues (n = 5), which were immunohistochemically assessed for AhR expression. Significantly stronger AhR staining was observed for hepatocellular carcinoma tissues than for cancer adjacent normal hepatic tissues (P = 0.003) and normal hepatic tissues (P = 0.004). In addition, AhR expression was associated with T stage (P = 0.03). The results from this study suggest that an increase in AhR expression is associated with hepatocellular carcinoma progression and may have a potential role in the treatment of hepatocellular carcinoma.

Rutin inhibits oleic acid induced lipid accumulation via reducing lipogenesis and oxidative stress in hepatocarcinoma cells

Excessive lipid accumulation within liver has been proposed to cause obesity, hyperlipidemia, diabetes, and fatty liver disease. Rutin, a common dietary flavonoid that is consumed in fruits, vegetables, and plant-derived beverages, has various biological functions, including antioxidant, anti-inflammatory, and anticancer effects. However, a hypolipidemic effect of rutin on fatty liver disease has not been reported. In this study, we examined the effect of rutin on reducing lipid accumulation in hepatic cells. Hepatocytes were treated with oleic acid (OA) containing with or without rutin to observe the lipid accumulation by Nile red stain. The result showed rutin suppressed OA-induced lipid accumulation and increased adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) activity in hepatocytes. The expression of critical molecule involved in lipid synthesis, sterol regulatory element binding proteins-1 (SREBP-1), was attenuated in rutin-treated cells. Moreover, long-term incubation of rutin inhibited the transcriptions of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGCR), glycerol-3-phosphate acyltransferase (GPAT), fatty acid synthase (FAS), and acetyl-coenzyme carboxylase (ACC). Besides, we also found out the antioxidative effect of rutin by increasing the expression of peroxisome proliferator-activated receptor (PPAR)-α and antioxidative enzymes. Taken together, our findings suggest rutin could attenuate lipid accumulation by decreasing lipogenesis and oxidative stress in hepatocyte.

Induction of human fetal hemoglobin via the NRF2 antioxidant response signaling pathway

Although hematopoietic stem cell transplantation and gene therapy have the potential to cure β-thalassemia and sickle cell disease, they are not currently available to most people with these diseases. In the near term, pharmacologic induction of fetal hemoglobin (HbF) may offer the best possibility for safe, effective, and widely available therapy. In an effort to define new pathways for targeted drug development for HbF induction, we evaluated the nuclear factor erythroid 2-related factor 2 (NRF2) antioxidant response element signaling pathway. We found that 3 well-known activators of this pathway increased γ-globin mRNA at nontoxic doses in K562 cells. Tert-butylhydroquinone (tBHQ), the most active of these compounds, increased cellular levels and nuclear translocation of NRF2 and binding of NRF2 to the γ-globin promoter. siRNA knockdown of NRF2 inhibited γ-globin induction by tBHQ. When tested in human primary erythroid cells, tBHQ induced NRF2 binding to the γ-globin promoter, increased γ-globin mRNA and HbF, and suppressed β-globin mRNA and HbA, resulting in a > 3-fold increase in the percentage of HbF. These results suggest that drugs that activate the NRF2/antioxidant response element signaling pathway have the potential to induce therapeutic levels of HbF in people with β-hemoglobinopathies.

Oltipraz upregulates the nuclear factor (erythroid-derived 2)-like 2 [corrected](NRF2) antioxidant system and prevents insulin resistance and obesity induced by a high-fat diet in C57BL/6J mice

AIMS/HYPOTHESIS

We investigated whether oltipraz, a nuclear respiratory factor 2 alpha subunit (NRF2) activator, improves insulin sensitivity and prevents the development of obesity in mice.

METHODS

C57BL/6J mice were fed with a low-fat diet (10% of energy as fat), a high-fat diet (HFD) (45% of energy as fat) or a HFD with oltipraz for 28 weeks. The effects of oltipraz on body weight, fat content, glucose disposal, insulin signalling, metabolic profiles and endogenous NRF2 functional status in the three groups of mice were investigated.

RESULTS

Oltipraz prevented or significantly attenuated the effect of HFD on glucose disposal, body weight and fat gain. Impairment of protein kinase B/Akt phosphorylation in this HFD-fed mouse model in response to intraperitoneal insulin injection was observed in adipose tissue, but not in the muscles, accompanied by inhibition of AMP-activated protein kinase signalling and activation of p70S6 kinase, as well as reduced GLUT4 content. These defects were attenuated by oltipraz administration. Nuclear content of NRF2 in adipose tissue was reduced by HFD feeding, associated with increased Keap1 mRNA expression and reduced production of haem oxygenase-1 and superoxide dismutase, increased protein oxidation, decreased plasma reduced:oxidised glutathione ratio and the appearance of macrophage marker F4/80. These defects were also restored by oltipraz. Finally, oltipraz attenuated HFD-induced inducible nitric oxide synthase overproduction.

CONCLUSIONS/INTERPRETATION

Impairment of the endogenous redox system is important in the development of obesity and insulin resistance in chronic HFD feeding. NRF2 activation represents a potential novel approach in the treatment and prevention of obesity and diabetes.