Kyle S. McCommis, Ph.D.
Washington University School of Medicine, St. Louis, MO

The obesity epidemic has resulted in a dramatic escalation in the number of individuals with hepatic fat accumulation or steatosis. When not combined with excessive alcohol consumption, the broad term for this spectrum of disease is referred to as non-alcoholic fatty liver disease (NAFLD). A significant proportion of individuals with simple steatosis will progress to the severe form of the disease known as non-alcoholic steatohepatitis (NASH), involving hepatocyte damage, inflammation, and fibrosis. If left untreated, NASH can lead to more severe forms of liver disease such as cirrhosis, hepatocellular carcinoma, liver failure, and eventually necessitate liver transplantation. Due to this large clinical burden, research efforts have greatly expanded to better understand NAFLD pathogenesis and the mechanisms underlying the transition to NASH. Likewise, there are currently no approved agents for treating NAFLD/NASH, thus efforts to identify therapeutic targets and progress drug development have intensified in recent years.

Metabolic syndrome and insulin resistance are the most significant risk factors for the development of NAFLD and NASH. Indeed, 60-75% of type 2 diabetic subjects have NAFLD.^1,2 Many aspects of the pathophysiology of insulin resistance are directly causal to NAFLD development (Figure 1). Insulin-resistant adipose tissue is unable to appropriately inhibit lipolysis, and therefore, plasma free fatty acid delivery to the liver is increased. In exquisite labeling experiments, plasma free fatty acids were shown to comprise the source of ~60% of liver triglyceride (TAG) in NAFLD subjects.³ Inadequate insulin signaling also results in increased insulin secretion from pancreatic β-cells and hyperinsulinemia, which can stimulate hepatic de novo lipogenesis.⁴ Whether derived from free fatty acids or de novo lipogenesis, these lipids are largely converted to acyl-CoAs by acyl-CoA synthetases, complexed to glycerol-3-phosphate to create diacylglyceride, and lastly, a third acyl-CoA is added by diacylglycerol acyltransferase (DGAT) to form TAG. This TAG, as well as other lipids and sterols, are then stored inside lipid droplets. These lipid-bound organelles are thought to safely store and control the hydrolysis of these lipids. While somewhat controversial, accumulation of other lipid intermediate species such as diacylglycerols, ceramides, and acyl-CoAs in the liver has been linked to worsening insulin resistance. Other potential mechanisms for fat accumulation in hepatocytes include decreased fat oxidation and decreased fat secretion in the form of very low-density lipoprotein (VLDL) particles. Defects in fat secretion do not appear to be a driver of hepatic steatosis, as NAFLD subjects display greater VLDL secretion both basally and after “suppression” by insulin.⁵ Fatty acid β-oxidation is decreased in animal models and humans with NAFLD/NASH.^6-8

Figure 1. Insulin resistance drives hepatic steatosis.

PPARs as Drug Targets

Because insulin resistance is a direct driver of hepatic lipid accumulation and NASH pathology, the insulin sensitizing thiazolidinedione compounds (TZDs) were one of the earliest classes of drugs to be tested in NASH. Rosiglitazone and pioglitazone have both been used in animal models and human trials of NASH and improve almost all aspects of NASH pathology.^9-15 These TZD compounds are agonists of the nuclear transcription factor peroxisome proliferator-activated receptor γ (PPARγ).¹⁶ PPARγ regulates a gene expression program for adipocyte differentiation and fatty acid storage. While lipid “sequestration” into adipose tissue could be protective against hyperlipidemia, this PPARγ activation with TZDs is associated with a number of side effects such as weight gain, edema, and bone mineral density loss and fracture risk,¹⁷ which have greatly diminished the clinical use of TZDs. Interestingly, pioglitazone is a much weaker agonist of PPARγ compared to rosiglitazone,¹⁶ but provides superior improvements in NASH histology, particularly with regards to fibrosis improvement.¹⁸ These results have opened the door to the possibility that TZDs could have additional target(s) responsible for their pharmacology. Indeed, it was recently observed that TZDs can bind and inhibit the mitochondrial pyruvate carrier (MPC), which regulates pyruvate transport across the inner mitochondrial membrane into the mitochondrial matrix.^19-21 PPARγ-sparing TZD compounds that retain full ability to bind the MPC (MSDC-0602K and MSDC-0160) have been developed.^19,22 MSDC-0602K was recently shown to improve NASH pathology in a mouse model²³ and is currently in a Phase 2B clinical trial for NASH. Inhibiting the MPC may improve NASH pathology by improving insulin sensitivity, enhancing fatty acid oxidation, attenuating the upregulated flux of carbon into the tricarboxylic acid (TCA) cycle,^6,24-26 and reducing hepatic stellate cell activation.²³

Non-TZD compounds have also been developed, which agonize other PPAR isoforms such as PPARα and PPARδ, with or without PPARγ activation. In theory, these compounds may combine the insulin-sensitizing effects of targeting PPARγ with the increased fat oxidation effects of PPARα and the anti-inflammatory and fatty acid oxidation effects of PPARδ. Saroglitazar (PPARα/δ agonist) and IVA337/lanifibranor (pan-PPAR agonist) are currently in Phase 2, while elafibranor (PPARα/δ agonist) is in Phase 3 trials for NASH.

Non-PPAR Drug Targets

A number of compounds for other targets that improve insulin sensitivity are currently being developed for NASH. Agonists of the farnesoid X receptor (FXR) have been observed to improve insulin sensitivity and provide anti-inflammatory and antifibrotic effects in diabetic patients with NASH.²⁷ Obeticholic acid is a synthetic bile acid analog, which is the predominant compound for FXR agonism. However, a trial for obeticholic acid in NASH was terminated before completion due to low treatment efficacy and side effects such as pruritis and modest LDL cholesterol elevations.²⁸ FXR agonism for NASH will likely depend on whether newer synthetic agonists lacking the bile acid structure display benefits without similar side effects. Synthetic derivatives of fibroblast growth factors FGF19 and FGF21 can also regulate hepatic FXR and PPARγ coactivator 1 (PGC-1) activity and lead to improved insulin sensitivity and improvements in NAFLD/NASH²⁹ but require larger, longer term trials. These pharmacologic agents that likely have hepatocyte-specific actions in addition to their overall improved insulin sensitivity are depicted in Figure 2. The glucagon-like peptide-1 (GLP-1) receptor agonist liraglutide predominantly promotes insulin secretion from pancreatic β-cells but can also improve insulin sensitivity.³⁰ GLP-1 receptor agonists provide additional metabolic effects, such as weight loss, that could be beneficial in NASH. However, these require further clinical testing. Inhibitors of DGAT-1 or DGAT-2 could potentially improve NASH by preventing TAG formation and forcing enhanced fatty acid oxidation. Indeed, pharmacologic or genetic inhibition of DGAT-1 or -2 in animal models has been shown to improve hepatic steatosis and fibrosis, as well as hepatic and global insulin sensitivity.^31-35 Intriguingly, DGAT-2 inhibition was reported to improve steatosis but exacerbate hepatic injury in the methionine-choline deficient mouse model of NASH.³⁶ Nevertheless, reducing DGAT expression or activity is still actively being explored to treat NASH, with both small molecule inhibitors and antisense oligonucleotides in clinical development. Lastly, analogs of the small secreted peptide encoded in the mitochondrial genome known as MOTS-c have been developed, which produce effects similar to insulin action, namely inhibition of adipose tissue lipolysis. These MOTS-c analogs (CB4209 and CB4211) improved NASH histology in a mouse model³⁷ and will likely progress to clinical trials in the near future.

Figure 2. Hepatic targets of insulin-sensitizing therapies in development for NASH.

Conclusions and Future Outlook

As insulin resistance and derangements in lipid metabolism are so intricately linked to the development of NAFLD, it is likely that targeting insulin sensitivity will provide the broadest spectrum of pharmacology for treating this disease. While research efforts and compound development are active for downstream pathology such as cell stress/death, inflammation, or antifibrotic compounds, it is hard to envision successful therapeutics that do not also address the core mechanisms of hepatic lipid accumulation. However, the presence and degree of hepatic fibrosis is the number one predictor of adverse outcomes in NASH,³⁸ and therefore treatment strategies will need to prevent or reverse fibrosis. Deciphering how altered metabolism results in hepatocellular stress and death, inflammation, and increased fibrogenesis requires greater research effort. Better understanding this transition from steatosis to NASH, as well as how therapeutics reverse this pathology, remain the ultimate goals for this area of research.

References

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About the Author

Kyle S. McCommis, Ph.D.

Dr. McCommis is a research scientist who integrates molecular biology and physiology to study metabolic disease. In addition to better understanding disease pathophysiology, another aspect of his lab has been to test the efficacy and investigate mechanisms of novel therapeutics. His research training began in cardiovascular physiology, and his projects in the lab still involve mitochondrial metabolism in heart failure. Since joining Washington University in 2013, Kyle has also been focused on hepatic mitochondrial metabolism in relation to both the pathology and treatment of diabetes and fatty liver disease. He is currently an Assistant Professor of Medicine and will be opening a lab in the Saint Louis University Department of Biochemistry and Molecular Biology in May 2019.