TGF-beta signaling in an in vivo model of NASH

Abstract

A burgeoning area of focus within liver disease research is centered on the concomitant muscle atrophy present in end stage liver disease patients which shows a correlation to severity of hepatic fibrosis and transplant survival outcomes. Of particular interest, nonalcoholic steatohepatitis (NASH) is a form of liver disease that is characterized as the hepatic manifestation of metabolic syndrome. If left untreated, the disease can progress to the state of cirrhosis and hepatocellular carcinoma requiring transplant. Concordant with increasing global prevalence of obesity, NASH is projected to become the leading cause for liver transplants by 2020. Due to a lack of therapeutic options, these patients represent a large unmet medical need in the western world. A major hurdle to therapeutic research is the lack of a quick, reproducible, and cost effective in vivo model that recapitulates the plethora of pathologies and their molecular underpinnings manifested by this disorder. Our studies attempted to validate and expand upon a two-hit model of NASH, which incorporated both the integral comorbidities associated with metabolic challenges of obesity along with liver injury. The two-hit model manifests not only the hepatic morphohistological characteristics of the disease, but also incorporates the obligatory muscle atrophy. To further elaborate on the potential direct link between liver and skeletal muscle and remove any confounding issues associated with the model, in vitro administration of hepatotoxins representing various pathologies associated with liver disease, were used to recapitulate the liver-muscle endocrine signaling that exists in vivo. Our data shows that a variety of hepatoxins can elicit hepatocellular damage which releases factors that inhibits myotube size in vitro. The two hit model also preserves many of conserved molecular underpinnings observed in clinical hepatic fibrosis. Of particular interest, the TGFβ superfamily has been demonstrated to play an important regulatory role in the progression of fibrosis in NASH patients. TGFβ, Activin A, and Follistatin are members of the highly conserved family that are increased in NASH patients. Furthermore, these proteins have a well-studied role in muscle health, regeneration, and mass that has been hypothesized to be conserved between liver and muscle tissues. Surprisingly, novel expression of the myokine and negative regulator of muscle mass Gdf8 (myostatin) was increased in our in vivo model as well. Our studies focused on the molecular interactions of these TGFβ superfamily members and their role on liver disease progression. Through specific inhibition of these proteins (Activin A and Gdf8), we demonstrated that they appear to play key individual roles in the progression of the concomitant muscle atrophy observed in NASH patients. Interestingly, superior efficacy was gained with the treatment of a pan inhibitor of these proteins (Activin A, B, Gdf8 etc.) via a soluble decoy receptor (ActRIIB-Fc), suggesting an additional unaccounted for ligand. Activin B, was found to be increased in two separate in vivo models of liver fibrosis (two-hit model and BDL), has been implicated in regulating muscle mass. Our data suggest a pivotal role for several members of the TGFβ superfamily in NASH associated muscle atrophy. Therapies designed to treat liver fibrosis and the resultant decrements in muscle mass and force must account for these agents which will require pan inhibition of TGFβ superfamily ligands that signal through the ActRIIB receptor.