The TGF-beta family's role in regulating the crosstalk and repair between liver and skeletal muscle

Benjamin Craig Yaden, Purdue University

Abstract

Liver and muscle share many commonalties in regards to their ability to regulate metabolism and to rapidly regenerate following injury. Recent reports support the notion that these organs may communicate amongst each other during times of stress and disease. This theory has already been exemplified in certain liver diseases such as cirrhosis, where patients exhibit a form of accelerated skeletal muscle mass atrophy known as sarcopenia. The transforming growth factor beta (TGFβ) superfamily has also been shown to have profound effects on liver and skeletal muscle especially in periods of growth, injury, remodeling and even regulating metabolism. Studies presented in this thesis demonstrate that liver and skeletal muscle are highly sensitive to signaling from this family, specifically Activin A, Activin B and growth differentiation factor 8 (Gdf8), where they have been shown to play a key role in the tissue repair. During skeletal muscle injury, Activin A exhibited a transient local increase in protein expression and its selective inhibition affects the timing of tissue remodeling by modifying the inflammatory milieu and activating muscle precursor (Pax7/Myod1-positive) cells, leading to accelerated muscle repair. Through the use of a re-engineered form of Follistatin, additional efficacy was produced as a result of inhibiting multiple TGFβ members, such as Gdf8 and Activins A and B. These same members were found to promote a fibrogenic response in hepatic stellate LX-2 cells, which serve as an in vitro model to measure fibrosis of the liver, similar to the proven pro-fibrotic effects of TGFβ 1 and 2. Synonymous to the efficacy data produced in skeletal muscle injury was also revealed in liver injury upon inhibition of Activin A. Inhibition of Activin A in both acute and chronic carbon tetrachloride (CCl4)-induced liver injury ameliorated fibrosis. Likewise, treatment with a non-selective inhibitor (similar to Follistatin), which was generated by fusing a protein consisting of the extracellular ligand-binding domain of Activin type IIB receptor with the Fc portion of mouse immunoglobulin G (ActRIIB-Fc), was also able to reduce the onset of liver fibrosis in both settings. Lastly, we investigated the consequences of both acute and chronic liver injury on skeletal muscle to delineate the interplay between liver and skeletal muscle. We demonstrated that CCl4 induced acute and chronic liver injury significantly decreased muscle mass. As stated before, inhibition of Activin A antibody ameliorated liver fibrosis, however was unable protect against skeletal muscle atrophy. Treatment with ActRIIB-Fc thwarted the liver fibrosis and attenuated the concomitant muscle atrophy. Through inhibition of multiple TGFβ members, including Activin A, B and Gdf8, ActRIIB-Fc was able to accomplish protection by similar means employed by the Follistatin variant. To further evaluate the putative interplay, acute liver injury was assessed to examine the etiology of this relationship. Proximate to liver injury, markers associated with the early phases of myogenesis such as Pax7, Myod1 and Lif were found to be down regulated. In concurrence with this, circulating levels of Activin B and Gdf8 were also found to be significantly increased early in liver injury, potentially contributing to the systemic atrophy. Finally, based on these data we tested the impact of liver injury on skeletal muscle repair. We found that liver injury markedly impaired skeletal muscle regeneration following cardiotoxin injury while treatment with ActRIIB-Fc significantly improved muscle recovery. In conclusion, these data identify a common subset of TGFβ family members modulating both liver and skeletal muscle injury and define a cross-talk between liver and muscle that is mediated by the TGFβ family.

Degree

Ph.D.

Advisors

Dai, Purdue University.

Subject Area

Biology|Pharmacology

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