MRTF-A Mediated TGF-β Signaling Pathway To Regulate The Epigenetic Mechanism Of Liver Fibrosis

Background&Aims:Fibrosis following injury is a common adaptive response in the liver,which can lead to irreparable and life-threatening cirrhosis and hepatocellular carcinoma without effectual intervention.Transforming growth factor(TGF-β)induced activation of portal fibroblast cells(PFs)or hepatic stellate cells(HSCs)serves as a primary cause for liver fibrosis following different types of injury.HSCs are involved in most types of liver fibrosis while PFs are considered to play a more dominant role in cholestatic injury induced liver fibrosis.The molecular mechanisms underlying fibrogenic response in the liver remain poorly understood.We studied the role of a transcriptional modulator,myocardin related transcription factor A(MRTF-A),in this process.Methods:Liver fibrosis was induced in wild type(WT)and MRTF-A deficient(KO)mice by bile duct ligation(BDL)or thioacetamide(TAA)injection.Expression of mRNA and protein was measured by real-time PCR,Western blotting,and immunohistochemistry.Protein binding to DNA was assayed by chromatin immunoprecipitation(ChIP).Knockdown of endogenous proteins was mediated by either small interfering RNA(siRNA)or short hairpin RNA(shRNA),carried by lentiviral particles.Results:MRTF-A deficiency ameliorated BDL and TAA-induced liver fibrosis in mice.Depletion of MRTF-A alleviated pro-fibrogenic transcription induced by TGF-β in HSCs and PFs.TGF-β stimulated the binding of MRTF-A on the promoters of pro-fibrogenic genes and promoted the interaction between MRTF-A and SMAD3.While SMAD3 was necessary for MRTF-A occupancy on the gene promoters,MRTF-A depletion impaired SMAD3 binding reciprocally.MRTF-A silencing in HSCs impacted the chromatin structure by reducing the deposition of methylated histone H3K4 on the promoters of pro-fibrogenic genes.Further analyses revealed that MRTF-A interacted with and recruited a histone methyltransferase complex(COMPASS),including ASH2,WDR5,and SET1,to the promoters of pro-fibrogenic genes in response to TGF-β treatment.Over-expression of COMPASS components enhanced the transactivation of pro-fibrogenic gene promoters by TGF-β in an MRTF-A-dependent manner.Knockdown of individual members of this complex significantly weakened the binding of SMAD3 and down-regulated the activation of portal fibroblast cells.Conclusion:We have identified an epigenetic pathway that dictates TGF-β induced pro-fibrogenic transcription in PFs and HSCs thereby providing novel insights for the development of therapeutic solutions to treat liver fibrosis.