Preliminary Study On Epigenetic Mechanisms In Liver Injury

Chronic inflammation, inflicted by the spillover of pro-inflammatory mediators, links metabolic dysfunction to nonalcoholic steatohepatitis(NASH). The epigenetic maneuverings that underscore accelerated synthesis of pro-inflammatory mediators in response to nutritional inputs are not clearly defined. Here we report that the ATP-dependent chromatin remodeling proteins Brahma-related gene 1(Brg1) and Brahma(Brm) were up-regulated in vitro in cultured hepatocytes treated with free fatty acid or glucose and in vivo in animal models of NASH. Occupancy of Brg1 and Brm on the promoter regions of pro-inflammatory genes was increased in vitro in cells and ex vivo in liver tissues. Estradiol suppressed the induction and recruitment of Brg1/Brm by palmitate. Recruitment of Brg1 and Brm relied on nuclear factor kappa B/p65; reciprocally, Brg1 and Brm contributed to the stabilization of p65 binding. Importantly, overexpression of Brg1/Brm enhanced, whereas knockdown of Brg1/Brm attenuated, the induction of pro-inflammatory mediators in hepatocytes challenged with excessive nutrient. Mechanistically, Brg1 and Brm were involved in the maintenance of a chromatin microenvironment marked by active histone modifications and friendly to the access of the general transcriptional machinery. Finally, depletion of Brg1/Brm by short hairpin RNA attenuated the release of pro-inflammatory mediators in the liver and significantly ameliorated hepatic pathology in NASH mice. Conclusion: Our data illustrate a Brg1-dependent pathway that connects the epigenetic regulation of pro-inflammatory genes to the pathogenesis of NASH and point to a potential druggable target in the therapeutic intervention of NASH.Background & Aims: Activation of hepatic stellate cells(HSCs) represents a key process in liver injury and, in the absence of intervention, leads to irreversible cirrhosis contributing significantly to the mortality of patients with liver disease. A missing link in the current understanding of HSC activation is the involvement of the epigenetic machinery. We investigated the role of the myocardin related transcription factor A(MRTF-A) in HSC activation.Methods: Liver fibrosis was induced in wild type(WT) and MRTF-A deficient(KO) mice by CCl4 injection. Expression of m RNA and protein was measured by real-time PCR, Western blotting, and immunohistochemistry. Protein binding to DNA was assayed by chromatin immunoprecipitation(Ch IP). Knockdown of endogenous proteins was mediated by either small interfering RNA(si RNA) or short hairpin RNA(sh RNA), carried by lentiviral particles.Results: KO mice exhibited resistance to CCl4-induced liver fibrosis compared to WT littermates. The expression of activated HSC signature genes was suppressed in the absence of MRTF-A. Ch IP assays revealed that MRTF-A deficiency led to the erasure of key histone modifications, associated with transcriptional activation, such as H3K4 di- and tri-methylation, on the promoter regions of fibrogenic genes. Mechanistically, MRTF-A recruited a histone methyltransferase complex(COMPASS) to the promoters of fibrogenic genes to activate transcription. Silencing of individual COMPASS components dampened transactivation of fibrogenic genes in vitro andblocked liver fibrosis in mice. Oestradiol suppressed HSC activation by dampening the expression and binding activity of COMPASS.Conclusions: Our data illustrate a novel mechanism that connects MRTF-A dependent histone H3K4 methylation to HSC activation.