Histone methyltransferase ASH1 orchestrates fibrogenic gene transcription during myofibroblast transdifferentiation

Abstract

Transdifferentiation of hepatic stellate cells (HSCs) to a myofibroblast-like phenotype is the pivotal event in liver fibrosis. The dramatic change in phenotype associated with transdifferentiation is underpinned by a global change in gene expression. Orchestrated changes in gene expression take place at the level of chromatin packaging which is regulated by enzymatic activity of epigenetic regulators that in turn affect histone modifications. Using expression profiling of epigenetic regulators in quiescent and activated primary HSCs we found a number of histone methyltranferases including MLL1, MLL5, Set1 and ASH1 to be highly up-regulated during transdifferentiation of HSCs. All of these histone methyltranferases regulate methylation of lysine 4 of histone H3, which is a signature of actively transcribed genes. We therefore postulated that one or more of these enzymes may be involved in positively influencing expression of profibrogenic genes. Conclusion: We find that ASH1 directly binds to the regulatory regions of alpha smooth muscle actin (αSMA), collagen I, tissue inhibitor of metalloproteinase-1 (TIMP1) and transforming growth factor beta1 (TGFβ1) in activated HSCs while depletion of ASH1 caused broad suppression of fibrogenic gene expression. We also discovered that MeCP2 positively regulates ASH1 expression and therefore identify ASH1 as a key transcriptional activator component of the MeCP2 epigenetic relay pathway that orchestrates coordinated induction of multiple profibrogenic genes. (Hepatology 2012;56:1129–1139)

Epigenetic Modifications in Hepatic Stellate Cells Contribute to Liver Fibrosis

“…The suppression of PPAR-γ is demonstrated to be a fatal action which is mainly regulated by epigenetic modifications. Second, methyl-CpG binding protein 2 (MeCP2), which participates in the regulation of PPAR-γ, can also be recruited to promoters in other master genes (such as inhibitor of nuclear factor-κB protein-α gene) and important modulating genes (such as the histone named H3K27 and methyltransferases named ASH1) (Tao et al 2011;Perugorria et al 2012). The epigenetic modulation dominated by PPAR-γ and MeCP2 mediates HSC differentiation in the following pathways.…”

“…A recent research tested the alternation of three H3K4 methyltransferases: MLL5, Set1 and ASH1 (Perugorria et al 2012). All of them were prominently up-regulated during myogenic HSC differentiation, and ASH1 was interestingly found to be positively regulated by MeCP2.…”

“…In the following pathway, ASH1 promoted the methylation of H3K4 promoter. As a result, there was increased methylated H3K4 recruited to the promoters of α-SMA, collagen I, TIMP-1 and TGFβ-1 genes, whose products are closely related with liver fibrosis (Perugorria et al 2012).…”

“…Only limited researches conduct primary culture of HSC and complete an in vitro examination within a cell line (Perugorria et al 2012), because generally accepted extraction methods require massive liver tissue of more than 50 g. Accordingly, we can only use human excisional tissues in surgery, while incapable of acquiring clinical samples in large quantities. Therefore it cannot be applied to clinical diagnosis.…”

“…Considering the direct regulating function of TIMP-3 to MMPs and ECM, methylation of TIMP-3 is highly predicted existing in liver fibrosis. Studies have also showed that the H3K4 methyltransferase ASH1 can recruit to the regulatory regions of TIMP-1 and further mediate cell differentiation (Perugorria et al 2012). However, direct epigenetic modification evidence of TIMP on liver fibrosis still needs further investigation.…”