Mechanisms Of Epigenetic Regulating The Expression Of Mir-146a In Breast Cancer Cells

Some miRNAs can inhibit the expression of oncogenes, acting as tumor suppressor genes, usually silenced in the malignant process. As a corepressor involved in transcription factor complex, HDAC mediates these miRNAs inactivation. Inhibition of HDAC activity by HDACi can reactivate silenced genes.Treatment SK-BR-3 cells with HDACi TSA, the expression of miRNAs have been detected, among which seven miRNAs increased and five decreased. Especially, miR-146a is silenced and can not be detected without TSA, but increased afterwards. It's demonstrated that miR-146a targeting CXCR4 3`-UTR represses its expression by inhibiting translation. Over-expressing miR-146a can reduce tumor cell invasion mediated by the chemotaxis of SDF-1 and inhibit tumor cell growth and proliferation, which establishes its role as a tumor suppressor gene.In order to find out the mechanisms for control of miR-146a gene expression, the transcription start site of miR-146a gene is identified through 5`-RACE experiment for analyzing the binding state of regulatory factors in its promoter region. From the chromatin immunoprecipitation assays, it's confirmed that the acetylation of histone H3K56 is closely related to the expression of miR-146a. Without TSA treatment, the H3K56 acetylation is almost undetectable in the promoter region of miR-146a followed by the gene silencing; with TSA, the level of the H3K56 acetylation is significantly increased, along with the activation of gene expression.Further experiments show CBP and P300 are histone H3K56 acetyltransferase, acting as coactivators, and involved in the activation of miR-146a gene expression. HDAC1, a histone H3K56 deacetylase and a corepressor, leads to the silencing of miR-146a gene. When cells treated with TSA, HDAC1 departs from the promoter region, meanwhile CBP and P300 move toward this region, which mediate H3K56 acetylation, then increased Polâ…¡accumulation in the promoter region to trigger gene transcription. Finally, it explains how epigenetic mechanisms regulate the expression of miR-146a.There is a diversity of epigenetic regulation of miR-146a gene in different tumor cells. During the observation of the breast, stomach, liver, lung, ovarian and endometrial cancer cells, TSA alone or its combination with demethylating drugs 5-Aza-CdR can induce up-regulation of miR-146a expression, which offers a new strategy to restore the function of tumor suppressor genes and provides evidence to support HDACi used as the anti-tumor drugs.Treatment with Herceptin, induced elevated level of miR-146a led to decreased expression of CXCR4, which offers new evidence for the effect of Herceptin as an anti-tumor drug in clinic cancer therapy.