H3K9 trimethylation silences Fas expression to confer colon carcinoma immune escape and chemoresistance (IRM6P.654)

Abstract

Abstract The Fas-FasL effector mechanism plays a key role in cancer immune surveillance by host T cells, but metastatic human colorectal carcinoma (CRC) often uses silencing of Fas expression as a mechanism of immune evasion. The molecular mechanism of FAS transcriptional silencing in human CRC is unknown. We performed genome-wide ChIP-Sequencing analysis and determined that the level of H3K9me3 in the FAS promoter region is significantly higher in metastatic than in primary human CRC cells, and is inversely correlated with the level of Fas expression. We discovered that verticillin A is a selective inhibitor of histone methyltransferases and inhibits H3K9 trimethylation in human CRC cells. Fas expression is abolished in metastatic human CRC cells and verticillin A treatment effectively restored Fas expression in these metastatic CRC cells. Furthermore, verticillin A exhibited greater efficacy than Decitabine and Vorinostat in overcoming metastatic human CRC resistance to FasL-induced apoptosis. Interestingly, verticillin A also effectively overcomes metastatic human CRC resistance to 5-Fluorouracil in vitro and in vivo. Using an orthotopic CRC mouse model, we demonstrated that tumor-infiltrating cytotoxic T lymphocytes are FasL+ and FasL-mediated cancer immune surveillance is essential for CRC inhibition in vivo. Our findings determine that H3K9 trimethylation of the FAS promoter is a dominant mechanism underlying FAS silencing and resultant CRC immune evasion and metastasis.