DNA methylation in the upstream CpG island of the GPER locus and its relationship with GPER expression in colon cancer cell lines.

The G-protein coupled estrogen receptor (GPER), a proposed tumor suppressor, relays short-term non-genomic responses in target cells and tissues. It frequently undergoes down-modulation in primary tumors of the breast, ovary, and endometrium. Liu and co-workers recently reported loss of GPER expression in colorectal cancer and attributed it to DNA methylation-dependent silencing. We hypothesized that GPER expression is inversely correlated with methylation in the upstream CpG island (upCpGi) in the GPER locus. Methylation in the upCpGi was analysed by bisulfite sequencing and correlated with GPER expression in a panel of colon cancer cell lines. Eight downstream CpGs of the upCpGi was differentially methylated across the cell lines. Methylation in this differentially methylated region (DMR) correlated inversely with GPER expression. Two cell lines, namely SW620 and COLO-320DM, were compared in terms of their viability in response to varying concentrations of G1, a GPER specific agonist. SW-620 cells, which had the least methylated DMR and the highest level of GPER expression, showed significant loss of viability with 1 µM G1. COLO-320DM, which had the most methylated DMR and the lowest level of GPER expression, did not show a significant response to 1 µM G1. At 5 µM G1, SW620 cells showed a greater reduction in viability than COLO-320DM cells. DNA methylation in the DMR is inversely correlated with GPER expression. DNA methylation-dependent silencing of GPER may be, at least in part, the underlying reason behind the loss of estrogen's oncoprotective effect via GPER in the colon.