RESUMEN
Silencing of gene expression by methylation of CpG islands in regulatory elements is frequently observed in cancer. However, an influence of the most common oncogenic signalling pathways onto DNA methylation has not yet been investigated thoroughly. To address this issue, we identified genes suppressed in HRAS-transformed rat fibroblasts but upregulated after treatment with the demethylating agent 5-Aza-2-deoxycytidine and with the MEK1,2 inhibitor U0126. Analysis of gene expression by microarray and Northern blot analysis revealed the MEK/ERK target genes clusterin, matrix metalloproteinase 2 (Mmp2), peptidylpropyl isomerase C-associated protein, syndecan 4, Timp2 and Thbs1 to be repressed in the HRAS-transformed FE-8 cells in a MEK/ERK- and methylation-dependent manner. Hypermethylation of putative regulatory elements in HRAS-transformed cells as compared to immortalized fibroblasts was detected within a CpG island 14.5 kb upstream of clusterin, within the clusterin promoter and within a CpG island of the Mmp2 promoter by bisulphite sequencing. Furthermore, hypermethylation of the clusterin promoter was observed 10 days after induction of HRAS in immortalized rat fibroblasts and a clear correlation between reduced clusterin expression and hypermethlyation could also be observed in distinct rat tissues. These results suggest that silencing of individual genes by DNA methylation is controlled by oncogenic signalling pathways, yet the mechanisms responsible for initial target gene suppression are variable.