Promoter CpG Hypermethylation and Downregulation of Caveolin Expression in Human Colon Cancers

Background/Aims: Abnormal reduction of caveolins has been found in many human cancers while its overexpression also correlates with increased metastatic progression of some tumors. To elucidate the possible implication of caveolin abnormality in human colon tumorigenesis, the expression and mutational status of caveolins was explored. Methods: We investigated 11 human colon cancer cell lines, 49 primary carcinoma tissues, and its matched normal colonic tissues. Both mRNA and protein levels of caveolins (cav-1, cav-2) were evaluated by quantitative RT-PCR and immunoblotting. Effect of cav-1 expression on tumor growth was tested using cell counting and colony formation assay. Cav-1 expression was restored in nonexpressing cells, whereas cav-1 expression was inhibited by siRNA-mediated knockdown in expressing cells. Methylation status of 38 CpG sites was evaluated by bisulfite DNA sequencing. Results: Low expression of cav-1 transcript was found in 54.5% of cancer cell lines, whereas 45.5% of those showed strong expression. Expression level of cav-1 protein was very low in majority of cancer cell lines except two cell lines. Approximately 47% and 10% of primary carcinomas exhibited significant reduction and elevation in cav-1 expression, respectively. Cav-2 expression also showed down- and up-regulation in 28% and 3% of primary tumors, respectively. Cav-1 transcript was re-expressed in nonexpressing cells by 5-aza-dC treatment. Restoration of cav-1 inhibited growth of cav-1-negative cells and reduced phospho-Erk level, whereas ectopic overexpression of cav-1 further stimulated cav-1-expressing cells and activated p53 and p21. Conclusions: Caveolin undergoes epigenetic silencing in a considerable proportion of human colon cancers by aberrant promoter CpG hypermethylation. Also, cav-1 acts two opposite functions as a growth suppressor or growth stimulator in colon cancers.