Dipeptidyl peptidase IV (DPPIV), a candidate tumor suppressor gene in melanomas is silenced by promoter methylation

Dipeptidyl peptidase IV (DPPIV), a serine protease is expressed by normal melanocytes but not by melanomas, the malignant counterpart. DPPIV is encoded by a gene that contains a 5 CpG island spanning a transcriptional regulatory region. Previously we have demonstrated that DPPIV abrogates growth factor independence and functions as a tumor suppressor gene in melanomas. In this study we show that loss of DPPIV occurs at RNA level and demethylating agent, 5-aza-2'-deoxycytidine (5-AZA-Cdr) treatment of DPPIV negative melanoma cell lines results in increase of DPPIV mRNA, protein, and enzyme activities. By using sodium bisulfite genomic DNA modifications, PCR, and sequencing we confirmed that DPPIV gene promoter is methylated in eight out of ten melanoma cell lines tested. Further more, 5-AZA-Cdr induced increases in DPPIV levels correlated with growth inhibition and apoptosis in melanoma cells. All together these findings suggest that frequent downregulation of DPPIV expression in melanoma can be attributed, in large part, to aberrant promoter hypermethylation and this loss of DPPIV may be a critical event contributing to melanoma development.