LOSS OF AP-2 RESULTS IN UP-REGULATION OF MCAM MUC18 AND AN INCREASE IN TUMOR-GROWTH AND METASTASIS OF HUMAN-MELANOMA CELLS/

MCAM/MUC18 is a cell-surface glycoprotein of 113 kDa, originally ident ified as a melanoma antigen, whose expression is associated with tumor progression and the development of metastatic potential. We have prev iously shown that enforced expression of MCAM/MUC18 in primary cutaneo us melanoma led to increased tumor growth and metastatic potential in nude mice. The mechanism for up-regulation of MCAM/MUC18 during melano ma progression is unknown. Here we show that up-regulation of MCAM/MUC 18 expression in highly metastatic cells correlates with loss of expre ssion of the transcription factor AP-2. The MCAM/MUC18 promoter contai ns four binding sites for AP-2, and electrophoretic mobility shift ass ay gels demonstrated that the AP-2 protein bound directly to the MCAM/ MUC18 promoter. Transfection of AP-2 into highly metastatic A375SM mel anoma cells (AP-a-negative and MCAM/MUC18-poposive) inhibited MCAM/MUC 18 promoter-driven chloramphenicol acetyltransferase reporter gene in a dose-dependent manner. MCAM/MUC18 mRNA and protein expression were d own-regulated in AP-2-transfected but not in control cells. In additio n, re-expression of AP-2 in A375SM cells inhibited their tumorigenicit y and metastatic potential in nude mice. These results indicate that t he expression of MCAM/MUC18 is regulated by AP-2 and that enforced AP- 2 expression suppresses tumorigenicity and metastatic potential of hum an melanoma cells, possibly by dawn-regulating MCAM/MUC18 gene express ion. Since AP-2 also regulates other genes that are involved in the pr ogression of human melanoma such as c-KIT, E-cadherin, MMP-2, and p21( WAF-1), We propose that loss of AP-2 is a crucial event in the develop ment of malignant melanoma.