Role of the initiator element in the regulation of the melanoma cell adhesion molecule gene

The melanoma cell adhesion molecule is a membrane glycoprotein whose expres sion is associated with tumor progression and the development of metastatic potential. The mechanisms for upregulation of the melanoma cell adhesion m olecule during melanoma progression are still poorly understood. In this st udy, we show further evidence that melanoma cell adhesion molecule expressi on is tightly regulated at the transcriptional level. Using a combination o f chloramphenicol acetyl transferase reporter assays and DNA mobility shift experiments, we investigated the role played by three putative melanoma ce ll adhesion molecule regulatory elements, namely the initiator sequence, th e SCA element, and the ASp element. The SCA and the ASp boxes can potential ly interact with the transcription factors Sp1 and AP-2, Sp1 binding to bot h sites was confirmed, but only the SCA sequence could form a complex with AP-2. AP-2-driven downregulation of the melanoma cell adhesion molecule pro moter, however, did not depend only on a functional SCA element. The pyrimi dine-rich CTCACTTG initiator, which overlaps the RNA start site, was essent ial for promoter function and was shown to interact with proteins related t o basic helix-loop-helix transcription factors. Binding in nonmetastatic me lanoma cells was induced by cAMP. In metastatic cells, however, binding was constitutive, but could be markedly decreased upon treatment with phorbol esters, As melanoma cell adhesion molecule expression is modulated by cAMP and phorbol ester signaling, these results suggest that the initiator is th e central element that mediates cAMP and phorbol ester sensitivity and init iates melanoma cell adhesion molecule overexpression in melanomas.