Role of the Initiator Element in the Regulation of the Melanoma Cell Adhesion Molecule Gene

Abstract

The melanoma cell adhesion molecule is a membrane glycoprotein whose expression is associated with tumor progression and the development of metastatic potential. The mechanisms for upregulation of the melanoma cell adhesion molecule during melanoma progression are still poorly understood. In this study, we show further evidence that melanoma cell adhesion molecule expression is tightly regulated at the transcriptional level. Using a combination of chloramphenicol acetyl transferase reporter assays and DNA mobility shift experiments, we investigated the role played by three putative melanoma cell adhesion molecule regulatory elements, namely the initiator sequence, the SCA element, and the ASp element. The SCA and the ASp boxes can potentially interact with the transcription factors Sp1 and AP-2. Sp1 binding to both 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 promoter, however, did not depend only on a functional SCA element. The pyrimidine-rich CTCACTTG initiator, which overlaps the RNA start site, was essential for promoter function and was shown to interact with proteins related to basic helix-loop-helix transcription factors. Binding in nonmetastatic melanoma 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 the central element that mediates cAMP and phorbol ester sensitivity and initiates melanoma cell adhesion molecule overexpression in melanomas.