Human melastatin 1 (TRPM1) is regulated by MITF and produces multiple polypeptide isoforms in melanocytes and melanoma.

Melastatin 1 (MLSN1), originally identified as melanoma metastasis suppressor, represents a TRPM subfamily of transient receptor potential (TRP) proteins which serve diverse biological roles in a wide variety of cell types. Down-regulation of MLSN1 expression in human cutaneous melanoma, as indicated by in situ hybridization, appears to be a prognostic marker for melanoma metastasis. However, the exact physiological function(s) of MLSN1, the mechanism(s) involved in the regulation of its expression and its role in melanoma tumour progression are not yet clear. In this study, we identified a 654 bp upstream sequence of MLSN1, containing four E boxes (E1-E4), including an 11 bp M box (E4), that is sufficient for melanocyte-specific transcription and activation by the melanocyte transcription factor MITF (a bHLH-zip factor). Deletion analysis showed that the two distal E boxes (E3 and E4) in the MLSN1 promoter are required for both its activation by MITF and its constitutive activity in melanoma cells. Western blot analysis using polyclonal rabbit anti-human MLSN1 antibodies identified several polypeptides, presumably generated by both alternative splicing of MLSN1 messenger RNA (mRNA) and proteolytic cleavage, in both melanocytes and metastatic melanoma cells. Thus, multiple mechanisms appear to regulate MLSN1 expression in melanocytes and melanoma cells.