THE POU DOMAIN TRANSCRIPTION FACTOR BRN-2 - ELEVATED EXPRESSION IN MALIGNANT-MELANOMA AND REGULATION OF MELANOCYTE-SPECIFIC GENE-EXPRESSION

Previous work has shown that melanoma cell lines express a distinct oc tamer binding protein. Given the role of octamer-binding proteins in c ell differentiation and development, the role this factor is a key iss ue in understanding melanocyte differentiation and transformation. Usi ng a proteolytic clipping assay, we show that the melanoma-specific oc tamer factor is Brn-2/N-Oct3, a POU domain protein previously known to be expressed in adult brain and in the developing nervous system. N-O ct3 mRNA was detected in a range of human melanoma cell lines and was around 10-fold elevated compared to normal human melanocytes while mRN A for Brn-2 was also detected in a mouse melanoblast cell line. Expres sion of Brn-2/N-Oct3, in melanoma cells in cotransfection assays activ ated the expression of the MHC class II DR alpha promoter but represse d the activity of the melanocyte-specific tyrosinase promoter. Repress ion correlated with Brn-2/N-Oct3 binding in a mutually exclusive fashi on with basic-helix-loop-helix-leucine-zipper (bHLH-LZ) transcription factor USF in vitro and with Brn-2 expression preventing activation of the tyrosinase promoter by the bHLH-LZ factor Microphthalmia in vivo. The potential role of Brn-2/N-Oct3 in melanocyte differentiation and gene expression is discussed.