MITF-M plays an essential role in transcriptional activation and signal transduction in Xiphophorus melanoma

The teleost Xiphophorus provides a genetically well-described model system to study the molecular processes underlying melanoma formation. As transcriptional deregulation is a widespread phenomenon in many tumors, we have studied the regulation of melanoma-specific gene expression in this fish. A central regulator of melanocyte specific gene expression, which is also a marker for melanomas, is the transcription factor microphthalmia-associated transcription factor (MITF). One of its targets, the tyrosinase gene, codes for a key enzyme in the melanin synthesis pathway. We could show that the promoter of the medaka tyrosinase gene is highly active in the Xiphophorus melanoma cell line PSM (platyfish–swordtail melanoma) but not in non-melanoma cells. Functional dissection of the promoter revealed that three E-boxes are essential for its pigment cell-specific activity. These binding sites for basic helix-loop-helix transcription factors are recognized by a nuclear protein from the melanoma cell line PSM, most likely MITF, as its exogenous delivery could activate the promoter in non-melanoma cells. The use of specific signalling inhibitors demonstrated that the activity of the tyrosinase promoter is negatively regulated by the melanoma-inducing receptor tyrosine kinase Xmrk in PSM cells. This repression is mediated by MAPkinase and dependent on E-box integrity, again implicating the involvement of MITF. The cumulative evidence indicates that in Xiphophorus, Xmrk suppresses differentiation signals relayed by MITF as part of the transformation process finally resulting in melanoma formation.