The ndcrophtbalinia (mi) gene appeals essential foi pigment cell development and/or survival, based on its mutation in mi mice. It has also been linked to the human disorder Waardenburg Syndrome.

Germline mutations at loci encoding the transcription factor Microphthalmia (Mi), the cytokine receptor c-Kit, or its ligand Steel factor (S1) result in strikingly similar defects in mast cell and melanocyte development. Here we describe a biochemical link between Kit signalling and the activity of Mi. Stimulation of melanoma cells with S1 results in activation of MAP kinase, which in turn phosphorylates Mi at a consensus target serine. This phosphorylation upregulates Mi transactivation of the tyrosinase pigmentation gene promoter. In addition to modulating pigment production, such signalling may regulate the expression of genes essential for melanocyte survival and development. The pathway represents a new application of the general MAP kinase machinery in transducing a signal between a tissue-specific receptor at the cell surface and a tissue-specific transcription factor in the nucleus.

Melanocyte differentiation characterized by an increased melanogenesis, is stimulated by α-melanocyte–stimulating hormone through activation of the cAMP pathway. During this process, the expression of tyrosinase, the enzyme that controls melanin synthesis is upregulated. We previously showed that cAMP regulates transcription of the tyrosinase gene through a CATGTG motif that binds microphthalmia a transcription factor involved in melanocyte survival. Further, microphthalmia stimulates the transcriptional activity of the tyrosinase promoter and cAMP increases the binding of microphthalmia to the CATGTG motif. These observations led us to hypothesize that microphthalmia mediates the effect of cAMP on the expression of tyrosinase. The present study was designed to elucidate the mechanism by which cAMP regulates microphthalmia function and to prove our former hypothesis, suggesting that microphthalmia is a key component in cAMP-induced melanogenesis. First, we showed that cAMP upregulates the transcription of microphthalmia gene through a classical cAMP response element that is functional only in melanocytes. Then, using a dominant-negative mutant of microphthalmia, we demonstrated that microphthalmia is required for the cAMP effect on tyrosinase promoter. These findings disclose the mechanism by which cAMP stimulates tyrosinase expression and melanogenesis and emphasize the critical role of microphthalmia as signal transducer in cAMP-induced melanogenesis and pigment cell differentiation.

Osteoclasts are multinucleated hematopoietic cells essential for bone resorption. Macrophage colony-stimulating factor (M-CSF) is critical for osteoclast development and function, although its nuclear targets in osteoclasts are largely unknown. Mitf and TFE3 are two closely related helix-loop-helix (HLH) transcription factors previously implicated in osteoclast development and function. We demonstrate that cultured Mitf(mi/mi) osteoclasts are immature, mononuclear, express low levels of TRAP, and fail to mature upon M-CSF stimulation. In addition, M-CSF induces phosphorylation of Mitf and TFE3 via a conserved MAPK consensus site, thereby triggering their recruitment of the coactivator p300. Furthermore, an unphosphorylatable mutant at the MAPK consensus serine is specifically deficient in formation of multinucleated osteoclasts, mimicking the defect in Mitf(mi/mi) mice. These results identify a signaling pathway that appears to coordinate cytokine signaling with the expression of genes vital to osteoclast development.