Oncogenic BRAF Regulates Melanoma Proliferation through the Lineage Specific Factor MITF
Oncogenic BRAF Regulates Melanoma Proliferation through the Lineage Specific Factor MITF
The Microphthalmia-associated transcription factor (MITF) is an important regulator of cell-type specific functions in melanocytic cells. MITF is essential for the survival of pigmented cells, but whereas high levels of MITF drive melanocyte differentiation, lower levels are required to permit proliferation and survival of melanoma cells. MITF is phosphorylated by ERK, and this stimulates its activation, but also targets it for degradation through the ubiquitin-proteosome pathway, coupling MITF degradation to its activation. We have previously shown that because ERK is hyper-activated in melanoma cells in which BRAF is mutated, the MITF protein is constitutively down-regulated. Here we describe another intriguing aspect of MITF regulation by oncogenic BRAF in melanoma cells. We show oncogenic BRAF up-regulates MITF transcription through ERK and the transcription factor BRN2 (N-Oct3). In contrast, we show that in melanocytes this pathway does not exist because BRN2 is not expressed, demonstrating that MITF regulation is a newly acquired function of oncogenic BRAF that is not performed by the wild-type protein. Critically, in melanoma cells MITF is required downstream of oncogenic BRAF because it regulates expression of key cell cycle regulatory proteins such as CDK2 and CDK4. Wild-type BRAF does not regulate this pathway in melanocytes. Thus, we show that oncogenic BRAF exerts exquisite control over MITF on two levels. It downregulates the protein by stimulating its degradation, but then counteracts this by increasing transcription through BRN2. Our data suggest that oncogenic BRAF plays a critical role in regulating MITF expression to ensure that its protein levels are compatible with proliferation and survival of melanoma cells. We propose that its ability to appropriate the regulation of this critical factor explains in part why BRAF is such a potent oncogene in melanoma.
- Massachusetts Institute of Technology United States
- Institute of Cancer Research United Kingdom
- Whitehead Institute for Biomedical Research United States
- University of Salford United Kingdom
Proto-Oncogene Proteins B-raf, Microphthalmia-Associated Transcription Factor, Transcription, Genetic, Science, Q, Cell Cycle, Cyclin-Dependent Kinase 2, R, Cyclin-Dependent Kinase 4, Down-Regulation, Gene Expression Regulation, Neoplastic, Cell Line, Tumor, Medicine, Humans, Melanocytes, Extracellular Signal-Regulated MAP Kinases, Melanoma, Research Article, Cell Proliferation, Signal Transduction
Proto-Oncogene Proteins B-raf, Microphthalmia-Associated Transcription Factor, Transcription, Genetic, Science, Q, Cell Cycle, Cyclin-Dependent Kinase 2, R, Cyclin-Dependent Kinase 4, Down-Regulation, Gene Expression Regulation, Neoplastic, Cell Line, Tumor, Medicine, Humans, Melanocytes, Extracellular Signal-Regulated MAP Kinases, Melanoma, Research Article, Cell Proliferation, Signal Transduction
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