BRAF oncogene-induced autophagy as melanoma tumor suppressor mechanism
Activating mutations in NRAS and BRAF are found frequently in cutaneous melanomas. Because concurrent mutations of both BRAF and RAS are extremely rare, it is thought that transformation by RAS and BRAF occurs through a common mechanism. Also, there is evidence for a relationship of synthetic lethality between NRAS and BRAF oncogenes that leads to selection against cells with a hyperactive mitogen-activated protein kinase (MAPK) pathway. However, it is not known whether the hyperactivation of the MAPK pathway by overexpression of either oncogene alone could also inhibit melanoma tumorigenesis. Research in my laboratory showed that in melanoma cells with oncogenic BRAF (mBRAF), high levels of mBRAF induce hyperactivation of ERK and senescence-like phenotype and trigger autophagy by inhibiting the mammalian target of rapamycin complex (mTORC) signaling. Growth inhibition and cell death caused by high mBRAF levels can be partially rescued by downregulation of BRAF protein or inhibition of autophagy, but not by inhibition of the MAPK or apoptotic pathways. Quantitative immunohistochemical analysis (AQUA) of human melanomas and cell lines showed a significant positive correlation between the levels of BRAF protein and autophagy marker light chain 3 (LC3). We are currently investigating mechanisms of autophagy induction by BRAF oncogene.
Melanoma transdifferentiation and tumor progression: crosstalk between BRAF and Notch signaling
My laboratory has previously identified expression of neuronal marker MAP2 in melanoma. This marker is (a) activated in cutaneous primary melanoma and (b) inversely associated with melanoma tumor progression. We also showed that ectopic expression of MAP2 in metastatic melanoma cells inhibits cell growth by inducing mitotic spindle defects and apoptosis. However, molecular mechanisms of regulation of MAP2 gene expression in melanoma are not understood. Recently, we showed that in melanoma cells neuronal marker MAP2 expression is induced by the demethylating agent 5-aza-2'-cytidine, and MAP2 promoter is progressively methylated during melanoma progression, indicating that epigenetic mechanisms are involved in silencing of MAP2 in melanoma. Because MAP2 promoter activity levels in melanoma cell lines also correlated with activating mutation in BRAF, a gene that is highly expressed in neurons, we hypothesized that BRAF signaling is involved in MAP2 expression. We showed that hyperactivation of BRAF-MEK signaling activates MAP2 expression in melanoma cells by two independent mechanisms, promoter demethylation or down-regulation of neuronal transcription repressor HES1. We are investigating the role of HES1 target genes in melanoma transdifferentiation and melanoma tumor progression.
A working model of melanoma transdifferentiation and crosstalk between BRAF and Notch