Synergy between MITF and Wnt in Melanoma

PLOPER, D

Malibu, California

Encuentro; UCLA Department of Biological Chemistry Retreat; 2012

UCLA Department of Biological Chemistry, UCLA

The Wnt signaling pathway is has a fundamental role in animal development and human disease. It functions by regulating the transcription of target genes and stabilizing many proteins phosphorylated by GSK3. We discovered that the MiT family of transcription factors, including the melanoma oncogene MITF and the lysosomal master regulator TFEB, had the highest phylogenetic conservation of putative GSK3 phosphorylation sites, potentially placing them under the control of Wnt signaling. We found that MITF levels correlated with many lysosomal genes in melanoma. Over-expression of MITF caused a significant increase in vesicular structures marked by the expression Rab7, LAMP1, and CD63 (late endosomal proteins). However, these late endolysosomes were less active in proteolysis. Nevertheless, they were able to accumulate Axin1, phospho-LRP6, phospho-beta-catenin, and GSK3 when cells were treated with Wnt. The accumulation of components of the Wnt pathway increased Wnt signaling, presumably by augmenting pool of MVBs available for sequestration of the signalosome/destruction-complex. MITF protein itself was also stabilized by Wnt signaling, mediated by novel GSK3 phosphorylations. This positive-feedback loop may be important in proliferative stages of melanoma development, and underscore the importance of misregulated endolysosomal biogenesis for Wnt signaling and maladies such as cancer or neurodegenerative diseases.