Indexed on: 25 May '15Published on: 25 May '15Published in: Pharmacological Research
Canonical Wnt signaling influences cellular fate and proliferation through inhibition of Glycogen Synthase Kinase (GSK3) and the subsequent stabilization of its many substrates, most notably β-Catenin, a transcriptional co-activator. MITF, a melanoma oncogene member of the microphthalmia family of transcription factors (MiT), was recently found to contain novel GSK3 phosphorylation sites and to be stabilized by Wnt. Other MiT members, TFEB and TFE3, are known to play important roles in cellular clearance pathways by transcriptionally regulating the biogenesis of lysosomes and autophagosomes via activation of CLEAR elements in gene promoters of target genes. Recent studies suggest that MITF can also upregulate many lysosomal genes. MiT family members are dysregulated in cancer and are considered oncogenes, but the underlying oncogenic mechanisms remain unclear. Here we review the role of MiT members, including MITF, in lysosomal biogenesis, and how cancers overexpressing MITF, TFEB or TFE3 could rewire the lysosomal pathway, inhibit cellular senescence, and activate Wnt signaling by increasing sequestration of negative regulators of Wnt signaling in multivesicular bodies (MVBs). Microarray studies suggest that MITF expression inhibits macroautophagy. In melanoma the MITF-driven increase in MVBs generates a positive feedback loop between MITF, Wnt, and MVBs.