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Proc Natl Acad Sci U S A. 2015 Feb 3;112(5):E420-9. doi: 10.1073/pnas.1424576112. Epub 2015 Jan 20.

MITF drives endolysosomal biogenesis and potentiates Wnt signaling in melanoma cells.

Author information

  • 1Howard Hughes Medical Institute and Department of Biological Chemistry.
  • 2Department of Medicine, Division of Hematology-Oncology.
  • 3Department of Molecular and Medical Pharmacology, Crump Institute for Molecular Imaging, and.
  • 4Jonsson Comprehensive Cancer Center, University of California, Los Angeles, CA 90095.
  • 5Department of Molecular and Medical Pharmacology, Crump Institute for Molecular Imaging, and Jonsson Comprehensive Cancer Center, University of California, Los Angeles, CA 90095.
  • 6Department of Medicine, Division of Hematology-Oncology, Jonsson Comprehensive Cancer Center, University of California, Los Angeles, CA 90095.
  • 7Howard Hughes Medical Institute and Department of Biological Chemistry, ederobertis@mednet.ucla.edu.

Abstract

Canonical Wnt signaling plays an important role in development and disease, regulating transcription of target genes and stabilizing many proteins phosphorylated by glycogen synthase kinase 3 (GSK3). We observed that the MiT family of transcription factors, which includes the melanoma oncogene MITF (micropthalmia-associated transcription factor) and the lysosomal master regulator TFEB, had the highest phylogenetic conservation of three consecutive putative GSK3 phosphorylation sites in animal proteomes. This finding prompted us to examine the relationship between MITF, endolysosomal biogenesis, and Wnt signaling. Here we report that MITF expression levels correlated with the expression of a large subset of lysosomal genes in melanoma cell lines. MITF expression in the tetracycline-inducible C32 melanoma model caused a marked increase in vesicular structures, and increased expression of late endosomal proteins, such as Rab7, LAMP1, and CD63. These late endosomes were not functional lysosomes as they were less active in proteolysis, yet were able to concentrate Axin1, phospho-LRP6, phospho-β-catenin, and GSK3 in the presence of Wnt ligands. This relocalization significantly enhanced Wnt signaling by increasing the number of multivesicular bodies into which the Wnt signalosome/destruction complex becomes localized upon Wnt signaling. We also show that the MITF protein was stabilized by Wnt signaling, through the novel C-terminal GSK3 phosphorylations identified here. MITF stabilization caused an increase in multivesicular body biosynthesis, which in turn increased Wnt signaling, generating a positive-feedback loop that may function during the proliferative stages of melanoma. The results underscore the importance of misregulated endolysosomal biogenesis in Wnt signaling and cancer.

KEYWORDS:

MITF; Wnt-STOP; lysosome; melanoma; multivesicular body

Comment in

PMID:
25605940
PMCID:
PMC4321275
DOI:
10.1073/pnas.1424576112
[PubMed - indexed for MEDLINE]
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