title: MITF drives endolysosomal biogenesis and potentiates Wnt signaling in melanoma cells.
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.
authors.affiliation: Howard Hughes Medical Institute and Department of Biological Chemistry.
authors.forename: Diego
authors.initials: D
authors.lastname: Ploper
authors.affiliation: Howard Hughes Medical Institute and Department of Biological Chemistry.
authors.forename: Vincent F
authors.initials: VF
authors.lastname: Taelman
authors.affiliation: Department of Medicine, Division of Hematology-Oncology.
authors.forename: Lidia
authors.initials: L
authors.lastname: Robert
authors.affiliation: Howard Hughes Medical Institute and Department of Biological Chemistry.
authors.forename: Brian S
authors.initials: BS
authors.lastname: Perez
authors.affiliation: Department of Molecular and Medical Pharmacology, Crump Institute for Molecular Imaging, and.
authors.forename: Björn
authors.initials: B
authors.lastname: Titz
authors.affiliation: Jonsson Comprehensive Cancer Center, University of California, Los Angeles, CA 90095.
authors.forename: Hsiao-Wang
authors.initials: HW
authors.lastname: Chen
authors.affiliation: Department of Molecular and Medical Pharmacology, Crump Institute for Molecular Imaging, and Jonsson Comprehensive Cancer Center, University of California, Los Angeles, CA 90095.
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Graeber
authors.affiliation: Jonsson Comprehensive Cancer Center, University of California, Los Angeles, CA 90095.
authors.forename: Erika
authors.initials: E
authors.lastname: von Euw
authors.affiliation: Department of Medicine, Division of Hematology-Oncology, Jonsson Comprehensive Cancer Center, University of California, Los Angeles, CA 90095.
authors.forename: Antoni
authors.initials: A
authors.lastname: Ribas
authors.affiliation: Howard Hughes Medical Institute and Department of Biological Chemistry, ederobertis@mednet.ucla.edu.
authors.forename: Edward M
authors.initials: EM
authors.lastname: De Robertis
mesh_terms: D045744:Cell Line, Tumor; D011992:Endosomes / Q000502:physiology*; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D008247:Lysosomes / Q000502:physiology*; D051739:Microphthalmia-Associated Transcription Factor / Q000502:physiology*; D010766:Phosphorylation; D015398:Signal Transduction / Q000502:physiology*; D051153:Wnt Proteins / Q000378:metabolism*
chemical_list: C497234:MITF protein, human; D051739:Microphthalmia-Associated Transcription Factor; D051153:Wnt Proteins; D038362:Glycogen Synthase Kinase 3
keywords: MITF; Wnt-STOP; lysosome; melanoma; multivesicular body