The effect of protein kinase C and Beta-catenin inhibitors on uveal melanoma cells
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PURPOSE: Uveal melanoma (UM) is the most common intraocular malignancy in adults with an incidence of six per one million individuals each year. Globe conserving treatments are currently the standard of care, but unfortunately, despite successful local control, a substantial mortality risk exists due to eventual emergence of distant metastasis, which is invariably lethal. There is therefore an unmet need for novel, effective, targeted therapies for metastatic UM. Somatic mutations in the G-protein α subunits, Gαq and Gα, are present in a mutually exclusive pattern in approximately 80% of UMs, and they abolish the GTPase activity, resulting in a constitutively active protein. We have previously demonstrated that GNAQ-mutant (GNAQ^mt) UMs are addicted to the oncogenic effect of the mutant GNAQ protein and dissected the GNAQ pathway in an attempt to identify druggable targets. Our findings that the mutant GNAQ protein activates the PKC/PKD axis, which activates beta-catenin (β-Catenin), prompted us to investigate the role of PKC and β-Catenin in GNAQ^mt UM. EXPERIMENTAL DESIGN: The GNAQ^mt UM cell lines Mel202 and OMM1.3 were treated with either the PKC inhibitor bisindolylmaleimide I (BIM) alone, the Wnt/β-Catenin inhibitors FH535 or cardamonin alone, the Wnt/β-Catenin activator Wnt-3a alone, or siRNAs for β-Catenin in combination with BIM, and their viability was assessed with the MTT assay. Levels of β-Catenin, phosphorylated AKT, ERK1/2, caspase 3 and LC3BII were assessed with western blotting. β-Catenin mRNA levels were assessed with microarray analysis and RT-PCR. RESULTS: GNAQ^mt UM cells are very sensitive to PKC inhibition and respond with a decrease in cell viability that involves autophagy and cleavage and translocation of LC3BII in autophagosomes, but not caspase activation. PKC inhibition results in the upregulation of β-Catenin protein, but not mRNA levels, through a post-translational mechanism that involved the phosphorylation and activation of AKT, but not ERK1/2. β-Catenin inhibition by either small molecule inhibitors or siRNA resulted in a dose-dependent increase of cell proliferation, whereas β-Catenin activation by Wnt-3a had the opposite effects, resulting in a decrease in cell viability. CONCLUTIONS: Our study demonstrates that PKC is a mediator of the oncogenic effect of mutant Gα protein in UM through the Wnt-3/β-Catenin signaling pathway. These results open exciting opportunities for the development of personalized targeted therapies for UM in a genotype-dependent fashion.