Abstract
Melanoma tumors driven by BRAF mutations often do not respond to BRAF/MEK/ERK pathway inhibitors currently used in treatment. One documented mechanism of resistance is upregulation of SOX2, a transcription factor that is essential for tumor growth and expansion, particularly in melanoma tumors with BRAF mutations. Targeting transcription factors pharmacologically has been elusive for drug developers, limiting treatment options. Here we show that ubiquitin-specific peptidase 9, X-linked (Usp9x), a deubiquitinase (DUB) enzyme controls SOX2 levels in melanoma. Usp9x knockdown in melanoma increased SOX2 ubiquitination, leading to its depletion, and enhanced apoptotic effects of BRAF inhibitor and MEK inhibitors. Primary metastatic melanoma samples demonstrated moderately elevated Usp9x and SOX2 protein expression compared to tumors without metastatic potential. Usp9x knockdown, as well as inhibition with DUB inhibitor, G9, blocked SOX2 expression, suppressed in vitro colony growth, and induced apoptosis of BRAF-mutant melanoma cells. Combined treatment with Usp9x and mutant BRAF inhibitors fully suppressed melanoma growth in vivo. Our data demonstrate a novel mechanism for targeting the transcription factor SOX2, leveraging Usp9x inhibition. Thus, development of DUB inhibitors may add to the limited repertoire of current melanoma treatments.
Highlights
Recent progress in targeting mutant pathways in metastatic melanoma has led to many improvements in treatment and patient survival
We confirm that protein levels of transcription factor (TF) SOX2 were induced in mutant BRAF melanoma cell lines, A375 and SK-Mel28, treated with BRAFi, vemurafenib, and MEKi, PD0325901 (Figure 1A)
We show that protein levels of TF SOX2 were induced dose dependent in mutant BRAF melanoma cell lines, A375, treated with BRAFi, vemurafenib, and MEKi, PD0325901 (Figure 1B)
Summary
Recent progress in targeting mutant pathways in metastatic melanoma has led to many improvements in treatment and patient survival. Combination of BRAF inhibitors (BRAFi, vemurafenib) and MEK inhibitors (MEKi, PD0325901) extended median progression-free survival from 7 to 11 months as compared to vemurafenib alone [1]. Additional research is needed to define other cellular targets and effective treatment strategies in both newly diagnosed and kinase BRAF and MEK inhibitor-resistant melanoma patients. Several mechanisms for resistance to BRAFi have been described, including many genetic alterations that reactivate MAPK signaling such as NRAS mutations [3], MEK mutations or mutant BRAF amplification [4]. BRAFi and MEKi combination therapy does not prevent acquired resistance, which can emerge via similar genetic mechanisms as arise during monotherapy [5, 6]. Identification of alternate biological pathways that contribute to resistance may lead to the design of more effective combination therapies [8]
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