MITF Upregulation Sensitizes Melanoma Cells to Antifolate Treatment

The presence of distinct subpopulations of differentiated and stem cell–like cells within a tumor contributes to phenotypic heterogeneity and the outgrowth of therapy-resistant cells. Microphthalmia-associated transcription factor (MITF) is a critical regulator of melanocyte differentiation, and decreased MITF expression in melanoma generates tumor-initiating cells with enhanced invasive potential that may contribute to drug resistance and metastasis. Sáez-Ayala and colleagues hypothesized that upregulation of MITF would eliminate these invasive cells and induce melanoma cell differentiation, thereby limiting phenotypic heterogeneity and increasing therapeutic sensitivity. To test this idea, the authors designed a two-step therapeutic strategy consisting of stimulation of MITF expression followed by inhibition of the essential enzyme dihydrofolate reductase (DHFR) using the antifolate prodrug, 3-O-(3,4,5-trimethoxybenzoyl)-(–)-epicatechin (TMECG), which is activated by expression of the melanocyte-specific gene tyrosinase (TYR). Treatment with methotrexate (MTX) stimulated MITF expression in melanoma cells, resulting in decreased invasiveness, increased expression of prodifferentiation genes including TYR, and TYR-driven processing of TMECG. Furthermore, MTX-induced MITF expression enhanced the sensitivity of melanoma cells to TMECG, as combined MTX/TMECG treatment synergistically and selectively induced apoptosis in melanoma cell lines as well as BRAF inhibitor– and MEK inhibitor–resistant patient-derived cells, irrespective of BRAF or TP53 mutation status. In addition, dual MTX/TMECG treatment effectively suppressed melanoma growth and metastasis in vivo without significant toxicity or acquisition of resistance. This cell type–specific antitumor effect was mediated by depletion of dTTP upon DHFR inhibition by MTX/TMECG, which promoted increased S-phase–associated DNA double-strand break formation and subsequent E2F transcription factor 1 (E2F1)-driven, p53-independent cell death. The results of these preclinical studies identify this two-step, directed phenotype-switching approach as an effective and specific antimelanoma therapeutic strategy and suggest that this drug combination may overcome resistance to BRAF and MEK inhibitors.

Sáez-Ayala M, Montenegro MF, Sánchez-del-Campo L, Fernández-Pérez MP, Chazarra S, Freter R, et al. Directed phenotype switching as an effective antimelanoma strategy. Cancer Cell 2013;24:105–19.

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