Directed Phenotype Switching as an Effective Antimelanoma Strategy

Abstract

Therapeutic resistance in melanoma and other cancers arises via irreversible genetic, and dynamic phenotypic, heterogeneity. Here, we use directed phenotype switching in melanoma to sensitize melanoma cells to lineage-specific therapy. We show that methotrexate (MTX) induces microphthalmia-associated transcription factor (MITF) expression to inhibit invasiveness and promote differentiation-associated expression of the melanocyte-specific Tyrosinase gene. Consequently, MTX sensitizes melanomas to a tyrosinase-processed antifolate prodrug 3-O-(3,4,5-trimethoxybenzoyl)-(-)-epicatechin (TMECG), that inhibits the essential enzyme DHFR with high affinity. The combination of MTX and TMECG leads to depletion of thymidine pools, double-strand DNA breaks, and highly efficient E2F1-mediated apoptosis in culture and in vivo. Importantly, this drug combination delivers an effective and tissue-restricted antimelanoma therapy in vitro and in vivo irrespective of BRAF, MEK, or p53 status.

Phenotype Switching: Tumor Cell Plasticity as a Resistance Mechanism and Target for Therapy

“…Given that melanoma cells can resort to phenotype switching as a therapy escape mechanism, it should be considered as a direct therapeutic target. The potential success is illustrated by a recent study (40), in which methotrexate was used to increase cellular levels of MITF, thereby forcing melanoma cells to adopt a proliferative, more differentiated phenotype associated with expression of the melanocyte-specific TYR gene. This in turn sensitized the cells to a TYR-processed prodrug, which induced apoptosis in all melanoma cells (but not in nontransformed melanocytes), irrespective of their mutational status (40).…”

The MITF family of transcription factors: Role in endolysosomal biogenesis, Wnt signaling, and oncogenesis

“…4) that further affects endolysomal trafficking and Wnt signaling during the proliferative stages of melanoma growth [8]. One possibility is that this regulatory loop might be manipulated via GSK3 inhibitors to entice differentiation (through phenotype-switching caused by increased MITF levels) [93][94][95] of the slowly dividing stem-like cells that make melanomas so refractory to treatment.…”

“…Melanomas progress through cell switching between these two cellular phenotypes that coexist in tumors in vivo [94]. Recently, MITF-driven phenotype switching has been proposed as of possible therapeutic value [95]. Induction of MITF causes expression of high levels of the enzyme Tyrosinase, the rate limiting enzyme in the synthesis of melanin.…”

“…Induction of MITF causes expression of high levels of the enzyme Tyrosinase, the rate limiting enzyme in the synthesis of melanin. An irreversible inhibitor of dihydrofolate reductase (DHFR) has been engineered as a drug precursor (called TMECG) that is active only after being processed specifically by tyrosinase [95]. Post-translational Wnt/STOP signaling, through GSK3 inhibition, stabilizes MITF [8], and MITF is also transcriptionally upregulated by ␤-Catenin [96].…”

The epigenetic modifier EZH2 controls melanoma growth and metastasis through silencing of distinct tumour suppressors

“…7l). Notably, an increase in MITF has previously been connected to loss of invasive capacity 45 , while genes such as TWIST1 and ZEB1 are well-established drivers of NCSC delamination as much as of melanoma EMT and metastasis 6,46 . Finally, when inoculating B16-F10 cells intravenously into C57Bl/6 mice, RNAi and importantly also GSK503 treatment drastically reduced lung nodule counts (Fig.…”