Vitamin C sensitizes melanoma to BET inhibitors

Bromodomain and extra-terminal inhibitors (BETi) are promising cancer therapies, yet prominent side effects of BETi at effective doses have been reported in Phase I clinical trials. Here we screened a panel of small molecules targeting epigenetic modulators against human metastatic melanoma cells. Cells were pretreated with or without ascorbate (vitamin C), which promotes DNA demethylation and subsequently changes the sensitivity to drugs. Top hits were structurally unrelated BETi including JQ1, I-BET151, CPI-203, and BI-2536. Ascorbate enhanced the efficacy of BETi by decreasing acetylation of histone H4, but not H3, while exerting no effect on the expression of BRD proteins. Histone acetyltransferase 1 (HAT1), which catalyzes H4K5ac and H4K12ac, was downregulated by ascorbate mainly via the TET-mediated DNA hydroxymethylation pathway. Loss of H4ac, especially H4K5ac and H4K12ac, disrupted the interaction between BRD4 and H4 by which ascorbate and BETi blocked the binding of BRD4 to acetylated histones. Co-treatment with ascorbate and JQ1 induced apoptosis and inhibited proliferation of cultured melanoma cells. Ascorbate deficiency as modeled in Gulo-/- mice diminished the treatment outcome of JQ1 for melanoma tumorgraft. In contrast, ascorbate supplementation lowered the effective dose of JQ1 needed to successfully inhibit melanoma tumors in mice. Based on our findings, future clinical trials with BETi should consider ascorbate levels in patients. Furthermore, ascorbate supplementation might help reduce the severe side effects that arise from BETi therapy by reducing the dosage necessary for treatment.

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