Treatment of doxorubicin resistant MCF7/Dx cells with nitric oxide causes histone glutathionylation and reversal of drug resistance.

Acquired drug resistance was found to be suppressed in doxorubicin resistant breast cancer cells (MCF7/Dx cells) after a pre-treatment with nitrosoglutathione (GSNO). The effect was accompanied by enhanced protein glutathionylation and accumulation of doxorubicin in the nucleus. Among the glutathionylated proteins we identified three members of the histone family; this is the first time that histone glutathionylation has been reported. Formation of the potential nitric oxide donor dinitrosyl diglutathionyl iron complex, bound to glutathione transferase (GST P1-1), was observed in both MCF7/Dx cells and drug-sensitive MCF7 cells, to a similar extent. In contrast, histone glutathionylation was found to be markedly increased in the resistant MCF7/Dx cells, which also showed 14-fold higher amount of GST P1-1 and increased glutathione concentration compared to MCF7 cells. These results suggest that the increased cytotoxic effect of combined doxorubicin and GSNO treatment involves the glutathionylation of histones, through a mechanism that requires high GSH levels and increased expression of GST P1-1. Because of the critical role of histones in the regulation of gene expression, the implication of this finding may go beyond the phenomenon of doxorubicin resistance.