γ-Glutamylcysteine synthetase mediates the c-Myc-dependent response to antineoplastic agents in melanoma cells

This study aims to investigate the role of γ-glutamylcysteine synthetase (γ-GCS), the rate-limiting enzyme for glutathione (GSH) synthesis, in the c-Myc-dependent response to antineoplastic agents. We found that specific c-Myc inhibition depleted cells of GSH by directly reducing the gene expression of both heavy and light subunits of the γ-GCS enzyme and increased their susceptibility to antineoplastic drugs with different mechanisms of action, such as cisplatin (CDDP), staurosporine (STR), and 5-fluorouracil (5-FU). The effect caused by c-Myc inhibition on CDDP and STR response, but not to 5-FU treatment, is directly linked to the impairment of the γ-GCS expression, because up-regulation of γ-GCS reverted drug sensitivity, whereas the interference of GSH synthesis increased drug susceptibility as much as after c-Myc down-regulation. The role of γ-GCS in the c-Myc-directed drug response depends on the capacity of drugs to trigger reactive oxygen species (ROS) production. Indeed, although 5-FU exposure did not induce any ROS, CDDP- and STR-induced oxidative stress enhanced the recruitment of c-Myc on both γ-GCS promoters, thus stimulating GSH neosynthesis and allowing cells to recover from ROSinduced drug damage. In conclusion, our data demonstrate that the γ-GCS gene is the downstream target of c-Myc oncoprotein, driving the response to ROS-inducing drugs. Thus, γ-GCS impairment might specifically sensitize high c-Myc tumor cells to chemotherapy.