Received for publication June 1, 2007.
Revised July 12, 2007.
Accepted for publication July 12, 2007.

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

Abstract

The paper aims at investigating the role of -glutamylcysteine synthetase (-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 -GCS enzyme, and increased their susceptibility to antineoplastic drugs with different mechanisms of action, 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 -GCS expression, since up-regulation of -GCS reverted drug sensitivity, while the interference of GSH synthesis increased drug susceptibility as much as following c-Myc down-regulation. The role of -GCS in the c-Myc-directed drug response depends on the capacity of drugs to trigger reactive oxygen species (ROS) production. Indeed, while 5-FU exposure did not induce any ROS, CDDP- and STR-induced oxidative stress enhanced the recruitment of c-Myc on both -GCS promoters, thus stimulating GSH neo-synthesis and allowing cells to recover from ROS-induced drug damage.In conclusion, our data demonstrate that -GCS gene is the down-stream target of c-Myc oncoprotein, driving the response to ROS-inducing drugs. Thus,-GCS impairment might specifically sensitize high c-Myc tumour cells to chemotherapy.

Key words: Mechanisms of cell killing/apoptosis, Oncogenes