Thiol-related mechanisms of resistance in a murine tumor model

The aim of these studies was to provide evidence supporting thiol-related alterations as a mechanism of resistance in the cyclophosphamide (CP)-resistant subline, KHT-rcp/iv, and to elucidate the mechanism by which the observed elevation in GSH is achieved.

The response of the parental and resistant cells to three anticancer agents (adriamycin, cisplatin, and ionizing radiation), previously associated with thiol-related resistance, was determined using a clonogenic cell survival assay. Further, using spectrophotometric techniques, the activities of several glutathione (GSH)-related enzymes; glutathione S-transferase (GST), glutathione peroxidase (GSH-PX), glutathione reductase (GSR), gamma-glutamylcysteine synthetase (GCS) and gamma-glutamyl transpeptidase (GGT) were also measured.

The studies indicated that the CP-resistant cells were cross-resistant to adriamycin, cisplatin and ionizing radiation. In addition, the KHT-rcp/iv cells exhibited 2-3-fold increases in GST, GSR and GCS activity. Since GCS catalyzes the rate-limiting step of GSH synthesis, increased activity of this enzyme may be one mechanism by which intracellular GSH levels may be upregulated in treatment refractory tumor cells. No significant difference in GSH-PX activity was observed between the two cell lines, while GGT activity could not be detected in either cell line.

Taken together, these results suggest that thiols could play a significant role in the acquired resistance and cross-resistance to chemotherapeutic agents. Modulation of these enzymes may be an effective means of enhancing anticancer therapies.