THIOL-RELATED MECHANISMS OF RESISTANCE IN A MURINE TUMOR-MODEL

Purpose: The aim of these studies was to provide evidence supporting t hiol-related alterations as a mechanism of resistance in the cyclophos phamide (CP)-resistant subline, KHT-rcp/iv, and to elucidate the mecha nism by which the observed elevation in GSH is achieved. Methods and M aterials: The response of the parental and resistant cells to three an ticancer agents (adriamycin, cisplatin, and ionizing radiation), previ ously associated with thiol-related resistance, was determined using a clonogenic cell survival assay. Further, using spectrophotometric tec hniques, the activities of several glutathione (GSH)related enzymes; g lutathione S-transferase (GST), glutathione peroxidase (GSH-PX), gluta thione reductase (GSR), gamma-glutamylcysteine synthetase (GCS) and ga mma-glutamyl transpeptidase (GGT) were also measured. Results: The stu dies indicated that the CP-resistant cells were cross-resistant to adr iamycin, cisplatin and ionizing radiation. In addition, the KHT-rcp/iv cells exhibited 2-3-fold increases in GST, GSR and GCS activity. Sinc e GCS catalyzes the rate-limiting step of GSH synthesis, increased act ivity of this enzyme may be one mechanism by which intracellular GSH l evels may be upregulated in treatment refractory tumor cells. No signi ficant difference in GSH-PX activity was observed between the two cell lines, while GGT activity could not be detected in either cell line. Conclusion: Taken together, these results suggest that thiols could pl ay a significant role in the acquired resistance and cross-resistance to chemotherapeutic agents. Modulation of these enzymes may be an effe ctive means of enhancing anticancer therapies.