Glutathione-independent mechanism of apoptosis inhibition by curcumin in rat thymocytes.

Curcumin (CUR) is a natural yellow dye with antioxidant and scavenging properties present in Curcuma species. It is widely used as an anti-inflammatory, anti-mutagenic and chemopreventive agent. In addition to its inhibitory effect on proliferation, CUR has recently been shown to block dexamethasone-induced programmed cell death (apoptosis) of rat thymocytes. Because cellular thiols seem to play a role in redox regulation of apoptosis, the mechanism of the anti-apoptotic effect of CUR was studied by examining the levels of glutathione and acid-soluble sulfhydryl groups. CUR was shown to prevent the glutathione loss occurring in dexamethasone-treated thymocytes, enhancing intracellular glutathione content at 8 hr to 192% of that of nontreated cells. A 60% increase in acid-soluble sulfhydryl groups was also observed. In the presence of L-buthionine S,R-sulfoximine (BSO, an inhibitor of glutathione synthesis), intracellular glutathione content of thymocytes treated with dexamethasone and CUR fell to 31% and that of the acid-soluble sulfhydryl groups to 23% of control after 8 hr. Unexpectedly, the electrophoretic and flow cytometric studies of DNA fragmentation demonstrated that apoptosis did not occur even after 20 hr of incubation with buthionine S,R-sulfoximine and dexamethasone, while control thymocytes and the cells treated only with buthionine S,R-sulfoximine showed DNA fragmentation at a level corresponding to spontaneous apoptosis. These results show that CUR treatment elevated the concentrations of glutathione and nonprotein sulfhydryl groups, thus preventing their decrease in apoptotic thymocytes. Coadministration of L-buthionine S,R-sulfoximine and CUR did not affect the anti-apoptotic effect of CUR suggesting a glutathione-independent mechanism of cell protection.