GLUTATHIONE DEPLETION BY L-BUTHIONINE SULFOXIMINE ANTAGONIZES TAXOL CYTOTOXICITY

Taxol is a naturally occurring chemotherapeutic agent that is active a gainst a variety of tumors. Taxol is believed to act by binding tightl y to microtubules and preventing their disaggregation. Others have sho wn that depletion of cellular glutathione results in the disaggregatio n of microtubules, presumably by allowing the oxidation of some or all of the cysteine residues in tubulins. We studied the effect of glutat hione (GSH) depletion by L-buthionine sulfoximine (L-BSO) on taxol cyt otoxicity in two human tumor lines. After a 24-h incubation in 5 mM L- BSO, the breast adenocarcinoma line MCF-7 and the lung adenocarcinoma line A549 were exposed to varying concentrations of taxol for 24 h. GS H levels were undetectable in cells treated with L-BSO. At the highest concentrations of taxol (50 nM), control MCF-7 cells had 10% cell sur vival and control A549 cells had only 1% cell survival as assessed by clonogenic assay. Pretreatment with 5 mM L-BSO resulted in a 3-fold in crease in survival of MCF-7 cells and a 10-fold increase in survival o f A549 cells. Pretreatment with L-BSO had no effect on taxol uptake in to A549 or MCF-7 cells, as assessed by measurement of binding of [H-3] taxol to cells. Following exposure to 37 nM taxol for 24 h, both cell lines had over 80% of their population in G2/M and bromodoxyuridine la beling showed that taxol markedly reduced the percentage of cells in S phase. L-BSO pretreatment had no effect on the cell cycle in either c ell line in the absence of taxol. However, in cells treated with taxol , L-BSO increased the percentage of cells in S phase by 3-fold in both cell lines. We conclude that depletion of cellular GSH by L-BSO resul ts in resistance to taxol in MCF-7 and A549 cells. Resistance to taxol mediated by GSH depletion is not due to alterations in cellular uptak e of taxol by L-BSO. L-BSO increased the S-phase fraction of taxol-tre ated cells in both cell lines. These data suggest that GSH depletion i nterferes with cell cycle changes induced by taxol. The alteration in taxol-induced cell cycle effects may account for the resistance to tax ol produced by L-BSO.