GSH plays an important role in cellular defense against a wide variety of toxic electrophiles via the formation of
thioether conjugates. We studied the role of GSH in murine
tumor cell defense against a novel class of sulfhydryl-reactive
antineoplastics, the
sesquiterpene lactones (SL). Incubation of P815
mastocytoma cells with any of the four SL tested (
vernolepin,
helenalin,
elephantopin, and
eriofertopin) for 1 h resulted in 70-97% depletion of GSH. The importance of GSH resynthesis upon exposure of
tumor cells to SL was evaluated with the use of
buthionine sulfoximine (BSO), a selective, nontoxic inhibitor of
gamma-glutamylcysteine synthetase. Inhibition of GSH synthesis with 0.2 mM BSO markedly enhanced SL-mediated cytolysis of four murine tumor cell lines. A 6- to 34-fold reduction in the amount of SL causing 50% lysis was obtained with BSO. Addition of BSO to P815cells either during or immediately after a 1-h pulse with 10 micrograms/ml of
vernolepin increased cytolysis from less than 3% to 78-82%. However, a 1.5-h delay in the addition of BSO to such cells, which allowed for substantial resynthesis of GSH, reduced cytolysis to 30%. Recovery of GSH synthetic capacity after BSO treatment correlated with loss of the synergistic effect of BSO on lysis by
vernolepin. BSO did not augment cytolysis by six other
antineoplastics (
doxorubicin,
mitomycin C,
vinblastine,
cytosine arabinoside,
maytansine, and 1,3-bis-[2-chloroethyl]-1-nitrosourea [
BCNU]). Of these, only
BCNU depleted cellular GSH. Lysis by
jatrophone, another GSH-depleting
antitumor agent, was increased 21-fold by BSO. Since prolonged incubation with BSO alone results in near-complete GSH depletion without loss of cell viability, SL-mediated cytolysis is probably not a result of GSH depletion. We have demonstrated, however, a critical role for GSH synthetic capacity as a determinant of
tumor cell susceptibility to cytolysis by SL. GSH also plays an important role in cellular defense against oxidative injury.
Vernolepin, acting as a GSH-depleting agent, markedly sensitized
tumor cells to lysis by H2O2 (greater than 6.5-fold increase with 20 micrograms/ml of
vernolepin). These findings suggest the possibility that the coordinated deployment of sulfhydryl-reactive
antitumor agents, BSO, and oxidative injury might constitute an effective chemotherapeutic strategy.