Differential non-redox inhibitory effects of glutathione against protein kinase C isozyme family members.

Glutathione (GSH) and the GSH metabolic precursor N-acetylcysteine (NAC) are potent antioxidants that have clear potential either as cancer chemopreventive agents or as lead compounds for new cancer chemopreventive agents. The potential efficacy of GSH and NAC in clinical cancer chemoprevention is suggested by their antagonism of tumor promotion in animal models. Protein kinase C (PKC) is an isozyme family that plays a critical role in phorbol ester-mediated tumor promotion. We recently found that GSH and NAC exert direct inhibitory effects against a purified PKC isozyme mixture by a mechanism that did not involve their antioxidant properties. In this report, we characterize non-redox inhibitory effects of glutathiones on PKC isozymes that have been shown to produce partially or fully transformed phenotypes in mammalian cells. We show that GSH, NAC, and oxidized GSH analogs exert potent inhibition of the isozyme cPKC-ç and are somewhat less effective against cPKC- 1. In contrast, the oncogenic isozyme nPKC-â was unaffected by NAC, and it was inhibited by GSH and oxidized GSH analogs very modestly. Our results suggest that the potential impact of non-redox GSH/NAC-mediated PKC inhibition on cellular responses to tumor promoters and indeed, on cell growth regulation in general, may depend upon the pattern of PKC isozyme expression in the cells.