Sulfhydryl oxidation down-regulates T-cell signaling and inhibits tyrosine phosphorylation of phospholipase C gamma 1.

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Abstract
Early events in both T-cell receptor (CD3)- and CD4-induced signal transduction pathways include tyrosine phosphorylation of protein substrates, the generation of phosphatidylinositol-phosphate breakdown products, and the mobilization of intracellular Ca2+. Oxidative stress in T cells mediated by sulfhydryl-reactive nonpolar maleimides was shown previously to down-regulate both receptor-mediated Ca2+ mobilization and interleukin 2 production. Here we show that N-ethylmaleimide suppresses both CD3- and CD4-induced Ca2+ responses in human T cells correlating with a reduction in the level of phospholipase C gamma 1 (PLC gamma 1) tyrosine phosphorylation. The inhibition of tyrosine phosphorylation of PLC gamma 1 and additional protein substrates was observed at concentrations of N-ethylmaleimide above 20 microM, whereas lower concentrations of oxidant appeared to increase tyrosine kinase activity following cell stimulation. Sulfhydryl oxidation did not directly affect the catalytic activity of PLC gamma 1, since immunopurified enzyme from N-ethylmaleimide-treated T cells was fully active. Although N-ethylmaleimide treatment of T cells did not cause a direct effect on total pp56lck kinase activity measured in vitro, the interaction between CD4 and pp56lck was oxidation-sensitive in vivo. However, CD3-induced signaling was inhibited at N-ethylmaleimide concentrations lower than that required for CD4/pp56lck dissociation, suggesting that CD3-associated tyrosine kinase activity involves acutely sensitive regulatory thiols. In addition to chemically induced sulfhydryl oxidation, naturally regulated cellular redox states appear to dictate the potential for T-cell responsiveness, since degranulating human peripheral blood neutrophils inhibited CD3-induced Ca2+ mobilization in T lymphocytes. These data indicate that signal transduction in T cells involves the activation of PLC gamma 1 by tyrosine phosphorylation through an oxidation-sensitive intermediate between surface receptors and tyrosine kinases, perhaps including the interaction between CD4 and pp56lck.