Engagement of the TCR (CD3-Ti) by Ag/MHC, CD3 mAb, or lectin mitogen stimulates the very early tyrosine phosphorylation of several cellular substrates including TCR-zeta. The T cell specific protein-tyrosine kinase (PTK), p56lck, has been implicated in the tyrosine phosphorylation of TCR-zeta. However, the significance of this event with regard to CD3-Ti signal transduction remains unclear. Herein, we have investigated the effect of the selective PTK inhibitor genistein (4',5,7-trihydroxyisoflavone) on cellular events associated with activation via CD3-Ti triggering. Genistein inhibited the T cell PTK, p56lck, in a dose-dependent fashion with an ID50 = 40 microM. Genistein also inhibited CD3 mAb or PHA-induced TCR-zeta chain phosphorylation in intact peripheral blood T cells. Genistein blocked the expression of IL-2 and IL-2R (CD25) in T cells stimulated with PHA/PMA or CD3 mAb/PMA, but did not inhibit the de novo expression of the CD69 early activation Ag, which is induced primarily by a PKC-dependent pathway. IL-2 and CD25 expression induced by calcium ionophore A23187 and PMA was largely refractory to inhibition by genistein, suggesting an effect of the drug on calcium-dependent pathways stimulated via CD3-Ti triggering. In this last regard, genistein partially inhibited the CD3 mAb-induced rise in [Ca2+]i but did not inhibit PHA- or CD3 mAb-induced phosphatidylinositol hydrolysis. Consequently, protein-tyrosine phosphorylation does not appear to be a prerequisite for CD3-Ti-mediated activation of phosphatidylinositol-specific phospholipase C activity and PIP2 hydrolysis. An alternative role for PTK in CD3-Ti signal transduction is suggested.

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