The protein tyrosine kinase inhibitor herbimycin A, but not genistein, specifically inhibits signal transduction by the T cell antigen receptor.

Several lines of evidence implicate a regulatory tyrosine phosphorylation in the activation of phospholipase C (PLC) by the T cell antigen receptor (TCR). These include studies using inhibitors of protein tyrosine kinases (PTKs). In Jurkat T cells expressing the heterologous human muscarinic receptor (HM1), PLC activity can be induced by either the TCR or HM1. HM1 activates PLC via a guanine nucleotide binding protein. We have studied the selectivity of the effects of the PTK inhibitors, herbimycin A and genistein, in this system. The results indicate that these inhibitors have different mechanisms of action, and suggest that herbimycin A, but not genistein, is a specific inhibitor of PTKs in T cells. Herbimycin A markedly inhibited both the resting and induced levels of phosphotyrosine-containing proteins, including the gamma 1 isozyme of PLC and the zeta chain of the TCR, and prevented activation of PLC by anti-TCR mAb. Herbimycin A did not inhibit activation of PLC by HM1. Genistein had a much less pronounced effect than herbimycin A on the appearance of tyrosine phosphoproteins. Moreover, genistein inhibited activation of PLC by both the TCR and HM1, and inhibition was only partial. Genistein was cytotoxic and markedly inhibited protein synthesis in both Jurkat cells and human peripheral lymphocytes. Herbimycin A was not cytotoxic. These findings confirm the role of a regulatory tyrosine phosphorylation in activation of PLC by the TCR. Herbimycin A was a selective inhibitor of a subclass of PTKs in Jurkat cells. In contrast, inhibition of signal transduction and later events in T cells by genistein may be due to effects other than direct inhibition of PTK activity.