Modulation of the cytotoxic activity of tumor necrosis factor by protein tyrosine kinase and protein tyrosine phosphatase inhibitors.

Abstract

The mechanism by which tumor necrosis factor (TNF) inhibits cell growth is not known. The importance of tyrosine phosphorylation in mediating the cytotoxicity of TNF has been suggested from previous studies. In this report, we investigated the effects of both tyrosine kinase (TK) and protein tyrosine phosphatase (PTP) inhibitors on the cytotoxic effects of TNF. TNF-induced changes in cellular PTPase activity were also examined. Incubation of tumor cells with genistein or tyrphostin, known TK inhibitors, protected against TNF cytotoxicity in a dose-dependent manner. Protection by TK inhibitors was observed at early time points after the start of TNF incubation. Preincubation of tumor cells with sodium orthovanadate, a known PTPase inhibitor, also protected against TNF cytotoxicity only at early time points after addition of TNF. Activation of total cellular PTPase activity by TNF was observed at early times of TNF incubation only in TNF-sensitive cell lines. Immunoblot analysis revealed that TNF enhanced tyrosyl phosphorylation of the epidermal growth factor receptor (EGFR) only in TNF-sensitive cell lines. No other substrates were tyr-phosphorylated after addition of TNF. The results suggest that both cellular PTKs and PTPases play a significant role in orchestrating the early events in the cytotoxic response of TNF. The nature and types of PTKs and PTPases involved in this process need further investigation.