ACTIVATION OF EPIDERMAL GROWTH-FACTOR RECEPTOR TYROSINE PHOSPHORYLATION BY TUMOR-NECROSIS-FACTOR CORRELATES WITH LOSS OF CYTOTOXIC ACTIVITY

TNF induces cytotoxicity in human tumor cells through a receptor-media ted process with unknown signaling characteristics. Evidence suggests that overexpression of transmembrane growth factor receptors with intr insic tyrosine kinase activity may suppress the antiproliferative or c ytotoxic activity of TNF, suggesting antagonism between these two sign aling pathways in tumor cells. To investigate TNF cytotoxic signal tra nsduction, ME-180 cervical carcinoma cell variants were isolated that expressed complete cytotoxic sensitivity (ME-180S) or resistance (ME-1 80R) to TNF but identical levels of p55 TNF receptor expression. ME-18 0R cells expressed threefold higher EGFR than the ME-180S cell line an d TNF treatment stimulated EGFR tyrosine phosphorylation only in resis tant cells. Activation of tyrosine phosphorylation in ME-180R cells wa s TNF concentration dependent and maximally stimulated (three- to-five -fold) after 10-15 minutes of treatment. Other tyrosine phosphoprotein s were not affected by TNF incubation demonstrating specific TNF-stimu lated tyrosine phosphomodulation of EGFR. Pretreatment with the tyrosi ne kinase inhibitor tryphostin before incubation with TNF resulted in partial reversal of TNF cytotoxic resistance in ME-180R cells and enha nced TNF responsiveness in ME-180S cells, suggesting a ''protective'' role for tyrosine phosphorylation in TNF-induced cytotoxicity. Togethe r these results demonstrate that TNF-mediated tyrosine phosphorylation is differentially regulated in sensitive and resistant tumor cells an d mag play a critical role in the cytotoxic signaling process through differential expression or regulation of tyrosine protein kinases and phosphatases.