EXOGENOUS PHOSPHOTYROSINE MODULATES EPIDERMAL GROWTH-FACTOR RECEPTOR TYROSINE PHOSPHORYLATION

Protein tyrosine phosphatases (PTPases) are potential tumor suppressor proteins which reverse the effects of protein tyrosine kinases (PTKs). We hypothesized that the induction of PTPase activity by the nutritional agent O-phospho-L-tyrosine (P-Tyr), a broad PTPase substrate, could potentially enhance total cellular PTPase activity and inhibit cell growth. In this study, we report that P-Tyr inhibited the growth of MDA-MB 468 cells in a dose-dependent fashion. P-Tyr incubation increased total cellular PTPase activity in MDA-MB 468 breast carcinoma cells. The increase of PTPase activity, as measured by a standard radioactive assay for PTPases, occurred within 10 min of P-Tyr incubation and was dependent on the concentration and time of incubation with P-Tyr. The increased PTPase activity in P-Tyr treated cells was also evident from a non-isotopic PTPase assay involving the dephosphorylation of epidermal growth factor receptor (EGFR). Epidermal growth factor (EGF)-mediated tyrosine phosphorylation of EGFR was decreased in situ in a time- and dose-dependent manner in P-Tyr-treated cells. Orthovanadate (100 muM for 4 h) inhibited this decrease, implicating the role of cellular PTPase in P-Tyr-mediated control of EGFR tyrosine phosphorylation. Further, EGFR kinase activity was found to be decreased in P-Tyr-treated cells. We conclude that P-Tyr may inhibit cell growth by decreasing cellular tyrosine phosphorylation. Both a decrease in activity of the EGFR kinase and increases in PTPase activity may have accounted for the growth inhibiting property of P-Tyr.