The Epidermal Growth Factor Receptor from Prostate Cells is Dephosphorylated by a Prostate-specific Phosphotyrosyl Phosphatase

Human prostatic acid phosphatase (PAcP) has been found to have phosphotyrosyl-protein phosphatase activity (H. C. Li, J. Chernoff, L. B. Chen, and A. Kirschonbaun, Eur. J. Biochem. 138:45-51, 1984; M.-F. Lin and G. M. Clinton, Biochem. J. 235:351-357, 1986) and has been suggested to negatively regulate phosphotyrosine levels, at least in part, by inhibition of tyrosine protein kinase activity (M.-F. Lin and G. M. Clinton, Adv. Protein Phosphatases 4:199-228, 1987; M.-F. Lin, C. L. Lee, and G. M. Clinton, Mol. Cell. Biol. 6:4753-4757, 1986). We investigated the molecular interaction of PAcP with a specific tyrosine kinase, the epidermal growth factor (EGF) receptor, from prostate carcinoma cells. Of several proteins phosphorylated in membrane vesicles from prostate carcinoma cells, PAcP selectively dephosphorylated the EGF receptor. The prostate EGF receptor was more efficiently dephosphorylated by PAcP than by another phosphotyrosyl phosphatase, potato acid phosphatase. Further characterization of the interaction of PAcP with the EGF receptor revealed that the optimal rate of dephosphorylation occurred at neutral rather than at acid pH. Thus, the enzyme that we formerly referred to as PAcP we now call prostatic phosphotyrosyl-protein phosphatase. Hydrolysis of phosphate from tyrosine residues in the immunoprecipitated EGF receptor catalyzed by purified prostatic phosphotyrosyl-protein phosphatase caused a 40 to 50% decrease in the receptor tyrosine kinase activity with angiotensin as the substrate. In contrast, autophosphorylation of the receptor was associated with an increase in tyrosine kinase activity.