Tyrosine phosphatases as targets in metal-induced signaling in human airway epithelial cells

We previously showed that exposure to metal-laden combustion particles disr egulates protein tyrosine phosphate homeostasis in human airway epithelial cells (HAEC). More recently, we reported that exposure to certain metal ion s activates mitogen-activated protein kinases in HAEC. To study the mechani sm responsible, we examined the effects of arsenic (As), vanadium (V), and zinc (Zn) on tyrosine phosphate catabolism in BEAS S6 cells or cultured hum an bronchial epithelial cells. Western blots and immunocy-tochemical analys es showed that exposure to noncytotoxic levels of As, V, or Zn resulted in increased levels of protein phosphotyrosines in HAEC. Tyrosine phosphatase activity, measured against [P-32]-labeled PolyGlu:Tyr, was markedly inhibit ed in cells treated with V or Zn but was unaffected by exposure to As. Fast performance liquid chromatography fractionation and subsequent in-gel phos phatase activity assay of HAEC protein extracts revealed the presence of nu merous tyrosine phosphatases, of varying molecular weights, that were effec tively inhibited by exposure to V or Zn ions. As had no discernible effect on these enzymes. The protein tyrosine phosphatase PTP1B, immunoprecipitate d from HAEC, was similarly inhibited by V and Zn but not by As ions. These data show that V and Zn may induce tyrosine phosphate accumulation by inhib iting dephosphorylation and implicate kinase activation as the mechanism in HAEC exposed to As. These findings suggest that metal exposure can activat e signaling pathways through multiple mechanisms.