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MECHANISM OF PROTEIN TYROSINE PHOSPHATASE INHIBITION IN HUMAN AIRWAY EPITHELIAL CELLS (HAEC) EXPOSED TO ZN2+
TAL, T., R. A. SILBAJORIS, P. A. BROMBERG, L. M. GRAVES, W. WU, AND J. M. SAMET. MECHANISM OF PROTEIN TYROSINE PHOSPHATASE INHIBITION IN HUMAN AIRWAY EPITHELIAL CELLS (HAEC) EXPOSED TO ZN2+. Presented at Society of Toxicology Annual Meeting, San diego, CA, March 05 - 09, 2006.
A number of studies have implicated zinc in the toxicity of ambient particulate matter (PM) inhalation. We previously showed that exposure to Zn2+ inhibits protein tyrosine phosphatase (PTP) activity and leads to activation of epidermal growth factor receptor (EGFR) signaling in HAEC. We hypothesized that Zn2+-induced EGFR activation is a consequence of PTP inhibition and that it occurs through a direct attack on the catalytic site of the PTP. We measured PTP enzymatic activity in recombinant PTPlB and in HAEC cell lysates following treatment with Zn2+. In both cases, treatment with Zn2+ was found to impair PTP activity as measured by hydrolysis of 32P-labeled PolyGlu-P-Tyr. Moreover, this inhibition was reversible when followed by exposure to dithiothreitol (DTT).To elucidate the mechanism by which Zn2+ inhibits PTP(s) which oppose EGFR kinase activity in human bronchial epithelium, we treated HAEC with non-cytotoxic levels of ZnSO4 (0-50 uM for 20min) followed by exposure to Zn2+ chelators and antioxidants and measured the state of EGFR phosphorylation. Western blots showed that treatment with the thiol antioxidant DTT reversed Zn2+-induced EGFR phosphorylation in HAEC. To distinguish between the reductive and chelating properties of DTT in mediating this effect, we tested structurally unrelated antioxidants and the Zn2+ chelating agent TPEN. TPEN effectively reversed phosphorylation of EGFR induced by Zn2+. In contrast, Zn2+ exposure followed by treatment with the anti-oxidants N-acetyl cysteine and lipoic acid did not reverse Zn2+ mediated EGFR phosphorylation. These data show that Zn2+-induced activation of EGFR in HAEC is the result of a non-oxidant inhibition of tyrosine phosphatase activity which function to dephosphorylate EGFR in opposition to baseline EGFR kinase activity. These findings suggest a mechanism for the toxicity of
Zn2+ inhalation. THIS ABSTRACT OF A PROPOSED PRESENTATION DOES NOT NECESSARILY REFLECT EPA POLICY.