Suppression of Plk3 expression by Ni(II) may contribute to nickel carcinogenesis in the lung

Abstract

Exposure to nickel (Ni) compounds is known to induce human lung malignancies. Research shows that Ni induces lung tumorigenesis mainly through epigenetic and metabolic pathways. Polo‐like kinase 3 (Plk3) is a protein kinase that has been implicated in lung tumorigenesis. In this study, we examined the effect of Ni on Plk3 in A549 lung carcinoma cells. We find that NiCl2 lowers the Plk3 protein level in time‐ and dose‐dependent manners. A549 cells treated with cycloheximide in the presence or absence of NiCl2 exhibited significantly reduced levels and the half‐life of the Plk3 protein. In addition, A549 cells treated with MG132 partially reversed the effect of NiCl2 on the Plk3 protein level. Our experiments also revealed that majority of the endogenous Plk3 protein is located in the cytoplasm where degradation of Plk3 appears to occur. Transfection of A549 cells with FLAG‐tagged ubiquitin (Ub) followed by treatment with NiCl2 and/or MG132, immunoprecipitation with an anti‐FLAG antibody, and Western blot with an anti‐Plk3 antibody showed that Plk3 was poly‐ubiquitinated and that the ubiquitination was significantly enhanced by NiCl2 and/or MG132 treatments. Furthermore, real‐time PCR analysis showed that the Plk3 mRNA level was also reduced by NiCl2 treatments. Together, our data show that NiCl2 suppresses Plk3 expression by lowering the PLK3 mRNA level and promoting proteasome‐mediated degradation of the Plk3 protein. Given the potential role of Plk3 as a tumor suppressor in the lung, suppression of Plk3 expression could be one of the mechanisms that contribute to Ni‐induced lung carcinogenesis and tumor progression.