Induction of c-fos Proto-oncogene in Mesangial Cells by Cadmium

Abstract

Cadmium is mitogenic under some circumstances and has been shown to cause accumulation of transcripts for several proto-oncogenes in a variety of cells, but the mechanism(s) remain to be delineated. Here we show that CdCl 2 causes an increase in c-fos mRNA within 30 min of exposure of mesangial cells. At 10 M Cd 2 , this increase persists for at least 8 h in both rat and human cells. The half-life of c-fos mRNA is the same whether it accumulates following 4 h of treatment with Cd 2 or is induced transiently by phorbol ester. Cycloheximide, which stabilizes the transcript, causes a synergistic increase when administered with CdCl 2 . Nuclear run-on analysis confirms that Cd 2 causes transcriptional activation of the c-fos gene. Calmodulin and Ca 2 /calmodulin-dependent kinase, and classical protein kinase C (PKC) isoforms represent two Ca 2 -dependent signaling pathways that can lead to induction of c-fos, and Cd 2 has been shown to activate both calmodulin and PKC in vitro, possibly by virtue of the similar ionic radii of Cd 2 and Ca 2 . Therefore, we investigated the effect of Cd 2 on these pathways in vivo. 10 M CdCl 2 did not increase total PKC activity or Ca 2 /calmodulin-dependent kinase II activity and inhibited the latter at higher concentrations, ruling out either pathway in the Cd 2 -dependent induction of c-fos. However, Cd 2 did lead to a sustained activation of the Erk family mitogenactivated protein kinases (MAPK) that correlated with induction of c-fos. A specific inhibitor of the MAPK kinases, PD98059, partially inhibited the induction of c-fos by Cd 2 . We conclude that Cd 2 induces c-fos at least in part by causing a sustained activation of MAPK independent of its ability to activate PKC and calmodulin in vitro.