Cadmium-induced apoptosis in rat kidney epithelial cells involves decrease in nuclear factor-kappa B activity

Renal epithelial cells undergo apoptosis upon exposure to cadmium (Cd). Transcription factors, such as nuclear factor-kappa B (NF-kappa B), mediate the expression of a number of genes involved in apoptosis. The present study was designed to examine the involvement of this transcription factor in Cd-induced apoptosis. Rat kidney proximal tubular epithelial cells, NRK-52E, were incubated with up to 20 mu M CdCl2 in serum-free medium for 5 h followed by incubation in serum-containing medium (without Cd) for an additional 12 h. The cells accumulated 582 +/- 19 ng Cd/mg protein after 5-h exposure to 20 mu M Cd. As a result of Cd exposure, the DNA-binding activity of the p65 subunit of NF-kappa B was decreased in a concentration- and time-dependent manner. The activity of tumor necrosis factor-alpha-induced inhibitor of kappa B (I kappa B) kinase alpha was also inhibited by Cd. In addition, the phosphorylation of I kappa B-alpha and NF-kappa B p65, as well as the levels of NF-kappa B target gene products, cIAP-1 and cIAP-2, were reduced. Pretreatment of the cells with the antioxidant U83836E or butylated hydroxytoluene preserved the DNA-binding activity and blocked the Cd-induced decease in I kappa B-alpha phosphorylation. Cd exposure caused the activation of caspase-3, -7, and -9 and DNA fragmentation. By flow cytometry, 14.6 and 30.5% apoptosis was detected at 6 and 12 h after stopping the Cd exposure. Overexpression of NF-kappa B p65 by transient transfection protected the cells from the Cd-induced apoptosis. Conversely, attenuation of NF-kappa B activity by pretreatment with SN50, an NF-kappa B nuclear translocation inhibitor, potentiated apoptosis. These results suggest that Cd-induced apoptosis involves suppression of NF-kappa B activity which may be mediated by oxidative stress.