Nuclear Factor B Activity Determines the Sensitivity of Kidney Epithelial Cells to Apoptosis: Implications for Mercury-Induced Renal Failure

Nuclear factor kappa B (NF-kappaB) is a thiol-dependent transcriptional factor that promotes cell survival and protects cells from apoptotic stimuli. Numerous studies have demonstrated increased sensitivity to apoptosis associated with inhibition of NF-kappaB activation in various cell types. We have previously demonstrated that mercuric ion (Hg(2+)), one of the strongest thiol-binding agents known, impairs NF-kappaB activation and DNA binding at low microM concentrations in kidney epithelial cells. In the present studies we investigated the hypothesis that inhibition of NF-kappaB activation by Hg(2+) and other selective NF-kappaB inhibitors would increase the sensitivity of kidney epithelial (NRK52E) cells to apoptogenic agents to which these cells are normally resistant. Fewer than 10% of untreated cells in culture were found to be apoptotic when evaluated by DNA fragmentation (TUNEL) assay. Treatment of cells with Hg(2+) in concentrations up to 5 microM or with tumor necrosis factor-alpha (TNF) (300 units/ml) did not significantly increase the proportion of apoptotic cells, compared with untreated controls. However, when TNF was given following Hg(2+) pretreatment (0.5 to 5 microM for 30 min), the proportion of cells undergoing apoptosis increased by 2- to 6-fold over that seen in untreated controls. Kidney cells pretreated with specific NF-kappaB inhibitors (Bay11-7082 or SN50) prior to TNF also showed a significant increase in apoptosis. Increased sensitivity to apoptotic cell death following these treatments was significantly attenuated in cells transfected with a p65 expression vector. In studies in vivo, rats pretreated by intraperitoneal injection with Hg(2+) (0.75 mg/kg) 18 h prior to administration of bacterial lipopolysaccharide (LPS) (10 mg/kg) displayed impaired NF-kappaB activation and an increased mitochondrial cytochrome c release in kidney cortical cells. These findings are consistent with the view that prevention of NF-kappaB activity in vitro or in vivo enhances the sensitivity of kidney cells to apoptotic stimuli to which these cells are otherwise resistant. Since apoptosis is known to play a seminal role in the pathogenesis of renal failure caused by toxicant injury to tubular cells, the present findings suggest that inhibition of NF-kappaB activity may define a molecular mechanism underlying the pathogenesis of Hg(2+) toxicity in kidney cells.