Inhibition of NF-κB-dependent Bcl-xL expression by clusterin promotes albumin-induced tubular cell apoptosis

Apoptosis and inflammation, important contributors to the progression of chronic kidney disease, can be influenced by clusterin ( a secreted glycoprotein that regulates apoptosis) and nuclear factor-kappa B (NF-kappa B, a transcription factor modifying the expression of inflammatory genes). We studied proteinuria-induced renal disease and its influence on clusterin-mediated apoptosis. Exposure of cultured mouse proximal tubule epithelial cells to bovine serum albumin (BSA) resulted in activation of NF-kappa B and activator protein-1 (AP-1) within hours followed by a decline in their activation, decreased activation of extracellular signal-regulated kinases (ERK1/2), decreased cell-associated antiapoptotic Bcl-xL protein but increased apoptosis. Clusterin progressively increased in the media over a 3 day period. Clusterin siRNA blocked protein production, increased NF-kappa B activation, and significantly increased cellular Bcl-xL protein, thereby reducing spontaneous and BSA-induced apoptosis. An siRNA to the NF-kappa B inhibitor I kappa B alpha had similar results. BSA-stimulated NF-kappa B activation reciprocally decreased AP-1 activity by preventing ERK1/2 phosphorylation. These in vitro studies suggest that clusterin inhibits NF-kappa B-mediated antiapoptotic effects by the apparent stabilization of I kappa B alpha switching from promoting inflammation to apoptosis during proteinuria.