Expression, regulation, and function of inhibitor of apoptosis family genes in rat mesangial cells

Background. The inhibitor of apoptosis (IAP) family of proteins regulates programmed cell death triggered by various stimuli. The purpose of this investigation was to examine the expression, regulation, and function of IAP genes in cultured rat mesangial cells. Methods. Basal and inducible expression of c-IAP1, c-IAP2, XIAP, and TIAP mRNAs was examined in mesangial cells, isolated glomeruli, and other cell lines under unstimulated and tumor necrosis factor-α (TNF-α)-stimulated conditions. To examine a role of nuclear factor-κB (NF-κB) in the regulation of IAPs, expression of IAPs in NF-κB-inactive mesangial cells was compared with that in wild-type cells. To investigate roles of IAPs in mesangial cell apoptosis, NF-κB-inactive cells were stably supertransfected with c-IAP1 or c-IAP2, and the susceptibility of these cells to TNF-α-induced apoptosis was evaluated quantitatively. Results. Substantial, constitutive expression of c-IAP2, XIAP, and TIAP was observed in serum-deprived rat mesangial cells and c-IAP2 and XIAP in isolated normal rat glomeruli. In response to TNF-α, expression of c-IAP1 and c-IAP2 was induced in HeLa cells and ECV304 endothelial cells, but not in mesangial cells. In contrast to previous reports on other cell types, the expression of IAPs in rat mesangial cells was independent of NF-κB; that is, expression levels of IAPs in NF-κB-inactive cells were same as those in NF-κB-active cells under both unstimulated and TNF-α-stimulated conditions. Even without the induction of IAPs, NF-κB-active mesangial cells were more resistant to TNF-α-induced apoptosis than NF-κB-inactive cells. Interestingly, overexpression of either c-IAP1 or c-IAP2 completely compensated for the lack of resistance to apoptosis in NF-κB-inactive cells. Conclusions. IAPs are constitutively expressed in cultured rat mesangial cells and isolated normal rat glomeruli. IAPs can contribute to the survival of rat mesangial cells, but unexpectedly, these molecules are not involved in the TNF-α-induced, NF-κB-dependent cytoprotection in this cell type.