Dexamethasone inhibits TNP-alpha-induced apoptosis and IAP protein downregulation in MCF-7 cells

1 Exposure of human mammary carcinoma cell line MCF-7 to TNF-alpha lends to apoptotic cell death within 24 h. In search for apoptosis-preventing signa ls. we identified glucocorticoids as potent death-preventing compounds. Ten nM dexamethasone provided a significant protective effect whereas 100 nM d examethasone roughly blocked 80-90% of TNF-alpha -induced apoptosis. 2 Surprisingly, dexamethasone exerted a protective effect even when supplie d several hours after TNF-alpha. This points to a powerful inhibition of ev en advanced apoptotic processes by dexamethasone. 3 To further pinpoint the anti-apoptotic glucocorticoid action, we investig ated the expression levels of several members of the inhibitors of apoptosi s (IAPs) family of proteins in response to TNF-alpha and dexamethasone. IAP proteins directly block caspase pratt ase activities including caspase-3, caspase-7, and caspase-9. Exposure of MCF-7 cells to TNF caused an extensiv e downregulation of cIAP1, cIAP2, and XIAP protein levels. The decline of t he IAP protein levels temporally paralleled the appearance of apoptotic DNA fragments which started 12-14 h following TNF-alpha addition and maximal e ffects were seen within 24 h. 4 Coincubation of cells with TNF-alpha and dexamethasone potently blocked c IAP1, cIAP2, and XIAP downregulation. 5 TNF-alpha -mediated IAP protein dow nregulation was not affected by proteasome inhibitors like lactacystin, ALL N or ALLM, whereas it was blocked by the broad-spectrum caspase inhibitor Z -VAD-fmk which also prevented TNF-alpha -induced apoptotic cell death. Thes e data suggest that inhibition of IAP downregulation mediated by a caspase proteolytic activity constitutes the antiapoptotic action of glucocorticoid s in MCF-7 carcinoma cells.