Proteolytic processing of the X-linked inhibitor of apoptosis protein during Fas-mediated apoptosis.

Abstract

Apoptosis is a natural cellular function which is tightly controlled along its multiple pathways by positive and negative regulator proteins. Among the negative regulators are the gene family called the IAPs (inhibitor of apoptosis proteins) which were initially identified in baculoviruses. To date, five vertebrate members of this family have been identified (NAIP, HIAP1, HIAP2, XIAP and survivin). The X-linked inhibitor of apoptosis protein (XIAP), is proteolytically processed during the course of apoptosis in Jurkat lymphoma cells when triggered with anti-Fas monoclonal antibodies and cycloheximide (CHX). Native 55 kDa XIAP undergoes proteolytic processing producing a single 30 kDa fragment containing the carboxy terminus of the protein. In vitro cell-free studies, the type I calpain isoform is capable of cleaving XIAP. Calpains are calcium-dependent cysteine proteases, which are activated during apoptosis. Calpain and 26S proteasome specific inhibitors are ineffective in blocking both XIAP cleavage and cell death in Fas/CHX-stimulated Jurkat cells when used individually. However, when used together these two classes were able to block death, but not XIAP cleavage. Caspase specific inhibitors block both cell death and XIAP cleavage. In this study, I have shown that XIAP cleavage is likely induced by a member of the caspase family and that the event occurs upstream of calpain and 26S proteasome activation during the apoptotic cascade. The proteolysis of XIAP during Fas/CHX mediated apoptosis may represent an important step in the propagation of the apoptotic cascade.