Transforming Growth Factor b Induces Caspase 3-independent Cleavage of aII-Spectrin (a-Fodrin) Coincident with Apoptosis*

Transforming growth factor b (TGF-b) is a potent growth inhibitor and inducer of cell death in B-lymphocytes and is essential for immune regulation and maintenance of self-tolerance. In this report the mouse immature B cell line, WEHI 231, was used to examine the mechanisms involved in TGF-b-mediated apoptosis. Induction of apoptosis is detected as early as 8 h after TGF-b administration. Coincident with the onset of apoptosis, the cytoskeletal actin-binding protein, aII-spectrin (a-fodrin) is cleaved into 150-, 115-, and 110-kDa fragments. The broad spectrum caspase inhibitor (BocD-fmk (BD-fmk)) completely abolished TGF-b-induced apoptosis and aII-spectrin cleavage. Caspase 3, although present in WEH1 231 cells, was not activated by TGF-b, nor was its substrate, poly(ADP-ribose) polymerase. These results identify aII-spectrin as a novel substrate that is cleaved during TGF-b-induced apoptosis. Our data provide the first evidence of calpain and caspase 3-independent cleavage of aII-spectrin during apoptosis and suggests that TGF-b induces apoptosis and aII-spectrin cleavage via a potentially novel caspase. This report also provides the first direct evidence of caspase 3 activation in WEH1 231 cells and indicates that at least two distinct apoptotic pathways exist.