Transforming growth factor beta induces caspase 3-independent cleavage of alpha II-spectrin (alpha-fodrin) coincident with apoptosis

Transforming growth factor beta (TGF-beta) is a potent growth inhibitor and inducer of cell death in B-lymphocytes and is essential for immune regulat ion 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-bet a-mediated apoptosis. Induction of apoptosis is detected as early as 8 h af ter TGF-P administration. Coincident with the onset of apoptosis, the cytos keletal actin-binding protein, alpha II-spectrin (cy-fodrin) is cleaved int o 150-, 115-, and 110-kDa fragments. The broad spectrum caspase inhibitor ( Boc-D-fmk (BD-fmk)) completely abolished TGF-P-induced apoptosis and alpha II-spectrin cleavage. Caspase 3, although present in WEH1 231 cells, was no t activated by TGF-P, nor was its substrate, poly(ADP-ribose) polymerase. T hese results identify alpha II-spectrin as a novel substrate that is cleave d during TGF-P-induced apoptosis. Our data provide the first evidence of ca lpain and caspase 3-independent cleavage of alpha II-spectrin during apopto sis and suggests that TG;F-P induces apoptosis and alpha II-spectrin cleava ge via a potentially novel caspase. This report also provides the first dir ect evidence of caspase 3 activation in WEH1 231 cells and indicates that a t least two distinct apoptotic pathways exist.