Cleavage of Bax is mediated by caspase-dependent or -independent calpain activation in dopaminergic neuronal cells: Protective role of Bcl-2

Two cysteine protease families, caspase and calpain, are known to participate in cell death. We investigated whether a stress-specific protease activation pathway exists, and to what extent Bcl-2 plays a role in preventing drug-induced protease activity and cell death in a dopaminergic neuronal cell line, MN9D. Staurosporine (STS) induced caspase-dependent apoptosis while a dopaminergic neurotoxin, MPP⁺ largely induced caspase-independent necrotic cell death as determined by morphological and biochemical criteria including cytochrome c release and fluorogenic caspase cleavage assay. At the late stage of both STS- and MPP⁺-induced cell death, Bax was cleaved into an 18-kDa fragment. This 18-kDa fragment appeared only in the mitochondria-enriched heavy membrane fraction of STS-treated cells, whereas it was detected exclusively in the cytosolic fraction of MPP⁺-treated cells. This proteolytic cleavage of Bax appeared to be mediated by calpain as determined by incubation with [³⁵S]methionine-labelled Bax. Thus, cotreatment of cells with calpain inhibitor blocked both MPP⁺- and STS-induced Bax cleavage. Intriguingly, overexpression of baculovirus-derived inhibiting protein of caspase, p35 or cotreatment of cells with caspase inhibitor blocked STS- but not MPP⁺-induced Bax cleavage. This appears to indicate that calpain activation may be either dependent or independent of caspase activation within the same cells. However, cotreatment with calpain inhibitor rescued cells from MPP⁺-induced but not from STS-induced neuronal cell death. In these paradigms of dopaminergic cell death, overexpression of Bcl-2 prevented both STS- and MPP⁺-induced cell death and its associated cleavage of Bax. Thus, our results suggest that Bcl-2 may play a protective role by primarily blocking drug-induced caspase or calpain activity in dopaminergic neuronal cells.