Bcl-2 and Bcl-XL Block Thapsigargin-Induced Nitric Oxide Generation c-Jun NH2-Terminal Kinase Activity and Apoptosis

摘要

The proteins Bcl-2 and Bcl-XL prevent apoptosis, but their mechanism of action is unclear. We examined the role of Bcl-2 and Bcl-XL in the regulation of cytosolic Ca²⁺, nitric oxide production (NO), c-Jun NH2-terminal kinase (JNK) activation, and apoptosis in Jurkat T cells. Thapsigargin (TG), an inhibitor of the endoplasmic reticulum-associated Ca²⁺ ATPase, was used to disrupt Ca²⁺ homeostasis. TG acutely elevated intracellular free Ca²⁺ and mitochondrial Ca²⁺ levels and induced NO production and apoptosis in Jurkat cells transfected with vector (JT/Neo). Buffering of this Ca²⁺ response with 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid tetra(acetoxymethyl) ester (BAPTA-AM) or inhibiting NO synthase activity with N^G-nitro-l-arginine methyl ester hydrochloride (l-NAME) blocked TG-induced NO production and apoptosis in JT/Neo cells. By contrast, while TG produced comparable early changes in the Ca²⁺ level (i.e., within 3 h) in Jurkat cells overexpressing Bcl-2 and Bcl-XL (JT/Bcl-2 or JT/Bcl-XL), NO production, late (36-h) Ca²⁺ accumulation, and apoptosis were dramatically reduced compared to those in JT/Neo cells. Exposure of JT/Bcl-2 and JT/Bcl-XL cells to the NO donor, S-nitroso-N-acetylpenacillamine (SNAP) resulted in apoptosis comparable to that seen in JT/Neo cells. TG also activated the JNK pathway, which was blocked by l-NAME. Transient expression of a dominant negative mutant SEK1 (Lys→Arg), an upstream kinase of JNK, prevented both TG-induced JNK activation and apoptosis. A dominant negative c-Jun mutant also reduced TG-induced apoptosis. Overexpression of Bcl-2 or Bcl-XL inhibited TG-induced loss in mitochondrial membrane potential, release of cytochrome c, and activation of caspase-3 and JNK. Inhibition of caspase-3 activation blocked TG-induced JNK activation, suggesting that JNK activation occurred downstream of caspase-3. Thus, TG-induced Ca²⁺ release leads to NO generation followed by mitochondrial changes including cytochrome c release and caspase-3 activation. Caspase-3 activation leads to activation of the JNK pathway and apoptosis. In summary, Ca²⁺-dependent activation of NO production mediates apoptosis after TG exposure in JT/Neo cells. JT/Bcl-2 and JT/Bcl-XL cells are susceptible to NO-mediated apoptosis, but Bcl-2 and Bcl-XL protect the cells against TG-induced apoptosis by negatively regulating Ca²⁺-sensitive NO synthase activity or expression.