Bcl-X _L interacts with Apaf-1 and inhibits Apaf-1-dependent caspase-9 activation

Abstract

Recent studies indicate that Caenorhabditis elegans CED-4 interacts with and promotes the activation of the death protease CED-3, and that this activation is inhibited by CED-9. Here we show that a mammalian homolog of CED-4, Apaf-1, can associate with several death proteases, including caspase-4, caspase-8, caspase-9, and nematode CED-3 in mammalian cells. The interaction with caspase-9 was mediated by the N-terminal CED-4-like domain of Apaf-1. Expression of Apaf-1 enhanced the killing activity of caspase-9 that required the CED-4-like domain of Apaf-1. Furthermore, Apaf-1 promoted the processing and activation of caspase-9 in vivo. Bcl-X L , an antiapoptotic member of the Bcl-2 family, was shown to physically interact with Apaf-1 and caspase-9 in mammalian cells. The association of Apaf-1 with Bcl-X L was mediated through both its CED-4-like domain and the Cterminal domain containing WD-40 repeats. Expression of Bcl-X L inhibited the association of Apaf-1 with caspase-9 in mammalian cells. Significantly, recombinant Bcl-X L purified from Escherichia coli or insect cells inhibited Apaf-1-dependent processing of caspase-9. Furthermore, Bcl-X L failed to inhibit caspase-9 processing mediated by a constitutively active Apaf-1 mutant, suggesting that Bcl-X L regulates caspase-9 through Apaf-1. These experiments demonstrate that Bcl-X L associates with caspase-9 and Apaf-1, and show that Bcl-X L inhibits the maturation of caspase-9 mediated by Apaf-1, a process that is evolutionarily conserved from nematodes to humans.

The opposing roles of the Akt and c-Myc signalling pathways in survival from CD95-mediated apoptosis

“…However, there is no obvious molecular mechanism linking anti-apoptotic Bcl-2 family members with the classical type 1 CD95/ FADD/caspase-8 pathway. Although it has been suggested that Bcl-2 and Bcl-X L directly (Boise and Thompson, 1997) or indirectly (Chinnaiyan et al, 1997;Han et al, 1997;Ng et al, 1997;Hu et al, 1998) bind caspase-8 and so modulate its activation (Medema et al, 1998), a recent report indicates that, in a system in which Bcl-X L e ectively suppresses CD95-induced apoptosis, Bcl-X L neither interacts with caspase-8 nor inhibits its recruitment or activation (Medema et al, 1998). This ®nding seems to leave little room for a role for Bad as an intermediary modulating activation of caspase-8 by FADD.…”

Living with death: the evolution of the mitochondrial pathway of apoptosis in animals

“…[16][17][18][19] As noted, CED-4 lacks the WD-domain repeats present in APAF-1 that are necessary for cytochrome c binding, and the release of CED-4 from sequestration by CED-9 appears to be sufficient to cause formation of the nematode apoptosome. Furthermore, despite early reports to the contrary 20,21 it is now accepted that mammalian Bcl-2 homologs do not functionally interact with APAF-1. [22][23][24] Given these significant differences, it is intriguing that the antiapoptotic functions of both Bcl-2 and CED-9 involve the mitochondria.…”

Signal transduction pathways that regulate cell survival and cell death

“…As mentioned before, in C. elegans CED-9 directly interacts with CED-4 preventing it from activating caspase CED-3. In mammalian cells, Bcl-x L has been reported to interact with the mammalian CED-4 homolog Apaf-1 and by that mechanism might prevent Apaf-1 from activating downstream caspases (Hu et al, 1998).…”