ApoReview-Introduction to Apoptosis:Page 14 of 26 There has been quite some debate about how the Bel-2 family controls apoptosis:one model proposes that Bel-2 members might caspase activation []whereas another mode claims that they mainly act by guarding mitochondrial integrity [Wang.2001].In support of the first model,the worm Bel-2 orthologue ced-9 binds to the Apaf-1-like adaptor protein ced-4 and prevents it from activating the caspase ced-3 unless the BH3-only protein Egl-1 displaces ced-4 as shown in Fig.3 [Conradt.19981.In contrast.the mammalian ced-4 homologue Apaf-1 obviously does not interact with Bel-2-like proteins [Moriishi,199]but is activated by cytosolic cytochrome c(see Fig.5).and it is the release of cytochrome c from the mitochondria that can be controlled by Bel-2 [Kluck,1997:Yang.1997].Therefore it appears likely,that the central function of mammalian Bel-2 family members is to guard mitochondrial integrity and to control the release of mitochondrial proteins into the eytoplasm [Cory,2002]. How then is mitochondrial integrity affected by proapoptotic Bel-2 family members?Central to this question are Bax and Bak,even though inactivation of the Bax gene alone affected apoptosis only slightly and disruption of Bak alone did not show any effect.However,the double knockout of Bax and Bak resulted in dramatic impairment of apoptosis during development in many tissues with superfluous cells accumulating in the hematopoietic system and in the brain.Additionally,cells derived from those Bax-Bak-/-mice are insensitive to treatment with e.g.etoposide or irradiation [Lindsten,2000.Wei,2001].Bax is a cytosolic monomer in viable cells but during apoptosis changes its conformation,integrates into the outer mitochondrial membrane and oligomerizes [Nechushtan, 2001].Although the mechanism is controversial,Bax and Bak oligomers are believed to provoke or contribute to the permeabilization of the outer mitochondrial membrane (PT),either by forming channels by themselves [Antonsson,2000]or by interacting with components of the PT pore such as VDAC [Tsujimoto,2000] In contrast,antiapoptotic Bel-2 members sequester proapoptotic Bel-2 members by bindig to their BH3 domains and thereby utimately prevent Bax or Bak activation/oligomerization and inhibit mitochondrial proapoptotic events:overexpression of Bel-2 or Bel-X potently inhibits apoptosis in response to many cytotoxic insults,among others by suppressing the generation of ROS. stabilizing v,preventing PT and consequently blocking the release of e.g cytochrome[Reed 199].Besides eliciting its antiapoptotic effects on the mitochondrial level by indirectly controlling the activation of the apoptosome,Bcl-2 also appears to inhibit apoptotic pathways that are independent of Apaf-1/caspase-9 and which might depend on caspase-7 as a central effector [Marsden,2002].In this context one might even expect the existence of another but up to now unidentified Apaf-1 homologue that can be directly controlled by Bel-2/Bel-XL [Puthalakath,2002]. ApoReview - Introduction to Apoptosis: Page 14 of 26 There has been quite some debate about how the Bcl-2 family controls apoptosis: one model proposes that Bcl-2 members might directly control caspase activation [Strasser, 2000], whereas another model claims that they mainly act by guarding mitochondrial integrity [Wang, 2001]. In support of the first model, the worm Bcl-2 orthologue ced-9 binds to the Apaf-1-like adaptor protein ced-4 and prevents it from activating the caspase ced-3 unless the BH3-only protein Egl-1 displaces ced-4 as shown in Fig. 3 [Conradt, 1998]. In contrast, the mammalian ced-4 homologue Apaf-1 obviously does not interact with Bcl-2-like proteins [Moriishi, 1999] but is activated by cytosolic cytochrome c (see Fig. 5), and it is the release of cytochrome c from the mitochondria that can be controlled by Bcl-2 [Kluck, 1997; Yang, 1997]. Therefore it appears likely, that the central function of mammalian Bcl-2 family members is to guard mitochondrial integrity and to control the release of mitochondrial proteins into the cytoplasm [Cory, 2002]. How then is mitochondrial integrity affected by proapoptotic Bcl-2 family members? Central to this question are Bax and Bak, even though inactivation of the Bax gene alone affected apoptosis only slightly and disruption of Bak alone did not show any effect. However, the double knockout of Bax and Bak resulted in dramatic impairment of apoptosis during development in many tissues with superfluous cells accumulating in the hematopoietic system and in the brain. Additionally, cells derived from those Bax -/- Bak -/- mice are insensitive to treatment with e.g. etoposide or irradiation [Lindsten, 2000; Wei, 2001]. Bax is a cytosolic monomer in viable cells but during apoptosis changes its conformation, integrates into the outer mitochondrial membrane and oligomerizes [Nechushtan, 2001]. Although the mechanism is controversial, Bax and Bak oligomers are believed to provoke or contribute to the permeabilization of the outer mitochondrial membrane (PT), either by forming channels by themselves [Antonsson, 2000] or by interacting with components of the PT pore such as VDAC [Tsujimoto, 2000]. In contrast, antiapoptotic Bcl-2 members sequester proapoptotic Bcl-2 members by bindig to their BH3 domains and thereby ultimately prevent Bax or Bak activation/ oligomerization and consequently inhibit mitochondrial proapoptotic events: overexpression of Bcl-2 or Bcl-XL potently inhibits apoptosis in response to many cytotoxic insults, among others by suppressing the generation of ROS, stabilizing ∆ψ, preventing PT and consequently blocking the release of e.g. cytochrome c [Reed, 1998]. Besides eliciting its antiapoptotic effects on the mitochondrial level by indirectly controlling the activation of the apoptosome, Bcl-2 also appears to inhibit apoptotic pathways that are independent of Apaf-1/caspase-9 and which might depend on caspase-7 as a central effector [Marsden, 2002]. In this context one might even expect the existence of another but up to now unidentified Apaf-1 homologue that can be directly controlled by Bcl-2/Bcl-XL [Puthalakath, 2002]