The endoplasmic reticulum gateway to apoptosis by Bcl-XL modulation of the InsP3R

Abstract

Members of the Bcl-2 protein family modulate outer mitochondrial membrane permeability to control apoptosis1 ,2 . However, these proteins also localize to the endoplasmic reticulum (ER), the functional significance of which is controversial 3, 4. Here we provide evidence that anti-apoptotic Bcl-2 proteins regulate the inositol 1,4,5-trisphosphate receptor (InsP 3 R) ER Ca 2+ release channel resulting in increased cellular apoptotic resistance and enhanced mitochondrial bioenergetics. Anti-apoptotic Bcl-X L interacts with the carboxyl terminus of the InsP 3 R and sensitizes single InsP 3 R channels in ER membranes to low [InsP 3 ], enhancing Ca 2+ and InsP 3 -dependent regulation of channel activity in vitro and in vivo, reducing ER Ca 2+ content and stimulating mitochondrial energetics. The proapoptotic proteins Bax and tBid antagonize this effect by blocking the biochemical interaction of Bcl-X L with the InsP 3 R. These data support a novel model in which Bcl-X L is a direct effector of the InsP 3 R, increasing its sensitivity to InsP 3 and enabling ER Ca 2+ release to be more sensitively coupled to extracellular signals. As a consequence, cells are protected against apoptosis by a more sensitive and dynamic coupling of ER to mitochondria through Ca 2+ -dependent signal transduction that enhances cellular bioenergetics and preserves survival.A central feature of molecular models of apoptosis is the control of outer mitochondrial membrane permeability by Bcl-2-related proteins. The pro-apoptotic Bcl-2-related proteins Bax and Bak are required to initiate cytochrome c release from mitochondria in response to diverse apoptotic stimuli 1,5 . Anti-apoptotic properties of Bcl-2 and Bcl-X L have been attributed to their ability to antagonize Bax/Bak by forming heterodimers that prevent their oligomerization and apoptosis initiation 6 . Pro-and anti-apoptotic Bcl-2 proteins also localize to the ER 3,7 , and it is now recognized that the ER has an important role in regulating apoptosis 8,9 . The ER is thought to contribute to apoptosis through its role as the principle Ca 2+ storage organelle in cells [8][9][10][11] . At physiological levels, Ca 2+ released from the ER during

Bcl-2 and Ca2+ homeostasis in the endoplasmic reticulum

“…Recombinant expression (or addition to the patch pipette) of Bax prevented the effect of Bcl-X L , both in terms of its binding to the IP3R and of capacity of modifying the sensitivity to IP3. 79 The results differ from a previous paper by the Distelhorst group that showed that Bcl-2 reduces the opening probability of IP3Rs inserted into lipid bilayers. 53 As to the mechanism, the Foskett-Thompson paper proposes a direct modulatory role on IP3R (through protein-protein interaction between the two proteins) rather than a phosphorylation event, based on the timing after Bcl-X L addition and the experimental conditions employed (nonpermissive for phosphorylation).…”

“…80 Finally, the Foskett-Thompson paper, by looking at sensitivity to apoptosis of DT40 cells in which all three IP3R isoforms had been deleted, shows that basal activation of the IP3R not only partially depletes the Ca 2 þ store but also provides a 'survival' signal per se (possibly by activating mitochondrial metabolism), given that the KO cells are more sensitive to apoptotic challenges than the wild-type cells. 79 Also in this case, given the robust evidence for the role of mitochondrial Ca 2 þ uptake in facilitating PTP opening and proapoptotic mitochondrial changes, it seems fair to conclude that Bcl-2 globally reprogrammes ER Ca 2 þ release and mitochondrial Ca 2 þ loading, as necessary for optimally activating apoptosis (in terms of metabolic requirements as well as prompt release of caspase cofactors into the cytosol).…”

Expression of anti‐apoptosis genes alters lactate metabolism of Chinese Hamster Ovary cells in culture

“…In a second role, Bcl-XL by its association with VDAC, has been shown to regulate flow of ATP/ADP in and out of mitochondria (Vander Heiden et al, 1999). Additionally, Bcl-XL was shown to bind the inositol 1,3,5-triosephosphate receptor in the endoplasmic reticulum resulting in increased cellular apoptotic resistance and enhanced mitochondrial bioenergetics (White et al, 2005). Consequently, there is now a clear relationship established between the cellular energetics and anti-apoptosis proteins in the mitochondria and other cellular compartments including the endoplasmic reticulum.…”

“…The central metabolic role of the mitochondria was initially considered distinct from its contribution to apoptosis. While the functionality of the apoptotic proteins still holds true, it is now evident that these proteins also play roles in the metabolic processes of the mitochondria (see Majors et al, 2007 for a review; White et al, 2005). Furthermore, apoptosis pathway proteins in other organelles within the cell may modulate both apoptosis and metabolism.…”

The permeability transition and BCL-2 family proteins in apoptosis: co-conspirators or independent agents?

“…A number of studies have demonstrated that members of the BCL-2 family reside not only in the OMM but also in the ER where they have opposing actions in regulating the transfer of ER Ca 2 þ to mitochondria; antiapoptotic members reduce ER Ca 2 þ and pro-apoptotic members promote Ca 2 þ mobilization from the ER to mitochondria during apoptosis, perhaps by regulating of the activity of the ER inositol trisphosphate receptor. [43][44][45] Thus, the action of the PT and BCL-2 family members may be harmonized through the ability of BCL-2 proteins to regulate the levels of ER Ca 2 þ , the key trigger of the mitochondrial PT.…”