An interaction between Bcl-xL and the voltage-dependent anion channel (VDAC) promotes mitochondrial Ca2+ uptake.

The role of the antiapoptotic protein Bcl-xL in regulating mitochondrial Ca(2+) ([Ca(2+)]mito) handling was examined in wild-type (WT) and Bcl-xL knock-out (Bcl-xL-KO) mouse embryonic fibroblast cells. Inositol 1,4,5-trisphosphate-generating agonist evoked cytosolic Ca(2+) transients that produced a larger [Ca(2+)]mito uptake in WT cells compared with Bcl-xL-KO. In permeabilized cells, stepping external [Ca(2+)] from 0 to 3 µm also produced a larger [Ca(2+)]mito uptake in WT; moreover, the [Ca(2+)]mito uptake capacity of Bcl-xL-KO cells was restored by re-expression of mitochondrially targeted Bcl-xL. Bcl-xL enhancement of [Ca(2+)]mito uptake persisted after dissipation of the mitochondrial membrane potential but was absent in mitoplasts lacking an outer mitochondrial membrane. The outer membrane-localized voltage-dependent anion channel (VDAC) is a known Ca(2+) permeability pathway that directly interacts with Bcl-xL. Bcl-xL interacted with VDAC1 and -3 isoforms, and peptides based on the VDAC sequence disrupted Bcl-xL binding. Peptides reduced [Ca(2+)]mito uptake in WT but were without effect in Bcl-xL-KO cells. In addition, peptides reduced [Ca(2+)]mito uptake in VDAC1 and VDAC3 knock-out but not VDAC1 and -3 double knock-out mouse embryonic fibroblast cells, confirming that Bcl-xL interacts functionally with VDAC1 and -3 but not VDAC2. Thus, an interaction between Bcl-xL and VDAC promotes matrix Ca(2+) accumulation by increasing Ca(2+) transfer across the outer mitochondrial membrane.

The Bcl-2 proteins Noxa and Bcl-xL co-ordinately regulate oxidative stress-induced apoptosis.

Because the detailed molecular mechanisms by which oxidative stress induces apoptosis are not completely known, we investigated how the complex Bcl-2 protein network might regulate oxidative stress-induced apoptosis. Using MEFs (mouse embryonic fibroblasts), we found that the endogenous anti-apoptotic Bcl-2 protein Bcl-xL prevented apoptosis initiated by H(2)O(2). The BH3 (Bcl-2 homology 3)-only Bcl-2 protein Noxa was required for H(2)O(2)-induced cell death and was the single BH3-only Bcl-2 protein whose pro-apoptotic activity was completely antagonized by endogenous Bcl-xL. Upon H(2)O(2) treatment, Noxa mRNA displayed the greatest increase among BH3-only Bcl-2 proteins. Expression levels of the anti-apoptotic Bcl-2 protein Mcl-1 (myeloid cell leukaemia sequence 1), the primary binding target of Noxa, were reduced in H(2)O(2)-treated cells in a Noxa-dependent manner, and Mcl-1 overexpression was able to prevent H(2)O(2)-induced cell death in Bcl-xL-deficient MEF cells. Importantly, reduction of the expression of both Mcl-1 and Bcl-xL caused spontaneous cell death. These studies reveal a signalling pathway in which H(2)O(2) activates Noxa, leading to a decrease in Mcl-1 and subsequent cell death in the absence of Bcl-xL expression. The results of the present study indicate that both anti- and pro-apoptotic Bcl-2 proteins co-operate to regulate oxidative stress-induced apoptosis.

ER stress modulates cellular metabolism.

Changes in metabolic processes play a critical role in the survival or death of cells subjected to various stresses. In the present study, we have investigated the effects of ER (endoplasmic reticulum) stress on cellular metabolism. A major difficulty in studying metabolic responses to ER stress is that ER stress normally leads to apoptosis and metabolic changes observed in dying cells may be misleading. Therefore we have used IL-3 (interleukin 3)-dependent Bak-/-Bax-/- haemopoietic cells which do not die in the presence of the ER-stress-inducing drug tunicamycin. Tunicamycin-treated Bak-/-Bax-/- cells remain viable, but cease growth, arresting in G1-phase and undergoing autophagy in the absence of apoptosis. In these cells, we used NMR-based SIRM (stable isotope-resolved metabolomics) to determine the metabolic effects of tunicamycin. Glucose was found to be the major carbon source for energy production and anabolic metabolism. Following tunicamycin exposure, glucose uptake and lactate production are greatly reduced. Decreased 13C labelling in several cellular metabolites suggests that mitochondrial function in cells undergoing ER stress is compromised. Consistent with this, mitochondrial membrane potential, oxygen consumption and cellular ATP levels are much lower compared with untreated cells. Importantly, the effects of tunicamycin on cellular metabolic processes may be related to a reduction in cell-surface GLUT1 (glucose transporter 1) levels which, in turn, may reflect decreased Akt signalling. These results suggest that ER stress exerts profound effects on several central metabolic processes which may help to explain cell death arising from ER stress in normal cells.

Activation of the proapoptotic Bcl-2 protein Bax by a small molecule induces tumor cell apoptosis.

The proapoptotic Bcl-2 protein Bax by itself is sufficient to initiate apoptosis in almost all apoptotic paradigms. Thus, compounds that can facilitate disruptive Bax insertion into mitochondrial membranes have potential as cancer therapeutics. In our study, we have identified small-molecule compounds predicted to associate with the Bax hydrophobic groove by a virtual-screen approach. Among these, one lead compound (compound 106) promotes Bax-dependent but not Bak-dependent apoptosis. Importantly, this compound alters Bax protein stability in vitro and promotes the insertion of Bax into mitochondria, leading to Bax-dependent permeabilization of the mitochondrial outer membrane. Furthermore, as a single agent, compound 106 inhibits the growth of transplanted tumors, probably by inducing apoptosis in tumors. Our study has revealed a compound that activates Bax and induces Bax-dependent apoptosis, which may lead to the development of new therapeutic agents for cancer.

Noxa couples lysosomal membrane permeabilization and apoptosis during oxidative stress.

The exact roles of lysosomal membrane permeabilization (LMP) in oxidative stress-triggered apoptosis are not completely understood. Here, we first studied the temporal relation between LMP and mitochondrial outer membrane permeabilization (MOMP) during the initial stage of apoptosis caused by the oxidative stress inducer H2O2. Despite its essential role in mediating apoptosis, the expression of the BH3-only Bcl-2 protein Noxa was dispensable for LMP. In contrast, MOMP was dependent on Noxa expression and occurred downstream of LMP. When lysosomal membranes were stabilized by the iron-chelating agent desferrioxamine, H2O2-induced increase in DNA damage, Noxa expression, and subsequent apoptosis were abolished by the inhibition of LMP. Importantly, LMP-induced Noxa expression increase was mediated by p53 and seems to be a unique feature of apoptosis caused by oxidative stress. Finally, exogenous iron loading recapitulated the effects of H2O2 on the expression of BH3-only Bcl-2 proteins. Overall, these data reveal a Noxa-mediated signaling pathway that couples LMP with MOMP and ultimate apoptosis during oxidative stress.

Distinct roles of mitochondria- and ER-localized Bcl-xL in apoptosis resistance and Ca2+ homeostasis.

Bcl-2 proteins are major regulators of cellular responses to intrinsic and extrinsic apoptotic stimuli. Among them, overexpression of the antiapoptotic protein Bcl-x(L) modulates intracellular Ca(2+) homeostasis and organelle-specific apoptotic signaling pathways. However, the specific activities of Bcl-x(L) at mitochondria and the endoplasmic reticulum (ER) have not been fully defined. To further explore this, we generated mouse embryonic fibroblast (MEF) cell lines deficient in Bcl-x(L) expression (Bcl-x-KO). Deficiency in Bcl-x(L) expression did not induce compensatory changes in the expression of other Bcl-2 proteins, and Bcl-x-KO MEF cells showed increased sensitivity to various apoptotic stimuli compared with wild-type MEF cells. Targeting Bcl-x(L) at mitochondria but not at the ER restored apoptosis protection in Bcl-x-KO MEF cells to the degree observed in wild-type MEF cells. However, expression of ER-targeted Bcl-x(L) but not mitochondrially targeted Bcl-x(L) was required to restore Ca(2+) homeostasis in Bcl-x-KO MEF cells. Of importance, ER-targeted Bcl-x(L) was able to protect cells against death stimuli in the presence of endogenous Bcl-x(L). These data indicate that mitochondrial Bcl-x(L) can regulate apoptosis independently of ER Bcl-x(L) and that when localized exclusively at the ER, Bcl-x(L) impinges on Ca(2+) homeostasis but does not affect apoptosis unless Bcl-x(L) is present in additional cellular compartments.