Contribution of membrane localization to the apoptotic activity of PUMA

The BH3-only protein PUMA plays an important role in the activation of apoptosis in response to p53. In different studies, PUMA has been described to function by either directly activating the pro-apoptotic proteins Bax and Bak, or by neutralizing anti-apoptotic members of the Bcl2 family. We have examined the contribution of regions of PUMA other than the BH3 domain to its localization and function. Although the hydrophobic domain in the C-terminus of PUMA is necessary for efficient mitochondrial localization of PUMA itself, PUMA proteins lacking this region can still induce apoptosis and localize to the mitochondria through binding to Bcl2. Even a nuclear localization signal (NLS)-tagged PUMA protein retains apoptotic activity and can be efficiently relocalized from the nucleus after interaction with ectopically expressed Bcl2, underscoring the efficiency of this interaction. Interestingly, unlike the Bcl2 interaction, the binding of PUMA to Bax is severely compromised by the loss of the C-terminal domain of PUMA. However, since the loss of the C-terminus does not compromise the ability of PUMA to induce cell death, our results indicate that the key apoptotic function of PUMA is through interaction with anti-apoptotic proteins such as Bcl2.     

Identification of the protein-protein contact site and interaction mode of human VDAC1 with Bcl-2 family proteins

Bcl-2 family of proteins plays differential roles in regulation of mitochondria-mediated apoptosis, by either promoting or inhibiting the release of apoptogenic molecules from mitochondria to cytosol. Bcl-2 family proteins modulate the mitochondrial permeability through interaction with adenine nucleotide translocator (ANT), voltage-dependent anion channel (VDAC), ADP/ATP exchange, or oxidative phosphorylation during apoptosis. Although the mitochondrial homeostasis is affected by the relative ratio of pro- and anti-apoptotic Bcl-2 family members, the molecular mechanism underlying the release of mitochondrial intermembrane proteins remains elusive. Here we reported the biochemical evidence that both pro-apoptotic Bax and anti-apoptotic Bcl-X(L) might simultaneously contact the putative loop regions of human VDAC1, and the existence of VDAC1-Bax-Bcl-X(L) tertiary complex in vitro suggested that VDAC1 channel conformation and mitochondrial permeability could be determined by the delicate balance between Bax and Bcl-X(L).     

Phosphorylation switches Bax from promoting to inhibiting apoptosis thereby increasing drug resistance

Akt is a pro‐survival kinase frequently activated in human cancers and is associated with more aggressive tumors that resist therapy. Here, we connect Akt pathway activation to reduced sensitivity to chemotherapy via Akt phosphorylation of Bax at residue S184, one of the pro‐apoptotic Bcl‐2 family proteins required for cells to undergo apoptosis. We show that phosphorylation by Akt converts the pro‐apoptotic protein Bax into an anti‐apoptotic protein. Mechanistically, we show that phosphorylation (i) enables Bax binding to pro‐apoptotic BH3 proteins in solution, and (ii) prevents Bax inserting into mitochondria. Together, these alterations promote resistance to apoptotic stimuli by sequestering pro‐apoptotic activator BH3 proteins. Bax phosphorylation correlates with cellular resistance to BH3 mimetics in primary ovarian cancer cells. Further, analysis of the TCGA database reveals that 98% of cancer patients with increased BAX levels also have an upregulated Akt pathway, compared to 47% of patients with unchanged or decreased BAX levels. These results suggest that in patients, increased phosphorylated anti‐apoptotic Bax promotes resistance of cancer cells to inherent and drug‐induced apoptosis.     

Bcl—2家族蛋白与细胞凋亡

Bcl 2家族蛋白是在细胞凋亡过程中起关键性作用的一类蛋白质。在线粒体上 ,Bcl 2家族蛋白通过与其他凋亡蛋白的协同作用 ,调控线粒体结构与功能的稳定性 ,发挥着细胞凋亡“主开关”的作用。Bcl 2家族包括两类蛋白质 :一类是抗凋亡蛋白 ,另一类是促凋亡蛋白。在细胞凋亡时 ,Bcl 2家族中的促凋亡蛋白成员发生蛋白质的加工修饰 ,易位到线粒体的外膜上 ,引起细胞色素c、凋亡诱导因子等其他促凋亡因子的释放 ,导致细胞凋亡 ;而平时被隔离在线粒体等细胞器内的该家族的抗凋亡蛋白成员则抑制细胞色素c和凋亡诱导因子等促凋亡因子的释放 ,具有抑制细胞凋亡的功能。但一旦这类抗凋亡蛋白成员与激活的促凋亡蛋白发生相互作用后 ,便丧失了对细胞凋亡的抑制作用 ,造成线粒体等细胞器的功能丧失和细胞器内促凋亡因子的释放 ,导致细胞凋亡。现以Bcl 2家族调控细胞凋亡的最新研究进展为基础 ,对Bcl 2家族成员及其蛋白质结构、分布和调控细胞凋亡的分子机制进行综述。     

SUMOylation of MCL1 protein enhances its stability by regulating the ubiquitin-proteasome pathway

In cancers, apoptosis evasion through dysregulation of pro-apoptotic and anti-apoptotic intracellular signals is a recurring event. Accordingly, selective inhibition of specific proteins represents an exciting therapeutic opportunity. Myeloid cell leukemia 1 (MCL1) is an anti-apoptotic protein of the BCL-2 family, which is overexpressed in many cancers. Here, we demonstrate that MCL1 can be modified by the small ubiquitin-like modifier (SUMO) at K234 and K238 sites. The SUMOylation of MCL1 can improve its stability by inhibiting the MCL1 ubiquitin-proteasome pathway mediated by the Tripartite motif-containing 11 (TRIM11, a novel MCL1 ubiquitin E3 ligase that we identify in this study). Moreover, SUMOylation of MCL1 increases the proliferation of cancer cells by inhibiting apoptosis. These results suggest that the SUMOylation of MCL1 may play a significant role in the regulation of its function.     

TOM20-mediated transfer of Bcl2 from ER to MAM and mitochondria upon induction of apoptosis

In this work, we have explored the subcellular localization of Bcl2, a major antiapoptotic protein. In U251 glioma cells, we found that Bcl2 is localized mainly in the ER and is translocated to MAM and mitochondria upon induction of apoptosis; this mitochondrial transfer was not restricted to the demonstrator cell line, even if cell-specific modulations exist. We found that the Bcl2/mitochondria interaction is controlled by TOM20, a protein that belongs to the protein import machinery of the mitochondrial outer membrane. The expression of a small domain of interaction of TOM20 with Bcl2 potentiates its anti-apoptotic properties, which suggests that the Bcl2–TOM20 interaction is proapoptotic. The role of MAM and TOM20 in Bcl2 apoptotic mitochondrial localization and function has been confirmed in a yeast model in which the ER–mitochondria encounter structure (ERMES) complex (required for MAM stability in yeast) has been disrupted. Bcl2–TOM20 interaction is thus an additional player in the control of apoptosis.Subject terms: Cancer, Apoptosis, Mitochondria     

Bcl-2 proteins and mitochondria--specificity in membrane targeting for death

The localization and control of Bcl-2 proteins on mitochondria is essential for the intrinsic pathway of apoptosis. Anti-apoptotic Bcl-2 proteins reside on the outer mitochondrial membrane (OMM) and prevent apoptosis by inhibiting the activation of the pro-apoptotic family members Bax and Bak. The Bcl-2 subfamily of BH3-only proteins can either inhibit the anti-apoptotic proteins or directly activate Bax or Bak. How these proteins interact with each other, the mitochondrial surface and within the OMM are complex processes we are only beginning to understand. However, these interactions are fundamental for the transduction of apoptotic signals to mitochondria and the subsequent release of caspase activating factors into the cytosol. In this review we will discuss our knowledge of how Bcl-2 proteins are directed to mitochondria in the first place, a crucial but poorly understood aspect of their regulation. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.     

Bcl-2 proteins and mitochondria—Specificity in membrane targeting for death

The localization and control of Bcl-2 proteins on mitochondria is essential for the intrinsic pathway of apoptosis. Anti-apoptotic Bcl-2 proteins reside on the outer mitochondrial membrane (OMM) and prevent apoptosis by inhibiting the activation of the pro-apoptotic family members Bax and Bak. The Bcl-2 subfamily of BH3-only proteins can either inhibit the anti-apoptotic proteins or directly activate Bax or Bak. How these proteins interact with each other, the mitochondrial surface and within the OMM are complex processes we are only beginning to understand. However, these interactions are fundamental for the transduction of apoptotic signals to mitochondria and the subsequent release of caspase activating factors into the cytosol. In this review we will discuss our knowledge of how Bcl-2 proteins are directed to mitochondria in the first place, a crucial but poorly understood aspect of their regulation. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.     

Regulation of hexokinase binding to VDAC

Hexokinase isoforms I and II bind to mitochondrial outer membranes in large part by interacting with the outer membrane voltage-dependent anion channel (VDAC). This interaction results in a shift in the susceptibility of mitochondria to pro-apoptotic signals that are mediated through Bcl2-family proteins. The upregulation of hexokinase II expression in tumor cells is thought to provide both a metabolic benefit and an apoptosis suppressive capacity that gives the cell a growth advantage and increases its resistance to chemotherapy. However, the mechanisms responsible for the anti-apoptotic effect of hexokinase binding and its regulation remain poorly understood. We hypothesize that hexokinase competes with Bcl2 family proteins for binding to VDAC to influence the balance of pro-and anti-apoptotic proteins that control outer membrane permeabilization. Hexokinase binding to VDAC is regulated by protein kinases, notably glycogen synthase kinase (GSK)-3β and protein kinase C (PKC)-ɛ. In addition, there is evidence that the cholesterol content of the mitochondrial membranes may contribute to the regulation of hexokinase binding. At the same time, VDAC associated proteins are critically involved in the regulation of cholesterol uptake. A better characterization of these regulatory processes is required to elucidate the role of hexokinases in normal tissue function and to apply these insights for optimizing cancer treatment.     

Prohibitin (PHB) inhibits apoptosis in rat granulosa cells (GCs) through the extracellular signal-regulated kinase 1/2 (ERK1/2) and the Bcl family of proteins

Mammalian ovarian follicular development is tightly regulated by crosstalk between cell death and survival signals, which include both endocrine and intra-ovarian regulators. Whether the follicle ultimately ovulates or undergoes atresia is dependent on the expression and actions of factors promoting follicular cell proliferation, differentiation or apoptosis. Prohibitin (PHB) is a highly conserved, ubiquitous protein that is abundantly expressed in granulosa cells (GCs) and associated with GC differentiation and apoptosis. The current study was designed to characterize the regulation of anti-apoptotic and pro-apoptotic factors in undifferentiated rat GCs (gonadotropin independent phase) governed by PHB. Microarray technology was initially employed to identify potential apoptosis-related genes, whose expression levels within GCs were altered by either staurosporine (STS) alone or STS in presence of ectopically over-expressed PHB. Next, immunoblot studies were performed to examine the expression patterns of selective Bcl-2 family members identified by the microarray analysis, which are commonly regulated in the intrinsic-apoptotic pathway. These studies were designed to measure protein levels of Bcl2 family in relation to expression of the acidic isoform (phosphorylated) PHB and the components of MEK-Erk1/2 pathway. These studies indicated that over-expression of PHB in undifferentiated GCs inhibit apoptosis which concomitantly results in an increased level of the anti-apoptotic proteins Bcl2 and Bclxl, reduced release of cytochrome c from mitochondria and inhibition of caspase-3 activity. In contrast, silencing of PHB expression resulted in change of mitochondrial morphology from the regular reticular network to a fragmented form, which enhanced sensitization of these GCs to the induction of apoptosis. Collectively, these studies have provided new insights on the PHB-mediated anti-apoptotic mechanism, which occurs in undifferentiated GCs through a PHB → Mek-Erk1/2 → Bcl/Bcl-xL pathway and may have important clinical implications.     

A functional role for nicotine in Bcl2 phosphorylation and suppression of apoptosis

Nicotine is not only a major component in tobacco but is also a survival agonist that inhibits apoptosis induced by diverse stimuli including chemotherapeutic drugs. However, the intracellular mechanism(s) involved in nicotine suppression of apoptosis is unclear. Bcl2 is a potent antiapoptotic protein and tumor promotor that is expressed in both small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC) cells. It is possible that nicotine may regulate Bcl2 to stimulate cell survival. Here we report that nicotine can induce Bcl2 phosphorylation exclusively at the serine 70 site in association with prolonged survival of SCLC H82 cells expressing wild-type but not the phosphorylation-deficient S70A mutant Bcl2 after treatment with chemotherapeutic agents (i.e. cisplatin or VP-16). Nicotine induces activation of PKC alpha and the MAPKs ERK1 and ERK2, which are physiological Bcl2 kinases. Furthermore, ET-18-OCH3, a specific phospholipase C (PLC) inhibitor, blocks nicotine-stimulated Bcl2 phosphorylation and promotes apoptosis, suggesting that PLC may be involved in nicotine activation of Bcl2 kinases. Using a genetic approach, the gain-of-function S70E mutant, which mimics Ser(70) site phosphorylation in the flexible loop domain, potently enhances chemoresistance in SCLC cells. Thus, nicotine-induced cell survival results, at least in part, from a mechanism that involves Bcl2 phosphorylation. Therefore, novel therapeutic strategies for lung cancer in which Bcl2 is expressed may be used to abrogate the anti-apoptotic activity of Bcl2 by inhibiting multiple upstream nicotine-activated pathways.     

Bcl-2 on the endoplasmic reticulum regulates Bax activity by binding to BH3-only proteins

Bcl-2 family members have been shown to be key mediators of apoptosis as either pro- or anti-apoptotic factors. It is thought that both classes of Bcl-2 family members act at the level of the mitochondria to regulate apoptosis, although the founding anti-apoptotic family member, Bcl-2 is localized to the endoplasmic reticulum (ER), mitochondrial, and nuclear membranes. In order to better understand the effect of Bcl-2 localization on its activity, we have utilized a Bcl-2 mutant that localizes only to the ER membrane, designated Bcl-2Cb5. Bcl-2Cb5 was expressed in MDA-MB-468 cells, which protected against apoptosis induced by the kinase inhibitor, staurosporine. Data presented here show that Bcl-2Cb5 inhibits this process by blocking Bax activation and cytochrome c release. Furthermore, we show that Bcl-2Cb5 can inhibit the activation of a constitutively mitochondrial mutant of Bax, indicating that an intermediate between Bcl-2 on the ER and Bax on the mitochondria must exist. We demonstrate that this intermediate is likely a BH3-only subfamily member. Data presented here show that Bcl-2Cb5 can sequester a constitutively active form of Bad (Bad3A) from the mitochondria and prevent it from activating Bax. These data suggest that Bcl-2 indirectly protects mitochondrial membranes from Bax, via BH3-only proteins.     

Anti-apoptotic activity of Bcl-2 is enhanced by its interaction with RTN3

Bcl-2 is known as a critical inhibitor of apoptosis triggered by a broad range of stimuli, mainly acting on the mitochondria. It can interact with many members of the Bcl-2 family, influence mitochondrial membrane permeability and modulate cell apoptosis. RTN3, a member of the reticulon (RTN) family, was predominantly localized on the endoplasmic reticulum (ER). Its N- and C-termini, both facing the cytoplasm, can recruit some proteins to the ER to modulate some physiological functions. We found that RTN3, which does not belong to the Bcl-2 family, can interact with Bcl-2 on the ER. In normal HeLa cells, ectopic overexpressed Bcl-2 could reduce the cell apoptosis induced by overexpressed RTN3. When the HeLa cells stably expressing Bcl-2 were treated with tunicamycin, endogenous RTN3 increased in the cell microsomal fraction. This change increased the Bcl-2 in microsomal fractions and also in the mitochondrial fractions where the anti-apoptotic activity of Bcl-2 mainly acts. These results suggest that RTN3 could bind with Bcl-2 and mediate its accumulation in mitochondria, which modulate the anti-apoptotic activity of Bcl-2.     

VDAC, a multi-functional mitochondrial protein as a pharmacological target

Regulation of mitochondrial physiology requires an efficient exchange of molecules between mitochondria and the cytoplasm via the outer mitochondrial membrane (OMM). The voltage-dependent anion channel (VDAC) lies in the OMM and forms a common pathway for the exchange of metabolites between the mitochondria and the cytosol, thus playing a crucial role in the regulation of metabolic and energetic functions of mitochondria. VDAC is also recognized to function in mitochondria-mediated apoptosis and in apoptosis regulation via interaction with anti-apoptotic proteins, namely members of Bcl-2 family, and the pro-survival protein, hexokinase, overexpressed in many cancer types. Thus, VDAC appears to be a convergence point for a variety of cell survival and cell death signals, mediated by its association with various ligands and proteins. In this article, we review mammalian VDAC, specifically focusing on VDAC1, addressing its functions in cell life and the regulation of apoptosis and its involvement in several diseases. Additionally, we provide insight into the potential of VDAC1 as a rational target for novel therapeutics.     

DHA induces apoptosis by altering the expression and cellular location of GRP78 in colon cancer cell lines

n-3 polyunsaturated fatty acids exert growth-inhibitory and pro-apoptotic effects in colon cancer cells. We hypothesized that the anti-apoptotic glucose related protein of 78kDa (GRP78), originally described as a component of the unfolded protein response in endoplasmic reticulum (ER), could be a molecular target for docosahexaenoic acid (DHA) in these cells. GRP78 total and surface overexpression was previously associated with a poor prognosis in several cancers, whereas its down-regulation with decreased cancer growth in animal models. DHA treatment induced apoptosis in three colon cancer cell lines (HT-29, HCT116 and SW480), and inhibited their total and surface GRP78 expression. The cell ability to undergo DHA-induced apoptosis was inversely related to their level of GRP78 expression. The transfection of the low GRP78-expressing SW480 cells with GRP78-GFP cDNA significantly induced cell growth and inhibited the DHA-driven apoptosis, thus supporting the essential role of GRP78 in DHA pro-apoptotic effect. We suggest that pERK1/2 could be the first upstream target for DHA, and demonstrate that, downstream of GRP78, DHA may exert its proapoptotic role by augmenting the expression of the ER resident factors ERdj5 and inhibiting the phosphorylation of PKR-like ER kinase (PERK), known to be both physically associated with GRP78, and by activating caspase-4. Overall, the regulation of cellular GRP78 expression and location is suggested as a possible route through which DHA can exert pro-apoptotic and antitumoral effects in colon cancer cells.     

Regulation of mitochondrial metabolism: yet another facet in the biology of the oncoprotein Bcl-2

The Bcl-2 (Bcl is B-cell lymphocytic-leukaemia proto-oncogene) family comprises two groups of proteins with distinct functional biology in cell-fate signalling. Bcl-2 protein was the first member to be discovered and associated with drug resistance in human lymphomas. Since then a host of other proteins such as Bcl-xL, Bcl-2A1 and Mcl-1 with similar anti-apoptotic functions have been identified. In contrast, the pro-apoptotic Bcl-2 proteins contain prototypic effector proteins such as Bax and Bak, and the BH3 (Bcl-2 homology)-only proteins comprising Bak, Bid, Bim, Puma and Noxa. A complex interplay between the association of pro-apoptotic and anti-apoptotic proteins with each other determines the sensitivity of cancer cells to drug-induced apoptosis. The canonical functional of Bcl-2 in terms of apoptosis inhibition is its ability to prevent mitochondrial permeabilization via inhibiting the translocation and oligomerization of pro-apoptotic proteins such as Bax; however, more recent evidence points to a novel mechanism of the anti-apoptotic activity of Bcl-2. Overexpression of Bcl-2 increases mitochondrial oxygen consumption and in doing so generates a slight pro-oxidant intracellular milieu, which promotes genomic instability and blocks death signalling. However, in the wake of overt oxidative stress, Bcl-2 regulates cellular redox status thereby preventing excessive build-up of ROS (reactive oxygen species), which is detrimental to cells and tissues. Taken together, the canonical and non-canonical activities of Bcl-2 imply a critical involvement of this protein in the processes of tumour initiation and progression. In the present paper we review these functionally distinct outcomes of Bcl-2 expression with implications for the chemotherapeutic management of cancers.     

The endoplasmic reticulum gateway to apoptosis by Bcl-X(L) modulation of the InsP3R

Members of the Bcl-2 protein family modulate outer mitochondrial membrane permeability to control apoptosis. However, these proteins also localize to the endoplasmic reticulum (ER), the functional significance of which is controversial. 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 pro-apoptotic 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.