Regulation of anti-apoptotic BCL2-proteins by non-canonical interactions: the next step forward or two steps back?

MCL1 binding to the reverse BH3 motif of P18INK4C couples cell survival to cell proliferation

Commitment to cell cycle entry and cellular duplication is a tightly coordinated and regulated process. Once initiated, a series of multiple checkpoints ensure both accurate genomic replication and chromosomal separation. In the event of unsuccessful cell division, parallel pathways exist that induce the cell to undergo programmed cell death, or apoptosis. At the center of such stress-induced, intrinsic apoptotic regulation lies the BCL2 family of pro- and anti-apoptotic regulatory proteins. In a proliferative state the balance of pro- and anti-apoptotic signaling proteins would be expected to favor an excess population of anti-apoptotic members. While the anti-apoptotic BCL2 family member, MCL1, has been identified to oversee mitotic progression, direct communication between the BCL2 family and cell proliferation has not been observed. In this study, we demonstrate a direct protein–protein interaction between MCL1 and the G₁/S checkpoint protein, P18INK4C. This interaction is mediated by a reverse BH3 (rBH3) motif located in P18INK4C’s C-terminal ankyrin repeat. MCL1 is further shown to decrease P18INK4C expression and thereby regulate cell cycle entry in a retinoblastoma (RB1)-dependent manner. Our findings establish a mechanism for translation independent and direct communication between the BCL2 family regulation of apoptosis and CDK4/6-RB regulation of early G₁/S transition during cellular division/growth.

Genetic Aberrations Associated with Photodynamic Therapy in Colorectal Cancer Cells

Photodynamic therapy (PDT) is a cancer treatment modality that utilizes three components: light (λ 650-750 nm), a photosensitizer (PS) and molecular oxygen, which upon activation renders the modality effective. Colorectal cancer has one of the highest incident rates as well as a high mortality rate worldwide. In this study, a zinc (Zn) metal-based phthalocyanine (ZnPcSmix) PS was used to determine its efficacy for the treatment of colon adenocarcinoma cells (DLD-1 and Caco-2). Photoactivation of the PS was achieved by laser irradiation at a wavelength of 680 nm. Dose responses were performed to establish optimal PS concentration and irradiation fluence. A working combination of 20 µM ZnPcSmix and 5 J/cm² was used. Biochemical responses were determined after 1 or 24 h incubation post-treatment. Since ZnPcSmix is localized in lysosomes and mitochondria, mitochondrial destabilization analysis was performed monitoring mitochondrial membrane potential (MMP). Cytosolic acidification was determined measuring hydrogen peroxide (H₂O₂) levels in the cytoplasm. Having established apoptotic cell death induction, an apoptosis PCR array was performed to establish the apoptotic mechanism. In DLD-1 cells, expression of genes included 3 up-regulated and 20 down-regulated genes while in Caco-2 cells, there were 16 up-regulated and 22 down-regulated genes. In both cell lines, in up-regulated genes, there was a combination of pro- and anti-apoptotic genes that were significantly expressed. Gene expression results showed that more tumorigenic cells (DLD-1) went through apoptosis; however, they exhibit increased risk of resistance and recurrence, while less tumorigenic Caco-2 cells responded better to PDT, thus being suggestive of a better prognosis post-PDT treatment. In addition, the possible apoptotic mechanisms of cell death were deduced based on the genetic expression profiling of regulatory apoptotic inducing factors.

PI3K-Akt Signal Transduction Molecules Maybe Involved in Downregulation of Erythroblasts Apoptosis and Perifosine Increased Its Apoptosis in Chronic Mountain Sickness

Background

Chronic mountain sickness (CMS) has a higher incidence in the plateau region. The one of its principal characters is excessive erythrocytosis. The PI3K-Akt pathway plays an important role in the process of erythropoiesis, and could downregulate apoptosis by regulating apoptosis-related molecules. In this paper, we explored the change in apoptosis of erythroblasts and the effect of the PI3K-Akt signal pathway on erythroblasts apoptosis in CMS.

Material/Methods

A total of 22 CMS and 20 non-CMS participants were involved in this study. Bone marrow mononuclear cells were cultured and treated with celecoxib and perifosine in vitro for 72 hours. The apoptotic rate, the mRNA expressions of Akt, Bcl-xl, and caspase-9, and the protein expressions of Akt, p-Akt, Bcl-xl, and caspase-9 were determined by flow cytometry, quantitative RT-PCR, and western-blot technique.

Results

The apoptotic rate of cultured erythroblasts was lower in the CMS group than in the non-CMS group. It was increased after perifosine intervention. The mRNA and protein expressions of Akt and Bcl-xl were higher and caspase-9 was lower in the CMS group than the non-CMS group. Perifosine induced decreased Bcl-xl mRNA and proteins and p-Akt proteins, and increased caspase-9 mRNA and proteins in vitro. In the CMS group, the hemoglobin concentration was correlated with apoptotic rate negatively and with Bcl-xl mRNA positively in erythroblasts; the erythroblasts apoptotic rate was negatively associated with the Akt mRNA and Bcl-xl mRNA.

Conclusion

The erythroblasts apoptosis was downregulated and the PI3K-Akt signal pathway appeared to be involved in the mechanism of decreased erythroblasts apoptosis in CMS.

The Bcl-2 family: structures, interactions and targets for drug discovery

Two phylogenetically and structurally distinct groups of proteins regulate stress induced intrinsic apoptosis, the programmed disassembly of cells. Together they form the B cell lymphoma-2 (Bcl-2) family. Bcl-2 proteins appeared early in metazoan evolution and are identified by the presence of up to four short conserved sequence blocks known as Bcl-2 homology (BH) motifs, or domains. The simple BH3-only proteins bear only a BH3-motif and are intrinsically disordered proteins and antagonize or activate the other group, the multi-motif Bcl-2 proteins that have up to four BH motifs, BH1-BH4. Multi-motif Bcl-2 proteins are either pro-survival or pro-apoptotic in action and have remarkably similar α-helical bundle structures that provide a binding groove formed from the BH1, BH2, and BH3-motifs for their BH3-bearing antagonists. In mammals a network of interactions between Bcl-2 members regulates mitochondrial outer membrane permeability (MOMP) and efflux of cytochrome c and other death inducing factors from mitochondria to initiate the apoptotic caspase cascade, but the molecular events leading to MOMP are uncertain. Dysregulation of the Bcl-2 family occurs in many diseases and pathogenic viruses have assimilated pro-survival Bcl-2 proteins to evade immune responses. Their role in disease has made the Bcl-2 family the focus of drug design attempts and clinical trials are showing promise for 'BH3-mimics', drugs that mimic the ability of BH3-only proteins to neutralize selected pro-survival proteins to induce cell death in tumor cells. This review focuses on the structural biology of Bcl-2 family proteins, their interactions and attempts to harness them as targets for drug design.

Regulation of ceramide channel formation and disassembly: Insights on the initiation of apoptosis

Sphingolipid research has surged in the past two decades and has produced a wide variety of evidence supporting the role of this class of molecules in mediating cellular growth, differentiation, senescence, and apoptosis. Ceramides are a subgroup of sphingolipids (SLs) that are directly involved in the process of initiation of apoptosis. We, and others, have recently shown that ceramides are capable of the formation of protein-permeable channels in mitochondrial outer membranes under physiological conditions. These pores are indeed good candidates for the pathway of release of pro-apoptotic proteins from the mitochondrial intermembrane space (IMS) into the cytosol to initiate intrinsic apoptosis. Here, we review recent findings on the regulation of ceramide channel formation and disassembly, highlighting possible implications on the initiation of the intrinsic apoptotic pathway.

Dimethyl fumarate induces apoptosis of hematopoietic tumor cells via inhibition of NF-κB nuclear translocation and down-regulation of Bcl-xL and XIAP

Dimethyl fumarate (DMF) is a fumaric acid ester that is used to treat psoriasis and multiple sclerosis. Recently, DMF was found to exhibit anti-tumor effects. However, the molecular mechanisms underlying these effects have not been elucidated. In this study, we investigated the mechanism of DMF-induced apoptosis in different human hematopoietic tumor cell lines. We found that DMF induced apoptosis in different human hematopoietic tumor cell lines but it did not affect the normal human B lymphocyte cell line RPMI 1788. We also observed a concurrent increase in caspase-3 activity and in the number of Annexin-V-positive cells. Furthermore, an examination of the survival signals, which are activated by apoptotic stimuli, revealed that DMF significantly inhibited nuclear factor-κB (NF-κB) p65 nuclear translocation. In addition, DMF suppressed B-cell lymphoma extra-large (Bcl-xL) and X-linked inhibitor of apoptosis (XIAP) expression whereas Bcl-2, survivin, Bcl-2-associated X protein (Bax), and Bim levels did not change. These results indicated that DMF induced apoptosis by suppressing NF-κB activation, and Bcl-xL and XIAP expression. These findings suggested that DMF might have potential as an anticancer agent that could be used in combination therapy with other anticancer drugs for the treatment of human hematopoietic tumors.

The structural biology of BH3-only proteins

B-cell lymphoma-2 (Bcl-2) homology-3 (BH3)-only proteins are considered members of the Bcl-2 family, though they bear little sequence or structural identity with the multi-BH motif prosurvival or proapoptotic Bcl-2 proteins like Bcl-2 or Bax. They are better considered a separate phylogenetic entity. In combination, results from biophysical, biochemical, cell biological, and animal studies in conjunction with structural investigations have elucidated the function and mechanism of action of these proteins. Either by antagonizing prosurvival Bcl-2 proteins or directly activating proapoptotic Bcl-2 proteins (Bax or Bak) they initiate apoptosis. BH3-only proteins are intrinsically disordered and fold and bind into a groove provided by their cognate receptor Bcl-2 family proteins. Our detailed molecular understanding of BH3-only protein action has aided the development of novel chemical entities that initiate cell death by mimicking the properties of BH3-only proteins.

Structural biology of the Bcl-2 family and its mimicry by viral proteins

Intrinsic apoptosis in mammals is regulated by protein–protein interactions among the B-cell lymphoma-2 (Bcl-2) family. The sequences, structures and binding specificity between pro-survival Bcl-2 proteins and their pro-apoptotic Bcl-2 homology 3 motif only (BH3-only) protein antagonists are now well understood. In contrast, our understanding of the mode of action of Bax and Bak, the two necessary proteins for apoptosis is incomplete. Bax and Bak are isostructural with pro-survival Bcl-2 proteins and also interact with BH3-only proteins, albeit weakly. Two sites have been identified; the in-groove interaction analogous to the pro-survival BH3-only interaction and a site on the opposite molecular face. Interaction of Bax or Bak with activator BH3-only proteins and mitochondrial membranes triggers a series of ill-defined conformational changes initiating their oligomerization and mitochondrial outer membrane permeabilization. Many actions of the mammalian pro-survival Bcl-2 family are mimicked by viruses. By expressing proteins mimicking mammalian pro-survival Bcl-2 family proteins, viruses neutralize death-inducing members of the Bcl-2 family and evade host cell apoptosis during replication. Remarkably, structural elements are preserved in viral Bcl-2 proteins even though there is in many cases little discernible sequence conservation with their mammalian counterparts. Some viral Bcl-2 proteins are dimeric, but they have distinct structures to those observed for mammalian Bcl-2 proteins. Furthermore, viral Bcl-2 proteins modulate innate immune responses regulated by NF-κB through an interface separate from the canonical BH3-binding groove. Our increasing structural understanding of the viral Bcl-2 proteins is leading to new insights in the cellular Bcl-2 network by exploring potential alternate functional modes in the cellular context. We compare the cellular and viral Bcl-2 proteins and discuss how alterations in their structure, sequence and binding specificity lead to differences in behavior, and together with the intrinsic structural plasticity in the Bcl-2 fold enable exquisite control over critical cellular signaling pathways.

Protect and serve: Bcl-2 proteins as guardians and rulers of cancer cell survival

It is widely accepted that anti-apoptotic Bcl-2 family members promote cancer cell survival by binding to their pro-apoptotic counterparts, thereby preventing mitochondrial outer membrane permeabilization (MOMP) and cytotoxic caspase activation. Yet, these proteins do not only function as guardians of mitochondrial permeability, preserving it, and maintaining cell survival in the face of acute or chronic stress, they also regulate non-apoptotic functions of caspases and biological processes beyond MOMP from diverse subcellular localizations and in complex with numerous binding partners outside of the Bcl-2 family. In particular, some of the non-canonical effects and functions of Bcl-2 homologs lead to an interplay with E2F-1, NFκB, and Myc transcriptional pathways, which themselves influence cancer cell growth and survival. We thus propose that, by feedback loops that we currently have only hints of, Bcl-2 proteins may act as rulers of survival signaling, predetermining the apoptotic threshold that they also directly scaffold. This underscores the robustness of the control exerted by Bcl-2 homologs over cancer cell survival, and implies that small molecules compounds currently used in the clinic to inhibit their mitochondrial activity may be not always be fully efficient to override this control.

Decoding and unlocking the BCL-2 dependency of cancer cells

Cancer cells are subject to many apoptotic stimuli that would kill them were it not for compensatory prosurvival alterations. BCL-2-like (BCL-2L) proteins contribute to such aberrant behaviour by engaging a network of interactions that is potent at promoting survival but that is also fragile: inhibition of a restricted number of interactions may suffice to trigger cancer cell death. Currently available and novel compounds that inhibit these interactions could be efficient therapeutic agents if this phenotype of BCL-2L dependence was better understood at a molecular, cellular and systems level and if it could be diagnosed by relevant biomarkers.

BAK activation is necessary and sufficient to drive ceramide synthase-dependent ceramide accumulation following inhibition of BCL2-like proteins

Determining mechanistic details about how drugs kill cancer cells is critical for predicting which cancers will respond to given therapeutic regimens and for identifying effective combinations of drugs that more potently kill cancer cells while sparing normal cells. The BCL2 family of proteins and bioactive sphingolipids are intricately linked during apoptotic cell death. In fact, many chemotherapeutic drugs are known to cause accumulation of the pro-apoptotic sphingolipid ceramide; however, the mechanism by which this occurs is not completely understood. In the present study we demonstrate that direct inhibition of anti-apoptotic BCL2 proteins with ABT-263 is sufficient to induce C(16)-ceramide synthesis in multiple cell lines, including human leukaemia and myeloma cells. ABT-263 activates CerS (ceramide synthase) activity only in cells expressing BAK or in cells capable of activating BAK. Importantly, recombinant BAK is sufficient to increase in vitro CerS activity in microsomes purified from Bak-KO (knockout) cells and activated BAK more potently activates CerS than inactive BAK. Likewise, ABT-263 addition to wild-type, but not Bak-deficient, microsomes increases CerS in vitro activity. Furthermore, we present a feed-forward model by which BAK activation of CerS by chemotherapeutic drugs leads to elevated ceramide levels that result in synergistic channel formation by ceramide (or one of its metabolites) and BAX/BAK.

Upregulation of miR-150* and miR-630 induces apoptosis in pancreatic cancer cells by targeting IGF-1R

MicroRNAs have been implicated in many critical cellular processes including apoptosis. We have previously found that apoptosis in pancreatic cancer cells was induced by adamantyl retinoid-related (ARR) molecule 3-Cl-AHPC. Here we report that 3-Cl-AHPC-dependent apoptosis involves regulating a number of microRNAs including miR-150* and miR-630. 3-Cl-AHPC stimulated miR-150* expression and caused decreased expression of c-Myb and IGF-1R in the pancreatic cancer cells. 3-Cl-AHPC-mediated reduction of c-Myb resulted in diminished binding of c-Myb with IGF-1R and Bcl-2 promoters, thereby causing repression of their transcription and protein expression. Over-expression of miR-150* also resulted in diminished levels of c-Myb and Bcl-2 proteins. Furthermore, the addition of the miRNA inhibitor 2′-O-methylated miR-150 blocked 3-Cl-AHPC-mediated increase in miR-150* levels and abrogated loss of c-Myb protein. Knockdown of c-Myb in PANC-1 cells resulted in enhanced apoptosis both in the presence or absence of 3-Cl-AHPC confirming the anti-apoptotic property of c-Myb. Overexpression of miR-630 also induced apoptosis in the pancreatic cancer cells and inhibited target protein IGF-1R mRNA and protein expression. Together these results implicate key roles for miR-150* and miR-630 and their targeting of IGF-1R to promote apoptosis in pancreatic cancer cells.

Beclin 1 interactome controls the crosstalk between apoptosis, autophagy and inflammasome activation: impact on the aging process

Autophagy and apoptosis are crucial cellular housekeeping and tissue survival mechanisms. There is emerging evidence of important crosstalk between apoptosis and autophagy which can be linked to inflammasome activation. Beclin 1 is a platform protein which assembles an interactome consisting of diverse proteins which control the initiation of autophagocytosis and distinct phases in endocytosis. Recent studies have demonstrated that the anti-apoptotic Bcl-2 family members can interact with Beclin 1 and inhibit autophagy. Consequently, impaired autophagy can trigger inflammasome activation. Interestingly, the hallmarks of the ageing process include a decline in autophagy, increased resistance to apoptosis and a low-grade inflammatory phenotype. Age-related stresses, e.g. genotoxic, metabolic and environmental insults, enhance the expression of NF-κB-driven anti-apoptotic Bcl-2 proteins which repress the Beclin 1-dependent autophagy. Suppression of autophagocytosis provokes inflammation including NF-κB activation which further potentiates anti-apoptotic defence. In a context-dependent manner, this feedback defence mechanism can enhance the aging process or provoke tumorigenesis or cellular senescence. We will review the role of Beclin 1 interactome in the crosstalk between apoptosis, autophagy and inflammasomes emphasizing that disturbances in Beclin 1-dependent autophagy can have a crucial impact on the aging process.

Multipolar functions of BCL-2 proteins link energetics to apoptosis

Classical apoptotic cell death is now sufficiently well understood to be interrogated with mathematical modeling and manipulated with targeted drugs for clinical benefit. However, a biological black hole has emerged with the realization that apoptosis regulators are functionally multipolar. BCL-2 family proteins appear to have much greater effects on cells than can be explained by their known roles in apoptosis. Although these effects may be observable simply because the cell is not dead, the general assumption is that BCL-2 proteins have undiscovered biochemical activities. Conversely, these as yet uncharacterized day-jobs also may underlie their profound effects on cell survival, challenging current assumptions about classical apoptosis. Even their sub-mitochondrial localizations remain controversial. Here we attempt to integrate seemingly conflicting information with the prospect that BCL-2 proteins themselves may be the critical crosstalk between life and death.