Differential targeting of prosurvival Bcl-2 proteins by their BH3-only ligands allows complementary apoptotic function

Diverse pathways mediate chemotherapy-induced cell death in acute lymphoblastic leukemia cell lines

Cancer cell resistance to chemotherapy may be mediated by defects in apoptotic pathways. A prior study showed that in vivo apoptosis of Acute Lymphoblastic Leukemia (ALL) blasts in response to chemotherapy could occur through diverse pathways including both p53-dependent and -independent mechanisms. In this study we investigated the apoptotic response in more detail by using a panel of ALL cell lines that differed in respect to p53 status. Upon exposure to a uniform stimulus, expression of apoptotic proteins, including the effector caspase-3, varied among ALL cell lines partly depending on p53 transcriptional activity and caspase-8 activation. Although the expression and contribution to apoptosis differed among known members of the apoptotic pathway, apoptosis was universally mediated by mitochondrial depolarization. The NFkappaB pathway was activated in response to chemotherapy but NFkappaB inhibition appeared to not influence chemosensitivity. This study further documents the highly variable nature of cell death programs in ALL and provides the foundation for cell death pathway modulation to improve ALL cure rates without increasing chemotherapy-related toxicity.

Apoptosis and melanoma: how new insights are effecting the development of new therapies for melanoma

PURPOSE OF REVIEW

Melanoma has proven resistant to most available chemotherapy and immunotherapy. Despite a range of different biochemical targets, most agents kill cancer cells by induction of apoptosis.

RECENT FINDINGS

Investigation of this process has provided insights into the resistance mechanisms in cancer cells and to development of a range of new agents that target apoptosis pathways. These include agents which inhibit antiapoptotic B cell lymphoma-2 family proteins and inhibitor of apoptosis proteins. In addition, a range of signal pathway inhibitors have become available that are able to inhibit signal pathways known to be associated with resistance to apoptosis.

SUMMARY

Evaluation of most of these reagents are at a preclinical level but studies on some pathway inhibitors have passed from phase II into phase III studies. Similarly, evaluation of antisense reagents are at an advanced stage. These early trials show much promise and suggest this approach to development of new therapies will lead to much needed advances in treatment of this disease.

tBid elicits a conformational alteration in membrane-bound Bcl-2 such that it inhibits Bax pore formation

During initiation of apoptosis, Bcl-2 family proteins regulate the permeability of mitochondrial outer membrane. BH3-only protein, tBid, activates pro-apoptotic Bax to release cytochrome c from mitochondria. tBid also activates anti-apoptotic Bcl-2 in the mitochondrial outer membrane, changing it from a single-spanning to a multispanning conformation that binds the active Bax and inhibits cytochrome c release. However, it is not known whether other mitochondrial proteins are required to elicit the tBid-induced Bcl-2 conformational alteration. To define the minimal components that are required for the functionally important Bcl-2 conformational alteration, we reconstituted the reaction using purified proteins and liposomes. We found that purified tBid was sufficient to induce a conformational alteration in the liposome-tethered, but not cytosolic Bcl-2, resulting in a multispanning form that is similar to the one found in the mitochondrial outer membrane of drug-treated cells. Mutations that abolished tBid/Bcl-2 interaction also abolished the conformational alteration, demonstrating that a direct tBid/Bcl-2 interaction at the membrane is both required and sufficient to elicit the conformational alteration. Furthermore, active Bax also elicited the Bcl-2 conformational alteration. Bcl-2 mutants that displayed increased or decreased activity in the conformational alteration assay showed corresponding activities in inhibiting pore formation by Bax in vitro and in preventing apoptosis in vivo. Thus, there is a strong correlation between the direct interaction of membrane-bound Bcl-2 and tBid with activation of Bcl-2 in vitro and in vivo.

p53 and Nur77/TR3 - transcription factors that directly target mitochondria for cell death induction

The complex apoptotic functions of the p53 tumor suppressor are central to its antineoplastic activity in vivo. Conversely, p53 function is altered or attenuated in one way or another in the majority of human cancers. Besides its well-understood action as a transcriptional regulator of multiple apoptotic genes, p53 also exerts a direct pro-apoptotic role at the mitochondria by engaging in protein-protein interactions with anti- and pro-apoptotic Bcl2 family members, thereby executing the shortest known circuitry of p53 death signaling. Nur77, also known as TR3 or NGFI-B, is a unique transcription factor belonging to the orphan nuclear receptor superfamily. Even more extreme than p53, Nur77 can exert opposing biological activities of survival and death. Its activities are regulated by subcellular distribution, expression levels, protein modification and heterodimerization with retinoid X receptor. In cancer cells, Nur77 functions in the nucleus as an oncogenic survival factor, but becomes a potent killer when certain death stimuli induce its migration to mitochondria, where it binds to Bcl2 and conformationally converts it to a killer that triggers cytochrome c release and apoptosis. This review focuses on their unexpected transcription-independent pro-death programs at mitochondria and highlights the remarkable mechanistic similarities between them. Moreover, an accumulating body of evidence provides ample rationale to further investigate how these mitochondrial p53 and Nur77 pathways could become exploitable targets for new cancer therapeutics.

Subversion of the Bcl-2 life/death switch in cancer development and therapy

The Bcl-2 protein family, which largely determines commitment to apoptosis, has central roles in tumorigenesis and chemoresistance. Its three factions of interacting proteins include the BH3-only proteins (e.g., Bim, Puma, Bad, Noxa), which transduce diverse cytotoxic signals to the mammalian pro-survival proteins (Bcl-2, Bcl-x(L), Bcl-w, Mcl-1, A-1), whereas Bax and Bak, when freed from pro-survival constraint, provoke the mitochondrial permeabilization that triggers apoptosis. We have discovered unexpected specificity in their interactions. Only Bim and Puma, which mediate multiple cytotoxic signals, engage all the pro-survival proteins. Noxa and Bad instead bind subsets and cooperate in killing, indicating that apoptosis requires neutralization of different pro-survival subsets. Furthermore, Mcl-1 and Bcl-x(L), but not Bcl-2, directly sequester Bak in healthy cells, and Bak is freed only when BH3-only proteins neutralize both its guards. BH3-only proteins such as Bim are tumor suppressors and mediate many of the cytotoxic signals from anticancer agents. Hence, compounds mimicking them may prove valuable for therapy. Indeed, the recently described ABT-737 is a promising "BH3 mimetic" of Bad. We find that, like Bad, ABT-737 kills cells efficiently only if Mcl-1 is absent or down-regulated. Thus, manipulation of apoptosis by targeting the Bcl-2 family has exciting potential for cancer treatment.

IgE-receptor activation induces survival and Bfl-1 expression in human mast cells but not basophils

BACKGROUND

The contribution of mast cells to the pathology of allergic diseases are facilitated by their long life span in tissue and ability to regranulate. Bcl-2 genes are one of the main regulators of cell death and survival. The aim of this study was to elucidate the mechanisms responsible for mast cell survival in allergy.

METHODS

Bcl-2 family gene expression in human mast cells and basophils was analyzed by ribonuclease protection assay and by reverse-transcriptase polymerase chain reaction. Cell survival was measured by mixing cells with the vital dye, trypan blue, and the number of living cells was enumerated. Apoptotic cells were measured by a Cell Death Detection ELISA.

RESULTS

We found that cross-linking of FcepsilonRI on human cord blood cultured mast cells (CBCMCs) promoted cell survival and induced expression of the pro-survival gene Bfl-1. CBCMCs were found to express both Bfl-1 and Bfl-1S, two splicing variants of Bfl-1. Bfl-1 induction was mediated through Syk, PI3-kinase and intracellular calcium mobilization, since piceatannol, wortmannin and EDTA, respectively, significantly reduced Bfl-1 expression levels. In contrast to CBCMCs, no evidence was found for Bfl-1 expression and survival promotion in human basophils.

CONCLUSIONS

Immunoglobulin E (IgE)-dependent activation-induced mast cell survival was correlated with Bfl-1 gene upregulation, providing a possible explanation for mast cell longevity in allergic reactions.

Cell death provoked by loss of interleukin-3 signaling is independent of Bad, Bim, and PI3 kinase, but depends in part on Puma

Growth and survival of hematopoietic cells is regulated by growth factors and cytokines, such as interleukin 3 (IL-3). When cytokine is removed, cells dependent on IL-3 kill themselves by a mechanism that is inhibited by overexpression of Bcl-2 and is likely to be mediated by proapoptotic Bcl-2 family members. Bad and Bim are 2 such BH3-only Bcl-2 family members that have been implicated as key initiators in apoptosis following growth factor withdrawal, particularly in IL-3-dependent cells. To test the role of Bad, Bim, and other proapoptotic Bcl-2 family members in IL-3 withdrawal-induced apoptosis, we generated IL-3-dependent cell lines from mice lacking the genes for Bad, Bim, Puma, both Bad and Bim, and both Bax and Bak. Surprisingly, Bad was not required for cell death following IL-3 withdrawal, suggesting changes to phosphorylation of Bad play only a minor role in apoptosis in this system. Deletion of Bim also had no effect, but cells lacking Puma survived and formed colonies when IL-3 was restored. Inhibition of the PI3 kinase pathway promoted apoptosis in the presence or absence of IL-3 and did not require Bad, Bim, or Puma, suggesting IL-3 receptor survival signals and PI3 kinase survival signals are independent.

Apoptosis mediated by p53 in rat neural AF5 cells following treatment with hydrogen peroxide and staurosporine

AF5 neural cells derived from fetal rat mesencephalic tissue were immortalized with a truncated SV40 LT vector lacking the p53-inactivating domain to maintain long-term cultures with a p53-responsive phenotype. This study examined p53 function in producing programmed cell death in propagating AF5 neural cells after exposure to hydrogen peroxide (H2O2) and the kinase inhibitor staurosporine (STSP). Concentration-dependent exposure of AF5 cells to 0-800 mM H2O2 and STSP at 0-1000 nM revealed increasing cytotoxicity from MTS cell viability assays. Apoptosis occurred at 400 mM H2O2 as evidenced by subG1 DNA and Annexin V flow cytometry analyses and cellular immunofluorescence staining with propidium iodide, anti-Annexin V and DAPI. DNA fragmentation, caspase-3/7 activity and cytochrome c release into cytosol also confirmed H2O2-mediated apoptotic events. p53 protein levels were increased over 24 h by H2O2 in a coordinated fashion with mdm2 expression. p53 activation by H2O2 was evidenced by elevated Ser15 phosphorylation, increased luciferase p53 reporter activity and upregulation of the downstream p53 targets p21(waf1) and apoptotic proteins, bax, Noxa and PUMA. STSP exposure produced apoptosis demonstrated by DNA fragmentation, caspase-3/7 activity, cytochrome c release and over 24 h was accompanied by sustained increase in p53 and Ser15 phosphorylation, rise in p21(waf1) and bax and a transient increase in p53 reporter activity but without Annexin V binding. These findings demonstrate that AF5 cells undergo apoptosis in response to H2O2-mediated oxidative stress and signal pathway disruption by STSP that therefore would be useful in studies related to p53-dependent neuronal cell death and neurodegeneration.

Bcl-2 family members and apoptosis, taken to heart

Loss of myocardial cells via apoptosis has been observed in many cardiovascular diseases and has been shown to contribute to the initiation and progression of heart failure. The Bcl-2 family members are important regulators of the mitochondrial pathway of apoptosis. These proteins decide whether the mitochondria should initiate the cell death program and release proapoptotic factors such as cytochrome c. The Bcl-2 proteins consist of anti- and proapoptotic members and play a key role in regulating apoptosis in the myocardium. The antiapoptotic proteins have been demonstrated to protect against various cardiac pathologies, whereas the antiapoptotic proteins have been reported to contribute to heart disease. This review summarizes the current understanding of the role of Bcl-2 proteins in the heart.

PUMA sensitizes lung cancer cells to chemotherapeutic agents and irradiation

PURPOSE

Lung cancer, the leading cause of cancer mortality worldwide, is often diagnosed at late stages and responds poorly to conventional therapies, including chemotherapy and irradiation. A great majority of lung tumors are defective in the p53 pathway, which plays an important role in regulating apoptotic response to anticancer agents. PUMA was recently identified as an essential mediator of DNA damage-induced and p53-dependent apoptosis. In this study, we investigated whether the regulation of PUMA by anticancer agents is abrogated in lung cancer cells and whether PUMA expression suppresses growth of lung cancer cells and/or sensitizes lung cancer cells to chemotherapeutic agents and irradiation through induction of apoptosis.

EXPERIMENTAL DESIGNS

The expression of PUMA was examined in lung cancer cells with different p53 status treated with chemotherapeutic agents. An adenovirus expressing PUMA (Ad-PUMA), alone or in combination with chemotherapeutic agents or gamma-irradiation, was used to treat lung cancer cells. The growth inhibitory and apoptotic effects of PUMA in vitro and in vivo were examined. The mechanisms of PUMA-mediated growth suppression and apoptosis were investigated through analysis of caspase activation and release of mitochondrial apoptogenic proteins. The cytotoxicities of PUMA on cancer and normal/nontransformed cells were compared. The efficacy of PUMA and p53 in suppressing the growth of lung cancer cells was also compared.

RESULTS

We showed that the induction of PUMA by chemotherapeutic agents is abolished in p53-deficient lung cancer cells. PUMA expression resulted in potent growth suppression of lung cancer cells and suppressed xenograft tumor growth in vivo through induction of apoptosis. Low dose of Ad-PUMA significantly sensitized lung cancer cells to chemotherapeutic agents and gamma-irradiation through induction of apoptosis. The effects of PUMA are mediated by enhanced caspase activation and release of cytochrome c and apoptosis-inducing factor into the cytosol. Furthermore, PUMA seems to be selectively toxic to cancer cells and more efficient than p53 in suppressing lung cancer cell growth.

CONCLUSIONS

Our findings indicate that PUMA is an important modulator of therapeutic responses of lung cancer cells and is potentially useful as a sensitizer in lung cancer therapy.

Modulation of Nr-13 antideath activity by peptide aptamers

Tumor cells are characterized by deregulated proliferation and resistance to proapoptotic stimuli. The Bcl-2 family of antiapoptotic proteins is overexpressed in a large number of chemoresistant tumors. Downregulation or inhibition of antiapoptotic proteins might result in the sensitization of cancer cells to chemotherapeutic agents. In the present study, we took advantage of the peptide aptamer strategy to target Nr-13, a Bcl-2 antiapoptotic protein involved in neoplastic transformation by the Rous sarcoma virus. We isolated peptide aptamers that behave as Nr-13 regulators, in vitro and in mammalian cells in culture. Some of these aptamers have potential proapoptotic activities. These data suggest that peptide aptamers targeting the Bcl-2 family of apoptosis inhibitors may be useful for the development of anticancer molecules.

The mitochondrial protein Bak is pivotal for gliotoxin-induced apoptosis and a critical host factor of Aspergillus fumigatus virulence in mice

Aspergillus fumigatus infections cause high levels of morbidity and mortality in immunocompromised patients. Gliotoxin (GT), a secondary metabolite, is cytotoxic for mammalian cells, but the molecular basis and biological relevance of this toxicity remain speculative. We show that GT induces apoptotic cell death by activating the proapoptotic Bcl-2 family member Bak, but not Bax, to elicit the generation of reactive oxygen species, the mitochondrial release of apoptogenic factors, and caspase-3 activation. Activation of Bak by GT is direct, as GT triggers in vitro a dose-dependent release of cytochrome c from purified mitochondria isolated from wild-type and Bax- but not Bak-deficient cells. Resistance to A. fumigatus of mice lacking Bak compared to wild-type mice demonstrates the in vivo relevance of this GT-induced apoptotic pathway involving Bak and suggests a correlation between GT production and virulence. The elucidation of the molecular basis opens new strategies for the development of therapeutic regimens to combat A. fumigatus and related fungal infections.

Functional characterization of the Bcl-2 gene family in the zebrafish

Members of the Bcl-2 protein family control the intrinsic apoptosis pathway. To evaluate the importance of this family in vertebrate development, we investigated it in the zebrafish (Danio rerio). We found that the zebrafish genome encodes structural and functional homologs of most mammalian Bcl-2 family members, including multi-Bcl-2-homology (BH) domain proteins and BH3-only proteins. Apoptosis induction by gamma-irradiation required zBax1 and zPuma, and could be prevented by overexpression of homologs of prosurvival Bcl-2 family members. Surprisingly, zebrafish Bax2 (zBax2) was homologous to mammalian Bax by sequence and synteny, yet demonstrated functional conservation with human Bak. Morpholino knockdown of both zMcl-1a and zMcl-1b revealed their critical role in early embryonic zebrafish development, and in the modulation of apoptosis activation through the extrinsic pathway. These data indicate substantial functional similarity between zebrafish and mammalian Bcl-2 family members, and establish the zebrafish as a relevant model for studying the intrinsic apoptosis pathway.

FKHRL1-mediated expression of Noxa and Bim induces apoptosis via the mitochondria in neuroblastoma cells

Protein kinase-B (PKB) and its target, the forkhead transcription factor like 1 (FKHRL1)/FoxO3a, have been suggested as regulators of neurotrophin-mediated cell survival in neuronal cells. We analyzed human neuroblastoma cells and found that FKHRL1 was phosphorylated, suggesting its inactivation. To study FKHRL1 function, we infected SH-EP and NB15 cells with a 4OH-tamoxifen-regulated FKHRL1(A3)ER(tm) transgene. Activation of FKHRL1 promoted cytochrome-c release and caspase-dependent apoptosis. FKHRL1 induced TRAIL and the BH3-only proteins Noxa and Bim, implicating both extrinsic and intrinsic death pathways. However, expression of dnFADD did not inhibit FKHRL1-induced cell death, whereas Bcl2 protected against apoptosis. This excluded the death-receptor pathway and suggested that cell death decision is regulated by Bcl2-rheostat. Importantly, RNAi knockdown of Noxa or Bim decreased apoptosis, indicating that Noxa and Bim cooperate to mediate FKHRL1-induced cell death. We conclude that Noxa and Bim establish a connection between FKHRL1 and mitochondria, and that both BH3-only proteins are critically involved in FKHRL1-induced apoptosis in neuroblastoma.

BH3-only proapoptotic Bcl-2 family members Noxa and Puma mediate neural precursor cell death

Neural precursor cells (NPCs) are highly sensitive to genotoxic injury, which triggers activation of the intrinsic mitochondria-dependent apoptotic pathway. This pathway is typically initiated by members of the BH3 (Bcl-2 homology 3)-only subgroup of the Bcl-2 (B-cell CLL/lymphoma 2) protein family, which are positioned upstream in the apoptotic pathway to respond to specific death stimuli. We have shown previously that NPCs deficient in the tumor suppressor protein p53 show significantly less death after exposure to genotoxic injury or to staurosporine (STS), a broad kinase inhibitor and potent apoptosis inducer. p53 has been shown to regulate the expression of both Noxa and Puma, two BH3-only proteins, although their involvement in p53-dependent cell death appears to be cell-type and stimulus specific. A systematic comparison of the relative contributions of Noxa and Puma to NPC apoptosis has not yet been performed. We hypothesized that p53-dependent transcription of Noxa and Puma leads to death in telencephalic NPCs exposed to genotoxic stress. We found that genotoxic injury induces a rapid p53-dependent increase in expression of Noxa and Puma mRNA in telencephalic NPCs. Furthermore, deficiency of either Noxa or Puma inhibited DNA damage-induced caspase-3 activation and cell death in telencephalic NPCs in vitro. However, only Puma deficiency protected telencephalic ventricular zone NPCs from death in vivo. In contrast to genotoxic injury, STS produced a p53-independent increase in Noxa and Puma expression, but neither Noxa nor Puma was required for STS-induced NPC death. Together, these experiments identify Noxa and Puma as important regulators of genotoxin-induced telencephalic NPC death.

A survival guide to early T cell development

The survival of immature T cell precursors is dependent on both thymus-derived extrinsic signals and self-autonomous pre-TCR-mediated signals. While the role of cytokines and the pre-TCR in promoting thymocyte survival has been well established, the relationship between pro- and anti-apoptotic signaling cascades remains poorly defined. Recent studies have established a link between cell survival and growth factor-mediated maintenance of cellular metabolism. In this regard, the Notch signaling pathway has emerged as more than an inducer of T lineage commitment and differentiation, but also as a potent trophic factor, promoting the survival and metabolic state of pre-T cells. In this review, we describe current concepts of the intracellular signaling pathways downstream of cell intrinsic and extrinsic factors that dictate survival versus death outcomes during early T cell development.

Cyclooxygenase-2 Inhibition Induces Apoptosis Signaling via Death Receptors and Mitochondria in Hepatocellular Carcinoma

Inhibition of cyclooxygenase (COX)-2 elicits chemopreventive and therapeutic effects in solid tumors that are coupled with the induction of apoptosis in tumor cells. We investigated the mechanisms by which COX-2 inhibition induces apoptosis in hepatocellular carcinoma (HCC) cells. COX-2 inhibition triggered expression of the CD95, tumor necrosis factor (TNF)-R, and TNF-related apoptosis-inducing ligand (TRAIL)-R1 and TRAIL-R2 death receptors. Addition of the respective specific ligands further increased apoptosis, indicating that COX-2 inhibition induced the expression of functional death receptors. Overexpression of a dominant-negative Fas-associated death domain mutant reduced COX-2 inhibitor-mediated apoptosis. Furthermore, our findings showed a link between COX-2 inhibition and the mitochondrial apoptosis pathway. COX-2 inhibition led to a rapid down-regulation of myeloid cell leukemia-1 (Mcl-1), an antiapoptotic member of the Bcl-2 family, followed by translocation of Bax to mitochondria and cytochrome c release from mitochondria. Consequently, overexpression of Mcl-1 led to inhibition of COX-2 inhibitor-mediated apoptosis. Furthermore, blocking endogenous Mcl-1 function using a small-interfering RNA approach enhanced COX-2 inhibitor-mediated apoptosis. It is of clinical importance that celecoxib acted synergistically with chemotherapeutic drugs in the induction of apoptosis in HCC cells. The clinical relevance of these results is further substantiated by the finding that COX-2 inhibitors did not sensitize primary human hepatocytes toward chemotherapy-induced apoptosis. In conclusion, COX-2 inhibition engages different apoptosis pathways in HCC cells stimulating death receptor signaling, activation of caspases, and apoptosis originating from mitochondria.

Bak functionally complements for loss of Bax during p14ARF-induced mitochondrial apoptosis in human cancer cells

In contrast to the initial notion that the biological activity of p14(ARF) strictly depends on a functional mdm-2/p53 signaling axis, we recently demonstrated that p14(ARF) mediates apoptosis in a p53/Bax-independent manner. Here, we show that p14(ARF) induces breakdown of the mitochondrial membrane potential and cytochrome c release before triggering caspase-9- and caspase-3/7-like activities in p53/Bax-deficient DU145 prostate cancer cells expressing wild-type Bak. Re-expression of Bax in these cells failed to further enhance p14(ARF)-induced apoptosis, suggesting that p14(ARF)-induced apoptosis primarily depends on Bak but not Bax in these cells. To further define the role of Bak and Bax in p14(ARF)-induced mitochondrial apoptosis, we employed short interference RNA for the knockdown of bak in isogeneic, p53 wild-type HCT116 colon cancer cells either proficient or deficient for Bax. There, combined loss of Bax and Bak attenuated p14(ARF)-induced apoptosis whereas single loss of Bax or Bak was only marginally effective, as in the case of DU145. Notably, HCT116 cells deficient for Bax and Bak failed to release cytochrome c and showed attenuated activation of caspase-9 (LEHDase) and caspase-3/caspase-7 (DEVDase) upon p14(ARF) expression. These data indicate that p14(ARF) triggers apoptosis via a Bax/Bak-dependent pathway in p53-proficient HCT116, whereas Bax is dispensable in p53-deficient DU145 cells. Nevertheless, a substantial proportion of p14(ARF)-induced cell death proceeds in a Bax/Bak-independent manner. This is also the case for inhibition of clonogenic growth that occurs, at least in part, through an entirely Bax/Bak-independent mechanism.

Therapeutic peptides: Targeting the mitochondrion to modulate apoptosis

For many years, medical drug discovery has extensively exploited peptides as lead compounds. Currently, novel structures of therapeutic peptides are derived from active pre-existing peptides or from high-throughput screening, and optimized following a rational drug design approach. Molecules of interest may prove their ability to influence the disease outcome in animal models and must respond to a set of criteria based on toxicity studies, ease of administration, the cost of their synthesis, and logistic for clinical use to validate it as a good candidate in a therapeutic perspective. This applies to the potential use of peptides to target one central intracellular organelle, the mitochondrion, to modulate (i.e. activate or prevent) apoptosis. Putative mitochondrial protein targets and the strategies already elaborated to correct the defects linked to these proteins (overexpression, inactivation, mutation..., etc.) are described, and recent advances that led or may lead to the conception of therapeutic peptides via a specific action on these mitochondrial targets in the future are discussed.

An internal EELD domain facilitates mitochondrial targeting of Mcl-1 via a Tom70-dependent pathway

Mcl-1 functions at an apical step in many regulatory programs that control cell death. Although the mitochondrion is one major subcellular organelle where Mcl-1 functions, the molecular mechanism by which Mcl-1 is targeted to mitochondria remains unclear. Here, we demonstrate that Mcl-1 is loosely associated with the outer membrane of mitochondria. Furthermore, we demonstrate that Mcl-1 interacts with the mitochondrial import receptor Tom70, and such interaction requires an internal domain of Mcl-1 that contains an EELD motif. A Tom70 antibody that blocks Mcl-1-Tom70 interaction blocks mitochondrial import of Mcl-1 in vitro. Furthermore, Mcl-1 is significantly less targeted to mitochondria in Tom70 knockdown than in the control cells. Similar targeting preference is also observed for the DM mutant of Mcl-1 whose mutation at the EELD motif markedly attenuates its Tom70 binding activity. Together, our results indicate that the internal EELD domain facilitates mitochondrial targeting of Mcl-1 via a Tom70-dependent pathway.

Pim-3, a proto-oncogene with serine/threonine kinase activity, is aberrantly expressed in human pancreatic cancer and phosphorylates bad to block bad-mediated apoptosis in human pancreatic cancer cell lines

Pancreatic cancer still remains a serious health problem with <5% 5-year survival rate for all stages. To develop an effective treatment, it is necessary to identify a target molecule that is crucially involved in pancreatic tumor growth. We previously observed that Pim-3, a member of the proto-oncogene Pim family that expresses serine/threonine kinase activity, was aberrantly expressed in human and mouse hepatomas but not in normal liver. Here, we show that Pim-3 is also expressed in malignant lesions of the pancreas but not in normal pancreatic tissue. Moreover, Pim-3 mRNA and protein were constitutively expressed in all human pancreatic cancer cell lines that we examined and colocalized with the proapoptotic protein Bad. The ablation of endogenous Pim-3 by small hairpin RNA transfection promoted apoptosis, as evidenced by increases in a proportion of cells in the sub-G(1) fraction of the cell cycle and in phosphatidyl serine externalization. A proapoptotic molecule, Bad, was phosphorylated constitutively at Ser(112) but not Ser(136) in human pancreatic cancer cell lines and this phosphorylation is presumed to represent its inactive form. Phosphorylation of Bad and the expression of an antiapoptotic molecule, Bcl-X(L), were reduced by the ablation of endogenous Pim-3. Thus, we provide the first evidence that Pim-3 can inactivate Bad and maintain the expression of Bcl-X(L) and thus prevent apoptosis of human pancreatic cancer cells. This may contribute to the net increase in tumor volume or tumor growth in pancreatic cancer.

Herceptin sensitizes ErbB2-overexpressing cells to apoptosis by reducing antiapoptotic Mcl-1 expression

PURPOSE

Monoclonal antibodies, such as herceptin and trastuzumab, against the epidermal growth factor receptor ErbB2 (also known as HER2/neu) are an effective therapy for breast cancer patients with overexpression of ErbB2. Herceptin, in combination with standard chemotherapy, such as taxol or etoposide, gives a synergistically apoptotic response in breast tumors.

EXPERIMENTAL DESIGN

The mechanism underlying this synergy between chemotherapy and herceptin treatment is not well understood. Herein, we have determined that addition of herceptin, sensitized breast cancer cell lines MDA-MB-231 and MCF-7 to etoposide- or taxol-induced apoptosis.

RESULTS

This treatment resulted in reduced expression of ErbB2 and the antiapoptotic Bcl-2 family member Mcl-1 in MDA-MB-231 cells. Using antisense oligonucleotides against Mcl-1, MDA-MB-231 cells were rendered sensitive to etoposide-induced apoptosis similar to herceptin, but combined treatment of antisense against Mcl-1 and herceptin failed to give a significant increase in apoptosis. In 29 human breast tumors immunostained for ErbB2 and Mcl-1, we found that when ErbB2 was overexpressed, there was a corresponding increase in Mcl-1 expression.

DISCUSSION

Using murine fibroblasts that express human ErbB2, but no other ErbB family member (NE2), these cells showed resistance to both taxol- and etoposide-induced apoptosis compared with parental cells. In addition, NE2 cells preferentially express the antiapoptotic Bcl-2 family member Mcl-1 compared with parental cells, and treatment with herceptin reduces Mcl-1 expression. Taken together, these results suggest that herceptin sensitizes ErbB2-overexpressing cells to apoptosis by reducing antiapoptotic Mcl-1 protein levels.

Evidence that membrane insertion of the cytosolic domain of Bcl-xL is governed by an electrostatic mechanism

Signals from different cellular networks are integrated at the mitochondria in the regulation of apoptosis. This integration is controlled by the Bcl-2 proteins, many of which change localization from the cytosol to the mitochondrial outer membrane in this regulation. For Bcl-xL, this change in localization reflects the ability to undergo a conformational change from a solution to integral membrane conformation. To characterize this conformational change, structural and thermodynamic measurements were performed in the absence and presence of lipid vesicles with Bcl-xL. A pH-dependent model is proposed for the solution to membrane conformational change that consists of three stable conformations: a solution conformation, a conformation similar to the solution conformation but anchored to the membrane by its C-terminal transmembrane domain, and a membrane conformation that is fully associated with the membrane. This model predicts that the solution to membrane conformational change is independent of the C-terminal transmembrane domain, which is experimentally demonstrated. The conformational change is associated with changes in secondary and, especially, tertiary structure of the protein, as measured by far and near-UV circular dichroism spectroscopy, respectively. Membrane insertion was distinguished from peripheral association with the membrane by quenching of intrinsic tryptophan fluorescence by acrylamide and brominated lipids. For the cytosolic domain, the free energy of insertion (DeltaG degrees x) into lipid vesicles was determined to be -6.5 kcal mol(-1) at pH 4.9 by vesicle binding experiments. To test whether electrostatic interactions were significant to this process, the salt dependence of this conformational change was measured and analyzed in terms of Gouy-Chapman theory to estimate an electrostatic contribution of DeltaG degrees el approximately -2.5 kcal mol(-1) and a non-electrostatic contribution of DeltaG degrees nel approximately -4.0 kcal mol(-1) to the free energy of insertion, DeltaG degrees x. Calcium, which blocks ion channel activity of Bcl-xL, did not affect the solution to membrane conformational change more than predicted by these electrostatic considerations. The lipid cardiolipin, that is enriched at mitochondrial contact sites and reported to be important for the localization of Bcl-2 proteins, did not affect the solution to membrane conformational change of the cytosolic domain, suggesting that this lipid is not involved in the localization of Bcl-xL in vivo. Collectively, these data suggest the solution to membrane conformational change is controlled by an electrostatic mechanism. Given the distinct biological activities of these conformations, the possibility that this conformational change might be a regulatory checkpoint for apoptosis is discussed.

Human Bcl-2 cannot directly inhibit the Caenorhabditis elegans Apaf-1 homologue CED-4, but can interact with EGL-1

Although the anti-apoptotic activity of Bcl-2 has been extensively studied, its mode of action is still incompletely understood. In the nematode Caenorhabditis elegans, 131 of 1090 somatic cells undergo programmed cell death during development. Transgenic expression of human Bcl-2 reduced cell death during nematode development, and partially complemented mutation of ced-9, indicating that Bcl-2 can functionally interact with the nematode cell death machinery. Identification of the nematode target(s) of Bcl-2 inhibition would help clarify the mechanism by which Bcl-2 suppresses apoptosis in mammalian cells. Exploiting yeast-based systems and biochemical assays, we analysed the ability of Bcl-2 to interact with and regulate the activity of nematode apoptosis proteins. Unlike CED-9, Bcl-2 could not directly associate with the caspase-activating adaptor protein CED-4, nor could it inhibit CED-4-dependent yeast death. By contrast, Bcl-2 could bind the C. elegans pro-apoptotic BH3-only Bcl-2 family member EGL-1. These data prompt us to hypothesise that Bcl-2 might suppress nematode cell death by preventing EGL-1 from antagonising CED-9, rather than by inhibiting CED-4.

BCL-2 in the crosshairs: tipping the balance of life and death

The discovery of B-cell lymphoma-2 (BCL-2) over 20 years ago revealed a new paradigm in cancer biology: the development and persistence of cancer can be driven by molecular roadblocks along the natural pathway to cell death. The subsequent identification of an expansive family of BCL-2 proteins provoked an intensive investigation of the interplay among these critical regulators of cell death. What emerged was a compelling tale of guardians and executioners, each participating in a molecular choreography that dictates cell fate. Ten years into the BCL-2 era, structural details defined how certain BCL-2 family proteins interact, and molecular targeting of the BCL-2 family has since become a pharmacological quest. Although many facets of BCL-2 family death signaling remain a mechanistic mystery, small molecules and peptides that effectively target BCL-2 are eliminating the roadblock to cell death, raising hopes for a medical breakthrough in cancer and other diseases of deregulated apoptosis.

BCL2 family in DNA damage and cell cycle control

Individual BCL2 family members couple apoptosis regulation and cell cycle control in unique ways. Antiapoptotic BCL2 and BCL-x(L) are antiproliferative by facilitating G0. BAX is proapoptotic and accelerates S-phase progression. The dual functions in apoptosis and cell cycle are coordinately regulated by the multi-domain BCL2 family members (MCL-1) and suggest that survival is maintained at the expense of proliferation. The role of BH3-only molecules in cell cycle is more variable. BAD antagonizes both the cell cycle and antiapoptotic functions of BCL2 and BCL-x(L) through BH3 binding. BID has biochemically separable functions in apoptosis and S-phase checkpoint, determined by post-translational modification. p53-induced PUMA is known only to have apoptotic function. Inhibition of apoptosis is oncogenic, whereas promotion of cell cycle arrest is tumor suppressive. Paradoxically, selected BCL2 family members can be both oncogenic and tumor suppressive. Which of the dual functions predominates is lineage specific and context dependent.

Loss of the tissue-specific proapoptotic BH3-only protein Nbk/Bik is a unifying feature of renal cell carcinoma

We report for the first time inactivation of a tissue-specific Bcl-2 homology domain 3 (BH3)-only protein as a common aspect in human cancer. In detail, we show that loss of the BH3-only protein natural born killer (Nbk)/Bcl-2-interacting killer (Bik) is a common feature of clear-cell renal cell carcinoma (RCC). While strong Nbk expression is found in the renal tubuli and the epithelial lining of the glomerula, a consistent loss of Nbk expression was observed in primary RCC tissue and RCC cell lines. Mutation of Nbk is, however, rare, whereas deletion of the Nbk gene at 22q13.2 is frequent. In addition to loss of heterozygosity (LOH), DNA methylation mediates transcriptional silencing of the Nbk gene. The conditional restoration of Nbk/Bik expression led to apoptotic death of RCC but not of nonmalignant renal epithelia. A broader expression analysis of RCC cell lines for BH3-only proteins revealed that loss of Nbk coincides with failure to express Bim, whereas Puma, Bid and BNIP3 are readily detectable and, in case of Puma, inducible by p53. These data delineate a role for defects in BH3-only proteins as tumor suppressors in RCC and may explain at the same time the impressive clinical apoptosis resistance of RCC.

Control of Peripheral T Cell Survival: A Delicate Division of Labor between Cytokines and Costimulatory Molecules

Throughout their life span, T lymphocytes are confronted with diverse and often extreme circumstances: they either are in a resting state or proliferate vigorously and find themselves in lymphoid organs, blood stream, or peripheral tissues. Under all these conditions, their survival depends on extracellular factors that counteract apoptotic cell death and sustain metabolism. In the different phases of T cell life and throughout the antigen-dependent T cell response, different cytokine receptors and costimulatory receptors provide the input to direct these processes, via intracellular signaling pathways that have been elucidated in significant detail.

Bcl2's flexible loop domain regulates p53 binding and survival

p53 not only functions as a transcription factor but also has a direct, apoptogenic role at the mitochondria. We have discovered that DNA damage-induced p53-Bcl2 binding is associated with decreased Bcl2-Bax interaction and increased apoptotic cell death in a mechanism regulated by Bcl2's flexible loop regulatory domain (FLD), since purified p53 protein can disrupt the Bcl2/Bax complex by directly binding to a negative regulatory region of the FLD (amino acids [aa] 32 to 68). Deletion of the negative regulatory region (Delta32-68) abolishes Bcl2-p53 binding and enhances Bcl2's antiapoptotic function. Conversely, removal of a positive regulatory region (aa 69 to 87) of the FLD, which contains the Bcl2 phosphorylation site(s) T69, S70, and S87, enhances Bcl2-p53 binding and significantly abrogates Bcl2's survival activity. The phospho-mimetic T69E/S70E/S87E (EEE) but not the nonphosphorylatable T69A/S70A/S87A (AAA) Bcl2 mutant displays a reduced capacity to bind p53 and potently inhibits p53-induced cytochrome c release from isolated mitochondria. Furthermore, the FLD-only aa32-87 and aa32-68 peptides but not the aa69-87 peptide can directly bind p53 in vitro. p53-induced cytochrome c release occurs through a mechanism involving Bax's integral insertion into the outer mitochondrial membrane. Either DNA damage to cells or expression of p53 selectively targeted to the mitochondria results in Bcl2-p53 binding followed by exposure of Bcl2's BH3 domain in association with inactivation of Bcl2's antiapoptotic function, indicating a conformational change in Bcl2 can occur upon direct ligation of p53. Thus, Bcl2's FLD contains both positive and negative regulatory regions which functionally regulate Bcl2's antiapoptotic activity by affecting Bax or p53 binding.

The Noxa/Mcl-1 Axis Regulates Susceptibility to Apoptosis under Glucose Limitation in Dividing T Cells

Throughout lymphocyte development, cellular persistence and expansion are tightly regulated by survival and apoptosis. Within the Bcl-2 family, distinct apoptogenic BH3-only members like Bid, Bim, and Puma appear to function in specific cell death pathways. We found that naive human T cells after mitogenic activation, apart from expected protective Bcl-2 members, also rapidly upregulate the BH3-only protein Noxa in a p53-independent fashion. The specific role of Noxa became apparent during glucose limitation and involves interaction with the labile Bcl-2 homolog Mcl-1. Knockdown of Noxa or Mcl-1 results in protection or susceptibility, respectively, to apoptosis induced by glucose deprivation. Declining Mcl-1 levels and apoptosis induction are inversely correlated to Noxa levels and prevented by readdition of glucose. We propose that the Noxa/Mcl-1 axis is an apoptosis rheostat in dividing cells, in a selective pathway that functions to restrain lymphocyte expansion and can be triggered by glucose deprivation.

Mitochondrial outer membrane permeabilization during apoptosis: the innocent bystander scenario

Mitochondrial outer membrane permeabilization (MOMP) is considered the 'point of no return' as this event is responsible for engaging the apoptotic cascade in numerous cell death pathways. MOMP is directly governed by a subset of the BCL-2 family of proapoptotic proteins, which induce disruptions in the outer mitochondrial membrane (OMM) and subsequent release of death-promoting proteins like cytochrome c. The proposal here is centered on our hypothesis that MOMP is dictated by an interaction between the cytosol and the OMM, and although proteins of the OMM may be important in the process, the 'decision' to undergo apoptosis originates within the cytosol with no participation (in terms of yes, no and when) by mitochondria.

Mitochondrially targeted wild-type p53 suppresses growth of mutant p53 lymphomas in vivo

The complex apoptotic functions of the p53 tumor suppressor are central to its antineoplastic activity in vivo. Besides its well-known action as a transcriptional activator of apoptotic genes, p53 exerts a direct proapoptotic role at the mitochondria via protein-protein interactions with Bcl2 family members, thus executing the shortest known circuitry of p53 death signaling. We recently reported that exclusive delivery of p53 to mitochondria exerts a significant in vivo tumor suppressor activity in p53-null lymphomas. However, it was unknown whether mitochondrially targeted p53 has suppressor activities in tumors harboring missense mutants, which constitute the vast majority of p53 alterations in human tumors. Here, we show that targeting p53 to mitochondria does confer a significant growth disadvantage in B-lymphomas expressing various point mutants of p53, resulting in efficient apoptosis induction in vitro and in vivo in mice.

Mitochondria primed by death signals determine cellular addiction to antiapoptotic BCL-2 family members

We show that the antiapoptotic proteins BCL-2, BCL-XL, MCL-1, BFL-1, and BCL-w each bear a unique pattern of interaction with a panel of peptides derived from BH3 domains of BH3-only proteins. Cellular dependence on an antiapoptotic protein for survival can be decoded based on the pattern of mitochondrial sensitivity to this peptide panel, a strategy that we call BH3 profiling. Dependence on antiapoptotic proteins correlates with sequestration of activator BH3-only proteins like BID or BIM by antiapoptotic proteins. Sensitivity to the cell-permeable BCL-2 antagonist ABT-737 is also related to priming of BCL-2 by activator BH3-only molecules. Our data allow us to distinguish a cellular state we call "primed for death," which can be determined by BH3 profiling and which correlates with dependence on antiapoptotic family members for survival.

Mitochondrial DNA mutations cause resistance to opening of the permeability transition pore

The age-related accumulation of mitochondrial DNA mutations has the potential to impair organ function and contribute to disease. In support of this hypothesis, accelerated mitochondrial mutagenesis is pathogenic in the mouse heart, and there is an increase in myocyte apoptosis. The current study sought to identify functional alterations in cell death signaling via mitochondria. Of particular interest is the mitochondrial permeability transition pore, opening of which can initiate cell death, while pore inhibition is protective. Here, we show that mitochondria from transgenic mice that develop mitochondrial DNA mutations have a marked inhibition of calcium-induced pore opening. Temporally, inhibited pore opening coincides with disease. Pore inhibition also correlates with an increase in Bcl-2 protein integrated into the mitochondrial membrane. We hypothesized that pore inhibition was mediated by mitochondrial Bcl-2. To test this hypothesis, we treated isolated mitochondria with Bcl-2 antagonistic peptides (derived from the BH3 domain of Bax or Bid). These peptides released the inhibition to pore opening. The data are consistent with a Bcl-2-mediated inhibition of pore opening. Thus, mitochondrial DNA mutations induce an adaptive-protective response in the heart that inhibits opening of the mitochondrial permeability pore.

At the gates of death

Apoptosis that proceeds via the mitochondrial pathway involves mitochondrial outer membrane permeabilization (MOMP), responsible for the release of cytochrome c and other proteins of the mitochondrial intermembrane space. This essential step is controlled and mediated by proteins of the Bcl-2 family. The proapoptotic proteins Bax and Bak are required for MOMP, while the antiapoptotic Bcl-2 proteins, including Bcl-2, Bcl-xL, Mcl-1, and others, prevent MOMP. Different proapoptotic BH3-only proteins act to interfere with the function of the antiapoptotic Bcl-2 members and/or activate Bax and Bak. Here, we discuss an emerging view, proposed by Certo et al. in this issue of Cancer Cell, on how these interactions result in MOMP and apoptosis.

BH3-only proteins in cell death initiation, malignant disease and anticancer therapy

Induction of apoptosis in tumour cells, either by direct activation of the death receptor pathway using agonistic antibodies or recombinant ligands, or direct triggering of the Bcl-2-regulated intrinsic apoptosis pathway by small molecule drugs, carries high hopes to overcome the shortcomings of current anticancer therapies. The latter therapy concept builds on a more detailed understanding of how Bcl-2-like molecules maintain mitochondrial integrity and how BH3-only proteins and Bax/Bak-like molecules can undermine it. Means to unleash the apoptotic potential of BH3-only proteins in tumour cells, or bypass the need for BH3-only proteins by blocking possible interactions of Bcl-2-like prosurvival molecules with Bax and/or Bak allowing their direct activation, constitute interesting options for the design of novel anticancer therapies.

Bim, Bad and Bmf: intrinsically unstructured BH3-only proteins that undergo a localized conformational change upon binding to prosurvival Bcl-2 targets

All BH3-only proteins, key initiators of programmed cell death, interact tightly with multiple binding partners and have sequences of low complexity, properties that are the hallmark of intrinsically unstructured proteins (IUPs). We show, using spectroscopic methods, that the BH3-only proteins Bim, Bad and Bmf are unstructured in the absence of binding partners. Detailed sequence analyses are consistent with this observation and suggest that most BH3-only proteins are unstructured. When Bim binds and inactivates prosurvival proteins, most residues remain disordered, only the BH3 element becomes structured, and the short alpha-helical molecular recognition element can be considered to behave as a 'bead on a string'. Coupled folding and binding is typical of many IUPs that have important signaling roles, such as BH3-only proteins, as the inherent structural plasticity favors interaction with multiple targets. This understanding offers promise for the development of BH3 mimetics, as multiple modes of binding are tolerated.

Coupling endoplasmic reticulum stress to the cell-death program: a novel HSP90-independent role for the small chaperone protein p23

The endoplasmic reticulum (ER) is the principal organelle for the biosynthesis of proteins, steroids and many lipids, and is highly sensitive to alterations in its environment. Perturbation of Ca(2+) homeostasis, elevated secretory protein synthesis, deprivation of glucose or other sugars, altered glycosylation and/or the accumulation of misfolded proteins may all result in ER stress, and prolonged ER stress triggers cell death. Studies from multiple laboratories have identified the roles of several ER stress-induced cell-death modulators and effectors through the use of biochemical, pharmacological and genetic tools. In the present work, we describe the role of p23, a small chaperone protein, in preventing ER stress-induced cell death. p23 is a highly conserved chaperone protein that modulates HSP90 activity and is also a component of the steroid receptors. p23 is cleaved during ER stress-induced cell death; this cleavage, which occurs close to the carboxy-terminus, requires caspase-3 and/or caspase-7, but not caspase-8. Blockage of the caspase cleavage site of p23 was associated with decreased cell death induced by ER stress. Immunodepletion of p23 or inhibition of p23 expression by siRNA resulted in enhancement of ER stress-induced cell death. While p23 co-immunoprecipitated with the BH3-only protein PUMA (p53-upregulated modulator of apoptosis) in untreated cells, prolonged ER stress disrupted this interaction. The results define a protective role for p23, and provide further support for a model in which ER stress is coupled to the mitochondrial intrinsic apoptotic pathway through the activities of BH3 family proteins.

PUMA Dissociates Bax and Bcl-X(L) to induce apoptosis in colon cancer cells

PUMA is a BH3-only Bcl-2 family protein that plays an essential role in DNA damage-induced apoptosis. PUMA interacts with anti-apoptotic Bcl-2 and Bcl-X(L) and is dependent on Bax to induce apoptosis. In this study, we investigated how the interactions of PUMA with the antiapoptotic proteins coordinate with Bax to initiate apoptosis in HCT116 colon cancer cells. We found that Bcl-X(L) was most effective among several antiapoptotic proteins in suppressing PUMA-induced apoptosis and PUMA-dependent apoptosis induced by the DNA-damaging agent adriamycin. Mutant Bcl-X(L) that cannot interact with Bax was unable to protect cells from PUMA-mediated apoptosis. Knockdown of Bcl-X(L) by RNA interference significantly enhanced PUMA-mediated apoptosis in HCT116 cells but not in PUMA-knockout cells. Furthermore, Bax was found to be dissociated preferentially from Bcl-X(L) in HCT116 cells but not in the PUMA-knockout cells, in response to PUMA induction and adriamycin treatment. PUMA inhibited the association of Bax and Bcl-X(L) in vitro by directly binding to Bcl-X(L) through its BH3 domain. Finally, we found that wild-type Bax, but not mutant Bax deficient in either multimerization or mitochondrial localization, was able to restore PUMA-induced apoptosis in the BAX-knockout cells. Together, these results indicate that PUMA initiates apoptosis in part by dissociating Bax and Bcl-X(L), thereby promoting Bax multimerization and mitochondrial translocation.

Reversible membrane interaction of BAD requires two C-terminal lipid binding domains in conjunction with 14-3-3 protein binding

BAD is a Bcl-2 homology domain 3 (BH3)-only proapoptotic member of the Bcl-2 protein family that is regulated by phosphorylation in response to survival factors. Binding of BAD to mitochondria is thought to be exclusively mediated by its BH3 domain. We show here that BAD binds to lipids with high affinities, predominantly to negatively charged phospholipids, such as phosphatidylserine, phosphatidic acid, and cardiolipin, as well as to cholesterol-rich liposomes. Two lipid binding domains (LBD1 and LBD2) with different binding preferences were identified, both located in the C-terminal part of the BAD protein. BAD facilitates membrane translocation of Bcl-XL in a process that requires LBD2. Integrity of LBD1 and LBD2 is also required for proapoptotic activity in vivo. Phosphorylation of BAD does not affect membrane binding but renders BAD susceptible to membrane extraction by 14-3-3 proteins. BAD can be removed efficiently by 14-3-3zeta, -eta, -tau and lesxs efficiently by other 14-3-3 isoforms. The assembled BAD.14-3-3 complex exhibited high affinity for cholesterol-rich liposomes but low affinity for mitochondrial membranes. We conclude that BAD is a membrane-associated protein that has the hallmarks of a receptor rather than a ligand. Lipid binding is essential for the proapoptotic function of BAD in vivo. The data support a model in which BAD shuttles in a phosphorylation-dependent manner between mitochondria and other membranes and where 14-3-3 is a key regulator of this relocation. The dynamic interaction of BAD with membranes is tied to activation and membrane translocation of Bcl-XL.

E2F1-induced apoptosis: turning killers into therapeutics

The cellular transcription factor E2F1 is part of an anti-tumor safeguard mechanism: it engages cell-death pathways either alone or in cooperation with p53 to protect organisms from the development of tumors. E2F1 activates downstream factors, which in turn produce secondary changes in gene expression that trigger apoptosis. Although the mechanisms are incompletely understood, several studies have demonstrated that E2F1 is involved in many different aspects of programmed cell death depending on the cellular background. Here, these findings are highlighted in the context of the most recent follow-up studies that have used apoptotic E2F1 genes as new therapeutics or drug targets, thereby providing insight into the basic mechanisms of E2F1-induced apoptosis and its possible clinical implications.

Auto-activation of the apoptosis protein Bax increases mitochondrial membrane permeability and is inhibited by Bcl-2

Interactions among Bcl-2 family proteins mediated by Bcl-2 homology (BH) regions transform apoptosis signals into actions. The interactions between BH3 region-only proteins and multi-BH region proteins such as Bax and Bcl-2 have been proposed to be the dominant interactions required for initiating apoptosis. Experimental evidence also suggests that both homo- and hetero-interactions are mediated primarily by the BH3 regions in all Bcl-2 family proteins and contribute to commitment to or inhibition of apoptosis. We found that a peptide containing the BH3 helix of Bax was not sufficient to activate recombinant Bax to permeabilize mitochondria. However, an extended peptide containing the BH3 helix and additional downstream sequences activated Bax to permeabilize mitochondria and liposomes. Bcl-2 inhibited the membrane-permeabilizing activity of peptide-activated Bax. This activity of Bcl-2 was inhibited by the extended but not the BH3-only peptide despite both peptides binding to Bcl-2 with similar affinity. Further, membrane-bound Bax activation intermediates directly activated soluble Bax further permeabilizing the membrane. Bcl-2 inhibited Bax auto-activation. We therefore propose that Bax auto-activation amplifies the initial death signal produced by BH3-only proteins and that Bcl-2 functions as an inhibitor of Bax auto-activation.

BH3 peptidomimetics potently activate apoptosis and demonstrate single agent efficacy in neuroblastoma

The major impediment to cure for many malignancies is the development of therapy resistance with resultant tumor progression. Genetic alterations leading to subversion of inherent apoptosis pathways are common themes in therapy resistance. Bcl-2 family proteins play a critical role in regulating mitochondrial apoptosis that governs chemotherapeutic effects, and defective engagement of these pathways contributes to treatment failure. We have studied the efficacy of BH3 peptidomimetics consisting of the minimal death, or BH3, domains of the proapoptotic BH3-only proteins Bid and Bad to induce apoptosis using neuroblastoma (NB) as a model system. We demonstrate that BH3 peptides, modified with an arginine homopolymer for membrane transduction (called r8-BidBH3 and r8-BadBH3, respectively), potently induce apoptosis in NB cells, including those with MYCN amplification. Cell death is caspase 9 dependent, consistent with a requirement for the intrinsic mitochondrial pathway. Substitutions at highly conserved residues within the r8-BidBH3 peptide abolish apoptotic efficacy supporting activity through specific BH domain interactions. Concomitant exposure to r8-BadBH3 and r8-BidBH3 at sublethal monotherapy doses revealed potent synergy consistent with a competitive displacement model, whereby BH3 peptides displace sequestered BH3 proteins to induce cell death. Further, BH3 peptides demonstrate antitumor efficacy in a xenograft model of NB in the absence of additional genotoxic or trophic stressors. These data provide proof of principle that targeted re-engagement of apoptosis pathways may be of therapeutic utility, and BH3-like compounds are attractive lead agents to re-establish therapy-induced apoptosis in refractory malignancies.

Mcl-1 is essential for the survival of synovial fibroblasts in rheumatoid arthritis

Mcl-1 is a Bcl-2-family, antiapoptotic molecule that is critical for the survival of T and B lymphocytes and macrophages; however, its role in nonhemopoietic cells remains to be fully elucidated. The current study focuses on the role of Mcl-1 in rheumatoid arthritis (RA). Mcl-1 was strongly expressed in the synovial lining and was increased in the sublining fibroblasts of patients with RA, compared with control synovial tissue. The expression of Mcl-1 in sublining fibroblasts correlated with the degree of inflammation and TNF-alpha, and IL-1beta treatment of cultured synovial fibroblasts resulted in the increased expression of Mcl-1 at the mRNA and protein levels. Mcl-1 was critical for the survival of RA synovial fibroblasts, because the forced reduction of Mcl-1 using a Mcl-1 antisense-expressing adenoviral vector induced apoptotic cell death, which was mediated through Bax, Bak, and Bim. These observations document a critical role for Mcl-1 in protecting against apoptosis in RA and suggest that Mc1-1 is a potential therapeutic target in this disease.

The stability and anti-apoptotic function of A1 are controlled by its C terminus

Most Bcl-2 family members can localize to intracellular membranes via hydrophobic sequences within their C-terminal portion. We found that the C terminus of the anti-apoptotic family member A1 did not function as a membrane anchor. Instead, this stretch of the protein rendered A1 highly unstable by mediating its polyubiquitination and rapid proteasomal degradation. Moreover, the domain did not only function independently of its position within the A1 protein but when transferred could even destabilize unrelated proteins like enhanced green fluorescent protein and caspase-3. A1 was, however, much more stable in the presence of the Bcl-2 homology-only protein BimEL, suggesting that direct interaction of A1 with pro-apoptotic members of the Bcl-2 family strongly reduces its rate of turnover. We further show that the C-terminal end of A1 also contributes to the anti-apoptotic capacity of the protein. In conclusion, our data demonstrate that the C terminus serves a dual function by controlling the stability of A1 and by amplifying the capacity of the protein to protect cells against apoptosis.

Dissecting p53-dependent apoptosis

The complexity of the p53 protein, coupled with the vast cellular responses to p53, is simply astonishing. As new isoforms, functional domains and protein-protein interactions are described; each morsel of information forces us to think (and re-think) about how it 'fits' into the current p53 paradigm. One aspect of p53 signaling that is under refinement is the mechanism(s) leading to apoptosis. Here we discuss what is known about p53-induced apoptosis, what proteins and protein-protein interactions are responsible for regulating apoptosis, how can this cascade be genetically dissected, and what pharmacological tools are available to modulate p53-dependent apoptosis. While everything may not comfortably fit into our understanding of p53, all of these data will certainly broaden our viewpoint on the complexity and significance of the p53-induced apoptotic pathway. Here, our discussion is primarily focused on the works presented at the 12th International p53 Workshop, except where appropriate background is required.

Two distinct steps of Bak regulation during apoptotic stress signaling: different roles of MEKK1 and JNK1

Stress-activated protein (SAP) kinases and the mitochondrial pro-apoptotic Bcl-2 protein Bak are important regulators of apoptosis. Reduced expression of Bak increases cellular resistance to the anticancer agent cisplatin, and we report here that mouse embryo fibroblasts deficient in the SAP kinase jnk1 are highly resistant to apoptosis induced by cisplatin. When human melanoma cells were treated with cisplatin, Bak function was found to be regulated in two distinct steps by two SAP kinases, MEKK1 and JNK1. The first of these steps involves MEKK1-controlled conformational activation of Bak. The second step leads to formation of 80-170 kDa Bak complexes correlating with apoptosis, and is controlled by JNK1. Inhibition of MEKK1 blocked the initial Bak conformational activation but did not block JNK1 activation, and deficiency in, or inhibition of, JNK1 did not prevent conformational activation of Bak. Furthermore, inducible expression of a constitutively active form of MEKK1 led to Bak conformational activation, but not to 80-170 kDa complexes. Consequently, apoptosis was delayed unless JNK was exogenously stimulated, indicating that Bak conformational activation is not necessarily an apoptotic marker. The two-step regulation of Bak revealed here may be important for tight control of mitochondrial factor release and apoptosis.

Androgen and its receptor promote Bax-mediated apoptosis

Androgen and its receptor (AR) have been reported to have pro- or antiapoptotic functions. However, the underlying molecular mechanism is incompletely understood. We report here that androgen and AR promote Bax-mediated apoptosis in prostate cancer cells. UV irradiation and ectopic expression of Bax induce apoptosis in AR-positive, but not AR-negative prostate cancer cells. UV- and Bax-induced apoptosis is abrogated in AR-positive cells that express small interference RNA (siRNA) of AR and is sensitized by reintroduction of AR into AR-negative cells. Although AR is able to promote Bax-mediated apoptosis independently of androgen, the promotion by AR can be further potentiated by androgen via AR-dependent transcription activation. AR is essential for the translocation of Bax to mitochondria in UV- or Bax-induced apoptosis. Inhibition of Bax expression by Bax siRNA suppresses UV-induced apoptosis in AR-positive cells. In addition, introduction of AR into AR-negative prostate cancer cells upregulates expression levels of the BH3-only protein Noxa, whereas inhibition of Noxa expression reduces the promotion by AR on UV-induced apoptosis. Thus, our results reveal a novel cross talk between the androgen/AR hormonal signaling pathway and the intrinsic apoptotic death pathway that determines the sensitivity of stress-induced apoptosis in prostate cancer cells.

Mcl-1 Interacts with Truncated Bid and Inhibits Its Induction of Cytochrome c Release and Its Role in Receptor-mediated Apoptosis

Engagement of death receptors such as tumor necrosis factor-R1 and Fas brings about the cleavage of cytosolic Bid to truncated Bid (tBid), which translocates to mitochondria to activate Bax/Bak, resulting in the release of cytochrome c. The mechanism underlying the activation, however, is not fully understood. Here, we have identified the anti-apoptotic Bcl-2 family member Mcl-1 as a potent tBid-binding partner. Site-directed mutagenesis reveals that the Bcl-2 homology (BH)3 domain of tBid is essential for binding to Mcl-1, whereas all three BH domains (BH1, BH2, and BH3) of Mcl-1 are required for interaction with tBid. In vitro studies using isolated mitochondria and recombinant proteins demonstrate that Mcl-1 strongly inhibits tBid-induced cytochrome c release. In addition to its ability to interact directly with Bax and Bak, tBid also binds Mcl-1 and displaces Bak from the Mcl-1-Bak complex. Importantly, overexpression of Mcl-1 confers resistance to the induction of apoptosis by both TRAIL and tumor necrosis factor-alpha in HeLa cells, whereas targeting Mcl-1 by RNA interference sensitizes HeLa cells to TRAIL-induced apoptosis. Therefore, our study demonstrates a novel regulation of tBid by Mcl-1 through protein-protein interaction in apoptotic signaling from death receptors to mitochondria.