Human Melanoma Cells under Endoplasmic Reticulum Stress Are More Susceptible to Apoptosis Induced by the BH3 Mimetic Obatoclax

Past studies have shown that melanoma cells have largely adapted to endoplasmic reticulum (ER) stress, and this is associated with up-regulation of the antiapoptotic proteins Bcl-2 and Mcl-1. In this report, we show that the BH3 mimetic obatoclax potently overcomes resistance of melanoma cells to apoptosis induced by ER stress. Obatoclax, as a single agent at nanomolar concentrations, was relatively ineffective in the induction of apoptosis in melanoma cells, but treatment with obatoclax at these concentrations in combination with the ER stress inducer tunicamycin (TM) or thapsigargin markedly enhanced apoptotic cell death. This was primarily because of the inhibition of Mcl-1 by obatoclax, in that cotreatment with TM and another BH3 mimetic ABT737, which does not antagonize Mcl-1, caused only minimal increases in apoptosis. Moreover, overexpression of Mcl-1 inhibited apoptosis to greater degrees than overexpression of Bcl-2. In addition to direct inhibition of Mcl-1 by obatoclax, the combination of obatoclax and TM caused strong up-regulation of the BH3-only protein Noxa. Small RNA interference knockdown of Noxa partially inhibited apoptosis induced by cotreatment with obatoclax and TM. Similarly, knockdown of Bak also blocked induction of apoptosis by the compounds. The Mcl-1/Bak interaction seemed to be disrupted more efficiently in melanoma cells cotreated with obatoclax and TM. Taken together, these results identify obatoclax as a potent agent that overcomes resistance of melanoma cells to ER stress-induced apoptosis and seem to have important implications in the use of BH3 mimetics in the treatment of melanoma.     

The BH3 mimetic ABT-737 targets selective Bcl-2 proteins and efficiently induces apoptosis via Bak/Bax if Mcl-1 is neutralized

Since apoptosis is impaired in malignant cells overexpressing prosurvival Bcl-2 proteins, drugs mimicking their natural antagonists, BH3-only proteins, might overcome chemoresistance. Of seven putative BH3 mimetics tested, only ABT-737 triggered Bax/Bak-mediated apoptosis. Despite its high affinity for Bcl-2, Bcl-x(L), and Bcl-w, many cell types proved refractory to ABT-737. We show that this resistance reflects ABT-737's inability to target another prosurvival relative, Mcl-1. Downregulation of Mcl-1 by several strategies conferred sensitivity to ABT-737. Furthermore, enforced Mcl-1 expression in a mouse lymphoma model conferred resistance. In contrast, cells overexpressing Bcl-2 remained highly sensitive to ABT-737. Hence, ABT-737 should prove efficacious in tumors with low Mcl-1 levels, or when combined with agents that inactivate Mcl-1, even to treat those tumors that overexpress Bcl-2.     

抑制USP9x表达增强食管癌Ec9706-R细胞的放射敏感性

背景与目的：泛素特异性蛋白酶9x（ubiquitin-specific protease 9x,USP9x）与多种肿瘤的发生、发展以及肿瘤细胞的放射抗拒相关。研究发现,USP9x的表达与食管鳞状细胞癌的浸润深度和淋巴结转移相关,但其食管癌细胞放射抗拒作用尚未见报道。探究USP9x对放射抗拒食管癌Ec9706-R细胞放射敏感性的作用及其机制。方法：首先通过实时荧光定量聚合酶链反应（real-time fluorescent quantitative polymerase chain reaction,RTFQ-PCR）和蛋白质印迹法（Western blot）检测放射线照射后Ec9706-R细胞及其亲本细胞Ec-9706中USP9x和抗髓样细胞白血病-1（myeloid cell leukemia-1,Mcl-1）mRNA表达和蛋白水平。然后将Ec9706-R细胞随机分成3组：放射（irradiation,IR）组、IR+对照siRNA组（IR+si-NC组,转染Control siRNA）和IR+USP9x siRNA组（IR+si-USP9x组,转染USP9x siRNA）,各组细胞均使用一定量的6 MV-X射线照射。噻唑蓝（methyl thiazolyl tetrazolium,MTT）比色法检测不同剂量（0、2、4、6和8 Gy）6 MV-X射线照射下各组细胞的活力。Transwell、流式细胞术、RTFQ-PCR和Western blot分别检测3组细胞在6 Gy照射下的细胞迁移、凋亡、Mcl-1 mRNA表达和蛋白水平以及增殖细胞核抗原（proliferating cell nuclear antigen,PCNA）和DNA损伤修复相关基因核苷酸切除修复交叉互补基因1（excision repair cross-complementing gene 1,ERCC1）蛋白水平。结果：放射后Ec9706-R和Ec-9706细胞中USP9x和Mcl-1 mRNA表达和蛋白水平均增加,Ec9706-R细胞尤为显著（P<0.05）。与IR组相比,IR+si-USP9x组中细胞活力、迁移细胞数目、PCNA、ERCC1和Mcl-1的表达均降低,细胞凋亡增加（P<0.05）。但是与IR组相比,IR+si-NC组中上述指标均无显著变化（P>0.05）。结论：抑制USP9x表达能够增强放射抗拒食管癌Ec9706-R细胞的放射敏感性,这可能是通过下调Mcl-1的表达发挥作用的。     

BH3 Mimetics Reactivate Autophagic Cell Death in Anoxia-Resistant Malignant Glioma Cells

Here, we investigated the specific roles of Bcl-2 family members in anoxia tolerance of malignant glioma. Flow cytometry analysis of cell death in 17 glioma cell lines revealed drastic differences in their sensitivity to oxygen withdrawal (<0.1% O₂). Cell death correlated with mitochondrial depolarization, cytochrome C release, and translocation of green fluorescent protein (GFP)-tagged light chain 3 to autophagosomes but occurred in the absence of caspase activation or phosphatidylserine exposure. In both sensitive and tolerant glioma cell lines, anoxia caused a significant up-regulation of BH3-only genes previously implicated in mediating anoxic cell death in other cell types (BNIP3, NIX, PUMA, and Noxa). In contrast, we detected a strong correlation between anoxia resistance and high expression levels of antiapoptotic Bcl-2 family proteins Bcl-xL, Bcl-2, and Mcl-1 that function to neutralize the proapoptotic activity of BH3-only proteins. Importantly, inhibition of both Bcl-2 and Bcl-xL with the small-molecule BH3 mimetics HA14-1 and BH3I-2′ and by RNA interference reactivated anoxia-induced autophagic cell death in previously resistant glioma cells. Our data suggest that endogenous BH3-only protein induction may not be able to compensate for the high expression of antiapoptotic Bcl-2 family proteins in anoxia-resistant astrocytomas. They also support the conjecture that BH3 mimetics may represent an exciting new approach for the treatment of malignant glioma.     

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.     

Noxa up-regulation and Mcl-1 cleavage are associated to apoptosis induction by bortezomib in multiple myeloma

Targeting the ubiquitin-proteasome pathway has emerged as a potent anticancer strategy. Bortezomib, a specific proteasome inhibitor, has been approved for the treatment of relapsed or refractory multiple myeloma. Multiple myeloma cell survival is highly dependent on Mcl-1 antiapoptotic molecules. In a recent study, proteasome inhibitors induced Mcl-1 accumulation that slowed down their proapoptotic effects. Consequently, we investigated the role of Bcl-2 family members in bortezomib-induced apoptosis. We found that bortezomib induced apoptosis in five of seven human myeloma cell lines (HMCL). Bortezomib-induced apoptosis was associated with Mcl-1 cleavage regardless of Mcl-1L accumulation. Furthermore, RNA interference mediated Mcl-1 decrease and sensitized RPMI-8226 HMCL to bortezomib, highlighting the contribution of Mcl-1 in bortezomib-induced apoptosis. Interestingly, an important induction of Noxa was found in all sensitive HMCL both at protein and mRNA level. Concomitant to Mcl-1 cleavage and Noxa induction, we also found caspase-3, caspase-8, and caspase-9 activation. Under bortezomib treatment, Mcl-1L/Noxa complexes were highly increased, Mcl-1/Bak complexes were disrupted, and there was an accumulation of free Noxa. Finally, we observed a dissociation of Mcl-1/Bim complexes that may be due to a displacement of Bim induced by Noxa. Thus, in myeloma cells, the mechanistic basis for bortezomib sensitivity can be explained mainly by the model in which the sensitizer Noxa can displace Bim, a BH3-only activator, from Mcl-1, thus leading to Bax/Bak activation.     

Enhanced killing of melanoma cells by simultaneously targeting Mcl-1 and NOXA

By deciphering the dysregulation of apoptosis in melanoma cells, new treatment approaches exploiting aberrant control mechanisms regulating cell death can be envisioned. Among the Bcl-2 family, a BH3-only member, NOXA, functions in a specific mitochondrial-based cell death pathway when melanoma cells are exposed to a proteasome inhibitor (e.g., bortezomib). Some therapeutic agents, such as bortezomib, not only induce proapoptotic Bcl-2 family members and active conformational changes in Bak and Bax but also are associated with undesirable effects, including accumulation of antiapoptotic proteins, such as Mcl-1. To enhance the bortezomib-mediated killing of melanoma cells, the apoptotic pathway involving NOXA was further investigated, leading to identification of an important target (i.e., the labile Bcl-2 homologue Mcl-1 but not other survival proteins). To reduce Mcl-1 levels, melanoma cells were pretreated with several different agents, including Mcl-1 small interfering RNA (siRNA), UV light, or the purine nucleoside analogue fludarabine. By simultaneously triggering production of NOXA (using bortezomib) as well as reducing Mcl-1 levels (using siRNA, UV light, or fludarabine), significantly enhanced killing of melanoma cells was achieved. These results show binding interactions between distinct Bcl-2 family members, such as NOXA and Mcl-1, in melanoma cells, paving the way for novel and rational therapeutic combination strategies, which target guardians of the proapoptotic Bak- and Bax-mediated pathways, against this highly aggressive and often fatal malignancy.     

JNJ-26481585 primes rhabdomyosarcoma cells for chemotherapeutics by engaging the mitochondrial pathway of apoptosis

Rhabdomyosarcoma (RMS) is a common soft-tissue sarcoma in childhood with a poor prognosis, highlighting the need for new treatment strategies. Here we identify a synergistic interaction of the second-generation histone deacetylase inhibitor (HDACI) JNJ-26481585 and common chemotherapeutic drugs (i.e. Doxorubicin, Etoposide, Vincristine, Cyclophosphamide and Actinomycin D) to trigger apoptosis in RMS cells. Importantly, JNJ-26481585/Doxorubicin cotreatment also significantly suppresses long-term clonogenic survival of RMS cells and tumor growth in vivo in a preclinical RMS model. Mechanistically, JNJ-26481585/Doxorubicin cotreatment causes upregulation of the BH3-only proteins Bim and Noxa as well as downregulation of the antiapoptotic proteins Mcl-1 and Bcl-x_L. These changes in the ratio of pro- and antiapoptotic Bcl-2 proteins contribute to JNJ-26481585/Doxorubicin-mediated apoptosis, since knockdown of Bim or Noxa significantly inhibits cell death. Also, JNJ-26481585 and Doxorubicin cooperate to stimulate activation of Bax and Bak, which is required for JNJ-26481585/Doxorubicin-induced apoptosis, since silencing of Bax or Bak protects against apoptosis. Consistently, overexpression of Bcl-2 significantly reduces JNJ-26481585/Doxorubicin-mediated apoptosis. JNJ-26481585/Doxorubicin cotreatment leads to caspase activation and caspase-dependent apoptosis, since the broad-range caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD.fmk) rescues cells from apoptosis. In conclusion, the second-generation HDACI JNJ-26481585 cooperates with chemotherapeutics to engage mitochondrial apoptosis in RMS cells, demonstrating that JNJ-26481585 represents a promising strategy for chemosensitization of RMS.     

Puma(*)Mcl-1 interaction is not sufficient to prevent rapid degradation of Mcl-1

Although Puma (p53 upregulated modulator of apoptosis) was known as a principal mediator of cell death in response to diverse apoptotic signals, the molecular mechanism underlying its proapoptotic regulation remains largely uncharacterized. Here we reported that myeloid cell leukemia-1 (Mcl-1), an anti-apoptotic member of the Bcl-2 family with a rapid turnover rate, interacts with Puma. The Puma/Mcl-1 interaction was verified by both yeast two-hybrid assay and co-immuno-precipation studies. Their binding sites were mapped to BH3 (Bcl-2 homology) domain of Puma and BH1 domain of Mcl-1, respectively. Mcl-1 and Puma was shown to colocalize at the mitochondria by immunostaining. The level of Mcl-1 was increased when coexpressed with Puma, indicating Puma is able to stabilize Mcl-1. Puma binding to Mcl-1 via its BH3 domain is the prerequisite for this effect, which is further supported by the finding that Puma mutant lacking BH3 domain no longer promotes Mcl-1 protein stability. This Puma-enhanced Mcl-1 stabilization was validated in vivo under non-overexpression conditions. We also showed that BH1 domain is essential for Mcl-1 to inhibit Puma-induced apoptosis, since Mcl-1 mutant lacking BH1 domain completely abrogates its protective function. In addition, we concluded that binding of Puma to BH1 domain of Mcl-1 is necessary, but not sufficient to prevent rapid degradation of Mcl-1. In addition to PEST (proline, glutamic acid, serine, and threonine) and BH1 domain, some additional degradation signal is expected to reside in the C-terminal region of Mcl-1. In conclusion, our results provide the first evidence that the interaction between Mcl-1 and Puma may represent a novel mechanism by which Mcl-1 prevents apoptosis by increasing its stability through binding to Puma.     

Apoptosis induction in human melanoma cells by inhibition of MEK is caspase-independent and mediated by the Bcl-2 family members PUMA, Bim, and Mcl-1.

PURPOSE: Given that inhibitors of mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinase (MEK) are being introduced into treatment for melanoma, the present study was carried out to better understand the mechanism by which they may induce apoptosis of melanoma cells. EXPERIMENTAL DESIGN: A panel of human melanoma cell lines and fresh melanoma isolates was assessed for their sensitivity to apoptosis induced by the MEK inhibitor U0126. The apoptotic pathways and regulatory mechanisms involved were examined by use of the inhibitor and small interfering RNA (siRNA) techniques. RESULTS: Inhibition of MEK induced apoptosis in the majority of melanoma cell lines through a mitochondrial pathway that was associated with the activation of Bax and Bak, release of mitochondrial apoptogenic proteins, and activation of caspase-3. However, apoptosis was independent of caspases and instead was associated with mitochondrial release of AIF as shown by the inhibition of apoptosis when AIF was knocked down by siRNA. Inhibition of MEK resulted in the up-regulation of the BH3-only proteins PUMA and Bim and down-regulation of the antiapoptotic protein Mcl-1. These changes were critical for the induction of apoptosis by U0126 as siRNA knockdown of PUMA or Bim inhibited apoptosis, whereas siRNA knockdown of Mcl-1 increased apoptosis particularly in the apoptosis-resistant cell lines. CONCLUSIONS: Apoptosis of melanoma cells induced by the inhibition of the MEK/ERK pathway is mediated by the up-regulation/activation of PUMA and Bim and down-regulation of Mcl-1. Release of AIF rather than the activation of caspases seems to be the mediator of apoptosis. Our results suggest that cotargeting Mcl-1 and the MEK/ERK pathway may further improve treatment results in melanoma.     

Hyperforin induces apoptosis of chronic lymphocytic leukemia cells through upregulation of the BH3-only protein Noxa

We previously reported that hyperforin, a phloroglucinol purified from Hypericum perforatum, induces the mitochondrial pathway of caspase-dependent apoptosis in chronic lymphocytic leukemia (CLL) cells ex vivo, and that this effect is associated with upregulation of Noxa, a BH3-only protein of the Bcl-2 family. Here, we investigated the role of this upregulation in the pro-apoptotic activity of hyperforin in the cells of CLL patients and MEC-1 cell line. We found that the increase in Noxa expression is a time- and concentration-dependent effect of hyperforin occurring without change in Noxa mRNA levels. A post-translational regulation is suggested by the capacity of hyperforin to inhibit proteasome activity in CLL cells. Noxa silencing by siRNA reduces partially hyperforin-elicited apoptosis. Furthermore, treatment with hyperforin, which has no effect on the expression of the prosurvival protein Mcl-1, induces the interaction of Noxa with Mcl-1 and the dissociation of Mcl-1/Bak complex, revealing that upregulated Noxa displaces the proapoptotic protein Bak from Mcl-1. This effect is accompanied with Bak activation, known to allow the release of apoptogenic factors from mitochondria. Our data indicate that Noxa upregulation is one of the mechanisms by which hyperforin triggers CLL cell apoptosis. They also favor that new agents capable of mimicking specifically the BH3-only protein Noxa should be developed for apoptosis-based therapeutic strategy in CLL.     

The BH3-only protein Noxa is stimulated during apoptosis of chronic lymphocytic leukemia cells triggered by M2YN, a new plant-derived extract

Deficiency of apoptosis is a hallmark of chronic lymphocytic leukemia (CLL) cells. M2Yn is a natural extract from plants of central Asia, identified for its antiangiogenic properties and its ability to block the migration of malignant cells. Here, we report that in vitro treatment of cells derived from CLL patients with M2Yn results in internucleosomal DNA fragmentation, phosphatidylserine externalization, mitochondrial membrane depolarization, caspase-3 activation and cleavage of the caspase substrate PARP-1. The extents of these effects depend on the patients and are mostly comparable to those of flavopiridol or hyperforin, two known plant-derived apoptosis inducers of CLL cells. M2Yn does not modulate Mcl-1 expression, while downregulation of this antiapoptotic protein is involved in the action of flavopiridol. By contrast, M2Yn, like hyperforin, upregulates the Noxa protein, possibly by inhibiting proteasomal activity. This BH3-only protein is known to trigger the activation of the pro-apoptotic protein Bak through displacement of the Mcl-1/Bak complex at the mitochondrial membrane, as actually observed here in M2Yn-treated cells. Our data, therefore, show that M2Yn can induce the caspase-dependent mitochondrial pathway of apoptosis in CLL cells via a mechanism resembling that of hyperforin. Our data also confirm that the BH3-only protein Noxa is a relevant target for CLL therapy.     

Cleavage of Mcl-1 by caspases impaired its ability to counteract Bim-induced apoptosis

Mcl-1 is an antiapoptotic member of the Bcl-2 family that can promote cell viability. We report here that Mcl-1 is a new substrate for caspases during induction of apoptosis. Mcl-1 cleavage occurs after Asp127 and Asp157 and generates four fragments of 24, 19, 17 and 12 kDa in both intact cells and in vitro, an effect prevented by selective caspase inhibitors. As a consequence, the resulting protein that lacks the first 127 or 157 amino acids contains only the BH1-BH3 domains of Bcl-2 family members. Mutation of Asp127 and Asp157 abolishes the generation of the 24 and 12 kDa fragments and that of the 19 and 17 kDa fragments, respectively. Interestingly, when expressed in HeLa cells Mcl-1 wt and Mcl-1 Delta127 showed a markedly different intracellular distribution. Mcl-1 wt colocalized with alpha-Tubulin near the internal face of the plasma membrane, while Mcl-1 Delta127 coassociated with Bim-EL at the mitochondrial level. Coimmunoprecipitation experiments also demonstrated that Mcl1 Delta127 exhibited increased binding to Bim when compared to Mcl-1 wt. Finally, Mcl-1 wt unlike Mcl-1 Delta127 inhibited Bim-EL-induced caspase activation. Altogether, our findings demonstrate that cleavage of Mcl-1 by caspases modifies its subcellular localization, increases its association with Bim and inhibits its antiapoptotic function.     

Mcl-1 Phosphorylation defines ABT-737 resistance that can be overcome by increased NOXA expression in leukemic B cells

ABT-737 is a small molecule Bcl-2 homology (BH)-3 domain mimetic that binds to the Bcl-2 family proteins Bcl-2 and Bcl-xL and is currently under investigation in the clinic. In this study, we investigated potential mechanisms of resistance to ABT-737 in leukemia cell lines. Compared with parental cells, cells that have developed acquired resistance to ABT-737 showed increased expression of Mcl-1 in addition to posttranslational modifications that facilitated both Mcl-1 stabilization and its interaction with the BH3-only protein Bim. To sensitize resistant cells, Mcl-1 was targeted by two pan-Bcl-2 family inhibitors, obatoclax and gossypol. Although gossypol was effective only in resistant cells, obatoclax induced cell death in both parental and ABT-737-resistant cells. NOXA levels were increased substantially by treatment with gossypol and its expression was critical for the gossypol response. Mechanistically, the newly generated NOXA interacted with Mcl-1 and displaced Bim from the Mcl-1/Bim complex, freeing Bim to trigger the mitochondrial apoptotic pathway. Together, our findings indicate that NOXA and Mcl-1 are critical determinants for gossypol-mediated cell death in ABT-737-resistant cells. These data therefore reveal novel insight into mechanisms of acquired resistance to ABT-737.     

Life, death, BH3 profiles, and the salmon mousse

New drugs that neutralize the antiapoptotic members of the Bcl-2 family hold promise for rational cancer therapies, both alone and in combination with other agents. An understanding of how and why such agents may trigger apoptosis on their own, and how resistance to these drugs can occur, depends on the complexity of the Bcl-2 family interactions that control mitochondrial outer membrane permeabilization (MOMP). By extracting mitochondria from tumor cells and exposing them to peptides corresponding to the regulatory BH3-only proteins, MOMP predicts not only which cells will undergo apoptosis in response to Bcl-2 antagonists, but also why other cells may be resistant.     

TGFbeta-mediated apoptosis of Burkitt's lymphoma BL41 cells is associated with the relocation of mitochondrial BimEL

In this study, we showed that the transforming growth factor beta (TGFbeta)-mediated apoptosis of Burkitt's lymphoma BL41 cells is dependent on the BH3-only protein Bim. In contrast to what has been observed with other cell types, TGFbeta activation did not promote Bim upregulation in BL41 cells, but instead resulted in Bim release from the mitochondria. Indeed, Bim levels were high in healthy BL41 cells, in which they dimerized with the Bcl-2-like protein Mcl-1 at the mitochondrial surface. In healthy and TGFbeta-activated BL41 cells, unlike in epithelial cells or hepatocytes, Bim did not associate with Bcl-2 or Bcl-xL. TGFbeta activation of BL41 cells triggered the p38-dependent activation of caspase-8, causing the cleavage of Mcl-1 and the transfer of Bim from the mitochondria to the cytoskeleton. In addition to mitochondrial activation, this relocation of Bim may facilitate the complete demise of a cell death that is beyond the commitment point to apoptosis and may represent a hallmark of the TGFbeta-mediated apoptosis of human lymphoma B cells.     

The Bcl-2 apoptotic switch in cancer development and therapy

Impaired apoptosis is both critical in cancer development and a major barrier to effective treatment. In response to diverse intracellular damage signals, including those evoked by cancer therapy, the cell's decision to undergo apoptosis is determined by interactions between three factions of the Bcl-2 protein family. The damage signals are transduced by the diverse 'BH3-only' proteins, distinguished by the BH3 domain used to engage their pro-survival relatives: Bcl-2, Bcl-x(L), Bcl-w, Mcl-1 and A1. This interaction ablates pro-survival function and allows activation of Bax and Bak, which commit the cell to apoptosis by permeabilizing the outer membrane of the mitochondrion. Certain BH3-only proteins (e.g. Bim, Puma) can engage all the pro-survival proteins, but others (e.g. Bad, Noxa) engage only subsets. Activation of Bax and Bak appears to require that the BH3-only proteins engage the multiple pro-survival proteins guarding Bax and Bak, rather than binding to the latter. The balance between the pro-survival proteins and their BH3 ligands regulates tissue homeostasis, and either overexpression of a pro-survival family member or loss of a proapoptotic relative can be oncogenic. Better understanding of the Bcl-2 family is clarifying its role in cancer development, revealing how conventional therapy works and stimulating the search for "BH3 mimetics" as a novel class of anticancer drugs.