Bim-targeted cancer therapy: a link between drug action and underlying molecular changes

Bim, a proapoptotic protein, up-regulated via transcription factor E2F1-dependent mechanism, functions as a prosurvival molecule in cancer

Proapoptotic Bcl-2 homology 3-only protein Bim plays an important role in Bax/Bak-mediated cytochrome c release and apoptosis. Here, we provide evidence for a novel prosurvival function of Bim in cancer cells. Bim was constitutively overexpressed in multiple prostate and breast cancer cells as well as in primary tumor cells. Quantitative real time PCR analysis showed that Bim was transcriptionally up-regulated. We have identified eight endogenous E2F1-binding sites on the Bim promoter using in silico analysis. Luciferase assay demonstrated that Bim expression was E2F1-dependent as mutation of the E2F1-binding sites on the Bim promoter inhibited luciferase activities. In support, E2F1 silencing led to the loss of Bim expression in cancer cells. Bim primarily localized to mitochondrial and cytoskeleton-associated fractions. Bim silencing or microinjection of anti-Bim antibodies into the cell cytoplasm resulted in cell rounding, detachment, and subsequent apoptosis. We observed up-regulation of prosurvival proteins Bcl-xL and Mcl-1, which sequester Bim in cancer cells. In addition, a phosphorylated form of Bim was also elevated in cancer cells. These findings suggest that the constitutively overexpressed Bim may function as a prosurvival molecule in epithelial cancer cells, and phosphorylation and association with Bcl-xL/Mcl-1 block its proapoptotic functions.

MiR-494 is regulated by ERK1/2 and modulates TRAIL-induced apoptosis in non-small-cell lung cancer through BIM down-regulation

MicroRNAs (miRNAs) have an important role in the development of chemosensitivity or chemoresistance in different types of cancer. Activation of the ERK1/2 pathway is a major determinant of diverse cellular processes and cancer development and is responsible for the transcription of several important miRNAs. Here we show a link between the ERK1/2 pathway and BIM expression through miR-494. We blocked ERK1/2 nuclear activity through the overexpression of an ERK1/2 natural interactor, the protein PED/PEA15, and we performed a microRNA expression profile. miR-494 was the most down-regulated microRNA after ERK1/2 inactivation. Moreover, we found that miR-494 induced Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) resistance in non-small-cell lung cancer (NSCLC) through the down-modulation of BIM. Elucidation of this undiscovered ERK1/2 pathway that regulates apoptosis and cell proliferation through miR-494 in NSCLC will greatly enhance our understanding of the mechanisms responsible for TRAIL resistance and will provide an additional arm for the development of anticancer therapies.

BHLH transcription factor DEC2 regulates pro-apoptotic factor Bim in human oral cancer HSC-3 cells

DEC1 (BHLHE40/Stra13/Sharp2) and DEC2 (BHLHE41/Sharp1) are basic helix-loop-helix (bHLH) transcription factors that are involved in the regulation of apoptosis, cell proliferation, circadian rhythms and the response to hypoxia. We previously showed the functional effects of DEC1 and DEC2 on apoptosis in human breast cancer MCF-7 cells. However, the roles of DEC1 and DEC2 in oral cancer are poorly understood. We examined whether DEC1 and DEC2 are involved in the regulation of apoptosis in human oral cancer HSC-3 and CA9-22 cells. The expression of DEC2 was upregulated by cis-diamminedichloroplatinum (II) (cisplatin: CDDP) treatment in HSC-3 cells, whereas CDDP treatment had little effects on the expression of DEC2 in CA9-22 cells. We showed that DEC2 overexpression inhibits pro-apoptotic factor Bim and inhibits apoptosis induced by CDDP in HSC-3 cells, whereas it had little effects on apoptosis in CA9-22 cells. DEC1 overexpression had little effects on apoptosis induced by CDDP in these cells. We also found that CDDP upregulated the amounts of DEC2 in the nucleus in HSC-3 cells. These results suggest that DEC2 has anti-apoptotic effects on apoptosis induced by CDDP in HSC-3 cells.

Expression of Bim, Noxa, and Puma in non-small cell lung cancer

Background

The BH3-only members of the Bcl-2 protein family have been proposed to play a key role in the control of apoptosis and in the initiation of the apoptotic pathways. In this study, we evaluated the expression of Bim, Noxa, and Puma in non-small cell lung cancer (NSCLC).

Methods

A total of 135 surgically resected NSCLCs were immunohistochemically assessed for Bim, Noxa, and Puma expression. The immunoscores were determined, and then its correlation with either the clinicopathological variables or the survival outcomes were analyzed.

Results

Immunohistochemical reactivity for Bim, Noxa, and Puma was detected in the cytoplasm of the tumor cells. Bim expression was associated with several clinicopathological factors, including sex (p < 0.001), smoking habit (p = 0.03), pathological histology (p = 0.001), pathological T stage (p = 0.03), pathological disease stage (p = 0.02), and differentiation of tumor (p < 0.001). Multivariate logistic regression analysis showed a significant correlation between low Bim expression and squamous cell carcinoma (p = 0.04), in addition to a correlation between high Bim expression and well differentiated tumors (p = 0.02). Analysis of cellular biological expression demonstrated a link between low Bim expression and high Ki67. While Noxa expression was also shown to be correlated with both smoking habit (p = 0.02) and the pathological histology (p = 0.03), there was no strong association observed between the expression and the clinical features when they were examined by a multivariate logistic regression analysis. No correlations were noted between Puma expression and any of the variables. Our analyses also indicated that the expression levels of the BH3-only proteins were not pertinent to the survival outcome.

Conclusions

The current analyses demonstrated that Bim expression in the NSCLCs was associated with both squamous cell carcinoma histology and tumor proliferation.

CDK inhibitors upregulate BH3-only proteins to sensitize human myeloma cells to BH3 mimetic therapies

BH3 mimetic drugs induce cell death by antagonizing the activity of antiapoptotic Bcl-2 family proteins. Cyclin-dependent kinase (CDK) inhibitors that function as transcriptional repressors downregulate the Bcl-2 family member Mcl-1 and increase the activity of selective BH3 mimetics that fail to target this protein. In this study, we determined whether CDK inhibitors potentiate the activity of pan-BH3 mimetics directly neutralizing Mcl-1. Specifically, we evaluated interactions between the prototypical pan-CDK inhibitor flavopiridol and the pan-BH3 mimetic obatoclax in multiple myeloma (MM) cells in which Mcl-1 is critical for survival. Coadministration of flavopiridol and obatoclax synergistically triggered apoptosis in both drug-naïve and drug-resistant MM cells. Mechanistic investigations revealed that flavopiridol inhibited Mcl-1 transcription but increased transcription of Bim and its binding to Bcl-2/Bcl-xL. Obatoclax prevented Mcl-1 recovery and caused release of Bim from Bcl-2/Bcl-xL and Mcl-1, accompanied by activation of Bax/Bak. Whether administered singly or in combination with obatoclax, flavopiridol also induced upregulation of multiple BH3-only proteins, including BimEL, BimL, Noxa, and Bik/NBK. Notably, short hairpin RNA knockdown of Bim or Noxa abrogated lethality triggered by the flavopiridol/obatoclax combination in vitro and in vivo. Together, our findings show that CDK inhibition potentiates pan-BH3 mimetic activity through a cooperative mechanism involving upregulation of BH3-only proteins with coordinate downregulation of their antiapoptotic counterparts. These findings have immediate implications for the clinical trial design of BH3 mimetic-based therapies that are presently being studied intensively for the treatment of diverse hematopoietic malignancies, including lethal multiple myeloma.

Role of Bcl-2 in tumour cell survival and implications for pharmacotherapy

OBJECTIVES

Bcl-2 is a protein that inhibits apoptosis, leading to cell survival. The Bcl-2 family has six different anti-apoptotic proteins, three pro-apoptotic proteins that are similar in structure, and other integrating proteins that function as promotors or inhibitors in the progression of apoptosis. In this discussion paper, we provide an overview of apoptosis, the role of Bcl-2 in normal cellular and molecular processes, and the role of Bcl-2 in tumour cell survival. It focuses primarily on anti-apoptotic Bcl-2, its activation in cancer, the manner in which it regulates the intrinsic and extrinsic mechanisms of apoptosis, and its broad molecular interactions with other critical proteins in the cell. Certain cancer treatments are reviewed and related directions for the future are presented.

KEY FINDINGS

Apoptosis is common to all organisms - for eukaryotes it is a normal process of development and regeneration. The rate at which apoptosis occurs is critical to the survival of the organism, as too much can lead to the onset of degenerative diseases such as dementia, and too little may lead to cancer. FKBP-38 is a binding protein that has been discovered to be upregulated in highly aggressive cancers and binds to Bcl-2 rather than the pro-apoptotics to induce a state of hyper-mitosis. A short binding protein (Nur-77) provides new insights into Bcl-2 'masking'. Nurr-77 binds to Bcl-2 and exposes the BH3 domain, transforming it from a cancer promoter to an unorthodox cancer inhibitor. This presents in itself an interesting and exciting opportunity - increasing the rate of apoptosis in neoplastic cells that are usually protected by Bcl-2 activity at the mitochondria.

SUMMARY

Development of drugs in the form of BH3-only and BH123 mimetic drugs provide a interesting avenue for cancer therapy for the future. Drugs that can either promote, or mimic anti-IAP activity such as Smac/Diablo would certainly be productive, thereby inducing apoptosis. Medicinal usage which can effectively suppress FKBP38 in Bcl-2-dependent cancers would provide further arsenal to combat apoptotic irregularities, particularly a treatment that is more dominant than kinetin riboside. WAVE-1 inhibitors may effectively suppress the phosphorylation of Bcl-2, thereby potentially reducing hyper-mitosis and increasing apoptosis. Recent findings shed molecular light on PDT, namely ER stress, and potential for anti-cancer therapy via either apoptosis or autophagy. A drug that can effectively upregulate Nurr-77, thereby masking the anti-apoptotic properties of Bcl-2, would indeed be life-saving for cancer patients.

Critical role of p53 upregulated modulator of apoptosis in benzyl isothiocyanate-induced apoptotic cell death

Benzyl isothiocyanate (BITC), a constituent of edible cruciferous vegetables, decreases viability of cancer cells by causing apoptosis but the mechanism of cell death is not fully understood. The present study was undertaken to determine the role of Bcl-2 family proteins in BITC-induced apoptosis using MDA-MB-231 (breast), MCF-7 (breast), and HCT-116 (colon) human cancer cells. The B-cell lymphoma 2 interacting mediator of cell death (Bim) protein was dispensable for proapoptotic response to BITC in MCF-7 and MDA-MB-231 cells as judged by RNA interference studies. Instead, the BITC-treated MCF-7 and MDA-MB-231 cells exhibited upregulation of p53 upregulated modulator of apoptosis (PUMA) protein. The BITC-mediated induction of PUMA was relatively more pronounced in MCF-7 cells due to the presence of wild-type p53 compared with MDA-MB-231 with mutant p53. The BITC-induced apoptosis was partially but significantly attenuated by RNA interference of PUMA in MCF-7 cells. The PUMA knockout variant of HCT-116 cells exhibited significant resistance towards BITC-induced apoptosis compared with wild-type HCT-116 cells. Attenuation of BITC-induced apoptosis in PUMA knockout HCT-116 cells was accompanied by enhanced G2/M phase cell cycle arrest due to induction of p21 and down regulation of cyclin-dependent kinase 1 protein. The BITC treatment caused a decrease in protein levels of Bcl-xL (MCF-7 and MDA-MB-231 cells) and Bcl-2 (MCF-7 cells). Ectopic expression of Bcl-xL in MCF-7 and MDA-MB-231 cells and that of Bcl-2 in MCF-7 cells conferred protection against proapoptotic response to BITC. Interestingly, the BITC-treated MDA-MB-231 cells exhibited induction of Bcl-2 protein expression, and RNA interference of Bcl-2 in this cell line resulted in augmentation of BITC-induced apoptosis. The BITC-mediated inhibition of MDA-MB-231 xenograft growth in vivo was associated with the induction of PUMA protein in the tumor. In conclusion, the results of the present study indicate that Bim-independent apoptosis by BITC in cancer cells is mediated by PUMA.

Therapeutic resistance resulting from mutations in Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR signaling pathways

Chemotherapy remains a commonly used therapeutic approach for many cancers. Indeed chemotherapy is relatively effective for treatment of certain cancers and it may be the only therapy (besides radiotherapy) that is appropriate for certain cancers. However, a common problem with chemotherapy is the development of drug resistance. Many studies on the mechanisms of drug resistance concentrated on the expression of membrane transporters and how they could be aberrantly regulated in drug resistant cells. Attempts were made to isolate specific inhibitors which could be used to treat drug resistant patients. Unfortunately most of these drug transporter inhibitors have not proven effective for therapy. Recently the possibilities of more specific, targeted therapies have sparked the interest of clinical and basic researchers as approaches to kill cancer cells. However, there are also problems associated with these targeted therapies. Two key signaling pathways involved in the regulation of cell growth are the Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR pathways. Dysregulated signaling through these pathways is often the result of genetic alterations in critical components in these pathways as well as mutations in upstream growth factor receptors. Furthermore, these pathways may be activated by chemotherapeutic drugs and ionizing radiation. This review documents how their abnormal expression can contribute to drug resistance as well as resistance to targeted therapy. This review will discuss in detail PTEN regulation as this is a critical tumor suppressor gene frequently dysregulated in human cancer which contributes to therapy resistance. Controlling the expression of these pathways could improve cancer therapy and ameliorate human health.

Increased expression of the pro-apoptotic protein BIM, a mechanism for cAMP/protein kinase A (PKA)-induced apoptosis of immature T cells

The second messenger cAMP is proapoptotic for numerous cell types, but the mechanism for this proapoptotic action is not defined. Here, we use murine CD4(+)/CD8(+) S49 lymphoma cells and isolated thymocytes to assess this mechanism. In WT S49 cells, cAMP acts via protein kinase A (PKA) to induce G(1) phase cell cycle arrest and apoptosis. Treatment of WT and cAMP-Deathless (D-) S49 cells, which lack cAMP-promoted apoptosis, with the PKA agonist 8-(4-chlorophenylthio)-cAMP (CPT-cAMP) differentially regulates transcripts for numerous proapoptotic and antiapoptotic proteins. In contrast, kin-S49 cells (which lack PKA) show no cAMP-promoted changes in transcript expression. In this study, we use knockdown and overexpression approaches to define the role in cAMP/PKA-promoted apoptosis of the proapoptotic factor BIM (Bcl-2 interacting mediator of cell death), whose expression prominently increases in response to CPT-cAMP treatment of WT but not D- or kin- S49 cells. Conditional expression of BimL, one of the three major forms of Bim, increases apoptosis of WT, D-, and kin-S49 cells, whereas inhibition of cAMP-mediated induction of Bim isoforms by shRNAi attenuates CPT-cAMP-mediated apoptosis of WT S49 cells. Bim protein levels increase in subpopulations of CPT-cAMP-treated cells that undergo apoptosis. Thymic CD4(+)/CD8(+) cells isolated from Bim(-/-) mice corroborated the requirement of Bim expression for cAMP-promoted apoptosis. Thus, up-regulation of Bim appears to be an important determinant of cAMP/PKA-mediated apoptosis in immature T cells and may be a mechanism for such apoptosis in other cell types as well.

Bim contributes to phenethyl isothiocyanate-induced apoptosis in breast cancer cells

Phenethyl isothiocyanate (PEITC) is a highly promising cancer chemopreventive constituent of cruciferous vegetables (e.g., watercress) with in vivo efficacy in experimental rodent cancer models. Research thus far implicates apoptosis induction in cancer chemopreventive response to PEITC, but the mechanism of proapoptotic effect is not fully understood. The present study demonstrates that p53 upregulated modulator of apoptosis (PUMA)-independent apoptosis by PEITC is mediated by B-cell lymphoma 2 interacting mediator of cell death (Bim). Exposure of a cell line (BRI-JM04) derived from spontaneously developing mammary tumor of a MMTV-neu transgenic mouse to pharmacological concentrations of PEITC resulted in decreased cell viability coupled with apoptosis induction, characterized by release of histone-associated DNA fragments into the cytosol and cleavage of poly-(ADP-ribose)-polymerase and procaspase-3. The PEITC-induced apoptosis in BRI-JM04 cells was associated with up-regulation of Bak, PUMA, and Bim (long and short forms of Bim), increased S65 phosphorylation of BimEL (extra-long form), and down-regulation of Bcl-xL and Bcl-2. On the other hand, a non-tumorigenic human mammary epithelial cell line (MCF-10A) was significantly more resistant to PEITC-induced apoptosis compared with BRI-JM04 despite induction of Bax and PUMA due to concomitant overexpression of anti-apoptotic proteins, including Bcl-xL, Bcl-2, and Mcl-1. Wild-type HCT-116 cells and its isogenic PUMA knockout variant exhibited comparable sensitivity to PEITC-induced apoptosis. On the other hand, small interfering RNA knockdown of Bim protein imparted partial but statistically significant protection against PEITC-induced apoptosis in BRI-JM04, MCF-7, and MDA-MB-231 cells. In conclusion, the present study provides novel insight into the mechanism of PEITC-induced apoptosis involving Bim.

The Mef2A transcription factor coordinately regulates a costamere gene program in cardiac muscle

The Mef2 family of transcription factors regulates muscle differentiation, but the specific gene programs controlled by each member remain unknown. Characterization of Mef2A knock-out mice has revealed severe myofibrillar defects in cardiac muscle indicating a requirement for Mef2A in cytoarchitectural integrity. Through comprehensive expression analysis of Mef2A-deficient hearts, we identified a cohort of dysregulated genes whose products localize to the peripheral Z-disc/costamere region. Many of these genes are essential for costamere integrity and function. Here we demonstrate that these genes are directly regulated by Mef2A, establishing a mechanism by which Mef2A controls the costamere. In an independent model system, acute knockdown of Mef2A in primary neonatal cardiomyocytes resulted in profound malformations of myofibrils and focal adhesions accompanied by adhesion-dependent programmed cell death. These findings indicate a role for Mef2A in cardiomyocyte survival through regulation of costamere integrity. Finally, bioinformatics analysis identified over-represented transcription factor-binding sites in this network of costamere promoters that may provide insight into the mechanism by which costamere genes are regulated by Mef2A. The global control of costamere gene expression adds another dimension by which this essential macromolecular complex may be regulated in health and disease.

Bim: guardian of tissue homeostasis and critical regulator of the immune system, tumorigenesis and bone biology

One of the most important roles of apoptosis is the maintenance of tissue homeostasis. Impairment of apoptosis leads to a number of pathological conditions. In response to apoptotic signals, various proteins are activated in a pathway and signal-specific manner. Recently, the pro-apoptotic molecule Bim has attracted increasing attention as a pivotal regulator of tissue homeostasis. The Bim expression level is strictly controlled in both transcriptional and post-transcriptional levels. This control is dependent on cell, tissue and apoptotic stimuli. The phenotype of Bim-deficient mice is a systemic lupus erythematosus-like autoimmune disease with an abnormal accumulation of hematopoietic cells. Bim is thus a critical regulator of hematopoietic cells and immune system. Further studies have revealed the critical roles of Bim in various normal and pathological conditions, including bone homeostasis and tumorigenesis. The current understanding of Bim signaling and roles in the maintenance of tissue homeostasis is reviewed in this paper, focusing on the immune system, bone biology and tumorigenesis to illustrate the diversified role of Bim.

Targeting the translational apparatus to improve leukemia therapy: roles of the PI3K/PTEN/Akt/mTOR pathway

It has become apparent that regulation of protein translation is an important determinant in controlling cell growth and leukemic transformation. The phosphoinositide 3-kinase (PI3K)/phosphatase and tensin homologue deleted on chromosome ten (PTEN)/Akt/mammalian target of rapamycin (mTOR) pathway is often implicated in sensitivity and resistance to therapy. Dysregulated signaling through the PI3K/PTEN/Akt/mTOR pathway is often the result of genetic alterations in critical components in this pathway as well as mutations at upstream growth factor receptors. Furthermore, this pathway is activated by autocrine transformation mechanisms. PTEN is a critical tumor suppressor gene and its dysregulation results in the activation of Akt. PTEN is often mutated, silenced and is often haploinsufficient. The mTOR complex1 (mTORC1) regulates the assembly of the eukaryotic initiation factor4F complex, which is critical for the translation of mRNAs that are important for cell growth, prevention of apoptosis and transformation. These mRNAs have long 5'-untranslated regions that are G+C rich, rendering them difficult to translate. Elevated mTORC1 activity promotes the translation of these mRNAs via the phosphorylation of 4E-BP1. mTORC1 is a target of rapamycin and novel active-site inhibitors that directly target the TOR kinase activity. Although rapamycin and novel rapalogs are usually cytostatic and not cytotoxic for leukemic cells, novel inhibitors that target the kinase activities of PI3K and mTOR may prove more effective for leukemia therapy.

Bortezomib interacts synergistically with belinostat in human acute myeloid leukaemia and acute lymphoblastic leukaemia cells in association with perturbations in NF-κB and Bim

Interactions between the histone deacetylase inhibitor belinostat and the proteasome inhibitor bortezomib were investigated in acute myeloid leukaemia (AML) and acute lymphoblastic leukaemia (ALL) cells. Co-administration of sub-micromolar concentrations of belinostat with low nanomolar concentrations of bortezomib sharply increased apoptosis in both AML and ALL cell lines and primary blasts. Synergistic interactions were associated with interruption of both canonical and non-canonical nuclear factor (NF)-κB signalling pathways, e.g. accumulation of the phosphorylated (S32/S36) form of IκBα, diminished belinostat-mediated RelA/p65 hyperacetylation (K310), and reduced processing of p100 into p52. These events were accompanied by down-regulation of NF-κB-dependent pro-survival proteins (e.g. XIAP, Bcl-xL). Moreover, belinostat/bortezomib co-exposure induced up-regulation of the BH3-only pro-death protein Bim. Significantly, shRNA knock-down of Bim substantially reduced the lethality of belinostat/bortezomib regimens. Administration of belinostat ± bortezomib also induced hyperacetylation (K40) of α-tubulin, indicating histone deacetylase inhibitor 6 inhibition. Finally, in contrast to the pronounced lethality of belinostat/bortezomib toward primary leukaemia blasts, equivalent treatment was relatively non-toxic to normal CD34(+) cells. Together, these findings indicate that belinostat and bortezomib interact synergistically in both cultured and primary AML and ALL cells, and raise the possibilities that up-regulation of Bim and interference with NF-κB pathways contribute to this phenomenon. They also suggest that combined belinostat/bortezomib regimens warrant further attention in acute leukaemias.

SIRT1 deficiency downregulates PTEN/JNK/FOXO1 pathway to block reactive oxygen species-induced apoptosis in mouse embryonic stem cells

Silent mating type information regulation 2 homolog 1 (SIRT1) plays a critical role in reactive oxygen species-triggered apoptosis in mouse embryonic stem (mES) cells. Here, we investigated a possible role for the PTEN/Akt/JNK pathway in the SIRT1-mediated apoptosis pathway in mES cells. Akt was activated by removal of anti-oxidant 2-mercaptoethanol in SIRT1(-/-) mES cells. Since PTEN is a negative regulator of Akt and its activity can be modulated by acetylation, we investigated if SIRT1 deacetylated PTEN to downregulate Akt to trigger apoptosis in anti-oxidant-free culture conditions. PTEN was hyperacetylated and excluded from the nucleus in SIRT1(-/-) mES cells, consistent with enhanced Akt activity. SIRT1 deficiency enhanced the acetylation/phosphorylation level of FOXO1 and subsequently inhibited the nuclear localization of FOXO1. Cellular acetylation levels were enhanced by DNA-damaging agent, not by removal of anti-oxidant. c-Jun NH2-terminal kinase (JNK) was activated by removal of anti-oxidant in SIRT1-dependent manner. Although p53 acetylation was stronger in SIRT1(-/-) mES cells, DNA-damaging stress activated phosphorylation and enhanced cellular levels of p53 irrespective of SIRT1, whereas removal of anti-oxidant slightly activated p53 only with SIRT1. Expression levels of Bim and Puma were increased in anti-oxidant-free culture conditions in an SIRT1-dependent manner and treatment with JNK inhibitor blocked induction of Bim expression. DNA-damaging agent activated caspase3 regardless of SIRT1. Our data support an important role for SIRT1 in preparing the PTEN/JNK/FOXO1 pathway to respond to cellular reactive oxygen species.

Emerging Bcl-2 inhibitors for the treatment of cancer

INTRODUCTION

Bcl-2 family proteins are a component of the antiapoptotic machinery and are overexpressed in different malignancies. Accordingly, their enhanced expression has been attributed to the observed chemoresistance in most of the cancers. Therefore, targeting Bcl-2 family members becomes an important and attractive approach towards cancer therapy and is currently a very rapidly evolving area of research. This article highlights the numerous advancements that have been made in the design and synthesis of small molecule inhibitors (SMI) of pro-survival Bcl-2 proteins.

AREAS COVERED

This review comprehensively describes the progress made over the last 2 decades on this subject, including the clinical status of SMIs of Bcl-2 family proteins. Newer insights on the status of our knowledge on SMIs of Bcl-2 family proteins, their most beneficial application as well as current and future directions in this field are discussed.

EXPERT OPINION

Targeting Bcl-2 family proteins using SMI strategies is gaining momentum, with the emergence of certain new classes of inhibitors in Phase I and II clinical settings. In view of the tremendous progress toward the development of such inhibitors, this innovative approach certainly holds promise and has the potential to become a future mainstay for cancer therapy. The stage is set for the next generation of SMIs, for not only Bcl-2 proteins but also for Mcl-1. Other emerging molecules in the apoptotic machinery will also be explored and targeted.