Cancer cells show deviant behavior that induces apoptotic signaling. To survive, cancer cells typically acquire changes enabling evasion of death signals. One way they do this is by increasing the expression of anti-apoptotic BCL-2 proteins. Anti-apoptotic BCL-2 family proteins antagonize death signaling by forming heterodimers with pro-death proteins. Heterodimer formation occurs through binding of the pro-apoptotic protein's BH3 domain into the hydrophobic cleft of anti-apoptotic proteins. The BH3 mimetics are small molecule antagonists of the anti-apoptotic BCL-2 members that function as competitive inhibitors by binding to the hydrophobic cleft. Under certain conditions, antagonism of anti-apoptotic BCL-2 family proteins can unleash pro-death molecules in cancer cells. Thus, the BH3 mimetics are a new class of cancer drugs that specifically target a mechanism of cancer cell survival to selectively kill cancer cells.
For decades, cytotoxic chemotherapy, along with surgery and radiation, has been one of the three important modalities used to treat cancer. As most chemotherapeutics were discovered by empirical screens, the molecular mechanisms of how they kill cells are poorly understood. An ideal chemotherapeutic drug would target only neoplastic cells, signaling their removal from the body without damaging adjacent cells. However, conventional chemotherapy is associated with many toxicities, largely because of the presence in normal cells of the targets of most conventional agents, that is, DNA and microtubules. A truly cancer-selective therapy must target a molecule or property that is selectively present in cancer cells to avoid toxicity to normal cells.
For a cancerous cell to develop, it must obtain the ability to surmount essential checkpoints that would normally send a deregulated cell to its demise. Cancer cells attain the capacity to evade the body's own immune response and to grow in a stressful environment where both oxygen and nutrients are limited. To maintain a high proliferation rate, tumor cells generate their own growth signal while simultaneously becoming insensitive to growth-inhibitory effects (Hanahan and Weinberg, 2000; Johnstone et al., 2002). The cells obtain mutations in key cellular proteins that allow the developing cancer to grow, although breaking certain cellular rules, such as genomic instability, oncogene activation, loss of adhesion, along with breach of cell cycle checkpoints. Each of the breaches mentioned can cause activation of the mitochondrial (intrinsic) apoptotic pathway. The oncogene c-myc has been shown to activate apoptosis (Evan et al., 1992). Similarly, anoikis or loss of adhesion leads to the activation of pro-apoptotic factors (Puthalakath et al., 1999, 2001). Cancer cells have a unique requirement to overcome death signaling engendered by these behaviors. Targeting the mechanisms cancers use to escape apoptosis offers the possibility, therefore, of a wide therapeutic index.
The BCL-2 family
The BCL-2 family of proteins governs whether a cell continues to live or instead commits to death through the mitochondrial apoptotic pathway. Interactions between pro- and anti-apoptotic BCL-2 family proteins control the decision-making process at the mitochondrion, modulating cell sensitivity to death (Wei et al., 2000; Letai et al., 2002; Kim et al., 2006). BCL-2, BCL-XL, MCL-1, BCL-W and BFL-1 are the anti-apoptotic members of the family (Cleary and Sklar, 1985; Tsujimoto et al., 1985; Boise et al., 1993; Kozopas et al., 1993; Choi et al., 1995; Gibson et al., 1996). They share sequence homology in four α- helical BCL-2 homology domains (BH) BH1-4, whereas the multidomain pro-apoptotic proteins BAX, BAK and BOK, the promoters of apoptosis, share homology in the domains BH1 through BH3 (Oltvai et al., 1993; Chittenden et al., 1995). BOK expression is largely limited to reproductive tissues, and less is known about its function, though it is widely supposed that it behaves much like BAX and BAK (Hsu et al., 1997). Lastly, the group sharing the least homology is the BH3-only pro-death group, which includes PUMA, NOXA, BIM, BID, HRK, BMF, BAD and BIK (Boyd et al., 1995; Yang et al., 1995; Wang et al., 1996; Inohara et al., 1997; O’Connor et al., 1998; Oda et al., 2000; Nakano and Vousden, 2001). They are so named because they show homology only in the BH3 domain. The BH3 domain, along with the presence of BAX and BAK, are absolutely essential for the death function of the BH3-only proteins (Zha and Reed, 1997; Cheng et al., 2001; Wei et al., 2001).
Death signals emanating from diverse cellular locations caused by DNA damage, growth factor deprivation and oncogene activation are directed through the mitochondrial pathway by activation (transcriptional or post-translational modifications) of BH3-only proteins (Evan et al., 1992; Oda et al., 2000; Nakano and Vousden, 2001). In viable cells, BAX and BAK exist as monomers; however, on receiving death signals from the activated BH3-only proteins, they homo-oligomerize and insert into the mitochondrial membrane (Eskes et al., 2000; Korsmeyer et al., 2000; Wei et al., 2001). The insertion of homo-oligomerized BAX and BAK causes mitochondrial outer membrane permeabilization (MOMP). Permeabilization is followed by release of cytochrome c and other pro-apoptotic factors from the mitochondria. Permeabilization of the mitochondrial outer membrane is the key step in commitment to death through the mitochondrial apoptotic pathway (Figure 1).
There is general agreement that BAX and BAK require activation before effecting permeabilization of the mitochondrion. Although, some reports emphasize the importance of activator BH3 proteins in facilitating this transition (Wei et al., 2000; Kuwana et al., 2002; Letai et al., 2002; Kim et al., 2006), other reports stress the neutralization of the anti-apoptotic members (Willis et al., 2005, 2007). The former describes a select few of the BH3-only proteins termed activators (BIM, tBID and perhaps PUMA) that have the ability to bind directly to BAX and BAK causing their activation (Letai et al., 2002; Kuwana et al., 2005; Kim et al., 2006). In the latter case, BH3-only proteins are sensitizers that cause death by binding the anti-apoptotic BCL-2 proteins causing a release of pro-apoptotic proteins (Figure 2) (Willis et al., 2005, 2007).
After its release, cytochrome c (Korsmeyer et al., 2000) binds in a complex with APAF-1, caspase-9 and dATP/dADP to form the apoptosome (Zou et al., 1999). From the apoptosome platform, the downstream effectors of death, the caspases, are activated (Zou et al., 1997, 1999; Acehan et al., 2002).
Apoptotic road blocks in cancer
Delineation of the mitochondrial apoptotic pathway leads to an important question: How does a cancer cell survive the apoptotic signaling events activated by its aberrant behavior? One mechanism is to select for a block in apoptotic signaling at the mitochondria by increasing the expression of anti-apoptotic BCL-2 members. There are many mechanisms known by which BCL-2 is expressed at high levels in cancer cells. BCL-2 was originally cloned from the breakpoint of the chromosomal translocation t(14; 18) in patients with follicular lymphoma (Cleary and Sklar, 1985; Tsujimoto et al., 1985). The translocation placed BCL-2 next to enhancer elements of the immunoglobulin promoter causing an increased expression of BCL-2. The BCL-2 translocation occurs in approximately 90% of follicular lymphomas and in about one-third of diffuse large B cell lymphomas (Weiss et al., 1987). However, BCL-2 overexpression has also been linked to gene amplification (Rao et al., 1998), hypermethylation of the BCL-2 gene (Hanada et al., 1993) or chromosomal deletions causing the loss of micro-RNAs involved in the silencing of BCL-2 (Cimmino et al., 2005). A series of genetic studies in mice really emphasized the oncogenic potential of BCL-2 (Vaux et al., 1988; Strasser et al., 1990; Letai et al., 2004). Work examining the combined action of c-myc and BCL-2 in mice showed that for neoplastic progression, there is a requirement for the deregulated cellular proliferation to be coupled with a compensatory inhibition of apoptosis.
Reciprocally, cancer cells can evade death through mutations that inactivate the effector arm of apoptosis, such as the mutations of BAX observed in some solid tumors and hematopoietic malignancies (Meijerink et al., 1995, 1998; Rampino et al., 1997). An especially common defect of cancers is a mutation in the tumor suppressor gene p53 (Greenblatt et al., 1994), which in turn has a negative effect on the activation of apoptosis, as the BH3-only proteins NOXA and PUMA are transcriptional targets of p53 (Oda et al., 2000; Nakano and Vousden, 2001; Jeffers et al., 2003; Shibue et al., 2003; Villunger et al., 2003). However, there are also indirect mechanisms for thwarting p53 function, including mutations in ATM, a kinase involved in the phosphorylation and activation of p53 (Banin et al., 1998).
A recent paper segregated each of these blocks in apoptosis into three classes. In a Class A block, there is a loss of activator BH3-only proteins. A Class B block is caused by a failure to activate the effector arm of apoptosis through loss or inactivation of BAX or BAK. Finally, increased expression of an inhibitor protein such as BCL-2/MCL-1 causes a class C block (Deng et al., 2007) (Figure 3). A new technique called BH3 profiling can detect the class of apoptotic block a cancer cell has evolved to evade apoptosis. BH3 profiling exploits the binding pattern of sensitizer BH3 proteins to the anti-apoptotic proteins (Letai et al., 2002; Certo et al., 2006) (Figure 2). An example of how this works is if the cancer cell has selected for upregulation of MCL-1 to avoid apoptosis, the sensitizer BH3 peptide NOXA would cause the release of cytochrome c from the isolated mitochondria, whereas the BAD peptide would be ineffective at diagnosing an MCL-1 dependent Class C block. BH3 profiling has been used to categorize diffuse large cell lymphoma cell lines into each of the three classes of apoptotic blocks, and it has shown that membership in the class C block correlates with increased drug sensitivity (Del Gaizo Moore et al., 2007; Deng et al., 2007).
How does increased expression of an anti-apoptotic correlate with enhanced drug sensitivity? From the results generated by overexpression studies in cell lines, it is easy to envision that increased expression of BCL-2 would provide additional opposition against chemotherapy (Miyashita and Reed, 1993; Martinou et al., 1994; Zhang and Insel, 2001; Takahashi et al., 2003). However, as mentioned earlier, during oncogenesis the abnormal phenotype of oncogene activation and genomic instability acquired for deregulated growth initiates apoptotic signals in the form of activated BH3-only proteins. A compensatory increased level of anti-apoptotic protein is selected to buffer the death signals generated. Therefore, the anti-apoptotic proteins are filled with death initiator signals or are ‘primed’ for death and the cancer cell is often dependent or addicted to the anti-apoptotic protein (Letai et al., 2004; Certo et al., 2006; Del Gaizo Moore et al., 2008). The relatively ‘full’ state of these selected anti-apoptotic proteins does not afford for the binding of many subsequent pro-apoptotic proteins. This is in contrast to the case of forced overexpression in cell lines, in which most of the overexpressed anti-apoptotic proteins are ‘empty’ and therefore competent to sequester subsequent pro-death signaling molecules.
Design of BCL-2 targeting drugs
Once it was understood that cancer cells depended on BCL-2 or related anti-apoptotic proteins for survival, it became apparent that this could be of great practical significance as a mechanism to specifically target cancer cells (Letai et al., 2004). One of the first strategies used to target the BCL-2 family was the use of antisense oligonucleotides to knockdown the mRNA expression of BCL-2 (Cotter et al., 1994). Oblimersen (Genasense) from Genta (Connell Drive, Berkley Heights, NJ, USA) is an 18-mer phosphorothioated antisense oligonucleotide directed against the open reading frame of BCL-2 (Klasa et al., 2002). The clinical effect of the drug has been modest in clinical trials. (O’Brien et al., 2005), and it is not clear how well the drug lowers BCL-2 protein levels in cells in vivo.
The next strategy that emerged was to antagonize the function of BCL-2 rather than to reduce its levels. This was approached mechanistically following the delineation of the crystal structure of BCL-XL, which revealed that the BH1- BH3 domains formed a hydrophobic groove (Muchmore et al., 1996), where the α-helix of the BH3-only proteins bind (Sattler et al., 1997). The structural analysis of BCL-XL bound to the BAK BH3 peptide (Sattler et al., 1997) was a proof-of-concept experiment indicating that it could be possible to create small molecules that bound to the hydrophobic groove of BCL-XL, inhibiting its anti-apoptotic function. Since this preliminary experiment, numerous small molecules with varying degrees of specificity and effectiveness have been synthesized and the mechanism by which they induce apoptosis is depicted in (Figure 4).
A number of natural compounds that function as cell permeable, small molecule mimics of the BH3 domain were found using a library-screening process including Tetrocarcin A, Antimycin and gossypol (Nakashima et al., 2000; Tzung et al., 2001; Kitada et al., 2003). Gossypol is found in cottonseeds. It is a natural polyphenol capable of displacing BH3 peptides from BCL-XL with a dissociation constant in the low micromolar range (Kitada et al., 2003; Becattini et al., 2004). An altered version of gossypol without the two reactive aldehyde groups has been synthesized. However, the new compound termed apogossypol has actually reduced affinity for BCL-XL (Becattini et al., 2004). TW-37 is another gossypol derivative that was designed using the structure of the BIM BH3 domain as a model; it binds with high affinity to BCL-2, BCL-XL and MCL-1 (Verhaegen et al., 2006). Two molecules were discovered from a chemical library screen, tested for the ability to disrupt the BCL-XL/BAK BH3 complex and they were named BH3I-1 and BH3I-2 (Degterev et al., 2001). An alternative approach to drug discovery is the use of computer aided design of a ligand based on the structure of a receptor. This method enabled the discovery of the organic compound HA14-1 (Wang et al., 2000). In general, most of these molecules show a biochemical affinity for certain anti-apoptotic BCL-2 proteins, linked to their ability to kill certain cells, and in some cases even improve the survival of xenografts (Degterev et al., 2001; Kitada et al., 2003; Mohammad et al., 2007). However, the mechanism of cell killing is often unclear, and it is not clear whether the primary event is antagonism of anti-apoptotic BCL-2 proteins (van Delft et al., 2006).
Of the compounds discovered to date, most evidence supports killing through direct interaction with the BCL-2 family for the BH3 mimetic ABT-737 (Abbott Laboratories, Abott Park, IL, USA). ABT-737 was ineffective at activating apoptosis in cells doubly deficient of BAX and BAK, showing that the mechanism of action is solely through the BCL-2 family (van Delft et al., 2006). ABT-737 was discovered by Abott using a strategy of combining screening using nuclear magnetic resonance, structure-based design and combinatory chemical synthesis. The strategy was named ‘SAR by NMR’ (Oltersdorf et al., 2005). The lead compound ABT-737 mimicked the BH3 domain of BAD and bound selectively to BCL-2, BCL-XL and BCL-W. It is important to note that the binding efficiency for ABT-737 to the subset of the anti-apoptotic members was is in the nanomolar range, hence making in vivo therapeutically significant concentrations attainable. The crystal structure of ABT-737 bound to BCL-XL shows the chloro-biphenyl and thio-phenyl moieties bind to the p2 and p4 pockets of the hydrophobic groove of BCL-XL (Lee et al., 2007). ABT-737, similar to the BAD peptide, binds with poor affinity to MCL-1 and BFL-1 with a dissociation constant in the micro-molar range (Figure 2) (Oltersdorf et al., 2005).
Obatoclax is a synthetic derivative of prodiginines (GX015-070) from Gemin X Biotech (Avenue du Parc, Montreal, Quebec, Canada) and is a pan-Bcl-2 inhibitor (Reed, 2003; Nguyen et al., 2007; Trudel et al., 2007a). However, the affinity for binding to the BCL-2 family is inferior to ABT-737, again with dissociation constants in hundreds of nanomolar range. TW-37 has also been shown to bind to BCL-2, BCL-XL and MCL-1 so it could potentially be a pan-BCL-2 mimetic (Verhaegen et al., 2006). Similarly, to many other putative BH3 mimetics, obatoclax is not entirely dependent on BAX and BAK for its method of apoptosis induction (Konopleva et al., 2008). A compound that binds with high affinity to all anti-apoptotic proteins is almost certainly going to be more toxic to cancer cells. Furthermore, such a drug will not stimulate selection for expression of alternative anti-apoptotic BCL-2 family proteins, as none would escape inhibition. However, the concern is that there would be greater toxicity to normal cells, so it is not clear whether an improved therapeutic index would be obtained by a pan-BCL-2 family inhibitor. Our observation that cancer cells are far more likely to be ‘primed’ than normal cells lends some hope to such an approach, but true confirmation would require extensive in vivo testing.
Clinical use of BH3 mimetics
Of the small molecule inhibitors of BCL-2 described thus far, four are at present in clinical trials—Genasense, TW-37, obatoclax and ABT-263. In in vitro experiments, ABT-737 has monotherapy toxicity to leukemia and lymphoma, and at higher concentrations it is also able to induce apoptosis in multiple myeloma, glioma and small cell lung cancer cell lines (Oltersdorf et al., 2005; Chauhan et al., 2007; Trudel et al., 2007b; Tagscherer et al., 2008). Numerous studies including work carried out by our laboratory have shown that primary cells from patients with chronic lymphocytic leukemia (Del Gaizo Moore et al., 2007), acute myeloid leukemia (Konopleva et al., 2006), acute lymphocytic leukemia (Del Gaizo Moore et al., 2008), along with B-cell neoplasms such as follicular lymphoma and marginal zone lymphoma (Vogler et al., 2008), are extremely sensitive to ABT-737, with widespread apoptosis when treated with concentrations in the nanomolar range. Similarly, obatoclax, the pan-BCL-2 inhibitor, shows single agent efficiency in the killing of multiple myeloma (Trudel et al., 2007a), AML cell lines and primary samples. However, it also causes cell cycle arrest independent of apoptosis (Trudel et al., 2007a; Konopleva et al., 2008). Although TW-37 has been shown to bind BCL-2, BCL-XL and MCL-1, the mechanism by which it induces apoptosis has not yet been elucidated. It causes a reduction in proliferation in pancreatic and B-cell lymphomas, as well as reducing the tumor size in xenograft models; however, whether this is BCL-2 dependent needs to be clarified (Mohammad et al., 2007; Wang et al., 2008).
To enhance its clinical potential, ABT-737 has been modified to make the drug orally available. ABT-263 binds to serum proteins resulting in a longer oral half life. The tumoricidal action of ABT-263 was shown in xenograft models of small cell lung cancer and acute lymphoblastic leukemia cancer cells, where ABT-263 caused complete regression of the tumors (Shoemaker et al., 2008; Tse et al., 2008). The drug is now in clinical trials for chronic lymphocytic leukemia, lymphoma and small cell lung cancer (Wilson et al., 2008). It is noted that ABT-263 binds to BCL-2 with affinity <100 picomolar, making it several logs more potent than the other small molecules detailed above.
Intrinsic and acquired resistance therapy
The BH3 mimetics are often not very effective as single killing agents against some of the epithelial cancers, such as the pancreatic, ovarian and breast cancers (Witham et al., 2007; Huang and Sinicrope, 2008; Kutuk and Letai, 2008). However, the other niche for these drugs is in combination therapy, where the BCL-2 antagonist serves to inhibit BCL-2-mediated resistance, enabling killing by conventional chemotherapy. Numerous examples exist in the literature of an enhanced apoptotic response when the BH3 mimetics are combined with traditional therapies to treat various cancers such as melanoma (Verhaegen et al., 2006), pancreatic (Huang and Sinicrope, 2008), glioma (Tagscherer et al., 2008), breast (Kutuk and Letai, 2008), multiple myeloma (Trudel et al., 2007b) and B-cell malignant models (Mohammad et al., 2007; Tse et al., 2008). Thus, it seems that the conventional therapies provide the additional priming event required to enable the Bcl-2 antagonists to kill the above examples of intrinsically resistant cancers. Resistance to conventional chemotherapy may also develop over time (acquired resistance). On initial treatment, there is a dramatic response, that tapers over time owing to the acquisition of resistance through various mechanisms, including the alteration of expression of Bcl-2 family members. A recent report, showed the use of the BH3 mimetic ABT-737 in the treatment of acquired resistance in cancer cells. It was effective at resensitizing drug-resistant breast cancer cell lines to apoptosis through paclitaxel. This study highlighted an important factor during the acquisition of resistance against the chemotherapeutic drug, although in vitro, the breast cancer cell lines selected for increasing the expression of BCL-2 family members to gain resistance. ABT-737 in combination with other chemotherapeutics was efficient in overcoming this altered BCL-2 balance to reinstate killing (Kutuk and Letai, 2008), emphasizing the potential effectiveness of the use of BH3 mimetics in combination therapy.
Why are some cells resistant and other cells sensitive to ABT-737? Two conditions must be met for ABT-737 to kill cells. First, BCL-2 (or BCL-XL and BCL-W) needs to be primed with death-initiating signals. These signals may be BH3 activators such as BIM, or perhaps activated monomeric BAX or BAK. These pro-death proteins are then released on ABT-737 binding to the hydrophobic groove of BCL-2, enabling the activation of apoptosis (Figure 4) (Del Gaizo Moore et al., 2007). Second, the displaced death-initiating signal (for example, BIM or BAX) needs to exceed the quantity of empty MCL-1 and BFL-1, which are not targeted by ABT-737. Excess empty MCL-1 or BFL-1 would otherwise sequester displaced pro-death proteins so that apoptosis would not be activated. High levels of MCL-1 or BFL-1 have been shown to correlate with resistance to ABT-737 (Konopleva et al., 2006; van Delft et al., 2006; Lin et al., 2007; Tagscherer et al., 2008). The resistance caused by MCL-1 can be overcome by inactivation through knockdown (Lin et al., 2007) or through addition of agents that decrease MCL-1, such as seliciclib (Chauhan et al., 2007; Chen et al., 2007; Nguyen et al., 2007). However, it is worth noting that cells that have either lost or mutated BAX or BAK, which fall into a class B block of apoptosis, are also resistant to BH3 mimetics. Similarly, cells with reduced levels of BH3-only activator proteins (Class A) will also be unable to respond to BH3 mimetics (Deng et al., 2007). Thus, resistance to the BH3 mimetics is not solely dependent on the expression levels of MCL-1/BFl-1. Utilization of the BH3 profiling tool facilitates identification of the apoptotic block used by cancer cells or of the block acquired on resistance to chemotherapy. Therefore, this tool would aid in the recognition of cancers that are likely to be susceptible to either pan- or selective BH3 mimetics, enabling a personalized approach to treatment (Del Gaizo Moore et al., 2007, 2008; Deng et al., 2007).
Huge inroads have been made in delineating the apoptotic pathway over the last two decades, in particular the mechanism by which the BCL-2 family functions through selective protein-protein interactions to control mitochondrial apoptosis. Simultaneously, an understanding of how cancer cells evade death at the molecular level has been achieved with the BCL-2 family playing a starring role in death avoidance. The stage is set for the next generation of therapeutics, the small-molecule inhibitors of anti-apoptotic BCL-2 proteins, to be tested and fully exploited clinically, either as single agents or in combination therapy.
Conflict of interest
T Ni Chonghaile has declared no conflict of interest. A Letai acted as a consultant/an advisor for Abbott Laboratories (Abbott Park, IL, USA) in April 2009, has equity ownership/stock options in Eutropics Pharmaceuticals Inc. (Boston, MA, USA) and has submitted patent applications for BH3 profiling.
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Chonghaile, T., Letai, A. Mimicking the BH3 domain to kill cancer cells. Oncogene 27, S149–S157 (2008). https://doi.org/10.1038/onc.2009.52
- BH3 mimetic
- drug resistance and ABT-737
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