Ending the prolonged life of cancer cells
Gerhard Wagner1
1. The author is in the Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, USA. Email: wagner@hms.harvard.edu
The fate of cells depends on a delicate balance of pro- and anti-apoptotic proteins. Small-molecule inhibitors of anti-apoptotic protein-protein interactions promote apoptosis in cancer cells.
Convincing tumor cells to commit suicide is an attractive new strategy in cancer therapy, an approach that is made possible by the increasingly detailed understanding of the mechanisms of apoptosis. Cells contain proteins designed to prevent apoptosis, and small molecules that inhibit these anti-apoptotic proteins could be promising anti-cancer agents1, 2. A report by Oltersorf et al.3, published recently in Nature, now describes the successful development of compounds that bind the anti-apoptotic proteins of the Bcl-2 family with sub-nanomolar affinities and have potent anti-tumor activities in cancer cell lines and animal models.
Programmed cell death was first described 40 years ago4 and later termed apoptosis. Mitochondria and associated proteins have key roles in determining the onset of apoptosis5. Death signals originating from death receptors or resulting from cellular conditions affect the homeostasis between pro- and anti-apoptotic proteins and cause mitochondria to initiate downstream apoptotic events that result in the activation of executioner caspases (caspase-3, caspase-6 and caspase-7), which cleave multiple substrates and coordinate cell destruction. In the mitochondrial pathway, the central trigger of apoptosis is the release of cytochrome c (Fig. 1), which stimulates the assembly of the heptameric apoptosome6. This process activates pro-caspase-9, which, in turn, activates downstream executioner caspases.
Figure 1: Mechanisms of mitochondrial cytochrome c release in apoptosis.
Figure 1 : Mechanisms of mitochondrial cytochrome c release in apoptosis. Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com
Before apoptosis, cytochrome c (green circles) resides in the space between the inner and outer mitochondrial membranes and Bax and Bak are found in the outer membrane, presumably as monomers. Death signals that activate caspase-8 lead to cleavage of the Bid protein, and its C-terminal cleavage product, tBid, induces Bax and/or Bak oligomerization and formation of the cytochrome c release channel. Bcl-2 and Bcl-xL maintain cellular homeostasis by sequestering free Bax, Bak and Bid. Binding of the apoptosis-sensing proteins Bad or Bik to Bcl-2 and/or Bcl-xL inhibits the pro-survival function of Bcl-2 and Bcl-xL, triggering the release of monomeric Bax and Bak and formation of the cytochrome c channels. Compound ABT-737 mimics this function.
Full figure and legend (16K) Figures, schemes & tables index
What are the mechanisms of mitochondrial cytochrome c release? According to current understanding, a channel in the mitochondrial outer membrane is formed by oligomerization of Bax, Bak or both7 (Fig. 1). Signals from death receptors activate caspase-8, which cleaves the pro-apoptotic protein Bid8. This cleavage step initiates oligomerization of Bax and Bak and formation of the cytochrome c release channel in the outer mitochondrial membrane. The process is regulated by the anti-apoptotic proteins Bcl-2, Bcl-xL and Bcl-w, which bind to Bid, Bim, Bax and Bak and prevent formation of the release channel. The pro-apoptotic proteins Bad and Bik block the action of Bcl-2−type proteins but cannot directly activate Bax or Bad oligomerization. Thus, the decision to start apoptosis depends on a delicate balance of pro- and anti-apoptotic protein concentrations.
How could this fine balance of apoptosis regulation be exploited for cancer therapy? One approach, which involves drugs that mimic tBid and cause apoptosis directly, has been explored by stabilizing the BH3 helix of Bid with a hydrocarbon 'staple'. Although this approach killed leukemia cells and inhibited human leukemia xenographs9, no small-chemical agents that replicate the channel-inducing activity of Bid have been reported. Other researchers have targeted the anti-apoptotic activity of Bcl-2 and Bcl-xL1, 2. Using different strategies, such as high-throughput screening of compound libraries or virtual screening of compound data bases, small molecules have been found that inhibit Bcl-xL, Bcl-2 or both, but only with micromolar affinities.
The recent work of Oltersorf et al.3 has applied the repertoire of drug design tools such as SAR by NMR (structure-activity relationships by nuclear magnetic resonance)10 and fragment fusion to obtain small-molecule inhibitors of Bcl-2 proteins that have sub-nanomolar affinities. In addition, the authors have used structure-based methods to overcome complications of drug absorption by human serum albumin (HSA). They found that the original compound bound tightly to HSA, which seriously reduced its activity. By solving the structures of the drug bound to both Bcl-xL and HSA, the authors showed that it was partially solvent exposed in the Bcl-xL complex, but tightly surrounded by lipophilic residues in the HSA complex. Using this information, they synthesized a structural variant in which the polarity of these solvent-exposed areas was increased. The resulting compound, ABT-737, still binds tightly to Bcl-xL but has greatly reduced HSA affinity. ABT-737 was shown to be effective in killing a variety of cancer cell lines and reducing solid tumors in animal models, while no increase in caspase-3 activation was observed in normal tissues.
The work of Oltersorf et al.3 supports the view that most of the binding energies of protein complexes may come from a few hydrophobic hot spots11 rather than uniformly from a large binding area. If small molecules can be found that bind these hot spots, they may well interfere with the formation of large binding interfaces. Several small ligands could even be linked to generate tightly binding and potent inhibitors for relatively flat protein surfaces. The work presented by Oltersdorf et al.3 describes a recent paradigm shift in drug discovery: potent inhibitors of protein-protein interactions can be found and developed into drug-like compounds.
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References
1. Wang, J.L. et al. Proc. Natl. Acad. Sci. USA 97, 7124–7129 (2000). | Article | PubMed | ChemPort |
2. Degterev, A. et al. Nat. Cell Biol. 3, 173–182 (2001). | Article | PubMed | ISI | ChemPort |
3. Oltersdorf, T. Nature, published online 15 May 2005 (10.1038/nature03579). | Article | PubMed |
4. Lockshin, R.A. & Williams, C.M. J. Insect. Physiol. 11, 123–133 (1965). | Article | PubMed | ChemPort |
5. Kroemer, G., Zamzami, N. & Susin, S.A. Immunol. Today 18, 44–51 (1997). | Article | PubMed | ISI | ChemPort |
6. Li, P. et al. Cell 91, 479–489 (1997). | Article | PubMed | ISI | ChemPort |
7. Wei, M.C. et al. Science 292, 727–730 (2001). | Article | PubMed | ISI | ChemPort |
8. Li, H., Zhu, H., Xu, C.J. & Yuan, J. Cell 94, 491–501 (1998). | Article | PubMed | ISI | ChemPort |
9. Walensky, L.D. et al. Science 305, 1466–1470 (2004). | Article | PubMed | ISI | ChemPort |
10. Shuker, S.B., Hajduk, P.J., Meadows, R.P. & Fesik, S.W. Science 274, 1531–1534 (1996). | Article | PubMed | ISI | ChemPort |
11. Clackson, T. & Wells, J.A. Science 267, 383–386 (1995). | PubMed | ISI | ChemPort |
[da Nature Chemical Biology http://www.nature.com/nchembio/journal/v1/n1/full/nchembio0605-8.html ]
Gerhard Wagner1
1. The author is in the Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, USA. Email: wagner@hms.harvard.edu
The fate of cells depends on a delicate balance of pro- and anti-apoptotic proteins. Small-molecule inhibitors of anti-apoptotic protein-protein interactions promote apoptosis in cancer cells.
Convincing tumor cells to commit suicide is an attractive new strategy in cancer therapy, an approach that is made possible by the increasingly detailed understanding of the mechanisms of apoptosis. Cells contain proteins designed to prevent apoptosis, and small molecules that inhibit these anti-apoptotic proteins could be promising anti-cancer agents1, 2. A report by Oltersorf et al.3, published recently in Nature, now describes the successful development of compounds that bind the anti-apoptotic proteins of the Bcl-2 family with sub-nanomolar affinities and have potent anti-tumor activities in cancer cell lines and animal models.
Programmed cell death was first described 40 years ago4 and later termed apoptosis. Mitochondria and associated proteins have key roles in determining the onset of apoptosis5. Death signals originating from death receptors or resulting from cellular conditions affect the homeostasis between pro- and anti-apoptotic proteins and cause mitochondria to initiate downstream apoptotic events that result in the activation of executioner caspases (caspase-3, caspase-6 and caspase-7), which cleave multiple substrates and coordinate cell destruction. In the mitochondrial pathway, the central trigger of apoptosis is the release of cytochrome c (Fig. 1), which stimulates the assembly of the heptameric apoptosome6. This process activates pro-caspase-9, which, in turn, activates downstream executioner caspases.
Figure 1: Mechanisms of mitochondrial cytochrome c release in apoptosis.
Figure 1 : Mechanisms of mitochondrial cytochrome c release in apoptosis. Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com
Before apoptosis, cytochrome c (green circles) resides in the space between the inner and outer mitochondrial membranes and Bax and Bak are found in the outer membrane, presumably as monomers. Death signals that activate caspase-8 lead to cleavage of the Bid protein, and its C-terminal cleavage product, tBid, induces Bax and/or Bak oligomerization and formation of the cytochrome c release channel. Bcl-2 and Bcl-xL maintain cellular homeostasis by sequestering free Bax, Bak and Bid. Binding of the apoptosis-sensing proteins Bad or Bik to Bcl-2 and/or Bcl-xL inhibits the pro-survival function of Bcl-2 and Bcl-xL, triggering the release of monomeric Bax and Bak and formation of the cytochrome c channels. Compound ABT-737 mimics this function.
Full figure and legend (16K) Figures, schemes & tables index
What are the mechanisms of mitochondrial cytochrome c release? According to current understanding, a channel in the mitochondrial outer membrane is formed by oligomerization of Bax, Bak or both7 (Fig. 1). Signals from death receptors activate caspase-8, which cleaves the pro-apoptotic protein Bid8. This cleavage step initiates oligomerization of Bax and Bak and formation of the cytochrome c release channel in the outer mitochondrial membrane. The process is regulated by the anti-apoptotic proteins Bcl-2, Bcl-xL and Bcl-w, which bind to Bid, Bim, Bax and Bak and prevent formation of the release channel. The pro-apoptotic proteins Bad and Bik block the action of Bcl-2−type proteins but cannot directly activate Bax or Bad oligomerization. Thus, the decision to start apoptosis depends on a delicate balance of pro- and anti-apoptotic protein concentrations.
How could this fine balance of apoptosis regulation be exploited for cancer therapy? One approach, which involves drugs that mimic tBid and cause apoptosis directly, has been explored by stabilizing the BH3 helix of Bid with a hydrocarbon 'staple'. Although this approach killed leukemia cells and inhibited human leukemia xenographs9, no small-chemical agents that replicate the channel-inducing activity of Bid have been reported. Other researchers have targeted the anti-apoptotic activity of Bcl-2 and Bcl-xL1, 2. Using different strategies, such as high-throughput screening of compound libraries or virtual screening of compound data bases, small molecules have been found that inhibit Bcl-xL, Bcl-2 or both, but only with micromolar affinities.
The recent work of Oltersorf et al.3 has applied the repertoire of drug design tools such as SAR by NMR (structure-activity relationships by nuclear magnetic resonance)10 and fragment fusion to obtain small-molecule inhibitors of Bcl-2 proteins that have sub-nanomolar affinities. In addition, the authors have used structure-based methods to overcome complications of drug absorption by human serum albumin (HSA). They found that the original compound bound tightly to HSA, which seriously reduced its activity. By solving the structures of the drug bound to both Bcl-xL and HSA, the authors showed that it was partially solvent exposed in the Bcl-xL complex, but tightly surrounded by lipophilic residues in the HSA complex. Using this information, they synthesized a structural variant in which the polarity of these solvent-exposed areas was increased. The resulting compound, ABT-737, still binds tightly to Bcl-xL but has greatly reduced HSA affinity. ABT-737 was shown to be effective in killing a variety of cancer cell lines and reducing solid tumors in animal models, while no increase in caspase-3 activation was observed in normal tissues.
The work of Oltersorf et al.3 supports the view that most of the binding energies of protein complexes may come from a few hydrophobic hot spots11 rather than uniformly from a large binding area. If small molecules can be found that bind these hot spots, they may well interfere with the formation of large binding interfaces. Several small ligands could even be linked to generate tightly binding and potent inhibitors for relatively flat protein surfaces. The work presented by Oltersdorf et al.3 describes a recent paradigm shift in drug discovery: potent inhibitors of protein-protein interactions can be found and developed into drug-like compounds.
Top of page
References
1. Wang, J.L. et al. Proc. Natl. Acad. Sci. USA 97, 7124–7129 (2000). | Article | PubMed | ChemPort |
2. Degterev, A. et al. Nat. Cell Biol. 3, 173–182 (2001). | Article | PubMed | ISI | ChemPort |
3. Oltersdorf, T. Nature, published online 15 May 2005 (10.1038/nature03579). | Article | PubMed |
4. Lockshin, R.A. & Williams, C.M. J. Insect. Physiol. 11, 123–133 (1965). | Article | PubMed | ChemPort |
5. Kroemer, G., Zamzami, N. & Susin, S.A. Immunol. Today 18, 44–51 (1997). | Article | PubMed | ISI | ChemPort |
6. Li, P. et al. Cell 91, 479–489 (1997). | Article | PubMed | ISI | ChemPort |
7. Wei, M.C. et al. Science 292, 727–730 (2001). | Article | PubMed | ISI | ChemPort |
8. Li, H., Zhu, H., Xu, C.J. & Yuan, J. Cell 94, 491–501 (1998). | Article | PubMed | ISI | ChemPort |
9. Walensky, L.D. et al. Science 305, 1466–1470 (2004). | Article | PubMed | ISI | ChemPort |
10. Shuker, S.B., Hajduk, P.J., Meadows, R.P. & Fesik, S.W. Science 274, 1531–1534 (1996). | Article | PubMed | ISI | ChemPort |
11. Clackson, T. & Wells, J.A. Science 267, 383–386 (1995). | PubMed | ISI | ChemPort |
[da Nature Chemical Biology http://www.nature.com/nchembio/journal/v1/n1/full/nchembio0605-8.html ]
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