Curing Cancer with p53
http://www.100md.com
《新英格兰医药杂志》
The activity of the p53 tumor-suppressor protein has a key role in controlling both cancer and aging: underactivity encourages the growth of cancer, and overactivity can accelerate the aging process. In most tumor cells, the p53 protein is inactivated by mutation, whereas in some others its function is blocked in other ways. Restoring the function of p53 to tumor cells is the dream of many investigators,1 and Snyder and colleagues2 show how well this can work by demonstrating that a p53-activating molecule can save the lives of mice carrying very aggressive disseminated cancer. What are the prospects for developing this treatment for clinical use?
Nine years ago biochemical studies of p53 showed that its in vitro activity as a DNA-binding protein could be boosted by the addition of small peptides derived from the C-terminal of p53.3 Although the exact mechanism of this activation remains the subject of debate, the ability of the peptides to activate some mutant p53 proteins of the type found in human tumors encouraged investigators to devise ways to test the peptides in vivo. To do so, they have had to overcome three obstacles: peptides are readily degraded by ubiquitous proteases, cells do not normally take up peptides efficiently, and the peptide activators are not very potent.
One solution to these problems has been to use the peptides simply as proof of concept and to seek as substitutes conventional drug-like molecules that would mimic their action. Despite occasional encouraging signs, this approach has yet to yield a candidate molecule suitable for clinical development.
A second approach has been to tackle the challenges of the peptide activators head on. In this respect, the study by Snyder et al. represents a big step forward. They tackled the problem of degradation through the use of peptides composed of d–amino acids. These are the optical isomers of l–amino acids of which all our native proteins are made. Fortunately, these d–amino acid peptides are resistant to proteases, yet (in inverse sequence) they were still able to activate p53. Adding other amino acids to the peptide in the form of a protein-transduction domain solved the problem of transport. This domain binds to the cell surface and is then internalized by a recently clarified mechanism with the use of lipid-raft–dependent pinocytosis.4
The optimized peptide was able to activate both wild-type and mutant p53 in tumor cells in culture and to promote both apoptosis and sustained arrest of growth — two well-described features of the p53 response. It also had a far greater ability to limit the growth of tumor cells than normal cells, perhaps because the tumor-cell environment is intrinsically programmed to promote a response to p53 activity. In contrast, normal, undamaged cells are not primed to respond to p53 activity and contain only minute amounts of inactive p53.
Snyder et al. then tested the 34-amino-acid peptide, called RI-TATp53C', in animal models of cancer. It is with this crucial set of tests that the authors obtained such encouraging results. When injected into the peritoneal cavity, the peptide was able to slow the growth of subcutaneous tumors in immunocompetent mice. But the really dramatic results came from models of disseminated disease. The peptide prolonged survival from an average of 11 days to 70 days in a mouse model of peritoneal carcinomatosis. And it effected a complete cure in a mouse model of terminal peritoneal lymphoma with severe combined immunodeficiency, with 50 percent of treated mice alive after 150 days. In contrast, untreated mice and mice given a placebo all had to be killed within 35 days owing to progressive disease.
Although it will be important for others to confirm and extend these results in preclinical models, a crucial question is whether RI-TATp53C' should be tested in the clinic. This process would be expensive, requiring the synthesis of hundreds of grams of the peptide so that safety and activity could be evaluated in humans. Any improvements in the potency of the peptide could reduce these demands, and the classic dilemma in drug development is when to stop improving a molecule and instead focus on clinical development.
Snyder et al. delivered the peptide directly into the peritoneal cavity; a critical issue is the extent to which this approach is acceptable for the treatment of terminal peritoneal cancer in humans. Perhaps the biggest effect of this study will be in its establishment of strategy. If the work is replicated, reactivation of mutant p53 will be accepted as an approach that can cure disseminated cancers. Such progress will encourage the further investment needed to support clinical trials. Twenty-five years after its discovery, the p53 protein may be on the road toward helping patients.
Dr. Lane reports holding equity in Cyclacel.
Source Information
From the Department of Surgery and Molecular Oncology, University of Dundee, Dundee, United Kingdom.
References
Vogelstein B, Lane D, Levine AJ. Surfing the p53 network. Nature 2000;408:307-310.
Snyder EL, Meade BM, Saenz CC, Dowdy SF. Treatment of terminal peritoneal carcinomatosis by a transducible p53-activating peptide. PLoS Biol 2004;2:186-93. (Also available at http://biology.plosjournals.org/plosonline/?request=get-document&doi=10.1371/journal.pbio.0020036.)
Hupp TR, Sparks A, Lane DP. Small peptides activate the latent sequence-specific DNA binding function of p53. Cell 1995;83:237-245.
Wadia JS, Stan RV, Dowdy SF. Transducible TAT-HA fusogenic peptide enhances escape of TAT-fusion proteins after lipid raft macropinocytosis. Nat Med 2004;10:310-315.(David Lane, Ph.D.)
Nine years ago biochemical studies of p53 showed that its in vitro activity as a DNA-binding protein could be boosted by the addition of small peptides derived from the C-terminal of p53.3 Although the exact mechanism of this activation remains the subject of debate, the ability of the peptides to activate some mutant p53 proteins of the type found in human tumors encouraged investigators to devise ways to test the peptides in vivo. To do so, they have had to overcome three obstacles: peptides are readily degraded by ubiquitous proteases, cells do not normally take up peptides efficiently, and the peptide activators are not very potent.
One solution to these problems has been to use the peptides simply as proof of concept and to seek as substitutes conventional drug-like molecules that would mimic their action. Despite occasional encouraging signs, this approach has yet to yield a candidate molecule suitable for clinical development.
A second approach has been to tackle the challenges of the peptide activators head on. In this respect, the study by Snyder et al. represents a big step forward. They tackled the problem of degradation through the use of peptides composed of d–amino acids. These are the optical isomers of l–amino acids of which all our native proteins are made. Fortunately, these d–amino acid peptides are resistant to proteases, yet (in inverse sequence) they were still able to activate p53. Adding other amino acids to the peptide in the form of a protein-transduction domain solved the problem of transport. This domain binds to the cell surface and is then internalized by a recently clarified mechanism with the use of lipid-raft–dependent pinocytosis.4
The optimized peptide was able to activate both wild-type and mutant p53 in tumor cells in culture and to promote both apoptosis and sustained arrest of growth — two well-described features of the p53 response. It also had a far greater ability to limit the growth of tumor cells than normal cells, perhaps because the tumor-cell environment is intrinsically programmed to promote a response to p53 activity. In contrast, normal, undamaged cells are not primed to respond to p53 activity and contain only minute amounts of inactive p53.
Snyder et al. then tested the 34-amino-acid peptide, called RI-TATp53C', in animal models of cancer. It is with this crucial set of tests that the authors obtained such encouraging results. When injected into the peritoneal cavity, the peptide was able to slow the growth of subcutaneous tumors in immunocompetent mice. But the really dramatic results came from models of disseminated disease. The peptide prolonged survival from an average of 11 days to 70 days in a mouse model of peritoneal carcinomatosis. And it effected a complete cure in a mouse model of terminal peritoneal lymphoma with severe combined immunodeficiency, with 50 percent of treated mice alive after 150 days. In contrast, untreated mice and mice given a placebo all had to be killed within 35 days owing to progressive disease.
Although it will be important for others to confirm and extend these results in preclinical models, a crucial question is whether RI-TATp53C' should be tested in the clinic. This process would be expensive, requiring the synthesis of hundreds of grams of the peptide so that safety and activity could be evaluated in humans. Any improvements in the potency of the peptide could reduce these demands, and the classic dilemma in drug development is when to stop improving a molecule and instead focus on clinical development.
Snyder et al. delivered the peptide directly into the peritoneal cavity; a critical issue is the extent to which this approach is acceptable for the treatment of terminal peritoneal cancer in humans. Perhaps the biggest effect of this study will be in its establishment of strategy. If the work is replicated, reactivation of mutant p53 will be accepted as an approach that can cure disseminated cancers. Such progress will encourage the further investment needed to support clinical trials. Twenty-five years after its discovery, the p53 protein may be on the road toward helping patients.
Dr. Lane reports holding equity in Cyclacel.
Source Information
From the Department of Surgery and Molecular Oncology, University of Dundee, Dundee, United Kingdom.
References
Vogelstein B, Lane D, Levine AJ. Surfing the p53 network. Nature 2000;408:307-310.
Snyder EL, Meade BM, Saenz CC, Dowdy SF. Treatment of terminal peritoneal carcinomatosis by a transducible p53-activating peptide. PLoS Biol 2004;2:186-93. (Also available at http://biology.plosjournals.org/plosonline/?request=get-document&doi=10.1371/journal.pbio.0020036.)
Hupp TR, Sparks A, Lane DP. Small peptides activate the latent sequence-specific DNA binding function of p53. Cell 1995;83:237-245.
Wadia JS, Stan RV, Dowdy SF. Transducible TAT-HA fusogenic peptide enhances escape of TAT-fusion proteins after lipid raft macropinocytosis. Nat Med 2004;10:310-315.(David Lane, Ph.D.)