| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
© Oxford University Press 2006.
NEWS |
Histone Deacetylase Inhibitors Sit at Crossroads of Diet, Aging, Cancer
Tumor cells can harbor abnormalities not only in DNA but also in the chemical structure of protein complexes called histones, which keep DNA precisely coiled and tightly condensed. Until recently, these so-called epigenetic changes have not gotten much attention from drug developers. But now, with early clinical trials beginning to show promise, a whole new class of anticancer drugs called histone deacetylase (HDAC) inhibitors is poised to become the first to specifically target epigenetic abnormalities.
The most advanced of the HDAC inhibitors is a compound called SAHA (suberoylanilide hydroxamic acid, now dubbed vorinostat), which has completed phase II testing. Following on the heels of SAHA's early promise, other pharmaceutical companies are now pursuing HDAC inhibitors as treatments. But investigators are also beginning to realize that there is much more to be uncovered. New research is identifying a web of molecular interactions that link HDAC activity to diet, premalignant cell changes, aging, and development of a variety of diseases, including cancer.
"I think they are a great class of drugs," said Kevin Kelly, D.O., Yale University, New Haven, Conn., who has led phase I trials of SAHA. "We are starting to be able to manipulate tumors and make them more sensitive to other drugs. This is just the tip of the iceberg."
SAHA was first described by Paul Marks, M.D., president emeritus of Memorial Sloan Kettering Cancer Center in New York, as a compound that drives cancer cells to growth arrest. It was later found to inhibit HDACs, which are master regulators of gene expression. Inside cells, HDACs modify gene transcription by changing the chemical structure of histones, inducing changes to their shape that alter the gene transcription machinery's access to genes.
In phase I studies, responses to SAHA alone were modest, with one complete and one partial response reported in 39 patients. No major adverse events were reported in patients receiving oral SAHA, which seems to be better tolerated and has better bioavailability than the intravenous form. Since these studies were completed, other trials have begun using SAHA in combination with other classes of chemotherapy drugs.
"As a single agent, SAHA probably has minimal activity," said Kelly. He added that in some cancers, such as cutaneous T-cell lymphoma, the drug may be hitting a crucial gene in the progression pathway.
SAHA's maker, Merck & Co, Whitehouse Station, N.J., conducted a phase II trial that included patients with advanced, refractory cutaneous T-cell lymphoma, an aggressive form of non-Hodgkin lymphoma. In that study, presented at the May 2005 annual meeting of the American Society of Clinical Oncology, study leaders reported partial responses in eight of 33 patients receiving SAHA. Merck also is pursuing clinical studies with SAHA in diffuse large B-cell lymphoma, multiple myeloma, and malignant pleural mesothelioma, and solid tumors, such as breast and colorectal cancer. Merck has reported that it plans to file a new-drug application for SAHA in early 2006.
The next step in the evolution of HDAC inhibitors will be to see if designing more specific inhibitors targeted to one particular HDAC will increase their specificity and efficacy, Kelly said.
"That could be good or bad," he said. "Our experience with tyrosine kinase inhibitors has been that if they are too specific, that can be bad. Our big challenge down the road is to learn how to use these drugs and how to use them in combination."
Exploring Mechanisms
While SAHA advances in clinical trials, investigators continue to explore mechanisms by which HDAC and HDAC inhibitors work. One question still to be answered is how and why the drugs target cancer cells while for the most part sparing normal cells.
"It is increasingly evident that the effect on histone and chromatin structure and in turn the alteration in transcription of specific genes is probably part of the anticancer effects of these agents, but certainly not the whole story," said Marks. "For example we discovered that relatively few genes are altered in their transcription by SAHA."
SAHA also appears to interfere with other proteins that contribute to cancer cells' demise. For example, Marks and his colleagues showed HDAC inhibitors directly bind thioredoxin-binding protein (TBP2). This binding prevents TBP2 from detoxifying oxygen free radicals, which build up in the cell, facilitating cell death.
"For reasons that we do not yet completely understand, this [binding] doesn't happen in normal cells, only in transformed cells," Marks said. "Right now it appears that the mechanisms for reversing the effects of HDAC inhibitors are much more intact in normal cells. You get accumulated acetylated histones in normal and cancer cells, but [the effect] doesn't last in normal cells."
While virtually all the current clinical studies with HDAC inhibitors focus on potent inhibitors of class I and II HDACs, there is a third class that is not affected by SAHA and its derivatives. Class III HDACs are beginning to emerge as factors linking aging, cancer, and diet.
Stephen Baylin, M.D., and his colleagues at Kimmel Cancer Center, Johns Hopkins University, Baltimore, were studying a protein called hypermethylated in cancer 1 (HIC1), a transcriptional repressor that is epigenetically inactivated in human cancers. The researchers were looking for proteins that interact with HIC1 when they discovered that it forms a complex with SIRT1, a class III deacetylase known to be linked to longevity in organisms such as flies and yeast.
Further research, published in the Nov. 4, 2005, issue of Cell, demonstrated that when HIC1 is deactivated in tumor cells, expression of SIRT1 increases, which in turn deacetylates and inactivates p53. The result is that the tumor cells survive, bypassing apoptosis.
"[SIRT1] has a fascinating sort of potential as a master protein," said Baylin. "Our model would say that if you lose HIC1, and SIRT1 is too high constitutively, you are going to cripple potentially p53, and since that's an important tumor suppressor gene you may be sending cells down a defective p53 pathway."
|
Baylin said the results from his experiments point to a homeostatic loop in which SIRT1, p53, and other proteins as yet unidentified provide a delicate balance that allows cells damaged by environmental insults such as oxidative stress or DNA modifications to be put in cell cycle arrest while damage is repaired. Moderately increased levels of SIRT1, as seen in experiments with animals fed a calorie-restricted diet, seem to allow cells to better survive under stress.
"Our results suggest that while SIRT1 may be a longevity gene, you may pay a price for this because it looks like that, if anything, SIRT1 is oncogenic," he said.
New research from the Baylin lab, now in press, shows that if SIRT1 is blocked with small-interfering RNA, transcriptionally repressed genes that contribute to cell cycle arrest come back on. This finding suggests that SIRT1 is silencing genes in the cancer pathway, Baylin said.
"All of our evidence points to the fact that HIC1 gets silenced in the preinvasive stage of cancer," he said. "It could be causing this network of events, this partial disruption of p53, and these cells have now learned to survive because of an increase in SIRT1." If this supposition turns out to be correct, he said, "then these are target points for those early risk status changes that you would want to damp down. And if you could damp them down, the mathematics on how much cancer you might prevent would be pretty substantial."
Broccoli Sprout Redux
Meanwhile, other studies are beginning to focus on dietary HDAC inhibitors. "We are just starting to learn about different HDACs within a class, and what are the downstream effectors to changes to that specific HDAC," said Roderick Dashwood, Ph.D., chief of the cancer chemoprevention program at the Linus Pauling Institute, Oregon State University in Corvallis.
Dashwood and others are studying compounds found in the diet that can inhibit HDACs moderately at biologically relevant concentrations. He and colleagues Emily Ho, Ph.D., and Melinda Myzk, Ph.D., became interested in HDACs while studying the biological effects of sulforaphane, a plant product found at high levels in broccoli, and especially broccoli sprouts.
Sulforaphane has been well studied for its ability to boost phase II detoxifying agents. But Dashwood noticed that some of sulforaphane's effects could not be explained by detoxification alone. He and his colleagues have found evidence of inhibition of HDAC activity in colon and prostate cancer cells in animals administered sulforaphane. Further studies showed that the parent compound had no inhibitory effect, but its metabolite SFN-cysteine substantially inhibited HDAC activity.
"Normal cells are being exposed to these agents like sulforaphane every day," said Dashwood. "They are modulating HDAC activity maybe 20% or 25%. The consequence in our view is that the genes are being derepressed by HDACs and are being primed, as it were, for a toxic insult to the normal cell. Genes that mediate cell cycle arrest and apoptosis, like p21 and BAX, might be more rapidly turned on so the cell can quickly respond."
Dashwood and his colleagues are now working to show that blood concentrations of sulforaphane achieved through diet can affect HDAC activity.
"We still don't know what concentrations are achieved in people," he said. But he added that a study just completed demonstrated that when volunteers are fed sulforaphane-rich broccoli sprouts, highly significant inhibition of HDAC can be measured in peripheral blood mononuclear cells—in some cases within 3 hours of consumption.
"I'm really excited by this whole area of dietary HDAC inhibition," he said. "The big black hole for many years in chemoprevention has been what about those agents that act after an initial mutation [in normal cells]. For me and for others in the chemoprevention field, the regulation of transcription factor access to the DNA through chromatin changes is an exciting area likely to expand significantly in the future."
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||