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© Oxford University Press 2006.
NEWS |
Researchers Target Unfolded Protein Response in Cancerous Tumor Growth
Cancer cells, in their relentless drive to survive, hijack many normal processes: cell cycle signaling, angiogenesis, glucose metabolism, cell death, multidrug resistance. In the last few years, researchers have shown how another cell survival mechanism—the unfolded protein response, or UPR—might also belong on the list.
"We think it's going to be really important in the context of tumor growth," said Alan Diehl, Ph.D., of the University of Pennsylvania in Philadelphia. Recently, groups at Stanford University in Palo Alto, Calif., and Wake Forest University in Winston-Salem, N.C., have shown that tumors depend on an intact UPR for growth. And studies of a drug used for multiple myeloma, bortezomib (Velcade), strongly suggest that it kills tumors by causing stress in the cell while inhibiting part of the UPR. These and other studies have sparked a surge of interest in the UPR in cancer, as well as in other diseases.
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UPR research "has grown tremendously," says Randal Kaufman, Ph.D., of the University of Michigan in Ann Arbor. "Almost when anyone looks hard enough, they ... find that [the UPR] gets activated in their favorite disease."
Cracking the UPR Code
The cancer link is new, but the UPR is a well-known phenomenon. It's a reaction to stress in the endoplasmic reticulum (ER), a large, billowy organelle where proteins destined to be secreted are folded and processed. An accumulation of unfolded or misfolded proteins within the ER, as well as outside stresses like nutrient and oxygen deprivation, trigger the UPR, leading to transcription of proteins in the nucleus that help cells cope with the stress. These coping mechanisms include increasing chaperone proteins and facilitating degradation of unfolded proteins.
The UPR story began in the mid-1970s, when the lab of Ira Pastan, M.D., at the National Cancer Institute found two proteins that appeared when cells were starved of glucose. Pastan dubbed them GRP78 (for "glucose-related protein, 78 Daltons") and GRP94. Then in the early 1980s, the lab of Amy Lee, Ph.D., at the University of Southern California in Los Angeles, isolated cDNA clones for these proteins and linked their transcription to ER stress. This communication between the ER and the nucleus was a black box that intrigued a small but dedicated group of researchers during the 1980s and 1990s. The key question is, how can ER stress lead to gene transcription in the nucleus? Says Lee, "It took us a long, long time to crack the code."
In the late 1980s, Hugh Pelham, Ph.D., at the MRC Laboratory in Cambridge, England, and the team of Joe Sambrook, Ph.D., and Mary-Jane Gething, Ph.D., then at the University of Texas–Southwestern in Dallas, separately found that GRP78 was identical to BiP, a protein discovered in 1983 that binds immunoglobulin heavy chains on the way to secretion. The two lines of investigation—stress response and chaperone proteins—merged, and in 1992 Sambrook and Gething coined the term "unfolded protein response."
In 1998 Lee in California and Kazutoshi Mori, Ph.D., at the HSP Research Institute in Kyoto, Japan, separately discovered the promoter element common to UPR mammalian target genes, making it possible to work backward and identify molecular pathways leading to their transcription and to find the key proteins. Others were identified based on yeast UPR pathways.
There are now three known pathways in mammals, all regulated by GRP78. Two work through the transcription factors XBP1 and ATF6. The third lowers the rate of protein translation, slowing the influx of new proteins into the stressed ER. It works through PERK, a kinase. The UPR can also trigger programmed cell death through a separate pathway.
Thanks to the UPR, "cells can survive a certain low level of chronic ER stress very well, but if it's too much over a short period of time ... they activate a death pathway," explained Kaufman.
The Cancer Connection
The first solid clue linking the UPR and cancer appeared in 1996, when Lee's lab injected GRP78-knockdown fibrosarcoma cells into mice. Tumors either didn't form or they quickly regressed. The implication: Inhibiting GRP78 "has a major effect on tumor growth," in Lee's words.
In 2004 Stanford University's Albert Koong, M.D., Ph.D., showed that XBP-1–knockout cells could not grow into tumors in mice. "That directly implicates XBP-1 as being an important part of tumor growth," said Koong.
Then in 2005, Constantinos Koumenis, Ph.D., at Wake Forest showed that tumors derived from PERK-positive transformed cells grew much more rapidly than PERK-negative tumors in nude mice. Finally, several groups have shown that blocking the UPR makes tumor cells more sensitive to chemotherapy, both in vitro and in vivo. (JNCI, Vol. 96, No. 17, p. 1300–10, Sept. 1, 2004.)
Hypoxia triggers the UPR. But the usual suspect in oxygen deficiency, the transcription factor HIF (hypoxia-inducible factor)—an important and widely studied "master switch" of tumor adaptation to the hypoxic microenvironment—does not seem to be involved. The UPR promotes tumor growth independently of HIF. The hypoxia field "has been very HIF-centric," said Koong. That's now changing. "With the enthusiasm for research in [hypoxia], I think more and more researchers are looking at these HIF-independent pathways."
The cancer–UPR link has already found clinical application, thanks to bortezomib. The drug's antitumor mechanism is under intense investigation, because the drug could work against solid tumors, not just multiple myeloma, and many new clinical trials are under way. Evidence is growing that bortezomib kills tumors at least partly by inhibiting the UPR. In 2003, the Harvard lab of Laurie Glimcher, M.D., showed that bortezomib prevented formation of the spliced form of XBP-1 in myeloma cells, disrupting the UPR and causing cell death. Then, last December, the lab of David McConkey, Ph.D., at the University of Texas M. D. Anderson Cancer Research Center in Houston, showed that bortezomib blocked the PERK arm of the UPR in pancreatic cancer cells.
A "very cool piece of work," said Diehl. "The data are very striking."
McConkey thinks that bortezomib both adds to ER stress by keeping misfolded proteins from being degraded and blocking the UPR's ability to slow down the influx of new, unfolded proteins into the ER—thus compounding the overall pressure on the tumor cell. The cell can't handle the protein overload in the ER and undergoes apoptosis. "The fact that bortezomib (creates) this unusual ER stress and disrupts the UPR is good news for therapy," he said. It also may mean less toxicity, because cancer cells usually have more ER stress than normal cells.
If the drug's selectivity is due to the higher rates of protein synthesis or secretion in cancer cells—a plausible theory—it may work not only in myeloma but also in other cancers originating in secretory tissue, including breast, prostate, ovarian, and pancreatic cancers. McConkey has begun a clinical trial of bortezomib in combination with carboplatin in pancreatic cancer. He is also eager to try other UPR-specific drugs, when they appear. "I'm very enthusiastic about the idea of developing PERK inhibitors, or [other] things that disrupt the UPR," he said.
Targeting the UPR
Several academic groups are now looking for drugs that target the UPR. PERK and IRE1 (which processes the spliced version of XBP-1) are the obvious candidates, because both are kinases and thus viable drug targets. Diehl, for example, is screening for PERK inhibitors. "Based on David's work, and Costas's work, and some preliminary data of our own ... that's where I put my money," he said. Lee's group is looking for compounds that act on GRP78, and Koong and Kaufman are screening for inhibitors of the IRE1 and XBP-1 pathway.
"These are very druggable pathways because the reactions are very specific," said Kaufman. XBP-1, for example, is IRE1's only known target, and PERK appears to be almost as specific.
Industry is starting to pay attention. "It's still a little too preliminary for a lot of companies to invest, but there's a lot of interest," Kaufman said.
These same investigators also express caution. "You can always develop small-molecule inhibitors of kinases," observed Diehl. "The fundamental question of whether you can modulate one of the UPR targets and have a direct impact on tumorigenesis has to be tested in an animal model, before you can make any bold statements."
One concern is that the three UPR pathways may be redundant, and so it might be necessary to inhibit all of them to get an antitumor effect. Another potential pitfall is that the UPR, although it usually protects cells, can also trigger cell death. So a UPR cancer drug, in theory, runs the risk of blocking apoptosis and thus could help tumors survive. That outcome can be avoided, in Lee's view. "A cancer drug should block the protective branch of the UPR but leave the apoptotic branch intact," she said. The experimental evidence, she pointed out, so far shows that blocking UPR pathways stops tumor growth—not the opposite.
Meanwhile, other groups are looking for drugs to block the UPR death pathways, as a way to treat protein misfolding and protein secretion disorders, including neurodegenerative disease, diabetes, and autoimmune disorders. Kaufman's lab is trying to unravel how the UPR activates survival pathways, as opposed to death pathways, to better target one or the other, depending on the disease to be treated.
The cancer field, meanwhile, is waiting for specific UPR inhibitors to appear and to be tested. Their potential remains unknown. "This is still a relatively young field," said Diehl. "These are new ideas .... I could speculate, but there's very little data at this point to say that there's going to be a component of this pathway that we can clearly target as a potential antitumor strategy."
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