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ONCOLOGY Cell Protein Potently Blocks Enzyme Linked to CancerTelomerase Inhibitor Suggests New Cancer Therapy The Greek myth of the three Fates who spin, measure, and inevitably cut the thread of human life may have its biological equivalent in telomeres, the repeated patterns of nucleotides knitted to the ends of our chromosomes: they slowly unravel at each cell division, ensuring death to each normal cell.
Xiao Zhen Zhou (left) and Kun Ping Lu were surprised when their newly discovered protein turned off telomerase in cancer cells. Photo by Graham Ramsay But cancer cells somehow escape this fate, spinning themselves a new life by rebuilding their telomeres. The vast majority manage this feat by switching on the enzyme telomerase. Since the discovery of the link between telomerase and cancer, scientists have been hunting for ways to turn the enzyme off, in hopes that such a therapy could render cancer cells mortal. Now, two HMS researchers have discovered the first endogenous protein that potently inhibits telomerase, suggesting a potential new cancer therapy and a genetic link between telomeres, cancer, and cell division. Cellular Clockwork"The length of the telomere is like a mitotic clock," said Kun Ping Lu, HMS assistant professor of medicine at Beth Israel Deaconess Medical Center and senior author of the study, which appears in the Nov. 2 Cell. Just as the sand in an hourglass marks passing time, the telomere's length marks the number of completions of the cell cycle, but the exact relationship between the two events is unclear. Lu and first author Xiao Zhen Zhou, an HMS instructor in medicine working in Lu's lab, thought there might be a functional link in the protein Pin2, which Lu had identified and studied for years and that seemed to have a role in both cell cycle control and telomere length. Pin2 is downregulated in tumors and helps to regulate the cell cycle by binding to proteins like the mitotic checkpoint ATM kinase. Though it affects telomere length, Pin2 does not directly inhibit telomerase, suggesting an intermediate mechanism between the two.To further determine the link between Pin2 and telomere length, Zhou performed a yeast two-hybrid screen to detect proteins that interact with Pin2. Out of millions of clones screened, she came up with six known and four unknown genes, including one they called PinX1. What distinguished the PinX1 protein from the rest was that it colocalizes with Pin2 at the telomeres and nucleolus. "I thought, maybe this protein also can regulate telomere length," Zhou said. But neither of the researchers anticipated the protein's dramatic effect on cells. Zhou developed tumor cell populations that stably expressed PinX1, antisense PinX1, or the protein's small C-terminal domain (PinX1-C), the binding site for Pin2. The cells appeared normal at first, but after many population doublings, the PinX1-C cells began to diminish in number and grow flat and bloated, characteristic of cancer cells in crisis. The cells expressing full-length PinX1 exhibited similar though less dramatic signs of crisis. The team analyzed telomerase expression and found it was lowered in PinX1 cells and undetectable in PinX1-C cells. "The first time we saw this inhibition in vitro, we actually did not believe it. We thought that maybe they were just artifacts," said Lu. After all, several small molecules have been studied as potential telomerase inhibitors, but no endogenous protein has had this effect. Further studies determined that telomere length in the cells was indeed shortening and that both PinX1 and its C-terminus could bind directly to the catalytic subunit of telomerase. The C-terminus of the protein being more potent than its full length suggests that alterations can modulate the protein's effect. Tumor SwitchAny protein that directly turns off telomerase might also function as a tumor suppressor, so Lu and Zhou began studying the tumorigenicity of the protein in mice. When injected with tumor cells expressing PinX1 or PinX1-C, mice showed no sign of growing cancerous masses. The results were consistent with the in vitro studies: both PinX1 and PinX1-C inhibited telomerase activity and sent cancer cells into crisis. In stark contrast, when mice were injected with cells expressing antisense PinX1, which elevated telomerase levels, tumors bloomed aggressively in the animals within a week. Initial studies on human tumor samples also have found depleted levels of PinX1 in many types of human cancer.Not only does the discovery of PinX1 herald a possible new strategy against cancer, it helps bolster the very notion that targeting telomerase can suppress tumors, a theory that is still being debated. "This describes the first telomerase inhibitor that causes cancer cells to go into crisis," said Lu. And as a mechanistic link between Pin2 and telomerase, PinX1 helps fill in the workings of the cell cycle's mitotic clock. Another reason that PinX1 may prove to be a very important link in a cancer cell's path to immortality is its location at 8p23, a chromosomal region that is frequently mutated in cancers and has been scrutinized by researchers in search of a putative tumor suppressor. As striking as the results of these studies are, however, more studies are necessary to characterize the protein's function. The product of a completely new gene with no obvious motif to suggest its action, PinX1 leaves many questions unanswered. Lu's lab is now working on developing a knockout mouse as a model for further studies. --Courtney Humphries |
