By David N. Leff
Down through the years, the long arms of the law have devised many devices to execute a criminal: Electrocution, hanging, lethal injection, the guillotine - to name the main ones.
Surgeons, too, have their methods for terminating a tumor with extreme prejudice: by scalpel, by radiation, by chemotherapy, by gene therapy, and by starvation, which cuts off the blood supply cancers need to grow.
The human body has its own natural system for nipping a malignant growth in the bud - tumor-suppressing oncogenes, and their protein executioners. Best known of these is p53; to date the number of tumor suppressors is pushing 16.
But all too often, these anticancer guardian genes go over to the enemy, and promote the tumors they were designed by nature to prevent. One way the traitors work is to make sure the newborn malignancy gets plenty of nourishing blood to launch it on its career of growth, invasion and metastasis. This process of angiogenesis - proliferation of budding blood vessels - is one of the trendiest targets for stopping cancer in its tracks. Anti-angiogenesis is the current buzzword; endostatin, angiostatin and thrombospondin rank high on the list of putative tumor stranglers.
But keep your eye on a tumor suppressor called PTEN.
This four-letter word stands for "phosphatase and tensin" - as coined by one of its early discoverers in the mid-1990s. Tensin is a cytoskeletal protein homologous in part to the PTEN gene product. (See BioWorld Today, March 28, 1997, p. 1.)
A paper in the current Proceedings of the National Academy of Sciences (PNAS) dated March 27, 2001, bears the title: "PTEN controls tumor-induced angiogenesis." It was submitted to PNAS by academy member Michael Wigler, a molecular biologist at the Cold Spring Harbor Laboratory in New York.
"PTEN expression blocks tumor formation," Wigler told BioWorld Today. "It does so apparently without affecting the basic fundamentals of cell division. PTEN has negligible effects on the growth rate of tumor cells in vitro, and their DNA synthesis within the heart of the tumor. So we're left with a bit of a puzzle as to why the tumor doesn't grow aggressively. This paper offers some evidence and hypothesis that it's angiogenesis, but I don't know if it's sufficient to explain the biology.
"PTEN does have effects on tumor vascularization," Wigler pointed out, "but I don't know how that explains why in one case the tumor continues to expand aggressively, and in the other it doesn't."
Cell's Second Messenger May Be Key
He cited two hypotheses as to what PTEN, which is a protein phosphatase, does at the biochemical level: "One is that it dephosphorylates proteins. The other is that it degrades the cell's second messenger. In this paper," Wigler said, "I believe we showed that mutant PTEN retains protein phosphate activity, but loses its ability to degrade secondary messenger. And our in vitro and in vivo experiments would suggest that the progressive tumor growth is a function of the second messenger."
As the paradigmatic malignancy in their experiments, the paper's co-authors fixed on a particularly aggressive brain tumor - glioblastoma multiforme. It's the commonest primary tumor of the central nervous system, and the commonest solid tumor in children.
"Mutations of the PTEN tumor suppressor, which resides on chromosome 10," Wigler observed, "accompany progression of brain tumors from benign to the most malignant forms. This progression is associated with the induction of angiogenesis by a process termed the angiogenic switch. Therefore, we tested whether PTEN regulates tumor progression by modulating angiogenesis.
"We took a human glioblastoma tumor cell," Wigler recounted, "that was growing in culture and lacked PTEN. We made two parallel cell lines. One was the parental line; the other, we derived from the parental line by forced expression of the PTEN gene. We reinserted the wild-type gene into it and compared the tumorigenicity of those two cell lines."
The co-authors introduced virulent glioblastoma cells stereotactically into the frontal cerebral cortex of nude mice, as well as subcutaneously under the skin of a flank, to monitor growing tumor volume. Of animals implanted with the PTEN-positive cells, 90 percent survived at 40 days. Of those implanted with the PTEN-minus parental cells, 100 percent were dead at 27 days.
"When the cells expressed PTEN," Wigler went on, "they still formed tumors, but these remained small. When they didn't express PTEN, they formed aggressive tumors that killed the host." This, he pointed out, "was evidence that PTEN regulates tumor-induced angiogenesis and the progression of gliomas to a malignancy phenotype via second-messenger signals."
From these experimental results, Wigler drew three conclusions: "No. 1, that PTEN blocks the aggressive growth of tumors; No. 2, PTEN blocks angiogenesis; No. 3, the exact pathophysiology remains to be explained, but is likely to be occurring at the tumor margin - the boundary between the growing tumor mass and its host."
Indiana Group Tapped For In Vivo Mouse Work
The PNAS paper's senior author is hematologist/oncologist Donald Burden, at the Indiana School of Medicine in Indianapolis. "Basically," Wigler observed, "all the in vivo work was from Indiana. We had done our own in vivo work, which actually preceded this paper, but we didn't want to publish it. And then we created the cell lines, characterized them in vitro, and then provided them to Burden."
Despite the paper's tentative findings, Wigler said, "It may focus interest on the huge question: Why are some tumors aggressive and other tumors are not? The lazy mind doesn't think of that as an unsolved problem," he observed. "But the active mind realizes that it is unsolved. One thing, hopefully," Wigler continued, "people will come up with ways of addressing it."
He and his Cold Spring Harbor group are seeking, in ongoing research, "to understand why expression of PTEN blocks the aggressive growth of the tumor. We don't consider that a solved problem. There may be some new mechanism there from which we can learn.
"And the other thing it suggests," Wigler concluded, "is a recent possibility that drugs that actually inhibit the enzyme that makes the second messenger in that PTEN pathway might be effective in retarding the progression of aggressive glioblastomas." n