17 December 1998 CapCure5\Sellers.MS-- 890 words
PTEN, newly-discovered tumor suppressor,
controls PI3 kinase/AKT and other pathways
Lake Tahoe, NV--In less than 2 years, a newly-discovered tumor suppressor gene has become a major target for cancer therapy. And as a bonus, it also explains a rare hereditary disease.
"PTEN is important because it's mutated in a wide range of tumors," said William R. Sellers, MD, instructor of medicine at Dana-Farber Cancer Institute, Boston.
"Tumor suppressor genes rank in importance according to the number of tumors in which they're implicated," said Dr. Sellers jocularly. p53, rb, and p16 are the most commonly mutated tumor suppressor genes, he noted.
"PTEN seems to be catching up, especially for certain tumors, including prostate cancer," said Dr. Sellers, one of several investigators describing their PTEN work at CapCURE'S 5th Annual Scientific Retreat this September.
"In the first year that researchers have been looking for it, they found the PTEN mutation in approximately 15% of primary tumors," said Dr. Sellers, "and with better analytic technique we might find it in more."
"But even if it's just found in 15% of tumors, it might have significance for a much larger number, because it might lead us to abnormalities in other places in that pathway," Dr. Sellers explained. "With the rb pathway in glioblastoma, for example, you can have tumors with rb mutations, or tumors without rb mutations. But the tumors without rb mutations often have mutations in other parts upstream in the same pathway, such as p16 and CDK4.
Once PTEN was identified as a tumor suppressor, it was also quickly identified as the defective gene in Cowden's disease, a multiple hamartoma syndrome associated with increased incidence of breast and thyroid cancer. (For reasons that are unclear, germline mutations have different expressions than somatic mutations.)
Cells have several pathways that keep them under control. These pathways destroy the cells by apoptosis when they are no longer needed, or when they become abnormal. PTEN has been emerging as an important tumor suppressor responsible for apoptosis. When it was first discovered on chromosome 10q23 by comparing normal to transformed cells (Science, 275(5308):1943-7, 28 March 1997), it also turned out to be mutated and inactivated in LNCaP and DU145 prostate cancer cell lines.
Actually, the mechanism of PTEN, and other tumor suppressor genes, is not just apoptosis, CapCURE researchers heard repeatedly. Sometimes PTEN kills cells by apoptosis, but sometimes it stops cell growth and leaves them quiescent.
One of the therapeutic strategies that CapCURE researchers are working on is to characterize these pathways, in normal and transformed cells. Is an oncogene overactive? Or is a tumor suppressor gene inactive?
Then, pharmaceutical companies can develop drugs to repair the functioning of the oncogene or suppressor by screening compounds for their ability to substitute for the missing function.
"For oncogenes, you could develop drugs that would directly antagonize the oncogenes, said Dr. Sellers. The recent success is trastuzumab (Herceptin), the monoclonal antibody to her-2/neu for treatment of breast cancer. "For tumor suppressors, since the gene product is absent, you would have to either replace the gene," he said, "or develop drugs that inhibit other parts of the pathway that are either overactivated or unregulated because of the absence of the tumor suppressor."
Dr. Sellers worked on a pathway known as PI3 kinase/AKT. "We found that cells that lack PTEN could be arrested in the cell cycle by putting PTEN back in," he said.
"AKT is an oncogene, with a history similar to the Rous sarcoma virus, c-src," said Dr. Sellers. Like c-src, AKT was first found in a chicken retrovirus. Like c-src, AKT is a normal gene that was taken up by a virus. Like c-src, AKT can cause cancers through somatic mutations.
AKT is regulated up and down. It's regulated up by a protein known as PI3 kinase, and regulated down by PTEN. The PI3 kinase turns on a phosphorylation switch, while the PTEN turns off a phosphorylation switch. The PI3 kinase adds a phosphate group to an intermediate compound, which activates AKT. PTEN, which is a phosphatase, removes that phosphate group from the intermediate compound, and de-activates AKT.
"We used a renal carcinoma cell line, 786-O, which lacks PTEN protein," said Dr. Sellers. "Right after PTEN was discovered, researchers started looking for altered PTEN function in standard cell lines." He found it in 786-O. "My lab found that, in Western blots, 786-O had no PTEN at all," he said. "We don't know yet what the mutation is." LNCaP had a 2-base pair deletion, and DU145 had a Met substituted for Leu.
"A common experiment in tumor suppressor studies is to find out what happens when you put a tumor suppressor back in," said Dr. Sellers. "We introduced the PTEN gene into this line by transfection with plasmids."
With the PTEN gene restored, "We found the cells arrested in G1," said Dr. Sellers.
"The more important finding," said Dr. Sellers, "was that we had mutations in PTEN that were still protein phosphatases but no longer able to act as lipid phosphatases."
The PI3 kinase/AKT pathway actually has 3 steps. PI3 kinase adds a phosphate group to a lipid compound, phosphotidylinositol, which activates AKT. "We found that some PTEN mutations would remove the phosphates from proteins but not from lipids."