Putting the Brakes on
|Contact: Lynn Yarris, [email protected]|
A potential new opportunity for the treatment of breast cancer has been identified in a study by researchers with Lawrence Berkeley National Laboratory. The study, which involved special lines of human breast cells, provides important new insight into the relationship between a tumor-suppressing protein called p53 and telomeres, the structures that protect the ends of chromosomes and enable tumor cells to grow and divide indefinitely.
"We have shown a new role for the p53 protein in the transformation of normal breast cells into immortal cancer cells," says Martha Stampfer, a biologist with Berkeley Lab's Life Sciences Division, who led this study along with colleague Paul Yaswen. "The p53 protein, which is retained in most breast tumors, but is lost more frequently in other cancers, can act as a sort of emergency brake to slow or even stop the development of cancer in its early phase."
Says Yaswen, "Even when it's not blocking cell division, the p53 protein seems to have a role in regulating a gene expression that is critical for the continuous growth of most tumor cells. Mutation or loss of p53 can propel precancerous cells from an indolent, genetically unstable, slow-growing state to a rapidly growing and evolving state that leads to a full-blown cancer."
Stampfer and Yaswen are coauthors of a paper reporting this research which appeared in the August 15 issue of the journal Oncogene. Other coauthors of the Oncogene paper were Berkeley Lab's James Garbe, Tarlochan Nijjar, and Don Wigington, plus Karen Swisshelm of the University of Washington.
Stampfer has been a pioneer in the development of finite life span human mammary epithelial cells (HMEC) cultures and immortally transformed lines for research, having started her work back in 1976. Normal HMEC, like most normal human cells, have a limited capacity for replication. After about 50 to 80 divisions, irreversible senescence sets in, an HMEC stops dividing into new cells or dies. This stringently enforced finite lifespan, called "replicative senescence," is thought to have arisen in long-lived organisms, such as humans, as a means of preventing cancer.
Immortalized cells, on the other hand, undergo a transformation that enables them to overcome senescence. Their growth and division continues unchecked, a critical step down the road to malignancy and invasive cancer.
"The long-term objective of our program has been to develop and characterize HMEC systems for use in a wide variety of studies on human cell biology and carcinogenesis," Stamper says. "The goal is to understand the normal growth control processes in HMEC and to determine how these processes may be altered as a result of immortal and malignant transformation."
Towards this end, Stampfer and Yaswen have used their cultured HMEC lines to study several important defects including the loss of p53 protein which is known to play a role in the immortal transformation of normal cells.
"We found that p53 plays a role in cancer suppression both before and after cells become immortalized," says Stampfer. "The role of p53 after immortalization was not known because in previous studies the p53 gene had been inactivated before the transformation to immortality."
Stampfer, Yaswen and their colleagues found that the presence of p53 in newly immortalized cells prevented the cells from expressing high levels of telomerase, the enzyme that maintains the telomeres. Without sufficient levels of telomerase, cell replication slows to a crawl even though the cells are characterized as immortal.
"Most breast cancers do have p53 present, which may explain why breast cancer is less aggressive than many other cancers," says Stampfer. "Even after breast cells have become immortalized, they may still have difficulty continuing to replicate because p53 is acting to slow the process."
Previous studies by Stampfer and Yaswen at Berkeley Lab, and by researchers at other institutions, have established that for a cell to become immortalized it must overcome two barriers. The first of these barriers is mediated by a tumor-suppressing protein called retinoblastoma and is easily overcome in cultured HEMC, says Stampfer.
The second barrier is mediated by the length of cell telomeres. Once the telomeres have become too short, a cell loses the ability to make accurate copies of its genome. If the shortening of the telomeres continues -- in the absence of sufficient telomerase, a cell loses some of its telomeres with each successive division -- the cell stops dividing altogether or dies. This second barrier, Stampfer says, is much more difficult to overcome in culture.
"What we're seeing is a dysfunctional stage that sets in as the telomere becomes short," says Stampfer. "Dysfunctional telomeres introduce genomic instability before the cell actually becomes immortal, and this gives rise to genetic errors that can lead to immortality and malignancy."
The observations of Stampfer and Yaswen and their Berkeley Lab research group are consistent with clinical observations that at the early or "pre-invasive" stages of cancer the telomeres of cells in a troubled area are very short. Furthermore, the loss or absence of p53 is associated with patients facing the worst prognosis.
"Our findings suggest that even though newly immortal cells have gained the potential to express telomerase, their very short telomeres and low level of telomerase activity may render them extremely sensitive to telomerase inhibitors, especially when p53 is still present," says Yaswen.
Stampfer and Yaswen believe this extreme sensitivity points to a potential new opportunity for therapeutic intervention. Since most normal cells appear to have no continuous need for telomerase, such a therapy should have few side effects.