Braman Institute Scientists Discover Mechanism That Promotes Cell Mobility
For some time, researchers have known that protein p27 is an important inhibitor of the cell division that spreads cancer. Understanding the full mechanism and extent of that role has taken much longer. Now, scientists at the Braman Family Breast Cancer Institute at the Sylvester Comprehensive Cancer Center have solved part of the puzzle, discovering that p27 can severely misbehave when in bad company. In fact, it can lose its restraining action on cell growth and bind to other molecules to promote cell motility, one of the first steps in the spread of cancer. In other words, Dr. Jeckyl becomes Mr. Hyde.
Joyce M. Slingerland, M.D., Ph.D., director of the Braman Family Breast Cancer Institute at Sylvester at the University of Miami Miller School of Medicine, led a team of researchers in collaboration with David Helfman, Ph.D., professor of cell biology and anatomy at Sylvester. Their findings were published in the May 19 issue of the Proceedings of the National Academy of Sciences (PNAS) journal.
The growth inhibitor p27 is a key regulator of cell division and motility, and resides in the nucleus. In that position, it regulates cell cycle progression. However, when p27 localization is shifted out into the cell’s cytoplasm, it takes on a different role, promoting cell motility and tumor spread. “Earlier work showed that p27 can play a role in cell motility,” says Slingerland, “but how this happened was really not clear.”
Like many proteins, p27 has several phosphorylation sites — positions where phosphates can be added on. Phosphorylation can change a protein’s shape and function and alter its ability to bind to other proteins. In this study, Slingerland and her team took a close look at p27 phosphorylation at its T198 site. They had already discovered that phosphorylation at T198 mislocalizes p27 to the cytoplasm. This study showed that phosphorylation at T198 helps p27 to partner with the RhoA protein.
The protein RhoA (Ras homolog, member A) regulates a cell’s structure, much like scaffolding gives an object shape. When RhoA action is interrupted, the cell changes its shape and alters its adhesion to its environment and to other cells, leaving it more motile. Scientists knew that the protein p27 interacted with RhoA, inhibiting its adhesive quality. “In this paper, we found that T198 phosphorylation is a trigger for p27 binding to RhoA,” says Slingerland.
PI3Kinase is activated in a majority of human cancers. RSK1 is one of three pathways turned on by the PI3Kinase. This study discovered that RSK1 is one of the factors phosphorylating p27 at T198, leading p27 to bind to RhoA. This interaction prevents RhoA’s natural cell adhesion, which subsequently increases cell motility. In this work, Slingerland’s group showed that RSK1 phosphorylates p27 at its T198 amino acid site, which retains p27 in the cytoplasm and promotes its binding and inhibition of RhoA. In turn, this alters RhoA’s effect on cell structure, making the cell more motile, which is a key step in the progression toward metastasis.
In this research, Slingerland describes p27 as a “good molecule gone bad,” winding up in the wrong place, surrounded by molecules that subvert its normal effect to restrain cell growth and causing it to gain the ability to promote cell motility. This increase in motility “might be the first critical step in acquiring the potential to survive in an abnormal environment and move around and cause metastasis.”
Much of the research in Slingerland’s lab focuses on molecular pathways regulating the p27 protein. In June 2008, she led a study which uncovered a new pathway that keeps p27 locked in the cytoplasm, shutting down its ability to inhibit cell division. That research was published in the journal Molecular Cell.
“This work in PNAS is another building block,” said Slingerland, “in learning how p27 can affect a cell’s proliferation and motility.” The next step is to find out whether this increased motility translates into a real ability of these cells to form metastasis, using mouse models of breast cancer. Slingerland says, “We hope to determine exactly what role this increased motility plays in metastasis.”