Michael Cleary, MD, the Linhard Family Professor in Pediatric Cancer Biology

School of Medicine

A protein crucial to healthy cell growth now appears also to maintain stockpiles of bone marrow stem cells by preventing the cells from dividing too often and exhausting themselves, according to a new study from the School of Medicine. The finding could lead to better techniques for growing stem cells in culture and for understanding the mechanism of some cancers.

The protein, called Pbx1, regulates the embryonic development of many tissues and organs by promoting the growth of semi-specialized cells. The findings suggest the protein also plays a role in keeping bone marrow stem cells in their resting phase, crucial for supporting their ability to self-renew. Pbx1 is one of the few known factors involved in regulating stem cells' quiescence.

"It was interesting for us to find how this works, and it was totally unexpected," said first author Francesca Ficara, PhD, postdoctoral scholar. Her work with Michael Cleary, MD, the Linhard Family Professor in Pediatric Cancer Biology, was published in the May 8 issue of Cell Stem Cell.

Bone marrow stem cells, also known as hematopoietic stem cells, give rise to all types of blood cells. Through an individual's lifetime they additionally replenish their own stores by creating cells that are identical to themselves. But this requires a delicate balancing act. "This ability to self-renew needs to be tightly regulated," Ficara said. "If hematopoietic stem cells self-renew too much, it can give rise to cancer. On the other hand, if they're not able to self-renew enough, the pool will become exhausted over time, and there will be no stem cells left to do the job of making blood cells."

At any one time, only a small fraction of bone marrow stem cells is in their active cell cycle. Most are instead in hibernation, waiting to be called upon to proliferate again. Maintaining a viable pool of stem cells, then, depends on strictly regulating how cells enter and exit their quiescent phase. Work from Ficara and Cleary, who is also a member of the Stanford Cancer Center, shows that Pbx1 plays a critical role in this gatekeeping process.

The researchers first removed Pbx1 from hematopoietic stem cells in mice. At first, the stem cell populations were normal. Over time, the pool declined. The researchers' data suggest the stem cells had exited quiescence early and differentiated more than normal. "They proliferated too much, and the pool of stem cells exhausted itself," Ficara said.

What's more, bone marrow grafts from these mice slowly failed in other mice over time. This showed that without Pbx1, hematopoietic stem cells lose their ability to self-renew. "At the molecular level we could see that the stem cells were not behaving like stem cells," Ficara said.

Their work also suggests a possible mechanism for this regulation. Many of the genes dependent on Pbx1 turn out to belong to the pathway of transforming growth factor beta, a protein also associated with stem cell quiescence. It's plausible that Pbx1 controls quiescence and self-renewal of stem cells by affecting their response to TGF-beta.

More broadly, the results suggest that Pbx1 has a dual role regulating cell cycling, by either encouraging or discouraging cell growth under different circumstances. This has larger implications because Pbx1 has also been found as the product of an oncogene in some childhood leukemias. "What is interesting is that we are starting to understand through this work the function of a protein that is also mutated in some cancers," Ficara said.

Other Stanford authors include Mark Murphy, PhD, and Min Lin, PhD, both postdoctoral scholars. This work was supported by the American Society of Hematology, the American-Italian Cancer Foundation, and the National Institutes of Health.