University of Washington scientists have discovered that the way cells degrade sugars plays a rather central role in reprogramming adult cells into pluripotent stem cells.
Julie Mathieu, a postdoctoral research fellow at the University of Washington, Wenyu Zhou, a postdoctoral scholar at Stanford University, and Hannele Ruhola-Baker, UW professor of Biochemistry, teamed up to address this problem. Their findings might have implications for the way pluripotent stem cells are made from adult cells for regenerative purposes in the future.
Reprogramming adult cells to induced pluripotent stem cells requires genetically engineering the cells with four different genes (now known as the Yamanaka factors after Shinya Yamanaka who discovered them). These four genes – Oct4, Klf4, Sox2 and c-Myc – causes the cells to de-differentiate into embryonic stem cell-like cells.
However, during reprogramming, cells change the way they make their energy. The reprogramming cells shut down the oxygen-utilizing part of metabolism and switch to a fermentative kind of metabolism that does not require the presence of oxygen.
This metabolism shift seems to mimic, in part, the metabolism of embryonic cells, which have to survive and grow in low-oxygen environments. Cancer cells also perform a similar shift in their metabolism.
Ruhola-Baker and her gang examined the function of two proteins that are known to control the use of oxygen: HIF or hypoxia-induced factor-1α and -2α. These two proteins regulate several genes involved in energy metabolism. When Ruhola-Baker and her colleagues made adult cells that lacked a functional copy of either HIF-1α or HIF-2α, these cells were unable to be reprogrammed into induced pluripotent stem cells.
When Ruhola-Baker and her team used these proteins to examine gain-of-function experiments, the results were very different. Stabilization of HIF-1α throughout the reprogramming process increased the formation of induced pluripotent stem cells from adult cells. However, stabilization of HIF-2α during the later stages of reprogramming inhibited the reprogramming process. The HIF-2α-mediated inhibition of reprogramming occurs in the presence or absence of extra HIF-1α. Thus HIF-1α increases reprogramming at all stages, but HIF-2α increases the efficiency of the early stages of reprogramming but inhibits reprogramming at the later stages.
“HIF-2α is like Darth Vader, originally a Jedi who falls to the dark side,” said Ruhola-Baker. “HIF-1α, the good guy, is beneficial for reprogramming throughout the process. HIF-2α, if not eliminated, turns bad in the middle and represses pluripotency.”
How does HIF-2α repress reprogramming to pluripotency? By increasing the production of a protein called TRAIL, which stands for Tumor necrosis factor-related apoptosis-inducing ligand. TRAIL has been known to cancer researchers for some time, since it causes some cancer cells to self-destruct. TRAIL has to be inhibited in order for reprogramming to occur. When HIF-2α activates TRAIL, it inhibits reprogramming.
Zhoi said that their data suggests that TRAIL and other members of this protein family might be alternate between playing good cop/bad cop during stem cell development.
Practically speaking, it might be possible to use HIF-1α to increase the number of stem cells derived from adult cells in a particular experiment. This might decrease the degree to which the cells are genetically manipulated in order to form induced pluripotent stem cells.
Alternatively, since activated HIF-1α is a marker for aggressive cancers, these data might also have implications for treating cancer. Blocking HIF-1α function or stimulating HIF-2α might interfere with particular cancers. The treatment possibilities are very intriguing.