Induced pluripotent stem cells

Credit: James Thomson

Proteins that regulate energy metabolism are essential for stem cell formation, according to a study published in Cell Stem Cell.

The researchers showed that hypoxia-induced factor 1α and 2α (HIF1α and HIF2α)—2 proteins that control how cells metabolize glucose—play a key role in the formation of stem cells.

The findings may advance our understanding of stem cell development, but they also suggest the proteins might be targets for new cancer therapies.

Julie Mathieu, PhD, of the University of Washington in Seattle, and her colleagues conducted this research, creating induced pluripotent stem cells (iPSCs) by reprogramming mature human tissue fibroblasts.

During reprogramming, the cells must go through a stage in which they shut down the metabolic pathway they use to generate energy from glucose that requires the presence of oxygen in mitochondria. The cells shift over to the glycolytic pathway, which generates less energy but does not require the presence of oxygen.

This shift may take place because in nature, embryonic and tissue stem cells often must survive in hypoxic conditions. This transition to a glycolytic state is of particular interest to cancer researchers because, as normal cells are transformed into cancer cells, they too go through a glycolytic phase.

For their study, Dr Mathieu and her colleagues focused on the function of HIF1α and HIF2α in this process. The researchers showed that each protein is required for iPSC generation.

To tease out the impact of HIF1α and 2α on cellular processes in more detail, the team stabilized the proteins in an active form and tested what each protein could do alone.

They found that when HIF1α was stabilized, the cells went into the glycolytic state and produced more iPSCs than normal. However, when the researchers activated HIF2α, the cells failed to develop into stem cells.

“This was a big surprise,” Dr Mathieu said. “These proteins are very similar, but HIF1α gives you lots of stem cells [and] HIF2α, none.”

If stabilized together, HIF2α won the battle, repressing all stem cell formation.

Further investigation revealed that HIF2α does indeed promote the shift to glycolysis in an early stage of the cells’ reprogramming. But if it persists too long, it has the opposite effect, blocking the progression to the stem cell state.

“HIF2α is like Darth Vader, originally a Jedi who falls to the dark side,” said study author Hannele Ruohola-Baker, PhD, also of the University of Washington.

“While HIF1α, the good guy, is beneficial for reprogramming throughout the process, HIF2α, if not eliminated, turns bad in the middle and represses pluripotency.”

HIF2α does this, in part, by upregulating production of the protein TRAIL, which is known to, among other things, induce apoptosis.

These findings suggest there may be proteins of other families that are playing alternating “good guy/bad guy” roles during stem cell development, according to study author Wenyu Zhou, PhD, of Stanford University in California.

“It is very intriguing that HIF2α has the capacity to both promote and repress pluripotency, doing so at different stages in a cellular reprogramming process,” she said.

The findings have implications for stem cell research, Dr Mathieu said. First, they indicate that it may be possible to use HIF1α to greatly increase the number of stem cells in a culture.

And second, they suggest it may be possible to induce stem cell formation with HIF proteins alone or in combination with other stimulating factors without inserting genes at the start of the reprogramming process.

But the findings may also have implications for cancer research. Both HIF1α and 2α are known to play a role in normal cells’ transformation to cancer stem cells. And the presence of activated HIF1α is known to be a marker for aggressive disease.

So the researchers believe it might be possible to interfere with cancer development by either blocking the effect of HIF1α in malignant cells early in the process or stimulating the effect of HIF2 at a later stage.

Proteins appear necessary for stem cell formation