Constitutive stabilization of hypoxia-inducible factor alpha selectively promotes the self-renewal of mesenchymal progenitors and maintains mesenchymal stromal cells in an undifferentiated state

With the increasing use of culture-expanded mesenchymal stromal cells (MSCs) for cell therapies, factors that regulate the cellular characteristics of MSCs have been of major interest. Oxygen concentration has been shown to influence the functions of MSCs, as well as other normal and malignant stem cells. However, the underlying mechanisms of hypoxic responses and the precise role of hypoxia-inducible factor-1α (Hif-1α), the master regulatory protein of hypoxia, in MSCs remain unclear, due to the limited span of Hif-1α stabilization and the complex network of hypoxic responses. In this study, to further define the significance of Hif-1α in MSC function during their self-renewal and terminal differentiation, we established adult bone marrow (BM)-derived MSCs that are able to sustain high level expression of ubiquitin-resistant Hif-1α during such long-term biological processes. Using this model, we show that the stabilization of Hif-1α proteins exerts a selective influence on colony-forming mesenchymal progenitors promoting their self-renewal and proliferation, without affecting the proliferation of the MSC mass population. Moreover, Hif-1α stabilization in MSCs led to the induction of pluripotent genes (oct-4 and klf-4) and the inhibition of their terminal differentiation into osteogenic and adipogenic lineages. These results provide insights into the previously unrecognized roles of Hif-1α proteins in maintaining the primitive state of primary MSCs and on the cellular heterogeneities in hypoxic responses among MSC populations. A transcription factor known to mediate the effects of low cellular levels of oxygen helps maintain the primitive state of the stem cells that give rise to bone, cartilage and fat. A protein called hypoxia-inducible factor 1α (HIF-1α) is the master regulator of tissue hypoxia, or low oxygen, and is only transiently expressed in the mesenchymal stem cells (MSCs), where only few of them have stem cell properties. Il-Hoan Oh and his colleagues at the Catholic University of Korea therefore established a culture of MSCs that stably expresses high levels of HIF-1α over prolonged periods. Using this system, they showed that HIF-1α selectively promotes proliferation of immature cell population in MSCs and drives expression of pluripotency genes. This drives MSCs toward a state of self-renewal of immature cells promoting their primitive state.