Hypoxia mediates low cell-cycle activity and increases the proportion of long-term–reconstituting hematopoietic stem cells during in vitro culture

Objective

Recent evidence suggests that hematopoietic stem cells (HSCs) in the bone marrow (BM) are located in areas where the environment is hypoxic. Although previous studies have demonstrated positive effects by hypoxia, its role in HSC maintenance has not been fully elucidated, neither has the molecular mechanisms been delineated. Here, we have investigated the consequence of in vitro incubation of HSCs in hypoxia prior to transplantation and analyzed the role of hypoxia-inducible factor (HIF)−1α.

Materials and Methods

HSC and progenitor populations isolated from mouse BM were cultured in 20% or 1% O₂, and analyzed for effects on cell cycle, expression of cyclin-dependent kinase inhibitors genes, and reconstituting ability to lethally irradiated mice. The involvement of HIF-1α was studied using methods of protein stabilization and gene silencing.

Results

When long-term FLT3⁻CD34⁻ Lin⁻Sca-1⁺c-Kit⁺ (LSK) cells were cultured in hypoxia, cell numbers were significantly reduced in comparison to normoxia. This was due to a decrease in proliferation and more cells accumulating in G₀. Moreover, the proportion of HSCs with long-term engraftment potential was increased. Whereas expression of the cyclin-dependent kinase inhibitor genes p21^cip1, p27^Kip1, and p57^Kip2 increased in LSK cells by hypoxia, only p21^cip1 was upregulated in FLT3⁻CD34⁻LSK cells. We could demonstrate that expression of p27^Kip1 and p57^Kip2 was dependent of HIF-1α. Surprisingly, overexpression of constitutively active HIF-1α or treatment with the HIF stabilizer agent FG-4497 led to a reduction in HSC reconstituting ability.

Conclusions

Our results imply that hypoxia, in part via HIF-1α, maintains HSCs by decreasing proliferation and favoring quiescence.