Hypoxia inhibits mesenchymal stem cell proliferation through HIF1[alpha]-dependent regulation of P27

Stem cells have inherent properties of self-renewal and differentiation. High percentage of transplanted stem cells (95 %) die into ischemic heart due to unfavorable environment where in hypoxia considered among major contributing factors. Hypoxia inducible factor-1 [alpha] is a well-known transcription factor which robustly induced during hypoxia and an essential factor for adaptation under lower oxygen tension. The effects of hypoxia onto stem cells and its cell cycle are poorly understood. Mesenchymal stem cells were isolated from adult male Fischer-344 rats bone marrow and kept under hypoxia (1 % O.sub.2). Cell survival and proliferation were studied using MTT and CFSE assay which showed reduced proliferation rate with an arrest in G.sub.0/G.sub.1 phase of cell cycle using flow cytometry. Western blot analysis revealed an increase in expressions of HIF-1[alpha] along with P53, a tumor suppressor gene. Cyclin-dependent kinase inhibitor (CDKI), p27 was significantly increased, in concordance with the findings, cyclin D1 was also reduced under hypoxia and forestalling S phase entry. In addition, loss of function study with HIF-1[alpha] knockdown revealed progression of cell cycle even under hypoxia. Knocking down p27 abrogated the hypoxia-induced G.sub.1 checkpoint, suggesting a key regulator of G.sub.1/S transition in hypoxic cells. Hypoxia could cause HIF-1[alpha]-dependent increase in the expression of p27 leading to cell cycle arrest in G.sub.0/G.sub.1 phase. The demonstration of the molecular mechanism of hypoxia-induced G.sub.1/S regulation provides insight into a fundamental response of stem cells to low oxygen tension.