Human bone marrow mesenchymal stem cells (MSC) are multipotent progenitors that generate osteoblasts, chondrocytes, adipocytes, myoblasts and the bone marrow stromal cells that support hematopoiesis. Although the bone marrow microenvironment is hypoxic, little is known about the maintenance and response of MSC and their bone marrow stromal progeny to hypoxia. Using cDNA microarray hybridization technologies, we show, for the first time, that a total of 231 mRNAs in cultured MSCs are regulated by short exposures (4–48hrs) to hypoxia. These include known hypoxia-responsive genes, such as BHLHB2, PGK1, GLUT-1 and VEGF. Interestingly, we demonstrate that a significant proportion of genes involved in cell growth, proliferation or survival are also regulated by hypoxia in these cells. Amongst these, we have identified the centromeric protein CENP-F as a novel gene up-regulated in cultured MSCs by hypoxia. This up-regulation results in an increased level of CENP-F protein. The hypoxic stimulus also enhances cell division in the bone marrow MSC-derived stromal cells, resulting in a doubling in cell number within 24hrs. This contrasts with the effects of hypoxia on mature endothelial cells (EC), where recruitment into cell cycle is unchanged. Our demonstration that hypoxia promotes cell division and cell proliferation in the bone marrow stromal progeny of human MSC may have wider implications for the regulated growth and survival of other stem cells in hypoxic microenvironmental niches.

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