The Value of Culturing MSCs under Hypoxic Conditions
The recent concept of optimal oxygen levels for growing and expanding stem cells will probably have a major impact on adult stem cell therapeutics. Although arterial blood oxygen saturation is 100%, tissue consumption of oxygen leads to very low tissue oxygen concentrations. Hypoxia is, in a sense, normoxia for bone marrow cells because normally these cells live in an oxygen concentration of 1–6%. When MSCs grown and expanded under ambient, or 21%, oxygen, they have different secretory and functional patterns compared to MSCs grown under hypoxic (5%) conditions. Studies also suggest that normoxia-grown MSCs are dysfunctional compared to MSCs grown under chronic hypoxic conditions.
Under normoxic conditions, HIF-1-alpha is rapidly degraded through the ubiquitin system, leading to very low intracellular HIF-1-alpha levels. In the presence of hypoxia, the degradative pathway is shut off. HIF-1-alpha levels increase, forming a heterodimer with HIF-1-beta; this molecule, along with p300, leads to the transcription of multiple genes, including VEGF, FGF, and their receptors, as well as receptors expressed on the cell surface involved in homing, migration, and engraftment—such as CXCR4, CXCR7, and CX3CR1. These receptors home to injured tissues because such tissues express distress signals, i.e., the ligands of these chemokine receptors, including SDF-1 and fractalkine. When stem cells detect tissue distress, they home to that tissue, migrate into it, and engraft. This homing and engraftment mechanism is critical to the healing process. Thus, the hypoxia-induced increase in chemokine receptors should enhance homing and engraftment.
Such data suggest that growing MSCs under chronic hypoxic conditions might result in an MSC with more robust healing properties. In vitro these cells express higher levels of many molecules associated with healing, and the cells are more effective in migrating toward various growth factors and cytokines.
That such in vitro data relate to in vivo activities is suggested by a paper (Tang et al., Circ Res 2009) examining the effects of culturing cardiac-derived progenitor cells for six hours under normoxic versus hypoxic conditions in a mouse model of AMI. The investigators found that cells exposed to hypoxia home and engraft into the ischemic myocardium to a much greater extent than cells exposed to normoxia. Importantly, mice treated with cells exposed to hypoxia had significantly greater recovery of cardiac function than the hearts of mice treated with cells exposed to normoxia.