Physiological Oxygen Tension Enhances Competence and Functional Properties of Murine Cardiac Mesenchymal Cells

Cardiac mesenchymal cells (CMCs), a newly-discovered and promising type of progenitor cells, are effective in improving cardiac function in rodents after myocardial infarction. Stem/progenitor cells are usually cultured at atmospheric O₂ tension (21%); however, the physiologic O₂ tension in the heart is ~5%, raising the concern that 21% O₂ may cause toxicity due to oxidative stress. Thus, we compared mouse CMCs cultured at 21% or 5% O₂ beginning at passage 2. At passage 5, CMCs underwent severe hypoxic stress (1% O₂ for 24 h). Compared with CMCs cultured at 21% O₂, culture at 5% O₂ consistently improved cell morphology throughout 5 passages, markedly decreased cell size, increased cell number, shortened cell doubling time, and dramatically reduced lactate dehydrogenase release from CMCs into culture media after hypoxic stress. Furthermore, culture at 5% O₂ increased telomerase activity and telomere length, implying that 21% O₂ tension impairs telomerase activity, resulting in telomere shortening and decreased cell proliferation. Thus far, almost all preclinical and clinical studies of cell therapy for the heart disease have used atmospheric (21%) O₂ to culture cells. Our data challenge this paradigm. Our results demonstrate that, compared with 21% O₂, 5% O₂ tension greatly enhances the competence and functional properties of CMCs. The increased proliferation rate at 5% O₂ means that target numbers of CMCs can be achieved with much less time and cost. Furthermore, since this increased proliferation may continue in vivo after CMC transplantation, and since cells grown at 5% O₂ are markedly resistant to severe hypoxic stress, and thus may be better able to survive after transplantation into scarred regions of the heart where O₂ is very low, culture at 5% O₂ may enhance the reparative properties of CMCs (and possibly other cell types). In conclusion, our data support a change in the methods used to culture CMCs and possibly other progenitor cells.