Abstract 1543: Factors Secreted by Adult Human Bone Marrow Stem/Progenitor Cells Activate and Protect Adult Cardiac Stem/Progenitor Cells
The mechanism(s) of cardiac repair by adult non-hematopoietic bone marrow stem/progenitor cells such as mesenchymal stem cells (MSCs) are controversial. MSCs injected into injured myocardium may improve cardiac function by differentiating into or fusing with cardiomyocytes, but the reported efficiency of differentiation or cell fusion is low. Recently, it was shown that factors secreted by MSCs can increase the survival of cardiomyocytes in culture during exposure to hypoxia. We report that factors secreted by MSCs may also stimulate cardiac repair by activating or protecting adult cardiac stem/progenitor cells. Human MSCs were isolated from bone marrow aspirates by plastic adherence and by magnetic-activated cell sorting using the p75 neurotrophin receptor (p75MSCs). MSCs, p75MSCs and dermal fibroblasts were expanded in serum-containing medium. Serum-free conditioned medium (CdM) was prepared by washing the cells and then incubating them for 48 hrs in serum free medium (SFM). C-kit-positive adult cardiac stem cells (CSCs) from rat heart were exposed to unconcentrated CdM or SFM without any additional supplements. Proliferation assays demonstrated that CdM from MSCs, p75MSCs, and fibroblasts all significantly induced the proliferation of cardiac progenitor cells (CPCs) while the number of CPCs incubated in control SFM decreased. The percentage of BrdU-positive CPCs in the CdM was significantly higher than in SFM-treated cells, and immunoblotting demonstrated that Ki67 was expressed in CPCs treated with CdM but not in those treated with SFM. Incubation in CdM led to the phosphorylation of STAT3 in CPCs, and the JAK2/STAT3-pathway inhibitor, AG490, completely blocked CdM-induced CPC proliferation. The CdM-expanded CPCs remained multipotent and stained for several cardiac cell lineage markers including alpha-sarcomeric actin, alpha-smooth muscle actin, and von Willebrand Factor. In further experiments, we found that CdM also significantly increased the survival of CPCs exposed to hypoxia (1% oxygen) for 48 hrs. Following cardiac injury, in addition to rescuing mature cardiomyocytes, factors secreted by adult bone marrow stem/progenitor cells may repair the heart through the activation and protection of resident CSCs and CPCs.