Cardioprotection by cord blood mesenchymal stem cells through activation of Akt, ERK and STAT3 signaling

Objectives: Cardiac stem cell therapy may contribute to functional improvement of the infarcted heart, even in the absence of de novo formation of contractile cells. Saving ischemically damaged cardiomyocytes from cell death by paracrine survival stimuli is one proposed mode of action. We aimed to investigate the stem cell triggered augmentation of cardiomyocyte survival signaling in a cell culture ischemia model.

Methods: Human umbilical cord blood-mesenchymal stem cells (CB-MSC) were expanded and conditioned culture medium was prepared. Cultivated murine HL-1 cardiomyocytes were subjected to simulated ischemia (combined oxygen glucose serum deprivation) in CB-MSC conditioned medium or control medium. After treatment the percentage of dead cells, overall viability and phosphorylation of kinases Akt and ERK and transcription factor STAT3 were quantified. Experiments were repeated in the presence of inhibitors of Akt (Wortmannin), ERK (UO126) and STAT3 (Stattic) phosphorylation.

Results: In the presence of CB-MSC conditioned medium the fraction of dead HL-1 cardiomyocytes was lowered by 10.4 ± 3.7% and overall viability was enhanced by 12.7 ± 3.9%, as compared to control medium (p < 0.05, each). Protein phosphorylation of Akt, ERK and STAT3 was elevated by factor 1.4, 3.0, and 12.4, respectively. Blocking neither PI3K dependent Akt nor MEK dependent ERK phosphorylation affected cell viability. However, combined inhibition of both pathways abolished the protective effect of CBMSC conditioned medium. Although inhibition of STAT3 phosphorylation lowered cell viability in control and conditioned medium drastically, a beneficial stem cell effect was retained.

Conclusions: Factors released by CB-MSCs protect cardiomyocyte-like HL-1 cells from the deleterious impact of oxygen glucose serum deprivation. This effect is associated with enhanced phosphorylation of cell survival promoting kinases Akt and ERK and transcription factor STAT3. CB-MSC dependent Akt and ERK activation appears to exert cardioprotection in a compensatory manner. STAT3 also seems to be a crucial but not exclusive mediator of the beneficial stem cell effects. Clearly, the differenzial paracrine stem cell actions on ischemic cardiomyocytes require further investigation in order to optimize the efficacy of cardiac cell therapy.