The Effects of Mesenchymal Stem Cells on C-kit Up-regulation and Cell-cycle Re-entry of Neonatal Cardiomyocytes Are Mediated by Activation of Insulin-like Growth Factor 1 Receptor

C-kit-positive neonatal cardiomyocytes (NCMs) contribute to myocardial regeneration. However, the myocardium itself cannot give rise to a robust regenerative response owing to the limited numbers of c-kit-positive resident stem cells present in the heart. It has been shown that mesenchymal stem cells (MSCs) can enhance cardiac repair via the release of paracrine factors such as insulin-like growth factor (IGF-1). We investigated whether the increased expression of c-kit in NCMs mediates the beneficial effects of MSCs on cardiac repair. MSCs and NCMs were prepared from Lewis rats and co-cultured in a Transwell system, which allowed the diffusion of secreted factors but prevented cell contact between the two cell types. The proliferation of NCMs was determined by BrdU assay. The expression of c-kit was assessed by real-time PCR and flow cytometry. The apoptosis rate of NCMs in response to hypoxia was determined by flow cytometry. We found that the expression of c-kit in NCMs was increased by paracrine factors released by MSCs. The effect of paracrine factors on c-kit expression was attenuated by IGF-1 receptor-neutralizing antibody. Furthermore, we found that increased c-kit expression requires IGF-1 receptor activation via the phosphatidylinositol 3 kinase/Akt-mediated pathway. These findings provide a new paradigm for the biological effects of IGF-1 and have significant implications for understanding the beneficial effects of MSCs on myocardial regeneration.