Mesenchymal stem cells (MSCs) contribute myocardial regeneration, and the beneficial effects may be mediated by paracrine factors produced by MSCs. C-kit positive neonatal cardiomyocytes (NCMs) contribute to myocardial regeneration, but they do not give a robust regenerative response since low expression of c-kit. Cell-cycle reentry of NCMs and insulin-like growth factor (IGF-1) improve myocardial function in infarcted hearts. MSCs and NCMs were prepared from Lewis rats, and cocultured in two chambers which allowed the diffusion of secreted factors from upper chamber to lower chamber, but prevented cell contacts. MSCs secreted significant amount of IGF-1 (159.6 ± 34.4 pg/ug DNA at 24 h, 285.3 ± 28.5 pg/ug DNA at 48 h, and 358.3 ± 39.9 pg/ug DNA at 72 h), whereas the amount of IGF-1 in conditioned medium from NCMs was undetectable assessed by IGF-1 ELISA. Using flow cytometry, we found that the secreted factors by MSCs increased c-kit protein expression, which was attenuated by IGF-1 receptor neutralizing antibody (IGF-1R Ab) and phosphatidylinositol 3 (PI3) kinase inhibitor LY 294002 (NCM vs MSC/NCM vs MSC/NCM+IGF-1R Ab vs MSC/NCM+ LY294002= 1.5 ± 0.6 % vs 5.5 ± 0.3 % vs 1.9 ± 0.6% vs 2.1 ± 0.5%) assessed by flow cytometry. The cytokinesis of NCMs was increased when cocultured with MSC analyzed by calcein fluorescence intensity (3.1 ± 0.5 fold increase, p<0.02). As determined by BrdU assay, the DNA synthesis of NCMs was significantly increased when cocultured with MSC compared to NCM alone (1.8 ± 0.3 fold increase at 48 h, 2.6 ± 0.2 fold increae at 72 h), which was attenuated by IGF-1R Ab and by PI3 kinase inhibitor. To confirm the paracrine effects of MSCs are mediated by IGF-1 signaling and PI3/Akt pathway, we performed in vitro Akt kinase assay using GSK-3 fusion protein as substrate, and found that co-culture system increased the activity of Akt kinase in NCMs, and the IGF-1R Ab and PI3 kinase inhibitor dose-dependent blocked the ability of co-culture system to increase Akt kinase activity. Our results demonstrate that the paracrine effects of MSC on c-kit up-regulation and cell-cycle reentry of NCM are mediated by IGF-1R activation through PI3 kinase/Akt - mediated pathway. These findings provide a new paradigm for the biological effects of IGF-1 on myocardial regeneration.
This research has received full or partial funding support from the American Heart Association, AHA South Central Affiliate (Arkansas, New Mexico, Oklahoma & Texas).
A Caveat Emptor for myocardial regeneration: Mechanical without electrical recovery will not suffice