Background:
Insulin-like growth factor (IGF-1) is a potent pro-survival cytokine that is not robustly expressed by human cardiac stem cells (CSCs). Previously, we have shown that paracrine engineering of CSCs with IGF-1 improves cell-mediated cardiac repair. Here, we explore the mechanisms underlying IGF-1 enhanced cardiac repair by CSCs.
Methods/Results:
Sub-culture of isolated c-Kit+, CD90+ and lineage negative cells (c-Kit-/CD90-) demonstrated that the natural low level production of IGF-1 by CSCs (149±16 pg/ml*mg) is secreted by all 3 sub-populations. After culture in hypoxic reduced serum media, lentiviral mediated over-expression of IGF-1 enhanced proliferation (population doubling time: 1.4±1.7 vs.-0.9±1.2 and -1.9±2.4 days, respectively; p≤0.01), expression of pro-survival transcripts (AKT, ERK and MAPK pathways) and pro-survival proteins (Bcl-2, Bcl-x, HIF-1a; p≤0.01) while decreasing expression of apoptotic markers (3.5±0.9 and 3.7±0.9 fold less annexin V; p≤0.01) as compared to GFP- and non-transduced CSCs. The high expression of the IGF-1 (79±3%) or the insulin receptor (61±5%) on CSCs suggests that autocrine pro-survival pre-conditioning underlies these effects. Direct and indirect co-culture of CSCs with neonatal rat ventricular cardiomyocytes (NRVMs) within hypoxic conditions demonstrated that IGF-1 promoted indirect myocardial repair by increasing NRVM viability and pro-survival signaling (Bcl2+; p≤0.01) while reducing apoptosis (annexin V+; p≤0.05) as compared GFP- or non-transduced CSCs. Transplant of CSCs genetically engineered to over-express IGF-1 into immunodeficient mice one week after infarction boosted IGF-1 content within infarcted tissue by 2.9±0.2 fold (p=0.004) and long-term engraftment (+4 weeks human alu content increased by 9.1±4 fold; p=0.05) while reducing myocardial apoptosis (3.4±0.3 and 2.5±0.5 fold reduction expression of Bax and p53, respectively; p<0.05) and long-term myocardial scarring (+ 4 weeks 2.2±0.4 fold less; p=0.01) as compared to GFP-transduced CSCs.
Conclusions:
Transplantation of IGF-1 enriched CSCs enhances cardiac repair by boosting transplant cell survival and reducing myocardial apoptosis to improve myocardial function and salvage of damaged myocardium.
Increases in Free, Unbound Insulin-like Growth Factor I Enhance Insulin Responsiveness in Human Hepatoma G2 Cells in Culture