Background: Our group has previously shown that growth hormone releasing hormone receptor agonists (GHRHR-A) improve cardiac performance in heart failure models and reverse remodeling. This effect was associated with an increase in the number of c-kit+ cardiac stem cells (CSCs), suggesting that this agonist might have an effect on these cells.
Methods and Results: We investigated the expression of GHRH receptor (GHRHR) in CSCs of different species by flow cytometry analysis. GHRH-R is expressed in 96-98% of CSCs isolated from mouse, rat and porcine. Results were compared to GHRHR expression in HeLa and MCF7, and T47D cell lines, positive and negative controls, respectively. To determine if GHRHR activation can improve CSCs self-renewal, we tested the effect of agonists on porcine CSCs proliferation. The rate of cell division was increased 2-fold with JI38 (GHRHR-A) treatment (3.4 ± 0.7) vs. vehicle control (1.7 ± 0.2) (p<0.05). Pre-treatment of CSCs with the GHRHR antagonist MIA-602, showed a trend toward reversal of the JI38 agonistic effect on proliferation rate (2.2 ± 0.6). These studies were further extended to other GHRHR agonists. In addition to JI38, MR356 and MR409, both of which showed significant increase in CSCs proliferation relative to vehicle control, by 20 ± 5.7%, 37 ± 8.5% and 36 ± 12.2%, respectively (p<0.05). The protective effect of JI38 on porcine CSCs survival was determined under oxidative stress generated by hydrogen peroxide exposure. Pre-treatment of CSCs with JI38 prior to peroxide exposure significantly reduced cell death by 33 ± 2.2% (p<0.02). Similar effects were observed for MR356, which decreased cell death by 12 ± 8.6% (p<0.03). Furthermore, we found that the effect of GHRHR-A on CSCs proliferation was completely reversed by inhibitors of the ERK, PI3K and Akt pathways (p<0.05).
Conclusion: These findings confirm for the first time the expression of GHRHR in CSCs. GHRHR-A promotes CSCs proliferation and enhance survival. GHRHR-A effects on CSCs proliferation are mediated through activation of ERK, PI3K and AKT pathways. Accordingly, activation of GHRHR signaling pathways represents a novel therapeutic approach to protect and stimulate endogenous CSC population, promoting cardiac repair.