Ghrelin protects heart against ERS-induced injury and apoptosis by activating AMP-activated protein kinase.

Ghrelin, the endogenous ligand of growth hormone secretagogue receptor (GHS-R), is a cardioprotective peptide. In our previous work, we have revealed that ghrelin could protect heart against ischemia/reperfusion (I/R) injury by inhibiting endoplasmic reticulum stress (ERS), which contributes to many heart diseases. In current study, using both in vivo and in vitro models, we investigated how ghrelin inhibits myocardial ERS. In the in vivo rat heart injury model induced by isoproterenol (ISO), we found that exogenous ghrelin could alleviate heart dysfunction, reduce myocardial injury and apoptosis and inhibit the excessive myocardial ERS induced by ISO. More importantly, the activation of AMP-activated protein kinase (AMPK) was observed. To explore the role of AMPK activation in ERS inhibition by ghrelin, we set up two in vitro ERS models by exposing cultured rat cardiomyocytes to tunicamycin(Tm) or dithiothreitol (DTT). In both models, compared with Tm or DTT treatment alone, pre-incubation cardiomyocytes with ghrelin significantly activated AMPK, reversed the upregulation of the ERS markers, C/EBP-homologous protein (CHOP) and cleaved caspase-12, and reduced apoptosis of cardiomyocytes. Further, we found that the ERS inhibitory and anti-apoptotic actions induced by ghrelin were blocked by an AMPK inhibitor. To investigate how ghrelin activates AMPK, selective antagonist of GHS-R1a and inhibitor of Ca(2+)/Calmodulin-dependent protein kinase kinase (CaMKK) were added, respectively, before ghrelin pre-incubation, and we found that AMPK activation was prevented and the ERS inhibitory and anti-apoptotic actions of ghrelin were blocked. In conclusion, ghrelin could protect heart against ERS-induced injury and apoptosis, at least partially through a GHS-R1a/CaMKK/AMPK pathway.