Des-Acyl Ghrelin Has Specific Binding Sites and Different Metabolic Effects from Ghrelin in Cardiomyocytes

The current study aimed to compare the effects of the peptide hormone ghrelin and des-G, its unacylated isoform, on glucose and fatty acid uptake and to identify des-G-specific binding sites in cardiomyocytes. In the murine HL-1 adult cardiomyocyte line, ghrelin and des-G had opposing metabolic effects: des-G increased medium-chain fatty acid uptake (BODIPY fluorescence intensity), whereas neither ghrelin alone nor in combination with des-G did so. Ghrelin inhibited the increase in glucose uptake normally induced by insulin (rate of 2-[3H]deoxy-d-glucose incorporation), but des-G did not; des-G was also able to partially reverse the inhibitory effect of ghrelin. In HL-1 cells and primary cultures of neonatal rat cardiomyocytes, des-G but not ghrelin increased insulin-induced translocation of glucose transporter-4 from nuclear to cytoplasmic compartments (immunohistochemistry and quantitative confocal analysis). AKT was phosphorylated by insulin but not affected by ghrelin or des-G, whereas neither AMP-activated protein kinase nor phosphatase and tensin homolog deleted from chromosome 10 was phosphorylated by any treatments. HL-1 and primary-cultured mouse and rat cardiomyocytes each possessed two independent specific binding sites for des-G not recognized by ghrelin (radioreceptor assays). Neither ghrelin nor des-G affected viability (dimethylthiazol diphenyltetrazolium bromide assays), whereas both isoforms were equally protective against apoptosis. Therefore, in cardiomyocytes, des-G binds to specific receptors and has effects on glucose and medium-chain fatty acid uptake that are distinct from those of ghrelin. Real-time PCR indicated that expression levels of ghrelin O-acyltransferase RNA were comparable between HL-1 cells, human myocardial tissue, and human and murine stomach tissue, indicating the possibility of des-G conversion to ghrelin within our model. Des-acyl ghrelin has specific binding sites that are distinct from those of ghrelin, and these two ghrelin isoforms have opposing metabolic effects on cardiomyocytes.