Submitted on October 23, 2006
Accepted on November 9, 2006
Ghrelin has Novel Vascular Actions that Mimic Only PI 3-kinase-dependent Insulin-stimulated Production of Nitric Oxide (NO) but Not MAP-kinase-dependent Insulin-stimulated Secretion of ET-1 From Endothelial Cells
Micaela Iantorno1, Hui Chen1, Jeong-a Kim2, Manfredi Tesauro3, Davide Lauro4, Carmine Cardillo5, and Michael J. Quon6*
1 NCCAM, NIH, Bethesda, Maryland, United States
2 NCCAM, NIH, Bethesda, Maryland, United States; Bethesda, Maryland, United States
3 Internal Medicine, University of Rome, Tor Vergata, Rome, Italy
4 Internal Medicine, University of Rome, Tor Vergata, Italy
5 Internal Medicine, Catholic University, Rome, Italy
6 NCCAM, NIH, Bethesda,, Maryland, United States
* To whom correspondence should be addressed. E-mail: quonm{at}nih.gov.
Ghrelin is an orexigenic hormone secreted by the stomach. In patients with metabolic syndrome, intra-arterial ghrelin administration acutely improves their endothelial dysfunction. Therefore, we hypothesized that ghrelin activates endothelial nitric oxide synthase (eNOS) in vascular endothelium resulting in increased production of NO using signaling pathways shared in common with the insulin receptor. Similar to insulin, ghrelin acutely stimulated increased production of nitric oxide (NO) in bovine aortic endothelial cells (BAEC) in primary culture (assessed using NO-specific fluorescent dye DAF-2) in a time- and dose-dependent manner. Production of NO in response to ghrelin (100 nM, 10 min) in human aortic endothelial cells (HAEC) was blocked by pre-treatment of cells with L-NAME (NOS inhibitor), wortmannin (PI 3-kinase inhibitor), or (D-Lys3)-GHRP-6 (selective antagonist of ghrelin receptor GHSR-1a), as well as by knock-down of GHSR-1a using siRNA (but not by MEK inhibitor PD98059). Moreover, ghrelin stimulated increased phosphorylation of Akt (Ser473) and eNOS (Akt phosphorylation site Ser1179) that was inhibitable by knock-down of GHSR-1a using siRNA or by pre-treatment of cells with wortmannin but not with PD98059. Ghrelin also stimulated phosphorylation of MAP-kinase in BAEC. However, unlike insulin, ghrelin did not stimulate MAP-kinase-dependent secretion of the vasoconstrictor ET-1 from BAEC. We conclude that ghrelin has novel vascular actions to acutely stimulate production of NO in endothelium using a signaling pathway that involves GHSR-1a, PI 3-kinase, Akt, and eNOS. Our findings may be relevant to developing novel therapeutic strategies to treat diabetes and related diseases characterized by reciprocal relationships between endothelial dysfunction and insulin resistance.
