Growth hormone secretagogue receptor GHSR is a member of G

Growth hormone secretagogue receptor (GHSR) is a member of G-protein-coupled receptor family [3]. GHSR is predominantly expressed in human pituitary, rhypothalamus and hippocampus [4]. GHSR is also expressed in peripheral tissues including lungs, liver, kidney, and adrenal gland. In cardiovascular system, it is found in vascular smooth muscle cells (SMCs) and endothelial cells [5]. Ghrelin, the endogenous ligand of GHSR, is a peptide hormone with 28 Lomustine mg mainly secreted by the gastrointestinal system [6]. Two subtypes of GHSR, GHSR-1a and GHSR-1b, have been identified. GHSR-1a is considered as the active subtype. Whereas GHSR-1b binds neither ghrelin nor synthetic ligands GHSs [7]. GHSR plays crucial roles in promoting growth hormone secretion, regulation of food intake, and other physiological processes such as insulin secretion, glucose and lipid metabolism [8]. In addition, GHSR has been shown to exert beneficial effects on cardiovascular system by improving heart function, inhibiting ventricular remodeling, and attenuating ischemia-reperfusion (I/R) injury [9], [10], [11]. More studies revealed that GHSR is involved in the cellular processes such as proliferation, migration and apoptosis [12], [13], [14], [15]. In addition, its ligand ghrelin inhibits the expression of proinflammatory cytokines such as IL-1β, IL-6, IL-8, TNF- α and MCP-1 [16], [17]. These observations imply that GHSR may play an important role in the vascular homeostasis. However, the function and mechanism of GHSR in restenosis after vascular injury remain to be identified.
Methods
Results
Discussion Previous studies suggested that GHSR is expressed in a wide range of tissues including vasculature. GHSR is detected in aorta, coronary, pulmonary, arcuate arteries and saphenous vein [24], [25]. In addition, GHSR is significantly up-regulated in both atherosclerotic coronary arteries and saphenous vein grafts with advanced intimal thickening, compared with normal vessels [24]. However, in low-density lipoprotein receptor-null (LDLR−/−) mice, GHSR deficiency did not affect high-fat, high-cholesterol western-type diet induced atherosclerosis [26]. Compared with GHSR+/+/LDLR−/− mice, T cells and macrophages are increased, while SMCs in atherosclerotic plaques were less in GHSR−/−/LDLR−/− mice [27]. Here, we showed that neointimal formation was significantly suppressed in GHSR−/− mice than WT mice after vascular injury. Our finding indicated an important role of GHSR in restenosis. Ghrelin inhibits TNF-α-induced NF-κB activation and monocyte adhesion, VCAM-1 expression [28], [29]. However, several inconsistent reports showed that ghrelin treatment increased the expression of ICAM-1, and promoted NF-κB activation [30], [31]. The recent study also points out that these is a significantly reduced pro-inflammatory cytokines in LPS-stimulated macrophages in vitro from GHSR−/− mice than WT mice [32]. Ghrelin inhibited the proliferation of SMCs probably mediated by GHSR [33]. Our study further clearly demonstrated that the effect of GHSR on restenosis development. In vivo, neointimal. Ghrelin has been reported to promote cells proliferation, migration and to inhibit cell apoptosis, and the underlying mechanisms may be involved in GHSR-mediated activation of MAPK and PI3K/Akt signaling pathways. Meanwhile the effect is significantly extenuated by pretreatment of cells with a GHSR inhibitor ([D-Lys3]-GHRH-6), PI3K inhibitor (LY294002) or ERK1/2 inhibitor (PD98059) [13], [15], [34]. In addition to the signaling pathways described above, ghrelin also regulates the proliferation and differentiation in a wide variety of cell types ranging from 3T3-L1 adipocytes, human embryonic stem cells to HepG2 cells [35], [36], [37]. In the present study, we found that the knockdown GHSR suppressed Akt and ERK1/2 phosphorylation and the activation of signaling pathway in cultured SMCs.
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