The aim of this article is to show,after shortly focusing on ghrelin, ghrelin target cells and relative pathways,how a reduction of T cell-derived Ghrelin enhances proinflammatory cytokine expression,clarifying implications for age-associated increases in inflammation.
Short focus on Ghrelin and his targets
Ghrelin is a 28 amino acid peptide and hormone that is produced mainly byP/D1 cells of the human stomach and epsilon cells of the pancreas;
Ghrelin effect is anyway many-sided and more complex in comparison with intially beliefs. Target cells and organs of this peptide are many:
Coronary Endothelial cells and myocardiocytes,about which we also know the pathways:
Pituitary Cells (GH release)
VentroMedial Hypothalamus (food intake increase/fasting)
NPY Neurons in the Arcuate Nucleus (orexigenic effect and control of energy homeostasis)
Endothelial cells (vascular actions)
Ghrelin is a potent stimulator of growth hormone from the anterior pituitary gland.The ghrelin receptor is a G protein-coupled receptor, known as the growth hormone secretagogue receptor. Ghrelin binds to the GHSR1a splice-variant of this receptor which is present in high density in the hypothalamus, pituitary as well as vagal afferent cell bodies and vagal afferent endings throughout the gastro-intestinal tract.
It has been reported that ghrelin is also expressed in T cells and exerts prothymic and anti-inflammatory effects. Acylated-bioactive ghrelin is expressed in human T cells and preferentially segregates within the lipid raft domains upon TCR ligation. The RNA interference (RNAi)–mediated down-regulation of ghrelin in primary human T cells activates IkB, and increases Th1 cytokines and IL-17 secretion. Ghrelin expression declines with increasing age in spleen and T cells and exogenous ghrelin administration in old mice reduces proinflammatory cytokines. These findings demonstrate that ghrelin functions in an autocrine and paracrine capacity to regulate proinflammatory cytokine expression in human and murine T cells and may contribute in regulating “inflamm-aging.”
Ghrelin, originally thought to be stimulator of GH axis and food intake also exerts potent inhibitory effects on proinflammatory mediators via its action on T cells, monocytes,3 and endothelial cells.Recent evidence suggests that leptin, which inhibits food intake,can promote inflammation,and is also produced from T cells. Interestingly, leptin neutralization in T cells with monoclonal antibodies promotes regulatory T cell (Treg) proliferation and protects against experimental autoimmune encephalomyelitis (EAE).
Many previous reports suggest that apart from cytokines and chemokines, T cells may also express certain hormones. Emerging evidence supports this long-held view that ligands and receptors of neuroendocrine origin may also directly regulate immune function.T cell-derived ghrelin expression declines in the thymus with age. In addition, mice with ablation of ghrelin and ghrelin-receptor (GHSR) display accelerated thymic involution, whereas ghrelin supplementation promotes thymopoiesis in aged mice and protects against sepsis.
In this study, was tested the hypothesis that the bioactive ghrelin within T cells serves as a regulator of proinflammatory cytokines and investigated whether T cell–expressed ghrelin is functionally significant.
Ghrelin expression and function in human T cell
Ghrelin is a 28–amino acid acylated peptide that is predominantly produced from gut but is also known to be widely expressed in various cell and tissue subtypes including immune cells.Given that exogenous ghrelin regulates T-cell function,it has been sought to determine whether T cell–expressed ghrelin is functionally significant. Human T cells constitutively expressed acylated ghrelin with cytoplasmic as well as membrane localization (Figure 1A). The nonacylated or des-acyl form of ghrelin circulates in 3 to 5 times higher concentrations than the bioactive acylated ghrelin peptide.Presence of acylated ghrelin in T cells clearly suggests a potential functional role of this peptide. Next has been quantified the acylated ghrelin expression and observed that greater than 70% of the human T cells express active ghrelin protein (Figure 1B). The acylated ghrelin was found to localize in lipid raft domains in activated T cells (Figure 1C,D). Quantitation of ghrelin in subcellular compartment of T cells by ELISA revealed that compared with cytoplasmic fractions, significantly higher concentration (P< .05) of ghrelin was detected in lipid rafts (Figure 1E). Our present findings suggest that acylated ghrelin may efficiently interact with GHSR within lipid rafts upon TCR ligation.Acylated proteins have a proclivity to localize in sphingolipid-rich membrane microdomains,hence it is likely that unique acyl moiety in ghrelin may promote its trafficking to lipid rafts.
Next has been examined the functional relevance of ghrelin expression in T cells using RNA interference and observed that compared with control scrambled siRNA sequences, the ghrelin-specific siRNAs caused significant knockdown of ghrelin protein expression in primary human T cells (Figure 1F). The activated T cells where ghrelin expression was reduced displayed a significant increase in TNFα, IFNγ, and IL-2 (Figure 1G) with no changes in cellular proliferation (data not shown). Interestingly, ghrelin knockdown led to significant increase in IL-17 production from T cells (Figure 1G). These findings raise the possibility of a potential role of endogenous ghrelin in regulation of Th17 cell function.Given that NFkB is a key transcription factor important for regulation of inflammation,we next examined the impact of ghrelin knockdown on a regulator of NFkB signaling, IkB. Compared with the control-transfected T cells, ghrelin knockdown caused increased phosphorylation of IkB, whereas ghrelin supplementation inhibited pIkB. These results suggest ghrelin expression in T cells may play an important role in regulating the activation of NFkB. These data suggest that T cell–derived ghrelin may play a key role in regulating the basal and activation- induced proinflammatory cytokine secretion.
Age-related reduction in ghrelin expression and inflammation
Although circulating ghrelin levels are not significantly reduced in 10-month-old mice, caloric restriction—a prolongevity intervention—enhances ghrelin levels in middle-aged animals. Interestingly, intrathymic ghrelin expression declines with age and is associated with age-related thymic involution.Thus the reduction of T cell–derived ghrelin in a tissue microenvironment may play a role in regulating the inflammatory cytokines. We observed that quite similar to thymus,ghrelin and GHSR mRNA expression increases at 4 months and then progressively declines with age (Figure 2A-C).
Given that RNAi-mediated reduction of ghrelin in T cells resulted in an increase in proinflammatory cytokines, we hypothesized that ghrelin infusion in aging may reduce the proinflammatory mediators. Ghrelin was infused in the old mice using subcutaneously implanted osmotic minipumps for a period of 2 weeks. Compared with sham-infused mice, ghrelin led to a significant reduction in IL-6, TNF-α, IL-1β, KC, GMCSF, IL-12, and IL-17 in the serum of aging mice.
Taken together, our results suggest that ghrelin expression in T cells plays an important role in inhibiting proinflammatory cytokines. These observations raise the possibility that ghrelin expression in tissue microenvironments may regulate local inflammatory state. Given that ghrelin inhibits inflammation, increases food intake, and promotes thymic function,these results suggest that synthetic ghrelin agonists may have potential therapeutic value in age-related inflammatory diseases.