Unacylated ghrelin: a gut-limb connection.

Ghrelin is a gastric hormone that acts on pituitary and hypothalamus to stimulate growth hormone release regulating adiposity and appetite. Its activity is thought to be dependent on acylation. Ghrelin is O-octanoylated at Ser3 (acylated ghrelin [AG]) by ghrelin O-acyltransferase (GOAT), a posttranslational modification required for mediating its action through growth hormone secretagogue receptor 1a (GHS-R1a). However, as only 5–20% of circulating ghrelin is acylated, unacylated ghrelin (UnAG, also called des-acyl ghrelin) remains the major circulating form (1). UnAG has long been considered as a degradation product of ghrelin. Hence, UnAG has not received much attention regarding its biological significance and possible therapeutic effects. In recent years however, there has been increasing interest in deciphering the biological actions of UnAG in an attempt to use it as a therapeutic tool (2). A few years ago in a very well-performed study, Delhanty et al. (2) showed rapid effects of UnAG on genome-wide expression patterns in the adipose tissues, muscles, and livers of GHSR1a knockout mice. These studies established the notion that UnAG has GHS-R1a– independent actions (3). Over the past few years, various groups have demonstrated effects of UnAG on ghrelin antagonism, insulin sensitivity, metabolism, muscle regeneration, and b-cell protection (2,4,5). In this issue of Diabetes, Togliatto et al. (6) demonstrated the novel actions of UnAG. Using glucose-intolerant ob/ob mice, a model of peripheral arterial disease, they showed that UnAG has a protective effect on muscle and endothelial cells subjected to ischemia. Such actions were mediated through normalization of sirtuin 1 (SIRT1) and superoxide dismutase 2 (SOD-2) expressions. They further demonstrated that the process also causes SIRT1-mediated p53 and histone 3 lysine 56 deacetylation and reduced vascular cell adhesion molecule 1 (VCAM-1) production and recruitment of inflammatory cells. Finally, through a series of experiments, they showed that all such effects of UnAG are orchestrated through micro-RNA-126 (miR-126). Interestingly, treatment with UnAG did not change large vessel perfusion and the effects were independent of neovascularization. The protective effects of UnAG were, however, results of the expansion of resident stem cells (i.e., satellite cells and antioxidant effects on endothelial cells). This group earlier showed that UnAG mediated nitric oxide–dependent mobilization of endothelial progenitor cells in the bone marrow (7). They also previously demonstrated that UnAG promotes skeletal muscle regeneration via SOD-2–mediated miR-221/ 222 expression (8). Glucose-induced oxidative stress is a major initiating factor of tissue damage in the context of chronic diabetes complications (9). In this study, UnAG-induced activation of endothelial oxidative defense appears to play a major role in the prevention of cellular damage (6). Beneficial effects of UnAG treatment also were shown to be mediated through miR-126 upregulation. Such upregulation, by reducing reactive oxygen species and augmenting SIRT1 activity, ultimately prevented cellular senescence. These results are also in keeping with previous studies by other investigators who have shown that UnAG protects microvascular endothelial cells from apoptosis through SIRT1 (10). Furthermore, in the endothelial cells and in other organs affected by chronic diabetes complications, accelerated senescence and SIRT1 downregulation, at least in part mediated by other miRs, have been demonstrated (11,12). miR alterations play important regulatory roles in several, if not all, physiological and pathological processes (13). Here, Togliatto et al. (6) showed that alteration of miR-126 plays a key role in the regulation SIRT1 as well as SOD-2 and VCAM-1. However, as one miR may target multiple transcripts and one transcript may be regulated by multiple miRs, identification of one miR for the treatment of a systemic disease is a challenge. Nevertheless, such approaches provide opportunities to develop RNAbased therapeutics to target specific disease-causing pathway(s) (Fig. 1).