Glucose and Weight Control in Mice with a Designed Ghrelin O-Acyltransferase Inhibitor

Abstract

Ghrelin is a gastric peptide hormone that stimulates weight gain in vertebrates. The biological activities of ghrelin require octanoylation of the peptide on Ser3, an unusual post-translational modification that is catalyzed by the enzyme ghrelin O-acyltransferase (GOAT). Here, we describe the design, synthesis, and characterization of GO-CoA-Tat, a peptide-based bisubstrate analog that antagonizes GOAT. GO-CoA-Tat potently inhibits GOAT in vitro, in cultured cells, and in mice. Intraperitoneal administration of GO-CoA-Tat improves glucose tolerance and reduces weight gain in wild-type mice but not in ghrelin-deficient mice, supporting the concept that its beneficial metabolic effects are due specifically to GOAT inhibition. In addition to serving as a research tool for mapping ghrelin actions, GO-CoA-Tat may help pave the way for clinical targeting of GOAT in metabolic diseases.The persistent rise in the proportion of overweight individuals in Western society over the past 30 years has been associated with substantial excess morbidity and is widely recognized as a major public health concern. To address this problem, intensive efforts are underway to ‡ To whom correspondence should be addressed. pcole@jhmi.edu. * These authors contributed equally to this work. † These authors contributed equally to this work. clarify nutrient-hormone interactions contributing to weight gain. Starting with the isolation of leptin (1), a series of hormones acting centrally and peripherally to influence body mass have been discovered. Among these, the gastric peptide hormone acyl ghrelin has generated considerable interest as an important stimulus for weight gain (2-5) and modulator of glucose homeostasis (6-8). Various strategies in therapeutic development have been devised to antagonize acyl ghrelin (9,10), although none has yet emerged as clinically beneficial. Acyl ghrelin has an unusual Ser3 octanoylation; only acylated ghrelin can bind and activate the growth hormone secretagogue receptor (GHSR-1a). The cDNA for the enzyme responsible for this esterification, GOAT, has recently been cloned (11,12). GOAT has been suggested as a potential therapeutic target for modulating weight gain and glucose control, but thishas not yet been directly tested (9,13). An acyl ghrelin product analog Dap-ghrelin blocks GOAT activity in a microsomal assay (14).We designed bisubstrate analog GO-CoA-Tat based on the theory that if GOAT uses a ternary complex mechanism which templates octanoyl-CoA and ghrelin peptide, then linking the two substrates with a non-cleavable bridge could combine the binding energies of the individual ligands without the entropic loss associated with forming the ternary complex (Fig. 1A). A related strategy has been used for other peptide modifying enzymes including histone acetyltransferases (HAT) and protein kinases (15,16). Since we were uncertain about the ghrelin peptide length needed for recognition by GOAT, we selected amino acids 1-10 for coupling to octanoyl-CoA, to maximize inclusion of highly conserved...

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“…On the other hand, ghrelin injected peripherally inhibited the glucosestimulated secretion of ghrelin [28], an inhibitory effect that has also been described in humans after intravenous injection of ghrelin resulting in increased blood glucose levels [7]. In line with these findings, an inhibition of GOAT by a novel GOAT antagonist, GOCoATat increases glucose-stimulated insulin secretion resulting in lower glucose levels under conditions of a glucose tolerance test in mice [3] pointing to an anti-hyperglycemic effect of GOAT inhibition and therefore decreased acyl ghrelin signaling.…”

Does des-acyl ghrelin improve glycemic control in obese diabetic subjects by decreasing acylated ghrelin levels?

“…The administration of GO-CoA-Tat improved glucose tolerance and reduced weight gain in wild-type (WT) mice (24). Interestingly, GO-CoA-Tat did not suppress weight gain in ghrelin-deficient mice that lack both AG and DAG, suggesting that these animals are less responsive because of their DAG deficiency (24). Reports suggesting that high DAG levels might be linked to positive metabolic effects include a report by Cederberg et al (25).…”

“…Interestingly, the degree of AG suppression induced in mice by GO-CoA-Tat was less than that observed in the present study during DAG administration in humans. The administration of GO-CoA-Tat improved glucose tolerance and reduced weight gain in wild-type (WT) mice (24). Interestingly, GO-CoA-Tat did not suppress weight gain in ghrelin-deficient mice that lack both AG and DAG, suggesting that these animals are less responsive because of their DAG deficiency (24).…”

“…Recently, Barnett et al (24) have characterized GO-CoA-Tat, a peptide-based GOAT antagonist. Interestingly, the degree of AG suppression induced in mice by GO-CoA-Tat was less than that observed in the present study during DAG administration in humans.…”

Obesity vaccines

“…The GO-CoA-Tat was shown to decrease serum levels of acyl ghrelin and prevent body weight gain; additionally, also demonstrated to increase insulin secretion and improve glucose tolerance. 69 …”