Synthetic Triterpenoid Inhibition of Human Ghrelin O-Acyltransferase: The Involvement of a Functionally Required Cysteine Provides Mechanistic Insight into Ghrelin Acylation

Abstract

The peptide hormone ghrelin plays a key role in regulating hunger and energy balance within the body. Ghrelin signaling presents a promising and unexploited target for development of small-molecule therapeutics to treat obesity, diabetes, and other health conditions. Inhibition of ghrelin O-acyltransferase (GOAT), which catalyzes an essential octanoylation step in ghrelin maturation, offers a potential avenue for controlling ghrelin signaling. Through screening a small molecule library, we have identified a class of synthetic triterpenoids that efficiently inhibit ghrelin acylation by the human isoform of GOAT (hGOAT). These compounds function as covalent reversible inhibitors of hGOAT, providing the first evidence for involvement of a nucleophilic cysteine residue in substrate acylation by a MBOAT family acyltransferase. Surprisingly, the mouse form of GOAT does not exhibit susceptibility to cysteine modifying electrophiles revealing an important distinction in the activity and behavior between these closely related GOAT isoforms. This study establishes these compounds as potent small molecule inhibitors of ghrelin acylation and provides a foundation for the development of novel hGOAT inhibitors as therapeutics targeting diabetes and obesity.

Obesity, energy balance and spermatogenesis

“…In recent years, there is also an increased interest in developing and optimising new compounds that control the levels of hormones dysregulated during obesity. For instance, inhibition of ghrelin O-acyltransferase (GOAT), which catalyses an essential octanoylation step in ghrelin maturation, has been described as a potential avenue for controlling ghrelin signalling and new small-molecule GOAT inhibitors have already been synthesised (McGovern et al 2016, McGovern-Gooch et al 2017). It will be crucial to test how those new compounds may affect the HPT axis and also their direct effect on testicular cells.…”

Decreased Plasma Octanoylated Ghrelin Levels in Mice by Oleanolic Acid

“…In this study, we did not examine the effect of oleanolic acid on the enzymatic activities of GOAT, furin, PC1/3, and PC2. Oleanolic acid does not inhibit GOAT activity 23 , but its effect on other PC activities have not been evaluated. Further studies should investigate the effect of oleanolic acid on PC activities to elucidate the mechanism underlying the inhibition of octanoylated ghrelin production.…”

The octanoylated energy regulating hormone ghrelin: An expanded view of ghrelin’s biological interactions and avenues for controlling ghrelin signaling

“…However, the predicted topology of GOAT places these two residues on opposite sides of the ER membrane, suggesting that both cannot be directly involved in catalysis (Taylor et al, 2013). A recent study provided evidence that GOAT contains a functionally essential cysteine residue, as N-ethylmaleimide treatment inhibits the human isoform of GOAT (McGovern-Gooch et al, 2017). Our understanding of the structural and mechanistic basis for GOAT-catalyzed ghrelin octanoylation has expanded rapidly in the last decade, but many of the aspects of GOAT enzymatic function remain to be determined.…”

“…In the most recent report of small molecule GOAT inhibitors, a class of molecules derived from synthetic triterpenoids was found to inhibit the human GOAT (hGOAT) ortholog (McGovern-Gooch et al, 2017). In screening a NIH-provided library of structurally diverse small molecules, two derivatives of 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid (CDDO) were identified as hGOAT inhibitors with IC 50 values in the low micromolar range as assessed by a microsomal hGOAT activity assay ( Figure 5) (Darling et al, 2013).…”