Metabolic regulation of ghrelin O-acyl transferase (GOAT) expression in the mouse hypothalamus, pituitary, and stomach

Abstract

Ghrelin acts as an endocrine link connecting physiological processes regulating food intake, body composition, growth, and energy balance. Ghrelin is the only peptide known to undergo octanoylation. The enzyme mediating this process, ghrelin O-acyltransferase (GOAT), is expressed in the gastrointestinal tract (GI; primary source of circulating ghrelin) as well as other tissues. The present study demonstrates that stomach GOAT mRNA levels correlate with circulating acylated-ghrelin levels in fasted and diet-induced obese mice. In addition, GOAT was found to be expressed in both the pituitary and hypothalamus (two target tissues of ghrelin's actions), and regulated in response to metabolic status. Using primary pituitary cell cultures as a model system to study the regulation of GOAT expression, we found that acylated-ghrelin, but not desacyl-ghrelin, increased GOAT expression. In addition, growth-hormone-releasing hormone (GHRH) and leptin increased, while somatostatin (SST) decreased GOAT expression. The physiologic relevance of these later results is supported by the observation that pituitary GOAT expression in mice lacking GHRH, SST and leptin showed opposite changes to those observed after in vitro treatment with the corresponding peptides. Therefore, it seems plausible that these hormones directly contribute to the regulation of pituitary GOAT. Interestingly, in all the models studied, pituitary GOAT expression paralleled changes in the expression of a dominant spliced-variant of ghrelin (In2-ghrelin) and therefore this transcript may be a primary substrate for pituitary GOAT. Collectively, these observations support the notion that the GI tract is not the only source of acylated-ghrelin, but in fact locally produced des-acylated-ghrelin could be converted to acylated-ghrelin within target tissues by locally active GOAT, to mediate its tissue-specific effects.

Figure 1

Circulating total- and acylated-ghrelin and stomach, hypothalamus and pituitary GOAT mRNA levels in: mice fed ad libitum or fasted for 12-, 24-, and 48-h (Panel A and D), mice fed a low-fat diet (LFD control group) or high-fat diet (HFD obese group) (Panel B and E) and lean controls (/?) and obese (ob/ob) mice (Panel C and F). Values are shown as mean ± SEM (n=5–8 mice/group) and expressed as percent of the respective control group (set at 100%). Group means that do not share a common letter (a,b) significantly differ (p<0.05; Panel A and D). Asterisks (*, p<0.05, **, p<0.01) indicate values that significantly differ from lean control LFD or/? values. The tissue-specific regulation of ghrelin mRNA levels in these mice models as previously reported (Kineman et al., 2007; Luque et al., 2007a; Luque et al., 2007c) are shown below each graph of tissue-specific GOAT expression and are provided as the relative changes (=, ↑, ↓) as compared to controls.

Figure 2

Top panel: Effect of 24-h treatment with acylated-ghrelin, growth hormone releasing hormone (GHRH), somatostatin (SST), neuropeptide Y (NPY), leptin, insulin and IGF-I on GOAT mRNA levels in mouse primary pituitary cell cultures. Data represent the mean ± SEM (n=3–7 experiment performed in different cell preparations; 3–4 wells/treatment). Bottom panel: GOAT mRNA expression levels in pituitaries of growth hormone releasing hormone knockout (GHRH-KO), somatostatin knockout (SST-KO), neuropeptide Y knockout (NPY-KO) and ob/ob (leptin deficient) mice. Values are shown as mean ± SEM and expressed as percent of intact control mice (set at 100%; n=5–9 mice/group). Asterisks (*, p<0.05, **, p<0.01, ***, p<0.001) indicate values that significantly differ from respective controls (shown by the dotted line set at 100%).