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Ghrelin O-Acyl Transferase: Bridging Ghrelin and Energy Homeostasis

Ghrelin O-Acyl Transferase: Bridging Ghrelin and Energy Homeostasis Hindawi Publishing Corporation International Journal of Peptides Volume 2011, Article ID 217957, 5 pages doi:10.1155/2011/217957 Review Article Ghrelin O-Acyl Transferase: Bridging Ghrelin and Energy Homeostasis Andrew Shlimun and Suraj Unniappan Laboratory of Integrative Neuroendocrinology, Department of Biology, York University, Toronto, ON, Canada M3J 1P3 Correspondence should be addressed to Suraj Unniappan, suraju@yorku.ca Received 3 May 2011; Accepted 20 July 2011 Academic Editor: Lloyd D. Fricker Copyright © 2011 A. Shlimun and S. Unniappan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Ghrelin O-acyl transferase (GOAT) is a recently identified enzyme responsible for the unique n-acyl modification of ghrelin, a multifunctional metabolic hormone. GOAT structure and activity appears to be conserved from fish to man. Since the acyl modification is critical for most of the biological actions of ghrelin, especially metabolic functions, GOAT emerged as a very important molecule of interest. The research on GOAT is on the rise, and several important results reiterating its significance have been reported. Notable among these discoveries are the identification of GOAT tissue expression patterns, effects on insulin secretion, blood glucose levels, feeding, body weight, and metabolism. Several attempts have been made to design and test synthetic compounds that can modulate endogenous GOAT, which could turn beneficial in favorably regulating whole body energy homeostasis. This paper will focus to provide an update on recent advances in GOAT research and its broader implications in the regulation of energy balance. 1. Introduction 2. Discovery and Characterization of GOAT Ghrelin is a gut hormone discovered in 1999 by Dr. Kojima Yang et al. [5] first identified 16 MBOAT protein sequences and colleagues in the laboratory of Dr. Kangawa [1]. It is from the mouse genome. They found the 11 putative catalytic the first known endogenous ligand of the growth hormone regions in these BOATs highly conserved among the 16 secretagogue receptor 1a (GHS-R1a), which is now known as sequences, and it all contained the asparagine and histidine the ghrelin receptor [2]. Since its discovery, ghrelin has been residues thought to be involved in the catalysis. Three differ- identified and functions characterized in a large number of ent murine endocrine cell lines (AtT-20, INS-1, and MIN-6) animals [3, 4]. A unique aspect of ghrelin, the only known were transfected with the preproghrelin sequence and were orexigenic hormone from the gut, is the presence of a post- found to produce acylated ghrelin when cotransfected with translational acyl modification, mainly the attachment of the GOAT sequence. Mutation of the third serine to alanine the octanoyl group to the third serine residue of the N- prevented the acylation of ghrelin by GOAT, indicating that terminal region of the peptide [3, 4]. Several variations of the third serine is indeed the amino acid to which the moiety this modification were found [3, 4], while the mechanisms is attached. It was also determined that both asparagine that result in this modification, which is critical for many in position 307 and histidine in position 338 of mouse biological actions of ghrelin remained elusive. A decade of GOAT are essential for the catalytic activity of this enzyme waiting ended in 2008 when two research teams indepen- (Figure 1). These results provide the first published [5] dently identified the membrane bound O-acyl transferase evidence for GOAT. (MBOAT) responsible for ghrelin acylation and named it the Meanwhile, Gutierrez et al. [6] performed gene-silencing ghrelin o-acyl Transferase (GOAT) [5, 6]. experiments to determine whether a member of the MBOAT 2 International Journal of Peptides ∗ ∗ Ghrelin gene—6 exons and 5 introns Human Pig Transcription Cow Dog Preproghrelin mRNA Rat Mouse Translation Chicken Precursor Zebrafish Signal peptide Ghrelin C-terminal peptide Figure 1: Alignment of amino acid sequences in the highly con- Prohormone convertases, served catalytic regions of GOAT. The online sequence analysis tool carboxypeptidase E, Posttranslational processing available at the following link was used to generate the figure: GOAT, and and modifications http://www.protocol-online.org/tools/sms2/color align prop.html. amidating enzymes The proposed catalytic residues (asparagine and histidine) of (exclusively in fish) GOAT are marked by asterisks. Partial sequences of GOAT Mature ghrelin were obtained from full-length amino acid sequences obtained (acylated form) from the GenBank. Accession numbers of sequences are human (NP 001094386.1), pig (NP 001177352.1), cow (NP 001179186.1), dog (NP 001188260.1), rat (NP 001100787.2), mouse (NP 001119786.1), chicken (NP 001186218.1), and zebrafish (NP Figure 2: Scheme showing formation of mature, acylated ghrelin 001116416.1). from ghrelin gene. Ghrelin gene in humans is comprised of six exons and five introns. Upon transcription, the preproghrelin mRNA is formed, which translates to produce the precursor peptide that contains a signal peptide, mature peptide (ghrelin), and the family could mediate the acylation of ghrelin in human C-terminal peptide. After posttranslational cleavage and processing medullary thyroid carcinoma (TT) cells. They discovered by various enzymes including the prohormone convertases, car- that the silencing of GOAT (MBOAT4), but not other boxypeptidase E, amidating enzymes, and GOAT, the amidated, MBOAT sequences, results in the attenuation of ghrelin acylated mature ghrelin is formed. Amidated ghrelin is currently octanoylation [6]. The GOAT gene has been localized to reported only in fish. the 8p12 region of the human chromosome 8. Further experiments by Gutierrez et al. [6] determined that only cotransfection of preproghrelin with GOAT, not with other MBOATs, yielded third serine octanoylated ghrelin in HEK- 2.1. Tissue Distribution and Regulation of GOAT Expression. 293 cells. The supplementation of the HEK-293 cell medium The discoverers of GOAT found highest levels of GOAT with lipids from acetate to tetradecanoic acid resulted in mRNA expression in the stomach and intestine followed the GOAT enabled modification of ghrelin with fatty acids by the testis of mice [5] and the pancreas and stomach up to tetradecanoic acid. A very high sequence similarity of humans [6]. A comprehensive reverse transcription- was found among GOAT proteins from humans to zebrafish polymerase chain reaction (RT-PCR) tissue distribution [5, 6]. Interestingly, zebrafish, rat, and mouse GOAT were studybySakataet al. [9] was in agreement with this initial able to acylate human ghrelin [6]. Octanoylated ghrelin finding. GOAT mRNA expression was detected primarily was undetectable in the blood of GOAT knockout mice, in the mouse stomach and intestine, while other MBOAT providing strong confirmatory evidence for the critical role family of enzymes were found in several other tissues in of GOAT in acylating ghrelin. Together, the pioneering addition to the gastrointestinal tract. This result was fur- research by Yang et al. [5]and Gutierrez et al. [6]led to the ther confirmed by double label in situ hybridization and discovery of GOAT, the sole mediator of the unique acylation immunohistochemistry that found GOAT mRNA expression found in ghrelin (Figure 2). in ghrelin immunopositive cells and neighboring cells within More recent in vitro studies by Ohgusu and colleagues [7] the gastric mucosa. Together, these results indicate that the found that recombinant GOAT can acylate a short peptide, primary source of GOAT is gastrointestinal cells that are pos- the N-terminal region of ghrelin consisting of just four itive for ghrelin. Stengel et al. [10]for thefirst timereported amino acids (GSSF). This short peptide is considered as the colocalization of ghrelin and GOAT immunoreactivity in bioactive core of ghrelin, and it provides further support to the stomach mucosa of rats and mice. More GOAT positive the notion that this fragment could elicit many physiological ghrelin cells were found in mice (∼95%) stomach compared processes regulated by the full-length mature ghrelin. GOAT to rats (∼56%). The nonghrelin positive GOAT cells in rat also has a preference for n-hexanoyl coA compared to n- gastric mucosa were positive for histidine decarboxylase, octanoyl CoA. Yang et al. [8] proposed that preproghrelin which helps in vitamin B6 processing. Western blot analyses is octanoylated in the membranes of endoplasmic reticulum found two bands, one at ∼50 kDa, the expected size of GOAT and the acylation occurs after the signal peptide is removed protein, and a second one at ∼100 kDa, which was consid- from the preproghrelin. These results provide further sup- ered as a dimer. Western blots of plasma detected GOAT in port for the four amino acid bioactive core of ghrelin, circulation, and blood levels of GOAT were elevated in mice intracellular location of ghrelin modification, and multiple and rats fasted for 24 hours. Intraperitoneal injections of third serine modifications of ghrelin. lipopolysaccharide (LPS) resulted in a significant reduction International Journal of Peptides 3 in plasma acylated ghrelin and caused a corresponding de- stomach GOAT mRNA in mice [17]. No differences in gastric crease in plasma GOAT levels, while a small increase in gut GOAT mRNA expression were found in ad libitum fed leptin GOAT was detected [11]. This suggests that ghrelin-GOAT deficient ob/ob mice compared to wild-type controls [17]. system may be involved in the infection-induced decrease Meanwhile, Gahete et al. [15] found that plasma acylated in food intake. While species-specific variations exist in the ghrelin levels were increased in fasted mice, and this increase expression of GOAT in the gut, it is now clear that stomach coincided with an elevation in the gut, hypothalamic, and and intestine are major sources of GOAT. pituitary GOAT mRNA expression. While accepting that In agreement with the original studies, An et al. [12] discrepancies exist between physiological status of animals found abundant expression of GOAT mRNA and protein in and species used, it is interesting that GOAT expression in the whole pancreas, isolated islets and INS-1 cells. Immuno- rodents in general is modulated by the metabolic status. localization studies found that GOAT-positive cells are The importance of GOAT on metabolism was further mainly present in the periphery of rat islets and no GOAT- elucidated using GOAT knockout mice, which lack acyl insulin colocalization was found, especially in the beta cell ghrelin in their circulation. These mice have normal body rich core of the islets. Gonzalez ´ et al. [13], using RT-PCR weight compared to wild-type control mice, but they gained analysis, reported GOAT mRNA expression in the stomach, significantly less weight and had reduced fat mass when fed pancreas, hypothalamus, ovary, serum, placenta, muscle, on a high-fat diet for eight weeks [17]. The enrichment of heart, and adrenal glands of rats. There were no differences diet with fatty acids resulted in a leaner body mass for the in the expression of GOAT mRNA in the stomach mucosa GOAT knockout mice, and this reduction was attributed of male rats at postnatal days 10, 25, and 60 [13]. GOAT to increased energy expenditure not alterations in feed expression was also detected in murine cartilage explants, intake. No differences in glucose homeostasis were found human primary chondrocytes, and in both human and between the knockout and wild-type mice. Transgenic mice mouse chondrocyte cells lines [14]. The expression of GOAT overexpressing human GOAT in the liver, when fed with mRNA in murine chondrogenic cell line ATDC-5 showed a medium chain fatty acid containing diet, produced more gradual increase as the cells differentiated, with the lowest acylated ghrelin and had a transient increase in food intake. levels being detected in the early stages of differentiation. LPS Overall, GOAT links the lipid intake into whole body energy significantly reduced GOAT mRNA expression in cultured balance. Zhao et al. [18] determined the body weight and cartilage cells [14]. GOAT mRNA was detected in the blood glucose levels in the GOAT knockout mice. While hypothalamus and pituitary of mice [15]. It was also found ad libitum feeding on regular or high-fat diets showed that GOAT mRNA expression in the cultured primary no difference in the metabolic phenotype, calorie-restricted pituitary cells of mice was increased by acyl ghrelin, leptin, GOAT knockout mice lost more weight and had low blood and growth hormone releasing hormone, while somatostatin glucose levels compared to wild-type controls. The adverse decreased its expression. No effects were found for neuropep- effects of GOAT absence on glucose levels were reversed upon tide Y and des-acyl ghrelin on GOAT mRNA expression [15]. exogenous administration of acylated ghrelin or growth GOAT mRNA expression was found increasing with age in hormone administration. Overall, the outcome of this the stomach of rats, while it was found inhibited by a decrease genetic approach highlights the significance of GOAT in in testosterone [16]. Collectively, from all studies available acylating ghrelin and regulating the role of ghrelin on energy to date, it appears that the gastroenteropancreatic tissues, balance. It is important to consider the possibility of GOAT which play a major role in postprandial satiety and glucose modulating peptides other than ghrelin. Therefore, whatever homeostasis, are the most abundant sources of GOAT in effects resulting from GOAT alterations are possibly not mammals. It is also important to note that several metabolic exclusively due to the absence of acylated ghrelin, as other hormones modulate GOAT expression. metabolically relevant peptides could be affected due to theabsence of GOAT.A newreview[19]is now available 2.2. GOAT and Energy Balance. To date, two lines of evi- that compares the effects of ghrelin and GOAT knock-out mice. The readers are encouraged to review it for a further dences are available indicating the involvement of GOAT in regulating energy homeostasis. The first set of data is discussion on the above aspect. originated from GOAT mRNA expression studies during var- Modulating GOAT activity could potentially affect bio- ious metabolic states. The second type of results arises from logical actions of ghrelin and this aspect could be targeted to studies directly testing the effects of GOAT by perturbing design pharmacological approaches to regulate energy intake the endogenous GOAT. Chronic food deprivation for 21 days and body weight. A successful attempt in this direction was resulted in a significant increase in ghrelin mRNA expression made when Yang et al. [8] discovered that the octanoylated in the gastric mucosa of rats and a corresponding increase short fragment of ghrelin comprised of N-terminal 5 amino in GOAT mRNA in the same tissue [13]. Intraperitoneal acids (GSSFL) could inhibit GOAT activity. This GOAT injections of leptin, a satiety signal that relays the status inhibitory activity was further enhanced when an amidated of long-term energy reserves to the brain, also caused a short peptide was used (GSSFL-NH ). It was also reported significant increase in GOAT mRNA expression in 48 fasted that a modified ghrelin fragment with the third serine rats but not in ad libitum fed rats. These results indicate replaced with an octanoylated diaminopropionic acid could that malnutrition and leptin are two regulatory factors that enhance the GOAT inhibitory activity to 45-fold. Such determine GOAT mRNA expression. In contrast, fasting inhibitors could therefore be used to attenuate active ghrelin for 12, 24, or 36 hours caused a significant reduction in and its effects on metabolism. 4 International Journal of Peptides In this regard, other GOAT-specific antagonists have also could emerge as potential treatment or preventative strategy been designed and functionally characterized [20]. GO-CoA- for diabetes. Similarly, tweaking GOAT biology may reap Tat, a bisubstrate analogue that acts as a GOAT inhibitor, rewards in the march towards curbing overweight and obe- was found to decrease acylated ghrelin production from cells sity. While several interesting results have been obtained to stably transfected with preproghrelin and GOAT. Similarly, date, GOAT research is still in its infancy, and future studies GO-CoA-Tat administration significantly reduced acylated hold the key to further our understanding of this protein and ghrelin production in vivo in mice fed with medium chain enhancing its utilization for the benefit of humans. fatty acid rich food. In addition, daily administration of GO-CoA-Tat prevented body weight gain in mice fed on a Acknowledgments high-fat diet. GOAT inhibition in pancreatic beta cells using the GO-CoA-Tat caused a significant increase in glucose- S. Unniappan is a Canadian Institutes of Health Research stimulated insulin secretion, suggesting an inhibitory role for (CIHR) New Investigator and a recipient of the Early Inves- ghrelin in this process. Similar increases in glucose-induced tigator Award from the Ontario Ministry of Research and insulin secretion were also found in mice treated with the Innovation (MRI). The research in the laboratory of S. GOAT antagonist. The expression of mRNA encoding UCP2, Unniappan is funded by operating grants from the Natural a potent inhibitor of insulin secretion, was suppressed 20- Sciences and Engineering Research Council (NSERC) and fold in GO-CoA-Tat-treated islets. In agreement with these the CIHR and equipment/infrastructure grants from the islet effects, An et al. [21]reportedthatGOATmRNA expres- NSERC, Canada Foundation for Innovation and the Ontario sion in islet beta-like INS-1 cells is inhibited by insulin. This MRI. effect of insulin was blocked when cells were treated with wortmannin that inhibits PI3 kinase/Akt pathway. From this References result, it is clear that the inhibitory effects of insulin on GOAT are mediated via the insulin receptor signaling pathways. [1] M. Kojima, H. Hosoda, Y. Date, M. Nakazato, H. Matsuo, and Insulin was also found to reduce GOAT promoter activity in K. Kangawa, “Ghrelin is a growth-hormone-releasing acylated vitro. In addition, it was also determined that the mammalian peptide from stomach,” Nature, vol. 402, no. 6762, pp. 656– target of rapamycin (mTOR) regulates transcription and 660, 1999. translation of GOAT. Inhibition of mTOR signaling using [2] A.P.Davenport, T.I.Bonner, S. M. Foord etal.,“International rapamycin significantly increased GOAT mRNA in INS-1 union of pharmacology. LVI. Ghrelin receptor nomenclature, cells in a concentration and time dependent manner. Sim- distribution, and function,” Pharmacological Reviews,vol.57, no. 4, pp. 541–546, 2005. ilarly, intraperitoneal injection of rapamycin that reduced [3] M. Kojima and K. Kangawa, “Ghrelin: from gene to physiolog- mTOR increased GOAT mRNA expression in the pancreas ical function,” Results and Problems in Cell Differentiation,vol. of C57BL/6 mice. In contrast, leucine, an activator of mTOR, 50, pp. 185–205, 2010. attenuated GOAT mRNA expression in mice pancreas. Over- [4] H. Kaiya, M. Miyazato, K. Kangawa, R. E. Peter, and S. Unniap- expressing tuberculosis sclerosis complex 1 and 2 (TSC1 and pan, “Ghrelin: a multifunctional hormone in non-mammalian TSC2), the negative regulators of mTOR action, increased vertebrates,” Comparative Biochemistry and Physiology,vol. GOAT activity in INS-1 cells. Together, these results provide 149, no. 2, pp. 109–128, 2008. further evidence for an important role for the endogenous [5] J. Yang, M. S. Brown, G. Liang, N. V. Grishin, and J. L. Gold- GOAT on glucose homeostasis and insulin secretion, and stein, “Identification of the acyltransferase that octanoylates intracellular mechanisms that mediate these effects. ghrelin, an appetite-stimulating peptide hormone,” Cell,vol. 132, no. 3, pp. 387–396, 2008. [6] J. A.Gutierrez, P.J. Solenberg,D.R.Perkins et al., “Ghrelin octanoylation mediated by an orphan lipid transferase,” 3. Perspectives Proceedings of the National Academy of Sciences of the United States of America, vol. 105, no. 17, pp. 6320–6325, 2008. Currently, GOAT is the only known enzyme that is capable in acylating ghrelin. It is now clear that GOAT is a critical [7] H. Ohgusu, K. Shirouzu, Y. Nakamura et al., “Ghrelin O- acyltransferase (GOAT) has a preference for n-hexanoyl-CoA component in making bioactive ghrelin, and thus mediating over n-octanoyl-CoA as an acyl donor,” Biochemical and the physiological functions of ghrelin. Within a short period Biophysical Research Communications, vol. 386, no. 1, pp. 153– since its discovery, enough strong evidence have been 158, 2009. collected to support a clear and very important role for [8] J.Yang, T. J. Zhao, J. L.Goldstein,and M. S. Brown, “Inhi- GOAT in metabolic physiology. GOAT expression is tissue bition of ghrelin O-acyltransferase (GOAT) by octanoylated specific, is modulated by a number of factors, and appears to pentapeptides,” Proceedings of the National Academy of Sciences vary among organisms. It is especially interesting that GOAT of the United States of America, vol. 105, no. 31, pp. 10750– is regulated by the metabolic status of animals. Whether 10755, 2008. GOAT expression is altered in metabolic pathophysiology in [9] I. Sakata, J. Yang, C. E. Lee et al., “Colocalization of ghrelin O- humans remain unknown. Another fascinating discovery is acyltransferase and ghrelin in gastric mucosal cells,” American the development of GOAT inhibitors and its success in pre- Journal of Physiology, vol. 297, no. 1, pp. E134–E141, 2009. venting weight gain, stimulating insulin secretion and reduc- [10] A. Stengel, M. Goebel, L. Wang, Y. Tache, ´ G. Sachs, and N. ing blood glucose levels. If these findings are translatable, W. G. Lambrecht, “Differential distribution of ghrelin-O- inhibition of endogenous GOAT using specific inhibitors acyltransferase (GOAT) immunoreactive cells in the mouse International Journal of Peptides 5 and rat gastric oxyntic mucosa,” Biochemical and Biophysical Research Communications, vol. 392, no. 1, pp. 67–71, 2010. [11] A. Stengel, M. Goebel, L. Wang, J. R. Reeve, Y. Tache, ´ and N. W. G. Lambrecht, “Lipopolysaccharide differentially decreases plasma acyl and desacyl ghrelin levels in rats: potential role of the circulating ghrelin-acylating enzyme GOAT,” Peptides,vol. 31, no. 9, pp. 1689–1696, 2010. [12] W. An, Y. Li, G. Xu et al., “Modulation of ghrelin o-acyltrans- ferase expression in pancreatic islets,” Cellular Physiology and Biochemistry, vol. 26, no. 4-5, pp. 707–716, 2010. [13] C. R. Gonza´lez,M. J.Vazq ´ uez, M. Lop ´ ez, and C. Dieguez ´ , “Influence of chronic undernutrition and leptin on GOAT mRNA levels in rat stomach mucosa,” Journal of Molecular Endocrinology, vol. 41, no. 5-6, pp. 415–421, 2008. [14] R. Go´mez,F.Lago, J. J. Go´ mez-Reino, C. Dieguez, and O. Gualillo, “Expression and modulation of ghrelin O-acyltrans- ferase in cultured chondrocytes,” Arthritis and Rheumatism, vol. 60, no. 6, pp. 1704–1709, 2009. [15] M. D. Gahete, J. Cor ´ doba-Chaco´n,R. Salvatori,J.P.Castano ˜ , R. D. Kineman, and R. M. Luque, “Metabolic regulation of ghrelin O-acyl transferase (GOAT) expression in the mouse hypothalamus, pituitary, and stomach,” Molecular and Cellular Endocrinology, vol. 317, no. 1-2, pp. 154–160, 2010. [16] O. Al-Massadi, A.B.Crujeiras, R.C. Gonza´lez et al., “Age, sex, and lactating status regulate ghrelin secretion and GOAT mRNA levels from isolated rat stomach,” American Journal of Physiology, vol. 299, no. 3, pp. E341–E350, 2010. [17] H. Kirchner, J. A. Gutierrez, P. J. Solenberg et al., “GOAT links dietary lipids with the endocrine control of energy balance,” Nature Medicine, vol. 15, no. 7, pp. 741–745, 2009. [18] T. J. Zhao,G. Liang,R. L.Li et al.,“Ghrelin O-acyltransferase (GOAT) is essential for growth hormone-mediated survival of calorie-restricted mice,” Proceedings of the National Academy of Sciences of the United States of America, vol. 107, no. 16, pp. 7467–7472, 2010. [19] K. Kang, E. Zmuda, and M. W. Sleeman, “Physiological role of ghrelin as revealed by the ghrelin and GOAT knockout mice,” Peptides. In press. [20] B. P. Barnett, Y. Hwang, M. S. Taylor et al., “Glucose and weight control in mice with a designed ghrelin O-acyltrans- ferase inhibitor,” Science, vol. 330, no. 6011, pp. 1689–1692, [21] W. An, Y. Li, G. Xu et al., “Modulation of ghrelin o-acyltrans- ferase expression in pancreatic islets,” Cellular Physiology and Biochemistry, vol. 26, no. 4-5, pp. 707–716, 2010. 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Ghrelin O-Acyl Transferase: Bridging Ghrelin and Energy Homeostasis

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Copyright © 2011 Andrew Shlimun and Suraj Unniappan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Abstract

Hindawi Publishing Corporation International Journal of Peptides Volume 2011, Article ID 217957, 5 pages doi:10.1155/2011/217957 Review Article Ghrelin O-Acyl Transferase: Bridging Ghrelin and Energy Homeostasis Andrew Shlimun and Suraj Unniappan Laboratory of Integrative Neuroendocrinology, Department of Biology, York University, Toronto, ON, Canada M3J 1P3 Correspondence should be addressed to Suraj Unniappan, suraju@yorku.ca Received 3 May 2011; Accepted 20 July 2011 Academic Editor: Lloyd D. Fricker Copyright © 2011 A. Shlimun and S. Unniappan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Ghrelin O-acyl transferase (GOAT) is a recently identified enzyme responsible for the unique n-acyl modification of ghrelin, a multifunctional metabolic hormone. GOAT structure and activity appears to be conserved from fish to man. Since the acyl modification is critical for most of the biological actions of ghrelin, especially metabolic functions, GOAT emerged as a very important molecule of interest. The research on GOAT is on the rise, and several important results reiterating its significance have been reported. Notable among these discoveries are the identification of GOAT tissue expression patterns, effects on insulin secretion, blood glucose levels, feeding, body weight, and metabolism. Several attempts have been made to design and test synthetic compounds that can modulate endogenous GOAT, which could turn beneficial in favorably regulating whole body energy homeostasis. This paper will focus to provide an update on recent advances in GOAT research and its broader implications in the regulation of energy balance. 1. Introduction 2. Discovery and Characterization of GOAT Ghrelin is a gut hormone discovered in 1999 by Dr. Kojima Yang et al. [5] first identified 16 MBOAT protein sequences and colleagues in the laboratory of Dr. Kangawa [1]. It is from the mouse genome. They found the 11 putative catalytic the first known endogenous ligand of the growth hormone regions in these BOATs highly conserved among the 16 secretagogue receptor 1a (GHS-R1a), which is now known as sequences, and it all contained the asparagine and histidine the ghrelin receptor [2]. Since its discovery, ghrelin has been residues thought to be involved in the catalysis. Three differ- identified and functions characterized in a large number of ent murine endocrine cell lines (AtT-20, INS-1, and MIN-6) animals [3, 4]. A unique aspect of ghrelin, the only known were transfected with the preproghrelin sequence and were orexigenic hormone from the gut, is the presence of a post- found to produce acylated ghrelin when cotransfected with translational acyl modification, mainly the attachment of the GOAT sequence. Mutation of the third serine to alanine the octanoyl group to the third serine residue of the N- prevented the acylation of ghrelin by GOAT, indicating that terminal region of the peptide [3, 4]. Several variations of the third serine is indeed the amino acid to which the moiety this modification were found [3, 4], while the mechanisms is attached. It was also determined that both asparagine that result in this modification, which is critical for many in position 307 and histidine in position 338 of mouse biological actions of ghrelin remained elusive. A decade of GOAT are essential for the catalytic activity of this enzyme waiting ended in 2008 when two research teams indepen- (Figure 1). These results provide the first published [5] dently identified the membrane bound O-acyl transferase evidence for GOAT. (MBOAT) responsible for ghrelin acylation and named it the Meanwhile, Gutierrez et al. [6] performed gene-silencing ghrelin o-acyl Transferase (GOAT) [5, 6]. experiments to determine whether a member of the MBOAT 2 International Journal of Peptides ∗ ∗ Ghrelin gene—6 exons and 5 introns Human Pig Transcription Cow Dog Preproghrelin mRNA Rat Mouse Translation Chicken Precursor Zebrafish Signal peptide Ghrelin C-terminal peptide Figure 1: Alignment of amino acid sequences in the highly con- Prohormone convertases, served catalytic regions of GOAT. The online sequence analysis tool carboxypeptidase E, Posttranslational processing available at the following link was used to generate the figure: GOAT, and and modifications http://www.protocol-online.org/tools/sms2/color align prop.html. amidating enzymes The proposed catalytic residues (asparagine and histidine) of (exclusively in fish) GOAT are marked by asterisks. Partial sequences of GOAT Mature ghrelin were obtained from full-length amino acid sequences obtained (acylated form) from the GenBank. Accession numbers of sequences are human (NP 001094386.1), pig (NP 001177352.1), cow (NP 001179186.1), dog (NP 001188260.1), rat (NP 001100787.2), mouse (NP 001119786.1), chicken (NP 001186218.1), and zebrafish (NP Figure 2: Scheme showing formation of mature, acylated ghrelin 001116416.1). from ghrelin gene. Ghrelin gene in humans is comprised of six exons and five introns. Upon transcription, the preproghrelin mRNA is formed, which translates to produce the precursor peptide that contains a signal peptide, mature peptide (ghrelin), and the family could mediate the acylation of ghrelin in human C-terminal peptide. After posttranslational cleavage and processing medullary thyroid carcinoma (TT) cells. They discovered by various enzymes including the prohormone convertases, car- that the silencing of GOAT (MBOAT4), but not other boxypeptidase E, amidating enzymes, and GOAT, the amidated, MBOAT sequences, results in the attenuation of ghrelin acylated mature ghrelin is formed. Amidated ghrelin is currently octanoylation [6]. The GOAT gene has been localized to reported only in fish. the 8p12 region of the human chromosome 8. Further experiments by Gutierrez et al. [6] determined that only cotransfection of preproghrelin with GOAT, not with other MBOATs, yielded third serine octanoylated ghrelin in HEK- 2.1. Tissue Distribution and Regulation of GOAT Expression. 293 cells. The supplementation of the HEK-293 cell medium The discoverers of GOAT found highest levels of GOAT with lipids from acetate to tetradecanoic acid resulted in mRNA expression in the stomach and intestine followed the GOAT enabled modification of ghrelin with fatty acids by the testis of mice [5] and the pancreas and stomach up to tetradecanoic acid. A very high sequence similarity of humans [6]. A comprehensive reverse transcription- was found among GOAT proteins from humans to zebrafish polymerase chain reaction (RT-PCR) tissue distribution [5, 6]. Interestingly, zebrafish, rat, and mouse GOAT were studybySakataet al. [9] was in agreement with this initial able to acylate human ghrelin [6]. Octanoylated ghrelin finding. GOAT mRNA expression was detected primarily was undetectable in the blood of GOAT knockout mice, in the mouse stomach and intestine, while other MBOAT providing strong confirmatory evidence for the critical role family of enzymes were found in several other tissues in of GOAT in acylating ghrelin. Together, the pioneering addition to the gastrointestinal tract. This result was fur- research by Yang et al. [5]and Gutierrez et al. [6]led to the ther confirmed by double label in situ hybridization and discovery of GOAT, the sole mediator of the unique acylation immunohistochemistry that found GOAT mRNA expression found in ghrelin (Figure 2). in ghrelin immunopositive cells and neighboring cells within More recent in vitro studies by Ohgusu and colleagues [7] the gastric mucosa. Together, these results indicate that the found that recombinant GOAT can acylate a short peptide, primary source of GOAT is gastrointestinal cells that are pos- the N-terminal region of ghrelin consisting of just four itive for ghrelin. Stengel et al. [10]for thefirst timereported amino acids (GSSF). This short peptide is considered as the colocalization of ghrelin and GOAT immunoreactivity in bioactive core of ghrelin, and it provides further support to the stomach mucosa of rats and mice. More GOAT positive the notion that this fragment could elicit many physiological ghrelin cells were found in mice (∼95%) stomach compared processes regulated by the full-length mature ghrelin. GOAT to rats (∼56%). The nonghrelin positive GOAT cells in rat also has a preference for n-hexanoyl coA compared to n- gastric mucosa were positive for histidine decarboxylase, octanoyl CoA. Yang et al. [8] proposed that preproghrelin which helps in vitamin B6 processing. Western blot analyses is octanoylated in the membranes of endoplasmic reticulum found two bands, one at ∼50 kDa, the expected size of GOAT and the acylation occurs after the signal peptide is removed protein, and a second one at ∼100 kDa, which was consid- from the preproghrelin. These results provide further sup- ered as a dimer. Western blots of plasma detected GOAT in port for the four amino acid bioactive core of ghrelin, circulation, and blood levels of GOAT were elevated in mice intracellular location of ghrelin modification, and multiple and rats fasted for 24 hours. Intraperitoneal injections of third serine modifications of ghrelin. lipopolysaccharide (LPS) resulted in a significant reduction International Journal of Peptides 3 in plasma acylated ghrelin and caused a corresponding de- stomach GOAT mRNA in mice [17]. No differences in gastric crease in plasma GOAT levels, while a small increase in gut GOAT mRNA expression were found in ad libitum fed leptin GOAT was detected [11]. This suggests that ghrelin-GOAT deficient ob/ob mice compared to wild-type controls [17]. system may be involved in the infection-induced decrease Meanwhile, Gahete et al. [15] found that plasma acylated in food intake. While species-specific variations exist in the ghrelin levels were increased in fasted mice, and this increase expression of GOAT in the gut, it is now clear that stomach coincided with an elevation in the gut, hypothalamic, and and intestine are major sources of GOAT. pituitary GOAT mRNA expression. While accepting that In agreement with the original studies, An et al. [12] discrepancies exist between physiological status of animals found abundant expression of GOAT mRNA and protein in and species used, it is interesting that GOAT expression in the whole pancreas, isolated islets and INS-1 cells. Immuno- rodents in general is modulated by the metabolic status. localization studies found that GOAT-positive cells are The importance of GOAT on metabolism was further mainly present in the periphery of rat islets and no GOAT- elucidated using GOAT knockout mice, which lack acyl insulin colocalization was found, especially in the beta cell ghrelin in their circulation. These mice have normal body rich core of the islets. Gonzalez ´ et al. [13], using RT-PCR weight compared to wild-type control mice, but they gained analysis, reported GOAT mRNA expression in the stomach, significantly less weight and had reduced fat mass when fed pancreas, hypothalamus, ovary, serum, placenta, muscle, on a high-fat diet for eight weeks [17]. The enrichment of heart, and adrenal glands of rats. There were no differences diet with fatty acids resulted in a leaner body mass for the in the expression of GOAT mRNA in the stomach mucosa GOAT knockout mice, and this reduction was attributed of male rats at postnatal days 10, 25, and 60 [13]. GOAT to increased energy expenditure not alterations in feed expression was also detected in murine cartilage explants, intake. No differences in glucose homeostasis were found human primary chondrocytes, and in both human and between the knockout and wild-type mice. Transgenic mice mouse chondrocyte cells lines [14]. The expression of GOAT overexpressing human GOAT in the liver, when fed with mRNA in murine chondrogenic cell line ATDC-5 showed a medium chain fatty acid containing diet, produced more gradual increase as the cells differentiated, with the lowest acylated ghrelin and had a transient increase in food intake. levels being detected in the early stages of differentiation. LPS Overall, GOAT links the lipid intake into whole body energy significantly reduced GOAT mRNA expression in cultured balance. Zhao et al. [18] determined the body weight and cartilage cells [14]. GOAT mRNA was detected in the blood glucose levels in the GOAT knockout mice. While hypothalamus and pituitary of mice [15]. It was also found ad libitum feeding on regular or high-fat diets showed that GOAT mRNA expression in the cultured primary no difference in the metabolic phenotype, calorie-restricted pituitary cells of mice was increased by acyl ghrelin, leptin, GOAT knockout mice lost more weight and had low blood and growth hormone releasing hormone, while somatostatin glucose levels compared to wild-type controls. The adverse decreased its expression. No effects were found for neuropep- effects of GOAT absence on glucose levels were reversed upon tide Y and des-acyl ghrelin on GOAT mRNA expression [15]. exogenous administration of acylated ghrelin or growth GOAT mRNA expression was found increasing with age in hormone administration. Overall, the outcome of this the stomach of rats, while it was found inhibited by a decrease genetic approach highlights the significance of GOAT in in testosterone [16]. Collectively, from all studies available acylating ghrelin and regulating the role of ghrelin on energy to date, it appears that the gastroenteropancreatic tissues, balance. It is important to consider the possibility of GOAT which play a major role in postprandial satiety and glucose modulating peptides other than ghrelin. Therefore, whatever homeostasis, are the most abundant sources of GOAT in effects resulting from GOAT alterations are possibly not mammals. It is also important to note that several metabolic exclusively due to the absence of acylated ghrelin, as other hormones modulate GOAT expression. metabolically relevant peptides could be affected due to theabsence of GOAT.A newreview[19]is now available 2.2. GOAT and Energy Balance. To date, two lines of evi- that compares the effects of ghrelin and GOAT knock-out mice. The readers are encouraged to review it for a further dences are available indicating the involvement of GOAT in regulating energy homeostasis. The first set of data is discussion on the above aspect. originated from GOAT mRNA expression studies during var- Modulating GOAT activity could potentially affect bio- ious metabolic states. The second type of results arises from logical actions of ghrelin and this aspect could be targeted to studies directly testing the effects of GOAT by perturbing design pharmacological approaches to regulate energy intake the endogenous GOAT. Chronic food deprivation for 21 days and body weight. A successful attempt in this direction was resulted in a significant increase in ghrelin mRNA expression made when Yang et al. [8] discovered that the octanoylated in the gastric mucosa of rats and a corresponding increase short fragment of ghrelin comprised of N-terminal 5 amino in GOAT mRNA in the same tissue [13]. Intraperitoneal acids (GSSFL) could inhibit GOAT activity. This GOAT injections of leptin, a satiety signal that relays the status inhibitory activity was further enhanced when an amidated of long-term energy reserves to the brain, also caused a short peptide was used (GSSFL-NH ). It was also reported significant increase in GOAT mRNA expression in 48 fasted that a modified ghrelin fragment with the third serine rats but not in ad libitum fed rats. These results indicate replaced with an octanoylated diaminopropionic acid could that malnutrition and leptin are two regulatory factors that enhance the GOAT inhibitory activity to 45-fold. Such determine GOAT mRNA expression. In contrast, fasting inhibitors could therefore be used to attenuate active ghrelin for 12, 24, or 36 hours caused a significant reduction in and its effects on metabolism. 4 International Journal of Peptides In this regard, other GOAT-specific antagonists have also could emerge as potential treatment or preventative strategy been designed and functionally characterized [20]. GO-CoA- for diabetes. Similarly, tweaking GOAT biology may reap Tat, a bisubstrate analogue that acts as a GOAT inhibitor, rewards in the march towards curbing overweight and obe- was found to decrease acylated ghrelin production from cells sity. While several interesting results have been obtained to stably transfected with preproghrelin and GOAT. Similarly, date, GOAT research is still in its infancy, and future studies GO-CoA-Tat administration significantly reduced acylated hold the key to further our understanding of this protein and ghrelin production in vivo in mice fed with medium chain enhancing its utilization for the benefit of humans. fatty acid rich food. In addition, daily administration of GO-CoA-Tat prevented body weight gain in mice fed on a Acknowledgments high-fat diet. GOAT inhibition in pancreatic beta cells using the GO-CoA-Tat caused a significant increase in glucose- S. Unniappan is a Canadian Institutes of Health Research stimulated insulin secretion, suggesting an inhibitory role for (CIHR) New Investigator and a recipient of the Early Inves- ghrelin in this process. Similar increases in glucose-induced tigator Award from the Ontario Ministry of Research and insulin secretion were also found in mice treated with the Innovation (MRI). The research in the laboratory of S. GOAT antagonist. The expression of mRNA encoding UCP2, Unniappan is funded by operating grants from the Natural a potent inhibitor of insulin secretion, was suppressed 20- Sciences and Engineering Research Council (NSERC) and fold in GO-CoA-Tat-treated islets. In agreement with these the CIHR and equipment/infrastructure grants from the islet effects, An et al. [21]reportedthatGOATmRNA expres- NSERC, Canada Foundation for Innovation and the Ontario sion in islet beta-like INS-1 cells is inhibited by insulin. This MRI. effect of insulin was blocked when cells were treated with wortmannin that inhibits PI3 kinase/Akt pathway. From this References result, it is clear that the inhibitory effects of insulin on GOAT are mediated via the insulin receptor signaling pathways. [1] M. Kojima, H. Hosoda, Y. Date, M. Nakazato, H. Matsuo, and Insulin was also found to reduce GOAT promoter activity in K. Kangawa, “Ghrelin is a growth-hormone-releasing acylated vitro. In addition, it was also determined that the mammalian peptide from stomach,” Nature, vol. 402, no. 6762, pp. 656– target of rapamycin (mTOR) regulates transcription and 660, 1999. translation of GOAT. Inhibition of mTOR signaling using [2] A.P.Davenport, T.I.Bonner, S. M. Foord etal.,“International rapamycin significantly increased GOAT mRNA in INS-1 union of pharmacology. LVI. Ghrelin receptor nomenclature, cells in a concentration and time dependent manner. Sim- distribution, and function,” Pharmacological Reviews,vol.57, no. 4, pp. 541–546, 2005. ilarly, intraperitoneal injection of rapamycin that reduced [3] M. Kojima and K. Kangawa, “Ghrelin: from gene to physiolog- mTOR increased GOAT mRNA expression in the pancreas ical function,” Results and Problems in Cell Differentiation,vol. of C57BL/6 mice. In contrast, leucine, an activator of mTOR, 50, pp. 185–205, 2010. attenuated GOAT mRNA expression in mice pancreas. Over- [4] H. Kaiya, M. Miyazato, K. Kangawa, R. E. Peter, and S. Unniap- expressing tuberculosis sclerosis complex 1 and 2 (TSC1 and pan, “Ghrelin: a multifunctional hormone in non-mammalian TSC2), the negative regulators of mTOR action, increased vertebrates,” Comparative Biochemistry and Physiology,vol. GOAT activity in INS-1 cells. Together, these results provide 149, no. 2, pp. 109–128, 2008. further evidence for an important role for the endogenous [5] J. Yang, M. S. Brown, G. Liang, N. V. Grishin, and J. L. Gold- GOAT on glucose homeostasis and insulin secretion, and stein, “Identification of the acyltransferase that octanoylates intracellular mechanisms that mediate these effects. ghrelin, an appetite-stimulating peptide hormone,” Cell,vol. 132, no. 3, pp. 387–396, 2008. [6] J. A.Gutierrez, P.J. Solenberg,D.R.Perkins et al., “Ghrelin octanoylation mediated by an orphan lipid transferase,” 3. Perspectives Proceedings of the National Academy of Sciences of the United States of America, vol. 105, no. 17, pp. 6320–6325, 2008. Currently, GOAT is the only known enzyme that is capable in acylating ghrelin. It is now clear that GOAT is a critical [7] H. Ohgusu, K. Shirouzu, Y. Nakamura et al., “Ghrelin O- acyltransferase (GOAT) has a preference for n-hexanoyl-CoA component in making bioactive ghrelin, and thus mediating over n-octanoyl-CoA as an acyl donor,” Biochemical and the physiological functions of ghrelin. Within a short period Biophysical Research Communications, vol. 386, no. 1, pp. 153– since its discovery, enough strong evidence have been 158, 2009. collected to support a clear and very important role for [8] J.Yang, T. J. Zhao, J. L.Goldstein,and M. S. Brown, “Inhi- GOAT in metabolic physiology. GOAT expression is tissue bition of ghrelin O-acyltransferase (GOAT) by octanoylated specific, is modulated by a number of factors, and appears to pentapeptides,” Proceedings of the National Academy of Sciences vary among organisms. It is especially interesting that GOAT of the United States of America, vol. 105, no. 31, pp. 10750– is regulated by the metabolic status of animals. Whether 10755, 2008. GOAT expression is altered in metabolic pathophysiology in [9] I. Sakata, J. Yang, C. E. Lee et al., “Colocalization of ghrelin O- humans remain unknown. Another fascinating discovery is acyltransferase and ghrelin in gastric mucosal cells,” American the development of GOAT inhibitors and its success in pre- Journal of Physiology, vol. 297, no. 1, pp. E134–E141, 2009. venting weight gain, stimulating insulin secretion and reduc- [10] A. Stengel, M. Goebel, L. Wang, Y. Tache, ´ G. Sachs, and N. ing blood glucose levels. If these findings are translatable, W. G. Lambrecht, “Differential distribution of ghrelin-O- inhibition of endogenous GOAT using specific inhibitors acyltransferase (GOAT) immunoreactive cells in the mouse International Journal of Peptides 5 and rat gastric oxyntic mucosa,” Biochemical and Biophysical Research Communications, vol. 392, no. 1, pp. 67–71, 2010. [11] A. Stengel, M. Goebel, L. Wang, J. R. Reeve, Y. Tache, ´ and N. W. G. Lambrecht, “Lipopolysaccharide differentially decreases plasma acyl and desacyl ghrelin levels in rats: potential role of the circulating ghrelin-acylating enzyme GOAT,” Peptides,vol. 31, no. 9, pp. 1689–1696, 2010. [12] W. An, Y. Li, G. Xu et al., “Modulation of ghrelin o-acyltrans- ferase expression in pancreatic islets,” Cellular Physiology and Biochemistry, vol. 26, no. 4-5, pp. 707–716, 2010. [13] C. R. Gonza´lez,M. J.Vazq ´ uez, M. Lop ´ ez, and C. Dieguez ´ , “Influence of chronic undernutrition and leptin on GOAT mRNA levels in rat stomach mucosa,” Journal of Molecular Endocrinology, vol. 41, no. 5-6, pp. 415–421, 2008. [14] R. Go´mez,F.Lago, J. J. Go´ mez-Reino, C. Dieguez, and O. Gualillo, “Expression and modulation of ghrelin O-acyltrans- ferase in cultured chondrocytes,” Arthritis and Rheumatism, vol. 60, no. 6, pp. 1704–1709, 2009. [15] M. D. Gahete, J. Cor ´ doba-Chaco´n,R. Salvatori,J.P.Castano ˜ , R. D. Kineman, and R. M. Luque, “Metabolic regulation of ghrelin O-acyl transferase (GOAT) expression in the mouse hypothalamus, pituitary, and stomach,” Molecular and Cellular Endocrinology, vol. 317, no. 1-2, pp. 154–160, 2010. [16] O. Al-Massadi, A.B.Crujeiras, R.C. Gonza´lez et al., “Age, sex, and lactating status regulate ghrelin secretion and GOAT mRNA levels from isolated rat stomach,” American Journal of Physiology, vol. 299, no. 3, pp. E341–E350, 2010. [17] H. Kirchner, J. A. Gutierrez, P. J. Solenberg et al., “GOAT links dietary lipids with the endocrine control of energy balance,” Nature Medicine, vol. 15, no. 7, pp. 741–745, 2009. [18] T. J. Zhao,G. Liang,R. L.Li et al.,“Ghrelin O-acyltransferase (GOAT) is essential for growth hormone-mediated survival of calorie-restricted mice,” Proceedings of the National Academy of Sciences of the United States of America, vol. 107, no. 16, pp. 7467–7472, 2010. [19] K. Kang, E. Zmuda, and M. W. Sleeman, “Physiological role of ghrelin as revealed by the ghrelin and GOAT knockout mice,” Peptides. In press. [20] B. P. Barnett, Y. Hwang, M. S. Taylor et al., “Glucose and weight control in mice with a designed ghrelin O-acyltrans- ferase inhibitor,” Science, vol. 330, no. 6011, pp. 1689–1692, [21] W. An, Y. Li, G. Xu et al., “Modulation of ghrelin o-acyltrans- ferase expression in pancreatic islets,” Cellular Physiology and Biochemistry, vol. 26, no. 4-5, pp. 707–716, 2010. 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