FP30-1 Tryptophan stimulates ghrelin secretion by the ghrelin-producing cell line, MGN3-1 cells in vitro

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: FP30-Central Regulation of Appetite & Feeding
Basic
Monday, June 17, 2013: 10:45 AM-11:15 AM
Presentation Start Time: 10:45 AM
Room 304 (Moscone Center)

Poster Board MON-676
Hiroyuki Koyama*1, Hiroshi Iwakura2, Mika Bando3, Hiroshi Hosoda4, Kiminori Hosoda5, Takashi Akamizu6, Kenji Kangawa7 and Kazuwa Nakao1
1Kyoto University Graduate School of Medicine, Kyoto, Japan, 2Kyoto University, Kyoto, Japan, 3Department of Human Health Sciences, 4National Cardiovascular Center, Higashiosaka-Shi, Japan, 5Kyoto Univ/ Grad Sch of Med, Kyoto, Japan, 6Wakayama Medical University, Wakayama, Japan, 7Natl Cereb and Cardiovasc Ctr Re, Osaka, Japan
Plasma ghrelin level is elevated by fasting and suppressed by re-feeding, while the level is low in obese and high in lean subjects, suggesting that plasma ghrelin level is regulated by acute and chronic energy status. The precise mechanism by which ghrelin secretion is controlled, however, has not yet been completely understood due to the lack of appropriate in vitro assay system for ghrelin secretion. Recently, we have established a ghrelin-secreting cell line MGN3-1, and found that oxytocin and catecholamine stimulate ghrelin secretion by MGN3-1 cells in vitro, while insulin and somatostatin suppress it. Given that ghrelin cell is located in the stomach, certain nutrients may have an influence on the ghrelin secretion in addition to these neurotransmitters or peptide hormones. Among nutrients, some amino acids stimulate the secretion of gut peptide hormones including gastrin, CCK or GLP-1 by gastrointestinal cells. In this study, we examined the effects of amino acids, especially tryptophan and phenylalanine, known to stimulate CCK, on ghrelin secretion by MGN3-1 cells in vitro. At the level of 1 mM, tryptophan, but not phenylalanine, stimulated ghrelin secretion by MGN3-1 cells, without affecting ghrelin and GOAT mRNA levels. At the level of 10 mM, pheylalanine also stimulated ghrelin secretion by MGN3-1 cells but with lesser extent. The stimulatory effect of tryptophan on ghrelin secretion by MGN3-1 cells was not accompanied by intra-cellular cAMP elevation, and addition of chelerythrine, a PKC inhibitor, completely blocked the stimulatory effect of tryptophan on ghrelin secretion, indicating that the stimulatory effect of tryptophan on ghrelin secretion is mediated by Gq-PKC signaling pathway, and not by cAMP pathway. Calcium sensing receptor (CaSR) is a Gq and Gi coupled receptor, which acts as a Ca sensor in the calcium-regulating organs (e.g. parathyroid or renal tubules). Besides its Ca-sensing activity, recent evidences suggest that CaSR is also expressed in gastrointestinal endocrine cells, where it participates in amino acid sensing to regulate the secretion of gastrin, CCK or GLP-1, raising a possibility that CaSR is also involved in ghrelin secretion. Actually, CaSR mRNA and protein was detected in MGN3-1 cells by RT-PCR, Western blot analysis and immunocytochemistry. Further, CaSR antagonist NPS2143 significantly attenuated the stimulatory effect of tryptophan on ghrelin secretion. These results indicate that tryptophan directly stimulates ghrelin secretion by ghrelin-producing cells through activating CaSR-Gq-PKC pathway. In summary, tryptophan stimulated ghrelin secretion by MGN3-1 cells. A specific CaSR antagonist blocked the tryptophan-induced responses. These results reveal that CaSR functions as a tryptophan receptor to induce ghrelin secretion by MGN3-1 cells.

Nothing to Disclose: HK, HI, MB, HH, KH, TA, KK, KN

*Please take note of The Endocrine Society's News Embargo Policy at http://www.endo-society.org/endo2013/media.cfm

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