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Some of the actions of leptin depend on cholecystokinin (CCK). However, it is unknown whether leptin modulates the release of CCK. Here, we demonstrate in vitro that leptin induces the phosphorylation of extracellular signal-related kinase (ERK)-1/2 proteins and increases CCK release (EC 50 ‫؍‬ 0.23 nmol/l) in CCK-secreting STC-1 cells. We showed that rat duodenal juice contains leptin that circulates free and bound to macromolecules, suggesting that leptin has a lumenal action on the intestine. In vivo in the rat, duodenal infusion of leptin increased plasma CCK at levels comparable to those induced by feeding. Moreover, meal-induced increases in plasma CCK were markedly reduced in obese fa/fa rats, whereas the mobilization of the gastric leptin pool was similar in lean and obese Zucker rats. L eptin, the ob gene product, was initially reported to be produced by adipose cells (1). It is released into the bloodstream and transported across the blood-brain barrier into the hypothalamus, where it activates specific leptin receptors (2,3) and regulates energy homeostasis by altering energy intake and expenditure (4 -6). Leptin regulates food intake by mechanisms involving cross-talk between hypothalamic leptin receptors and various neuropeptides involved in the control of feeding. The leptin receptor (Ob-R) is a member of the gp130 family of cytokine receptors. It occurs in several isoforms resulting from the alternative splicing of the db leptin receptor gene (2,3). It is currently thought that the long isoform, Ob-Rb, can activate the signal transducers and activators of transcription (STAT) pathways, whereas both Ob-Rb and the short isoform (Ob-Ra) can transduce signals through insulin receptor substrates and through mitogen-activated protein kinase (MAPK) pathways (7).The signals that arise from the upper gastrointestinal tract upon feeding are transmitted to the brain by the vagus nerve. These signals are key components in the control of meal-induced satiety. Cholecystokinin (CCK) is secreted from duodenal endocrine I cells and typically functions as one of these short-term satiety signals (8,9). Interestingly, the leptin-induced inhibition of food intake (10) and the stimulation of pancreatic exocrine secretions (11) can be blocked by a CCK-1 receptor antagonist. These data suggest that endogenous CCK is involved in these effects, operating through CCK-1 receptors. However, it is not currently known whether leptin directly modulates the release of CCK.Leptin is also produced by the stomach (12-14) and is mainly secreted into the gastric juice after CCK in rats (12,15) and after secretin or vagal stimulation in humans (14,16). Some of the stomach-derived leptin is not fully degraded by proteolysis, indicating that it reaches the intestine in an active form and thus can initiate biological processes controlling functions of the intestinal tract. Indeed, lumenal leptin increases the activity of the brush border proton-dependent transporter, PepT1, which enhances the intestinal absorption of oligopeptides (17). T...

In the present study, the effect of H(+)/peptide transporter (PepT1)-mediated N-formylmethionyl-leucyl-phenylalanine (fMLP) transport on inflammation in vivo in the rat small intestine, which expresses high PepT1 levels, and in the rat colon, which does not express PepT1, were investigated using myeloperoxidase (MPO) activity and histological analysis. We found that 10 microM fMLP perfusion in the jejunum for 4 h significantly increased MPO activity and altered the architecture of jejunal villi. In contrast, 10 microM fMLP perfusion in the colon for 4 h did not induce any inflammation. In addition, we have shown that 50 mM Gly-Gly alone did not affect basal MPO activity but completely inhibited the MPO activity induced by 10 microM fMLP in the jejunum. Together, these experiments demonstrate that 1) the differential expression of PepT1 between the small intestine and the colon plays an important role in epithelial-neutrophil interactions and 2) the inhibition of fMLP uptake by jejunal epithelial cells (expressing PepT1) reduces the neutrophil ability to move across the epithelium, in agreement with our previously published in vitro study. This report constitutes the first in vivo study showing the implication of a membrane transporter (PepT1) in intestinal inflammation.

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Leptin has been suggested to be involved in tissue injury and/or mucosal defence mechanisms. Here, we studied the effects of leptin on colonic mucus secretion and rat mucin 2 (rMuc2) expression. Wistar rats and ob/ ob mice were used. Secretion of mucus was followed in vivo in the rat perfused colon model. Mucus secretion was quantified by ELISA, and rMuc2 mRNA levels were quantified by real-time RT PCR. The effects of leptin alone or in association with protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) inhibitors on mucin secreted by human mucus-secreting HT29-MTX cells were determined. Leptin was detected in the rat colonic lumen at substantial levels. Luminal perfusion of leptin stimulates mucus-secreting goblet cells in a dose-dependent manner in vivo in the rat. Leptin (10 nmol/l) increased mucus secretion by a factor of 3.5 and doubled rMuc2 mRNA levels in the colonic mucosa. There was no damage to mucosa 24 h after leptin, but the number of stained mucus cells significantly increased. Leptin-deficient ob/ ob mice have abnormally dense mucus-filled goblet cells. In human colonic goblet-like HT29-MTX cells expressing leptin receptors, leptin increased mucin secretion by activating PKC- and PI3K-dependent pathways. This is the first demonstration that leptin, acting from the luminal side, controls the function of mucus-secreting goblet cells. Because the gel layer formed by mucus at the surface of the intestinal epithelium has a barrier function, our data may be relevant physiologically in defence mechanisms of the gastrointestinal tract.