Antral mucosa expresses functional leptin receptors coupled to STAT-3 signaling, which is involved in the control of gastric secretions in the rat

Leptin and the Blood-Brain Barrier: Curiosities and Controversies

Leptin for over 25 years has been a central theme in the study of appetite, obesity, and starvation. As the major site of leptin production is peripheral, and the site of action of greatest interest is the hypothalamus, how leptin accesses the central nervous system (CNS) and crosses the blood-brain barrier (BBB) has been of great interest. We review here the ongoing research that addresses fundamental questions such as the sites of leptin resistances in obesity and other conditions, the causes of resistances and their relations to one another, the three barrier sites of entry into the CNS, why recent studies using suprapharmacological doses cannot address these questions but give insight into nonsaturable entry of leptin into the CNS, and how that might be useful in using leptin therapeutically. The current status of the controversy of whether the short form of the leptin receptor acts as the BBB leptin transporter and how obesity may transform leptin transport is reviewed. Review of these and other topics summarizes in a new appreciation of what leptin may have actually evolved to do and what physiological role leptin resistance may play. © 2021 American Physiological Society. Compr Physiol 11:1-19, 2021.

Evaluation of a modified rat model for functional dyspepsia

BACKGROUND/AIM

The purpose of this study was to establish a modified rat model with functional dyspepsia (FD) and analyze the changes in gastrointestinal motility and brain-gut peptide levels in serum and brain-gut axis.

MATERIALS AND METHODS

Male Wistar rats were divided into control group (Con) and FD model group. FD model was established by stimulating semi-starvation rats via tail damping, provocation, and forced exercise fatigue until gastrointestinal motility disorder appeared, and then levels of motilin, leptin, cholecystokinin (CCK), and vasoactive intestinal peptide (VIP) were detected in serum by enzyme linked immunosorbent assay and in duodenum, antrum, and hypothalamus by immunohistochemistry, reverse transcriptase-polymerase chain reaction, and Western blot.

RESULTS

The results showed rates of intestinal propulsion and gastric emptying slowed down markedly compared to Con (P < 0.05), the gastrointestinal electric activity attenuated, and migrating motor complex (MMC) interrupted in the model group. The levels of leptin and VIP markedly increased, but motilin decreased as compared to the Con (P < 0.05) in serum and in the above tissues. It is interesting that the level of CCK decreased in the antrum and duodenum but increased in the hypothalamus as compared to Con (P < 0.05).

CONCLUSIONS

The modified rat model meets the diagnostic criteria of FD and can be used as a method for studying FD in animals.

The relationship of human milk leptin and macronutrients with gastric emptying in term breastfed infants

BackgroundInfants breastfed on demand exhibit a variety of feeding patterns and self-regulate their nutrient intake, but factors influencing their gastric emptying (GE) are poorly understood. Despite research into appetite regulation properties of leptin, there is limited information about relationships between human milk leptin and infant GE.MethodsGastric volumes were calculated from ultrasound scans of infants' stomachs (n=20) taken before and after breastfeeding, and then every 12.5 min (median; range: 3-45 min) until the next feed. Skim milk leptin and macronutrient concentrations were measured and doses were calculated.ResultsThe leptin concentration was (mean±SD) 0.51±0.16 ng/ml; the leptin dose was 45.5±20.5 ng per feed. No relationships between both concentration and dose of leptin and time between the feeds (P=0.57; P=1, respectively) or residual stomach volumes before the subsequent feed (P=0.20; P=0.050) were found. Post-feed stomach volumes (GE rate) were not associated with leptin concentration (P=0.77) or dose (P=0.85).ConclusionGE in term breastfed infants was not associated with either skim milk leptin concentration or dose. Further investigation with inclusion of whole-milk leptin and other hormones that affect gastrointestinal activity is warranted.

Cross-talk between adipose and gastric leptins for the control of food intake and energy metabolism

The understanding of the regulation of food intake has become increasingly complex. More than 20 hormones, both orexigenic and anorexigenic, have been identified. After crossing the blood-brain barrier, they reach their main site of action located in several hypothalamic areas and interact to balance satiety and hunger. One of the most significant advances in this matter has been the discovery of leptin. This hormone plays fundamental roles in the control of appetite and in regulating energy expenditure. In accordance with the lipostatic theory stated by Kennedy in 1953, leptin was originally discovered in white adipose tissue. Its expression by other tissues was later established. Among them, the gastric mucosa has been shown to secrete large amounts of leptin. Both the adipose and the gastric tissues share similar characteristics in the synthesis and storage of leptin in granules, in the formation of a complex with the soluble receptor and a secretion modulated by hormones and energy substrates. However while adipose tissue secretes leptin in a slow constitutive endocrine way, the gastric mucosa releases leptin in a rapid regulated exocrine fashion into the gastric juice. Exocrine-secreted leptin survives the extreme hydrolytic conditions of the gastric juice and reach the duodenal lumen in an intact active form. Scrutiny into transport mechanisms revealed that a significant amount of the exocrine leptin crosses the intestinal wall by active transcytosis. Leptin receptors, expressed on the luminal and basal membrane of intestinal epithelial cells, are involved in the control of nutrient absorption by enterocytes, mucus secretion by goblet cells and motility, among other processes, and this control is indeed different depending upon luminal or basal stimulus. Gastric leptin after transcytosis reaches the central nervous system, to control food intake. Studies using the Caco-2, the human intestinal cell line, in vitro allowed analysis of the mechanisms of leptin actions on the intestinal mucosa, identification of the mechanisms of leptin transcytosis and understanding the modulation of leptin receptors by nutrients and hormones. Exocrine-secreted gastric leptin thus participates in a physiological axis independent in terms of time and regulation from that of adipose tissue to rapidly control food intake and nutrient absorption. Adipocytes and gastric epithelial cells are two cell types the metabolism of which is closely linked to food intake and energy storage. The coordinated secretion of adipose and gastric leptins ensures proper management of food processing and energy storage.

Roles of hormones in taste signaling

Proper nutrition, avoidance of ingesting substances that are harmful to the whole organism, and maintenance of energy homeostasis are crucial for living organisms. Additionally, mammals possess a sophisticated system to control the types and content of food that we swallow. Gustation is a vital sensory skill for determining which food stuffs to ingest and which to avoid, and for maintaining metabolic homeostasis. It is becoming apparent that there is a strong link between metabolic control and flavor perception. Although the gustatory system critically influences food preference, food intake, and metabolic homeostasis, the mechanisms for modulating taste sensitivity by metabolic hormones are just now being explored. It is likely that hormones produced in the tongue influence the amounts and types of food that we eat: the hormones that we associate with appetite control, glucose homeostasis and satiety, such as glucagon-like peptide-1, cholecystokinin, and neuropeptide Y are also produced locally in taste buds. In this report, we will provide an overview of the peptidergic endocrine hormone factors that are present or are known to have effects within the gustatory system, and we will discuss their roles, where known, in taste signaling.

Leptin signaling: A key pathway in immune responses

Leptin is a hormone whose central role is to regulate endocrine functions and to control energy expenditure. After the discovery that leptin can also have pro-inflammatory effects, several studies have tried to address - at the molecular level - the pathways involved in leptin-induced modulation of the immune functions in normal and pathologic conditions. The signaling events influenced by leptin after its binding to the leptin receptor have been under scrutiny in the past few years, and considerable experimental work has elucidated the consequences of leptin effects on immune cells. This review examines the biochemistry, function and regulation of leptin signaling in view of possible intervention on this molecule for a better management and therapy of immune-mediated diseases.

Development of a pharmacologically improved peptide agonist of the leptin receptor

Leptin, a hormone produced by adipose tissue, regulates energy balance in the hypothalamus and is involved in fertility, immune response and carcinogenesis. The existence of disorders related to leptin deficit and leptin overabundance calls for the development of drugs activating or inhibiting the leptin receptor (ObR). We synthesized four proposed receptor-binding leptin fragments (sites I, IIa and IIb, III), their reportedly antagonist analogs, and a peptide chimera composed of the two discontinuous site II arms. To assess the pharmacological utility of leptin fragments, we studied the peptides' ability to stimulate the growth of ObR-positive and ObR-negative cells. The combined site II construct and site III derivatives selectively reversed leptin-induced growth of ObR-positive cells at mid-nanomolar concentrations. However, these peptides appeared to be partial agonists/antagonists as they activated cell growth in the absence of exogenous leptin. A designer site III analog, featuring non-natural amino acids at terminal positions to decrease proteolysis and a blood-brain barrier (BBB) penetration-enhancing carbohydrate moiety, proved to be full agonist to ObR, i.e., stimulated proliferation of different ObR-positive but not ObR-negative cells in the presence or absence of leptin. This glycopeptide bound to isolated ObR on solid-phase assays and activated ERK-1/2 signaling in ObR-positive MCF-7 cells at 100-500 nM concentrations. The glycopeptide was stable in mouse serum, readily crossed endothelial/astrocyte cell layers in a cellular BBB model, and was distributed into the brain of Balb/c mice after intraperitoneal administration. These characteristics suggest a potential pharmaceutical utility of the designer site III glycopeptide in leptin-deficient diseases.

STAT3 activation regulates growth, inflammation, and vascularization in a mouse model of gastric tumorigenesis

BACKGROUND & AIMS

The gp130(757F/F) mouse is a well-characterized and robust model of distal gastric tumorigenesis displaying many of the characteristics of human intestinal type gastric cancer. Key to the development of tumors in this model, and in many examples of human tumor development, is hyperactivation of the transcription factor STAT3. This study addressed the requirement for STAT3 activation in tumor initiation and characterized some of the genes downstream of STAT3 required for tumor development. Furthermore, the interaction among STAT3, the microbial environment, and tumorigenesis was evaluated.

METHODS

The role of STAT3 in gastric tumor development was assessed in detail in gp130(757F/Y757F):STAT3(+/-) mice displaying reduced STAT3 activity. Tumor size was quantified morphologically, and the effects on endocrine cell populations, neovascularization, and inflammatory cell infiltration as well as the outcome of STAT3 activation on transcription of a number of genes relevant in growth and inflammation were quantified.

RESULTS

Loss of one STAT3 allele in gp130(757F/F) mice reduced the frequency and rate of tumor development because of inhibition of proliferation-induced glandular hyperplasia. There was also a concomitant reduction in the degree of inflammatory infiltration and cytokine and chemokine expression, angiogenesis, and expression of metalloproteinases and growth factors. Antimicrobial treatment of gp130(757F/F) mice slowed tumor growth coincident with reduced macrophage and neutrophil infiltration.

CONCLUSIONS

Activation of STAT3 and the microbial environment are pivotal for gastric tumor initiation and development in the gp130(757F/F) mouse, thus supporting the notion that STAT3 activation may play a role in human gastric cancer development.

Leptin and cancer

The prevalence of obesity has markedly increased over the past two decades, especially in the industrialized countries. While the impact of excess body weight on the development of cardiac disease and diabetes has been well documented, the link between obesity and carcinogenesis is just being recognized. This review will focus on the link between leptin, a cytokine that is elevated in obese individuals, and cancer development. First, we briefly discuss the biological functions of leptin and its signaling pathways. Then, we summarize the effects of leptin on different cancer types in experimental cellular and animal models. Next, we analyze epidemiological data on the relationship between obesity and the presence of cancer or cancer risk in patients. Finally, leptin as a target for cancer treatment and prevention will be discussed.

Secretion of soluble leptin receptors by exocrine and endocrine cells of the gastric mucosa

Leptin is a hormone secreted by the gastric mucosa into the lumen of the stomach. It is present in its intact form in the intestine where it regulates nutrient absorption and intestinal mucosa integrity. We have identified the binding protein that protects leptin from the harsh conditions of the gastric juice. Immunoprecipitations and Western blot analyses demonstrated that leptin is present in the gastric mucosa and the gastric juice, bound to a protein corresponding to the extracellular domain of the leptin receptor. In the absence of this soluble receptor, leptin is rapidly degraded. Immunocytochemistry on rat gastric mucosa identified the cells and intracellular compartments involved in secretion of this complex. Leptin receptor extracellular domain and leptin are present along the rough endoplasmic reticulum-Golgi-granules secretory pathways and form a complex in the secretory granules of Chief and specific endocrine cells. The long-form membrane leptin receptor OB-Rb, the protease activator furin, and proprotein convertase 7 were found in Chief cell granules but not in those of endocrine cells. The shedding of the receptor occurs in the immature granules. It is concluded that in the immature secretory granules of Chief cells, furin activates proprotein convertase 7 that, in turn, cleaves the extracellular portion of membrane-bound leptin receptors. Leptin bound to its soluble receptor forms a complex that is resistant to the gastric juice. Endocrine cells, on the other hand, generate a soluble leptin receptor by mechanisms different from those of the exocrine cells.

Endocrine and exocrine secretion of leptin by the gastric mucosa

Leptin is a hormone that plays important roles in nutritional status and in obesity. By means of immunocytochemistry, two populations of leptin-secreting cells were found in the lower half of the gastric mucosa. One consists of numerous large cells located around the gastric pits, the Chief epithelial cells, whereas the second refers to much smaller cells, strongly stained, few in number, and scattered between the gastric pits, the endocrine cells. By double immunostaining, leptin and pepsinogen were colocalized in the Chief cells, whereas the endocrine cells were positive only for leptin. Immunoelectron microscopy showed that leptin is present along the rough endoplasmic reticulum-Golgi-granules secretory pathways of the Chief and endocrine cells. On the other hand, leptin-receptor (long and short forms) immunolabelings, although absent in the gastric epithelial cell plasma membranes, were present in enterocytes at the level of their apical and basolateral membranes. Duodenal, jejunal, and ileal enterocytes displayed similar labelings for the leptin receptor. Thus, exocrine and endocrine secretions of leptin together with the presence of leptin receptors on enterocyte plasma membranes constitute a gastroenteric axis that coordinates the role played by leptin in the digestive tract.

Mechanisms of action of leptin in preventing gastric ulcer

AIM: To investigate the effects of leptin (1-20 μg/kg) on acidified ethanol (AE)- and indomethacin (Indo)-induced gastric lesions in rats and compare it with ranitidine, lanso-prazole, and omeprazole and to determine its mechanisms of actions.

METHODS: Gastric ulcers, which were approximately 1 mm in width, formed in the glandular portion of the gastric mucosa produced by oral administration of either AE or Indo were taken as ulcer index. The inhibitory effect of subcutaneous administration of leptin, two proton pump inhibitors (PPIs) lansoprazole and omeprazole, or H₂-receptor antagonist ranitidine 30 min before AE or Indo was evaluated. A radioimmunoassay was used to determine the PGE₂ concentration in the homogenate of the glandular portion of the stomach. We performed histological study of the glandular stomach for the evaluation of total, acidic, and sulfated mucus content.

RESULTS: Subcutaneous administration of leptin, two PPIs lansoprazole and omeprazole or H₂-receptor antagonist ranitidine 30 min before AE or Indo produced a dose-dependent and reproducible inhibition of gastric ulcers (GUs). This inhibition was found to be more potent than other antagonists used. In N^G-nitro L-arginine methyl ester (L-NAME)-pretreated animals, the ulcer prevention ability of leptin in AE-induced ulcer was significantly reduced, compared to rats without L-NAME pretreatment. However, the ulcer prevention ability of leptin was not altered by L-NAME treatment in Indo-induced ulcers. Leptin produced a dose-dependent increase in PGE₂ level in the gastric glandular tissues. Leptin also increased mucus secretion.

CONCLUSION: The results of the present study show that leptin inhibits GU formation by AE or Indo in a dose-dependent and reproducible manner in rats. The results also suggest that leptin prevents ulcer formation by increasing the activities of the cyclo-oxygenase and/or nitric oxide pathways and by increasing mucus secretion.

Implication of STAT3 Signaling in Human Colonic Cancer Cells during Intestinal Trefoil Factor 3 (TFF3) – and Vascular Endothelial Growth Factor–Mediated Cellular Invasion and Tumor Growth

Signal transducer and activator of transcription (STAT) 3 is overexpressed or activated in most types of human tumors and has been classified as an oncogene. In the present study, we investigated the contribution of the STAT3s to the proinvasive activity of trefoil factors (TFF) and vascular endothelial growth factor (VEGF) in human colorectal cancer cells HCT8/S11 expressing VEGF receptors. Both intestinal trefoil peptide (TFF3) and VEGF, but not pS2 (TFF1), activate STAT3 signaling through Tyr705 phosphorylation of both STAT3α and STAT3β isoforms. Blockade of STAT3 signaling by STAT3β, depletion of the STAT3α/β isoforms by RNA interference, and pharmacologic inhibition of STAT3α/β phosphorylation by cucurbitacin or STAT3 inhibitory peptide abrogates TFF- and VEGF-induced cellular invasion and reduces the growth of HCT8/S11 tumor xenografts in athymic mice. Differential gene expression analysis using DNA microarrays revealed that overexpression of STAT3β down-regulates the VEGF receptors Flt-1, neuropilins 1 and 2, and the inhibitor of DNA binding/differentiation (Id-2) gene product involved in the neoplastic transformation. Taken together, our data suggest that TFF3 and the essential tumor angiogenesis regulator VEGF165 exert potent proinvasive activity through STAT3 signaling in human colorectal cancer cells. We also validate new therapeutic strategies targeting STAT3 signaling by pharmacologic inhibitors and RNA interference for the treatment of colorectal cancer patients.

High leptin level is accompanied with decreased long leptin receptor transcript in histamine deficient transgenic mice

Leptin is a multifunctional cytokine and hormone that primarily acts in the hypothalamus and plays a key role in regulation of food intake and energy expenditure. Leptin acts through its receptor (OBR), the product of db gene that activates the Jak/STAT pathway predominantly. To exert its functions, leptin interacts with histamine as well. Histamine is a downstream effector of leptin as its release, metabolism is enhanced by leptin and hypothalamic histamine reduces food intake. In a bi-directional regulatory loop histamine also influences leptin concentration by inhibiting its expression. In this study we demonstrate that histamine deficiency elevates serum leptin level and decreases full-length leptin receptor isoform with a slight increase of the short one and results in mild late onset obesity. These observation can help to elucidate further the bi-lateral interaction of leptin and histamine, and therefore provide useful data to understand the pathomechanism of obesity.

Effects of insulin, leptin, and glucagon on ghrelin secretion from isolated perfused rat stomach

Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor, was originally purified from the rat stomach. Although ghrelin has been recognized as an important regulator of energy metabolism, the regulation of the ghrelin secretion is largely unknown. Here, we examined the direct effects of insulin, leptin, and glucagon on the release of ghrelin from the isolated rat stomach. The isolated pancreas-spleen-duodenum deprived preparation of rat stomach was used. After a baseline control infusion into the left gastric artery, insulin, leptin, or glucagon were infused for 15 min at concentrations of 0.1, 1, and 10 nM. The levels of immunoreactive ghrelin in the venous effluents were measured with a radioimmunoassay. Insulin and leptin inhibited ghrelin secretion dose-dependently (total amount of ghrelin release: insulin at 1 nM, 73.5+/-7.3% of the control infusion; leptin at 1 nM, 81.8+/-2.5% of the control infusion; n=5, P<0.05), while glucagon increased it dose-dependently (total amount of ghrelin released at 10 nM was 143.9+/-19.3% of the control infusion; n=5, P<0.01). These results indicate that the ghrelin responses observed in vivo could be due to direct effects of multiple hormonal signals on the stomach.

Gastric secretion

PURPOSE OF REVIEW

The purpose of this chapter is to summarize and place into perspective the past year's literature regarding the regulation of gastric exocrine and endocrine secretion.

RECENT FINDINGS

To prevent acid and pepsin from overwhelming mucosal defense mechanisms and causing injury, the secretion of gastric acid is precisely regulated by a variety of central (eg, neuropeptide Y, corticotropin-releasing factor, and neuromedin U) and peripheral (eg, gastrin, histamine, acetylcholine, somatostatin, cholecystokinin, calcitonin gene-related peptide, leptin, and parietal cell) pathways. These pathways regulate the acid-producing parietal cell directly and/or indirectly by regulating the secretion of histamine from enterochromaffin-like cells, gastrin from G cells, and somatostatin from D cells. Recently, genetically engineered mouse models have been used to reevaluate the neural, hormonal, and paracrine pathways that physiologically regulate acid secretion.

SUMMARY

An improved understanding of the pathways and mechanisms regulating gastric acid secretion should lead to the development of novel therapies to prevent and treat acid-peptic disorders as well as circumvent the adverse effects of currently used antisecretory medications such as the acid rebound observed after discontinuation of proton pump inhibitors.

Serum leptin level in patients with functional dyspepsia

BACKGROUND/GOAL: Previous studies have shown that leptin plays a major role in the amount of food consumption. Recently, leptin and its receptors have been found in the human gastric mucosa. The aim of this study was to seek any possible correlation between serum leptin level and subtypes and pathological findings in functional dyspepsia.

MATERIALS AND METHODS

In a prospective study, we randomly select 44 patients as dysmotility-like and ulcer-like dyspepsia (according to ROME II criteria) in two equivalent groups and compared them with 22 healthy volunteers control group who matched the patients in relation to age, sex and body mass index. From each patient, a fasting blood sample for leptin level and two antral biopsies for evaluating the intensity of gastritis and Helicobacter pylori infection were provided and compared with the control group.

RESULTS

Compared to the control group, serum leptin level was significantly higher in patients with dysmotility-like dyspepsia (P < 0.05). Leptin level were also significantly correlated with the presence of gastritis and H. pylori infection (P < 0.05).

CONCLUSION

Leptin may have a role in the pathogenesis of the dysmotility variety of non-ulcer dyspepsia through mechanisms other than H. pylori infection. Further studies based on gastric leptin immunohistochemistry may need correlation between symptoms of functional dyspepsia and gastric leptin expression.

Disruption of STAT3 signaling leads to tumor cell invasion through alterations of homotypic cell-cell adhesion complexes

STAT3 is frequently overexpressed and constitutively activated by tyrosine phosphorylation during malignant transformation. Despite the clear importance of STAT3 in cell proliferation and survival in diverse human cancers, its possible contribution to tumor cell adhesion, motility and invasion remains hypothetical. We therefore compared the transforming properties of STAT3wt, its constitutively activated dimeric form STAT3C, and the dominant negative mutant STAT3-Y705F in human colorectal HCT8/S11 cancer cells. Both STAT3wt and STAT3C exert a permissive action to the proinvasive activity of the scatter factor HGF in HCT8/S11 cells. In contrast, the monomeric and cytoplasmic mutant Y705F induces a constitutive invasive phenotype through Wnt/Rho-independent and EGFR/PI3-kinase-dependent pathways. Accordingly, Y705F decreases cell-cell homotypic adhesions, and increases cell motility and scattering, as well as lamellipodia-type cellular extensions. STAT3-Y705F-transfected HCT8/S11 cells display an increased tyrosine phosphorylation of the cell-cell adhesion regulator beta-catenin and its dissociation from the invasion suppressor E-cadherin at cell-cell contacts. Our data imply that both invasion promoter and repressor genes are controlled by the canonical STAT3 transcription pathways. Disruption of this cascade by Y705F reveals the proinvasive potential of altered forms of STAT3 as a persistent signaling adaptor in cytokine/transforming growth factor receptor scaffolds and oncogenic pathways.

Ghrelin, appetite, and gastric motility: the emerging role of the stomach as an endocrine organ

Recent progress in the field of energy homeostasis was triggered by the discovery of adipocyte hormone leptin and revealed a complex regulatory neuroendocrine network. A late addition is the novel stomach hormone ghrelin, which is an endogenous agonist at the growth hormone secretagogne receptor and is the motilin-related family of regulatory peptides. In addition to its ability to stimulate GH secretion and gastric motility, ghrelin stimulates appetite and induces a positive energy balance leading to body weight gain. Leptin and ghrelin are complementary, yet antagonistic, signals reflecting acute and chronic changes in energy balance, the effects of which are mediated by hypothalamic neuropeptides such as neuropeptide Y and agouti-related peptide. Endocrine and vagal afferent pathways are involved in these actions of ghrelin and leptin. Ghrelin is a novel neuroendocrine signal possessing a wide spectrum of biological activities that illustrates the importance of the stomach in providing input into the brain. Defective ghrelin signaling from the stomach could contribute to abnormalities in energy balance, growth, and associated gastrointestinal and neuroendocrine functions.

Expression of leptin receptor (Ob-R) isoforms and signal transducers and activators of transcription (STATs) mRNAs in the mouse taste buds

Leptin is a hormone that regulates food intake, energy expenditure and body weight. Our previous studies have demonstrated that the taste organ is a new peripheral target for leptin in mice. Leptin selectively inhibits the responses of taste nerves and receptor cells to sweet substances without affecting responses to sour, salty, and bitter substances. Still, there is no convincing evidence for the existence of leptin receptors (Ob-Rs) in taste receptor cells, especially the functional isoform Ob-Rb. We investigated the expression of 5 different Ob-R isoforms (a-e) and 6 STAT (signal transducers and activators of transcription) members in mouse taste cells. STATs are considered to be involved in the leptin signaling via Ob-Rb. Semiquantitative RT-PCR analysis showed that Ob-Rb was expressed in the taste buds of the fungiform and circumvallate papillae, but not so clearly in the surrounding epithelial tissue. The expression pattern among the three different tissues was similar to that of the taste cell specific G-protein, alpha-gustducin. The other Ob-R isoforms were widely detected in either the taste papillae or the epithelial tissue. Among 6 STAT members, STAT3 showed the highest relative abundance of mRNA in the taste buds. Consistently, in situ hybridization analysis showed that while Ob-Rb and STAT3 signals were detected in some taste bud cells, the signals were not clearly observed in the epithelial tissue cells. In conclusion, the present study provides evidence of the existence of the leptin receptor, Ob-Rb, and STAT3 in the mouse taste bud cells. This finding further confirms the involvement of leptin in the control of taste sensitivities to sweet substances in mice.

Helicobacter pylori persistence: biology and disease

Helicobacter pylori are bacteria that have coevolved with humans to be transmitted from person to person and to persistently colonize the stomach. Their population structure is a model for the ecology of the indigenous microbiota. A well-choreographed equilibrium between bacterial effectors and host responses permits microbial persistence and health of the host but confers risk of serious diseases, including peptic ulceration and gastric neoplasia.

The many lives of leptin

Leptin is a 16,000-Da protein which is secreted by fat but acts within the brain to regulate adiposity. Our Peptides Classic addressed the mystery of how such a large molecule could negotiate the blood-brain barrier (BBB), a structure which normally excludes proteins from the brain. We found that leptin was transported across the BBB by a saturable transport system. This finding was important to understanding how satiety-related peptides and proteins worked, but it was also important to the concept that the BBB is a regulatory interface important in brain-body communication. Obesity in humans and many animals is associated with a leptin resistant state rather than a leptin deficiency. Subsequent work has shown that a defect in the BBB transport of leptin is key in producing and reinforcing this state of resistance. Leptin is pluripotent and the concept of it being primarily an adipostat is being discarded for more encompassing views. Consideration of the BBB data would favor the view that ancestral levels of leptin were much lower than those currently considered normal and are consistent with leptin acting as a metabolic switch, informing the brain when fat reserves are adequate to direct energy expenditures towards activities other than seeking calories.

Helicobacter pylori persistence: biology and disease

Helicobacter pylori are bacteria that have coevolved with humans to be transmitted from person to person and to persistently colonize the stomach. Their population structure is a model for the ecology of the indigenous microbiota. A well-choreographed equilibrium between bacterial effectors and host responses permits microbial persistence and health of the host but confers risk of serious diseases, including peptic ulceration and gastric neoplasia.

Leptin reduces the development of the initial precancerous lesions induced by azoxymethane in the rat colonic mucosa

BACKGROUND & AIMS

Recent studies suggest that leptin, a hormone involved in food intake regulation, released into the circulation and gastrointestinal juice, may be a growth factor for intestine and may be involved in carcinogenesis; however, data are contradictory. This study investigates in rat colonic mucosa (1) the effects of hyperleptinemia on epithelial cell proliferation and development of aberrant crypts, earliest preneoplastic lesions, and (2) whether luminal leptin affects cell proliferation.

METHODS

Leptin (1 mg/kg/d) or vehicle was administered systemically by miniosmotic pump in Fischer 344 rats either for 7 days (BrdU-labeling indices study) or 23 days (azoxymethane-induced colonic lesions study). The effects of injections or continuous infusion of leptin into the colon were also studied.

RESULTS

In systemic leptin-treated rats, plasma leptin levels were 4- to 5-fold increased (P < 0.008 to P < 0.001); labeling indices were higher in proximal colon than in pair-fed control rats (P = 0.006) but unaffected in distal colon. Unexpectedly, in azoxymethane-treated rats, leptin significantly inhibited aberrant crypt foci formation in the middle and distal colon compared with controls (P = 0.006). Under these conditions, plasma insulin levels were reduced by 41%-58%, but gastrin levels were unchanged. In controls, luminal immunoreactive leptin reached the colon. A 3.6-fold increase in intraluminal leptin had no effect on epithelial cell proliferation.

CONCLUSIONS

This study provides the first evidence that leptin reduces the development of chemically induced precancerous lesions in colon, perhaps through decreased insulinemia, and thus does not support an important role for leptin in carcinogenesis promotion. Moreover, the study indicates that leptin is not a potent growth factor for normal intestine.

Gastric cancer development in mice lacking the SHP2 binding site on the IL-6 family co-receptor gp130

BACKGROUND AND AIMS

We have developed a mouse model of gastric cancer that resembles human intestinal-type adenocarcinoma. The aim of this study was to determine the identity and temporal changes in mediators of IL-6 signaling regulating tumor development.

METHODS

gp130(757F/F) Mice that lack the SHP2-binding site on the IL-6 family receptor gp130 and have increased STAT 3 activity and wild-type littermates were used. Cohorts were assessed by quantitative histology and immunohistochemistry for gastric cell phenotype and proliferation markers from 4 to 40 weeks of tumor development. Northern blotting and in situ hybridization were used to quantify expression of the tumor suppressor TFF1 and the mitogens gastrin and Reg I. Expression of epidermal growth factor receptor (EGFr) and its ligands was measured by RT-PCR analysis. Age-matched differences in gene expression profiles were tested by ANOVA.

RESULTS

Hyperplastic antral tumors with inflammation and ulceration were evident in gp130(757F/F) mice at 4 weeks of age and reached maximum size by 20 weeks. Tumor progression was marked by gastritis, atrophy, intestinal metaplasia, dysplasia, and submucosal invasion after 30 weeks. Both TFF1 and gastrin expression were progressively inhibited during tumorigenesis, whereas Reg I was stimulated. The EGFr and its ligands transforming growth factor (TGF)-alpha and heparin-binding EGF had increased expression corresponding to maximal tumor growth.

CONCLUSIONS

gp130(757F/F) Mice rapidly develop distal stomach tumors, with loss of SHP2/Erk/AP-1 transcriptional regulation exemplified by decreased TFF1 expression and increased STAT1/3 regulated genes such as Reg I. Tumor development occurs in a hypogastrinemic environment. Balanced IL-6 signaling is required for maintaining gastric homeostasis.

Partial cloning and localization of leptin and leptin receptor in the mammary gland of the Egyptian water buffalo

Originally an overall metabolic control was attributed to the leptin hormone, which is produced mainly by the adipose tissue. Recently, leptin gene expression was demonstrated in several additional peripheral tissues. Furthermore, several isoforms of leptin receptor were found both in the central nervous system and in the peripheral tissues. Using reverse transcription and polymerase chain reaction analysis we demonstrate that leptin is expressed both in the adipose tissue and in the lactating mammary gland tissue of Egyptian water buffalo. Our results show that, short and long isoforms of leptin receptor are expressed in buffalo mammary gland tissue. We have partially cloned the buffalo leptin and its short and long isoforms of receptor, which show a high sequence homology to previously published sequences of other mammalian species especially to that of other ruminants. Localization of leptin and its receptor mRNA transcripts, as determined by in situ hybridization procedure, revealed that leptin and its receptor transcripts are expressed specifically in the alveolar epithelial cells of the mammary gland. These morphological data support that leptin could also act as an autocrine and paracrine mediator for mammary gland metabolism and as a facilitator of alveolar epithelial cell activity during lactation.

Hyperleptinemia prevents increased plasma ghrelin concentration during short-term moderate caloric restriction in rats

BACKGROUND & AIMS

Ghrelin is an orexigenic hormone secreted by the stomach. Increased plasma ghrelin concentration was reported during diet-induced weight loss in obese humans, suggesting that ghrelin contributes to adaptive increment in appetite associated with caloric restriction. Leptin reduces spontaneous food intake and body weight in rodents. The current study tested the hypothesis that increased plasma leptin prevents the potential increase in plasma ghrelin concentration during moderate caloric restriction in lean rats.

METHODS

Six-month-old male rats (body weight, 367 +/- 9 grams) were randomly assigned to one of the following treatments (8 rats each) for 1 week: (1) leptin subcutaneous infusion to induce moderate hyperleptinemia and moderate caloric restriction (-26% of ad libitum), (2) vehicle infusion and pair feeding, and (3) vehicle infusion and ad libitum feeding.

RESULTS

Leptin-treated (-19 +/- 5 grams) and pair-fed (-19 +/- 2) rats lost weight compared with ad libitum-fed rats (-3 +/- 1, P < 0.05). Compared with control (6.8 +/- 0.7 ng/mL), plasma leptin was higher in leptin-treated (18.6 +/- 0.9 ng/mL, P < 0.01) rats and lower in pair-fed rats (4.3 +/- 0.4 ng/mL, P < 0.05). Plasma ghrelin was substantially higher in calorie-restricted than control rats (2505 +/- 132 pg/mL vs. 1790 +/- 134 pg/mL, P < 0.01), and leptin treatment (1625 +/- 117 pg/mL) completely prevented this change. Plasma ghrelin concentration was negatively correlated with body weight changes in calorie-restricted and control (r = -0.75, P < 0.01) but not in leptin-treated rats (P > 0.8).

CONCLUSIONS

Moderate hyperleptinemia prevents an increase of plasma ghrelin during moderate short-term caloric restriction. Satiety-inducing effects of leptin include suppression of gastric orexigenic signals and disruption of a potential feedback mechanism between body weight changes and plasma ghrelin in lean adult rats.

Role of leptin in the regulation of glucagon-like peptide-1 secretion

Glucagon-like peptide-1 (GLP-1), released from intestinal endocrine L cells, is a potent insulinotropic hormone. GLP-1 secretion is diminished in obese patients. Because obesity is linked to abnormal leptin signaling, we hypothesized that leptin may modulate GLP-1 secretion. Leptin significantly stimulated GLP-1 secretion (by up to 250% of control) from fetal rat intestinal cells, a mouse L cell line (GLUTag), and a human L cell line (NCI-H716) in a dose-dependent manner (P < 0.05-0.001). The long form of the leptin receptor was shown to be expressed, and leptin induced the phosphorylation of STAT3 in the three cell types. The leptin receptor was also expressed by rodent and human intestinal L cells, and leptin (1 mg/kg i.p.) significantly stimulated GLP-1 secretion in rats and ob/ob mice. To determine the effect of leptin resistance on GLP-1 secretion, C57BL/6 mice were fed a high-fat (45%) or low-fat (10%) diet for 8 weeks. Mice on the high-fat diet became obese; developed glucose intolerance, hyperinsulinemia, and hyperleptinemia; and were leptin resistant. Mice on the high-fat diet also had twofold lower basal plasma GLP-1 and a diminished GLP-1 response to oral glucose, by 28.5 +/- 5.0% (P < 0.05). These results show for the first time that leptin stimulates GLP-1 secretion from rodent and human intestinal L cells, and they suggest that leptin resistance may account for the decreased levels of GLP-1 found in obese humans.

Gastric secretion

Overlapping neural, hormonal, and paracrine pathways finely regulate gastric acid secretion. In rats and guinea pigs, most of the intrinsic neural innervation to the gastric mucosa originates in the myenteric plexus. In contrast, human stomachs have a clearly defined submucosal plexus that contains a variety of transmitters including nitric oxide, vasoactive intestinal peptide (VIP), gastrin-releasing peptide (GRP), substance P, and calcitonin gene-related peptide (CGRP). Although GRP is known to participate in meal-stimulated acid secretion by releasing gastrin in a variety of laboratory animals, recent studies were unable to demonstrate a role for endogenous GRP in meal-stimulated gastrin secretion in humans. Pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the secretin-glucagon-VIP family, has been localized to gastric mucosal neurons and may participate in vagally mediated acid secretion. Two novel peptides, ghrelin and leptin, have been localized to the stomach. Peripheral administration of ghrelin stimulates and of leptin inhibits acid secretion. The binding of secretagogues to parietal cells generates changes in second messengers that regulate the translocation and activation of the proton pump, HK-ATPase. In resting cells, HK-ATPase is contained within cytoplasmic tubulovesicles in an inactive form. At stimulation, the tubulovesicles fuse with the apical canaliculi and the HK-ATPase is incorporated into the apical membrane where it actively pumps H ions in exchange for K. Acute infection with Helicobacter pylori results in hypochlorhydria, whereas chronic infection can cause either hypo- or hyperchlorhydria, depending on the distribution of the infection and the degree of corpus gastritis. Recent studies suggest that inflammatory cytokines, produced in response to the organism, can play a role in the perturbations in acid and gastrin secretion induced by H. pylori.

Modulation of exocrine pancreatic secretion by leptin through CCK(1)-receptors and afferent vagal fibres in the rat

In this report, we determined whether leptin could modify the exocrine pancreatic secretion of anaesthetized rats in vivo. Intravenous injection of recombinant murine leptin resulted in a time- and dose-dependent stimulation of exocrine pancreatic secretion, maximally observed with 30 nmol/kg of leptin. This stimulation of pancreatic water, bicarbonate, and protein output was abolished by atropine, hexamethonium, L364,718 ([3S(-)-N-(1,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine]), a cholecystokinin CCK(1) receptor antagonist or perivagal capsaicin pretreatment, but unaffected by the CCK(2) receptor antagonist L365,260 ([3R(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin-3yl)-N'-(3-methylphenyl)urea]). In addition, the physiological dose of 3 nmol/kg leptin, ineffective per se, potentiated the secretory effect of 45 pmol/kg of cholecystokinin octapeptide (CCK-8) on exocrine pancreatic secretion. Furthermore, intraperitoneal leptin induced a rapid increase in plasma CCK levels in vivo in the rat. In conclusion, exogenous leptin can modulate exocrine pancreatic secretion through mechanisms involving CCK(1) receptors and capsaicin-sensitive afferent fibres in the rat. Whether this may have a physiological relevance in the postprandial regulation of exocrine pancreatic secretion and thus in nutrient digestion will require further investigations.