Genetic ablation of carbonic anhydrase IX disrupts gastric barrier function via claudin-18 downregulation and acid backflux.

AIM: This study aimed to explore the molecular mechanisms for the parietal cell loss and fundic hyperplasia observed in gastric mucosa of mice lacking the carbonic anhydrase 9 (CAIX). METHODS: We assessed the ability of CAIX-knockout and WT gastric surface epithelial cells to withstand a luminal acid load by measuring the pHi of exteriorized gastric mucosa in vivo using two-photon confocal laser scanning microscopy. Cytokines and claudin-18A2 expression was analysed by RT-PCR. RESULTS: CAIX-knockout gastric surface epithelial cells showed significantly faster pHi decline after luminal acid load compared to WT. Increased gastric mucosal IL-1ß and iNOS, but decreased claudin-18A2 expression (which confer acid resistance) was observed shortly after weaning, prior to the loss of parietal and chief cells. At birth, neither inflammatory cytokines nor claudin-18 expression were altered between CAIX and WT gastric mucosa. The gradual loss of acid secretory capacity was paralleled by an increase in serum gastrin, IL-11 and foveolar hyperplasia. Mild chronic proton pump inhibition from the time of weaning did not prevent the claudin-18 decrease nor the increase in inflammatory markers at 1 month of age, except for IL-1ß. However, the treatment reduced the parietal cell loss in CAIX-KO mice in the subsequent months. CONCLUSIONS: We propose that CAIX converts protons that either backflux or are extruded from the cells rapidly to CO2 and H2 O, contributing to tight junction protection and gastric epithelial pHi regulation. Lack of CAIX results in persistent acid backflux via claudin-18 downregulation, causing loss of parietal cells, hypergastrinaemia and foveolar hyperplasia.

The effect of pregnane X receptor agonists on postprandial incretin hormone secretion in rats and humans.

We recently showed that pregnane X receptor (PXR) agonists cause hyperglycaemia during oral glucose tolerance test (OGTT) in rats and healthy volunteers (Rifa-1 study). We now aimed to determine if the secretion of incretin hormones, especially glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP), are affected by PXR agonists since these gut-secreted hormones are major regulators of postprandial glucose metabolism. The Rifa-2 study had a one-phase, open-label design. Twelve subjects were given 600 mg of rifampicin a day for a week. OGTT with glucose, insulin, and incretin hormone measurements was performed before and after the rifampicin dosing. Incretins and insulin were analysed in previously collected rat OGTT samples after pregnenolone 16α-carbonitrile (PCN) or control treatment for 4 days. Rifampicin treatment did not affect glucose, insulin, GLP-1, GIP, glucagon, and peptide YY levels statistically significantly. Incremental AUCs (AUCincr) of glucose and insulin tended to increase (41% increase in glucose AUCincr, P = 0.21, 95% confidence interval (CI) of the difference -47, 187; 24% increase in insulin AUCincr, P = 0.084, CI of the difference -110, 1493). Glucagon AUC was increased in women (53% increase, P = 0.028) and decreased in men (19% decrease, P < 0.001) after rifampicin dosing. In combined analysis of human Rifa-1 and Rifa-2 studies, glucose AUCincr was elevated by 63% (P = 0.010) and insulin AUCincr by 37% (P = 0.011). PCN increased rat insulin level at 60 min time point but did not affect incretin and insulin AUCs statistically significantly. In conclusion, PXR agonists do not affect the secretion of incretin hormones. The regulation of glucagon secretion by PXR may be sexually dimorphic in humans. The mechanism of disrupted glucose metabolism induced by PXR activation requires further study.

Neuropeptide Y in the noradrenergic neurones induces obesity and inhibits sympathetic tone in mice.

AIM: Neuropeptide Y (NPY) co-localized with noradrenaline in central and sympathetic nervous systems seems to play a role in the control of energy metabolism. In this study, the aim was to elucidate the effects and pathophysiological mechanisms of increased NPY in catecholaminergic neurones on accumulation of body adiposity. METHODS: Transgenic mice overexpressing NPY under the dopamine-beta-hydroxylase promoter (OE-NPY(DßH) ) and wild-type control mice were followed for body weight gain and body fat content. Food intake, energy expenditure, physical activity, body temperature, serum lipid content and markers of glucose homoeostasis were monitored. Thermogenic and lipolytic responses in adipose tissues, and urine catecholamine and tissue catecholamine synthesizing enzyme levels were analysed as indices of sympathetic tone. RESULTS: Homozygous OE-NPY(DßH) mice showed significant obesity accompanied with impaired glucose tolerance and insulin resistance. Increased adiposity was explained by neither increased food intake or fat absorption nor by decreased total energy expenditure or physical activity. Adipocyte hypertrophy and decreased circulating lipid levels suggested decreased lipolysis and increased lipid uptake. Brown adipose tissue thermogenic capacity was decreased and brown adipocytes filled with lipids. Enhanced response to adrenergic stimuli, downregulation of catecholamine synthesizing enzyme expressions in the brainstem and lower adrenaline excretion supported the notion of low basal catecholaminergic activity. CONCLUSION: Increased NPY in catecholaminergic neurones induces obesity that seems to be a result of preferential fat storage. These results support the role of NPY as a direct effector in peripheral tissues and an inhibitor of sympathetic activity in the pathogenesis of obesity.

Characterization of sleep-wake patterns in a novel transgenic mouse line overexpressing human prepro-orexin/hypocretin.

AIM: Orexin/hypocretin peptides are expressed in the lateral hypothalamus and involved in the regulation of autonomic functions, energy homeostasis and arousal states. The sleep disorder narcolepsy, which is characterized by excessive daytime sleepiness and occurrence of sudden rapid eye movement (REM) sleep, is associated with a loss of orexin neurones. Our study investigated the effects of orexins on sleep-wake patterns in a novel transgenic mouse line overexpressing the human prepro-orexin (hPPO) gene under the control of its endogenous promoter. METHODS: Orexin overexpression was investigated by PCR, Southern and Western blotting as well as immunohistochemistry. Polysomnographic recordings were performed for analyses of sleep-wake patterns and for electroencephalographic activity during 24 h baseline and during and after 6 h of sleep deprivation (SD). RESULTS: Transgenic hPPO mice had increased expression of human prepro-orexin (hPPO) and orexin-A in the hypothalamus. Transgene expression decreased endogenous orexin-2 receptors but not orexin-1 receptors in the hypothalamus without affecting orexin receptor levels in the basal forebrain, cortex or hippocampus. Transgenic mice compared with their wild type littermates showed small but significant differences in the amount of waking and slow wave sleep, particularly during the light-dark transition periods, in addition to a slight reduction in REM sleep during baseline and during recovery sleep after SD. CONCLUSION: The hPPO-overexpressing mice show a small reduction in REM sleep, in addition to differences in vigilance state amounts in the light/dark transition periods, but overall the sleep-wake patterns of hPPO-overexpressing mice do not significantly differ from their wild type littermates.

Functions of orexins in peripheral tissues.

Orexin A (OXA) and orexin B were originally isolated as hypothalamic peptides regulating sleep, wakefulness and feeding. However, growing evidence suggests that orexins have major functions also in the peripheral tissues. Central orexigenic pathways originating from medulla activate the hypothalamus-pituitary axis and can influence the sympathetic tone. Orexins and their receptors are widely dispersed throughout the intestine, where orexin receptors are regulated by the nutritional status, affect insulin secretion and intestinal motility. Although the primary source of the peptide has not been elucidated, OXA is detected in plasma and its level varies in response to the metabolic state. In this review, we focus on the current knowledge on peripheral functions of orexins and discuss possible endocrine, paracrine and neurocrine roles.