Pilot Study Measuring the Novel Satiety Hormone, Pro-Uroguanylin, in Adolescents With and Without Obesity

Uroguanylin prevents hepatic steatosis, mitochondrial dysfunction and fibrosis in obesity-associated NAFLD

BACKGROUND

The biological mediators supporting the resolution of liver steatosis, inflammation and fibrosis after bariatric surgery in patients with obesity and NAFLD remain unclear. We sought to analyze whether uroguanylin and guanylin, two gut hormones involved in the regulation of satiety, food preference and adiposity, are involved in the amelioration of obesity-associated NAFLD after bariatric surgery.

METHODS

Proguanylin (GUCA2A) and prouroguanylin (GUCA2B) were measured in 214 participants undergoing bariatric surgery with biopsy-proven NAFLD diagnosis. Pathways involved in lipid metabolism, mitochondrial network and fibrogenesis were evaluated in liver biopsies (n = 137). The effect of guanylin and uroguanylin on these metabolic functions was assessed in HepG2 hepatocytes and LX-2 hepatic stellate cells (HSC) under lipotoxic and profibrogenic conditions.

RESULTS

Plasma and hepatic expression of GUCA2B were decreased in obesity-associated NAFLD. Both GUCA2A and GUCA2B levels were increased after sleeve gastrectomy and Roux-en-Y gastric bypass in parallel to the improved liver function. The liver of patients with type 2 diabetes showed impaired mitochondrial β-oxidation, biogenesis, dynamics as well as increased fibrosis. Uroguanylin diminished the lipotoxicity in palmitate-treated HepG2 hepatocytes, evidenced by decresased steatosis and lipogenic factors, as well as increased mitochondrial network expression, AMPK-induced β-oxidation and oxygen consumption rate. Additionally, uroguanylin, but not guanylin, reversed HSC myofibroblast transdifferentiation as well as fibrogenesis after TGF-β1 stimulation.

CONCLUSIONS

Uroguanylin constitutes a protective factor against lipotoxicity, mitochondrial dysfunction and fibrosis. Increased GUCA2B levels might contribute to improve liver injury in patients with obesity-associated NAFLD after bariatric surgery.

Reduced Plasma Guanylin Levels Following Enterotoxigenic Escherichia coli-Induced Diarrhea

The intestinal peptide hormones guanylin (GN) and uroguanylin (UGN) interact with the epithelial cell receptor guanylate cyclase C to regulate fluid homeostasis. Some enterotoxigenic Escherichia coli (ETEC) produce heat-stable enterotoxin (ST), which induces diarrhea by mimicking GN and UGN. Plasma concentrations of prohormones of GN (proGN) and UGN (proUGN) are reportedly decreased during chronic diarrheal diseases. Here we investigate whether prohormone concentrations also drop during acute diarrhea caused by ST-producing ETEC strains TW10722 and TW11681. Twenty-one volunteers were experimentally infected with ETEC. Blood (n = 21) and urine (n = 9) specimens were obtained immediately before and 1, 2, 3, and 7 days after ETEC ingestion. Concentrations of proGN and proUGN were measured by ELISA. Urine electrolyte concentrations were measured by photometry and mass spectrometry. Ten volunteers developed diarrhea (D group), and eleven did not (ND group). In the D group, plasma proGN, but not proUGN, concentrations were substantially reduced on days 2 and 3, coinciding with one day after diarrhea onset. No changes were seen in the ND group. ETEC diarrhea also seemed to affect diuresis, the zinc/creatinine ratio, and sodium and chloride secretion levels in urine. ETEC-induced diarrhea causes a reduction in plasma proGN and could potentially be a useful marker for intestinal isotonic fluid loss.

The Intestinal Tract Brush Border in Young Children Uniformly Expresses Guanylate Cyclase C

The present study examined staining of guanylate cyclase C (GCC/GUCY2C) in the small and large intestines of children younger than age 7 years. Normal intestinal tissue from children aged 0 to 7 years was stained using GCC, uroguanylin, and villin antibodies and scored for staining intensity. A subset underwent quantitative real-time polymerase chain reaction. Data were analyzed using t test of independent means, descriptive statistics, and logistic regression. Four hundred sixty-four specimens underwent immunohistochemistry; 291 specimens underwent real-time polymerase chain reaction. GCC, villin, and uroguanylin were detected across age groups and anatomic sites. No significant differences were identifiable across age groups. GUCY2C and uroguanylin mRNA was detected in all samples, with no variability of statistical significance of either target-to-villin normalization between any age cohorts. A gradient of expression of GCC across age groups does not seem to exist.

Neurohormonal Changes in the Gut-Brain Axis and Underlying Neuroendocrine Mechanisms following Bariatric Surgery

Obesity is a complex, multifactorial disease that is a major public health issue worldwide. Currently approved anti-obesity medications and lifestyle interventions lack the efficacy and durability needed to combat obesity, especially in individuals with more severe forms or coexisting metabolic disorders, such as poorly controlled type 2 diabetes. Bariatric surgery is considered an effective therapeutic modality with sustained weight loss and metabolic benefits. Numerous genetic and environmental factors have been associated with the pathogenesis of obesity, while cumulative evidence has highlighted the gut-brain axis as a complex bidirectional communication axis that plays a crucial role in energy homeostasis. This has led to increased research on the roles of neuroendocrine signaling pathways and various gastrointestinal peptides as key mediators of the beneficial effects following weight-loss surgery. The accumulate evidence suggests that the development of gut-peptide-based agents can mimic the effects of bariatric surgery and thus is a highly promising treatment strategy that could be explored in future research. This article aims to elucidate the potential underlying neuroendocrine mechanisms of the gut-brain axis and comprehensively review the observed changes of gut hormones associated with bariatric surgery. Moreover, the emerging role of post-bariatric gut microbiota modulation is briefly discussed.

High plasma and lingual uroguanylin as potential contributors to changes in food preference after sleeve gastrectomy

BACKGROUND

The biological mediators supporting long-term weight loss and changes in dietary choice behaviour after sleeve gastrectomy remain unclear. Guanylin and uroguanylin are gut hormones involved in the regulation of satiety, food preference and adiposity. Thus, we sought to analyze whether the guanylin system is involved in changes in food preference after sleeve gastrectomy in obesity.

METHODS

Proguanylin (GUCA2A) and prouroguanylin (GUCA2B) were determined in patients with severe obesity (n = 41) as well as in rats with diet-induced obesity (n = 48), monogenic obesity (Zucker fa/fa) (n = 18) or in a food choice paradigm (normal diet vs high-fat diet) (n = 16) submitted to sleeve gastrectomy. Lingual distribution and expression of guanylins (GUCA2A and GUCA2B) and their receptor GUCY2C as well as the fatty acid receptor CD36 were evaluated in the preclinical models.

RESULTS

Circulating concentrations of GUCA2A and GUCA2B were increased after sleeve gastrectomy in patients with severe obesity as well as in rats with diet-induced and monogenic (fa/fa) obesity. Interestingly, the lower dietary fat preference observed in obese rats under the food choice paradigm as well as in patients with obesity after sleeve gastrectomy were negatively associated with post-surgical GUCA2B levels. Moreover, sleeve gastrectomy upregulated the low expression of GUCA2A and GUCA2B in taste bud cells of tongues from rats with diet-induced and monogenic (fa/fa) obesity in parallel to a downregulation of the lingual lipid sensor CD36.

CONCLUSIONS

The increased circulating and lingual GUCA2B after sleeve gastrectomy suggest an association between the uroguanylin-GUCY2C endocrine axis and food preference through the regulation of gustatory responses.

Receptor Guanylyl Cyclase C and Cyclic GMP in Health and Disease: Perspectives and Therapeutic Opportunities

Receptor Guanylyl Cyclase C (GC-C) was initially characterized as an important regulator of intestinal fluid and ion homeostasis. Recent findings demonstrate that GC-C is also causally linked to intestinal inflammation, dysbiosis, and tumorigenesis. These advances have been fueled in part by identifying mutations or changes in gene expression in GC-C or its ligands, that disrupt the delicate balance of intracellular cGMP levels and are associated with a wide range of clinical phenotypes. In this review, we highlight aspects of the current knowledge of the GC-C signaling pathway in homeostasis and disease, emphasizing recent advances in the field. The review summarizes extra gastrointestinal functions for GC-C signaling, such as appetite control, energy expenditure, visceral nociception, and behavioral processes. Recent research has expanded the homeostatic role of GC-C and implicated it in regulating the ion-microbiome-immune axis, which acts as a mechanistic driver in inflammatory bowel disease. The development of transgenic and knockout mouse models allowed for in-depth studies of GC-C and its relationship to whole-animal physiology. A deeper understanding of the various aspects of GC-C biology and their relationships with pathologies such as inflammatory bowel disease, colorectal cancer, and obesity can be leveraged to devise novel therapeutics.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

Mutational landscape of receptor guanylyl cyclase C: Functional analysis and disease-related mutations

Guanylyl cyclase C (GC-C) is the receptor for the heat-stable enterotoxin, which causes diarrhea, and the endogenous ligands, guanylin and uroguanylin. GC-C is predominantly expressed in the intestinal epithelium and regulates fluid and ion secretion in the gut. The receptor has a complex domain organization, and in the absence of structural information, mutational analysis provides clues to mechanisms of regulation of this protein. Here, we review the mutational landscape of this receptor that reveals regulatory features critical for its activity. We also summarize the available information on mutations in GC-C that have been reported in humans and contribute to severe gastrointestinal abnormalities. Since GC-C is also expressed in extra-intestinal tissues, it is likely that mutations thus far reported in humans may also affect other organ systems, warranting a close observation of these patients in future.

Activation of brown adipose tissue in diet-induced thermogenesis is GC-C dependent

Uroguanylin (UGN) is released from the intestine after a meal. When applied in brain ventricles, UGN increases expression of markers of thermogenesis in brown adipose tissue (BAT). Therefore, we determine the effects of its receptor, guanylate cyclase C (GC-C), on mouse interscapular BAT (iBAT) activity during diet-induced thermogenesis (DIT). The activation of iBAT after a meal is diminished in GC-C KO mice, decreased in female wild type (WT) mice, and abolished in old WT animals. The activation of iBAT after a meal is the highest in male WT animals which leads to an increase in GC-C expression in the hypothalamus, an increase in iBAT volume by aging, and induction of iBAT markers of thermogenesis. In contrast to iBAT activation after a meal, iBAT activation after a cold exposure could still exist in GC-C KO mice and it is significantly higher in female WT mice. The expression of GC-C in the proopiomelanocortin neurons of the arcuate nucleus of the hypothalamus but not in iBAT suggests central regulation of iBAT function. The iBAT activity during DIT has significantly reduced in old mice but an intranasal application of UGN leads to an increase in iBAT activity in a dose-dependent manner which is in strong negative correlation to glucose concentration in blood. This activation was not present in GC-C KO mice. Our results suggest the physiological role of GC-C on the BAT regulation and its importance in the regulation of glucose homeostasis and the development of new therapy for obesity and insulin resistance.

Postprandial regulation of prouroguanylin in humans of a healthy weight and those who are overweight or with obesity

Uroguanylin is a peptide gut hormone proposed to have a role in signalling post meal satiety. Uroguanylin circulates as its pro-hormone, prouroguanylin. There has been limited investigation of the regulation of prouroguanylin by food; therefore we investigated prouroguanylin regulation following meals. In separate experiments we investigated the effects of high calorie (1451 kcal) and medium calorie (725 kcal), high fat meals, on plasma prouroguanylin concentrations. We then examined the effect of a 722.5 kcal high carbohydrate breakfast on prouroguanylin concentrations, comparing the response in healthy weight adults versus those who are overweight/ with obesity. The 1451 kcal meal increased prouroguanylin concentrations, versus fasting at 60 (P < 0.05), 90 (P < 0.01) and 120 (P < 0.001) minutes. After the 725 kcal meal hormone concentrations rose more slowly and were significant versus fasting concentrations at 120 min (P < 0.01). The high carbohydrate breakfast 722.5 kcal, led to an initial suppression of hormone concentrations at 30 min. post meal (P < 0.05) followed by an increase in concentrations until they were significant versus fasting at 120 min. (P < 0.01). Participants overweight/ with obesity had lower fasting prouroguanylin concentrations (P < 0.05), but post meal concentrations did not differ between the groups. Our results suggest there is a delayed increase in prouroguanylin concentrations following, large and regular sized mixed macronutrient meals rich in fat or carbohydrate. Fasting levels are suppressed in people who are overweight/ with obesity, but the post meal response remains intact. There may be potential to target post meal release of prouroguanylin in obesity.

Two distinct GUCY2C circuits with PMV (hypothalamic) and SN/VTA (midbrain) origin

Guanylyl cyclase C (GUCY2C) is the afferent central receptor in the gut-brain endocrine axis regulated by the anorexigenic intestinal hormone uroguanylin. GUCY2C mRNA and protein are produced in the hypothalamus, a major center regulating appetite and metabolic homeostasis. Further, GUCY2C mRNA and protein are expressed in the ventral midbrain, a principal structure regulating hedonic reward from behaviors including eating. While GUCY2C is expressed in hypothalamus and midbrain, its precise neuroanatomical organization and relationship with circuits regulating satiety remain unknown. Here, we reveal that hypothalamic GUCY2C mRNA is confined to the ventral premammillary nucleus (PMV), while in midbrain it is produced by neurons in the ventral tegmental area (VTA) and substantia nigra (SN). GUCY2C in the PMV is produced by 46% of neurons expressing anorexigenic leptin receptors, while in the VTA/SN it is produced in most tyrosine hydroxylase-immunoreactive neurons. In contrast to mRNA, GUCY2C protein is widely distributed throughout the brain in canonical sites of PMV and VTA/SN axonal projections. Selective stereotaxic ablation of PMV or VTA/SN neurons eliminated GUCY2C only in their respective canonical projection sites. Conversely, specific anterograde tracer analyses of PMV or VTA/SN neurons confirmed distinct GUCY2C-immunoreactive axons projecting to those canonical locations. Together, these findings reveal two discrete neuronal circuits expressing GUCY2C originating in the PMV in the hypothalamus and in the VTA/SN in midbrain, which separately project to other sites throughout the brain. They suggest a structural basis for a role for the GUCY2C-uroguanylin gut-brain endocrine axis in regulating homeostatic and behavioral components contributing to satiety.

Blunted Evoked Prouroguanylin Endocrine Secretion in Chronic Constipation

OBJECTIVES

Prouroguanylin (ProUGN) in the intestine is cleaved to form uroguanylin (UGN), which stimulates guanylate cyclase C (GUCY2C), inducing cyclic guanosine monophosphate signaling. Paracrine release regulates fluid secretion, contributing to bowel function, whereas endocrine secretion evoked by eating forms a gut-brain axis, controlling appetite. Whereas hormone insufficiency contributes to hyperphagia in obesity, its contribution to the pathophysiology of constipation syndromes remains unexplored. Here, we compared circulating ProUGN and UGN in healthy subjects and in patients with chronic idiopathic constipation (CIC) and patients with irritable bowel syndrome with constipation (IBS-C).

METHODS

Circulating ProUGN and UGN levels were measured in 60 healthy subjects, 53 patients with CIC, and 54 patients with IBS-C. After an overnight fast, the participants ingested a standardized meal; blood samples were drawn at fasting and at 30, 60, and 90 minutes thereafter, and hormone levels were quantified by enzyme-linked immunosorbent assay.

RESULTS

Fasting ProUGN levels were >30% lower in patients with CIC and those with IBS-C compared with healthy subjects regardless of age, sex, or disease state. After eating, ProUGN levels increased compared with fasting levels, although the rate of change was slower and maximum levels were lower in patients with CIC and those with IBS-C. Similarly, fasting UGN levels were lower in patients with CIC and those with IBS-C compared with healthy subjects. However, unlike ProUGN levels, UGN levels did not increase after eating.

DISCUSSION

These observations support a novel pathophysiologic model in which CIC and IBS-C reflect a contribution of ProUGN insufficiency dysregulating intestinal fluid and electrolyte secretion.

TRANSLATIONAL IMPACT

This study suggests that CIC and IBS-C can be treated by oral GUCY2C hormone replacement. Indeed, these observations provide a mechanistic framework for the clinical utility of oral GUCY2C ligands like plecanatide (Trulance) and linaclotide (Linzess) to treat CIC and IBS-C.

Uroguanylin Improves Leptin Responsiveness in Diet-Induced Obese Mice

The gastrointestinal-brain axis is a key mediator of the body weight and energy homeostasis regulation. Uroguanylin (UGN) has been recently proposed to be a part of this gut-brain axis regulating food intake, body weight and energy expenditure. Expression of UGN is regulated by the nutritional status and dependent on leptin levels. However, the exact molecular mechanisms underlying this UGN-leptin metabolic regulation at a hypothalamic level still remains unclear. Using leptin resistant diet-induced obese (DIO) mice, we aimed to determine whether UGN could improve hypothalamic leptin sensitivity. The present work demonstrates that the central co-administration of UGN and leptin potentiates leptin’s ability to decrease the food intake and body weight in DIO mice, and that UGN activates the hypothalamic signal transducer and activator of transcription 3 (STAT3) and phosphatidylinositide 3-kinases (PI3K) pathways. At a functional level, the blockade of PI3K, but not STAT3, blunted UGN-mediated leptin responsiveness in DIO mice. Overall, these findings indicate that UGN improves leptin sensitivity in DIO mice.

Circulating Pro-Uroguanylin Levels In Children And Their Relation To Obesity, Sex And Puberty

Uroguanylin is a 16 amino acid peptide that constitutes a key component of the gut- brain axis with special relevance in body weight regulation. In childhood and adolescence, periods of life with notable metabolic changes; limited data exist, with measurements of pro-uroguanylin in adolescence but not in prepubertal children. This study investigates pro-uroguanylin circulating levels in children with obesity and its relationship with obesity, sex and pubertal development. We analyzed circulating prouroguanylin levels in 117 children (62) and adolescents (55), including 73 with obesity and 44 with normal weight. The pro-uroguanylin concentration is higher in lean girls during pre-puberty versus lean boys (1111 vs 635, p < 0.001). During puberty, pro-uroguanylin levels are higher in lean males with respect to lean females (1060 vs 698, p < 0.01). In girls, a negative correlation exists between pro-uroguanylin and age, Tanner stage, weight, height, BMI (body mass index), waist circumference and plasma levels of leptin and testosterone; a positive correlation was found between pro-uroguanylin and free triiodothyronine. In boys, a positive correlation was found between pro-uroguanylin and BMI and waist circumference and a negative correlation was found with high density lipoprotein-cholesterol. We conclude that a sexual dimorphism exists in circulating pro-uroguanylin levels with respect to BMI. Uroguanylin presents also an opposed circulating pattern during puberty in both sexes.