Serotonin Regulates De Novo Lipogenesis in Adipose Tissues through Serotonin Receptor 2A

Exploring the contribution of mammary-derived serotonin on liver and pancreas metabolism during lactation

During lactation, the murine mammary gland is responsible for a significant increase in circulating serotonin. However, the role of mammary-derived serotonin in energy homeostasis during lactation is unclear. To investigate this, we utilized C57/BL6J mice with a lactation and mammary-specific deletion of the gene coding for the rate-limiting enzyme in serotonin synthesis (TPH1, Wap-Cre x TPH1FL/FL) to understand the metabolic contributions of mammary-derived serotonin during lactation. Circulating serotonin was reduced by approximately 50% throughout lactation in Wap-Cre x TPH1FL/FL mice compared to wild-type mice (TPH1FL/FL), with mammary gland and liver serotonin content reduced on L21. The Wap-Cre x TPH1FL/FL mice had less serotonin and insulin immunostaining in the pancreatic islets on L21, resulting in reduced circulating insulin but no changes in glucose. The mammary glands of Wap-Cre x TPH1FL/FL mice had larger mammary alveolar areas, with fewer and smaller intra-lobular adipocytes, and increased expression of milk protein genes (e.g., WAP, CSN2, LALBA) compared to TPH1FL/FL mice. No changes in feed intake, body composition, or estimated milk yield were observed between groups. Taken together, mammary-derived serotonin appears to contribute to the pancreas-mammary cross-talk during lactation with potential implications in the regulation of insulin homeostasis.

Gut microbiota dysbiosis and decreased levels of acetic and propionic acid participate in glucocorticoid-induced glycolipid metabolism disorder

Long-term/high-dose glucocorticoid (GC) use results in glycolipid metabolism disorder, which severely limits its clinical application. The role of the gut microbiota and its metabolites in GC-induced glycolipid metabolism disorder remains unclear. Our previous human study found that obvious gut microbiota dysbiosis characterized by an increasing abundance of Proteobacteria and a decreased abundance of Lachnospiraceae and Faecalibacterium were observed in patients with endogenous hypercortisolism. In this study, we established a mouse model of GC-induced glycolipid metabolism disorder (Dex group) and found that the relative abundances of Proteobacteria and Parasuttrerella were increased, while the abundances of Lachnospiraceae, Faecalibacterium, and Lachnospiraceae_NK4A136_group were decreased significantly in the Dex group. Compared with the control group, serum total short-chain fatty acids (SCFAs), acetic acid, propionic acid, and GLP-1 levels were all decreased in the Dex group. The mRNA expression of the GPR41 receptor and Pcsk1 in the colon was significantly decreased in the Dex group. Furthermore, GC-induced glycolipid metabolism disorder could be alleviated by depletion of the gut microbiota or fecal bacteria transplantation with control bacteria. The abundances of Lachnospiraceae_NK4A136_group and the serum GLP-1 levels were significantly increased, while the abundances of Proteobacteria and Parasutterella were significantly decreased after fecal bacteria transplantation with control bacteria. Our work indicates that gut microbiota dysbiosis and decreased levels of serum acetic acid and propionic acid may participate in GC-induced glycolipid metabolism disorder. These findings may provide novel insights into the prevention and treatment of GC-induced metabolic disorders.IMPORTANCEThe role of the gut microbiota in glucocorticoid (GC)-induced glycolipid metabolism disorder remains unclear. In our study, gut microbiota dysbiosis characterized by an increased abundance of Proteobacteria/Parasuttrerella and a decreased abundance of Lachnospiraceae_NK4A136_group was observed in mice with GC-induced glycolipid metabolism disorder. Some bacteria were shared in our previous study in patients with endogenous hypercortisolism and the mouse model used in the study. Furthermore, the depletion of the gut microbiota and fecal bacteria transplantation with control bacteria could alleviate GC-induced glycolipid metabolism disorder. Plasma acetic acid, propionic acid, and GLP-1 and the mRNA expression of the GPR41 receptor and Pcsk1 in the colon were decreased significantly in mice with GC-induced glycolipid metabolism disorder, which indicated that the gut microbiota/SCFA/GPR41/GLP-1 axis may participate in GC-induced glycolipid metabolism disorder. Our findings indicate that the gut microbiota may serve as a novel therapeutic target for GC-related metabolic disorders.

Serotonin transporter-deficient mice display enhanced adipose tissue inflammation after chronic high-fat diet feeding

Introduction

Serotonin is involved in leukocyte recruitment during inflammation. Deficiency of the serotonin transporter (SERT) is associated with metabolic changes in humans and mice. A possible link and interaction between the inflammatory effects of serotonin and metabolic derangements in SERT-deficient mice has not been investigated so far.

Methods

SERT-deficient (Sert -/-) and wild type (WT) mice were fed a high-fat diet, starting at 8 weeks of age. Metabolic phenotyping (metabolic caging, glucose and insulin tolerance testing, body and organ weight measurements, qPCR, histology) and assessment of adipose tissue inflammation (flow cytometry, histology, qPCR) were carried out at the end of the 19-week high-fat diet feeding period. In parallel, Sert -/- and WT mice received a control diet and were analyzed either at the time point equivalent to high-fat diet feeding or as early as 8-11 weeks of age for baseline characterization.

Results

After 19 weeks of high-fat diet, Sert -/- and WT mice displayed similar whole-body and fat pad weights despite increased relative weight gain due to lower starting body weight in Sert -/-. In obese Sert -/- animals insulin resistance and liver steatosis were enhanced as compared to WT animals. Leukocyte accumulation and mRNA expression of cytokine signaling mediators were increased in epididymal adipose tissue of obese Sert -/- mice. These effects were associated with higher adipose tissue mRNA expression of the chemokine monocyte chemoattractant protein 1 and presence of monocytosis in blood with an increased proportion of pro-inflammatory Ly6C+ monocytes. By contrast, Sert -/- mice fed a control diet did not display adipose tissue inflammation.

Discussion

Our observations suggest that SERT deficiency in mice is associated with inflammatory processes that manifest as increased adipose tissue inflammation upon chronic high-fat diet feeding due to enhanced leukocyte recruitment.

Linking serotonin homeostasis to gut function: Nutrition, gut microbiota and beyond

Serotonin (5-HT) produced by enterochromaffin (EC) cells in the digestive tract is crucial for maintaining gut function and homeostasis. Nutritional and non-nutritional stimuli in the gut lumen can modulate the ability of EC cells to produce 5-HT in a temporal- and spatial-specific manner that toning gut physiology and immune response. Of particular interest, the interactions between dietary factors and the gut microbiota exert distinct impacts on gut 5-HT homeostasis and signaling in metabolism and the gut immune response. However, the underlying mechanisms need to be unraveled. This review aims to summarize and discuss the importance of gut 5-HT homeostasis and its regulation in maintaining gut metabolism and immune function in health and disease with special emphasis on different types of nutrients, dietary supplements, processing, and gut microbiota. Cutting-edge discoveries in this area will provide the basis for the development of new nutritional and pharmaceutical strategies for the prevention and treatment of serotonin homeostasis-related gut and systematic disorders and diseases.

The Chain-Mediating Effect of Obesity, Depressive Symptoms on the Association between Dietary Quality and Cardiovascular Disease Risk

In order to explore the relationship between the Healthy Eating Index (HEI-2015) and cardiovascular disease (CVD), and the mediating role of obesity and depressive symptoms, we used the data from the 2011-2018 National Health and Nutrition Examination Survey (NHANES) for further study. A total of 12,644 participants were included in the study. The HEI was derived using NHANES personal food data and USDA Food Pattern Equivalence Database (FPED) dietary data. The risk of cardiovascular disease was determined using the Framingham Heart Study's multifactorial calculation tool. The weighted multiple logistic regression model was used to explore the association between the HEI-2015 and CVD, and the generalized structural equation was used to explore the mediating effects of obesity and depression, respectively and jointly. Higher HEI-2015 scores were associated with a lower risk of CVD compared to lower quartiles. Obesity, depressive symptoms, and their chain effects all played significant mediating roles in the association between the HEI-2015 and CVD, with proportional mediations of 9.03%, 2.23% and 0.25%, respectively. Our results suggest that higher dietary quality is associated with a lower risk of CVD, mediated by obesity, depressive symptoms, and the chain effect of obesity and depressive symptoms.

Metabolic and Molecular Response to High-Fat Diet Differs between Rats with Constitutionally High and Low Serotonin Tone

Maintaining energy balance is a complex physiological function whose dysregulation can lead to obesity and associated metabolic disorders. The bioamine serotonin (5HT) is an important regulator of energy homeostasis, with its central and peripheral pools influencing energy status in opposing ways. Using sublines of rats with constitutionally increased (high-5HT) or decreased (low-5HT) whole-body 5HT tone, we have previously shown that under standard diet constitutionally higher 5HT activity is associated with increased body weight, adiposity, and impaired glucose homeostasis. Here, we investigated the response of 5HT sublines to an obesogenic diet. Consistent with previous findings, high-5HT animals fed a standard diet had poorer metabolic health. However, in response to a high-fat diet, only low-5HT animals increased body weight and insulin resistance. They also showed more pronounced changes in blood metabolic parameters and the expression of various metabolic genes in hypothalamus and adipose tissue. On the other hand, high-5HT animals appeared to be protected from major metabolic disturbances of the obesogenic diet. The results suggest that constitutionally low 5HT activity is associated with higher susceptibility to harmful effects of a high-energy diet. High-5HT subline, which developed less adverse metabolic outcomes on hypercaloric diets, may prove useful in understanding metabolically healthy obesity in humans.

Modulation of adipose tissue metabolism by microbial-derived metabolites

Obesity and its complications, including type 2 diabetes, cardiovascular disease, and certain cancers, have posed a significant burden on health and healthcare systems over the years due to their high prevalence and incidence. Gut microbial derivatives are necessary for the regulation of energy metabolism and host immunity, as well as for maintaining homeostasis of the intestinal environment. Gut flora metabolites may be a link between gut microbes and diseases, such as obesity, and help understand why alterations in the microbiota can influence the pathophysiology of human disease. This is supported by emerging evidence that microbial-derived metabolites, such as short-chain fatty acids, bile acids, tryptophan, trimethylamine-N-oxide, and lipopolysaccharides, can be beneficial or detrimental to the host by affecting organs outside the gut, including adipose tissue. Adipose tissue is the largest lipid storage organ in the body and an essential endocrine organ that plays an indispensable role in the regulation of lipid storage, metabolism, and energy balance. Adipose tissue metabolism includes adipocyte metabolism (lipogenesis and lipolysis), thermogenesis, and adipose tissue metabolic maladaptation. Adipose tissue dysfunction causes the development of metabolic diseases, such as obesity. Here, we review the current understanding of how these microbial metabolites are produced and discuss both established mechanisms and the most recent effects of microbial products on host adipose tissue metabolism. We aimed to identify novel therapeutic targets or strategies for the prevention and treatment of obesity and its complications.

Traumatic brain injury alters the gut-derived serotonergic system and associated peripheral organs

Most efforts to understand the pathology of traumatic brain injury (TBI) have been centered on the brain, ignoring the role played by systemic physiology. Gut-derived serotonin is emerging as a major regulator of systemic homeostasis involving various organs and tissues throughout the body. Here, we shed light on the roles occupied by gut-derived serotonin and its downstream metabolic targets in the systemic pathogenesis of TBI. Male C57BL/6J mice were subjected to a fluid percussion injury (FPI) and RT-qPCR was used to examine mRNA levels in intestine, liver, and adipose tissue. In the intestinal tract, TBI transiently downregulated enteric neuronal markers Chat and Nos1 in the duodenum and colon, and altered colonic genes related to synthesis and degradation of serotonin, favoring an overall serotonin downregulation. There also was a decrease in serotonin fluorescence intensity in the colonic mucosa and reduced circulating blood serotonin levels, with concurrent alterations in serotonin-associated gene expression in downstream tissues after TBI (i.e., upregulation of serotonin receptor Htr2a and dysregulation of genes associated with lipid metabolism in liver and adipose). Levels of commensal bacterial species were also altered in the gut and were associated with TBI-mediated changes in the colonic serotonin system. Our findings suggest that TBI alters peripheral serotonin homeostasis, which in turn may impact gastrointestinal function, gut microbiota, and systemic energy balance. These data highlight the importance of building an integrative view of the role of systemic physiology in TBI pathogenesis to assist in the development of effective TBI treatments.

Serotonin in the regulation of systemic energy metabolism

Serotonin is a well‐known neurotransmitter that is synthesized from the amino acid, tryptophan. To date, more than 14 different serotonin receptors have been discovered; they exist universally in our body and enable diverse biological functions in different organs. Central serotonin regulates mood and behavior, and impacts the systemic energy balance by decreasing appetite. A number of drugs that modulate central serotonin function (e.g., fenfluramine, sibutramine and lorcaserin) were approved and used as anti‐obesity drugs, but then later withdrawn due to adverse cardiovascular and carcinogenic effects. Over the past decade, the role of peripheral serotonin in regulating systemic energy metabolism has been extensively explored using tissue‐specific knockout animal models. By inhibiting the action of serotonin in liver and adipose tissues, hepatic steatosis was improved and lipid accumulation was mitigated, respectively. Recent findings show that modulation of the serotonergic system is a promising therapeutic target for metabolic diseases. This review summarizes the role of serotonin in regulating energy metabolism in different organs, and discusses the potential of serotonin modulation for treating metabolic diseases.

Synthesis and biological evaluation of tyrosine derivatives as peripheral 5HT2A receptor antagonists for nonalcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD), attributed to excessive fat accumulation in the liver, is reportedly prevalent worldwide. NAFLD is one of the leading causes of chronic liver disease, including non-alcoholic steatohepatitis (NASH), cirrhosis, and hepatic cellular carcinoma (HCC). The peripheral roles of serotonin (5-hydroxytryptamine, 5HT) were found to regulate hepatic lipid metabolism. Among serotonin receptor subtypes, 5HT_2A receptor is known to regulate hepatic lipid metabolism. Hepatic lipid accumulation and hepatic triglyceride (TG) were reduced in liver-specific 5HT_2A receptor knockout (5HT_2A receptor LKO) mice upon high-fat diet (HFD) feeding. In the present study, we explored a series of new peripherally acting 5HT_2A receptor antagonists. Compound 14a displayed good in vitro activity, with an IC₅₀ value of 0.17 nM. Compound 14a exhibited good microsomal stability, no significant CYP and hERG inhibition, and 5HT receptor subtype selectivity. The brain-to-plasma ratio of 14a was below the lower limit of quantification, indicating limited blood-brain barrier (BBB) penetration. HFD-fed 14a treated mice showed decreased liver steatosis and lobular inflammation. These results demonstrate the potential of newly synthesized peripheral 5HT_2A receptor antagonists for treating NAFLD.

Maternal Metabolic State and Fetal Sex and Genotype Modulate Methylation of the Serotonin Receptor Type 2A Gene (HTR2A) in the Human Placenta

The serotonin receptor 2A gene (HTR2A) is a strong candidate for the fetal programming of future behavior and metabolism. Maternal obesity and gestational diabetes mellitus (GDM) have been associated with an increased risk of metabolic and psychological problems in offspring. We tested the hypothesis that maternal metabolic status affects methylation of HTR2A in the placenta. The prospective study included 199 pairs of mothers and healthy full-term newborns. Genomic DNA was extracted from feto-placental samples and analyzed for genotypes of two polymorphisms (rs6311, rs6306) and methylation of four cytosine residues (−1665, −1439, −1421, −1224) in the HTR2A promoter region. Placental HTR2A promoter methylation was higher in male than female placentas and depended on both rs6311 and rs6306 genotypes. A higher maternal pre-gestational body mass index (pBMI) and, to a lesser extent, diagnosis of GDM were associated with reduced HTR2A promoter methylation in female but not male placentas. Higher pBMI was associated with reduced methylation both directly and indirectly through increased GDM incidence. Tobacco use during pregnancy was associated with reduced HTR2A promoter methylation in male but not female placentas. The obtained results suggest that HTR2A is a sexually dimorphic epigenetic target of intrauterine exposures. The findings may contribute to a better understanding of the early developmental origins of neurobehavioral and metabolic disorders associated with altered HTR2A function.

Genome-wide survey and functional analysis reveal TCF21 promotes chicken preadipocyte differentiation by directly upregulating HTR2A

BACKGROUND/AIM

Previously, we showed that transcription factor 21 (TCF21) promotes chicken preadipocyte differentiation. However, the genome-wide TCF21 binding sites and its downstream target genes in chicken adipogenesis were unknown.

METHODS

ChIP-Seq and RNA-Seq were used to screen candidate targets of TCF21. qPCR and luciferase reporter assay were applied to verify the sequencing results. Western blotting, oil red-O staining and pharmacological treatments were performed to investigate the function of 5-hydroxytryptamine receptor 2A (HTR2A), one of the bonafide direct downstream binding targets of TCF21.

RESULTS

A total of 94 candidate target genes of TCF21 were identified. ChIP-qPCR, RT-qPCR, and luciferase reporter assay demonstrated that HTR2A is one of the bonafide direct downstream binding targets of TCF21. HTR2A expression in adipose tissue was upregulated in fat line broilers. Also, the abundance of HTR2A gradually increased during the adipogenesis process. Interestingly, pharmacological enhancement or inhibition of HTR2A promoted or attenuated the differentiation of preadipocytes, respectively. Furthermore, HTR2A inhibition impaired the TCF21 promoted adipogenesis.

CONCLUSIONS

We profiled the genome-wide TCF21 binding sites in chicken differentiated preadipocytes revealing HTR2A as the direct downstream target of TCF21 in adipogenesis.

Inhibiting serotonin signaling through HTR2B in visceral adipose tissue improves obesity-related insulin resistance

Insulin resistance is a cornerstone of obesity-related complications such as type 2 diabetes, metabolic syndrome, and nonalcoholic fatty liver disease. A high rate of lipolysis is known to be associated with insulin resistance, and inhibiting adipose tissue lipolysis improves obesity-related insulin resistance. Here, we demonstrate that inhibition of serotonin (5-hydroxytryptamine [5-HT]) signaling through serotonin receptor 2B (HTR2B) in adipose tissues ameliorates insulin resistance by reducing lipolysis in visceral adipocytes. Chronic high-fat diet (HFD) feeding increased Htr2b expression in epididymal white adipose tissue, resulting in increased HTR2B signaling in visceral white adipose tissue. Moreover, HTR2B expression in white adipose tissue was increased in obese humans and positively correlated with metabolic parameters. We further found that adipocyte-specific Htr2b-knockout mice are resistant to HFD-induced insulin resistance, visceral adipose tissue inflammation, and hepatic steatosis. Enhanced 5-HT signaling through HTR2B directly activated lipolysis through phosphorylation of hormone-sensitive lipase in visceral adipocytes. Moreover, treatment with a selective HTR2B antagonist attenuated HFD-induced insulin resistance, visceral adipose tissue inflammation, and hepatic steatosis. Thus, adipose HTR2B signaling could be a potential therapeutic target for treatment of obesity-related insulin resistance.

Serotonergic Regulation of Hepatic Energy Metabolism

The liver is a vital organ that regulates systemic energy metabolism and many physiological functions. Nonalcoholic fatty liver disease (NAFLD) is the commonest cause of chronic liver disease and end-stage liver failure. NAFLD is primarily caused by metabolic disruption of lipid and glucose homeostasis. Serotonin (5-hydroxytryptamine [5-HT]) is a biogenic amine with several functions in both the central and peripheral systems. 5-HT functions as a neurotransmitter in the brain and a hormone in peripheral tissues to regulate systemic energy homeostasis. Several recent studies have proposed various roles of 5-HT in hepatic metabolism and inflammation using tissue-specific knockout mice and 5-HT-receptor agonists/antagonists. This review compiles the most recent research on the relationship between 5-HT and hepatic metabolism, and the role of 5-HT signaling as a potential therapeutic target in NAFLD.

Single or combined ablation of peripheral serotonin and p21 limit adipose tissue expansion and metabolic alterations in early adulthood in mice fed a normocaloric diet

Identifying the fundamental molecular factors that drive weight gain even in the absence of hypercaloric food intake, is crucial to enable development of novel treatments for the global pandemic of obesity. Here we investigated both adipose tissue-specific and systemic events that underlie the physiological weight gain occurring during early adulthood in mice fed a normocaloric diet. In addition, we used three different genetic models to identify molecular factors that promote physiological weight gain during normocaloric and hypercaloric diets. We demonstrated that normal physiological weight gain was accompanied by an increase in adipose tissue mass and the presence of cellular and metabolic signatures typically found during obesity, including adipocyte hypertrophy, macrophage recruitment into visceral fat and perturbed glucose metabolism. At the molecular level, this was associated with an increase in adipose tissue tryptophan hydroxylase 1 (Tph1) transcripts, the key enzyme responsible for the synthesis of peripheral serotonin. Genetic inactivation of Tph1 was sufficient to limit adipose tissue expansion and associated metabolic alterations. Mechanistically, we discovered that Tph1 inactivation resulted in down-regulation of cyclin-dependent kinase inhibitor p21Waf1/Cip1 expression. Single or double ablation of Tph1 and p21 were equally effective in preventing adipocyte expansion and systemic perturbation of glucose metabolism, upon both normocaloric and hypercaloric diets. Our results suggest that serotonin and p21 act as a central molecular determinant of weight gain and associated metabolic alterations, and highlights the potential of targeting these molecules as a pharmacologic approach to prevent the development of obesity.

The Mechanism of Secretion and Metabolism of Gut-Derived 5-Hydroxytryptamine

Serotonin, also known as 5-hydroxytryptamine (5-HT), is a metabolite of tryptophan and is reported to modulate the development and neurogenesis of the enteric nervous system, gut motility, secretion, inflammation, sensation, and epithelial development. Approximately 95% of 5-HT in the body is synthesized and secreted by enterochromaffin (EC) cells, the most common type of neuroendocrine cells in the gastrointestinal (GI) tract, through sensing signals from the intestinal lumen and the circulatory system. Gut microbiota, nutrients, and hormones are the main factors that play a vital role in regulating 5-HT secretion by EC cells. Apart from being an important neurotransmitter and a paracrine signaling molecule in the gut, gut-derived 5-HT was also shown to exert other biological functions (in autism and depression) far beyond the gut. Moreover, studies conducted on the regulation of 5-HT in the immune system demonstrated that 5-HT exerts anti-inflammatory and proinflammatory effects on the gut by binding to different receptors under intestinal inflammatory conditions. Understanding the regulatory mechanisms through which 5-HT participates in cell metabolism and physiology can provide potential therapeutic strategies for treating intestinal diseases. Herein, we review recent evidence to recapitulate the mechanisms of synthesis, secretion, regulation, and biofunction of 5-HT to improve the nutrition and health of humans.

Metabolic Disturbances in Rat Sublines with Constitutionally Altered Serotonin Homeostasis

Central and peripheral serotonin (5HT) have opposing functions in the regulation of energy homeostasis. Both increasing 5HT signaling in the brain and decreasing 5HT signaling in the periphery have been proposed as potential treatments for obesity. This study investigates the relationship between constitutionally high or low 5HT activity and systemic net energy balance. Two sublines of rats with high and low whole-body 5HT tone, obtained by selective breeding for platelet 5HT parameters, were examined for fat accumulation in different white adipose tissue (WAT) depots, glucose/insulin tolerance, blood metabolic parameters, and expression of various metabolic genes. High-5HT animals, unlike their low-5HT counterparts, developed widespread intra-abdominal obesity associated with glucose and insulin intolerance, which worsened with age. They also had elevated blood glucose and lipid parameters but showed no significant changes in circulating leptin, resistin, and adipsin levels. Surprisingly, adiponectin levels were increased in plasma but reduced in the WAT of high-5HT rats. A limited number of metabolic genes belonging to different functional classes showed differential expression in WAT of high-5HT compared to low-5HT rats. Overall, a constitutive increase in 5HT tone is associated with a positive energy balance acting through subtle dysregulation of a broad spectrum of metabolic pathways.

A Systems Biology Approach to Investigating the Interaction between Serotonin Synthesis by Tryptophan Hydroxylase and the Metabolic Homeostasis

Obesity has become a global public health and economic problem. Obesity is a major risk factor for a number of complications, such as type 2 diabetes, cardiovascular disease, fatty liver disease, and cancer. Serotonin (5-hydroxytryptamine [5-HT]) is a biogenic monoamine that plays various roles in metabolic homeostasis. It is well known that central 5-HT regulates appetite and mood. Several 5-HT receptor agonists and selective serotonin receptor uptake inhibitors (SSRIs) have shown beneficial effects on appetite and mood control in clinics. Although several genetic polymorphisms related to 5-HT synthesis and its receptors are strongly associated with obesity, there is little evidence of the role of peripheral 5-HT in human metabolism. In this study, we performed a systemic analysis of transcriptome data from the Genotype-Tissue Expression (GTEX) database. We investigated the expression of 5-HT and tryptophan hydroxylase (TPH), the rate-limiting enzyme of 5-HT biosynthesis, in the human brain and peripheral tissues. We also performed differential gene expression analysis and predicted changes in metabolites by comparing gene expressions of tissues with high TPH expression to the gene expressions of tissues with low TPH expression. Our analyses provide strong evidence that serotonin plays an important role in the regulation of metabolic homeostasis in humans.

Peripheral Selective Oxadiazolylphenyl Alanine Derivatives as Tryptophan Hydroxylase 1 Inhibitors for Obesity and Fatty Liver Disease

Tryptophan hydroxylase 1 (TPH1) has been recently suggested as a promising therapeutic target for treating obesity and fatty liver disease. A new series of 1,2,4-oxadiazolylphenyl alanine derivatives were identified as TPH1 inhibitors. Among them, compound 23a was the most active in vitro, with an IC₅₀ (half-maximal inhibitory concentration) value of 42 nM, showed good liver microsomal stability, and showed no significant inhibition of CYP and hERG. Compound 23a inhibited TPH1 in the peripheral tissue with limited BBB penetration. In high-fat diet-fed mice, 23a reduced body weight gain, body fat, and hepatic lipid accumulation. Also, 23a improved glucose intolerance and energy expenditure. Taken together, compound 23a shows promise as a therapeutic agent for the treatment of obesity and fatty liver diseases.

Risperidone Exacerbates Glucose Intolerance, Nonalcoholic Fatty Liver Disease, and Renal Impairment in Obese Mice

Risperidone, a second-generation antipsychotic drug used for schizophrenia treatment with less-severe side effects, has recently been applied in major depressive disorder treatment. The mechanism underlying risperidone-associated metabolic disturbances and liver and renal adverse effects warrants further exploration. This research explores how risperidone influences weight, glucose homeostasis, fatty liver scores, liver damage, and renal impairment in high-fat diet (HFD)-administered C57BL6/J mice. Compared with HFD control mice, risperidone-treated obese mice exhibited increases in body, liver, kidney, and retroperitoneal and epididymal fat pad weights, daily food efficiency, serum triglyceride, blood urea nitrogen, creatinine, hepatic triglyceride, and aspartate aminotransferase, and alanine aminotransferase levels, and hepatic fatty acid regulation marker expression. They also exhibited increased insulin resistance and glucose intolerance but decreased serum insulin levels, Akt phosphorylation, and glucose transporter 4 expression. Moreover, their fatty liver score and liver damage demonstrated considerable increases, corresponding to increases in sterol regulatory element-binding protein 1 mRNA, fatty acid-binding protein 4 mRNA, and patatin-like phospholipid domain containing protein 3 expression. Finally, these mice demonstrated renal impairment, associated with decreases in glutathione peroxidase, superoxide dismutase, and catalase levels. In conclusion, long-term administration of risperidone may exacerbate diabetes syndrome, nonalcoholic fatty liver disease, and kidney injury.

Pancreatic Sirtuin 3 Deficiency Promotes Hepatic Steatosis by Enhancing 5-Hydroxytryptamine Synthesis in Mice With Diet-Induced Obesity

Sirtuin 3 (SIRT3) is a protein deacetylase regulating β-cell function through inhibiting oxidative stress in obese and diabetic mice, but the detailed mechanism and potential effect of β-cell-specific SIRT3 on metabolic homeostasis, and its potential effect on other metabolic organs, are unknown. We found that glucose tolerance and glucose-stimulated insulin secretion were impaired in high-fat diet (HFD)-fed β-cell-selective Sirt3 knockout (Sirt3 f/f;Cre/+) mice. In addition, Sirt3 f/f;Cre/+ mice had more severe hepatic steatosis than Sirt3 f/f mice upon HFD feeding. RNA sequencing of islets suggested that Sirt3 deficiency overactivated 5-hydroxytryptamine (5-HT) synthesis as evidenced by upregulation of tryptophan hydroxylase 1 (TPH1). 5-HT concentration was increased in both islets and serum of Sirt3 f/f;Cre/+ mice. 5-HT also facilitated the effect of palmitate to increase lipid deposition. Treatment with TPH1 inhibitor ameliorated hepatic steatosis and reduced weight gain in HFD-fed Sirt3 f/f;Cre/+ mice. These data suggested that under HFD feeding, SIRT3 deficiency in β-cells not only regulates insulin secretion but also modulates hepatic lipid metabolism via the release of 5-HT.