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function (e.g., fenfluramine, sibutramine and lorcaserin) were approved and used as anti-obesity drugs, but then later withdrawn due to adverse cardiovascular and carcinogenic effects.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.

INTRODUCTION

Serotonin (5‐hydroxytryptamine [5‐HT]) is a monoamine that exerts diverse functions in both the central nervous system and peripheral organs. 5‐HT is known as a neurotransmitter in the brain that modulates mood, sleep, behavior, appetite and so on . For synthesis of 5‐HT, the amino acid, tryptophan, is converted to 5‐hydroxtryptophan (5‐HTP) by the rate‐limiting enzyme, tryptophan hydroxylase (TPH), and then to 5‐HT by aromatic acid decarboxylase. In the early 2000s, two isoforms of TPH were identified and found to be expressed in a mutually exclusive pattern: TPH1 is expressed in peripheral non‐neuronal tissues, and TPH2 is expressed in the central nervous system and peripheral neuronal tissues .

Most of the 5‐HT in the periphery is synthesized by enterochromaffin cells of the gut. 5‐HT acts locally in the gut and enters the circulation, where >95% of it is taken up by platelets. In damaged tissues, the circulating platelets secrete 5‐HT, leading to blood vessel contraction and coagulation. However, 5‐HT exerts other biological functions, including promotion of liver regeneration and inhibition of bone formation. 5‐HT is also produced in other organs (e.g., pancreatic β‐cells and adipocytes), where it acts locally.

5‐HT exerts its biological functions through different mechanisms. To date, seven 5‐HT receptor (HTR) families have been identified. Most of the identified HTRs are G‐protein‐coupled receptors; an exception is HTR3, which is a ligand‐gated ion channel. 5‐HT acts as a pan‐agonist to these receptors, whereas certain chemical species, including phospholipids (e.g., phosphatidylinositol 4‐phosphate) can modulate the activity of G protein‐coupled receptor‐type HTRs . 5‐HT can act intracellularly through serotonylation of cytoplasmic proteins, which directly modulates their functions . 5‐HT also binds to histones to modify histone codes and, thereby, epigenetically regulate gene expression . Furthermore, 5‐HT is an indole derivative that acts to scavenge oxidative stress molecules in the cellular environment.

In the present review, we discuss the recent findings on the metabolic roles of 5‐HT and its potential as a therapeutic target for metabolic disorders (Figure 1).

Figure 1

Serotonin (5‐hydroxytryptamine [5‐HT]) in the regulation of systemic energy metabolism. Peripheral 5‐HT synthesis and central 5‐HT synthesis are independently regulated by tryptophan hydroxylase (TPH) 1 and 2, respectively, as 5‐HT cannot cross the blood–brain barrier. Central 5‐HT decreases appetite through HTR2C. Peripheral 5‐HT is majorly synthesized by enterochromaffin cells of the gut; from there, it is taken up by platelets and enters systemic circulation. 5‐HT is also produced by pancreatic β‐cells and adipocytes to regulate systemic energy metabolism in a cell‐autonomous manner. 5‐HT from the gut and pancreas goes into portal circulation and regulates metabolism in the liver, thereby involving in inter‐organ crosstalk of systemic metabolism through an endocrine manner. GH, growth hormone; GHR, growth hormone receptor; HTR, 5‐hydroxytryptamine receptor; PL, placental lactogen; PRL, prolactin; PRLR, prolactin receptor. [Colour figure can be viewed at wileyonlinelibrary.com]

5‐HT IN PANCREATIC Β‐CELLS

A pancreatic islet comprises different types of cells that secrete hormones (e.g., insulin, glucagon and somatostatin) to regulate systemic metabolism. 5‐HT is produced and secreted by human and rodent pancreatic endocrine cells. The presence of biogenic amines in pancreatic islets was first identified in the 1960s, and subsequent studies showed that pancreatic islets exhibit exocytotic efflux of 5‐HT under a hyperglycemic environment . Recent transcriptomic analyses of human pancreatic islets showed that human β‐cells express genes encoding factors required for 5‐HT synthesis (TPH1, TPH2 and aromatic acid decarboxylase) and HTRs . TPH1 protein is expressed in human α‐, β‐ and δ‐cells, whereas β‐cells are the predominant cells producing 5‐HT in the pancreas . In this context, 5‐HTP, a precursor of 5‐HT, was used to trace the endocrine pancreas in positron emission tomography imaging . At the subcellular level in β‐cells, 5‐HT was found in granules with insulin, similar to its known role as a neurotransmitter in the central nervous system, but it was also found to be covalently bound to cytoplasmic proteins and histones in the nucleus , , .

5‐HT is robustly produced in β‐cells during the perinatal period, pregnancy and lactation , , . Dramatic β‐cell mass expansion occurs during these periods alongside increases in the circulating levels of reproductive hormones, including prolactin and placental lactogen. Given that prolactin and placental lactogen are somatomammotrophic hormones that majorly act as growth factors, this suggests that 5‐HT might contribute to the physiological regulation of β‐cell mass in response to hormonal changes. During pregnancy and lactation, prolactin and placental lactogen binds to the prolactin receptor and induce the phosphorylation of STAT5 to transcriptionally upregulate Tph1, leading to the production of 5‐HT in β‐cells. Unlike pregnancy and lactation, growth hormone stimulates Tph1 expression during the perinatal period. Elimination of Tph1 from β‐cells during lactation decreased β‐cell proliferation and mass by approximately 30% . Elimination of Tph1 from β‐cells during the perinatal period decreased β‐cell proliferation and mass by >50%, and the impairment of β‐cell mass expansion during this period led to impaired glucose tolerance in adulthood . Overall, 5‐HT is essential for the abilities to attain proper β‐cell mass and regulate glucose levels throughout the lifespan.

The early studies on the role of 5‐HT in insulin secretion should be interpreted with caution, because they largely depended on in vitro experiments carried out using pharmacological agonists and antagonists that affect multiple receptors. 5‐HT is thought to stimulate insulin secretion , but its exact function in this context varies by the activated receptor type, the intra/extracellular location of the 5‐HT and so on. Regarding the receptor types, HTR2B is a Gq protein‐coupled receptor that is most abundantly expressed in human and rodent β‐cells. Activation of HTR2B stimulates insulin secretion by modulating the intracellular Ca2+ flux and enhancing mitochondrial respiration . HTR3 is a ligand‐gated cation channel that depolarizes membrane potential and increases membrane excitability to potentiate insulin secretion , . Activation of the Gi protein‐coupled receptor, HTR1D, was shown to inhibit insulin secretion and contribute to the postpartum regression of β‐cells , .

5‐HT can regulate β‐cell function in a receptor‐independent manner. Paulmann et al. showed that intracellular 5‐HT binds to small GTPases in a process called serotonylation, which potentiates the exocytosis of insulin granules by β‐cells. Other work showed that intracellular 5‐HT protects β‐cells from oxidative stress; 5‐HTP and 5‐HT are indole derivatives that chemically scavenge reactive oxygen species, improving β‐cell survival and insulin secretory function .

Although 5‐HT exerts a metabolically beneficial role in β‐cells, it might play alternative roles in other peripheral metabolic organs (see below for details). For example, 5‐HT secreted from β‐cells might enter portal circulation and be delivered to the liver. Ming et al. showed that 5‐HT overexpression in β‐cells (Sirt3 knockout [KO] in their model) induced hepatic steatosis through activation of Srebp1c, and that this phenotype was rescued by inhibition of 5‐HT synthesis (using PCPA) or HTR2A (using sarpogrelate). This is consistent with our finding that gut‐derived 5‐HT accelerates hepatic steatosis through activation of HTR2A and SREBP1c through portal circulation . These results suggest that, similar to insulin, β‐cell‐derived 5‐HT might act as an endocrine signal to exert metabolic effects in distant organs.

5‐HT IN ADIPOSE TISSUE

Adipose tissue is anatomically classified as visceral white adipose tissue (VAT), subcutaneous white adipose tissue (SAT) and brown adipose tissue (BAT). In 2012, Sumara et al. reported that circulating 5‐HT from the gut (gut‐derived serotonin [GDS]) functions in adipose tissues. Expression of Tph1 in the duodenum and plasma GDS in mice is upregulated during fasting. Elevated GDS communicates through HTR2B in white adipocytes to promote lipolysis by increasing the phosphorylation and activity of hormone‐sensitive lipase.

5‐HT can be produced in all three adipose tissue types and it can modulate the differentiation and function of white and brown adipose tissues in either an autocrine or a paracrine manner , . In VAT, 5‐HT regulates lipolysis through HTR2B signaling. Htr2b expression in VAT was found to be elevated by chronic HFD feeding; this enhancement increased 5‐HT signaling through hormone‐sensitive lipase‐activated, HTR2B‐stimulated lipolysis. Inhibition of 5‐HT signaling in VAT reportedly improved peripheral insulin resistance by reducing circulating free fatty acids levels . 5‐HT also regulates de novo lipogenesis in VAT through HTR2A. HTR2A antagonist treatment inhibited lipid accumulation in 3 T3‐L1 adipocytes . HFD‐fed Htr2a fat‐specific KO mice have lower lipid buildup in white adipose tissues and were reported to resist obesity .

In BAT, 5‐HT modulates the differentiation and thermogenesis of brown adipocytes. 5‐HT treatment blocked the differentiation of immortalized mouse brown HIB‐1B preadipocytes. Furthermore, in differentiated HIB‐1B adipocytes, 5‐HT reduced the gene expression of markers for thermogenesis and differentiation . 5‐HT signaling in BAT regulates thermogenesis through HTR3 signaling in diet‐induced‐obesity model mice. Mice with inducible Tph1 KO in adipose tissues showed activated adaptive thermogenesis in BAT and suppression of lipogenesis in VAT. When Htr3a KO mice received a high‐fat diet (HFD), their energy expenditure and BAT thermogenesis were increased, whereas their weight gain was decreased .

5‐HT also regulates beige adipocyte formation in the SAT. Mice treated with chemical inhibitors of 5‐HT synthesis (e.g., PCPA or LP533401) and HFD‐fed whole‐body Tph1 KO mice showed increased beige adipocyte formation in the SAT . Animal studies showed that both circulating and fat‐derived 5‐HT might play roles in the development of beige adipocytes. Zhang et al. reported that mast cell‐derived 5‐HT suppresses SAT thermogenesis. The inhibition of 5‐HT synthesis in mast cells leads to increased beige adipocyte formation and energy metabolism in mice. Mice with inducible Tph1 KO in adipose tissues also showed increased beige adipocyte development in the inguinal WAT . However, we do not yet know which receptors influence the browning of subcutaneous WAT. Both HFD‐fed Htr2a fat‐specific KO mice and Htr2b fat‐specific KO mice failed to show beige adipocyte formation in the SAT , . More research is required to determine the mechanism(s) underlying these phenomena.

5‐HT is also important for the physiological weight gain seen with aging in adult mice. From 2 to 6 months‐of‐age, normal physiological weight gain is accompanied by a rise in adipose tissue mass. A study showed that Tph1 and Htr2b expression in VAT are higher in 6‐month‐old mice than in 2‐month‐old mice, and genetic silencing of Tph1 is sufficient to restrict adipose tissue expansion in adult mice .

5‐HT IN THE LIVER

The liver is a vital organ that governs systemic energy metabolism and regulates various other physiological processes, such as macronutrient metabolism, immunomodulation, lipid metabolism and cholesterol homeostasis. Hepatocytes cannot manufacture 5‐HT, but circulating 5‐HT plays important roles in the liver, such as by impacting hepatic regeneration, metabolism and vascular activities .

Platelets are the major storage site for circulating 5‐HT, and platelet‐derived 5‐HT regulates liver regeneration . In a mouse model of partial hepatectomy (PH), thrombocytopenia reduced the initial cellular proliferation seen in regenerating liver. Liver regeneration was also suppressed in whole‐body Tph1 KO mice, and this suppression was rescued by 5‐HT precursor supplementation. The overexpression of Htr2a and Htr2b in the liver increased liver regeneration, whereas antagonists of HTR2A and HTR2B inhibited this process. These findings suggest that platelet‐derived 5‐HT regulates liver regeneration through HTR2A and HTR2B signaling . A clinical study further found that a low preoperative intraplatelet 5‐HT level was associated with an increased incidence of postoperative liver dysfunction .

5‐HT also activates liver regeneration through other pathways. In older animals, liver regeneration is reduced. However, 5‐HT triggered hepatic regeneration in aged mice after PH . 2,5‐Dimethoxy‐4‐iodoamphetamine, an HTR2 agonist, increased serum vascular endothelial growth factor levels in old mice, and reduced age‐related pseudocapillarization of old liver and enhanced hepatic sinusoidal blood flow via a vascular endothelial growth factor‐dependent pathway . In addition, Fang et al. reported that 5‐HT promoted liver regeneration by activating extracellular signal‐regulated kinase–Yes‐associated protein signaling. The levels of phosphorylated extracellular signal‐regulated kinase and Yes‐associated protein were enhanced after PH, but this was inhibited in the livers of whole‐body Tph1 KO mice. In human samples obtained from patients undergoing hemi‐hepatectomies, portal vein 5‐HT levels were significantly associated with Yes‐associated protein expression . Although 5‐HT‐induced hepatocyte proliferation is an important response for liver regeneration after PH, this feature of 5‐HT has been linked to unfavorable biological effects, such as tumorigenesis .

Non‐alcoholic fatty liver disease (NAFLD) is characterized by excessive lipid accumulation in the liver (hepatic steatosis), which is the hepatic manifestation of metabolic syndrome. Similar to the situation in adipocytes, 5‐HT influences lipid metabolism in hepatocytes and has emerged as a new therapeutic target for treating NAFLD. Studies showed that GDS exacerbates hepatic steatosis through HTR2A signaling , . In the HFD‐induced mouse model of fatty liver, both gut‐specific Tph1 KO mice and liver‐specific Htr2a KO mice were found to resist hepatic steatosis progression without alteration of systemic energy homeostasis . Serum 5‐HT levels were found to correlate with the quantitative ultrasonography score of NAFLD severity in a human investigation (r = 0.7045, P = 0.0001) . Both tryptophan‐free diet feeding and LP533401 treatment markedly reduced accumulation of triglycerides in the liver and improved serum aspartate transaminase and aspartate aminotransferase levels in fat‐sucrose diet‐induced non‐alcoholic steatohepatitis (NASH) model rats . 5‐HT also regulates the activity of hepatic stellate cells. 5‐HT signaling through HTR2B in activated hepatic stellate cells induced transforming growth factor 1 production and inhibited hepatocyte proliferation . 5‐HT also increased the expression of microribonucleic acid 221/222, which is a putative biomarker for the progression of human liver fibrosis .

Gut barrier disruption is the critical step in NASH development. Increased intestinal permeability has been significantly associated with NASH in patients . Many studies have shown that 5‐HT plays roles in gut barrier homeostasis and NASH . HTR3A antagonist treatment increased the expression of tight‐junction proteins in the duodenum, decreased endotoxin influx into the liver, and reduced hepatic inflammation and fat storage in ob/ob mice . A clinical study supported these findings from animal studies: HTR3A antagonist treatment was linked with lower 28‐day (hazard ratio 0.18, 95% confidence interval 0.10–0.34, P = 0.001) and 90‐day (hazard ratio 0.21, 95% confidence interval 0.13–0.33, P = 0.001) mortality of liver failure patients in a Chinese population .

5‐HT IN THE CENTRAL NERVOUS SYSTEM

In addition to its functions in the periphery, 5‐HT is involved in various physiological and pathological processes of the central nervous system. As a neurotransmitter, 5‐HT regulates neuronal activity and various cognitive functions; accordingly, HTR‐targeting medications are commonly utilized in psychiatry and neurology .

In the brain, 5‐HT regulates the endocrine system and energy metabolism by modulating the hypothalamic–pituitary–adrenal axis , . HTR2C is a G protein‐coupled receptor that is highly expressed in the hypothalamus and brain stem. 5‐HT modulates upstream corticotropin‐releasing hormone signaling circuits through activating HTR2C in the hypothalamic paraventricular nucleus . HTR2C is also expressed on pro‐opiomelanocortin neurons and functions to regulate energy‐balance characteristics, such as hyperphagia, sensitivity to diet‐induced obesity, locomotor hyperactivity, insulin resistance and insensitivity to the anorectic effects of 5‐HT agonists . HTR2C KO mice acquired hyperphagia and obesity, whereas treatment with an HTR2C agonist was found to reduce food intake in mice, which contributed to its anorexigenic effects .

Central 5‐HT appears to promote BAT and beige adipocyte thermogenic activity by altering the sympathetic outflow to these tissues. The depletion of serotonergic neurons leads to thermoregulation loss, steatosis, and >50% decreases of UCP1 protein expression levels in BAT and SAT. These mice also showed increases in the blood levels of glucose, free fatty acids and triglycerides . 5‐HT administration reduces sympathetic nerve activity in BAT through a gamma‐aminobutyric acid‐mediated input to the dorsomedial hypothalamus in rats . Clinical studies have supported these findings: Several 5‐HT modulating drugs, including sibutramine, fluoxetine, and amitriptyline, have shown significant effects on energy expenditure in humans.

5‐HT AS A THERAPEUTIC TARGET FOR METABOLIC DISEASES

Supplementation of 5‐HTP and 5‐HT has anti‐obesity and anti‐diabetic effects , and a number of 5‐HT‐modulating drugs have been developed and approved as anti‐obesity drugs. The 5‐HT and noradrenaline reuptake inhibitor, sibutramine, was approved by the Food and Drug Administration in 1997, but withdrawn in 2010, because it was found to be associated with an increased risk of cardiovascular events. The HTR2C agonist, lorcaserin, was approved in 2012 and did not show adverse cardiovascular effects, but the Food and Drug Administration recommended that it be withdrawn due to a possible increase in the risk of cancer . Currently, phentermine/topiramate and naltrexone/bupropion survive in the market as anti‐obesity medications that modulate 5‐HT to a lesser extent.

Unlike the central nervous system, mice lacking 5‐HT in the periphery (Tph1 −/−) show anti‐obesity phenotypes. Lexicon Pharmaceuticals developed TPH inhibitors that do not cross the blood–brain barrier (e.g., LP‐533401), with the goal of specifically blocking peripheral 5‐HT synthesis. Telotristat was the first marketed TPH inhibitor approved for carcinoid syndrome. A series of 1,2,4‐oxadiazolylphenyl alanine derivatives were found to reduce the levels of 5‐HT, blood glucose and adiposity, and thus might have potential as therapeutics for anti‐obesity and the treatment of NAFLD .

5‐HT induces lipogenesis in the liver and adipose tissues through HTR2A, and its antagonism might be a potential therapeutic strategy for metabolic diseases. Atypical antipsychotics (e.g., clozapine) antagonize HTR2A, but can cause fatal and non‐fatal adverse effects, including agranulocytosis, cardiomyopathy, metabolic syndrome and so on. Structural derivatives of pimavanserin, which is an HTR2A antagonist that has been approved for Parkinson's disease, improved hepatic steatosis and reduced fat mass while possessing blood–brain barrier‐impermeable moieties . Given that more than 10 types of HTRs are ubiquitously expressed throughout the body, efforts to target 5‐HT for the treatment of metabolic diseases might require organ‐specific delivery to avoid adverse events.

CONCLUSIONS

Since 2010, researchers have increasingly shed light on the role of peripheral serotonin in regulating systemic energy metabolism. Most of the 5‐HT in the body is synthesized by enterochromaffin cells of the gut. However, 5‐HT is also produced from different metabolic organs, and it is known to exert biological functions in auto‐, para‐ and endocrine manners. In pancreatic β‐cells, 5‐HT induces proliferation and expansion of the β‐cell mass. In adipose tissues, 5‐HT promotes lipogenesis and inhibits adaptive thermogenesis. In the liver, 5‐HT induces lipogenesis and gluconeogenesis and activates hepatic stellate cells. The 5‐HT‐modulating drugs introduced to date, such as sibutramine and lorcaserin, have focused on central 5‐HT. These drugs showed some promise for treating metabolic diseases, but they could not overcome the adverse effects arising from their ability to act on multiple HTRs in multiple organs. Going forward, the continued development of 5‐HT‐modulating drugs that act on specific organs and/or specific HTRs should be a promising strategy to treat metabolic diseases.

DISCLOSURE

The authors declare no conflict of interest.