Central serotonin (5-HT) is an anorexigenic neurotransmitter in the brain. However, accumulating evidence suggests peripheral 5-HT may affect organismal energy homeostasis. Here we show 5-HT regulates white and brown adipose tissue function. Pharmacological inhibition of 5-HT synthesis leads to inhibition of lipogenesis in epididymal white adipose tissue (WAT), induction of browning in inguinal WAT and activation of adaptive thermogenesis in brown adipose tissue (BAT). Mice with inducibleTph1KO in adipose tissues exhibit a similar phenotype as mice in which 5-HT synthesis is inhibited pharmacologically, suggesting 5-HT has localized effects on adipose tissues. In addition,Htr3aKO mice exhibit increased energy expenditure and reduced weight gain when fed a high-fat diet. Treatment with an Htr2a antagonist reduces lipid accumulation in 3T3-L1 adipocytes. These data suggest important roles for adipocyte-derived 5-HT in controlling energy homeostasis.

Nonalcoholic fatty liver disease (NAFLD) is increasing in worldwide prevalence, closely tracking the obesity epidemic, but specific pharmaceutical treatments for NAFLD are lacking. Defining the key molecular pathways underlying the pathogenesis of NAFLD is essential for developing new drugs. Here we demonstrate that inhibition of gut-derived serotonin synthesis ameliorates hepatic steatosis through a reduction in liver serotonin receptor 2A (HTR2A) signaling. Local serotonin concentrations in the portal blood, which can directly travel to and affect the liver, are selectively increased by high-fat diet (HFD) feeding in mice. Both gut-specific Tph1 knockout mice and liver-specific Htr2a knockout mice are resistant to HFD-induced hepatic steatosis, without affecting systemic energy homeostasis. Moreover, selective HTR2A antagonist treatment prevents HFD-induced hepatic steatosis. Thus, the gut TPH1-liver HTR2A axis shows promise as a drug target to ameliorate NAFLD with minimal systemic metabolic effects.

Jun Namkung, Hail Kim, and Sangkyu Park. Mol. Cells 2015;38:1023-8. https://doi.org/10.14348/molcells.2015.0258

Recent studies have shown that sodium-glucose co-transporter 2 (SGLT2) inhibitors reduce the risk of heart failure (HF)-associated hospitalization and mortality in patients with diabetes. However, it is not clear whether SGLT2 inhibitors have a cardiovascular benefit in patients without diabetes. We aimed to determine whether empagliflozin (EMPA), an SGLT2 inhibitor, has a protective role in HF without diabetes. Cardiomyopathy was induced in C57BL/6J mice using intraperitoneal injection of doxorubicin (Dox). Mice with HF were fed a normal chow diet (NCD) or an NCD containing 0.03% EMPA. Then we analyzed their phenotypes and performed in vitro experiments to reveal underlying mechanisms of the EMPA's effects. Mice fed NCD with EMPA showed improved heart function and reduced fibrosis. In vitro studies showed similar results. Phloridzin, a non-specific SGLT inhibitor, did not show any protective effect against Dox toxicity in H9C2 cells. SGLT2 inhibitor can cause increase in blood ketone levels. Beta hydroxybutyrate (βOHB), which is well known ketone body associated with SGLT2 inhibitor, showed a protective effect against Dox in H9C2 cells and in Dox-treated mice. These results suggest elevating βOHB might be a convincing mechanism for the protective effects of SGLT2 inhibitor. SGLT2 inhibitors have a protective effect in Dox-induced HF in mice. This implied that SGLT2 inhibitor therapy could be a good treatment strategy even in HF patients without diabetes.

Obesity is defined as excessive fat mass and is a major cause of many chronic diseases such as diabetes, cardiovascular disease, and cancer. Increasing energy expenditure and regulating adipose tissue metabolism are important targets for the treatment of obesity. Serotonin (5-hydroxytryptophan [5-HT]) is a monoamine metabolite of the essential amino acid tryptophan. Here, we demonstrated that 5-HT in mature adipocytes regulated energy expenditure and lipid metabolism. Tryptophan hydroxylase 1 (TPH1) is the rate-limiting enzyme during 5-HT synthesis in non-neural peripheral tissues. We generated adipose tissue-specific Tph1 knockout (Tph1 FKO) mice and adipose tissue-specific serotonin receptor 2A KO (Htr2a FKO) mice and analyzed their phenotypes during high-fat diet (HFD) induced obesity. Tph1 FKO mice fed HFD exhibited reduced lipid accumulation, increased thermogenesis, and resistance to obesity. In addition, Htr2a FKO mice fed HFD showed reduced lipid accumulation in white adipose tissue and resistance to obesity. These data suggest that the inhibition of serotonin signaling might be an effective strategy in obesity.

Namkung J, et al. Diabetes Metab J. 2018 Jan;42:e14. https://doi.org/10.4093/dmj.2017.0084