Weight Loss After RYGB Is Independent of and Complementary to Serotonin 2C Receptor Signaling in Male Mice

ORIGINAL RESEARCH Weight Loss After RYGB Is Independent of and Complementary to Serotonin 2C Receptor Signaling in Male Mice Jill S. Carmody, Nadia N. Ahmad, Sriram Machineni, Scott Lajoie, and Lee M. Kaplan Obesity, Metabolism, and Nutrition Institute and Gastrointestinal Unit (J.S.C., N.N.A., S.M., S.L., L.M.K.), Massachusetts General Hospital, Boston, Massachusetts 02114; and Department of Medicine (J.S.C., N.N.A., S.M., L.M.K.), Harvard Medical School, Boston, Massachusetts 02115 Roux-en-Y gastric bypass (RYGB) typically leads to substantial, long-term weight loss (WL) and diabetes remission, although there is a wide variation in response to RYGB among individual patients. Defining the pathways through which RYGB works should aid in the development of less invasive anti-obesity treatments, whereas identifying weight-regulatory pathways unengaged by RYGB could facilitate the development of therapies that complement the beneficial effects of surgery. Activation of serotonin 2C receptors (5-HT2CR) by serotonergic drugs causes WL in humans and animal models. 5-HT2CR are located on neurons that activate the melanocortin-4 receptors, which are essential for WL after RYGB. We therefore sought to determine whether 5-HT2CR signaling is also essential for metabolic effects of RYGB or whether it is a potentially complementary pathway, the activation of which could extend the benefits of RYGB. Diet-induced obese male mice deficient for the 5-HT2CR and their wild-type littermates underwent RYGB or sham operation. Both groups lost similar amounts of weight after RYGB, demonstrating that the improved metabolic phenotype after RYGB is 5-HT2CR independent. Consistent with this hypothesis, wild-type RYGB-treated mice lost additional weight after the administration of the serotonergic drugs fenfluramine and meta-chlorophenylpiperazine but not the nonserotonergic agent topiramate. The fact that RYGB does not depend on 5-HT2CR signaling suggests that there are important WL mechanisms not fully engaged by surgery that could potentially be harnessed for medical treatment. These results suggest a rational basis for designing medical-surgical combination therapies to optimize clinical outcomes by exploiting complementary physiological mechanisms of action. (Endocrinology 156: 3183–3191, 2015) T he high prevalence of obesity and the clinical impact of its related comorbidities have created an urgent need for more effective weight-loss therapies. Gastrointestinal weight loss procedures, such as Roux-en-Y gastric bypass (RYGB), remain the most effective treatments for patients with severe obesity, producing substantial long-term weight loss and frequent remission of diabetes (1–3). The clinical improvements seen after RYGB are brought about by changes in metabolic physiology, though the precise cellular and molecular mechanisms of action, have only begun to be elucidated (4). Dissecting the energy regula- tory pathways underlying surgical weight loss, and differentiating those pathways engaged by RYGB from those that are not, could reveal new and effective anti-obesity targets, thus facilitating the development of less invasive therapies that mimic the therapeutic benefits of surgery. Alternatively, identifying weight and metabolism regulatory pathways that are not used by RYGB may reveal therapeutic targets that complement surgery and thus enhance clinical outcomes after surgical intervention. The weight-loss response to RYGB in human patients is highly variable. It displays a normal distribution and is largely ISSN Print 0013-7227 ISSN Online 1945-7170 Printed in USA Copyright © 2015 by the Endocrine Society Received March 12, 2015. Accepted June 8, 2015. First Published Online June 11, 2015 Abbreviations: BW, body weight; DIO, diet-induced obese; HFD, high-fat diet; HOMA-IR, homeostasis model assessment-insulin resistance; 5-HT2CR, serotonin 2C receptor; mCPP, meta-chlorophenylpiperazine; MC4R, melanocortin-4 receptor; POMC, proopiomelanocortin; RYGB, Roux-en-Y gastric bypass; WT, wild type. doi: 10.1210/en.2015-1226 Endocrinology, September 2015, 156(9):3183–3191 press.endocrine.org/journal/endo 3183 3184 Carmody et al RYGB Induced Weight Loss Is 5-HT2CR Independent biologically driven, demonstrated by the heritability and genetic predicators of weight-loss response (5, 6). Given this wide variation in outcomes and the invasive nature of surgery, means of enhancing weight loss in those patients who experience less complete responses would be highly valuable. With respect to metabolic pathways that are required for RYGB action, we have recently shown in animals models that weight loss and decreased fat mass after RYGB are dependent on intact signaling through the melanocortin-4 receptor (MC4R) (7). Recent clinical cases have confirmed this observation in human patients homozygous for mutations in the MC4R gene interfering with the efficacy of RYGB and another bariatric operation, vertical sleeve gastrectomy (Kaplan, L.M., unpublished data). MC4R-containing neurons in the hypothalamus receive signals from the proopiomelanocortin (POMC) neurons that regulate their activity. Several recent studies have demonstrated that activation of the serotonin 2C receptors (5-HT2CR) located on these POMC neurons strongly influence energy balance and glucose regulation (8 –12). For example, mice lacking the 5-HT2CR only in the POMC neurons develop obesity when fed a high-fat diet and display glucoregulatory defects, demonstrating that 5-HT2CR-bearing melanocortinergic signaling neurons are critical for normal energy balance and glucose regulation (8). More generally, mice lacking the 5-HT2CR globally have increased body weight compared with wild-type mice (13). It has been long known that stimulation of serotonin receptors can suppress food intake and reduce weight, making these receptors the target of several anti-obesity agents (14). Unfortunately, despite their efficacy, the nonselective nature of several earlier serotonergic anti-obesity medications, including fenfluramine and dexfenfluramine, has been associated with adverse cardiovascular effects that have made them clinically unacceptable (15, 16). One of the few pharmacological agents currently available to treat patients with obesity is lorcaserin, a selective agonist for the 5-HT2CR (17–19). Activation of this receptor has been shown to cause weight loss and improve insulin sensitivity independent of weight, without the adverse cardiovascular effects observed with the nonspecific agonists (9, 19). Given the broad requirement of the MC4R for RYGBinduced metabolic improvements, it is likely that one or more inputs into the melanocortin pathway are also necessary to achieve RYGB outcomes. In this study, we sought to determine the role of the 5-HT2CR in these effects. We theorized that if 5-HT2CR signaling was an essential input into the melanocortin system after RYGB, then mice deficient for the 5-HT2CR would exhibit attenuated w (...truncated)