5-HT Obesity Medication Efficacy via POMC Activation is Maintained During Aging

B R I E F R E P O R T 5-HT Obesity Medication Efficacy via POMC Activation is Maintained During Aging Luke K. Burke, Barbora Doslikova, Giuseppe D’Agostino, Alastair S. Garfield, Gala Farooq, Denis Burdakov, Malcolm J. Low, Marcelo Rubinstein, Mark L. Evans, Brian Billups, and Lora K. Heisler Department of Pharmacology (L.K.B., B.D., G.D., A.S.G., G.F., D.B., B.B., L.K.H.) and Wellcome Trust/ Medical Research Council Institute of Metabolic Science (M.L.E.), University of Cambridge, Cambridge, CB2 0QQ, United Kingdom; Rowett Institute of Nutrition and Health (G.D., L.K.H.), University of Aberdeen, Aberdeen, AB21 9SB, United Kingdom; Department of Molecular and Integrative Physiology (M.J.L., M.R.), University of Michigan Medical School, Ann Arbor, Michigan 48105; and Instituto de Investigaciones en Ingeniería Genética y Biología Molecular (M.R.), Consejo Nacional de Investigaciones Científicas y Técnicas, 1428 Buenos Aires, Argentina The phenomenon commonly described as the middle-age spread is the result of elevated adiposity accumulation throughout adulthood until late middle-age. It is a clinical imperative to gain a greater understanding of the underpinnings of age-dependent obesity and, in turn, how these mechanisms may impact the efficacy of obesity treatments. In particular, both obesity and aging are associated with rewiring of a principal brain pathway modulating energy homeostasis, promoting reduced activity of satiety pro-opiomelanocortin (POMC) neurons within the arcuate nucleus of the hypothalamus (ARC). Using a selective ARC-deficient POMC mouse line, here we report that former obesity medications augmenting endogenous 5-hydroxytryptamine (5-HT) activity D-fenfluramine and sibutramine require ARC POMC neurons to elicit therapeutic appetite-suppressive effects. We next investigated whether age-related diminished ARC POMC activity therefore impacts the potency of 5-HT obesity pharmacotherapies, lorcaserin, D-fenfluramine, and sibutramine and report that all compounds reduced food intake to a comparable extent in both chowfed young lean (3–5 months old) and middle-aged obese (12–14 months old) male and female mice. We provide a mechanism through which 5-HT anorectic potency is maintained with age, via preserved 5-HT–POMC appetitive anatomical machinery. Specifically, the abundance and signaling of the primary 5-HT receptor influencing appetite via POMC activation, the 5-HT2CR, is not perturbed with age. These data reveal that although 5-HT obesity medications require ARC POMC neurons to achieve appetitive effects, the anorectic efficacy is maintained with aging, findings of clinical significance to the global aging obese population. (Endocrinology 155: 3732–3738, 2014) diposity commonly accumulates with age from early adulthood (20 years old in humans) through late middle-age (65 years old in humans). This increase in adiposity has significant health implications; age-related obesity represents the primary cause of metabolic syndromes (1). Understanding the mechanisms underpinning age-associ- A ated adiposity has clinical implications both for obesity treatment and the prevention of chronic obesity-related comorbidities in the aging population (2). To date, medications targeting 5-hydroxytryptamine (5-HT; serotonin) pathways have been among the most clinically successful obesity treatments (3). These include ISSN Print 0013-7227 ISSN Online 1945-7170 Printed in U.S.A. This article has been published under the terms of the Creative Commons Attribution License (CC-BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Copyright for this article is retained by the author(s). Author(s) grant(s) the Endocrine Society the exclusive right to publish the article and identify itself as the original publisher. Received March 11, 2014. Accepted July 12, 2014. First Published Online July 22, 2014 Abbreviations: ARC, arcuate nucleus of the hypothalamus; EGFP, enhanced green fluorescent protein; 5-HT, 5-hydroxytryptamine; 5-HTR, 5-HT receptor; mTOR, mammalian target of rapamycin; NA, noradrenaline; POMC, pro-opiomelanocortin. 3732 endo.endojournals.org Endocrinology, October 2014, 155(10):3732–3738 doi: 10.1210/en.2014-1223 doi: 10.1210/en.2014-1223 D-fenfluramine, which stimulates 5-HT release/blocks 5-HT reuptake, and sibutramine, which blocks 5-HT/noradrenaline (NA) reuptake. However, both compounds have been withdrawn from clinical use due to off-target effects (3, 4). Transgenic technology has aided in the identification of 5-HT receptors (5-HTRs) responsible for the therapeutic effect of 5-HT obesity medications; an effect principally mediated via Gq-coupled 5-HT2CRs (3). The therapeutic potential of selective activation of 5-HT2CRs for obesity treatment was realized in the summer of 2012 with the U.S. Food and Drug Administration approval of 5-HT2CR agonist lorcaserin (Arena Pharmaceuticals) and its release in 2013. Over the last decade, a primary mechanism through which 5-HT action, via 5-HT2CR activation, achieves effects on appetite and body weight has been revealed. Specifically, D-fenfluramine and 5-HT2CR agonists chiefly elicit appetitive effects through activation of pro-opiomelanocortin (POMC) neurons (5–10). Brain POMC, a critical mediator of energy balance, is abundantly expressed in the arcuate nucleus of the hypothalamus (ARC), and a smaller popula- endo.endojournals.org 3733 tion is localized in the nucleus of the solitary tract (11, 12). Although the 5-HT2CR–POMC axis represents a critical target in the clinical treatment of obesity, recent reports demonstrate that endogenous ARC POMC regulation of energy balance is impaired with aging. In middle-aged mice (12 months old, comparable to a human 40 years old), ARC POMC neurons display an age-dependent upregulation of mammalian target of rapamycin (mTOR) signaling, which functions to reduce POMC neuronal activity, an effect reversed by mTOR inhibitor rapamycin (13). Furthermore, genetic ablation of endogenous inhibitors of mTOR promoted aged characteristics in ARC POMC neurons of young mice (14, 15). Middle-aged mice also display increased inhibitory agouti-related protein drive onto ARC POMC neurons, which functionally reduced POMC firing rate and decreased resting membrane potential compared with younger mice (16). Given the importance of POMC in mediating 5-HT’s appetitive effect, we investigated whether age-associated perturbed ARC POMC activity thereby reduces 5-HT compound anorectic potency. Materials and Methods Animals Mice on a C57BL/6 background expressing enhanced green fluorescent protein (EGFP) under the control of POMC regulatory elements (POMC-EGFP mice) (8, 17), ARC-specific POMC-knockout mice (POMCfneo/fneo) (12), and wild-type littermates were maintained on a 12-hour light, 12-hour dark cycle (lights on at 7:00 AM) with ad libitum access to chow diet and water unless otherwise stated. (...truncated)