Neuropeptide Y potentiates beta-adrenergic stimulation of lipolysis in 3T3-L1 adipocytes
Highlights
► NPY potentiates isoproterenol-stimulated lipolysis in 3T3-L1 adipocytes. ► NPY potentiation occurs upstream of adenylyl cyclase activation. ► NPY potentiation is mediated in part through enhanced phosphorylation of hormone sensitive lipase. ► NPY does not alter the expression of several key lipolytic and lipogenic enzymes/proteins.
Introduction
Neuropeptide Y (NPY) belongs to a family of structurally related peptides, which includes peptide YY and pancreatic polypeptide. It is synthesized as a prepropeptide that is subsequently cleaved into its active 36 amino acid peptide [1]. Functions of NPY are mediated by five distinct receptor subtypes (known as Y1, Y2, Y4, Y5, and Y6), which belong to the G protein-coupled receptor superfamily [2]. NPY is produced in both the central and peripheral nervous systems and is co-stored with norepinephrine in some sympathetic neurons [3]. NPY is one of the most abundant neuropeptides, and mediates a number of physiological effects, including regulation of neuroendocrine systems, blood pressure, anxiety, memory retention and feeding [2]. In the hypothalamus, NPY is the most potent appetite-stimulating signal, and hypothalamic NPY expression is upregulated in obese humans and animal models of obesity [4], [5]. Chronic infusion of NPY into the hypothalamus of rats and mice leads to hyperphagia and obesity [6].
Therefore, NPY was thought to contribute to the pathogenesis of obesity mainly through its potent stimulation of appetite in the hypothalamus. Recently, this dogma has been challenged by the identification of adipose tissue as a novel peripheral site of NPY biosynthesis by us [7] and others [8], [9]. Importantly, we also found that NPY expression in adipose tissue was upregulated dramatically in an early-life programmed rat model of visceral adiposity and obese Zucker rats [7], [10]. Furthermore, Kuo and colleagues reported a similar upregulation of NPY expression in adipose tissue of a diet-induced mouse model of obesity when subjected to cold stress [8]. Collectively, the up-regulated NPY expression in the adipose tissues of several distinct rodent models of obesity underscores an important peripheral role for NPY in the pathogenesis of obesity.
Indeed, we and others have demonstrated that NPY promotes adipogenesis in vitro [7], [8]. NPY has also been shown to stimulate angiogenesis, another fundamental process that underlies adipose tissue expansion. Importantly, NPY delivery via a pellet within adipose tissue led to increases in both the weight and volume of adipose tissue in mice [8]. However, the effects of NPY on adipocytes are poorly understood. Therefore, the present study was designed to investigate the effects of NPY on adipocyte lipolysis and gene expression using 3T3-L1 adipocytes as an in vitro model system because these cells have been shown to recapitulate many of the characteristics of adipocytes [11]. This study uncovers a novel role for NPY in potentiating β-adrenergic agonist stimulated lipolysis.
Section snippets
3T3-L1 cultures and differentiation
The murine preadipocyte 3T3-L1 cell line was obtained from the American Type Culture Collection (Manassas, VA). 3T3-L1 cells were cultured and differentiated, as previously described [7]. Briefly, they were cultured in growth medium, consisting of DMEM (Sigma) supplemented with 50 U/ml penicillin, 50 μg/ml streptomycin (Invitrogen) and 10% fetal bovine serum (Sigma). Cultures were maintained in a humidified incubator at 5% CO2 and 37 °C. The medium was replaced every 2 to 3 days. At the 2nd day
Effects of NPY on lipolysis
Obesity results from a chronic imbalance between energy storage (i.e., lipogenesis) and energy expenditure (i.e., lipolysis) in adipose tissue. Since NPY expression is upregulated in visceral adipose tissue of our early-life programmed rat model of increased visceral adiposity [7], we hypothesized that NPY may act directly on adipocytes to inhibit lipolysis and/or stimulate lipogenesis thereby contributing to the pathogenesis of increased visceral adiposity. As a first step in examining this
Discussion
Adipose tissue is a novel site of NPY biosynthesis, and NPY acts locally to promote adipogenesis thereby contributing to the pathogenesis of obesity [7], [8]. Despite these important observations, the role of NPY in regulating adipocyte metabolism is poorly understood. The present findings reveal a novel role for NPY in potentiating β-adrenergic stimulation of lipolysis. Furthermore, our data suggest that this potentiation likely occurs upstream of adenylyl cyclase activation, and is mediated
Acknowledgments
This work was supported by the Heart and Stroke Foundation of Ontario (Grant-In-Aid, NA-6049). R.L. was a recipient of the Department of Obstetrics and Gynaecology Graduate Research Scholarship and the CIHR Master's Award: Frederick Banting and Charles Best Canada Graduate Scholarship.
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