J Phys Fitness Sports Med, 1(2): 351-356 (2012) JPFSM: Short Review Article Effect of physical exercise on lipolysis in white adipocytes Junetsu Ogasawara1*, Takuya Sakurai1, Takako Kizaki1, Kazuto Takahashi2, Hitoshi Ishida2, Tetsuya Izawa3, Koji Toshinai4, Norihiko Nakano5 and Hideki Ohno1 1 Department of Molecular Predictive Medicine and Sport Science, Kyorin University, School of Medicine,6-20-2 Shinkawa, Mitaka, Tokyo 181-8611, Japan 2 Third Department of Internal Medicine, Kyorin University, School of Medicine,6-20-2 Shinkawa, Mitaka, Tokyo 181-8611, Japan 3 Department of Sports Biochemistry, Faculty of Health and Sports Science, Doshisha University, Tataramiyakodani, Kyotanabe, Kyoto 610-0394, Japan 4 Neurology, Respirology, Endocrinology, and Metabolism, Division of Internal Medicine, Faculty of Medicine, University of Miyazaki, 5200 Kihara, Kiyotake, Miyazaki 889-1692, Japan 5 Aino Institute of Regeneration and Rehabilitation, Aino University, 4-5-4 Higashiohara, Ibaraki, Osaka 567-0012, Japan Received: April 27, 2012 / Accepted: June 12, 2012 Abstract Fatty acids are derived from the hydrolysis of triacylglycerol (TG) found in white adipose tissue, muscle tissue and circulating lipoproteins. The mobilization of free fatty acids (FFA) from white adipose tissue contributes to about 50% of the FFA utilized during moderate- intensity exercise. The delivery of FFA from white adipose tissue is improved by hormone- stimulated lipolytic events in white adipocytes (WA). Thus, the lipolysis in WA that provides fuel for metabolism has been a highly conserved function throughout the course of evolution. This short review outlines our current understanding of the molecular regulation of TG lipases via the lipolytic cascade in WA, as well as provides an account of our recent findings concern- ing changes in the lipolytic molecules of WA that result from acute and habitual exercise. Keywords : Lipolysis, HSL, ATGL, perilipin 1, CGI-58, white adipocyte In this short review, the known regulatory mechanism(s) Introduction of lipolytic responses in WA, such as the lipolytic cas- The central contribution of white adipocytes (WA) to cade and the molecular behavior of lipases and regulatory the regulation of energy homeostasis is due to the storage proteins, are outlined. Then, the authors’ recent findings and cleavage capacity of lipids as well as to their function are introduced concerning exercise-induced molecular as endocrine organs that secrete many adipokines.1) Li- changes in lipolytic factors in WA. polysis is a catabolic event involving stored intracellular triacylglycerol (TG) that releases free fatty acids (FFA) Role of the lipolytic cascade in the control of lipolysis and glycerol to provide fuel for whole-body metabolism. Thus, any alteration of lipolysis in adipocytes has an im- Lipolysis in WA is regulated by a multifaceted phenom- portant effect on energy partitioning under environmental enon that is subject to distinct temporal controls, includ- conditions. Indeed, lipolysis in WA is known to be activat- ing physical exercise. Hormonal activation of lipolysis in ed by environmental changes, such as nutrient depletion adipocytes is known to be mediated via a cAMP-depen- (fasting) or an increase in energy demand (e.g., exercise). dent signal transduction process3,4) (Fig. 1). The stimula- While the cellular bases of the dynamic hydrolysis tion of G-protein coupled receptors (GPCRs), i.e., β1-, action of TG are well documented, the process of eluci- β2- and β3-adrenergic receptors, induces a conformational dating the molecular factors regulating these processes change in the Gα subunit of the heterotrimeric G protein continues to evolve. It is widely accepted that hierarchical (Gαβγ) that leads to GDP release and GTP binding. Acti- action via alteration in intracellular cAMP is the mecha- vated Gαs leads to the activation of adenylyl cyclase (AC) nism by which hormone-stimulated and non-hormone- and production of cAMP. On the other hand, stimulation 5) stimulated lipolysis in WA (i.e., lipolytic cascade) pro- of GPCRs, i.e., α2-drenergic receptor, adenosine recep- ceeds. However, recent studies have indicated that the tor,6) and prostaglandin E2 receptor,7) which stimulate trafficking of lipases and regulatory proteins is essential Gαi, causes the inactivation of AC and reduces the pro- for full lipolysis as final steps in the lipolytic cascade.2) duction of cAMP, resulting in an attenuation of the lipo- lytic response. In addition, insulin attenuates intracellular *Correspondence: [email protected] cAMP production through increases in phosphodiester- 352 JPFSM: Ogasawara J, et al. Stimulatory actions E1Ͳ,E2Ͳ,E3ͲAR FA Glycerol GsD GE GJ GTP GsD GDP MGL AC Mg䊶ATP P MG GiD HSL Translocation cAMP FA GiD GE P DG FA PDEͲ3B PKA GJ HSL TG PKB/AKT P Perilipin1 ATGL Inhibitory actions CGIͲ58 PI3ͲK Lipiddroplet D2ͲAR IRSͲ1 AdenosineͲR NicotinicacidͲR Nucleus InsulinͲR Figure1 Fig. 1 The lipolytic cascade for white adipocytes Lipolysis in white adipocytes is regulated by diverse stimulatory and inhibitory actions through GPCRs that exist on the plasma membrane. Ligand binding to stimulatory GPCRs provokes AD activation via the action of Gsα, resulting in the PKA-mediated phosphorylation of lipase and regulator proteins, and thereby the activation of lipolysis. On the other hand, insulin and inhibi- tory ligands attenuate lipolysis via a reduction in the cAMP-mediated activation of PKA. -AR, adrenergic receptor;-R, receptor; PKA, cAMP-dependent protein kinase A; TG, triacylglycerol; DG, diacylglycerol; MG, monoacylglycerol; IRS-1, insulin re- ceptor substrate-1;PI3-K, phosphatidil inositol 3 kinase; PKB, PDE-3B, phophodiesterase-3B; MGL, monoglyceride lipase. ase-3B (PDE-3B) activity, which changes cAMP to AMP glycerol (DG) and monoacylglycerol (MG) and glycerol. via activation of protein kinase B/AKT. An increased HSL exhibits DG hydrolase activity that is 10- and 5-fold intracellular cAMP level phosphorylates and activates greater than its activity against TG and MG, respec- cAMP-dependent protein kinase A (PKA),8,9) which, in tively,14) and is considered to be the major lipase mediat- turn, phosphorylates hormone-sensitive lipase (HSL), a ing hormone-stimulated lipolysis in adipocytes.15) HSL rate-limiting enzyme of lipolysis, and activates hydrolysis activation is regulated by reversible phosphorylation by of TG in adipocytes10) via the translocation of HSL on the protein kinases. The phosphorylation of HSL at Ser563, lipid droplet surface.11) Under activation of PKA, perilipin Ser659 and Ser660, by cAMP-dependent protein kinase 1, which is a dominant target of PKA activation,12) is also (PKA), is known to enhance its enzymatic activity; and phosphorylated and acts as a scaffold protein, resulting in extracellular-regulated kinase (ERK) increases HSL activ- facilitated lipolysis by allowing the subcellular localiza- ity via phosphorylation at Ser600 in 3T3-L1 adipocytes,16) tion of HSL to lipid droplets.13) Taken together, lipolytic although this has not been observed in either human or responses provide a conceptual network that is regulated murine primary adipocytes. On the other hand, insulin by the PKA-mediated trafficking of phosphorylated pro- has an inhibitory effect on the cAMP-dependent and -in- teins to lipid droplet surfaces. dependent activation of HSL.17,18) AMP activated protein kinase (AMPK) is known to attenuate the enzymatic ac- tivity of HSL through an increase in the phosphorylation Lipase control of lipolysis via the sequential hydrolysis of HSL at Ser565.19) In addition, glycogen synthase ki- of TG nase-420) and Ca2+/calmodulin-dependent kinaseⅡ21) have The hydrolysis of TG in adipocytes is regulated by also been reported to cause phosphorylation of HSL at specific enzymes, and results in the liberation of fatty Ser563 and Ser565, respectively. Thus, the phosphoryla- acids (FA) at several steps, with the production of diacyl- tion of HSL is known to play a central role in the regula- JPFSM: Regulation of lipolysis in white adipocytes 353 tion of enzyme activity, and is closely associated with the the enzymatic activity of HSL. However, the activation of catabolism of adipocytes. PKA in response to β-AR causes up to a 100-fold induc- Until recently, HSL was considered to be the only, and tion of FA and glycerol release in intact adipocytes, in- therefore the rate-limiting, enzyme for the lipolytic ca- dicating the possibility that other mechanisms contribute tabolism of intracellular TG in adipocytes and nonadipose to hormone-induced lipolysis. The discovery of perilipin cells, because of its capacity to hydrolyze both TG and 1 provided proof that this was indeed the case.30) Perili- DG as substrates. However, studies focusing on white pin 1 is expressed mostly in white adipose tissue and in adipocytes of HSL knockout mice have revealed that no steroidogenic tissue.12) Perilipin 1 can be phosphorylated complete loss of activity in the hydrolysis of TG occurs at as many as six sites (Ser81, Ser222, Ser276, Ser433, in mice adipocytes,22-24) which indicates that, in addition Ser492 and Ser517) by PKA31-34) and is a major target of to HSL, other lipases are involved in the TG degradation PKA phosphorylation in adipocytes.12) Previous studies of adipocytes. In 2004, three groups independently pub- have demonstrated that perilipin 1 is a multifunctional lished the discovery of an enzyme that could hydrolyze protein that is capable of reducing basal lipolysis by TG,25-27) and Zimmermann et al. named it