A family of Acrp30/adiponectin structural and functional paralogs

^{Whitehead Institute for Biomedical Research, Cambridge, MA 02142;}^{Division of Respiratory Disease, Children's Hospital, Boston, MA 02115;}^{Department of Pediatrics, Harvard Medical School, Boston, MA 02115; and}^{Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02142}

^{To whom correspondence should be addressed. E-mail:}ude.tim.iw@hsidol.

Contributed by Harvey F. Lodish, May 29, 2004

Abstract

Biochemical, genetic, and animal studies in recent years have established a critical role for the adipokine Acrp30/adiponectin in controlling whole-body metabolism, particularly by enhancing insulin sensitivity in muscle and liver, and by increasing fatty acid oxidation in muscle. We describe a widely expressed and highly conserved family of adiponectin paralogs designated as C1q/tumor necrosis factor-α-related proteins (CTRPs) 1–7. In the present study, we focus on mCTRP2, the mouse paralog most similar to adiponectin. At nanomolar concentrations, bacterially produced mCTRP2 rapidly induced phosphorylation of AMP-activated protein kinase, acetyl-CoA carboxylase, and mitogen-activated protein kinase in C2C12 myotubes, which resulted in increased glycogen accumulation and fatty acid oxidation. The discovery of a family of adiponectin paralogs has implications for understanding the control of energy homeostasis and could provide new targets for pharmacologic intervention in metabolic diseases such as diabetes and obesity.

Abstract

Adipose tissue plays an active role in monitoring and controlling whole-body metabolism by secreting a variety of bioactive molecules collectively termed adipokines (1). Acrp30/adiponectin is one such adipokine discovered in a screen to identify novel secreted proteins induced upon adipocyte differentiation (2). Adiponectin is composed of four distinct domains: a signal peptide at the N terminus, a short variable region, a collagenous domain, and a C-terminal globular domain homologous to C1q. The crystal structure of adiponectin globular domain reveals a striking resemblance to the structure of tumor necrosis factor (TNF)-α (3). Adiponectin belongs to a growing family of proteins, all of which contain a C-terminal globular C1q-like domain of ≈135 aa. Most of them also contain a variable number of “Gly-X-Y” (where X and Y represent any amino acid) collagenous repeats. Including the seven adiponectin paralogs described here, there are currently 25 proteins belonging to the C1q/TNF-α superfamily. Among these proteins are the Siberian chipmunk hibernating proteins HP20, 25, and 27; serum levels of these three proteins are dramatically reduced during hibernation (4).

Adiponectin is expressed exclusively by differentiated adipocytes and its expression is induced ≈100-fold during adipocyte differentiation (2). Serum levels of adiponectin correlate inversely with insulin sensitivity. In healthy adults, adiponectin circulates in serum at a high concentration (1.9–17.0 μg/ml). Adiponectin levels are significantly reduced in a variety of obese and insulin-resistant states in mice (5), monkeys (6), and humans (7). Diabetic humans with coronary artery disease have even lower plasma levels of adiponectin than those without (8). Mutations and polymorphisms in the adiponectin gene are associated with reduced serum levels of adiponectin, in part due to effects on protein secretion, multimerization, and/or stability (9–11). Weight loss, caloric restriction, or thiazolidinedione treatment increase adiponectin levels in human (12–14) and mice (15, 16), and this increase correlates with increased insulin sensitivity.

Injection of a proteolytically generated globular C-terminal form of recombinant adiponectin into mice significantly reduced levels of plasma-free fatty acids and glucose after a high-fat meal (17). Furthermore, long-term administration of adiponectin to mice fed a high-fat diet caused profound weight loss by enhancing free fatty acid oxidation in muscles without affecting food intake. Subsequently, Yamauchi et al. (18) demonstrated that recombinant adiponectin could restore insulin sensitivity to insulin-resistant obese (ob/ob), diabetic (db/db), or lipoatrophic mice by increasing β-oxidation of fatty acids in muscle. Full-length, but not the globular C-terminal domain of adiponectin produced in mammalian cells, enhanced the ability of insulin to suppress gluconeogenesis and glucose release by primary rat hepatocytes (19). Many recent studies (summarized in ref. 20) have confirmed and extended these initial findings and corroborated the current notion that adiponectin acts as an insulin sensitizer in vivo, exerting its effects on the liver to suppress glucose output while acting on muscle to increase glucose uptake and fatty acid oxidation.

Differences in activity initially attributed to full-length or globular adiponectin can be ascribed to the different oligomeric forms of adiponectin (11, 21–23). In serum, adiponectin exists as trimers, hexamers, and high molecular weight species and the proportion of these oligomeric forms changes according to metabolic status and disease states (24). These different oligomeric forms possess distinct signaling properties; hexameric and high molecular weight forms of adiponectin induce NF-κB activation, whereas the trimeric forms of adiponectin induce AMP-activated protein kinase (AMPK) activation in muscle (21, 22). In muscle, AMPK activation results in increased glucose uptake (25, 26) and glycogen accumulation (27). In addition, activated AMPK phosphorylates and inhibits acetylCoA carboxylase (ACC), leading to decreased fatty acid synthesis and a concomitant increase in β-oxidation of fatty acid (26). Beyond its role in controlling glucose and lipid metabolism, additional functions of adiponectin have been suggested, including antiinflammatory (28, 29), antiatherosclerotic (30), and pro- (31) or antiangiogenic (32) properties.

Here, we describe a family of adiponectin paralogs. We show that at least one paralog, mouse CTRP2 (mCTRP2), possesses similar biologic properties as adiponectin in enhancing glycogen accumulation and fatty acid oxidation in C2C12 myotubes by activating the AMPK signaling pathway. We discuss important implications for metabolic control orchestrated by adiponectin and its paralogs.

Numbers indicate percent amino acid identities.

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Acknowledgments

This work was supported in part by U.S. Public Health Service Grant R37DK47618 (to H.F.L.) and mentor-based grants from the American Diabetes Association (to H.F.L.). T.-S.T. is supported by a fellowship from the Ares-Serono Foundation and the American Diabetes Association. C.H. was supported by a training grant from the National Institutes of Health to the Division of Respiratory Diseases at The Children's Hospital and is the recipient of a Charles Hood Award from The Medical Foundation. G.W.W. is supported by a fellowship from the Ruth L. Kirschstein National Service Award.

Acknowledgments

Notes

Abbreviations: TNF, tumor necrosis factor; CTRP, C1q/TNF-α-related protein; mCTRP, mouse CTRP; hCTRP, human CTRP; AMPK, AMP-activated protein kinase; HA, hemagglutinin; ACC, acetyl-CoA carboxylase; MAPK, mitogen-activated protein kinase.

Notes

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