Human Protein Reference Database (HPRD) is an object database that integrates a wealth of information relevant to the function of human proteins in health and disease. Data pertaining to thousands of protein-protein interactions, posttranslational modifications, enzyme/substrate relationships, disease associations, tissue expression, and subcellular localization were extracted from the literature for a nonredundant set of 2750 human proteins. Almost all the information was obtained manually by biologists who read and interpreted >300,000 published articles during the annotation process. This database, which has an intuitive query interface allowing easy access to all the features of proteins, was built by using open source technologies and will be freely available at http://www.hprd.org to the academic community. This unified bioinformatics platform will be useful in cataloging and mining the large number of proteomic interactions and alterations that will be discovered in the postgenomic era.

Skeletal muscle plays important roles in whole-body glucose and fatty acid metabolism. However, muscle also secretes cytokines and growth factors (collectively termed myokines) that can potentially act in an autocrine, a paracrine, and/or an endocrine manner to modulate metabolic, inflammatory, and other processes. Here, we report the identification and characterization of myonectin, a novel myokine belonging to the C1q/TNF-related protein (CTRP) family. Myonectin transcript was highly induced in differentiated myotubes and predominantly expressed by skeletal muscle. Circulating levels of myonectin were tightly regulated by the metabolic state; fasting suppressed, but refeeding dramatically increased, its mRNA and serum levels. Although mRNA and circulating levels of myonectin were reduced in a diet-induced obese state, voluntary exercise increased its expression and circulating levels. Accordingly, myonectin transcript was up-regulated by compounds (forskolin, epinephrine, ionomycin) that raise cellular cAMP or calcium levels. In vitro, secreted myonectin forms disulfide-linked oligomers, and when co-expressed, forms heteromeric complexes with other members of the C1q/TNF-related protein family. In mice, recombinant myonectin administration reduced circulating levels of free fatty acids without altering adipose tissue lipolysis. Consistent with this, myonectin promoted fatty acid uptake in cultured adipocytes and hepatocytes, in part by up-regulating the expression of genes (CD36, FATP1, Fabp1, and Fabp4) that promote lipid uptake. Collectively, these results suggest that myonectin links skeletal muscle to lipid homeostasis in liver and adipose tissue in response to alterations in energy state, revealing a novel myonectin-mediated metabolic circuit.

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.

The insulin-sensitizing hormone, adiponectin, belongs to the expanding C1q/TNF (tumour necrosis factor) family of proteins. We recently identified a family of adiponectin paralogues designated as CTRP (C1q/TNF-related protein) 1–7, and in the present study describe CTRP10. In the present study, we show that CTRP1, CTRP2, CTRP3, CTRP5 and CTRP7 transcripts are expressed predominantly by adipose tissue. In contrast, placenta and eye expressed the highest levels of CTRP6 and CTRP10 transcripts respectively. Expression levels of CTRP1, CTRP2, CTRP3, CTRP6 and CTRP7 transcripts are up-regulated in 8-week-old obese (ob/ob) mice relative to lean controls. Treatment of mice with a PPAR-γ (peroxisome-proliferator-activated receptor-γ) agonist, rosiglitazone, increased the expression of CTRP1 and decreased CTRP6 transcript levels. All CTRPs are secreted glycoproteins when expressed in mammalian cells. CTRP1, CTRP2, CTRP3, CTRP5 and CTRP6 circulate in the blood and are potential endocrine hormones; their serum levels vary according to the sex and genetic background of mice. Importantly, serum levels of CTRP1 and CTRP6 are increased in adiponectin-null mice. Like adiponectin, all secreted CTRP proteins form trimers as their basic structural units. CTRP3, CTRP5, CTRP6 and CTRP10 trimers are further assembled into higher-order oligomeric complexes via disulfide bonding mediated by their N-terminal cysteine residues. Besides forming homo-oligomers, CTRP1/CTRP6, CTRP2/CTRP7 and adiponectin/CTRP2 are secreted as heterotrimers, thus providing a mechanism to potentially generate functionally distinct ligands. Functional characterization of one such family member, CTRP1, showed that it specifically activates Akt and p44/42-MAPK (mitogen-activated protein kinase) signalling pathways in differentiated mouse myotubes. Moreover, injection of recombinant CTRP1 into mice significantly reduced their serum glucose levels. Thus at least CTRP1 may be considered a novel adipokine. In summary, these molecular, biochemical and functional data provide an important framework to further address the physiological functions and mechanisms of the action of this family of secreted glycoproteins in normal and disease states.

Adiponectin is a major insulin‐sensitizing, multimeric hormone derived from adipose tissue that acts on muscle and liver to regulate whole‐body glucose and lipid metabolism. Here, we describe a novel and highly conserved paralog of adiponectin designated as Clq/TNF‐related protein (CTRP) 9. Of all the CTRP paralogs, CTRP9 shows the highest degree of amino acid identity to adiponectin in its globular C1q domain. CTRP9 is expressed predominantly in adipose tissue and females expresses higher levels of the transcript than males. Moreover, its expression levels in ob/ob mice changed in an age‐dependent manner, with significant up‐regulation in younger mice. CTRP9 is a secreted glycoprotein with multiple post‐translational modifications in its collagen domain that include hydroxylated prolines and hydroxylated and glycosylated lysines. It is se‐creted as multimers (predominantly trimers) from transfected cells and circulates in the mouse serum with levels varying according to sex and metabolic state of mice. Furthermore, CTRP9 and adiponectin can be secreted as heterooligomers when cotrans‐fected into mammalian cells, and in vivo , adiponectin/ CTRP9 complexes can be reciprocally coimmuno‐precipitated from the serum of adiponectin and CTRP9 transgenic mice. Biochemical analysis demonstrates that adiponectin and CTRP9 associate via their globular C1q domain, and this interaction does not require their conserved N‐terminal cysteines or their collagen domains. Furthermore, we show that adiponectin and CTRP9 form heterotrimers. In cultured myotubes, CTRP9 specifically activates AMPK, Akt, and p44/42 MAPK signaling pathways. Adenovirus‐mediated over‐expression of CTRP9 in obese (ob/ob) mice signifi‐cantly lowered serum glucose levels. Collectively, these results suggest that CTRP9 is a novel adipokine, and further study of CTRP9 will yield novel mechanistic insights into its physiological and metabolic function.— Wong, G. W., Krawczyk, S. A., Kitidis‐Mitrokostas, C., Ge, G., Spooner, E., Hug, C., Gimeno, R., Lodish, H. F. Identification and characterization of CTRP9, a novel secreted glycoprotein from adipose tissue that reduces serum glucose in mice and forms heterotrimers with adiponectin. FASEB J. 23, 241‐258 (2009)