Functional bioactive recombinant acylation stimulating protein is distinct from C3a anaphylatoxin

The Controversial C5a Receptor C5aR2: Its Role in Health and Disease

After the discovery of the C5a receptor C5aR1, C5aR2 is the second receptor found to bind C5a and its des-arginine form. As a heptahelical G protein-coupled receptor but devoid of the intracellular Gα signal, C5aR2 is special and confusing. Ramifications and controversies about C5aR2 are under debate since its identification, from putative ligands and cellular localization to intracellular signals and pathological roles in inflammation and immunity. The ruleless and even conflicting pro- or anti-inflammatory role of C5aR2 in animal models of diverse diseases makes one bewildered. This review summarizes reports on C5aR2, tries to clear up available evidence on these four controversial aspects, and delineates C5aR2 function(s). It also summarizes available toolboxes for C5aR2 study.

Nicotinic α7 receptor inhibits the acylation stimulating protein‑induced production of monocyte chemoattractant protein‑1 and keratinocyte‑derived chemokine in adipocytes by modulating the p38 kinase and nuclear factor‑κB signaling pathways

Obesity is associated with chronic low-grade inflammation, which is characterized by increased infiltration of macrophages into adipose tissue. Acylation stimulating protein (ASP) is an adipokine derived from the immune complement system, which constitutes a link between adipocytes and macrophages, and is involved in energy homeostasis and inflammation. The purpose of the present study was to preliminarily investigate in vitro, whether functional α7nAChR in adipocytes may suppress ASP-induced inflammation and determine the possible signaling mechanism. Studies have reported associations between the expression of α7 nicotinic acetylcholine receptor (α7nAChR) and obesity, insulin resistance and diabetes. Additionally, α7nAChRs are important peripheral mediators of chronic inflammation, which is a key contributor to health problems in obesity. The primary aim of the present study was to evaluate the impact of exogenous ASP and α7nAChR on macrophage infiltration in adipose tissue and to examine the potential underlying molecular mechanism. Western blot analysis revealed that recombinant ASP increased the expression levels of monocyte chemoattractant protein-1 (MCP-1) and keratinocyte-derived chemokine (KC) by 3T3-L1 adipocytes. However, nicotine significantly inhibited the production of ASP-induced cytokines via the stimulation of α7nAChR. It was also found that α7nAChR inhibited the ASP-induced activation of p38 kinase and nuclear factor-κB (NF-κB), and the production of MCP-1 and KC. These data indicated that α7nAChR caused the inhibition of ASP-induced activation of p38 kinase and NF-κB to inhibit the production of MCP-1 and KC.

C5aR and C5L2 act in concert to balance immunometabolism in adipose tissue

Recent studies suggested that the immunometabolic receptors; C5aR and C5L2, constitutively self-associate into homo-/heterodimers and that acylation stimulating protein (ASP/C3adesArg) or C5a treatment of adipocytes increased their colocalization. The present study evaluates the C5aR contribution in adipocytes to the metabolic and immune responses elicited by ligand stimulation. The effects of C5a, ASP, and insulin on cytokine production, triglyceride synthesis (TGS), and key signaling pathways were evaluated in isolated primary adipocytes and cultured 3T3-L1 differentiated adipocytes. In addition, mRNA expression of IRS1 and PGC1α was compared in adipose tissue samples from WT vs. C5aRKO mice. Both C5a and ASP directly increased MCP-1 (238±4%; P<0.001, and 377±2% vs. basal 100%; P<0.001, respectively) and KC (413±11%; P<0.001, and 529±16%; P<0.001 vs. basal 100%, respectively) secretion, TGS (131±1%; P<0.001, and 152±6%; P<0.001, vs. basal 100% respectively), and Akt/NFκB phosphorylation pathways in adipocytes. However, in C5aRKO adipocytes, C5a effects were disrupted, while stimulatory effects of ASP were mostly maintained. Addition of C5a completely blocked ASP signaling and activity in both C5aRKO and WT adipocytes as well as 3T3-L1 adipocytes. Furthermore, C5aRKO adipocytes revealed impaired insulin stimulation of cytokine production, with partial impairment of signaling and TGS stimulation, consistent with decreased IRS1 and PGC1α mRNA expression in adipose tissue. These observations indicate the importance of C5aR in adipose tissue metabolism and immunity, which may be regulated through heterodimerization with C5L2.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.

Relationship of C5L2 receptor to skeletal muscle substrate utilization

Objective

To investigate the role of Acylation Stimulating Protein (ASP) receptor C5L2 in skeletal muscle fatty acid accumulation and metabolism as well as insulin sensitivity in both mice and human models of diet-induced insulin resistance.

Design and Methods

Male wildtype (WT) and C5L2 knockout (KO) mice were fed a low (LFD) or a high (HFD) fat diet for 10 weeks. Intramyocellular lipid (IMCL) accumulation (by oil red O staining) and beta-oxidation HADH enzyme activity were determined in skeletal muscle. Mitochondria were isolated from hindleg muscles for high-resolution respirometry. Muscle C5L2 protein content was also determined in obese type 2 diabetics and age- and BMI matched men.

Results

IMCL levels were increased by six-fold in C5L2KO-HFD compared to WT-HFD mice (p<0.05) and plasma insulin levels were markedly increased in C5L2KO-HFD mice (twofold, p<0.05). Muscle HADH activity was elevated in C5L2KO-LFD mice (+75%, p<0.001 vs. WT-LFD) and C5L2KO-HFD displayed increased mitochondrial fatty acid oxidative capacity compared to WT-HFD mice (+23%, p<0.05). In human subjects, C5L2 protein content was reduced (−48%, p<0.01) in type 2 diabetic patients when compared to obese controls. Further, exercise training increased C5L2 (+45%, p = 0.0019) and ASP (+80%, p<0.001) in obese insulin-resistant men.

Conclusion

The results suggest that insulin sensitivity may be permissive for coupling of C5L2 levels to lipid storage and utilization.

C5L2 and C5aR interaction in adipocytes and macrophages: insights into adipoimmunology

Obesity is associated with inflammation characterized by increased infiltration of macrophages into adipose tissue. C5aR-like receptor 2 (C5L2) has been identified as a receptor for acylation-stimulating protein (ASP) and the inflammatory factor C5a, which also binds C5aR. The present study examines the effects of ligands ASP and C5a on interactions between the receptors C5L2 and C5aR in 3T3-L1 adipocytes and J774 macrophages. BRET experiments indicate that C5L2 and C5aR form homo- and heterodimers in transfected HEK 293 cells, which were stable in the presence of ligand. Cell surface receptor levels of C5L2 and C5aR increased during 3T3-L1 adipocyte differentiation; both receptors are also highly expressed in J774 macrophages. Using confocal microscopy to evaluate endogenous receptors in adipocytes following stimulation with ASP or C5a, C5L2 is internalized with increasing perinuclear colocalization with C5aR. There is little C5a-dependent colocalization in macrophages. While adipocyte-conditioned medium (ACM) increased C5L2-C5aR colocalization in macrophages, this was blocked by C5a. ASP stimulation increased Akt (Ser(473)) phosphorylation in both cell types; C5a induced slight Akt phosphorylation in adipocytes with less effect in macrophages. ASP, but not C5a, increased fatty acid uptake/esterification in adipocytes. C5L2-C5aR homodimerization versus heterodimerization may thus contribute to differential responses obtained following ASP vs C5a stimulation of adipocytes and macrophages, providing new insights into the complex interaction between these two cell types within adipose tissue. Studying the mechanisms involved in the differential responses of C5L2-C5aR activation based on cell type will further our understanding of inflammatory processes in obesity.

Recombinant C3adesArg/acylation stimulating protein (ASP) is highly bioactive: a critical evaluation of C5L2 binding and 3T3-L1 adipocyte activation

C5L2 is a recently identified receptor for C5a/C5adesArg, C3a and C3adesArg (ASP). C5a/C5adesArg bind with high affinity, with no identified activation. By contrast, some studies demonstrate C3a/ASP binding/activation to C5L2; others do not. Our aim is to critically evaluate ASP/C3adesArg-C5L2 binding and bioactivity. Cell-associated fluorescent-ASP (Fl-ASP) binding to C5L2 increased from transiently transfected<stably transfected<Fl-ASP-sorted C5L2-HEK for both human C5L2 and mouse C5L2. Transfected C5L2-CHO cells had similar results. Endogenous C5L2 expression increased from 3T3-L1 preadipocytes<3T3-L1 adipocytes<primary mouse adipocytes. Non-transfected cells+/-Fl-ASP demonstrated background fluorescence only. In adherent C5L2-HEK (Fl-ASP sorted) and 3T3-L1 cells, blocking with 10% fetal calf serum, protamine sulfate or ovalbumin prevented (125)I-ASP non-specific binding (NSB, no cells), while albumin increased NSB. Binding to non-transfected HEK was comparable to NSB. Optimal specific binding was obtained at 20 degrees C (vs. 4 degrees C) in PBS or serum-free medium with K(d) 83.7+/-23.7 nM (C5L2-HEK), 66+/-15 nM (C5L2-CHO) and 76+/-14.3 nM (3T3-L1 preadipocytes); (125)I-C5a binding had greater affinity. Fl-ASP-C5L2 binding was comparable and concentration dependent (K(d) 31 nM (direct binding) and IC(50) 35 nM (competition binding) regardless of conditions). Recombinant ASP (rASP) produced in modified Escherichia coli Origami (DE3) (allowing folding and disulphide bridge formation), purified under non-denaturing conditions demonstrated 10x greater bioactivity vs. proteolytically derived plasma ASP for triglyceride synthesis and fatty acid uptake in 3T3-L1 adipocytes and preadipocytes while adipose tissue from C5L2 KO mice was non-responsive. rASP stimulation of adipocyte BODIPY-fatty acid uptake demonstrated EC(50) 115+/-93 nM and maximal stimulation of 413+/-33%, p<0.001. ASP binding has distinct characteristics that lead to C5L2 activation and increased bioactivity.

A complement-dependent balance between hepatic ischemia/reperfusion injury and liver regeneration in mice

Massive liver resection and small-for-size liver transplantation pose a therapeutic challenge, due to increased susceptibility of the remnant/graft to ischemia reperfusion injury (IRI) and impaired regeneration. We investigated the dual role of complement in IRI versus regeneration in mice. Complement component 3 (C3) deficiency and complement inhibition with complement receptor 2-complement receptor 1-related protein y (CR2-Crry, an inhibitor of C3 activation) provided protection from hepatic IRI, and while C3 deficiency also impaired liver regeneration following partial hepatectomy (PHx), the effect of CR2-Crry in this context was dose dependent. In a combined model of IRI and PHx, either C3 deficiency or high-dose CR2-Crry resulted in steatosis, severe hepatic injury, and high mortality, whereas low-dose CR2-Crry was protective and actually increased hepatic proliferative responses relative to control mice. Reconstitution experiments revealed an important role for the C3a degradation product acylation-stimulating protein (ASP) in the balance between inflammation/injury versus regeneration. Furthermore, liver regeneration was dependent on the putative ASP receptor, C5L2. Several potential mechanisms of hepatoprotection and recovery were identified in mice treated with low-dose CR2-Crry, including enhanced IL-6 expression and STAT3 activation, reduced hepatic ATP depletion, and attenuated oxidative stress. These data indicate that a threshold of complement activation, involving ASP and C5L2, promotes liver regeneration and suggest a balance between complement-dependent injury and regeneration.

C5a- and ASP-mediated C5L2 activation, endocytosis and recycling are lost in S323I-C5L2 mutation

C5L2, a G-protein-coupled receptor (GPCR), has been identified as an ASP (C3adesArg) and C5a receptor. Controversy exists regarding both ligand binding and functionality. ASP activation of C5L2 is proposed to regulate fat storage. C5L2 is also proposed as a decoy receptor for C5a, an inflammatory mediator, based on absence of Ca(2+) or chemotaxis changes.

AIMS

(i) to evaluate C5L2 receptor activation and recycling using recombinant ASP (rASP) and rC5a and (ii) assess receptor trafficking of S323I-C5L2 mutation previously identified in a family and demonstrated to have altered functionality.

RESULTS

stably transfected C5L2-HEK cells were sorted using fluorescent-ASP (Fluos-ASP) binding. Following 2-h serum-free pretreatment, C5L2 was typically localized to the cell-surface. beta-Arrestin-2-GFP transiently transfected C5L2-HEK cells demonstrated rASP and rC5a-dependent beta-arrestin-2-GFP translocation, which showed time-dependent intracellular colocalization with C5L2. Without ligand or C5L2 transfection, no translocation was identified at any time point. Ligand-dependent (rASP and rC5a) C5L2 endocytosis was time-dependent with a 1-h nadir, and was clathrin- and cholesterol-dependent. Transiently transfected Rab-GFP proteins (Rabs 5, 7 and 11) demonstrated time-dependent colocalization of Rab5, Rab7, and Rab11 with C5L2. In contrast to C5L2, a large proportion of stably transfected S323I-C5L2 did not localize to the cell-surface. While S323I-C5L2 was competent for Fluos-ASP and (125)I-ASP binding, although at a reduced level, there was no ligand-mediated receptor phosphorylation. Further, there was no ligand-mediated activation of beta-arrestin-2-GFP translocation, and no downstream functional activation of glucose transport or triglyceride synthesis.

CONCLUSION

C5L2 is a functional metabolic receptor, and serine 323 is important for ASP induced functionality.

Identification of a Novel C5L2 Variant (S323I) in a French Canadian Family With Familial Combined Hyperlipemia

Objective— A functional acylation stimulating protein (ASP) receptor, C5L2, has been recently identified in ASP-responsive cells. Impaired ASP-mediated triglyceride synthesis has previously been described in a subset of hyperapolipoprotein B/familial combined hyperlipidemia subjects.

Methods and Results— DNA sequencing of C5L2 coding region in 61 unrelated probands identified a heterozygous variant (G968→T) in 1 subject, resulting in Ser323→Ile substitution in the carboxyl terminal region. This variant was not detected in 2176 additional chromosomes by restriction fragment length polymorphism or fluorescence polarization genotyping. Eight family members of the proband were identified with one altered (+/−)C5L2 allele. Nine other family members had the wild-type (+/+)C5L2 sequence. The abnormal allele was associated with increased plasma triglyceride, plasma cholesterol, low-density lipoprotein (LDL) cholesterol, apolipoprotein B and ASP. Of 23 subjects tested in cell-based ASP bioactivity assays, those with C5L2(+/−) variant (n=2) had a 50% reduction in ASP-stimulated triglyceride synthesis, glucose transport and marked reduction in maximal binding (B max ). By contrast, a C5L2(+/+) family member responded normally, as did hyperapolipoprotein B normal ASP subjects compared with C5L2(+/+) controls (n=6).

Conclusion— The S323I variant may alter C5L2 function and might be one molecular basis contributing to familial combined hyperlipidemia.

C5L2 is a functional receptor for acylation-stimulating protein

C5L2 binds acylation-stimulating protein (ASP) with high affinity and is expressed in ASP-responsive cells. Functionality of C5L2 has not yet been demonstrated. Here we show that C5L2 is expressed in human subcutaneous and omental adipose tissue in both preadipocytes and adipocytes. In mice, C5L2 is expressed in all adipose tissues, at levels comparable with other tissues. Stable transfection of human C5L2 cDNA into HEK293 cells results in ASP stimulation of triglyceride synthesis (TGS) (193 +/- 33%, 5 microM ASP, p < 0.001, where basal = 100%) and glucose transport (168 +/- 21%, 10 microM ASP, p < 0.001). C3a similarly stimulates TGS (163 +/- 12%, p < 0.001), but C5a and C5a des-Arg have no effect. The ASP mechanism is to increase Vmax of glucose transport (149%) and triglyceride (TG) synthesis activity (165%) through increased diacylglycerolacyltransferase activity (200%). Antisense oligonucleotide down-regulation of C5L2 in human skin fibroblasts decreases cell surface C5L2 (down to 54 +/- 4% of control, p < 0.001, comparable with nonimmune background). ASP response is coordinately lost (basal TGS = 14.6 +/- 1.6, with ASP = 21.0 +/- 1.4 (144%), with ASP + oligonucleotides = 11.0 +/- 0.8 pmol of TG/mg of cell protein, p < 0.001). In mouse 3T3-L1 preadipocytes, antisense oligonucleotides decrease C5L2 expression to 69.5 +/- 0.5% of control, p < 0.001 (comparable with nonimmune) with a loss of ASP stimulation (basal TGS = 22.4 +/- 2.9, with ASP = 39.6 +/- 8.8 (177%), with ASP + oligonucleotides = 25.3 +/- 3.0 pmol of TG/mg of cell protein, p < 0.001). C5L2 down-regulation and decreased ASP response correlate (r = 0.761, p < 0.0001 for HSF and r = 0.451, p < 0.05 for 3T3-L1). In HEK-hC5L2 expressing fluorescently tagged beta-arrestin, ASP induced beta-arrestin translocation to the plasma membrane and formation of endocytic complexes concurrently with increased phosphorylation of C5L2. This is the first demonstration that C5L2 is a functional receptor, mediating ASP triglyceride stimulation.

Novel roles for acylation stimulating protein/C3adesArg: a review of recent in vitro and in vivo evidence

Recent experimental evidence is shedding more light on the physiological actions of acylation-stimulating protein (ASP)/C3adesArg. The role of ASP in regulating lipid metabolism has primarily focused on its participation in the stimulation of triglyceride synthesis (TGS) and glucose transport. Although there is no doubt that ASP, an adipocyte-produced hormone, plays a key physiological role, accumulating evidence suggests that the effects of ASP go beyond its acute effects on lipid metabolism. In this review, we present novel findings of ASP/C3adesArg effects on preadipocyte differentiation. In 3T3-L1 and 3T3-F442A cells, ASP can substitute for insulin and enhance differentiation as measured by intracellular lipid droplet accumulation, clonal expansion, and increased expression of differentiation markers. Specifically, ASP increased basal TGS by 250% after 9 days differentiation, with similar effects induced by insulin. With ASP treatment, expression of C/EBPdelta was up-regulated early in differentiation (day 2) and decreased thereafter. Expression of PPARgamma and late markers of differentiation, such as adipsin and diacylglycerol acyltransferase-1, were also increased. Effects on clonal expansion were indicated by a twofold increase in [(3)H] thymidine incorporation in 3T3-L1 cells compared to treatment with IBMX + DX alone. Further, the effects of ASP extended beyond adipose tissue to endocrine effects on hormone secretion of insulin (pancreatic cells); cytokines TNFalpha, IL-1beta, and IL-6 (myeloid cells); prolactin, growth hormone, and adrenocorticotropin (pituitary cells). Finally, the potential implication of C5L2, the newly discovered ASP receptor, and its expression profile in various tissues are discussed relative to ASP function.

Lack of complement factor C3, but not factor B, increases hyperlipidemia and atherosclerosis in apolipoprotein E-/- low-density lipoprotein receptor-/- mice

OBJECTIVE

To investigate the effect of complement deficiency on atherogenesis and lipidemia, we used mice deficient in the third complement component (C3-/-) or factor B (FB-/-).

METHODS AND RESULTS

Complement-deficient mice were crossed with mice deficient in both apolipoprotein E and the low-density lipoprotein receptor (Apoe-/- LDLR-/-). The percent lesion area in the aorta at 16 weeks, determined by en face analysis, was 84% higher in C3-/- mice than in controls (11.8%+/-0.4% versus 6.4%+/-0.8%, mean+/-SEM, P<0.00005). The C3-/- mice also had 58% higher serum triglyceride levels (P<0.05) and a more proatherogenic lipoprotein profile, with significantly more low-density lipoprotein cholesterol and very-low-density lipoprotein triglycerides than control mice. The C3-/- mice weighed 13% less (P<0.01) and had a lower body fat content (3.5%+/-1.0% versus 13.1%+/-3.0%, P<0.01). There were no differences between FB-/- mice and controls.

CONCLUSIONS

Complement activation by the classical or lectin pathway exerts atheroprotective effects, possibly through the regulation of lipid metabolism.

Acylation-stimulating protein (ASP)/complement C3adesArg deficiency results in increased energy expenditure in mice

Acylation-stimulating protein (ASP) acts as a paracrine signal to increase triglyceride synthesis in adipocytes. In mice, C3 (the precursor to ASP) knock-out (KO) results in ASP deficiency and leads to reduced body fat and leptin levels yet they are hyperphagic. In the present study, we investigated the mechanism for this energy repartitioning. Compared with wild-type (WT) mice, male and female C3(-/-) ASP-deficient mice had elevated oxygen consumption (VO2) in both the active (dark) and resting (light) phases of the diurnal cycle: +8.9% males (p < 0.05) +9.4% females (p < 0.05). Increased physical activity (movement) was observed during the dark phase in female but not in male KO animals. Female WT mice moved 16.9 +/- 2.4 m whereas KO mice moved 30.1 +/- 5.4 m, over 12 h, +78.4%, p < 0.05). In contrast, there was no difference in physical activity in male mice, but a repartitioning of dietary fat following intragastric fat administration was noted. This was reflected by increased fatty acid oxidation in liver and muscle in KO mice, with increased UCP2 (inguinal fat) and UCP3 (muscle) mRNA expression (p = 0.005 and 0.036, respectively). Fatty acid uptake into brown adipose tissue (BAT) and white adipose tissue (WAT) was reduced as reflected by a decrease in the fatty acid incorporation into lipids (BAT -68%, WAT -29%. The decrease of FA incorporation was normalized by intraperitoneal administration of ASP at the time of oral fat administration. These results suggest that ASP deficiency results in energy repartitioning through different mechanisms in male and female mice.

The chemoattractant receptor-like protein C5L2 binds the C3a des-Arg77/acylation-stimulating protein

The orphan receptor C5L2 has recently been described as a high affinity binding protein for complement fragments C5a and C3a that, unlike the previously described C5a receptor (CD88), couples only weakly to G(i)-like G proteins (Cain, S. A., and Monk, P. N. (2002) J. Biol. Chem. 277, 7165-7169). Here we demonstrate that C5L2 binds the metabolites of C4a and C3a, C4a des-Arg(77), and C3a des-Arg(77) (also known as the acylation-stimulating protein or ASP) at a site distinct from the C5a binding site. The binding of these metabolites to C5L2 does not stimulate the degranulation of transfected rat basophilic leukemia cells either through endogenous rat G proteins or when co-transfected with human G(alpha 16). C3a des-Arg(77)/ASP and C3a can potently stimulate triglyceride synthesis in human skin fibroblasts and 3T3-L1 preadipocytes. Here we show that both cell types and human adipose tissue express C5L2 mRNA and that the human fibroblasts express C5L2 protein at the cell surface. This is the first demonstration of the expression of C5L2 in cells that bind and respond to C3a des-Arg(77)/ASP and C3a. Thus C5L2, a promiscuous complement fragment-binding protein with a high affinity site that binds C3a des-Arg(77)/ASP, may mediate the acylation-stimulating properties of this peptide.

Critical review of acylation-stimulating protein physiology in humans and rodents

In the last few years, there has been increasing interest in the physiological role of acylation-stimulating protein (ASP). Recent studies in rats and mice, in particular in C3 (-/-) mice that are ASP deficient, have advanced our understanding of the role of ASP. Of note, the background strain of the mice influences the phenotype of delayed postprandial triglyceride clearance in ASP-deficient mice. Administration of ASP in all types of lean and obese mice studied to date, however, enhances postprandial triglyceride clearance. On the other hand, regardless of the background strain, ASP-deficient mice demonstrate reduced body weight, reduced leptin and reduced adipose tissue mass, suggesting that ASP deficiency results in protection against development of obesity. In humans, a number of studies have examined the relationship between ASP, obesity, diabetes and dyslipidemia as well as the influence of diet, exercise and pharmacological therapy. While many of these studies have small subject numbers, interesting observations may help us to better understand the parameters that may influence ASP production and ASP action. The aim of the present review is to provide a comprehensive overview of the recent literature on ASP, with particular emphasis on those studies carried out in rodents and humans.

Acylation-stimulating protein (ASP) deficiency induces obesity resistance and increased energy expenditure in ob/ob mice

Acylation-stimulating protein (ASP) acts as a paracrine signal to increase triglyceride synthesis in adipocytes. ASP administration results in more rapid postprandial lipid clearance. In mice, C3 (the precursor to ASP) knockout results in ASP deficiency and leads to reduced body fat and leptin levels. The protective potential of ASP deficiency against obesity and involvement of the leptin pathway were examined in ob/ob C3(-/-) double knockout mice (2KO). Compared with age-matched ob/ob mice, 2KO mice had delayed postprandial triglyceride and fatty acid clearance; associated with decreased body weight (4-17 weeks age: male: -13.7%, female: -20.6%, p < 0.0001) and HOMA (homeostasis model assessment) index (-37.7%), suggesting increased insulin sensitivity. By contrast, food intake in 2KO mice was +9.1% higher over ob/ob mice (p < 0.001, 2KO 5.1 +/- 0.2 g/day, ob/ob 4.5 +/- 0.2 g/day, wild type 2.6 +/- 0.1 g/day). The hyperphagia/leanness was balanced by a 28.5% increase in energy expenditure (oxygen consumption: 2KO, 131 +/- 8.9 ml/h; ob/ob, 102 +/- 4.5 ml/h; p < 0.01; wild type, 144 +/- 8.9 ml/h). These results suggest that the ASP regulation of energy storage may influence energy expenditure and dynamic metabolic balance.

A proteomic approach for identification of secreted proteins during the differentiation of 3T3-L1 preadipocytes to adipocytes

We have undertaken a systematic proteomic approach to purify and identify secreted factors that are differentially expressed in preadipocytes versus adipocytes. Using one-dimensional gel electrophoresis combined with nanoelectrospray tandem mass spectrometry, proteins that were specifically secreted by 3T3-L1 preadipocytes or adipocytes were identified. In addition to a number of previously reported molecules that are up- or down-regulated during this differentiation process (adipsin, adipocyte complement-related protein 30 kDa, complement C3, and fibronectin), we identified four secreted molecules that have not been shown previously to be expressed differentially during the process of adipogenesis. Pigment epithelium-derived factor, a soluble molecule with potent antiangiogenic properties, was found to be highly secreted by preadipocytes but not adipocytes. Conversely, we found hippocampal cholinergic neurostimulating peptide, neutrophil gelatinase-associated lipocalin, and haptoglobin to be expressed highly by mature adipocytes. We also used liquid chromatography-based separation followed by automated tandem mass spectrometry to identify proteins secreted by mature adipocytes. Several additional secreted proteins including resistin, secreted acidic cysteine-rich glycoprotein/osteonectin, stromal cell-derived factor-1, cystatin C, gelsolin, and matrix metalloprotease-2 were identified by this method. To our knowledge, this is the first study to identify several novel secreted proteins by adipocytes by a proteomic approach using mass spectrometry.

Acylation stimulating protein (ASP) acute effects on postprandial lipemia and food intake in rodents

BACKGROUND

In vitro studies have shown that acylation stimulating protein (ASP) stimulates triglyceride (TG) synthesis and storage in adipocytes. We have previously demonstrated that intraperitoneal (i.p.) injection of ASP in C57BL/6J mice accelerated TG clearance following an orally-administered fat load as well as reducing postprandial glucose levels.

RESULTS

In the present study, we first examined the effect of i.p. and intracerebroventricular (i.c.v.) injection of ASP on food intake in Sprague-Dawley rats. Intraperitoneal injection resulted in a short-term increase in food intake (maximum increase 29.3% within the first hour, P<0.025) decreasing thereafter as compared to vehicle alone. i.c.v. Administration of a comparable dose of ASP resulted in a similar but delayed increase in food intake with a maximum at 2-4 h, suggesting that the actions of ASP are peripherally mediated. However, there was no significant difference in 24 h food intake with either i.p. or i.c.v. injection. We also examined the effects of ASP on TG clearance in two obese mouse strains with different metabolic profiles: ob/ob (C57BL/6J-Lep(ob)) and db/db (C57BLKS/J-Lepr(db)). In a crossover design, the response to an oral fat load was determined with and without i.p. injection of exogenous ASP. In ob/ob mice, there was a 44% greater clearance of postprandial TG (area under the curve (AUC)=245+/-49 control vs 138+/-43 mg/dl h with ASP; P<0.05 by RM ANOVA). The db/db mice showed a greater response, with a 62% decrease in postprandial TG (AUC=4080+/-1489 control vs 1540+/-719 mg/dl h with ASP; P=0.004 by RM ANOVA). In addition there were decreases in postprandial glucose and non-esterified fatty acid (NEFA) levels in response to ASP.

CONCLUSION

These results are the first to report that ASP can increase food intake in rats and also enhance postprandial TG clearance in obese animals. These data therefore support previous in vitro evidence pointing to ASP as a regulator of lipid metabolism.

Acylation stimulating protein (ASP) deficiency alters postprandial and adipose tissue metabolism in male mice

Acylation stimulating protein (ASP) is a potent stimulator of triglyceride synthesis in adipocytes. In the present study, we have examined the effect of an ASP functional knockout (ASP(-/-)) on lipid metabolism in male mice. In both young (14 weeks) and older (26 weeks) mice there were marked delays in postprandial triglyceride clearance (80% increase at 14 weeks and 120% increase at 26 weeks versus wild type (+/+)). Postprandial nonesterified fatty acids were also increased in ASP(-/-) mice versus ASP(+/+) mice by 37% (low fat 10% Kcal) and by 73% (high fat 40% Kcal) diets, although there were no differences in fasting lipid levels. The ASP(-/-) mice had moderately increased energy intake (16% +/- 2% p < 0.0001) and reduced feed efficiency (33% increase in calories/g of body weight gained on low fat diet) versus wild type. The ASP(-/-) mice also had modest changes in insulin/glucose metabolism (30% to 40% decrease in insulin.glucose product), implying increased insulin sensitivity. As well, there were decreases in leptin (29% shift in leptin to body weight ratio) and up to a 26% decrease in specific adipose tissue depots versus the wild type mice on both low fat and high fat diets. These results demonstrate that ASP plays an important role in adipose tissue metabolism and fat partitioning.

Enhanced triglyceride clearance with intraperitoneal human acylation stimulating protein in C57BL/6 mice

Acylation stimulating protein (ASP), a novel adipocyte-derived autocrine protein, stimulates triglyceride synthesis and glucose transport in vitro in human and murine adipocytes. In vitro, chylomicrons increase ASP and precursor complement C3 production in adipocytes. Furthermore, in vivo, ASP production from human adipose tissue correlates positively with triglyceride clearance postprandially. The aim of the present study was to determine if intraperitoneally injected ASP accelerated triglyceride clearance in vivo after a fat load in C57Bl/6 mice. ASP increased the triglyceride clearance with a reduction of the triglyceride area under the curve over 6 h (AUC(0-6)) from 102.6 +/- 30.0 to 61.0 +/- 14.5 mg. dl(-1). h(-1) (P < 0.05), especially in the latter postprandial period (AUC(3-6); 56.2 +/- 18.0 vs. 24.9 +/- 8.9 mg. dl(-1). h(-1), P < 0.025). ASP also reduced plasma glucose both in the mice with accelerated plasma triglyceride clearance and in those with relatively delayed triglyceride clearance (P < 0.025). Therefore, ASP alters postprandial triglyceride and glucose metabolism.

A critical evaluation of the putative role of C3adesArg (ASP) in lipid metabolism and hyperapobetalipoproteinemia

The acylation stimulating protein, ASP is a small, basic serum protein capable of stimulating triglyceride synthesis in cultured fibroblasts and adipocytes. Sequence analysis of ASP has shown that ASP is identical to C3adesArg the inactive fragment of the complement anaphylatoxin peptide, C3a. It has been proposed that C3adesArg (ASP) can be generated by mature adipocytes secreting the three complement proteins: complement protein C3, factor B and factor D (adipsin). There have also been indications that adipocytes may express a specific C3adesArg (ASP)-receptor that is distinct from the recently cloned C3a-receptor. This suggests that C3adesArg (ASP) acts as an adipocyte autocrine and that it plays a central role in the metabolism of adipose tissue. Based on these observations a hypothesis for the etiology of hyperapobetalipoproteinemia (hyperapoB) has been proposed. Hyperapobetalipoproteinemia (hyperapoB), is a familial lipoprotein disorder characterized by increased hepatic secretion of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) particles. If C3adesArg (ASP) function in the adipose tissue is impaired, a reduced rate of triglyceride synthesis will follow, generating an increased flux of fatty acids to the liver. In response to an increased flow of fatty acids, the liver will increase its production of VLDL particles yielding the phenotype of hyperapoB. This review critically assesses this hypothesis and the potential role of C3adesArg (ASP) as a major determinant for triglyceride synthesis in the light of data collected in vitro and in vivo.

Plasma acylation stimulating protein, adipsin and lipids in non-obese and obese populations

BACKGROUND

Acylation stimulating protein (ASP) is a potent stimulator of TG synthesis in human adipocytes.

DESIGN

In the present study, we have analysed plasma ASP and adipsin levels and their relationships to plasma lipids in non-obese and obese groups.

RESULTS

The results show that the frequency distribution of ASP is skewed but that of adipsin is normal in both groups. In the non-obese population, the mean levels of plasma ASP and adipsin were 20.2 nmol L-1 (median) and 66.6 +/- 19 nmol L-1 (mean) respectively. No difference was observed between men and women for each of the parameters. In the obese population, the median plasma ASP was increased by 246% (69.9 nmol L-1) and adipsin by 31% (87.0 +/- 22.7 nmol L-1) above that of the control group. Although the levels for men and women were not statistically different for adipsin, the median ASP plasma concentration was 1.9-fold higher in obese women than in obese men (71.8 nmol L-1 vs. 37.6 nmol L-1, P < 0.05). Best subset regression analysis provided a model with variables that best predict plasma ASP [r2 = 0.160, P < 0.008 for body mass index (BMI), P < 0.05 for triacylglycerol (TG), P < 0.03 for free fatty acid (FFA)] and plasma adipsin (r2 = 0.057, P < 0.017 for BMI) in a non-obese population. In obese subjects, the model was different for plasma ASP (P = NS for any of the variables) and plasma adipsin (r2 = 0.356, P < 0.008 for FFA, P < 0.0002 for BMI, P < 0.02 for age). There was no correlation between ASP and adipsin in either the non-obese or the obese group.

CONCLUSION

The present data suggest involvement of the ASP/adipsin pathway in the pathogenesis of obesity.

Genetic deficiency of acylation stimulating protein (ASP(C3ades-Arg)) does not cause hyperapobetalipoproteinemia in mice

The acylation stimulating protein (ASP) is a 76-amino acid peptide that has been proposed as a potent mediator of triglyceride synthesis and, when functionally impaired, as a major cause of hyperapobetalipoproteinemia (HyperapoB). Purification and sequence analysis of ASP from human sera have revealed that ASP is identical to the complement C3-derived activation peptide C3ades-Arg. Because C3 is the precursor for C3ades-Arg and therefore ASP, a deficiency in C3 would be predicted to result in a phenotype characteristic of HyperapoB. To test this hypothesis in vivo, the current study was undertaken in which ASP(C3ades-Arg)-deficient mice were used as a model system. No significant differences were found in the triglyceride, cholesterol, or free fatty acid concentrations in the plasma of fasted normal and ASP(C3ades-Arg)-deficient animals. In addition, plasma lipoprotein analyses indicated that the very low density lipoprotein, low density lipoprotein, and high density lipoprotein cholesterol and triglyceride concentrations as well as the apolipoprotein B-48 and B-100 levels were not significantly different in the plasma of ASP(C3ades-Arg)-deficient and wild type mice. Furthermore, when challenged with an oral fat load, the ASP(C3ades-Arg)-deficient mice showed no impaired ability to clear triglycerides and free fatty acids from their circulation when compared with their wild-type littermates. Collectively, these results indicate that ASP(C3ades-Arg) deficiency does not cause HyperapoB in mice and that the physiological importance of impaired ASP(C3ades-Arg) function as a cause of hyperapobetalipoproteinemia needs to be reevaluated.

Regional specificity of ASP binding in human adipose tissue

Obesity, in particular omental (OM) adiposity, is associated with diabetes and cardiovascular disease. Thus site-specific regulation of fat storage is important to understand. Acylation-stimulating protein (ASP) is a potent stimulator of glucose transport and triglyceride synthesis in adipocytes. In the present study, we characterized receptor binding of 125I-labeled ASP to human adipocyte plasma membranes from paired OM and subcutaneous (SC) sites in normal (N) and obese (O) male (M) and female (F) subjects (n = 24). Overall, specific binding of 125I-ASP was in the order of SC > OM and O > N (in SC tissue, particularly in F). Receptor affinity of 125I-ASP was higher [lower dissociation constant (Kd)] in SC than in OM (63.6 +/- 16.2 vs. 160.7 +/- 38.6 nM, P < 0.02), especially in F (37.0 +/- 11.1 F-N and 26.3 +/- 6.7 nM F-O) and lower (higher Kd) in male OM (291.8 +/- 116.8 M-N and 149.4 +/- 56.4 M-O). The greater binding and higher affinity of 125I-ASP binding to SC suggests that ASP may be an important factor in maintaining regional adipose tissue mass. Conversely, lower binding and receptor affinity in male OM adipose tissue may contribute to the fatty acid imbalance and metabolic complications associated with this syndrome, by reducing the efficiency of adipose fatty acid trapping by the ASP pathway.