Role of specificity protein-1, PPARγ, and pituitary protein transcription factor-1 in transcriptional regulation of the murine CORS-26 promoter

New Insights Into Implications of CTRP3 in Obesity, Metabolic Dysfunction, and Cardiovascular Diseases: Potential of Therapeutic Interventions

Adipose tissue, as the largest endocrine organ, secretes many biologically active molecules circulating in the bloodstream, collectively termed adipocytokines, which not only regulate the metabolism but also play a role in pathophysiological processes. C1q tumor necrosis factor (TNF)-related protein 3 (CTRP3) is a member of C1q tumor necrosis factor-related proteins (CTRPs), which is a paralog of adiponectin. CTRP3 has a wide range of effects on glucose/lipid metabolism, inflammation, and contributes to cardiovascular protection. In this review, we comprehensively discussed the latest research on CTRP3 in obesity, diabetes, metabolic syndrome, and cardiovascular diseases.

C1q/TNF-related protein-3 and glucose homeostasis

Adipokines are cytokines produced by adipocytes that may mediate inflammatory processes, whilst adipocyte-derived proteins may have the converse effect. C1q/TNF-related protein-3 or CTRP3 is a novel adipokine that is expressed and released by most types of human tissues including adipose tissue. This adipokine, considered as an adiponectin, can normalize blood glucose by several mechanisms. In addition, it can modulate the expression/secretion of other cytokine and adipokines leading to lower insulin resistance in peripheral tissues. Beneficial effects of CTRP3 against hyperglycemia-induced complications in the kidney and eye have been reported. In this review, we have presented the latest findings on the in vitro and in vivo hypoglycemic effects of CTRP3, followed by the findings on the preventive/therapeutic effects of CTRP3 adipokines against diabetes related complications.

Serum levels of CTRP3 in diabetic nephropathy and its relationship with insulin resistance and kidney function

Background

C1q TNF related protein 3 (CTRP3) is an adipokine secreted from adipose tissue. Previous studies have suggested that CTRP3 improves insulin sensitivity and reduces inflammation. Human studies have evaluated circulating levels of this adipokine in patients with diabetes mellitus (DM), diabetic retinopathy, metabolic syndrome, and coronary artery diseases. However, circulating levels of this adipokine in patients with diabetic nephropathy have not been evaluated. The present study aimed to assess serum levels of CTRP3 in patients with type 2 diabetes mellitus (T2DM) and diabetic nephropathy (T2DM-NP) and its relationship with metabolic and inflammatory markers.

Methods

This cross-sectional study was performed on 55 controls, 54 patients with T2DM, and 55 patients with T2DM-NP. Serum levels of CTRP3, adiponectin, TNF-α, and IL-6 were measured by ELISA technique.

Results

Serum levels of CTRP3 were significantly lower in patients with T2DM (257.61 ± 69.79 ng/mL, p < 0.001) and T2DM-NP (222.03 ± 51.99 ng/mL, p < 0.001) compared to controls (328.17 ± 80.73 ng/mL), and those with T2DM-NP compared to T2DM group. CTRP3 was independently associated with HOMA-IR (r = -0.327, p < 0.05) and adiponectin (r = 0.436, p < 0.01) in T2DM group. In T2DM-NP patients, CTRP3 independently was associated with eGFR (r = 0.428, p < 0.01) and HOMA-IR (r = -0.436, p < 0.01). Furthermore, CTRP3 revealed a ability to differentiate T2DM-NP patients from controls (area under curve (95% confidence interval): 0.881 (0.820–0.943) and p < 0.001).

Conclusion

Decreased serum levels of CTRP3 in patients with T2DM and diabetic nephropathy and its association with pathologic mechanism in these patients suggested a possible role for CTRP3 in pathogenesis of diabetic nephropathy; nevertheless, further studies are required in this regard.

Dietary epicatechin improves survival and delays skeletal muscle degeneration in aged mice

We recently reported that epicatechin, a bioactive compound that occurs naturally in various common foods, promoted general health and survival of obese diabetic mice. It remains to be determined whether epicatechin extends health span and delays the process of aging. In the present study, epicatechin or its analogue epigallocatechin gallate (EGCG) (0.25% w/v in drinking water) was administered to 20-mo-old male C57BL mice fed a standard chow. The goal was to determine the antiaging effect. The results showed that supplementation with epicatechin for 37 wk strikingly increased the survival rate from 39 to 69%, whereas EGCG had no significant effect. Consistently, epicatechin improved physical activity, delayed degeneration of skeletal muscle (quadriceps), and shifted the profiles of the serum metabolites and skeletal muscle general mRNA expressions in aging mice toward the profiles observed in young mice. In particular, we found that dietary epicatechin significantly reversed age-altered mRNA and protein expressions of extracellular matrix and peroxisome proliferator–activated receptor pathways in skeletal muscle, and reversed the age-induced declines of the nicotinate and nicotinamide pathway both in serum and skeletal muscle. The present study provides evidence that epicatechin supplementation can exert an antiaging effect, including an increase in survival, an attenuation of the aging-related deterioration of skeletal muscles, and a protection against the aging-related decline in nicotinate and nicotinamide metabolism.—Si, H., Wang, X., Zhang, L., Parnell, L. D., Ahmed, B., LeRoith, T., Ansah, T.-A., Zhang, L., Li, J., Ordovás, J. M., Si, H., Liu, D., Lai, C.-Q. Dietary epicatechin improves survival and delays skeletal muscle degeneration in aged mice.

C1q/TNF-Related Protein 3 (CTRP3) Function and Regulation

As the largest endocrine organ, adipose tissue secretes many bioactive molecules that circulate in blood, collectively termed adipokines. Efforts to identify such metabolic regulators have led to the discovery of a family of secreted proteins, designated as C1q tumor necrosis factor (TNF)-related proteins (CTRPs). The CTRP proteins, adiponectin, TNF-alpha, as well as other proteins with the distinct C1q domain are collectively grouped together as the C1q/TNF superfamily. Reflecting profound biological potency, the initial characterization of these adipose tissue-derived CTRP factors finds wide-ranging effects upon metabolism, inflammation, and survival-signaling in multiple tissue types. CTRP3 (also known as CORS26, cartducin, or cartonectin) is a unique member of this adipokine family. In this review we provide a comprehensive overview of the research concerning the expression, regulation, and physiological function of CTRP3. © 2017 American Physiological Society. Compr Physiol 7:863-878, 2017.

Association of C1q/TNF-Related Protein-3 (CTRP3) and CTRP13 Serum Levels with Coronary Artery Disease in Subjects with and without Type 2 Diabetes Mellitus

C1q/TNF-Related Protein-3 (CTRP3) and CTRP13 are two newly discovered adipokines regulating glucose and lipid metabolism. But their role in type 2 diabetes mellitus (T2DM) and coronary artery disease (CAD) is still in infancy. The aim of this study was to investigate the associations of gene expression and serum levels of CTRP3 and CTRP13 with CAD, metabolic and inflammatory markers in patients with and without T2DM. Serum levels of CTRP3, CTRP13, adiponectin and inflammatory cytokines and their gene expression in peripheral blood mononuclear cells (PBMCs) were determined in 172 subjects categorized as group I (without T2DM and CAD), group II (with CAD but no T2DM), group III (with T2DM but no CAD) and group IV (with T2DM and CAD). Serum levels and gene expression of CTRP3, CTRP13 and adiponectin in the group I were higher compared to other groups. Inflammatory cytokines in the control group were lower than other groups too. CTRP3 serum levels have an independent association with BMI, smoking and CTRP3 gene expression; also CTRP13 serum levels has an independent association with BMI, HDL-C, insulin, HOMA-IR, HbA1c and TNF-α. Decreased serum levels of CTRP3 and CTRP13 were also associated with CAD. It appears that the decreased levels of CTRP3 and especially CTRP13 were associated with increased risk of T2DM and CAD. These findings suggest an emerging role of these adipokines in the pathogenesis of CAD, but further studies are necessary to establish this concept.

CTRP family: linking immunity to metabolism

It is well known that infectious and inflammatory diseases such as sepsis and severe inflammatory response syndrome are accompanied by metabolic alterations such as insulin resistance. Conversely, metabolic diseases such as visceral obesity and type 2 diabetes are characterized by high levels of proinflammatory cytokines. Metabolism and immunity are linked by proteins of dual function. Adiponectin, a member of the C1q/TNF-related protein (CTRP) family, has attracted much interest because of its anti-inflammatory and insulin-sensitizing effects. To date, 15 additional CTRP family members have been identified that might also play a role in metabolism and immunity. This review focuses on the biochemistry and pleiotropic physiological functions of CTRPs as new molecular mediators connecting inflammatory and metabolic diseases.

C1q/TNF-related protein-3 (CTRP-3) is secreted by visceral adipose tissue and exerts antiinflammatory and antifibrotic effects in primary human colonic fibroblasts

BACKGROUND

The adipokine CTRP-3 (C1q/TNF-related protein-3) belongs to the C1q/TNF-related protein family which antagonizes the effects of lipopolysaccharide (LPS). The aim was to investigate the antiinflammatory and antifibrotic role of CTRP-3 in Crohn's disease (CD).

METHODS

Mesenteric adipose tissue (MAT) of patients with CD or colonic cancer (CC) was resected. Human primary colonic lamina propria fibroblasts (CLPF) were isolated from controls and CD patients. Concentrations of chemokines and cytokines in the supernatants were measured by enzyme-linked immunosorbent assay (ELISA). Expression of connective tissue growth factor (CTGF), collagen I, and collagen III was analyzed by real-time polymerase chain reaction (PCR). Recombinant CTRP-3 expressed in insect cells was used for stimulation experiments.

RESULTS

CTRP-3 is synthesized and secreted by MAT resected from patients with CD, ulcerative colitis (UC), CC, and sigma diverticulitis as well as by murine and human mature adipocytes. CTRP-3 had no effect on the basal secretion of MCSF, MIF, or RANTES in MAT of CD and control patients. LPS-stimulation (10 ng/mL) significantly increased IL-8 release in CLPF of CD patients and, to a lesser extent, in cells of controls and of fibrotic CD tissue. CTRP-3 significantly and dose-dependently reduced LPS-induced IL-8 secretion in CLPF within 8 hours after LPS exposure, whereas LPS-induced IL-6 and TNF release was not affected. CTRP-3 inhibited TGF-β production and the expression of CTGF and collagen I in CLPF, whereas collagen III expression remained unchanged.

CONCLUSIONS

CTRP-3 exerts potent antiinflammatory and antifibrotic effects in CLPF by antagonizing the LPS pathway and by targeting the TGF-β-CTGF-collagen I pathway.

C1q/TNF-related protein-3 represents a novel and endogenous lipopolysaccharide antagonist of the adipose tissue

Proteins secreted by adipocytes (adipokines) play an important role in the pathophysiology of type 2 diabetes mellitus and the associated chronic and low-grade state of inflammation. It was the aim to characterize the antiinflammatory potential of the new adipocytokine, C1q/TNF-related protein-3 (CTRP-3), which shows structural homologies to the pleiotropic adipocytokine adiponectin. mRNA and protein expression of CTRP-3 was analyzed by RT-PCR and Western blot. Recombinant CTRP-3 and small interfering RNA-based strategies were used to investigate the effect of CTRP-3 on toll-like receptor (TLR) ligand, lipopolysaccharide (LPS)-, and lauric acid-induced chemokine release of monocytes and adipocytes. Together with complex ELISA-based techniques, a designed TLR4/myeloid differentiation protein-2 fusion molecule shown to bind LPS was used to prove the ability of CTRP-3 to act as endogenous LPS antagonist. CTRP-3 is synthesized in monocytes and adipocytes. The recombinant protein dose-dependently inhibits the release of chemokines in monocytes and adipocytes that were induced by lauric acid, LPS, and other TLR ligands in vitro and ex vivo. CTRP-3 inhibits monocyte chemoattractant protein-1 release in adipocytes, whereas small interfering RNA-mediated knockdown of CTRP-3 up-regulates monocyte chemoattractant protein-1 release, reduces lipid droplet size, and decreases intracellular triglyceride concentration in adipocytes, causing a dedifferentiation into a more proinflammatory and immature phenotype. By using a designed TLR4/MD-2 fusion molecule, it is shown by different techniques that CTRP-3 specifically and effectively inhibits the binding of LPS to its receptor, TLR4/MD-2. CTRP-3 inhibits three basic and common proinflammatory pathways involved in obesity and type 2 diabetes mellitus (adipo-inflammation) by acting as an endogenous LPS antagonist of the adipose tissue.

Low-abundant adiponectin receptors in visceral adipose tissue of humans and rats are further reduced in diabetic animals

BACKGROUND AND AIMS

Adipose tissue is an endocrine organ that releases various proteins that may also exert autocrine/paracrine effects. The antidiabetic adipokine adiponectin acts through two receptors, AdipoR1 and AdipoR2, but so far mainly mRNA expression has been measured in adipocytes and adipose tissues. Therefore, we aimed to analyze AdipoR1 and AdipoR2 proteins in adipocytes and paired samples of subcutaneous and visceral adipocytes/adipose tissue.

METHODS

AdipoR1 and AdipoR2 mRNA and protein expression were determined in adipocytes and paired samples of subcutaneous and visceral adipose tissue of humans and rats.

RESULTS

AdipoR1 and AdipoR2 proteins were similarly abundant in preadipocytes and mature adipocytes despite an induction of mRNA expression during differentiation. Differentiation of 3T3-L1 cells in the presence of palmitic acid did not alter adiponectin receptor proteins but metformin and fenofibrate upregulated AdipoR2 within 24 h of incubation. AdipoR2 protein was significantly lower in human visceral compared to subcutaneous fat, and both receptors were reduced in visceral adipocytes. In rat tissues both receptors were reduced in visceral fat. In diabetic animals AdipoR2 protein, but not mRNA, was lower in both fat depots compared to similarly obese rats with normal glucose disposal. AdipoR1 was only reduced in subcutaneous adipose tissue of diabetic animals where mRNA expression was induced.

CONCLUSIONS

These data indicate that mRNA expression is not suitable to predict adiponectin receptor protein. Low adiponectin receptors in visceral adipocytes and adipose tissue and further suppression in adipose tissue of insulin-resistant animals indicate disturbed adiponectin bioactivity.

Adiponectin-stimulated CXCL8 release in primary human hepatocytes is regulated by ERK1/ERK2, p38 MAPK, NF-kappaB, and STAT3 signaling pathways

Adiponectin is believed to exert hepatoprotective effects and induces CXCL8, a chemokine that functions as a survival factor, in vascular cells. In the current study, it is demonstrated that adiponectin also induces CXCL8 expression in primary human hepatocytes but not in hepatocellular carcinoma cell lines. Knock down of the adiponectin receptor (AdipoR) 1 or AdipoR2 by small-interfering RNA indicates that AdipoR1 is involved in adiponectin-stimulated CXCL8 release. Adiponectin activates nuclear factor (NF)-kappaB in primary hepatocytes and pharmacological inhibition of NF-kappaB, the p38 mitogen-activated protein kinase, and extracellular signal-regulated kinase (ERK) 1/ERK2 reduces adiponectin-mediated CXCL8 secretion. Furthermore, adiponectin also activates STAT3 involved in interleukin (IL)-6 and leptin-mediated CXCL8 induction in primary hepatocytes. Inhibition of JAK2 by AG-490 does not abolish adiponectin-stimulated CXCL8, indicating that this kinase is not involved. Pretreatment of primary cells with "STAT3 Inhibitor VI," however, elevates hepatocytic CXCL8 secretion, demonstrating that STAT3 is a negative regulator of CXCL8 in these cells. In accordance with this assumption, IL-6, a well-characterized activator of STAT3, reduces hepatocytic CXCL8. Therefore, adiponectin-stimulated induction of CXCL8 seems to be tightly controlled in primary human hepatocytes, whereas neither NF-kappaB, STAT3, nor CXCL8 are influenced in hepatocytic cell lines. CXCL8 is a survival factor, and its upregulation by adiponectin may contribute to the hepatoprotective effects of this adipokine.

Elevated adiponectin serum levels in patients with chronic alcohol abuse rapidly decline during alcohol withdrawal

BACKGROUND

Adiponectin is a circulating protein with hepatoprotective effects.

AIMS

To study the relationship of excessive alcohol consumption and serum adiponectin levels (SAL).

PATIENTS AND METHODS

The SAL were determined in (i) heavy drinkers without advanced liver damage during the course of alcohol withdrawal, (ii) patients with chronic hepatitis C virus (HCV) infection, (iii) patients with alcohol-associated cirrhosis, and (iv) healthy volunteers that consumed excessive amounts of alcohol for only a short period of time. Further, primary human hepatocytes (PHH) and adipocytes were incubated in vitro with alcohol or serum of patients.

RESULTS

Patients with chronic alcohol consumption had significantly higher SAL than HCV-patients with comparable degrees of liver damage. In alcoholics, but not in HCV patients, SAL positively correlated with serum levels of aminotransferases. Further, SAL correlated with the amount of alcohol consumption but declined during the course of alcohol abstinence. After short-term excessive alcohol consumption SAL were not elevated in healthy individuals. Adiponectin mRNA was detectable in adipocytes but not in hepatocytes, and alcohol failed to induce adiponectin in both cell types. In contrast, serum of active drinkers induced adiponectin expression in adipocytes while serum from the same individuals collected after alcohol withdrawal had no effect.

CONCLUSIONS

Alcohol exhibits a specific effect on SAL that is dose and time dependent, and correlates with the degree of hepatic damage. Alcohol does not seem to affect adiponectin expression directly in adipocytes but potentially via mediators systemically released as a result of the chronic alcohol intake.

Characterization of the transcriptional regulation of the regulator of G protein signaling 2 (RGS2) gene during 3T3-L1 preadipocyte differentiation

Adipocyte differentiation is a complex process involving several signaling pathways. Molecular mechanisms regulating the very early stage of adipocyte differentiation is not fully appreciated yet. Several inducible genes at the early stage of preadipocyte differentiation have been identified, including the regulator of G protein signaling 2 (RGS2), a member of the RGS protein superfamily. This study aimed to clarify the precise induction profile of RGS2 and to determine the essential transcription element(s) regulating RGS2 expression in differentiating 3T3-L1 preadipocytes. RGS2 mRNA expression was elevated immediately at 1 h after differentiation initiation and it remained high until the late stage of differentiation. The putative promoter sequence (approximately 3,000 bp) of the mouse RGS2 gene was isolated and the RGS2 promoter activity was significantly upregulated 3 h after inducing differentiation. The primary signaling pathway leading to RGS2 transcriptional activation appeared to be cAMP-dependent. Sequential deletion and site-directed mutagenesis strategies demonstrate that the RGS2 promoter sequence truncated down to 78 bp in size retained full inducibility by the differentiation stimuli. Mutation of a Sp1 site within the 78 bp region significantly blocked promoter activity. In addition, high expression of Sp1 transcription factor was noted prior to and paralleling the differentiation process. Taken together, our data suggest that RGS2 transcription is immediately induced via a cAMP-dependent pathway after initiation of 3T3-L1 differentiation and the RGS2 mRNA level remains consistently high throughout the differentiation progression. A Sp1 site within RGS2 promoter appeared to be a crucial response element to regulate RGS2 transcription.

Effects of the new C1q/TNF-related protein (CTRP-3) "cartonectin" on the adipocytic secretion of adipokines

BACKGROUND

Cartonectin (collagenous repeat-containing sequence of 26-kDa protein; CORS-26) was described as a new adipokine of the C1q/TNF molecular superfamily C1q/TNF-related protein-3 (CTRP-3), secreted by the adipocytes of mice and humans. The receptor and function of cartonectin are unknown and the recombinant protein is not commercially available.

OBJECTIVE

To investigate the effects of recombinant cartonectin on the secretion of adipokines such as adiponectin, leptin, and resistin from adipocytes of human and murine origin. The effect of the BMI of the adipocyte donor was also investigated.

METHODS AND PROCEDURES

Human adipocytes from pooled lean and preobese healthy individuals and murine 3T3-L1 adipocytes were used for stimulation experiments. Recombinant cartonectin was expressed in insect H5 cells. Adipokine secretion was measured using enzyme-linked immunosorbent assay. In addition, western blot analysis and luciferase reporter gene assays were employed.

RESULTS

Cartonectin (1, 10, 50, and 250 ng/ml) in higher doses stimulates the secretion of adiponectin and resistin from murine adipocytes. This effect is not caused by an induction of peroxisome proliferator-activated receptor-gamma (PPAR-gamma) protein expression, as confirmed by western blot analysis. Also, luciferase reporter gene assay revealed that cartonectin failed to induce luciferase activity at the peroxisome proliferator-activated receptor responsive element site containing the adiponectin/luciferase promoter fragment. Human adipocytes from lean individuals secrete higher amounts of adiponectin and leptin when compared with adipocytes of individuals with a preobesity BMI (25-30 kg/m(2)). Cartonectin failed to stimulate adiponectin or leptin secretion from human adipocytes, irrespective of the BMI value.

DISCUSSION

Cartonectin is a new adipokine that differentially regulates the secretion of classical adipokines, with marked differences between the human and the murine systems. These effects are species-dependent, while basal adipokine secretion is influenced by the BMI.

Adipose tissue as an immunological organ: Toll-like receptors, C1q/TNFs and CTRPs

Adipose tissue has long been regarded as a mostly resting tissue that is dedicated solely to energy storage and release. However, in recent years, this view has changed dramatically following new insights into the metabolic and immunological functions of preadipocytes and adipocytes. There are several lines of evidence for the involvement of adipose tissue in innate and acquired immune responses. First, adipocytes are potent producers of proinflammatory cytokines, such as interleukin-6 and tumor necrosis factor (TNF), and chemokines. Furthermore, adipocytes secrete high amounts of adipokines, such as leptin, adiponectin and resistin, that regulate monocyte/macrophage function, and also secrete molecules associated with the innate immune system, such as the C1qTNF-related protein superfamily. Finally, preadipocytes and adipocytes express a broad spectrum of functional Toll-like receptors and the former can convert into macrophage-like cells. Collectively, these data clearly establish the role of adipose tissue as a new member of the immune system.

Regulation and function of collagenous repeat containing sequence of 26-kDa protein gene product "cartonectin"

OBJECTIVE

Collagenous repeat containing sequence of 26-kDa protein (CORS-26) was identified as a new gene transcript expressed in cartilage with unknown function. It was the aim of this study to investigate expression, regulation, and function of CORS-26 in adipocytes.

RESEARCH METHODS AND PROCEDURES

CORS-26 mRNA and protein expression was studied by reverse transcriptase-polymerase chain reaction, Western blot analysis, and quantitative real-time polymerase chain reaction. Transcriptional regulation was studied by electrophoretic mobility shift assay and luciferase reporter gene assay. The adipocytic secretion of adiponectin and resistin was measured by enzyme-linked immunosorbent assay.

RESULTS

CORS-26 mRNA is absent in 3T3-L1 preadipocytes and adipocytes after 48 hours of differentiation. CORS-26 mRNA was induced from Day 4 to Day 9 of adipocyte differentiation. CORS-26 protein was induced in mature adipocytes. Peroxisome proliferator-activated receptor (PPAR) gamma (but not PPARalpha) in nuclear extracts prepared from adipocytes was shown to bind specifically to a putative peroxisome proliferator response element-one-half-site located at -641/-596 bp. Increasing doses of the ligands troglitazone (1, 10, 20 microM) and fenofibrate (50, 100, 200 microM) but not 15-deoxy-prostaglandin (J(2)) (0.5, 1.0, 2.5 microM) resulted in a significant reduction of both promoter activity and the amount of mRNA expression. Recombinant CORS-26 significantly stimulated the adipocytic secretion of adiponectin and resistin in a dose-dependent manner.

DISCUSSION

The mRNA and protein expression profile puts CORS-26 in the adipocytokine family. Cartonectin is negatively regulated by exogenous, but not endogenous, PPARgamma ligands. Because CORS-26 up-regulates adipokine secretion, it might be involved in metabolic and immunologic pathways.

Role of adipose tissue as an inflammatory organ in human diseases

Reviews on the inflammatory role of adipose tissue outside the field of metabolism are rare. There is increasing evidence provided by numerous basic research studies from nearly all internal medicine subspecializations that adipocytes and adipocytokines are involved in primary inflammatory processes and diseases. Therefore, it is the aim of the present review to discuss and to summarize the current knowledge on the inflammatory role of adipocytokines and special types of regional adipocytes such as retroorbital, synovial, visceral, subdermal, peritoneal, and bone marrow adipocytes in internal medicine diseases. Future clinical and therapeutic implications are discussed.