Recombinant CTRP1 facilitated the secretion of IL-6, TNF-α, IL-1β, and monocyte chemoattractant protein-1 (MCP-1) in primary human macrophages stimulated with ox-LDL (45). CTRP1 also dramatically increased the mRNA levels of IL-6, ICAM-1, and MCP-1 in human aortic smooth muscle cells (hASMCs) (46). Lu et al. found that CTRP1 activated the p38 mitogen-activated protein kinase (MAPK)/nuclear factor (NF)-kB signaling pathway to promote the expression of adhesion molecules and synthesis of inflammatory cytokines (ICAM-1, VCAM-1 and E-selectin), leading to increased interaction between human peripheral blood monocytes and human ECs. These authors further demonstrated that loss of CTRP1 in apoE^-/- mice reduced atherosclerotic lesion area, along with a significant decrease in ICAM-1, VCAM-1, and E-selectin expression and macrophage infiltration within the plaques (10).

CTRP3 expression was inhibited in ApoE^-/- mice compared to control mice. CTRP3 alleviates ox-LDL-induced inflammatory response by reducing pro-inflammatory factors CRP, TNF-α, IL-6, CD40, and CD40L in mouse aortic endothelial cells stimulated with ox-LDL. CTRP3 also inhibits ox-LDL induced endothelial inflammation by promoting phosphatidylinositol-3 kinase protein kinase B/endothelial nitric oxide synthase (PI3K/Akt/eNOS) pathway (47). Over-expression of CTRP3 elevated cell activity and decreased lactated hydrogenase release, accompanied by a marked reduction in cell apoptosis induced by ox-LDL (47). Furthermore, CTRP3 exhibited potent anti-inflammatory properties in adipocytes by inhibiting the binding of lipopolysaccharides (LPS) to toll-like receptor 4 (TLR4) (48).

Adenovirus-mediated hypothalamic CTRP4 over-expression suppressed hypothalamic inflammation induced by high-fat diet in mice, which restored the impaired leptin signaling and decreased food intake (49). Additionally, CTRP4 over-expression alleviated the inflammatory cytokine storm by demoting of TLR4 internalization, which leading to NF-κB activation (50). Therefore, we can conclude that CTRP4 over-expression acts as an anti-inflammatory factor.

CTRP5 exerts its pro-inflammatory effects by promoting the transport and oxidation of LDL by increasing 12/15-lipoxygenase (LOX) expression. CTRP5 facilitated the growth, migration, and inflammation of VSMCs through multiple pathways, leading to in-stent restenosis after coronary stent implantation (51).

CTRP6-overexpressing mice or CTRP6-treated adipocytes had reduced insulin-stimulated Akt phosphorylation and glucose uptake. In addition, CTRP6 promoted a chronic state of low-level inflammation. On the contrary, CTRP6 deficiency reduced circulating inflammatory cytokines and pro-inflammatory macrophages in adipose tissue, while enhancing the activation of insulin-stimulated Akt in adipose tissue (22). Therefore, we speculated that CTRP6 was a novel metabolic/immune regulator linking obesity with adipose tissue inflammation and insulin resistance.

CTRP9 could stabilize the mature plaques by reducing pro-inflammatory cytokines (IL-6, TNF-a, INF-γ and MCP-1) both in THP-1 macrophage foam cells (52) and the macrophages in the ApoE^-/- mice model (53). In addition, CTRP9 prevented adverse remodeling in the ischemic mouse heart by reducing MMP9 activation, which was associated with plaque vulnerability (54).

Previous study showed CTRP12 could reduce the expression of pro-inflammatory cytokines and decrease macrophage accumulation within adipose tissue in obese mice. Clinical study reported that CTRP12 inhibited the secretion of inflammatory cytokines IL-6 and TNF-α in CAD patient (28, 55). These results indicate CTRP12 has anti-inflammatory and insulin sensitizing effects in the development and deterioration of CAD.

CTRP13 inhibited the proliferation and migration of macrophages by down regulating lipid uptake, and then inhibited the plaque formation and AS development. Furthermore, CTRP13 delayed inflammatory responses during AS by promoting CD36-degradation through autophagy-lysosome pathways in macrophages, and thereby reduced the number of macrophages in lesions (56).

Hyperglycemia is a well-known risk factor of AS. Plasma CTRP1 levels are higher in T2DM than controls in male. CTRP1 plays an important role in regulating body energy homeostasis and sensitivity to insulin, loss of CTRP1 disrupts glucose and lipid homeostasis (15). CTRP1 also improves glucose metabolism and insulin resistance in obese and STZ-induced diabetic mice. CTRP1 up regulates the protein level of leptin in blood, thermogenic gene expression in brown adipose tissue, and the gene expression responsible for lipolysis and glycolysis in white adipose tissue, thus reducing food intake and enhancing energy expenditure (59). Moreover, CTRP1 knockout mice fed a high-fat diet showed reduced liver and serum triglyceride and cholesterol levels due in part to increased hepatic AMP-activated protein kinase activation and decreased expression of lipid synthesis genes (15).

Recent study showed a novel mechanistic insight into its pro-atherosclerotic action. CTRP1 attenuated miR-424-5p levels and then augmented FoxO1 expression in the nucleus, which led to the reduced expression of ATP binding cassette transporter A1 (ABCA1). The primary function of ABCA1 is to mediate cholesterol efflux to apolipoprotein A-I (apoA-I) for generation of nascent high-density lipoprotein (HDL) particles. Briefly, CTRP1 decreased ABCA1 expression and promoted lipid accumulation through the miR-424-5p/FoxO1 pathway in THP-1 macrophage-derived foam cells (60).

CTRP2, as the most similar to biological activities to those of adiponectin (61), is important in the regulation of whole body metabolism. Previous studies have revealed that mice over expressing CTRP2 exhibited improved insulin resistance and were better able to cope with acute lipid challenges than the control mice (33). On the contrary, Lei et al. found that the plasma TG and VLDL-TG in CTRP2 knockout mice were significantly elevated, and the absence of CTRP2 promoted hepatic TG secretion (16). Thus, we speculate that CTRP2 exerts its effects on the progression of AS through modulating glucose and lipid metabolism.

Peterson et al. found that administration of recombinant CTRP3 to ob/ob mice could significantly reduce blood glucose levels by activating the Akt signaling pathway and inhibiting gluconeogenic enzymes in the liver (18).

CTRP4 is a novel nutrient-responsive central regulator of food intake and energy balance (62). Serum CTRP4 levels are increased in leptin-deficient obese (ob/ob) mice. Central administration of recombinant CTRP4 inhibited food intake by inducing the activation of signal transducer and activator of transcription 3 (STAT3) signaling (63), and then altered the whole-body energy balance in both chow-fed and high-fat diet-fed mice. Serum CTRP4 concentrations decreased in patients with newly diagnosed T2DM (64), indicating CTRP4 is negatively associated with the risk of T2DM.

CTRP5 activated signal transducer and activator of transcription 6 (STAT6) signaling, which in turn up-regulated the expression of 12/15-lipoxygenase (LOX). 12/15-LOX is a key enzyme which mediates LDL trafficking and oxidation. Genetic or pharmacological inhibition of 12/15-LOX dramatically reduced the deposition of ox-LDL in the sub-endothelial space and the development of AS. In short, CTRP5 is a novel pro-atherogenic cytokine, which promotes transcytosis and oxidation of LDL in endothelial cells via up regulating 12/15-LOX (65).

In obese humans and Metabolic Syndrome (MetS) patients, circulating CTRP7 levels were significantly elevated and positively correlated with BMI, glucose, insulin, insulin resistance index, hemoglobin A1c, and triglyceride levels, which may be a novel biomarker related to metabolic diseases (23, 24). Expression of CTRP7 in liver was also significantly upregulated in obese humans, and was positively correlated with gluconeogenic genes. In mice, the expression of CTRP7 was differentially modulated in various tissues by fasting and refeeding, and by diet-induced obesity (66). Bioinformatics analysis revealed that CTRP7 was closely related to metabolism-related genes and signal pathways, further illustrating the association of CTRP7 with whole-body metabolism.

In an AMPK/mTOR signaling pathway-dependent manner, CTRP9 promotes cholesterol efflux and inhibits foam cell formation by activating autophagy in ox-LDL-induced THP-1 macrophages (67).

CTRP11 is mainly expressed in white and brown adipose, and its expression is acutely regulated by changes in metabolic state. Impaired adipogenesis was caused by a CTRP11-mediated decrease in p42/44-MAPK signaling and inhibition of mitotic clonal expansion. These results implicate that CTRP11 is a novel secreted regulator of adipogenesis (68). Interestingly, CTRP11 deficiency affects metabolic parameters in a sexually dimorphic manner. Significantly higher fasting serum ketones and reduced physical activity were only found in Ctrp11-KO female mice, which can be reversed by refeeding (69). Although it is unclear whether sex hormones directly modulate CTRP expression levels, these sexually dimorphic patterns are observed in several other CTRP family members, such as CTRP5, CRTP9 (70), CTRP11, CTRP13 (71) and adiponectin.

In apoE^-/- mice fed a Western diet, CTRP12 reduced the area of atherosclerotic lesion by increasing the plasma level of HDL-C, promoting reverse cholesterol transport (RCT) and alleviating inflammatory response (29). CTRP12 also directly activated the PI3K-Akt signaling pathway to inhibit gluconeogenesis and promote glucose uptake in the obese and diabetic mouse (72).

CTRP13 is a secreted adipokine that can ameliorate abnormal glucose and lipid metabolism (56). CTRP13 has been verified to stimulate glucose uptake in adipocytes, myotubes, and hepatocytes in vitro by activating the AMPK signaling pathway. CTRP13 diminishes lipid-induced insulin resistance in hepatocytes through inhibiting the SAPK/JNK stress signaling that damages the insulin signaling pathway. In addition, CTRP13 reduces glucose output in hepatocytes by inhibiting the mRNA expression of gluconeogenic enzymes, glucose-6-phosphatase and the cytosolic form of phosphoenolpyruvate carboxykinase. Taken together, these results indicate that CTRP13 plays an important role in glucose homeostasis (71).

Previous studies showed that upregulation of CD36 inhibited cholesterol efflux through the activation of PKCθ (73). Additionally, CTRP13 inhibited AS via autophagy- lysosome-dependent degradation of CD36, leading to the increase of cholesterol efflux in macrophage (56). Furthermore, CTRP13 hydrolyzed cholesterol droplets stored in macrophages, which attenuates cholesterol influx and promotes reverse cholesterol transport, thus inhibiting the formation of foam cells by decreasing the uptake of Ox-LDL (56) and the progression of AS (74, 75).

CTRP15 over-expression significantly decreased atherosclerotic plaque lesions through increasing reverse cholesterol transport (RCT) efficiency and circulating HDL-C levels in ApoE^-/- mice. Consistently, in vitro, over-expression of CTRP15 inhibited intracellular lipid accumulation and promoted cholesterol efflux from macrophages (76). Mechanism study verified that CTRP15 enhanced RCT efficiency and increased plasma HDL-C levels via the T-cadherin/miR-101-3p/ABCA1 pathway. Targeting CTRP15 may serve as a novel and promising therapeutic strategy for atherosclerotic diseases.

Endothelial hyper-permeability is a main determinant factor that contributes to the accelerated development of atherosclerotic lesions at hemodynamically disturbed sites. CTRP1 expression was significantly elevated in vascular endothelial cells under disturbed flow compared to steady laminar flow in mouse aorta (80). The activation of vascular endothelial growth factor receptor 2 (VEGFR2) by CTRP1 might be related to vascular hyper-permeability. CTRP1 is a mechanically sensitive pro-inflammatory factor that mediates disturbed flow-induced vascular barrier dysfunction. Inhibition of CTRP1 may inhibit the pathogenesis of AS at early stage.

Over-expressed CTRP3 caused a decrease in Angiotensin II (AngII), ICAM-1, and VCAM-1 expression, which regulated the balance between ET-1 and NO. Incremental CTRP3 increased the expression of p-PI3K, p-Akt and p-eNOS, indicating that CTRP3 facilitated the activation of PI3K/Akt/eNOS pathway (47). CTRP3 ameliorated uric acid-induced endothelial inflammation and oxidative stress, possibly by inhibiting TLR4-mediated inflammation and down-regulating oxidative stress (81). Globular form CTRP5 is a novel molecule that leads to vascular EC dysfunction through Nox1-mediated mitochondrial apoptosis in diabetes, which indicates that interventions blocking gCTRP5 may protect diabetic EC function (82). AngII has been regarded as a major contributor to the incidence of vascular endothelial dysfunction (83). Over-expression of CTRP6 improved peroxisome proliferator-activated receptor gamma (PPARγ) activation, which caused a significant decrease in AngII expression, and vascular endothelial inflammation and apoptosis (83). On the contrary, silencing CTRP6 inhibited PPARγ activation and exacerbated AngII-mediated vascular endothelial dysfunction and apoptosis.

CTRP9 exerts a significant protective role in endothelial cells. CTRP9 attenuates palmitic acid-induced endothelial cell senescence via increasing autophagy (84). Sun, H et al. found that CTRP9 treatment reversed ox-LDL-evoked decreases in antioxidant enzymes as well as eNOS. CTRP9 ameliorates ox-LDL-induced endothelial dysfunction via activation of proliferator-activated receptor γ co-activator 1α (PGC1-α)/adenosine monophosphate-activated protein kinase (AMPK)-mediated antioxidant enzyme induction (85). CTRP9 also exerts vasculoprotective effects via the adiponectin receptor 1/AMPK/eNOS dependent/NO mediated signaling pathway (86). Moreover, CTRP9 might protect endothelial oxidative damage via AdipoR1-SIRT1-PGC1-alpha signaling pathway (87) and inhibit endothelial cell senescence through the AMPKα/KLF4 signaling pathway under high glucose (88). The endothelial cells generate more ROS production under a high glucose environment, along with decreased mitochondrial biogenesis. In contrary, the treatment of CTPR9 significantly increased the activity of cytochrome c oxidase, indicating an induction of mitochondrial biogenesis (87).

Previous study showed CTRP13 supplement rescued the impaired endothelium-dependent relaxation ex vivo in the db/db mouse aortae and in high glucose-treated mouse aortae. CTRP13 preserves endothelial function in diabetic mice by increasing GTP cyclohydrolase 1 (GCH1) expression and tetrahydrobiopterin (BH4) levels to ameliorate eNOS coupling (89). More importantly, CTRP13 rescued high glucose-induced inhibition of protein kinase A (PKA) activity. GCH1 transcription was activated by the phosphorylation and recruitment of PPARα, thus improved the endothelial relaxation. Together, these results suggested that CTRP13 preserves endothelial function in diabetic mice by regulating GCH1/BH4 axis-dependent eNOS coupling.

CTRP14 is synthesized and secreted mainly by the brain and adipose tissues. The globular domain of C1ql1/Ctrp14 and C1ql4/Ctrp11 proteins directly stimulate the angiogenesis of endothelial cells activation of ERK1/2 signal pathway (90). However, Guan et al. illustrated that CTRP14 was largely dispensable for AS formation in ApoE-deficient (apoE^-/-) mice and does not improve atherosclerotic plaque formation in the aorta (91).