C1q/TNF-related protein 1: a novel link between visceral fat and athero-inflammation

This editorial refers to ‘C1q/TNF-related protein 1: an adipokine marking and promoting atherosclerosis’†, by L. Lu et al., on page 1762. Approximately 30% of the global population are overweight or obese, posing a major threat to healthcare in developed and developing countries. Obesity decreases life expectancy, mainly by increasing the risk of type 2 diabetes and cardiovascular disease. A mechanistic explanation for these intertwined metabolic and cardiovascular risks has been offered by adipokines, a large and diverse group of molecules secreted by adipose tissue that affect both glucose metabolism and cardiovascular function. Importantly, the perivascular depot of adipose tissue is situated around the majority of the arterial tree, and a large number of studies have shown that local adipokine expression controls vascular function and dysfunction.1 Well-known adipokines include cytokines such as interleukin-1β and tumour necrosis factor α (TNFα) (resulting in the term adipose tissue-derived cytokines), leptin, and fatty acids. Among the most intriguing adipokines are the 16-member family of C1q/TNF-related proteins (CTRPs) that includes the intensively studied adiponectin,2 also synthesized in arterial perivascular adipose tissue (PVAT).3 The CTRPs share the globular adiponectin domain in their molecular structure, which binds and activates the adiponectin receptors AdipoR1 and AdipoR2, widely expressed in the cardiovascular system.4 CTRPs circulate as monomers but also as heterotrimers, enabling diverse biological effects.2 The adipokine CTRP1 (C1q/tumour necrosis factor-related protein 1) is produced in adipose tissue (yellow area) as well by oxidized LDL (OxLDL)- or interleukin-1β (IL-1β)-stimulated macrophages and endothelial cells in the atherosclerotic plaque (purple area). In the plaque, CTRP1 activates endothelial cells and macrophages themselves to produce leucocyte adhesion molecules and tumour necrosis factor α (TNFα), which enhance leucocyte entry and inflammatory activation. An increase in CTRP1 levels in blood is associated with increased coronary artery disease risk. Plasma CTRP1 originates from visceral adipose tissue as well as from monocytes and from areas with ongoing inflammation. MAPK, mitogen-acivated protein kinase; NF-κB, nuclear factor-κB; VCAM, vascular cell adhesion molecule; M1, macrophage. Since their discovery, several CTRPs have been linked to control of glucose metabolism. Adiponectin, paradoxically decreased in obese individuals, has well-known insulin-sensitizing effects. Moreover, its globular form increases tissue perfusion5 by enhancing nitric oxide (NO) synthesis and decreasing endothelin activity in vascular endothelium.4,6 Similarly, injection of CTRP1 induced glucose uptake in mice,2 while CTRP3, CTRP9, and CTRP12/adipolin have also been shown to improve glucose metabolism. However, CTRP1 levels are increased in type 2 diabetes, contradicting a mechanistic role in insulin resistance.7 In contrast to the beneficial metabolic effects of CTRPs described above, evidence regarding the relationship between CTRPs and coronary artery disease (CAD) is conflicting. High adiponectin levels have been found to decrease risk of incident myocardial infarction,8 but have also been linked with increased cardiovascular mortality in prospective studies.9,10 Levels of CTRP3 are decreased in patients with CAD or angina pectoris.11 In a mechanistic study on human endothelial cells, CTRP3, CTRP5, and CTRP9 but not the other group members enhanced endothelial nitric oxide synthase (eNOS) activity by activating AdipoR1 and 5′AMP-activated protein kinase (AMPK), with CTRP9 showing the strongest effect.12 CTRP1 was shown to reduce damage after a myocardial infarction in mice13 and to decrease platelet aggregation in primates, suggesting a vasoprotective effect. In contrast, CTRP1 levels are increased in CAD in man, positively correlate with blood pressure, and negatively correlate with HDL levels. The differences between CTRPs were further illustrated by association of high levels of CTRP1, but not CTRP3, with low collateralization.14 In summary, different CTRPs probably have different roles in regulation of cardiovascular function, dependent on the consistency of heterotrimers. Elevated levels of CTRP1 may well contribute to increased cardiovascular risk of type 2 diabetes. In this issue, Lu and colleagues15 show the involvement of CTRP1 in atherosclerosis and CAD by three approaches. First, they demonstrate in a large cohort of human patients that circulating levels of CTRP1 protein are elevated in patients with stable CAD. This finding not only confirms data from two recent observational studies on small numbers of patients in a much larger cohort, but also extends them by showing that an increased CTRP1 level is also detected in circulating mononuclear cells of the groups of CAD patients and that in atherosclerotic plaques the amount of CTRP1 protein (corrected for cell content by β-actin) exceeds that in non-atherosclerotic, matched subjects. The elevated CTRP1 was not related to the degree of obesity (body mass index), which points to a predominant inflammatory/cardiovascular effect, which is explained by the fact that CTRP1 is secreted by vascular cells and leucocytes as well as adipose tissue. Multivariable analysis indicated CRPT1 as an independent risk factor for CAD. In a second set of experiments, the investigators show that human monocytes and endothelial cells in culture both express CTRP1 and respond to CTRP1 (Figure 1). This response involves activation of nuclear factor-κB (NF-κB) and p38MAPK (mitogen-activated protein kinase) and enhances inflammation-stimulating action of the cells. This is reflected by increased expression of leucocyte adhesion molecules and synthesis of inflammatory cytokines and chemokines. From the immunohistochemical staining of atherosclerotic arteries, it is clear that CTRP1 co-localizes with macrophages and also with part of the endothelial cells of plaque microvessels. Together with the in vitro finding, it suggests that this localization reflects local generation, independent of the circulating CTRP1, although an additional contribution of blood-borne CTRP1 cannot be fully excluded. Thus, exposure of macrophages and endothelial cells in the atherosclerotic plaque to CTRP1 contributes to the local inflammatory process by facilitating enhanced recruitment of leucocytes and increased production of inflammatory cytokines. To underpin the contribution of CTRP1 to the atherosclerotic process, Lu et al. performed a third set of experiments, in which they not only confirmed the presence of CTRP1 in atherosclerotic plaques of apoE-deficient mice, but also provided proof for a causal involvement of CTRP1 in plaque development. This was obtained by showing that an increase of CTRP1 protein or deletion of the CTRP1 gene in apoE-deficient mice promoted or reduced the progression of atherosclerosis, respectively. One the one hand, an increase of CTRP1 protein was achieved by intraperitoneal injection of recombinant CTRP1 protein and caused a 1.5-fold increase in plaque area in apoE-deficient mice. On the other hand, deletion of the CTRP1 gene caused a substantial reduction of atherosclerotic plaque area in apoE-deficient mice (as well as in fat-fed control mice) and was accompanied by a decrease in vascular cell adhesion molecule 1 (VCAM1) and other leucocyte adhesion molecules E-selectin and intercellular adhesion molecule-1 (ICAM-1), as well as in the presence of TNFα, which fits with an action on endothelial cells and macrophages. The cellular mechanism by which CTRP1 changes cellular signalling and subsequent gene transcription is less clear. It is generally thought that adiponectin and the CRTP family members act via activation of adiponectin receptors AdipoR1 and AdipoR2. Lu et al.15 indicate that the p38MAPK/NF-κB pathway is involved in the effect of CTRP1. As they also show that CTRP1 has a strong synergistic effect with TNFα, which also affects these pathways, it is difficult to discriminate between direct and TNFα-mediated indirect effects of CTRP1. Several options regarding CTRP1 signalling in atheroma can thus be considered. First, CTRP1 may act as an antagonist of adiponectin for AdipoR1. Activation of AdipoR1 stimulates the AMPK and phosphatidylinositol 3-kinase (PI3 K)/Akt pathways and subsequent production of NO, which can suppress NF-κB activation.16 Thus, competitive inhibition would result in a decrease of NO production and stimulate inflammatory activation of cells. However, data showing that deletion of adiponectin did not significantly affect the progression of atherosclerosis in mice17 make this explanation less plausible. Activation of AdipoR2 by CTRP1 may be an interesting alternative, as a recent study of Lindgren et al.18 has shown that atherosclerosis is suppressed in AdipoR2-deficient hypercholesterolaemic mice. This would imply that AdipoR2 when occupied by CTRP1 can also activate pathways other than the PI3 K/Akt pathway. This needs confirmation in future studies. A third possibility is that a still unknown receptor is present to which CTRP1 can bind. Yamauchi and Kadowaki4 already hinted at the possible existence of an AdipoRX that is involved in NF-κB activation. This intriguing possibility raises the question of which part of CTRP1 may bind to this receptor, which requires additional studies using CTRP1 variants in which specific domains are mutated or deleted. Overall, the present studies further underline the perspective of CTRP1 as an important mediator in cardiovascular and inflammatory responses, in addition to its metabolic effects. It is likely that other members of the CTRP family also participate in or contribute to the delicate regulatory balance of inflammation-mediating cytokines. From the study of Lu et al., it is clear that CTRP1 worsens atherosclerosis in mice. The human data also point to an elevation of CTRP1 in CAD. However, before this can be translated into clinical interventions, it is important to realize that inflammation is a two-edged sword in atherosclerosis. While inflammation is pivotal in plaque progression, it also has a counterbalancing effect as it also drives beneficial vascular adaptation. Nevertheless, the present study provides important evidence for a contribution of the CTRP1 member of the CTRP family to cardiovascular disease, opening up the perspective for delicately influencing the inflammatory process that accompanies atherosclerosis. In an age of population-wide weight gain, this perspective may prove particularly useful. E.C.E. is supported by a VIDI grant of the Netherlands Organisation for Scientific Research. Conflict of interest: none declared.