C-reactive protein levels are associated with the progression of atherosclerotic lesions in rabbits

C-reactive protein and cardiovascular disease: From animal studies to the clinic (Review)

C-reactive protein (CRP) and cardiovascular disease (CVD) have long been important research topics. CRP is an acute phase protein, while CVD is an inflammatory condition. The association between CRP and CVD remains controversial and has been attracting increasing attention. Traditionally, the main marker of CVD is considered to be low-density lipoprotein cholesterol. However, due to its unique characteristics, CRP may represent a novel marker or a new therapeutic target for CVD. Clinical studies have demonstrated that CRP is a predictor of CVD, but whether it is directly involved in the development and progression of CVD has yet to be fully elucidated. Recent clinical studies have demonstrated that lowering plasma CRP levels may reduce the incidence of CVD. The aim of the present review was to investigate the association between CRP and CVD, particularly atherosclerosis, from laboratory animal studies to clinical research.

Functionality of C-Reactive Protein for Atheroprotection

C-reactive protein (CRP) is a pentameric molecule made up of identical monomers. CRP can be seen in three different forms: native pentameric CRP (native CRP), non-native pentameric CRP (non-native CRP), and monomeric CRP (mCRP). Both native and non-native CRP execute ligand-recognition functions for host defense. The fate of any pentameric CRP after binding to a ligand is dissociation into ligand-bound mCRP. If ligand-bound mCRP is proinflammatory, like free mCRP has been shown to be in vitro, then mCRP along with the bound ligand must be cleared from the site of inflammation. Once pentameric CRP is bound to atherogenic low-density lipoprotein (LDL), it reduces both formation of foam cells and proinflammatory effects of atherogenic LDL. A CRP mutant, that is non-native CRP, which readily binds to atherogenic LDL, has been found to be atheroprotective in a murine model of atherosclerosis. Thus, unlike statins, a drug that can lower only cholesterol levels but not CRP levels should be developed. Since non-native CRP has been shown to bind to all kinds of malformed proteins in general, it is possible that non-native CRP would be protective against all inflammatory states in which host proteins become pathogenic. If it is proven through experimentation employing transgenic mice that non-native CRP is beneficial for the host, then using a small-molecule compound to target CRP with the goal of changing the conformation of endogenous native CRP would be preferred over using recombinant non-native CRP as a biologic to treat diseases caused by pathogenic proteins such as oxidized LDL.

Evolution of C-Reactive Protein

C-reactive protein (CRP) is an evolutionarily conserved protein. From arthropods to humans, CRP has been found in every organism where the presence of CRP has been sought. Human CRP is a pentamer made up of five identical subunits which binds to phosphocholine (PCh) in a Ca²⁺-dependent manner. In various species, we define a protein as CRP if it has any two of the following three characteristics: First, it is a cyclic oligomer of almost identical subunits of molecular weight 20–30 kDa. Second, it binds to PCh in a Ca²⁺-dependent manner. Third, it exhibits immunological cross-reactivity with human CRP. In the arthropod horseshoe crab, CRP is a constitutively expressed protein, while in humans, CRP is an acute phase plasma protein and a component of the acute phase response. As the nature of CRP gene expression evolved from a constitutively expressed protein in arthropods to an acute phase protein in humans, the definition of CRP became distinctive. In humans, CRP can be distinguished from other homologous proteins such as serum amyloid P, but this is not the case for most other vertebrates and invertebrates. Literature indicates that the binding ability of CRP to PCh is less relevant than its binding to other ligands. Human CRP displays structure-based ligand-binding specificities, but it is not known if that is true for invertebrate CRP. During evolution, changes in the intrachain disulfide and interchain disulfide bonds and changes in the glycosylation status of CRP may be responsible for different structure-function relationships of CRP in various species. More studies of invertebrate CRP are needed to understand the reasons behind such evolution of CRP. Also, CRP evolved as a component of and along with the development of the immune system. It is important to understand the biology of ancient CRP molecules because the knowledge could be useful for immunodeficient individuals.

Transcriptomic analysis of the liver of cholesterol-fed rabbits reveals altered hepatic lipid metabolism and inflammatory response

Rabbits are a suitable animal model for atherosclerosis due to their sensitivity to dietary cholesterol. Moreover, rabbits have lipoprotein profiles that are more similar to humans than those of other laboratory animals. However, little is known about the transcriptomic information related to atherosclerosis in rabbits. We aimed to determine the changes in the livers of rabbits fed a normal chow diet (control) or high cholesterol diet (HCD) by histological examinations and RNA sequencing analysis. Compared with the control group, the lipid levels and small LDL subfractions in plasma were increased, and aortic atherosclerotic plaques were formed in the HCD group. Most importantly, HCD resulted in lipid accumulation and inflammation in the livers. Transcriptomic analysis of the liver showed that HCD induces 1183 differentially expressed genes (DEGs) that mainly participate in the regulation of inflammation and lipid metabolism. Furthermore, the signaling pathways involved in inflammation and lipid metabolism were enriched by KEGG pathway analysis. In addition, hepatic DEGs of the HCD group were further validated by real-time PCR. These results suggest that HCD causes liver lipid accumulation and inflammatory response. Although the relationships between these hepatic changes and atherogenesis need further investigation, these findings provide a fundamental framework for future research on human atherosclerosis using rabbit models.

Dietary restriction slightly affects glucose homeostasis and delays plasma cholesterol removal in rabbits with dietary lipid lowering

Dietary restriction (DR) has been reported to have beneficial effects on atherosclerotic progression as well as lipid and glucose metabolism, but little is known about whether these effects can be enhanced or weakened by dietary lipid lowering. Here, after 12 weeks of high-cholesterol diet feeding, hypercholesterolemic rabbits were fed with either a standard chow diet ad libitum (AL) or a standard chow diet with DR for 16 weeks of dietary lipid lowering. We found that the DR group exhibited a loss of body weight, smaller internal organs, and reduced fat mass, while the AL group accumulated more subcutaneous fat than the baseline group. DR treatment slightly worsened glucose tolerance but enhanced insulin sensitivity, and a slight effect of DR on insulin secretion was also observed. After dietary cholesterol withdrawal, rabbits showed persistent lowering of total cholesterol and triglycerides in plasma. However, the DR group had significantly higher plasma total cholesterol than the AL group at most time points during weeks 7 to 16 of lipid lowering. Although both the AL and DR groups developed more severe atherosclerosis than the baseline group, DR did not improve atherosclerotic progression or the accumulation of macrophages and smooth muscle cells. We conclude that DR affected glucose and lipid metabolism but did not ameliorate atherosclerosis in rabbits when associated with lipid lowering by dietary cholesterol withdrawal.

Elevations in C-reactive protein and endothelin-1 system activity in humans

AIMS

C-reactive protein (CRP) is an inflammatory cytokine that has been shown to be an independent predictor of future atherothrombotic events. Hyperactivity of endothelin-1 (ET-1), a potent vasoconstrictor peptide produced by the endothelium, is linked with cardiovascular disease development and progression. ET-1 is sensitive to inflammatory stimuli, though the influence of CRP on ET-1 system activity is unknown. We tested the hypothesis that ET-1-mediated vasoconstrictor tone is enhanced in adults with elevated plasma CRP concentrations.

MATERIALS AND METHODS

Sixty non-obese adults (43-70years) were studied: 20 with hsCRP<1.0mg/L (low CRP; 0.5±0.1mg/L); 20 with hsCRP 1.0-3.0mg/L (moderate CRP; 2.0±0.1mg/L); and 20 with hsCRP>3.0mg/L (high CRP; 6.3±0.5mg/L). Forearm blood flow (FBF; plethysmography) was determined in response to intra-arterial infusions of ET-1 (5pmol/min for 20min) and selective ETA receptor blockade (BQ-123, 100nmol/min for 60min).

KEY FINDINGS

In response to ET-1, FBF decreased ~10% in the low (-10.0±2.3%), moderate (-10.7±4.0%), and high (-6.6±5.2%) CRP groups, with no significant differences between groups. Additionally, all groups demonstrated a marginal, though significant (~10%), vasodilator response to BQ-123; however, there were no differences in the FBF response to BQ-123 across CRP groups. There were no significant correlations between plasma CRP concentrations and peak FBF response to either ET-1 or BQ-123.

SIGNIFICANCE

These results indicate that ET-1 system activity is not influenced by elevations in CRP. Enhanced ET-1 system activity may not be involved in the increased cardiovascular disease risk associated with elevations in plasma CRP concentrations.

Combined use of probucol and cilostazol with atorvastatin attenuates atherosclerosis in moderately hypercholesterolemic rabbits

Background

Atherosclerotic cardiovascular disease is one of the major diseases that seriously impacts human health. Combined drug therapy may be efficacious in delaying the occurrence of cardiovascular events.

Aim

The current study was designed to investigate whether combined use of probucol (an anti-oxidant agent) with cilostazol (a platelet aggregation inhibitor) would increase the inhibitory effect of statins (a lipid-lowering agent) on atherosclerosis in moderately hypercholesterolemic rabbits.

Methods and Results

Thirty Japanese white rabbits were fed with a high cholesterol diet for 12 weeks, which was supplemented with either 0.005 % atorvastatin alone or 0.005 % atorvastatin plus 0.3 % probucol and 0.3 % cilostazol. Except for high-density lipoprotein cholesterol, no difference was found in plasma lipids among vehicle, statin, and the combined treatment group. However, atherosclerotic lesions were significantly reduced by statin treatment compared with vehicle. Moreover, we found that the anti-atherogenic effect of statin was further enhanced by the combined treatment, which was due to increased anti-inflammatory and anti-oxidant properties.

Conclusions

These data demonstrated that combined drug treatment exhibits potent athero-protective effects via pleiotropic functions, such as anti-inflammatory and anti-oxidative stress, which is independent of the lipid-lowering effect.

Recognition functions of pentameric C-reactive protein in cardiovascular disease

C-reactive protein (CRP) performs two recognition functions that are relevant to cardiovascular disease. First, in its native pentameric conformation, CRP recognizes molecules and cells with exposed phosphocholine (PCh) groups, such as microbial pathogens and damaged cells. PCh-containing ligand-bound CRP activates the complement system to destroy the ligand. Thus, the PCh-binding function of CRP is defensive if it occurs on foreign pathogens because it results in the killing of the pathogen via complement activation. On the other hand, the PCh-binding function of CRP is detrimental if it occurs on injured host cells because it causes more damage to the tissue via complement activation; this is how CRP worsens acute myocardial infarction and ischemia/reperfusion injury. Second, in its nonnative pentameric conformation, CRP also recognizes atherogenic low-density lipoprotein (LDL). Recent data suggest that the LDL-binding function of CRP is beneficial because it prevents formation of macrophage foam cells, attenuates inflammatory effects of LDL, inhibits LDL oxidation, and reduces proatherogenic effects of macrophages, raising the possibility that nonnative CRP may show atheroprotective effects in experimental animals. In conclusion, temporarily inhibiting the PCh-binding function of CRP along with facilitating localized presence of nonnative pentameric CRP could be a promising approach to treat atherosclerosis and myocardial infarction. There is no need to stop the biosynthesis of CRP.

Add-on effect of probucol in atherosclerotic, cholesterol-fed rabbits treated with atorvastatin

Objective

Lowering the blood concentration of low-density lipoprotein (LDL) cholesterol is the primary strategy employed in treating atherosclerotic disorders; however, most commonly prescribed statins prevent cardiovascular events in just 30% to 40% of treated patients. Therefore, additional treatment is required for patients in whom statins have been ineffective. In this study of atherosclerosis in rabbits, we examined the effect of probucol, a lipid-lowering drug with potent antioxidative effects, added to treatment with atorvastatin.

Methods and Results

Atherosclerosis was induced by feeding rabbits chow containing 0.5% cholesterol for 8 weeks. Probucol 0.1%, atorvastatin 0.001%, and atorvastatin 0.003% were administered solely or in combination for 6 weeks, beginning 2 weeks after the start of atherosclerosis induction. Atorvastatin decreased the plasma concentration of non-high-density lipoprotein cholesterol (non-HDLC) dose-dependently; atorvastatin 0.003% decreased the plasma concentration of non-HDLC by 25% and the area of atherosclerotic lesions by 21%. Probucol decreased the plasma concentration of non-HDLC to the same extent as atorvastatin (i.e., by 22%) and the area of atherosclerotic lesions by 41%. Probucol with 0.003% atorvastatin decreased the plasma concentration of non-HDLC by 38% and the area of atherosclerotic lesions by 61%. Co-administration of probucol with atorvastatin did not affect the antioxidative effects of probucol, which were not evident on treatment with atorvastatin alone, such as prevention of in vitro LDL-oxidation, increase in paraoxonase-1 activity of HDL, and decreases in plasma and plaque levels of oxidized-LDL in vivo.

Conclusions

Probucol has significant add-on anti-atherosclerotic effects when combined with atorvastatin treatment; suggesting that this combination might be beneficial for treatment of atherosclerosis.

C-reactive protein in human atherogenesis: facts and fiction

The role of C-reactive protein (CRP) in atherosclerosis is controversially discussed. Whereas initial experimental studies suggested a pathogenic role for CRP in atherogenesis, more recent genetic data from Mendelian randomization trials failed to provide evidence for a causative role of CRP in cardiovascular disease. Also, experimental results from laboratories all over the world were indeed contradictory, partly because of species differences in CRP biology and partly because data were not accurately evaluated. Here we summarize the published data from experimental work with mainly human material in order to avoid confusion based on species differences in CRP biology. Experimental work needs to be reevaluated after reconsideration of some traditional rules in research: (1) in order to understand a molecule's role in disease it may be helpful to be aware of its role in physiology; (2) it is necessary to define the disease entity that experimental CRP research deals with; (3) the scientific consensus is as follows: do not try to prove your hypothesis. Specific CRP inhibition followed by use of CRP inhibitors in controlled clinical trials may be the only way to prove or disprove a causative role for CRP in cardiovascular disease.