Rat C-reactive protein causes a charge modification of LDL and stimulates its degradation by macrophages

Relevance of lipoproteins, membranes, and extracellular vesicles in understanding C-reactive protein biochemical structure and biological activities

Early purification protocols for C-reactive protein (CRP) often involved co-isolation of lipoproteins, primarily very low-density lipoproteins (VLDLs). The interaction with lipid particles was initially attributed to CRP’s calcium-dependent binding affinity for its primary ligand—phosphocholine—the predominant hydrophilic head group expressed on phospholipids of most lipoprotein particles. Later, CRP was shown to additionally express binding affinity for apolipoprotein B (apo B), a predominant apolipoprotein of both VLDL and LDL particles. Apo B interaction with CRP was shown to be mediated by a cationic peptide sequence in apo B. Optimal apo B binding required CRP to be surface immobilized or aggregated, treatments now known to structurally change CRP from its serum soluble pentamer isoform (i.e., pCRP) into its poorly soluble, modified, monomeric isoform (i.e., mCRP). Other cationic ligands have been described for CRP which affect complement activation, histone bioactivities, and interactions with membranes. mCRP, but not pCRP, binds cholesterol and activates signaling pathways that activate pro-inflammatory bioactivities long associated with CRP as a biomarker. Hence, a key step to express CRP’s biofunctions is its conversion into its mCRP isoform. Conversion occurs when (1) pCRP binds to a membrane surface expressed ligand (often phosphocholine); (2) biochemical forces associated with binding cause relaxation/partial dissociation of secondary and tertiary structures into a swollen membrane bound intermediate (described as mCRP_m or pCRP*); (3) further structural relaxation which leads to total, irreversible dissociation of the pentamer into mCRP and expression of a cholesterol/multi-ligand binding sequence that extends into the subunit core; (4) reduction of the CRP subunit intrachain disulfide bond which enhances CRP’s binding accessibility for various ligands and activates acute phase proinflammatory responses. Taken together, the biofunctions of CRP involve both lipid and protein interactions and a conformational rearrangement of higher order structure that affects its role as a mediator of inflammatory responses.

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.

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.

Atherosclerosis-related functions of C-reactive protein

C-reactive protein (CRP) is secreted by hepatocytes as a pentameric molecule made up of identical monomers, circulates in the plasma as pentamers, and localizes in atherosclerotic lesions. In some cases, localized CRP was detected by using monoclonal antibodies that did not react with native pentameric CRP but were specific for isolated monomeric CRP. It has been reported that, once CRP is bound to certain ligands, the pentameric structure of CRP is altered so that it can dissociate into monomers. Accordingly, the monomeric CRP found in atherosclerotic lesions may be a stationary, ligand-bound, by-product of a ligand-binding function of CRP. CRP binds to modified forms of low-density lipoprotein (LDL). The binding of CRP to oxidized LDL requires acidic pH conditions; the binding at physiological pH is controversial. The binding of CRP to enzymatically-modified LDL occurs at physiological pH; however, the binding is enhanced at acidic pH. Using enzymatically-modified LDL, CRP has been shown to prevent the formation of enzymatically-modified LDL-loaded macrophage foam cells. CRP is neither pro-atherogenic nor atheroprotective in ApoE⁻(/)⁻ and ApoB¹⁰⁰(/)¹⁰⁰Ldlr ⁻(/)⁻ murine models of atherosclerosis, except in one study where CRP was found to be slightly atheroprotective in ApoB¹⁰⁰(/)¹⁰⁰Ldlr ⁻(/)⁻ mice. The reasons for the ineffectiveness of human CRP in murine models of atherosclerosis are not defined. It is possible that an inflammatory environment, such as those characterized by acidic pH, is needed for efficient interaction between CRP and atherogenic LDL during the development of atherosclerosis and to observe the possible atheroprotective function of CRP in animal models.

C-reactive protein-bound enzymatically modified low-density lipoprotein does not transform macrophages into foam cells

The formation of low-density lipoprotein (LDL) cholesterol-loaded macrophage foam cells contributes to the development of atherosclerosis. C-reactive protein (CRP) binds to atherogenic forms of LDL, but the role of CRP in foam cell formation is unclear. In this study, we first explored the binding site on CRP for enzymatically modified LDL (E-LDL), a model of atherogenic LDL to which CRP binds. As reported previously, phosphocholine (PCh) inhibited CRP-E-LDL interaction, indicating the involvement of the PCh-binding site of CRP in binding to E-LDL. However, the amino acids Phe66 and Glu81 in CRP that participate in CRP-PCh interaction were not required for CRP-E-LDL interaction. Surprisingly, blocking of the PCh-binding site with phosphoethanolamine (PEt) dramatically increased the binding of CRP to E-LDL. The PEt-mediated enhancement in the binding of CRP to E-LDL was selective for E-LDL because PEt inhibited the binding of CRP to another PCh-binding site-ligand pneumococcal C-polysaccharide. Next, we investigated foam cell formation by CRP-bound E-LDL. We found that, unlike free E-LDL, CRP-bound E-LDL was inactive because it did not transform macrophages into foam cells. The function of CRP in eliminating the activity of E-LDL to form foam cells was not impaired by the presence of PEt. Combined data lead us to two conclusions. First, PEt is a useful compound because it potentiates the binding of CRP to E-LDL and, therefore, increases the efficiency of CRP to prevent transformation of macrophages into E-LDL-loaded foam cells. Second, the function of CRP to prevent formation of foam cells may influence the process of atherogenesis.

The connection between C-reactive protein and atherosclerosis

The connection between C-reactive protein (CRP) and atherosclerosis lies on three grounds. First, the concentration of CRP in the serum, which is measured by using highly sensitive (a.k.a. 'hs') techniques, correlates with the occurrence of cardiovascular disease. Second, although CRP binds only to Fcgamma receptor-bearing cells and, in general, to apoptotic and damaged cells, almost every type of cultured mammalian cells has been shown to respond to CRP treatment. Many of these responses indicate proatherogenic functions of CRP but are being reinvestigated using CRP preparations that are free of endotoxins, sodium azide, and biologically active peptides derived from the protein itself. Third, CRP binds to modified forms of low-density lipoprotein (LDL), and, when aggregated, CRP can bind to native LDL as well. Accordingly, CRP is seen with LDL and damaged cells at the atherosclerotic lesions and myocardial infarcts. In experimental rats, human CRP was found to increase the infarct size, an effect that could be abrogated by blocking CRP-mediated complement activation. In the Apob (100/100) Ldlr (-/-) murine model of atherosclerosis, human CRP was shown to be atheroprotective, and the importance of CRP-LDL interactions in this protection was noted. Despite all this, at the end, the question whether CRP can protect humans from developing atherosclerosis remains unanswered.

Chlamydia pneumoniae infections augment atherosclerotic lesion formation: a role for serum amyloid P

Multiple reports have demonstrated an association between Chlamydia pneumoniae (Cpn) and cardiovascular disease. In this study we evaluated the effect of Cpn infections on early lesion progression in C57BL/6J mice. Since plaque formation in these mice does not develop past the initial stage, we thought these mice might be a better model for unravelling the effect of Cpn infection on early lesion type progression. C57BL/6J mice were fed an atherogenic diet and injected 10 times with 5 x 10(7) IFU Cpn or mock. At sacrifice, lesion number, size and type were analysed. To study the role of Cpn in inflammation, serum amyloid P (SAP) in plasma was determined as well as T-cells, macrophages and SAP in the lesions. In the aortic sinus of both groups, type 2 lesions were found. Cpn infection resulted in a 2.2-fold increase in total lesion size (Cpn: 10821+/-2429 microm(2)vs mock: 5022+/-1348 microm(2); p=0.04). No difference in lesion number was observed. Also, Cpn infection increased SAP in the lesions from 1.10(-4)+/-0.1.10(-4) SAP-positive cells/lesion area to 10.10(-4)+/-1.10(-4) SAP-positive cells/lesion area (p=0.05). The influx of T-lymphocytes and macrophages in the lesions as well as SAP plasma levels were not different between groups. Multiple Cpn infections resulted in a significant increase in total lesion size of C57BL/6J mice. Increase in total SAP-positive area in infected mice suggests a role for this acute-phase protein in lesion enlargement.

Effect of plasma C-reactive protein levels in modulating the risk of coronary heart disease associated with small, dense, low-density lipoproteins in men (The Quebec Cardiovascular Study)

This purpose of this study was to investigate how plasma C-reactive protein (CRP), a nonspecific acute-phase reactant, modulates the risk of coronary heart disease (CHD) associated with the small, dense, low-density lipoprotein (LDL) phenotype. LDL particle size and plasma CRP were measured in the Quebec Cardiovascular Study cohort of 2,025 men free of CHD at baseline, among whom 103 had a first CHD event during a 5-year follow-up period. Plasma CRP levels were measured using the Behring Latex-Enhanced (highly sensitive) CRP assay. LDL particle size phenotype was characterized using 2% to 16% polyacrylamide gradient gel electrophoresis. There were weak but significant associations between plasma CRP levels and features of LDL size, such as the proportion of LDL with a diameter <255 A (r = 0.09, p <0.001) and LDL peak particle size (r = -0.09, p <0.001). Variations in plasma CRP levels modulated the risk of CHD associated with small LDL peak particle size (relative risk 4.3 vs 2.5 in men with high vs low plasma CRP levels, respectively) and with an elevated proportion of LDL <255 A (relative risk 6.6 vs 3.0). Thus, increased plasma CRP levels further elevate the risk of CHD associated with having small, dense LDL particles.

Proteins of rat serum IV. Time-course of acute-phase protein expression and its modulation by indomethacine

Changes in the concentration of major serum proteins were monitored from day 0 to day 4 in three experimental groups: rats injected with turpentine, rats receiving the turpentine shot and daily doses of indomethacine, and rats given indomethacine alone. In inflamed animals, peak changes for acute-phase reactants, evaluated by two-dimensional electrophoresis (2-DE), were usually observed between 48 and 72 h after the phlogistic stimulus. By itself, indomethacine was found to affect the synthesis of most proteins (except one of the thiostatin variants and ceruloplasmin); the changes in serum levels, whether positive or negative, were the same as upon inflammation (except for kallikrein-binding protein), but their extent and/or timing usually differed. When inflamed animals were given indomethacine, a clear-cut difference in the concentration of some proteins was observed versus inflamed rats not given medication, at 24 h after the start of the treatments. Proteins mainly affected were alpha2-macroglobulin, alpha2-HS-glycoprotein, C-reactive protein and kallikrein-binding protein.

Serum amyloid P component associates with high density lipoprotein as well as very low density lipoprotein but not with low density lipoprotein

Serum amyloid P component (SAP) is a glycoprotein in human plasma. We previously showed that SAP is specifically localized in human atherosclerotic lesions, suggesting that SAP may play a role in atherogenesis. In this study, the interactions between human SAP and high density lipoprotein (HDL), low density lipoprotein (LDL) and very low density lipoprotein (VLDL) were investigated by using a solid phase plate assay. Biotinylated SAP bound to immobilized HDL and VLDL in a calcium-dependent, saturable manner. The SAP-HDL and SAP-VLDL bindings reached saturation at 4 nM and 16 nM of SAP, respectively. The bindings were inhibited by native SAP in a dose-dependent manner. No binding between SAP and LDL was found in the presence of calcium or EDTA, which indicates the specificity of SAP-lipoproteins interactions. These results suggest that the function of SAP is related to its capability to interact with lipoproteins and this may have important implications in atherosclerosis and in amyloidosis.

Structure and function of the pentraxins

Over the past two years, the three-dimensional structure of the serum amyloid P component was defined by X-ray diffraction, the first such visualization of a pentraxin. Binding sites for calcium, ligands and complement were identified. New fusion proteins with amino acid sequence homology to the pentraxins were described, and new insights were gained into pentraxin phylogeny, biosynthesis, ligands, complement activation, leukocyte reactivity and biological functions in vivo.