Alterations in the structure of apolipoprotein B-100 determine the behaviour of LDL towards thromboplastin

Aggregation and fusion of low-density lipoproteins in vivo and in vitro

Low-density lipoproteins (LDLs, also known as 'bad cholesterol') are the major carriers of circulating cholesterol and the main causative risk factor of atherosclerosis. Plasma LDLs are 20- to 25-nm nanoparticles containing a core of cholesterol esters surrounded by a phospholipid monolayer and a single copy of apolipoprotein B (550 kDa). An early sign of atherosclerosis is the accumulation of LDL-derived lipid droplets in the arterial wall. According to the widely accepted 'response-to-retention hypothesis', LDL binding to the extracellular matrix proteoglycans in the arterial intima induces hydrolytic and oxidative modifications that promote LDL aggregation and fusion. This enhances LDL uptake by the arterial macrophages and triggers a cascade of pathogenic responses that culminate in the development of atherosclerotic lesions. Hence, LDL aggregation, fusion, and lipid droplet formation are important early steps in atherogenesis. In vitro, a variety of enzymatic and nonenzymatic modifications of LDL can induce these reactions and thereby provide useful models for their detailed analysis. Here, we summarize current knowledge of the in vivo and in vitro modifications of LDLs leading to their aggregation, fusion, and lipid droplet formation; outline the techniques used to study these reactions; and propose a molecular mechanism that underlies these pro-atherogenic processes. Such knowledge is essential in identifying endogenous and exogenous factors that can promote or prevent LDL aggregation and fusion in vivo and to help establish new potential therapeutic targets to decelerate or even block these pathogenic reactions.

Generation in human plasma of misfolded, aggregation-prone electronegative low density lipoprotein

Human plasma contains small amounts of a low density lipoprotein in which apoprotein is misfolded. Originally identified and isolated by means of anion-exchange chromatography, this component was subsequently described as electronegative low density lipoprotein (LDL)(-), with increased concentrations associated with elevated cardiovascular disease risk. It has been recognized recently as the trigger of LDL amyloidogenesis, which produces aggregates similar to subendothelial droplets observed in vivo in early atherogenesis. Although LDL(-) has been produced in vitro through various manipulations, the mechanisms involved in its generation in vivo remain obscure. By using a more physiological model, we demonstrate spontaneous, sustained and noticeable production of LDL(-) during incubation of unprocessed human plasma at 37 degrees C. In addition to a higher fraction of amyloidogenic LDL(-), LDL purified from incubated plasma contains an increased level of lysophospholipids and free fatty acids; analysis of LDL lipids packing shows their loosening. As a result, during plasma incubation, lipid destabilization and protein misfolding take place, and aggregation-prone particles are generated. All these phenomena can be prevented by inhibiting calcium-dependent secretory phospholipases A2. Our plasma incubation model, without removal of reaction products, effectively shows a lipid-protein interplay in LDL, where lipid destabilization after lipolysis threatens the apoprotein's structure, which misfolds and becomes aggregation-prone.

Tissue factor upregulation drives a thrombosis-inflammation circuit in relation to cardiovascular complications

The extrinsic coagulation is recognized as an 'inducible' signalling cascade resulting from tissue factor (TF) upregulation by exposure to clotting zymogen FVII upon inflammation or tissue injury. Following the substantial initiation, an array of proteolytic activation generates mediating signals (active serine proteases: FVIIa, FXa and FIIa) that lead to hypercoagulation with fibrin overproduction manifesting thrombosis. In addition, TF upregulation plays a central role in driving a thrombosis-inflammation circuit. Coagulant mediators (FVIIa, FXa and FIIa) and endproduct (fibrin) are proinflammatory, eliciting tissue necrosis factor, interleukins, adhesion molecules and many other intracellular signals in different cell types. Such resulting inflammation could ensure 'fibrin' thrombosis via feedback upregulation of TF. Alternatively, the resulting inflammation triggers platelet/leukocyte/polymononuclear cell activation thus contributing to 'cellular' thrombosis. TF is very vulnerable to upregulation resulting in hypercoagulability and subsequent thrombosis and inflammation, either of which presents cardiovascular risks. The prevention and intervention of TF hypercoagulability are of importance in cardioprotection. Blockade of inflammation reception and its intracellular signalling prevents TF expression from upregulation. Natural (activated protein C, tissue factor pathway inhibitor, or antithrombin III) or pharmacological anticoagulants readily offset the extrinsic hypercoagulation mainly through FVIIa, FXa or FIIa inhibition. Therefore, anticoagulants turn off the thrombosis-inflammation circuit, offering not only antithrombotic but anti-inflammatory significance in the prevention of cardiovascular complications.

Novel anticoagulant activity of polyamino acid offsets bacterial endotoxin-induced extrinsic hypercoagulation: downregulation of monocytic tissue factor-dependent FVII activation

The extrinsic hypercoagulation often resulting from sepsis could contribute to disseminated intravascular coagulation and cardiovascular complications. The effective prevention and intervention remained largely complex and unclear. In a cell model of human leukemia THP-1 monocytes following bacterial endotoxin (LPS) exposure, we show the novel anticoagulant ability of polyamino acid (polyAA) to suppress the extrinsic hypercoagulation. LPS-induced monocytic tissue factor (mTF) procoagulation was readily offset by poly-L-lysine (PLK), poly-L-arginine (PLR), or poly-L-ornithine (POR) included in single-stage clotting assays. IC50 was estimated at 0.35, 0.30, or 0.58 microM for PLR, POR, or PLK, respectively, whereas, poly-L-asparatic acid (PLD) remained ineffective. In a separate approach, inclusion of cationic polyAA in human plasma significantly prolonged prothrombin time, confirming the depressed extrinsic coagulation. In chromogenic assays dissecting the extrinsic pathway, we further determined the inhibitory site(s). PLK, PLR, or POR significantly inhibited LPS-induced FVII activation, which was consistent with the diminished FVIIa formation shown on Western blotting analysis. In contrast, polyAA did not show any additional effect on either FVIIa/FXa amidolytic activities or mTF/FVIIa-catalyzed FX activation. Nor did polyAA show any effect on FVII activation directly catalyzed by FXa. Taken together, PLK, PLR, or POR preferentially inhibited mTF-dependent FVII activation, accounting for their novel anticoagulant activities. PolyAA might present the specific antagonists to arrest the extrinsic hypercoagulation following inflammation.

One site on the apoB-100 specifically binds 17-beta-estradiol and regulates the overall structure of LDL

The major protein component (apoB-100) of low-density lipoprotein (LDL) is known as a multipotential molecule the several functional regions of which can all be affected by key structural modifications driven by specific domains. Based on our previous report on structural and conformational modifications of apoB-100 in the presence of 17-beta-estradiol (E2), we characterized the interaction between E2 and the apoB-100 and further explored the induced alterations in terms of the structural arrangement of the whole LDL particle. We report evidence for the existence on apoB-100 of a single specific and saturable binding site for E2, the occupancy of which modifies the overall structure of the protein, inducing an increase in the alpha-helix fraction. As a consequence, the structure of the LDL particle is deeply perturbed, with a change in the arrangement of both the outer shell and lipid core and an overall volume shrinkage. The evidence of a regulation of apoB-100 structure by a physiological ligand opens new perspectives in the study of the biological addressing of the LDL particle and suggests a novel rationale in the search for mechanisms underlying the beneficial role of E2 in decreasing the risk of early lesions in atherosclerosis.

Possible role of Marcks in the cellular modulation of monocytic tissue factor-initiated hypercoagulation

The enhanced extrinsic tissue factor (TF)-initiated coagulation, often resulting from sepsis, could lead to disseminated intravascular coagulation presenting cardiovascular complications. Using model human leukaemia THP-1 monocytes, we studied monocytic TF (mTF) hypercoagulation and its regulation. After an 8 h exposure to bacterial endotoxin [lipopolysaccharide (LPS); 100 ng/ml], mTF activity was significantly upregulated as the result of the enhanced mTF synthesis. Thereafter, LPS induction declined, exhibiting a "quiescent-desensitizing' phenomenon. Such diminished LPS induction was,however,associated with sustained LPS-enhanced mTF synthesis, revealing the possible occurrence of a post-translational downregulation. It was noted that LPS desensitization was accompanied by the increased expression of myristoylated alanine-rich C kinase substrate (Marcks). In contrast, A23187 (20 micromol/l) or Quin-2AM (20 micromol/l) drastically activated mTF activity without detectable effect on mTF synthesis; both of which showed that sustained functional upregulation during 24 h culture did not enhance Marcks expression. These inverse correlations between mTF activity upregulation and Marcks expression suggested that Marcks could be inhibitory. Marcks phosphorylation site domain (151-175) (Marcks PSD) readily inhibited mTF-dependent FVII activation and diminished FVIIa formation in LPS-challenged cells. As a result, Marcks PSD offset LPS-induced mTF hypercoagulation upon inclusion in the single-stage clotting assays. The anticoagulant activity was confirmed by showing that Marcks PSD significantly blocked rabbit brain thromboplastin (rbTF) procoagulation and inhibited rbTF-dependent FVII activation as well as FVIIa formation. Our study suggests that Marcks expression plays a role in a novel cellular modulation to downregulate mTF hypercoagulation.

The role of the C-terminal domain in the inhibitory functions of tissue factor pathway inhibitor

Tissue factor pathway inhibitor (TFPI) inhibits the activity of coagulation factors VIIa and Xa through Kunitz domains, thereby inhibiting the activity of tissue factor. However, it has been shown that the C-terminal of this inhibitor is essential for the maximal anticoagulant activity of TFPI. We have investigated the endogenous ability of the C-terminal of TFPI to influence coagulation. A synthetic peptide corresponding to residues 254-265 within the C-terminal of TFPI was prepared and shown to be capable of inhibiting tissue factor pathway by preventing the activation of factor VII. Mutational analysis of the peptide revealed the identity of the key lysine residues.

Comparison of the inhibitory effects of ApoB100 and tissue factor pathway inhibitor on tissue factor and the influence of lipoprotein oxidation

The procoagulant activity of tissue factor is regulated by circulating inhibitors such as tissue factor pathway inhibitor (TFPI) and LDL. These 2 inhibitors also readily associate making the distinction between their activities difficult. We have examined the relative contributions of intact and C-terminal truncated TFPI and ApoB100. By following the inhibitory potential of the preparations, over a period of 120 minutes, it was demonstrated that TFPI and LDL-resembling particles inhibited tissue factor at different rates. TFPI was found to be a short, fast-acting inhibitor, whereas the action of LDL-resembling particles was more prolonged but slower. The oxidation of LDL has been closely associated with the development of cardiovascular disease, including atherosclerosis and thrombosis. Positively charged amino acids, particularly lysine residues, are prone to alterations via the formation of adducts by lipid peroxidation products. These residues are important in the inhibition of tissue factor activity by ApoB100. They also play an important role in the inhibitory Kunitz domains of TFPI. We have shown that the decline in the ability of LDL to inhibit tissue factor was as a result of modifications in LDL arising from oxidation. By examining the effects of oxidation on full-length and C-terminal truncated TFPI bound to LDL-resembling particles, we found that TFPI is only affected when in close association with ApoB100. C-terminal truncated TFPI was not affected significantly by oxidation. Finally, chemical modification of lysine and arginine residues reduced the overall inhibition of tissue factor by TFPI. We propose that TFPI and LDL act separately to inhibit tissue factor in vivo. However, the oxidation of LDL can alter both the endogenous activity of ApoB100 and reduce that of closely associated TFPI, compromising normal hemostasis.

Identification of a domain in apolipoprotein B-100 that inhibits the procoagulant activity of tissue factor

The ability of low-density lipoprotein (LDL) to inhibit the procoagulant activity of tissue factor is mediated by a direct protein-protein interaction involving apolipoprotein (apo) B-100. A lysine-rich sequence within apo B-100 (residues 3121-3217), which we have termed lysine-rich apo B-100-derived (KRAD)-98 peptide, may be responsible for its activity. Within this region, residues 3147-3160 (KRAD-14) contain an exceptionally high proportion of positive amino acids. Both recombinant KRAD-98 and KRAD-14 peptides inhibited the procoagulant activity of tissue factor by preventing the activation of factor VII. KRAD-14 also inhibited the prothrombinase components, factors Xa and V. In comparison with the parent protein (apo B-100), KRAD-14 peptide displayed a 20-fold enhancement in the rate of inhibition, whereas KRAD-98 peptide exhibited a rate closer to that of apo B-100. Mutational analysis of KRAD-14 peptide revealed three adjacent amino acids, alteration of which greatly reduced the inhibitory potential of this peptide. A peptide derived from tissue factor (residues 58-66) was found to act co-operatively with tissue factor itself, but also augmented the inhibition of tissue-factor activity by apo B-100. In conclusion, LDL may be a physiological regulator of haemostatic mechanisms through the interactions of lysine-rich domains of apo B-100 with tissue factor.