Apolipoprotein composition and particle size affect HDL degradation by chymase: effect on cellular cholesterol efflux

Mast cell stabilizers: from pathogenic roles to targeting therapies

Mast cells (MCs) are bone-marrow-derived haematopoietic cells that are widely distributed in human tissues. When activated, they will release tryptase, histamine and other mediators that play major roles in a diverse array of diseases/disorders, including allergies, inflammation, cardiovascular diseases, autoimmune diseases, cancers and even death. The multiple pathological effects of MCs have made their stabilizers a research hotspot for the treatment of related diseases. To date, the clinically available MC stabilizers are limited. Considering the rapidly increasing incidence rate and widespread prevalence of MC-related diseases, a comprehensive reference is needed for the clinicians or researchers to identify and choose efficacious MC stabilizers. This review analyzes the mechanism of MC activation, and summarizes the progress made so far in the development of MC stabilizers. MC stabilizers are classified by the action mechanism here, including acting on cell surface receptors, disturbing signal transduction pathways and interfering exocytosis systems. Particular emphasis is placed on the clinical applications and the future development direction of MC stabilizers.

Characterization of novel truncated apolipoprotein A-I in human high-density lipoprotein generated by sequential treatment with myeloperoxidase and chymase

High-density lipoprotein (HDL) cholesterol is a well-known biomarker, which has been associated with reduction in the risk of cardiovascular diseases (CVD). However, some HDL anti-atherosclerotic functions may be impaired without altered HDL-cholesterol (HDL-C) level via its dysfunctional proteins or other physiological reactions in vivo. We previously showed that activated mast cell-derived chymase could modestly cleave apolipoprotein A-I (apoA-I) in HDL3, and further easily cleave lipid-free apoA-I. In contrast, myeloperoxidase (MPO) secreted by macrophages, the main cell type in atherosclerotic plaques, could oxidize HDL proteins, which might modify their tertiary structures, increasing their susceptibility to other enzymes. Here we focused on the co-modification and impact of chymase and MPO, usually secreted during inflammation from cells with possible co-existence in atheromas, on HDL. Only after sequential treatment with MPO and then chymase, two novel truncated apoA-I fragments were generated from HDL. One fragment was 16.5 kDa, and the cleavage site by chymase after MPO modification was the C-terminal of Tyr100 in apoA-I, cross-validated by three different mass spectrometry methods. This novel apoA-I fragment can be trapped in HDL particles to avoid kidney glomerular filtration and has a specific site for antibody generation for ELISA tests. As such, its quantification can be useful in predicting patients with CVD having normal HDL-C levels.

Reconstituted high density lipoprotein (rHDL), a versatile drug delivery nanoplatform for tumor targeted therapy

rHDL is a synthesized drug delivery nanoplatform exhibiting excellent biocompatibility, which possesses most of the advantages of HDL. rHDL shows almost no toxicity and can be degraded to non-toxic substances in vivo. The severe limitation of the application of various antitumor agents is mainly due to their low bioavailability, high toxicity, poor stability, etc. Favorably, antitumor drug-loaded rHDL nanoparticles (NPs), which are known as an important drug delivery system (DDS), help to change the situation a lot. This DDS shows an outstanding active-targeting ability towards tumor cells and improves the therapeutic effect during antitumor treatment while overcoming the shortcomings mentioned above. In the following text, we will mainly focus on the various applications of rHDL in tumor targeted therapy by describing the properties, preparation, receptor active-targeting ability and antitumor effects of antineoplastic drug-loaded rHDL NPs.

Lipoproteins and lipids in cardiovascular disease: from mechanistic insights to therapeutic targeting

With cardiovascular disease being the leading cause of morbidity and mortality worldwide, effective and cost-efficient therapies to reduce cardiovascular risk are highly needed. Lipids and lipoprotein particles crucially contribute to atherosclerosis as underlying pathology of cardiovascular disease and influence inflammatory processes as well as function of leukocytes, vascular and cardiac cells, thereby impacting on vessels and heart. Statins form the first-line therapy with the aim to block cholesterol synthesis, but additional lipid-lowering drugs are sometimes needed to achieve low-density lipoprotein (LDL) cholesterol target values. Furthermore, beyond LDL cholesterol, also other lipid mediators contribute to cardiovascular risk. This review comprehensively discusses low- and high-density lipoprotein cholesterol, lipoprotein (a), triglycerides as well as fatty acids and derivatives in the context of cardiovascular disease, providing mechanistic insights into their role in pathological processes impacting on cardiovascular disease. Also, an overview of applied as well as emerging therapeutic strategies to reduce lipid-induced cardiovascular burden is provided.

Serum levels of 9α,11β-PGF2 and apolipoprotein A1 achieve high predictive power as biomarkers of anaphylaxis

Anaphylaxis is a life-threatening hypersensitivity reaction. To identify biomarkers for the condition, we assessed serum levels of apolipoprotein (Apo)A and ApoE. We found a reduction of both lipoproteins in anaphylactic mice as well as in orally challenged food allergic patients. We then compared patients after acute anaphylaxis with several control groups (nonallergic, history of allergen-triggered anaphylaxis, acute cardiovascular/febrile reactions). In this unpaired setting, ApoE levels were unaltered, while ApoA1 was reduced in the anaphylactic group. Although unable to discriminate between anaphylaxis and cardiovascular/febrile reactions, ROC curve analysis revealed a reasonably high area under the curve (AUC) of 0.91 for ApoA1. Serum 9α,11ß-PGF₂ , recently identified as a suitable biomarker for anaphylaxis, outperformed ApoA1 with AUC=0.95. Intriguingly however its power further increased upon combination of both mediators reaching AUC=1_. Our data suggest that ApoA1 combined with 9α,11ß-PGF₂ represents a useful composite biomarker of anaphylaxis, achieving superior diagnostic power over either factor alone.

Development of a new bioactivatable fluorescent probe for quantification of apolipoprotein A-I proteolytic degradation in vitro and in vivo

BACKGROUND AND AIMS

The potential benefits of high-density lipoproteins (HDL) against atherosclerosis are attributed to its major protein component, apolipoprotein A-I (apoA-I). Most of the apoA-I in the vascular wall appears to be in its lipid-poor form. The latter, however, is subjected to degradation by proteases localized in atherosclerotic plaques, which, in turn, has been shown to negatively impact its atheroprotective functions. Here, we report the development and in vivo use of a bioactivatable near-infrared full-length apoA-I-Cy5.5 fluorescent probe for the assessment of apoA-I-degrading proteolytic activities.

METHODS

Fluorescence quenching was obtained by saturation of Cy5.5 fluorophore molecules on apoA-I protein. ApoA-I cleavage led to near-infrared fluorescence enhancement. In vitro proteolysis of the apoA-I probe by a variety of proteases including serine, cysteine, and metalloproteases resulted in an up to 11-fold increase in fluorescence (n = 5, p ≤ 0.05).

RESULTS

We detected activation of the probe in atherosclerotic mice aorta sections using in situ zymography and showed that broad-spectrum protease inhibitors protected the probe from degradation, resulting in decreased fluorescence (-54%, n = 6 per group, p ≤ 0.0001). In vivo, the injected probe showed stronger fluorescence emission in the aorta of human apoB transgenic Ldlr^-/- atherosclerotic mice (ATX) as compared to wild-type mice. In vivo observations were confirmed by ex vivo aorta imaging quantification where a 10-fold increase in fluorescent signal in ATX mice (p ≤ 0.05 vs. control mice) was observed.

CONCLUSIONS

The use of this probe in different applications may help to assess new molecular mechanisms of atherosclerosis and may improve current HDL-based therapies by enhancing apoA-I functionality.

Network-Based Analysis on Orthogonal Separation of Human Plasma Uncovers Distinct High Density Lipoprotein Complexes

High density lipoprotein (HDL) particles are blood-borne complexes whose plasma levels have been associated with protection from cardiovascular disease (CVD). Recent studies have demonstrated the existence of distinct HDL subspecies; however, these have been difficult to isolate and characterize biochemically. Here, we present the first report that employs a network-based approach to systematically infer HDL subspecies. Healthy human plasma was separated into 58 fractions using our previously published three orthogonal chromatography techniques. Similar local migration patterns among HDL proteins were captured with a novel similarity score, and individual comigration networks were constructed for each fraction. By employing a graph mining algorithm, we identified 183 overlapped cliques, among which 38 were further selected as candidate HDL subparticles. Each of these 38 subparticles had at least two literature supports. In addition, GO function enrichment analysis showed that they were enriched with fundamental biological and CVD protective functions. Furthermore, gene knockout experiments in mouse model supported the validity of these subparticles related to three apolipoproteins. Finally, analysis of an apoA-I deficient human patient's plasma provided additional support for apoA-I related complexes. Further biochemical characterization of these putative subspecies may facilitate the mechanistic research of CVD and guide targeted therapeutics aimed at its mitigation.

Cholesterol efflux and reverse cholesterol transport

Both alterations of lipid/lipoprotein metabolism and inflammatory events contribute to the formation of the atherosclerotic plaque, characterized by the accumulation of abnormal amounts of cholesterol and macrophages in the artery wall. Reverse cholesterol transport (RCT) may counteract the pathogenic events leading to the formation and development of atheroma, by promoting the high-density lipoprotein (HDL)-mediated removal of cholesterol from the artery wall. Recent in vivo studies established the inverse relationship between RCT efficiency and atherosclerotic cardiovascular diseases (CVD), thus suggesting that the promotion of this process may represent a novel strategy to reduce atherosclerotic plaque burden and subsequent cardiovascular events. HDL plays a primary role in all stages of RCT: (1) cholesterol efflux, where these lipoproteins remove excess cholesterol from cells; (2) lipoprotein remodeling, where HDL undergo structural modifications with possible impact on their function; and (3) hepatic lipid uptake, where HDL releases cholesterol to the liver, for the final excretion into bile and feces. Although the inverse association between HDL plasma levels and CVD risk has been postulated for years, recently this concept has been challenged by studies reporting that HDL antiatherogenic functions may be independent of their plasma levels. Therefore, assessment of HDL function, evaluated as the capacity to promote cell cholesterol efflux may offer a better prediction of CVD than HDL levels alone. Consistent with this idea, it has been recently demonstrated that the evaluation of serum cholesterol efflux capacity (CEC) is a predictor of atherosclerosis extent in humans.

Structure of HDL: particle subclasses and molecular components

A molecular understanding of high-density lipoprotein (HDL) will allow a more complete grasp of its interactions with key plasma remodelling factors and with cell-surface proteins that mediate HDL assembly and clearance. However, these particles are notoriously heterogeneous in terms of almost every physical, chemical and biological property. Furthermore, HDL particles have not lent themselves to high-resolution structural study through mainstream techniques like nuclear magnetic resonance and X-ray crystallography; investigators have therefore had to use a series of lower resolution methods to derive a general structural understanding of these enigmatic particles. This chapter reviews current knowledge of the composition, structure and heterogeneity of human plasma HDL. The multifaceted composition of the HDL proteome, the multiple major protein isoforms involving translational and posttranslational modifications, the rapidly expanding knowledge of the HDL lipidome, the highly complex world of HDL subclasses and putative models of HDL particle structure are extensively discussed. A brief history of structural studies of both plasma-derived and recombinant forms of HDL is presented with a focus on detailed structural models that have been derived from a range of techniques spanning mass spectrometry to molecular dynamics.

Mast cell function: a new vision of an old cell

Since first described by Paul Ehrlich in 1878, mast cells have been mostly viewed as effectors of allergy. It has been only in the past two decades that mast cells have gained recognition for their involvement in other physiological and pathological processes. Mast cells have a widespread distribution and are found predominantly at the interface between the host and the external environment. Mast cell maturation, phenotype and function are a direct consequence of the local microenvironment and have a marked influence on their ability to specifically recognize and respond to various stimuli through the release of an array of biologically active mediators. These features enable mast cells to act as both first responders in harmful situations as well as to respond to changes in their environment by communicating with a variety of other cells implicated in physiological and immunological responses. Therefore, the critical role of mast cells in both innate and adaptive immunity, including immune tolerance, has gained increased prominence. Conversely, mast cell dysfunction has pointed to these cells as the main offenders in several chronic allergic/inflammatory disorders, cancer and autoimmune diseases. This review summarizes the current knowledge of mast cell function in both normal and pathological conditions with regards to their regulation, phenotype and role.

Microenvironmentally controlled secondary structure motifs of apolipoprotein A-I derived peptides

The structure of apolipoprotein A-I (apoA-I), the major protein of HDL, has been extensively studied in past years. Nevertheless, its corresponding three-dimensional structure has been difficult to obtain due to the frequent conformational changes observed depending on the microenvironment. Although the function of each helical segment of this protein remains unclear, it has been observed that the apoA-I amino (N) and carboxy-end (C) domains are directly involved in receptor-recognition, processes that determine the diameter for HDL particles. In addition, it has been observed that the high structural plasticity of these segments might be related to several amyloidogenic processes. In this work, we studied a series of peptides derived from the N- and C-terminal domains representing the most hydrophobic segments of apoA-I. Measurements carried out using circular dichroism in all tested peptides evidenced that the lipid environment promotes the formation of α-helical structures, whereas an aqueous environment facilitates a strong tendency to adopt β-sheet/disordered conformations. Electron microscopy observations showed the formation of amyloid-like structures similar to those found in other well-defined amyloidogenic proteins. Interestingly, when the apoA-I peptides were incubated under conditions that promote stable globular structures, two of the peptides studied were cytotoxic to microglia and mouse macrophage cells. Our findings provide an insight into the physicochemical properties of key segments contained in apoA-I which may be implicated in disorder-to-order transitions that in turn maintain the delicate equilibrium between both, native and abnormal conformations, and therefore control its propensity to become involved in pathological processes.

Apolipoprotein A-II is a key regulatory factor of HDL metabolism as appears from studies with transgenic animals and clinical outcomes

The structure and metabolism of HDL are linked to their major apolipoproteins (apo) A-I and A-II. HDL metabolism is very dynamic and depends on the constant remodeling by lipases, lipid transfer proteins and receptors. HDL exert several cardioprotective effects, through their antioxidant and antiinflammatory capacities and through the stimulation of reverse cholesterol transport from extrahepatic tissues to the liver for excretion into bile. HDL also serve as plasma reservoir for C and E apolipoproteins, as transport vehicles for a great variety of proteins, and may have more physiological functions than previously recognized. In this review we will develop several aspects of HDL metabolism with emphasis on the structure/function of apo A-I and apo A-II. An important contribution to our understanding of the respective roles of apo A-I and apo A-II comes from studies using transgenic animal models that highlighted the stabilizatory role of apo A-II on HDL through inhibition of their remodeling by lipases. Clinical studies coupled with proteomic analyses revealed the presence of dysfunctional HDL in patients with cardiovascular disease. Beyond HDL cholesterol, a new notion is the functionality of HDL particles. In spite of abundant literature on HDL metabolic properties, a major question remains unanswered: which HDL particle(s) confer(s) protection against cardiovascular risk?

Pharmaceutical stabilization of mast cells attenuates experimental atherogenesis in low-density lipoprotein receptor-deficient mice

Mast cells (MCs) contribute to atherogenesis by releasing pro-inflammatory mediators to activate vascular cells and other inflammatory cells. This study examined whether MC activation or stabilization affects diet-induced atherosclerosis in low-density lipoprotein receptor-deficient (Ldlr(-/-)) mice. When Ldlr(-/-) mice consumed an atherogenic diet for 3 or 6 months, MC activation with compound 48/80 (C48/80) increased aortic arch intima and total lesion areas, and plasma total cholesterol, LDL, and triglyceride levels, whereas MC stabilization with cromolyn reduced these parameters. There were significant differences in arch intima and total lesion areas, and plasma total cholesterol, LDL, and triglyceride levels between C48/80-treated and cromolyn-treated mice. To examine a therapeutic application of cromolyn in atherosclerosis, we fed Ldlr(-/-) mice an atherogenic diet for 3 months followed by giving mice cromolyn for additional 3 months. Cromolyn did not affect aortic arch intima area, but significantly reduced lipid deposition in the thoracic-abdominal aortas. In aortic arches, however, cromolyn treatment significantly reduced lesion contents of Mac-3(+) macrophages, CD4(+) T cells, activated MCs, and lesion cell proliferation. While plasma total cholesterol and LDL levels increased and high-density lipoprotein (HDL) levels decreased from 3 months to 6 months of an atherogenic diet, cromolyn treatment decreased significantly plasma total cholesterol, LDL, and triglyceride levels and increased HDL levels above those of 3-month time point. These observations demonstrate that MC stabilization reduces lesion inflammation, ameliorates plasma lipid profiles, and may serve as a potential therapy for this cardiovascular disease.

Pegylation of high-density lipoprotein decreases plasma clearance and enhances antiatherogenic activity

RATIONALE

Infusions of apolipoprotein AI (apoAI), mimetic peptides, or high-density lipoprotein (HDL) remain a promising approach for the treatment of atherosclerotic coronary disease. However, rapid clearance leads to a requirement for repeated administration of large amounts of material and limits effective plasma concentrations.

OBJECTIVE

Because pegylation of purified proteins is commonly used as a method to increase their half-life in the circulation, we determined whether pegylation of apoAI or HDL would increase its plasma half-life and in turn its antiatherogenic potential.

METHODS AND RESULTS

Initial pegylation attempts using lipid-poor apoAI showed a marked tendency to form multi-pegylated (PEG) species with reduced ability to promote cholesterol efflux from macrophage foam cells. However, pegylation of human holo-HDL or reconstituted phospholipid/apoAI particles (rHDL) led to selective N-terminal monopegylation of apoAI with full preservation of cholesterol efflux activity. The plasma clearance of PEG-rHDL was estimated after injection into hypercholesterolemic Apoe-/- mice; the half-life of pegylated PEG-apoAI after injection of PEG-rHDL was increased ≈7-fold compared with apoAI in nonpegylated rHDL. In comparison with nonpegylated rHDL, infusion of PEG-rHDL (40 mg/kg) into hypercholesterolemic Apoe-/- mice led to more pronounced suppression of bone marrow myeloid progenitor cell proliferation and monocytosis, as well as reduced atherosclerosis and a stable plaque phenotype.

CONCLUSIONS

We describe a novel method for effective monopegylation of apoAI in HDL particles, in which lipid binding seems to protect against pegylation of key functional residues. Pegylation of apoAI in rHDL markedly increases its plasma half-life and enhances antiatherogenic properties in vivo.

Identification of sites in apolipoprotein A-I susceptible to chymase and carboxypeptidase A digestion

MCs (mast cells) adversely affect atherosclerosis by promoting the progression of lesions and plaque destabilization. MC chymase cleaves apoA-I (apolipoprotein A-I), the main protein component of HDL (high-density lipoprotein). We previously showed that C-terminally truncated apoA-I (cleaved at the carboxyl side of Phe²²⁵) is present in normal human serum using a newly developed specific mAb (monoclonal antibody). In the present study, we aimed to identify chymase-induced cleavage sites in both lipid-free and lipid-bound (HDL₃) forms of apoA-I. Lipid-free apoA-I was preferentially digested by chymase, at the C-terminus rather than the N-terminus. Phe²²⁹ and Tyr¹⁹² residues were the main cleavage sites. Interestingly, the Phe²²⁵ residue was a minor cleavage site. In contrast, the same concentration of chymase failed to digest apoA-I in HDL₃; however, a 100-fold higher concentration of chymase modestly digested apoA-I in HDL₃ at only the N-terminus, especially at Phe³³. CPA (carboxypeptidase A) is another MC protease, co-localized with chymase in severe atherosclerotic lesions. CPA, in vitro, further cleaved C-terminal Phe²²⁵ and Phe²²⁹ residues newly exposed by chymase, but did not cleave Tyr¹⁹². These results indicate that several forms of C-terminally and N-terminally truncated apoA-I could exist in the circulation. They may be useful as new biomarkers to assess the risk of CVD (cardiovascular disease).

Mast cells and metabolic syndrome

Mast cells are critical effectors in the development of allergic diseases and in many immunoglobulin E-mediated immune responses. These cells exert their physiological and pathological activities by releasing granules containing histamine, cytokines, chemokines, and proteases, including mast cell-specific chymase and tryptase. Like macrophages and T lymphocytes, mast cells are inflammatory cells, and they participate in the pathogenesis of inflammatory diseases such as cardiovascular complications and metabolic disorders. Recent observations suggested that mast cells are involved in insulin resistance and type 2 diabetes. Data from animal models proved the direct participation of mast cells in diet-induced obesity and diabetes. Although the mechanisms by which mast cells participate in these metabolic diseases are not fully understood, established mast cell pathobiology in cardiovascular diseases and effective mast cell inhibitor medications used in pre-formed obesity and diabetes in experimental models offer hope to patients with these common chronic inflammatory diseases. This article is part of a Special Issue entitled: Mast cells in inflammation.

Extracellular modifications of HDL in vivo and the emerging concept of proteolytic inactivation of preβ-HDL

PURPOSE OF REVIEW

Both quantity and quality of the circulating HDL particle matter for the optimal antiatherogenic potential of HDL. This review summarizes various mechanisms capable of inducing extracellular modifications of HDL and reducing the function of HDL subclasses as cholesterol acceptors. Special emphasis is laid on the proteolytic inactivation of lipid-poor preβ-migrating HDL (preβ-HDL).

RECENT FINDINGS

HDL particles can undergo functional inactivation in vivo. During atherogenesis, different cell types in the arterial intima release enzymes into the intimal fluid, potentially capable of causing structural and chemical modifications of the various components present in the lipid core or in the polar surface of the HDL particles. Enzymatic oxidation, lipolysis and proteolysis, and nonenzymatic glycosylation are among the HDL modifications that adversely affect HDL functionality. Proteolysis of preβ-HDL by various proteases present in the arterial intima has emerged as a potential mechanism that impairs the efficiency of HDL to promote cholesterol efflux from macrophage foam cells, the mast cell-derived neutral protease chymase being a prime example of such impairment. A paradigm of proteolytic inactivation of preβ-HDL in vivo is emerging.

SUMMARY

Several extracellular enzymes present in the arterial intima may compromise various cardioprotective functions of HDL. Observations on proteolysis of specific lipid-poor HDL subpopulations in vivo constitute the basis for future studies evaluating the actual impact of proteolytic microenvironments on the initiation and progression of atherosclerotic lesions.

Detection of chymase-digested C-terminally truncated apolipoprotein A-I in normal human serum

In atherosclerotic artery walls, mast cells, an inflammatory cell, are activated and secrete some proteases including chymase. Chymase, a chymotrypsin-like protease, cleaves the C-terminus of apolipoprotein A-I (apoA-I) at Phe225. This cleavage reduces the ability of apoA-I to promote the efflux of cellular cholesterol. The aim of this study is to detect C-terminally truncated apoA-I in normal human serum. For this purpose, we generated a monoclonal antibody that specifically recognizes C-terminally truncated apoA-I by immunizing mice with a peptide that corresponds to human apoA-I amino acid residues 216-225. The monoclonal antibody, termed 16-4 mAb, selectively reacted with recombinant C-terminally truncated apoA-I, but not recombinant full-length apoA-I. A two-dimensional electrophoresis analysis also indicated that only two out of six spots that contained apoA-I fragments and had a molecular mass of 26 kDa after chymase digestion reacted with the 16-4 mAb. We detected an extremely small amount of C-terminally truncated apoA-I in normal human serum by concentrating the serum through affinity chromatography using a 16-4 mAb-conjugated resin, and then performing Western blot analysis. The 16-4 mAb could be useful to examine whether C-terminally truncated apoA-I is associated with the progression of atherosclerosis.

In vitro effects of Echis carinatus venom on the human plasma proteome

Echis carinatus venom (EV) is a complex mixture of toxins that contribute to its lethality. EV proteolytic activity was analyzed by zymography, chromogenic assays, and SDS-PAGE. To understand the molecular mechanism of the envenomation, we investigated the in vitro effect of EV on human plasma proteins. We looked for EV protein substrates and their proteolytic fragments. We analyzed EV proteolytic activity on standard proteins such as prothrombin or fibrinogen. To set up the optimal EV:plasma protein ratio conditions, plasma was incubated with EV (treated plasma), depleted of abundant proteins, and subjected to SDS-PAGE. Samples from control and treated plasma were also analyzed by 2-DE/MALDI-TOF MS, leading to the identification of four classes of plasma proteins cleaved by EV: proteases, protease inhibitors, binding proteins, and transporters. EV mainly proteolyzes entire proteins but can also act on physiological fragments. In summary, the physiological effects of EV proteases involve other important processes in addition to blood coagulation; complement activation and hemoglobin metabolism are also affected. In particular, the cleavage of protease inhibitors appears to be the mechanism through which the venom neutralizes the body's defenses.

Mast cells promote atherosclerosis by inducing both an atherogenic lipid profile and vascular inflammation

Accumulating in vitro and in vivo studies have proposed a role for mast cells in the pathogenesis of atherosclerosis. Here, we studied the role of mast cells in lipoprotein metabolism, a key element in the atherosclerotic disease. Male mice deficient in low-density lipoprotein receptors and mast cells on a Western diet for 26 weeks had significantly less atherosclerotic changes both in aortic sinus (55%, P = 0.0009) and in aorta (31%, P = 0.049), as compared to mast cell-competent littermates. Mast cell-deficient female mice had significantly less atherosclerotic changes in aortic sinus (43%, P = 0.011). Furthermore, we found a significant positive correlation between the extent of atherosclerosis and the number of adventitial/perivascular mast cells in aortic sinus of mast cell-competent mice (r = 0.615, P = 0.015). Serum cholesterol and triglyceride levels were significantly lower in both male (63%, P = 0.0005 and 57%, P = 0.004) and female (73%, P = 0.00009 and 54%, P = 0.007) mast cell-deficient mice, with a concomitant decrease in atherogenic apoB-containing particles and serum prebeta-high-density lipoprotein and phospholipid transfer protein activity in both male (69% and 24%) and female (74% and 54%) mast cell-deficient mice. Serum soluble intercellular adhesion molecule was decreased in both male (32%, P = 0.004) and female (28%, P = 0.003) mast cell-deficient mice, whereas serum amyloid A was similar between mast cell-deficient and competent mice. In conclusion, mast cells participate in the pathogenesis of atherosclerosis in ldlr(-/-) mice by inducing both an atherogenic lipid profile and vascular inflammation.

Mast cell-dependent proteolytic modification of HDL particles during anaphylactic shock in the mouse reduces their ability to induce cholesterol efflux from macrophage foam cells ex vivo

OBJECTIVE

We have found previously that proteolytic modification of HDL by mast cell chymase in vitro reduces cholesterol efflux from cultured macrophage foam cells. Here, we evaluated whether mast cell-dependent proteolysis of HDL particles may occur in vivo, and whether such modification would impair their function in inducing cellular cholesterol efflux ex vivo.

METHODS

Systemic activation of mast cells in the mouse was achieved by intraperitoneal injection of a high dose of the mast cell-specific noncytotoxic degranulating agent, compound 48/80. Serum and intraperitoneal fluid were then evaluated for degradation of HDL apolipoproteins and for their potential to act as cholesterol acceptors from cultured mouse macrophage foam cells.

RESULTS

Lysates of isolated mouse peritoneal mast cells containing active chymase partially proteolyzed apoA-I in alpha- and prebeta-HDL particles in mouse serum in vitro, and, when injected into the mouse peritoneal cavity, the lysates also degraded endogenous apoA-I in peritoneal fluid in vivo. Systemic activation of mast cells in mast cell-competent mice, but not in mast cell-deficient (W-sash c-kit mutant) mice, reduced the ability of serum and intraperitoneal fluid derived from these animals to promote efflux of cellular cholesterol. This inhibitory effect was related to mast cell-dependent proteolytic degradation of apoA-I, apoA-IV, and apoE, i.e., the HDL-associated apolipoproteins that are efficient inducers of cholesterol efflux.

CONCLUSION

The present results document a role for extracellular mast cell-dependent proteolysis in the generation of dysfunctional HDL, and suggest an inhibitory role for mast cells in the initial step of reverse cholesterol transport in vivo.

Integrated analytical approach in veal calves administered the anabolic androgenic steroids boldenone and boldione: urine and plasma kinetic profile and changes in plasma protein expression

Surveillance of illegal use of steroids hormones in cattle breeding is a key issue to preserve human health. To this purpose, an integrated approach has been developed for the analysis of plasma and urine from calves treated orally with a single dose of a combination of the androgenic steroids boldenone and boldione. A quantitative estimation of steroid hormones was obtained by LC-APCI-Q-MS/MS analysis of plasma and urine samples obtained at various times up to 36 and 24 h after treatment, respectively. These experiments demonstrated that boldione was never found, while boldenone alpha- and beta-epimers were detected in plasma and urine only within 2 and 24 h after drug administration, respectively. Parallel proteomic analysis of plasma samples was obtained by combined 2-DE, MALDI-TOF-MS and muLC-ESI-IT-MS/MS procedures. A specific protein, poorly represented in normal plasma samples collected before treatment, was found upregulated even 36 h after hormone treatment. Extensive mass mapping experiments proved this component as an N-terminal truncated form of apolipoprotein A1 (ApoA1), a protein involved in cholesterol transport. The expression profile of ApoA1 analysed by Western blot analysis confirmed a significant and time dependent increase of this ApoA1 fragment. Then, provided that further experiments performed with a growth-promoting schedule will confirm these preliminary findings, truncated ApoA1 may be proposed as a candidate biomarker for steroid boldenone and possibly other anabolic androgens misuse in cattle veal calves, when no traces of hormones are detectable in plasma or urine.

ApoA-I cleaved by transthyretin has reduced ability to promote cholesterol efflux and increased amyloidogenicity

A fraction of plasma transthyretin (TTR) circulates in HDL through binding to apolipoprotein A-I (apoA-I). Moreover, TTR is able to cleave the C terminus of lipid-free apoA-I. In this study, we addressed the relevance of apoA-I cleavage by TTR in lipoprotein metabolism and in the formation of apoA-I amyloid fibrils. We determined that TTR may also cleave lipidated apoA-I, with cleavage being more effective in the lipid-poor prebeta-HDL subpopulation. Upon TTR cleavage, discoidal HDL particles displayed a reduced capacity to promote cholesterol efflux from cholesterol-loaded THP-1 macrophages. In similar assays, TTR-containing HDL from mice expressing human TTR in a TTR knockout background had a decreased ability to perform reverse cholesterol transport compared with similar particles from TTR knockout mice, reinforcing the notion that cleavage by TTR reduces the ability of apoA-I to promote cholesterol efflux. As amyloid deposits composed of N-terminal apoA-I fragments are common in the atherosclerotic intima, we assessed the impact of TTR cleavage on apoA-I aggregation and fibrillar growth. We determined that TTR-cleaved apoA-I has a high propensity to form aggregated particles and that it formed fibrils faster than full-length apoA-I, as assessed by electron microscopy. Our results show that apoA-I cleavage by TTR may affect HDL biology and the development of atherosclerosis by reducing cholesterol efflux and increasing the apoA-I amyloidogenic potential.

Human umbilical cord blood-derived mast cells: a unique model for the study of neuro-immuno-endocrine interactions

Findings obtained using animal models have often failed to reflect the processes involved in human disease. Moreover, human cultured cells do not necessarily function as their actual tissue counterparts. Therefore, there is great demand for sources of human progenitor cells that may be directed to acquire specific tissue characteristics and be available in sufficient quantities to carry out functional and pharmacological studies. Acase in point is the mast cell, well known for its involvement in allergic reactions, but also implicated in inflammatory diseases. Mast cells can be activated by allergens, anaphylatoxins, immunoglobulin-free light chains, superantigens, neuropeptides, and cytokines, leading to selective release of mediators. These could be involved in many inflammatory diseases, such as asthma and atopic dermatitis, which worsen by stress, through activation by local release of corticotropin-releasing hormone or related peptides. Umbilical cord blood and cord matrix-derived mast cell progenitors can be separated magnetically and grown in the presence of stem cell factor, interleukin-6, interleukin-4, and other cytokines to yield distinct mast cell populations. The recent use of live cell array, with its ability to study such interactions rapidly at the single-cell level, provides unique new opportunities for fast output screening of mast cell triggers and inhibitors.

The critical role of mast cells in allergy and inflammation

Mast cells are well known for their involvement in allergic and anaphylactic reactions, but recent findings implicate them in a variety of inflammatory diseases affecting different organs, including the heart, joints, lungs, and skin. In these cases, mast cells appear to be activated by triggers other than aggregation of their IgE receptors (FcepsilonRI), such as anaphylatoxins, immunoglobulin-free light chains, superantigens, neuropeptides, and cytokines leading to selective release of mediators without degranulation. These findings could explain inflammatory diseases, such as asthma, atopic dermatitis, coronary inflammation, and inflammatory arthritis, all of which worsen by stress. It is proposed that the pathogenesis of these diseases involve mast cell activation by local release of corticotropin-releasing hormone (CRH) or related peptides. Combination of CRH receptor antagonists and mast cell inhibitors may present novel therapeutic interventions.

A Unique Protease-sensitive High Density Lipoprotein Particle Containing the Apolipoprotein A-IMilano Dimer Effectively Promotes ATP-binding Cassette A1-mediated Cell Cholesterol Efflux

Carriers of the apolipoprotein A-I(Milano) (A-I(M)) variant present with severe reductions of plasma HDL levels, not associated with premature coronary heart disease (CHD). Sera from 14 A-I(M) carriers and matched controls were compared for their ability to promote ABCA1-driven cholesterol efflux from J774 macrophages and human fibroblasts. When both cell types are stimulated to express ABCA1, the efflux of cholesterol through this pathway is greater with A-I(M) than control sera (3.4 +/- 1.0% versus 2.3 +/- 1.0% in macrophages; 5.2 +/- 2.4% versus 1.9 +/- 0.1% in fibroblasts). A-I(M) and control sera are instead equally effective in removing cholesterol from unstimulated cells and from fibroblasts not expressing ABCA1. The A-I(M) sera contain normal amounts of apoA-I-containing prebeta-HDL and varying concentrations of a unique small HDL particle containing a single molecule of the A-I(M) dimer; chymase treatment of serum degrades both particles and abolishes ABCA1-mediated cholesterol efflux. The serum content of chymase-sensitive HDL correlates strongly and significantly with ABCA1-mediated cholesterol efflux (r = 0.542, p = 0.004). The enhanced capacity of A-I(M) serum for ABCA1 cholesterol efflux is thus explained by the combined occurrence in serum of normal amounts of apoA-I-containing prebeta-HDL, together with a unique protease-sensitive, small HDL particle containing the A-I(M) dimer, both effective in removing cell cholesterol via ABCA1.

Mast Cell Proteases

Mast cells (MCs) are traditionally thought of as a nuisance for its host, for example, by causing many of the symptoms associated with allergic reactions. In addition, recent research has put focus on MCs for displaying harmful effects during various autoimmune disorders. On the other hand, MCs can also be beneficial for its host, for example, by contributing to the defense against insults such as bacteria, parasites, and snake venom toxins. When the MC is challenged by an external stimulus, it may respond by degranulation. In this process, a number of powerful preformed inflammatory "mediators" are released, including cytokines, histamine, serglycin proteoglycans, and several MC-specific proteases: chymases, tryptases, and carboxypeptidase A. Although the exact effector mechanism(s) by which MCs carry out their either beneficial or harmful effects in vivo are in large parts unknown, it is reasonable to assume that these mediators may contribute in profound ways. Among the various MC mediators, the exact biological function of the MC proteases has for a long time been relatively obscure. However, recent progress involving successful genetic targeting of several MC protease genes has generated powerful tools, which will enable us to unravel the role of the MC proteases both in normal physiology as well as in pathological settings. This chapter summarizes the current knowledge of the biology of the MC proteases.

Apolipoprotein A-I breakdown is induced by thrombolysis in coronary patients

BACKGROUND

The outcome of percutaneous coronary intervention (PCI) is apparently worse in patients receiving a prior thrombolytic therapy ('facilitated PCI'). Recombinant tissue-type plasminogen activator (rt-PA) can degrade circulating high-density lipoproteins (HDL) bound apolipoprotein A-I (apoA-I), thus possibly reducing the vascular protective activity. There have never been reports of the detection of apolipoprotein breakdown products in the circulation.

AIM

We studied the potential interactions between the protein components of HDL and tenecteplase, infused as thrombolytic therapy.

METHODS

Sera from a total of 40 patients with acute myocardial infarction (AMI), unstable angina (UA), and dilative cardiomyopathy (controls) were investigated. AMI patients underwent either immediate PCI or were treated with tenecteplase thrombolysis.

RESULTS

Products of extensive proteolysis of apoA-I were found in many acute coronary patients treated with tenecteplase, and in some AMI patients before starting the treatment (time 0). These were not detected in controls, UA patients as well as AMI patients undergoing immediate PCI. Small pre-beta-HDLs were selectively degraded.

CONCLUSION

Significant apoA-I degradation occurs in AMI patients after thrombolytic treatment. This finding may provide a potential mechanism for the apparent reduction of benefit of facilitated versus nonfacilitated PCI.

Mast cell proteases: Physiological tools to study functional significance of high density lipoproteins in the initiation of reverse cholesterol transport

The extracellular fluid of the intima is rich in lipid-poor species of high density lipoproteins (HDL) that promote efficient efflux of cholesterol from macrophages. Yet, during atherogenesis, cholesterol accumulates in macrophages, and foam cells are formed. We have studied proteolytic modification of HDL by mast cell proteases as a potential mechanism of reduced cholesterol efflux from foam cells. Mast cells are present in human atherosclerotic lesions and, when activated, they expel cytoplasmic granules that are filled with heparin proteoglycans and two neutral proteases, chymase and tryptase. Both proteases were found to specifically deplete in vitro the apoA-I-containing prebeta-migrating HDL (prebeta-HDL) and other lipid-poor HDL particles that contain only apoA-IV or apoE. These losses led to inhibition of the high-affinity component of cholesterol efflux from macrophage foam cells facilitated by the ATP-binding cassette transporter A1 (ABCA1). In contrast, the diffusional component of efflux promoted by alpha-HDL particles was not changed after proteolysis. Mast cell proteases are providing new insights into the role of extracellular proteolysis of HDL as an inhibiting principle of the initial steps of reverse cholesterol transport in the atherosclerotic intima, where many types of protease-secreting cells are present.

The double jeopardy of HDL

The ability of high-density lipoprotein (HDL) to promote cholesterol efflux is thought to be important in its protection against cardiovascular disease. Anti-inflammatory properties of HDL have emerged as additional properties that may also be important. HDL appears to have evolved as part of the innate immune system functioning to inhibit inflammation in the absence of an acute phase response (APR) but functioning to increase inflammation in the presence of an APR. Inbred strains of mice that are genetically susceptible to atherosclerosis have pro-inflammatory HDL, while inbred strains that are resistant to atherosclerosis have anti-inflammatory HDL. In one small study, humans with coronary heart disease (CHD) or CHD equivalents had pro-inflammatory HDL prior to statin therapy and about half continued to have pro-inflammatory HDL after statin therapy despite a profound decrease in plasma lipids. Pro-inflammatory HDL was relatively weak in its ability to promote cholesterol efflux while anti-inflammatory HDL was better in promoting cholesterol efflux. In other studies, oxidative alterations of the major protein of HDL, apolipoprotein A-I (apoA-I), impaired the ability of the apoA-I to promote cholesterol efflux. Thus, HDL structure and function may be more important than HDL-cholesterol levels in predicting risk for cardiovascular disease.

Apolipoproteins A-I and A-II are potentially important effectors of innate immunity in the teleost fish Cyprinus carpio

We have previously shown that high density lipoprotein is the most abundant protein in the carp plasma and displays bactericidal activity in vitro. Therefore the aim of this study was to analyze the contribution of its principal apolipoproteins, apoA-I and apoA-II, in defense. Both apolipoproteins were isolated by a two step procedure involving affinity and gel filtration chromatography and were shown to display bactericidal and/or bacteriostatic activity in the micromolar range against Gram-positive and Gram-negative bacteria, including some fish pathogens. In addition, a cationic peptide derived from the C-terminal region of carp apoA-I was synthesized and shown to possess antimicrobial activity (EC(50) = 3-6 micro m) against Planococcus citreus. This peptide was also able to potentiate the inhibitory effect of lysozyme in a radial diffusion assay at subinhibitory concentrations of both effectors. Finally, limited proteolysis of HDL-associated apoA-I with chymotrypsin in vitro was shown to generate a major truncated fragment, which indicates that apoA-I peptides liberated in vivo through a regulated proteolysis could also be involved in innate immunity.

Transthyretin, a new cryptic protease

Transthyretin (TTR) is a plasma homotetrameric protein that acts physiologically as a transporter of thyroxine (T(4)) and retinol, in the latter case through binding to retinol-binding protein (RBP). A fraction of plasma TTR is carried in high density lipoproteins by binding to apolipoprotein AI (apoA-I). We further investigated the nature of the TTR-apoA-I interaction and found that TTR from different sources (recombinant and plasmatic) is able to process proteolytically apoA-I, cleaving its C terminus after Phe-225. TTR-mediated proteolysis was inhibited by serine protease inhibitors (phenylmethanesulfonyl fluoride, Pefabloc, diisopropyl fluorophosphate, chymostatin, and N(alpha)-p-tosyl-l-phenylala-nine-chloromethyl ketone), suggesting a chymotrypsin-like activity. A fluorogenic substrate corresponding to an apoA-I fragment encompassing amino acid residues 223-228 (Abz-ESFKVS-EDDnp) was used to characterize the catalytic activity of TTR, including optimum reaction conditions (37 degrees C and pH 6.8) and catalytic constant (K(m) = 29 microm); when complexed with RBP, TTR activity was lost, whereas when complexed with T(4), only a slight decrease was observed. Cell lines expressing TTR were able to degrade Abz-ESFKVS-EDDnp 2-fold more efficiently than control cells lacking TTR expression; this effect was reversed by the presence of RBP in cell culture media, therefore proving a TTR-specific proteolytic activity. TTR can act as a novel plasma cryptic protease and might have a new, potentially important role under physiological and/or pathological conditions.

Depletion of pre-beta-high density lipoprotein by human chymase impairs ATP-binding cassette transporter A1- but not scavenger receptor class B type I-mediated lipid efflux to high density lipoprotein

The ATP-binding cassette transporter A1 (ABCA1) mediates the efflux of cellular unesterified cholesterol and phospholipid to lipid-poor apolipoprotein A-I. Chymase, a protease secreted by mast cells, selectively cleaves pre-beta-migrating particles from high density lipoprotein (HDL)(3) and reduces the efflux of cholesterol from macrophages. To evaluate whether this effect is the result of reduction of ABCA1-dependent or -independent pathways of cholesterol efflux, in this study we examined the efflux of cholesterol to preparations of chymase-treated HDL(3) in two types of cell: 1) in J774 murine macrophages endogenously expressing low levels of scavenger receptor class B, type I (SR-BI), and high levels of ABCA1 upon treatment with cAMP; and 2) in Fu5AH rat hepatoma cells endogenously expressing high levels of the SR-BI and low levels of ABCA1. Treatment of HDL(3) with the human chymase resulted in rapid depletion of pre-beta-HDL and a concomitant decrease in the efflux of cholesterol and phospholipid (2-fold and 3-fold, respectively) from the ABCA1-expressing J774 cells. In contrast, efflux of free cholesterol from Fu5AH to chymase-treated and to untreated HDL(3) was similar. Incubation of HDL(3) with phospholipid transfer protein led to an increase in pre-beta-HDL contents as well as in ABCA1-mediated cholesterol efflux. A decreased cholesterol efflux to untreated HDL(3) but not to chymase-treated HDL(3) was observed in ABCA1-expressing J774 with probucol, an inhibitor of cholesterol efflux to lipid-poor apoA-I. Similar results were obtained using brefeldin and gliburide, two inhibitors of ABCA1-mediated efflux. These results indicate that chymase treatment of HDL(3) specifically impairs the ABCA1-dependent pathway without influencing either aqueous or SR-BI-facilitated diffusion and that this effect is caused by depletion of lipid-poor pre-beta-migrating particles in HDL(3). Our results are compatible with the view that HDL(3) promotes ABCA1-mediated lipid efflux entirely through its lipid-poor fraction with pre-beta mobility.