Magnus Bäck

Matrix Metalloproteinases in Atherothrombosis

The metalloproteinases (MMPs, matrixins) are zinc-containing endopeptidases involved in the metabolism of extracellular matrix as well as in the cleavage of other proteins. The MMP family currently consists of 28 enzymes with somewhat different activities. The members are in part categorized into groups according to either structure or preferred substrates and referred to as collagenases, gelatinases, stromelysins, matrilysins, and membrane-bound MMPs. The proteinase activities exerted by 11 of the 28 MMPs have been implicated in some of the biologic processes associated with atherosclerosis and its ischemic clinical manifestations such as myocardial infarction and stroke. For example, several of the MMPs are locally expressed within human atherosclerotic lesions. However, association studies of subclinical atherosclerosis have generated contradictory results in the role of MMP activities. In addition, circulating MMP levels as well as genetic variations within the genes encoding the different enzymes have been associated with both an increased and decreased cardiovascular risk. Finally, experimental studies of hyperlipemic mice and vascular injury have suggested some of the MMPs function as modulators of atherogenesis, vascular remodeling, and plaque rupture.

5-Lipoxygenase–Activating Protein A Potential Link Between Innate and Adaptive Immunity in Atherosclerosis and Adipose Tissue Inflammation

Transforming growth factor-&bgr; (TGF-&bgr;) is a major antiinflammatory mediator in atherosclerosis. Transgenic ApoE−/− mice with a dominant-negative TGF&bgr; type II receptor (dnTGF&bgr;RII) on CD4+ and CD8+ T cells display aggravated atherosclerosis. The aim of the present study was to elucidate the mechanisms involved in this enhanced inflammatory response. Gene array analyses identified the 5-lipoxygenase–activating protein (FLAP) among the most upregulated genes in both the aorta and adipose tissue of dnTGF&bgr;RII transgenic ApoE−/− mice compared with their ApoE−/− littermates, a finding that was confirmed by real-time quantitative RT-PCR. Aortas from the former mice in addition produced increased amounts of the lipoxygenase product leukotriene B4 after ex vivo stimulation. FLAP protein expression in both the aorta and adipose tissue was detected in macrophages, but not in T cells. Four weeks of treatment with the FLAP inhibitor MK-886 (10 mg/kg in 1% tylose delivered by osmotic pumps) significantly reduced atherosclerotic lesion size and T-cell content. Finally, FLAP mRNA levels were upregulated approximately 8-fold in adipose tissue derived from obese ob/ob mice. In conclusion, the results of the present study suggest a key role for mediators of the 5-lipoxygenase pathway in inflammatory reactions of atherosclerosis and metabolic disease.

Leukotriene Signaling in Atherosclerosis and Ischemia

IntroductionThe inflammatory process of atherosclerosis is associated with several pathophysiological reactions within the vascular wall. The arachidonic acid released by phospholipase A2 serves as substrate for the production of a group of lipid mediators known as the leukotrienes, which induce pro-inflammatory signaling through activation of specific BLT and CysLT receptors.DiscussionLeukotriene signaling has been implicated in early lipid retention and foam cell accumulation, as well as in the development of intimal hyperplasia and advanced atherosclerotic lesions. Furthermore, the association of leukotrienes with degradation of extracellular matrix has suggested a role in atherosclerotic plaque rupture. Finally, studies of either myocardial or cerebral ischemia and reperfusion indicate that leukotriene signaling in addition may be involved in the development of ischemic injury.ConclusionBoth leukotriene synthesis inhibitors and leukotriene receptor antagonists have been suggested to induce beneficial effects at different stages of the atherosclerosis process.

Update on leukotriene, lipoxin and oxoeicosanoid receptors: IUPHAR Review 7

The endogenous ligands for the LT, lipoxin (LX) and oxoeicosanoid receptors are bioactive products produced by the action of the lipoxygenase family of enzymes. The LT receptors BLT1 and BLT2, are activated by LTB4 and the CysLT1 and CysLT2 receptors are activated by the cysteinyl‐LTs, whereas oxoeicosanoids exert their action through the OXE receptor. In contrast to these pro‐inflammatory mediators, LXA4 transduces responses associated with the resolution of inflammation through the receptor FPR2/ALX (ALX/FPR2). The aim of the present review is to give a state of the field on these receptors, with focus on recent important findings. For example, BLT1 receptor signalling in cancer and the dual role of the BLT2 receptor in pro‐ and anti‐inflammatory actions have added more complexity to lipid mediator signalling. Furthermore, a cross‐talk between the CysLT and P2Y receptor systems has been described, and also the presence of novel receptors for cysteinyl‐LTs, such as GPR17 and GPR99. Finally, lipoxygenase metabolites derived from ω‐3 essential polyunsaturated acids, the resolvins, activate the receptors GPR32 and ChemR23. In conclusion, the receptors for the lipoxygenase products make up a sophisticated and tightly controlled system of endogenous pro‐ and anti‐inflammatory signalling in physiology and pathology.

International Union of Basic and Clinical Pharmacology. LXXXIV: Leukotriene Receptor Nomenclature, Distribution, and Pathophysiological Functions

The seven-transmembrane G protein-coupled receptors activated by leukotrienes are divided into two subclasses based on their ligand specificity for either leukotriene B4 or the cysteinyl leukotrienes (LTC4, LTD4, and LTE4). These receptors have been designated BLT and CysLT receptors, respectively, and a subdivision into BLT1 and BLT2 receptors and CysLT1 and CysLT2 receptors has been established. However, recent findings have also indicated the existence of putative additional leukotriene receptor subtypes. Furthermore, other ligands interact with the leukotriene receptors. Finally, leukotrienes may also activate other receptor classes, such as purinergic receptors. The aim of this review is to provide an update on the pharmacology, expression patterns, and pathophysiological roles of the leukotriene receptors as well as the therapeutic developments in this area of research.

Differential inflammatory activity across human abdominal aortic aneurysms reveals neutrophil-derived leukotriene B4 as a major chemotactic factor released from the intraluminal thrombus

Development and progression of acquired abdominal aortic aneurysms (AAAs) have been associated with different inflammatory mediators. The aim of the present study was to elucidate the topology and the potential mechanisms linking the leukotriene pathway to human AAAs. Human aneurysmal lesions were obtained from 24 patients undergoing surgery, and the intraluminal thrombus was separated from the vascular wall. Histological examination revealed major expression of the leukotriene‐producing enzymes 5‐lipoxygenase and LTA4 hydrolase, as well as the two receptors for leukotriene B4 (BLT1R and BLT2R), corresponding to neutrophils in the luminal part of the thrombus. In contrast, in the vascular wall, the leukotriene pathway mainly localized in macrophage‐rich adventitial areas. Furthermore, conditioned media of the intraluminal thrombus contained significantly higher concentrations of leukotriene B4 than that derived from the vascular wall, which were significantly correlated to other neutrophil‐derived mediators, such as elastase/a1‐antitrypsin complexes, myeloperoxidase, and MMP9/NGAL complexes. Finally, the neutrophil‐chemotactic activity of the conditioned media from the intraluminal thrombus exhibited major inhibition by antagonists of the leukotriene B4 receptors. Taken together, these results indicate neutrophil‐derived leukotriene B4 as a major neutrophil chemotactic factor released from the intraluminal thrombus of human AAAs and suggest that targeting BLT receptors may represent a potential medical therapeutic strategy in the prevention of AAA progression in humans.— Houard, X., Ollivier, V., Louedec, L., Michel, J.‐B., Back, M. Differential inflammatory activity across human abdominal aortic aneurysms reveals neutrophil‐derived leukotriene B4 as a major chemotactic factor released from the intraluminal thrombus. FASEBJ. 23, 1376–1383 (2009)

Biomechanical factors in the biology of aortic wall and aortic valve diseases

The biomechanical factors that result from the haemodynamic load on the cardiovascular system are a common denominator of several vascular pathologies. Thickening and calcification of the aortic valve will lead to reduced opening and the development of left ventricular outflow obstruction, referred to as aortic valve stenosis. The most common pathology of the aorta is the formation of an aneurysm, morphologically defined as a progressive dilatation of a vessel segment by more than 50% of its normal diameter. The aortic valve is exposed to both haemodynamic forces and structural leaflet deformation as it opens and closes with each heartbeat to assure unidirectional flow from the left ventricle to the aorta. The arterial pressure is translated into tension-dominated mechanical wall stress in the aorta. In addition, stress and strain are related through the aortic stiffness. Furthermore, blood flow over the valvular and vascular endothelial layer induces wall shear stress. Several pathophysiological processes of aortic valve stenosis and aortic aneurysms, such as macromolecule transport, gene expression alterations, cell death pathways, calcification, inflammation, and neoangiogenesis directly depend on biomechanical factors.

Leukotriene receptors in atherosclerosis.

Leukotriene‐forming enzymes are expressed within atherosclerotic lesions and locally produced leukotrienes exert pro‐inflammatory actions within the vascular wall by means of cell surface receptors of the BLT and CysLT receptor subtypes. The migration and accumulation of inflammatory cells that follow leukotriene receptor activation have been implicated in atherosclerosis initiation and progression. Leukotriene receptors are in addition expressed on endothelial and vascular smooth muscle cells, associated with intimal hyperplasia in early atherosclerosis and restenotic lesions after angioplasty. Taken together, recent evidence suggests that leukotriene receptors may be a potential target in the treatment of atherosclerosis and in the prevention of restenosis after coronary interventions.

Novel methodologies for biomarker discovery in atherosclerosis.

Identification of subjects at increased risk for cardiovascular events plays a central role in the worldwide efforts to improve prevention, prediction, diagnosis, and prognosis of cardiovascular disease and to decrease the related costs. Despite their high predictive value on population level, traditional risk factors fail to fully predict individual risk. This position paper provides a summary of current vascular biomarkers other than the traditional risk factors with a special focus on the emerging -omics technologies. The definition of biomarkers and the identification and use of classical biomarkers are introduced, and we discuss the limitations of current biomarkers such as high sensitivity C-reactive protein (hsCRP) or N-terminal pro-brain natriuretic peptide (NT-proBNP). This is complemented by circulating plasma biomarkers, including high-density lipoprotein (HDL), and the conceptual shift from HDL cholesterol levels to HDL composition/function for cardiovascular risk assessment. Novel sources for plasma-derived markers include microparticles, microvesicles, and exosomes and their use for current omics-based analytics. Measurement of circulating micro-RNAs, short RNA sequences regulating gene expression, has attracted major interest in the search for novel biomarkers. Also, mass spectrometry and nuclear magnetic resonance spectroscopy have become key complementary technologies in the search for new biomarkers, such as proteomic searches or identification and quantification of small metabolites including lipids (metabolomics and lipidomics). In particular, pro-inflammatory lipid metabolites have gained much interest in the cardiovascular field. Our consensus statement concludes on leads and needs in biomarker research for the near future to improve individual cardiovascular risk prediction.

Functional characteristics of cysteinyl-leukotriene receptor subtypes.

Cysteinyl-leukotrienes, i.e. leukotriene (LT) C4, D4 and E4, are inflammatory mediators and potent airway- and vasoconstrictors. Two different cysteinyl-leukotriene receptors, CysLT1 and CysLT2, have been cloned and functionally characterised using potent CysLT1 receptor antagonists and the dual CysLT1/CysLT2 receptor antagonist BAY u9773. However, the rank order of potency of the cysteinyl-leukotrienes at the CysLT receptors differs between tissues and studies, and a CysLT receptor classification based on agonist selectivity has not been established. In addition, the existence of more than two receptor subtypes for cysteinyl-leukotrienes has been suggested.