The significance of platelet-derived growth factors for proliferation of vascular smooth muscle cells

Extracellular Vesicles as Drivers of Immunoinflammation in Atherothrombosis

Atherosclerotic cardiovascular disease is the leading cause of morbidity and mortality all over the world. Extracellular vesicles (EVs), small lipid-bilayer membrane vesicles released by most cellular types, exert pivotal and multifaceted roles in physiology and disease. Emerging evidence emphasizes the importance of EVs in intercellular communication processes with key effects on cell survival, endothelial homeostasis, inflammation, neoangiogenesis, and thrombosis. This review focuses on EVs as effective signaling molecules able to both derail vascular homeostasis and induce vascular dysfunction, inflammation, plaque progression, and thrombus formation as well as drive anti-inflammation, vascular repair, and atheroprotection. We provide a comprehensive and updated summary of the role of EVs in the development or regression of atherosclerotic lesions, highlighting the link between thrombosis and inflammation. Importantly, we also critically describe their potential clinical use as disease biomarkers or therapeutic agents in atherothrombosis.

Ginsenosides Rb2 and Rd2 isolated from Panax notoginseng flowers attenuate platelet function through P2Y12-mediated cAMP/PKA and PI3K/Akt/Erk1/2 signaling

Saponins derived from Panax notoginseng root are widely used as herbal medicines and dietary supplements due to their wide range of health benefits. However, the effects of those from Panax notoginseng flowers (PNF) on platelet function and thrombus formation remain largely unknown. Using a series of platelet function assays, we found that G-Rb2 and G-Rd2, among the ten PNF saponin monomers, significantly inhibited human platelet aggregation and activation induced by adenosine diphosphate (ADP) in vitro. The 50% inhibitory concentration (IC50) of G-Rb2 and G-Rd2 against ADP-induced platelet aggregation was 85.5 ± 4.5 μg mL-1 and 51.4 ± 4.6 μg mL-1, respectively. Mechanistically, G-Rb2 and G-Rd2 could effectively modulate platelet P2Y12-mediated signaling by up-regulating cAMP/PKA signaling and down-regulating PI3K/Akt/Erk1/2 signaling pathways. Co-incubation of the P2Y12 antagonist cangrelor with either G-Rb2 or G-Rd2 did not show significant additive inhibitory effects. G-Rb2 and G-Rd2 also substantially suppressed thrombus growth in a FeCl3-induced murine arteriole thrombosis model in vivo. Interestingly, G-Rd2 generally exhibited more potent inhibitory effects on platelet function and thrombus formation than G-Rb2. Thus, our data suggest that PNF-derived G-Rb2 and G-Rd2 effectively attenuate platelet hyperactivity through modulating signaling pathways downstream of P2Y12, which indicates G-Rb2 and G-Rd2 may play important preventive roles in thrombotic diseases.

Current understanding of intimal hyperplasia and effect of compliance in synthetic small diameter vascular grafts

Despite much effort, synthetic small diameter vascular grafts still face limited success due to vascular wall thickening known as intimal hyperplasia (IH). Compliance mismatch between graft and native vessels has been proposed to be one of a key mechanical factors of synthetic vascular grafts that could contribute to the formation of IH. While many methods have been developed to determine compliance both in vivo and in vitro, the effects of compliance mismatch still remain uncertain. This review aims to explain the biomechanical factors that are responsible for the formation and development of IH and their relationship with compliance mismatch. Furthermore, this review will address the current methods used to measure compliance both in vitro and in vivo. Lastly, current limitations in understanding the connection between the compliance of vascular grafts and the role it plays in the development and progression of IH will be discussed.

Effects of fibroblast extracellular matrix calcification on platelet adhesion in vitro

Calcified atherosclerotic lesions are more prone to rupture during angioplasty than non-calcified lesions and are associated with an increased risk of thrombotic complications following angioplasty. This study investigates the possible role of extracellular matrix (ECM) calcification for platelet adhesion. Human cultured fibroblasts (CRL-1635) were subjected to beta-glycerophosphate (10 mM) for 10 to 16 days. Calcification was visualized by von Kossa staining and quantified by the O-cresolphthalein complexone method. Adhesion of calcein-labelled platelets was measured by fluorescence microscopy at static conditions and in a parallel-flow chamber at a shear rate of 1000 s(-1). beta-glycerophosphate treatment resulted in a marked calcification of the ECM. In parallel, a small, albeit significant increase in platelet adhesion under static conditions was observed. In contrast, at flow conditions, the area covered by thrombi was significantly lower when calcified ECM was used. The number of thrombi was not significantly different which is compatible with a smaller thrombus size. Taken together, it appears unlikely that calcification of atherosclerotic lesions contributes to thrombotic complications by an increased platelet adhesion.

Complete Downmodulation of P-Selectin Glycoprotein Ligand in Monocytes Undergoing Apoptosis

Objectives— Apoptotic monocytes release membrane microparticles which may play a major role in thrombogenicity through a P-selectin glycoprotein ligand (PGSL-1)–mediated mechanism. We have studied systematically the regulation of PSGL-1 expression and function in apoptotic monocytic cells.

Methods and Results— PSGL-1 expression (flow cytometry, immunofluorescence microscopy, immunoblot) was virtually abolished in apoptotic monocytes by proteolytic shedding. This was accompanied by a complete loss of PSGL-1–mediated platelet–leukocyte (flow cytometry) and leukocyte–endothelial cell (parallel plate flow chamber) interactions. Systematic screening of protease inhibitors combined with knock-out and siRNA experiments characterized the PSGL-1-cleaving enzyme as an N-ethylmaleimide-inhibitable metalloproteinase of the ADAM family.

Conclusions— Downmodulation of PGSL-1 in apoptotic monocytes may prevent ectopic cell clearance in the peripheral vasculature to reduce local inflammatory and proliferative responses. Depletion of PSGL-1 expression on apoptotic microparticles may also act as a molecular switch to modulate their thrombogenic activity.

Effect of chemical stabilizers of hypoxia-inducible factors on early lung development

Low oxygen stimulates pulmonary vascular development and airway branching and involves hypoxia-inducible factor (HIF). HIF is stable and initiates expression of angiogenic factors under hypoxia, whereas normoxia triggers hydroxylation of the HIF-1α subunit by prolyl hydroxylases (PHDs) and subsequent degradation. Herein, we investigated whether chemical stabilization of HIF-1α under normoxic (20% O2) conditions would stimulate vascular growth and branching morphogenesis in early lung explants. Tie2-LacZ (endothelial LacZ marker) mice were used for visualization of the vasculature. Embryonic day 11.5 (E11.5) lung buds were dissected and cultured in 20% O2 in the absence or presence of cobalt chloride (CoCl2, a hypoxia mimetic), dimethyloxalylglycine (DMOG; a nonspecific inhibitor of PHDs), or desferrioxamine (DFO; an iron chelator). Vascularization was assessed by X-gal staining, and terminal buds were counted. The fine vascular network surrounding the developing lung buds seen in control explants disappeared in CoCl2- and DFO-treated explants. Also, epithelial branching was reduced in the explants treated with CoCl2 and DFO. In contrast, DMOG inhibited branching but stimulated vascularization. Both DFO and DMOG increased nuclear HIF-1α protein levels, whereas CoCl2 had no effect. Since HIF-1α induces VEGF expression, the effect of SU-5416, a potent VEGF receptor (VEGFR) blocker, on early lung development was also investigated. Inhibition of VEGFR2 signaling in explants maintained under hypoxic (2% O2) conditions completely abolished vascularization and slightly decreased epithelial branching. Taken together, the data suggest that DMOG stabilization of HIF-1α during early development leads to a hypervascular lung and that airway branching proceeds without the vasculature, albeit at a slower rate.

The C-terminal peptide of thrombospondin-1 stimulates distinct signaling pathways but induces an activation-independent agglutination of platelets and other cells

A peptide from the C-terminal domain of thrombospondin-1 (4N1-1) has been proposed to stimulate platelet aggregation by a novel mechanism involving both an activation-independent agglutination and an activation-dependent, glycoprotein (GP) IIb/IIIa-mediated aggregation which involves GPVI signaling but does not involve CD47. The present study demonstrates that 4N1-1 stimulated a different pattern of signal transduction pathways than the GPVI agonist convulxin. Furthermore, 4N1-1-induced platelet aggregation was activation-independent and not dependent on GPVI or GPIIb/IIIa. Interestingly, 4N1-1 also stimulated activation-independent agglutination of different megakaryocytic and non-megakaryocytic cells. 4N1-1-induced cell agglutination but not platelet signaling was inhibited by anti-CD47 antibodies.

Activation of the murine EP3 receptor for PGE2 inhibits cAMP production and promotes platelet aggregation

The importance of arachidonic acid metabolites (termed eicosanoids), particularly those derived from the COX-1 and COX-2 pathways (termed prostanoids), in platelet homeostasis has long been recognized. Thromboxane is a potent agonist, whereas prostacyclin is an inhibitor of platelet aggregation. In contrast, the effect of prostaglandin E2 (PGE2) on platelet aggregation varies significantly depending on its concentration. Low concentrations of PGE2 enhance platelet aggregation, whereas high PGE2 levels inhibit aggregation. The mechanism for this dual action of PGE2 is not clear. This study shows that among the four PGE2 receptors (EP1-EP4), activation of EP3 is sufficient to mediate the proaggregatory actions of low PGE2 concentration. In contrast, the prostacyclin receptor (IP) mediates the inhibitory effect of higher PGE2 concentrations. Furthermore, the relative activation of these two receptors, EP3 and IP, regulates the intracellular level of cAMP and in this way conditions the response of the platelet to aggregating agents. Consistent with these findings, loss of the EP3 receptor in a model of venous inflammation protects against formation of intravascular clots. Our results suggest that local production of PGE2 during an inflammatory process can modulate ensuing platelet responses.

PDGF-induced Akt phosphorylation does not activate NF-kappa B in human vascular smooth muscle cells and fibroblasts

A recent report suggested that platelet-derived growth factor (PDGF) activates nuclear factor-kappa B (NF-kappa B) by phosphorylation of the protein kinase Akt [Romashkova and Makarov, Nature 401 (1999) 86-90]. The present study investigates the role of Akt in the activation of NF-kappa B by tumor necrosis factor-alpha (TNF alpha, 10 ng/ml) and PDGF-BB (20 ng/ml) in human vascular smooth muscle cells (SMC), skin and foreskin fibroblasts. TNF alpha stimulated serine phosphorylation and degradation of the inhibitory protein I kappa B alpha and strongly induced nuclear NF-kappa B translocation and binding activity. PDGF did not induce serine phosphorylation or degradation of I kappa B alpha and did not enhance binding activity of NF-kappa B. In contrast, stimulation with PDGF resulted in a marked phosphorylation of Akt, but no Akt phosphorylation occurred after stimulation with TNF alpha. These data suggest that Akt phosphorylation is not involved in NF-kappa B activation in human SMC and fibroblasts.