Anitaben Tailor

Hypercholesterolemia promotes inflammation and microvascular dysfunction: role of nitric oxide and superoxide

Relatively brief periods (days) of hypercholesterolemia can exert profound effects on endothelium-dependent functions of the microcirculation, including dilation of arterioles, fluid filtration across capillaries, and regulation of leukocyte recruitment in postcapillary venules. Hypercholesterolemia appears to convert the normal anti-inflammatory phenotype of the microcirculation to a proinflammatory phenotype. This phenotypic change appears to result from a decline in nitric oxide (NO) bioavailability that results from a reduction in NO biosynthesis, inactivation of NO by superoxide (O(2)(*)(-)), or both. A consequence of the hypercholesterolemia-induced microvascular responses is an enhanced vulnerability of the microcirculation to the deleterious effects of ischemia and other inflammatory conditions. Hence, therapeutic strategies that are directed towards preventing the early microcirculatory dysfunction and inflammation caused by hypercholesterolemia may prove effective in reducing the high mortality associated with ischemic tissue diseases. Agents that act to maintain the normal balance between NO and reactive oxygen species (ROS) in vascular endothelial cells may prove particularly useful in this regard.

Oxidative stress promotes blood cell-endothelial cell interactions in the microcirculation

Oxidative stress occurs when the production of reactive oxygen species (ROS) exceeds the capacity of the cell to detoxify these potentially injurious oxidants using endogenous antioxidant defense systems. Conditions associated with oxidative stress include ischemia/reperfusion, hypercholesterolemia, diabetes, and hypertension. The adhesion of circulating blood cells (leukocytes, platelets) to vascular endothelium is a key element of the pro-inflammatory and prothrombogenic phenotype assumed by the vasculature in these and other disease states that are associated with an oxidative stress. There is a growing body of evidence that links the blood cell-endothelial cell interactions in these conditions to the enhanced production of ROS. Potential enzymatic sources of ROS within the microcirculation include xanthine oxidase, NAD(P)H oxidase, and nitric oxide synthase. ROS can promote a pro-inflammatory/prothrombogenic phenotype within the microvasculature by a variety of mechanisms, including the inactivation of nitric oxide, the activation of redox-sensitive transcription factors (e.g., nuclear factor-ξB) that govern the expression of endothelial cell adhesion molecules (e.g., P-selectin), and the activation of enzymes (e.g., phospholipase A2) that produce leukocyte-stimulating inflammatory mediators (e.g., platelet-activating factor). The extensively documented ability of different oxidant-ablating interventions to attenuate blood cell-endothelial cell interactions underscores the importance of ROS in mediating the dysfunctional microvascular responses to oxidative stress.

Hypercholesterolemia Promotes P-Selectin–Dependent Platelet–Endothelial Cell Adhesion in Postcapillary Venules

Objective—The objectives of this study were to determine whether hypercholesterolemia promotes platelet–endothelial cell (P/E) adhesion in murine postcapillary venules and define the contributions of endothelial or platelet associated P-selection to hypercholesterolemia-induced P/E interactions. Methods and Results—Wild-type (WT) or P-selectin deficient (P-sel−/−) platelets were isolated and labeled with the fluorochrome CFSE and administered to either WT or P-sel−/− mice placed on a normal diet (ND) or high cholesterol diet (HCD). Intravital videomicroscopy was used to quantify platelet saltation and firm adhesion. HCD-WT mice exhibited a time-dependent increase in P/E cell interactions (relative to ND-WT). Flow cytometry revealed an increased expression of P-selectin on circulating platelets of HCD-WT mice at 2 weeks compared with ND-WT mice. When WT platelets were monitored in HCD-P-sel−/− mice, P/E adhesion was dramatically reduced. However, when P-sel−/− platelets were monitored in HCD-WT recipients, P/E adhesive interactions were reduced even further, comparable to ND-WT mice. Conclusions—These results indicate that elevated cholesterol levels promote P/E adhesion in postcapillary venules and that whereas both endothelial and platelet P-selectin contribute to hypercholesterolemia-induced recruitment of platelets, platelet-associated P-selectin seems to play a more important role in producing the prothrombogenic phenotype in venules.

Platelet–Vessel Wall Interactions in the Microcirculation

Platelet adhesion in the microcirculation is being reported and studied in a growing number of animal models of human disease. The adhesion molecules utilized by platelets to attach to the walls of microscopic blood vessels have been defined in most model systems, with P‐selectin‐PSGL‐1, GPIIb/IIIa‐fibrinogen‐ICAM‐1, and vWF‐GPIbα interactions serving as major adhesion pathways. The binding of platelets to adherent leukocytes appears to contribute significantly to the platelet–vessel wall interactions elicited in many models of disease. Shear forces generated by the movement of blood through the microcirculation exert an important influence on platelet adhesion. Physiologic regulation of platelet adhesion is mediated by both nitric oxide and superoxide, with the former inhibiting and the latter promoting platelet adhesion. The interactions between platelets and the walls of microscopic blood vessels appear to have important implications in the initiation and/or progression of tissue injury associated with different experimental models of human disease.

Dexamethasone inhibition of leucocyte adhesion to rat mesenteric postcapillary venules: role of intercellular adhesion molecule 1 and KC

BACKGROUND A previous study showed that the glucocorticoid dexamethasone, at doses of 100 μg/kg and above, inhibited leucocyte adhesion to rat mesenteric postcapillary venules activated with interleukin 1β (IL-1β), as assessed by videomicroscopy. AIMS To identify whether the adhesion molecule, intercellular adhesion molecule 1 (ICAM-1), or the chemokine KC could be targeted by the steroid to mediate its antiadhesive effect. METHODS Rat mesenteries were treated with IL-1β (20 ng intraperitoneally) and the extent of leucocyte adhesion measured at two and four hours using intravital microscopy. Rats were treated with dexamethasone, and passively immunised against ICAM-1 or KC. Endogenous expression of these two mediators was validated by immunohistochemistry, ELISA, and the injection of specific radiolabelled antibodies. RESULTS Dexamethasone greatly reduced IL-1β induced leucocyte adhesion, endothelial expression of ICAM-1 in the postcapillary venule, and release of the mast cell derived chemokine KC. Injection of specific antibodies to the latter mediators was also extremely effective in downregulating (>80%) IL-1β induced leucocyte adhesion. CONCLUSIONS Induction by IL-1β of endogenous ICAM-1 and KC contributes to leucocyte adhesion to inflamed mesenteric vessels. Without excluding other possible mediators, these data clearly show that dexamethasone interferes with ICAM-1 expression and KC release from mast cells, resulting in suppression of leucocyte accumulation in the bowel wall, which is a prominent feature of several gastrointestinal pathologies.

Analysis of the protection afforded by annexin 1 in ischaemia–reperfusion injury: focus on neutrophil recruitment

Ischaemia-reperfusion injury underlies many of the most important cardiovascular diseases such as myocardial infarction, thrombotic stroke, embolic vascular occlusions and peripheral vascular insufficiency. Neutrophils feature prominently in this inflammatory component of post-ischaemic injury. Experimental therapies, shown to reduce neutrophil-mediated ischaemia-reperfusion injury include neutrophil depletion, direct inhibitors of neutrophil activators, antibodies against neutrophil adhesion molecules and the endothelial adhesion molecules. However, aside from these approaches, it is increasingly recognised that glucocorticoids are potent inhibitors of neutrophil-mediated injury. The anti-inflammatory actions of glucocorticoid include the activation of classical cytoplasmic receptors leading to changes in gene transcription as well as the induction of regulatory proteins, such as annexin 1. Annexin 1 is a potent inhibitor of neutrophil extravasation in vivo. Administration of the annexin 1 or peptides derived from its N-terminal domain, reduce neutrophil extravasation in models of acute inflammation. In addition, as reviewed by this article, annexin 1 protects against ischaemia-reperfusion in the heart and mesenteric microcirculation, as well as in multiple organ failure associated with splanchnic ischaemia-reperfusion. Such findings would suggest annexin 1 is a novel anti-inflammatory agent with a potential for the treatment of cardiovascular pathologies associated with neutrophil activation and recruitment.

Hypercholesterolemia promotes leukocyte-dependent platelet adhesion in murine postcapillary venules.

Objective: Leukocyte‐platelet aggregates form in blood during the development of cardiovascular diseases, including atherosclerosis. The study determined whether leukocytes contribute to the platelet adhesion induced by hypercholesterolemia in postcapillary venules.

Dexamethasone inhibits leukocyte emigration in rat mesenteric post-capillary venules: an intravital microscopy study.

The effect of subcutaneous administration of dexamethasone (DEX) on interleukin‐lβ (IL‐lβ, 20 ng i.p., – 2 h) and platelet‐activating factor (PAF, 100 nM in superfusion) ‐induced leukocyte interaction with the endothelium of rat mesenteric post‐capillary venules was studied. DEX produced a dose‐dependent inhibition of IL‐lβ‐induced leukocyte extravasation in the rat mesenteric vascular bed, with a calculated ED50 of 40 μg/kg and a maximal effect of 80–100% inhibition at 0.1 mg/kg. IL‐lβ‐induced cell adhesion to post‐capillary venules was only partially inhibited by the steroid, with a calculated ED50 of 480 μg/kg and a maximal effect of 40–60% inhibition. Furthermore, the steroid inhibited leukocyte emigration, but not adhesion, caused by superfusion of the mesenteric vascular bed with PAF. A doubling of leukocyte emigration time (from 226 to 552 s) was observed after treatment of rats with DEX. Administration for 5 days of a dose of 10 μg/kg DEX (which was inactive when given as a single injection) resulted again in a selective inhibition of IL‐lβ‐induced leukocyte emigration, without effect on cell adhesion. These data demonstrate a preferential susceptibility of the leukocyte emigration process to the inhibitory action of DEX. J. Leukoc. Biol. 62: 301–308; 1997.