Cholesterol Enhances Thrombin-Induced Release of Fibroblast Growth Factor-2 in Human Vascular Smooth Muscle Cells

Functional role of protease activated receptors in vascular biology

Protease activated receptors (PARs) are a small family of G protein-coupled receptors (GPCR) mediating the cellular effects of some proteases of the coagulation system, such as thrombin, or other proteases, such as trypsin or metalloproteinase 1. As the prototype of PARs, PAR1 is a seven transmembrane GPCR that, upon cleavage by thrombin, unmasks a new amino-terminus able to bind intramolecularly to PAR1 itself thus inducing signaling. In the vascular system, thrombin and other proteases of the coagulation-fibrinolysis system, such as plasmin, factor VIIa and factor Xa, activated protein C, are considered physiologically relevant agonists, and PARs appear to largely account for the cellular effects of these enzymes. In the vasculature, PARs are expressed on platelets, endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). In the vessel wall, under physiological conditions, PARs are mainly expressed in ECs and participate in the regulation of vascular tone, by inducing endothelium-dependent relaxation. PAR activation on ECs promotes conversion of these cells into a proinflammatory phenotype, causes increase of vascular permeability, and the exposure/secretion of proteins and cytokines mediating the local accumulation of platelets and leukocytes. These effects contribute to the vascular consequences of sepsis and of diseases such as acute lung injury and acute respiratory distress syndrome. In normal arteries PARs are to a much lesser amount expressed on VSMCs. However, in conditions associated with endothelial dysfunction, PARs mediate contraction, proliferation, migration, hypertrophy of VSMCs and their production of extracellular matrix, thereby contributing to the pathophysiology of atherosclerosis and hypertension. Inhibition of protease-PAR interaction might thus become a potential therapeutic target in various vascular diseases.

Cholesterol induces apoptosis-associated loss of the activated leukocyte cell adhesion molecule (ALCAM) in human monocytes

The activated leukocyte cell adhesion molecule (ALCAM/CD166) is associated with cell migration and leukocyte invasion into the vessel wall. This study investigates the impact of cholesterol loading on the expression of ALCAM, as compared with P-selectin glycoprotein ligand-1 (PSGL-1), vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) in monocytic U937 cells and human primary monocytes. Cells were enriched with cholesterol by incubation with a cyclodextrin-cholesterol complex. Expression of adhesion molecules and apoptosis was determined by flow cytometry. Migration was quantified by chemotaxis toward serum. Incubation with cholesterol (10-100 μg/ml) for 16 h caused a concentration-dependent increase in apoptosis. Enhanced apoptosis was associated with reduction of ALCAM by >70%. While PSGL-1 was affected similarly, expression of VCAM-1 was markedly increased by cholesterol and ICAM-1 levels were not regulated. The nonselective caspase/apoptosis inhibitor Q-VD-OPh partially prevented cholesterol-modulated alteration of adhesion molecule expression. Migration of cholesterol-rich monocytic cells toward serum was greatly reduced. This effect was partially restored by Q-VD-OPh and was dependent on ALCAM as shown by ALCAM-neutralizing antibodies. In conclusion, cholesterol-induced apoptosis in monocytes is accompanied by reduced expression of ALCAM and attenuated monocyte migration. This may restrain monocytes at cholesterol-rich sites and thereby expedite vascular lesion formation.

Protein targets of inflammatory serine proteases and cardiovascular disease

Serine proteases are a key component of the inflammatory response as they are discharged from activated leukocytes and mast cells or generated through the coagulation cascade. Their enzymatic activity plays a major role in the body's defense mechanisms but it has also an impact on vascular homeostasis and tissue remodeling. Here we focus on the biological role of serine proteases in the context of cardiovascular disease and their mechanism(s) of action in determining specific vascular and tissue phenotypes. Protease-activated receptors (PARs) mediate serine protease effects; however, these proteases also exert a number of biological activities independent of PARs as they target specific protein substrates implicated in vascular remodeling and the development of cardiovascular disease thus controlling their activities. In this review both PAR-dependent and -independent mechanisms of action of serine proteases are discussed for their relevance to vascular homeostasis and structural/functional alterations of the cardiovascular system. The elucidation of these mechanisms will lead to a better understanding of the molecular forces that control vascular and tissue homeostasis and to effective preventative and therapeutic approaches.

Thrombin increases inflammatory cytokine and angiogenic growth factor secretion in human adipose cells in vitro

Background

Abdominal obesity is associated with pro-thrombotic and inflammatory states. Therefore, the purpose of this study was to examine the expression of thrombin receptors (PAR1 and PAR4) human adipose tissue and whether thrombin stimulates an inflammatory cytokine and growth factor profile in human adipose tissue.

Methods

Human adipose tissue, isolated preadipocytes and differentiated adipocytes were used in this study. PAR1 and PAR4 mRNA and protein were detected by RT-PCR and immunoblot analysis in both adipose tissue and adipose microvessels. In separate studies, IL-1β, IL-6, MCP-1, TNF-α, IL-10, FGF-2, VEGF, and PDGF production were measured from adipose tissue (n = 5), adipocytes (n = 5), and preadipocytes (n = 3) supernatants with and without thrombin (1 or 10 U/ml; 24 hrs) treatment.

Results

Thrombin increased cytokine secretion of IL-1β, IL-6, MCP-1 and TNF-α and growth factor secretion of VEGF from adipocytes along with MCP-1 and VEGF from preadipocytes. The direct thrombin inhibitor lepirudin given in conjunction with thrombin prevented the thrombin-mediated increase in cytokine and growth factor secretion.

Conclusion

Here we show that thrombin PAR1 and PAR4 receptors are present and that thrombin stimulates inflammatory cytokine generation and growth factor release in human adipose tissue and cells in vitro. These data suggest that thrombin may represent a molecular link between obesity and associated inflammation.

Secretion without Golgi

A growing number of proteins devoid of signal peptides have been demonstrated to be released through the non-classical pathways independent of endoplasmic reticulum and Golgi. Among them are two potent proangiogenic cytokines FGF1 and IL1alpha. Stress-induced transmembrane translocation of these proteins requires the assembly of copper-dependent multiprotein release complexes. It involves the interaction of exported proteins with the acidic phospholipids of the inner leaflet of the cell membrane and membrane destabilization. Not only stress, but also thrombin treatment and inhibition of Notch signaling stimulate the export of FGF1. Non-classical release of FGF1 and IL1alpha presents a promising target for treatment of cardiovascular, oncologic, and inflammatory disorders.