Expression of Adhesion Molecules

Oxidatively modified LDL up-regulates the surfece expression of VCAM-1 and intracellular adhesion molecule-1 (ICAM-1) in cultured endothelial cells, promoting the interactions between both cell types (Kume et al., 1992). This may play a pivotal role in the development of atherosclerosis by promoting the penetration of circulating monocytes into the suben-dothelial space whilst inhibiting the mobility of resident macrophages. It has been previously demonstrated that ICAM-1, E-selectin, and VCAM-1 are up-regulated in the microvasculature of rheumatoid but not control synovium (Corkill et al., 1991 Koch et al., 1991). The association between ox-LDL and increased expression of adhesion molecules in the inflamed synovium has yet to be studied. 

There is a long-standing hypothesis that the microvasculature plays a pathological role in forms of chronic inflammatory polyarthritis, particularly RA (Rothschild and Masi, 1982). One of the proposed mechanisms of vascular damage in connective tissue disease is the direct action of a cytotoxic serum factor inducing endothelial cell damage. Blake et al. (1985) have su ested that the vascular abnormalities associated with RA may be linked to oxidized lipoproteins because they are cytotoxic to endothelial cells. 

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Frequently, the EAR is followed by a late phase response 4-6 h later and it is caused by the pulmonary sequestration of eosinophils, neutrophils, mast cells, and T-lymphocytes. This leukocyte recruitment depends on mast cell-derived mediators such as TNFa and various chemokines, as well as on the expression of adhesion molecules on leukocytes (e.g. VLA-4, CD11/18) and vascular endothelial cells (e.g. VCAM-1, ICAM-1, E-selectin). Products of these leukocytes have several functions First, they cause the second phase of bron-choconstriction, mucus secretion, and airway swelling second, they cause tissue destruction third, they launch and entertain the chronic inflammation. 

The vascular endothelium plays an important role in regulation of vascular tone and permeability. Dilatation of arterioles to increase blood flow and constriction of endothelial cells of postcapillary venules causing exsudation of plasma constituents illustrates the complex nature of this cell type. Moreover, by expression of adhesion molecules and secretion of chemokines endothelial cells play an important role in the recruitment of leukocytes to the inflamed area. Endothelial cells express two basic types of adhesion molecules on their surface ... 

The cell-to-cell interaction following the expression of adhesion molecules (ICAM-1, VCAM-1 and selectin) in endothelial cells induced by cytokines treatment has been reported to be blocked by hydroflavones and flavanols. Apigenin, the most potent flavone tested in this study, inhibited the expression... 

In collaboration with PA Baeuerle (Freibuig), active NF-xB has been detected immunohistochemically in rheumatoid synovium by using a polyclonal antibody directed against the Rel-A NLS subunit of NF-xB (M.L. Kus, unpublished observations). The antibody employed in these studies was considered to be activity specific because IxB sterically masks the NLS sequence. NF-xB activation by ROM in the rheumatoid joint may orchestrate some of the chronic inflammatory processes characteristic of this disease. It is plausible that lL-1 and TNFo generated in the inflamed synovium as well as the up-regulated expression of adhesion molecules may be under the influence of NF-xB. 

Figure 22.2 Injury to endothelial cells, adhesion of monocytes and entry into subendothelial space. Injury to the cells by various factors activates genes for expression of adhesion molecules on the luminal surface of the cells. Monocytes attach to these molecules and then enter the subendothelial space. Here they are activated to form macrophages.

In chronic inflammatory disorders there is a general over-activation of the immune system, the cause of which is often unknown. In all of these diseases, an increased number of infiltrated immune cells can be found in the inflamed tissues. Furthermore, increased expression of adhesion molecules on the endothelium can often be observed, although the adhesion molecules are expressed differentially in the various diseases. 

Activation of endothelial cells leads to changes in endothehal ceU properties such as loss of vascular integrity, expression of adhesion molecules, antithrombotic to prothrombotic phenotype changes, cytokine production and the upregulation of HLA molecules. All these diverse effects can be attributed to the activation of transcription factors [44]. Of the presently known transcription factors, NFkB is believed to be one of the most important in the regulation of endothehal cell activation. After a stimulus at the cell surface which is caused by e.g. 

In most, if not all, chronic inflammatory diseases endothelial cells are prominently involved in the disease process. This is demonstrated by an increased expression of adhesion molecules and production of cytokines, and their pro-angiogenic behaviour. This leads to continuous recruitment of leucocytes into the inflamed area, without (detectable) antigen present in the affected tissue, resulting in a vicious circle of tissue damage and leucocyte recruitment. Targeting inhibitory agents (in)to the endothelial cell may interrupt in this process by controlling the activation status of this cell type. 

A more selective inhibition of NFkB can be achieved by transfecting cells with DNA coding for the natural inhibitor IkBo or a mutant IkB protein that lacks 36 N-terminal amino acids, and consequently becomes proteolysis resistant. In this way expression of adhesion molecules and monocyte adhesion and transmigration can be inhibited [87,88], The potentials and limitations of these latter types of therapy are however not fully understood as yet. Different transfection systems (adenoviral, retroviral, non-viral) are available for gene delivery purposes, all with their own potentials and restrictions. 

Inhibitors of IkBo phosphorylation have been described which irreversibly inhibit cytokine-induced phosphorylation without affecting constitutive phosphorylation. One such compound (Bay 11-7083 ((E)3-[4-f-butylphenyl)-sulfonyl]-2-propenenitrile)) was found to be effective in two animal models of inflammation after intraperitoneal administration [89]. In addition to the effect it has on the expression of adhesion molecules in pro-inflammatory responses, inhibition of the transcription factor NFkB will also have an effect on angiogenesis. Endothelial cells can produce growth factors and cytokines which have pro-angiogenic effects. Some of these factors, e.g. IL-8, TNFa and MCP-1 are known to be produced via NFkB-mediated endothelial cell activation [90,91]. The importance of NFKB-mediated responses in pro-angiogenic endothelium was reflected in studies in which the NFkB inhibitor PDTC decreased retinal neovascularization in the eye of mice [92]. 

A selective method of preventing the expression of adhesion molecules or cytokines is the use of antisense oligonucleotides. These oligonucleotides are short sequences of nucleic acids complementary to mRNA sequences of specific proteins of interest. If delivered to the cytoplasmic compartment of cells these oligonucleotides are able to form a complex with their target mRNA. In this way the translation of mRNA into protein by ribosomes is inhibited. The subsequent mRNA degradation by RNAse H results in reduced expression of the protein (see also Chapter 5 for a description of antisense ohgonucleotides as therapeutic modalities). 

Cytokines, including tumour necrosis factor (TNF) and interferon-y, favour the secretion of numerous chemokines and the expression of adhesion molecules by endothelial cells. The mechanisms of action of the principle drugs used in MS, and in priority beta interferons, are the following (1) inhibition of the expression of major histocompatibility complex class II molecules, (2) inhibition of metal-loproteases, (3) induction of immunosuppressor cytokines. 

Gupta, B. and Ghosh, B., Curcuma longa inhibits TNF-alpha induced expression of adhesion molecules on human umbilical vein endothelial cells, Int. J. Immunopharmacol, 21, 745, 1999. 

Kumar, A., Dhawan, S., Hardegen, N.J., and Aggarwal, B.B., Curcumin (diferuloylmethane) inhibition of tumor necrosis factor (TNF)-mediated adhesion of monocytes to endothelial cells by suppression of cell surface expression of adhesion molecules and of nuclear factor-KB activation, Biochem. Pharmacol, 55, 775, 1998. 

NO also reduces endothelial adhesion of monocytes and leukocytes, key features of the early development of atheromatous plaques. This effect is due to the inhibitory effect of NO on the expression of adhesion molecules on the endothelial surface. In addition, NO may act as an antioxidant, blocking the oxidation of low-density lipoproteins and thus preventing or reducing the formation of foam cells in the vascular wall. Plaque formation is also affected by NO-dependent reduction in endothelial cell permeability to lipoproteins. The importance of eNOS in cardiovascular disease is supported by experiments showing increased atherosclerosis in animals deficient in eNOS by pharmacologic inhibition. Atherosclerosis risk factors, such as smoking, hyperlipidemia, diabetes, and hypertension, are associated with decreased endothelial NO production, and thus enhance atherogenesis. 

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