The Role of the Cellular Adhesion Molecule F11R/JAM-A in Atherosclerosis

Abstract

Atherosclerosis, the greatest cause of death world-wide, is initiated by the adherence of platelets and leukocytes to an inflamed endothelium which ultimately results in the formation of atherosclerotic plaque. Rupture of these plaques may be complicated by intraluminal thrombus formation, which presents clinically as myocardial infarction, stroke and possibly death. The development of agents that prevent adhesion of platelets and leukocytes to an inflamed endothelium may prevent atherosclerotic plaque formation and the associated significant morbidity and mortality. Biomarkers able to distinguish stable from unstable plaques would be useful in determining risk and preventing events of myocardial infarction or stroke. Towards this end, we have focused on the F11R/JAM-A protein, which is recognized as a major adhesion molecule that plays an important role in platelet adhesion to cytokine activated endothelium. However, the exact functions of the F11 receptor (F11R/JAM-A)/Junctional Adhesion Molecule-A (JAM-A) in the initiation, development and stability of atherosclerotic plaques has yet to be fully elucidated. Data from our laboratory and that of others demonstrate that F11R/JAM-A contributes to the development and growth of atherosclerotic plaques through its ability to cause the adherence of platelets and leukocytes to an inflamed endothelium. Atherosclerotic plaque has increased expression of the F11R/JAM-A molecule in regions of plaque instability, regions shown to be vulnerable to rupture. Additionally, increasing levels of a soluble form of F11R/JAM-A (sF11R/JAM-A) have been shown to correlate with worsening coronary artery disease (CAD) score predicting serious illness and death. The hypothesis of this thesis is that the increased level of serum sF11R/JAM-A is due to an increased expression of F11R/JAM-A on the surface of inflamed endothelium and that expression of F11R/JAM-A on SMC in plaques is involved in SMC migration and proliferation. Data presented in this thesis indicate that there is increased expression of F11R/JAM-A mRNA and protein in the atherosclerotic plaques of arteries obtained from human coronary artery disease (CAD) patients as well as in the arteries obtained from atherosclerotic-prone ApoE deficient mice. The significantly increased expression of F11R/JAM-A has been localized to specific regions of the blood vessel at sites containing atherosclerotic plaques. One possible explanation for the increased levels of F11R/JAM-A found in atherosclerotic plaques is the upregulation of the F11R/JAM-A gene with enhanced expression of F11R/JAM-A on the luminal surface of inflamed endothelial cells, as well as other cells that comprise the region encompassing the atherosclerotic plaque. Cultured human endothelial cells obtained from the aorta and umbilical vein demonstrated an increased F11R/JAM-A mRNA level when treated with inflammatory cytokines, TNFα and INFγ to mimic vascular inflammatory conditions, thus indicating that the F11R/JAM-A gene is upregulated in endothelial cells in response to inflammatory cytokines. Experiments using siRNA technology to inhibit the expression of F11R/JAM-A determined inhibition in the expression of F11R/JAM-A results in a reduction in the ability of platelets to adhere to the inflamed endothelium, thus confirming that F11R/JAM-A has a critical role in platelet/endothelial cell interactions. The presence of F11R/JAM-A in atherosclerotic plaque of coronary arteries has been identified on SMC, macrophage/foam cells, platelets and EC. Results presented here are the first report of F11R/JAM-A expression on intimal SMC in atherosclerotic plaque of coronary arteries. In culture F11R/JAM-A expression was induced on HASMC under inflammatory conditions. Experiments utilizing F11R/JAM-A gene silencing found F11R/JAM-A to be involved in SMC migration and proliferation. The presence of F11R/JAM-A correlated with increases in β1 integrin levels in SMC which has been found to increase following F11R/JAM-A dimerization on epithelial cells. The presence of SMC in the cap of atherosclerotic plaque is important for plaque stability. Findings presented here support a role for F11R/JAM-A is involved in the migration and proliferation of SMC to form the cap. These studies helped to elucidate the mechanisms by which F11R/JAM-A influences plaque initiation and plaque stability. The clinical significance of these studies lies in the hypothesis that agents aimed at preventing increased F11R/JAM-A expression on the surface of inflamed endothelium or to prevent adhesion of platelets and leukocytes to inflamed endothelium should be developed to prevent atherosclerotic plaque formation. F11R/JAM-A may also be developed as an agent to promote plaque stability and prevention and treatment of restenosis. Additionally, the sF11R/JAM-A biomarker may be developed as a diagnostic tool to indicate plaque stability.