Hypertension-induced inflammation: from cellular mechanisms to disease

The work presented in this thesis focused on high intraluminal pressure and its role in vascular inflammation. This was done in the context of hypertension and its contribution to the progression of atherosclerosis, the major underlying pathology in coronary artery disease. While a large body of work have explored the role of immune cells in the pathogenesis of hypertension, very few studies have investigated the opposite; whether hypertension plays a role in the aetiology of inflammation. Studies which have previously attempted to investigate the effect of high blood pressure and inflammation have been thwarted by the difficulty of trying to separate the consequences of the various neurohumoral factors associated with hypertension compared to those induced by the increase in pressure itself. This is particularly true of in vivo protocols. To address this, we developed a customised ex vivo vessel chamber that enabled the recording of leukocyte adhesion to the endothelium in real time, in intact vessels under various pressures. Using this technique, vessels exposed to high intraluminal pressure demonstrated greater leukocyte adhesion, adhesion molecule gene expression and endothelial microparticle production. Interestingly, we found this to be independent of the renin-angiotensin system. Several mechanisms involved in this process are presented in Chapters 4 & 5. In Chapter 4, the production of reactive oxygen species (ROS), endothelial nitric oxide synthase (eNOS), arginase, and the transcription factor, nuclear factor κB (NFκB), all known to be implicated in both hypertension and vascular inflammation, were explored as possible mechanisms. High intraluminal pressure was shown to increase ROS production, arginase II expression and activity as well as NFκB expression, while eNOS was unaltered by acute increases in pressure. Furthermore, functional studies indicated a possible role for NADPH oxidase and the mitochondria as the sources of the ROS mediating response. To tease out the mechanical forces responsible for the pressure-induced response, in Chapter 5, we observed the effect of different shear rates on leukocyte adhesion in pressurised vessels. High intraluminal pressure (circumferential stretch) together with low shear stress produced the greatest response, while high stress reduced the effect of high pressure. Caveolae, membrane invaginations that have been shown to mediate many intracellular signalling pathways, were explored as possible mechanosensors involved in pressure-induced inflammatory response. Cav1-/- (Caveolin-1) mice and Cav1 KD cells had reduced leukocyte adhesion and adhesion molecule gene expression in response to TNFα stimulation. These results suggest caveolae play an important role in the inflammatory response. Furthermore, when the effect of pressure on caveolae structure was further explored, we showed, for the first time, that high intraluminal pressure reduced caveolae number. We speculate this may be due to the dissociation of the caveolae proteins Cav1 and cavin-1. In Chapter 6, we examined the effect of hypertension on atherosclerosis in an in vivo setting. Hypertensive atherosclerotic prone BPHxApoe-/- placed on a high fat diet for 12 weeks demonstrated no change in plaque size. However, detrimental changes in plaque morphology with increased lipid and macrophage content and reduced collagen were observed suggestive of reduced plaque stability. We also demonstrated that blockade of P-selectin, which mediates recruitment of leukocytes to the endothelium, partially improved this stability. In conclusion, the studies described in this thesis provide evidence that high intraluminal pressure induces vascular inflammation. We demonstrated that the mechanical forces exerted by high pressure promote caveolae flattening, NADPH oxidase dependent ROS production, arginase II activation and NFκB translocation, which all contribute to endothelial activation, adhesion molecule expression and enhanced leukocyte adhesion, all hallmarks of inflammation. We also showed in vivo that hypertension may result in atherosclerotic plaque instability, which may be partially improved by blocking leukocyte recruitment. Therefore, this thesis has established fundamental concepts on how high intraluminal pressure can alter cellular biological responses that may lead to new paradigms in the management of the increased risk of cardiovascular complications seen in hypertensive individuals.