Investigation of the mechanisms underlying the development of atherosclerosis.

It has become increasingly evident that low serum levels of testosterone experienced by aging males are associated with cardiovascular disease. Clinical trials have shown that testosterone replacement therapy (TRT) can improve symptoms of cardiovascular disease and reduce the inflammatory burden, evident in the early stages of atherosclerosis. Thus, a potential role of testosterone as an anti-inflammatory agent has emerged. The over-recruitment and activation of leukocytes characteristic of early atherosclerosis is considered the driving force behind atheroma development and is regulated by the concerted activities of several cytokines, chemokines and adhesion molecules, expressed by vascular endothelial and smooth muscle cells. The chemokines CCL2 and CX[3]CL1, which influence the migration of these leukocytes to sites of inflammation, have been implicated in disease progression. Whether testosterone has a modulatory effect on these orchestrating inflammatory molecules remains largely unknown.The aim of this thesis was to determine whether testosterone influences vascular inflammation as part of its beneficial effects on atherosclerosis, using a combined in vitro and in vivo approach. Primary human aortic endothelial and smooth muscle cells in vitro were investigated for CX[3]CL1 and CCL2 expression under pro-inflammatory conditions, as a model of vascular inflammation. Androgen treatment of these vascular cells, with or without AR blockade, was studied to determine potential anti-inflammatory effects. In addition, the testicular feminised (Tfm) mouse, which expresses low endogenous testosterone and a nonfunctional androgen receptor (AR), was used to assess the in vivo effect of androgen status upon atheroma formation, serum lipids and inflammatory mediators, to identify actions of testosterone on these pathways.Findings from this thesis confirmed that CX[3]CL1 and CCL2 are involved in vascular inflammation associated with atherosclerosis evident by the in vitro up-regulation by pro-inflammatory cytokines, and also that CX[3]CL1 was present, along with CX[3]CR1, in early fatty streaks in the aortic root of Tfm mice. A high-cholesterol diet was associated with increased fatty streak formation in the aortic root of wildtype mice, an effect that was significantly amplified by the low endogenous testosterone and a non-functional AR (in the Tfm mouse). Physiological TRT reduced fatty streak formation in the Tfm mouse but did not have significant effects on circulating cytokines. No significant modulation of CX[3]CL1 or CCL2 expression by testosterone treatment in vitro was observed, although further work is required to definitively confirm this finding.This thesis has demonstrated that physiological concentrations of testosterone can inhibit fatty streak formation, via an AR-independent mechanism in Tfm mice, although not through systemic or local anti-inflammatory actions. The mechanisms by which TRT may confer cardiovascular benefits to men with hypotestosteronemia and cardiovascular risk requires further investigation.