Mechanisms of glucocorticoid-mediated inhibition of angiogenesis

Abstract

Angiogenesis, the formation of new blood vessels from pre-existing vascular beds, is a requirement of embryonic development but only occurs in a limited number of discrete processes in the healthy adult. In contrast, abnormal angiogenesis is central to many pathological processes including: tumour growth, diabetic retinopathy and arthritis. Consequently, pharmacological manipulation of angiogenesis has great clinical potential.Angiogenesis is inhibited by glucocorticoids and this is exploited clinically for the treatment of proliferating capillary haemangiomas in children. Despite this, the exact mechanism(s) through which glucocorticoids inhibit angiogenesis is (are) unknown. Whilst glucocorticoids can act directly on the vessel wall their effects on individual cell types and on molecular signalling remain unclear. The work in this thesis explores the hypothesis that glucocorticoid-mediated inhibition of angiogenesis is the result of direct modulation of growth factor signalling within the vascular endothelial cells.A well-characterised 2-dimensional in vitro model of human endothelial tube formation was introduced. Glucocorticoids were shown to inhibit tube formation in this model via stimulation of glucocorticoid receptors and this process was not influenced by intra-cellular glucocorticoid metabolism by ll(3-hydroxysteroid dehydrogenases. This demonstration that glucocorticoids inhibit angiogenesis by acting directly on the endothelium is consistent with, and extends, observations of glucocorticoid-mediated angiostasis in rodent aortic rings and during cutaneous wound healing. Molecular and biochemical assays suggested that glucocorticoids inhibit tube formation by altering the balance of pro- and anti-angiogenic factor activity. Time-lapse imaging of tube formation, combined with assays of endothelial cell migration and proliferation, indicated that glucocorticoids reduce tube formation, rather than accelerating degradation of existing tubes, by preventing morphological changes in the cells but do not inhibit cell division or migration.In conclusion, these studies demonstrate that glucocorticoids can inhibit angiogenesis by directly inhibiting morphological changes required for tube formation by endothelial cells but without altering migration or proliferation.