Assessment of HIF-1α expression and release following endothelial injury in-vitro and in-vivo

Background: Endothelial injury is an early and enduring feature of cardiovascular disease. Inflammation and hypoxia may be responsible for this, and are often associated with the up-regulation of several transcriptional factors that include Hypoxia Inducible Factor-1 (HIF-1). Although it has been reported that HIF-1 alpha is detectable in plasma, it is known to be unstable. Our aim was to optimize an assay for HIF-1 alpha to be applied to in vitro and in vivo applications, and to use this assay to assess the release kinetics of HIF-1 alpha following endothelial injury. Methods: An ELISA for the measurement of HIF-1 alpha in cell-culture medium and plasma was optimized, and the assay was used to determine the best conditions for sample collection and storage. The results of the ELISA were validated using Western blotting and immunohistochemistry (IHC). In vitro, a standardized injury was produced in a monolayer of rat aortic endothelial cells (RAECs) and intracellular HIF-1 alpha was measured at intervals over 24 h. In vivo, a rat angioplasty model was used. The right carotid artery was injured using a 2F Fogarty balloon catheter. HIF-1 alpha was measured in the plasma and in the arterial tissue (0, 1, 2, 3 and 5 days post injury). Results: The HIF-1 alpha ELISA had a limit of detection of 2.7 pg/mL and was linear up to 1000 pg/ mL. Between and within-assay, the coefficient of variation values were less than 15%. HIF-1 alpha was unstable in cell lysates and plasma, and it was necessary to add a protease inhibitor immediately after collection, and to store samples at -80 degrees C prior to analysis. The dynamics of HIF-1 alpha release were different for the in vitro and in vivo models. In vitro, HIF-1 alpha reached maximum concentrations approximately 2 h post injury, whereas peak values in plasma and tissues occurred approximately 2 days post injury, in the balloon injury model. Conclusion: HIF-1 alpha can be measured in plasma, but this requires careful sample collection and storage. The carotid artery balloon injury model is associated with the transient release of HIF-1 alpha into the circulation that probably reflects the hypoxia induced in the artery wall.