The Acute Phase Reactant Orosomucoid-1 Is a Bimodal Regulator of Angiogenesis with Time- and Context-Dependent Inhibitory and Stimulatory Properties

Background: Tissues respond to injury by releasing acute phase reaction (APR) proteins which regulate inflammation and angiogenesis. Among the genes upregulated in wounded tissues are tumor necrosis factor-alpha (TNF alpha) and the acute phase reactant orosomucoid-1 (ORM1). ORM1 has been shown to modulate the response of immune cells to TNF alpha, but its role on injury- and TNF alpha-induced angiogenesis has not been investigated. This study was designed to characterize the role of ORM1 in the angiogenic response to injury and TNF alpha. Methods and Results: Angiogenesis was studied with in vitro, ex vivo, and in vivo angiogenesis assays. Injured rat aortic rings cultured in collagen gels produced an angiogenic response driven by macrophage-derived TNF alpha. Microarray analysis and qRT-PCR showed that TNF alpha and ORM1 were upregulated prior to angiogenic sprouting. Exogenous ORM1 delayed the angiogenic response to injury and inhibited the proangiogenic effect of TNF alpha in cultures of aortic rings or isolated endothelial cells, but stimulated aortic angiogenesis over time while promoting VEGF production and activity. ORM1 inhibited injury- and TNF alpha-induced phosphorylation of MEK1/2 and p38 MAPK in aortic rings, but not of NF kappa B. This effect was injury/TNF alpha-specific since ORM1 did not inhibit VEGF-induced signaling, and cell-specific since ORM1 inhibited TNF alpha-induced phosphorylation of MEK1/2 and p38 MAPK in macrophages and endothelial cells, but not mural cells. Experiments with specific inhibitors demonstrated that the MEK/ERK pathway was required for angiogenesis. ORM1 inhibited angiogenesis in a subcutaneous in vivo assay of aortic ring-induced angiogenesis, but stimulated developmental angiogenesis in the chorioallantoic membrane (CAM) assay. Conclusion: ORM1 regulates injury-induced angiogenesis in a time- and context-dependent manner by sequentially dampening the initial TNF alpha-induced angiogenic response and promoting the downstream stimulation of the angiogenic process by VEGF. The context-dependent nature of ORM1 angioregulatory function is further demonstrated in the CAM assay where ORM1 stimulates developmental angiogenesis without exerting any inhibitory activity.