Increased ischemia-induced angiogenesis in the staggerer mouse, a mutant of the nuclear receptor Rorα

Ror alpha is an orphan nuclear receptor. In homozygous staggerer mutant mice (Rora(sg/sg)), a deletion within the Rora gene leads to an overexpression of inflammatory cytokines. Because inflammation and hypoxia are 2 key stimuli of ischemia-induced angiogenesis, we studied the role of Ror alpha in this setting. Ischemia was induced by ligation of the right femoral artery in C57BL/6 Rora(+/+) and Rora(sg/sg) mice. After 3 and 28 days, angiogenesis was evaluated by microangiography, measurement of capillary density using immunohistochemistry (anti-CD31), and measurement of blood flow by laser Doppler imaging. At day 3, angiographic score and blood flow were similar in Rora(sg/sg) mice and in Rora(+/+) littermates. Conversely, at day 28, Rora(sg/sg) mice showed a significant 2-fold increase in angiographic score and a 3-fold increase in capillary density within the ischemic hindlimb compared with control. Functionally, this coincided with a significant rise in leg perfusion in Rora(sg/sg) mice (0.83 +/-0.05 for ischemic/nonischemic leg perfusion ratio) compared with Ror(+/+) mice (0.66 +/-0.04, P <0.05). In addition, more extensive angiogenesis in Rora(sg/sg) mice correlated with an increased expression of eNOS protein by 83 +/- 12% and 71 +/- 24% at 3 and 28 days, respectively (P <0.05), whereas the level of the antiangiogenic cytokine IL-12 was significantly reduced by 38 +/- 10% at day 28 (P <0.05). Conversely, no changes in VEGF expression were observed. Our study identifies for the first time a new role for Rora as a potent negative regulator of ischemia-induced angiogenesis.