Role of VEGF-A. (A) Both 5-FU and Ang-1 induce VEGF-A expression in wild-type mice. Wild-type mice received a single intravenous injection of AdAng-1 (10⁹ pfu), AdNull (10⁹ pfu), or 5-FU (250 mg/kg). Introduction of Ang-1 and myelosuppression induced by 5-FU were followed by a robust increase in plasma VEGF-A levels, with maximum levels detected at day 10 after treatment with Ang-1 or 5-FU (n = 3, P < .001 on day 10). After 20 days, at the time where steady-state hematopoiesis was reached, VEGF-A levels reverted to normal. Data are displayed as mean ± standard deviation. (B) Introduction of VEGF-A induces up-regulation of Tie2 expression in the BM vasculature. Tie2-LacZ animals were injected with a single tail-vein injection of AdVEGF-A (1.5 × 10⁸ pfu per injection, n = 4). At day 4 after injection, mice were killed and femurs obtained for histology. β-Gal staining showed exceptionally strong LacZ activity in the BM vasculature of treated animals, demonstrating that VEGF-A is a primary factor supporting the emergence of Tie2-positive endothelial cells.^53,54