Baicalein potently suppresses angiogenesis induced by vascular endothelial growth factor through the p53/Rb signaling pathway leading to G1/S cell cycle arrest.

Vascular endothelial growth factor (VEGF) is a key modulator of angiogenesis. Recent studies have shown that VEGF stimulates endothelial cell growth and modulates the cell cycle by reactivation of G0 cells and by reducing the duration of the G1 phase. This study examined the effect of baicalein, a well-known flavonoid, on VEGF-induced angiogenesis and further investigated the role of cell cycle regulators on the antiangiogenic effects of baicalein. Classic in vivo and in vitro models, including a rat aortic ring model, a wound healing model and a tube formation model were used to evaluate angiogenesis in vivo and in vitro. Baicalein exerted marked inhibition of angiogenesis, significantly inhibited migration of human umbilical vein endothelial cells (HUVECs), suppressed tube formation and reduced new blood vessel growth inducted by VEGF. Baicalein reduced phosphorylation of VEGF receptor 2 and extracellular signal-regulated protein kinase, two major signaling elements modulating endothelial cell proliferation. Baicalein also inhibited colony formation by HUVECs, further confirming the suppression of proliferation. Cell cycle analysis demonstrated that baicalein-treated HUVECs were arrested in the G1/S phase. Baicalein also induced a decline in the expression of G1-related proteins that normally promote transition from the G1 phase to the S phase, including cyclin D, cyclin E, cdk-4, cdk-6 and p-Rb. In contrast, several proteins upstream of cdks and cyclins, including p16, p21, p27 and p53, were up-regulated by baicalein, indicating that baicalein may inhibit angiogenesis, at least in part, by effects on the p53/Rb signaling pathway. Baicalein could exert antitumor effects by inhibiting VEGF-induced angiogenesis and endothelial cell proliferation.