Inhibition of squamous cell carcinoma angiogenesis by direct interaction of retinoic acid with endothelial cells

Retinoic acid (RA) is a multifunctional drug that is particularly effective at preventing the development of multiple primary oral squamous cell carcinomas. A portion of this activity is due to the inhibition of tumor angiogenesis. It has been thought that RA influences tumor angiogenesis only via its interactions with the tumor cells themselves. Here, we test the hypothesis that the drug can also block neovascularization by directly inhibiting the angiogenic activity of normal endothelial cells. Clinically achievable doses of RA rapidly caused large- and small-vessel endothelial cells to become refractory to stimulation of migration either by tumor-conditioned media or purified angiogenic factors (a-fibroblast growth factor (aFGF), bFGF, vascular endothelial GF, platelet-derived GF, TGF beta-1, and IL-8). However, RA had little effect on their proliferation. Inhibition of migration was complete within 3 hours and was reversed 36 hours after drug removal. The migration of human oral keratinocytes was not sensitive to RA, whereas the migration of fibroblasts and vascular smooth muscle cells was inhibited. To determine if systemic RA affected neovascularization, rats were given 1 mg/kg/day of all-trans RA and their angiogenic potential was tested by implanting pellets of tumor-conditioned media into their avascular corneas. This treatment rendered the rats unable to mount a neovascular response in their corneas. These data demonstrate that RA directly affects endothelial cells, rapidly and reversibly inhibiting their ability to migrate toward a variety of stimuli in vitro and halting the formation of new vessels in vivo. These direct effects on vascular cells seem likely to contribute to the success of RA as a chemopreventive agent for oral squamous cell carcinoma.