Fig. 6. Mechanism by retinoic acid regulates endothelial cell cycle progression. Total protein was isolated from control or RA-treated endothelial cells, immunoprecipitated with antibodies against Cdk4, p21 or p27, and subjected to western analyses. Complex formation between cyclin D1 or D2 and Cdk4 was significantly reduced in RA-treated endothelial cells, whereas complex formation between RA-induced p21 and Cdk4, as well as cyclins D1 and D2, was greatly enhanced (A). There were lower levels of phosphorylated Rb protein in RA-treated endothelial cells, compared with controls (B).