We previously demonstrated that progesterone (P4) up-regulated p53 expression， which in turn increased p21 and p27 expression， and finally resulted in proliferation inhibition in human umbilical vein endothelial cells (HUVEC). While a direct transcriptional activation of p21 by p53 protein has been clearly elucidated， the mechanism by which p53 induces p27 expression has not been documented. In this study， we identified three putative p53 protein binding domains at the p27 promoter. Luciferase assay showed that the activity of ectopically introduced p27 promoter constructs containing the potential p53 protein binding region was significantly increased by P4. Immunoblotting analysis indicated that P4 increased the level of p53 protein. Treatment with pifithrin-α-HBr (PFTα)， a specific blocker of p53-responsive gene transactivation， reduced the P4-increased p27 promoter activity and p27 protein expression. Transfection with dominant-negative mutants of p53 (C135Y， R175H and R248 W) abolished the P4-increased p27 promoter activity. Moreover， deletion or TCCT nucleotide sequence fill-in at the core site of any of p53 protein binding domains led to the irresponsiveness of the p27 promoter to P4 treatment. Interestingly， immunoprecipitation and chromatin-immunoprecipitation analyses demonstrated that P4 increased the complex of p53-P4 receptor (PR) protein in the nucleus and the assembly of PR protein to the p53 protein binding region of the p27 promoter. Ectopic co-overexpression of p53 and PR-A constructs further augmented the P4-increased p27 promoter activity. Taken together， the results from the present study suggest that P4-increased p53 expression might directly up-regulate p27 transactivation， and PR-A protein might promote this effect by forming complex with p53 protein.