Spatial repression of PCNA by p53 during kidney development.

Transcriptional repression is a key mechanism for the spatial specification of gene expression and cell fate determination. During kidney development, proliferating cell nuclear antigen (PCNA) is expressed in the nephrogenic zone and is downregulated rapidly as renal epithelial cells enter terminal differentiation and acquire functional characteristics. Our laboratory reported that the transcription factor p53 stimulates the terminal differentiation of renal epithelial cells by means of transcriptional activation of renal function genes (Saifudeen Z, Dipp S, and El-Dahr SS. J Clin Invest 109: 1021-1030, 2002). Because p53-induced growth arrest correlates with downregulation of PCNA gene expression, we examined the impact of p53 inactivation on PCNA expression in mice and evaluated the effect of p53 on PCNA transcription. Immunohistochemistry revealed that the transition from nephrogenesis to terminal epithelial cell differentiation correlates with accumulation of the transcription factor p53. Importantly, the spatially restricted pattern of PCNA expression is disrupted in kidneys of p53-deficient pups, in which there was a redistribution of PCNA expression into the differentiation zone (without a change in total kidney PCNA content) and distortion of the tubular architecture. Electrophoretic mobility shift assays revealed that the binding of kidney nuclear extracts to the p53 response elements in human and rat PCNA promoters is developmentally regulated. Transient transfection assays performed in p53-deficient HeLa cells revealed that exogenous p53 strongly represses transcription from human PCNA promoter-reporter constructs. Interestingly, deletion of the p53-binding site confers enhanced responsiveness to p53-mediated repression, suggesting that transcriptional repression of PCNA by p53 is achieved by a mechanism other than direct DNA binding. On the basis of these results, we propose the hypothesis that p53-mediated transcriptional repression plays a role in the spatial restriction of PCNA gene expression during normal renal development.