Differential binding of the transcription factors Sp1, AP-1, and NFI to the promoter of the human alpha5 integrin gene dictates its transcriptional activity.

PURPOSE Damage to the corneal epithelium results in the massive secretion of fibronectin (FN) shortly after injury and induces the expression of its integrin receptor alpha5beta1. The authors reported previously that FN induces alpha5 expression in human corneal epithelial cells and rabbit corneal epithelial cells by altering the binding of the transcription factor (TF) Sp1 to a regulatory element from the alpha5 promoter that it is also flanked by binding sites for the TFs NFI and AP-1. Here, they assessed the function of NFI and AP-1 on alpha5 gene expression and evaluated the contribution of FN to their overall regulatory influence. METHODS TF binding to the alpha5 promoter was evaluated in vitro by electrophoretic mobility shift assays and in vivo by ligation-mediated PCR or chromatin immunoprecipitation. TFs expression was monitored by Western blot, whereas their influence was assessed by transfection and RNAi analyses. RESULTS Coexpression of Sp1, NFI, and AP-1 was demonstrated in all cell types, and each TF was shown to bind efficiently to the alpha5 promoter. Whereas both AP-1 and Sp1 activated expression directed by the alpha5 promoter, NFI functioned as a potent repressor of that gene. Interestingly, FN could either promote or repress alpha5 promoter activity in a cell density-dependent manner by differentially altering the ratio of these TFs. CONCLUSIONS These results suggest that alpha5 gene expression is likely dictated by subtle alterations in the nuclear ratio of TFs that either repress (NFI) or activate (Sp1 and AP-1) alpha5 transcription in corneal epithelial cells.