Dual roles for proxl in the regulation of the chicken β3B1-crystallin promoter

Purpose. Lens fiber cell differentiation is marked by the onset of β3B1-crystallin expression and is controlled by the cooperative action of a set of transcription factors including Prox1, an atypical homeodomain protein. Previously, the authors reported that Prox1 directly interacts with the OL2 element found in the chicken βB1-crystallin basal promoter to activate the expression of this gene. Here they mapped the location of activating and repressing sequences of the full-length chicken βB1-crystallin promoter (-432/+30) in lens epithelial cells, annular pad cells, and intact lens and characterized Prox1- binding sites found in this region. Methods. Transfection analysis and transgenic mice were used to characterize upstream regions of the chicken βB1-crystallin gene. DNaseI footprinting and chromatin immunoprecipitation was performed to identify Prox1-binding sites, and transfection analyses were used to characterize these sites functionally. Results. Sequences between - 152 and - 432 of the chicken βB1-crystallin promoter mediated either promoter activation or repression, depending on the stage of lens differentiation tested. Two new Prox1-binding sites were found in this region that bound Prox1 more avidly than the OL2 element. However, neither binding site conferred Prox1-mediated activation on a heterologous promoter; instead, each allowed Prox1 to repress promoter function. Conclusions. The function of the upstream region of the chicken βB1-crystallin promoter changes depending on cellular context. These data suggest that Prox1 function as a transcriptional activator could be regulated at the DNA level based on the characteristics of the responsive elements.