Differentiation of retinal pigment epithelial cells in vitro uncovers silencer activity in the FGF-5 gene promoter

Differentiated retinal pigment epithelial (RPE) cells in vive express basal levels of FGF-5, a secreted member of the FGF gene family. RPE cells proliferate in response to pathological events, resulting in a transient increase in FGF-5 gene expression. The goal of this study is to identify cis- acting sequences in the FGF-5 gene promoter which upregulate FGF-5 gene expression when differentiated RPE cells enter the cell cycle and proliferate. In vitro cultures of RPE cells were transfected with various FGF-5 promoter/luciferase deletion constructs, using methods specifically optimized for proliferating and differentiated RPE cells. A proximal promoter/enhancer whose activity is not cell-context dependent was identified between FGF-5 sequences - 314 and +48. In addition, a silencer element (- 1256/-883) was identified in the distal region which is active only in differentiated RPE cells. When tested in a heterologous system, the same element had silencer activity in differentiated cells. Two small regions in the distal FGF-5 gene promoter, -1195/-1173 and -984/-967 were able to specifically bind to nuclear proteins from differentiated RPE cells but not from proliferating RPE cells as evidenced by gel mobility shift assays. Therefore, FGF-5 gene expression in the RPE may be regulated by the formation of differentiation-specific complexes.