ABSTRACT
Both all-trans-retinoic acid (ATRA) and pigment epithelial-derived factor (PEDF) regulate cell proliferation and differentiation. Treatment of human Y-79 retinoblastoma and A-RPE 19 pigment epithelial cells with ATRA increased the levels of PEDF protein and RNA. Endothelial cells from bovine retina and human umbilical cord expressed PEDF and the levels were also increased by ATRA. Mouse Müller glial cells and rat C6 glioma cells showed at least a 2.5 fold increase in PEDF RNA levels after ATRA treatment, as measured by quantitative PCR. The PEDF promoter contains a retinoic acid receptor element (RARE). Plasmids containing a PEDF promoter regulating a luciferase gene were transfected into D407 and C6 cells and the luciferase activity measured after incubation in the presence or absence of ATRA. In both cell types ATRA increased the level of luciferase activity suggesting the RARE is functional. Dexamethasone was also effective at increasing PEDF RNA levels in both mouse Muller glial cells and C6 rat glioma cells. To test the effects of PEDF on retinoic acid function, expression of retinoic acid receptors in Y-79 and A-RPE 19 cells was measured by PCR. In Y79 cells, PEDF treatment increased the expression levels of RARalpha and RXRgamma receptors and in the A-RPE 19 cells it resulted in a decrease in expression of the RARbeta and RXRbeta receptors. This study clearly indicates an interaction between PEDF and ATRA. The cell differentiation activities of PEDF may operate through mechanisms orchestrated by retinoids, and the converse may also be true. The differentiation, anti-mitotic, and apoptotic actions of PEDF and ATRA may utilize parallel pathways that converge at key junctional transduction molecules to coordinate cellular quiescence and maintain tissue mass in the presence of signals that stimulate abnormal cell proliferation. It will be an interesting therapeutic strategy to co-administer PEDF and retinoic acid in developing protocols for neovascular diseases in the eye and in cancer.
