PARACRINE EFFECTS OF PHOSPHORYLATED AND EXCRETED FGF1 BY RETINAL PIGMENTED EPITHELIAL-CELLS

We have recently shown that both inhibition of endogenous Fibroblast g rowth factor (FGF) synthesis in non dividing lens epithelial cells (Re naud et al. J. Biol. Chem 1996, 271 : 2801-2811) and inhibition of sec reted FGF1 in confluent quiescent retinal pigmented epithelial (RPE) c ells (Guillonneau et al., Exp. Cell. Res. 1997, in press) induce rapid cell apoptosis. In addition, FGF2-stimulated release of endogenous FG F1 is associated with reduced apoptosis in RPE cells. We now show that a single addition of exogenous FGF2 to RPE cells induces after 4 days of culture, a great accumulation of FGF1 within the cells. Concomitan tly we observe that FGF1 was released into the extracellular medium. S ecreted FGF1 from RPE cells, purified from culture medium and added to either Go-arrested RPE or RMG cells at low plating density induced ce ll proliferation, whereas when it is added once to serum-depleted conf luent RPE and RMG cells it prevented apoptosis. Both endogenous and se creted FGF1 are phosphorylated. In addition, FGF2 stimulated the produ ction and release of phosphorylated FGF1 by RPE cells. We show that th is secreted form of phosphorylated FGF1 binds to the high affinity tyr osine kinase receptors of RPE and RMG cells on retinal sections and to heparan sulfate proteoglycan in RPE cell extracellular matrix. In con trast to non-phosphorylated FGF1, phosphorylated secreted FGF1 was not degraded after internalization but accumulated within RPE and RMG cel ls, and is rapidly translocated to the nucleus suggesting a role in si gnal transduction and gene expression pathways. These results show tha t exogenous FGF2 activities might be mediated indirectly by phosphoryl ation and that secretion of FGF1 may function as a paracrine trophic f actor for retinal cells.