DIFFERENTIAL REGULATION OF INDUCIBLE NITRIC-OXIDE SYNTHASE BY FIBROBLAST GROWTH-FACTORS AND TRANSFORMING GROWTH-FACTOR-BETA IN BOVINE RETINAL PIGMENTED EPITHELIAL-CELLS - INVERSE CORRELATION WITH CELLULAR PROLIFERATION

Bovine retinal pigmented epithelial (RPE) cells express, after activat ion with interferon gamma (IFN-gamma) and lipopolysaccharide (LPS), an inducible nitric oxide synthase (NOS). Experiments were done to inves tigate the effects of the transforming growth factor beta1, epidermal growth factor, and fibroblast growth factors (FGFs), which are abundan t in the retina, on NOS activity. Transforming growth factor beta1 sli ghtly increases the production of nitrite, an oxidation product of NO, induced by LPS plus IFN-gamma, whereas acidic and basic FGFs markedly inhibit the nitrite release due to LPS/IFN-gamma in a concentration-d ependent manner, and epidermal growth factor did not modify LPS/IFN-ga mma-induced NOS activity. The growth factors alone did not stimulate n itrite release. We have attempted to elucidate the mechanism of FGF in hibition. Results with heparin, suramin, and tyrphostin suggest involv ement of the high-affinity receptor for FGF in its inhibition of LPS/I FN-gamma-stimulated NOS activity. Continued stimulation of RPE cells w ith LPS/IFN-gamma was essential for the induction of NO synthesis, and maximal inhibition was obtained when FGF was present during stimulati on with LPS/IFN-gamma, suggesting that FGF inhibits NOS induction. Fur thermore, an antiproliferative action of NO was demonstrated by an inv erse correlation between the amounts of nitrite or citrulline produced in response to different stimuli (LPS/IFN-gamma or LPS/IFN-gamma with growth factors) and the level of cellular proliferation. Similar inhi bition of growth was obtained when RPE cells were incubated with an NO donor, sydnonimide. Because NO acts as a cytotoxic compound in the re tina, FGF, by inhibiting the induction of NOS in RPE cells, may have b eneficial effects in protecting the retina from cytokine and endotoxin -mediated tissue damage.