Evaluation of fibroblast growth factor signaling during lens fiber cell differentiation.

PURPOSE Previous studies have implicated members of the fibroblast growth factor (FGF) and insulin-like growth factor (IGF) families as stimulators of lens fiber cell differentiation in rodent and chicken embryo lenses, respectively. In the present study, the role of FGFs in fiber cell differentiation and epithelial cell proliferation in chicken embryos was examined. METHODS Lenses were injected on embryonic day (E)3 with replication-defective retroviruses that express full-length or truncated FGF receptor (FGFR)-1 or a secreted form of FGF1. Lens epithelial explants were cultured in defined medium or medium supplemented with FGFs or vitreous humor, in the presence or absence of the FGF receptor antagonist SU5402. Explants were also cultured in vitreous humor that had been depleted of heparin-binding growth factors. Cell elongation was measured optically and protein accumulation by densitometry and Western blot analysis. RESULTS Lens fiber cell differentiation was not inhibited in cells infected with virus expressing truncated FGFR1. Epithelial cells infected with virus encoding a secreted form of FGF1 did not differentiate into ectopic fiber cells. Viral transduction of FGFR1, truncated FGFR1, or FGF1 did not appreciably alter the proliferation of lens epithelial cells. Bovine vitreous humor stimulated chicken embryo lens epithelial cells to elongate and express markers of lens fiber cell differentiation. Bovine vitreous humor, but not FGF2, protected lens epithelial cells from apoptosis. Depleting vitreous humor of heparin-binding growth factors or treatment of lens cells with SU5402 did not inhibit the initial, rapid phase of lens cell elongation. Both treatments, used separately or together, reduced but did not prevent the expression of later markers of fiber cell differentiation. CONCLUSIONS Fiber differentiation factors that are not members of the FGF family are present in chicken and mammalian vitreous humor. The factors that stimulate fiber cell differentiation in avian and mammalian eyes are similar.