Abstract
Purpose:
Fibroblast growth factors (FGFs) are well established as primary inducers of fiber differentiation. In vitro studies have shown that a relatively high dose of FGF, comparable to vitreous humor, can induce lens fiber differentiation, as can a lower dose of FGF when combined with other ocular-derived growth factors, including platelet-derived growth factor (PDGF), insulin-like growth factor 1 (IGF1) or epidermal growth factor (EGF). In the lens, these ‘co-growth factors’ act primarily as mitogens, and here we attempt to better characterise the role they play in lens fiber differentiation.
Methods:
Rat lens epithelial explants were cultured with either bovine vitreous (v/v 50%), a high dose of FGF2 (100 ng/ml), IGF1 (50 ng/ml), EGF (5 ng/ml) or a combination of IGF1 and EGF (IGF1/EGF) for up to 5-days. A selective FGF receptor (FGFR) antagonist, SU5402, was used for inhibitory studies. Immunolabeling was used to localize fiber differentiation markers including β-crystallin and Prox1, as well as downstream intracellular signaling molecules including the phosphorylation of fibroblast receptor substrate 2 alpha (FR2Sα). Explants were also stained with periodic acid-Schiff to examine cell morphology.
Results:
IGF1 or EGF both stimulated lens cell proliferation; however, in combination, IGF1 and EGF synergistically induced a lens fiber differentiation response, with the development of multicellular lentoid bodies expressing β-crystallin and Prox1, similar to features of vitreous- and FGF-induced lens fiber differentiation. Most interestingly, IGF1/EGF-induced lens fiber differentiation was dependent on FGFR-signaling, with downstream phosphorylation of FR2Sα. This IGF1/EGF-fiber differentiation response was also inhibited in the presence of SU5402.
Conclusions:
IGF1/EGF-induced lens fiber differentiation appears to directly involve FGFR-mediated signaling. This finding supports an essential requirement for FGFR-signaling in lens fiber differentiation, and highlights a putative novel mechanism for the activation of FGFR-signaling via other independent ocular growth factors, such as IGF1 and EGF.