Growth factors involved in aqueous humour-induced lens cell proliferation

Abstract

Lens epithelial cell proliferation is regulated by growth factors in the aqueous humour of the eye. Although the lens fibre cell-differentiating factors are well defined, the factors in aqueous that promote lens cell proliferation are not. Mitogens present in aqueous primarily signal through the MAPK/ERK and PI3-K/Akt pathways. By characterising the signalling pathways involved in lens cell proliferation, we aim to identify the factors in aqueous that regulate this process in vivo. Using rat lens epithelial explants, 5'-2'-bromo-deoxyuridine and H(3)-thymidine incorporation were used to compare the effects of aqueous, insulin-like growth factor (IGF-1), platelet-derived growth factor (PDGF-A), epidermal growth factor (EGF) and fibroblast growth factor (FGF-2) on lens cell proliferation. Western blotting was employed to characterise ERK1/2 and Akt signalling induced by these mitogens. The above assays were also repeated in the presence of selective receptor inhibitors. Similar to aqueous, FGF induced a sustained ERK1/2 signalling profile (up to 6 h), unlike IGF, PDGF and EGF that induced a transient activation of ERK1/2. In the presence of a FGF receptor (FGFR) inhibitor, the sustained aqueous-induced ERK1/2 signalling profile was perturbed, resembling the transient IGF-, PDGF- or EGF-induced profile. In the presence of other growth factor receptor inhibitors, aqueous maintained its sustained, 6 h, ERK1/2 signalling profile, although ERK1/2 phosphorylation at earlier time periods was reduced. No one-specific receptor inhibitor could block aqueous-induced lens cell proliferation; however, combinations of inhibitors could, providing FGFR signalling was blocked. Multiple growth factors are likely to regulate lens cell proliferation in vivo, with a key role for FGF in aqueous-induced signalling and lens cell proliferation.

The lens growth process

“…In either case, proliferation requires activation of the ERK1/2 and PI3K/Akt signaling pathways (Iyengar et al, 2006). No single growth factor receptor inhibitor (including SU5402, an inhibitor of FGF receptors) is able to block completely the endogenous growth promoting activity of aqueous humor (Iyengar et al, 2009), suggesting that the control of cell proliferation in the lens epithelium may involve signaling by several mitogens, operating through multiple receptor tyrosine kinases.…”

Understanding the role of growth factors in embryonic development: insights from the lens

“…Adapted from Chen et al [42]. induced cell proliferation also appears to be due to a combination [134] of growth factor signals, of which FGF plays a central role. Analysis of aqueous-induced responses in lens epithelial explants typically showed induction of ERK phosphorylation for up to 6 h. This could be mimicked by a 'low' dose (5 ng ml 21 ) of FGF but not by PDGF, IGF or EGF, which only stimulated ERK phosphorylation for up to 1 h. By specifically blocking FGF, PDGF and IGF receptors, we assayed the contribution of each of these growth factors to the aqueous-induced ERK1/2 phosphorylation profile and showed there are at least two phases of ERK1/2 phosphorylation, with the early phase dependent on IGF and/or PDGF and the later phase dependent on FGF [134].…”

ERK1/2 signaling is required for the initiation but not progression of TGFβ-induced lens epithelial to mesenchymal transition (EMT)

“…ERK1/2 is a multifunctional signaling molecule, and in the lens it has been shown to be correlated with cell proliferation and differentiation, dependent on its duration of activity and intensity (Lovicu et al, 2001; Le & Musil, 2001). The lens epithelium is exposed to a multitude of aqueous-derived growth factors in situ such as FGF, IGF, PDGF and EGF, that when applied to lens epithelial explants in vitro are able to induce different ERK1/2 signaling profiles that can all lead to cell proliferation (Iyengar et al , 2007; Iyengar et al , 2009). RTK-antagonists, Spry, Spred and Sef, regulate ERK1/2 signaling and have been identified in the lens (Boros et al , 2006; Shin et al , 2012; Zhao et al , 2015).…”