Regulation of Growth Factor Signalling in The Eye and Its Implications for Normal Lens Biology and Pathology

Fibroblast Growth Factor (FGF) is thought to control normal behaviour of lens cells and sustain lens growth by regulating the epithelial cell proliferation and fibre differentiation. FGF acts via receptor tyrosine kinases (RTKs) to induce ERK/MAPK signalling pathway. Transforming Growth Factor Beta (TGF) potently induces an epithelial-to-mesenchymal transition (EMT) that underlies fibrotic forms of cataract. TGF activates the Smads signalling, which is reported to interplay with ERK/MAPK signalling pathway. The ERK/MAPK signalling is tightly regulated to ensure the appropriate biological outcome, one level of negative regulation is thought to be achieved through RTK antagonists, namely, Sprouty (Spry), Spred and Sef that all expressed in the lens. In this study, we tested different negative regulators and compared their ability to block FGF-induced cell proliferation, fibre differentiation and TGF-induced EMT in rat lens epithelial cells. Rat lens epithelial cells were transfected with either a reporter vector (pLXSG), or with plasmids also coding for different RTK antagonists (i.e. pLXSG-Spry1, pLXSG-Spry2, pLXSG-Spred1, pLXSG-Spred2 and pLXSG-Spred3) and then treated with a differentiating dose of FGF or TGF. The percentage of transfected cells that underwent FGF-induced fibre differentiation were significantly reduced in the presence of RTK antagonists. Moreover, the percentage of cells that underwent TGF-induced EMT was markedly decreased with overexpression of different RTK-antagonists. In order to further investigate the inhibitory function of RTK antagonists, we established an alternative transfection system using recombinant adenoviral plasmids. We examined and compared different RTK inhibitors including Spry1, Spry2 and Spred2; and showed overexpression of Spry2 or Spred2 exhibited inhibitory effects on ERK phosphorylation under the stimulation of a proliferating dose of FGF, consistent with significant reduction in the percentages of epithelial cell proliferation. In addition, we indicated that Spry1, Spry2 and Spred2 were able to modulate the TGF signalling through the interaction between the ERK/MAPK signalling and Smad signalling pathways, thus the features of TGF-induced EMT were also attenuated