Differential Activation of MAPK/ERK during FGF- and IGF-induced Lens Cell Proliferation

Abstract

Abstract: : Purpose: The vertebrate lens grows throughout life via the proliferation of epithelial cells and their subsequent differentiation into elongated lens fibres. The tight regulation of lens cell proliferation is critical for proper lens functioning and is controlled by the presence of mitogens in the ocular media including FGF, PDGF and IGF. The endogenous mitogen(s) that regulate lens cell proliferation in vivo are yet to be elucidated. Recent studies from our laboratory have shown a role for the extracellular signal regulated kinase (ERK) pathway in FGF-induced lens cell proliferation (Lovicu and McAvoy, 2001. Development). To further understand the signalling pathways downstream of lens mitogens, the present study aimed to characterise the pattern of phosphorylated ERK in IGF- and FGF-induced lens cell proliferation. Methods: Rat lens epithelial explants were treated with either IGF-1 (50ng/ml) or FGF-2 (5ng /ml) from 5mins up to 48hrs. Cell proliferation was assayed using immunolabelling for incorporation of BrdU. Activation of ERK was assayed using immunofluorescence and Western blotting. Blocking experiments were performed using UO126 (a specific inhibitor of ERK activation). Results: IGF was shown to induce cell proliferation within 24hrs whereas FGF only induced comparable levels of cell proliferation after 48hrs. This difference can be correlated with the differences in the mode that these growth factors activate ERK. Immunolabelling revealed that IGF induced ERK activation within 5mins, with maximal levels attained within 10 to 20mins in culture. Exposure of lens cells to IGF for 10mins was sufficient to induce comparable levels of cell proliferation to that of cells exposed to IGF for up to 24hrs. FGF did not induce ERK activation within 5 mins, however did induce maximal ERK activation by 10 to 20mins, which was sustained for up to 4hrs. In contrast to IGF, a short-term pulse of FGF was insufficient to induce cell proliferation after 24 or 48hrs. Blocking experiments using UO126 demonstrated that both FGF- and IGF-induced cell proliferation is dependent on the activation of ERK. Conclusions: Characterising the differences in patterns of ERK phosphorylation elicited by these growth factors may facilitate the identification of the endogenous mitogens responsible for lens cell proliferation in vivo.

Keywords: signal transduction • growth factors/growth factor receptors • proliferation