Differential activation of phosphatidylinositol 3-kinase signaling during proliferation and differentiation of lens epithelial cells.
OBJECTIVE
To investigate whether phosphatidylinositol 3-kinase (PI-3K) signaling is involved in lens epithelial cell proliferation and differentiation promoted by growth factors.


METHODS
Proliferation of rabbit lens epithelial cells grown in culture was measured with a DNA-binding fluorescent dye in a proliferation assay. Primary cultures of embryonic chicken lens epithelial cells that develop lentoids were used for differentiation-related studies, and delta-crystallin synthesis in these cultures was determined by metabolic labeling with [(35)S]methionine. Immunoprecipitation and immunoblot analyses were also used.


RESULTS
The PI-3K inhibitors wortmannin and LY294002 blocked the insulin-, insulin-like growth factor (IGF)-1-, and fibroblast growth factor (FGF)-2-promoted cell proliferation in rabbit lens epithelial cells. Inhibition of PI-3K activity by these inhibitors unexpectedly increased the synthesis of early differentiation marker protein delta-crystallin in chicken lens epithelial cells. Insulin and IGF-1 stimulated activation of PI-3K in proliferating and differentiating cultures. FGF-2 showed no direct effect on PI-3K activation. Platelet-derived growth factor (PDGF) did not induce significant proliferation or increased expression of delta-crystallin, but stimulated PI-3K. The presence of FGF-2 in proliferating rabbit lens epithelial cells enhanced the IGF-1-, but not the PDGF-mediated PI-3K activation, suggesting a possible integration of FGF-2 signals with IGF-1. Whereas there was a gradual decrease in insulin/IGF-1-mediated activation of PI-3K and its downstream target Akt, with progression of differentiation in chicken lens epithelial cells, Erk2 phosphorylation induced by these growth factors was not decreased; rather, it remained increased in early stages of differentiation.


CONCLUSIONS
The results reveal significant differences in the modulation of PI-3K signaling by different growth factors during proliferation in rabbit lens epithelial cells and differentiation in chicken lens epithelial cells and demonstrate that regulation of the PI-3K pathway plays a key role in these processes. A balance between the nonactivation of PI-3K and the activation of Erk2 may be necessary during early stages of epithelial cell transformation.
D000818:Animals; D015153:Blotting, Western; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology; D002455:Cell Division / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D002642:Chick Embryo; D002645:Chickens; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004847:Epithelial Cells / Q000166:cytology* / Q000201:enzymology*; D016222:Fibroblast Growth Factor 2 / Q000494:pharmacology; D007328:Insulin / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D007908:Lens, Crystalline / Q000166:cytology*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D011817:Rabbits; D015398:Signal Transduction / Q000502:physiology*; D038207:delta-Crystallins / Q000096:biosynthesis
D004791:Enzyme Inhibitors; D016222:Fibroblast Growth Factor 2; D007328:Insulin; D007334:Insulin-Like Growth Factor I; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D038207:delta-Crystallins