Insulin and IGF-I affect the protein composition of the lens fibre cell with possible consequences for cataract

Explanted newborn rat lens epithelial cells were cultured with various concentrations of EGF-2 and/or insulin or IGF-I for 8-20 days. The accumulation of alpha A-, alpha beta-, beta A3/1-, beta B2- and gamma A-F-crystallin was measured. During culture with insulin only, i.e, in the absence of fibre cell differentiation, alpha A- and alpha beta-crystallin accumulated to the same level as found in differentiating cells. Culture of epithelial cells with IGF-T led to an increase in alpha beta-crystallin, but not in alpha beta-crystallin. The addition of insulin under differentiation conditions (in the presence of 25 ng ml(-1) EGF-2) augmented the accumulation of alpha A-crystallin 1.5-fold, the accumulation of beta B2-crystallin two-fold and the accumulation of gamma A-F-crystallin five-fold over that found with FGF-2 only. The accumulation of alpha B- and beta A3/1-crystallin was not affected by insulin in the presence of FGF-2. Adding IGF-I to fibre cells differentiating in the presence of 25 ng ml(-1) FGF-2 resulted in a Ii-fold increase (of questionable statistical significance) in both alpha A- and alpha B-crystallin and a two to three-fold increase in gamma A-F-crystallin compared to cells cultured with FGE-2 only, no significant effect of IGF-I on the accumulation of beta A3/1- or beta B2-crystallin was found. Comparison of the levels of mRNA and protein suggests that insulin acts to increase the level of transcription. Our results show that the response of fibre cells to insulin or IGF-I differs. Hence, even though half the maximum dosage required for the insulin effect was rather high (between 0.1 and > 5 mu g), the effect of insulin cannot be merely transmitted by the IGF-I receptor. Our data further predict that insulin or IGF-I increases the overall ratio of beta- and gamma-crystallin to alpha-crystallin in the fibre cell, which could predispose the lens to cataract. (C) 2000 Academic Press.