Insulin-like growth factor I and insulin regulate delta-crystallin gene expression in developing lens

Normal development of the chicken embryo requires insulin and insulin receptors. Insulin and also insulin-like growth factor I (IGF-I) can stimulate embryonic growth when applied in vivo at the beginning of organogenesis (Girbau, M., Gomez, J. A., Lesniak, M. A., and De Pablo, F. (1987) Endocrinology 121, 1477-1482). In the present work we chose the developing eye lens, an avascular organ composed of a single cell type, to characterize further the specific effects of insulin and IGF-I upon cell differentiation and gene expression. Epithelial cells (before terminal differentiation) and fiber cells (terminally differentiated) were cultured in the presence of the hormones. delta-Crystallin mRNA steady-state levels as well as nuclear delta-crystallin gene transcription were measured. Either insulin or IGF-I (0.1-10 ng/ml) increased (2-4-fold) delta-crystallin mRNA in epithelial and fiber lens cells from day 6 embryos. The effect of insulin was largely blocked by the Fab fragment of anti-insulin receptor antibody (B-10). By contrast, as it had been shown for metabolic actions in other systems, bivalent B-10 IgG itself mimicked insulin action, i.e. it induced an increase on delta-crystallin mRNA levels. Thus, insulin appears to act through its own receptor in regulating the levels of delta-crystallin mRNA. There was a differential transcriptional component in insulin and IGF-I effects on delta-crystallin gene expression. IGF-I induction of transcription, as measured by nuclear run-on assay, is greater than insulin induction (approximately 2.5-fold versus 1.4-fold) and faster. The delta-crystallin gene will provide the opportunity to analyze the action of insulin and IGF-I on the expression of a structural protein marker of cell differentiation during early embryonic development.