REGULATION BY INSULIN-LIKE GROWTH-FACTOR (ICF) BINDING-PROTEINS OF IGF-II-STIMULATED GLYCOGENESIS IN CULTURED FETAL-RAT HEPATOCYTES

We previously showed that when added to fresh medium, insulin-like gro wth factor (IGF)-II stimulates glycogen synthesis in cultured 18-day-o ld fetal rat hepatocytes. In the present study, we investigated the in fluence of 24-h culture-conditioned medium on IGF-II- and insulin-indu ced glycogenesis. The stimulatory effect of IGF-II (2.9-fold) on [C-14 ]glucose incorporation into glycogen over 3 h was dose dependently inh ibited by conditioned medium, whereas that of insulin (3.2-fold) was u naffected. Western ligand and immunoblot analysis of the conditioned m edia revealed IGF binding protein (IGFBP)-1, IGFBP-2, and IGFBP-4, wit h a predominance of IGFBP-1. IGFBP-3 was not detected. Preincubation o f conditioned medium with IGF-II for 4 h at 4 C restored the glycogeni c effect of newly added IGF-II. Preincubation of fresh medium with rec ombinant IGFBP-1 or IGFBP-3 in the presence of IGF-II suppressed IGF-I I stimulation of glycogen synthesis. IGFBPs alone had no effect on gly cogenesis. Des(l-G)IGF-II and R(6)IGF-II, structural analogs of IGF-II that have weak affinity for the IGFBPs, elicited maximal stimulation, near that of IGF-II (2.8-fold and 3.1-fold vs. 3.0-fold for IGF-II), whether tested in fresh or conditioned medium. The inhibitory effect o f conditioned medium on IGF-II-induced glycogenesis is therefore media ted by the IGFBPs via sequestration of IGF-II. This suggests that the IGFBPs, particularly IGFBP-1, produced by fetal rat hepatocytes in cul ture may play a role in regulating glycogenesis.