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IGFBP-1 and IGFBP-2 mRNAs are increased in the livers of streptozotocin-diabetic rats. A corresponding increase is observed in transcription of the IGFBP-1 but not the IGFBP-2 gene, indicating that the increase in steady-state levels of IGFBP-2 mRNA is a post-transcriptional effect. IGFBP-1 and IGFBP-2 mRNAs also differ in the rapidity of their response to insulin treatment: hepatic IGFBP-1 mRNA is normalized within 1 h, IGFBP-2 mRNA decreases more slowly. These differences suggest that IGFBP-2 may provide more chronic adaptation to metabolic change than IGFBP-1.

Insulin-like growth factor-binding protein-1 (IGFBP-1) can inhibit or potentiate IGF action. The biological activity of IGFBP-1 is determined by many factors, including its abundance in tissues and plasma, posttranslational modifications, and localization. IGFBP-1 levels in human plasma are highly regulated. They are increased after acute fasting and in diabetes, and are rapidly reversed by refeeding and insulin treatment, respectively. Similarly, IGFBP-1 mRNA is increased in the liver of severely diabetic and ketotic rats and decreased after 4 days of insulin treatment. Insulin rapidly decreases IGFBP-1 mRNA and IGFBP-1 transcription in rat hepatoma cells. The present study asks whether the increase in IGFBP-1 mRNA in diabetic rat liver reflects increased gene transcription, whether insulin decreases IGFBP-1 mRNA through a transcriptional or posttranscriptional mechanism, and whether this decrease is sufficiently rapid to account for the dynamic fluctuations in plasma IGFBP-1. Rats were injected ip with 100 mg/kg streptozotocin and used 7 days later when they were hyperglycemic and failed to gain weight, but were not ketotic. Hepatic IGFBP-1 mRNA levels were 13.6 +/- 5.3-fold greater in diabetic than control liver and decreased to the low levels in nondiabetic controls within 1 h after insulin treatment. In run-on transcription assays, IGFBP-1 transcription was 12.6 +/- 1.5-fold greater in nuclei from diabetic than control liver and decreased to low control levels by 1 h after insulin injection. Normalization of hepatic IGFBP-1 mRNA in insulin-treated diabetic animals did not require restoration of euglycemia. IGFBP-1 mRNA and IGFBP-1 gene transcription also were increased in the kidney of diabetic ketotic rats. We propose that the dynamic regulation of IGFBP-1 gene transcription in diabetes and after insulin treatment, by determining the availability of IGFBP-1 in tissues and plasma, may be a critical factor in the modulation of IGF action.

The insulin-like growth factor-binding proteins (IGFBPs) are thought to determine the distribution of IGF-I and IGF-II between the blood and tissue compartments and to modulate their biological activities. A dynamic metabolic role for one of the IGFBPs, IGFBP-1, is suggested by the fact that plasma IGFBP-1 was increased after fasting and diabetes and rapidly decreased by refeeding or insulin treatment, respectively. IGFBP-1 mRNA also is increased in the livers of diabetic rats and decreased by insulin treatment. To understand the molecular basis for this regulation, we have examined the effects of insulin on IGFBP-1 and IGFBP-1 mRNA in the H4-II-E cell line derived from the well differentiated H35 rat hepatoma. IGFBP-1, identified by ligand blotting and immunoblotting, is the major IGFBP in H4-II-E cells. Incubation of H4-II-E cells with insulin for 24 h decreased IGFBP-1 in the culture medium by approximately 50%. Inhibition was observed at physiological concentrations of insulin (ED50, less than 0.5 nM), but not at higher concentrations of IGF-II. These results, together with the fact that H4-II-E cells do not possess IGF-I receptors with which insulin might cross-react, suggest that insulin acts via the insulin receptor. Insulin inhibited IGFBP-1 in the medium by 80% in the absence of glucose, suggesting that the inhibition is a direct effect of insulin; glucose exerted a smaller independent effect in the absence of insulin. Insulin decreased IGFBP-1 mRNA in H4-II-E cells by 50% within 1 h and by 90% after 2-12 h of incubation. Nuclear run-on transcription assays indicated a corresponding decrease in the rate of IGFBP-1 gene transcription. Pretreatment of H4-II-E cells with dexamethasone stimulated IGFBP-1 transcription and increased steady state IGFBP-1 mRNA; stimulation was abolished by insulin treatment, indicating that inhibition by insulin was dominant over induction by dexamethasone. Thus, insulin, acting through the insulin receptor, rapidly decreases the abundance of IGFBP-1 mRNA in H4-II-E cells. Regulation occurs at least in part at the level of gene transcription. We propose that regulation of IGFBP-1 synthesis is an important component of the regulation of IGFBP-1 by insulin in vivo.

A synthetic insulin-like compound consisting of the A-chain of insulin extended at its carboxyl terminus with the hexapeptide "D-domain" of insulin-like Growth Factor II, linked via disulfide bonds to a B-chain corresponding to the "B-domain" of insulin-like Growth Factor I, has been examined for insulin-like metabolic activity and for mitogenic activity. The synthetic material (A27 insulin/BIGF-I) is less potent than insulin in metabolic assays, and less potent than both insulin and IGF-I in mitogenic assays. It is proposed that neither the "D-domain" nor the "B-domain" of the IGFs is a major contributor to mitogenic activity. Their presence in the same molecule does not result in significant growth-promoting activity.

Two synthetic insulin-like compounds consisting of the B-chain of insulin linked via disulfide bonds to A chains corresponding to the A-domain or the A- and D-domains of insulin-like growth factor I (IGF-I) have been evaluated for mitogenic activity and for binding to IGF receptors and IGF carrier proteins. Both compounds are 3- to 5-fold more potent mitogens than insulin, and have a comparably increased affinity for the type I IGF receptor that mediates these mitogenic effects in chick embryo fibroblasts. Neither compound interacts with IGF carrier proteins. These results indicate that the A-domain of IGF-I is importantly involved in its growth-promoting properties.