No abstract

In biological fluids, the insulin-like growth factors (IGFs) are associated with binding proteins (IGFBPs), which modify IGF distribution and action. Circulating IGFs are bound predominantly to IGFBP-3, of apparent hepatic origin, but regulation of IGFBP-3 has been difficult to dissect because of the lack of systems suitable for examining hepatic production of IGFBP-3 in vitro. In the present studies, IGFBP-3 expression was identified primarily in hepatic nonparenchymal cells, particularly Kupffer and sinusoidal endothelial cells. Coculture with hepatocytes enhanced the stability of nonparenchymal cells to express IGFBP-3 in vitro. IGFBP-3 in conditioned medium had apparent mol wt of 150-300 kilodaltons, suggesting formation of a ternary complex with IGFs and the acid-labile subunit. Expression and secretion of IGFBP-3 were hormonally responsive and strongly correlated (r = 0.79; P < 0.001), with 2- to 3-fold stimulation by added insulin or IGF-I (both P < 0.05), but not by added GH alone. Our findings suggest that GH may act indirectly to promote IGFBP-3 generation in vivo via increasing both the secretion of insulin and the hepatic production of IGF-I; in patients with diabetes mellitus, reduced circulating levels of IGFBP-3 despite high levels of GH may result from both insulin deficiency and inadequate hepatic production of IGF-I. Coculture of hepatic nonparenchymal and parenchymal cells should be useful for further analysis of the mechanism of IGFBP-3 regulation.

Short stature and decreased growth velocity are prominent features of endogenous and pharmacological glucocorticoid excess in children. Underlying processes may involve direct cellular effects or defective generation of insulin-like growth factors (IGFs) and/or IGF-binding proteins (IGFBPs), which modulate IGF-stimulated events and regulate growth. To evaluate potential mechanisms, we investigated the impact of dexamethasone (dex) on hepatic expression of IGFBP-3, the major carrier protein for IGFs. Using cocultured hepatic parenchymal and nonparenchymal cells, dex at 10(-8) and 10(-6) M decreased IGFBP-3 secretion by 67 +/- 9% and 73 +/- 9%, respectively (both P < 0.05 vs. no dex). In a separate experiment, IGFBP-3 messenger RNA (mRNA) expression was decreased by 84 +/- 2% and 75 +/- 2% (both P < 0.05 vs. no dex). In combined studies, levels of IGFBP-3 protein in conditioned medium were strongly correlated with the abundance of IGFBP-3 mRNA (r = 0.75; P < 0.01), consistent with regulation at a pretranslational level. After inhibition of transcription, levels of IGFBP-3 mRNA decreased 85% and 86% over 24 h in cells cultured in 10(-6) M and no dex, respectively; the t1/2 was 13.6 h at 10(-6) M and 12.6 h with no dex, indicating that dex had no effect on IGFBP-3 mRNA stability. To evaluate transcriptional effects, the rate of IGFBP-3 gene transcription was measured by incorporation of [alpha-32P]UTP into preinitiated message in isolated nuclei and fell 78% after the addition of 10(-6) M dex for 48 h (compared to cells cultured in 10(-9) M dex), an inhibition of a magnitude similar to the effects on protein release and mRNA abundance. We conclude that dex may reduce the production of IGFBP-3 by inhibiting IGFBP-3 gene transcription.

One of the major mechanisms by which insulin modulates glucose homeostasis is through regulation of gene expression. Therefore, reduced expression of transcription factors that are required for insulin-regulated gene expression may contribute to insulin resistance. We recently identified insulin response element-binding protein-1 (IRE-BP1) as a transcription factor that binds and transactivates multiple insulin-responsive genes, but the regulation of IRE-BP1 in vivo is largely unknown. In this study, we show that IRE-BP1 interacts with the insulin response sequence of the IGF-I, IGFBP-1, and IGFBP-3 genes using chromatin immunoprecipitation assay. Furthermore, activation by IRE-BP1 is sequence specific and mimics that of the insulin effect on gene transcription. Tissue expression of IRE-BP1 is 50- to 200-fold higher in classical insulin target compared with nontarget tissues in lean animals, with a significantly reduced level of expression in the skeletal muscle and adipose tissue in obese and diabetic animals. In the liver, IRE-BP1 is localized to the nucleus in lean rats but is sequestered to the cytoplasm in obese and diabetic animals. Cytoplasmic sequestration appears to be related to inhibition of insulin-mediated phosphatidylinositol-3 kinase signaling. Therefore, in diabetes and obesity, the mechanisms involved in reducing the transactivation of the insulin response sequence by IRE-BP1 include decreased gene transcription and nuclear exclusion to prevent DNA binding. Our study supports the notion that IRE-BP1 may be relevant to the action of insulin in vivo and may play a role in the development of insulin resistance and diabetes.

Normal reproductive functioning may require secretion of LH independently of FSH. Variation in GnRH pulse frequency and inhibin negative feedback are mechanisms for differential gonadotropin regulation; however, the first instance of differential regulation in rats is during fetal development, prior to the establishment of GnRH connections, when LH accumulates appreciably 2-4 d prior to FSH. Pituitary adenylate cyclase activating polypeptide (PACAP) can differentially regulate the gonadotropins in vitro by stimulating alpha-subunit transcription, lengthening LHbeta transcripts and decreasing FSHbeta mRNA levels, probably through stimulation of follistatin transcription. These experiments are the first to examine whether PACAP influences gonadotroph function in perinatal pituitaries. In vivo, pituitary PACAP mRNA and peptide levels were high at embryonic d 19 and declined by 94 and 85%, respectively, after parturition. This was accompanied by a decrease of 65 and 96% in total follistatin and follistatin-288 mRNAs. These changes were temporally associated with a 20- and 6.5-fold rise in FSHbeta and GnRH receptor mRNAs, respectively, with no significant increase in LHbeta mRNA. In pituitary cell cultures from fetal and postnatal male rats, PACAP mRNA levels were likewise highest in fetal cultures in which the PACAP 6-38 antagonist decreased alpha-subunit and increased FSHbeta mRNA. PACAP 6-38 also reduced basal and GnRH-stimulated LH secretion with little effect on FSH. These data support the hypothesis that PACAP expressed at high levels in the fetal pituitary stimulates alpha-subunit expression and LH secretion and restrains FSH synthesis relative to LH and that a decline in PACAP allows for the neonatal rise in FSH and GnRH receptor because follistatin is decreased.