A selective primary culture of fetal rat hepatocytes was established in our laboratory in order to elucidate the molecular mechanisms of action of different factors and conditions on insulin-like growth factor (IGF)-I and -II gene expression during the perinatal period of the rat. In this model we report that, in a serum-free condition and the presence of non-stimulatory doses of insulin, 5-20 mM glucose evoked an increase of IGF-I and -II mRNA abundance. Glucose regulated in a parallel manner IGF peptide secretion, and an excellent correlation was observed between IGF-I and -II mRNA and IGF-I and -II peptide levels in the conditioned media in response to the carbohydrate. The experiment with 2-deoxyglucose suggests that glucose 6-phosphate, but not its further metabolism, is necessary for the induction of IGF transcript abundance in cultured fetal hepatocytes. Finally, the glucose-induced rise in IGF-II mRNA, the main IGF in fetal stages, was mediated by stimulation of gene transcription and increased transcript stability. The results support the idea that IGFs belong to a family of genes that are positively regulated by glucose. Insulin-like growth factors (IGFs)1 -I and -II are cell growth regulators that originate largely in the liver (1). Hepatic production of IGFs appears to be regulated at pretranslational levels, as indicated by the strong correlations between circulating IGFs and abundance of hepatic IGF-I and -II mRNA (2-4). Secretion of both IGFs in adult animals is mainly regulated by growth hormone (GH), but nutritional status and serum insulin concentration are important factors involved in the regulation of IGF synthesis and secretion (1-4). Previous work "in vivo" has shown that a balanced insulin/nutrients ratio regulates IGFs secretion during the perinatal period of the rat, when the IGF response to GH is not yet well established (2). We have recently demonstrated that refeeding and insulin treatment of undernourished and diabetic neonatal rats, respectively, lead to a recovery of serum and liver mRNA expression of IGF-I and -II without a prior increase in serum GH (3). These in vivo experiments strongly suggest that during the perinatal period of the rat, IGFs regulation is GH-independent, supporting the role of insulin and nutrients. However, in experiments in vivo the simultaneous fluctuations of fuels and hormones that occur in diabetic and undernourished animals make it difficult to demonstrate specific regulation. It seemed appropriate, therefore, to investigate "in vitro" the underlying mechanisms for IGFs regulation. The in vitro system that most closely resembles normal developing liver is the primary culture of fetal hepatocytes (5-7). Since IGF expression in primary fetal cultures is limited and subject to plastic substratum-induced changes in the differentiation state of the liver cells, very scant data are available in the literature about the IGF response of fetal hepatocytes in culture to different conditions (8 -11). To overcome these difficulties, a selective primary culture of...

During perinatal development, insulin and nutrients, rather than GH, regulate the IGF system. A selective primary culture of fetal rat hepatocytes has been established in our laboratory to elucidate the molecular mechanism of action of the above regulatory factors on IGF-I and -II gene expression during the late fetal period of the rat. In this model we have previously reported a regulatory role for glucose on IGF-I and -II synthesis and secretion. In the same experimental model, we now report that doses of insulin (0.1-5 microM) within the physiological range in rat fetuses during the last stages of gestation evoke an increase of IGF-I and -II mRNA abundance. Insulin regulated in a parallel manner IGF peptide secretion, and an excellent correlation was observed between IGF-I and -II mRNA and IGF-I and -II peptide levels in the conditioned media in response to the hormone. Finally, the insulin-induced rise in IGF-I and -II mRNA was not mediated by stimulation of gene transcription but by increased transcript stability. The results support the hypothesis that insulin plays a major role in IGF regulation at immature stages of development.

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Changes in the renal synthesis and concentration of insulin-like growth factors (IGFs) and their serum-binding proteins (IGFBPs) reported in insulin-deficient diabetes mellitus may be implicated in the alterations of the kidney function and morphology accompanying this disease. Most research on this subject has been carried out in adult animals, as well as in peripubertal rats, when the regulation of the IGF system is fully dependent on serum growth hormone (GH). However, relevant differences in the regulatory pathways of IGFs between adult and neonatal periods have been described. To examine the response of the IGF/IGFBP system of neonatal kidney to diabetes, renal IGF-I and -II and IGFBP-1, -2, and -3 concentration and mRNA expression were determined in streptozotocin-induced diabetic rat neonates. Diabetic neonates exhibited a kidney weight-to-body weight ratio higher than that of control rats, together with decreased IGF-I and increased IGF-II renal concentration. Because kidney mRNA expression of both IGFs decreased, the elevated renal IGF-II might result from increased uptake from circulation. Insulin treatment recovered the altered IGFs to control values, indicating the insulin-dependent regulation of IGFs in the neonatal kidney. Elevated levels of the IGFBP-1 and -2 in the kidney of diabetic neonates did not result from changes in their kidney mRNA transcript expression, suggesting also a possible uptake from circulation.