Moira S. Lewitt

Developmental regulation of circulating insulin-like growth factor-binding proteins in normal pregnancies and in pre-eclampsia.

The insulin-like growth factors (IGFs) and their binding properties (IGFBPs) are believed to play important roles in the growth and development of the human fetus. They have been implicated in the pathophysiology of pre-eclampsia. In this study we have characterized the developmental regulation, in normal and pre-eclamptic pregnancies, of IGFs and IGFBPs in maternal serum, neonatal serum and amniotic fluid. In neonatal cord serum IGFBP-1, -2 and -6 decreased with increasing gestational age. In contrast, the ternary complex and its components, IGF-I, IGFBP-3 and ALS increased with gestation. We show that while ALS is an important limiting factor for ternary complex formation in the fetal circulation, there is a fraction of IGFBP-3 which is unable to form this complex. IGFs and IGFBPs in the maternal and fetal circulation were similar in normal and pre-eclamptic pregnancies.

Interaction of insulin, glucocorticoids, and protein kinase C in the regulation of insulin-like growth factor-binding protein-1 production by H4IIE rat hepatoma cells

A sensitive RIA was used to examine regulation of IGFBP‐1 in H4IIE rat hepatoma cells. IGFBP‐1 was stimulated up to tenfold by dexamethasone and corticosterone, and this stimulation was abolished by RU486. The effect of dexamethasone increased with time in culture. Phorbol 12‐myristate 13‐acetate (PMA) stimulated IGFBP‐1 up to fourfold with a maximal effect in short‐term culture. Dexamethasone and PMA were additive in stimulating IGFBP‐1. Under basal conditions IGFBP‐1 production was linearly related to cell density: however, stimulation by dexamethasone was greatest in confluent cells, and PMA had a greater effect in sparse cultures. Insulin inhibited IGFBP‐1 up to 80%, and this effect diminished with time in culture but was unaffected by cell density. Dexamethasone was stimulatory in the presence of a maximal inhibitory concentration of insulin, and insulin was inhibitory in the presence of maximal dexamethasone from 3–48 h in culture, regardless of cell density. PMA abolished the inhibitory action of insulin on IGFBP‐1 secretion and mRNA expression during incubation periods of less than 4 h and not during longer incubations. PMA did not influence the stability of IGFBP‐1 mRNA. We conclude that, in rat H4IIE cells, dexamethasone and PMA stimulate IGFBP‐1 by independent mechanisms and speculate that when protein kinase C is activated the inhibitory action of insulin is blocked.

Cytochalasin B stimulates insulin-like growth factor-binding protein-1 production by Hep G2 cells.

Hep G2 cells were used to study the early sequence of events regulating production of insulin-like growth factor-binding protein-1 (IGFBP-1). Cytochalasin B (100 microM) specifically inhibited 2-deoxyglucose uptake by Hep G2 cells and stimulated IGFBP-1 production 2-fold. Insulin (300 nM) did not stimulate hexose uptake but inhibited IGFBP-1 production more than 50%. A change in IGFBP-1 secretion was observed as early as 2 h after a 15-min or 2-h pulse exposure to either effector. In contrast to IGFBP-1, albumin production was diminished in the presence of cytochalasin B and increased by insulin. From these results we conclude that IGFBP-1 synthesis is (i) stimulated by transient inhibition of cellular glucose uptake and further stimulated by long-term glucose deprivation, and (ii) inhibited by transient exposure to insulin with further inhibition on long-term exposure. These effects are consistent with the dynamic regulation of IGFBP-1 by nutritional status.