Nutritional regulation of insulin-like growth factor-binding protein gene expression in the ovine fetus and pregnant ewe

Abstract

The factors controlling the synthesis and degradation of the insulin-like growth factor-binding proteins (IGFBPs) during pregnancy are poorly understood. To clarify the roles of nutritional factors in the regulation of fetal and maternal IGFBP production, we examined the effects of fasting, refeeding, and glucose administration on plasma IGFBP concentrations and hepatic IGFBP mRNA levels in fetal lambs and pregnant ewes (n = 24). Maternal fasting for 3 days in late gestation stimulated a 50-100% increase in maternal plasma BP-1 concentrations (P < 0.05) and a 2- to 3-fold increase in fetal plasma BP-1 (P < 0.05), as determined by densitometric analysis of Western ligand blots. Fasting also stimulated a 40-70% increase in maternal plasma BP-2 concentrations (P < 0.05), but had no significant effect on fetal plasma BP-2 levels. Levels of hepatic BP-1 mRNA in the fetus and pregnant ewe during fasting paralleled plasma BP-1 levels, suggesting that fasting modulates fetal and maternal plasma BP concentrations at least in part through effects on hepatic gene expression. The effects of fasting on both mRNA and plasma levels of BP-1 and BP-2 were reversed by 3 days of refeeding and were prevented by glucose infusion during fasting. When ewes were made hyperglycemic by the infusion of hypertonic glucose, plasma BP-1 and BP-2 concentrations varied inversely with blood glucose concentrations. In addition, hyperglycemia reduced maternal liver BP-1 and BP-2 mRNA levels and fetal BP-1 mRNA levels by 50-65%. Direct administration of hypertonic glucose to the fetus decreased fetal plasma BP-1 levels acutely and reduced fetal BP-1 mRNA levels by 57%, but had no effect on fetal plasma BP-2 or fetal hepatic BP-2 mRNA levels. These findings indicate that glucose and other nutritional factors regulate gene expression and plasma levels of BP-1 and BP-2 in the pregnant ewe and BP-1 in the fetal lamb. The changes in expression of these IGFBPs during fasting and hyperglycemia may play roles in adaptation of the pregnant mother and fetus to metabolic stress.