The physiological role of IGF-II remains unclear but there is evidence for a role in postnatal growth, the growth of the thymus and bone homeostasis. Glucocorticoids have many effects that are opposite to the effects of IGF-II such as growth retardation, osteoporosis and thymic involution. We therefore wondered whether IGF-II overexpression in transgenic mice might counteract some of the growth inhibitory effects of the glucocorticoid, dexamethasone (DXM). In a dose-finding study in normal mice, 20 microg DXM/day caused a significant growth delay. The various organs had a different susceptibility to the growth inhibitory effects of DXM. Most affected were thymus and spleen, followed by liver, skeletal muscle and lumbar vertebrae. The weights of the kidney, tibia, and humerus were not significantly diminished. In a second experiment, the effects of DXM in normal and IGF-II-transgenic animals were compared. The IGF-II serum levels in the transgenic animals were more than 40-fold increased compared with control mice and were decreased by 35% in the DXM-treated group. IGF-I serum levels were identical in both mouse strains and rose slightly after DXM administration in controls. Transgenic mice had higher levels of IGF binding protein species of apparent molecular masses of 41.5 kDa, 30 kDa, and 26.5 kDa. DXM reduced the 24 kDa band in both mice strains. In addition it reduced the bands at 38.5 kDa and 26.5 kDa but only in the transgenic animals. The effect of DXM on body growth was similar in normal and IGF-II-transgenic mice. The weight reduction of the various organs caused by DXM was similar in both types of mice except for the skeleton. The weight of the tibia and the humerus were significantly higher in the DXM-treated transgenic mice. In conclusion, we speculate that overexpression of IGF-II in mice partially protects bone from the osteopenic effects of glucocorticoids.

Abstract

The ontogeny of serum insulin-like growth factors (IGFs)-I and -II and their binding proteins (IGFBPs) was studied in normal and dwarf Snell mice. IGF-I concentrations in serum of normal mice increased between 4 and 8 weeks of age; dwarf mice had very low serum IGF-I levels. In both normals and dwarfs, serum IGF-II levels were highest soon after birth and dropped steadily thereafter. Western ligand blots of serum IGFBPs with ¹²⁵I-IGF-II as tracer revealed the expected bands 41·5, 38·5, 30–32 and 24 kDa. In normal mice the IGFBP-3 doublet was already detectable at 2 weeks of age, and its intensity increased with age. In dwarf mice the IGFBP-3 doublet was hardly detectable.

The changes of IGFs and their IGFBPs were studied in sera of dwarf mice after treatment with growth hormone (GH) and/or thyroxine (T₄) for 4 weeks. In spite of a comparable growth response obtained using these hormones, serum IGF-I was increased only by GH treatment; a small but significant decrease of serum IGF-II was obtained following GH or T₄ treatment. An increase of the IGFBP-3 doublet was only obtained with GH; T₄ and GH+T₄ had no effect. The rise of IGFBP-3 after GH treatment was accompanied by the formation of the IGFBP 150 kDa complex, as measured by neutral gel chromatography. The size distribution of ¹²⁵ I-IGF-II was restored to normal, while with ¹²⁵I-IGF-I only a small peak at 150 kDa was observed. Elution profiles of sera after treatment with T₄ or GH+T₄ were identical to those of dwarf controls.

The presence of the IGFBPs was investigated in media conditioned by liver and lung explants of normal and dwarf animals. In culture media of liver explants from normal mice, bands at 30–32 and 24 kDa predominated; the intensity of the IGFBP-3 doublet was relatively low. In dwarfs the 30–32 kDa predominated. In culture media of the lung from normal mice the IGFBP-3 doublet and the 24 kDa band were clearly visible; in dwarf mice IGFBPs could not be detected. We were unable to identify the 150 kDa IGFBP-complex in this medium using the size distribution of ¹²⁵I-IGFs on neutral gel chromatography after incubation with the conditioned media. This was in contrast to data obtained with normal serum.

Our data suggest that serum IGFBP-3 and IGF-I are regulated by GH and not by T₄. In dwarf Snell mice, serum IGF-II is down regulated by GH as well as T₄. The 150 kDa IGFBP complex is absent in dwarfs and, when induced by GH, seems to have a high affinity for IGF-II.

Journal of Endocrinology (1994) 143, 191–198