Co-administration of IGF-binding protein-3 differentially inhibits the IGF-I-induced total body and organ growth of Snell dwarf mice

Human insulin-like growth factor (IGF) binding protein-1 inhibits IGF-I-stimulated body growth but stimulates growth of the kidney in snell dwarf mice

The actions of insulin-like growth factor-I (IGF-I) are modulated by IGF binding proteins (IGFBPs). The effects of IGFBP-1 in vivo are insufficiently known, with respect to inhibitory or stimulatory actions on IGF-induced growth of specific organs. Therefore, we studied the effects of IGFBP-1 on IGF-I-induced somatic and organ growth in pituitary-deficient Snell dwarf mice. Human GH, IGF-I, IGFBP-1, and a preequilibrated combination of equimolar amounts of IGF-I and IGFBP-1 were administered sc during 4 weeks. Treatment with IGF-I alone induced a significant increase in body length (108% of control) and weight (112%) as well as an increase in weight of the submandibular salivary glands (135%), kidneys (124%), femoral muscles (111%), testes (129%), and spleen (126%) compared with saline-treated controls. IGFBP-1 alone induced a significant increase in weight of the kidneys (152% of control). Coadministration of IGF-I with IGFBP-1 neutralized the stimulating effects of IGF-I on body length and weight as well as on the femoral muscles and testes. In contrast, the weights of the submandibular salivary glands (143%) were not significantly different from those of IGF-I-treated animals, whereas the weights of the kidneys (171%) and spleen (156%) were significantly increased compared with IGF-I-treated mice. The effect of IGFBP-1 plus IGF-I on kidney weight was not significantly greater than the effect of IGFBP-1 alone. Western ligand blotting showed induction of the IGFBP-3 doublet as well as IGFBPs with molecular masses of 24 kDa, most probably IGFBP-4, by human GH, IGF-I alone, and IGF-I in combination with IGFBP-1. Our data show that coadministration of IGFBP-1 inhibits IGF-I-induced body growth of GH-deficient mice but significantly stimulates the growth promoting effects of IGF-I on the kidneys and the spleen. These data warrant further investigation because differences in concentrations of IGFBP-1 occurring in vivo may influence IGF-I-induced anabolic processes.

Insulin-like growth factor (IGF) II induced changes in expression of IGF binding proteins in lymphoid tissues of hIGF-II transgenic mice

Overexpression of human insulin-like growth factor II (IGF-II) in transgenic mice does not result in increased overall body growth. The IGF-II overexpression, however, specifically causes growth of the thymus and not of the spleen. We address the question whether the observed differences in growth induction in lymphoid tissues by IGF-II can be related to differences in local IGF binding protein (IGFBP) production, using nonradioactive in situ hybridization and Northern blot analysis. IGFBP-2, -4, and -5 are expressed in both lymphoid tissues of normal mice. The spleen additionally expresses IGFBP-3 and IGFBP-6. IGFBP-1 expression was not detected. Although the expression pattern of the IGFBPs did not change upon IGF-II overexpression, the level of expression changed in a specific manner for each IGFBP. In both the thymus and the spleen of transgenic mice, IGFBP-2 and -5 gene expression was slightly increased, whereas the level of IGFBP-4 expression was not altered. In the spleen, IGFBP-6 expression was not altered by IGF-II overexpression, whereas IGFBP-3 expression was strongly increased. The differences in IGFBP expression, and the difference in response of these IGFBPs to IGF-II overexpression in thymus and spleen suggests an important role of these proteins in growth regulation of both lymphoid tissues. We speculate that an increase of IGFBP-3 expression together with changes in expression of other IGFBPs, inhibits IGF-II stimulated growth in the spleen by an autocrine-/paracrine pathway.

Effects of calcium depletion and repletion on serum insulin-like growth factor I and binding protein levels in weanling rats

Previous studies in a weanling rat model indicated that dietary calcium depletion not only stimulated osteoclastic resorption but also inhibited bone formation. The present study sought to test whether the depletion-associated inhibition of bone formation is related to a reduction in serum insulin-like growth factor-I (IGF-I) and/or an increase in its binding proteins (IGFBPs). Twenty male weanling rats were divided into two weight-matched groups. The study group was subjected to a semisynthetic diet deficient in calcium (0.02% calcium) for 28 days, while the control group was pair-weighed on the same diet but containing 0.62% calcium. After the depletion phase, all rats were fed the same calcium-containing diet for an additional 14 days. Serum samples were obtained from each animal on a weekly basis and assayed for IGF-I and IGFBPs. During depletion, there was no statistically significant difference in serum IGF-I level between the study group and the control group. In contrast, the study group showed a statistically significant increase in several serum IGFBPs with apparent molecular size of 30-38 kD (IGFBP-3), 26-28 kD (IGFBP-1, -2, -5, and/or -6), and 24-25 kD (IGFBP-4), respectively, compared to the control group. There was no difference in nutritional intakes between the two groups of rats during depletion. During repletion, there was also no significant difference in serum IGF-I level between the control and study group. However, during the first 7 days of repletion, serum IGFBP-3 and the 26-28 kD IGFBP of the study group was significantly less than those of the control group, which then returned to the control level after 2 weeks of repletion. In summary: (1) calcium depletion in weanling rats increased several serum IGFBPs without an effect on IGF-I; and (2) calcium repletion induced an acute reduction in serum IGFBP-3. In conclusion, these findings represent the first evidence that the depletion-related inhibition of bone formation in the rat may be associated with an increase in several serum IGFBPs, which may act to inhibit the osteogenic actions of IGFs.

Insulin-like growth factor binding proteins and their role in controlling IGF actions

The insulin-like growth factor binding proteins (IGFBPs) are a family of six proteins that bind to insulin-like growth factor-I and -II with very high affinity. Because their affinity constants are between two- and 50-fold greater than the IGF-I receptor, they control the distribution of the IGFs among soluble IGFBPs in interstitial fluids, IGFBPs bound to cell surfaces or extracellular matrix (ECM) and cell surface receptors. Although there are six forms of insulin-like growth factor binding proteins, most interstitial fluids contain only three or four forms, and usually only one or two predominate. The proteins differ significantly in their biochemical characteristics, and this accounts for many of the differences that have been observed in their biological actions. Several different types of protease cleave these binding proteins. Proteolytic cleavage generally inactivates the binding proteins or reduces their ability to bind to IGF-I or -II substantially. Several cell types have been shown to secrete these proteases; therefore, the factors that regulate protease activity can control binding protein actions indirectly. Other post-translational modifications, such as glycosylation and phosphorylation, have been shown to alter IGF binding protein activity. While binding protein actions have been studied extensively in vitro, many of the in vivo activities of these proteins remain to be defined.