The role of the IGF axis in IGFBP-1 and IGF-I induced renal enlargement in Snell dwarf mice.

Insulin-like growth factor (IGF) binding protein-1 (IGFBP-1) is generally believed to inhibit IGF action in the circulation. In contrast, IGFBP-1 has been reported to interact with cell surfaces and enhance IGF-I action locally in some tissues. Renal IGFBP-1 levels are found elevated in various conditions characterized by renal growth (e.g. diabetes mellitus, hypokalemia). To test whether IGFBP-1 is a renotropic factor, IGFBP-1 was administered alone or in combination with IGF-I to Snell dwarf mice, an in vivo model without compensatory feedback effects on growth hormone (GH) secretion. In three control groups of Snell dwarf mice, placebo, GH or IGF-I was administered. Compared with placebo, kidney weight increased in all treated groups, however, with different effects on kidney morphology. Administration of IGF-I, alone or in combination with IGFBP-1, tended to increase glomerular volume, while no changes were seen in the other groups. Administration of IGFBP-1 or IGFBP-1+IGF-I both caused dilatation of the thin limbs of Henle's loop, while GH or IGF-I administration had no visible effect. Furthermore, IGF-I administration resulted in an increased mean number of nuclei per cortical area and renal weight, whereas GH, IGF-I+IGFBP-1 or IGFBP-1 caused a decreased renal nuclei number. In situ hybridization and immunohistochemistry showed specific changes of the renal IGF system expression patterns in the different groups. Particularly, IGFBP-1 administration resulted in extensive changes in the mRNA expression of the renal IGF system, whereas the other administration regimen resulted in less prominent modifications. In contrast, administration of IGFBP-1 and IGFBP-1+IGF-I resulted in identical changes in the protein expression of the renal IGF system. Our results indicate that IGFBP-1, alone or in combination with IGF-I, demonstrated effects on the renal tubular system that differ from the effects of IGF-I.

Gene expression of the insulin-like growth factor system during postnatal development of the rat pituitary gland.

Insulin-like growth factors I and II (IGFI and II) are synthesized by anterior pituitary cells and participate in cellular growth and differentiation, as well as the control of pituitary hormone secretion. Type 1 and 2 IGF receptors (IGFR1 and IGFR2) and the six IGF binding proteins (IGFBPs), which modulate IGF effects, are expressed in the anterior pituitary gland. We used in situ hybridization to analyse the temporal expression pattern of IGFI and II, IGFR1 and 2 and IGFBP1-6 in the anterior pituitary gland during postnatal development in both male and female rats (10, 20, 30, 40 and 60 days of age). We found all of the components of the IGF system to be expressed in the anterior pituitary gland, with each having a specific temporal pattern of expression. In addition, there exist differences between the sexes in the expression of some components of the IGF system. These data emphasize that in the anterior pituitary gland the IGF system is under tight regulation during postnatal life when this gland continues to develop. The distinct temporal expression of each member of the IGF system may indicate specific roles in the development and physiology of the anterior pituitary gland.

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.

Generation of antisera to mouse insulin-like growth factor binding proteins (IGFBP)-1 to -6: comparison of IGFBP protein and messenger ribonucleic acid localization in the mouse embryo.

The insulin-like growth factor (IGF) system is an important regulator of fetal growth and differentiation. IGF bioavailability is modulated by IGF binding proteins (IGFBPs). We have generated six different antisera, directed to synthetic peptide fragments of mouse IGFBP-1 through -6. The specificity of the produced antisera was demonstrated by enzyme-linked immunosorbent assay, Western blotting, and by immunohistochemistry on sections of mouse embryos of 13.5 days post coitum. Specificity for the IGFBP-2 through -6 antisera also was confirmed immunohistochemically in liver and lung of corresponding gene deletion (knock-out) mutant mice and wild-type litter mates. Immunohistochemistry and messenger RNA (mRNA) in situ hybridization on sections of mouse embryos of 13.5 days post coitum revealed tissue-specific expression patterns for the six IGFBPs. The only site of IGFBP-1 protein and mRNA production was the liver. IGFBP-2, -4, and -5 protein and mRNA were detected in various organs and tissues. IGFBP-3 and -6 protein and mRNA levels were low. In several tissues, such as lung, liver, kidney, and tongue, more than one IGFBP (protein and mRNA) could be detected. Differences between mRNA and protein localization were extensive for IGFBP-3, -5, and -6, suggesting that these IGFBPs are secreted and transported. These results confirm the different spatial localization of the IGFBPs, on the mRNA and protein level. The overlapping mRNA and protein localization for IGFBP-2 and -4, on the other hand, may indicate that these IGFBPs also function in an auto- or paracrine manner.

The IGF system during fetal-placental development of the mouse.

Insulin-like growth factors (IGF-I and -II) promote cellular mitosis and differentiation and have been implicated in fetal and placental growth. Together with the IGF receptors and IGF binding proteins (IGFBPs) they form a complex network, with tissue specific activity. This review will discuss the data generated to elucidate the functions of the IGF system during mouse development.

mRNA expression patterns of the IGF system during mouse limb bud development, determined by whole mount in situ hybridization.

During limb development the primary limb bud requires various signals to differentiate. Insulin-like growth factor (IGF)-I and IGF-II serve as ubiquitous cellular growth promoters and are modulated by their binding proteins (IGFBPs), which inhibit or augment IGF bioavailability. This is the first study to give a complete overview of the mRNA expression patterns of Igf-1, Igf-2, type 1 Igf receptor (Igf1r) and six Igf binding proteins (IGFBP-1-6) in embryonic mouse limbs, at various stages of development, by whole mount in situ hybridization (ISH). Our results show that all the members of the Igf system, except Igfbp-1 and -6, have specific spatio-temporal mRNA expression patterns. IGFBP-2 and -5 are found in the apical ectodermal ridge (AER), and IGF-I and IGFBP-4 in the region of the zone of polarizing activity (ZPA). IGF-II and IGF1R are found in regions of pre-cartilage formation. At 13.5 days post coitus (dpc) the IGF system colocalizes with apoptosis areas; IGFBP-2, -4 and -5 are found in the interdigital zone, while IGFBP-3 and IGF-I border this region. Furthermore, IGFBP-3, -4 and -5 are found in the phalangeal joint areas, at an early stage of joint formation. This supports the hypothesis that the IGF system may be involved in chondrogenic differentiation of mesenchyme and the regulation of apoptosis in the developing limb.

Gene expression of the insulin-like growth factor system during mouse kidney development.

Expression of the insulin-like growth factor (IGF) system was investigated in mouse renal development and physiology, using non radioactive in situ hybridization and quantitative RT-PCR. IGF-I mRNA levels increased after birth and were confined to distal tubules and peritubular capillaries in the outer medulla. IGF-II mRNA levels were high in developing kidneys and peaked after birth. The type I receptor mRNA expression pattern mostly parallelled those of IGF-I and IGF-II. The IGF binding proteins (IGFBP's) showed weak mRNA expression for IGFBP-1 and -6. High fetal mRNA levels were measured for IGFBP-2, showing a similar profile in time as observed for IGF-II. Low fetal IGFBP-3 and -5 mRNA levels increased after birth. IGFBP-2, -4 and -5 mRNA expression was localized to differentiating cells. In the mature kidney predominant expression was confined to proximal tubules (IGFBP-4), thin limbs of Henle's Loop (IGFBP-2), glomerular mesangial cells (IGFBP-5) and peritubular capillaries of the medulla (IGFBP-5). IGFBP-3 mRNA was exclusively expressed in endothelial cells of the renal capillary system. Distinct mRNA expression for each member of the IGF system may point to specific roles in development and physiology of the mouse kidney.

IGF, type I IGF receptor and IGF-binding protein mRNA expression in kidney and liver of potassium-depleted and normal rats infused with IGF-I.

Dietary potassium (K) depletion is known to reduce body weight gain and organ growth, except for kidney which increases in weight. This renal hypertrophy is preceded by increased renal IGF-I levels. In the present study, we investigated IGF-I and -II, type I IGF receptor and IGF-binding protein (IGFBP) mRNA expression in liver and kidney of K-depleted and normal rats infused with vehicle or recombinant human IGF-I. Body weight gain was almost completely arrested in K-depleted rats without any stimulatory effect of IGF-I infusion. Both absolute and relative kidney weight (kidney weight/body weight) were significantly increased in K-depleted rats and this was further enhanced by IGF-I infusion. In contrast, relative liver weight was comparable in the different groups and unaffected by IGF-I infusion. IGF-I mRNA expression was significantly lower in kidney and liver of K-depleted animals whereas type I IGF receptor levels were unchanged. In contrast, in kidney, K depletion increased IGFBP-1 and -2 mRNA expression with no additional effect of IGF-I infusion. In liver of K-depleted animals, IGFBP-1 mRNA expression was increased whereas increased IGFBP-1 and -2 mRNA expression was observed when these animals were infused with IGF-I. These observations may point towards a differential mode of action of the IGFBPs. In kidney increased IGFBP-1 and -2 mRNA expression may enhance IGF-I bioavailability with subsequent kidney growth. In liver, with clearly detectable type I IGF receptor mRNA expression, increased IGFBP levels may protect from IGF-I-induced organ growth by decreasing IGF-I bioavailability.