Expression and regulation of insulin-like growth factor binding protein-1 in the rat uterus throughout estrous cycle

The role of Ca²⁺-stimulated adenosine 5-triphosphatase (Ca²⁺-ATPase) in Ca²⁺ sequestering of rat liver nuclei was investigated. Ca²⁺-ATPase activity was calculated by subtracting Mg²⁺-ATPase activity from (Ca²⁺–Mg²⁺)-ATPase activity. Ca²⁺ uptake and release were determined with a Ca²⁺ electrode. Nuclear Ca²⁺-ATPase activity increased linearly in the range of 10–40 M Ca²⁺ addition. With those concentrations, Ca²⁺ was completely taken up by the nuclei dependently on ATP (2 mM). Nuclear Ca²⁺-ATPase activity was decreased significantly by the presence of arachidonic acid (25 and 50 M), nicotinamide-adenine dinucleotide (NAD⁺; 2 mM) and zinc sulfate (2.5 and 5.0 M). These reagents caused a significant decrease in the nuclear Ca²⁺ uptake and a corresponding elevation in Ca²⁺ release from the nuclei. Moreover, calmodulin (10 g/ml) increased significantly nuclear Ca²⁺-ATPase activity, and this increase was not seen in the presence of trifluoperazine (10 M), an antogonist of calmodulin. The present findings suggest that Ca²⁺-ATPase plays a role in Ca²⁺ sequestering by rat liver nuclei, and that calmodulin is an activator. Moreover, the inhibition of Ca²⁺-ATPase may evoke Ca²⁺ release from the Ca²⁺-loaded nuclei.     

Differential expression of insulin-like growth factor-I and insulin-like growth factor binding protein-1 in the diabetic rat

Summary Multiple factors contribute to the growth retardation which is a characteristic feature of uncontrolled diabetes. In this report we have examined the effects of streptozotocin-induced (STZ) diabetes on expression of insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-1 (IGFBP-1) in various tissues. As early as 7 days after STZ administration there was a modest reduction in IGF-I mRNA abundance. The reduction (10–30%) was of similar magnitude in each of the 7 tissues examined; liver, kidney, lung, diaphragm, quadraceps, heart and adipose tissue. However, the reduction achieved statistical significance only in the lung (p < 0.05) and diaphragm (p < 0.01). A further reduction in IGF-I mRNA abundance was seen in many tissues, 32 and 91 days after STZ administration. In contrast to the decrease in IGF-I mRNA, IGFBP-1 mRNA was significantly increased in the liver and kidney of diabetic rats. IGFBP-1 mRNA was detectable at only very low levels in other tissues but was increased in diabetic rats compared non-diabetic rats. In diabetic rats, a highly significant correlation (R = 0.75, p < 0.001) between hepatic IGFBP-1 mRNA and glucose was observed whereas there was no significant correlation between serum glucose and hepatic IGF-I mRNA abundance (R = 0.24, p = NS). Treatment of diabetic rats with insulin resulted in a small, non significant increase in hepatic and renal IGF-I mRNA and a significant decrease in renal IGFBP-1 mRNA abundance. The observations reported here are consistent with the hypothesis that diminished IGF-I expression and inhibition of available IGF-1 by increased levels of IGFBP-1 may explain the impaired growth seen in diabetic animals.     

Serum insulin-like growth factor-I, insulin-like growth factor binding protein-3, and the pubertal growth spurt in the female rhesus monkey

While there is good evidence suggesting IGF-I links to pubertal development and crown-rump length growth among rhesus monkeys, linkages between IGF-I and other measures of morphological growth have not been established. In this study, the pubertal growth spurt in a number of morphological characteristics of female rhesus monkeys is related to serum endocrine status of insulin-like growth factor-I (IGF-I) and its binding protein, insulin-like growth factor binding protein-3 (IGFBP-3), to test the hypothesis that elevations in IGF-I and IGFBP-3 coincide with the time of greatest growth rate of different morphological characteristics. A longitudinal study of pubertal growth among four female rhesus monkeys was carried out across a 3-year period. Morphometric measurements included weight, crown-rump length, foot-length, and skinfolds at five sites (biceps, triceps, abdominal, subscapular, and suprailiac). These measures were taken as being representative of total mass, skeletal growth of the trunk and head, limb length, and body fatness, respectively. Measurements were carried out as closely as possible to 3-monthly, with interpolations being performed to standardise the data to exactly 3-monthly intervals for all individuals. Blood samples were taken at time of morphometry. Elevations in serum IGF-I and IGFBP-3 took place in a manner similar to that of humans, and across the period associated with onset of puberty. Mean 3-monthly gain in crown-rump length and foot length showed significant peaks across the measurement period, while mean 3-monthly gains in weight and sum of five skinfolds did not. Greatest foot length gain occurred on average between 3-3.5 years of age, while crown-rump length gain was greatest between 3.75-4 years of age. Periods of greatest gain in crown-rump length and foot length took place across the period of elevated serum IGF-I levels, which was between 3-4.5 years of age. Significant elevations in IGF-I and IGFBP-3 were not coincident with greatest gains in foot length or crown-rump length. Thus the hypothesis does not hold true for the two measures showing significant peaks in 3-monthly gain across the measurement period. The nature of the endocrine impact on macaque morphology remains unclear, although this may be fundamental to the understanding of the variation in the pubertal growth spurt and its influence on morphology at maturity both within and across primate species.     

Differential dissociation kinetics explain the binding preference of insulin-like growth factor binding protein-6 for insulin-like growth factor-II over insulin-like growth factor-I

Insulin-like growth factor binding protein-6 binds insulin-like growth factor-II with a marked preferential affinity over insulin-like growth factor-I. The kinetic basis of this binding preference was studied using surface plasmon resonance. Binding of insulin-like growth factor-I and insulin-like growth factor-II to immobilized insulin-like growth factor binding protein-6 fitted a two-site binding kinetic model. Insulin-like growth factor-I and insulin-like growth factor-II association rates were similar whereas the dissociation rate was approximately 60-fold lower for insulin-like growth factor-II, resulting in a higher equilibrium binding affinity for insulin-like growth factor-II. The equilibrium binding affinities of a series of insulin-like growth factor-II mutants were also explained by differential dissociation kinetics. O-glycosylation had a small effect on the association kinetics of insulin-like growth factor binding protein-6. The insulin-like growth factor binding properties of insulin-like growth factor binding protein-6 are explained by differential dissociation kinetics.     

Regulation of insulin-like growth factor binding protein-6 expression in the reproductive tract throughout the estrous cycle and during the development of the placenta in the ewe

Insulin-like growth factors (IGF-I and IGF-II) are essential for normal uterine development and have been particularly implicated in fetal and placental growth. A family of six IGF binding proteins enhance or attenuate IGF-stimulated cell proliferation. In this study we have used in situ hybridization to map the distribution of IGFBP-6, one of the lesser known of the IGFBPs, in sections of the uterus collected from cyclic, anestrous, and ovariectomized nonpregnant ewes and from the uterus and placenta of early pregnant (13-55 days) and unilaterally pregnant ewes. In nonpregnant ewes IGFBP-6 mRNA (measured as arbitrary optical density units from autoradiographs) was abundant in the periepithelium and caruncles, with lower levels in the endometrial stroma and myometrium. In most regions IGFBP-6 mRNA showed cyclic variations with concentrations maximal around ovulation and the early luteal phase. In addition, 16 out of 25 ewes expressed IGFBP-6 mRNA in their endometrial glands between estrus and Day 2. Measurements of IGFBP-6 mRNA were high in anestrous ewes (equivalent values to ovulation) but low in ovariectomized ewes (equivalent values to mid to late luteal phase). In pregnant ewes IGFBP-6 mRNA was found in similar regions to those recorded during the cycle. In the periepithelium and caruncular stroma IGFBP-6 mRNA levels were higher during early pregnancy than in the midluteal phase. In the unilateral pregnant ewes there was no difference in IGFBP-6 mRNA measured between pregnant and nonpregnant horns. In conclusion, IGFBP-6 mRNA is differentially regulated during the estrous cycle and pregnancy and may be functionally important in modulating IGF activity in the uterus and placenta by virtue of its strong affinity and ability to regulate IGF-II mediated actions.     

Expression and localization of transient receptor potential channels in the bovine uterus epithelium throughout the estrous cycle

Transient receptor potential (TRP) channels are expressed in the endometrium but it is unknown if they are modulated through the estrous cycle (EC). This study was undertaken to identify the modulation of the TRPC gene and protein isoforms in bovine uterine epithelium, as a model for human, throughout the EC. Changes in the expression of TRPC genes in bovine uterine epithelium throughout the EC were measured using Real-Time PCR, while immunohistochemistry and immunocytochemistry were used to determine the localization of these channels. Out of the 7 members of the TRPC family, TRPC1, 2, 3, 4 and 6 genes were expressed in bovine uterine epithelial tissue and TRPC 5 and 7 were not. Gene expression levels of all TRPC isoforms underwent cyclical changes throughout the EC. Moreover, cyclical changes were detected in the protein levels of TRPC1 and TRPC6 throughout the EC. These findings show that TRPC channels are modulated through the EC and therefore may have a role in reproductive events.

     

Insulin-like growth factor-I is internalized after binding to the type I insulin-like growth factor receptor

Receptor-mediated endocytosis may represent an important mechanism whereby peptide hormones exert their biological effects. The ability of recombinant insulin-like growth factor (IGF)-I to be internalized by cultured cells was evaluated in BRL-3A2 cells, a rat liver-derived cell line which lacks insulin receptors. Since recombinant IGF-I does not bind to the Type II IGF receptor, all specific binding of 125I-IGF-I in BRL-3A2 cells represents binding to the Type I receptor. Exposure of BRL-3A2 cells to IGF-I resulted in a rapid 50% downregulation of Type I IGF receptors. Only one-half of these binding sites were sensitive to treatment with trypsin, a phenomenon which indicates that the peptide and its receptor were internalized after the cells were exposed to IGF-I. In conclusion, these experiments demonstrate that IGF-I can be internalized by cultured cells via the Type I IGF receptor, and suggest that IGF hormone action may be exerted by receptor-mediated endocytosis.     

Endometrial mRNA expression of oestrogen receptor alpha, progesterone receptor and insulin-like growth factor-I (IGF-I) throughout the bovine oestrous cycle

This study characterized endometrial expression of mRNAs of oestrogen and progesterone receptors (ER, PR) and insulin-like growth factor-I (IGF-I) during the oestrous cycle. Seven Holstein heifers that showed standing oestrus on the same day (day 0) were selected and blood samples for oestradiol (E2) and progesterone (P4) determinations by RIA were taken daily until day 23. Endometrial samples were taken by transcervical biopsies on days 0, 5, 12 and 19 for mRNA determination by solution hybridization. The highest endometrial mRNA levels of ERalpha and PR were observed at oestrus and a decline was observed already at day 5, which then decreased progressively at the end of the luteal phase. IGF-I mRNA levels were higher at day 0 and 5 than at day 12. At day 19, mRNA levels of ERalpha, PR and IGF-I were the lowest in heifers that were at the end of their luteal phase (n=4), but were high again in heifers which P4 levels were basal (n=3). The temporal changes in mRNA endometrial expression of ERalpha, PR and IGF-I and their relation to the changes in steroid concentrations during the bovine oestrus cycle are described.     

Influence of smoking on serum and milk of mothers, and their infants' serum insulin-like growth factor-I and insulin-like growth factor binding protein-3 levels

OBJECTIVE: To investigate the serum and milk in active-smoking and nonsmoking mothers, and their infants' insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-3 (IGFBP-3) levels. DESIGN AND SETTING: A cohort study conducted at a tertiary medical center. Subjects: Forty-four mothers (age range: 21-34 years) and their newborns (7 days old) were enrolled in the study. Mothers were interviewed and classified according to their smoking status into one of two groups: the active-smoking mothers (n = 21) and the nonsmoking mothers (n = 23). RESULTS: There was no difference noted in either IGF-I, IGFBP-3 or IGF-I/IGFBP-3 ratios in serum and milk of mothers, and their infants' serum samples according to maternal smoking. CONCLUSION: This study demonstrated that maternal smoking (5-10 cigarettes/day) did not influence the maternal and infant serum levels of IGF-I and IGFBP-3 as well as the breast milk levels of these peptides.     

The roles of tyrosines 24, 31, and 60 in the high affinity binding of insulin-like growth factor-I to the type 1 insulin-like growth factor receptor

A series of insulin-like growth factor I (IGF-I) structural analogs in which one or more of the three tyrosine residues were replaced with nonaromatic residues were produced and their binding properties characterized. The single point mutations, [Leu24]IGF-I, [Ala31]IGF-I, and [Leu60]IGF-I result in an 18-, 6-, or 20-fold loss in affinity, respectively, for the type 1 IGF receptor. Multiple mutations, [Ala31,Leu60]IGF-I, [Leu24, Ala31]IGF-I, [Leu24, Leu60]IGF-I, or [Leu24, Ala31, Leu60]IGF-I result in a 520-, 240-, 1200-, or greater than 1200-fold loss in affinity, respectively, at the type 1 IGF receptor. In contrast, none of the analogs display greater than a 2-fold loss in affinity for the acid-stable human serum binding proteins. At the insulin receptor, [Ala31]IGF-I and [Leu24]IGF-I are equipotent to and 5-fold less potent than IGF-I, whereas [Leu60]IGF-I and the multiple mutation analogs are inactive up to 10 microM. Analogs [Leu24]IGF-I, [Ala31]IGF-I, and [Leu24, Ala31]IGF-I are equipotent to IGF-I at the type 2 IGF receptor, whereas all analogs containing Leu60 demonstrate little measurable affinity at this receptor. Thus, Tyr24, Tyr31, and Tyr60 are involved in the high affinity binding of IGF-I to the type 1 IGF receptor, while Tyr60 is important for maintaining binding to the type 2 IGF receptor.     

Interaction of insulin-like growth factor binding protein-3 with latent transforming growth factor-beta binding protein-1

Insulin-like growth factor binding protein-3 (IGFBP-3) inhibits the replication and promotes apoptosis in various cell lines in an IGF-independent manner. We utilized a yeast two-hybrid system to identify binding partners for IGFBP-3 in a mouse embryo cDNA library. A partial cDNA encoding mouse latent transforming growth factor beta (TGF-) binding protein-1 (LTBP-1) was identified. This cDNA encoded a mouse LTBP-1 mRNA fragment corresponding to amino acid residues 1160–1712. Analysis of C-terminal deleted mutants indicated that the IGFBP-3 interacting domain resides in the 552 residue C-terminal fragment encoding amino acids 831–1383. The interaction of IGFBP-3 with recombinant human LTBP-1 immobilized on nitrocellulose was also demonstrated. Neither binding of IGF-I to IGFBP-3 nor binding of latency associated protein (LAP) with LTBP-1 inhibited the interaction of IGFBP-3 with LTBP-1. Furthermore the large latent complex, ¹²⁵I-TGF-/LAP/LTBP-1 was able to bind to immobilized IGFBP-3. These data demonstrate that IGFBP-3 can bind to LTBP-1 and provide a potential mechanism whereby IGFBP-3 can interact with the TGF- system.     

Transferrin binds insulin-like growth factors and affects binding properties of insulin-like growth factor binding protein-3.

In the circulation, most of the insulin-like growth factors (IGFs) are bound to a ternary 150 kDa complex with IGF-binding protein (IGFBP)-3 and the acid labile subunit. In the current study, we identify transferrin (Tf) by mass spectrometry, and immunoprecipitation as a component of a major IGF-binding fraction separated from human plasma. IGF ligand blotting, cross-linkage experiments and surface plasmon resonance spectrometry have been used to demonstrate the capability of Tf to bind IGFs specifically. In combination with Tf, IGFBP-3 showed a 5-fold higher affinity for IGF-II than IGFBP-3 alone. The data suggest that Tf may play an important role in regulating IGF/IGFBP-3 functions.     

Insulin-like growth factor-I and insulin-like growth factor binding protein-3 levels of children living in an iodine- and selenium-deficient endemic goiter area

Serum insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-3 (IGFBP-3) levels were investigated in 31 children living in an endemic goiter area and 33 healthy subjects living in an nonendemic area. Serum IGF-I and IGFBP-3 levels of iodine- and selenium-deficient children were found to be lower than those of control subjects (p<0.001). There was a positive correlation between the IGF-I with chronological age and body mass index. There was also positive correlation between the IGF-I and IGFBP-3. No significant difference was found between the goitrous and nongoitrous children. These results suggest that IGF-I and IGFBP-3 levels are affected by thyroid dysfunction as a result of iodine and selenium deficiency. However, IGF-I and IGFBP-3 levels are not associated with goiter.     

Estrogen induces insulin-like growth factor-I expression in the rat uterus

The inability to convincingly demonstrate a mitogenic effect of estrogen on isolated uterine cells in culture suggests that autocrine or paracrine growth factors may be important in the estrogen-induced uterine proliferative response. Here we report that uterine expression of insulin-like growth factor-I (IGF-I), an important mediator of GH action, is increased after 17 beta-estradiol (5 micrograms/100 g bw, ip) administration to ovariectomized prepubertal rats. An increase in uterine IGF-I mRNA abundance, approximately 14-fold above untreated controls, was apparent 6 h after estrogen administration and the level achieved exceeded that seen in the uterus from intact mature rats during diestrus. In contrast to the increase in IGF-I expression in the uterus, no significant change in serum IGF-I concentration or hepatic or renal IGF-I mRNA abundance was demonstrable after 17 beta-estradiol injection of ovariectomized prepubertal rats. The increase in uterine IGF-I expression, was similar in both pituitary-intact and hypophysectomized, ovariectomized rats. We believe this is the first report of induction of IGF-I expression by estrogen in vivo. As such, the finding expands the role and significance of IGF-I as a mediator of growth beyond that related to GH.