IGF-I In Response Research Articles

Insulin-like Growth Factor (IGF) Signaling Requires αvβ3-IGF1-IGF Type 1 Receptor (IGF1R) Ternary Complex Formation in Anchorage Independence, and the Complex Formation Does Not Require IGF1R and Src Activation

Integrin αvβ3 plays a role in insulin-like growth factor 1 (IGF1) signaling (integrin-IGF1 receptor (IGF1R) cross-talk) in non-transformed cells in anchorage-dependent conditions. We reported previously that IGF1 directly binds to αvβ3 and induces αvβ3-IGF1-IGF1R ternary complex formation in these conditions. The integrin-binding defective IGF1 mutant (R36E/R37E) is defective in inducing ternary complex formation and IGF signaling, whereas it still binds to IGF1R. We studied if IGF1 can induce signaling in anchorage-independent conditions in transformed Chinese hamster ovary cells that express αvβ3 (β3-CHO) cells. Here we describe that IGF1 signals were more clearly detectable in anchorage-independent conditions (polyHEMA-coated plates) than in anchorage-dependent conditions. This suggests that IGF signaling is masked by signals from cell-matrix interaction in anchorage-dependent conditions. IGF signaling required αvβ3 expression, and R36E/R37E was defective in inducing signals in polyHEMA-coated plates. These results suggest that αvβ3-IGF1 interaction, not αvβ3-extracellular matrix interaction, is essential for IGF signaling. Inhibitors of IGF1R, Src, AKT, and ERK1/2 did not suppress αvβ3-IGF-IGF1R ternary complex formation, suggesting that activation of these kinases are not required for ternary complex formation. Also, mutations of the β3 cytoplasmic tail (Y747F and Y759F) that block β3 tyrosine phosphorylation did not affect IGF1R phosphorylation or AKT activation. We propose a model in which IGF1 binding to IGF1R induces recruitment of integrin αvβ3 to the IGF-IGF1R complex and then β3 and IGF1R are phosphorylated. It is likely that αvβ3 should be together with the IGF1-IGF1R complex for triggering IGF signaling.

Physical Training Leads to Improved Fitness in CKD Despite no Change in Circulating Bioactive IGF-I

Chronic kidney disease (CKD) is characterized by abnormalities in the insulin-like growth factor (IGF) system. The measurement of bioactive IGF-I in response to physical activity may provide further insight into the therapeutic potential of exercise training in CKD patients. PURPOSE: The purpose of the present study was to assess changes in the IGF system, including bioactive IGF-I,in CKD patients following a training program. METHODS: Patients (N = 21) with CKD (stage 2-4) were randomized into matched treatment and control groups. The treatment group participated in 48 weeks of progressive exercise training and dietary counseling. The control group received only usual care. Fitness measurements and blood samples for analyses of immunoreactive IGF-I, IGF-II, IGFBP-1 and -2, and bioactive IGF-I were taken at baseline, 24 weeks, and 48 weeks. Bioactive IGF-1 was assessed using the kinase receptor activation assay (KIRA). RESULTS: There were no significant differences in any of the components of the IGF system (all p- values >.05). Immunoreactive IGF-I levels correlated significantly with bioactive IGF-I at baseline (r =.50, p =.02) and at 48 weeks (r =.64, p =.01). Both VO2peak (p =.03) and total treadmill time (TT) (p =.00) increased over 48 weeks in the treatment group. CONCLUSIONS: Despite improvements in physical performance in this CKD patient population, a 48-week training program did not affect either immunoreactive or bioactive circulating IGF-I. These data suggest that if the IGF-I system is involved or mediating these positive exercise-induced adaptations in CKD patients; changes are not reflected by blood/serum measurements. Funded by the Baystate Medical Center Incubator Fund.Table

Early serum IGF-I response to oral protein supplements in elderly women with a recent hip fracture

In patients with recent hip fracture, reduced serum IGF-I in relation to protein undernutrition is frequent. Elevation of circulating IGF-I in response to a daily oral supplement of 20 g of casein was observed after 6 months. This study determined if the response to casein as compared to whey protein can be observed as early as after one week. 45 women were randomized after recent hip fracture in 3 groups receiving a preparation of 20 g of casein, an isocaloric supplement of 20 g of whey protein or an isocaloric supplement of 15 g of whey protein combined with 5 g of essential amino acids (a.a.). A similar significant elevation of serum IGF-I was already observed after 7 days for casein (+37.3 microg/L), whey (+29.4) and for whey+a.a. (+34.3). From day 7-28, no further significant rise in IGF-I was recorded. After one week of protein supplementation, the percent increase of IGF-I was of similar magnitude to that previously observed after 6 months of protein supplementation. It suggests that in hip fracture patients, long-term effects of various protein preparations on IGF-I could be predicted from changes observed as early as 7 days after the onset of supplementation.

Role of the integrin αVβ3 in mediating increased smooth muscle cell responsiveness to IGF-I in response to hyperglycemic stress

Role of the integrin αVβ3 in mediating increased smooth muscle cell responsiveness to IGF-I in response to hyperglycemic stress

Estrogen-dependent responses of the mammary fat pad in prepubertal dairy heifers

Ovaries are absolutely required for development of the mammary parenchyma (PAR) in cattle, reflecting estrogen-dependent epithelial cell proliferation. However, the estrogen receptor (ER) that mediates the mammary estrogen effects, ERalpha, is absent in proliferating epithelial cells. In the mouse, this discrepancy is explained in part by the ability of the mammary fat pad (MFP) to synthesize epithelial cell mitogens such as IGF-I in response to estrogen. Consistent with a similar role for the bovine MFP, 30% of its fibroblasts and adipocytes were immunoreactive for ERalpha in prepubertal dairy heifers. To assess estrogen-dependent gene expression in the MFP, 16 prepubertal dairy heifers were randomly assigned to a 2x2 factorial. The first factor was ovarian status, with heifers undergoing bilateral ovariectomy or left intact at 4.6 months of age. The second factor was applied 30 days after surgery and consisted of injection of estrogen or excipient. After 3 days of injection, heifers were administered an intrajugular bolus of bromodeoxyuridine (BrdU) and slaughtered 2 h later. The estrogen injection, but not ovarian status, caused significant increases in the fraction of epithelial cells labeled with BrdU and produced tissue-specific effects on gene expression. In the PAR, estrogen injection increased IGF-I gene expression by twofold despite reductions of 50% or more in ERalpha mRNA abundance and the fraction of epithelial cells immunoreactive for ERalpha. The estrogen-dependent increase in IGF-I mRNA was greater in the MFP, presumably because estrogen failed to downregulate ERalpha expression in this mammary compartment. Finally, estrogen-responsiveness of the MFP appears unique among the bovine fat depots as estrogen injection did not induce IGF-I expression in its s.c. counterpart. Our data demonstrate that the bovine MFP is highly responsive to exogenous estrogen, consistent with a role for this tissue compartment in communicating its effects on epithelial cell proliferation.

Pregnancy-Associated Plasma Protein-A Is Involved in Matrix Mineralization of Human Adult Mesenchymal Stem Cells and Angiogenesis in the Chick Chorioallontoic Membrane

Pregnancy-associated plasma protein A (PAPP-A) is an IGF binding protein 4 protease that can function to increase local IGF-I bioavailability. Aside from its assumed role during pregnancy, in vitro and in vivo studies have indicated roles for PAPP-A in IGF-I-mediated wound healing, vascular repair, and bone formation. Because bone morphogenetic protein 2 (BMP-2) is known to up-regulate Igf-I gene expression, we hypothesized that PAPP-A may be involved in BMP-2 mechanisms in bone formation. To test this hypothesis, we quantified gene expression of Papp-A in response to BMP-2 treatment and runt-related transcription factor 2, Osterix, and Igf-I in response to PAPP-A protein treatment in human adult mesenchymal stem cells. Our results demonstrate that BMP-2 directly up-regulated Papp-A gene and protein expression. Purified PAPP-A protein directly up-regulated runt-related transcription factor 2 and Igf-I gene expression but not Osterix. When added in combination with recombinant human BMP-2, PAPP-A increased matrix mineralization in the absence of dexamethasone. PAPP-A further demonstrated an angiogenic effect in the chick chorioallontoic membrane, which implicates a critical developmental role and possible therapeutic potential. Our findings suggest that PAPP-A functions in the formation of mineralized tissues through direct up-regulation of key genes. Furthermore, PAPP-A is involved in the formation of new blood vessels, which is essential for proper bone regeneration.

Invited commentary

The growth hormone and insulin-like growth factor I (GH-IGF-I) axis plays an important role in cardiac development and plays a trophic role on the heart. There is a high density of growth hormone receptors in cardiomyocytes, and locally produced IGF-I in response to exogenous growth hormone (GH) has been shown to improve contractility and leads to increased synthesis of contractile myofilaments and hypertrophy.Growth hormone and insulin-like growth factor I axis has also been implicated in heart failure from a variety of etiologies both experimentally and in the clinical setting, where a putative role has been suggested. A number of clinical trials now completed and currently underway are attempting to determine the potential therapeutic benefits of growth hormone supplementation in adults with symptoms of congestive heart failure but who are not growth hormone deficient. The rationale for this approach is based on the observation that a relative growth hormone insensitivity develops as heart failure progresses, with high serum GH levels and low IGF-I levels. In children with dilated cardiomyopathy, growth hormone administration is proposed as a therapy that could promote hypertrophy and increase in muscle mass. Clinical trials are ongoing in pediatric patients with cardiomyopathy to evaluate the impact of exogenous growth hormone on cardiac function and symptoms of congestive heart failure.The report by Tsai and associates showing similar findings in a group of children with isolated ventricular septal defect is consistent with previous reports describing the growth hormone insensitivity in children with congenital heart defects and symptoms of heart failure. However, the observation that GH and IGF-I levels gradually return towards normal following surgical repair is important because it supports the theory that GH insensitivity is a secondary phenomenon associated with heart failure, rather than a primary defect contributing to the progression to heart failure due to an inability to adapt to the increase in cardiac workload. To resolve this controversy more definitively, however, further studies monitoring GH-IGF-I serum levels before and after surgical repair, as well as correlating serum levels with actual tissue levels of these growth factors, may be necessary. Until further insight is gained into the mechanism of growth hormone insensitivity in children with congenital heart defects and symptoms of heart failure, a proposal for therapeutic GH supplementation in these patients may be premature. The growth hormone and insulin-like growth factor I (GH-IGF-I) axis plays an important role in cardiac development and plays a trophic role on the heart. There is a high density of growth hormone receptors in cardiomyocytes, and locally produced IGF-I in response to exogenous growth hormone (GH) has been shown to improve contractility and leads to increased synthesis of contractile myofilaments and hypertrophy. Growth hormone and insulin-like growth factor I axis has also been implicated in heart failure from a variety of etiologies both experimentally and in the clinical setting, where a putative role has been suggested. A number of clinical trials now completed and currently underway are attempting to determine the potential therapeutic benefits of growth hormone supplementation in adults with symptoms of congestive heart failure but who are not growth hormone deficient. The rationale for this approach is based on the observation that a relative growth hormone insensitivity develops as heart failure progresses, with high serum GH levels and low IGF-I levels. In children with dilated cardiomyopathy, growth hormone administration is proposed as a therapy that could promote hypertrophy and increase in muscle mass. Clinical trials are ongoing in pediatric patients with cardiomyopathy to evaluate the impact of exogenous growth hormone on cardiac function and symptoms of congestive heart failure. The report by Tsai and associates showing similar findings in a group of children with isolated ventricular septal defect is consistent with previous reports describing the growth hormone insensitivity in children with congenital heart defects and symptoms of heart failure. However, the observation that GH and IGF-I levels gradually return towards normal following surgical repair is important because it supports the theory that GH insensitivity is a secondary phenomenon associated with heart failure, rather than a primary defect contributing to the progression to heart failure due to an inability to adapt to the increase in cardiac workload. To resolve this controversy more definitively, however, further studies monitoring GH-IGF-I serum levels before and after surgical repair, as well as correlating serum levels with actual tissue levels of these growth factors, may be necessary. Until further insight is gained into the mechanism of growth hormone insensitivity in children with congenital heart defects and symptoms of heart failure, a proposal for therapeutic GH supplementation in these patients may be premature.

Incubation Temperature Affects Plasma Insulin-Like Growth Factors in Embryos from Selected Lines of Turkeys

An experiment was conducted to test the hypothesis that incubator temperature may affect circulating insulin-like growth factors (IGF-I and IGF-II). In prior studies, growth of turkey embryos was altered by increasing incubator temperatures. Interestingly, the embryonic growth of a growth-selected line (F) was reduced, whereas embryos from an egg-production-selected line (E) did not alter embryonic growth but altered organogenesis. Growth of the F and E lines was altered experimentally in the current study by increasing incubator temperature from 36.8 to 37.2 C during the last 3 d of incubation. Embryonic blood samples were taken and analyzed for glucose, glucagon, IGF-I, and IGF-II concentrations.Increased incubator temperature elevated embryonic plasma glucose concentrations of all treatments compared to controls, which was accompanied by increased plasma glucagon concentration only in the E line embryos. Line and treatment interacted to affect IGF-I and IGF-II concentrations of embryo and hatchlings. Line E embryos increased IGF-I in response to the higher temperature, but controls did not; F embryos altered IGF-II in response to treatment, but controls did not. Alterations in IGF-I in E corresponded to growth responses, whereas IGF-II in F corresponded to metabolic responses. We concluded that changes in turkey embryo growth rates to incubator temperature involved changes in IGF-I. Additionally, IGF-II and glucagon are involved in intermediary metabolism during higher temperature exposure.

Substitution of Specific Amino Acids in Insulin-like Growth Factor (IGF) Binding Protein 5 Alters Heparin Binding and Its Change in Affinity for IGF-I in Response to Heparin

Heparin binding to insulin-like growth factor (IGF)-binding protein 5 (IGFBP-5) leads to a 17-fold decrease in its affinity for IGF-I, and a region that contains several basic amino acids (Arg201-Arg218) may be involved in this affinity shift. In the present study, mutagenesis was used to analyze the effect of substitutions for basic amino acids in the Arg201-Arg218 region of IGFBP-5 on heparin-binding and the heparin-induced affinity shift. Nine mutant forms were prepared. Their association constants (Ka) for IGF-I were similar to native IGFBP-5. When 10 microg/ml of heparin was added, the Ka of native IGFBP-5 decreased 17-fold, and the Ka of the K134A/R136A mutant decreased 16-fold. In contrast, substitutions for specific basic amino acids in the Arg2O1-Arg218 region decrease the affinity shift to 1.1-3.2-fold. Lys 211 was especially important. When a mutant containing that single substitution was tested, heparin caused only a 2.5-fold reduction in IGF-I affinity. Affinity cross-linking studies showed that heparin was equipotent in inhibiting the formation of 125I-IGF-I-K134A/Rl36A mutant complexes compared to native IGFBP-5. In contrast, heparin had minimal effects on the formation of complexes between 125I-IGF-I and the other mutants. The heparin-binding activity of each mutant was determined. Four mutants, R201A/K202N, K202A/K206A/R207A, R201A/K202N/K206N/K208N, and K211N/R214A/K217A/R218A, had reduced heparin binding compared to native IGFBP-5. The other five mutants, including the K21IN mutant, showed no change in heparin binding. The four mutants with reduced heparin binding could be dissociated from heparin-Sepharose with much lower NaCl concentrations, indicating that they had reduced affinity. These findings suggest that Arg201 Lys202, LysS206, and Arg214 are important for heparin binding. In contrast, LyS211 is not important for the binding of IGFBP-5 to heparin, but substitution for it reduced the heparin-induced affinity shift.

Substitution of Specific Amino Acids in Insulin-like Growth Factor (IGF) Binding Protein 5 Alters Heparin Binding and Its Change in Affinity for IGF-I in Response to Heparin

Substitution of Specific Amino Acids in Insulin-like Growth Factor (IGF) Binding Protein 5 Alters Heparin Binding and Its Change in Affinity for IGF-I in Response to Heparin

Modulation of plasma IGF-I in response to acute ethanol toxiciry

Modulation of plasma IGF-I in response to acute ethanol toxiciry

Differential regulation of IGF-I leader exon transcription.

We have shown that there is differential expression of the two IGF-I leader exons during physiological changes in GH and nutritional status in liver, with exon 2 being more responsive than exon 1. It may be that this exon is responsible for endocrine IGF-I synthesised in response to specific anabolic stimuli inducing subsequent whole body and skeletal muscle growth.