IGFBP-3 Proteolysis Research Articles

Alpha-1 Antitrypsin Inhibits Tumorigenesis and Progression of Colitis-Associated Colon Cancer through Suppression of Inflammatory Neutrophil-Activated Serine Proteases and IGFBP-3 Proteolysis.

Colitis-associated colon cancer (CAC) accompanies the massive infiltration of neutrophils during tumorigenesis and progression of CAC. Depletion of neutrophils in circulation results in significant inhibition of tumor incidence in CAC. However, the underlying mechanisms are largely unclear. In this study, we provide evidence for the crucial involvement of inflammatory neutrophil-activated serine proteases (NSPs) on the dysregulation of the anti-inflammatory and antitumor IGFBP-3/IGFBP-3R signaling axis in CAC using a chronic AOM/DSS mouse model. We also provide preclinical evidence for α1-antitrypsin (AAT) as a preventive and as a therapeutic for CAC. AAT administration not only prevented colitis-associated tumorigenesis but also inhibited established CAC. AOM/DSS treatment results in the significant activation of NSPs, leading to CAC through increased pro-inflammatory cytokines and decreased anti-inflammatory and antitumor IGFBP-3. Collectively, these data suggest that the NSPs proteolyze IGFBP-3, whereas AAT inhibits chronic colonic inflammation-induced NSP activity and subsequently suppresses IGFBP-3 proteolysis. Therefore, the anti-inflammatory and antitumor functions of the IGFBP-3/IGFBP-3R axis are restored. AAT mimicking small peptides also showed their inhibitory effects on NSP-induced IGFBP-3 proteolysis. These results suggest that targeting the NSP-IGFBP-3/IGFBP-3R axis using NSP inhibitors such as AAT and the AAT mimics and IGFBP-3R agonists could lead to novel approaches for the prevention and treatment of CAC.

Macrosomia and the IGF System: A Short Review

Introduction: Macrosomia in diabetic pregnancy is an ubiquitous finding and implies both maternal and fetal issues. Its incidence surpasses other critical obstetrical morbidity in diabetic pregnancy with increased placental size and polyhydramnios as comorbidity. The studies on the underlying factors contributing to macrosomia have focused on growth stimuli like IGF, glycemia, and insulin and their effects on regional organ function. IGF-I and –II in maternal serum are associated with birth weight and the bioavailabiliy is modulated by IGF- binding-proteins (IGFBP) and phosporylated isoforms of IGFBP and the presence of proteases and IGF split products. Human placental growth hormone regulates the effect of IGF-I in pregnancy.Methods: The literature on the IGF system′s possible effect on fetal growth in diabetic pregnancy is reviewed before our current studies and updated with respect to later findings. Regulation on bioavailability by proteolysis and phosporylation is described. The review discusses briefly the studies on pregnant women with type 1 diabetes with respect to vasculopathy.Results: The data show good correlation of IGF-1/2 levels with birth weight. No such correlation with IGFBP-1 and -3 is found. Fetal growth deviation in diabetic pregnancy is not uniform. In first trimester, growth delay is documented in fetal growth curves compared to non-diabetic controls. Accordingly, a shift in IGF-IGFBP regulation in second trimester is found with less proteolysis of IGFBPs and relative increase in total binding capacity. In third trimester, free IGF further increases as IGFBP binding decreases by proteolysis and modulation. The first trimester values may be used as predictor of fetal weight at term. Diabetic women show a two- to threefold increase in the in vitro IGFBP-3 proteolytic activity during pregnancy as compared to post-partum values.Conclusions: Prominent proteolysis of binding proteins and the modulation by insulin account for the mechanism leading to macrosomia. The two factors combined guarantee the stimulation by more bioavailable IGF and the glycemic levels reveal whether insulin dose are optimal. Structural factors of diabetes i.e. microangiopathy further modulate the sum of effects on fetal growth. In contrast to macrosomia, fetal growth restriction is associated with prominent maternal vascular endothelial complications, such as overt nephropathy, proliferative retinopathy, preeclampsia, and hypertension. In parallel to the growth inhibition, stimulatory effects are exerted by the GH axis including the placental growth hormones and occur probably as compensatory mechanism, similar to the increased IGFBP-3 proteolysis in late pregnancy found in growth retardation. This influence may in part explain the disproportionate growth in different tissue components occurring in the various stages of diabetic pregnancy.

Free dissociable IGF-I: Association with changes in IGFBP-3 proteolysis and insulin sensitivity after surgery

Patients receiving a carbohydrate drink (CHO) before major abdominal surgery display improved insulin sensitivity postoperatively and increased proteolysis of IGFBP-3 (IGFBP-3-PA) compared to patients undergoing similar surgery after overnight fasting. We hypothesized that serum IGFBP-3-PA increases bioavailability of circulating IGF-I and preserves insulin sensitivity in patients given CHO. Matched control study. At Karolinska University Hospital, patients given CHO before major elective abdominal surgery (CHO,n=8) were compared to patients undergoing similar surgical procedures after overnight fasting (FAST,n=10). Results from two different techniques for determination of free-dissociable IGF-I (fdIGF-I) were compared with changes in IGFBP-3-PA and insulin sensitivity. Postoperatively, CHO displayed 18% improvement in insulin sensitivity (hyperinsulinemic clamp) and increased IGFBP-3-PA vs. FAST. As determined by IRMA, fdIGF-I increased by 48±25% in CHO while fdIGF-I decreased by 13±18% in FAST (p<0.01 vs. CHO, when corrected for duration of surgery). However, fdIGF-I determined by ultra-filtration decreased similarly in both groups (-22±8% vs.-25±8%, p=0.8) and IGFBP-1 increased similarly in both groups. Patients with less insulin resistance after surgery demonstrated larger increases in fdIGF-I by IRMA (r=0.58, p<0.05). Fifty-three % of the variability of the changes in fdIGF-I by IRMA could be explained by changes in IGFBP-3-PA and total IGF-I levels (p<0.05), while IGFBP-1 did not contribute significantly. During conditions when serum IGF-I bioavailability is regulated by IGFBP-3 proteolysis, measurements of fdIGF-I by IRMA is of physiological relevance as it correlates with the associated changes in insulin sensitivity.

Papp-a2 modulates development of cranial cartilage and angiogenesis in zebrafish embryos

Pregnancy-associated plasma protein A2 (PAPP-A2, also known as pappalysin-2) is a large metalloproteinase that is known to be required for normal postnatal growth and bone development in mice. We here report the detection of zebrafish papp-a2 mRNA in the chordamesoderm, notochord and lower jaw of zebrafish ( Danio rerio ) embryos, and that papp-a2 -knockdown embryos display broadened axial mesoderm, notochord bends and severely reduced cranial cartilages. Genetic data link these phenotypes to insulin-like growth factor (Igf)-binding protein-3 (Igfbp-3) and bone morphogenetic protein (Bmp) signaling, and biochemical analysis show specific Igfbp-3 proteolysis by Papp-a2, implicating Papp-a2 in the modulation of Bmp signaling by Igfbp-3 proteolysis. Knockdown of papp-a2 additionally resulted in angiogenesis defects, strikingly similar to previous observations in embryos with mutations in components of the Notch system. Accordingly, we find that Notch signaling is modulated by Papp-a2 in vivo , and, furthermore, that human PAPP-A2 is capable of modulating Notch signaling independently of its proteolytic activity in cell culture. Based on these results, we conclude that Papp-a2 modulates Bmp and Notch signaling by independent mechanisms in zebrafish embryos. In conclusion, these data link pappalysin function in zebrafish to two different signaling pathways outside the IGF system.

Ternary complex formation and IGFBP-3 proteolytic activity during childhood: age-dependent changes.

IGF-I is mainly sequestered in a 150-kDa ternary complex with IGF binding protein (IGFBP)-3 and the acid-labile subunit. Data on complex formation and factors influencing formation have not been established. Dissociation of IGF-I from the ternary complex is in part regulated by proteolysis of IGFBP-3, which reduces its affinity for IGF-I. Short small for gestational age (SGA) children have lower IGF-I and IGFBP-3 levels compared with healthy peers. The objective of the study was to determine complex formation in healthy normal-statured children and assess variables influencing complex formation. Second, we determined complex formation in short SGA children. Complex formation was assessed using (125)I-hIGF-I column chromatography in 70 controls (40 boys), median age 10.6 years, and 40 short SGA children (25 boys), median age 8.6 years. IGFBP-3 was determined by Western immunoblotting. (125)I-hIGF-I complex formation showed an age-specific pattern in healthy controls. Variables positively influencing ternary complex formation were higher serum IGF-I levels compared with IGFBP-3 levels (P < .001) and lower serum IGF-II (P < .001) and IGFBP-1 levels (P < .001). In addition, a higher presence of proteolyzed IGFBP-3 negatively influenced 150-kDa complex formation (P = .006). At a young age, healthy children showed considerable IGFBP-3 proteolytic activity, which declined with aging (P < .001). IGFBP-3 proteolytic activity was negatively correlated with IGF-I levels (P < .001). Compared with healthy controls, short SGA children showed reduced IGF-I levels (-1.3 vs 0.1 SD score) and increased proteolyzed IGFBP-3 (35.1% vs 12.2%). Age-specific normative values for (125)I-hIGF-I 150-kDa ternary complex formation are presented. A decrease in IGF-I and an increase in IGF-II, IGFBP-1, and IGFBP-3 proteolytic activity associate with reduced (125)I-hIGF-I ternary complex formation. Our results suggest that in conditions in which IGF-I levels are low, such as young age and in short SGA children, IGFBP-3 proteolytic activity is increased to ensure IGF-I bioavailability.

Papp-a2 modulates development of cranial cartilage and angiogenesis in zebrafish embryos.

Pregnancy-associated plasma protein A2 (PAPP-A2, also known as pappalysin-2) is a large metalloproteinase that is known to be required for normal postnatal growth and bone development in mice. We here report the detection of zebrafish papp-a2 mRNA in the chordamesoderm, notochord and lower jaw of zebrafish (Danio rerio) embryos, and that papp-a2-knockdown embryos display broadened axial mesoderm, notochord bends and severely reduced cranial cartilages. Genetic data link these phenotypes to insulin-like growth factor (Igf)-binding protein-3 (Igfbp-3) and bone morphogenetic protein (Bmp) signaling, and biochemical analysis show specific Igfbp-3 proteolysis by Papp-a2, implicating Papp-a2 in the modulation of Bmp signaling by Igfbp-3 proteolysis. Knockdown of papp-a2 additionally resulted in angiogenesis defects, strikingly similar to previous observations in embryos with mutations in components of the Notch system. Accordingly, we find that Notch signaling is modulated by Papp-a2 in vivo, and, furthermore, that human PAPP-A2 is capable of modulating Notch signaling independently of its proteolytic activity in cell culture. Based on these results, we conclude that Papp-a2 modulates Bmp and Notch signaling by independent mechanisms in zebrafish embryos. In conclusion, these data link pappalysin function in zebrafish to two different signaling pathways outside the IGF system.

IGFBP-3 Inhibits Cytokine-Induced Insulin Resistance and Early Manifestations of Atherosclerosis

Metabolic syndrome is associated with visceral obesity, insulin resistance and an increased risk of cardiovascular diseases. Visceral fat tissue primarily consists of adipocytes that secrete cytokines leading to a state of systemic inflammation in obese conditions. One of the IGF-independent functions of IGFBP-3 is its role as an anti-inflammatory molecule. Our study in obese adolescents show a decrease in total IGFBP-3 levels and increase in proteolyzed IGFBP-3 in circulation when compared to their normal counterparts and establishes a positive correlation between IGFBP-3 proteolysis and adiposity parameters as well as insulin resistance. In human adipocytes, we show that IGFBP-3 inhibits TNF-α-induced NF-κB activity in an IGF-independent manner, thereby restoring the deregulated insulin signaling and negating TNF-α-induced inhibition of glucose uptake. IGFBP-3 further inhibits TNF-α, CRP and high glucose-induced NF-κB activity in human aortic endothelial cells (HAECs) and subsequently suppresses monocyte adhesion to HAEC through the IGFBP-3 receptor. In conclusion, these findings suggest that reduced levels of IGFBP-3 in circulation and reduced expression of IGFBP-3 in macrophages in obesity may result in suppression of its anti-inflammatory functions and therefore IGFBP-3 may present itself as a therapeutic for obesity-induced insulin resistance and for events occurring in the early stages of atherosclerosis.

Role of insulin-like growth factor binding protein-3 in glucose and lipid metabolism.

Insulin-like growth factor binding protein (IGFBP)-3 has roles in modulating the effect of IGFs by binding to IGFs and inhibiting cell proliferation in an IGF-independent manner. Although recent studies have been reported that IGFBP-3 has also roles in metabolic regulation, their exact roles in adipose tissue are poorly understood. In this review, we summarized the studies about the biological roles in glucose and lipid metabolism. IGFBP-3 overexpression in transgenic mice suggested that IGFBP-3 results in glucose intolerance, and insulin resistance. IGFBP-3 knockout (KO) mice exhibited normal insulin level and glucose response after glucose challenge. More recent study in IGFBP-3 KO mice with a high-fat diet demonstrated that IGFBP-3 KO mice exhibited elevated fasting glucose and insulin, but normal response to glucose challenge, suggesting that IGFBP-3 KO mice may induce insulin resistance even though preserved insulin sensitivity. In vitro and in vivo studies using 3T3-L1 adipocytes and rat, IGFBP-3 induced insulin resistance by inhibiting glucose uptake. In contrast, the reduced levels of IGFBP-3 in obesity might induce insulin resistance by suppression of IGFBP-3's anti-inflammatory function, suggesting IGFBP-3 has a protective effect on insulin resistance. Also, proteolysis of IGFBP-3 might contribute to the insulin resistance in obesity and type 2 diabetes mellitus. In addition, IGFBP-3 inhibited adipocyte differentiation, suggesting IGFBP-3 may contribute to the insulin insensitivity. Taken together, it is not yet certain that IGFBP-3 has a protective effect or enhancing effect on insulin resistance, and more studies will be needed to clarify the roles of IGFBP-3 in metabolic regulation.

Correlation between insulin-like growth factor binding protein-3 serum level and melanoma progression

Insulin-like growth factor (IGF) binding protein (IGFBP)-3 is the main carrier of circulating IGFs and the main modulator of their activity. IGFBP-3 controls cellular availability of IGFs, which cannot exert their pro-proliferative activity while bound to IGFBP-3. Proteolysis of IGFBP-3 is one mechanism to control IGF release. A reduction of serum IGFBP-3 levels and the associated increased availability of IGFs may represent a strategy whereby melanoma increases its metastatic potential. The aim of our study was to evaluate the correlation between the IGFBP-3 serum level and melanoma stage. The study included 41 patients, 24 male and 17 female, with median age of 60 years (range 24-80), affected by cutaneous melanoma. Blood samples were taken from each patient and IGFBP-3 serum levels were measured using Western blot analysis with commercial antibodies. Values were normalized using commercial IGFBP-3. The statistical analysis showed that full-size, glycosylated IGFBP-3 concentrations were significantly lower in the sera of patients with stage IV melanoma. Low serum levels of IGFBP-3 correlated with both disease progression and presence of disease at the time of sample collection. In patients who underwent follow-up visits with further collections of blood samples, the concentrations of glycosylated IGFBP-3 decreased only in those who showed progression of disease. Our study shows only preliminary results on a limited number of patients. We demonstrate that there is a significant inverse correlation between the serum concentration of full-size, glycosylated IGFBP-3 and disease progression in patients with melanoma.

Acute regulation of IGF-I by alterations in post-exercise macronutrients

This investigation sought to examine the contributions of exercise and nutrient replenishment on in vivo regulation of the insulin-like growth factor-I (IGF-I) axis components. Eight college-aged males completed three high-intensity interval training (HIIT) protocols followed by three post-exercise nutritional protocols: (1) placebo (EX); (2) carbohydrate only (CHO); and (3) essential amino acid/carbohydrate (EAA/CHO). Samples were analyzed for growth hormone (GH), free IGF-I, IGFBP-1, IGFBP-2, insulin, hematocrit, hemoglobin, serum leucine, matrix metalloproteinase-9 (MMP-9) proteolytic activity, and presence of IGFBP-3 protease activity. No evidence for IGFBP-3 proteolysis was observed. Significant increases in [free IGF-I] and [leucine] were observed in the EAA/CHO group only. Significant differences were noted in [IGFBP-1] and [IGFBP-2] across conditions. Significant increases in [GH] and MMP-9 activity were observed in all groups. These results indicate that post-exercise macronutrient ratio is a determinant of [free IGF-I], [IGFBP-1 and -2] and may play a role in modulating the IGF-I axis in vivo.

Regulation of prostate cancer cell proliferation by somatostatin receptor activation

Although some evidence supports the antitumoral effects of somatostatin (SRIF) and related agonists, the available data in prostate cancer (PCa) model systems and clinical studies are few, conflicting and not conclusive. This study investigated the effects of lanreotide and new mono- and bi-specific SRIF agonists on proliferation, ligand-driven SRIF receptor (sst) dimerization and secretory pattern of the IGF system in LNCaP cells, a model of androgen-dependent PCa. LNCaP expressed all sst(s), but sst(4). Among them, sst(1) and sst(3) were inversely regulated by serum concentration. sst(1)/sst(2) and sst(2)/sst(5) dimers were constitutively present and further stabilized by treatment with BIM-23704 (sst(1)/sst(2)) and BIM-23244 (sst(2)/sst(5)), respectively. Dose-response studies showed that lanreotide and BIM-23244 were significantly more potent in inhibiting LNCaP cell proliferation than BIM-23120 (sst(2)) and BIM-23206 (sst(5)) alone or in combination. Treatment with BIM-23926 [corrected] (sst(1)) markedly reduced cell proliferation, whereas exposure to BIM-23704 resulted in a lower cell growth inhibition. The antiproliferative effects of BIM-23244, lanreotide and BIM-23704 were unchanged, reduced and abolished by the sst(2) antagonist BIM-23627, respectively. All SRIF analogs caused a significant induction in p27(KipI) and p21 and down-regulation of protein expression of cyclin E, as well as reduced IGF-I and IGF-II secretion. In particular, the administration of exogenous IGF-I, at variance to IGF-II, counteracted the inhibitory effect on cell proliferation of these compounds. Moreover, SRIF agonists reduced endogenous IGFBP-3 proteolysis. These results show that, in LNCaP cells, activation of sst(1) and sst(2)/sst(5) results in relevant antiproliferative/antisecretive actions.

Interstitial IGF-I in exercising skeletal muscle in women

To study interstitial IGF-I concentrations in resting and exercising skeletal muscle in relation to the circulating components of the IGF-IGF binding protein (IGFBP) system. Seven women performed endurance exercise with 1 leg (Ex-leg) for 1 h. The resting leg (Rest-leg) served as a control. IGF-I was determined in microdialysate (MD) and was compared with veno-arterial (v-a) concentrations of circulating IGF-IGFBP components. Median (range) basal MD-IGF-I was 0.87 (0.4-1.5) microg/l or 0.4 (0.2)% of total-IGF-I (t-IGF-I) determined in arterial serum and in the same concentration range as free dissociable IGF-I (f-IGF-I). Rest-leg MD-IGF-I decreased, reaching significance after exercise. Ex-leg MD-IGF-I was unchanged during exercise and declined after exercise at the level of significance (P = 0.05). There was a release of f-IGF-I from the Ex-leg into the circulation at the end of and shortly after exercise. A small but significant increase in circulating IGFBP-1 was detected at the end of exercise and IGFBP-1 increased further after exercise. Although interleukin-6 (IL-6) has been associated with IGFBP-3 proteolysis, the circulating molecular forms of IGFBP-3 remained unchanged in spite of an IL-6 release from the muscle compartment. Circulating IGFBP-1 is related to interstitial IGF-I in resting muscle although the temporal relationship may not be simple. Further studies should explore the role of local release of IGF-I and its impact on IGF-I activity during contraction.

Normalization of the IGF-IGFBP Axis by Sustained Nightly Insulinization in Type 1 Diabetes

We sought to test the hypothesis that start of insulin glargine with sustained nightly insulin action results in changes in circulating concentrations of IGF-I and IGF binding proteins (IGFBPs) in adolescents with type 1 diabetes-changes that may support improvement of A1C. Twelve pubertal adolescents with type 1 diabetes and initially on NPH insulin were studied during 12 weeks of intensified treatment with glargine. Subnormal IGF-I SD scores on NPH (-1.8 +/- 0.4) rapidly increased and remained 54 +/- 9% elevated (P < 0.001) after 12 weeks on glargine. A1C decreased from 8.3 +/- 0.6% to a nadir of 6.9 +/- 0.3% (P = 0.002) at 6 weeks and correlated with changes in IGF-I (r = -0.64, P < 0.05). The increase in IGF-I did not suppress the mean overnight growth hormone (GH) secretion at 6 weeks. The mean overnight IGFBP-1 levels decreased (P = 0.035), supporting the hypothesis that the nightly hepatic insulin action was increased. Circulating IGF-I increased in the absence of changes in both GH secretion and GH receptor numbers (assessed by growth hormone binding protein), indicating that postreceptor mechanisms are involved. IGFBP-3 proteolysis was decreased. Increased hepatic insulin action after start of glargine was evident from a decrease in night time IGFBP-1 concentrations. This may improve GH postreceptor signaling, resulting in increased circulating IGF-I. We suggest that even in the absence of changes in GH, increased IGF-I and decreased IGFBP-1 support the improvement of metabolic control.

Short Stature in Children with Sickle Cell Anemia Correlates with Alterations in the IGF-I Axis

Children with sickle cell anemia (SCA) frequently have short stature. We propose that alterations in the IGF-I axis are involved in their growth failure. We investigated the IGF-I axis in children with SCA and height below the 25th percentile (n = 15) and compared it with that of children with SCA and height above the 50th percentile (n = 7). IGF-I and IGFBP-3 levels were assessed by RIA. IGFBP-3 proteolysis was assessed by a protease activity assay and by Western immunoblots. IGF-I and IGFBP-3 SDS were low for both groups. In the short statured patients, IGF-I SDS correlated with height velocity SDS (p = 0.018). IGFBP-3 SDS, when corrected for bone age, decreased with age (p = 0.0054). IGFBP-3 was proteolyzed in both groups although the short statured patients had lower levels of absolute intact IGFBP-3 when compared with the normally growing group (p = 0.028). We demonstrated that children with SCA have abnormalities in the IGF-I axis, which worsen with age.

Local changes in the insulin-like growth factor system in human skeletal muscle assessed by microdialysis and arterio-venous differences technique

IGF-I plays a direct role in whole body glucose homeostasis primarily by stimulating skeletal muscle glucose uptake. IGF-I is also involved in exercise induced muscle hypertrophy. Knowledge regarding local changes in muscle IGF-I bioavailability and its regulation by IGFBPs at rest and during exercise is limited. We have therefore explored changes in total IGF-I levels as well as circulating IGFBP levels and their post-translational modifications over an exercising leg. For the first time we have determined IGF-I levels in exercising skeletal muscle microdialysate in an attempt to assess local IGF-I bioavailability. Eighteen healthy young men performed one legged knee-extension exercise during 45 min. Blood samples were taken from the femoral artery and vein of the exercising leg. No significant differences between arterial and venous concentrations of total IGF-I or IGFBP-1 were detected over the leg at any time. IGF-I concentrations increased significantly during exercise in the artery but not in the vein. Total IGFBP-1 increased after exercise in both artery and vein. The increase in non-plus less phosphorylated forms of IGFBP-1 was less pronounced and did not reach statistical significance. The proportion of fragmented IGFBP-3 (IGFBP-3 proteolysis) assessed by Western immunoblotting did not change significantly during or after exercise. Although optimization and validation of IGF-I determinations in muscle microdialysate (md) will be required, our first results using this technique demonstrate a significant 2-fold increase in mdIGF-I collected during and after exercise. We conclude that determination of A-V-differences appears to be of limited value in the assessments of local muscle change in the IGF-system. A substantial release of IGF-I during short time is required to detect significant change in the large circulating store of IGF-I. We suggest that an optimized and validated microdialysis technique for determination of local IGF-I may be advantageous in future studies.

Glycosaminoglycans increase levels of free and bioactive IGF-I in vitro

It is unclear how IGFs become separated from their IGF-binding proteins (IGFBPs) in vivo. However, the IGFBPs possess binding sites for glycosaminoglycans (GAGs) and interaction with GAGs alters IGFBP ligand affinity. Accordingly, GAGs may control IGF bioavailability. To test this hypothesis, we investigated the effect of GAGs on serum levels of free and bioactive IGF-I, total IGF-I, and IGFBPs in vitro. Serum was incubated with increasing concentrations of six different GAGs (heparin, tinzaparin (Innohep), dermatan sulfate, heparan sulfate, non-anticoagulant (nac) heparin, and nac low-molecular weight heparin). To investigate for reversibility, heparin was co-incubated with protamine sulfate (PS). Finally, the effect of heparin was studied in serum from pregnant and post partum women, normal subjects and patients with type 1 diabetes. All GAGs increased free IGF-I in a dose-dependent manner (P<0.0001), whereas total IGF-I and IGFBP levels remained unchanged. However, the potency of the GAGs differed significantly (P<0.0001) and did not relate to their anti-coagulating activity. The effect of heparin on free IGF-I was fully reversed by PS. Heparin increased free and bioactive IGF-I in all tested sera (P<0.0001), but the increase was most pronounced in samples from pregnant women (P<0.0001). All tested GAGs stimulated the release of free and bioactive IGF-I in several types of serum, most likely by reversible interaction with the IGFBPs. The effect was most pronounced in pregnancy sera, which are characterized by extensive IGFBP-3 proteolysis. Our findings support the view that GAGs localized in the vessel wall and attached to the extracellular matrix control IGF-I tissue accessibility and bioactivity.

Acute Interleukin-6 Infusion Increases IGFBP-1 but Has No Short-Term Effect on IGFBP-3 Proteolysis in Healthy Men

Human conditions of elevated interleukin-6 (IL-6) and transgenic mice overexpressing IL-6 have increased proteolytic degradation of insulin-like growth factor binding protein (IGFBP)-3. In addition, IL-6 alters the hepatic expression of insulin-like growth factor-I (IGF-I) and the IGFBPs in vitro. The aim of the present study was to investigate whether moderately elevated IL-6 levels have short-term effects on circulating IGF-I, IGFBP-1 and IGFBP-3 proteolysis in vivo. Healthy men received a 3-h IL-6 (n = 6) or saline (n = 6) infusion and blood samples were collected prior to and up to 8 h after the start of infusion. Free IGF-I, total IGF-I, IGFBP-1, insulin and cortisol were measured using immunoassays. Serum IGFBP-3 proteolysis was analyzed by Western immunoblot and by in vitro degradation of ¹²⁵I-IGFBP-3. We found that IL-6 concentrations reaching approximately 100 pg/ml significantly increased IGFBP-1 after the end of infusion in the absence of changes in insulin. In addition, plasma levels of cortisol were increased in response to IL-6 during and after infusion compared to saline. There was no effect of IL-6 on IGFBP-3 proteolysis, total IGF-I or free dissociable IGF-I. These data suggest that moderately elevated levels of IL-6 such as in the post-operative state or after exercise may contribute to increased levels of IGFBP-1. Although this study does not exclude that high levels and/or prolonged exposure to IL-6 may induce IGFBP-3 proteolysis in sepsis or chronic inflammatory disease, it suggests that IL-6 released from exercising skeletal muscle is not directly involved in proteolysis of circulating IGFBP-3.

Nuclear Insulin-Like Growth Factor Binding Protein-3 Induces Apoptosis and Is Targeted to Ubiquitin/Proteasome–Dependent Proteolysis

Insulin-like growth factor binding protein-3 (IGFBP-3), the product of a tumor suppressor target gene, can modulate cell proliferation and apoptosis by IGF-I-dependent and IGF-I-independent mechanisms. IGFBP-3 controls the bioavailability of IGFs in the extracellular environment and is known to be subject to degradation by various extracellular proteases. Although nuclear localization and functions of IGFBP-3 have been described in the past, we show as the novel features of this study that the abundance of nuclear IGFBP-3 is directly regulated by ubiquitin/proteasome-dependent proteolysis. We show that IGFBP-3 degradation depends on an active ubiquitin-E1 ligase, specific 26S proteasome inhibitors can efficiently stabilize nuclear IGFBP-3, and the metabolic half-life of nuclear IGFBP-3 is strongly reduced relative to cytoplasmic IGFBP-3. Nuclear IGFBP-3 is highly polyubiquitinated at multiple lysine residues in its conserved COOH-terminal domain and stabilized through mutation of two COOH-terminal lysine residues. Moreover, we show that IGFBP-3, if ectopically expressed in the nucleus, can induce apoptotic cell death. These results suggest that ubiquitin/proteasome-mediated proteolysis of IGFBP-3 may contribute to down-regulation of apoptosis.

Inaccuracy of Insulin-Like Growth Factor (IGF) Binding Protein (IGFBP)-3 Assessment in the Diagnosis of Growth Hormone (GH) Deficiency from Childhood to Young Adulthood: Association to Low GH Dependency of IGF-II and Presence of Circulating IGFBP-3 18-Kilodalton Fragment

Poor sensitivity of IGF binding protein (IGFBP)-3 assessment in the work-up of GH deficiency (GHD) has been ascribed to the equal affinity of IGFBP-3 for IGF-I and IGF-II and to IGFBP-3 proteolysis. The objective of this study was to determine the IGF-II GH dependency and IGFBP-3 proteolysis in patients with GHD from childhood to young adulthood. This study was cross-sectional. This was a national multicenter study performed in university hospitals. One hundred thirty-one subjects (chronological age, 1.3-25 yr), 72 patients with GHD and 59 subjects with idiopathic short stature, were studied. IGF-I, IGF-II, and IGFBP-3 serum concentrations were measured by immunoradiometric assay. IGFBP-3 circulating forms were assessed by Western immunoblot (WIB) analysis. Main outcome measures were sensitivity and specificity of IGF-I, IGF-II, and IGFBP-3 measurements. Sensitivity and specificity of IGFBP-3 measurement were 27 and 100%, respectively. IGFBP-3 sensitivity was 46% in young adulthood. Sensitivity and specificity of IGF-I were 69 and 81%, respectively. Sensitivity and specificity of IGF-II assessment were 23 and 97%, respectively. IGFBP-3 WIB revealed the presence of the intact form and the major 29-kDa fragment in both GHD and subjects with idiopathic short stature. In patients with GHD, WIB showed the presence of an additional smaller IGFBP-3 fragment migrating at approximately 18 kDa. Our results suggest that in children and young adults with GHD, the low GH dependency of IGF-II together with IGFBP-3 proteolytic activity yielding the 18-kDa fragment concur to reduce the sensitivity of IGFBP-3 assessment, ultimately making it too inaccurate as a screening test in the work-up of GHD.

The effect of submaximal exercise on immuno- and bioassayable IGF-I activity in patients with GH-deficiency and healthy subjects

Objective Growth hormone (GH) increases during exercise, but the response of the insulin-like growth factor (IGF) system has not been as definitive. Therefore, we investigated the effect of the exercise-induced GH response on the circulating IGF-system in GH-deficient (GHD) and intact adults. Design Eight GHD adults were studied on 2 occasions, with (+GH) and without (−GH) GH administered (0.4 IU) during exercise (45 min of cycle ergometer exercise at the lactate threshold). Eight age-matched controls were only studied on one occasion. Blood samples were drawn at baseline, during and post-exercise. IGFBP-3 proteolysis was measured by an in vitro proteolytic activity assay, IGF-I bioactivity by novel IGF-I kinase receptor activation assay (KIRA) and other hormones by immunoassay. Results GH administration to GHD adults resulted in a serum GH peak similar to the exercise-stimulated GH response in GH intact controls, but exercise had only a small impact on the IGF system. IGF-I concentration was lower in controls but was only significantly lower than the +GH day. Neither IGF-I nor -II levels changed over time. IGFBP-1 demonstrated a time effect ( P < 0.01) in all groups, and a time × group interaction ( P < 0.01) with a rise at 75 min post-exercise, which was greater in the GHD subjects than controls. IGFBP-2 and -3 increased significantly ( P < 0.01) over time in the GHD subjects, but not in the controls. No change in IGFBP-3 proteolysis or IGF-I bioactivity was found during exercise or recovery in either group. Conclusion Submaximal exercise induced minor changes in IGFBP-1, -2 and -3, without affecting IGFBP-3 proteolysis and IGF-I bioavailability. Thus the metabolic status during submaximal exercise does not require a change in plasma IGF-I bioavailability. Administration of GH to GHD adults does not result in changes in proteolysis or bioavailability.