Insulin-like growth factor binding protein-3 is a novel mediator of apoptosis in insulin-secreting cells

Verapamil Prevents Decline of IGF-I in Subjects With Type 1 Diabetes and Promotes β-Cell IGF-I Signaling

Verapamil promotes functional β-cell mass and improves glucose homeostasis in diabetic mice and humans with type 1 diabetes (T1D). Now, our global proteomics analysis of serum from subjects with T1D at baseline and after 1 year of receiving verapamil or placebo revealed IGF-I as a protein with significantly changed abundance over time. IGF-I, which promotes β-cell survival and insulin secretion, decreased during disease progression, and this decline was blunted by verapamil. In addition, we found that verapamil reduces β-cell expression of IGF-binding protein 3 (IGFBP3), whereas IGFBP3 was increased in human islets exposed to T1D-associated cytokines and in diabetic NOD mouse islets. IGFBP3 binds IGF-I and blocks its downstream signaling, which has been associated with increased β-cell apoptosis and impaired glucose homeostasis. Consistent with the downregulation of IGFBP3, we have now discovered that verapamil increases β-cell IGF-I signaling and phosphorylation/activation of the IGF-I receptor (IGF1R). Moreover, we found that thioredoxin-interacting protein (TXNIP), a proapoptotic factor downregulated by verapamil, promotes IGFBP3 expression and inhibits the phosphorylation/activation of IGF1R. Thus, our results reveal IGF-I signaling as yet another previously unappreciated pathway affected by verapamil and TXNIP that may contribute to the beneficial verapamil effects in the context of T1D.

ARTICLE HIGHLIGHTS

Verapamil prevents the decline of IGF-I in subjects with type 1 diabetes (T1D). Verapamil decreases the expression of β-cell IGF-binding protein 3 (IGFBP3), whereas IGFBP3 is increased in human and mouse islets under T1D conditions. Verapamil promotes β-cell IGF-I signaling by increasing phosphorylation of IGF-I receptor and its downstream effector AKT. Thioredoxin-interacting protein (TXNIP) increases IGFBP3 expression and inhibits the phosphorylation/activation of IGF1R in β-cells. Regulation of IGFBP3 and IGF-I signaling by verapamil and TXNIP may contribute to the beneficial verapamil effects in the context of T1D.

Insulin/IGF-driven cancer cell-stroma crosstalk as a novel therapeutic target in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is unrivalled the deadliest gastrointestinal cancer in the western world. There is substantial evidence implying that insulin and insulin-like growth factor (IGF) signaling axis prompt PDAC into an advanced stage by enhancing tumor growth, metastasis and by driving therapy resistance. Numerous efforts have been made to block Insulin/IGF signaling pathway in cancer therapy. However, therapies that target the IGF1 receptor (IGF-1R) and IGF subtypes (IGF-1 and IGF-2) have been repeatedly unsuccessful. This failure may not only be due to the complexity and homology that is shared by Insulin and IGF receptors, but also due to the complex stroma-cancer interactions in the pancreas. Shedding light on the interactions between the endocrine/exocrine pancreas and the stroma in PDAC is likely to steer us toward the development of novel treatments. In this review, we highlight the stroma-derived IGF signaling and IGF-binding proteins as potential novel therapeutic targets in PDAC.

Combination of Recreational Soccer and Caloric Restricted Diet Reduces Markers of Protein Catabolism and Cardiovascular Risk in Patients with Type 2 Diabetes

BACKGROUND

Moderate calorie-restricted diets and exercise training prevent loss of lean mass and cardiovascular risk. Because adherence to routine exercise recommendation is generally poor, we utilized recreational soccer training as a novel therapeutic exercise intervention in type 2 diabetes (T2D) patients.

OBJECTIVE

We compared the effects of acute and chronic soccer training plus calorie-restricted diet on protein catabolism and cardiovascular risk markers in T2D.

DESIGN, SETTING AND SUBJECTS

Fifty-one T2D patients (61.1±6.4 years, 29 females: 22 males) were randomly allocated to the soccer+diet-group (SDG) or to the diet-group (DG). The 40-min soccer sessions were held 3 times per week for 12 weeks.

RESULTS

Nineteen participants attended 100% of scheduled soccer sessions, and none suffered any injuries. The SDG group showed higher levels of growth hormone (GH), free fatty acids and ammonia compared with DG. After 12 weeks, insulin-like growth factor binding protein (IGFPB)-3 and glucose levels were lower in SDG, whereas insulin-like growth factor (IGF)-1/ IGFBP-3 ratio increased in both groups. After the last training session, an increase in IGF-1/IGFBP-3 and attenuation in ammonia levels were suggestive of lower muscle protein catabolism.

CONCLUSIONS

Recreational soccer training was popular and safe, and was associated with decreased plasma glucose and IGFBP-3 levels, decreased ammoniagenesis, and increased lipolytic activity and IGF-1/IGFBP-3 ratio, all indicative of attenuated catabolism.

IGF binding protein-3 mediates stress-induced apoptosis in non-transformed mammary epithelial cells

Mammary epithelial cell (MEC) number is an important determinant of milk production in lactating dairy cows. IGF-I increases IGF binding protein-3 (IGFBP-3) production in these cells, which plays a role in its ability to enhance proliferation. In the present study, we show that the apoptotic factor anisomycin (ANS) also increases IGFBP-3 mRNA and protein in a dose- and concentration-dependent manner that mirrors activation of caspase-3 and -7, with significant increases in both IGFBP-3 protein and caspase activation observed by 3 h. Knock-down of IGFBP-3 with small interfering (si) RNA attenuated the ability of ANS to induce apoptosis, while knock-down of IGFBP-2, the other major IGFBP made by bovine MEC, had no effect. Reducing IGFBP-3 also decreased the ability of ANS to induce mitochondrial cytochrome c release, indicating its involvement in the intrinsic apoptotic pathway. In contrast, transfection with IGFBP-3 in the absence of ANS failed to induce apoptosis. Since both the mitogen IGF-I and the apoptotic inducer ANS increase IGFBP-3 production in MEC, we proposed that cellular localization might determine IGFBP-3 action. While both IGF-I and ANS stimulated the release of IGFBP-3 into conditioned media, only ANS induced nuclear localization of IGFBP-3. A pan-caspase inhibitor had no effect on ANS-induced nuclear localization of IGFBP-3, indicating that nuclear entry of IGFBP-3 precedes caspase activation. Treatment with IGF-I had no effect on ANS-induced nuclear localization, but did block ANS-induced apoptosis. In summary, our data indicate that IGFBP-3 plays a role in stress-induced apoptosis that may require nuclear localization in non-transformed MEC.

Human IGF binding protein-3 overexpression impairs glucose regulation in mice via an inhibition of insulin secretion

Human IGF binding protein-3 (hIGFBP-3) overexpression in mice causes hyperglycemia, but its effect on β-cell function is unknown. We compared wild-type mice with mice overexpressing hIGFBP-3 [phoshoglycerate kinase (PGK)BP3] and mutant (Gly⁵⁶/Gly⁸⁰/Gly⁸¹)hIGFBP-3 devoid of IGF binding affinity (PGKmBP3). Intraperitoneal glucose and insulin tolerance tests were performed, and glucose, IGFBP-3, IGF-I, and insulin were determined. Pancreatic sections were used for islet histomorphometry and stained with antibodies against insulin, glucagon, and hIGFBP-3. Pancreatic islets were isolated to determine the expression of IGFBP-3, and glucose-stimulated insulin secretion was measured using both islet batch incubation and perifusion. IGFBP-3 was expressed in β-cells but not in other islet cell types. Fasting glucose concentration was elevated in PGKBP3 mice (6.27 ± 0.31 mm) compared with PGKmBP3 mice (3.98 ± 0.36 mm) and wild-type mice (4.84 ± 0.07 mm). During glucose tolerance test, glucose declined more slowly in PGKBP3 and PGKmBP3 mice than in wild-type mice, and insulin secretion was impaired in PGKBP3 mice. During insulin tolerance test, insulin declined more slowly in both transgenic mice compared with wild-type mice. Insulin secretion in islets incubated with 3.3 mm glucose was similar among groups, but islet insulin response to 16.7 mm glucose alone, or with carbachol and cAMP enhancers, was reduced in PGKBP3 and PGKmBP3 mice compared with wild-type controls. ATP content, Akt phosphorylation, and phosphoglucose isomerase activity were reduced in islets from both transgenic mice. Thus, overexpression of hIGFBP-3 in mice delays in vivo insulin clearance and reduces glucose-stimulated insulin secretion in pancreatic islets by both IGF-dependent and IGF-independent mechanisms.

Insulin-like growth factor axis gene polymorphisms and clinical outcomes in pancreatic cancer

BACKGROUND & AIMS

Insulin-like growth factor (IGF)-axis mediated signaling pathways play an important role in pancreatic cancer development and progression. We examined whether IGF-axis gene variants are associated with clinical outcomes in pancreatic cancer.

METHODS

We retrospectively genotyped 41 single-nucleotide polymorphisms from 10 IGF-axis genes in 333 patients with localized pancreatic adenocarcinoma and validated the findings in 373 patients with advanced disease. Associations between genotype and overall survival (OS) were evaluated using multivariable Cox proportional hazard regression models.

RESULTS

IGF1 *8470T>C, IGF1R IVS2+46329T>C, IGFBP3 A32G, IRS1 G972R in patients with localized disease; IGF1R IVS20-3431A>G, IGF1R T766T, IGFBP3-202A>C, IRS1 IVS1+4315C>G, IRS1 G972R in patients with advanced disease; and IGF1R T766T, IGF2R L252V, IGFBP3 -202A>C, IRS1 IVS1+4315C>G, IRS1 G972R, IRS2 IVS1+5687T>C in all patients were significantly associated with OS (P<or=.007). Two haplotypes containing the variant allele of either IRS1 G972R or IVS1-10949G>A, and an IRS2 haplotype predicted worse OS (P<or=.002). A significant correlation between increased number of unfavorable genotypes and decreased OS was observed; patients with 0-1 (n=247), 2 (n=237), 3 (n=145), 4 (n=60), and 5-8 (n=17) unfavorable genotypes had median survival time of 24.2, 16.4, 14.4, 9.6, and 7.4 months, respectively (P<.001). Several single-nucleotide polymorphisms of IGF1R, IGF2R, and IRS1 gene were significantly associated with tumor response to therapy and disease stage.

CONCLUSIONS

These data suggest that individual genetic variations in the IGF axis pathway may predict worse survival in patients with pancreatic cancer. This information may identify population subgroups that could benefit from IGF(1)R-targeted agents.

The neurosurvival factor Humanin inhibits beta-cell apoptosis via signal transducer and activator of transcription 3 activation and delays and ameliorates diabetes in nonobese diabetic mice

Pancreatic beta-cell apoptosis is important in the pathogenesis and potential treatment of type 1 diabetes mellitus. We investigated whether Humanin, a recently described survival factor for neurons, could improve the survival of beta-cells and delay or treat diabetes in the nonobese diabetic (NOD) model. Humanin reduced apoptosis induced by serum starvation in NIT-1 cells and decreased apoptosis induced by cytokine treatment. Humanin induced signal transducer and activator of transcription 3 and extracellular signal-regulated kinase phosphorylation over a 24-hour time course. Specific inhibition of signal transducer and activator of transcription 3 resulted in nullifying the protective effect of Humanin. Humanin normalized glucose tolerance in NOD mice treated for 6 weeks, and their pancreata revealed decreased lymphocyte infiltration and severity. In addition, Humanin delayed/prevented the onset of diabetes in NOD mice treated for 20 weeks. In summary, Humanin treatment decreases cytokine-induced apoptosis in beta-cells in vitro and improved glucose tolerance and onset of diabetes in NOD mice in vivo. This indicates that Humanin may be useful for islet protection and survival in a spectrum of diabetes-related therapeutics.

Perspectives in mammalian IGFBP-3 biology: local vs. systemic action

Insulin-like growth factor (IGF) binding protein (IGFBP)-3 has traditionally been defined by its role as a binding protein and its association with IGF delivery and availability. Development of non-IGF binding IGFBP-3 analogs and the use of cell lines devoid of type 1 IGF receptors (IGF-R) have led to critical advances in the field of IGFBP-3 biology. These studies show that IGFBP-3 has IGF-independent roles in inhibiting cell proliferation in cancer cell lines. Nuclear transcription factor, retinoid X receptor (RXR)-alpha, and IGFBP-3 functionally interact to reduce prostate tumor growth and prostate-specific antigen in vivo. Moreover, IGFBP-3 inhibits insulin-stimulated glucose uptake into adipocytes independent of IGF. The purpose of this review is to highlight IGFBP-3 as a novel effector molecule and not just another "binding protein" by discussing its IGF-independent actions on metabolism and cell growth. Although this review presents studies that assume the role of IGFBP-3 as either an endocrine or autocrine/paracrine molecule, these systems may not exist as distinct entities, justifying the examination of IGFBP-3 in an integrated model. Also, we provide an overview of factors that regulate IGFBP-3 availability, including its production, methylation, and ubiquitination. We conclude with the role of IGFBP-3 in whole body systems and possible future applications of IGFBP-3 in physiology.

Longitudinal analysis of the insulin-like growth factor system in African-American and European American children and adolescents

CONTEXT

IGF-I and its binding proteins influence growth, development, and disease risk. Studies have revealed ethnic variations in the IGF system.

OBJECTIVE

This longitudinal study was undertaken to test the hypothesis that the ethnic differences in the IGF system exist throughout the pubertal transition, and these differences are mediated at least in part by inherent differences in insulin dynamics.

DESIGN

This was a longitudinal study. Annual evaluations were conducted for pubertal maturation, body composition, acute insulin response to glucose (AIRg), and reproductive-endocrine profile. Hormones and binding proteins were determined using standard assays, the AIRg during a frequently sampled iv glucose tolerance test, and body composition by dual-energy x-ray absorptiometry. Mixed model analyses were used to identify and characterize ethnic differences in the IGF system across the pubertal transition after adjusting for ethnicity, sex, age, maturation status, body composition, and reproductive hormones, and to identify the contribution of insulin to IGF binding protein (IGFBP)-1.

PARTICIPANTS

Subjects included African-American (AA) and European American children (n = 162 at baseline) aged 7-16 yr, evaluated across the pubertal transition.

MAIN OUTCOME MEASURES

Annual data on IGF-I, IGFBP-1, and IGFBP-3 were examined.

RESULTS

IGF-I was higher in AA children at pubertal stage 1 only (P < 0.001). However, IGFBP-3 and IGFBP-1 concentrations were lower in AAs through much of puberty (P < 0.05). The lower IGFBP-1 of AAs was in part explained by greater AIRg.

CONCLUSIONS

Our data suggest that the higher IGF-I and lower IGFBP-1 and IGFBP-3 levels in AAs as compared with European Americans during puberty suggest potential ethnic differences in circulating bioavailable IGF-I. In addition, higher AIRg in AAs may lead to greater bioavailable IGF-I. Whether these differences in the IGF system account for disparities in disease risk warrants further investigation.

Serum complexes of insulin-like growth factor-1 modulate skeletal integrity and carbohydrate metabolism

Serum insulin-like growth factor (IGF) -1 is secreted mainly by the liver and circulates bound to IGF-binding proteins (IGFBPs), either as binary complexes or ternary complexes with IGFBP-3 or IGFBP-5 and an acid-labile subunit (ALS). The purpose of this study was to genetically dissect the role of IGF-1 circulatory complexes in somatic growth, skeletal integrity, and metabolism. Phenotypic comparisons of controls and four mouse lines with genetic IGF-1 deficits-liver-specific IGF-1 deficiency (LID), ALS knockout (ALSKO), IGFBP-3 (BP3) knockout, and a triply deficient LID/ALSKO/BP3 line-produced several novel findings. 1) All deficient strains had decreased serum IGF-1 levels, but this neither predicted growth potential or skeletal integrity nor defined growth hormone secretion or metabolic abnormalities. 2) IGF-1 deficiency affected development of both cortical and trabecular bone differently, effects apparently dependent on the presence of different circulating IGF-1 complexes. 3) IGFBP-3 deficiency resulted in increased linear growth. In summary, each IGF-1 complex constituent appears to play a distinct role in determining skeletal phenotype, with different effects on cortical and trabecular bone compartments.

Mining the acute respiratory distress syndrome proteome: identification of the insulin-like growth factor (IGF)/IGF-binding protein-3 pathway in acute lung injury

To obtain a more complete protein profile of the airspace milieu in acute respiratory distress syndrome (ARDS) and to identify new mediators, we analyzed bronchoalveolar lavage fluid (BALF) by shotgun proteomics. Using BALF from three patients, we identified a total of 870 different proteins, a nearly 10-fold increase from previous reports. Among the proteins identified were known markers of lung injury, such as surfactant, proteases, and serum proteins. We also identified several biologically interesting proteins not previously identified in patients with ARDS, including insulin-like growth factor-binding protein-3 (IGFBP-3). Because of the known role of IGFBP-3 in regulating cell survival, we measured IGFBP-3 levels by enzyme-linked immunosorbent assay in ARDS BALF. Normal controls had low levels of IGFBP-3, whereas patients with early ARDS had a significant increase in IGFBP-3. The IGF pathway, acting through the type 1 IGF-receptor, repressed apoptosis of lung fibroblasts but not lung epithelial cells. Furthermore, depletion of IGF in ARDS BALF led to enhanced fibroblast apoptosis. Our data suggest that the IGFBP-3/IGF pathway is involved in the pathogenesis of lung injury, illustrating the power of shotgun proteomics to catalog proteins present in complex biological fluids, such as BALF, from which hypotheses can be developed and tested.

Novel actions of IGFBP-3 on intracellular signaling pathways of insulin-secreting cells

Understanding mechanisms underlying apoptotic destruction of insulin-secreting cells is critical to validate therapeutic targets for type 1 diabetes mellitus. We recently reported insulin-like growth factor binding protein-3 (IGFBP-3) as a novel mediator of apoptosis in insulin-secreting cells. In light of emerging IGF-independent roles for IGFBP-3, we investigated the mechanisms underlying actions of the novel, recombinant human mutant G(56)G(80)G(81)-IGFBP-3, which lacks intrinsic IGF binding affinity. Using the rat insulinoma RINm5F cell line, we report the first studies in insulin-secreting cells that IGFBP-3 selectively suppresses multiple, key intracellular phosphorelays. By immunoblot, we demonstrate that G(56)G(80)G(81)-IGFBP-3 suppresses phosphorylation of c-raf-MEK-ERK pathway and p38 kinase in time-dependent and dose-dependent manners. SAPK/JNK signaling was unaffected. These data delineate several novel intracellular sites of action for IGFBP-3 in insulin-secreting cells.