The insulin-like growth factor-I receptor stimulating activity (IRSA) in health and disease.

Determination of true IGF-I bioactivity in serum and other biological fluids is still a substantial challenge. The IGF-IR Kinase Receptor Activation assay (IGF-IR KIRA assay) is a novel tool to asses IGF-IR stimulating activity (IRSA) and has opened a new era in studying the IGF system. In this paper we discuss many studies showing that measuring IRSA by the IGF-IR KIRA assay often provides fundamentally different information about the IGF system than the commonly used total IGF-I immunoassays. With the IGF-IR KIRA assay phosphorylation of tyrosine residues of the IGF-IR is used as read out to quantify IRSA in unknown (serum) samples. The IGF-IR KIRA assay gives information about net overall effects of circulating IGF-I, IGF-II, IGFBPs and IGFBP-proteases on IGF-IR activation and seems especially superior to immunoreactive total IGF-I in monitoring therapeutic interventions. Although the IRSA as measured by the IGF-IR KIRA assay probably more closely reflects true bioactive IGF-I than measurements of total IGF-I in serum, the IGF-IR KIRA assay in its current form does not give information about all the post-receptor intracellular events mediated by the IGF-IR. Interestingly, in several conditions in health and disease IRSA measured by the IGF-IR KIRA assay is considerably higher in interstitial fluid and ascites than in serum. This suggests that both the paracrine (local) and endocrine (circulating) IRSA should be measured to get a complete picture about the role of the IGF system in health and disease.

Serum Insulin Bioassay Reflects Insulin Sensitivity and Requirements in Type 1 Diabetes.

Context: Insulin resistance could increase insulin requirements in type 1 diabetes (T1D). Current insulin immunoassays do not detect insulin analogs. Kinase insulin receptor (IR) activation (KIRA) bioassays specific for human IR isoforms A (IR-A) and B (IR-B) permit assessment of all circulating insulin bioactivity. We studied whether IR-A and IR-B KIRA assays are related to direct measures of insulin sensitivity or insulin doses in T1D. Design: We evaluated 31 adult patients with T1D (age 45.7 ± 1.6 years, body mass index 28.8 ± 0.7 kg/m2). Serum IR-A and IR-B bioactivities were measured by KIRA bioassays. Insulin sensitivity of glucose production (Ra) was measured by the euglycemic hyperinsulinemic clamp technique in which a low insulin dose (0.4 mU/kg/min for 240 minutes) was combined with D-[3-3H] glucose infusion to measure rates of Ra and utilization and insulin action on antilipolysis from suppression of serum free fatty acids. Results: Baseline circulating IR-A bioactivity was 53 ± 7 pmol/L, and IR-B bioactivity was 81 ± 11 pmol/L. Compared with baseline, insulin infusion significantly increased IR-A (P < 0.001) and IR-B (P < 0.001) bioactivities. Fasting IR-A and IR-B bioactivities were positively related to endogenous Ra (r = 0.44, P = 0.01 and r = 0.38, P < 0.05). Fasting IR-A (r = 0.43, P = 0.02) and IR-B (r = 0.47, P = 0.01) bioactivities were significantly correlated with insulin requirements and glycosylated hemoglobin (IR-A: r = 0.52, P = 0.002; IR-B: r = 0.48, P = 0.006). Conclusions: Circulating IR-A and IR-B bioactivities are associated with insulin resistance, high insulin requirements, and poor glycemic control in T1D. Measurement of IR bioactivity by KIRA assays provides a tool to assess the amount of biologically active insulin in groups of T1D patients treated with insulin analogs.

Changes in circulating IGF1 receptor stimulating activity do not parallel changes in total IGF1 during GH treatment of GH-deficient adults.

CONTEXT: Previously we demonstrated that IGF1 receptor stimulating activity (IGF1RSA) offers advantages in diagnostic evaluation of adult GH deficiency (GHD). It is unknown whether IGF1RSA can be used to monitor GH therapy. OBJECTIVE: To investigate the value of circulating IGF1RSA for monitoring GH therapy. DESIGN/METHODS: 106 patients (54 m; 52 f) diagnosed with GHD were included; 22 were GH-naïve, 84 were already on GH treatment and discontinued therapy 4 weeks before baseline values were established. IGF1RSA was determined by the IGF1R kinase receptor activating assay, total IGF1 by immunoassay (Immulite). GH doses were titrated to achieve total IGF1 levels within the normal range. RESULTS: After 12 months, total IGF1 and IGF1RSA increased significantly (total IGF1 from 8.1 (95% CI 7.3-8.9) to 14.9 (95% CI 13.5-16.4) nmol/l and IGF1RSA from 115 (95% CI 104-127) to 181 (95% CI 162-202) pmol/l). After 12 months, total IGF1 normalized in 81% of patients, IGF1RSA in 51% and remained below normal in more than 40% of patients in whom total IGF1 had normalized. CONCLUSIONS: During 12 months of GH treatment, changes in IGF1RSA did not parallel changes in total IGF1. Despite normalization of total IGF1, IGF1RSA remained subnormal in a considerable proportion of patients. At present our results have no short-term consequences for GH therapy of GHD patients. However, based on our findings we propose future studies to examine whether titrating GH dose against IGF1RSA results in a better clinical outcome than titrating against total IGF1.

Insulin analogs and cancer: a note of caution.

In view of the lifelong exposure and large patient populations involved, insulin analogs with an increased mitogenic effect in comparison to human insulin may potentially constitute a major health problem, since these analogs may possibly induce the growth of pre-existing neoplasms. At present, the available data suggest that insulin analogs are safe. In line with these findings, we observed that serum of diabetic patients treated with insulin analogs, compared to that of diabetic patients treated with human insulin, did not induce an increased phosphorylation of tyrosine residues of the insulin-like growth factor-I receptor (IGF-IR). However, the classical model of the IGF-IR signaling may be insufficient to explain (all) mitogenic effects of insulin analogs since also non-canonical signaling pathways of the IGF-IR may play a major role in this respect. Although phosphorylation of tyrosine residues of the IGF-IR is generally considered to be the initial activation step within the intracellular IGF-IR signaling pathway, it has been found that cells undergo a signaling switch under hyperglycemic conditions. After this switch, a completely different mechanism is utilized to activate the mitogenic (mitogen-activated protein kinase) pathways of the IGF-IR that is independent from tyrosine phosphorylation of the IGF-IR. At present it is unknown whether activation of this alternative intracellular pathway of the IGF-IR occurs during hyperglycemia in vivo and whether it is stronger in patients treated with (some) insulin analogs than in patients treated with human insulin. In addition, it is unknown whether the insulin receptors (IRs) also undergo a signaling switch during hyperglycemia. This should be investigated in future studies. Finally, relative overexpression of IR isoform A (IR-A) in (pre) cancer tissues may play a key role in the development and progression of human cancers during treatment with insulin (analogs). Further studies are required to unravel whether the IR-A is involved in the development of cancers and whether, in this respect (some) insulin analogs differ from human insulin.

IGF-IR Targeted Therapy: Past, Present and Future.

The IGF-I receptor (IGF-IR) has been studied as an anti-cancer target. However, monotherapy trials with IGF-IR targeted antibodies or with IGF-IR specific tyrosine kinase inhibitors have, overall, been very disappointing in the clinical setting. This review discusses potential reasons why IGF-I R targeted therapy fails to inhibit growth of human cancers. It has become clear that intracellular signaling pathways are highly interconnected and complex instead of being linear and simple. One of the most potent candidates for failure of IGF-IR targeted therapy is the insulin receptor isoform A (IR-A). Activation of the IR-A by insulin-like growth factor-II (IGF-II) bypasses the IGF-IR and its inhibition. Another factor may be that anti-cancer treatment may reduce IGF-IR expression. IGF-IR blocking drugs may also induce hyperglycemia and hyperinsulinemia, which may further stimulate cell growth. In addition, circulating IGF-IRs may reduce therapeutic effects of IGF-IR targeted therapy. Nevertheless, it is still possible that the IGF-IR may be a useful adjuvant or secondary target for the treatment of human cancers. Development of functional inhibitors that affect the IGF-IR and IR-A may be necessary to overcome resistance and to make IGF-IR targeted therapy successful. Drugs that modify alternative downstream effects of the IGF-IR, so called "biasing agonists," should also be considered.

Concentrations of insulin glargine and its metabolites during long-term insulin therapy in type 2 diabetic patients and comparison of effects of insulin glargine, its metabolites, IGF-I, and human insulin on insulin and igf-I receptor signaling.

We investigated 1) the ability of purified glargine (GLA), metabolites 1 (M1) and 2 (M2), IGF-I, and NPH insulin to activate the insulin receptor (IR)-A and IR-B and IGF-I receptor (IGF-IR) in vitro; 2) plasma concentrations of GLA, M1, and M2 during long-term insulin therapy in type 2 diabetic patients; and 3) IR-A and IR-B activation in vitro induced by serum from patients treated with GLA or NPH insulin. A total of 104 patients (age 56.3 ± 0.8 years, BMI 31.4 ± 0.5 kg/m(2), and A1C 9.1 ± 0.1% [mean ± SE]) were randomized to GLA or NPH insulin therapy for 36 weeks. Plasma concentrations of GLA, M1, and M2 were determined by liquid chromatography-tandem mass spectrometry assay. IR-A, IR-B, and IGF-IR autophosphorylation was induced by purified hormones or serum by kinase receptor activation assays. In vitro, M1 induced comparable IR-A, IR-B, and IGF-IR autophosphorylation (activation) as NPH insulin. After 36 weeks, M1 increased from undetectable (<0.2 ng/mL) to 1.5 ng/mL (0.9-2.1), while GLA and M2 remained undetectable. GLA dose correlated with M1 (r = 0.84; P < 0.001). Serum from patients treated with GLA or NPH insulin induced similar IR-A and IR-B activation. These data suggest that M1 rather than GLA mediates GLA effects and that compared with NPH insulin, GLA does not increase IGF-IR signaling during long-term insulin therapy in type 2 diabetes.

IGF-I bioactivity might reflect different aspects of quality of life than total IGF-I in GH-deficient patients during GH treatment.

CONTEXT: No relationship has been found between improvement in quality of life (QOL) and total IGF-I during GH therapy. AIM: Our aim was to investigate the relationship between IGF-I bioactivity and QOL in GH-deficient (GHD) patients receiving GH for 12 months. METHODS: Of 106 GHD patients, 84 on GH treatment discontinued therapy 4 weeks before establishing baseline values and 22 were GH-naive. IGF-I bioactivity was determined by IGF-I kinase receptor activation assay, total IGF-I by immunoassay (Immulite), and QOL by the disease-specific Question on Life Satisfaction Hypopituitarism (QLS-H) module and by the general SF-36 questionnaire (SF-36Q). RESULTS: IGF-I bioactivity increased after 6 months (-2.5 vs -1.9 SD, P < .001) and did not further increase after 12 months (-1.8 SD, P = .23); total IGF-I increased from -2.3 to -0.9 SD (P < .001) and to -0.6 SD (P = .005), respectively. QLS-H did not change over 12 months (-0.66 ± 0.16 to -0.56 ± 0.17 SD [P = .42] to -0.68 ± 0.17 SD [P = .22]). The mental component summary of the SF-36Q increased from 47.4 (38.7-52.8) to 50.2 (43.1-55.3) (P = .001) and did not further improve (49.4 [42.1-54.1], P = .19); the physical component summary did not change (47.5 [42.0-54.2] vs 47.0 [41.9-55.3], P = .91, vs 48.3 [39.9-55.4], P = .66). After 12 months, IGF-I bioactivity was related to QLS-H (r = 0.28, P = .01); total IGF-I was not (r = 0.10, P = .37). IGF-I bioactivity and total IGF-I were related to PCS (r = 0.35, P = .001; and r = 0.31, P = .003). CONCLUSION: IGF-I bioactivity remained subnormal after GH treatment and was positively related to QLS-H, whereas total IGF-I was not. This suggests that IGF-I bioactivity reflects different aspects of QOL than total IGF-I in GHD patients during GH treatment.

Circulating IgGs may modulate IGF-I receptor stimulating activity in a subset of patients with Graves' ophthalmopathy.

CONTEXT: There is a close association between levels of TSH binding inhibitory immunoglobulins (TBIIs) and Graves' ophthalmopathy (GO). In addition to the TSH receptor, the IGF-I receptor (IGF-IR) has been proposed to be a second autoantigen that plays a role in the pathogenesis of GO. OBJECTIVE: The aim was to study relationships between TBII and serum IGF-IR stimulating activity in relationship to age in patients with GO. METHODS: We performed a prospective study of 70 patients with GO (26 euthyroid, 39 subclinical hyperthyroid, 5 hyperthyroid; 8 males, 62 females; age, 47.9 ± 1.0 y). Patients were graded according to clinical activity score. IGF-IR stimulating activity was determined by IGF-IR kinase receptor activation assay; TBIIs were measured by immunoassay (Trak). Protein G magnetic beads were used to deplete serum of IgGs. RESULTS: TBII and clinical activity score were positively related (r = 0.30; P = .01). In subjects with TBII above mean +1 SD, IGF-IR stimulating activity was positively related to age (r = 0.43; P = .05), whereas such a relationship was absent for subjects with TBII below the mean +1 SD (r = -0.04; P = .81). Depletion of IgGs from sera of patients with both TBII above the mean +1 SD and IGF-IR stimulating activity above the mean -1 SD decreased IGF-IR stimulating activity, whereas depletion in patients with TBII above the mean +1 SD but IGF-IR stimulating activity below the mean -1 SD did not change IGF-IR stimulating activity. CONCLUSIONS: In subjects with TBII above the mean +1 SD, we observed an increase of IGF-IR stimulating activity with age. In a subgroup of these patients, depletion of IgGs significantly decreased IGF-IR stimulating activity, suggesting that, in a subset of patients with GO, IgGs may have IGF-IR stimulating activities.

Circulating insulin-like growth factors may contribute substantially to insulin receptor isoform A and insulin receptor isoform B signalling.

BACKGROUND: Only a fraction of circulating insulin-like activity is due to insulin itself. The aim of this study was to determine total serum insulin-like activity mediated via the insulin receptor isoform A (IR-A) and isoform B (IR-B) by using kinase receptor activation (KIRA) assays specific for the IR-A and IR-B. METHODS: The IR-A and IR-B KIRA assays use human embryonic kidney cells which have been transfected with the human IR-A or IR-B gene and quantify serum-mediated phosphorylation of the IR. RESULTS: Both IR KIRA assays were sensitive (detection limit 32 pmol/L) and precise (intra- and inter assay CV: <12% and <15%). The EC50s of insulin, IGF-I and IGF-II were 1.4, 11.2 and 6.7 nmol/L for the IR-A KIRA assay, and 1.3, 31.0 and 15.7 nmol/L for the IR-B KIRA assay. The operational range of both assays allowed for determination of total insulin-like activity in human serum. Analysis of serum samples showed that there was a significant positive correlation between serum insulin-like and immunoreactive insulin concentrations (IR-A: r = 0.56, p = 0.01, IR-B: r = 0.68, p = 0.001). Importantly, addition of IGF-I or IGF-II antibodies to human serum samples could substantially decrease the endpoint signal in both KIRA assays. CONCLUSIONS: We showed that serum IGF-I and IGF-II may substantially contribute to IR signalling. Since IR isoform specific KIRA assays also take into account the contribution of IGFs present in serum on IR signalling, they may help to gain more insight into the roles of IGF mediated IR-A and IR-B activation in health and disease.

Low circulating IGF-I bioactivity is associated with human longevity: findings in centenarians' offspring.

UNLABELLED: Centenarians' offspring represent a suitable model to study age-dependent variables (e.g. IGF-I) potentially involved in the modulation of the lifespan. The aim of the present study was to investigate the role of the IGF-I in human longevity. We evaluated circulating IGF-I bioactivity measured by an innovative IGF-I Kinase Receptor Activation (KIRA) Assay, total IGF-I, IGFBP-3, total IGF-II, insulin, glucose, HOMA2-B% and HOMA2-S% in 192 centenarians' offspring and 80 offspring-controls of which both parents died relatively young. Both groups were well-matched for age, gender and BMI with the centenarians' offspring. IGF-I bioactivity (p〈0.01), total IGF-I (p〈0.01) and the IGF-I/IGFBP-3 molar ratio (p〈0.001) were significantly lower in centenarians' offspring compared to offspring matched-controls. Serum insulin, glucose, HOMA2-B% and HOMA2-S% values were similar between both groups. In centenarians' offspring IGF-I bioactivity was inversely associated to insulin sensitivity. IN CONCLUSION: 1) centenarians' offspring had relatively lower circulating IGF-I bioactivity compared to offspring matched-controls; 2) IGF-I bioactivity in centenarians' offspring was inversely related to insulin sensitivity. These data support a role of the IGF-I/insulin system in the modulation of human aging process.

Insulin and its analogues and their affinities for the IGF1 receptor.

Insulin analogues have been developed in an attempt to achieve a more physiological replacement of insulin and thereby a better glycaemic control. However, structural modification of the insulin molecule may result in altered binding affinities and activities to the IGF1 receptor (IGF1R). As a consequence, insulin analogues may theoretically have an increased mitogenic action compared to human insulin. In view of the lifelong exposure and large patient populations involved, insulin analogues with an increased mitogenic effect in comparison to human insulin may potentially constitute a major health problem, since these analogues may possibly induce the growth of pre-existing neoplasms. This hypothesis has been evaluated extensively in vitro and also in vivo by using animal models. In vitro, all at present commercially available insulin analogues have lower affinities for the insulin receptor (IR). Although it has been suggested that especially insulin analogues with an increased affinity for the IGF1R (such as insulin glargine) are more mitogenic when tested in vitro in cells expressing a high proportion of IGF1R, the question remains whether this has any clinical consequences. At present, there are several uncertainties which make it very difficult to answer this question decisively. In addition, recent data suggest that insulin (or insulin analogues)-mediated stimulation of IRs may play a key role in the progression of human cancer. More detailed information is required to elucidate the exact mechanisms as to how insulin analogues may activate the IR and IGF1R and how this activation may be linked to mitogenesis.