A Novel In Vitro Assay Correlates Insulin Receptor Autoantibodies With Fasting Insulin in Type B Insulin Resistance

CONTEXT

Severe insulin resistance (IR) in the presence of insulin receptor autoantibodies (InsR-aAb) is known as type B insulin resistance (TBIR). Considerable progress in therapy has been achieved, but diagnosis and monitoring of InsR-aAb remains a challenge.

OBJECTIVE

This work aimed to establish a robust in vitro method for InsR-Ab quantification.

METHODS

Longitudinal serum samples from patients with TBIR at the National Institutes of Health were collected. A bridge-assay for InsR-aAb detection was established using recombinant human insulin receptor as bait and detector. Monoclonal antibodies served as positive controls for validation.

RESULTS

The novel assay proved sensitive, robust, and passed quality control. The measured InsR-aAb from TBIR patients was associated with disease severity, decreased on treatment, and inhibited insulin signaling in vitro. Titers of InsR-aAb correlated positively to fasting insulin in patients.

CONCLUSION

Quantification of InsR-aAb from serum samples via the novel in vitro assay enables identification of TBIR and monitoring of successful therapy.

Antagonistic Autoantibodies to Insulin-Like Growth Factor-1 Receptor Associate with Poor Physical Strength

Natural autoantibodies to the IGF1 receptor (IGF1R-aAb) have been described in relation to Graves' ophthalmopathy. Other physiological roles of natural IGF1R-aAb are not known. We hypothesized that IGF1R-aAb may be related to muscle development. Serum samples (n = 408) from young overweight subjects (n = 143) were collected during a lifestyle intervention study. Anthropometric parameters, along with leptin, IGF1 and IGF1R-aAb concentrations, were analyzed, and the subjects were categorized into positive or negative for IGF1R-aAb. Eleven out of 143 subjects (7.7%) were positive for IGF1R-aAb. Identified IGF1R-aAb were molecularly characterized and showed antagonistic activity in vitro impairing IGF1-mediated IGF1R activation. Mean body weight, height or age were similar between IGF1R-aAb-positive and -negative subjects, but IGF1 concentrations differed. Jumping ability, as well as right and left handgrip strengths, were lower in the IGF1R-aAb-positive as compared to the IGF1R-aAb-negative subjects. We conclude that natural IGF1R-aAb are detectable in apparently healthy subjects and are capable of antagonizing IGF1-dependent IGF1R activation. Moreover, the presence of IGF1R-aAb is associated with poor physical strength. Although the causality of this association is unclear, the data imply a potential influence of IGF1R autoimmunity on muscle development.

Selenium-Binding Protein 1 Indicates Myocardial Stress and Risk for Adverse Outcome in Cardiac Surgery

Selenium-binding protein 1 (SELENBP1) is an intracellular protein that has been detected in the circulation in response to myocardial infarction. Hypoxia and cardiac surgery affect selenoprotein expression and selenium (Se) status. For this reason, we decided to analyze circulating SELENBP1 concentrations in patients (n = 75) necessitating cardioplegia and a cardiopulmonary bypass (CPB) during the course of the cardiac surgery. Serum samples were collected at seven time-points spanning the full surgical process. SELENBP1 was quantified by a highly sensitive newly developed immunological assay. Serum concentrations of SELENBP1 increased markedly during the intervention and showed a positive association with the duration of ischemia (ρ = 0.6, p < 0.0001). Elevated serum SELENBP1 concentrations at 1 h after arrival at the intensive care unit (post-surgery) were predictive to identify patients at risk of adverse outcome (death, bradycardia or cerebral ischemia, "endpoint 1"; OR 29.9, CI 3.3-268.8, p = 0.00027). Circulating SELENBP1 during intervention (2 min after reperfusion or 15 min after weaning from the CPB) correlated positively with an established marker of myocardial infarction (CK-MB) measured after the intervention (each with ρ = 0.5, p < 0.0001). We concluded that serum concentrations of SELENBP1 were strongly associated with cardiac arrest and the duration of myocardial ischemia already early during surgery, thereby constituting a novel and promising quantitative marker for myocardial hypoxia, with a high potential to improve diagnostics and prediction in combination with the established clinical parameters.

Circulating levels of selenium-binding protein 1 (SELENBP1) are associated with risk for major adverse cardiac events and death

OBJECTIVE

Selenium-binding protein 1 (SELENBP1) is an intracellular protein with variable expression in response to cellular stress. As the selenium (Se) status is affected by inflammation and hypoxia, we hypothesized that SELENBP1 contributes to disease-specific Se metabolism. To test this hypothesis, a quantitative assay was developed and used to monitor SELENBP1 in patients with acute coronary syndrome (ACS).

MATERIALS AND METHODS

SELENBP1 was expressed, antibodies were generated and a luminometric immuno assay (LIA) was established and characterized. Serum samples were collected from controls (n = 37) and patients (n = 85) admitted to the Chest Pain Unit with suspected ACS. Blood samples were available from time of first medical contact in the ambulance, at admission to hospital, and after 2, 4, 6 and 12-36 h.

RESULTS

Circulating SELENBP1 was close to limit of detection in healthy controls and elevated in patients with suspected ACS. SELENBP1 was unrelated to other biomarkers of myocardial damage such as troponin T or aspartate aminotransferase. Serum SELENBP1 enabled a categorization of patients on first medical contact as either high-risk or low-risk for major adverse cardiac events (MACE) or death, when using 0.8 nmol/l as threshold. The odds-ratios (OR) for MACE and death were OR = 11 (95% CI: 2-49, p = 0.0022) and OR = 12 (2-74, p = 0.014), respectively.

CONCLUSIONS

Until now, SELENBP1 was mainly considered as an intracellular protein involved in Se metabolism and redox control. Our data indicate that SELENBP1 constitutes a circulating biomarker for cardiac events categorizing patients with suspected ACS at first medical contact into high-risk or low-risk for MACE and death, independent from and complimentary to current biomarkers.

A Nonradioactive DEHAL Assay for Testing Substrates, Inhibitors, and Monitoring Endogenous Activity

Iodotyrosine deiodinase (DEHAL1) is a crucial enzyme in iodine homeostasis. Unbound mono- and diiodotyrosines are indispensable byproducts of thyroid hormone biosynthesis. Their iodine needs to be recovered to avoid iodine deficiency, as observed in genetic defects in DEHAL1. Despite its importance, the enzyme is rarely studied. The deiodination process can be monitored by radioactive tracers or via techniques involving mass spectrometry. However, isotope-labeled molecules are expensive, not always commercially available, and their use is legally restricted, whereas mass spectrometry requires sophisticated, costly, and sensitive instrumentation. To circumvent these difficulties, we adapted the nonradioactive iodothyronine deiodinase assay to determine DEHAL1 activity by a colorimetric readout, based on the Sandell-Kolthoff reaction. DEHAL1 was recombinantly expressed and used to optimize the assay in microtiter format. We applied the setup to scenarios of alternative substrate screening or search for compounds potentially acting as endocrine disrupting compounds, without identifying novel readily accepted substrates or inhibitors yet. Next, the assay was adapted to ex vivo material, and activity was reliably determined from rodent kidney and other tissues. Analyzing two mouse models of hyperthyroidism, we observed a decreased renal Dehal1 activity and mRNA expression. Our results show that this nonradioactive DEHAL1 assay is suited to screen for potential endocrine disrupters and to monitor endogenous Dehal1 expression. We harmonized the assay protocols to enable iodothyronine deiodinase and DEHAL1 activity measurements from the same samples. Hereby, a more complete view on iodine metabolism by these predominant deiodinating activities can be obtained from a given sample by a similar process flow.

An Improved Nonradioactive Screening Method Identifies Genistein and Xanthohumol as Potent Inhibitors of Iodothyronine Deiodinases

BACKGROUND

Deiodinases (DIO1, 2, and 3) are key enzymes in thyroid hormone (TH) activation and inactivation with impact on energy metabolism, development, cell differentiation, and a number of other physiological processes. The three DIO isoenzymes thus constitute sensitive rate-limiting components within the TH axis, prone to dysregulation by endocrine disruptive compounds or disease state. In animal models and cell culture experiments, they serve as readout for local TH status and disarrangement of the hormonal axis. Furthermore, some human diseases are characterized by apparent deiodinase dysregulation (e.g., the low triiodothyronine syndrome in critical illness). Consequently, these enzymes are targets of interest for the development of pharmacological compounds with modulatory activities. Until now, the portfolio of inhibitors for these enzymes is limited. In the clinics, the DIO1-specific inhibitor propylthiouracil is in use for treatment of severe hyperthyroidism. Other well-known inhibitors (e.g., iopanoic acid or aurothioglucose) are nonselective and block all three isoenzymes. Furthermore, DIO3 was shown to be a potential oncogenic gene, which is strongly expressed in some tumors and might, in consequence, protect tumor tissue form differentiation by TH. With respect to its role in tumorigenesis, specific inhibitors of DIO3 as a potential target for anticancer drugs would be highly desirable. To this end, a flexible and convenient assay for high-throughput screening is needed. We recently described a nonradioactive screening assay, utilizing the classic Sandell-Kolthoff reaction as readout for iodide release from the substrate molecules. While we used murine liver as enzyme source, the assay was limited to murine DIO1 activity testing. Here, we describe the use of recombinant proteins as enzyme sources within the assay, expanding its suitability from murine Dio1 to human DIO1, DIO2, and DIO3.

METHODS

As proof-of-concept, deiodination reactions catalyzed by these recombinant enzymes were monitored with various nonradioactive substrates and confirmed by liquid chromatography-tandem mass spectrometry.

RESULTS

The contrast agent and known DIO inhibitor iopanoic acid was characterized as readily accepted substrate by DIO2 and Dio3. In a screening approach using established endocrine disrupting compounds, the natural food ingredient genistein was identified as a further DIO1-specific inhibitor, while xanthohumol turned out to potently block the activity of all three isoenzymes.

CONCLUSIONS

A rapid nonradioactive screening method based on the Sandell-Kolthoff reaction is suitable for identification of environmental, nutritive and pharmacological compounds modulating activities of human deiodinase enzymes.

Autoantibodies to the IGF1 receptor in Graves' orbitopathy

CONTEXT

Graves' disease (GD) is maintained by stimulating antibodies against the TSH receptor. Graves' orbitopathy (GO) is the main extrathyroidal manifestation of GD, potentially involving autoimmunity against the IGF1 receptor (IGF1R).

OBJECTIVE

We tested for autoantibodies against the IGF1R (IGF1R-Abs) in sera of GD patients and controls and elucidated their possible implication in the disease.

DESIGN

A diagnostic assay for IGF1R-Ab was established with recombinant human IGF1R as autoantigen. Serum samples or purified Ig preparations were analyzed for IGF1R binding and modulation of IGF1 signaling in vitro. A total of 108 consecutive GO patients represented on average by 5.4 separate serum samples per individual along with 92 healthy controls were analyzed.

RESULTS

IGF1R-Ab were detected in 10 serum samples from control subjects (11%) and in 60 samples (10%) from the GO patient serum bank. The positive patient samples were derived from 15 individuals yielding an IGF1R-Ab prevalence of 14% in GO. More than three consecutive samples were available from 11 of the 15 positive GO patients spanning an average disease period of 2 years. IGF1R-Ab concentrations were constantly elevated in these patients demonstrating relatively stable IGF1R-Ab expression over time. IGF1R-Ab failed to stimulate IGF1R autophosphorylation but instead inhibited IGF1-induced signaling in hepatocarcinoma HepG2 cells. Similarly, growth of MCF7 breast cancer cells was inhibited by IGF1R-Ab, supporting their classification as IGF1 antagonists.

CONCLUSIONS

Our data demonstrate the existence of IGF1R-Abs in humans but do not support the hypothesis that the IGF1R-Abs contribute to GO pathogenesis.

Is cyclic AMP formation desensitized in patients with end-stage renal failure?

1 Cyclic AMP formation has consistently been reported to be desensitized in various tissues including heart of animal models of end-stage renal failure (ESRF). In contrast, reports on desensitization of cAMP formation in ESRF patients remain contradictory. Whether this discrepancy results from a difference between human ESRF and its animal models or from the use of circulating blood cells in the human and various solid tissues in the animal studies, remains unclear. Therefore, we performed three studies with heart and platelets of ESRF patients undergoing haemodialysis or continuous ambulatory peritoneal dialysis and age- and gender-matched controls with normal renal function (n = 11-13 each). 2 In platelets from haemodialysis patients adenylyl cyclase activity in response to receptor-dependent and -independent agonists was reduced by approximately 30%, and this could be explained by an alteration at the level of adenylyl cyclase itself. However, no such desensitization was seen in platelets from peritoneal dialysis patients. 3 In hearts from ESRF patients undergoing haemodialysis, beta-adrenoceptor density and subtype distribution, cAMP formation in response to the beta-adrenoceptor agonist isoprenaline or various receptor-independent stimuli, were very similar to those in control patients but activity of G-protein-coupled receptor kinase was increased by approximately 20%. 4 We conclude that conflicting reports on the desensitization of cAMP formation between ESRF patients and ESRF animal models are not explained by the use of solid tissues in animal studies vs. circulating blood cells in patient studies. Rather desensitization of cAMP formation seems to be a less consistent feature of human ESRF than of its animal models.