Degradation of proinsulin C-peptide in kidney and placenta extracts by a specific endoprotease activity

Streptozotocin-Induced Diabetes in a Mouse Model (BALB/c) Is Not an Effective Model for Research on Transplantation Procedures in the Treatment of Type 1 Diabetes

Type 1 diabetes (T1D) is characterized by the destruction of over 90% of the β-cells. C-peptide is a parameter for evaluating T1D. Streptozotocin (STZ) is a standard method of inducing diabetes in animals. Eight protocols describe the administration of STZ in mice; C-peptide levels are not taken into account. The aim of the study is to determine whether the STZ protocol for the induction of beta-cell mass destruction allows for the development of a stable in vivo mouse model for research into new transplant procedures in the treatment of type 1 diabetes. Materials and methods: Forty BALB/c mice were used. The animals were divided into nine groups according to the STZ dose and a control group. The STZ doses were between 140 and 400 mg/kg of body weight. C-peptide was taken before and 2, 7, 9, 12, 14, and 21 days after STZ. Immunohistochemistry was performed. The area of the islet and insulin-/glucagon-expressing tissues was calculated. Results: Mice who received 140, 160, 2 × 100, 200, and 250 mg of STZ did not show changes in mean fasting C-peptide in comparison to the control group and to day 0. All animals with doses of 300 and 400 mg of STZ died during the experiment. The area of the islets did not show any differences between the control and STZ-treated mice in groups below 300 mg. The reduction of insulin-positive areas in STZ mice did not exceed 50%. Conclusions: Streptozotocin is not an appropriate method of inducing a diabetes model for further research on transplantation treatments of type 1 diabetes, having caused the destruction of more than 90% of the β-cell mass in BALB/c mice.

Development and Validation of a Novel LC-MS/MS Assay for C-Peptide in Human Serum

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Quantification of C-peptide without an antibody or multidimensional chromatography.

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High-throughput method with good comparability to reference measurement procedure.

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Proteolysis improves limit of detection over intact C-peptide.

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Glu-C is an important proteolytic enzyme for targeted proteomic workflows.

Introduction

 C-peptide is used as a marker of endogenous insulin secretion in the assessment of residual β-cell function in diabetes and in the diagnostic workup of hypoglycemia. Previously developed LC-MS/MS methods to quantify serum concentrations of C-peptide have monitored intact peptide, which ionizes poorly. As a result, methods have leveraged immunoaffinity enrichment or two-dimensional chromatography. In this study, we aimed to use proteolysis during sample preparation to enhance the sensitivity of traditional LC-MS/MS.

Methods

 Due to the absence of arginine and lysine residues in C-peptide, we utilized Glu-C as the proteolytic enzyme in the method. After protein precipitation using acetonitrile and solid phase extraction with mixed anion exchange, lower molecular weight polypeptides were reduced, alkylated, and proteolyzed. The two amino-terminal peptide fragments, EAEDLQVGQVE and LGGGPGAGSLQPLALE, were monitored using multiple reaction monitoring in positive ion mode (Acquity ULPC-Xevo TQ-S, Waters). The former peptide was used for quantification and the latter for quality assurance.

Results

 Glu-C was determined to be a reliable proteolytic enzyme with monotonic digestion kinetics. The assay was linear between 0.1 and 15 ng/mL and had a lower limit of quantification of 0.06 ng/mL. Total imprecision was 7.7 %CV and long-term imprecision at 0.16 ng/mL was 10.0%. Spike-recovery experiments demonstrated a mean recovery of 98.2 % (± 9.1 %) and the method compared favorably with a commercially available immunoassay and a reference measurement procedure.

Conclusion

 Protein precipitation with solid phase extraction and proteolysis with Glu-C is a robust sample preparation method for quantification of C-peptide in human serum by LC-MS/MS.

The Relationship Between β-cell Function Indices and Sudomotor Function in Chinese Patients with Type 2 Diabetes

PURPOSE

SUDOSCAN, a new non-invasive, quick, sensitive and quantitative technique, has been developed to detect diabetic peripheral neuropathy, and the latter is believed to be correlated with impaired β-cell function. The purpose of the present study was to investigate the associations between β-cell function indices and sudomotor function in Chinese type 2 diabetes.

METHODS

A total of 266 Chinese patients with type 2 diabetes were enrolled. Sudomotor function was assessed using electrochemical skin conductance of hands and feet. Pancreatic β-cell function was determined by homeostasis model assessment of β-cell function index, early-phase β-cell function indices and total β-cell function indices. Pearson correlation analysis and multiple linear stepwise regression analysis were carried out to explore the associations between β-cell function indices and sudomotor function.

RESULTS

Patients with lower early-phase β-cell function had lower electrochemical skin conductance levels of hands and feet and higher asymmetry ratio of hands and feet. Both Pearson correlation analysis and multiple linear stepwise regression analysis showed significantly positive relationships between early-phase β-cell function and electrochemical skin conductance levels of hands and feet, after controlling for potential confounders (P<0.05).

CONCLUSIONS

Impaired early-phase β-cell function was positively associated with sudomotor dysfunction in Chinese patients with type 2 diabetes. We speculated that impaired early-phase β-cell function may be associated with the incidence of sudomotor dysfunction in patients with T2DM.

Insulin resistance and endometrial cancer risk: A systematic review and meta-analysis

AIM

It has been suggested that chronic hyperinsulinemia from insulin resistance is involved in the etiology of endometrial cancer (EC). We performed a systematic review and meta-analysis to assess whether insulin resistance is associated with the risk of EC.

METHODS

We searched PubMed-Medline, Embase, Scopus, and Web of Science for articles published from database inception through 30th September 2014. We included all observational studies evaluating components defining insulin resistance in women with and without EC. Quality of the included studies was assessed by Newcastle-Ottawa scale. Random-effects models and inverse variance method were used to meta-analyze the association between insulin resistance components and EC.

RESULTS

Twenty-five studies satisfied our inclusion criteria. Fasting insulin levels (13 studies, n = 4088) were higher in women with EC (mean difference [MD] 33.94 pmol/L, 95% confidence interval [CI] 15.04-52.85, p = 0.0004). No differences were seen in postmenopausal versus pre- and postmenopausal subgroup analysis. Similarly, non-fasting/fasting C-peptide levels (five studies, n = 1938) were also higher in women with EC (MD 0.14 nmol/L, 95% CI 0.08-0.21, p < 0.00001). Homeostatic model assessment - insulin resistance (HOMA-IR) values (six studies, n = 1859) in EC patients were significantly higher than in women without EC (MD 1.13, 95% CI 0.20-2.06, p = 0.02). There was moderate-to-high heterogeneity among the included studies.

CONCLUSION

Currently available epidemiologic evidence is suggestive of significantly higher risk of EC in women with high fasting insulin, non-fasting/fasting C-peptide and HOMA-IR values.

The structure, molecular interactions and bioactivities of proinsulin C-peptide correlate with a tripartite molecule

Many biological roles have been assigned to proinsulin C-peptide over the years. Some appear surprisingly disparate and sometimes even contradictory, like chaperone-like actions and depository tendencies. This review summarizes recently reported biomolecular interactions of the peptide and presents how they correlate with structural and functional aspects into a partitioned molecular architecture. At the structural level, the C-peptide sequence and fold can be subdivided into three distinct parts ('tripartite'). At the functional level, its chaperone-like abilities, self-assembly, and membrane interactions, as well as interactions with relevant proteins can be separately ascribed to these three segments. At the biological level, the assignments are compatible with the suggested roles of C-peptide in granular insulin storage, chaperone-like activities on insulin oligomers, possible depository tendencies, and proposed receptor interactions. Finally, the assignments give interesting parallels to further bioactive peptides, including glucagon and neurotensin. Provided pharmaceutical and clinical trials are successfully completed, the present interpretations should supply mechanistic explanations on C-peptide as a bioactive compound of importance in health and diabetes.

C-peptide: new findings and therapeutic possibilities

Much new information on C-peptide physiology has appeared during the past 20 years. It has been shown that C-peptide binds specifically to cell membranes, elicits intracellular signaling via G-protein and Ca2+ -dependent pathways, resulting in activation and increased expression of endothelial nitric oxide synthase, Na+, K+ -ATPase and several transcription factors of importance for anti-inflammatory, anti-oxidant and cell protective mechanisms. Studies in animal models of diabetes and early clinical trials in patients with type 1 diabetes demonstrate that C-peptide in replacement doses elicits beneficial effects on early stages of diabetes-induced functional and structural abnormalities of the peripheral nerves, the kidneys and the retina. Much remains to be learned about C-peptide's mechanism of action and long-term clinical trials in type 1 diabetes subjects will be required to determine C-peptide's clinical utility. Nevertheless, even a cautious evaluation of the available evidence presents the picture of a bioactive endogenous peptide with therapeutic potential.

Physiological effects and therapeutic potential of proinsulin C-peptide

Connecting Peptide, or C-peptide, is a product of the insulin prohormone, and is released with and in amounts equimolar to those of insulin. While it was once thought that C-peptide was biologically inert and had little biological significance beyond its role in the proper folding of insulin, it is now known that C-peptide binds specifically to the cell membranes of a variety of tissues and initiates specific intracellular signaling cascades that are pertussis toxin sensitive. Although it is now clear that C-peptide is a biologically active molecule, controversy still remains as to the physiological significance of the peptide. Interestingly, C-peptide appears to reverse the deleterious effects of high glucose in some tissues, including the kidney, the peripheral nerves, and the vasculature. C-peptide is thus a potential therapeutic agent for the treatment of diabetes-associated long-term complications. This review addresses the possible physiologically relevant roles of C-peptide in both normal and disease states and discusses the effects of the peptide on sensory nerve, renal, and vascular function. Furthermore, we highlight the intracellular effects of the peptide and present novel strategies for the determination of the C-peptide receptor(s). Finally, a hypothesis is offered concerning the relationship between C-peptide and the development of microvascular complications of diabetes.

Stability of selected serum proteins after long-term storage in the Janus Serum Bank

BACKGROUND

Human serum from biobanks is frequently used in prospective epidemiological studies. Long-term storage may modify its composition. A better understanding of the stability of the serum components may improve the interpretation of future studies.

METHODS

The concentrations of selected proteins; immunoglobulins, carrier proteins and enzymes in samples stored at -25 degrees C for 25 years and 2 years were compared with 1-month-old samples. For each length of storage time, 130 specimens were randomly selected from apparently healthy male blood donors aged 40-49 years. We examined the distribution of values, compared dispersion and localization of central tendency, and established reference intervals for each component.

RESULTS

The study demonstrated non-significant or numerically small group differences in the concentrations of albumin, aspartate amino transferase, cystatin C, immunoglobulin E, immunoglobulin G, and sex hormone binding globulin. Mean values between fresh and 25-year-old samples suggested larger differences during storage for alanine amino transferase (-73.4%), creatinine kinase (-96.1%), insulin C-peptide (-98.7%), ferritin (-18.5%) and transferrin (+8.2%).

CONCLUSIONS

The findings showed that long-term storage can introduce a considerable bias for vulnerable components.