Molecular effects of proinsulin C-peptide

Understanding Insulin in the Age of Precision Medicine and Big Data: Under-Explored Nature of Genomics

Insulin is amongst the human genome's most well-studied genes/proteins due to its connection to metabolic health. Within this article, we review literature and data to build a knowledge base of Insulin (INS) genetics that influence transcription, transcript processing, translation, hormone maturation, secretion, receptor binding, and metabolism while highlighting the future needs of insulin research. The INS gene region has 2076 unique variants from population genetics. Several variants are found near the transcriptional start site, enhancers, and following the INS transcripts that might influence the readthrough fusion transcript INS-IGF2. This INS-IGF2 transcript splice site was confirmed within hundreds of pancreatic RNAseq samples, lacks drift based on human genome sequencing, and has possible elevated expression due to viral regulation within the liver. Moreover, a rare, poorly characterized African population-enriched variant of INS-IGF2 results in a loss of the stop codon. INS transcript UTR variants rs689 and rs3842753, associated with type 1 diabetes, are found in many pancreatic RNAseq datasets with an elevation of the 3'UTR alternatively spliced INS transcript. Finally, by combining literature, evolutionary profiling, and structural biology, we map rare missense variants that influence preproinsulin translation, proinsulin processing, dimer/hexamer secretory storage, receptor activation, and C-peptide detection for quasi-insulin blood measurements.

Correlation between serum C-peptide-releasing effects and the risk of elevated uric acid in type 2 diabetes mellitus

Our study aimed to investigate the C-peptide-releasing effect associated with the risk of elevated serum uric acid (SUA) levels in patients with type 2 diabetes mellitus (T2DM). In the cross-sectional study, 345 patients with T2DM hospitalized at the First Affiliated Hospital of Harbin Medical University were consecutively enrolled, and their baseline data were collected. The study design used two parameters for C-peptide releasing effects: the multiplication effect of 1 h postprandial C-peptide to fasting C-peptide ratio (1hCp/FCp) and 2hCp/FCp; the incremental effect of 1hCp minus FCp (1hΔCp) and 2hΔCp. The patients with T2DM in the upper quartiles of SUA had higher FCp, 1hCp, 1hΔCp, 2hCp, and 2hΔCp. Multiple linear regression analysis revealed that after adjusting all the confounding factors, the serum C-peptide including 1hCp (β = 5.14, p = 0.036), 1hΔCp (β = 7.80, p = 0.010), 2hCp (β = 4.27, p = 0.009) and 2hΔCp (β = 5.20, p = 0.005) were still positively correlated with SUA levels in patients with T2DM. In female patients, only the 2hCp (β = 4.78, p = 0.017) and 2hΔCp (β = 5.28, p = 0.019) were associated with SUA level; however, in male patients, no C-peptide parameter was associated with SUA levels in T2DM (all p > 0.05). Within a certain range, the elevated SUA levels might be associated with the better C-peptide incremental effect of islet β cell function in T2DM, especially in female patients.

C-Peptide as a Therapy for Type 1 Diabetes Mellitus

Diabetes mellitus (DM) is a complex metabolic disease affecting one-third of the United States population. It is characterized by hyperglycemia, where the hormone insulin is either not produced sufficiently or where there is a resistance to insulin. Patients with Type 1 DM (T1DM), in which the insulin-producing beta cells are destroyed by autoimmune mechanisms, have a significantly increased risk of developing life-threatening cardiovascular complications, even when exogenous insulin is administered. In fact, due to various factors such as limited blood glucose measurements and timing of insulin administration, only 37% of T1DM adults achieve normoglycemia. Furthermore, T1DM patients do not produce C-peptide, a cleavage product from insulin processing. C-peptide has potential therapeutic effects in vitro and in vivo on many complications of T1DM, such as peripheral neuropathy, atherosclerosis, and inflammation. Thus, delivery of C-peptide in conjunction with insulin through a pump, pancreatic islet transplantation, or genetically engineered Sertoli cells (an immune privileged cell type) may ameliorate many of the cardiovascular and vascular complications afflicting T1DM patients.

Biological activity versus physiological function of proinsulin C-peptide

Proinsulin C-peptide (C-peptide) has drawn much research attention. Even if the peptide has turned out not to be important in the treatment of diabetes, every phase of C-peptide research has changed our view on insulin and peptide hormone biology. The first phase revealed that peptide hormones can be subject to processing, and that their pro-forms may involve regulatory stages. The second phase revealed the possibility that one prohormone could harbor more than one activity, and that the additional activities should be taken into account in the development of hormone-based therapies. In the third phase, a combined view of the evolutionary patterns in hormone biology allowed an assessment of C-peptide´s role in physiology, and of how biological activities and physiological functions are shaped by evolutionary processes. In addition to this distinction, C-peptide research has produced further advances. For example, C-peptide fragments are successfully administered in immunotherapy of type I diabetes, and plasma C-peptide levels remain a standard for measurement of beta cell activity in patients. Even if the concept of C-peptide as a hormone is presently not supported, some of its bioactivities continue to influence our understanding of evolutionary changes of also other peptides.

Elucidation of a Copper Binding Site in Proinsulin C-peptide and Its Implications for Metal-Modulated Activity

The connecting peptide (C-peptide) is a hormone with promising health benefits in ameliorating diabetes-related complications, yet mechanisms remain elusive. Emerging studies point to a possible dependence of peptide activity on bioavailable metals, particularly Cu(II) and Zn(II). However, little is known about the chemical nature of the interactions, hindering advances in its therapeutic applications. This work uncovers the Cu(II)-binding site in C-peptide that may be key to understanding its metal-dependent function. A combination of spectroscopic studies reveal that Cu(II) and Zn(II) bind to C-peptide at specific residues in the N-terminal region of the peptide and that Cu(II) is able to displace Zn(II) for C-peptide binding. The data point to a Cu(II)-binding site consisting of 1N3O square-planar coordination that is entropically driven. Furthermore, the entire random coil peptide sequence is needed for specific metal binding as mutations and truncations reshuffle the coordinating residues. These results expand our understanding of how metals influence hormone activity and facilitate the discovery and validation of both new and established paradigms in peptide biology.

Novel Formulations of C-Peptide with Long-Acting Therapeutic Potential for Treatment of Diabetic Complications

The development and application of novel nanospheres based on cationic and anionic random amphiphilic polypeptides with prolonged stability were proposed. The random copolymers, e.g., poly(l-lysine-co-d-phenylalanine) (P(Lys-co-dPhe)) and poly(l-glutamic acid-co-d-phenylalanine) (P(Glu-co-dPhe)), with different amount of hydrophilic and hydrophobic monomers were synthesized. The polypeptides obtained were able to self-assemble into nanospheres. Such characteristics as size, PDI and ζ-potential of the nanospheres were determined, as well as their dependence on pH was also studied. Additionally, the investigation of their biodegradability and cytotoxicity was performed. The prolonged stability of nanospheres was achieved via introduction of d-amino acids into the polypeptide structure. The cytotoxicity of nanospheres obtained was tested using HEK-293 cells. It was proved that no cytotoxicity up to the concentration of 500 µg/mL was observed. C-peptide delivery systems were realized in two ways: (1) peptide immobilization on the surface of P(Glu-co-dPhe) nanospheres; and (2) peptide encapsulation into P(Lys-co-dPhe) systems. The immobilization capacity and the dependence of C-peptide encapsulation efficiency, as well as maximal loading capacity, on initial drug concentration was studied. The kinetic of drug release was studied at model physiological conditions. Novel formulations of a long-acting C-peptide exhibited their effect ex vivo by increasing activity of erythrocyte Na⁺/K⁺-adenosine triphosphatase.

Unraveling the aggregation propensity of human insulin C-peptide

Over the last 20 years, proinsulin C-peptide emerged as an important player in various biological events. Much time and effort has been spent in exploring all functional features of C-peptide and recording its implications in Diabetes mellitus. Only a few studies, though, have addressed C-peptide oligomerization and link this procedure with Diabetes. The aim of our work was to examine the aggregation propensity of C-peptide, utilizing Transmission Electron Microscopy, Congo Red staining, ATR-FTIR, and X-ray fiber diffraction at a 10 mg ml^-1 concentration. Our experimental work clearly shows that C-peptide self-assembles into amyloid-like fibrils and therefore, the aggregation propensity of C-peptide is a characteristic novel feature that should be related to physiological and also pathological conditions. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 108: 1-8, 2017.

C-peptide is independent associated with diabetic peripheral neuropathy: a community-based study

BACKGROUND

Because the relationship between C-peptide and diabetic peripheral neuropathy (DPN) is controversial, the aim of our study was to evaluate the relationship between C-peptide and DPN in community-based Chinese patients with type 2 diabetes.

METHODS

In total, 220 consecutive type 2 diabetic patients treated by our regional medical consortium were enrolled. DPN was assessed by clinical symptoms, signs, and electromyography.

RESULTS

Fasting C-peptide, 2-h postprandial C-peptide and ΔC-peptide (i.e., 2-h postprandial C-peptide minus the fasting C-peptide) serum concentrations in the non-DPN group were significantly higher than those in the clinical DPN group (all P ≤ 0.040) and the confirmed DPN group (all P < 0.002). The three C-peptide parameters were independently associated with DPN (all P < 0.05) after adjusting for age, sex, diabetes duration, smoking status, systolic pressure, body mass index, angiotensin-converting enzyme inhibitors/angiotensin receptor blocker use, fasting plasma glucose, HbA1c, triglyceride and estimated glomerular filtration rate. Compared with the ΔC-peptide quartile 1 (reference), patients in quartile 3 (odds ratio [OR], 0.110; 95% confidence interval [CI] 0.026-0.466; P = 0.003) and quartile 4 (OR, 0.012; 95% CI 0.026-0.559; P = 0.007) had a lower risk of DPN after adjusting for the confounders.

CONCLUSIONS

C-peptide was negatively associated with DPN in community-based Chinese type 2 diabetic patients in China.

The role of cullin 5-containing ubiquitin ligases

The suppressor of cytokine signaling (SOCS) box consists of the BC box and the cullin 5 (Cul5) box, which interact with Elongin BC and Cul5, respectively. SOCS box-containing proteins have ubiquitin ligase activity mediated by the formation of a complex with the scaffold protein Cul5 and the RING domain protein Rbx2, and are thereby members of the cullin RING ligase superfamily. Cul5-type ubiquitin ligases have a variety of substrates that are targeted for polyubiquitination and proteasomal degradation. Here, we review the current knowledge on the identification of Cul5 and the regulation of its expression, as well as the signaling pathways regulated by Cul5 and how viruses highjack the Cul5 system to overcome antiviral responses.

Relationship between serum C-peptide level and diabetic retinopathy according to estimated glomerular filtration rate in patients with type 2 diabetes

OBJECTIVE

To test the hypothesis that serum C-peptide level would relate to the risk of diabetic retinopathy (DR) in type 2 diabetic patients independently of estimated glomerular filtration rate (eGFR).

DESIGN

A total of 2,062 patients with type 2 diabetes were investigated in this cross-sectional study. Fasting C-peptide, 2-hour postprandial C-peptide, and ΔC-peptide (postprandial C-peptide minus fasting C-peptide) levels were measured. The patients were divided into two groups according to eGFR (ml∙min(-1)1.73m(-2)): patients without renal impairment (eGFR ≥60) and those with renal impairment (eGFR <60).

RESULTS

In subjects both with and without renal impairment, patients with DR showed lower levels of fasting C-peptide, postprandial C-peptide and ΔC-peptide. In multivariate analysis, serum C-peptide levels were significantly associated with DR (odds ratio [OR] of each standard deviation increase in the logarithmic value, 0.85; 95% confidence interval [CI], 0.78-0.92 for fasting C-peptide, P<0.001; OR, 0.87; 95% CI, 0.82-0.92 for postprandial C-peptide, P<0.001; OR, 0.88; 95% CI, 0.82-0.94 for ΔC-peptide, P<0.001) after adjustment for age, gender, and other confounding factors including eGFR.

CONCLUSIONS

Serum C-peptide levels are inversely associated with the prevalence of DR in type 2 diabetic patients independently of eGFR.

The role of insulin C-peptide in the coevolution analyses of the insulin signaling pathway: a hint for its functions

As the linker between the A chain and B chain of proinsulin, C-peptide displays high variability in length and amino acid composition, and has been considered as an inert byproduct of insulin synthesis and processing for many years. Recent studies have suggested that C-peptide can act as a bioactive hormone, exerting various biological effects on the pathophysiology and treatment of diabetes. In this study, we analyzed the coevolution of insulin molecules among vertebrates, aiming at exploring the evolutionary characteristics of insulin molecule, especially the C-peptide. We also calculated the correlations of evolutionary rates between the insulin and the insulin receptor (IR) sequences as well as the domain-domain pairs of the ligand and receptor by the mirrortree method. The results revealed distinctive features of C-peptide in insulin intramolecular coevolution and correlated residue substitutions, which partly supported the idea that C-peptide can act as a bioactive hormone, with significant sequence features, as well as a linker assisting the formation of mature insulin during synthesis. Interestingly, the evolution of C-peptide exerted the highest correlation with that of the insulin receptor and its ligand binding domain (LBD), implying a potential relationship with the insulin signaling pathway.

C-peptide: a new mediator of atherosclerosis in diabetes

Diabetes type 2 and insulin resistance are the risk factors for cardiovascular disease. It is already known that atherosclerosis is an inflammatory disease, and a lot of different factors are involved in its onset. C-peptide is a cleavage product of proinsulin, an active substance with a number of effects within different complications of diabetes. In this paper we discuss the role of C-peptide and its effects in the development of atherosclerosis in type 2 diabetic patients.

Association between serum C-peptide levels and chronic microvascular complications in Korean type 2 diabetic patients

This study evaluated the association between serum C-peptide levels and chronic vascular complications in Korean patients with type 2 diabetes. Data for 1,410 patients with type 2 diabetes were evaluated cross-sectionally. Fasting and postprandial 2-hour serum C-peptide levels were analyzed with respect to diabetic micro- and macrovascular complications. In the group of patients with lower fasting serum C-peptide quartile, the prevalences of diabetic retinopathy and neuropathy were significantly higher (P = 0.035, P < 0.001, respectively). In the group of patients with lower delta C-peptide (postprandial - fasting C-peptide) quartile, the prevalences of diabetic retinopathy, nephropathy, and neuropathy were significantly higher (P < 0.001 for all). Low delta C-peptide quartile was also associated with increased severity of retinopathy and nephropathy. The age- and sex-adjusted odds ratios (ORs) for retinopathy, neuropathy, and nephropathy in the lowest versus the highest delta C-peptide quartile were 6.45 (95% confidence interval 3.41-12.22), 3.01 (2.16-4.19), and 2.65 (1.71-4.12), respectively. After further adjustment for the duration of diabetes, type of antidiabetic therapy, mean hemoglobin A1c, body mass index, and blood pressure, the ORs were reduced to 2.83 (1.32-6.08), 1.68 (1.12-2.53), and 1.61 (1.05-2.47), respectively, but remained significant. No significant difference was observed in the prevalence of macrovascular complications with respect to fasting or delta C-peptide quartiles. These results suggest that low C-peptide level is associated with diabetic microvascular, but not macrovascular complications in patients with type 2 diabetes mellitus.

Factors associated with diabetic retinopathy in chinese patients with type 2 diabetes mellitus

Objective. To investigate the risk factors of DR in Chinese T2DM patients. Methods. 2009 patients with T2DM were included in this cross-sectional study. All patients underwent eye examination, and the DR stage was defined by an ophthalmologist. Correlation analysis was performed to evaluate the relation between DR and clinical variables. Logistic regression models were used to assess risk for those factors associated with DR. Results. A total of 597 T2DM patients (29.7%) had DR, of which 548 (27.3%) were nonproliferative diabetic retinopathy and 49 (2.4%) were proliferative diabetic retinopathy. Positive correlations were found between DR and duration of diabetes, systolic blood pressure (SBP), diastolic blood pressure, glycated hemoglobin, glycated albumin, 24 hurinary albumin excretion, peripheral atherosclerosis (PA), diabetes nephropathy (DN), diabetic peripheral neuropathy, and anemia. Negative correlations were found between DR and C-peptide and glomerular filtration rate. Logistic regression analysis revealed that duration of diabetes, SBP, DN, anemia, PA, and C-peptide were each independent risk factors of DR. Conclusion. The duration of diabetes, SBP, DN, anemia, and PA are positively associated with DR in Chinese T2DM patients, while C-peptide is negatively associated with DR. Monitoring and evaluation of these related factors will likely contribute to the prevention and treatment of DR.

Basal C-peptide Level as a Surrogate Marker of Subclinical Atherosclerosis in Type 2 Diabetic Patients

BACKGROUND

Recent studies have revealed that C-peptide induces smooth muscle cell proliferation and causes human atherosclerotic lesions in diabetic patients. The present study was designed to examine whether the basal C-peptide levels correlate with cardiovascular risk in type 2 diabetes mellitus (T2DM) patients.

METHODS

Data was obtained from 467 patients with T2DM from two institutions who were followed for four years. The medical findings of all patients were reviewed, and patients with creatinine >1.4 mg/dL, any inflammation or infection, hepatitis, or type 1 DM were excluded. The relationships between basal C-peptide and other clinical values were statistically analyzed.

RESULTS

A simple correlation was found between basal C-peptide and components of metabolic syndrome (MS). Statistically basal C-peptide levels were significantly higher than the three different MS criteria used in the present study, the Adult Treatment Panel III (ATP III) of the National Cholesterol Education Program's (NCEP's), World Health Organization (WHO), and the International Diabetes Federation (IDF) criteria (NCEP-ATP III, P=0.001; IDF, P<0.001; WHO, P=0.029). The multiple regression analysis between intima-media thickness (IMT) and clinical values showed that basal C-peptide significantly correlated with IMT (P=0.043), while the analysis between the 10-year coronary heart disease risk by the United Kingdom Prospective Diabetes Study risk engine and clinical values showed that basal C-peptide did not correlate with IMT (P=0.226).

CONCLUSION

Basal C-peptide is related to cardiovascular predictors (IMT) of T2DM, suggesting that basal C-peptide does provide a further indication of cardiovascular disease.

Total chemical synthesis of human proinsulin

A convergent synthetic strategy based on modern chemical ligation methods was used to make human proinsulin. The synthetic protein was characterized by LCMS, CD spectroscopy, and by 1D- and 2D-NMR spectroscopy. Synthetic human proinsulin had full biochemical activity in a receptor-binding assay.

Impact of temperature dependent sampling procedures in proteomics and peptidomics--a characterization of the liver and pancreas post mortem degradome

Little is known about the nature of post mortem degradation of proteins and peptides on a global level, the so-called degradome. This is especially true for nonneural tissues. Degradome properties in relation to sampling procedures on different tissues are of great importance for the studies of, for instance, post translational modifications and/or the establishment of clinical biobanks. Here, snap freezing of fresh (<2 min post mortem time) mouse liver and pancreas tissue is compared with rapid heat stabilization with regard to effects on the proteome (using two-dimensional differential in-gel electrophoresis) and peptidome (using label free liquid chromatography). We report several proteins and peptides that exhibit heightened degradation sensitivity, for instance superoxide dismutase in liver, and peptidyl-prolyl cis-trans isomerase and insulin C-peptides in pancreas. Tissue sampling based on snap freezing produces a greater amount of degradation products and lower levels of endogenous peptides than rapid heat stabilization. We also demonstrate that solely snap freezing related degradation can be attenuated by subsequent heat stabilization. We conclude that tissue sampling involving a rapid heat stabilization step is preferable to freezing with regard to proteomic and peptidomic sample quality.

C-peptide microheterogeneity in type 2 diabetes populations

PURPOSE

The purpose of this study was to investigate naturally occurring C-peptide microheterogeneity in healthy and type 2 diabetes (T2D) populations.

EXPERIMENTAL DESIGN

MS immunoassays capable of simultaneously detecting intact C-peptide and variant forms were applied to plasma samples from 48 healthy individuals and 48 individuals diagnosed with T2D.

RESULTS

Common throughout the entire sample set were three previously unreported variations of C-peptide. The relative contribution of one variant, subsequently identified as C-peptide (3-31), was found to be more abundant in the T2D population as compared to the healthy population. Dipeptidyl peptidase IV is suspected to be responsible for this particular cleavage product, which is consistent with the pathophysiology of T2D.

CONCLUSIONS AND CLINICAL RELEVANCE

C-peptide does not exist in the human body as a single molecular species. It is qualitatively more heterogeneous than previously thought. These results lay a foundation for future studies devoted to a comprehensive understanding of C-peptide and its variants in healthy and diabetic populations.

A possible role of insulin-like growth factor-II C-peptide in regulating the function of steroidogenic cells in adult frog adrenal glands

The sole structural determinant for the differential ability of the insulin-like growth factors (IGF-I and IGF-II) to induce autophosphorylation of specific insulin receptor (IR) tyrosine residues and activate downstream signaling molecules is the C domain. The IR is structurally related to the type I insulin-like growth factor receptor (IGF-IR). This study aimed to identify the presence of IGF receptors by which the IGF-II C-peptide could mediate its effects in the frog (Rana ridibunda) adrenal glands and to observe whether injection of IGF-II C-peptide affects the function of adrenal steroidogenic cells using light and transmission electron microscopy and by the evaluation of the immunoreactivity of steroidogenic acute regulatory protein (StAR). After IGF-II C-peptide injection, there was a reduction of StAR protein immunoreactivity levels, an accumulation of large lipid droplets in close contact with each other, and an induction of proliferation of the steroidogenic cells. These results indicate a possible role of IGF-II C-peptide in steroidogenic cell function and in induction of steroidogenesis. The detection in this study of IGF-I receptor (IGF-IR) immunoreactivity in frog adrenal glands also indicates that the metabolic and mitogenic effects of IGF-II C-peptide in these glands may occur via the IGF-IR.

Massive peptide sharing between viral and human proteomes

Thirty viral proteomes were examined for amino acid sequence similarity to the human proteome, and, in parallel, a control of 30 sets of human proteins was analyzed for internal human overlapping. We find that all of the analyzed 30 viral proteomes, independently of their structural or pathogenic characteristics, present a high number of pentapeptide overlaps to the human proteome. Among the examined viruses, human T-lymphotropic virus 1, Rubella virus, and hepatitis C virus present the highest number of viral overlaps to the human proteome. The widespread and ample distribution of viral amino acid sequences through the human proteome indicates that viral and human proteins are formed of common peptide backbone units and suggests a fluid compositional chimerism in phylogenetic entities canonically classified distantly as viruses and Homo sapiens. Importantly, the massive viral to human peptide overlapping calls into question the possibility of a direct causal association between virus-host sharing of amino acid sequences and incitement to autoimmune reactions through molecular recognition of common motifs.

Possible role of insulin-like growth factor-II C-peptide on catecholamine release and ultrastructural aspects of chromaffin cells in the adrenal gland of the frog

The present study was undertaken to demonstrate that insulin-like growth factor-II C-peptide (IGF-II C-peptide) affects the function of the adrenal gland of Rana ridibunda (Anura, Amphibia) by stimulating chromaffin cells. Previous studies have shown that insulin-like growth factors affect adrenal gland function in mammals. On the basis of these findings, frogs were injected with IGF-II C-peptide (2.5 microg/0.2 ml), whereas control animals were injected with Ringer solution (0.2 ml). The adrenal glands were removed at 12 and 48 h after injection and fixed, embedded in paraffin wax and Epon, and examined by immunohistochemistry and transmission electron microscopy to investigate whether there were structural changes and activation of chromaffin cells in the frog adrenal gland. Sections were stained with hematoxylin and eosin for overall tissue analysis and, in parallel, serotonin was localized using the streptavidin-biotin complex technique while dopamine beta-hydroxylase was shown by the peroxidase-antiperoxidase-3, 3'-diaminobenzidine tetrachloride method. After injection of IGF-II C-peptide, chromaffin cells released serotonin and synthesized dopamine beta-hydroxylase. The most pronounced effect of IGF-II C-peptide on the chromaffin cells was observed at 12h after injection. Our results indicate that there is a possible role of IGF-II C-peptide on chromaffin cell activity enhancing catecholamine release in the adrenal gland of the frog.

Peptidology: short amino acid modules in cell biology and immunology

Short amino acid motifs, either linear sequences or discontinuous amino acid groupings, can interact with specific protein domains, so exerting a central role in cell adhesion, signal transduction, hormone activity, regulation of transcript expression, enzyme activity, and antigen-antibody interaction. Here, we analyze the literature for such critical short amino acid motifs to determine the minimal peptide length involved in biologically important interactions. We report the pentapeptide unit as a common minimal amino acid sequence critically involved in peptide-protein interaction and immune recognition. The present survey may have implications in defining the dimensional module for peptide-based therapeutical approaches such as the development of novel antibiotics, enzyme inhibitors/activators, mimetic agonists/antagonists of neuropeptides, thrombolitic agents, specific anti-viral agents, etc. In such a therapeutical context, it is of considerable interest that low molecular weight peptides can easily cross biological barriers, are less susceptible to protease attacks, and can be administered at high concentrations. In addition, small peptides are a rational target for strategies aimed at antigen-specific immunotherapeutical intervention. As an example, specific short peptide fragments might be used to elicit antibodies capable of reacting with the full-length proteins containing the peptide fragment's amino acid sequence, so abolishing the risk of cross-reactivity.

Proinsulin C-peptide stimulates a PKC/IkappaB/NF-kappaB signaling pathway to activate COX-2 gene transcription in Swiss 3T3 fibroblasts

Proinsulin C-peptide causes multiple molecular and physiological effects, and improves renal and neuronal dysfunction in patients with diabetes. However, whether C-peptide controls the inhibitor kappaB (IkappaB)/NF-kappaB-dependent transcription of genes, including inflammatory genes is unknown. Here we showed that 1 nM C-peptide increased the expression of cyclooxygenase-2 (COX-2) mRNA and its protein in Swiss 3T3 fibroblasts. Consistently, C-peptide enhanced COX-2 gene promoter-activity, which was inhibited by GF109203X and Go6976, specific PKC inhibitors, and BAY11-7082, a specific nuclear factor-kappaB (NF-kappaB) inhibitor, accompanied by increased phosphorylation and degradation of IkappaB. These results suggest that C-peptide stimulates the transcription of inflammatory genes via activation of a PKC/IkappaB/NF-kappaB signaling pathway.

Proinsulin C-peptide elicits disaggregation of insulin resulting in enhanced physiological insulin effects

Using surface plasmon resonance (SPR) and electrospray mass spectrometry (ESI-MS), proinsulin C-peptide was found to influence insulin-insulin interactions. In SPR with chip-bound insulin, C-peptide mixed with analyte insulin increased the binding, while alone C-peptide did not. A control peptide with the same residues in random sequence had little effect. In ESI-MS, C-peptide lowered the presence of insulin hexamer. The data suggest that C-peptide promotes insulin disaggregation. Insulin/insulin oligomer muM dissociation constants were determined. Compatible with these findings, type 1 diabetic patients receiving insulin and C-peptide developed 66% more stimulation of glucose metabolism than when given insulin alone. A role of C-peptide in promoting insulin disaggregation may be important physiologically during exocytosis of pancreatic beta-cell secretory granulae and pharmacologically at insulin injection sites. It is compatible with the normal co-release of C-peptide and insulin and may contribute to the beneficial effect of C-peptide and insulin replacement in type 1 diabetics.

[Physiological effects of the connecting peptide]

Insulin-dependent diabetic (IDDM) patients present significantly altered Na,K-ATPase activity in several organs, including kidney. Particularly in kidney tubule, Na,K-ATPase alteration occurs together with changes in glomerular filtration rate, the first step of IDDM-induced renal failure. The latter is a major cause of morbidity and mortality in IDDM patients. The C-peptide of proinsulin is important for the biosynthesis of insulin but has for a long time been considered to be biologically inert. Recent studies have demonstrated that replacement of C-peptide to normal physiological concentrations in IDDM patients either on a short-term basis (1-3 hours) or on a prolonged administration (1-3 months) was accompanied by improvements in renal glomerular and tubular function. Animal studies have shown that most of the renal tubular effects of C-peptide may in part be explained by its ability to stimulate Na,K-ATPase activity. In conclusion, these combined findings indicate that C-peptide is a biologically active hormone. The possibility that C-peptide therapy in IDDM patients may be beneficial should be considered. The present review focuses on: 1) Making a point about C-peptide-induced tubular effects on the basis of clinical and experimental experiments, and 2) precising the molecular mechanisms involved in C-peptide-induced tubular Na,K-ATPase effects.

Human proinsulin C-peptide prevents proliferation of rat aortic smooth muscle cells cultured in high-glucose conditions

AIMS/HYPOTHESIS

Proinsulin C-peptide is involved in several biological activities. However, the role of C-peptide in vascular smooth muscle cells is unclear. We therefore investigated its effects, in vascular smooth muscle cells in high-glucose conditions.

METHODS

Rat aortic smooth muscle cells were cultured with 5.5 or 20 mmol/l glucose with or without C-peptide (1 to 100 nmol/l) for 3 weeks. Proliferation activities, the protein expression of platelet-derived growth factor (PDGF)-beta receptor, the phosphorylation of p42/p44 mitogen-activated protein (MAP) kinases, and glucose uptake were measured.

RESULTS

The proliferation activities increased approximately three-fold under high-glucose conditions (p<0.05). C-peptide suppressed hyperproliferation activities that were induced by high glucose. This happened in a dose-dependent manner from 1 to 100 nmol/l of C-peptide. C-peptide (10 and 100 nmol/l) inhibited the increased protein expression of PDGF-beta receptor and the phosphorylation of p42/p44 MAP kinases that had been induced by high glucose (p<0.05). Furthermore, 100 nmol/l of C-peptide augmented the impaired glucose uptake in the high-glucose conditions.

CONCLUSIONS/INTERPRETATION

These observations suggest that C-peptide could prevent diabetic macroangiopathy by inhibiting smooth muscle cell growth and ameliorating glucose utilisation in smooth muscle cells. C-peptide may thus be a novel agent for treating diabetic macroangiopathy in patients with type 1 and type 2 diabetes.

Solution structure of human proinsulin C-peptide

The C-peptide of proinsulin is important for the biosynthesis of insulin, but has been considered for a long time to be biologically inert. Recent studies in diabetic patients have stimulated a new debate about its possible regulatory role, suggesting that it is a hormonally active peptide. We describe structural studies of the C-peptide using 2D NMR spectroscopy. In aqueous solution, the NOE patterns and chemical shifts indicate that the ensemble is a nonrandom structure and contains substructures with defined local conformations. These are more clearly visible in 50% H2O/50% 2,2,2-trifluoroethanol. The N-terminal region (residues 2-5) forms a type I beta-turn, whereas the C-terminal region (residues 27-31) presents the most well-defined structure of the whole molecule including a type III'beta-turn. The C-terminal pentapeptide (EGSLQ) has been suggested to be responsible for chiral interactions with an as yet uncharacterized, probably a G-protein-coupled, receptor. The three central regions of the molecule (residues 9-12, 15-18 and 22-25) show tendencies to form beta-bends. We propose that the structure described here for the C-terminal pentapeptide is consistent with the previously postulated CA knuckle, believed to represent the active site of the C-peptide of human proinsulin.

Electroimmobilization of proinsulin C-peptide to a quartz crystal microbalance sensor chip for protein affinity purification

Proinsulin C-peptide was electroimmobilized to a quartz crystal microbalance sensor chip, localizing this low-pI peptide for covalent attachment to activated surface carboxyl groups. The resulting chip was used in a continuous flow biosensor to capture anti-C-peptide antibodies, which could subsequently be eluted in 5% formic acid between air bubbles for efficient recovery and mass spectrometric identification. The method is reproducible through repeated cycles, providing affinity purification of proteins under real-time monitoring of the binding and elution processes.

C-peptide increases the expression of vasopressin-activated calcium-mobilizing receptor gene through a G protein-dependent pathway

OBJECTIVE

Although an increasing number of reports suggest that physiological concentrations of C-peptide protect against the development of diabetic nephropathy, possibly through the modulation of Na-K pump activity, the intracellular pathways controlled by C-peptide are still unrecognized. C-peptide and vasopressin share similar intracellular effects including the activation of calcium influx and endothelial nitric oxide synthase. Both hormones stimulate also the activity of Na-K pump activity. Whether the activity of C-peptide is mediated by the recently identified vasopressin-activated calcium-mobilizing receptor (VACM-1) has never been previously investigated.

DESIGN AND METHODS

To clarify this issue, we evaluated the effect of C-peptide on VACM-1 RNA (measured by semiquantitative RT-PCR) and protein expression (measured by immunoblotting) in human skin fibroblasts (where a specific binding of C-peptide was demonstrated) and in human mesangial cells, the cellular target of diabetic nephropathy.

RESULTS

C-peptide-induced activation of VACM-1 was demonstrated in fibroblasts from six healthy individuals (0.51+/-0.1 vs 1.48+/-0.4, arbitrary units+/-s.e., P = 0.025). This finding was paralleled by an increased VACM-1 protein expression (5.64+/-1.0 vs 8.47+/-1.2, arbitrary units+/-s.e., P= 0.043). Similar results were confirmed in three independent cultures of human mesangial cells. VACM-1 activation in fibroblasts was insensitive to phosphatidylinositol-3-kinase inhibitor LY294002, but was inhibited by pertussis toxin, suggesting that activation of VACM-1 could be mediated by a G protein-coupled receptor.

CONCLUSIONS

This study demonstrates for the first time that C-peptide activates VACM-1, possibly through a G protein-coupled receptor. Further studies are needed to clarify whether VACM-1 is involved in the protective effect of C-peptide against the development of diabetic nephropathy.

Ligand-independent activation of peroxisome proliferator-activated receptor-gamma by insulin and C-peptide in kidney proximal tubular cells: dependent on phosphatidylinositol 3-kinase activity

Peroxisome proliferator-activated receptor gamma (PPARgamma) has key roles in the regulation of adipogenesis, inflammation, and lipid and glucose metabolism. C-peptide is believed to be inert and without appreciable biological functions. Recent studies suggest that C-peptide possesses multiple functions. The present study investigated the effects of insulin and C-peptide on PPARgamma transcriptional activity in opossum kidney proximal tubular cells. Both insulin and C-peptide induced a concentration-dependent stimulation of PPARgamma transcriptional activity. Both agents substantially augmented thiazolidinedione-stimulated PPARgamma transcriptional activity. Neither insulin nor C-peptide had any effect on the expression levels of PPARgamma. GW9662, a PPARgamma antagonist, blocked PPARgamma activation by thiazolidinediones but had no effect on either insulin- or C-peptide-stimulated PPARgamma transcriptional activity. Co-transfection of opossum kidney cells with dominant negative mitogen-activated protein kinase kinase significantly depressed basal PPARgamma transcriptional activity but had no effect on that induced by either insulin or C-peptide. Both insulin- and C-peptide-stimulated PPARgamma transcriptional activity were attenuated by wortmannin and by expression of a dominant negative phosphatidylinositol (PI) 3-kinase p85 regulatory subunit. In addition PI 3-kinase-dependent phosphorylation of PPARgamma was observed after stimulation by C-peptide or insulin. C-peptide effects but not insulin on PPARgamma transcriptional activity were abolished by pertussis toxin pretreatment. Finally both C-peptide and insulin positively control the expression of the PPARgamma-regulated CD36 scavenger receptor in human THP-1 monocytes. We concluded that insulin and C-peptide can stimulate PPARgamma activity in a ligand-independent fashion and that this effect is mediated by PI 3-kinase. These results support a new and potentially important physiological role for C-peptide in regulation of PPARgamma-related cell functions.

C-peptide and captopril are equally effective in lowering glomerular hyperfiltration in diabetic rats

BACKGROUND

C-peptide has been shown to reduce glomerular hyperfiltration, glomerular hypertrophy and urinary albumin excretion in type 1 diabetes, but its effect has not been compared with that of an angiotensin-converting enzyme inhibitor (ACEI) in the early stage of renal involvement in diabetes.

METHODS

Glomerular filtration rate (GFR) was measured in terms of inulin clearance and renal blood flow, using ultrasound technique, in four groups of streptozotocin-induced diabetic rats before and after a 60 min infusion of C-peptide (D-Cp), captopril (D-ACEI), C-peptide and captopril (D-Cp-ACEI) or placebo (D-placebo). In addition, a non-diabetic control group was studied before and after captopril infusion (C-ACEI).

RESULTS

GFR was 37-51% higher in the diabetic groups than in the control animals. GFR decreased after treatment in the D-Cp, D-ACEI and D-Cp-ACEI groups, but did not change in the D-placebo group. Blood flow increased by 26-32% in the three groups receiving captopril and by 5% in the diabetic groups treated with C-peptide alone or placebo. The increase in blood flow in the three ACEI-treated groups was significantly greater than in the D-placebo group. Filtration fraction fell significantly in all groups, but only in the combined D-Cp-ACEI group did it fall significantly more than in the D-placebo group.

CONCLUSIONS

C-peptide and captopril lower diabetes-induced glomerular hyperfiltration to a similar extent, but the influence of captopril on blood flow is greater than that of C-peptide, suggesting different mechanisms of action. No statistically significant additive effects of C-peptide and captopril were shown in this acute infusion study.

C-Peptide Colocalizes with Macrophages in Early Arteriosclerotic Lesions of Diabetic Subjects and Induces Monocyte Chemotaxis In Vitro

Objective— Increased levels of C-peptide, a cleavage product of proinsulin, circulate in patients with insulin resistance and early type 2 diabetes, a high-risk population for the development of a diffuse and extensive pattern of arteriosclerosis. This study tested the hypothesis that C-peptide might participate in atherogenesis in these patients.

Method and Results— We demonstrate significantly higher intimal C-peptide deposition in thoracic artery specimens from young diabetic subjects compared with matched nondiabetic controls as determined by immunohistochemical staining. C-peptide colocalized with monocytes/macrophages in the arterial intima of artery specimen from diabetic subjects. In vitro, C-peptide stimulated monocyte chemotaxis in a concentration-dependent manner with a maximal 2.3±0.4-fold increase at 1 nmol/L C-peptide. Pertussis toxin, wortmannin, and LY294002 inhibited C-peptide–induced monocyte chemotaxis, suggesting the involvement of pertussis toxin-sensitive G-proteins as well as a phosphoinositide 3-kinase (PI3K)-dependent mechanism. In addition, C-peptide treatment activated PI3K in human monocytes, as demonstrated by PI3K activity assays.

Conclusion— C-peptide accumulated in the vessel wall in early atherogenesis in diabetic subjects and may promote monocyte migration into developing lesions. These data support the hypothesis that C-peptide may play an active role in atherogenesis in diabetic patients and suggest a new mechanism for accelerated arterial disease in diabetes.

C-peptide improves adenosine-induced myocardial vasodilation in type 1 diabetes patients

Patients with type 1 (insulin-dependent) diabetes show reduced skeletal muscle blood flow and coronary vasodilatory function despite intensive insulin therapy and good metabolic control. Administration of proinsulin C-peptide increases skeletal muscle blood flow in these patients, but a possible influence of C-peptide on myocardial vasodilatory function in type 1 diabetes has not been investigated. Ten otherwise healthy young male type 1 diabetic patients (Hb A1c 6.6%, range 5.7-7.9%) were studied on two consecutive days during normoinsulinemia and euglycemia in a double-blind, randomized, crossover design, receiving intravenous infusion of C-peptide (5 pmol.kg-1.min-1) for 120 min on one day and saline infusion on the other day. Myocardial blood flow (MBF) was measured at rest and during adenosine administration (140 microg.kg-1.min-1) both before and during the C-peptide or saline infusions by use of positron emission tomography and [15O]H2O administration. Basal MBF was not significantly different in the patients compared with an age-matched control group, but adenosine-induced myocardial vasodilation was 30% lower (P < 0.05) in the patients. During C-peptide administration, adenosine-stimulated MBF increased on average 35% more than during saline infusion (P < 0.02) and reached values similar to those for the healthy controls. Moreover, as evaluated from transthoracal echocardiographic measurements, C-peptide infusion resulted in significant increases in both left ventricular ejection fraction (+5%, P < 0.05) and stroke volume (+7%, P < 0.05). It is concluded that short-term C-peptide infusion in physiological amounts increases the hyperemic MBF and left-ventricular function in type 1 diabetic patients.

Correlations between carbohydrate metabolism and corticotrop axis parameters in anorexia nervosa

The hypercortisolism is one of the hormonal features of anorexia nervosa (AN) in the undernutrition phase. This abnormality seems to be related to the nutritional factors because the weight restoration leads to the normalization of cortisol. We have investigated glycemia, plasma insulin and C-peptide like markers of carbohydrate metabolism, and, also, adrenocorticotrope hormone (ACTH), beta-endorphins and cortisol in basal and dynamics conditions in 142 patients with AN. Insulin negatively correlated with the values of cortisol at 16:00 h (r=-0.28, P<0.05, N=45); 04:00 h (r=-0.29, P<0.01, N=38). C-peptide negatively correlated with the values of cortisol at 08:00 h (r=-0.36, P<0.05, N=36); 12:00 h (r=-0.49, P<0.01, N=36); 16:00 h (r=-0.38, P<0.02, N=36); 20:00 h (r=-0.39, P<0.02, N=36); 04:00 h (r=-0.51, P<0.01, N=31); urinary free cortisol (r=-0.42, P<0.01, N=35); dexamethasone suppression test (DST; r=-0.52, P<0.01, N=30). C-peptide negatively correlated with the values of ACTH at 08:00 h (r=-0.33, P<0.05, N=37); 24:00 h (r=-0.38, P<0.05, N=32); 04:00 h (r=-0.49, P<0.01, N=31); DST (r=-0.45, P<0.02, N=25). We did not identify correlations between carbohydrate metabolism indices and beta-endorphins. These findings suggest that the nutritional factors may cause or maintain the hormonal changes of corticotrop axis in the undernutrition states.

Proinsulin C-peptide increases nitric oxide production by enhancing mitogen-activated protein-kinase-dependent transcription of endothelial nitric oxide synthase in aortic endothelial cells of Wistar rats

AIMS/HYPOTHESIS

Recent studies have suggested that proinsulin C-peptide improves vascular functions, possibly through nitric oxide (NO) production. To clarify the molecular mechanisms of vascular NO production induced by C-peptide, we examined the effects of C-peptide on NO production and NO synthase expression in rat aortic endothelial cells in connection with mitogen-activated protein kinase (MAPK) activation.

METHODS

Aortic endothelial cells were isolated from female Wistar rats, cultured to confluence, and serum-starved for 24 h before treatment with C-peptide. Nitric oxide production was measured by the DAF-2 fluorescence dye method and relative amounts of endothelial nitric oxide synthase (eNOS) protein and its mRNA were semi-quantified by western blot and RT-PCR analyses respectively. Activation of MAPK was estimated by western blot detection of activity-related phosphorylation and in vitro kinase assay.

RESULTS

Stimulation of cells with C-peptide for 3 h doubled NO production, which was suppressed by the NO synthase inhibitor, N(G)-nitro- L-arginine methyl ester (L-NAME). Stimulation also increased mRNA and protein contents of eNOS in a manner sensitive to the transcription inhibitor actinomycin D. It did not affect inducible NO synthase mRNA. C-peptide also induced rapid phosphorylation and activation of extracellular signal-regulated kinase (ERK, also known as p44/42MAPK), but not of p38MAPK. In cells pretreated with the ERK inhibitor PD98059 the C-peptide-elicited increase of NO production and eNOS was abrogated in a dose-dependent manner; suppression of ERK phosphorylation induced by C-peptide also occurred.

CONCLUSIONS/INTERPRETATION

Our results show that C-peptide increases NO production by increasing eNOS protein contents through ERK-dependent up-regulation of eNOS gene transcription. This could explain some actions of C-peptide on the vasculature, indicating a pivotal role for C-peptide in vascular homeostasis.

C-peptide makes a comeback

Proinsulin C-peptide was for long considered to be without biological activity of its own. New findings demonstrate, however, that it is capable of eliciting both molecular and physiological effects, suggesting that C-peptide is in fact a bioactive peptide. When administered in replacement doses to animal models or to patients with type 1 diabetes, C-peptide ameliorates diabetes-induced functional and structural changes in both the kidneys and the peripheral nerves. It augments blood flow in a number of tissues, notably skeletal muscle, myocardium, skin and nerve. These effects are thought to be mediated via a stimulatory influence on Na+,K(+)-ATPase and on endothelial nitric oxide synthase. Specific binding of C-peptide to cell membranes of intact cells and to detergent-solubilized cellular components has been demonstrated, indicating the existence of cell-surface binding sites for C-peptide. A number of intracellular responses are elicited by C-peptide, including a rise in Ca2+ concentration and activation of MAP-kinase signaling pathways. Many but not all of C-peptide's intracellular effects can be inhibited by pertussis toxin, supporting the notion that C-peptide may interact via a G-protein-coupled receptor. Additional data suggest that C-peptide may interact synergistically also in the insulin signaling pathway. Combined, the available observations show conclusively that C-peptide is biologically active, even though its molecular mechanism of action is not as yet fully understood. The possibility that replacement of C-peptide in patients with type 1 diabetes may serve to retard or prevent the development of long-term complications should be evaluated.

A novel mass spectrometric approach to the analysis of hormonal peptides in extracts of mouse pancreatic islets

Liquid chromatography mass spectrometry (LC-MS) is a valuable tool in the analysis of proteins and peptides. The combination of LC-MS with different fragmentation methods provides sequence information on components in complex mixtures. In this work, on-line packed capillary LC electrospray ionization Fourier transform ion cyclotron resonance MS was combined with two complementary fragmentation techniques, i.e. nozzle-skimmer fragmentation and electron capture dissociation, for the determination of hormonal peptides in an acid ethanol extract of mouse pancreatic islets. The most abundant peptides, those derived from proinsulin and proglucagon, were identified by their masses and additional sequence-tag information established their identities. Interestingly, the experiments demonstrated the presence of truncated C-peptides, des-(25-29)-C-peptide and des-(27-31)-C-peptide. These novel findings clearly illustrate the potential usefulness of the described technique for on-line sequencing and characterization of peptides in tissue extracts.

C-peptide has no effect on forearm blood flow during local hyperinsulinaemia in healthy humans

BACKGROUND

C-peptide increases forearm blood flow (FBF) in patients with Type 1 diabetes, probably by interaction with insulin, but not in healthy subjects. It is unclear if the vasodilating effect is sealed at normal fasting insulin concentrations.

METHODS

The effects of C-peptide alone and during local hyperinsulinaemia were studied in healthy young men. Subjects received intra-arterial insulin at 6 pmol min-1 (low dose) or placebo for 60 min with subsequent coinfusion of C-peptide at increasing doses of 2-60 pmol min-1 in a double-blind crossover study (n = 8). In control experiments insulin at 30 pmol min-1 (high dose) was coinfused with C-peptide (n = 3). FBF was measured by strain-gauge plethysmography.

RESULTS

Placebo had no effect on FBF (mean percentage change from baseline at 50 min -3.1%, 95% confidence interval [CI]-14.9, + 8.7). Insulin infusion slightly enhanced FBF by + 10.2% (95% CI -6.8, + 27.2; low dose) and + 17.6% (95% CI -38.8, + 74.0; high dose), respectively. The mean individual difference of the change in FBF between low-dose insulin and placebo was + 13.3% (95% CI -6.0, + 32.7; P = NS). Infusion of C-peptide increased local C-peptide concentrations from 1.8 +/- 0.1 ng ml-1 to 6.1 +/- 2.8 ng ml-1, but had no effect on FBF during placebo or hyperinsulinaemia (mean difference vs low dose insulin -16.0%, 95% CI -38.9, + 6.9).

CONCLUSION

The vasodilating effect of C-peptide seen in Type 1 diabetes is not detectable during fasting or hyperinsulinaemia in the forearm vasculature of healthy subjects. This suggests saturation of its vasodilating potency at insulin concentrations within the normal or in the supraphysiological range.