Physiological effects and therapeutic potential of proinsulin C-peptide

[Correlation between fasting C-peptide and serum uric acid in patients with type 2 diabetes mellitus]

OBJECTIVE

To explore the relationship between fasting C-peptide (F-CP) and serum uric acid (SUA) in patients with type 2 diabetes mellitus (T2DM).

METHODS

A total of 347 hospitalized patients with T2DM were stratified according to F-CP level to analyze the impact of increased F-CP levels on SUA level and the incidence of hyperuricemia (HUA). The patients with an elevated SUA level (>420 µmol/L) and a normal SUA level (≤420 µmol/L) were compared for general data, fasting C-peptide and other clinical indexes. Pearson or Spearman correlation analysis was used to analyze the correlation of SUA level with F-CP levels and other parameters. The risk factors of elevated SUA were analyzed by binary logistic regression, multiple regression analysis and hierarchical interaction analysis. The ROC curve was used to analyze the independent risk factors of elevated SUA and determine the corresponding cut-off values.

RESULTS

Compared with those with a normal SUA level, patients with elevated SUA had higher body mass index (BMI), waist-to-hip ratio, F-CP, postprandial 2hC peptide (2hP-CP), triglyceride (TG), homocysteine (HCY), serum creatinine (SCr) level (P<0.05), and a greater percentage of drinking (44.8% vs 32.6%, P=0.006), but had significantly lowered levels of HbA1c, high-density lipoprotein (HDL), and estimated glomerular filtration rate (eGFR) (P<0.05). SUA was found to be positively correlated with F-CP, 2hP-CP, BMI, waist-to-hip ratio, diastolic blood pressure, TG, HCY, SCr, smoking and drinking (P<0.05), and was negatively correlated with gender, age, age of disease onset, HbA1c, HDL and eGFR (P<0.05). SUA level and the incidence of hyperuricemia increasea significantly with F-CP level (P<0.05). F-CP was identified as an independent risk factor for elevated SUA, and gender did not affect the relationship between F-CP and SUA. ROC curve analysis showed that a F-CP level >1.260 ng/mL was associated with a significantly increased risk of hyperuricemia in T2DM patients.

CONCLUSION

F-CP is closely related with SUA and may be an independent risk factor of elevated SUA in patients with T2DM.

[Correlation between fasting C-peptide and serum uric acid in patients with type 2 diabetes mellitus]

OBJECTIVE

To explore the relationship between fasting C-peptide (F-CP) and serum uric acid (SUA) in patients with type 2 diabetes mellitus (T2DM).

METHODS

A total of 347 hospitalized patients with T2DM were stratified according to F-CP level to analyze the impact of increased F-CP levels on SUA level and the incidence of hyperuricemia (HUA). The patients with an elevated SUA level (>420 µmol/L) and a normal SUA level (≤420 µmol/L) were compared for general data, fasting C-peptide and other clinical indexes. Pearson or Spearman correlation analysis was used to analyze the correlation of SUA level with F-CP levels and other parameters. The risk factors of elevated SUA were analyzed by binary logistic regression, multiple regression analysis and hierarchical interaction analysis. The ROC curve was used to analyze the independent risk factors of elevated SUA and determine the corresponding cut-off values.

RESULTS

Compared with those with a normal SUA level, patients with elevated SUA had higher body mass index (BMI), waist-to-hip ratio, F-CP, postprandial 2hC peptide (2hP-CP), triglyceride (TG), homocysteine (HCY), serum creatinine (SCr) level (P<0.05), and a greater percentage of drinking (44.8% vs 32.6%, P=0.006), but had significantly lowered levels of HbA1c, high-density lipoprotein (HDL), and estimated glomerular filtration rate (eGFR) (P<0.05). SUA was found to be positively correlated with F-CP, 2hP-CP, BMI, waist-to-hip ratio, diastolic blood pressure, TG, HCY, SCr, smoking and drinking (P<0.05), and was negatively correlated with gender, age, age of disease onset, HbA1c, HDL and eGFR (P<0.05). SUA level and the incidence of hyperuricemia increasea significantly with F-CP level (P<0.05). F-CP was identified as an independent risk factor for elevated SUA, and gender did not affect the relationship between F-CP and SUA. ROC curve analysis showed that a F-CP level >1.260 ng/mL was associated with a significantly increased risk of hyperuricemia in T2DM patients.

CONCLUSION

F-CP is closely related with SUA and may be an independent risk factor of elevated SUA in patients with T2DM.

Transplantation of human fetal pancreatic progenitor cells ameliorates renal injury in streptozotocin-induced diabetic nephropathy

Background

Diabetic nephropathy (DN) is a severe complication of diabetes mellitus (DM). Pancreas or islet transplantation has been reported to prevent the development of DN lesions and ameliorate or reverse existing glomerular lesions in animal models. Shortage of pancreas donor is a severe problem. Islets derived from stem cells may offer a potential solution to this problem.

Objective

To evaluate the effect of stem cell-derived islet transplantation on DN in a rat model of streptozotocin-induced DM.

Methods

Pancreatic progenitor cells were isolated from aborted fetuses of 8 weeks of gestation. And islets were prepared by suspension culture after a differentiation of progenitor cells in medium containing glucagon-like peptide-1 (Glp-1) and nicotinamide. Then islets were transplanted into the liver of diabetic rats via portal vein. Blood glucose, urinary volume, 24 h urinary protein and urinary albumin were measured once biweekly for 16 weeks. Graft survival was evaluated by monitoring human C-peptide level in rat sera and by immunohistochemical staining for human mitochondrial antigen and human C-peptide in liver tissue. The effect of progenitor-derived islets on filtration membrane was examined by electron microscopy and real-time polymerase chain reaction (PCR). Immunohistochemical staining, real-time PCR and western blot were employed for detecting fibronectin, protein kinase C beta (PKCβ), protein kinase A (PKA), inducible nitric oxide synthase (iNOS) and superoxide dismutase (SOD).

Results

Islet-like clusters derived from 8th gestational-week human fetal pancreatic progenitors survived in rat liver. And elevated serum level of human C-peptide was detected. Blood glucose, 24 h urinary protein and urinary albumin were lower in progenitor cell group than those in DN or insulin treatment group. Glomerular basement membrane thickness and fibronectin accumulation decreased significantly while podocytes improved morphologically in progenitor cell group. Furthermore, receptor of advanced glycation end products and PKCβ became down-regulated whereas PKA up-regulated by progenitor cell-derived islets. And iNOS rose while SOD declined.

Conclusions

DN may be reversed by transplantation of human fetal pancreatic progenitor cell-derived islets. And fetal pancreatic progenitor cells offer potential resources for cell replacement therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-017-1253-1) contains supplementary material, which is available to authorized users.

A Practical Review of C-Peptide Testing in Diabetes

C-peptide is a widely used measure of pancreatic beta cell function. It is produced in equimolar amounts to endogenous insulin but is excreted at a more constant rate over a longer time. Methods of estimation include urinary and unstimulated and stimulated serum sampling. Modern assays detect levels of c-peptide which can be used to guide diabetes diagnosis and management. We explore the evidence behind the various tests available. We recommend the glucagon stimulation c-peptide testing owing to its balance of sensitivity and practicality. C-peptide levels are associated with diabetes type and duration of disease. Specifically a c-peptide level of less than 0.2 nmol/l is associated with a diagnosis of type 1 diabetes mellitus (T1DM). C-peptide level may correlate with microvascular and macrovascular complications and future use of insulin therapy, as well as likely response to other individual therapies. We explore the potential uses of c-peptide measurement in clinical practice.

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.

Targeting orphan G protein-coupled receptors for the treatment of diabetes and its complications: C-peptide and GPR146

G protein-coupled receptors (GPCRs) are the most abundant receptor family encoded by the human genome and are the targets of a high percentage of drugs currently in use or in clinical trials for the treatment of diseases such as diabetes and its associated complications. Thus, orphan GPCRs, for which the ligand is unknown, represent an important untapped source of therapeutic potential for the treatment of many diseases. We have identified the previously orphan GPCR, GPR146, as the putative receptor of proinsulin C-peptide, which may prove to be an effective treatment for diabetes-associated complications. For example, we have found a potential role of C-peptide and GPR146 in regulating the function of the retinal pigment epithelium, a monolayer of cells in the retina that serves as part of the blood-retinal barrier and is disrupted in diabetic macular oedema. However, C-peptide signalling in this cell type appears to depend at least in part on extracellular glucose concentration and its interaction with insulin. In this review, we discuss the therapeutic potential of orphan GPCRs with a special focus on C-peptide and GPR146, including past and current strategies used to 'deorphanize' this diverse family of receptors, past successes and the inherent difficulties of this process.

C-peptide antioxidant adaptive pathways in β cells and diabetes

In this review, we present findings that support autocrine cell protection by C-peptide in the context of clinical studies of type 1 diabetes (T1D), which universally measure C-peptide serum levels as a surrogate for β cell functional mass. Over the last decade, evidence has accumulated that supports models in which C-peptide, cosecreted with insulin by pancreatic β cells, acts on peripheral targets including the vascular endothelium to reduce oxidative stress and apoptosis subsequent to exposure to diabetic insults. In parallel, as assays have become more sensitive, C-peptide has been detected in the circulation of most subjects with T1D where higher C-peptide levels are associated with fewer and slower development of diabetic microvascular complications, consistent with antioxidant protection by C-peptide. Clinical trials investigating C-peptide-replacement therapy effects have demonstrated amelioration of T1D nephropathy and neuropathy. Recently, the antioxidant action of C-peptide was extended to the β cells secreting it, that is an autocrine mechanism. Autocrine protection has major implications for the treatment of diabetes because the more C-peptide secreted, the more protection provided to the same β cells resulting in a slower decay in β cell functional mass over the time course of disease. Why β cells evolved to cosecrete an antioxidant C-peptide hormone together with the glycaemia-lowering insulin hormone is explored in the context of proposed evolutionary advantages of physiologically transient oxidative stress and insulin resistance as an adaptation for survival through times of fuel scarcity. The importance of recognizing autocrine C-peptide protection of functional β cell mass in observational clinical studies, and its therapeutic implications in interventional C-peptide-replacement studies, will be discussed.

Long-Acting C-Peptide and Neuropathy in Type 1 Diabetes: A 12-Month Clinical Trial

OBJECTIVE

Lack of C-peptide in type 1 diabetes may be an important contributing factor in the development of microvascular complications. Replacement of native C-peptide has been shown to exert a beneficial influence on peripheral nerve function in type 1 diabetes. The aim of this study was to evaluate the efficacy and safety of a long-acting C-peptide in subjects with type 1 diabetes and mild to moderate peripheral neuropathy.

RESEARCH DESIGN AND METHODS

A total of 250 patients with type 1 diabetes and peripheral neuropathy received long-acting (pegylated) C-peptide in weekly dosages of 0.8 mg (n = 71) or 2.4 mg (n = 73) or placebo (n = 106) for 52 weeks. Bilateral sural nerve conduction velocity (SNCV) and vibration perception threshold (VPT) on the great toe were measured on two occasions at baseline, at 26 weeks, and at 52 weeks. The modified Toronto Clinical Neuropathy Score (mTCNS) was used to grade the peripheral neuropathy.

RESULTS

Plasma C-peptide rose during the study to 1.8-2.2 nmol/L (low dose) and to 5.6-6.8 nmol/L (high dose). After 52 weeks, SNCV had increased by 1.0 ± 0.24 m/s (P < 0.001 within group) in patients receiving C-peptide (combined groups), but the corresponding value for the placebo group was 1.2 ± 0.29 m/s. Compared with basal, VPT had improved by 25% after 52 weeks of C-peptide therapy (Δ for combined C-peptide groups: -4.5 ± 1.0 μm, placebo group: -0.1 ± 0.9 μm; P < 0.001). mTCNS was unchanged during the study.

CONCLUSIONS

Once-weekly subcutaneous administration of long-acting C-peptide for 52 weeks did not improve SNCV, other electrophysiological variables, or mTCNS but resulted in marked improvement of VPT compared with placebo.

Vascular Smooth Muscle as a Target for Novel Therapeutics

Cardiovascular disease is the principal cause of death in patients with type 2 diabetes (T2DM). Exposure of the vasculature to metabolic disturbances leaves a persistent imprint on vascular walls, and specifically on smooth muscle cells (SMC) that favours their dysfunction and potentially underlies macrovascular complications of T2DM. Current diabetes therapies and continued development of newer treatments has led to the ability to achieve more efficient glycaemic control. There is also some evidence to suggest that some of these treatments may exert favourable pleiotropic effects, some of which may be at the level of SMC. However, emerging interest in epigenetic markers as determinants of vascular disease, and a putative link with diabetes, opens the possibility for new avenues to develop robust and specific new therapies. These will likely need to target cell-specific epigenetic changes such as effectors of DNA histone modifications that promote or inhibit gene transcription, and/or microRNAs capable of regulating entire cellular pathways through target gene repression. The growing epidemic of T2DM worldwide, and its attendant cardiovascular mortality, dictates a need for novel therapies and personalised approaches to ameliorate vascular complications in this vulnerable population.

Efficacy of a long-acting C-peptide analogue against peripheral neuropathy in streptozotocin-diabetic mice

AIMS

To investigate the efficacy of a pegylated C-peptide (Peg-C-peptide) against indices of peripheral neuropathy in a mouse model of type 1 diabetes and to compare efficacy of this C-peptide analogue against that of the native molecule.

METHODS

C57Bl/6 mice were injected with two consecutive doses of streptozotocin (STZ) to induce type 1 diabetes. Mice were treated twice daily with native C-peptide [0.4-1.3 mg/kg subcutaneously (s.c.)] or twice weekly with Peg-C-peptide (0.1-1.3 mg/kg s.c.) for 20 weeks. Motor and sensory nerve conduction velocities, thermal and tactile responses and rate dependent H-wave depression were assessed after 20 weeks of diabetes. Foot skin intraepidermal fibres and corneal nerves were counted, and sciatic nerve substance P and plasma C-peptide levels were also determined.

RESULTS

After 5 months of STZ-induced diabetes, mice exhibited significant motor and sensory nerve conduction slowing, thermal hypoalgesia, tactile allodynia and attenuation of rate-dependent depression of the H reflex. These functional disorders were accompanied by nerve substance P depletion but not loss of small sensory fibres in the hind paw epidermis or the cornea. The efficacy of twice-daily treatment with native C-peptide in preventing these disorders was matched or exceeded by twice-weekly treatment with Peg-C-peptide. Both native and Peg-C-peptide also increased corneal nerve occupancy in the sub-basal nerve plexus of control rats.

CONCLUSIONS

These data identify actions of C-peptide against novel and clinically pertinent aspects of diabetic neuropathy in mice and also establish Peg-C-peptide as a long-acting therapeutic method of potential clinical value.

The Physiology of Proinsulin C-Peptide: Unanswered Questions and a Proposed Model

C-peptide is produced, processed, and secreted with insulin, and appears to exert separate but intimately related effects. In this review, we address the existence of the C-peptide receptor, the interaction between C-peptide and insulin, and the potential physiological significance of proinsulin C-peptide.

Mexico City normal weight children exposed to high concentrations of ambient PM2.5 show high blood leptin and endothelin-1, vitamin D deficiency, and food reward hormone dysregulation versus low pollution controls. Relevance for obesity and Alzheimer disease

Millions of Mexico, US and across the world children are overweight and obese. Exposure to fossil-fuel combustion sources increases the risk for obesity and diabetes, while long-term exposure to fine particulate matter (PM2.5) and ozone (O3) above US EPA standards is associated with increased risk of Alzheimer's disease (AD). Mexico City Metropolitan Area children are chronically exposed to PM2.5 and O3 concentrations above the standards and exhibit systemic, brain and intrathecal inflammation, cognitive deficits, and Alzheimer disease neuropathology. We investigated adipokines, food reward hormones, endothelial dysfunction, vitamin D and apolipoprotein E (APOE) relationships in 80 healthy, normal weight 11.1±3.2 year olds matched by age, gender, BMI and SES, low (n: 26) versus high (n:54) PM2.5 exposures. Mexico City children had higher leptin and endothelin-1 (p<0.01 and p<0.000), and decreases in glucagon-like peptide-1 (GLP 1), ghrelin, and glucagon (<0.02) versus controls. BMI and leptin relationships were significantly different in low versus high PM2.5 exposed children. Mexico City APOE 4 versus 3 children had higher glucose (p=0.009). Serum 25-hydroxyvitamin D<30 ng/mL was documented in 87% of Mexico City children. Leptin is strongly positively associated to PM 2.5 cumulative exposures. Residing in a high PM2.5 and O3 environment is associated with 12h fasting hyperleptinemia, altered appetite-regulating peptides, vitamin D deficiency, and increases in ET-1 in clinically healthy children. These changes could signal the future trajectory of urban children towards the development of insulin resistance, obesity, type II diabetes, premature cardiovascular disease, addiction-like behavior, cognitive impairment and Alzheimer's disease. Increased efforts should be made to decrease pediatric PM2.5 exposures, to deliver health interventions prior to the development of obesity and to identify and mitigate environmental factors influencing obesity and Alzheimer disease.

C-peptide as a Therapy for Kidney Disease: A Systematic Review and Meta-Analysis

C-peptide has intrinsic biological activity and may be renoprotective. We conducted a systematic review to determine whether C-peptide had a beneficial effect on renal outcomes. MEDLINE, EMBASE, and the Cochrane Central Databases were searched for human and animal studies in which C-peptide was administered and renal endpoints were subsequently measured. We identified 4 human trials involving 74 patients as well as 18 animal studies involving 35 separate experiments with a total of 641 animals. In humans, the renal effects of exogenously delivered C-peptide were only studied in type 1 diabetics with either normal renal function or incipient nephropathy. Pooled analysis showed no difference in GFR (mean difference, -1.36 mL/min/1.73 m2, p = 0.72) in patients receiving C-peptide compared to a control group, but two studies reported a reduction in glomerular hyperfiltration (p<0.05). Reduction in albuminuria was also reported in the C-peptide group (p<0.05). In diabetic rodent models, C-peptide led to a reduction in GFR (mean difference, -0.62 mL/min, p<0.00001) reflecting a partial reduction in glomerular hyperfiltration. C-peptide also reduced proteinuria (mean difference, -186.25 mg/day, p = 0.05), glomerular volume (p<0.00001), and mesangial matrix area (p<0.00001) in diabetic animals without affecting blood pressure or plasma glucose. Most studies were relatively short-term in duration, ranging from 1 hour to 3 months. Human studies of sufficient sample size and duration are needed to determine if the beneficial effects of C-peptide seen in animal models translate into improved long-term clinical outcomes for patients with chronic kidney disease. (PROSPERO CRD42014007472)