C-peptide corrects endoneurial blood flow but not oxidative stress in type 1 BB/Wor rats

The role of C-peptide in diabetes and its complications: an updated review

Worldwide, diabetes and its complications have seriously affected people's quality of life and become a serious public health problem. C-peptide is not only an indicator of pancreatic β-cell function, but also a biologically active peptide that can bind to cell membrane surface signaling molecules and activate downstream signaling pathways to play antioxidant, anti-apoptotic and inflammatory roles, or regulate cellular transcription through internalization. It is complex how C-peptide is related to diabetic complications. Both deficiencies and overproduction can lead to complications, but their mechanisms of action may be different. C-peptide replacement therapy has shown beneficial effects on diabetic complications in animal models when C-peptide is deficient, but results from clinical trials have been unsatisfactory. The complex pattern of the relationship between C-peptide and diabetic chronic complications has not yet been fully understood. Future basic and clinical studies of C-peptide replacement therapies will need to focus on baseline levels of C-peptide in addition to more attention also needs to be paid to post-treatment C-peptide levels to explore the optimal range of fasting C-peptide and postprandial C-peptide maintenance.

Study on risk factors of diabetic peripheral neuropathy and establishment of a prediction model by machine learning

BACKGROUND

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes. Predicting the risk of developing DPN is important for clinical decision-making and designing clinical trials.

METHODS

We retrospectively reviewed the data of 1278 patients with diabetes treated in two central hospitals from 2020 to 2022. The data included medical history, physical examination, and biochemical index test results. After feature selection and data balancing, the cohort was divided into training and internal validation datasets at a 7:3 ratio. Training was made in logistic regression, k-nearest neighbor, decision tree, naive bayes, random forest, and extreme gradient boosting (XGBoost) based on machine learning. The k-fold cross-validation was used for model assessment, and the accuracy, precision, recall, F1-score, and the area under the receiver operating characteristic curve (AUC) were adopted to validate the models' discrimination and clinical practicality. The SHapley Additive exPlanation (SHAP) was used to interpret the best-performing model.

RESULTS

The XGBoost model outperformed other models, which had an accuracy of 0·746, precision of 0·765, recall of 0·711, F1-score of 0·736, and AUC of 0·813. The SHAP results indicated that age, disease duration, glycated hemoglobin, insulin resistance index, 24-h urine protein quantification, and urine protein concentration were risk factors for DPN, while the ratio between 2-h postprandial C-peptide and fasting C-peptide(C2/C0), total cholesterol, activated partial thromboplastin time, and creatinine were protective factors.

CONCLUSIONS

The machine learning approach helped established a DPN risk prediction model with good performance. The model identified the factors most closely related to DPN.

Mechanistic approach towards diabetic neuropathy screening techniques and future challenges: A review

Diabetic neuropathy, also called peripheral diabetic neuropathy (PDN), is among the most significant diabetes health consequences, alongside diabetic nephropathy, diabetic cardiomyopathy and diabetic retinopathy. Diabetic neuropathy is the existence of signs and indications of peripheral nerve damage in patients with diabetes after other causes have been governed out. Diabetic neuropathy is a painful and severe complication of diabetes that affects roughly 20% of people. The development of diabetic neuropathy is regulated by blood arteries that nourish the peripheral nerves and metabolic problems such as increased stimulation of polyol pathway, loss of myo-inositol and enhanced non-enzymatic glycation. It's divided into four types based on where neurons are most affected: autonomic, peripheral, proximal, and focal, with each kind presenting different symptoms like numbing, gastrointestinal disorders, and heart concerns. Pharmacotherapy for neuropathic pain is complex and for many patients, effective treatment is lacking; as a result, scientific proof recommendations are crucial. As a result, the current demand is to give the most vital medications or combinations of drugs that work directly on the nerves to help diabetic neuropathy patients feel less pain without causing any adverse effects. In diabetic neuropathy research, animal models are ubiquitous, with rats and mice being the most typically chosen for various reasons. This review covers the epidemiology, clinical features, pathology, clinical symptom, mechanism of diabetic neuropathy development, diagnosis, screening models of animals, diabetic neuropathy pharmacotherapy.

Inflammatory biomarkers as a part of diagnosis in diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN), a chronic neurological complication of diabetes mellitus, remains scantily addressed area of research. Many lacunae in the temporal sequence between cause and effect of DPN still remain unfilled and therefore treatment of DPN remains unsatisfactory. This is largely due to the conventional glucocentric focus to resolve the problem. This focus over hyperglycemia should be shifted to consider, chronic low grade inflammation as the major determinant in DPN. Rapidly emerging evidences from recent studies suggest that chronic low grade inflammation leads to the activation of innate immune system response, loss of insulin signaling and insulin resistance, endoplasmic reticulum stress, mitochondrial stress, leading to production of kinases like protein kinase C, mitogen activated protein kinase and jun-N-terminal kinase, pro-inflammatory cytokines and inter leukins-1b, 2, 6 and 8, tumour necrosis factor-alpha and other chemokines, leading to DPN. These biomarkers can be early predictors of DPN and therefore should be the focus of work testing their clinical utility to identify high-risk individuals as well as perhaps to target interventions. In this paper, we would like to review all the aspects of DPN, laying greater emphasis on inflammatory biomarkers as a tool for early diagnosis of DPN and the possible research approaches to address it satisfactorily.

Regional anaesthesia in patients with diabetes

Diabetes is the most common metabolic condition worldwide and about 20% of surgical patients will have this condition. It is a major risk-factor for worse outcomes after surgery including mortality; infective and non-infective complications; and increased length of stay. However, diabetes is a modifiable risk-factor, and programs to improve medical management have the potential to reduce peri-operative complications and the risk of harm. Regional anaesthesia has well-documented benefits in promoting the restoration of function but there are legitimate concerns that the incidence of complications of regional anaesthesia in patients with diabetes is higher. The aim of this review is to explore in detail the various potential advantages and disadvantages of regional anaesthesia in patients with diabetes. This, in turn, will allow practitioners to undertake more informed shared decision-making and potentially modify their anaesthetic technique for patients with diabetes.

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.

[Molecular mechanisms of action and physiological effects of the proinsulin C-peptide (a systematic review)]

The C-peptide is a fragment of proinsulin, the cleavage of which forms active insulin. In recent years, new information has appeared on the physiological effects of the C-peptide, indicating its positive effect on many organs and tissues, including the kidneys, nervous system, heart, vascular endothelium and blood microcirculation. Studies on experimental models of diabetes mellitus in animals, as well as clinical trials in patients with diabetes, have shown that the C-peptide has an important regulatory effect on the early stages of functional and structural disorders caused by this disease. The C-peptide exhibits its effects through binding to a specific receptor on the cell membrane and activation of downstream signaling pathways. Intracellular signaling involves G-proteins and Ca2+-dependent pathways, resulting in activation and increased expression of endothelial nitric oxide synthase, Na+/K+-ATPase and important transcription factors involved in apoptosis, anti-inflammatory and other intracellular defense mechanisms. This review gives an idea of the C-peptide as a bioactive endogenous peptide that has its own biological activity and therapeutic potential.

Differential gene expression profiling of the sciatic nerve in type 1 and type 2 diabetic mice

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes mellitus (DM). The pathogenic mechanisms of DPN and the therapeutic interventions required may be distinct between type 1 (T1) and type 2 (T2) DM. However, the molecular mechanisms underlying the pathogenesis of DPN in both types of diabetes remain unclear. The aim of the current study was to identify the changes in genes and pathways associated with DPN in sciatic nerves of T1- and T2DM mice using bioinformatics analysis. The microarray profiles of sciatic nerves of T1DM (GSE11343) and T2DM (GSE27382) mouse models were downloaded from the Gene Expression Omnibus database to identify differentially expressed genes (DEGs) in each. DEGs in the two types of DM (with fold change ≥2 and P<0.05) were identified with BRB-ArrayTools. Gene Ontology (GO) term and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed using the Database for Annotation, Visualization and Integrated Discovery. A protein-protein interaction (PPI) network was constructed using the Search Tool for the Retrieval of Interacting Genes/Proteins and visualized using Cytoscape. Compared with control samples, 623 and 1,890 DEGs were identified in sciatic nerves of T1- and T2DM mice, respectively. Of these, 75 genes were coordinately dysregulated in the sciatic nerves of both models. Many DEGs unique to T1DM mice were localized to the nucleoplasm and were associated with regulation of transcription processes, while many unique to T2DM mice were localized at cell junctions and were associated with ion transport. In addition, certain DEGs may be associated with the different treatment strategies used for the two types of DM. This analysis provides insight into the functional gene sets and pathways operating in sciatic nerves in T1- and T2DM. The results should improve understanding of the molecular mechanisms underlying the pathophysiology of DPN, and provide information for the development of therapeutic strategies for DPN specific to each type of DM.

Prophylactic L-arginine and ibuprofen delay the development of tactile allodynia and suppress spinal miR-155 in a rat model of diabetic neuropathy

Diabetic neuropathy (DN) is a common complication of diabetes mellitus that is hardly reversible at the late stages. Since treatment of neuropathic pain is predominantly symptomatic, a prophylactic measure would be useful. Both ibuprofen and L-arginine exert antiallodynic effects on chronic constriction injury (CCI)-induced cold allodynia. Furthermore, ibuprofen is effective in CCI-induced mechanical allodynia. The aim of the study was to assess the antiallodynic effect of prophylactic ibuprofen and L-arginine in streptozotocin-induced DN in rats and to further investigate the role of spinal miR-155 and nitric oxide (NO) in this effect. Tactile allodynia was assessed weekly by von Frey filaments. Oral daily administration of ibuprofen, L-arginine and their combination, for 4 weeks starting 1 week after streptozotocin injection (ie, before the development of tactile allodynia), resulted in a significant decrease of tactile allodynia compared with the control diabetic group. This was evident in the fifth week of the experiment. The 3 treatments prevented the decrease in muscle fiber diameter and epidermal thickness, seen in the control diabetic group. Furthermore, ibuprofen, L-arginine and their combination prevented the increase in the spinal NO level and miRNA-155, seen in the control diabetic group. In conclusion, both ibuprofen and L-arginine delayed the development of behavioral and histologic changes of DN, with concomitant suppression of spinal miR-155 and NO level. L-arginine being tolerable may be useful prophylactically in diabetic patients.

Painful neuropathy: Mechanisms

Painful neuropathy, like the other complications of diabetes, is a growing healthcare concern. Unfortunately, current treatments are of variable efficacy and do not target underlying pathogenic mechanisms, in part because these mechanisms are not well defined. Rat and mouse models of type 1 diabetes are frequently used to study diabetic neuropathy, with rats in particular being consistently reported to show allodynia and hyperalgesia. Models of type 2 diabetes are being used with increasing frequency, but the current literature on the progression of indices of neuropathic pain is variable and relatively few therapeutics have yet been developed in these models. While evidence for spontaneous pain in rodent models is sparse, measures of evoked mechanical, thermal and chemical pain can provide insight into the pathogenesis of the condition. The stocking and glove distribution of pain tantalizingly suggests that the generator site of neuropathic pain is found within the peripheral nervous system. However, emerging evidence demonstrates that amplification in the spinal cord, via spinal disinhibition and neuroinflammation, and also in the brain, via enhanced thalamic activity or decreased cortical inhibition, likely contribute to the pathogenesis of painful diabetic neuropathy. Several potential therapeutic strategies have emerged from preclinical studies, including prophylactic treatments that intervene against underlying mechanisms of disease, treatments that prevent gains of nociceptive function, treatments that suppress enhancements of nociceptive function, and treatments that impede normal nociceptive mechanisms. Ongoing challenges include unraveling the complexity of underlying pathogenic mechanisms, addressing the potential disconnect between the perceived location of pain and the actual pain generator and amplifier sites, and finding ways to identify which mechanisms operate in specific patients to allow rational and individualized choice of targeted therapies.

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.

Alternatives to the Streptozotocin-Diabetic Rodent

The study of diabetic neuropathy has relied primarily on the use of streptozotocin-treated rat and mouse models of type 1 diabetes. This chapter will review the creation and use of other rodent models that have been developed in order to investigate the contribution of factors besides insulin deficiency to the development and progression of diabetic neuropathy as it occurs in obesity, type 1 or type 2 diabetes. Diabetic peripheral neuropathy is a complex disorder with multiple mechanisms contributing to its development and progression. Even though many animal models have been developed and investigated, no single model can mimic diabetic peripheral neuropathy as it occurs in humans. Nonetheless, animal models can play an important role in improving our understanding of the etiology of diabetic peripheral neuropathy and in performing preclinical screening of potential new treatments. To date treatments found to be effective for diabetic peripheral neuropathy in rodent models have failed in clinical trials. However, with the identification of new endpoints for the early detection of diabetic peripheral neuropathy and the understanding that a successful treatment may require a combination therapeutic approach there is hope that an effective treatment will be found.

Toxic stress, inflammation and symptomatology of chronic complications in diabetes

Diabetes affects at least 382 million people worldwide and the incidence is expected to reach 592 million by 2035. The incidence of diabetes in youth is skyrocketing as evidenced by a 21% increase in type 1 diabetes and a 30.5% increase in type 2 diabetes in the United States between 2001 and 2009. The effects of toxic stress, the culmination of biological and environmental interactions, on the development of diabetes complications is gaining attention. Stress impacts the hypothalamus-pituitary-adrenal axis and contributes to inflammation, a key biological contributor to the pathogenesis of diabetes and its associated complications. This review provides an overview of common diabetic complications such as neuropathy, cognitive decline, depression, nephropathy and cardiovascular disease. The review also provides a discussion of the role of inflammation and stress in the development and progression of chronic complications of diabetes, associated symptomatology and importance of early identification of symptoms of depression, fatigue, exercise intolerance and pain.

Evaluating peripheral nerve function in asymptomatic patients with type 2 diabetes or latent autoimmune diabetes of adults (LADA): results from nerve conduction studies

AIMS

To assess the involvement of peripheral nerve dysfunction in asymptomatic patients with latent autoimmune diabetes of adults (LADA) or type 2 diabetes (T2DM), and compare nerve conduction (NC) parameters between the two groups.

METHODS

A total of 1635 patients without symptoms and signs relevant to diabetic polyneuropathy (1275 with T2DM and 360 with LADA) were included and were further categorized into 3 groups according to diabetes duration: <5years, 5-14 years and ≥15years. All subjects underwent nerve conduction studies. Abnormal NC was defined as the abnormality of NC parameters in two or more anatomical nerves.

RESULTS

In both LADA and T2DM patients, the proportions of abnormal NC increased significantly with increasing durations (both P for trend <0.001). Specifically, abnormal NC was present in 18.5%, 38.8% and 66.7% of LADA patients with duration of <5 years, 5-14 years and ≥15 years, respectively. Those numbers were 24.8% (P=0.152, vs. LADA), 25.3% (P=0.023, vs. LADA) and 62.8% (P=0.723, vs. LADA) in T2DM. Regarding NC parameters, T2DM patients had higher composite Z-scores of latency than LADA patients within 5 years of duration (P=0.001). In patients with duration of 5-14years, the latency Z-scores were comparable between the two groups (P=0.164), whereas the Z-scores of amplitude were lower (the lower the worse) in LADA than in T2DM (P=0.035).

CONCLUSIONS

Peripheral nerve dysfunction is common in asymptomatic patients with LADA or T2DM. Findings of the study suggest that LADA and T2DM differ in the pattern of peripheral nerve involvement over diabetes duration.

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.

Association between serum C-peptide as a risk factor for cardiovascular disease and high-density lipoprotein cholesterol levels in nondiabetic individuals

Objective

Objective: Although serum C-peptide has increasingly received attention as a new and important risk factor for cardiovascular disease (CVD), the potential mechanisms remain unclear. This study aimed to investigate the association between serum C-peptide as a risk factor for CVD and high-density lipoprotein cholesterol (HDL-C) levels.

Methods

The present study included 13,185 participants aged ≥20 years. Serum C-peptide and HDL-C levels were measured according to a standard protocol. Stratified analysis of covariance was used to compare serum HDL-C levels between different quartiles of serum C-peptide levels. Logistic regression analysis was used to determine the association between serum C-peptide and HDL-C levels. Cox proportional hazard regression analysis was conducted to determine the hazard ratio of serum HDL-C for CVD-related mortality.

Results

The results of the ANCOVA analysis showed a significant linear trend between the mean serum HDL-C level and the different quartiles of serum C-peptide. Compared to the first quartile (25th percentile), the second, third, and fourth quartiles had gradual reduction in serum HDL-C levels. Logistic regression analyses showed a strong negative association between serum C-peptide levels and HDL-C levels; the p value for the linear trend was <0.001. In men, compared with the lowest quartile of the serum C-peptide level, the relative risk was 1.75, 2.79, and 3.07 for the upper three quartiles of the serum C-peptide level. The relative risk was 1.60, 2.61, and 3.67 for women. The results of the survival analysis showed that serum HDL-C levels were negatively associated with CVD-related death in both men and women.

Conclusion

Serum C-peptide as a risk factor for CVD was significantly and negatively associated with serum HDL-C levels in individuals without diabetes. These findings suggest that serum C-peptide levels association with CVD death can be caused, at least in part, by the low serum HDL-C level.

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.

C-peptide replacement therapy as an emerging strategy for preventing diabetic vasculopathy

Lack of C-peptide, along with insulin, is the main feature of Type 1 diabetes mellitus (DM) and is also observed in progressive β-cell loss in later stage of Type 2 DM. Therapeutic approaches to hyperglycaemic control have been ineffective in preventing diabetic vasculopathy, and alternative therapeutic strategies are necessary to target both hyperglycaemia and diabetic complications. End-stage organ failure in DM seems to develop primarily due to vascular dysfunction and damage, leading to two types of organ-specific diseases, such as micro- and macrovascular complications. Numerous studies in diabetic patients and animals demonstrate that C-peptide treatment alone or in combination with insulin has physiological functions and might be beneficial in preventing diabetic complications. Current evidence suggests that C-peptide replacement therapy might prevent and ameliorate diabetic vasculopathy and organ-specific complications through conservation of vascular function, as well as prevention of endothelial cell death, microvascular permeability, vascular inflammation, and neointima formation. In this review, we describe recent advances on the beneficial role of C-peptide replacement therapy for preventing diabetic complications, such as retinopathy, nephropathy, neuropathy, impaired wound healing, and inflammation, and further discuss potential beneficial effects of combined C-peptide and insulin supplement therapy to control hyperglycaemia and to prevent organ-specific complications.

Mechanism of diabetic neuropathy: Where are we now and where to go?

Neuropathy is the most common complication of diabetes. As a consequence of longstanding hyperglycemia, a downstream metabolic cascade leads to peripheral nerve injury through an increased flux of the polyol pathway, enhanced advanced glycation end‐products formation, excessive release of cytokines, activation of protein kinase C and exaggerated oxidative stress, as well as other confounding factors. Although these metabolic aberrations are deemed as the main stream for the pathogenesis of diabetic microvascular complications, organ‐specific histological and biochemical characteristics constitute distinct mechanistic processes of neuropathy different from retinopathy or nephropathy. Extremely long axons originating in the small neuronal body are vulnerable on the most distal side as a result of malnutritional axonal support or environmental insults. Sparse vascular supply with impaired autoregulation is likely to cause hypoxic damage in the nerve. Such dual influences exerted by long‐term hyperglycemia are critical for peripheral nerve damage, resulting in distal‐predominant nerve fiber degeneration. More recently, cellular factors derived from the bone marrow also appear to have a strong impact on the development of peripheral nerve pathology. As evident from such complicated processes, inhibition of single metabolic factors might not be sufficient for the treatment of neuropathy, but a combination of several inhibitors might be a promising approach to overcome this serious disorder. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00070.x, 2010)

Experimental diabetes in neonatal mice induces early peripheral sensorimotor neuropathy

Animal models of diabetes do not reach the severity of human diabetic neuropathy but relatively mild neurophysiological deficits and minor morphometric changes. The lack of degenerative neuropathy in diabetic rodent models seems to be a consequence of the shorter length of the axons or the shorter animal life span. Diabetes-induced demyelination needs many weeks or even months before it can be evident by morphometrical analysis. In mice myelination of the peripheral nervous system starts at the prenatal period and it is complete several days after birth. Here we induced experimental diabetes to neonatal mice and we evaluated its effect on the peripheral nerve 4 and 8 weeks after diabetes induction. Neurophysiological values showed a decline in sensory nerve conduction velocity at both time-points. Morphometrical analysis of the tibial nerve demonstrated a decrease in the number of myelinated fibers, fiber size and myelin thickness at both time-points studied. Moreover, aldose reductase and poly(ADP-ribose) polymerase activities were increased even if the amount of the enzyme was not affected. Thus, type 1 diabetes in newborn mice induces early peripheral neuropathy and may be a good model to assay pharmacological or gene therapy strategies to treat diabetic neuropathy.

Biology of diabetic neuropathy

More than half of all patients with diabetes develop neuropathic disorders affecting the distal sensory and/or motor nerves, or autonomic or cranial nerve functions. Glycemic control can decrease the incidence of neuropathy but is not adequate alone to prevent or treat the disease. This chapter introduces diabetic neuropathy with a morphological description of the disease then describes our current understanding of metabolic and molecular mechanisms that contribute to neurovascular dysfunctions. Key mechanisms include glucose and lipid imbalances and insulin resistance that are interconnected via oxidative stress, inflammation, and altered gene expression. These complex interactions should be considered for the development of new treatment strategies against the onset or progression of neuropathy. Advances in understanding the combined metabolic stressors and the novel study of epigenetics suggest new therapeutic targets to combat this morbid and intractable disease affecting millions of patients with type 1 or type 2 diabetes.

Animal models of diabetes-induced neuropathic pain

Neuropathy will afflict over half of the approximately 350 million people worldwide who currently suffer from diabetes and around one-third of diabetic patients with neuropathy will suffer from painful symptoms that may be spontaneous or stimulus evoked. Diabetes can be induced in rats or mice by genetic, dietary, or chemical means, and there are a variety of well-characterized models of diabetic neuropathy that replicate either type 1 or type 2 diabetes. Diabetic rodents display aspects of sensorimotor dysfunction such as stimulus-evoked allodynia and hyperalgesia that are widely used to model painful neuropathy. This allows investigation of pathogenic mechanisms and development of potential therapeutic interventions that may alleviate established pain or prevent onset of pain.

Formation of a salsolinol-like compound, the neurotoxin, 1-acetyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, in a cellular model of hyperglycemia and a rat model of diabetes

There are statistical data indicating that diabetes is a risk factor for Parkinson's disease (PD). Methylglyoxal (MG), a biologically reactive byproduct of glucose metabolism, the levels of which have been shown to be increase in diabetes, reacts with dopamine to form 1-acetyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (ADTIQ); this formation may provide further insight into the connection between PD and diabetes. In this study, we investigated the role of ADTIQ in these two diseases to determine in an aim to enhance our understanding of the link between PD and diabetes. To this end, a cell model of hyperglycemia and a rat model of diabetes were established. In the cell model of hyperglycemia, compared with the control group, the elevated glucose levels promoted free hydroxyl radical formation (p<0.01). An ADTIQ assay was successfully developed and ADTIQ levels were detected and quantified. The levels of its precursors, MG and dopamine (DA), were determined in both the cell model of hyperglycemia and the rat model of diabetes. The proteins related to glucose metabolism were also assayed. Compared with the control group, ADTIQ and MG levels were significantly elevated not only in the cell model of hyperglycemia, but also in the brains of rats with diabetes (p<0.01). Seven key enzymes from the glycolytic pathway were found to be significantly more abundant in the brains of rats with diabetes. Moreover, it was found that adenosine triphosphate (ATP) synthase and superoxide dismutase (SOD) expression levels were markedly decreased in the rats with diabetes compared with the control group. Therefore, ADTIQ expression levels were found to be elevated under hyperglycemic conditions. The results reported herein demonstrate that ADTIQ, which is derived from MG, the levels of which are increased in diabetes, may serve as a neurotoxin to dopaminergic neurons, eventually leading to PD.

Na+/H+ exchanger 1 inhibition reverses manifestation of peripheral diabetic neuropathy in type 1 diabetic rats

Evidence for an important role for Na(+)/H(+) exchangers in diabetic complications is emerging. The aim of this study was to evaluate whether Na(+)/H(+) exchanger 1 inhibition reverses experimental peripheral diabetic neuropathy. Control and streptozotocin-diabetic rats were treated with the specific Na(+)/H(+) exchanger 1 inhibitor cariporide for 4 wk after 12 wk without treatment. Neuropathy end points included sciatic motor and sensory nerve conduction velocities, endoneurial nutritive blood flow, vascular reactivity of epineurial arterioles, thermal nociception, tactile allodynia, and intraepidermal nerve fiber density. Advanced glycation end product and markers of oxidative stress, including nitrated protein levels in sciatic nerve, were evaluated by Western blot. Rats with 12-wk duration of diabetes developed motor and sensory nerve conduction deficits, thermal hypoalgesia, tactile allodynia, and intraepidermal nerve fiber loss. All these changes, including impairment of nerve blood flow and vascular reactivity of epineurial arterioles, were partially reversed by 4 wk of cariporide treatment. Na(+)/H(+) exchanger 1 inhibition was also associated with reduction of diabetes-induced accumulation of advanced glycation endproduct, oxidative stress, and nitrated proteins in sciatic nerve. In conclusion, these findings support an important role for Na(+)/H(+) exchanger 1 in functional, structural, and biochemical manifestations of peripheral diabetic neuropathy and provide the rationale for development of Na(+)/H(+) exchanger 1 inhibitors for treatment of diabetic vascular and neural complications.

Can C-peptide mediated anti-inflammatory effects retard the development of microvascular complications of type 1 diabetes?

Hyperglycemia is considered to be the major cause of microvascular complications of diabetes. Growing evidence highlights the importance of hyperglycemia-mediated inflammation in the initiation and progression of microvascular complications in type 1 diabetes. We hypothesize that lack of proinsulin C-peptide and lack of its anti-inflammatory properties contribute to the development of microvascular complications. Evidence gathered over the past 20 years shows that C-peptide is a biologically active peptide in its own right. It has been shown to reduce formation of reactive oxygen species and nuclear factor-κB activation induced by hyperglycemia, resulting in inhibition of cytokine, chemokine and cell adhesion molecule formation as well as reduced apoptotic activity. In addition, C-peptide stimulates and induces the expression of both Na⁺, K⁺-ATPase and endothelial nitric oxide synthase. Animal studies and small-scale clinical trials in type 1 diabetes patients suggest that C-peptide replacement combined with regular insulin therapy exerts beneficial effects on kidney and nerve dysfunction. Further clinical trials in patients with microvascular complications including measurements of inflammatory markers are warranted to explore the clinical significance of the aforementioned, previously unrecognized, C-peptide effects.

Intra-epidermal nerve fibers density and nociception in EPO-treated type 1 diabetic rats with peripheral neuropathy

Small-diameter nerve fibers, which subserve nociception, can be affected early in peripheral neuropathies, although their injury may not be detectable by routine neurophysiologic tests. On the other hand, skin biopsy has proved to be a reliable tool to examine nonmyelinated nerve fibers, as assessed by the quantification of intra-epidermal nerve fiber (IENF) density not only along with the degenerative process but, noteworthy, IENF density could be very helpful in evaluating drug efficacy such as erythropoietin (EPO) treatment.

C-peptide-stimulated nitric oxide production in a cultured pulmonary artery endothelium is erythrocyte mediated and requires Zn(2+)

BACKGROUND

C-peptide has been shown to stimulate the production of nitric oxide (NO) in aortic endothelial cells via activation of endothelial nitric oxide synthase (eNOS) through an increased calcium influx. Here, results obtained using cultured bovine pulmonary artery endothelial cells (bPAECs) suggest that C-peptide does not induce eNOS activation directly in cultured pulmonary artery endothelium. However, C-peptide has been shown to stimulate the release of ATP from erythrocytes, a well-documented stimulus of eNOS activity in the pulmonary endothelium. Therefore, studies were performed to examine if C-peptide can indirectly stimulate NO production in a cultured pulmonary endothelium that is erythrocyte mediated.

METHODS

NO production and free intracellular calcium changes were monitored in immobilized bPAECs using specific intracellular fluorescent probes after stimulation with adenosine triphosphate (ATP), calcium ionophore A23187, or C-peptide. A microfluidic device enabled immobilized bPAECs to interact with flowing erythrocytes in the presence and absence of C-peptide to determine the role of the erythrocyte in C-peptide-stimulated NO production in cultured bPAECs.

RESULTS

ATP and the calcium ionophore stimulate significant increases in both intracellular NO production and influx of free calcium in cultured bPAECs. In contrast, C-peptide, ranging from physiological to above physiological concentrations, was unable to stimulate NO production or calcium influx in the bPAECs. However, when erythrocytes were pre-incubated with a mixture containing physiological concentrations of C-peptide with Zn(2+) and haemodynamically pumped beneath bPAECs cultured on a microfluidic device, an 88.6 ± 7.5% increase in endothelial NO production was observed.

CONCLUSIONS

C-peptide does not affect NO production in bPAECs directly but can impact NO production through an erythrocyte-mediated mechanism. Furthermore, in the absence of Zn(2+), C-peptide does not stimulate this NO production directly or indirectly. These results suggest that C-peptide, in the presence of Zn(2+), may be a determinant in purinergic receptor signalling via its ability to stimulate the release of ATP from erythrocytes.

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.

Metanx alleviates multiple manifestations of peripheral neuropathy and increases intraepidermal nerve fiber density in Zucker diabetic fatty rats

Metanx is a product containing L-methylfolate, pyridoxal 5'-phosphate, and methylcobalamin for management of endothelial dysfunction. Metanx ingredients counteract endothelial nitric oxide synthase uncoupling and oxidative stress in vascular endothelium and peripheral nerve. This study evaluates Metanx on diabetic peripheral neuropathy in ZDF rats, a model of type 2 diabetes. Metanx was administered to 15-week-old ZDF and ZDF lean rats at either 4.87 mg ⋅ kg(-1) ⋅ day(-1) (a body weight-based equivalent of human dose) or 24.35 mg ⋅ kg(-1) ⋅ day(-1) by oral gavage two times a day for 4 weeks. Both doses alleviated hind limb digital sensory, but not sciatic motor, nerve conduction slowing and thermal and mechanical hypoalgesia in the absence of any reduction of hyperglycemia. Low-dose Metanx increased intraepidermal nerve fiber density but did not prevent morphometric changes in distal tibial nerve myelinated fibers. Metanx treatment counteracted endothelial nitric oxide synthase uncoupling, inducible nitric oxide synthase upregulation, and methylglyoxal-derived advanced glycation end product, nitrotyrosine, and nitrite/nitrate accumulation in the peripheral nerve. In conclusion, Metanx, at a body weight-based equivalent of human dose, increased intraepidermal nerve fiber density and improved multiple parameters of peripheral nerve function in ZDF rats. Clinical studies are needed to determine if Metanx finds use in management of diabetic peripheral neuropathy.

Animal models of peripheral neuropathies

Peripheral neuropathies are common neurological diseases, and various animal models have been developed to study disease pathogenesis and test potential therapeutic drugs. Three commonly studied disease models with huge public health impact are diabetic peripheral neuropathy, chemotherapy-induced peripheral neuropathy, and human immunodeficiency virus-associated sensory neuropathies. A common theme in these animal models is the comprehensive use of pathological, electrophysiological, and behavioral outcome measures that mimic the human disease. In recent years, the focus has shifted to the use of outcome measures that are also available in clinical use and can be done in a blinded and quantitative manner. One such evaluation tool is the evaluation of epidermal innervation with a simple skin biopsy. Future clinical trials will be needed to validate the translational usefulness of this outcome measure and validation against accepted outcome measures that rely on clinical symptoms or examination findings in patients.

C-peptide reduces high-glucose-induced apoptosis of endothelial cells and decreases NAD(P)H-oxidase reactive oxygen species generation in human aortic endothelial cells

AIMS/HYPOTHESIS

Reactive oxygen species (ROS) generated during hyperglycaemia are implicated in the development of diabetic vascular complications. High glucose increases oxidative stress in endothelial cells and induces apoptosis. A major source of ROS in endothelial cells exposed to glucose is the NAD(P)H oxidase enzyme. Several studies demonstrated that C-peptide, the product of proinsulin cleavage within the pancreatic beta cells, displays anti-inflammatory effects in certain models of vascular dysfunction. However, the molecular mechanism underlying this effect is unclear. We hypothesised that C-peptide reduces glucose-induced ROS generation by decreasing NAD(P)H oxidase activation and prevents apoptosis

METHODS

Human aortic endothelial cells (HAEC) were exposed to 25 mmol/l glucose in the presence or absence of C-peptide and tested for protein quantity and activity of caspase-3 and other apoptosis markers by ELISA, TUNEL and immunoblotting. Intracellular ROS were measured by flow cytometry using the ROS sensitive dye chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (CM-H(2)-DCDFA). NAD(P)H oxidase activation was assayed by lucigenin. Membrane and cytoplasmic levels of the NAD(P)H subunit ras-related C3 botulinum toxin substrate 1 (rho family, small GTP binding protein Rac1) (RAC-1) and its GTPase activity were studied by immunoblotting and ELISA. RAC-1 (also known as RAC1) gene expression was investigated by quantitative real-time PCR.

RESULTS

C-peptide significantly decreased caspase-3 levels and activity and upregulated production of the anti-apoptotic factor B cell CLL/lymphoma 2 (BCL-2). Glucose-induced ROS production was quenched by C-peptide and this was associated with a decreased NAD(P)H oxidase activity and reduced RAC-1 membrane production and GTPase activity.

CONCLUSIONS/INTERPRETATION

In glucose-exposed endothelial cells, C-peptide acts as an endogenous antioxidant molecule by reducing RAC-1 translocation to membrane and NAD(P)H oxidase activation. By preventing oxidative stress, C-peptide protects endothelial cells from glucose-induced apoptosis.

Diabetic neuropathy: cellular mechanisms as therapeutic targets

In patients with diabetes, nerve injury is a common complication that leads to chronic pain, numbness and substantial loss of quality of life. Good glycemic control can decrease the incidence of diabetic neuropathy, but more than half of all patients with diabetes still develop this complication. There is no approved treatment to prevent or halt diabetic neuropathy, and only symptomatic pain therapies, with variable efficacy, are available. New insights into the mechanisms leading to the development of diabetic neuropathy continue to point to systemic and cellular imbalances in metabolites of glucose and lipids. In the PNS, sensory neurons, Schwann cells and the microvascular endothelium are vulnerable to oxidative and inflammatory stress in the presence of these altered metabolic substrates. This Review discusses the emerging cellular mechanisms that are activated in the diabetic milieu of hyperglycemia, dyslipidemia and impaired insulin signaling. We highlight the pathways to cellular injury, thereby identifying promising therapeutic targets, including mitochondrial function and inflammation.

C-peptide and diabetic encephalopathy

With the changes of life style, diabetes and its complications have become a major cause of morbidity and mortality. It is reasonable to anticipate a continued rise in the incidence of diabetes and its complications along with the aging of the population, increase in adult obesity rate, and other risk factors. Diabetic encephalopathy is one of the severe microvascular complications of diabetes, characterized by impaired cognitive functions, and electrophysiological, neurochemical, and structural abnormalities. It may involve direct neuronal damage caused by intracellular glucose. However, the pathogenesis of this disease is complex and its diagnosis is not very clear. Previous researches have suggested that chronic metabolic alterations, vascular changes, and neuronal apoptosis may play important roles in neuronal loss and damaged cognitive functions. Multiple factors are responsible for neuronal apoptosis, such as disturbed insulin growth factor (IGF) system, hyperglycemia, and the aging process. Recent data suggest that insulin/C-peptide deficiency may exert a primary and key effect in diabetic encephalopathy. Administration of C-peptide partially improves the condition of the IGF system in the brain and prevents neuronal apoptosis in the hippocampus of diabetic patients. Those findings provide a basis for application of C-peptide as a potentially effective therapy for diabetes and diabetic encephalopathy.

Baicalein alleviates diabetic peripheral neuropathy through inhibition of oxidative-nitrosative stress and p38 MAPK activation

With the consideration of the multifactorial etiology of diabetic peripheral neuropathy, an ideal drug or drug combination should target at least several key pathogenetic mechanisms. The flavonoid baicalein (5,6,7-trihydroxyflavone) has been reported to counteract sorbitol accumulation, activation of 12/15-lipoxygenase, oxidative-nitrosative stress, inflammation, and impaired signaling in models of chronic disease. This study evaluated baicalein on diabetic peripheral neuropathy. Control and streptozotocin-diabetic C57Bl6/J mice were maintained with or without baicalein treatment (30 mg kg(-1) d(-1), i.p., for 4 weeks after 12 weeks without treatment). Neuropathy was evaluated by sciatic motor and hind-limb digital sensory nerve conduction velocities, thermal algesia (Hargreaves test), tactile response threshold (flexible von Frey filament test), and intraepidermal nerve fiber density (fluorescent immunohistochemistry with confocal microscopy). Sciatic nerve and spinal cord 12/15-lipoxygenase and total and phosphorylated p38 mitogen-activated protein kinase expression and nitrated protein levels were evaluated by Western blot analysis, 12(S)hydroxyeicosatetraenoic acid concentration (a measure of 12/15-lipoxygenase activity) by ELISA, and glucose and sorbitol pathway intermediate concentrations by enzymatic spectrofluorometric assays. Baicalein did not affect diabetic hyperglycemia, and alleviated nerve conduction deficit and small sensory nerve fiber dysfunction, but not intraepidermal nerve fiber loss. It counteracted diabetes-associated p38 mitogen-activated protein kinase phosphorylation, oxidative-nitrosative stress, and 12/15-lipoxygenase overexpression and activation, but not glucose or sorbitol pathway intermediate accumulation. In conclusion, baicalein targets several mechanisms implicated in diabetic peripheral neuropathy. The findings provide rationale for studying hydroxyflavones with an improved pharmacological profile as potential treatments for diabetic neuropathy and other diabetic complications.

Emerging drugs for diabetic neuropathy

IMPORTANCE OF THE FIELD

Diabetic neuropathy (DN) is a very common and disabling diabetes-related complication. DN is associated with significant morbidity and mortality. Diabetic peripheral neuropathy (DPN) can be painful in the earlier stages of the disease before becoming painless. Most of the currently available therapies are symptomatic (focusing on pain relief) rather than disease-modifying. With the exception of good glycemic control, there is currently no effective treatment to slow the progression of or reverse DPN.

AREAS COVERED IN THIS REVIEW

In this article, we review the epidemiology, pathogenesis, currently available and future treatments for DPN, and the potential development issues/challenges related to such new therapies. Literature search was performed using PubMed, Medline and Pharmaprojects from 1950 onwards. Search terms include a combination of terms such as diabetic neuropathy, pathogenesis, pathophysiology, mechanisms, treatment, therapy, oxidative/nitrosative stress, anti-oxidants, serotonin, nitrotyrosine, protein kinase C, aldose reductase, sodium channels, taurine, lipoic acid and poly (ADP-ribose) polymerase.

WHAT THE READER WILL GAIN

The reader will gain an overview of the epidemiology, clinical features and risk factors of DN. In addition, the reader will have a better understanding of the mechanisms that underpin the development of DPN and their relationships to the current and future therapies. The reader will also develop an insight into the limitations of the current approach to DPN treatment and the potential avenues for future research.

TAKE HOME MESSAGE

DN is a very common and disabling complication that currently has no effective treatments other than diabetes control. The pathogenesis of DPN is complex and multi-factorial. Several disease-modifying and symptomatic treatments are currently under development. Oxidative and nitrosative stress have been identified as key pathogenic factors in the development of DPN and new treatments target these pathways and/or their downstream consequences. Gene therapy and growth factors have also emerged as potential new therapies that target particular cellular compartments as opposed to being delivered systemically. The recognition of the difficulty in reversing established DN has focused efforts on slowing its progression.

Different roles of 12/15-lipoxygenase in diabetic large and small fiber peripheral and autonomic neuropathies

Up-regulation of 12/15-lipoxygenase, which converts arachidonic acid to 12(S)- and 15(S)-hydroxyeicosatetraenoic acids, causes impaired cell signaling, oxidative-nitrosative stress, and inflammation. This study evaluated the role for 12/15-lipoxygenase in diabetic large and small fiber peripheral and autonomic neuropathies. Control and streptozotocin-diabetic wild-type and 12/15-lipoxygenase-deficient mice were maintained for 14 to 16 weeks. 12/15-lipoxygenase gene deficiency did not affect weight gain or blood glucose concentrations. Diabetic wild-type mice displayed increased sciatic nerve 12/15-lipoxygenase and 12(S)-hydroxyeicosatetraenoic acid levels. 12/15-lipoxygenase deficiency prevented or alleviated diabetes-induced thermal hypoalgesia, tactile allodynia, motor and sensory nerve conduction velocity deficits, and reduction in tibial nerve myelinated fiber diameter, but not intraepidermal nerve fiber loss. The frequencies of superior mesenteric-celiac ganglion neuritic dystrophy, the hallmark of diabetic autonomic neuropathy in mouse prevertebral sympathetic ganglia, were increased 14.8-fold and 17.2-fold in diabetic wild-type and 12/15-lipoxygenase-deficient mice, respectively. In addition, both diabetic groups displayed small (<1%) numbers of degenerating sympathetic neurons. In conclusion, whereas 12/15-lipoxygenase up-regulation provides an important contribution to functional changes characteristic for both large and small fiber peripheral diabetic neuropathies and axonal atrophy of large myelinated fibers, its role in small sensory nerve fiber degeneration and neuritic dystrophy and neuronal degeneration characteristic for diabetic autonomic neuropathy is minor. This should be considered in the selection of endpoints for future clinical trials of 12/15-lipoxygenase inhibitors.

A Molecular Level Understanding of Zinc Activation of C-peptide and its Effects on Cellular Communication in the Bloodstream

Inspired by previous reports, our group has recently demonstrated that C-peptide exerts beneficial effects upon interactions with red blood cells (RBCs). These effects can be measured in RBCs obtained from animal models of both type 1 diabetes and type 2 diabetes, though to different extents. To date, the key metrics that have been measured involving C-peptide and RBCs include an increase in glucose uptake by these cells and a subsequent increase in adenosine triphosphate (ATP) release. Importantly, to date, our group has only been able to elicit these beneficial effects when the C-peptide is prepared in the presence of Zn2+. The C-peptide-induced release of ATP is of interest when considering that ATP is a purinergic signaling molecule known to stimulate the production of nitric oxide (NO) in the endothelium and in platelets. This NO production has been shown to participate in smooth muscle relaxation and subsequent vessel dilation. Furthermore, NO is a well-established platelet inhibitor. The objective of this review is to provide information pertaining to C-peptide activity on RBCs. Special attention is paid to the necessity of Zn2+ activation, and the origin of that activation in vivo. Finally, a mechanism is proposed that explains how C-peptide is exerting its effects on other cells in the bloodstream, particularly on endothelial cells and platelets, via its ability to stimulate the release of ATP from RBCs.

The beneficial effects of C-Peptide on diabetic polyneuropathy

Diabetic polyneuropathy (DPN) is a common complication in diabetes. At present, there is no adequate treatment, and DPN is often debilitating for patients. It is a heterogeneous disorder and differs in type 1 and type 2 diabetes. An important underlying factor in type 1 DPN is insulin deficiency. Proinsulin C-peptide is a critical element in the cascade of events. In this review, we describe the physiological role of C-peptide and how it provides an insulin-like signaling function. Such effects translate into beneficial outcomes in early metabolic perturbations of neural Na+/K+-ATPase and nitric oxide (NO) with subsequent preventive effects on early nerve dysfunction. Further corrective consequences resulting from this signaling cascade have beneficial effects on gene regulation of early gene responses, neurotrophic factors, their receptors, and the insulin receptor itself. This may lead to preventive and corrective results to nerve fiber degeneration and loss, as well as, promotion of nerve fiber regeneration with respect to sensory somatic fibers and small nociceptive nerve fibers. A characteristic abnormality of type 1 DPN is nodal and paranodal degeneration with severe consequences for myelinated fiber function. This review deals in detail with the underlying insulin-deficiency-related molecular changes and their correction by C-peptide. Based on these observations, it is evident that continuous maintenance of insulin-like actions by C-peptide is needed in peripheral nerve to minimize the sequences of metabolic and molecular abnormalities, thereby ameliorating neuropathic complications. There is now ample evidence demonstrating that C-peptide replacement in type 1 diabetes promotes insulin action and signaling activities in a more enhanced, prolonged, and continuous fashion than does insulin alone. It is therefore necessary to replace C-peptide to physiological levels in diabetic patients. This will have substantial beneficial effects on type 1 DPN.

Diabetic painful and insensate neuropathy: pathogenesis and potential treatments

Advanced peripheral diabetic neuropathy (PDN) is associated with elevated vibration and thermal perception thresholds that progress to sensory loss and degeneration of all fiber types in peripheral nerve. A considerable proportion of diabetic patients also describe abnormal sensations such as paresthesias, allodynia, hyperalgesia, and spontaneous pain. One or several manifestations of abnormal sensation and pain are described in all the diabetic rat and mouse models studied so far (i.e., streptozotocin-diabetic rats and mice, type 1 insulinopenic BB/Wor and type 2 hyperinsulinemic diabetic BBZDR/Wor rats, Zucker diabetic fatty rats, and nonobese diabetic, Akita, leptin- and leptin-receptor-deficient, and high-fat diet-fed mice). Such manifestations are 1) thermal hyperalgesia, an equivalent of a clinical phenomenon described in early PDN; 2) thermal hypoalgesia, typically present in advanced PDN; 3) mechanical hyperalgesia, an equivalent of pain on pressure in early PDN; 4) mechanical hypoalgesia, an equivalent to the loss of sensitivity to mechanical noxious stimuli in advanced PDN; 5) tactile allodynia, a painful perception of a light touch; and 5) formalin-induced hyperalgesia. Rats with short-term diabetes develop painful neuropathy, whereas those with longer-term diabetes and diabetic mice typically display manifestations of both painful and insensate neuropathy, or insensate neuropathy only. Animal studies using pharmacological and genetic approaches revealed important roles of increased aldose reductase, protein kinase C, and poly(ADP-ribose) polymerase activities, advanced glycation end-products and their receptors, oxidative-nitrosative stress, growth factor imbalances, and C-peptide deficiency in both painful and insensate neuropathy. This review describes recent achievements in studying the pathogenesis of diabetic neuropathic pain and sensory disorders in diabetic animal models and developing potential pathogenetic treatments.

Prevalence of diabetic peripheral neuropathy and relation to glycemic control therapies at baseline in the BARI 2D cohort

We evaluated the associations between glycemic therapies and prevalence of diabetic peripheral neuropathy (DPN) at baseline among participants in the Bypass Angioplasty Revascularization Investigation 2 Diabetes (BARI 2D) trial on medical and revascularization therapies for coronary artery disease (CAD) and on insulin-sensitizing vs. insulin-providing treatments for diabetes. A total of 2,368 patients with type 2 diabetes and CAD was evaluated. DPN was defined as clinical examination score >2 using the Michigan Neuropathy Screening Instrument (MNSI). DPN odds ratios across different groups of glycemic therapy were evaluated by multiple logistic regression adjusted for multiple covariates including age, sex, hemoglobin A1c (HbA1c), and diabetes duration. Fifty-one percent of BARI 2D subjects with valid baseline characteristics and MNSI scores had DPN. After adjusting for all variables, use of insulin was significantly associated with DPN (OR = 1.57, 95% CI: 1.15-2.13). Patients on sulfonylurea (SU) or combination of SU/metformin (Met)/thiazolidinediones (TZD) had marginally higher rates of DPN than the Met/TZD group. This cross-sectional study in a cohort of patients with type 2 diabetes and CAD showed association of insulin use with higher DPN prevalence, independent of disease duration, glycemic control, and other characteristics. The causality between a glycemic control strategy and DPN cannot be evaluated in this cross-sectional study, but continued assessment of DPN and randomized therapies in BARI 2D trial may provide further explanations on the development of DPN.

Cellular and physiological effects of C-peptide

In recent years, accumulating evidence indicates a biological function for proinsulin C-peptide. These results challenge the traditional view that C-peptide is essentially inert and only useful as a surrogate marker of insulin release. Accordingly, it is now clear that C-peptide binds with high affinity to cell membranes, probably to a pertussis-toxin-sensitive G-protein-coupled receptor. Subsequently, multiple signalling pathways are potently and dose-dependently activated in multiple cell types by C-peptide with the resulting activation of gene transcription and altered cell phenotype. In diabetic animals and Type 1 diabetic patients, short-term studies indicate that C-peptide also enhances glucose disposal and metabolic control. Furthermore, results derived from animal models and clinical studies in Type 1 diabetic patients suggest a salutary effect of C-peptide in the prevention and amelioration of diabetic nephropathy and neuropathy. Therefore a picture of Type 1 diabetes as a dual-hormone-deficiency disease is developing, suggesting that the replacement of C-peptide alongside insulin should be considered in its management.

Regulation of liver hepcidin expression by alcohol in vivo does not involve Kupffer cell activation or TNF-alpha signaling

Alcohol downregulates hepcidin expression in the liver leading to an increase in intestinal iron transport and liver iron storage. We have previously demonstrated that alcohol-mediated oxidative stress is involved in the inhibition of hepcidin transcription by alcohol in vivo. Kupffer cells and TNF-alpha play a key role in alcohol-induced liver injury. The aim of this study was to define their involvement in the regulation of hepcidin expression by alcohol. Kupffer cells were inactivated or depleted by employing gadolinium chloride and liposomes containing clodronate, respectively. Rats pair fed with the alcohol-Lieber-DeCarli diet for 6 wk and mice fed with 20% ethanol in the drinking water for 1 wk were used as experimental models. Interestingly, alcohol downregulated hepcidin expression in the livers of rats and mice independent of gadolinium chloride or clodronate treatment. One week of alcohol treatment was sufficient to induce a significant increase in TNF-alpha levels and phosphorylation of NF-kappaB subunit p65. The neutralization of TNF-alpha by specific antibodies inhibited p65 phosphorylation. However, neither the neutralization of TNF-alpha nor the lack of TNF-alpha receptor expression reversed alcohol-induced suppression of liver hepcidin expression. The level of alcohol-induced ROS in the liver was also undiminished following Kupffer cell inactivation or depletion. Our results demonstrate that alcohol-induced Kupffer cell activation and TNF-alpha signaling are not involved in the suppression of liver hepcidin expression by alcohol-mediated oxidative stress in vivo. Therefore, these findings suggest that alcohol acts within hepatocytes to suppress hepcidin expression and thereby influences iron homeostasis.

Alcohol and lipid metabolism

Many new mechanisms for alcoholic steatosis have been suggested by work reported in the last five years. These include alterations of transcriptional controls of lipid metabolism, better understanding of the effects of abnormal methionine metabolism on the endoplasmic reticulum stress response, unraveling of the basis for sensitization of the Kupffer cell to lipopolysaccharide, a better understanding of the role of cytokines and adipokines in alcoholic liver disease, and implication of the innate immune and complement systems in responses to alcohol. Much of this work has been facilitated by work with knockout mice. Undoubtedly, there are interrelationships among these various pathogenic mechanisms that ultimately will provide a more cohesive picture of how heavy alcohol use deranges hepatic lipid metabolism.

Intracellular signalling by C-peptide

C-peptide, a cleavage product of the proinsulin molecule, has long been regarded as biologically inert, serving merely as a surrogate marker for insulin release. Recent findings demonstrate both a physiological and protective role of C-peptide when administered to individuals with type I diabetes. Data indicate that C-peptide appears to bind in nanomolar concentrations to a cell surface receptor which is most likely to be G-protein coupled. Binding of C-peptide initiates multiple cellular effects, evoking a rise in intracellular calcium, increased PI-3-kinase activity, stimulation of the Na(+)/K(+) ATPase, increased eNOS transcription, and activation of the MAPK signalling pathway. These cell signalling effects have been studied in multiple cell types from multiple tissues. Overall these observations raise the possibility that C-peptide may serve as a potential therapeutic agent for the treatment or prevention of long-term complications associated with diabetes.

Metal-activated C-peptide facilitates glucose clearance and the release of a nitric oxide stimulus via the GLUT1 transporter

AIMS/HYPOTHESIS

Proinsulin C-peptide has been implicated in reducing complications associated with diabetes and also in improving blood flow. We hypothesised that incubation of erythrocytes with C-peptide would improve the ability of these cells to release ATP, a stimulus for nitric oxide production.

METHODS

Erythrocytes obtained from rabbits (n = 11) and both healthy and type 2 diabetic humans (n = 7) were incubated with C-peptide in the absence and presence of Fe2+ and Cr3+, and the resulting ATP release was measured via chemiluminescence. This release was also measured in the presence and absence of phloretin, an inhibitor of GLUT1, and also of mannose, a glycolysis inhibitor. To determine glucose transport, 14C-labelled glucose was added to erythrocytes in the presence and absence of the C-peptide-metal complex and the aforementioned inhibitors.

RESULTS

The release of ATP from the erythrocytes of patients with diabetes increased from 64 +/- 13 to 260 +/- 39 nmol/l upon incubation of the cells in C-peptide. The C-peptide activity was dependent upon binding to Fe2+, which was extended upon binding to Cr3+. The increase in ATP release from the erythrocytes is due to metal-activated C-peptide stimulation of glucose transfer into the erythrocytes via the GLUT1 transporter. In the presence of C-peptide complexed to Cr3+, the amount of glucose transferred into the erythrocyte increased by 31%.

CONCLUSIONS/INTERPRETATION

When complexed to Fe2+ or Cr3+, C-peptide has the ability to promote ATP release from erythrocytes. This release is due to an increase in glucose transport through GLUT1.

The effects of C-peptide on type 1 diabetic polyneuropathies and encephalopathy in the BB/Wor-rat

Diabetic polyneuropathy (DPN) occurs more frequently in type 1 diabetes resulting in a more severe DPN. The differences in DPN between the two types of diabetes are due to differences in the availability of insulin and C-peptide. Insulin and C-peptide provide gene regulatory effects on neurotrophic factors with effects on axonal cytoskeletal proteins and nerve fiber integrity. A significant abnormality in type 1 DPN is nodal degeneration. In the type 1 BB/Wor-rat, C-peptide replacement corrects metabolic abnormalities ameliorating the acute nerve conduction defect. It corrects abnormalities of neurotrophic factors and the expression of neuroskeletal proteins with improvements of axonal size and function. C-peptide corrects the expression of nodal adhesive molecules with prevention and repair of the functionally significant nodal degeneration. Cognitive dysfunction is a recognized complication of type 1 diabetes, and is associated with impaired neurotrophic support and apoptotic neuronal loss. C-peptide prevents hippocampal apoptosis and cognitive deficits. It is therefore clear that substitution of C-peptide in type 1 diabetes has a multitude of effects on DPN and cognitive dysfunction. Here the effects of C-peptide replenishment will be extensively described as they pertain to DPN and diabetic encephalopathy, underpinning its beneficial effects on neurological complications in type 1 diabetes.