C-Peptide replacement therapy in type 1 diabetes: are we in the trough of disillusionment?

Copper(II) Affects the Biochemical Behavior of Proinsulin C-peptide by Forming Ternary Complexes with Serum Albumin

Peptide hormones are essential signaling molecules with therapeutic importance. Identifying regulatory factors that drive their activity gives important insight into their mode of action and clinical development. In this work, we demonstrate the combined impact of Cu(II) and the serum protein albumin on the activity of C-peptide, a 31-mer peptide derived from the same prohormone as insulin. C-peptide exhibits beneficial effects, particularly in diabetic patients, but its clinical use has been hampered by a lack of mechanistic understanding. We show that Cu(II) mediates the formation of ternary complexes between albumin and C-peptide and that the resulting species depend on the order of addition. These ternary complexes notably alter peptide activity, showing differences from the peptide or Cu(II)/peptide complexes alone in redox protection as well as in cellular internalization of the peptide. In standard clinical immunoassays for measuring C-peptide levels, the complexes inflate the quantitation of the peptide, suggesting that such adducts may affect biomarker quantitation. Altogether, our work points to the potential relevance of Cu(II)-linked C-peptide/albumin complexes in the peptide's mechanism of action and application as a biomarker.

A homogeneous enzyme-free ratiometric immunoassay for the determination of C-peptide

In this study, a homogeneous enzyme-free ratiometric (HOMO- EF-RA) immunoassay was developed for the sensitive detection of C-peptide. In the immunoassay, there have been a miscible detection system by mixing with the fluorescent quantum dots conjugated antigen (QD-Ag conjugates) and the dylight dye conjugated antibody (DL-Ab conjugates). When connecting between Ag-QD conjugate and Ab-DL conjugate by specific recognition, the system emitted fluorescence resonant energy transfer (FRET). The target C-peptide can inhibit the connection and FRET formation between QD-Ag conjugates and DL-Ab conjugates, thus changing the dual fluorescence. By measuring the ratio dual fluorescence changes of the system, the content of C-peptide was evaluated without any enzyme used and multiple incubation and washing steps. This immunoassay realized the highly sensitive (as low as 0.12 ng mL^-1), selective and rapid (as less as 6 min) detection of C-peptide. Furthermore, the the simple and convenient immunoassay was applied successfully to the determination of C-peptide in real serum samples.

The Theory of Hyperlipidemic Memory of Type 1 Diabetes

A literature search was conducted to identify publications addressing the early phases of lipid phenotypes in children and adults with either type 1 diabetes or type 2 diabetes. Medline, EMBASE, and Ovid were searched using the following search terms: clinical remission, partial remission, partial clinical remission, honeymoon phase, C-peptide, type 1 or 2 diabetes, children, pediatric type 1 or 2 diabetes, and paediatrics type 1 or 2 diabetes, adults, adult type 1 or type 2 diabetes. Partial clinical remission (PR) of type 1 diabetes (T1D) is characterized by continued endogenous production of insulin and C-peptide following the diagnosis and the introduction of exogenous insulin therapy. PR is associated with improved glycemic control and reduced prevalence of diabetes complications. The theory of hyperglycemic memory was proposed to explain this concept of improved glycemic outcomes in remitters (those who experienced PR) versus non-remitters (those who did not experience PR). However, this theory is incomplete as it does not explain the dichotomy in early lipid phenotypes in T1D based on PR status, which is an understudied area in diabetology and lipidology. To fill this knowledge gap, we propose the Theory of Hyperlipidemic Memory of T1D. This theory is premised on our 5-year research on early post-diagnostic dichotomy in lipid phenotypes between remitters and non-remitters across the lifespan. It provides a more rigorous explanation for the differences in lifelong atherosclerotic cardiovascular disease (ASCVD) risk between remitters and non-remitters. We conducted 4 clinical studies in pediatric and adult subjects with diabetes mellitus to characterize the particulars of the hyperlipidemic memory. In the first investigation, we explored the impact of the presence or absence of PR on lipid parameters in children and adolescents with T1D. In the second, we investigated whether pubertal maturation influenced our findings in T1D; and whether these findings could be replicated in healthy, non-diabetic children and adolescents. In the third, we leveraged our findings from T1D and controls to investigate the mechanisms of early lipid changes in T2D by comparing the earliest lipid phenotype of subjects with type 2 diabetes (T2D) to those of remitters, non-remitters, and controls. In the fourth, we investigated the impact of PR on the earliest lipid phenotypes in adults with T1D and compared these early lipid data to those of T2D subjects and controls. This body of work across the lifespan in children, adolescents, and adults supports the Theory of Hyperlipidemic Memory. This new theory clarifies why PR largely determines the risks for early-phase dyslipidemia, mid-term microvascular disease risk, and long-term ASCVD risk in subjects with T1D.

Biological Activity of c-Peptide in Microvascular Complications of Type 1 Diabetes-Time for Translational Studies or Back to the Basics?

People with type 1 diabetes have an increased risk of developing microvascular complications, which have a negative impact on the quality of life and reduce life expectancy. Numerous studies in animals with experimental diabetes show that c-peptide supplementation exerts beneficial effects on diabetes-induced damage in peripheral nerves and kidneys. There is substantial evidence that c-peptide counteracts the detrimental changes caused by hyperglycemia at the cellular level, such as decreased activation of endothelial nitric oxide synthase and sodium potassium ATPase, and increase in formation of pro-inflammatory molecules mediated by nuclear factor kappa-light-chain-enhancer of activated B cells: cytokines, chemokines, cell adhesion molecules, vascular endothelial growth factor, and transforming growth factor beta. However, despite positive results from cell and animal studies, no successful c-peptide replacement therapies have been developed so far. Therefore, it is important to improve our understanding of the impact of c-peptide on the pathophysiology of microvascular complications to develop novel c-peptide-based treatments. This article aims to review current knowledge on the impact of c-peptide on diabetic neuro- and nephropathy and to evaluate its potential therapeutic role.

Future Perspective of Diabetic Animal Models

Objective

The need of today’s research is to develop successful and reliable diabetic animal models for understanding the disease susceptibility and pathogenesis. Enormous success of animal models had already been acclaimed for identifying key genetic and environmental factors like Idd loci and effects of microorganisms including the gut microbiota. Furthermore, animal models had also helped in identifying many therapeutic targets and strategies for immune-intervention. In spite of a quite success, we have acknowledged that many of the discovered immunotherapies are working on animals and did not have a significant impact on human. Number of animal models were developed in the past to accelerate drug discovery pipeline. However, due to poor initial screening and assessment on inequivalent animal models, the percentage of drug candidates who succeeded during clinical trials was very low. Therefore, it is essential to bridge this gap between pre-clinical research and clinical trial by validating the existing animal models for consistency.

Results and Conclusion

In this review, we have discussed and evaluated the significance of animal models on behalf of published data on PUBMED. Amongst the most popular diabetic animal models, we have selected six animal models (e.g. BioBreeding rat, “LEW IDDM rat”, “Nonobese Diabetic (NOD) mouse”, “STZ RAT”, “LEPR Mouse” and “Zucker Diabetic Fatty (ZDF) rat” and ranked them as per their published literature on PUBMED. Moreover, the vision and brief imagination for developing an advanced and robust diabetic model of 21^st century was discussed with the theme of one mice-one human concept including organs-on-chips.

Biological activity versus physiological function of proinsulin C-peptide

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

[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.

Navigating Two Roads to Glucose Normalization in Diabetes: Automated Insulin Delivery Devices and Cell Therapy

Incredible strides have been made since the discovery of insulin almost 100 years ago. Insulin formulations have improved dramatically, glucose levels can be measured continuously, and recently first-generation biomechanical "artificial pancreas" systems have been approved by regulators around the globe. However, still only a small fraction of patients with diabetes achieve glycemic goals. Replacement of insulin-producing cells via transplantation shows significant promise, but is limited in application due to supply constraints (cadaver-based) and the need for chronic immunosuppression. Over the past decade, significant progress has been made to address these barriers to widespread implementation of a cell therapy. Can glucose levels in people with diabetes be normalized with artificial pancreas systems or via cell replacement approaches? Here we review the road ahead, including the challenges and opportunities of both approaches.

The dual effect of C-peptide on cellular activation and atherosclerosis: Protective or not?

C-peptide is a cleavage product of proinsulin that acts on different type of cells, such as blood and endothelial cells. C-peptide biological effects may be different in type 1 and type 2 diabetes. Besides, there are further evidence for a functional interaction between C-peptide and insulin. In this way, C-peptide has ambiguous effects, acting as an antithrombotic or thrombotic molecule, depending on the physiological environment and disease conditions. Moreover, C-peptide regulates interaction of leucocytes, erythrocytes, and platelets with the endothelium. The beneficial effects include stimulation of nitric oxide production with its subsequent release by platelets and endothelium, the interaction with erythrocytes leading to the generation of adenosine triphosphate, and inhibition of atherogenic cytokine release. The undesirable action of C-peptide includes the chemotaxis of monocytes, lymphocytes, and smooth muscle cells. Also, C-peptide was related with increased lipid deposits and elevated smooth muscle cells proliferation in the vessel wall, contributing to atherosclerosis. Purpose of this review is to explore these dual roles of C-peptide on the blood, contributing at one side to haemostasis and the other to atherosclerotic process.

Ultrafiltration binding analyses of glycated albumin with a 3D-printed syringe attachment

Protein-ligand binding assays facilitate the understanding of biomolecular interactions. Classical equilibrium dialysis methods are often used for accurate determination of binding properties. While accurate, the long equilibration times associated with the technique (> 6 h) hinder throughput. Here, in an attempt to gather high-accuracy results while reducing total analysis time, a low pressure ultrafiltration method that relies on a simple membrane-containing syringe attachment was developed. A minimal portion (1-2%) of the solution containing the binding analytes of interest is driven through the membrane pores and collected for analysis. Specifically, the device was used to investigate the binding affinity between Zn²⁺ and either normal human serum albumin (nHSA) or a commercially purchased glycated human serum albumin (gHSA). Both of these ligand/protein-binding systems have implications in type 1 diabetes. The device was then used to investigate the binding between the various albumin types and C-peptide, the 31 amino acid peptide that is co-secreted with insulin from pancreatic β cells. Results for nHSA/Zn²⁺ binding obtained using the ultrafiltration method (K_d = 5.77 ± 0.19 × 10^-7 M) were statistically equivalent with results reported using other methods. Importantly, the amount of Zn²⁺ bound to the nHSA was significantly different from the gHSA (97 ± 2% protein bound vs. 91 ± 3%, respectively p < 0.05). The binding affinity of C-peptide to nHSA (K_d = 2.4 ± 0.3 × 10^-6 M) agreed with values reported in the literature using standard techniques. Unlike Zn²⁺ binding, the binding of C-peptide to nHSA was statistically equal to its binding to gHSA (77.7 ± 6.2 and 78.8 ± 7.4%, respectively), suggesting that C-peptide replacement therapy in people with T1D may be strongly dependent upon the characteristics of Zn²⁺ binding to human serum albumin. Graphical abstract ᅟ.

Selenium-coated nanostructured lipid carriers used for oral delivery of berberine to accomplish a synergic hypoglycemic effect

Diabetes mellitus is an incurable metabolic disorder that seriously threatens human health. At present, there is no effective medication available to defeat it. This work intended to develop selenium-coated nanostructured lipid carriers (SeNLCs) for enhancing the oral bioavailability and the curative effect of berberine, an antidiabetic phytomedicine. Berberine-loaded SeNLCs (BB-SeNLCs) were prepared by hot-melt dispersion/homogenization procedure followed by in situ reduction. BB-SeNLCs were characterized by particle size, morphology, entrapment efficiency (EE) and in vitro release. Pharmacokinetics of berberine solution, berberine-loaded NLCs (BB-NLCs) and BB-SeNLCs were studied in Sprague Dawley rats administered by oral gavage. The prepared BB-SeNLCs were around 160 nm in particle size with an EE of 90%. In addition, BB-SeNLCs exhibited a better sustained release of berberine compared to the plain NLCs. After oral administration, BB-SeNLCs greatly enhanced the oral bioavailability of berberine, which was approximately 6.63 times as much as that of berberine solution. The hypoglycemic effect of BB-SeNLCs was also significantly superior to that of BB-NLCs and berberine solution. It turned out that sustained drug release and good intestinal absorption, plus the synergy of selenium, were basically responsible for enhanced oral bioavailability and hypoglycemic effect. Our findings show that SeNLCs are promising nanocarriers for oral delivery of berberine to strengthen the antidiabetic action.