Low O2-induced ATP release from erythrocytes of humans with type 2 diabetes is restored by physiological ratios of C-peptide and insulin

C-peptide in diabetes: A player in a dual hormone disorder?

C-peptide, a byproduct of insulin synthesis believed to be biologically inert, is emerging as a multifunctional molecule. C-peptide serves an anti-inflammatory and anti-atherogenic role in type 1 diabetes mellitus (T1DM) and early T2DM. C-peptide protects endothelial cells by activating AMP-activated protein kinase α, thus suppressing the activity of NAD(P)H oxidase activity and reducing reactive oxygen species (ROS) generation. It also prevents apoptosis by regulating hyperglycemia-induced p53 upregulation and mitochondrial adaptor p66shc overactivation, as well as reducing caspase-3 activity and promoting expression of B-cell lymphoma-2. Additionally, C-peptide suppresses platelet-derived growth factor (PDGF)-beta receptor and p44/p42 mitogen-activated protein (MAP) kinase phosphorylation to inhibit vascular smooth muscle cells (VSMC) proliferation. It also diminishes leukocyte adhesion by virtue of its capacity to abolish nuclear factor kappa B (NF-kB) signaling, a major pro-inflammatory cascade. Consequently, it is envisaged that supplementation of C-peptide in T1DM might ameliorate or even prevent end-organ damage. In marked contrast, C-peptide increases monocyte recruitment and migration through phosphoinositide 3-kinase (PI-3 kinase)-mediated pathways, induces lipid accumulation via peroxisome proliferator-activated receptor γ upregulation, and stimulates VSMC proliferation and CD4+ lymphocyte migration through Src-kinase and PI-3K dependent pathways. Thus, it promotes atherosclerosis and microvascular damage in late T2DM. Indeed, C-peptide is now contemplated as a potential biomarker for insulin resistance in T2DM and linked to increased coronary artery disease risk. This shift in the understanding of the pathophysiology of diabetes from being a single hormone deficiency to a dual hormone disorder warrants a careful consideration of the role of C-peptide as a unique molecule with promising diagnostic, prognostic, and therapeutic applications.

The actions of C-peptide in HEK293 cells are dependent upon insulin and extracellular glucose concentrations

Connecting peptide, or C-peptide, is a part of the insulin prohormone and is essential for the proper folding and processing of the mature insulin peptide. C-peptide is released from the same beta cell secretory granules as insulin in equimolar amounts. However, due to their relative stabilities in plasma, the two peptides are detected in the circulation at ratios of approximately 4:1 to 6:1 (C-peptide to insulin), depending on metabolic state. C-peptide binds specifically to human cell membranes and induces intracellular signaling cascades, likely through an interaction with the G protein coupled receptor, GPR146. C-peptide has been shown to exert protective effects against the vascular, renal, and ocular complications of diabetes. The effects of C-peptide appear to be dependent upon the presence of insulin and the absolute, extracellular concentration of glucose. In this study, we employed HEK293 cells to further examine the interactive effects of C-peptide, insulin, and glucose on cell signaling. We observed that C-peptide's cellular effects are dampened significantly when cells are exposed to physiologically relevant concentrations of both insulin and C-peptide. Likewise, the actions of C-peptide on cFos and GPR146 mRNA expressions were affected by changes in extracellular glucose concentration. In particular, C-peptide induced significant elevations in cFos expression in the setting of high (25 mmol) extracellular glucose concentration. These data indicate that future experimentation on the actions of C-peptide should control for the presence or absence of insulin and the concentration of glucose. Furthermore, these findings should be considered prior to the development of C-peptide-based therapeutics for the treatment of diabetes-associated complications.

The Expression Regulatory Network in the Lung Tissue of Tibetan Pigs Provides Insight Into Hypoxia-Sensitive Pathways in High-Altitude Hypoxia

To adapt to a low-oxygen environment, Tibetan pigs have developed a series of unique characteristics and can transport oxygen more effectively; however, the regulation of the associated processes in high-altitude animals remains elusive. We performed mRNA-seq and miRNA-seq, and we constructed coexpression regulatory networks of the lung tissues of Tibetan and Landrace pigs. HBB, AGT, COL1A2, and EPHX1 were identified as major regulators of hypoxia-induced genes that regulate blood pressure and circulation, and they were enriched in pathways related to signal transduction and angiogenesis, such as HIF-1, PI3K-Akt, mTOR, and AMPK. HBB may promote the combination of hemoglobin and oxygen as well as angiogenesis for high-altitude adaptation in Tibetan pigs. The expression of MMP2 showed a similar tendency of alveolar septum thickness among the four groups. These results indicated that MMP2 activity may lead to widening of the alveolar wall and septum, alveolar structure damage, and collapse of alveolar space with remarkable fibrosis. These findings provide a perspective on hypoxia-adaptive genes in the lungs in addition to insights into potential candidate genes in Tibetan pigs for further research in the field of high-altitude adaptation.

Insights into the physiology of C-peptide

Current knowledge suggests a complex role of C-peptide in human physiology, but its mechanism of action is only partially understood. The effects of C-peptide appear to be variable depending on the target tissue, physiological environment, its combination with other bioactive molecules such as insulin, or depending on its concentration. It is apparent that C-peptide has therapeutic potential for the treatment of vascular and nervous damage caused by type 1 or late type 2 diabetes mellitus. The question remains whether the effect is mediated by the receptor, the existence of which is still uncertain, or whether an alternative non-receptor-mediated mechanism is responsible. The Institute of Endocrinology in Prague has been paying much attention to the issue of C-peptide and its metabolic effect since the 1980s. The RIA methodology of human C-peptide determination was introduced here and transferred to commercial production. By long-term monitoring of C-peptide oGTT-derived indices, the Institute has contributed to elucidating the pathophysiology of glucose tolerance disorders. This review summarizes the current knowledge of C-peptide physiology and highlights the contributions of the Institute of Endocrinology to this issue.

Do We Store Packed Red Blood Cells under "Quasi-Diabetic" Conditions?

Red blood cell (RBC) transfusion is one of the most common therapeutic procedures in modern medicine. Although frequently lifesaving, it often has deleterious side effects. RBC quality is one of the critical factors for transfusion efficacy and safety. The role of various factors in the cells’ ability to maintain their functionality during storage is widely discussed in professional literature. Thus, the extra- and intracellular factors inducing an accelerated RBC aging need to be identified and therapeutically modified. Despite the extensively studied in vivo effect of chronic hyperglycemia on RBC hemodynamic and metabolic properties, as well as on their lifespan, only limited attention has been directed at the high sugar concentration in RBCs storage media, a possible cause of damage to red blood cells. This mini-review aims to compare the biophysical and biochemical changes observed in the red blood cells during cold storage and in patients with non-insulin-dependent diabetes mellitus (NIDDM). Given the well-described corresponding RBC alterations in NIDDM and during cold storage, we may regard the stored (especially long-stored) RBCs as “quasi-diabetic”. Keeping in mind that these RBC modifications may be crucial for the initial steps of microvascular pathogenesis, suitable preventive care for the transfused patients should be considered. We hope that our hypothesis will stimulate targeted experimental research to establish a relationship between a high sugar concentration in a storage medium and a deterioration in cells’ functional properties during storage.

Multi-Omic Meta-Analysis of Transcriptomes and the Bibliome Uncovers Novel Hypoxia-Inducible Genes

Hypoxia is a condition in which cells, tissues, or organisms are deprived of sufficient oxygen supply. Aerobic organisms have a hypoxic response system, represented by hypoxia-inducible factor 1-α (HIF1A), to adapt to this condition. Due to publication bias, there has been little focus on genes other than well-known signature hypoxia-inducible genes. Therefore, in this study, we performed a meta-analysis to identify novel hypoxia-inducible genes. We searched publicly available transcriptome databases to obtain hypoxia-related experimental data, retrieved the metadata, and manually curated it. We selected the genes that are differentially expressed by hypoxic stimulation, and evaluated their relevance in hypoxia by performing enrichment analyses. Next, we performed a bibliometric analysis using gene2pubmed data to examine genes that have not been well studied in relation to hypoxia. Gene2pubmed data provides information about the relationship between genes and publications. We calculated and evaluated the number of reports and similarity coefficients of each gene to HIF1A, which is a representative gene in hypoxia studies. In this data-driven study, we report that several genes that were not known to be associated with hypoxia, including the G protein-coupled receptor 146 gene, are upregulated by hypoxic stimulation.

Specific Binding of Leptin to Red Blood Cells Delivers a Pancreatic Hormone and Stimulates ATP Release

Since its discovery in 1994, leptin continues to have new potential physiological roles uncovered, including a role in the regulation of blood flow. Leptin's role in regulating blood flow is not completely understood. Red blood cell (RBC)-derived ATP is a recognized stimulus of blood flow, and multiple studies suggest that C-peptide, a hormone secreted in equimolar amounts with insulin from the pancreatic β-cells, can stimulate that release when delivered by albumin and in combination with Zn²⁺. Here, we report leptin delivers C-peptide and Zn²⁺ to RBCs in a saturable and specific manner. We labeled leptin with technetium-99 m (^99mTc) to perform binding studies while using albumin to block the specific binding of ^99mTc-leptin in the presence or absence of C-peptide. Our results suggest that leptin has a saturable and specific binding site on the RBC ((K_d = 1.79 ± 0.46) × 10^-7 M) that is statistically equal to the binding affinity in the presence of 20 nM C-peptide ((K_d = 2.05 ± 0.20) × 10^-7 M). While the binding affinity between leptin and the RBC did not change with C-peptide, the moles of bound leptin did increase with C-peptide, suggesting a separate binding site on the cell for a leptin/C-peptide complex. The RBC-derived ATP increased in the presence of a leptin/C-peptide/Zn²⁺ addition, in a concentration-dependent manner. Control RBCs ATP release increased (71 ± 5.6%) in the presence of C-peptide and Zn²⁺, which increased further to (94 ± 5.6%) in the presence of Zn²⁺, C-peptide, and leptin.

A C-peptide complex with albumin and Zn2+ increases measurable GLUT1 levels in membranes of human red blood cells

People with type 1 diabetes (T1D) require exogenous administration of insulin, which stimulates the translocation of the GLUT4 glucose transporter to cell membranes. However, most bloodstream cells contain GLUT1 and are not directly affected by insulin. Here, we report that C-peptide, the 31-amino acid peptide secreted in equal amounts with insulin in vivo, is part of a 3-component complex that affects red blood cell (RBC) membranes. Multiple techniques were used to demonstrate saturable and specific C-peptide binding to RBCs when delivered as part of a complex with albumin. Importantly, when the complex also included Zn²⁺, a significant increase in cell membrane GLUT1 was measured, thus providing a cellular effect similar to insulin, but on a transporter on which insulin has no effect.

Difference in the duration of pediatric diabetic ketoacidosis: Comparison of new-onset to known type 1 diabetes

OBJECTIVE

To compare the duration (hours until HCO3-  ≥ 15 mmol/L) of diabetic ketoacidosis (DKA) episodes that are the first manifestation of new type 1 diabetes (NT1D) and those that are a complication in patients with previously diagnosed type 1 diabetes (PT1D).

METHODS

A multicenter retrospective cohort study was designed. The duration of DKA was measured from the start of the treatment. The primary outcome was the comparison of the time needed in each group to reach HCO3-  ≥ 15 mmol/L. The secondary outcomes were the comparison of the time to reach pH ≥ 7.3 and length of hospital stay in each group. Data were analyzed with a bivariate analysis of the variables vs primary outcome. Then, a regression model was analyzed. Results There were 305 episodes included (NT1D: 115, PT1D: 190). DKA in the NT1D group lasted longer (NT1D 20 (16-19) vs PT1D 12 (8-16), hours, P < .01) with a significant difference in each level of DKA severity. This group also took longer to reach pH ≥ 7.3 (NT1D 16 (12-22) vs PT1D 9 (6-12), hours, P < .01) and had a longer hospital stay (NT1D 9 (6-12) vs PT1D 7 (4-10), hours, P < .01).

CONCLUSION

The duration of DKA is longer in patients with NT1D regardless of characteristics like DKA severity, duration of symptoms, and type of treatments received.

Alteration of purinergic signaling in diabetes: Focus on vascular function

Diabetes is an important risk factor for the development of cardiovascular disease including atherosclerosis and ischemic heart disease. Vascular complications including macro- and micro-vascular dysfunction are the leading causes of morbidity and mortality in diabetes. Disease mechanisms at present are unclear and no ideal therapies are available, which urgently calls for the identification of novel therapeutic targets/agents. An altered nucleotide- and nucleoside-mediated purinergic signaling has been implicated to cause diabetes-associated vascular dysfunction in major organs. Alteration of both purinergic P1 and P2 receptor sensitivity rather than the changes in receptor expression accounts for vascular dysfunction in diabetes. Activation of P2X₇ receptors plays a crucial role in diabetes-induced retinal microvascular dysfunction. Recent findings have revealed that both ecto-nucleotidase CD39, a key enzyme hydrolyzing ATP, and CD73, an enzyme regulating adenosine turnover, are involved in the renal vascular injury in diabetes. Interestingly, erythrocyte dysfunction in diabetes by decreasing ATP release in response to physiological stimuli may serve as an important trigger to induce vascular dysfunction. Nucleot(s)ide-mediated purinergic activation also exerts long-term actions including inflammatory and atherogenic effects in hyperglycemic and diabetic conditions. This review highlights the current knowledge regarding the altered nucleot(s)ide-mediated purinergic signaling as an important disease mechanism for the diabetes-associated vascular complications. Better understanding the role of key receptor-mediated signaling in diabetes will provide more insights into their potential as targets for the treatment.

Reduced deformability contributes to impaired deoxygenation-induced ATP release from red blood cells of older adult humans

KEY POINTS

Red blood cells (RBCs) release ATP in response to deoxygenation, which can increase blood flow to help match oxygen supply with tissue metabolic demand. This release of ATP is impaired in RBCs from older adults, but the underlying mechanisms are unknown. In this study, improving RBC deformability in older adults restored deoxygenation-induced ATP release, whereas decreasing RBC deformability in young adults reduced ATP release to the level of that of older adults. In contrast, treating RBCs with a phosphodiesterase 3 inhibitor did not affect ATP release in either age group, possibly due to intact intracellular signalling downstream of deoxygenation as indicated by preserved cAMP and ATP release responses to pharmacological G_i protein activation in RBCs from older adults. These findings are the first to demonstrate that the age-related decrease in RBC deformability is a primary mechanism of impaired deoxygenation-induced ATP release, which may have implications for treating impaired vascular control with advancing age.

ABSTRACT

In response to haemoglobin deoxygenation, red blood cells (RBCs) release ATP, which binds to endothelial purinergic receptors and stimulates vasodilatation. This ATP release is impaired in RBCs from older vs. young adults, but the underlying mechanisms are unknown. Using isolated RBCs from young (24 ± 1 years) and older (65 ± 2 years) adults, we tested the hypothesis that age-related changes in RBC deformability (Study 1) and cAMP signalling (Study 2) contribute to the impairment. RBC ATP release during normoxia ( P O 2 ∼112 mmHg) and hypoxia ( P O 2 ∼20 mmHg) was quantified with the luciferin-luciferase technique following RBC incubation with Y-27632 (Rho-kinase inhibitor to increase deformability), diamide (cell-stiffening agent), cilostazol (phosphodiesterase 3 inhibitor), or vehicle control. The mean change in RBC ATP release from normoxia to hypoxia in control conditions was significantly impaired in older vs. young (∼50% vs. ∼120%; P < 0.05). RBC deformability was also lower in older vs. young as indicated by a higher RBC transit time (RCTT) measured by blood filtrometry (RCTT: 8.541 ± 0.050 vs. 8.234 ± 0.098 a.u., respectively; P < 0.05). Y-27632 improved RBC deformability (RCTT: 8.228 ± 0.083) and ATP release (111.7 ± 17.2%) in older and diamide decreased RBC deformability (RCTT: 8.955 ± 0.114) and ATP release (67.4 ± 11.8%) in young (P < 0.05), abolishing the age group differences (P > 0.05). Cilostazol did not change ATP release in either age group (P > 0.05), and RBC cAMP and ATP release to pharmacological G_i protein activation was similar in both groups (P > 0.05). We conclude that decreased RBC deformability is a primary contributor to age-related impairments in RBC ATP release, which may have implications for impaired vascular control with advancing age.

Investigation into the presence and functional significance of proinsulin C-peptide in the female germline†

Diabetes is associated with poor oocyte quality and the dysregulation of ovarian function and is thus a leading contributor to the increasing prevalence of female reproductive pathologies. Accordingly, it is well-established that insulin fulfills a key role in the regulation of several facets of female reproduction. What remains less certain is whether proinsulin C-peptide, which has recently been implicated in cellular signaling cascades, holds a functional role in the female germline. In the present study, we examined the expression of insulin, C-peptide, and its purported receptor; GPR146, within the mouse ovary and oocyte. Our data establish the presence of abundant C-peptide within follicular fluid and raise the prospect that this bioactive peptide is internalized by oocytes in a G-protein coupled receptor-dependent manner. Further, our data reveal that internalized C-peptide undergoes pronounced subcellular relocalization from the ooplasm to the pronuclei postfertilization. The application of immunoprecipitation analysis and mass spectrometry identified breast cancer type 2 susceptibility protein (BRCA2), the meiotic resumption/DNA repair protein, as a primary binding partner for C-peptide within the oocyte. Collectively, these findings establish a novel accumulation profile for C-peptide in the female germline and provide the first evidence for an interaction between C-peptide and BRCA2. This interaction is particularly intriguing when considering the propensity for oocytes from diabetic women to experience aberrant meiotic resumption and perturbation of traditional DNA repair processes. This therefore provides a clear imperative for further investigation of the implications of dysregulated C-peptide production in these individuals.

Reduced skeletal-muscle perfusion and impaired ATP release during hypoxia and exercise in individuals with type 2 diabetes

AIMS/HYPOTHESIS

Plasma ATP is a potent vasodilator and is thought to play a role in the local regulation of blood flow. Type 2 diabetes is associated with reduced tissue perfusion. We aimed to examine whether individuals with type 2 diabetes have reduced plasma ATP concentrations compared with healthy control participants (case-control design).

METHODS

We measured femoral arterial and venous plasma ATP levels with the intravascular microdialysis technique during normoxia, hypoxia and one-legged knee-extensor exercise (10 W and 30 W) in nine participants with type 2 diabetes and eight control participants. In addition, we infused acetylcholine (ACh), sodium nitroprusside (SNP) and ATP into the femoral artery to assess vascular function and ATP signalling.

RESULTS

Individuals with type 2 diabetes had a lower leg blood flow (LBF; 2.9 ± 0.1 l/min) compared with the control participants (3.2 ± 0.1 l/min) during exercise (p < 0.05), in parallel with lower venous plasma ATP concentration (205 ± 35 vs 431 ± 72 nmol/l; p < 0.05). During systemic hypoxia, LBF increased from 0.35 ± 0.04 to 0.54 ± 0.06 l/min in control individuals, whereas it did not increase (0.25 ± 0.04 vs 0.31 ± 0.03 l/min) in the those with type 2 diabetes and was lower than in the control individuals (p < 0.05). Hypoxia increased venous plasma ATP levels in both groups (p < 0.05), but the increase was higher in control individuals (90 ± 26 nmol/l) compared to those with type 2 diabetes (18 ± 5 nmol/l). LBF and vascular conductance were lower during ATP (0.15 and 0.4 μmol min^-1 [kg leg mass]^-1) and ACh (100 μg min^-1 [kg leg mass]^-1) infusion in individuals with type 2 diabetes compared with the control participants (p < 0.05), whereas there was no difference during SNP infusion.

CONCLUSIONS/INTERPRETATION

These findings demonstrate that individuals with type 2 diabetes have lower plasma ATP concentrations during exercise and hypoxia compared with control individuals, and this occurs in parallel with lower blood flow. Moreover, individuals with type 2 diabetes have a reduced vasodilatory response to infused ATP. These impairments in the ATP system are both likely to contribute to the reduced tissue perfusion associated with type 2 diabetes.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02001766.

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.

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

Type 1 diabetes is associated with such complications as blindness, kidney failure, and nerve damage. Replacing C-peptide, a hormone normally co-secreted with insulin, has been shown to reduce diabetes-related complications. Interestingly, after nearly 30 years of positive research results, C-peptide is still not being co-administered with insulin to diabetic patients. The following review discusses the potential of C-peptide as an auxilliary replacement therapy and why it's not currently being used as a therapeutic.

The structural and functional changes of blood cells and molecular components in diabetes mellitus

It is known fact that diabetes mellitus (DM) affects blood cells. Changes in the erythrocyte membrane, disorder in hemoglobin oxygen-binding and modification in mechanical characteristics, are effects of hyperglycemia on red blood cells. Altered susceptibility infection of patients with diabetes has been ascribed to a depression in the function of polymorphonuclear leukocytes. Neutrophil function in patients with diabetes with good glucose control is slightly different than in healthy ones. DM causes significant changes in lymphocytes metabolism and their functions. Patients with diabetes, presenting with acute coronary syndrome, are at higher risk of cardiovascular complications and recurrent ischemic events in comparison to non-diabetic counterparts. Various mechanisms, including endothelial dysfunction, platelet hyperactivity, and abnormalities in coagulation and fibrynolysis have been implicated for this increased atherothrombotic risk. There are many other alterations of blood cells due to DM. In the present review we focused on modifications of blood cells due to DM. Then, as a second point, we explored how the changes affect functions of red blood cells, white blood cells and platelets.

Augmentation of Muscle Blood Flow by Ultrasound Cavitation Is Mediated by ATP and Purinergic Signaling

BACKGROUND

Augmentation of tissue blood flow by therapeutic ultrasound is thought to rely on convective shear. Microbubble contrast agents that undergo ultrasound-mediated cavitation markedly amplify these effects. We hypothesized that purinergic signaling is responsible for shear-dependent increases in muscle perfusion during therapeutic cavitation.

METHODS

Unilateral exposure of the proximal hindlimb of mice (with or without ischemia produced by iliac ligation) to therapeutic ultrasound (1.3 MHz, mechanical index 1.3) was performed for 10 minutes after intravenous injection of 2×10⁸ lipid microbubbles. Microvascular perfusion was evaluated by low-power contrast ultrasound perfusion imaging. In vivo muscle ATP release and in vitro ATP release from endothelial cells or erythrocytes were assessed by a luciferin-luciferase assay. Purinergic signaling pathways were assessed by studying interventions that (1) accelerated ATP degradation; (2) inhibited P2Y receptors, adenosine receptors, or K_ATP channels; or (3) inhibited downstream signaling pathways involving endothelial nitric oxide synthase or prostanoid production (indomethacin). Augmentation in muscle perfusion by ultrasound cavitation was assessed in a proof-of-concept clinical trial in 12 subjects with stable sickle cell disease.

RESULTS

Therapeutic ultrasound cavitation increased muscle perfusion by 7-fold in normal mice, reversed tissue ischemia for up to 24 hours in the murine model of peripheral artery disease, and doubled muscle perfusion in patients with sickle cell disease. Augmentation in flow extended well beyond the region of ultrasound exposure. Ultrasound cavitation produced an ≈40-fold focal and sustained increase in ATP, the source of which included both endothelial cells and erythrocytes. Inhibitory studies indicated that ATP was a critical mediator of flow augmentation that acts primarily through either P2Y receptors or adenosine produced by ectonucleotidase activity. Combined indomethacin and inhibition of endothelial nitric oxide synthase abolished the effects of therapeutic ultrasound, indicating downstream signaling through both nitric oxide and prostaglandins.

CONCLUSIONS

Therapeutic ultrasound using microbubble cavitation to increase muscle perfusion relies on shear-dependent increases in ATP, which can act through a diverse portfolio of purinergic signaling pathways. These events can reverse hindlimb ischemia in mice for >24 hours and increase muscle blood flow in patients with sickle cell disease.

CLINICAL TRIAL REGISTRATION

URL: http://clinicaltrials.gov. Unique identifier: NCT01566890.

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.

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.

Mechanisms of C-peptide-mediated rescue of low O2-induced ATP release from erythrocytes of humans with type 2 diabetes

The circulating erythrocyte, by virtue of the regulated release of ATP in response to reduced oxygen (O2) tension, plays a key role in maintaining appropriate perfusion distribution to meet tissue needs. Erythrocytes from individuals with Type 2 diabetes (DM2) fail to release ATP in response to this stimulus. However, the administration of C-peptide and insulin at a 1:1 ratio was shown to restore this important physiological response in humans with DM2. To begin to investigate the mechanisms by which C-peptide influences low O2-induced ATP release, erythrocytes from healthy humans and humans with DM2 were exposed to reduced O2 in a thin-film tonometer, and ATP release under these conditions was compared with release during normoxia. We determined that 1) low O2-induced ATP release from DM2 erythrocytes is rescued by C-peptide in the presence and absence of insulin, 2) the signaling pathway activated by C-peptide in human erythrocytes involves PKC, as well as soluble guanylyl cyclase (sGC) and 3) inhibitors of cGMP degradation rescue low O2-induced ATP release from DM2 erythrocytes. These results provide support for the hypothesis that both PKC and sGC are components of a signaling pathway activated by C-peptide in human erythrocytes. In addition, since both C-peptide and phosphodiesterase 5 inhibitors rescue low O2-induced ATP release from erythrocytes of humans with DM2, their administration to humans with DM2 could aid in the treatment and/or prevention of the vascular disease associated with this condition.