Polyol pathway and protein kinase C activity of rat Schwannoma cells

Function and therapeutic value of astrocytes in diabetic cognitive impairment

Diabetic cognitive impairment (DCI) is a complex complication of diabetes in the central nervous system, and its pathological mechanism is still being explored. Astrocytes are abundant glial cells in central nervous system that perform diverse functions in health and disease. Accumulating excellent research has identified astrocyte dysfunction in many neurodegenerative diseases (such as Alzheimer's disease, aging and Parkinson's disease), and summarized and discussed its pathological mechanisms and potential therapeutic value. However, the contribution of astrocytes to DCI has been largely overlooked. In this review, we first systematically summarized the effects and mechanisms of diabetes on brain astrocytes, and found that the diabetic environment (such as hyperglycemia, advanced glycation end products and cerebral insulin resistance) mediated brain reactive astrogliosis, which was specifically reflected in the changes of cell morphology and the remodeling of signature molecules. Secondly, we emphasized the contribution and potential targets of reactive astrogliosis to DCI, and found that reactive astrogliosis-induced increased blood-brain barrier permeability, glymphatic system dysfunction, neuroinflammation, abnormal cell communication and cholesterol metabolism dysregulation worsened cognitive function. In addition, we summarized effective strategies for treating DCI by targeting astrocytes. Finally, we discuss the application of new techniques in astrocytes, including single-cell transcriptome, in situ sequencing, and prospected new functions, new subsets and new targets of astrocytes in DCI.

Moving toward a new horizon for the aldose reductase inhibitor epalrestat to treat drug-resistant cancer

Epalrestat (EPA) is a potent inhibitor of aldose reductases AKR1B1 and AKR1B10, used for decades in Japan for the treatment of diabetic peripheral neuropathy. This orally-active, brain-permeable small molecule, with a relatively rare and essential 2-thioxo-4-thiazolidinone motif, functions as a regulator intracellular carbonyl species. The repurposing of EPA for the treatment of pediatric rare diseases, brain disorders and cancer has been proposed. A detailed analysis of the mechanism of action, and the benefit of EPA to combat advanced malignancies is offered here. EPA has revealed marked anticancer activities, alone and in combination with cytotoxic chemotherapy and targeted therapeutics, in experimental models of liver, colon, and breast cancers. Through inhibition of AKR1B1 and/or AKR1B10 and blockade of the epithelial-mesenchymal transition, EPA largely enhances the sensitivity of cancer cells to drugs like doxorubicin and sorafenib. EPA has revealed a major anticancer effect in an experimental model of basal-like breast cancer and clinical trials have been developed in patients with triple-negative breast cancer. The repurposing of the drug to treat chemo-resistant solid tumors seems promising, but more studies are needed to define the best trajectory for the positioning of EPA in oncology.

Pathogenesis of Distal Symmetrical Polyneuropathy in Diabetes

Distal symmetrical polyneuropathy (DSPN) is a serious complication of diabetes associated with significant disability and mortality. Although more than 50% of people with diabetes develop DSPN, its pathogenesis is still relatively unknown. This lack of understanding has limited the development of novel disease-modifying therapies and left the reasons for failed therapies uncertain, which is critical given that current management strategies often fail to achieve long-term efficacy. In this article, the pathogenesis of DSPN is reviewed, covering pathogenic changes in the peripheral nervous system, microvasculature and central nervous system (CNS). Furthermore, the successes and limitations of current therapies are discussed, and potential therapeutic targets are proposed. Recent findings on its pathogenesis have called the definition of DSPN into question and transformed the disease model, paving the way for new research prospects.

Status and Trends of the Association Between Diabetic Nephropathy and Diabetic Retinopathy From 2000 to 2021: Bibliometric and Visual Analysis

Background: Diabetic nephropathy (DN) and diabetic retinopathy (DR) are microvascular complications of diabetes that share a similar pathogenesis and clinical relevance. The study aimed to visually analyze the research status and development trend of the relationship between DN and DR by means of bibliometrics and knowledge mapping.

Methods: Publications were collected from the Science Citation Index-Expanded of the Web of Science Core Collection between 2000 and 2021. CiteSpace, Alluvial Generator, and Microsoft Excel were used to analyze and present the data.

Results: A total of 3,348 publications were retrieved and 3,285 were included in the analysis after deduplication. The publications demonstrated an annually increasing trend. The results of the collaborative network analysis showed that the United States, Steno Diabetes Center, and Tien Y. Wong were the most influential country, institution and author, in this field of research, respectively. The analysis of references and keywords showed that the pathogenesis of DN and DR and their relationship with cardiovascular disease are research hotspots. The clinical relevance and drug therapy for DN and DR will become frontiers of future research in this field.

Conclusion: This study is the first to visualize the correlation between DN and DR using a bibliometric approach. This study provides a reference of research trends for scholars.

Insulin enhances neurite extension and myelination of diabetic neuropathy neurons

Background

The authors established an in vitro model of diabetic neuropathy based on the culture system of primary neurons and Schwann cells (SCs) to mimic similar symptoms observed in in vivo models of this complication, such as impaired neurite extension and impaired myelination. The model was then utilized to investigate the effects of insulin on enhancing neurite extension and myelination of diabetic neurons.

Methods

SCs and primary neurons were cultured under conditions mimicking hyperglycemia prepared by adding glucose to the basal culture medium. In a single culture, the proliferation and maturation of SCs and the neurite extension of neurons were evaluated. In a co-culture, the percentage of myelination of diabetic neurons was investigated. Insulin at different concentrations was supplemented to culture media to examine its effects on neurite extension and myelination.

Results

The cells showed similar symptoms observed in in vivo models of this complication. In a single culture, hyperglycemia attenuated the proliferation and maturation of SCs, induced apoptosis, and impaired neurite extension of both sensory and motor neurons. In a co-culture of SCs and neurons, the percentage of myelinated neurites in the hyperglycemia-treated group was significantly lower than that in the control group. This impaired neurite extension and myelination was reversed by the introduction of insulin to the hyperglycemic culture media.

Conclusions

Insulin may be a potential candidate for improving diabetic neuropathy. Insulin can function as a neurotrophic factor to support both neurons and SCs. Further research is needed to discover the potential of insulin in improving diabetic neuropathy.

Aldose Reductase and the Polyol Pathway in Schwann Cells: Old and New Problems

Aldose reductase (AR) is a member of the reduced nicotinamide adenosine dinucleotide phosphate (NADPH)-dependent aldo-keto reductase superfamily. It is also the rate-limiting enzyme of the polyol pathway, catalyzing the conversion of glucose to sorbitol, which is subsequently converted to fructose by sorbitol dehydrogenase. AR is highly expressed by Schwann cells in the peripheral nervous system (PNS). The excess glucose flux through AR of the polyol pathway under hyperglycemic conditions has been suggested to play a critical role in the development and progression of diabetic peripheral neuropathy (DPN). Despite the intensive basic and clinical studies over the past four decades, the significance of AR over-activation as the pathogenic mechanism of DPN remains to be elucidated. Moreover, the expected efficacy of some AR inhibitors in patients with DPN has been unsatisfactory, which prompted us to further investigate and review the understanding of the physiological and pathological roles of AR in the PNS. Particularly, the investigation of AR and the polyol pathway using immortalized Schwann cells established from normal and AR-deficient mice could shed light on the causal relationship between the metabolic abnormalities of Schwann cells and discordance of axon-Schwann cell interplay in DPN, and led to the development of better therapeutic strategies against DPN.

Schwann Cells as Crucial Players in Diabetic Neuropathy

Schwann cells maintain peripheral nerve structure and function by ensheathment of unmyelinated axons, myelination of myelinated axons, and secretion of neurotrophic factors, and these cells also play a crucial role in the pathogenic mechanisms of diabetic neuropathy. A decrease in unmyelinated and small myelinated axons appeared earlier than a decrease in large myelinated fibers in diabetic neuropathy. Electron microscopic studies of human diabetic neuropathy demonstrated edematous cell cytoplasm, aggregates of glycogen particles, and hyperplasia of the surrounding basal lamina in Schwann cells. Diabetic conditions also induces metabolic disorders, such as polyol pathway hyperactivity, activation of protein kinase C, and increased advanced glycosylation end products in Schwann cells, followed by the depletion of neurotrophic factor production.Cell transplantation using progenitor or stem cells is expected to cure diabetic neuropathy. Many studies demonstrated that the paracrine effect of abundant secreted factors from transplanted stem cells was crucial for the success of cell transplantation in diabetic neuropathy. Transplantation of progenitor or stem cells in diabetic animal models ameliorated impaired nerve conduction velocity, nerve blood flow, sensory disorders, and intraepidermal nerve fiber density, with an increase of myelin thickness. The supernatant from cultured dental pulp stem cells increased the proliferation and production of myelin-related protein in Schwann cells, suggesting that Schwann cells is the main target of cell transplantation for diabetic neuropathy.

Antiangiogenic cytokines as potential new therapeutic targets for resveratrol in diabetic retinopathy

Diabetes mellitus (DM) affects >350 million people worldwide. With many complications that can reduce the patient’s quality of life, vision loss is one of the most debilitating disorders it can cause. Active research in the field of diabetes includes microvascular complications in diabetic retinopathy (DR). Disturbances in the balance of pro-angiogenesis and anti-angiogenesis factors can lead to the progression of DR. The retinal pigment epithelium (RPE) is the outermost layer of the retina, and it is essential in maintaining the visual function. The RPE produces and secretes growth factors as well as protective agents which maintain structural integrity of the retina. Small natural molecules, such as resveratrol, may influence neurotrophic factors of the retina. The pigment epithelium-derived factor (PEDF) and thrombospondin-1 (TSP-1) are secreted by RPE cells. These two proteins inhibit angiogenesis and inflammation in RPE cells. An alteration of their production contributes to various eye diseases. There is a critical balance between two important factors secreted on opposite sides of the RPE: at the basal side, vascular endothelial growth factor (VEGF; acts on the choroidal endothelium) and, on the apical side, PEDF (acts on neurons and photoreceptors). Resveratrol inhibits VEGF expression in human adult RPE cells and limits the development of proliferative vitreoretinopathy, by attenuating transforming growth factor-β2-induced wound closure and cell migration. Possible new mechanisms could include PEDF and TSP-1 expression alterations under physiological and pathological conditions. Resveratrol is currently of interest due to its capacity to influence the cell’s secretory activity. Some limitations arise from its low bioavailability. Several drug delivery systems are currently tested, promising to improve tissue concentrations. This article reviews biological pathways involved in the pathogenesis of DR that could be influenced by resveratrol. A study of these pathways could identify new potential targets for the reduction of diabetic complications.

High Glucose-Induced PC12 Cell Death by Increasing Glutamate Production and Decreasing Methyl Group Metabolism

Objective. High glucose- (HG-) induced neuronal cell death is responsible for the development of diabetic neuropathy. However, the effect of HG on metabolism in neuronal cells is still unclear. Materials and Methods. The neural-crest derived PC12 cells were cultured for 72 h in the HG (75 mM) or control (25 mM) groups. We used NMR-based metabolomics to examine both intracellular and extracellular metabolic changes in HG-treated PC12 cells. Results. We found that the reduction in intracellular lactate may be due to excreting more lactate into the extracellular medium under HG condition. HG also induced the changes of other energy-related metabolites, such as an increased succinate and creatine phosphate. Our results also reveal that the synthesis of glutamate from the branched-chain amino acids (isoleucine and valine) may be enhanced under HG. Increased levels of intracellular alanine, phenylalanine, myoinositol, and choline were observed in HG-treated PC12 cells. In addition, HG-induced decreases in intracellular dimethylamine, dimethylglycine, and 3-methylhistidine may indicate a downregulation of methyl group metabolism. Conclusions. Our metabolomic results suggest that HG-induced neuronal cell death may be attributed to a series of metabolic changes, involving energy metabolism, amino acids metabolism, osmoregulation and membrane metabolism, and methyl group metabolism.

Glucotoxic Mechanisms and Related Therapeutic Approaches

Neuropathy is the earliest and commonest complication of diabetes. With increasing duration of diabetes, frequency and severity of neuropathy are worsened. Long-term hyperglycemia is therefore implicated in the development of this disorder. Nerve tissues require glucose energy to function and survive. Upon excessive glucose entry into the peripheral nerve, the glycolytic pathway and collateral glucose-utilizing pathways are overactivated and initiate adverse effects on nerve tissues. During hyperglycemia, flux through the polyol pathway, formation of advanced glycation end-products, production of free radicals, flux into the glucosamine pathway, and protein kinase C activity are all enhanced to negatively influence nerve function and structure. Suppression of these aberrant metabolic pathways has succeeded in prevention and inhibition of the development of neuropathy in animal models with diabetes. Satisfactory results were not attained, however, in patients with diabetes and further clinical trials are required. In this review, the author summarizes the hitherto proposed theories on the pathogenesis of diabetic neuropathy related to glucose metabolism and future prospects for the effective treatment of neuropathy.

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)

A brief review of in vitro models of diabetic neuropathy

The neuropathies of the peripheral, central and autonomic nervous systems are known to be caused by hyperglycemia, a consequence of the deregulation of glucose in diabetes. Several in vivo models such as streptozotocin-induced diabetic rats, mice and Chinese hamsters have been used to study the pathogenesis of diabetic neuropathy because of their resemblance to human pathology. However, these in vivo models have met with strong ethical oppositions. Further, the system complexity has inherent limitations of inconvenience of analyzing ephemeral molecular events and crosstalk of signal transduction pathways. Alternative in vitro models have been selected and put to effective use in diabetic studies. We critically review the use of these in vitro models such as primary cultures of dorsal root ganglia, Schwann cells and neural tissue as well as neural cell lines which have proved to be excellent systems for detailed study. We also assess the use of embryo cultures for the study of hyperglycemic effects on development, especially of the nervous system. These systems function as useful models to scrutinize the molecular events underlying hyperglycemia-induced stress in neuronal systems and have been very effectively used for the same. This comprehensive overview of advantages and disadvantages of in vitro systems that are currently in use will be of interest especially for comparative assessment of results and for appropriate choice of models for experiments in diabetic neuropathy.

Methylcobalamin effects on diabetic neuropathy and nerve protein kinase C in rats

BACKGROUND

Methyl-base-attached cobalamin (Methycobalamin) (MC) has a special affinity for nerve tissues to promote myelination and transport of axonal cytoskeleton. It is not known, however, how MC influences on peripheral nerve in experimental diabetic neuropathy.

MATERIALS AND METHODS

We studied the effects of MC on expressions and activities of protein kinase C (PKC) in peripheral nerve of streptozotocin-induced diabetic rats. Wistar rats, 8 weeks of age, were rendered diabetic by streptozotocin (40 mg kg(-1), iv) and followed for 16 weeks. A half of diabetic animals were treated with MC (10 mg kg(-1) per every other day, im) after the induction of diabetes. Normal Wistar rats were served as control.

RESULTS

At the end, untreated diabetic animals developed significant delay of nerve conduction velocity (NCV), and MC treatment normalized the NCV. Nerve PKC activity was significantly suppressed in untreated diabetic rats, while the activity was normalized in treated animals. While PKCα located in Schwann cells, PKCβΙα and βII distributed in axoplasm, vascular walls and macrophages. The decreased PKC activity in diabetic nerve was associated with reduced expression of membrane PKCα and increased membrane expression of PKCβII, and MC treatment corrected these changes. Diabetic nerve contained an increased number of macrophages and 8-hydroxydeoxyguanosine-positive cells in the endoneurium, the latter of which was significantly suppressed by MC treatment. Elevated nerve polyol levels in diabetic nerve were partially corrected by MC treatment.

CONCLUSIONS

This study suggested that correction of impaired neural signalling of PKC and oxidative stress-induced damage may be a major attribute to the beneficial effects of MC on diabetic nerve.

Effect of Jinmaitong serum on the proliferation of rat Schwann cells cultured in high glucose medium

OBJECTIVE

To investigate the effect of Jinmaitong (JMT) serum on the proliferation of rat Schwann cells (SCs) primarily cultured in high glucose medium.

METHOD

SCs were primarily cultured in Dulbecco's minmum essential medium (DMEM control), 50 mmol/L glucose medium (50 mmol/L Glu), 75 mmol/L glucose medium (75 mmol/L Glu), as well as 50 mmol/L glucose medium, with different concentrations of JMT serum (undiluted, 1:2 diluted and 1:8 diluted) and Neurotropin (Ntp), respectively. The proliferation of SCs under different conditions was detected by MTT.

RESULT

SCs grew exuberantly in DMEM within 24-72 h, but slowed down at 96 h. The proliferation of SCs was inhibited in 50 mmol/L Glu and 75 mmol/L Glu after cultures of 48, 72 and 96 h, which showed that both were significantly different compared to the control group (P<0.01). The inhibition was more significant in 75 mmol/L Glu than in 50 mmol/L Glu (P<0.05). Spearman's rho analysis revealed that the proliferation of SCs had a negative correlation with the concentration of glucose (r=-0.471, P<0.01). Excluding the time factor, partial correlation showed similar results (r=-0.679, P<0.01). After 48 h, the proliferation of SCs increased significantly in JMT1:2 and Ntp compared with 50 mmol/L Glu (control 0.437+or-0.019, 50 mmol/L Glu 0.367+or-0.035, JMT1:2 0.426+or-0.024, Ntp 0.422+or-0.013; P<0.01), and there were no statistically significant differences among the JMT groups, the Ntp group and the control group (P>0.05).

CONCLUSIONS

The proliferation of SCs was inhibited in high glucose medium, and the inhibition was reduced by different concentrations of JMT serum, especially at JMT1:2.

Metformin prevents methylglyoxal-induced apoptosis of mouse Schwann cells

Methylglyoxal (MG) is involved in the pathogenesis of diabetic complications via the formation of advanced glycation end products (AGEs) and reactive oxygen species (ROS). To clarify whether the antidiabetic drug metformin prevents Schwann cell damage induced by MG, we cultured mouse Schwann cells in the presence of MG and metformin. Cell apoptosis was evaluated using Hoechst 33342 nuclear staining, caspase-3 activity, and c-Jun-N-terminal kinase (JNK) phosphorylation. Intracellular ROS formation was determined by flow cytometry, and AMP-activated kinase (AMPK) phosphorylation was also examined. MG treatment resulted in blunted cell proliferation, an increase in the number of apoptotic cells, and the activation of caspase-3 and JNK along with enhanced intracellular ROS formation. All of these changes were significantly inhibited by metformin. No significant activation of AMPK by MG or metformin was observed. Taken together, metformin likely prevents MG-induced apoptotic signals in mouse Schwann cells by inhibiting the formation of AGEs and ROS.

PKC-ɛ inhibition: a new therapeutic approach for diabetic complications?

PKC comprises a superfamily of isoenzymes that is activated in response to various stimuli. Hyperglycaemia induces the activation of different PKC isoforms. However, the PKC-B isoform appears to be preferentially activated by high glucose levels and has been shown to be associated with diabetic vascular complications. In vitro and in vivo animal studies have shown that ruboxistaurin mesylate, a novel selective inhibitor of PKC-B ameliorates the biochemical and functional consequences of PKC activation and may have the potential to reduce the burden of vascular complications associated with diabetes. Results of the first phase-II and phase-III trials evaluating the efficacy of this compound on diabetic microvascular complications have been published recently. They confirm that this compound may favorably influence the evolution of diabetic microvascular complications.

Restoration of ultrastructural and biochemical changes in alloxan-induced diabetic rat sciatic nerve on treatment with Na3VO4 and Trigonella--a promising antidiabetic agent

Vanadium has been reported to have broad pharmacological activity both in vitro and in vivo. Vanadium compound, sodium orthovanadate, Na3VO4, is well known for its hypoglycaemic effects. However, Na3VO4 exerts these effects at relatively high doses (0.6 mg/ml) and exhibit several toxic effects. In the present study lower doses of Na3VO4 (0.2 mg/ml) are combined with Trigonella foenum graecum seed powder (TSP), another hypoglycaemic agent, to reduce its toxicity without compromising its antidiabetic potential. The efficacy of the lower doses of Na3VO4 has been investigated in restoring the altered glucose metabolism and histological structure in the sciatic nerves in 21 and 60 days alloxan diabetic rats. A portion of the glucose was found to be channelled from the normal glycolytic route to polyol pathway, evident by the reduced hexokinase activity and increased polyol pathway enzymes aldose reductase and sorbitol dehydrogenase activity causing accumulation of sorbitol and fructose in diabetic conditions. Ultrastructural observation of the sciatic nerve showed extensive demylination and axonal loss after eight weeks of diabetes induction. Blood glucose levels increased in diabetic rats were normalized with the lower dose of vanadium and Trigonella treatment. The treatment of the diabetic rats with vanadium and Trigonella prevented the activation of the polyol pathway and sugar accumulations. The sciatic nerves were also protected against the structural abnormalities found in diabetes with Trigonella foenum graecum as well as Na3VO4. Results suggest that lower doses of Na3VO4 may be used in combination with TSP as an efficient antidiabetic agent to effectively control the long-term complications of diabetes in tissues like peripheral nerve.

Altered protein kinase C activation associated with rat embryonic dysmorphogenesis

It has been suggested that protein kinase C (PKC) is involved in the etiology of diabetic complications. The aim of the present study was to investigate the putative involvement of different PKC isoforms (alpha, beta1, beta 2, gamma, delta, epsilon, and zeta) in the embryopathy of diabetic rat pregnancy. Embryos were collected from normal and diabetic rats and assayed for PKC activity, PKC mRNA levels, and PKC protein distribution on gestational d 10 and 11. Embryos of diabetic rats showed markers of increased activity of PKC-alpha, PKC-beta1, PKC-gamma, PKC-delta, and PKC-zeta compared with embryos of normal rats on d 10. In addition, the malformed embryos had further increased PKC-gamma, and PKC-delta activity markers compared with nonmalformed embryos of diabetic rats on gestational d 10. In contrast, maternal diabetes caused only two alterations in PKC activity markers on gestational d 11, i.e. both PKC-alpha and PKC-zeta were decreased in embryos of diabetic rats. We found increased mRNA levels of PKC-beta 1 and PKC-zeta on d 10 in embryos of diabetic rats and decreased mRNA levels of PKC-gamma on d 11 in embryos of diabetic rats. Malformed embryos from diabetic rats showed increased distribution of PKC-beta 1 and PKC-beta 2 protein in the tissue compared with nonmalformed embryos from diabetic rats and embryos from normal rats. We conclude that diabetic rat embryopathy may be associated with increased activity and enhanced tissue distribution of several PKC isoforms in early organogenesis.