Astroglial Pentose Phosphate Pathway Rates in Response to High-Glucose Environments

Abstract

ROS (reactive oxygen species) play an essential role in the pathophysiology of diabetes, stroke and neurodegenerative disorders. Hyperglycaemia associated with diabetes enhances ROS production and causes oxidative stress in vascular endothelial cells, but adverse effects of either acute or chronic high-glucose environments on brain parenchymal cells remain unclear. The PPP (pentose phosphate pathway) and GSH participate in a major defence mechanism against ROS in brain, and we explored the role and regulation of the astroglial PPP in response to acute and chronic high-glucose environments. PPP activity was measured in cultured neurons and astroglia by determining the difference in rate of 14CO2 production from [1-14C]glucose and [6-14C]glucose. ROS production, mainly H2O2, and GSH were also assessed. Acutely elevated glucose concentrations in the culture media increased PPP activity and GSH level in astroglia, decreasing ROS production. Chronically elevated glucose environments also induced PPP activation. Immunohistochemical analyses revealed that chronic high-glucose environments induced ER (endoplasmic reticulum) stress (presumably through increased hexosamine biosynthetic pathway flux). Nuclear translocation of Nrf2 (nuclear factor-erythroid 2 p45 subunit-related factor 2), which regulates G6PDH (glyceraldehyde-6-phosphate dehydrogenase) by enhancing transcription, was also observed in association with BiP (immunoglobulin heavy-chain-binding protein) expression. Acute and chronic high-glucose environments activated the PPP in astroglia, preventing ROS elevation. Therefore a rapid decrease in glucose level seems to enhance ROS toxicity, perhaps contributing to neural damage when insulin levels given to diabetic patients are not properly calibrated and plasma glucose levels are not adequately maintained. These findings may also explain the lack of evidence for clinical benefits from strict glycaemic control during the acute phase of stroke.

Diabetes Mellitus and Blood-Brain Barrier Dysfunction: An Overview

“…Takahashi et al [128] reported that acutely and chronically induced hyperglycemia increased PPP activity and glutathione (GSH) levels in astrocytic culture, in turn decreasing ROS levels. Higher PPP activity facilitates the regeneration of GSH such that cells are able to combat the higher oxidative stress level (protective role).…”

Glucose administration after traumatic brain injury exerts some benefits and no adverse effects on behavioral and histological outcomes

“…Although the mechanisms by which exogenous glucose improved outcomes are uncertain it is possible that increased blood levels of glucose, as well as increased lactate or pyruvate resulting from peripheral glucose metabolism, ensured that sufficient concentrations of these biofuels were available to meet the increased cerebral metabolic demands after TBI. This could include use of these fuels to meet energy demands within neurons and glia, or increased consumption of glucose by astrocytes to support glycolysis or pentose-phosphate pathway demands, with shuttling of lactate from astrocytes to neurons for use as fuel (Pellerin et al, 2007; Takahashi et al, 2012). Increased shunting of glucose to the pentose-phosphate pathway has been reported after TBI in rats (Bartnik et al, 2005, 2007) and humans (Dusick et al, 2007), and a neuroprotective activation of the pentose-phosphate pathway with increased glutathione production has been reported in mixed astrocyte-neuronal cultures exposed to high glucose levels (Takahashi et al, 2012).…”

“…This could include use of these fuels to meet energy demands within neurons and glia, or increased consumption of glucose by astrocytes to support glycolysis or pentose-phosphate pathway demands, with shuttling of lactate from astrocytes to neurons for use as fuel (Pellerin et al, 2007; Takahashi et al, 2012). Increased shunting of glucose to the pentose-phosphate pathway has been reported after TBI in rats (Bartnik et al, 2005, 2007) and humans (Dusick et al, 2007), and a neuroprotective activation of the pentose-phosphate pathway with increased glutathione production has been reported in mixed astrocyte-neuronal cultures exposed to high glucose levels (Takahashi et al, 2012). We are currently determining the effects of acute glucose treatments on pentose-phosphate pathway activation after experimental TBI.…”

A new HPLC-based assay for the measurement of fructosamine-3-kinase (FN3K) and FN3K-related protein activity in human erythrocytes

“…Together with a phosphatase, it can thus repair the protein glycation caused by the pentose phosphate pathway intermediates ribose-5-phosphate and erythrose-4-phosphate [13]. There is evidence that this pathway, i.e., glucose-6-phosphate-dehydrogenase, is upregulated in the diabetic state to regenerate the NADPH necessary for the defence against oxidative stress [14][15][16]. Therefore, the activity of FN3K-RP may be of importance.…”