Activation of PI 3-kinase by the hexosamine biosynthesis pathway

High Glucose Enhances Transient Receptor Potential Channel Canonical Type 6–Dependent Calcium Influx in Human Platelets via Phosphatidylinositol 3-Kinase–Dependent Pathway

Background— Transient receptor potential canonical type 6 (TRPC6) channels mediating 1-oleoyl-2-acetyl-sn-glycerol (OAG)–induced calcium entry have been identified on human platelets. In the present study we tested the hypothesis that hyperglycemia increases the expression of TRPC6 channels.

Methods and Results— Platelets from healthy control subjects and patients with type 2 diabetes mellitus were incubated with glucose and calcium influx was measured using the fluorescent dye technique. TRPC channel protein expression was investigated using immunofluorescence and fluorescence microscopy of single platelets. Administration of 25 mmol/L glucose significantly enhanced the OAG-induced calcium influx, which was attenuated by inhibitors of the phosphatidylinositol 3-kinase, wortmannin or LY294002. The glucose-enhanced and OAG-induced calcium influx was concentration- and time-dependent. Glucose significantly increased the TRPC6 protein expression in platelets to 131±12% (n=33; P <0.05), whereas the expression of TRPC1, TRPC3, TRPC4, or TRPC5 were unchanged. The glucose-induced TRPC6 expression was significantly attenuated in the presence of wortmannin or LY294002. Platelets from patients with type 2 diabetes mellitus showed increased TRPC6 expression compared to nondiabetic individuals ( P <0.05).

Conclusion— The study indicates that high glucose increases TRPC6 channel protein expression on the platelet surface which is mediated by a phosphatidylinositol 3-kinase–dependent pathway.

Glucosamine-induced increase in Akt phosphorylation corresponds to increased endoplasmic reticulum stress in astroglial cells

Increased glucose flux through the hexosamine biosynthetic pathway (HBP) is known to affect the activity of a number of signal transduction pathways and lead to insulin resistance. Although widely studied in insulin responsive tissues, the effect of increased HBP activity on largely insulin unresponsive tissues, such as the brain, remains relatively unknown. Herein, we investigate the effects of increased HBP flux on Akt activation in a human astroglial cells line using glucosamine, a compound commonly used to mimic hyperglycemic conditions by increasing HBP flux. Cellular treatment with 8 mM glucosamine resulted in a 96.8% +/- 24.6 increase in Akt phosphorylation after 5 h of treatment that remained elevated throughout the 9-h time course. Glucosamine treatment also resulted in modest increases in global levels of the O-GlcNAc protein modification. Increasing O-GlcNAc levels using the O-GlcNAcase inhibitor streptozotocin (STZ) also increased Akt phosphorylation by 96.8% +/- 11.0 after only 3 h although for a shorter duration than glucosamine; however, the more potent O-GlcNAcase inhibitors O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc) and 1,2-dideoxy-2'-propyl-alpha-D-glucopyranoso-[2,1-d]-Delta2'-thiazoline (NAGBT) failed to mimic the increases in phospho-Akt indicating that the Akt phosphorylation is not a result of increased O-GlcNAc protein modification. Further analysis indicated that this increased phosphorylation was also not due to increased osmotic stress and was not attenuated by N-acetylcysteine eliminating the potential role of oxidative stress in the observed phospho-Akt increases. Glucosamine treatment, but not STZ treatment, did correlate with a large increase in the expression of the endoplasmic reticulum (ER) stress marker GRP 78. Altogether, these results indicate that increased HBP flux in human astroglial cells results in a rapid, short-term phosphorylation of Akt that is likely a result of increased ER stress. The mechanism by which STZ increases Akt phosphorylation, however, remains unknown.

Cell signaling, the essential role of O-GlcNAc!

An increasing body of evidence points to a central regulatory role for glucose in mediating cellular processes and expands the role of glucose well beyond its traditional role(s) in energy metabolism. Recently, it has been recognized that one downstream effector produced from glucose is UDP-GlcNAc. Levels of UDP-GlcNAc, and the subsequent addition of O-linked beta-N-acetylglucosamine (O-GlcNAc) to Ser/Thr residues, is involved in regulating nuclear and cytoplasmic proteins in a manner analogous to protein phosphorylation. O-GlcNAc protein modification is essential for life in mammalian cells, highlighting the importance of this simple post-translational modification in basic cellular regulation. Recent research has highlighted key roles for O-GlcNAc serving as a nutrient sensor in regulating insulin signaling, the cell cycle, and calcium handling, as well as the cellular stress response.

Glucosamine induces rapid desensitization of glucose transport in isolated adipocytes by increasing GlcN-6-P levels

We have examined the hypothesis that glucosamine (GlcN) can rapidly induce insulin resistance through an allosteric mechanism. When insulin-treated adipocytes were exposed to 2mM GlcN, glucose uptake was rapidly reduced by approximately 60% with a T(1/2) of 2 min. We also observed an increase in intracellular GlcN-6-P (at 5 min) from undetectable levels to approximately 260 nmol/g. Continued GlcN treatment resulted in additional accumulation of GlcN-6-P (>1200 nmol/g at 2h), but caused no further decrease in glucose uptake. Although the acute inhibitory action of GlcN could be completely reversed by removing extracellular GlcN, a slow and progressive decrease in insulin-stimulated glucose transport was observed with longer treatment times (T(1/2) of 45 min, 62% loss by 5h). From these data, we conclude that: (1) GlcN elevates intracellular GlcN-6-P levels within minutes, resulting in desensitization of the glucose transport system through allosteric inhibition of hexokinase; (2) prolonged treatment elevates GlcN-6-P to levels that cannot be effectively lowered by cell washing; and (3) residual levels of GlcN-6-P continue to allosterically inhibit glucose uptake, resulting in a slower rate of desensitization that is temporally similar to glucose-induced desensitization, but mechanistically different.

Selective decrease of membrane-associated PKC-alpha and PKC-epsilon in response to elevated intracellular O-GlcNAc levels in transformed human glial cells

Increased flux through the hexosamine biosynthetic pathway (HBP) has been shown to affect the activity and translocation of certain protein kinase C (PKC) isoforms. It has been suggested that this effect is due to increases in the beta-O-linked N-acetylglucosamine (O-GlcNAc) modification. Herein, we demonstrate the effect of increasing the O-GlcNAc modification on the translocation of select PKC isozymes in a human astroglial cell line. Treating cells with either 8 mM d-glucosamine (GlcN), 5 mM streptozotocin (STZ), or 80 muM O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc) produced a significant increase in the O-GlcNAc modification on both cytosolic and membrane proteins; however, both the level and rate of O-GlcNAc increase varied with the compound. GlcN treatment resulted in a rapid, transient translocation of PKC-betaII that was maximal after 3 h (73+/-8%) and also produced a 48+/-15% decrease in membrane-associated PKC-epsilon after 9 h of treatment. Similar to GlcN treatment, STZ and PUGNAc treatment also resulted in decreased levels of PKC-epsilon in the membrane fraction. Significant decreases were seen as early as 5 h and, by 9 h of treatment, had decreased by 87+/-6% with STZ and 73+/-7% with PUGNAc. Unlike GlcN, both STZ and PUGNAc produced a decrease in PKC-alpha membrane levels by 9 h posttreatment (78+/-10% with STZ and 66+/-8% with PUGNAc) while neither compound produced any changes in PKC-betaII translocation. In addition, none of the three compounds affected membrane levels of PKC-iota. Altogether, these results demonstrate a novel link between increased levels of the O-GlcNAc modification and the regulation of specific PKC isoforms.

Peripheral insulin resistance develops in transgenic rats overexpressing phosphoenolpyruvate carboxykinase in the kidney

AIMS/HYPOTHESIS

To study the secondary consequences of impaired suppression of endogenous glucose production (EGP) we have created a transgenic rat overexpressing the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) in the kidney. The aim of this study was to determine whether peripheral insulin resistance develops in these transgenic rats.

METHODS

Whole body rate of glucose disappearance (R(d)) and endogenous glucose production were measured basally and during a euglycaemic/hyperinsulinaemic clamp in phosphoenolpyruvate carboxykinase transgenic and control rats using [6-(3)H]-glucose. Glucose uptake into individual tissues was measured in vivo using 2-[1-(14)C]-deoxyglucose.

RESULTS

Phosphoenolpyruvate carboxykinase transgenic rats were heavier and had increased gonadal and infrarenal fat pad weights. Under basal conditions, endogenous glucose production was similar in phosphoenolpyruvate carboxykinase transgenic and control rats (37.4+/-1.1 vs 34.6+/-2.6 micromol/kg/min). Moderate hyperinsulinaemia (810 pmol/l) completely suppressed EGP in control rats (-0.6+/-5.5 micromol/kg/min, p<0.05) while there was no suppression in phosphoenolpyruvate carboxykinase rats (45.2+/-7.9 micromol/kg/min). Basal R(d) was comparable between PEPCK transgenic and control rats (37.4+/-1.1 vs 34.6+/-2.6 micromol/kg/min) but under insulin-stimulated conditions the increase in R(d) was greater in control compared to phosphoenolpyruvate carboxykinase transgenic rats indicative of insulin resistance (73.4+/-11.2 vs 112.0+/-8.0 micromol/kg/min, p<0.05). Basal glucose uptake was reduced in white and brown adipose tissue, heart and soleus while insulin-stimulated transport was reduced in white and brown adipose tissue, white quadriceps, white gastrocnemius and soleus in phosphoenolpyruvate carboxykinase transgenic compared to control rats. The impairment in both white and brown adipose tissue glucose uptake in phosphoenolpyruvate carboxykinase transgenic rats was associated with a decrease in GLUT4 protein content. In contrast, muscle GLUT4 protein, triglyceride and long-chain acylCoA levels were comparable between PEPCK transgenic and control rats.

CONCLUSIONS/INTERPRETATION

A primary defect in suppression of EGP caused adipose tissue and muscle insulin resistance.