Upregulated expression of brain enzymatic markers of arachidonic and docosahexaenoic acid metabolism in a rat model of the metabolic syndrome

BMC Neuroscience Upregulated expression of brain enzymatic markers of arachidonic and docosahexaenoic acid metabolism in a rat model of the metabolic syndrome Ameer Y Taha 1 Fei Gao 1 Epolia Ramadan 1 Yewon Cheon 1 Stanley I Rapoport 1 Hyung-Wook Kim 0 1 0 Department of Environmental and Occupational Health Science, University of Washington , Box 3572341705 Pacific St, Seattle, WA 98195 , USA 1 Brain Physiology and Metabolism Section, National Institute on Aging, National Institutes of Health , Bethesda, MD 20892 , USA Background: In animal models, the metabolic syndrome elicits a cerebral response characterized by altered phospholipid and unesterified fatty acid concentrations and increases in pro-apoptotic inflammatory mediators that may cause synaptic loss and cognitive impairment. We hypothesized that these changes are associated with phospholipase (PLA2) enzymes that regulate arachidonic (AA, 20:4n-6) and docosahexaenoic (DHA, 22:6n-6) acid metabolism, major polyunsaturated fatty acids in brain. Male Wistar rats were fed a control or high-sucrose diet for 8 weeks. Brains were assayed for markers of AA metabolism (calcium-dependent cytosolic cPLA2 IVA and cyclooxygenases), DHA metabolism (calcium-independent iPLA2 VIA and lipoxygenases), brain-derived neurotrophic factor (BDNF), and synaptic integrity (drebrin and synaptophysin). Lipid concentrations were measured in brains subjected to high-energy microwave fixation. Results: The high-sucrose compared with control diet induced insulin resistance, and increased phosphorylatedcPLA2 protein, cPLA2 and iPLA2 activity and 12-lipoxygenase mRNA, but decreased BDNF mRNA and protein, and drebrin mRNA. The concentration of several n-6 fatty acids in ethanolamine glycerophospholipids and lysophosphatidylcholine was increased, as was unesterified AA concentration. Eicosanoid concentrations (prostaglandin E2, thromboxane B2 and leukotriene B4) did not change. Conclusion: These findings show upregulated brain AA and DHA metabolism and reduced BDNF and drebrin, but no changes in eicosanoids, in an animal model of the metabolic syndrome. These changes might contribute to altered synaptic plasticity and cognitive impairment in rats and humans with the metabolic syndrome. Arachidonic acid; Docosahexaenoic acid; BDNF; Brain; Polyunsaturated fatty acids (PUFA); Metabolic syndrome; Drebrin; Sucrose; Insulin resistance - Background The metabolic syndrome is a clinical disorder characterized by obesity, hypertension, dyslipidemia, glucose intolerance and peripheral inflammation [1-3], and is a risk factor for cognitive decline and mood disorders [4-8]. In rodent models of the metabolic syndrome, behavioral abnormalities have been linked to cerebral hypoglycemia [9] and increased cytokine production [10], and changes in brain lipid metabolism [11,12]. The brain is highly enriched with the polyunsaturated fatty acids (PUFAs), arachidonic acid (AA, 20:4n-6) and docosahexaenoic acid (DHA, 22:6n-3) [13-15], which mostly are esterified in the stereospecifically numbered-2 position of membrane phospholipids. AA and DHA are essential for mediating neuroreceptor signaling, while excessive AA is released during neuroinflammation and excitotoxicity [16-19]. Stimulation of AA signaling by glutamatergic, serotonergic, cholinergic or dopaminergic neuroreceptors, among others, triggers AA release by AA-selective Ca2+-dependent cytosolic phospholipase A2 (cPLA2 IVA) (reviewed in [19]). Unesterified AA is a precursor of prostaglandins, thromboxanes, leukotrienes, and related compounds that have important roles in regulating the brains neuroinflammatory response [1315,20-25]. Stimulation of DHA release from membrane phospholipid via DHA-selective calcium-independent iPLA2 type VIA is thought to be neuroprotective, and shows anti-inflammatory effects based on in vitro and in vivo studies [17,26-30]. Disturbed brain AA and DHA metabolism has been linked to a number of neurodegenerative diseases, including Alzheimers disease and bipolar disorder [31-33], which are more common in individuals with the metabolic syndrome [4-8]. Brain lipid metabolism is altered in the metabolic syndrome. In a rat model of intracerebroventricular streptozotocin-induced brain insulin resistance and hypoglycemia, cerebral cortex concentrations of ethanolamine glycerophospholipid (EtnGpl) and phosphatidylserine (PtdSer) were decreased, while concentrations of unesterified palmitate, stearate and AA were increased, suggesting increased PLA2-mediated membrane degradation [11,12]. Brain phospholipid concentration was reported reduced in a genetic mouse model of diabetes [34]. An increased hippocampal malonaldehyde concentration, a marker of PUFA oxidative degradation, was reported in the hippocampus of genetically obese and hypertensive rats [35]. Taken together, these studies suggest an effect of the metabolic syndrome on the enzymes that regulate brain PUFA metabolism, such as AA-selective cPLA2 IVA and iPLA2 VIA, which prefers DHA but also can release AA [36,37]. Unesterified AA can be converted to pro-inflammatory and pro-apoptotic secondary mediators, such as prostaglandin E2 (PGE2), thromboxane B2 (TXB2) 82 and leukotriene B4 (LXB4), via cyclooxygenase-2 (COX-2) or 5, 12 and 15 lipoxygenase (LOX) [17,38]. These eicosanoids can cause synaptic-dendritic injury by reducing brain levels of trophic factors, such as brain-derived neurotrophic factor (BDNF) [39,40]. In this regard, studies reported decreased BDNF and synaptic loss [41-43] in association with cognitive impairment and behavioral changes, in animal models of the metabolic syndrome [10,41,44]. Although iPLA2 VIA can regulate peripheral glucose-stimulated insulin secretion, apoptosis and mitochondrial fatty acid oxidation [45,46], its involvement in modulating brain lipid metabolism in the metabolic syndrome is not known [38]. In view of the reported changes in brain concentrations of phospholipids and PUFAs, and of neuronal loss in animal models of the metabolic syndrome [41-43], we hypothesized correlated disturbances in brain cPLA2 IVA and iPLA2 VIA expression, fatty acid concentrations, synaptic loss, BDNF, and PGE2, TXB2 and LXB4 concentrations. Such changes have been reported in animal models of neuroinflammation [47-49]. To test this hypothesis, we induced early-stage metabolic syndrome by feeding rats a high-sucrose diet for 8 weeks. In this model, feeding a high-sucrose diet induces time-dependent changes in insulin-resistance and in other markers of the metabolic syndrome after 8 weeks [50], without causing diabetic pathology, fatty liver or weight gain [50-52], which may independently alter brain lipid metabolism [53]. Insulin resistance can be induced in this model without changing the fat composition of the diet, thereby eliminating confounding effects of diet on brain fatty acid composition [53]. In sucrose and control diet fed rats maintained for 8 weeks, we examined brain 1) expression of enzymes (...truncated)