Kaizong Ma

Half-lives of docosahexaenoic acid in rat brain phospholipids are prolonged by 15 weeks of nutritional deprivation of n-3 polyunsaturated fatty acids

Male rat pups (21 days old) were placed on a diet deficient in n‐3 polyunsaturated fatty acids (PUFAs) or on an n‐3 PUFA adequate diet containing α‐linolenic acid (α‐LNA; 18 : 3n‐3). After 15 weeks on a diet, [4,5‐3H]docosahexaenoic acid (DHA; 22 : 6n‐3) was injected into the right lateral cerebral ventricle, and the rats were killed at fixed times over a period of 60 days. Compared with the adequate diet, 15 weeks of n‐3 PUFA deprivation reduced plasma DHA by 89% and brain DHA by 37%; these DHA concentrations did not change thereafter. In the n‐3 PUFA adequate rats, DHA loss half‐lives, calculated by plotting log10 (DHA radioactivity) against time after tracer injection, equaled 33 days in total brain phospholipid, 23 days in phosphatidylcholine, 32 days in phosphatidylethanolamine, 24 days in phosphatidylinositol and 58 days in phosphatidylserine; all had a decay slope significantly greater than 0 (p < 0.05). In the n‐3 PUFA deprived rats, these half‐lives were prolonged twofold or greater, and calculated rates of DHA loss from brain, Jout, were reduced. Mechanisms must exist in the adult rat brain to minimize DHA metabolic loss, and to do so even more effectively in the face of reduced n‐3 PUFA availability for only 15 weeks.

α-Linolenic acid does not contribute appreciably to docosahexaenoic acid within brain phospholipids of adult rats fed a diet enriched in docosahexaenoic acid

Adult male unanesthetized rats, reared on a diet enriched in both α‐linolenic acid (α‐LNA) and docosahexaenoic acid (DHA), were infused intravenously for 5 min with [1‐14C]α‐LNA. Timed arterial samples were collected until the animals were killed at 5 min and the brain was removed after microwaving. Plasma and brain lipid concentrations and radioactivities were measured. Within plasma lipids, > 99% of radioactivity was in the form of unchanged [1‐14C]α‐LNA. Eighty‐six per cent of brain radioactivity at 5 min was present as β‐oxidation products, whereas the remainder was mainly in ‘stable’ phospholipid or triglyceride as α‐LNA or DHA. Equations derived from kinetic modeling demonstrated that unesterified unlabeled α‐LNA rapidly enters brain from plasma, but that its incorporation into brain phospholipid and triglyceride, as in the form of synthesized DHA, is ≤ 0.2% of the amount that enters the brain. Thus, in rats fed a diet containing large amounts of both α‐LNA and DHA, the α‐LNA that enters brain from plasma largely undergoes β‐oxidation, and is not an appreciable source of DHA within brain phospholipids.

Dietary n-3 PUFA deprivation for 15 weeks upregulates elongase and desaturase expression in rat liver but not brain

Fifteen weeks of dietary n-3 PUFA deprivation increases coefficients of conversion of circulating α-linolenic acid (α-LNA; 18:3n-3) to docosahexaenoic acid (DHA; 22:6n-3) in rat liver but not brain. To determine whether these increases reflect organ differences in enzymatic activities, we examined brain and liver expression of converting enzymes and of two of their transcription factors, peroxisome proliferator-activated receptor α (PPARα) and sterol-regulatory element binding protein-1 (SREBP-1), in rats fed an n-3 PUFA “adequate” (4.6% α-LNA of total fatty acid, no DHA) or “deficient” (0.2% α-LNA, no DHA) diet for 15 weeks after weaning. In rats fed the deficient compared with the adequate diet, mRNA and activity levels of Δ5 and Δ6 desaturases and elongases 2 and 5 were upregulated in liver but not brain, but liver PPARα and SREBP-1 mRNA levels were unchanged. In rats fed the adequate diet, enzyme activities generally were higher in liver than brain. Thus, differences in conversion enzyme expression explain why the liver has a greater capacity to synthesize DHA from circulating α-LNA than does the brain in animals on an adequate n-3 PUFA diet and why liver synthesis capacity is increased by dietary deprivation. These data suggest that liver n-3 PUFA metabolism determines DHA availability to the brain when DHA is absent from the diet.

Disturbed Choline Plasmalogen and Phospholipid Fatty Acid Concentrations in Alzheimer's Disease Prefrontal Cortex

Alzheimers disease (AD) is a progressive neurodegenerative disorder characterized by brain deposition of senile (neuritic) plaques containing amyloid-β, neurofibrillary tangles, synaptic loss, neuroinflammation, and overexpression of arachidonic acid (AA, 20:4n-6) metabolizing enzymes. Lipid concentration changes have been reported in different brain regions, but often partially or as a percent of the total concentration. In this study, we measured absolute concentrations (per gram wet weight) of a wide range of lipids in postmortem prefrontal cortex (Brodmann area 9) from 10 AD patients and 9 non-AD controls. Mean total brain lipid, phospholipid, cholesterol, and triglyceride concentrations did not differ significantly between AD and controls. There was a significant 73% decrease in plasmalogen choline, but no difference in other measured phospholipids. Fatty acid concentrations in total phospholipid did not differ from control. However, docosahexaenoic acid (DHA, 22:6n-3) was reduced in ethanolamine glycerophospholipid and choline glycerophospholipid, but increased in phosphatidylinositol. AA was reduced in choline glycerophospholipid, but increased in phosphatidylinositol, while docosatetraenoic acid (22:4n-6), an AA elongation product, was reduced in total brain lipid, cholesteryl ester and triglyceride. These lipid changes, which suggest extensive membrane remodeling, may contribute to membrane instability and synaptic loss in AD and reflect neuroinflammation.

Dietary n-6 PUFA deprivation for 15 weeks reduces arachidonic acid concentrations while increasing n-3 PUFA concentrations in organs of post-weaning male rats

Few studies have examined effects of feeding animals a diet deficient in n-6 polyunsaturated fatty acids (PUFAs) but with an adequate amount of n-3 PUFAs. To do this, we fed post-weaning male rats a control n-6 and n-3 PUFA adequate diet and an n-6 deficient diet for 15 weeks, and measured stable lipid and fatty acid concentrations in different organs. The deficient diet contained nutritionally essential linoleic acid (LA,18:2n-6) as 2.3% of total fatty acids (10% of the recommended minimum LA requirement for rodents) but no arachidonic acid (AA, 20:4n-6), and an adequate amount (4.8% of total fatty acids) of alpha-linolenic acid (18:3n-3). The deficient compared with adequate diet did not significantly affect body weight, but decreased testis weight by 10%. AA concentration was decreased significantly in serum (-86%), brain (-27%), liver (-68%), heart (-39%), testis (-25%), and epididymal adipose tissue (-77%). Eicosapentaenoic (20:5n-3) and docosahexaenoic acid (22:6n-3) concentrations were increased in all but adipose tissue, and the total monounsaturated fatty acid concentration was increased in all organs. The concentration of 20:3n-9, a marker of LA deficiency, was increased by the deficient diet, and serum concentrations of triacylglycerol, total cholesterol and total phospholipid were reduced. In summary, 15 weeks of dietary n-6 PUFA deficiency with n-3 PUFA adequacy significantly reduced n-6 PUFA concentrations in different organs of male rats, while increasing n-3 PUFA and monounsaturated fatty acid concentrations. This rat model could be used to study metabolic, functional and behavioral effects of dietary n-6 PUFA deficiency.

Low liver conversion rate of α-linolenic to docosahexaenoic acid in awake rats on a high-docosahexaenoate-containing diet

We quantified the rates of incorporation of α-linolenic acid (α-LNA; 18:3n-3) into “stable” lipids (triacylglycerol, phospholipid, cholesteryl ester) and the rate of conversion of α-LNA to docosahexaenoic acid (DHA; 22: 6n-3) in the liver of awake male rats on a high-DHA-containing diet after a 5-min intravenous infusion of [1-14C]α-LNA. At 5 min, 72.7% of liver radioactivity (excluding unesterified fatty acid radioactivity) was in stable lipids, with the remainder in the aqueous compartment. Using our measured specific activity of liver α-LNA-CoA, in the form of the dilution coefficient λα-LNA-CoA, we calculated incorporation rates of unesterified α-LNA into liver triacylglycerol,phospholipid, and cholesteryl ester as 2,401, 749, and 9.6 nmol/s/g × 10−4, respectively, corresponding to turnover rates of 3.2, 8.7, and 2.9%/min and half-lives of 8–24 min. A lower limit for the DHA synthesis rate from α-LNA equaled 15.8 nmol/s/g × 10−4 (0.5% of the net in corporation rate). Thus, in rats on a high-DHA-containing diet, rates of β-oxidation and esterification of α-LNA into stable liver lipids are high, whereas its conversion to DHA is comparatively low and insufficient to supply significant DHA to the brain. High incorporation and turnover rates likely reflect a high secretion rate by liver of stable lipids within very low density lipoproteins.

Rat heart cannot synthesize docosahexaenoic acid from circulating α-linolenic acid because it lacks elongase-2

The extent to which the heart can convert alpha-linolenic acid (alpha-LNA, 18:3n-3) to longer chain n-3 PUFAs is not known. Conversion rates can be measured in vivo using radiolabeled alpha-LNA and a kinetic fatty acid model. [1-(14)C]alpha-LNA was infused intravenously for 5 min in unanesthetized rats that had been fed an n-3 PUFA-adequate [4.6% alpha-LNA, no docosahexaenoic acid (DHA, 22:6n-3)] or n-3 PUFA-deficient diet (0.2% alpha-LNA, nor DHA) for 15 weeks after weaning. Arterial plasma was sampled, as was the heart after high-energy microwaving. Rates of conversion of alpha-LNA to longer chain n-3 PUFAs were low, and DHA was not synthesized at all in the heart. Most alpha-LNA within the heart had been beta-oxidized. In deprived compared with adequate rats, DHA concentrations in plasma and heart were both reduced by >90%, whereas heart and plasma levels of docosapentaenoic acid (DPAn-6, 22:5n-6) were elevated. Dietary deprivation did not affect cardiac mRNA levels of elongase-5 or desaturases Delta6 and Delta5, but elongase-2 mRNA could not be detected. In summary, the rat heart does not synthesize DHA from alpha-LNA, owing to the absence of elongase-2, but must obtain its DHA entirely from plasma. Dietary n-3 PUFA deprivation markedly reduces heart DHA and increases heart DPAn-6, which may make the heart vulnerable to different insults.

Altered fatty acid concentrations in prefrontal cortex of schizophrenic patients

BACKGROUND Disturbances in prefrontal cortex phospholipid and fatty acid composition have been reported in patients with schizophrenia (SCZ), often as an incomplete lipid profile or a percent of total lipid concentration. In this study, we quantified absolute concentrations (nmol/g wet weight) and fractional concentrations (i.e. percent of total fatty acids) of several lipid classes and their constituent fatty acids in postmortem prefrontal cortex of SCZ patients (n = 10) and age-matched controls (n = 10). METHODS Lipids were extracted, fractionated with thin layer chromatography and assayed. RESULTS Mean total lipid, phospholipid, individual phospholipids, plasmalogen, triglyceride and cholesteryl ester concentrations did not differ significantly between the groups. Compared to controls, SCZ brains showed significant increases in several monounsaturated and polyunsaturated fatty acid absolute concentrations in cholesteryl ester. Significant increases or decreases occurred in palmitoleic, linoleic, γ-linolenic and n-3 docosapentaenoic acid absolute concentrations in total lipids, triglycerides or phospholipids. Changes in fractional concentrations did not consistently reflect absolute concentration changes. CONCLUSION These findings suggest disturbed prefrontal cortex fatty acid absolute concentrations, particularly within cholesteryl esters, as a pathological aspect of schizophrenia.

Lithium modifies brain arachidonic and docosahexaenoic metabolism in rat lipopolysaccharide model of neuroinflammation

Neuroinflammation, caused by 6 days of intracerebroventricular infusion of a low dose of lipopolysaccharide (LPS; 0.5 ng/h), stimulates brain arachidonic acid (AA) metabolism in rats, but 6 weeks of lithium pretreatment reduces this effect. To further understand this action of lithium, we measured concentrations of eicosanoids and docosanoids generated from AA and docosahexaenoic acid (DHA), respectively, in high-energy microwaved rat brain using LC/MS/MS and two doses of LPS. In rats fed a lithium-free diet, low (0.5 ng/h)- or high (250 ng/h)-dose LPS compared with artificial cerebrospinal fluid increased brain unesterified AA and prostaglandin E2 concentrations and activities of AA-selective Ca2+-dependent cytosolic phospholipase A2 (cPLA2)-IV and Ca2+-dependent secretory sPLA2. LiCl feeding prevented these increments. Lithium had a significant main effect by increasing brain concentrations of lipoxygenase-derived AA metabolites, 5- hydroxyeicosatetraenoic acid (HETE), 5-oxo-eicosatetranoic acid, and 17-hydroxy-DHA by 1.8-, 4.3- and 1.9-fold compared with control diet. Lithium also increased 15-HETE in high-dose LPS-infused rats. Ca2+-independent iPLA2-VI activity and unesterified DHA and docosapentaenoic acid (22:5n-3) concentrations were unaffected by LPS or lithium. This study demonstrates, for the first time, that lithium can increase brain 17-hydroxy-DHA formation, indicating a new and potentially important therapeutic action of lithium.

Whole-body synthesis-secretion rates of long-chain n-3 PUFAs from circulating unesterified α-linolenic acid in unanesthetized rats

Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), long-chain n-3 PUFAs important for brain and heart function, can be obtained from dietary fish products or by liver synthesis from alpha-linolenic acid (alpha-LNA). Their daily human dietary requirements are not clear, and their liver synthesis rates in humans and nonhumans are unknown. We estimated whole-body (presumably liver) synthesis rates in unanesthetized rats by infusing [U-(13)C]alpha-LNA intravenously for 2 h and measuring labeled and unlabeled n-3 PUFA in arterial plasma using negative chemical ionization GC-MS. Newly synthesized esterified [(13)C]DHA, [(13)C]EPA, and [(13)C]docosapentaenoic acid (DPA) appeared in arterial plasma after 60 min of infusion, then their concentrations rose in an S-shaped manner. Esterified concentration x plasma volume data were fit with a sigmoidal equation, whose peak first derivatives provided synthesis rates of unlabeled EPA, DPA, and DHA equal to 8.40, 6.27, and 9.84 mumol/day, respectively. The DHA synthesis rate exceeded the published daily rat brain DHA consumption rate by 30-fold, suggesting that liver synthesis from alpha-LNA could maintain brain DHA homeostasis were DHA absent from the diet. This stable isotope infusion method could be used to quantify whole-body DHA synthesis rates in human subjects.