The metabolism and distribution of docosapentaenoic acid (n-6) in rats and rat hepatocytes

Bioconversion of Docosapentaenoic Acid in Human Cell Lines, Caco-2, HepG2, and THP-1

Docosapentaenoic acids (DPAs) are long chain polyunsaturated fatty acids that exist as two major structural isomers: n-3 DPA and n-6 DPA. n-3 DPA is found in seal meat, salmon and abalone, and n-6 DPA is found in several marine microbial oil. We investigated the bioconversion of n-3 and n-6 DPAs in three different human cell lines, Caco-2, HepG2, and THP-1. n-3 DPA was converted to docosahexaenoic acid only in HepG2 cells. In contrast, retro-conversion to eicosapentaenoic acid (EPA) was observed in all three cell lines. n-6 DPA was also retro-converted to arachidonic acid (AA) in Caco-2 and HepG2 cells. EPA and AA were particularly elevated in Caco-2 cells, compared to HepG2 cells. Further, the retro-conversion of n-3 DPA led to a greater increase of EPA in the phospholipid fraction than in the neutral lipid fraction.

Metabolic fate of docosahexaenoic acid (DHA; 22:6n-3) in human cells: direct retroconversion of DHA to eicosapentaenoic acid (20:5n-3) dominates over elongation to tetracosahexaenoic acid (24:6n-3)

Docosahexaenoic acid (22:6n-3) supplementation in humans causes eicosapentaenoic acid (20:5n-3) levels to rise in plasma, but not in neural tissue where 22:6n-3 is the major omega-3 in phospholipids. We determined whether neuronal cells (Y79 and SK-N-SH) metabolize 22:6n-3 differently from non-neuronal cells (MCF7 and HepG2). We observed that (13) C-labeled 22:6n-3 was primarily esterified into cell lipids. We also observed that retroconversion of 22:6n-3 to 20:5n-3 was 5- to 6-fold greater in non-neural compared to neural cells and that retroconversion predominated over elongation to tetracosahexaenoic acid (24:6n-3) by 2-5-fold. The putative metabolic intermediates, (13) C-labeled 22:5n-3 and (13) C-labeled 24:5n-3, were not detected in our assays. Analysis of the expression of enzymes involved in fatty acid beta-oxidation revealed that MCF7 cells abundantly expressed the mitochondrial enzymes CPT1A, ECI1, and DECR1, whereas the peroxisomal enzyme ACOX1 was abundant in HepG2 cells, thus suggesting that the initial site of 22:6n-3 oxidation depends on the cell type. Our data reveal that non-neural cells more actively metabolize 22:6n-3 to 20:5n-3 via channeled retroconversion, while neural cells retain 22:6n-3.

Alternative sources of n-3 long-chain polyunsaturated fatty acids in marine microalgae

The main source of n-3 long-chain polyunsaturated fatty acids (LC-PUFA) in human nutrition is currently seafood, especially oily fish. Nonetheless, due to cultural or individual preferences, convenience, geographic location, or awareness of risks associated to fatty fish consumption, the intake of fatty fish is far from supplying the recommended dietary levels. The end result observed in most western countries is not only a low supply of n-3 LC-PUFA, but also an unbalance towards the intake of n-6 fatty acids, resulting mostly from the consumption of vegetable oils. Awareness of the benefits of LC-PUFA in human health has led to the use of fish oils as food supplements. However, there is a need to explore alternatives sources of LC-PUFA, especially those of microbial origin. Microalgae species with potential to accumulate lipids in high amounts and to present elevated levels of n-3 LC-PUFA are known in marine phytoplankton. This review focuses on sources of n-3 LC-PUFA, namely eicosapentaenoic and docosahexaenoic acids, in marine microalgae, as alternatives to fish oils. Based on current literature, examples of marketed products and potentially new species for commercial exploitation are presented.

Orally administered [¹⁴C]DPA and [¹⁴C]DHA are metabolised differently to [¹⁴C]EPA in rats

Previous studies have revealed that C20 PUFA are significantly less oxidised to CO₂ in whole-body studies compared with SFA, MUFA and C18 PUFA. The present study determined the extent to which three long-chain PUFA, namely 20:5n-3 EPA, 22:5n-3 docosapentaenoic acid (DPA) and 22:6n-3 DHA, were catabolised to CO₂ or, conversely, incorporated into tissue lipids. Rats were administered a single oral dose of 2·5 μCi [1-¹⁴C]DPA, [1-¹⁴C]EPA, [1-¹⁴C]DHA or [1-¹⁴C]oleic acid (18:1n-9; OA), and were placed in a metabolism chamber for 6 h where exhaled ¹⁴CO₂ was trapped and counted for radioactivity. Rats were euthanised after 24 h and tissues were removed for analysis of radioactivity in tissue lipids. The results showed that DPA and DHA were catabolised to CO₂ significantly less compared with EPA and OA (P<0·05). The phospholipid (PL) fraction was the most labelled for all three n-3 PUFA compared with OA in all tissues, and there was no difference between C20 and C22 n-3 PUFA in the proportion of label in the PL fraction. The DHA and DPA groups showed significantly more label than the EPA group in both skeletal muscle and heart. In the brain and heart tissue, there was significantly less label in the cholesterol fraction from the C22 n-3 PUFA group compared with the C20 n-3 PUFA group. The higher incorporation of DHA and DPA into the heart and skeletal muscle, compared with EPA, suggests that these C22 n-3 PUFA might play an important role in these tissues.

Altered essential fatty acid metabolism and composition in rat liver, plasma, heart and brain after microalgal DHA addition to the diet

To investigate the effect of docosahexaenoic acid (DHA) without other highly unsaturated fatty acids (HUFA) on n-3 and n-6 essential fatty acid (EFA) metabolism and fatty acid composition in mammals, a stable isotope tracer technique was used in adult rats fed diets with or without 1.3% of algal DHA in a base diet containing 15% of linoleic acid and 3% of alpha-linolenic acid over 8 weeks. The rats were administered orally a mixed oil containing 48 mg/kg body weight of deuterated linoleic and alpha-linolenic acids and euthanized at 4, 8, 24, 96, 168, 240, 360 and 600 h after administration of the isotopes. Fatty acid compositions and the concentrations of deuterated precursors and their respective metabolites were determined in rat liver, plasma, heart and brain as a function of time. DHA, docosapentaenoic acid and eicosapentaenoic acid in the n-3 EFA family were significantly increased in all organs tested in the DHA-fed group, ranging from 5% to 200% greater in comparison with the control group. The accumulation of the metabolites, deuterated-DHA and deuterated-docosapentaenoic acid n-6 was greatly decreased by 1.5- to 2.5-fold in the dietary DHA group. In summary, feeding preformed DHA led to a marked increase in n-3 HUFA content of rat organs at the expense of n-6 HUFA and also prevented the accumulation of newly synthesized deuterated end products. This is the first study which has isolated the effects of DHA on the de novo metabolism on both the n-6 and n-3 EFA pathways.

Artificial rearing with docosahexaenoic acid and n-6 docosapentaenoic acid alters rat tissue fatty acid composition

Docosahexaenoic acid (DHA; 22:6n-3) and n-6 docosapentaenoic acid (DPAn-6; 22:5n-6) are components of enriched animal feed and oil derived from Schizochytrium species microalgae. A one generation, artificial rearing model from day 2 after birth onward (AR) and a dam-reared control group (DAM) were used to examine DPAn-6 feeding on the fatty acid composition of various rat tissues at 15 weeks of age. Four AR diets were based on an n-3 fatty acid-deficient, 18:2n-6-based artificial milk with 22:6n-3 and/or 22:5n-6 added: AR-LA, AR-DHA, AR-DPAn-6, and AR-DHA+DPAn-6. The 22:6n-3 levels for the DAM, AR-DHA, and AR-DHA+DPAn-6 groups tended to be similar and higher than in the AR-LA and AR-DPAn-6 groups. The levels of 22:5n-6 tended to be higher only in the absence of dietary 22:6n-3. Adipose levels of 22:5n-6 was the only exception, as 22:5n-6 was significantly higher in AR-DHA+DPAn-6 than was observed in either the DAM or the AR-DHA group. There were no differences in 20:4n-6 levels within the tissues examined. In conclusion, 22:5n-6 replaces 22:6n-3 in the absence of 22:6n-3 only and does not appear to compete with 22:6n-3 in the presence of dietary 22:6n-3, suggesting that oils containing 22:5n-6 and 22:6n-3 may be a good dietary source of 22:6n-3.

Docosahexaenoic acid and n-6 docosapentaenoic acid supplementation alter rat skeletal muscle fatty acid composition

Background

Docosahexaenoic acid (22:6n-3, DHA) and n-6 docosapentaenoic acid (22:5n-6, DPAn-6) are highly unsaturated fatty acids (HUFA, ≥ 20 carbons, ≥ 3 double bonds) that differ by a single carbon-carbon double bond at the Δ19 position. Membrane 22:6n-3 may support skeletal muscle function through optimal ion pump activity of sarcoplasmic reticulum and electron transport in the mitochondria. Typically n-3 fatty acid deficient feeding trials utilize linoleic acid (18:2n-6, LA) as a comparison group, possibly introducing a lower level of HUFA in addition to n-3 fatty acid deficiency. The use of 22:5n-6 as a dietary control is ideal for determining specific requirements for 22:6n-3 in various physiological processes. The incorporation of dietary 22:5n-6 into rat skeletal muscles has not been demonstrated previously. A one generation, artificial rearing model was utilized to supply 22:6n-3 and/or 22:5n-6 to rats from d2 after birth to adulthood. An n-3 fatty acid deficient, artificial milk with 18:2n-6 was supplemented with 22:6n-3 and/or 22:5n-6 resulting in four artificially reared (AR) dietary groups; AR-LA, AR-DHA, AR-DPAn-6, AR-DHA+DPAn-6. A dam reared group (DAM) was included as an additional control. Animals were sacrificed at 15 wks and soleus, white gastrocnemius and red gastrocnemius muscles were collected for fatty acid analyses.

Results

In all muscles of the DAM group, the concentration of 22:5n-6 was significantly lower than 22:6n-3 concentrations. While 22:5n-6 was elevated in the AR-LA group and the AR-DPAn-6 group, 20:4n-6 tended to be higher in the AR-LA muscles and not in the AR-DPAn-6 muscles. The AR-DHA+DPAn-6 had a slight, but non-significant increase in 22:5n-6 content. In the red gastrocnemius of the AR-DPAn-6 group, 22:5n-6 levels (8.1 ± 2.8 wt. %) did not reciprocally replace the 22:6n-3 levels observed in AR-DHA reared rats (12.2 ± 2.3 wt. %) suggesting a specific preference/requirement for 22:6n-3 in red gastrocnemius.

Conclusion

Dietary 22:5n-6 is incorporated into skeletal muscles and appears to largely compete with 22:6n-3 for incorporation into lipids. In contrast, 18:2n-6 feeding tends to result in elevations of 20:4n-6 and restrained increases of 22:5n-6. As such, 22:5n-6 dietary comparison groups may be useful in elucidating specific requirements for 22:6n-3 to support optimal health and disease prevention.

Lipid and fatty acid profiles in the brain, liver, and stomach contents of neonatal rats: effects of hypoxia

Neonatal hypoxia leads to clinically significant fatty liver, presumably due to disturbances in lipid metabolism. To fully evaluate lipid metabolism, the present study analyzed the complete lipid profile of the brain, liver, and ingested stomach contents of 7-day-old rats exposed to hypoxia from birth. Hypoxia had negligible direct effects on lipid metabolism in the brain. Conversely, hypoxia exhibited direct effects on hepatic lipid metabolism that could not be fully explained by changes in dietary intake. Triacylglyceride concentration was significantly increased in the hypoxic liver but remained unchanged in the brain and stomach contents. Diacylglyceride concentration was increased in both the brain and liver, and this was associated with increased diacylglyceride in the stomach contents. Most n-3 and n-6 fatty acids were increased in the liver, but not in the brain, of hypoxic pups. These changes did not reflect those measured in the stomach contents. Saturated fatty acid concentrations were increased in both the hypoxic brain and liver, and these changes reflected those in the stomach contents. Hypoxia also increased total phospholipid concentration in the brain and stomach contents. We conclude that neonatal hypoxia indirectly affects specific lipid and fatty acid concentrations in the brain and liver through alterations in the absorbed stomach contents. Hypoxia also exhibits some direct affects through modulation of metabolic pathways in situ, mostly in the liver. In this respect, the neonatal brain exhibits tighter control on lipid homeostasis than the liver during neonatal hypoxia.