Hypotriacylglycerolemic component of fish oil

Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 57(4 & 5), 387-388

© HarcourtBrace & Co Ltd 1997

Hypotriacylglycerolemic component of fish oil L. Froyland, 1'2 H. Vaagenes, 1 D. K. Asiedu, 1 A. Garras, 1 a . Lie, 2 R. K. Berge ~ 1University of Bergen, Department of Clinical Biology, Division of Biochemistry, Haukeland Hospital, N-5021 Bergen, Norway 2Institute of Nutrition, Directorate of Fisheries, Strandgatten 229, N-5002 Bergen, Norway

Fish oil rich in omega-3 fatty acids have been shown to decrease plasma lipid levels but the underlying mechanism has not yet been fully elucidated. 1-5 This investigation was performed in order to further clarify the effects of purified ethyl esters of eicosapentaenoic acid (EPA-EE) and docosahexaenoic acid (DHA-EE) on lipid metabolism in rats. The animals were fed EPA-EE, DHA-EE, palmitic acid or corn off (1 g/kg/d) by oro-gastric intubation along with a chow background diet for 3 months. At the end, the animals were sacrificed. Plasma and liver lipids were measured, as well as lipid-related enzyme activities and mRNA levels. The fatty acid composition of plasma and different tissues was also determined.

This study shows that, compared to the corn oil control, EPA-EE and DHA-EE lowered plasma cholesterol level whereas only EPA-EE lowered the amount of plasma triacylglycerol (Table 1). In liver peroxisomes, both EE preparations increased fatty acyl-CoA oxidase (FAO) activities and neither altered 3-hydroxy-3methylglutaryl-CoA (HMG-CoA) reductase activities. Morphometric measurements of randomly selected hepatocytes revealed that the areal fraction occupied by fat vacuoles decreased in rats fed EPA-EE compared to controls whereas DHA-EE treatment led to an accumulation of fat droplets in the liver (Table 2). This has also been found in the heart. 6 Of the two omega-3 fatty acids,

Table I C h a n g e s in plasma lipids in rats after prolonged treatment (3 months) with palmitic acid, EPA-EE, DHA-EE and corn oil (control)

Plasma lipids (raM)

Control

EPA-EE

DHA-EE

Palmitic acid

Triacylglycerol Cholesterol

1.81 ~ 0.47 a 1.65 __+.0.22 a

1.05 + 0.46 b 0.95 +__0.14 b

1.74 _+ 0.60 a 1.08 _+ 0.23 b

1.51 _+ 0.30 a 1.52 _ 0.372

T h e plasma lipid values represent means + SD from six animals in each experimental group. M e a n s in a row with a different superscript are significantly different (ANOVA, P < 0.05).

Table 2 Morphometric analysis of fat droplets, peroxisomes and mitochondria in hepatocytes of normal and fatty acid a n a l o g u e treated rats (12 weeks)

Variables

Control

EPA-EE

DHA-EE

Palmitic acid

Fat droplets: Volume fraction (%)

0.9+_0.1 a

0.5 ___0.1 b

1.2___0.10

0.8+0.1 a

1.1 -+0.1 a

1.2_.-0.1 a

1.9-+0.2 b

1.1 -+0.1 ~

11.1 _+ 1.2 a

21.8 ± 2.3 b

17.2 _+ 1.6 °

15.7 _+ 1.4 °

Peroxisomes: Volume fraction (%)

Mitochondria: Volume fraction (%)

The analysis was performed on five micrographs from each animal (n = 6) per treatment group. M e a n s in a row with different superscript are significantly different (ANOVA, P < 0.05).

Correspondence to: Livar Froyland, Institute of Nutrition, Directorate of Fisheries, Strandgatten 229, N-5002 Bergen, Norway

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Table 3 Relative changes in eicosapentaenoic acid (20:5n-3), docosapentaenoic acid (22:5n-3) and docosahexaenoic acid (22:6n-3) in plasma and different tissues of rats treated with EPA-EE, DHA-EE and corn oil (corn) (control) for 3 months

Fatty acid

Treatment Plasma

Liver

Relative change Heart Kidney

Epididymal fat

20:5n-3

Corn EPA-EE DHA-EE

1.0 10.9 4.9

1.0 17.4 8.0

1.0 36.0 6.0

1.0 11.2 6.8

1.0 41.0 7.0

22:5n-3

Corn EPA-EE DHA-EE

1.0 5.0 2.1

1.0 5.3 1.5

1.0 4.4 0.7

1.0 5.3 2.0

1.0 8.3 3.3

22:6n-3

Corn EPA-EE DHA-EE

1.0 1.0 3.7

1.0 1.0 2.6

1.0 0.8 1.6

1.0 0.8 2.8

1.0 2.3 11.3

The values are based on the total fatty acid composition (mol%) and the control is set to 1.0.

DHA-EE especially revealed an increased v o l u m e fraction of peroxisomes (Table 2). At equal doses, DHA-EE seems to be a m o r e potent peroxisome proliferator t h a n EPA-EE. In rats fed EPA-EE, the v o l u m e fraction of mitochondria in the hepatocytes increased c o m p a r e d to controls, whereas DHA-EE feeding h a d less effect (Table 2). Another interesting finding in the present study was the difference in the relative incorporation of EPA-EE and DHA-EE in different rat tissues (Table 3). Feeding DHA-EE increased the a m o u n t of EPA in all tissues to a greater extent t h a n DHA-EE t r e a t m e n t increased the a m o u n t of DHA itself. This indicates that DHA-EE is retroconverted to EPA. However, in rats fed EPA-EE, no significant change in the content of DHA could be observed (Table 3). The fact that DHA is retroconverted to EPA implies that it will be difficult to study the unique effects of DHA. One should, therefore, consider to quantify the levels of EPA and DHA before and after feeding experiments with these two omega-3 fatty acids. Based on these results it seems likely that EPA and DHA possess different metabolic properties, at least in rats. However, a hypotriacylglycerolemic effect has b e e n reported with EPA, b u t not with DHA in h u m a n s / In liver microsomes, EPA-EE raised HMG-CoA reductase and acyl-CoA:cholesterol acyltransferase (ACAT) activities, whereas DHA-EE lowered the former and did not affect the latter. Neither p r o d u c t altered mRNA levels for HMG-CoA reductase, LDL-receptor or LDL-related protein-4. EPA-EE lowered plasma triacylglycerol reflecting lowered VLDL secretion, thus the cholesterol lowering effect in EPA-EE treated rats m a y be secondary to the hypotriacylglycerolemic effect, i.e. an increased fatty acid oxidation with a concomitant decreased triacylglycerol synthesis and release of lipoproteins. An inhibition of

HMG-CoA reductase activity in DHA-EE treated rats m a y contribute to the hypocholesterolemic effect. In s u m m a r y , EPA, and not DHA, seems to be the fatty acid primarily responsible for the triacylglycerolemic effects of fish off. Increased triacylglycerol clearance possibly due to an increased fatty acid oxidation concomitant with mitochondrial proliferation, m a y be the m e c h a n i s m underlying the hypotriacylglycerolemic effect of fish oil. In contrast, DHA, and not EPA, is the fatty acid primarily responsible for the peroxisome proliferating effect of fish off. REFERENCES

1. Harris W. S., Connor W. E., McMurry M. P. The comparative reductions of the plasma lipids and lipoproteins by dietary polyunsaturated fats: salmon off versus vegetable otis. Metabolism 1983; 32: 179-184. 2. Philipson B. E., Rothrock D. W., Connor W. E., Harris W. S., Illingsworth R. D. Reduction of plasma lipids, lipoproteins, and apoproteins by dietary fish oils in patients with hypertriglyceridemia. NEnglJMed 1985; 312: 1210-1216. 3. Wong S., Reardon M., Nestel P. Reduced triglyceride formation from long-chain polyenoic fatty acids in rat hepatocytes. Metabolism 1985; 34: 900-905. 4. Sanders T. A. B., Sullivan D. R., Reeve J., Thompson G. R. Triglyceride-lowering effect of marine polyunsaturates in patients with hypertriglyceridemia. Arteriosclerosis 1985; 5: 459-465. 5. Schmidt E. B., Dyerberg J. Omega-3 fatty acids. Current status in cardiovascular medicine. Drugs 1994; 47: 405-424. 6. Hexeberg S., Froyland L., Asiedu D. K., Demoz A., Berge R. K. Tetradecylthioacetic acid reduces the amount of lipid droplets, induces megamitochondria and increases the fatty acid oxidation in rat heart. JMol Cell Cardio11995; 27: 1851-1857. 7. Rambjor G. S., Wgtlen A. I., Windsor S. L., Harris W. S. Eicosapentaenoic acid is primarily responsible for hypotriglyceridemic effect of fish oil in humans. Lfpfds 1996; 31: S-45-S-49.

Prostaglandins, Leukotrienes and Essential Fatty Acids (1997) 57(4 & 4), 387-388

© Harcourt Brace & Co Ltd 1997