Omega 3 long chain fatty acid synthesis is regulated more by substrate levels than gene expression

The conversion of linoleic acid (LA) and alpha-linolenic acid (ALA) to long chain polyunsaturated fatty acids (LCPUFA) is known to involve desaturation and elongation steps. Although there is evidence that genes for these steps can be regulated by extremes of dietary PUFA, the degree to which there is meaningful regulation of LC PUFA levels in tissues by diet as a result of changes in expression of desaturase and elongase genes is unclear. In this study, we tested the effect of increasing ALA level s in diets of rats from 0.2% to 2.9% energy (en) against a constant LA level (1% en) on plasma and liver phospholipid LC PUFA content together with the expression of hepatic genes involved in PUFA metabolism, the desaturases FADS1 and FADS2, the elongases ELOV2 and ELOV5, and the transcription factors sterol regulatory element-binding protein-1 c (SREBP- 1c) and peroxisome proliferator�activated receptor alpha (PPAR a). The levels of plasma and liver eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA) increased in proportion to dietary ALA whereas docosahexaenoic acid (DHA) increased only up to 1% en ALA. A low PUFA (0.4%en) reference diet stimulated the expression of delta 6 desaturase (FADS 2) and elongase 2 (ELOVL2) when compared to higher PUFA diets. There was, however, no difference in the expression of any of the genes in rats, which were fed diets containing between 0.2% en and 2.9% en ALA and mRNA expression was unrelated to tissue/plasma LCPU FA content. These data suggest that the endogenous synthesis of n- 3 LCPU FA from the precursor ALA is regulated independently of changes in the expression of the synthetic enzymes or regulatory transcription factor, and provides evidence that n- 3 LC PUFA synthesis is regulated more by substrate competition for existing enzymes than by an increase in their mRNA expression. (Author`s abstract)
The regulation of LCPUFA metabolism is poorly understood but is thought to involve substrate competition, nutrients, and nutrient-related hormonal regulation. The pathway from 18 carbon unsaturates to 20 and 22 carbon LCPUFAs involves two desaturase enzymes, the delta 6 desaturase (D6D) and the delta 5 desaturase (D5D), which are coded for by the genes FADS2 and FADS1, respectively. Elongation of 18, 20 and 22 carbon compounds in the pathway occurs via elongase enzymes, which have a regulatory role on LCPUFA synthesis and may be transcribed from one or more genes (ELOVL2 and ELOVL5). Changes in expression levels of the desaturase and the elongase genes involve transcription factors such as peroxisome proliferator-activated receptor alpha (PPAR a) and sterol response element binding protein-1c (SREBP-1c). The activity of desaturase genes is related to the prevailing concentrations of PUFA, and these genes are upregulated in response to low dietary PUFA and suppressed at high PUFA/LCPUFA levels. The extent to which changes in the expression of these genes contribute to the regulation of PUFA metabolism within the physiological range is not known. The aim of this study was to determine whether changes in tissue LCPUFA levels, induced by ALA levels within the range that could reasonably be present in human diets, could be explained by the changes in gene expression of key hepatic enzymes involved in fatty acid metabolism. The results confirmed that a diet containing very low levels of PUFA (0.4% en) resulted in elevation in the expressions of FADS2 and ELOVL2 genes relative to higher PUFA diets. The range of dietary ALA levels tested, in conjunction with a sufficient amount of LA to avoid essential fatty acid (EFA) deficiency (1% en), resulted in the expected alterations in the level of n- 3 LCPUFA in plasma and tissue phospholipids. The liver and plasma fatty acid profiles reported also suggest that there was endogenous conversion of dietary ALA into EPA, DPA, and DHA. Given that the LCPUFA content of tissues was altered by the dietary treatments, and that there was a direct relationship between dietary ALA and the level of n- 3 LCPUFA in cells, endogenous conversion of ALA to LCPUFA is the most likely explanation for the change in tissue LCPUFA content. Although the relationship between dietary ALA content and the amount of product accumulated was essentially linear for EPA and DPA, for DHA there was a curvilinear relationship with a maximum at 1% en dietary ALA. The diets in which the dietary PUFA level was always above 1.2% en resulted in little or no change in the expression of genes that regulate the key enzymes in the fatty acid pathway FADS2, FADS1, ELOVL2, or ELOVL5. The mRNA levels of liver transcription factors PPARa and SREBP-1c had no direct correlation with the liver content of n- 3 LCPUFA induced by changes in dietary ALA.  In summary, this study confirms previous studies showing a direct relationship between dietary ALA and elevated EPA and DPA accumulation in rat plasma and liver. Alterations in DHA levels were more complex, with increasing dietary ALA elevating DHA but only up to a maximum at 1% en ALA. Very low PUFA diets may cause a shift in gene expression of D6D and elongase 2, but when dietary PUFA exceeded 1.2%en and when ALA was in the range 0.2�2.9%en in the diet, there was no effect on the mRNA expression of desaturase and elongase genes or transcription factors involved in hepatic lipid metabolism. There was no relationship between the level of mRNA expression of these hepatic genes and the concentration of PUFA or LCPUFA in either plasma or liver phospholipids. Therefore, at the levels of dietary PUFA used in this experiment, n- 3 LCPUFA levels would seem to be regulated more by competitive interaction between distinct substrates at different stages of LCPUFA biosynthesis and esterification into glycerolipids than alterations in the expression of key components of the lipid metabolism and LCPUFA biosynthetic pathway. (Editor`s comments)