Dietary intake of EPA&DHA induced changes in gene expression in the small intestine, including many metabolic genes. Especially genes involved in lipid catabolism were upregulated. In contrast, a large cluster of the genes engaged in cholesterol biosynthesis was downregulated. Importantly, all these effects were specifically induced by long-chain n-3 PUFA, EPA and DHA, as compared with their precursor ALA, and could not be detected in the colon for a selected set of genes.
It is tempting to suggest that increased catabolism of lipids induced by EPA&DHA in the small intestine contributes to the complex and beneficial effects of n-3 PUFA of marine origin. The small intestine mediates the entry of nutritional lipids and is one of the main sites of β-oxidation [32
]. Therefore, an increase in lipid oxidation in the intestine may exert a hypolipidemic effect, i.e. one of the most pronounced effects of EPA and DHA in mammals (reviewed in [1
]). This effect of EPA&DHA in the intestine is surprisingly similar to the enhanced lipid oxidation induced by diacylglycerols versus triacylglycerols [18
]. These two types of treatments both have relatively little effects in liver and muscle ([20
]; own unpublished data). In contrast, in white adipose tissue, intake of EPA&DHA induced genes of fatty acid oxidation, as well as quite specifically, mitochondrial biogenesis [8
]. Taken together, EPA and DHA orchestrate gene expression adaptations in many tissues, including the intestine.
Recent studies addressing the gene expression changes of intestinal tissue upon fish oil or fatty acids, focussed on barrier genes only [16
] or a focussed limited number of genes by qRT-PCR [34
], while here a whole genome approach was used. This allows for detection of changes not only in the most likely pathways, but also in pathways not foreseen. Our study supports the findings by Mori et al. [34
] for the majority of their selected genes analyzed (Cpt1a, Mod1, Pdk4, Hmgcs2, Cyp4a10
, and Acadm
), as well as for barrier gene expression ([16
], data not shown). Unexpectedly, we observed intestinal downregulation of cholesterol biosynthesis due to EPA&DHA, although this is in agreement with a similar effect observed in murine livers after tuna fish oil feeding [11
]. In addition, this might coincide with a possible increase in cholesterol absorption as observed from ScarB1
gene expression, even with identical cholesterol content of the diets. Intracellular homeostasis may decrease cholesterol biosynthesis to counteract increased influx. Maintenance of homeostasis is supported by non differential expression of Soat2
/ACAT, involved in cholesterol esterification and of other genes in cholesterol metabolism (HMG-CoA reductase, Npc1l1
, Mttp, Abcg5, Abcg8, Nr1h2
) and Nr1h3
)). The intestinal lack of regulation of Srebf1
/SREBP further strengthen the observations that PUFA regulation of SREBP that accounts for PUFA-mediated suppression of gene expression seems to be liver-specific [13
]. Furthermore, of the regulatory machine known to be induced by DHA and EPA (PPARs, LXRs, HNF4A, and SREBPs), only PPARα showed differential expression in murine small intestine. This is further supported by our promoter-analysis of the differentially regulated genes, which showed PPARα as the major transcription factor involved.
Moreover, most if not all tissues analysed thus far show an increased energy metabolism upon n-3 FFA, and our results support the notion of the beneficial effects of fish oils independent of its n-3 effect.
Genes engaged in lipid oxidation and ketogenesis are in general upregulated in small intestine [35
], liver [20
], and skeletal muscle [36
] by an increase in dietary fat content. When activated in the muscle, ketogenesis marks a metabolic disconnection between β-oxidation and tricarboxylic acid cycle and could lead to insulin resistance [36
]. In our study however, we compared diets with equal fat content in control and intervention groups, which only differed in their fatty acid composition. This implies that marine PUFAs specifically induce lipid catabolism in intestine. Furthermore, in comparison with another high fat Western diet [35
], we observed similar (up or down) gene expression regulation by EPA&DHA diet (e.g. Angplt4
, and Smpdl3
), as well as an inverse regulation (e.g. ApoC2
are upregulated by a high-fat diet [35
], but downregulated by EPA&DHA). Differences might be explained by the fatty acid content in the diets used.
Despite decreased adiposity due to EPA&DHA in the diet, these fatty acids have not affected total energy intake [8
], and also content of lipids in faeces was unaffected [22
]. This strongly suggests a higher energy expenditure in the animals exposed to EPA&DHA. The results presented here indicate that one of the organs that physiologically contribute to increased oxidation of fatty acids is the small intestine.