Therefore, the decreased fatty acid oxidation observed in n-3 PUFA-depleted mice may contribute to hepatic steatosis. The in vitro studies using PCLS also revealed a higher capacity to synthesise fatty acids and TG from labelled precursors in the livers of DEF certainly mice compared to those of CT mice. Microarray analysis confirmed a higher expression of all the enzymes involved in fatty acid synthesis in the livers of DEF mice compared to those of CT mice. The SCD-1 overexpression found in DEF mice could explain the increased MUFA content in hepatic PLs and, to a certain extent, the enhanced liver TG secretion observed under n-3 PUFA depletion, as an increased SCD-1 expression is associated with higher liver TG secretion in mice [24].

Numerous studies, both in vitro and in vivo, have demonstrated that n-3 PUFA supplementation decreases lipid synthesis [2] through inhibition of SREBP-1c gene expression and/or through the inhibition of the translocation of the active form of SREBP-1c in the nucleus [4]�C[6], [25]. The ER protein Insig2a keeps SREBP-1c inactive at the ER membranes, and some studies have reported that Insig2a overexpression leads to a reduction of SREBP-1c activation [26]. In our study, we showed that SREBP-1c expression and the nuclear level of the SREBP-1 protein both increased despite a higher Insig2a expression. Insulin is considered to be the classical driver of SREBP-1c activation, which largely explains carbohydrate-induced lipogenesis [20]. In DEF mice, we observed no changes in glycemia and insulinemia.

Euglycemic-hyperinsulinemic clamp revealed a lower inhibitory effect of insulin on hepatic glucose production in DEF mice compared to CT mice. These results demonstrate that n-3 PUFA depletion promotes hepatic insulin resistance. Therefore, SREBP-1c activation may be driven by an insulin-independent pathway. A recent study has suggested that the increased lipogenesis observed in an insulin-resistant state could result from ER stress [22]. To maintain ER function when the secretory pathway is compromised, cells have developed an adaptive mechanism called UPR [27], [28]. Activation of the UPR pathway occurs together with activation of SREBP-1c in the livers of obese ob/ob mice and in nutritional models of steatosis such as hyperhomocysteinaemia or alcohol-fed rodents [22], [29], [30]. IRE1��, ATF6 and PERK are three proximal sensors of ER stress [27], [28].

Some studies have pointed out the importance of IRE1��/X-box binding protein-1 (XBP-1) and PERK pathways activation for the stimulation of lipogenesis in genetic deletion models [31]. Here we did not observe any modifications in the expression and/or the protein content of PDI, GRP94, EDEM1, unspliced XBP-1, spliced XBP-1 or CHOP, all involved in the 3 branches of the UPR. Cilengitide Moreover, eukaryotic translation initiation factor 2�� (eIF2��) phosphorylation was similar between groups.