Elovl2 ablation demonstrates that systemic DHA is endogenously produced and is essential for lipid homeostasis in mice

Abstract

The potential role of endogenously synthesized PUFAs is a highly overlooked area. Elongation of very long-chain fatty acids (ELOVLs) in mammals is catalyzed by the ELOVL enzymes to which the PUFA elongase ELOVL2 belongs. To determine its in vivo function, we have investigated how ablation of ELOVL2, which is highly expressed in liver, affects hepatic lipid composition and function in mice. The Elovl2(-/-) mice displayed substantially decreased levels of 22:6(n-3), DHA, and 22:5(n-6), docosapentaenoic acid (DPA) n-6, and an accumulation of 22:5(n-3) and 22:4(n-6) in both liver and serum, showing that ELOVL2 primarily controls the elongation process of PUFAs with 22 carbons to produce 24-carbon precursors for DHA and DPAn-6 formation in vivo. The impaired PUFA levels positively influenced hepatic levels of the key lipogenic transcriptional regulator sterol-regulatory element binding protein 1c (SREBP-1c), as well as its downstream target genes. Surprisingly, the Elovl2(-/-) mice were resistant to hepatic steatosis and diet-induced weight gain, implying that hepatic DHA synthesis via ELOVL2, in addition to controlling de novo lipogenesis, also regulates lipid storage and fat mass expansion in an SREBP-1c-independent fashion. The changes in fatty acid metabolism were reversed by dietary supplementation with DHA.

Keywords: docosahexaenoic acid; elongase of very long-chain fatty acid 2; fatty acid elongase; liver steatosis; weight gain.

Fig. 1.

ELOVL2 is the sole elongase that elongates C22–C24 PUFA in liver. PUFA composition of phospholipid pool (A) and TG pool (B) from liver, as well as total fatty acid composition of serum (C) in wild-type and Elovl2^−/− animals fed standard chow diet. For the % mole composition of the remaining fatty acids, see supplementary Tables III, IV, and V, respectively. Biosynthesis of PUFAs in the liver (D) where ELOVL2 specifically elongates 22:5n-3 and 22:4n-6 fatty acids to form precursors (24:5n-3 and 24:4n-6) for DHA and DPAn-6 formation. Results shown are means ± SEM of 3–6 mice. Statistical significances are shown between wild-type and Elovl2^−/− mice (* P < 0.05, ** P < 0.01, *** P < 0.001, and **** P < 0.0001).

Fig. 2.

Upregulation of lipogenic gene expression does not result in hepatic lipid accumulation in Elovl2^−/− mice fed standard chow diet. Analysis of hepatic gene expression of SREBP-1c, FAS, and SCD1 (A) in wild-type and Elovl2^−/− mice fed standard chow diet. nSREBP-1c protein expression in hepatic nuclear extracts from wild-type and Elovl2^−/− animals fed standard chow diet (B). Microscopic overview (C) of liver tissue from wild-type and Elovl2^−/− mice fed standard chow diet and content of neutral lipids and phospholipids (D). Time course showing REE (E) and mean values of respiratory quotient (RQ) (F) during a 22 h period for wild-type and Elovl2^−/− mice fed standard chow diet. Results shown are means ± SEM of 3–6 mice (exception E and F). Statistical significances are shown between wild-type and Elovl2^−/−. * P < 0.05, ** P < 0.01, and *** P < 0.001.

Fig. 3.

Resistance of Elovl2^−/− mice to diet-induced liver steatosis. PUFA composition of phospholipid pool (A) and TG pool (B) from liver, and total fatty acid composition of serum (C) in wild-type and Elovl2^−/− animals fed high-fat diet for 12 weeks. For the % mole composition of remaining fatty acids, see supplementary Tables III–V. Relative hepatic gene expression of SREBP-1c, FAS, and SCD1 (D) in wild-type and Elovl2^−/− mice fed high-fat diet compared with values in wild-type mice fed chow diet (dotted line). Immunobloting of nSREBP-1c protein in hepatic nuclear extract in wild-type and Elovl2^−/− animals fed high-fat diet compared with values in wild-type mice fed chow diet (dotted line) (E). Microscopic overview (F) of liver tissue from wild-type and Elovl2^−/− fed high-fat diet for 12 weeks and of content of neutral lipids and phospholipids (G). Increase in body weight during 12 weeks of high-fat diet treatment presented as delta values for wild-type and Elovl2^−/− mice (H). Fat content in wild-type and Elovl2^−/− animals fed high-fat diet for 12 weeks measured at two time points: 4 weeks (W4) and 12 weeks (W12) (I). Results shown are means ± SEM of 3–6 mice. Statistical significances are shown between wild-type and Elovl2^−/− mice, as well as different time points (* P < 0.05, ** P < 0.01, *** P < 0.001, and **** P < 0.0001) and versus control values (mice fed chow diet) (# P < 0.05, ## P < 0.01 and ### P < 0.001, #### P < 0.0001).

Fig. 4.

DHA supplementation abolished resistance of weight gain in Elovl2^−/− mice. Total PUFA composition of liver (A) and serum (B) of wild-type and Elovl2^−/− animals fed standard chow diet followed by 2 weeks of high-fat diet (Chow/HF) or prefed for 2 weeks with DHA-enriched diet followed by 2 weeks of high-fat diet (DHA/HF). For the experimental overview, see supplementary Fig. I, and for the % mole composition of remaining fatty acids, see supplementary Tables VI and VII. Relative hepatic gene expression of SREBP-1c, FAS, and SCD1 (C) in wild-type and Elovl2^−/− mice fed for 2 weeks with DHA-enriched diet followed by 2 weeks of high-fat diet (DHA/HF) relative to values in wild-type mice fed chow diet (dotted line). Relative levels of nSREBP-1c protein in wild-type and Elovl2^−/− animals fed for 2 weeks with DHA-enriched diet followed by 2 weeks of high-fat diet (DHA/HF) relative to values in wild-type mice fed chow diet (dotted line) (D). Increase in body weight of wild-type and Elovl2^−/− animals fed standard chow diet followed by 2 weeks of high-fat diet (Chow/HF) or prefed for 2 weeks with DHA-enriched diet followed by 2 weeks of high-fat diet (DHA/HF), presented as mean of delta values after third week of treatment (E). Fat content in wild-type and Elovl2^−/− animals measured at two time points: after 2 weeks prefed with standard chow diet (Chow) or DHA-enriched diet (DHA) followed by 2 weeks of high-fat diet (Chow/HF and DHA/HF), respectively (F). Relative hepatic gene expression of PPARγ (G) and Pck1 (H) in wild-type and Elovl2^−/− mice fed standard chow diet (Chow), standard chow diet followed by 2 weeks of high-fat diet (Chow/HF), or 2 weeks with DHA-enriched diet followed by 2 weeks of high-fat diet (DHA/HF) relative to values in mice fed chow diet. Results shown are means ± SEM of 3–5 mice. Statistical significances are shown between wild-type and Elovl2^−/− mice, as well as different diets (* P < 0.05, ** P < 0.01, *** P < 0.001, and **** P < 0.0001) and versus control values (mice fed chow diet) (# P < 0.05, ## P < 0.01 and ### P < 0.001, #### P < 0.0001).