High fat feeding induces hepatic fatty acid elongation in mice

Abstract

Background: High-fat diets promote hepatic lipid accumulation. Paradoxically, these diets also induce lipogenic gene expression in rodent liver. Whether high expression of these genes actually results in an increased flux through the de novo lipogenic pathway in vivo has not been demonstrated.

Methodology/principal findings: To interrogate this apparent paradox, we have quantified de novo lipogenesis in C57Bl/6J mice fed either chow, a high-fat or a n-3 polyunsaturated fatty acid (PUFA)-enriched high-fat diet. A novel approach based on mass isotopomer distribution analysis (MIDA) following 1-(13)C acetate infusion was applied to simultaneously determine de novo lipogenesis, fatty acid elongation as well as cholesterol synthesis. Furthermore, we measured very low density lipoprotein-triglyceride (VLDL-TG) production rates. High-fat feeding promoted hepatic lipid accumulation and induced the expression of lipogenic and cholesterogenic genes compared to chow-fed mice: induction of gene expression was found to translate into increased oleate synthesis. Interestingly, this higher lipogenic flux (+74 microg/g/h for oleic acid) in mice fed the high-fat diet was mainly due to an increased hepatic elongation of unlabeled palmitate (+66 microg/g/h) rather than to elongation of de novo synthesized palmitate. In addition, fractional cholesterol synthesis was increased, i.e. 5.8+/-0.4% vs. 8.1+/-0.6% for control and high fat-fed animals, respectively. Hepatic VLDL-TG production was not affected by high-fat feeding. Partial replacement of saturated fat by fish oil completely reversed the lipogenic effects of high-fat feeding: hepatic lipogenic and cholesterogenic gene expression levels as well as fatty acid and cholesterol synthesis rates were normalized.

Conclusions/significance: High-fat feeding induces hepatic fatty acid synthesis in mice, by chain elongation and subsequent desaturation rather than de novo synthesis, while VLDL-TG output remains unaffected. Suppression of lipogenic fluxes by fish oil prevents from high fat diet-induced hepatic steatosis in mice.

Figure 1. Hepatic lipogenic gene expressions.

Data were calculated relative to the expression of 18S and normalized for expression levels of control mice on chow. Srebp-1c, sterol regulatory element binding protein 1c; Pgc-1β, peroxisome proliferator activated receptor gamma co-activator 1 beta; Acc, acetyl-CoA carboxylase; Fas, fatty acid synthase; Elovl6, fatty acid elongase 6; Scd1, stearoyl-CoA desaturase 1; Dgat, diacylglycerol acyltransferase; Gpat, glycerol-3-phosphate acyltransferase. White bars represent chow diet; black bars represent high-fat diet and grey bars represent high-fat/fish oil diet. Values are given as means±SEM for n = 6/7; * p<0.05 high-fat vs. chow; # p<0.05 high-fat/fish oil vs. high-fat.

Figure 2. Hepatic lipid content.

A, Hepatic triglyceride content. B, Hepatic cholesterol ester content. C, Hepatic free cholesterol content. D, Hepatic phospholipid content. White bars represent chow diet; black bars represent high-fat diet and grey bars represent high-fat/fish oil diet. Values are given as means±SEM for n = 6/7; * p<0.05 high-fat vs. chow; # p<0.05 high-fat/fish oil vs. high-fat.

Figure 3. Hepatic fatty acid synthesis.

A, Fractional palmitate synthesis from de novo lipogenesis (C16:0*). B, Absolute palmitate synthesis from de novo lipogenesis (C16:0*). C, Fractional stearate (C18:0) and oleate (C18:1) synthesis from elongation of de novo synthesized (C16:0*) and pre-existing (C16:0) palmitate. D, Absolute stearate (C18:0) and oleate (C18:1) synthesis from elongation of de novo synthesized (C16:0*) and pre-existing (C16:0) palmitate. White bars represent chow diet, black bars represent high-fat diet and grey bars represent high-fat/fish oil diet. Plain bars represent synthesis from elongation of de novo synthesized palmitate and dashed bars represent synthesis from elongation of pre-existing palmitate. Absolute synthesis rates are expressed as micrograms per gram liver per hour. Values are given as means±SEM for n = 5–7; * p<0.05 high-fat vs. chow; # p<0.05 high-fat/fish oil vs. high-fat.

Figure 4. Cholesterol metabolism.

A, Hepatic cholesterogenic gene expression. Cholesterogenic gene expression levels were calculated relative to the expression of 18S and normalized for expression levels of control mice on chow. Srebp-2, sterol regulatory element binding protein-2; Hmgs (cyto), 3-hydroxy-3-methylglutaryl-CoA synthase 1; Hmgr, 3-Hydroxy-3-methylglutaryl-CoA reductase; Acat, acyl-CoA:cholesterol acyltransferase. B, Fractional synthesis rates. White bars represent chow diet, black bars represent high-fat diet and grey bars represent high-fat/fish oil diet. Values are given as means±SEM for n = 5–7; * p<0.05 high-fat vs. chow; # p<0.05 high-fat/fish oil vs. high-fat.

Figure 5. Hepatic very low density lipoprotein (VLDL) secretion.

A, Plasma TG concentrations and B, VLDL-TG production rates. White bullets and bars represent chow diet, black bullets and bars represent high-fat diet and grey bullets and bars represent high-fat/fish oil diet. Values are given as means±SEM for n = 7–8; * p<0.05 high-fat vs. chow; # p<0.05 high-fat/fish oil vs. high-fat.