Do polyunsaturated fatty acids ameliorate hepatic steatosis in obese mice by SREPB-1 suppression or by correcting essential fatty acid deficiency.

Abstract

We read with interest the recent report by Sekiya et al. describing the potential of polyunsaturated fatty acids (PUFAs) to ameliorate hepatic steatosis in ob/ob mice.1 They show that dietary PUFAs decreased the mature form of sterol regulatory element-binding protein (SREBP)-1 in these mice, and reduced the expression of several lipogenic enzymes in the livers that may have led to decreased liver triglyceride content and plasma alanine aminotransferase levels (ALT). The authors conclude, “PUFAs ameliorate obesity-associated symptoms, such as hepatic steatosis and insulin resistance, presumably through both down-regulation of SREBP-1 and activation of peroxisome proliferators-activated receptor-α.” The model that Sekiya et al. employ consists of feeding heterozygous Lepob/+ C57BL/6J mice and their wild-type littermates a high-carbohydrate, fat-free chow diet. Experimental animals were supplemented with 15% triolein, 15% triolein plus 5% eicosapentaenoic acid (EPA), 20% fish oil, 0.125% fenofibrate, or 0.05% pioglitazone, for 7 days. Essential fatty acid deficiency (EFAD) can lead to hepatic steatosis, probably due to enhanced lipogenesis and impaired lipid transport.2-4 Essential fatty acid deficiency is typically determined by measuring the Mead acid (20:3n-9) / Arachidonic acid (20:4n-6) ratio (M/A) in serum or tissue. Generally, an M/A greater than 0.2 is associated with EFAD. The data presented by Sekiya et al. show that control animals had a M/A of 0.28, animals supplemented with triolein had a ratio of 0.22, and animals supplemented with pioglitazone had a ratio of 0.41. These animals all have EFAD and the highest degree of steatosis as determined by liver triglyceride content and ALT levels. Conversely, in the animals supplemented with triolein and EPA, and animals supplemented with fish oil, the M/A was 0.18 and 0.07, respectively. EFAD in these animals was thus prevented, as reflected in a decreased amount of liver triglycerides. We developed a model of hepatic steatosis and EFAD by giving wild-type C57BL/6 mice a liquid fat-free, high-carbohydrate diet (HCD) ad libitum for 19 days (Fig. 1). When animals were supplemented with 2.4 g/kg body-weight fish oil containing 7.5–16.9 mg of EPA and 8.6–18.5 mg of docosahexaenoic acid every other day, EFAD and hepatic steatosis were prevented (Fig. 1). This was confirmed by magnetic resonance imaging spectroscopy, revealing liver fat content of 7.2% ± 0.4%, compared to 3.4% ± 0.6% in mice fed a normal diet and 24.1% ± 1.7% in animals fed an HCD only. Furthermore, supplementation with a standard lipid solution containing soybean oil without PUFAs also prevented EFAD and diminished hepatic steatosis (Fig. 1), but only when double the amount of lipid was supplemented. The liver fat content of these animals was 8.1% ± 4.3%. It appears, therefore, that PUFAs are more efficient in preventing hepatic steatosis than standard lipid solutions, as lower doses are required to obtain comparable results. Histology of livers stained with hematoxylin & eosin (200×). (A) Control mice receiving normal rodent chow demonstrate normal liver architecture without evidence of hepatic steatosis. (B) Mice receiving a fat-free, high-carbohydrate diet (HCD)-only show marked microvesicular and macrovesicular steatosis predominantly in midzone and periportal hepatocytes. (C) Mice fed HCD supplemented with fish oil reveal normal liver histology without fatty liver changes. (D) Mice fed HCD supplemented with soybean oil have near normal histology with sparse microvacuoles in the midzone hepatocytes. In conclusion, although it is likely that the beneficial effect of PUFAs on hepatic steatosis in this model is due to SREBP-1 suppression, it cannot be ruled out that prevention of EFAD plays a significant role.