Effects of High Starch and Supplementation of an Olive Extract on the Growth Performance, Hepatic Antioxidant Capacity and Lipid Metabolism of Largemouth Bass (Micropterus salmoides)

Abstract

An 8-week feeding trial was conducted to investigate the effects of high-starch diets and the supplementation of an olive extract (OE) on the growth performance, liver health and lipid metabolism of largemouth bass (Micropterus salmoides). Four isonitrogenous and isolipidic diets were prepared: two basal diets containing low (9.0%) and high (14.4%) levels of starch (named as LS and HS), and 0.125% OE was supplemented to each basal diet (named LSOE and HSOE). The results show that high-starch diets had significant negative effects on growth performance, with lower FR, SGR and higher FCR, whereas OE significantly lowered FCR, determined by two-way ANOVA analysis. High-starch diets induced oxidative stress, inflammatory response and liver function injury, with significant increases in the content of plasmatic AKP, AST, ALT, hepatic SOD and MDA, and up-regulation of hepatic TNFα, IL1β, and TGFβ1 gene expression. In addition, a high-starch diet decreased the phosphorylation of AMPK and upregulated the expression of SREBP, together with higher hepatic liver lipid and HSI. The oxidative stress and lipid metabolism disorders indicate metabolic liver disease (MLD) of largemouth bass fed high-starch diets. Feeding on OE-supplemented diets increased the hepatic antioxidant capacity by decreasing the content of MDA and SOD. Fish fed the HSOE diet had an activated phosphorylation of JNK and decreased expression of pro-inflammatory IL1β compared with those fed the HS diet, which strongly indicated that the degree of inflammatory responses was reduced after OE supplementation. Interestingly, this study demonstrated that OE regulates hepatic lipid metabolism in fish by inhibiting the expression of hepatic lipogenesis genes (ACC1 and FASN) and promoting lipolysis (ATGL) and β-oxidation (CPT1α) to prevent TG accumulation. In conclusion, high-starch feed induced oxidative stress and lipid metabolic disorder of largemouth bass, while supplementation with OE improved its antioxidant capacity, anti-inflammatory responses and lipid metabolism. However, hepatic histopathological results suggested that OE supplementation could not completely repair the MLD caused by the high level of starch in largemouth bass.

Keywords: antioxidant; high starch; largemouth bass; lipid metabolism; metabolic liver disease.

Figure 1

Effects of different diets on hepatic histopathological and inflammatory responses of largemouth bass. (A) Four phenotypes of hepatic histopathological examination with symptoms from light to heavy by HE staining for histology examination. Inflammatory response signals of NF-κB were lower (marked by yellow arrows) and mainly (marked by green arrows) expressed in the nucleus (marked with DAPI in blue color) (bar = 15 μm), in which (I) no obvious abnormity, (II) fatty liver, (Ⅲ) nuclear dense tissue, and (IV) hepatic fibrosis symptoms were observed. (B) Statistical results of liver phenotypes (n = 12). Since the samples were damaged during the embedding process, the number of slices was less than 12 of the HS group.

Figure 2

Effects of different diets on hepatic proliferation and inflammatory responses of largemouth bass, (A) Western blot of P-ERK, ERK, P-JNK and JNK in the liver (n = 3). (B) Effects of different diets on the transcriptional levels of hepatic pro- and anti-inflammation-related genes (n = 8). Both one-way ANOVA and two-way ANOVA statistics were analyzed. Differences were regarded as significant when p < 0.05 (n = 8). Values marked with “a, b and c” are significantly different according to one-way ANOVA.

Figure 2

Effects of different diets on hepatic proliferation and inflammatory responses of largemouth bass, (A) Western blot of P-ERK, ERK, P-JNK and JNK in the liver (n = 3). (B) Effects of different diets on the transcriptional levels of hepatic pro- and anti-inflammation-related genes (n = 8). Both one-way ANOVA and two-way ANOVA statistics were analyzed. Differences were regarded as significant when p < 0.05 (n = 8). Values marked with “a, b and c” are significantly different according to one-way ANOVA.

Figure 3

Effects of different diets on the hepatic lipid metabolism of largemouth bass. (A) Western blot of P-AMPK and AMPK in the liver (n = 3). (B) Transcriptional levels of SREBP1 (n = 8). (C) Transcriptional levels of hepatic FA synthesis (ACC1 and FASN), TG hydrolysis (ATGL), and β-oxidation (CPT1α) related genes (n = 8). Both one-way ANOVA and two-way ANOVA statistics were analyzed. Differences were regarded as significant when p < 0.05 (n = 8). Values marked with “a, b and c” are significantly different according to one-way ANOVA.

Figure 3

Effects of different diets on the hepatic lipid metabolism of largemouth bass. (A) Western blot of P-AMPK and AMPK in the liver (n = 3). (B) Transcriptional levels of SREBP1 (n = 8). (C) Transcriptional levels of hepatic FA synthesis (ACC1 and FASN), TG hydrolysis (ATGL), and β-oxidation (CPT1α) related genes (n = 8). Both one-way ANOVA and two-way ANOVA statistics were analyzed. Differences were regarded as significant when p < 0.05 (n = 8). Values marked with “a, b and c” are significantly different according to one-way ANOVA.