Abstract
BackgroundPrevious studies in teleost have demonstrated the adaptive strategy to maintain hepatic lipid homeostasis within certain limit. The excess of fat-intake could induce abnormal lipid deposition in liver but not adipose tissue. However, the molecular mechanism between the impaired lipid homeostasis and the aggravated lipid deposition in liver has not been elucidated well in fish.MethodsFour isonitrogenous diets with different fat levels (2, 7, 12 and 17%) were formulated, named L2, L7, L12 and L17 respectively, and fed Chinese perch (44.50 ± 0.25 g) to apparent satiation for five weeks. Growth index, triglyceride concentrations and expression of genes involved in lipid metabolism were measured.ResultsThe maximal growth performance and food intake were observed in L12 group. The lipid content in liver and serum were comparable in L2, L7 and L12 groups, while they were increased significantly in L17 group. Histology analysis also demonstrated that mass lipid droplets emerged in hepatocyte and then induced hepatic steatosis in L17 group. Compared to L2 group, the lipolytic genes related to fatty acids (FAs) transport (lpl & hl) and FAs β-oxidation (cpt1 & cs) were increased in L7 and L12 group. Relative mRNA levels of the gluconeogenesis (pc, pepck & g6pase) were also increased, in contrast, the lipogenic genes (srebp1, accα & fas) were decreased. Compared to L12 group, L17 group had higher mRNA levels of the FAs transport and the lipogenesis. But the lipolytic genes related to FAs β-oxidation were steady and the mRNA levels of gluconeogenesis were down-regulated instead.ConclusionsWithin certain limit, the increase of dietary fat in L7 and L12 group was propitious to reduce the consumption of protein and improve growth performance in Chinese perch. It was due to the homeostasis of hepatic triglyceride (TG) pool and serum glucose through promoting the FAs β-oxidation and gluconeogenesis respectively. Both the increase of lipogenesis and the absence of FAs β-oxidation in L17 group could trigger the esterification of FAs, indeed, the inhibition of gluconeogenesis could also aggravate triglyceride accumulation in liver and induce hepatic steatosis.
Highlights
Previous studies in teleost have demonstrated the adaptive strategy to maintain hepatic lipid homeostasis within certain limit
Because the activation of hepatic mitochondrial oxidation could accelerate the degradation of free fatty acids (FFAs) via carnitine palmitoyltransferase I (CPT1) and release adenosine triphosphate (ATP) for providing energy through the tricarboxylic acid (TCA) cycle [22]
Effects of dietary fat levels on lipid depositional sites The lipid content (LC) of fish body was progressively elevated with the increased dietary fat from 2 to 17% (Fig. 1a)
Summary
Previous studies in teleost have demonstrated the adaptive strategy to maintain hepatic lipid homeostasis within certain limit. The increase of dietary fat level could improve the utilization of feed [6] and protect somehow against the metabolism of protein for energy [7, 8], especially in carnivorous fish species. High fat diets led to the increased fat deposition in fish body, induced metabolic impairments including fatty liver syndrome [9], abnormal oxidative status [10], and altered nutritional value, organoleptic and physical properties [6]. Different fish species have different tolerance on exogenous fat-intake, natural selection endows fish with the abilities to store lipid in different organs when dietary fat is abundant, on the other hand, it accelerates lipid mobilization for providing energy. Because the activation of hepatic mitochondrial oxidation could accelerate the degradation of free fatty acids (FFAs) via carnitine palmitoyltransferase I (CPT1) and release adenosine triphosphate (ATP) for providing energy through the tricarboxylic acid (TCA) cycle [22]
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