a1 Department of Basic Sciences and Aquatic Medicine, Aquaculture Protein Centre (a CoE), Norwegian School of Veterinary Science, PO Box 8146 Dep, NO-0033 Oslo, Norway
Inclusion of plant protein sources such as soyabean meal (SBM) in aquafeeds is associated with decreased lipid digestibility, reduced bile acid levels and hypocholesterolaemia. The mechanism for these metabolic abnormalities is unknown. The present study aimed at gaining further insight into how cholesterol and bile acid metabolism is modulated by SBM feeding by quantifying a number of mRNA species corresponding to key proteins involved in cholesterol and bile acid metabolism using quantitative real-time PCR. A 21 d feeding trial with sequential sampling at ten time points following initiation of 20 % SBM exposure was conducted on Atlantic salmon. A histological evaluation confirmed distal intestinal enteritis after 5 d of dietary exposure to the SBM, whereas diminished glycogen/lipid deposition was the only relevant finding observed in the liver. SBM inclusion resulted in reduced body pools of cholesterol and bile acids. Hepatic gene expression profiles revealed up-regulation of genes encoding rate-limiting enzymes in cholesterol (3-hydroxy-3-methyl-glutaryl-CoA reductase; HMGCR) and bile acid (cytochrome P4507A1 (CYP7A1)) biosynthesis, as well as up-regulation of their associated transcription factors (sterol regulatory element binding proteins 1 and 2, liver X receptor, farnesoid X receptor and PPAR isoforms). Hepatic gene expressions of cholesterol (ATP binding cassette G5 (ABCG5)) and bile acid (ATP binding cassette B11 (ABCB11)) transporters were, by and large, not influenced by the SBM, but distal intestinal expression patterns of ABCG5 and apical Na-dependent bile acid transporter indicated impaired cholesterol and bile acid reabsorption. In conclusion, hepatic gene expression profiles indicated that the capacity for cholesterol and bile acid synthesis was up-regulated, whereas the indicated impaired cholesterol and bile acid reabsorption probably occurred as a direct result of distal intestinal inflammation.
(Received January 11 2012)
(Revised April 20 2012)
(Accepted April 22 2012)
(Online publication May 30 2012)
Abbreviations: ABCG5, ATP binding cassette G5; ABCB11, ATP binding cassette B11; ACTB, β-actin; ASBT, apical Na-dependent bile acid transporter; DI, distal intestine; FM, fishmeal; FXR, farnesoid X receptor; HMGCR, 3-hydroxy-3-methyl-glutaryl-CoA reductase; LXR, liver X receptor; PCNA, proliferating cell nuclear antigen; PI, pyloric intestine; qPCR, quantitative real-time PCR; SBM, soyabean meal; SREBP, sterol regulatory element binding protein