FABP3 and FABP10 in Atlantic salmon (Salmo salar L.)- General effects of dietary fatty acid composition and life cycle variations

Abstract

The increased use of dietary plant oil supplementation combined with high dietary lipid loads challenges the lipid transport systems of cultivated fish species. Fatty acid binding proteins (FABPs) have been thoroughly studied as intracellular fatty acid transporters in vertebrates, but no data have been reported in Atlantic salmon. In the present study, comparative characterizations were performed, and dietary influence of plant oil supplementation on FABP3 and FABP10 expression was studied for several tissues in two separate dietary trials. In trial I, groups (6 fish each) were fed diets for 42 weeks (body mass 142 ± 1 to 1463 ± 83 g) (mean ± S.D.), containing graded levels of rapeseed oil substituting for fish oil using a linear regression design. In trial II, groups (3 fish each) were fed 100% fish oil or 100% plant oil for 22 months (0.160 ± 0.052 to 2523 ± 590 g) (mean ± S.D.) and sampled at regular intervals. Liver and muscle tissues appeared to express several FABPs possibly linked to different metabolic functions. FABPs mRNA expression did not change with dietary inclusion of 75% rapeseed oil, whereas FABP3 protein expression seemed to be affected by dietary rapeseed oil inclusion. Significant changes in red muscle FABP3 mRNA expression correlate to significant changes in total β-oxidation capacity during the energy consuming process of smoltification.

Introduction

The substitution of plant oils in diets is used increasingly in aquaculture, due to limited supplies of marine fish oils (FO) (Barlow, 2000). Plant oils have a different fatty acid composition than marine oils, and these differences are believed to challenge lipid metabolism and transport. Functionally, fatty acid-binding proteins (FABPs) are involved in lipid transport and metabolism (Ockner et al., 1972, Spener et al., 1989, Veerkamp et al., 1991, Veerkamp and Maatman, 1995, Massolini and Calleri, 2003), and exhibit differences in ligand specificity (Glatz and van der Vusse, 1990, Veerkamp et al., 1991) as well as affinity for specific fatty acids (Veerkamp et al., 1990). FABPs belong to the conserved multigene family of intracellular lipid binding proteins (iLBPs) that are individual genes arising from an ancestral iLBP gene through gene duplication and diversification (Schaap et al., 2002). The nomenclature for iLBPs suggested by Hertzel and Bernlohr (2000) will be used in this report (Table 1). Phylogenetic analysis suggests that FABP3 (Ando et al., 1998) and FABP10 (Di Pietro et al., 1997) diverged from an FABP progenitor before the divergence of fish and mammals. FABP3 has been characterized in heart ventricle of four Antarctic teleosts (Vayda et al., 1998), in rainbow trout (Oncorhynchus mykiss) (Ando et al., 1998) (Table 2) and in barnacle geese (Branta leucopsis) (Pelsers et al., 1999). FABP10 has been characterized in catfish (Rhamdia sapo) (Di Pietro et al., 1996, Di Pietro et al., 1997), zebrafish (Danio rerio) (Denovan-Wright et al., 2000), axolotl (Ambystoma mexicanum) (Di Pietro et al., 1999), argentine toad (Bufo arenarum) (Di Pietro et al., 2001, Di Pietro et al., 2003), lungfish (Lepidosiren paradoxus) (Di Pietro and Santome, 2001) and chicken (Gallus gallus) (Nichesola et al., 2004, Nolan et al., 2005) (Table 3).

Atlantic salmon both oxidize (Froyland et al., 2000, Torstensen et al., 2000, Stubhaug et al., 2005) and store lipids in red and white muscles (Zhou et al., 1995). Functionally, FABP3 is thought to function as a transport protein for mitochondrial β-oxidation in muscle tissues (Haunerland and Spener, 2004). However, FABP have also been suggested as a transport protein for lipid storage, for genetic variants of FABP3 in pigs (Gerbens et al., 1999, Zeng et al., 2005), and through FABP in vitro overexpression and antisense studies (Makowski and Hotamisligil, 2004 and references therein).

Liver docosahexaenoic acid (DHA) content, but not FABP10 expression, changed with dietary DHA enrichment in Tsaiya duck (Anas platyrhynchos) (Ko et al., 2004). Furthermore, Nolan et al. (2005) recently renamed FABP10 as bile acid binding protein (BABP) on its sequence identity to FABP6 (formerly known as BABP) (Nichesola et al., 2004). FABP6, belonging to the same iLBP subfamily as FABP10 (Haunerland and Spener, 2004), has not been characterized in fish.

Our focus was to characterize FABP3 and FABP10, and to evaluate whether dietary inclusion of rapeseed oil (RO) changed mRNA expression in metabolically active organs like liver, red and white muscle. Furthermore, FABP3 protein expression in heart, red and white muscle was examined. Since Atlantic salmon muscle is used for lipid storage and β-oxidation, FABP3 mRNA expression and its relation to changes in β-oxidation and total lipids between life cycle stages during production life cycle was examined. The possible involvement of liver FABPs as transport proteins to β-oxidation was also examined.