The aim of the present study was to develop a cell culture system for studying the proliferation and differentiation of preadipocytes isolated from Atlantic salmon adipose tissue. The expression of proliferating cell nuclear antigen (PCNA) was used as a marker for cell proliferation. The cells started to proliferate within 48 h after seeding and continued to proliferate throughout the culture period of 2 wk. Undifferentiated preadipocytes showed a fibroblast-like morphology with a homogeneous cytoplasm devoid of lipid droplets. At confluence, an exogenous lipid mixture was added to the cell cultures. The preadipocytes became larger and rounder during the subsequent days, and the cytoplasm gradually filled with lipid-rich droplets. These droplets were revealed by oil red O staining. Immunocytochemical staining showed that differentiated adipocytes expressed detectable levels of the three regulatory proteins associated with adipocyte differentiation: peroxisome proliferator-activated receptor gamma (PPARgamma), CCAAT/enhancer binding protein alpha (C/EBPalpha), and leptin. The cells also showed activity of glycerol-3-phosphate dehydrogenase (GPDH) (EC 1.1.1.8), a biochemical marker of adipocyte differentiation. The morphological and biochemical data presented here show that fish preadipocytes have properties that are similar to those of preadipocytes in mammals. We conclude therefore that salmon adipose tissue contains a sizable population of preadipocytes. Exogenous lipids promote the activation of adipose-related genes and induce the differentiation of fish preadipocytes in vitro.

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Relative gene expression pattern of fatty acid transport proteins (FATP and cd36), intracellular fatty acid-binding proteins (FABP3, FABP10 and FABP11), b-oxidationrelated genes [carnitine palmitoyl transferase II (CPTII), peroxisome proliferator-activated receptor b (PPARb), acyl-CoA oxidase (AOX), long-chain fatty acyl-CoA synthetase (FACS), acyl-CoA dehydrogenase (dehydrogenase)] and uncoupling protein 2 (UCP2) was assessed by RT-qPCR in Atlantic salmon muscle (red and white), liver, heart, myosepta and visceral fat. FABP11, a FABP isoform not previously described in Atlantic salmon, was highly expressed in visceral fat and myosepta and at the lower level in red muscle, white muscle, myosepta and heart. Furthermore, Atlantic salmon were fed either a diet containing fish oil (FO) or a complete replacement of FO with a vegetable oil blend (55% rapeseed oil, 30% palm oil and 15% linseed oil; VO) for the production cycle (27 months from start of feeding and until 4.5 kg mean weight). The expression of genes related to b-oxidation, fatty acid uptake and transport in the white muscle indicate (n = 3) significant down-regulation in VO fed Atlantic salmon and correlated with previously reported white muscle triacylglycerol stores and b-oxidation. FABP11 in visceral fat and myosepta was also down-regulated in VO fed fish. KEY WORDS

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