Increased Lipogenesis in the Liver of Seawater-Acclimated Nile Tilapia, Oreochromis niloticus

Abstract

Seawater (SW)-acclimatized Oreochromis niloticus (in 27-30 ppt) has been generated for more than 10 generations in a private farm in Thailand, and it was noted that the muscle of the fish was softer than that of the fish reared under freshwater (FW) (in 0-3 ppt). We hypothesized that high lipid content in the muscle might be the cause. Proximate analysis of the muscle from FW and O. niloticus reared in SW revealed significantly higher (P < 0.05) lipid content in the SW fish. The SW fish also had significantly higher hepatosomatic index (HIS), mesenteric adipose tissue somatic index, and lipid content in the liver (all, P < 0.05), compared to the FW fish. Liver histology of the 2 groups revealed hypertrophy of the hepatocytes and a significant increase in the percentage of apoptosis of the hepatocytes, as well as an increasing size of lipid droplets observed by Oil Red-O staining. In the liver, the genes accl, accα, and fas, which are related to lipogenesis, were expressed at significantly higher levels in the SW fish, compared to the FW fish. The genes dgat2, targeting the enzyme catalyzing the last step of triglyceride synthesis, and cd36, targeting a glycoprotein for the transportation of long-chain fatty acids across cell membrane, were also expressed at significantly higher levels in the SW fish. The only gene tested that did not show a difference between the FW and SW fish was scd, which targets the enzyme catalyzing saturated fatty acid to monounsaturated fatty acid. On the contrary, the gene gys1, targeting glycogen synthase, was down-regulated. All the data indicate that O. niloticus reared in SW adapted, or re-differentiated, themselves to cope with a high-salinity environment by increasing liver biosynthesis and storage of lipid.