Effects of hydroxyproline-taurine-cholesterol supplementation on the liver health and lipid metabolisms of turbot Scophthalmus maximus fed Chlorella vulgaris

Microalgae, such as Chlorella vulgaris (CV), possess significant potential as a protein source for aquafeed. However, it has been demonstrated that CV can cause liver damage in carnivorous fish. The present study was aimed at evaluating a combination of hydroxyproline, taurine, and cholesterol (HTC, in a ratio of 1:1:2) for the purpose of alleviating this issue. Seven experimental diets were formulated. The control diet utilized fishmeal as the exclusive protein source. Three CV diets were designed, in which CV substituted 45 %, 60 %, or 75 % of the fishmeal protein. Moreover, three HTC diets were prepared by incorporating 2 % of HTC into the CV diets. Each of the seven diets was fed to three groups of turbot with an initial average weight of 211.6 +/- 1.0 g. The results showed that CV at any assayed level reduced the growth performance and feed intake of turbot and induced inflammatory damage to the liver. CV also disrupted lipid metabolism, as indicated by the significantly lower levels of total triglyceride and total cholesterol in the plasma and bile acids in the liver (P < 0.05). The expression of cyp7 alpha 1, which encodes the rate-limiting enzyme for bile acid synthesis, was significantly reduced by CV (P < 0.05). Concurrent supplementation of HTC with CV completely restored bile acid metabolism. The bile acid levels and the expression of cyp7 alpha 1 in all HTC groups were at the same level as those in the control group (P > 0.05). HTC also increased the total triglyceride and total cholesterol levels in the plasma while reducing the liver lipid content. Moreover, HTC effectively reduced liver injury by alleviating the inflammatory condition and down -regulating the expression of inflammatory-related cytokines induced by CV. In conclusion, the present study provides evidence that the combination of hydroxyproline, taurine, and cholesterol has a hepatic protection function, possibly due to its ability to restore the bile acid status disturbed by CV.