ABSTRACT : The aim of our study was to research the effect of overfeeding on plasma parameters and mRNA expression of genes associated with hepatic lipogenesis in the Sichuan white goose and Landes goose. Fifty-four male Landes geese and 57 male Sichuan white geese were hatched on the same day under the same feeding conditions. After overfeeding for 14 days, (1) extrahepatic adipose tissues grew greatly in the Sichuan white geese, while more lipid accumulated in liver tissue in the Landes geese. (2) Sichuan white geese had a higher plasma concentration of triacylglycerols (TG), lipoproteins and insulin than the Landes geese. However, the Landes geese exhibited higher increase of plasma concentrations of TG, lipoproteins and insulin, with greater decrease of the diacylglycerol acyltransferase 2 (DGAT2) activity and DGAT2 mRNA level and a smller decrease of plasma glucose concentration. In addition, the mRNA level of MTP and LPL in liver was down- and up- regulated by overfeeding, respectively. (3) The correlations between the activity of LPL and the proportions of subcutaneous adipose tissue, abdominal adipose tissue, and liver weight, and the plasma concentration of VLDL were different in the two breeds. (4) The proportion of fatty liver weight was positively correlated to plasma concentrations of VLDL and TG in the overfed Sichuan white geese. Such a relationship did not exist in the Landes geese. (5) The activity of DGAT2 and its mRNA abundance in liver had significant negative correlations with the TG content in liver lipid and plasma insulin level in the Landes geese, while in the Sichuan white geese they had negative correlation (p>0.05) with TG concentration in liver lipid and had significant positive correlation with VLDL and TG concentrations in plasma. (Key Words : Fatty Liver, Goose, Lipids, Overfeeding, Hepatic Lipogenesis)
INTRODUCTION
Under natural conditions, birds, especially some wild waterfowl, are more likely to show non-pathological hepatic steatosis as a result of energy storage before migration (Pilo and George, 1983). This specific capability is used for the production of commercial fatty liver in waterfowl production. Little information is available concerning the mechanism of how fatty liver is induced by overfeeding dietary carbohydrate in palmipedes. Fatty liver induced by overfeeding results from intense lipogenesis which most exclusively occurs in the liver caused by dietary carbohydrate (Davail et al., 2000; Davail et al., 2003a; Molee et al., 2005). When overfeeding-induced lipogenesis intensity exceeds the hepatic capacity for lipoprotein synthesis, a large proportion of the triacylglycerols (TG) will be stored in the hepatocytes and may cause dramatic hepatic steatosis (Fournier et al., 1997; Hermier et al., 2003). During overfeeding, part of the newly synthesized TG are incorporated into hepatic lipoprotein, mainly very low density lipoproteins (VLDL) which can be secreted into blood and used (or stored) in extrahepatic tissues. Therefore, it appears that a liver imbalance between lipogenesis and blood secretion of lipids may mainly explain the hepatic fatty deposition particularly developed in some palmipedes. Interestingly, the susceptibility to fatty liver varies in different species, or even different breeds of palmipedes, even among a given group of birds (Hermier et al., 1991; Poujardieu et al., 1994; Hermier et al., 1999). It was indicated that the breed-related differences of fatty liver susceptibility relies, at least partly, on different genetics in hepatic lipid metabolism and, more specifically, in the channelling of fatty acids towards lipoprotein assembly and secretion. At a molecular level, previous research studied the mRNA expression of malic enzyme (ME) in the lipid synthesis pathway (Mourot et al., 2000; Liu et al., 2006). However, there was no report about the pathway of assembly and secretion of lipopretein in the liver of palmipedes, which is another aspect of lipid metabolism.
The present study was designed to elucidate and compare the effects of overfeeding on plasma parameters and mRNA expression of several genes associated with hepatic lipogenesis between Landes goose and Sichuan white goose. Landes goose is famous worldwide for its fatty liver production, and Sichuan white goose has a moderate capability for egg laying and meat production in China. Several plasma parameters (lipoproteins, TG, glucose and insulin) were measured to study their influence on hepatic lipid synthesis and secretion. Meanwhile, the related enzyme activity and gene expression of lipoprotein lipase (LPL), diacylglycerol acyltransferase 2 (DGAT2) and microsomal triglyceride transfer protein (MTP) were studied regarding their regulation of VLDL-TG assembly and secretion. This would be a strong asset to breeding of specialized strains of geese for fatty liver and production of the commercial foiegras.
MATERIALS AND METHODS
Animals and experimental design
Fifty-four male Landes geese and 57 male Sichuan white geese hatched on the same day were grown under natural conditions of light and temperature at the …
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