Abstract
To study the effect of dietary docosahexaenoic acid (DHA) enrichment on the expression of hepatic genes in pigs, weaned, crossbred pigs (30 d old) were fed diets supplemented with either 2% tallow or DHA oil for 18 d. Hepatic mRNA was extracted. Suppression subtractive hybridization was used to explore the hepatic genes that were specifically regulated by dietary DHA enrichment. After subtraction, we observed 288 cDNA fragments differentially expressed in livers from pigs fed either 2% DHA oil or 2% tallow for 18 d. After differential screening, 7 genes were found to be differentially expressed. Serum amyloid A protein 2 (SAA2) was further investigated because of its role in lipid metabolism. Northern analysis indicated that hepatic SAA2 was upregulated by dietary DHA enrichment (p<0.05). In a second experiment, feeding 10% DHA oil for 2d significantly increased the expression of SAA2 (compared to the 10% tallow group; p<0.05). The porcine SAA2 full length cDNA sequence was cloned and the sequence was compared to the human and mouse sequences. The homology of the SAA2 amino acid sequence between pig and human was 73% and between pig and mouse was 62%. There was a considerable difference in SAA2 sequences among these species. Of particular note was a deletion of 8 amino acids, in the pig compared to the human. This fragment is a specific characteristic for the SAA subtype that involved in acute inflammation reaction. Similar to human and mouse, porcine SAA2 was highly expressed in the liver of pigs. It was not detectable in the skeletal muscle, heart muscle, spleen, kidney, lung, and adipose tissue. These data suggest that SAA2 may be involved in mediation of the function of dietary DHA in the liver of the pig, however, the mechanism is not yet clear.
Highlights
Dietary polyunsaturated fatty acids (PUFA) can reduce blood lipid concentration (Rambjor et al, 1996; Harris et al., 1997; Liu et al, 2005), increase insulin sensitivity (Storlien et al, 1998; Suresh and Das, 2003), inhibit the growth of cancer cells (Rose, 1997), and inhibit expression of lipogenic genes (Kim et al, 1999) as well as increase expression of fatty acid oxidation genes (Brandt et al, 1998)to reduce body fat deposition
Dietary PUFA exert their function through expression of many genes and the gene list is increasing as research continues
Previous studies using high docosahexaenoic acid (DHA) algal oil in the diet demonstrated that dietary DHA enrichment suppresses expression of hepatic sterol regulatory element-binding protein1 (SREBP1) in the pig (Hsu et al, 2004; Liu et al, 2005)
Summary
Dietary polyunsaturated fatty acids (PUFA) can reduce blood lipid concentration (Rambjor et al, 1996; Harris et al., 1997; Liu et al, 2005), increase insulin sensitivity (Storlien et al, 1998; Suresh and Das, 2003), inhibit the growth of cancer cells (Rose, 1997), and inhibit expression of lipogenic genes (Kim et al, 1999) as well as increase expression of fatty acid oxidation genes (Brandt et al, 1998). Received February 26, 2006; Accepted May 30, 2006 synthase (FAS) and acetyl coenzyme A carboxylase to increase lipogenic activity and fat deposition (Brown and Goldstein, 1997; Shimano, 2001). These lipogenic genes are inhibited by dietary fish oils in rodents (Xu et al, 1999; 2001). Dietary PUFA reduces body fat deposition by reducing expression of lipogenic genes and increasing expression of fatty acid oxidation genes, PUFA may work through other pathways to regulate lipid metabolism
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