Freshwater Clam Extract Ameliorates Triglyceride and Cholesterol Metabolism through the Expression of Genes Involved in Hepatic Lipogenesis and Cholesterol Degradation in Rats

Abstract

The freshwater clam (Corbicula spp.) is a popular edible bivalve and has been used as a folk remedy for liver disease in Asia. As a Chinese traditional medicine, it is said that freshwater clam ameliorates alcoholic intoxication and cholestasis. In this study, to estimate the practical benefit of freshwater clam extract (FCE), we compared the effects of FCE and soy protein isolate (SPI) on triglyceride and cholesterol metabolism in rats. FCE and SPI lowered serum cholesterol, and FCE tended to reduce serum triglycerides. FCE enhanced fecal sterol excretion and hepatic mRNA levels of CYP7A1 and ABCG5 more substantially than SPI; however, both diets reduced hepatic cholesterol. Both of the diets similarly suppressed liver lipids improved Δ9-desaturated fatty acid profile, and FCE was associated with a reduction in FAS and SCD1 mRNA levels. Hepatic transcriptome analysis revealed that inhibition of lipogenesis-related gene expression may contribute to downregulation of hepatic triglycerides by FCE. FCE would have better potential benefits for preventing metabolic disorders, through greater improvement of metabolism of triglycerides and cholesterol, likely through a mechanism similar to SPI.

Figure 1

Hepatic lipids, index of hepatic elongation of palmitate and palmitoleate, and desaturation index of palmitate and stearate in rats fed FCE and SPI. In rats fed control diet, FCE, or SPI, hepatic lipids were extracted, and triglycerides, cholesterol, and phospholipids were quantified using commercial kits (a). Extracted hepatic lipids were converted to methyl ester, and the fatty acid profile was analyzed using gas chromatography/mass spectrometry. Then, fatty acid ratios were calculated to estimate the elongation of palmitate and palmitoleate (C18:0/C16:0, C18:1/C16:1) and desaturation of palmitate and stearate (C16:1/C16:0, C18:1/C18:0) (b). The statistical significance of differences among values was analyzed by ANOVA and then by Tukey's multiple-range test. Values with different letters indicate a statistically significant difference, P < .05. Each value is expressed as the mean ± SEM for 6 rats in each group (control (C), FCE (F), SPI (S)).

Figure 2

Hepatic levels of the genes involved in cholesterol metabolism, bile acid biosynthesis, and transport and biosynthesis of fatty acids in rats fed FCE and SPI. In the first experiment, the levels of mRNA for CYP7A1, ABCG5, FAS, SCD1, and ELOVL6 were measured by northern blotting. Each value is expressed as the mean ± SEM for 6 rats in each group (control (C), FCE (F), SPI (S)). The statistical significance of differences among values was analyzed by ANOVA and then by Tukey's multiple-range test. Values with different letters indicate a statistically significant difference, P < .05 (a). In the second experiment (control group (C) versus FCE group (F)), the levels of mRNA for ACACA, FABP2, FABP5, apo A-I, and SREBP-1 were measured by northern blotting. Each value is expressed as the mean ± SEM for 6 rats in each group. The statistical significance of differences among values was analyzed by Student's t-test. In each graph, * and *** indicate statistically significant difference at P < .05 and P < .001, respectively (b). ApoE was used as a normalization standard, as its levels were not significantly changed in our experiments. FCE, freshwater clam extract; SPI, soy protein extract; CYP7A1, cytochrome P-450 7A1; FAS, fatty acid synthase; ACACA, acetyl-coenzyme A carboxylase alpha; SCD1, stearoyl-coenzyme A desaturase 1; FABP2, fatty acid-binding protein 2 intestinal; FABP5, fatty acid-binding protein 5 epidermal; SREBP-1, sterol regulatory element-binding protein 1; ABCG5, ATP-binding cassette subfamily G, member 5; ELOVL6, elongation of very long-chain fatty acids protein 6; Apo A-I, apolipoprotein A-1.