Department of Bioscience and Biotechnology, Laboratory of Nutrition Chemistry, Faculty of Agriculture, Graduate School, Kyushu University, Fukuoka, Japan.
PLoS One. 2020 Mar 12;15(3):e0229669. doi: 10.1371/journal.pone.0229669. eCollection 2020.
Exogenously hypercholesterolemic (ExHC) rats develop diet-induced hypercholesterolemia (DIHC) when fed with dietary cholesterol. Previously, we reported that, under the high-sucrose-diet-feeding condition, a loss-of-function mutation in Smek2 results in low activity of fatty acid synthase (FAS) followed by the shortage of hepatic triacylglycerol content in ExHC rats and the onset of DIHC. However, the relationship between the Smek2 mutation and FAS dysfunction is still unclear. Here, we focused on carbohydrate metabolism, which provides substrates for FAS, and analyzed carbohydrate and lipid metabolisms in ExHC rats to clarify how the deficit of Smek2 causes DIHC. Male ExHC and SD rats were fed high-sucrose or high-starch diets containing 1% cholesterol for 2 weeks. Serum cholesterol levels of the ExHC rats were higher, regardless of the dietary carbohydrate. Hepatic triacylglycerol levels were higher in only the SD rats fed the high-sucrose diet. Moreover, the ExHC rats exhibited a diabetes-like status and accumulation of hepatic glycogen and low hepatic mRNA levels of liver-type phosphofructokinase (Pfkl), which encodes a rate-limiting enzyme for glycolysis. These results suggest that the glucose utilization, particularly glycolysis, is impaired in the liver of ExHC rats. To evaluate how the diet with extremely low glucose affect to DIHC, ExHC.BN-Dihc2BN, a congenic strain that does not develop DIHC, and ExHC rats were fed a high-fructose diet containing 1% cholesterol for 2 weeks. The serum cholesterol and hepatic triacylglycerol levels were similar in the strains. Results of water-soluble metabolite analysis with primary hepatocytes, an increase in fructose-6-phosphate and decreases in succinate, malate and aspartate in ExHC rats, support impaired glycolysis in the ExHC rats. Thus, the Smek2 mutation causes abnormal hepatic glucose utilization via downregulation of Pfkl expression. This abnormal glucose metabolism disrupts hepatic fatty acid synthesis and causes DIHC in the ExHC rats.
外源性高胆固醇血症（ExHC）大鼠在喂食含胆固醇的饮食后会发展为饮食诱导的高胆固醇血症（DIHC）。之前，我们报道过，在高蔗糖饮食喂养条件下，Smek2 的功能丧失突变导致脂肪酸合酶（FAS）活性降低，继而导致 ExHC 大鼠肝三酰甘油含量不足，并引发 DIHC。然而，Smek2 突变与 FAS 功能障碍之间的关系仍不清楚。在这里，我们专注于为 FAS 提供底物的碳水化合物代谢，并分析 ExHC 大鼠的碳水化合物和脂质代谢，以阐明 Smek2 的缺失如何导致 DIHC。雄性 ExHC 和 SD 大鼠喂食含 1%胆固醇的高蔗糖或高淀粉饮食 2 周。无论饮食中的碳水化合物如何，ExHC 大鼠的血清胆固醇水平都较高。只有喂食高蔗糖饮食的 SD 大鼠的肝三酰甘油水平较高。此外，ExHC 大鼠表现出类似糖尿病的状态，肝糖原积累，肝型磷酸果糖激酶（Pfkl）的 mRNA 水平降低，Pfkl 编码糖酵解的限速酶。这些结果表明，ExHC 大鼠的肝脏葡萄糖利用，特别是糖酵解受损。为了评估极低葡萄糖饮食对 DIHC 的影响，我们喂食 ExHC.BN-Dihc2BN 同源性大鼠（一种不会发展为 DIHC 的同基因品系）和 ExHC 大鼠高果糖含 1%胆固醇饮食 2 周。两种大鼠的血清胆固醇和肝三酰甘油水平相似。原代肝细胞水溶性代谢产物分析结果显示，ExHC 大鼠的果糖-6-磷酸增加，琥珀酸、苹果酸和天冬氨酸减少，支持 ExHC 大鼠糖酵解受损。因此，Smek2 突变通过下调 Pfkl 表达导致异常的肝葡萄糖利用。这种异常的葡萄糖代谢扰乱了肝脂肪酸合成，并导致 ExHC 大鼠发生 DIHC。
