Department of Clinical Nutrition and Food Management, Institute of Biomedical Sciences, Tokushima University Graduate School, Tokushma, Tokushima, Japan.
Department of Clinical Nutrition and Food Management, Institute of Biomedical Sciences, Tokushima University Graduate School, Tokushma, Tokushima, Japan.
J Lipid Res. 2024 Aug;65(8):100598. doi: 10.1016/j.jlr.2024.100598. Epub 2024 Jul 18.
All-trans retinoic acid (atRA), a metabolite of vitamin A, reduces hepatic lipid accumulation in liver steatosis model animals. Lipophagy, a new lipolysis pathway, degrades a lipid droplet (LD) via autophagy in adipose tissue and the liver. We recently found that atRA induces lipophagy in adipocytes. However, it remains unclear whether atRA induces lipophagy in hepatocytes. In this study, we investigated the effects of atRA on lipophagy in Hepa1c1c7 cells and the liver of mice fed a high-fat diet (HFD). First, we confirmed that atRA induced autophagy in Hepa1c1c7 cells by Western blotting and the GFP-LC3-mCherry probe. Next, we evaluated the lipolysis in fatty Hepa1c1c7 cells treated with the knockdown of Atg5, an essential gene in autophagy induction. Atg5-knockdown partly suppressed the atRA-induced lipolysis in fatty Hepa1c1c7 cells. We also found that atRA reduced the protein, but not mRNA, expression of Rubicon, a negative regulator of autophagy, in Hepa1c1c7 cells and the liver of HFD-fed mice. Rubicon-knockdown partly inhibited the atRA-induced lipolysis in fatty Hepa1c1c7 cells. In addition, atRA reduced hepatic Rubicon expression in young mice, but the effect of atRA on it diminished in aged mice. Finally, we investigated the mechanism underlying reduced Rubicon protein expression by atRA in hepatocytes. A protein synthesis inhibitor, but not proteasome or lysosomal inhibitors, significantly blocked the reduction of Rubicon protein expression by atRA in Hepa1c1c7 cells. These results suggest that atRA may promote lipophagy in fatty hepatocytes by reducing hepatic Rubicon expression via inhibiting protein synthesis.
全反式视黄酸（atRA），维生素 A 的代谢产物，可减少肝脂肪变性模型动物的肝脂质积聚。脂噬作用，一种新的脂解途径，通过脂肪组织和肝脏中的自噬作用降解脂滴（LD）。我们最近发现 atRA 可诱导脂肪细胞中的脂噬作用。然而，atRA 是否诱导肝细胞中的脂噬作用尚不清楚。在这项研究中，我们研究了 atRA 对 Hepa1c1c7 细胞和高脂肪饮食（HFD）喂养的小鼠肝脏中脂噬作用的影响。首先，我们通过 Western blot 和 GFP-LC3-mCherry 探针证实 atRA 诱导了 Hepa1c1c7 细胞中的自噬作用。接下来，我们评估了用 Atg5（自噬诱导的必需基因）敲低处理的富含脂肪的 Hepa1c1c7 细胞中的脂肪分解。Atg5 敲低部分抑制了富含脂肪的 Hepa1c1c7 细胞中 atRA 诱导的脂肪分解。我们还发现，atRA 降低了 Hepa1c1c7 细胞和 HFD 喂养小鼠肝脏中 Rubicon（自噬的负调控因子）的蛋白表达，但不影响其 mRNA 表达。Rubicon 敲低部分抑制了富含脂肪的 Hepa1c1c7 细胞中 atRA 诱导的脂肪分解。此外，atRA 降低了年轻小鼠肝脏中的 Rubicon 表达，但在老年小鼠中 atRA 对其的影响减弱。最后，我们研究了 atRA 在肝细胞中降低 Rubicon 蛋白表达的机制。蛋白合成抑制剂而非蛋白酶体或溶酶体抑制剂显著阻断了 Hepa1c1c7 细胞中 atRA 降低 Rubicon 蛋白表达的作用。这些结果表明，atRA 可能通过抑制蛋白合成降低肝 Rubicon 表达，从而促进富含脂肪的肝细胞中的脂噬作用。
