Therapeutic potential of nectandrin B, a nutmeg lignan, in nonalcoholic fatty liver disease: Anti‐lipogenic and hepatocyte‐protective effects through AMP‐activated protein kinase and Nrf2 activation

Abstract

Nonalcoholic fatty liver disease is one of the common causes of chronic liver diseases, which can progress to nonalcoholic steatohepatitis and liver cirrhosis. It is well known that oxidative stress contributes to the development and progression of nonalcoholic fatty liver disease. Nectandrin B is a tetrahydrofuran‐type lignan isolated from nutmeg extract. To date, the effects of nectandrin B on lipogenesis and oxidative stress in liver have not been elucidated. Liver X Receptor α (LXRα) and sterol regulatory element‐binding protein (SREBP)‐1c play a crucial role in hepatic de novo lipogenesis. In this study, we investigated the inhibitory effect of nectandrin B on hepatic lipogenesis mediated by LXRα‐SREBP‐1c pathway and the protective effect against oxidative injury induced by tert‐butylhydroperoxide (t‐BHP).Nectandrin B dramatically repressed the transcriptional activity of LXRα and expression of its target gene, SREBP‐1c induced by T0901317, a synthetic LXRα ligand. Nectandrin B also attenuated the increase in SREBP‐1c expression enhanced by insulin plus high glucose, physiological contributing factors to hepatic lipid accumulation. Fatty acid synthesis and acetyl‐coA carboxylase 1 are the main enzymes for de novo lipogenesis and the well‐known targets of LXRα‐SREBP‐1c signaling pathway. They were also suppressed by nectandrin B. Moreover, Oil Red O staining indicated that LXRα agonist‐induced lipid accumulation was notably attenuated by nectandrin B. Nectandrin B enhanced the activities of AMP‐activated protein kinase (AMPK) signaling pathway that has been known to inhibit the activities of LXRα‐SREBP‐1c. On the other hand, nectandrin B protected cell death induced by t‐BHP in both HepG2 cells and primary mouse hepatocytes. Nectandrin B also reduced t‐BHP‐induced production of reactive oxygen species, presumably via the increase in the glutathione level. Both protein and mRNA levels of Nrf2‐regulated antioxidant genes were induced by nectandrin B indicating its antioxidant effect. Interestingly, the cytoprotective effect of nectandrin B against t‐BHP‐induced oxidative damage was abolished by Nrf2 knockdown using Nrf2 specific siRNA thus confirming the important role of Nrf2. Nectandrin B promoted phosphorylation of extracellular signal‐regulated kinase (ERK) and AMPK‐mediated inhibitory phosphorylation of glycogen synthase kinase‐3β (GSK‐3β). Cytoprotection and antioxidant response element (ARE)‐luciferase activity stimulated by nectandrin B were inhibited by the overexpression of dominant‐negative mutant of MEK1 or AMPKα, or wild‐type GSK‐3β. These results suggest that ERK and AMPK‐GSK‐3β signaling are involved in the activation of Nrf2/ARE pathway by nectandrin B.Taken together, nectandrin B has anti‐lipogenic and hepatocyte‐protective abilities through AMPK and Nrf2 activation. We propose the potential of nectandrin B as a candidate for the treatment of nonalcoholic fatty liver disease and the prevention of disease progression.