Autophagy dysregulation caused by ApoM deficiency plays an important role in liver lipid metabolic disorder

Autophagy is thought to be a key mechanism in maintaining the balance of liver lipid metabolism. However, the relationship between apolipoprotein M (ApoM) and autophagy has not been reported, and the role of ApoM in triglyceride metabolism is still unclear. In this study, we investigated the correlation between ApoM and autophagy and liver triglyceride metabolism in ApoM-knockout animal and cellular models. First, we observed that spontaneous hepatic steatosis developed in the liver of adult ApoM^?/? mice, which was presented as the accumulation of large quantities of lipid droplets in hepatocytes under electron microscopy; Oil Red O staining showed significant accumulation of triglycerides. At the molecular level, the expression of lipid synthesis-associated proteins (primarily triglyceride synthesis) as well as acetyl-CoA carboxylase alpha (ACACA), fatty acid synthase (FASN) and sterol regulatory element-binding protein 1 (SREBP1) was upregulated. Moreover, lipid metabolic disorder and accumulation were accompanied by dysfunction in autophagy, which displayed predominantly as inhibition of the degradation pathway; for example, P62 protein accumulated and key proteins involved in the initiation of autophagy including ATG7, ATG5-12, Beclin1 and the LC3BII/LC3BI ratio were upregulated as a feedback response. When the autophagy dysfunction was ameliorated by the activation of autophagy pathways induced by starvation, the lipid metabolic disorder was corrected to a certain extent. This suggests that the autophagy dysfunction caused by the deficiency of ApoM is an important factor in hepatic steatosis (triglyceride accumulation). ApoM plays a key role in normal autophagy activity in the liver and thereby further regulates the metabolism of liver lipids, particularly triglycerides.