Integrated Action of Autophagy and Adipose Tissue Triglyceride Lipase Ameliorates Diet-Induced Hepatic Steatosis in Liver-Specific PLIN2 Knockout Mice.

John D Griffin,Eloy Bejarano,Xiang-Dong Wang,Andrew S Greenberg

doi:10.3390/cells10051016

Abstract

An imbalance in the storage and breakdown of hepatic lipid droplet (LD) triglyceride (TAG) leads to hepatic steatosis, a defining feature of non-alcoholic fatty liver disease (NAFLD). The two primary cellular pathways regulating hepatic TAG catabolism are lipolysis, initiated by adipose triglyceride lipase (ATGL), and lipophagy. Each of these processes requires access to the LD surface to initiate LD TAG catabolism. Ablation of perilipin 2 (PLIN2), the most abundant lipid droplet-associated protein in steatotic liver, protects mice from diet-induced NAFLD. However, the mechanisms underlaying this protection are unclear. We tested the contributions of ATGL and lipophagy mediated lipolysis to reduced hepatic TAG in mice with liver-specific PLIN2 deficiency (PLIN2LKO) fed a Western-type diet for 12 weeks. We observed enhanced autophagy in the absence of PLIN2, as determined by ex vivo p62 flux, as well as increased p62- and LC3-positive autophagic vesicles in PLIN2LKO livers and isolated primary hepatocytes. Increased levels of autophagy correlated with significant increases in cellular fatty acid (FA) oxidation in PLIN2LKO hepatocytes. We observed that inhibition of either autophagy or ATGL blunted the increased FA oxidation in PLIN2LKO hepatocytes. Additionally, combined inhibition of ATGL and autophagy reduced FA oxidation to the same extent as treatment with either inhibitor alone. In sum, these studies show that protection against NAFLD in the absence of hepatic PLIN2 is driven by the integrated actions of both ATGL and lipophagy.

Highlights

Hepatic triglyceride (TAG) is stored within cytoplasmic lipid droplets (LD), an organelle responsible for sequestering neutral lipid and regulating its release [1,2]
We find that liver-specific deficiency of perilipin 2 (PLIN2) increases autophagy in vivo and show that the integration action of lipophagy and adipose triglyceride lipase (ATGL)-mediated lipolysis is necessary for increased fatty acids (FA) oxidation following loss of PLIN2
To determine whether hepatic PLIN2 deficiency altered the expression of other LDassociated proteins, we first performed RT-PCR analysis, which demonstrated no difference in Plin1, Plin3, Plin4, Plin5, Fsp27 and Cideb mRNA expression between the two lines of mice during both LFD and Western-type diet (WTD) feeding (Figure 1C)

Summary

Introduction

Hepatic triglyceride (TAG) is stored within cytoplasmic lipid droplets (LD), an organelle responsible for sequestering neutral lipid and regulating its release [1,2]. Lipolysis is thought to be initiated by association of the cytosolic adipose triglyceride lipase (ATGL) with the LD surface, where it catalyzes the release of fatty acids (FA) from stored TAG, generating diacylglycerol and FA. Lipophagy occurs at the surface of the LD, where a network of proteins including Rab family members and autophagy-related family members facilitate the engulfment of LD lipid in specialized degradative vesicles, called autophagosomes. Breakdown of LD TAG contained within the autophagosome occurs when these lipid containing vesicles fuse with lysosomes, exposing their contents to the hydrolase, lysosomal acid lipase

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Conclusion