Lipid Droplet Size Determines Cooperation of Lipolysis and Lipophagy in Hepatocytes.
Authors: Schott Micah B. , Weller Shaun G. , Schulze Ryan , McNiven Mark A. 
Published: June 2021

Fatty liver disease affects 80–100 million Americans and is a precursor to irreversible and sometimes fatal liver damage. The initial stages of fatty liver disease are marked by the hepatocellular accumulation of fat‐storage organelles known as lipid droplets (LDs) that are degraded by two pathways: lipolysis that involves the recruitment of cytosolic lipases (ATGL, HSL) to the LD surface, and lipophagy that relies on autophagic membranes to target and traffic LDs for lysosomal degradation. Importantly, the mechanisms underlying a cooperation between these seemingly distinct pathways are not well understood. Previous studies have shown that the autophagic engulfment of cytosolic cargo is size‐restricted. Because of this, we HYPOTHESIZE that lipolysis and lipophagy occur as sequential processes initiated by cytosolic lipases to reduce LD size and allow for subsequent engulfment by autophagic membranes.

Methods

In AML12 hepatocytes, immunofluorescence microscopy and biochemical approaches were used to determine the enrichment of lipolysis vs lipophagy machinery on large vs. small LDs. Live‐cell microscopy was used for the first time to monitor LDs transitioning from lipolysis to lipophagic engulfment.

Results

Confocal microscopy of AML12 cells immunolabeled for adipose triglyceride lipase (ATGL) revealed a preferential association with large LDs with an average area of 4.0 μm2, seven‐fold larger than LDs not associated with ATGL. In striking comparison, LDs associated with components of the autophagic machinery (LC3) were two‐fold smaller in area (0.39 μm2). Consistent with this observation was the finding that siRNA knockdown of ATGL resulted in the persistence of large LDs (2.8 μm2), whereas knockdown of lysosomal acid lipase (LAL) left cells filled with many LDs of a smaller size (1.5 μm2). These observations were mimicked by inhibitors of ATGL (atglistatin) and lysosomal acidification (chloroquine). In support of these findings, differential centrifugation of AML12 homogenates allowed for separation of two distinct populations of LDs: a buoyant fraction of large LDs enriched in the cytosolic lipase ATGL, and a second collection of smaller, less buoyant LDs enriched in lysosomal enzymes such as Lamp2A and the autophagosome marker LC3‐II. Most interesting are time lapse movies of live cells showing engulfment of small LDs by a LAMP3‐positive late endosomal/lysosomal compartment, suggesting that these organelles play an important role in the direct targeting and engulfment of LDs for lipophagy.

Conclusion

Hepatocytes appear to catabolize LDs in a tandem stepwise process that first utilizes cytosolic lipases to substantially reduce LD size, thereby allowing for subsequent engulfment by autophagosomes as well as late endosomes and lysosomes.

Support or Funding Information

This study was generously supported by funding from NIAAA R01AA020735, NIDDK T32DK007352, and the Mayo Clinic Robert and Arlene Kogod Center on Aging.

This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.