Casey, Carol A

The liver stores substantial numbers of neutral lipid organelles termed lipid droplets (LDs) that accumulate within hepatocytes in response to chronic ethanol (EtOH) consumption leading to hepatic steatosis. Mass spectrometry analysis of LDs isolated from EtOH-damaged rat livers revealed a substantial reduction in the valosin-containing protein ATPase (VCP/p97) that acts to remove targeted proteins from cellular membranes for degradation. Experimental disruption of VCP function resulted in an increase in LD content in hepatocytes and mouse livers along with a marked increase in LD-associated hydroxysteroid dehydrogenase (HSD17β13) known to contribute to hepatic steatosis. Surprisingly, treatment of hepatocytes with the proteasome inhibitor MG132 had no effect on HSD17β13 levels, while a disruption of lysosome function and chaperone-mediated autophagy increased cellular HSD17β13 levels substantially. These findings provide new insights into the cellular mechanisms by which the liver regulates its lipid stores and how this is disrupted by chronic EtOH exposure.

Lipid droplet (LD) catabolism in hepatocytes is mediated by a combination of lipolysis and a selective autophagic mechanism called lipophagy, but the relative contributions of these seemingly distinct pathways remain unclear. We find that inhibition of lipolysis, lipophagy, or both resulted in similar overall LD content but dramatic differences in LD morphology. Inhibition of the lipolysis enzyme adipose triglyceride lipase (ATGL) resulted in large cytoplasmic LDs, whereas lysosomal inhibition caused the accumulation of numerous small LDs within the cytoplasm and degradative acidic vesicles. Combined inhibition of ATGL and LAL resulted in large LDs, suggesting that lipolysis targets these LDs upstream of lipophagy. Consistent with this, ATGL was enriched in larger-sized LDs, whereas lipophagic vesicles were restricted to small LDs as revealed by immunofluorescence, electron microscopy, and Western blot of size-separated LDs. These findings provide new evidence indicating a synergistic relationship whereby lipolysis targets larger-sized LDs to produce both size-reduced and nascently synthesized small LDs that are amenable for lipophagic internalization.

The liver performs numerous vital functions, including the detoxification of blood before access to the brain while simultaneously secreting and internalizing scores of proteins and lipids to maintain appropriate blood chemistry. Furthermore, the liver also synthesizes and secretes bile to enable the digestion of food. These diverse attributes are all performed by hepatocytes, the parenchymal cells of the liver. As predicted, these cells possess a remarkably well-developed and complex membrane trafficking machinery that is dedicated to moving specific cargos to their correct cellular locations. Importantly, while most epithelial cells secrete nascent proteins directionally toward a single lumen, the hepatocyte secretes both proteins and bile concomitantly at its basolateral and apical domains, respectively. In this Beyond the Cell review, we will detail these central features of the hepatocyte and highlight how membrane transport processes play a key role in healthy liver function and how they are affected by disease.