New insights into lipid droplet breakdown in alcohol-associated hepatic steatosis

Lipid droplets (LDs) are organelles composed of a core of neutral lipids, primarily triglycerides and sterol esters, which are surrounded by a monolayer of phospholipids containing embedded proteins (1). LDs are formed to store excess energy and prevent lipotoxicity (1). However, when LDs are dysregulated or accumulate excessively, they can cause damage to cellular membranes, lead to cell death, and trigger inflammation. These features are characteristic of alcohol-associated liver disease (ALD) and metabolic dysfunction–associated fatty liver disease (MASLD), two of the most prevalent chronic liver diseases worldwide (2, 3). Therefore, understanding the mechanisms that regulate hepatic LD homeostasis is crucial for developing better management strategies for ALD and MASLD.

p97, also known as valosin-containing protein (VCP), belongs to the AAA⁺ ATPase family and functions as a molecular chaperone while playing a crucial role in the ubiquitin–proteasome system. p97/VCP interacts with adaptor proteins to process ubiquitinated substrates, facilitating the degradation of proteins marked for removal (4). 17β-hydroxysteroid dehydrogenase 13 (HSD17β13) is a liver-specific, LD-associated NAD⁺-dependent oxidoreductase. Loss-of-function mutations in the HSD17β13 gene are associated with a reduced risk of chronic liver diseases, including MASLD, cirrhosis, and hepatocellular carcinoma (5). Whether and how p97/VCP regulates HSD17β13 in LD homeostasis, particularly in ALD, remains unknown.

In this Journal of Cell Biology issue, Sen et al. conducted a comprehensive study to characterize a novel role of p97/VCP in regulating hepatic LD catabolism and its implications in hepatic steatosis in ALD (6). They first conducted an unbiased proteomic screen using isolated LDs from ethanol-fed rat livers and found a remarkable decrease in p97/VCP, along with an increase in HSD17β13 levels on LDs. The changes in p97/VCP and HSD17β13 on the LDs were further validated through western blot analysis, which confirmed an almost complete depletion of p97/VCP, accompanied by a marked increase in HSD17β13 in isolated LD fractions, without affecting the total hepatic levels of p97/VCP and HSD17β13 in ethanol-fed rat livers. More importantly, the genetic knockdown of p97/VCP using siRNA or the pharmacological inhibition of p97/VCP by DBeQ was sufficient to increase the number of LDs in hepatocytes cultured under standard medium. Similarly, hepatocyte-specific deletion of p97/VCP in mouse livers also leads to the development of hepatic steatosis even when fed on a chow diet. The overexpression of HSD17β13 alone is sufficient to increase LDs in cultured hepatocytes by increased retention of LDs independent of its enzymatic activity, supporting a role of HSD17β13 in suppressing LD catabolism. Mechanistically, p97/VCP acts as a co-chaperone that facilitates the delivery of HSD17β3 to lysosomes through chaperone-mediated autophagy (CMA). This process occurs via its interaction with heat shock cognate protein 70 (HSC70) and LAMP2A in the lysosomes. Notably, HSD17β3 colocalizes with the lysosomal marker LAMP1, and knocking down Lamp2a leads to increased levels of HSD17β13. CMA substrate proteins contain the KFERQ motif, which is also found in the HSD17β13 protein. Mutating this motif at the carboxyl terminus disrupts the interaction between HSD17β13 and HSC70, decreasing HSD17β13 protein turnover. Moreover, cells treated with the lysosomal inhibitor chloroquine but not a proteasome inhibitor MG132 increased HSD17β13 levels, further supporting the idea that p97/VCP regulates the degradation of HSD17β13 from the LD surface via CMA rather than the proteasome.

This study has significantly advanced our understanding of ALD by describing a novel mechanism by which p97/VCP regulates LD homeostasis through the degradation of HSD17β13. However, several questions remain unanswered. Notably, it is intriguing that ethanol specifically depletes p97/VCP on the LDs without affecting the overall hepatic levels of p97/VCP. p97/VCP is primarily localized in the cytosol; a small fraction is also found on organelle membranes, including the endoplasmic reticulum (ER), Golgi apparatus, mitochondria, and endosomes (7). However, the mechanisms governing this localization are yet to be determined. Interestingly, knocking down UBXD8, an adaptor for p97/VCP localized in both the ER and LDs, resulted in LD accumulation. The overexpression of a UBXD8 mutant that cannot bind to p97/VCP similarly led to LD accumulation, suggesting that the loss of UBXD8 may be a key factor contributing to the depletion of p97/VCP on the LDs. Contrary to the findings here, in HeLa cells, the overexpression of UBXD8 increased recruitment of p97/VCP to LDs, which led to an increased number and size of LDs by inhibiting the activity of adipose triglyceride lipase (ATGL) via promoting its dissociation from its endogenous coactivator, CGI-58 (8). Perhaps knocking down UBXD8 in hepatocytes is more physiologically relevant than overexpressing UBXD8 in HeLa cells. Nonetheless, future studies should also determine the levels of p97/VCP on LDs and the changes in CGI-58 and ATGL activities in UBXD8 knockdown hepatocytes. Another critical unresolved question is whether UBXD8 levels decrease in human ALD livers and how ethanol reduces hepatic UBXD8 levels, which remains unclear. Since the accumulation of LDs is also a hallmark of MASLD, it is uncertain whether mice fed a high-fat diet would experience a similar depletion of p97/VCP and accumulation of HSD17β13 on LDs, or whether these changes are exclusive to alcohol consumption. Additionally, lysosomes play a vital role in LD catabolism through a process known as microlipophagy (9, 10). It remains to be seen whether the loss of p97/VCP and UBXD8 on LDs impairs lysosome–LD contact or the entrapment of LDs by lysosomes for degradation. Future studies are needed to clarify these critical questions further.

In conclusion, this study identifies a UBXD8-p97/VCP-HSD17β3 axis as a key regulator of LD homeostasis in ALD. This finding could lead to new strategies for reducing hepatic steatosis by enhancing the clearance of LDs through this novel catabolism pathway. Targeting this pathway may significantly impact treating ALD and MASLD (Fig. 1).