Lipid Droplet-Associated Hydrolase Promotes Lipid Droplet Fusion and Enhances ATGL Degradation and Triglyceride Accumulation

Young-Hwa Goo,Antoni Paul,Se-Hee Son

doi:10.1038/s41598-017-02963-y

Abstract

Lipid droplet (LD)-associated hydrolase (LDAH) is a newly identified LD protein abundantly expressed in tissues that predominantly store triacylglycerol (TAG). However, how LDAH regulates TAG metabolism remains unknown. We found that upon oleic acid loading LDAH translocalizes from the ER to newly formed LDs, and induces LD coalescence in a tubulin-dependent manner. LDAH overexpression and downregulation in HEK293 cells increase and decrease, respectively, TAG levels. Pulse and chase experiments show that LDAH enhances TAG biogenesis, but also decreases TAG turnover and fatty acid release from cells. Mutations in predicted catalytic and acyltransferase motifs do not influence TAG levels, suggesting that the effect is independent of LDAH’s enzymatic activity. However, a LDAH alternative-splicing variant missing 90 amino acids at C-terminus does not promote LD fusion or TAG accumulation, while it still localizes to LDs. Interestingly, LDAH enhances polyubiquitination and proteasomal degradation of adipose triglyceride lipase (ATGL), a rate limiting enzyme of TAG hydrolysis. Co-expression of ATGL reverses the changes in LD phenotype induced by LDAH, and both proteins counterbalance their effects on TAG stores. Together, these studies support that under conditions of TAG storage in LDs LDAH plays a primarily lipogenic role, inducing LD growth and enhancing degradation of ATGL.

Highlights

Tissues that predominantly store TAG, including liver and white and brown adipose tissues[15]
As we previously observed in cholesterol-laden cells, in both cell types LDAH readily localized to the Lipid droplet (LD) perimeter (Fig. 1A)[15]
Morphometric studies confirmed that LDAH overexpression considerably increases the average LD size, while it decreases the total number of LDs (Fig. 1B–E)

Summary

Introduction

Tissues that predominantly store TAG, including liver and white and brown adipose tissues[15]. In agreement with other groups we did not detect hydrolytic activity against TAG in vitro, but it was shown that LDAH is involved in LD clustering and fusion[14, 15]. Given that LDAH associates with TAG-rich LDs but does not seem to work as a TAG hydrolase, the aims of this study were to investigate its role in the biology of TAG-rich LDs and its impact on TAG homeostasis. The effects of the different LDAH splicing variants were compared, and mutants lacking the catalytic site as well as novel potential acyltransferase motifs we identified were studied. The main conclusion of these studies is that in contrast to what its lipase-like structure suggested, under metabolic conditions that promote TAG storage in LDs LDAH plays a lipogenic role

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Conclusion