Methods to Measure Lipophagy in Yeast

General autophagy-dependent and -independent lipophagic processes collaborate to regulate the overall level of lipophagy in yeast

ABBREVIATION

AP: autophagosome; ATG: autophagy related; CMA: chaperone-mediated autophagy; ESCRT: endosomal sorting complex required for transport; FA: fatty acid; LD: lipid droplet; Ld microdomains: liquid-disordered microdomains; NL: neutral lipid.

Identification of novel lipid droplet factors that regulate lipophagy and cholesterol efflux in macrophage foam cells

Macrophage autophagy is a highly anti-atherogenic process that promotes the catabolism of cytosolic lipid droplets (LDs) to maintain cellular lipid homeostasis. Selective autophagy relies on tags such as ubiquitin and a set of selectivity factors including selective autophagy receptors (SARs) to label specific cargo for degradation. Originally described in yeast cells, "lipophagy" refers to the degradation of LDs by autophagy. Yet, how LDs are targeted for autophagy is poorly defined. Here, we employed mass spectrometry to identify lipophagy factors within the macrophage foam cell LD proteome. In addition to structural proteins (e.g., PLIN2), metabolic enzymes (e.g., ACSL) and neutral lipases (e.g., PNPLA2), we found the association of proteins related to the ubiquitination machinery (e.g., AUP1) and autophagy (e.g., HMGB, YWHA/14-3-3 proteins). The functional role of candidate lipophagy factors (a total of 91) was tested using a custom siRNA array combined with high-content cholesterol efflux assays. We observed that knocking down several of these genes, including Hmgb1, Hmgb2, Hspa5, and Scarb2, significantly reduced cholesterol efflux, and SARs SQSTM1/p62, NBR1 and OPTN localized to LDs, suggesting a role for these in lipophagy. Using yeast lipophagy assays, we established a genetic requirement for several candidate lipophagy factors in lipophagy, including HSPA5, UBE2G2 and AUP1. Our study is the first to systematically identify several LD-associated proteins of the lipophagy machinery, a finding with important biological and therapeutic implications. Targeting these to selectively enhance lipophagy to promote cholesterol efflux in foam cells may represent a novel strategy to treat atherosclerosis.Abbreviations:ADGRL3: adhesion G protein-coupled receptor L3; agLDL: aggregated low density lipoprotein; AMPK: AMP-activated protein kinase; APOA1: apolipoprotein A1; ATG: autophagy related; AUP1: AUP1 lipid droplet regulating VLDL assembly factor; BMDM: bone-marrow derived macrophages; BNIP3L: BCL2/adenovirus E1B interacting protein 3-like; BSA: bovine serum albumin; CALCOCO2: calcium binding and coiled-coil domain 2; CIRBP: cold inducible RNA binding protein; COLGALT1: collagen beta(1-O)galactosyltransferase 1; CORO1A: coronin 1A; DMA: deletion mutant array; Faa4: long chain fatty acyl-CoA synthetase; FBS: fetal bovine serum; FUS: fused in sarcoma; HMGB1: high mobility group box 1; HMGB2: high mobility group box 2: HSP90AA1: heat shock protein 90: alpha (cytosolic): class A member 1; HSPA5: heat shock protein family A (Hsp70) member 5; HSPA8: heat shock protein 8; HSPB1: heat shock protein 1; HSPH1: heat shock 105kDa/110kDa protein 1; LDAH: lipid droplet associated hydrolase; LIPA: lysosomal acid lipase A; LIR: LC3-interacting region; MACROH2A1: macroH2A.1 histone; MAP1LC3: microtubule-associated protein 1 light chain 3; MCOLN1: mucolipin 1; NBR1: NBR1, autophagy cargo receptor; NPC2: NPC intracellular cholesterol transporter 2; OPTN: optineurin; P/S: penicillin-streptomycin; PLIN2: perilipin 2; PLIN3: perilipin 3; PNPLA2: patatin like phospholipase domain containing 2; RAB: RAB, member RAS oncogene family; RBBP7, retinoblastoma binding protein 7, chromatin remodeling factor; SAR: selective autophagy receptor; SCARB2: scavenger receptor class B, member 2; SGA: synthetic genetic array; SQSTM1: sequestosome 1; TAX1BP1: Tax1 (human T cell leukemia virus type I) binding protein 1; TFEB: transcription factor EB; TOLLIP: toll interacting protein; UBE2G2: ubiquitin conjugating enzyme E2 G2; UVRAG: UV radiation resistance associated gene; VDAC2: voltage dependent anion channel 2; VIM: vimentin.

PCSK9: A new participant in lipophagy in regulating atherosclerosis?

Proprotein convertase subtilisin kexin 9 (PCSK9) regulates lipid metabolism by degrading low-density lipoprotein receptor on the surface of hepatocytes. PCSK9-mediated lipid degradation is associated with lipophagy. Lipophagy is a process by which autophagosomes selectively sequester lipid-droplet-stored lipids and are delivered to lysosomes for degradation. Lipophagy was first discovered in hepatocytes, and its occurrence provides important fundamental insights into how lipid metabolism regulates cellular physiology and pathophysiology. Furthermore, PCSK9 may regulate lipid levels by affecting lipophagy. This review will discuss recent advances by which PCSK9 mediates lipid degradation via the lipophagy pathway and present lipophagy as a potential therapeutic target for atherosclerosis.

Lipophagy in nonliver tissues and some related diseases: Pathogenic and therapeutic implications

Lipid autophagy (lipophagy) is defined as a selective autophagy process in which some intracellular lipid droplets are selectively degraded by autophagic lysosomes pathway. The occurrence of lipophagy was first discovered in liver tissues. Additionally, abundant evidence indicated that the occurrence of hepatic lipophagy has been implicated in many liver diseases including fatty liver diseases, nonalcoholic fatty liver diseases, liver fibrosis, and liver cirrhosis. However, recent studies suggested that hepatic lipophagy occurs not only in liver tissue but also in other nonliver tissues and cells. Furthermore, the occurrence of lipophagy plays a crucial role in nonliver tissues and some related diseases. For instance, lipophagy relieves insulin resistance in adipose tissue from obesity patient with type 2 diabetes. Additionally, lipophagy has the ability to remit neurodegenerative diseases by reducing activity-dependent neurodegeneration in nervous tissue. Lipophagy decreases muscle lipid accumulation and accordingly improves lipid storage myopathy in muscle tissue. Moreover, lipophagy alleviates the malignancy and metastasis of cancer in clear renal cell carcinoma tissue. Lipophagy is also involved in other processes, such as spermatogenesis, osteoblastogenesis, and mucosal ulceration. In conclusion, targeting lipophagy may be a critical regulator and a new therapeutic strategy for nonliver tissues and some related diseases.