title: GBF1-Arf-COPI-ArfGAP-mediated Golgi-to-ER transport involved in regulation of lipid homeostasis.
abstract: Eukaryotic cells store neutral lipids and cholesteryl esters in cytoplasmic lipid droplets (LDs), which are generated from the endoplasmic reticulum (ER). Accumulating lines of evidence have indicated that Golgi-to-ER-retrograde transport mediated by COPI-coated vesicles under the control of Arf small GTPases is implicated in LD formation and utilization. However, the detailed mechanism underlying the regulation of lipid homeostasis by COPI-dependent transport has been poorly understood. Here we show that LD deposition and the cellular triacylglycerol content are significantly increased by siRNA-mediated depletion of not only β-COP (a subunit of the COPI coat complex) but also GBF1 (a guanine nucleotide exchange factor for Arfs), Arf4 and Arf5 (class II Arfs), and ArfGAP1-ArfGAP3 (GTPase-activating proteins for Arfs). Although a previous proteomic study suggested the presence of COPI subunits and Arfs on LDs, we have failed to show that components of the GBF1-Arf-COPI-ArfGAP retrograde transport machinery are directly associated with and closely apposed to LDs. Furthermore, although recent studies suggested that COPI-mediated transport and GBF1 participated in delivery of adipose triglyceride lipase (ATGL) onto the LD surface, we have found that depletion of β-COP or GBF1 does not affect association of ATGL with LDs or ATGL-mediated lipolysis. On the basis of these results, we propose other mechanisms how the GBF1-Arf-COPI-ArfGAP transport machinery is implicated in the regulation of lipid homeostasis.
authors.affiliation: Department of Physiological Chemistry, Graduate School of Pharmaceutical Sciences, Kyoto University, Japan.
authors.forename: Kouhei
authors.initials: K
authors.lastname: Takashima
authors.forename: Akina
authors.initials: A
authors.lastname: Saitoh
authors.forename: Shohei
authors.initials: S
authors.lastname: Hirose
authors.forename: Waka
authors.initials: W
authors.lastname: Nakai
authors.forename: Yumika
authors.initials: Y
authors.lastname: Kondo
authors.forename: Yasuaki
authors.initials: Y
authors.lastname: Takasu
authors.forename: Hideaki
authors.initials: H
authors.lastname: Kakeya
authors.forename: Hye-Won
authors.initials: HW
authors.lastname: Shin
authors.forename: Kazuhisa
authors.initials: K
authors.lastname: Nakayama
mesh_terms: D020727:ADP-Ribosylation Factors / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D002478:Cells, Cultured; D020755:Coat Protein Complex I / Q000378:metabolism*; D004285:Dogs; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D020690:GTPase-Activating Proteins / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism*; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D006367:HeLa Cells; D006706:Homeostasis; D006801:Humans; D050356:Lipid Metabolism; D008055:Lipids
chemical_list: D020755:Coat Protein Complex I; C120445:GBF1 protein, human; D020690:GTPase-Activating Proteins; D020662:Guanine Nucleotide Exchange Factors; D008055:Lipids; D020727:ADP-Ribosylation Factors