GBF1-Arf-COPI-ArfGAP-mediated Golgi-to-ER transport involved in regulation of lipid homeostasis.

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.