title: Rab18 dynamics in adipocytes in relation to lipogenesis, lipolysis and obesity.
abstract: Lipid droplets (LDs) are organelles that coordinate lipid storage and mobilization, both processes being especially important in cells specialized in managing fat, the adipocytes. Proteomic analyses of LDs have consistently identified the small GTPase Rab18 as a component of the LD coat. However, the specific contribution of Rab18 to adipocyte function remains to be elucidated. Herein, we have analyzed Rab18 expression, intracellular localization and function in relation to the metabolic status of adipocytes. We show that Rab18 production increases during adipogenic differentiation of 3T3-L1 cells. In addition, our data show that insulin induces, via phosphatidylinositol 3-kinase (PI3K), the recruitment of Rab18 to the surface of LDs. Furthermore, Rab18 overexpression increased basal lipogenesis and Rab18 silencing impaired the lipogenic response to insulin, thereby suggesting that this GTPase promotes fat accumulation in adipocytes. On the other hand, studies of the β-adrenergic receptor agonist isoproterenol confirmed and extended previous evidence for the participation of Rab18 in lipolysis. Together, our data support the view that Rab18 is a common mediator of lipolysis and lipogenesis and suggests that the endoplasmic reticulum (ER) is the link that enables Rab18 action on these two processes. Finally, we describe, for the first time, the presence of Rab18 in human adipose tissue, wherein the expression of this GTPase exhibits sex- and depot-specific differences and is correlated to obesity. Taken together, these findings indicate that Rab18 is involved in insulin-mediated lipogenesis, as well as in β-adrenergic-induced lipolysis, likely facilitating interaction of LDs with ER membranes and the exchange of lipids between these compartments. A role for Rab18 in the regulation of adipocyte biology under both normal and pathological conditions is proposed.
authors.affiliation: Department of Cell Biology, Physiology and Immunology, University of Córdoba, Córdoba, Spain.
authors.forename: Marina R
authors.initials: MR
authors.lastname: Pulido
authors.forename: Alberto
authors.initials: A
authors.lastname: Diaz-Ruiz
authors.forename: Yolanda
authors.initials: Y
authors.lastname: Jiménez-Gómez
authors.forename: Socorro
authors.initials: S
authors.lastname: Garcia-Navarro
authors.forename: Francisco
authors.initials: F
authors.lastname: Gracia-Navarro
authors.forename: Francisco
authors.initials: F
authors.lastname: Tinahones
authors.forename: José
authors.initials: J
authors.lastname: López-Miranda
authors.forename: Gema
authors.initials: G
authors.lastname: Frühbeck
authors.forename: Rafael
authors.initials: R
authors.lastname: Vázquez-Martínez
authors.forename: Maria M
authors.initials: MM
authors.lastname: Malagón
mesh_terms: D041721:3T3-L1 Cells; D017667:Adipocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D052242:Adiponectin / Q000378:metabolism; D000818:Animals; D015153:Blotting, Western; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D005260:Female; D006801:Humans; D007004:Hypoglycemic Agents / Q000494:pharmacology; D007124:Immunoenzyme Techniques; D007328:Insulin / Q000494:pharmacology; D050356:Lipid Metabolism; D050155:Lipogenesis; D008066:Lipolysis; D008297:Male; D051379:Mice; D009765:Obesity / Q000503:physiopathology*; D012333:RNA, Messenger / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D020691:rab GTP-Binding Proteins / Q000502:physiology*
chemical_list: D052242:Adiponectin; D007004:Hypoglycemic Agents; D007328:Insulin; D012333:RNA, Messenger; C083879:Rab18 protein, mouse; D020691:rab GTP-Binding Proteins