Atrophy of White Adipose Tissue Accompanied with Decreased Insulin-Stimulated Glucose Uptake in Mice Lacking the Small GTPase Rac1 Specifically in Adipocytes

Insulin-stimulated translocation of the fatty acid transporter CD36 to the plasma membrane is mediated by the small GTPase Rac1 in adipocytes

Cluster of differentiation 36 (CD36) is a scavenger receptor (SR), recognizing diverse extracellular ligands in various types of mammalian cells. Long-chain fatty acids (FAs), which are important constituents of phospholipids and triglycerides, also utilize CD36 as a predominant membrane transporter, being incorporated from the circulation across the plasma membrane in several cell types, including cardiac and skeletal myocytes and adipocytes. CD36 is localized in intracellular vesicles as well as the plasma membrane, and its distribution is modulated by extracellular stimuli. Herein, we aimed to clarify the molecular basis of insulin-stimulated translocation of CD36, which leads to the enhanced uptake of long-chain FAs, in adipocytes. To this end, we developed a novel exofacial epitope-tagged reporter to specifically detect cell surface-localized CD36. By employing this reporter, we demonstrate that the small GTPase Rac1 plays a pivotal role in insulin-stimulated translocation of CD36 to the plasma membrane in 3T3-L1 adipocytes. Additionally, phosphoinositide 3-kinase and the protein kinase Akt2 are shown to be involved in the regulation of Rac1. Downstream of Rac1, another small GTPase RalA directs CD36 translocation. Collectively, these results suggest that CD36 is translocated to the plasma membrane by insulin through mechanisms similar to those for the glucose transporter GLUT4 in adipocytes.

Regulation of De Novo Lipid Synthesis by the Small GTPase Rac1 in the Adipogenic Differentiation of Progenitor Cells from Mouse White Adipose Tissue

White adipocytes act as lipid storage, and play an important role in energy homeostasis. The small GTPase Rac1 has been implicated in the regulation of insulin-stimulated glucose uptake in white adipocytes. Adipocyte-specific rac1-knockout (adipo-rac1-KO) mice exhibit atrophy of subcutaneous and epididymal white adipose tissue (WAT); white adipocytes in these mice are significantly smaller than controls. Here, we aimed to investigate the mechanisms underlying the aberrations in the development of Rac1-deficient white adipocytes by employing in vitro differentiation systems. Cell fractions containing adipose progenitor cells were obtained from WAT and subjected to treatments that induced differentiation into adipocytes. In concordance with observations in vivo, the generation of lipid droplets was significantly attenuated in Rac1-deficient adipocytes. Notably, the induction of various enzymes responsible for de novo synthesis of fatty acids and triacylglycerol in the late stage of adipogenic differentiation was almost completely suppressed in Rac1-deficient adipocytes. Furthermore, the expression and activation of transcription factors, such as the CCAAT/enhancer-binding protein (C/EBP) β, which is required for the induction of lipogenic enzymes, were largely inhibited in Rac1-deficient cells in both early and late stages of differentiation. Altogether, Rac1 is responsible for adipogenic differentiation, including lipogenesis, through the regulation of differentiation-related transcription.