Nobuyuki Takenaka

A fraction of the insulin-stimulated uptake of long-chain fatty acids (FAs) is mediated by the FA translocase cluster of differentiation 36 (CD36) in white adipocytes. Intracellular vesicle-localized CD36 is redistributed to the plasma membrane following insulin stimulation, enhancing the uptake of long-chain FAs across the plasma membrane. We previously developed an epitope-tagged CD36 reporter, which enabled the visualization and quantification of the plasma membrane translocation of CD36. Herein, we demonstrate that the insulin-stimulated CD36 translocation is regulated by the phosphoinositide 3-kinase (PI3K)/Akt2/Rac1/RalA axis in adipocytes of subcutaneous white adipose tissue (WAT) in living mice. The uptake of long-chain FAs by insulin was completely abrogated in white adipocytes isolated from adipocyte-specific rac1-knockout (adipo-rac1-KO) mice. Correspondingly, the translocation of CD36 to the plasma membrane by insulin was also totally inhibited in Rac1-deficient white adipocytes. PI3K and Akt2 acted upstream of Rac1, and the guanin nucleotide exchange factor FLJ00068 served as a regulator for Rac1. Furthermore, the involvement of another small GTPase RalA was suggested by inhibitory effects of a dominant-negative mutant. Taken together, these results support the notion that insulin regulates the plasma membrane translocation of CD36 by mechanisms similar to those for the translocation of the glucose transporter GLUT4 in white adipocytes.

Insulin-stimulated glucose uptake in skeletal muscle is mediated by the glucose transporter GLUT4. The small GTPase Rac1 acts as a switch of signal transduction that regulates GLUT4 translocation to the plasma membrane following insulin stimulation. However, it remains obscure whether signaling cascades upstream and downstream of Rac1 in skeletal muscle are impaired by obesity that causes insulin resistance and type 2 diabetes. In an attempt to clarify this point, we investigated Rac1 signaling in the leptin-deficient (Lep^ob/ob) mouse model. Here, we show that insulin-stimulated GLUT4 translocation and Rac1 activation are almost completely abolished in Lep^ob/ob mouse skeletal muscle. Phosphorylation of the protein kinase Akt2 and plasma membrane translocation of the guanine nucleotide exchange factor FLJ00068 following insulin stimulation were also diminished in Lep^ob/ob mice. On the other hand, the activation of another small GTPase RalA, which acts downstream of Rac1, by the constitutively activated form of Akt2, FLJ00068, or Rac1, was partially abrogated in Lep^ob/ob mice. Taken together, we conclude that insulin-stimulated glucose uptake is impaired by two mechanisms in Lep^ob/ob mouse skeletal muscle: one is the complete inhibition of Akt2-mediated activation of Rac1, and the other is the partial inhibition of RalA activation downstream of Rac1.

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.