The Rho family GTPase Rac1 has been implicated in the regulation of glucose uptake in myoblast cell lines. However, no evidence for the role of Rac1 has been provided by a mouse model. The purpose of this study is to test the involvement of Rac1 in insulin action in mouse skeletal muscle. Intravenous administration of insulin indeed elicited Rac1 activation in gastrocnemius muscle, suggesting the involvement of Rac1 in this signaling pathway. We then examined whether insulin-stimulated translocation of the facilitative glucose transporter GLUT4 from its storage sites to the skeletal muscle sarcolemma depends on Rac1. We show that ectopic expression of constitutively activated Rac1, as well as intravenous administration of insulin, caused translocation of GLUT4 to the gastrocnemius muscle sarcolemma, as revealed by immunofluorescent staining of a transiently expressed exofacial epitope-tagged GLUT4 reporter. Of particular note, insulin-dependent, but not constitutively activated Rac1-induced, GLUT4 translocation was markedly suppressed in skeletal muscle-specific rac1-knockout mice compared to control mice. Immunogold electron microscopic analysis of endogenous GLUT4 gave similar results. Collectively, we propose a critical role of Rac1 in insulin-dependent GLUT4 translocation to the skeletal muscle sarcolemma, which has heretofore been predicted solely by cell culture studies.

Collectively, Rac1 activation specifically in membrane ruffles by the GEF FLJ00068 is sufficient for insulin induction of glucose uptake into skeletal-muscle cells.

Introduction c-Kit is a receptor tyrosine kinase (RTK), which constitutes a type III RTK subfamily with the receptors for platelet-derived growth factor (PDGF), colony-stimulating factor 1 (CSF-1), and flt-3 ligand. 1,2 The type III RTKs are characterized by an extracellular domain with 5 immunoglobulinlike domains and a cytoplasmic domain consisting of a kinase domain that is interrupted by a kinase insert. c-Kit (KIT) and its ligand stem cell factor (SCF) play an important role in hematopoiesis, melanogenesis, and gametogenesis, 3 as has been clearly shown by loss of function mutations of c-kit gene. In addition, c-kit gene product has been associated with various forms of neoplasms. Activating mutants of KIT, either in the juxtamembrane domain or the catalytic domain, were identified as the cause for transformation of hematopoietic stem cells, mast cells, and gastrointestinal stromal cells. [4][5][6][7][8][9][10] Thus, KIT/SCF has pleiotropic functions such as proliferation, survival, differentiation, and transformation. In this report, we focus on SCF/KITmediated cell migration, which is also a characteristic function of SCF in hematopoietic stem cells and mast cells, [11][12][13] and has critical roles in immunity, metastasis, and development.On ligand stimulation, KIT receptors dimerize, activate its intrinsic tyrosine kinase, and autophosphorylate. The phosphorylated KIT receptor generates binding sites for SH2 domaincontaining proteins, which include proteins of the p21Ras-mitogenactivated protein kinase (MAPK) pathway, 14 the p85 subunit of phosphatidylinositol 3Ј kinase (PI3K), 15 phospholipase C-␥ 1 , the Grb2 adaptor protein, 16 the Src family kinases (SFKs), 17 Cbl, CRKL, 19 SHP1, and SHP2. 20 Those proteins are subsequently activated or phosphorylated and further transduce signaling cascades that lead to various cellular responses. However, little is known about which signaling is essential for SCF-mediated migration. Recently, a few reports indicated that Lyn or p38 MAPK plays an important role, 21,22 but no comprehensive investigation has been done in which the tyrosine residue of KIT is involved in signal transduction, which is required for cell migration. In this study, we have converted all possible tyrosine (Y) residues on KIT cytoplasmic domain to phenylalanine (F) and introduced these YF substitute mutants on 293T cells or murine interleukin 3 (IL-3)-dependent BAF3 cells. We used these cell lines to elucidate signaling cascades that are important for SCF-mediated cell Supported in part by grants from the Japanese Ministry of Education, Culture, Sports, Science and Technology, the Japanese Ministry of Health, Labor and Welfare, and the Japan Society for Promotion of Science.Reprints: Yuzuru Kanakura, Department of Hematology and Oncology, Osaka University Graduate School of Medicine, 2-2, Yamada-oka, Suita, Osaka 565-0871, Japan; e-mail: kanakura@bldon.med.osaka-u.ac.jp.The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate thi...