|M.Sc Student||Hazan Brill Roni|
|Subject||The Role of GLUT4 in the IGF-1 Mediated Chondrogenesis|
|Department||Department of Medicine||Supervisors||Mr. Eli Sprecher|
|Dr. Gila Maor|
|Full Thesis text|
Introduction: Uncontrolled type 1 diabetic (DM1) patients frequently have impaired skeletal growth that improves upon insulin therapy. However the molecular mechanism is yet unclear. In skeletal growth centers glucose transporter 4 (GLUT4) and IGF-I receptor (IGF-IR) are co-regulated. Further, in the murine mandibular condyle, a model for endochondral ossification, IGF-1 modulates GLUT4 expression and function via IGF-IR signaling cascade. The purpose of the current study was to examine the importance of GLUT4 in IGF-I-mediated skeletal growth.
Methods and Results: Mandibular condyle-derived cells (MCDC) were preincubated with either insulin or IGF-I for up to 12 days. Both insulin and IGF-I enhanced proliferation and differentiation (e.g. DNA synthesis, collagen and aggrecan synthesis) and endochondral ossification process parallel the increase in GLUT4 gene expression. Compared to basal state, insulin and IGF-I pre-incubation increased 2-deoxyglucose uptake by 9.8-fold and 5.9-fold above the basal level respectively, resulting from GLUT4 translocation to the outer cell membrane. To determine the role of GLUT4 in IGF-I effect on bone growth, we used siRNA technique. GLUT4 gene expression was shut down by 56% after 48 hrs and gradually recovered thereafter. This abolished GLUT4 induced effects on both proliferation and differentiation. Surprisingly, IGF-I failed to stimulate type X collagen and aggrecan synthesis up to 5 days after GLUT4 levels were fully recovered (e.g. 12 days old MCDC cultures). However, insulin-induced growth was not affected by GLUT4 gene silencing.
Conclusions: Our data emphasizes the important role of GLUT4 in IGF-I induced skeletal growth. Thus, even temporary reduction in bone GLUT4 content, as probably occurs in uncontrolled DM1 patients, is “imprinted” in the “cell memory”, leading to potential long term impairments in skeletal growth. This phenomenon can be accounted for by the major role of glucose metabolism in skeletal growth centers as the main source for energy consumption (via glycolysis), and as an important structural component in aggrecan synthesis.