The insulin-sensitive glucose transporter (GLUT4) is involved in early bone growth in control and diabetic mice, but is regulated through the insulin-like growth factor I receptor.

Abstract

Children with uncontrolled type I (insulin-dependent) diabetes mellitus are characterized by a slow growth rate, which improves upon adequate therapy. While skeletal growth is an energy-consuming process involving high glucose utilization, the role of glucose transporters (GLUT) and their regulation in the bone formation process are not yet fully understood. Thus, we studied both in vivo and in vitro early endochondral bone formation in control and streptozotocin-induced young diabetic mice. Using in situ hybridization and immunohistochemistry techniques, we demonstrated the novel existence of the insulin-sensitive glucose transporter (GLUT4), as well as GLUT1, in juvenile-derived murine mandibular condyles and in the humeral growth plate-two models for endochondral bone formation. Insulin-like growth factor (IGF) I receptors (IGF-I-R), but not insulin receptors (IR), were shown to have cellular distribution similar to GLUT4, being more abundant in mature chondrocytes. Further, in the skeletal growth centers of streptozotocin-induced diabetic mice, GLUT4, IGF-I, and IGF-I and insulin receptor levels, but not GLUT1 were markedly reduced. The decrease in GLUT4 and in IGF-I and insulin receptors was associated with severe histological changes in the mandibular condyles and humeral growth plate. Insulin therapy restored IR levels to normalcy, whereas IGF-I-R and GLUT4 levels were only partially recovered. Thus, GLUT4 and IGF-I-R have a potential role in early bone growth in mice. Further, during early bone growth GLUT4 may be regulated through the IGF-I receptor rather than via the insulin receptor. We propose that skeletal growth retardation in type I diabetes may be associated with reduced expression of the GLUT4 and IGF-I receptor in the bone growth center.