THE ROLE OF INSULIN IN ACTIVATION OF 2 ENHANCERS IN THE MOUSE GLUT1 GENE

We identified two enhancer elements of the mouse GLUT1 gene responsive to serum, growth factor, and oncogenes; the first enhancer element (e nhancer-1) is located 2.7 kilobases upstream of the cap site of the ge ne, and the second one (enhancer-2) is located in the second intron of the gene (Murakami, T., Nishiyama, T., Shirotani, T., Shinohara, Y., Kan, M., Ishii, K., Kanai, F., Nakazuru, S., and Ebina, Y. (1992) J. B iol. Chem. 267, 9300-9306). In the present work, we describe the role of insulin in activation of these two enhancers. NIH/3T3 HIR3.5 cells, which express a large number of insulin receptors, were stably transf ormed by hybrid genes containing the enhancer(s) and promoter of GLUT1 gene and the coding region of chloramphenicol acetyltransferase (CAT) gene as a reporter gene. In stable transformants of the reporter gene without the enhancers, the CAT mRNA was not induced by insulin; howev er, in clones containing the reporter gene with enhancer-1, the CAT mR NA was induced by insulin at 30 min and reached a maximum at 1 h. In c lones transfected by the reporter gene with enhancer-2, the CAT mRNA w as induced at 1 h and reached a maximum at 3 h. To determine the early response element to insulin in enhancer-1, transformants of hybrid re porter genes containing truncated or mutated enhancer-1 were examined. The homologous sequence with the serum response element in enhancer-1 is essential for an early response to insulin.