To define the molecular mechanism(s) that activate insulin receptor gene transcription during cell differentiation, we tested nuclear extracts from BC3H-1 muscle cells for their binding to the 5'-flanking region of the human insulin receptor gene. DNA binding activity of nuclear extracts was low in undifferentiated BC3H-1 cells and increased significantly during differentiation. Gel retardation assays, combined with DNase I footprinting, showed that the increased insulin receptor gene transcription occurring during differentiation was directly correlated with the appearance of DNA binding proteins that specifically interacted with two AT-rich sequences of the regulatory region of the insulin receptor gene. Fibroblast growth factor, a known inhibitor of the transcription of muscle-specific DNA binding proteins, did not inhibit the appearance of these insulin receptor DNA binding proteins. When 3T3-L1 cells differentiate from preadipocytes to adipocytes, insulin receptor gene transcription significantly increases. In differentiated adipocytes, the same two insulin receptor DNA binding proteins markedly increased. Reporter gene analysis with the two AT-rich sequences demonstrated that both of these regions of the insulin receptor gene had the characteristics of promoter rather than enhancer elements. Thus, these proteins interacting with these AT-rich sequences may have major importance in regulating the expression of the insulin receptor in target tissues.
A Brunetti, D Foti, I D Goldfine