TRANSCRIPTION OF THE INSULIN GENE - TOWARDS DEFINING THE GLUCOSE-SENSITIVE CIS-ELEMENT AND TRANS-ACTING FACTORS

Previous work has shown that the sequence -196 to -247 of the rat insulin I gene mediates the stimulatory effect of glucose in fetal islets. We have used adult rat and human islets to delineate the glucose-sensitive cis-element to the sequence -193 to -227. In electrophoretic mobility shift assays, a 22 bp nucleotide corresponding to the sequence -206 to -227 bound all the nuclear proteins that could be bound by the entire minienhancer sequence -196 to -247. The rat insulin I sequence -206 to -227 formed three major complexes; in contrast, the corresponding human insulin sequence formed one single band with human and rat islet nuclear extracts, corresponding to the complex C1 of the rat insulin gene. Incubation of islets with varying glucose levels resulted in a dose-dependent increase in the intensity of the C1 band, while the other nuclear complexes formed with the insulin sequence, or the AP1 and SP1 binding activities used as control, were glucose insensitive. This is thus the first demonstration of a physiologic glucose-sensitive trans-acting factor for the insulin gene, whose further study may markedly enhance our understanding of the regulation of insulin biosynthesis in normal and diabetic beta cells. Furthermore, once cloned, the introduction of this glucose sensitive factor may enable the construction of truly physiologic artificial beta cells.