Differential regulation of the glucagon and insulin I gene promoters by the basic helix-loop-helix transcription factors E47 and BETA2.

The insulin and glucagon genes are expressed in the beta and alpha cells of the islets of Langerhans, respectively. The factors controlling their cell- and islet-specific expression are poorly known. Insulin-enhancer factor-1 (IEF1) has previously been shown to interact with the E boxes of the rat insulin I and II genes and was proposed to play a critical role in beta cell-specific expression. BETA2, a recently identified basic helix-loop-helix (bHLH) protein, binds with high affinity and transactivates the rat insulin II gene upon dimerization with the ubiquitous bHLH protein E47. We show here that the heterodimer E47/BETA2 also binds and transactivates the rat insulin I and glucagon genes and exhibits the same characteristics as IEF1. In transfection experiments, the E boxes of the insulin I and glucagon genes confer transcriptional activity in both insulin- and glucagon-producing cells, which is increased by overexpression of E47 and BETA2. However, overexpression of E47 inhibits only E box-mediated glucagon gene expression, whereas it activates insulin gene transcription, indicating that the E boxes of the insulin and glucagon genes display gene-specific characteristics. We conclude that the heterodimer E47/BETA2 represents an islet-specific factor that controls both insulin and glucagon gene transcription and that the E47/BETA2 ratio may be important for regulated gene expression.