Differential Regulation of the Glucagon and Insulin I Gene Promoters by the Basic Helix-Loop-Helix Transcription Factors E47 and BETA2

Eric Dumonteil,Beate Laser,Isabel Constant,Jacques Philippe

doi:10.1074/jbc.273.32.19945

Abstract

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.

Highlights

Glucagon and insulin are two major antagonist hormones secreted by the endocrine pancreas which control glucose homeostasis
The two E boxes E1 and E2 of the rat insulin I gene have been reported to play a major role in beta cell-specific expression and regulation by glucose; they interact with insulin enhancer factor 1 (IEF1), a protein complex composed of the ubiquitous basic helixloop-helix (bHLH) protein E12/E47 and an islet cell-specific factor (14 –16)
We suggested that the IEF1 complex, previously shown to represent a critical determinant of the beta cell-specific expression of the insulin gene, could interact with E3 of the glucagon gene and be involved in the islet-specific expression of the glucagon gene

Summary

Introduction

Glucagon and insulin are two major antagonist hormones secreted by the endocrine pancreas which control glucose homeostasis. Taken together with the fact that nuclear extracts from BHK-21 cells transfected with either the E47 or/and BETA2 cDNAs can form specific complexes with both E1 and E3 boxes, these results indicate that IEF1 by itself is not sufficient for transactivating the insulin and glucagon gene promoter in heterologous cells and that it likely requires additional cell-specific factors such as PDX1 (formally Idx1/ IPF1/STF1) or RIPE3b to be functional [17, 18].

Results

Conclusion