The Pan basic helix-loop-helix proteins are required for insulin gene expression

Abstract

The rat I insulin enhancer contains two principal regulatory elements, the Nir and Far motifs of an identical 9-base pair sequence, which function both as positive and negative cis-acting elements. The Nir and Far elements are targets for DNA-binding proteins, which play a predominant role in the selective transcription of the insulin gene in endocrine beta-cells. In vitro DNA-binding studies have demonstrated the ability of several helix-loop-helix (HLH) proteins, including the Pan/E2A proteins, upstream stimulating factor, human beta-HLH factor (rat beta-HLH factor), and E2-2/ITF-2, to bind the Nir and Far enhancer motifs. The presence of the aforementioned different HLH proteins in endocrine beta-cells, all of which display similar binding affinities for the Nir and Far elements in vitro, raises the question of which HLH proteins actively participate in the transcriptional regulation of the rat insulin I gene in pancreatic endocrine beta-cells. To investigate the specific role that Pan proteins play in regulating insulin gene expression, we have created endocrine beta-cell stable integrants that constitutively express Pan antisense transcripts that selectively inhibit endogenous Pan protein synthesis in differentiated beta-cells. We demonstrate that diminished Pan protein levels in beta-cells, caused by Pan antisense transcripts, accompanies a dramatic attenuation of rat insulin gene transcription. We also show that the decrease in Pan protein expression correlates with a specific reduction of the beta-endocrine-specific Nir and Far element-binding activity, insulin enhancer factor 1.(ABSTRACT TRUNCATED AT 250 WORDS)