Aims/hypothesis  

Protein hydrolysates (peptones) increase not only glucagon-like peptide-1 (GLP-1) secretion but also transcription of the proglucagon (PG) gene in the intestine. The critical physiological roles of gut-derived GLPs raised hope for their therapeutic use in several disorders, especially GLP-1 in diabetes. We aimed to investigate the molecular mechanisms involved in this nutrient–PG gene interaction.

Methods  

Wild-type and mutated PG promoter fragments fused to the luciferase reporter gene were transfected into enteroendocrine STC-1 cells, which were then either treated or not with peptones. Co-transfection with expression vectors of dominant-negative forms of cAMP response element binding protein (CREB) and protein kinase A (PKA) proteins were performed, as well as electrophoresis mobility shift assays.

Results  

Deletion analysis showed that the promoter region spanning between –350 and –292 bp was crucial for the transcriptional stimulation induced by peptones. Site-directed mutagenesis of the canonical cAMP response element (CRE_PG) and of the adjacent putative CRE site (CRE-like1) led to a dramatic inhibition of the promoter responsiveness to peptones. Over expression of a dominant-negative mutant of CREB or of PKA produced a comparable and selective inhibitory effect on the activity of transfected promoter fragment containing the –350/–292 sequence. EMSA showed that CREB and fra2 transcription factors bound to CRE_PG and CRE-like1 elements respectively, independently of peptone treatment.

Conclusions/interpretation  

Our report identified cis- and trans-regulatory elements implicated in the transcriptional control of PG gene by nutrients in enteroendocrine cells. It highlights the role of a previously unsuspected CRE-like1 element, and emphasises the importance of CRE-related sequences in the regulation of PG gene transcription in the intestine.