THE CAUDAL-RELATED HOMEODOMAIN PROTEIN CDX-2 3 REGULATES GLUCAGON GENE-EXPRESSION IN ISLET CELLS/

Glucagon gene transcription in the endocrine pancreas is regulated by at least four cis-acting DNA control elements. We showed previously th at G1 is critical for alpha cell-specific expression. G1 contains thre e AT-rich sequences important for promoter function, which represent c andidate binding sites for homeodomain transcription factors. Performi ng reverse transcription-polymerase chain reaction amplifications with degenerate oligonucleotide primers homologous to the Antennapedia hom eobox, cDNA clones corresponding to the caudal-related gene cdx-2/3 we re predominantly obtained from glucagon-producing cells and primary no n-beta cells. From RNase protection and polymerase chain reaction anal yses, cdx-2/3 turned out to be the only caudal-related gene that is ex pressed at significant levels in cells of the endocrine pancreas, Cdx- 2/3 binds with high affinity to an AT-rich motif of G1, which matches the consensus binding site of caudal-related proteins. In the glucagon -producing hamster cell line InR1G9, Cdx-2/3 is a subunit of complex B 3 formed on G1. Alternative splicing generates two cdx-2/3 transcripts in islet cells, coding for a full-length protein and an amino-termina lly truncated isoform. Although both isoforms bind G1 with similar aff inity, only the full-length Cdx-2/3 A protein activates glucagon gene transcription in non-glucagon-producing cells, transcriptional activat ion being dose-dependent. We therefore conclude that the caudal-relate d gene cdx-2/3 is implicated in the transcriptional control of glucago n gene expression in the alpha cells of the islets of Langerhans.