THE XENOPUS PROGLUCAGON GENE ENCODES NOVEL GLP-1-LIKE PEPTIDES WITH INSULINOTROPIC PROPERTIES

The proglucagon gene encodes several hormones that have key roles in t he regulation of metabolism, In particular, glucagon-like peptide (GLP -1), a potent stimulus of insulin secretion, is being developed as a t herapy for the treatment of non-insulin-dependent diabetes mellitus, T o define structural moieties of the molecule that convey its insulinot ropic activity, we have cloned and characterized the proglucagon gene from the amphibian, Xenopus laevis, Unexpectedly, these cDNAs were fou nd to encode three unique glucagon-like-1 peptides, termed xenGLP-1A, xenGLP-1B, and xenGLP-1C in addition to the typical proglucagon-derive d hormones glucagon and GLP-2. xenGLP-1A, -1B, and -1C were synthesize d and tested for their ability to bind and activate the human GLP-1 re ceptor (hGLP-IR), and to stimulate insulin release from rat pancreas, All three Xenopus GLP-1-like peptides bind effectively to the hGLP-1R and stimulate cAMP production. Surprisingly, xenGLP-1B(1-30) demonstra ted higher affinity for the hGLP-1R than hGLP-1 (IC50 of 1.1 +/- 0.4 n M vs, 4.4 +/- 1.0 nM, respectively, P < 0.02) and was equipotent to hG LP-1 in stimulating cAMP production (EC50 of 0.17 +/- 0.02 nM vs, 0.6 +/- 0.2 nM, respectively, P > 0.05), Further studies demonstrated that hGLP-1, xenGLP-1A, -1B, and -1C stimulate comparable insulin release from the pancreas. These results demonstrate that despite an average o f nine amino acid differences between the predicted Xenopus GLPs and h GLP-1, all act as hGLP-1R agonists.