Discovery of novel peptide/receptor interactions: identification of PHM-27 as a potent agonist of the human calcitonin receptor

Many naturally occurring Peptides exhibit a high degree of promiscuity across G-protein coupled receptor subtypes. The degree to which this phenomenon occurs, and its physiological significance is not well characterized. In addition, many 'orphan' Peptides exist for which there are no known receptors. Therefore, to identify novel interactions between biologically active Peptides and G-protein coupled receptors, a library of nearly 200 Peptides was screened against the human Calcitonin (hCTr), human Parathyroid Hormone (PTH1R), human Corticotropin Releasing Factor (CRF1), and the human Glucagon-like peptide (GLP1) receptors using a cell-based functional assay (Receptor Selection and Amplification Technology). Functional profiling revealed that the 'orphan peptide' PHM-27 selectively activated the hCTr; no activity was observed at the PTH1, CRF1, or GLP1 receptors. PHM-27 was a potent agonist at the hCTr, with similar efficacy as human Calcitonin, and a potency of 11 nM. These results were confirmed in cyclic AMP assays. Responses to Calcitonin and PHM-27 could be suppressed by the antagonist salmon Calcitonin (8-32). In competition binding studies, salmon Calcitonin (8-32), Calcitonin, and PHM-27 were each able to inhibit (125)I-calcitonin from cell membranes containing transiently expressed hCTr. These results indicate that the orphan peptide PHM-27 is a potent agonist at the hCTr.