EXPRESSION OF A NOVEL RECEPTOR FOR THE CALCITONIN PEPTIDE FAMILY AND A SALMON CALCITONIN-LIKE PEPTIDE IN THE ALPHA-THYROTROPIN THYROTROPH CELL-LINE

We have previously shown an increased incidence of alpha-subunit-produ cing thyrotroph tumors after salmon calcitonin (sCT) injection into ra ts. However, it is not clear whether the effects of CT are direct or i ndirect. Our hypothesis was that for sCT to act directly, it must have a binding site on thyrotrophs, The (alpha TSH cell line was used as a model for thyrotrophs. Receptor binding studies using alpha TSH membr anes revealed a high affinity binding site for sCT [IC50 = 0.97 +/- 0. 18 nM (n = 4); K-d = 5.45 +/- 0.43 nM (n = 3); binding capacity = 6.6 pmol/mg protein (n = 3)]. Rat GT did not compete with binding at this site. Receptor screening for other CT peptide family members revealed high specific binding for GT gene-related peptide (CGRP; IC50 = 0.25 /- 0.08 nM; n = 3) and islet amyloid polypeptide (IC50 = 4.36 +/- 1.1 nM; n = 3). This together with the absence of rat CT binding excluded a conventional CT-binding site, and, we propose a site similar to the CGRP subtype III receptor described in the rat nucleus so cumbens. Gua nosine 5'-O-(3-thiotriphosphate) (GTP gamma S) (20 mu M), reduced [I-1 25]CGRP binding to 38% of maximal, indicating that this site is G-prot ein coupled. Immunocytochemically, all of the cells displayed intense sCT-like immunoreactivity, which was totally abolished by preabsorptio n of the antibody with sCT. The presence of this receptor supports the hypothesis that sCT mediates tumorigenesis via a direct pituitary act ion and, together with the coexistence of a sCT-like peptide in these cells, provides evidence for a possible autocrine role of this peptide in the control of thyrotroph function.