We have previously shown an increased incidence of alpha-subunit-producing thyrotroph
tumors after
salmon calcitonin (sCT) injection into rats. However, it is not clear whether the effects of CT are direct or indirect. 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 membranes revealed a high affinity binding site for sCT [IC50 = 0.97 +/- 0.18 nM (n = 4); Kd = 5.45 +/- 0.43 nM (n = 3); binding capacity = 6.6 pmol/mg
protein (n = 3)]. Rat CT did not compete with binding at this site. Receptor screening for other CT
peptide family members revealed high specific binding for CT 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 accumbens.
Guanosine 5'O-(3-thiotriphosphate) (
GTP gamma S) (20 microM), reduced [125I]CGRP binding to 38% of maximal, indicating that this site is
G-protein coupled. Immunocytochemically, all of the cells displayed intense sCT-like immunoreactivity, which was totally abolished by preabsorption of the antibody with sCT. The presence of this receptor supports the hypothesis that sCT mediates
tumorigenesis via a direct pituitary action 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.