Expression cloning and receptor pharmacology of human calcitonin receptors from MCF-7 cells and their relationship to amylin receptors

Human breast cell carcinoma MCF-7 cells were found to bind I-125-labeled rat amylin (rAmylin) and the peptide amylin antagonist radioligand I-125-AC512 with high affinity. This high affinity binding possessed characteristics unique to the already defined high affinity binding site for amylin in the rat nucleus accumbens [Mol. Pharmacol. 44:493-497 (1993); J. Pharmacol. Exp. Ther. 270:779-787 (1994); Eur. J. Pharmacol. 262:133-141 (1994)], To further define this receptor, we report results of expression cloning studies from an MCF-7 cell library. We isolated two variants of a seven-transmembrane receptor that were identical to two previously described human calcitonin receptors (hCTR1 and hCTR2). These receptors were characterized by expression in different surrogate host cell systems, Transient expression of hCTR1 in COS cells yielded membranes that bound I-125-AC512 and I-125-salmon calcitonin with high affinity, but no high affinity binding was observed with I-125-human calcitonin (hCAL) or I-125-rAmylin. Stable expression of hCTR1 in HEK 293 cells produced similar data. In contrast, expression of hCTR2 in COS cells yielded membranes that bound I-125-AC512, I-125-hCAL, and I-125-rAmylin with high affinity. The agonists I-125-hCAL and I-125-rAmylin bound 65% and 1.5%, respectively, of the sites bound by the antagonist radioligand I-125-AC512 in this expression system. This pattern of binding was repealed in HEK 293 cells stably transfected with hCTR2 (I-125-hCAL = 24.8% B-max, I-125-rAmylin = 8% B-max). In both expression systems, the agonists hCAL and rAmylin were much more potent in displacing their radioligand counterparts than was the antagonist radioligand I-125-AC512. For example, the pK(i) value for displacement of I-125-AC512 by rAmylin was 7.2 in HEK 293 cells but rose to 9.1 when displacing I-125-rAmylin. Finally, hCTR2 was expressed in baculovirus-infected Ti ni cells. In this system, only specific binding to the antagonist I-125-AC512 and agonist I-125-hCAL was observed; no binding to I-125-rAmylin could be detected. These data are discussed in terms of two working hypotheses. The first is that amylin is a weak agonist far hCTR2 and that this receptor is unrelated to the amylin receptor found in this cell line. The second is that hCTR2 couples to different G proteins for calcitonin and amylin function in different cells. At present, these data cannot be used to disprove conclusively either hypothesis.