EXPRESSION OF 2 HUMAN SKELETAL CALCITONIN RECEPTOR ISOFORMS CLONED FROM A GIANT-CELL TUMOR OF BONE - THE FIRST INTRACELLULAR DOMAIN MODULATES LIGAND-BINDING AND SIGNAL-TRANSDUCTION

Two distinct calcitonin (CT) receptor (CTR)-encoding cDNAs (designated GC-2 and GC-10) were cloned and characterized from giant cell tumor of bone (GCT). Both GC-2 and GC-10 differ structurally from the human ovarian cell CTR (o-hCTR) that we cloned previously, but differ from each other only by the presence (GC-10) or absence (GC-2) of a predicted 16-amino acid insert in the putative first intracellular domain, Expression of all three CTR isoforms in COS cells demonstrated that GC-2 has a lower binding affinity for salmon (s) CT (K-d similar to 15 nM) than GC-10 or o-hCTR (K-d similar to 1.5 nM). Maximal stimulatory concentrations of CT resulted in a mean accumulation of cAMP in GC-2 transfected cells that was greater than eight times higher than in cells transfected with GC-10 after normalizing for the number of receptor-expressing cells, The marked difference in maximal cAMP response was also apparent after normalizing for receptor number, GC-2 also demonstrated a more potent ligand-mediated cAMP response compared with GC-1O for both human (h) and sCT (the ECS, values for GC-2 were similar to 0.2 nM for sCT and similar to 2 nM for hCT; EC(50) values for GC-10 were similar to 6 nM for sCT and similar to 25 nM for hCT), Reverse transcriptase PCR of GCT RNA indicated that GC-2 transcripts are more abundant than those encoding for GC-10. In situ hybridization on GCT tissue sections demonstrated CTR mRNA expression in osteoclast-like cells. We localized the human CTR gene to chromosome 7 in band q22. The distinct functional characteristics of GC-2 and GC-10, which differ in structure only in the first intracellular domain, indicate that the first intracellular domain of the CTR plays a previously unidentified role in modulating ligand binding and signal transduction via the G protein/adenylate cyclase system.