Expression of two human skeletal calcitonin receptor isoforms cloned from a giant cell tumor of bone

Two distinct calcitonin ( CT ) receptor (CTR)- encoding cDNAs( designated GC-2 and GC-l0) 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 binging affinity for salmon (s) CT ( Kd, ～15 nM) than GC-10 or o-hCTR (Kd ~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- 10 for both human(h) and sCT ( the EC50 values for GC-2 were ～0.2 nM for sCT and ～2 nM for hCT; EC50 values for GC-10 were ～6 nM for sCT and ～25nM 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