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 o f bone (GCT). Both GC-2 and GC-10 differ structurally from the human o varian cell CTR (o-hCTR) that we cloned previously, but differ from ea ch other only by the presence (GC-10) or absence (GC-2) of a predicted 16-amino acid insert in the putative first intracellular domain, Expr ession of all three CTR isoforms in COS cells demonstrated that GC-2 h as a lower binding affinity for salmon (s) CT (K-d similar to 15 nM) t han GC-10 or o-hCTR (K-d similar to 1.5 nM). Maximal stimulatory conce ntrations of CT resulted in a mean accumulation of cAMP in GC-2 transf ected cells that was greater than eight times higher than in cells tra nsfected with GC-10 after normalizing for the number of receptor-expre ssing cells, The marked difference in maximal cAMP response was also a pparent after normalizing for receptor number, GC-2 also demonstrated a more potent ligand-mediated cAMP response compared with GC-1O for bo th 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 simi lar to 6 nM for sCT and similar to 25 nM for hCT), Reverse transcripta se PCR of GCT RNA indicated that GC-2 transcripts are more abundant th an those encoding for GC-10. In situ hybridization on GCT tissue secti ons demonstrated CTR mRNA expression in osteoclast-like cells. We loca lized the human CTR gene to chromosome 7 in band q22. The distinct fun ctional characteristics of GC-2 and GC-10, which differ in structure o nly in the first intracellular domain, indicate that the first intrace llular domain of the CTR plays a previously unidentified role in modul ating ligand binding and signal transduction via the G protein/adenyla te cyclase system.