New insights into interactions between the human PTH/PTHrP receptor and agonist/antagonist binding

We prepared a polyclonal antiserum [Ab-(88-97)] against residues 88-97 of the NH₂-terminal tail of the human (h) parathyroid hormone (PTH)/PTH- related protein (PTHrP) receptor. Ab-(88-97) bound specifically to the receptor, as assessed by fluorescence-activated cell sorter analysis of HEK C21 cells, which stably express ~400,000 hPTH/PTHrP receptors per cell. Unlike PTH, Ab-(88-97) binding did not elicit either adenosine 3',5'-cyclic monophosphate or intracellular calcium concentration signaling responses in these cells. Incubation of C21 cells for 90 min at 4°C with hPTH-(1-34) plus antiserum reduced the Ab-(88-97) binding to the cells by up to 40-50% of control values in a PTH concentration-dependent fashion with a half-maximal effective concentration of ~5 nM. The decrease in Ab-(88-97) binding caused by hPTH-(1-34) was completely reversed by coincubation with hPTHrP-(7-34). We conclude that residues 88-97 of the hPTH/PTHrPR are involved, either directly or indirectly, in agonist but not antagonist binding to the receptor.