Dual signaling and ligand selectivity of the human PTH/PTHrP receptor

Parathyroid hormone (PTH) activates PTH/PTH-related peptide-related recepto rs (PTHRs) to stimulate both adenylyl cyclase (AC) and phospholipase C (PLC ). How these parallel signals mediate specific cellular and tissue response s to PTH, such as the complex anabolic versus catabolic actions of PTH on b one, remains unsettled. Previous studies of PTHR signaling and function emp loyed mainly rodent or other cell lines that express endogenous PTHRs and, possibly, alternate species of PTH receptors, To preclude confounding effec ts of such receptors, we stably expressed recombinant human PTHRs (hPTHRs) at different levels of surface density in LLC-PK1 porcine renal epithelial cells that lack endogenous PTH responsiveness. hPTH(1-34) induced concentra tion-dependent activation of both AC and PLC via transfected hPTHRs, Maxima l intensity of each signal increased with receptor density, but more hPTHRs : were required for PLC than for AC activation. Coupling to AC was saturate d at receptor densities too low to detect sustained PLC activation. hPTH(3- 34), found by others to be a PLC/protein kinase C (PKC)-selective peptide i n rat cells, did not activate PLC via human (or rat) PTHRs: under condition s (1 mu M peptide, 10(6) hPTHRs/cell) where hPTH(1-34) stimulated PLC sever al fold. Other cellular responses that require PKC activation in these cell s, such as sodium-dependent phosphate transport and cAMP-independent secret ion of plasminogen activator, were induced by PTH(1-34) but not by hPTH(3-3 4) or hPTH(7-34). We conclude that amino-truncated PTH analogs reported to activate PKC cannot directly activate phosphatidylinositol-specific PLC via the human or rat PTHR and therefore that PTH receptors may access alternat e, PLC-independent pathways of PKC activation in some target cells. The rel ative intensity of AC and PLC signaling via the hPTHR may be strongly regul ated by changes in its surface expression.