Hisashi Takasu

Dual Signaling and Ligand Selectivity of the Human PTH/PTHrP Receptor

Parathyroid hormone (PTH) activates PTH/PTH‐related peptide‐related receptors (PTHRs) to stimulate both adenylyl cyclase (AC) and phospholipase C (PLC). How these parallel signals mediate specific cellular and tissue responses to PTH, such as the complex anabolic versus catabolic actions of PTH on bone, remains unsettled. Previous studies of PTHR signaling and function employed mainly rodent or other cell lines that express endogenous PTHRs and, possibly, alternate species of PTH receptors. To preclude confounding effects 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 concentration‐dependent activation of both AC and PLC via transfected hPTHRs. Maximal intensity of each signal increased with receptor density, but more hPTHRs were required for PLC than for AC activation. Coupling to AC was saturated 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 in rat cells, did not activate PLC via human (or rat) PTHRs under conditions (1 μM peptide, 106 hPTHRs/cell) where hPTH(1–34) stimulated PLC severalfold. Other cellular responses that require PKC activation in these cells, such as sodium‐dependent phosphate transport and cAMP‐independent secretion of plasminogen activator, were induced by PTH(1–34) but not by hPTH(3–34) 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 alternate, PLC‐independent pathways of PKC activation in some target cells. The relative intensity of AC and PLC signaling via the hPTHR may be strongly regulated by changes in its surface expression.

Type-1 parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptors activate phospholipase C in response to carboxyl-truncated analogs of PTH(1-34).

The carboxyl(C)-truncated human (h) PTH (hPTH) analog hPTH(1–31), which activates adenylyl cyclase (AC), but not protein kinase C, in rat osteosarcoma cells, exerts an anabolic effect on rat bone in vivo similar to that of hPTH(1–34). It has been proposed, therefore, that this action of PTH(1–34) is mediated exclusively by stimulation of AC via the rat type-1 PTH/PTH-related peptide (PTHrP) receptor (PTH1R). To determine whether this selective signaling pattern also might be a property of the hPTH1R, we studied signal transduction via heterologously expressed hPTH1Rs in response to activation by hPTH(1–34), hPTH(1–31), and a C-truncated analog that does not increase rat bone mass in vivo, hPTH(1–30). In porcine LLC-PK1 cells that stably expressed recombinant hPTH1Rs, these three peptides activated AC identically (EC50 = 1–2 nm). In cells with comparable expression of rat PTH1Rs, AC activation by hPTH(1–34) and hPTH(1–31) again was identical, whereas full activation by hPTH(1–30) required higher concentrat...