Contributions of parathyroid hormone (PTH)/PTH-related peptide receptor signaling pathways to the anabolic effect of PTH on bone.

PTH regulates osteoblastic function by activating PTH/PTHrP receptors (PTH1Rs), which trigger several signaling pathways in parallel, including cAMP/protein kinase A (PKA) and, via both phospholipase-C (PLC)-dependent and PLC-independent mechanisms, protein kinase C (PKC). These signaling functions have been mapped to distinct domains within PTH(1-34), but their roles in mediating the anabolic effect of intermittent PTH in vivo are unclear. We compared the anabolic effects in mice of hPTH(1-34) with those of two analogs having restricted patterns of PTH1R signaling. [G(1),R(19)]hPTH(1-28) lacks the 29-34 domain of hPTH(1-34) needed for PLC-independent PKC activation, incorporates a Gly(1) mutation that prevents PLC activation, and stimulates only cAMP/PKA signaling. [G(1),R(19)]hPTH(1-34) retains the 29-34 domain and activates both cAMP/PKA and PLC-independent PKC. Human PTH(1-34) (40 microg/kg), [G(1),R(19)]hPTH(1-34) (120 microg/kg), and [G(1),R(19)]hPTH(1-28) (800 microg/kg), at doses equipotent in elevating blood cAMP at 10 min and cAMP-dependent gene expression in bone at 6 h after s.c. injection, were administered to 10-week-old female C57BL/6J mice 5 days/week for 4 weeks. Acute blood cAMP responses, retested after 4 weeks, were not reduced by the preceding PTH treatment. The three PTH peptides induced equivalent increases in distal femoral bone mineral density (BMD), and, by microCT analysis, distal femoral and vertebral bone volume and trabecular thickness and mid-femoral cortical endosteal apposition. [G(1),R(19)]hPTH(1-34) and hPTH(1-34) increased distal femoral BMD more rapidly and augmented total-body BMD and bone volume of proximal tibial trabeculi to a greater extent than did [G(1),R(19)]hPTH(1-28). We conclude that cAMP/PKA signaling is the dominant mechanism for the anabolic actions of PTH in trabecular bone and that PLC-independent PKC signaling, attributable to the PTH(29-34) sequence, appears to accelerate the trabecular response and augment BMD at some skeletal sites. PTH1R PLC signaling pathway is not required for an anabolic effect of intermittent PTH(1-34) on bone.