Background: Parathyroid hormone (PTH) regulates osteoblast function by binding to the PTH receptor 1 (PTHR1) to activate downstream signaling to induce expression of primary response genes (PRGs), which affect various aspects of the osteoblast phenotype. We previously identified PTH-induced PRGs in MC3T3-E1 cells, including mitogen-activated protein kinase (MAPK) phosphatase 1 (mkp1), which dephosphorylates members of the MAPK family. The aim of this study was to explore the molecular mechanisms of PTH's induction of mkp1 in primary mouse osteoblasts.
Methods: Northern and Western analyses were used to determine mkp1 mRNA and protein expression. In vivo experiments were also performed to determine PTH's effect on mkp1 in mouse calvariae and long bones.
Results: A total of 10 nM PTH and PTH-related protein (PTHrP) maximally induced mkp1 mRNA levels after 1 hour in osteoblasts. PTH also increased mkp1 protein expression, and induced mkp1 mRNA independent of new protein synthesis. PTHR1 triggers protein kinase A (PKA), PKC, and calcium pathways. Although PKA and PKC agonists induced mkp1 mRNA levels, only cyclic adenosine 3':5'-monophosphate (cAMP)-PKA inhibition blocked PTH-induced mkp1 mRNA levels. These data suggest that PTH-induced mkp1 mRNA levels are primarily mediated through the cAMP-PKA pathway. Further, prostaglandin E2 (PGE2), which activates cAMP-PKA and PKC, induced mkp1 mRNA to a greater extent than PGF2alpha and fluprostenol, which activate PKC signaling only. Finally, PTH maximally induced mkp1 mRNA levels in mouse calvariae and long bones in vivo at 0.5 hours.
Conclusions: mkp1's in vitro and in vivo induction in PTH-target tissues suggests its involvement in some of the effects of PTH on osteoblast function. mkp1 may be an important target gene in the anabolic effect of PTH on osteoblasts.