ABSTRACT
We have previously shown that differentiation of hypertrophic chondrocytes is delayed in mice expressing a mutated PTH/PTHrP receptor (PTHR) (called DSEL here) that stimulates adenylyl cyclase normally but fails to activate phospholipase C (PLC). To better understand the role of PLC signaling via the PTHR in skeletal and mineral homeostasis, we examined these mice fed a normal or calcium-deficient diet. On a standard diet, DSEL mice displayed a modest decrease in bone mass. Remarkably, when fed a low-calcium diet or infused with PTH, DSEL mice exhibited strikingly curtailed peritrabecular stromal cell responses and attenuated new bone formation when compared with Wt mice. Attenuated in vitro colony formation was also observed in bone marrow cells derived from DSEL mice fed a low-calcium diet. Furthermore, PTH stimulated proliferation and increased mRNAs encoding cyclin D1 in primary osteoblasts derived from Wt but not from DSEL mice. Our data indicate that PLC signaling through the PTHR is required for skeletal homeostasis.
Subject(s)
Bone and Bones/drug effects , Parathyroid Hormone/pharmacology , Receptor, Parathyroid Hormone, Type 1/physiology , Type C Phospholipases/physiology , Animals , Animals, Newborn , Bone Remodeling/drug effects , Bone Remodeling/genetics , Bone and Bones/metabolism , Bone and Bones/physiology , Calcium, Dietary/pharmacology , Cells, Cultured , Diet , Female , Male , Mice , Mice, Inbred C57BL , Mice, Transgenic , Osteoblasts/drug effects , Osteoblasts/metabolism , Osteoblasts/physiology , Parathyroid Hormone/metabolism , Receptor, Parathyroid Hormone, Type 1/genetics , Receptor, Parathyroid Hormone, Type 1/metabolism , Signal Transduction/drug effects , Signal Transduction/genetics , Signal Transduction/physiology , Type C Phospholipases/metabolismABSTRACT
Parathyroid hormone (PTH) suppresses Dickkopf 1 (Dkk1) expression in osteoblasts. To determine whether this suppression is essential for PTH-mediated Wnt signaling and bone formation, we examined mice that overexpress Dkk1 in osteoblasts (Dkk1 mice). Dkk1 mice were osteopenic due to abnormal osteoblast and osteoclast activity. When fed a low-calcium diet, and in two other models of hyperparathyroidism, these mice failed to develop the peritrabecular stromal cell response ("osteitis fibrosis") and new bone formation seen in wild-type mice. Despite these effects of Dkk1 overexpression, PTH still activated Wnt signaling in Dkk1 mice and in osteoblastic cells cultured from these mice. In cultured MC3T3E1 preosteoblastic cells, PTH dramatically suppressed Dkk1 expression, induced PKA-mediated phosphorylation of beta-catenin, and significantly enhanced Lef1 expression. Our findings indicate that the full actions of PTH require intact Wnt signaling but that PTH can activate the Wnt pathway despite overexpression of Dkk1.
