Continuous and intermittent exposure of neonatal rat calvarial cells to PTHrP (1-36) inhibits bone nodule mineralization in vitro by downregulating bone sialoprotein expression via the cAMP signaling pathway

The development and growth of the skeleton in the absence of parathyroid-hormone-related protein (PTHrP) is abnormal.  The shortening of appendicular bones in PTHrP gene null mice is explained by an effect of PTHrP on endochondral bone growth.  Whether or not PTHrP influences intramembranous ossification is less clear.  The purpose of this study was to determine the effect of exogenous PTHrP on intramembranous ossification in vitro.  Neonatal rat calvarial cells maintained in primary cell culture conditions that permit spontaneous formation of woven bone nodules by intramembranous ossification were studied. The expression of PTHrP, parathyroid hormone 1 receptor (PTH1R), and alkaline phosphatase (AP) by osteogenic cells in developing nodules and the effects of PTHrP (1-36) on nodule development was determined over 3-18 days. PTHrP and PTH1R were detected colonies of osteogenic cells on culture day three, and AP was detected on day six. PTHrP and its receptor were localized in pre-osteoblasts, osteoblasts, and osteocytes, and AP activity was detected in pre-osteoblasts and osteoblasts but not osteocytes. Continuous and intermittent exposure to PTHrP (1-36) decreased the number of mineralized bone nodules and bone sialoprotein (BSP) mRNA and protein, but had no effect on the number of AP-positive osteogenic cell colonies, cell proliferation, apoptosis, or osteopontin (OPN) mRNA. These results demonstrate that osteogenic cells that participate in the formation of woven bone nodules in vitro exhibit PTHrP and PTH1R before they demonstrate AP activity. Exogenous PTHrP (1-36) inhibits the mineralization of woven bone deposited during bone nodule formation in vitro, possibly by reducing the expression of BSP.