Parathyroid hormone-related protein regulates extracellular matrix gene expression in cementoblasts and inhibits cementoblast-mediated mineralizationin vitro

Parathyroid hormone-related protein (PTHrP) has been implicated in regulati ng tooth eruption and/or development. Formation of cementum, a mineralized tissue covering the tooth root surface, is a critical biological event for tooth root development. To test the hypothesis that PTHrP targets cementobl asts (CMs) and acts to regulate cementogenesis, CM cell lines were establis hed and their responsiveness to PTHrP stimulation was determined, in vitro. First, subclones were derived from two immortalized murine cell population s that contained CiMs; SV-CM/periodontal ligament (PDL) cells were obtained from the root surface of first mandibular molars of CD-1 mice and immortal ized with SV40 T-antigen (TAg), and OC-CM cell population was established f rom OC-TAg transgenic mice in which their cells harbor an osteocalcin (OC a nd/or OCN) promoter-driving immortal gene SV40 TAg. Based on our previous i n situ studies, CM subclones were identified as cells expressing bone sialo protein (BSP) and OCN transcripts, while PDL cell lines were designated as cells lacking BSP and OCN messenger RNA (mRNA). CMs exhibited a cuboidal ap pearance and promoted biomineralization, both in vitro and in vivo. In cont rast, PDL cells (PDL subclones) displayed a spindle-shaped morphology and l acked the ability to promote mineralized nodule formation, both in vitro an d in vivo. Next, using these subclones, the effect of PTHrP on cementogenes is was studied. CMs, not PDL cells, expressed PTH/PTHrP receptor mRNA and e xhibited PTHrP-mediated elevation in cyclic adenosine mono-phosphate (cAMP) levels and c-fos gene induction. PTHrP stimulation repressed mRNA expressi on of BSP and OCN in CMs and blocked CM-mediated mineralization, in vitro. Collectively, these data suggest that CMs possess PTH/PTHrP receptors and, thus, are direct targets for PTHrP action during cementogenesis and that PT HrP may serve as an important regulator of cementogenesis.