JunB as a downstream mediator of PTHrP actions in cementoblasts.
The role of AP-1 family members in the action of PTHrP was examined in cementoblasts. PTHrP increased mRNA and protein levels of all Fos members, but only one Jun member (JunB) was increased. Overexpression of JunB in cementoblasts mimicked actions of PTHrP to support osteoclastogenesis and inhibit cementoblast differentiation, suggesting that the actions of PTHrP on mesenchymal cells operate through JunB.


BACKGROUND
Cementoblasts are mesenchymal cells that share phenotypic features with osteoblasts in vitro; however, unlike osteoblasts, cementoblasts rarely support osteoclastogenesis in vivo. The osteoblast-mediated support of osteoclastogenesis involves PTH-induced reduction in osteoprotegerin (OPG) expression. PTH acts on osteoblastic cells through specific signaling pathways and transcription factors such as activator protein 1 (AP-1). The purpose of this study was to determine the impact of PTH-related protein (PTHrP) on AP-1 transcription factors in cementoblasts and the role of JunB in the actions of PTHrP.


METHODS
Cementoblastic cells were treated with PTHrP and evaluated for mRNA and protein levels of AP-1 family members. Stable transfectants of OCCM cells overexpressing JunB were evaluated for OPG production, ability to support osteoclastogenesis, and measures of proliferation and differentiation.


RESULTS
PTHrP treatment in vitro resulted in a time-dependent upregulation of mRNA and proteins for the Fos family members, but only JunB of the Jun family. OPG mRNA and protein levels were reduced by PTHrP in OCCM and were lower in JunB overexpressing cells than controls. In co-culture experiments, TRACP+ cells were increased with RANKL treatment in JunB overexpressing cells compared with controls. Cementoblast differentiation was reduced with overexpression of JunB as measured by a decrease in mineralized nodule formation and gene expression for bone sialoprotein and osterix. Measures of proliferation including cell number and cyclin D1 levels were increased in JunB overexpressing clones. In vivo, cementoblast implants exhibited a cementoblastoid nature with copious mineral-like matrix, whereas JunB-overexpressing implants were densely cellular with little mineralized matrix.


CONCLUSIONS
JunB was the only Jun family member increased by PTHrP, and its overexpression showed similar patterns of gene expression and OPG production as PTHrP treatment of controls. These data suggest that JunB may be a key mediator of PTHrP actions in cementoblasts.
Berry
Janice E
JE
Department of Periodontics and Oral Medicine, University of Michigan, Ann Arbor, Michigan 48109-1078, USA.
Ealba
Erin L
EL
Pettway
Glenda J
GJ
Datta
Nabanita S
NS
Swanson
Erica C
EC
Somerman
Martha J
MJ
McCauley
Laurie K
LK
D000818:Animals; D002352:Carrier Proteins / Q000494:pharmacology; D002454:Cell Differentiation / Q000187:drug effects / Q000235:genetics; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D018920:Coculture Techniques; D019938:Cyclin D1 / Q000032:analysis; D003739:Dental Cementum / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D015870:Gene Expression; D006023:Glycoproteins / Q000235:genetics / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000494:pharmacology; D051379:Mice; D053244:Osteoprotegerin; D044162:Parathyroid Hormone-Related Protein / Q000494:pharmacology*; D016760:Proto-Oncogene Proteins c-fos / Q000235:genetics / Q000378:metabolism; D016755:Proto-Oncogene Proteins c-jun / Q000235:genetics / Q000378:metabolism*; D053245:RANK Ligand; D012333:RNA, Messenger / Q000032:analysis / Q000378:metabolism; D053246:Receptor Activator of Nuclear Factor-kappa B; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000378:metabolism; D018124:Receptors, Tumor Necrosis Factor / Q000235:genetics / Q000378:metabolism; D012795:Sialoglycoproteins / Q000235:genetics; D000076182:Sp7 Transcription Factor; D018808:Transcription Factor AP-1 / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors / Q000235:genetics
D002352:Carrier Proteins; D006023:Glycoproteins; D008562:Membrane Glycoproteins; D053244:Osteoprotegerin; D044162:Parathyroid Hormone-Related Protein; D016760:Proto-Oncogene Proteins c-fos; D016755:Proto-Oncogene Proteins c-jun; D053245:RANK Ligand; D012333:RNA, Messenger; D053246:Receptor Activator of Nuclear Factor-kappa B; D018160:Receptors, Cytoplasmic and Nuclear; D018124:Receptors, Tumor Necrosis Factor; D012795:Sialoglycoproteins; D000076182:Sp7 Transcription Factor; C479466:Sp7 protein, mouse; C505687:Tnfrsf11a protein, mouse; C506031:Tnfrsf11b protein, mouse; C505689:Tnfsf11 protein, mouse; D018808:Transcription Factor AP-1; D014157:Transcription Factors; D019938:Cyclin D1