Parathyroid hormone stimulates fra-2 expression in osteoblastic cells in vitro and in vivo

PTH and PTH-related protein (PTHrP) are key mediators of skeletal developme nt and homeostasis through their activation of the PTH-1 receptor. Previous studies have found that several AP-1 family members are regulated by PTH, such as c-fos, fra-1, and c-jun. There are numerous genes in the bone micro environment that contain AP-1 sites, and different Fos family members are r eported to have opposing transcriptional activities at AP-1 sites. The purp ose of this study was to identify the effects of PTH on expression of the A P-1 protein complex member, fra-2, to extend our understanding of transcrip tional regulators of PTH action. PTH induction of fra-2 messenger RNA (mRNA ) levels in MC3T3-E1 preosteoblastic cells was maximal with 0.1 muM PTH (1- 34). The expression in vitro was greatest Ih after treatment and was presen t with N-terminal PTH but not PTH (7-34) or (53-84). Cycloheximide treatmen t induced fra-2 expression, and actinomycin D inhibited basal and PTHrP-ind uced expression. AP-1 protein in nuclear extracts of MC3T3-E1 cells was inc reased with PTH treatment at 3 h and consisted of high levels of Fra-a prot ein, as evidenced by a supershift in an electrophoretic mobility shift assa y and Western blot analysis. Up-regulation of steady-state fra-2 mRNA was a lso noted in vivo, where injection of PTH (1-34) (20 mug) resulted in a mor e-than-7-fold maximal increase in fra-2 mRNA expression in the calvaria of mice, after 1 h of treatment. These data add to the transcriptional mediato rs induced by PTH and suggest that the interplay of AP-1 family members wil l provide insight into regulatory pathways of PTH and PTHrP for their anabo lic and catabolic actions in bone.