PROTOONCOGENE C-FOS IS TRANSCRIPTIONALLY REGULATED BY PARATHYROID-HORMONE (PTH) AND PTH-RELATED PROTEIN IN A CYCLIC ADENOSINE MONOPHOSPHATE-DEPENDENT MANNER IN OSTEOBLASTIC CELLS

PTH and PTH-related protein (PTHrP) bind to the PTH-1 (PTH/ PTHrP) rec eptor and produce anabolic and catabolic effects in bone. To investiga te postreceptor mechanisms of action, MC3T3-E1 cells were induced to d ifferentiate to optimize PTH-1 receptor expression, and differentiated MC3T3-E1 cells were treated with varying doses of PTH (1-34) for I h. Northern blot analysis revealed a dose-dependent stimulation of stead y state c-ibs messenger RNA (mRNA), with measurable expression at dose s as low as 1 par PTH. The time course of c-fos mRNA induction was rap id, with peak levels detected at 30-45 min. Increased steady state c-i bs mRNA was due to increased transcription of the c-fos gene as demons trated by nuclear run-on assays and was dependent on the temporal diff erentiation state of the MC3T3-E1 cells. Stimulation of c-ibs mRNA was induced exclusively by N-terminal PTH and PTHrP (which is also respon sible for cAMP activation), and did not occur with PTH (7-34), (53-84) , or PTHrP (107-139). The effects of PTH (1-34) on c-fos stimulation w ere dependent on intracellular cAMP. Forskolin ia guanine-nucleotide-b inding protein (G(alpha)) agonist] stimulated c-fos mRNA, whereas 9-(t etrahydro-2-furyl) adenine (THFA) (a cAMP antagonist), 1,9 dideoxyfors kolin (a cAMP independent analog of forskolin), and phorbol 12-myrista te 13-acetate (a protein kinase C activator) did not. Furthermore, THF A inhibited the ability of PTH (1-34) to stimulate c-fos mRNA in a tim e-dependent manner. These findings indicate that c-fos is transcriptio nally regulated by PTH (1-34) in osteoblastic cells, and that cAMP is a mediator of PTH-stimulated c-fos induction. Several known bone-assoc iated proteins contain DNA binding sites in their promoter regions tha t recognize c-fos in conjunction with c-jun (AP-1 sites). Consequently , the induction of c-fos by PTH (1-34) in osteoblastic cells may be a sensitive indicator of PTH effects in vitro and in vivo, and provide v aluable information regarding mechanisms of PTH action in bone.