Differential regulation of PTH-induced primary response gene expression in osteoblasts.

摘要

Parathyroid hormone (PTH), a potent mediator of calcium homeostasis, stimulates bone formation when administrated intermittently as in treatment for osteoporosis, while continuous infusion of PTH leads to severe bone loss. These dual effects are caused by a regulated osteoblastic gene expression by PTH.;To understand molecular mechanisms for dual effects of PTH, we first investigated expression kinetics of primary response genes (PRGs) in primary mouse calvarial osteoblasts and MC3T3-E1 cells in response to brief or continuous PTH exposure. Based on the necessity of PTH exposure over I hour to induce maximal mRNA expression level, PRGs were classified into two PRG groups, one represented by Nurr1 and the other represented by RANKL. Continuous PTH was required to uphold RNA transcription rate and acetylated histone H4 near the transcription start site of RANKL. The time-course pattern of PTH-induced nucleosomal structure change and histone H4 acetylation near Nurr1 and RANKL transcription start sites corresponded to RNA transcription rate change of each genes, suggesting gene-specific transcriptional regulation through chromatin remodeling.;We further examined PTH signaling cascades responsible for differential regulation of Nurr1 and RANKL gene expression. PKA signaling was required for mRNA expression and RNA transcription of both Nurr1 and RANKL, while ERK signaling was required for RANKL only. PKA signaling also down regulated Nurr1 mRNA level after peak induction without affecting its transcription rate, possibly through mRNA degradation. ChIP assays showed PKA signaling to be crucial for histone acetylation, suggesting its involvement in transcriptional regulation of PRGs through chromatin remodeling. PTH-induced cytoskeleton rearrangement and Rho GTPase signaling was also required for RANKL mRNA expression, but not for Nurr1, in osteoblasts. As multiple crosstalks between PKA, ERK and Rho GTPase signaling were revealed in our study, further study will be required to clarify the role of each signaling cascade in PRG expression. Taken altogether, our study demonstrated differential expression of PRG groups in response to brief vs. continuous PTH, a novel transcriptional regulation mechanism through chromatin remodeling and distinctive contribution of PKA and ERK signaling to Nurr1 and RANKL expression.