Parathyroid Hormone Stimulates Receptor Activator of NFκB Ligand and Inhibits Osteoprotegerin Expression via Protein Kinase A Activation of cAMP-response Element-binding Protein

Abstract

Parathyroid hormone (PTH) stimulates osteoclast formation by binding to its receptor on stromal/osteoblastic cells and stimulating the production of receptor activator of NFB ligand (RANKL) and inhibiting the expression of osteoprotegerin (OPG). However, the mechanisms through which PTH regulates these genes remain unknown. Here we report that PTH stimulated RANKL gene transcription and increased RANKL mRNA stability in murine stromal/osteoblastic cells stably expressing human PTH/PTH-related protein receptor 1. PTH also potently suppressed OPG mRNA in these cells. Cycloheximide did not block the effects of PTH on RANKL but did inhibit the suppression of OPG mRNA. Activation of protein kinase A (PKA) was necessary and sufficient for the effect of PTH on both genes. Conditional expression of a dominant-negative form of the transcription factor CREB, but not c-fos or Runx2, significantly reduced PTH stimulation of RANKL. CREB activity was also required for full stimulation of RANKL by oncostatin M or 1,25-dihydroxyvitamin D 3 . Dominant-negative forms of CREB and c-fos reduced the suppression of OPG by PTH. These results demonstrate that PTH directly stimulates RANKL expression via a PKA-CREB pathway and that CREB may be a central regulator of RANKL expression. Furthermore, they suggest that PTH suppression of OPG involves CREB and c-fos. Parathyroid hormone (PTH)1 maintains calcium homeostasis in part by increasing the production of bone-resorbing osteoclasts to release calcium from the skeleton. It does so indirectly by binding to its G-protein-coupled receptor, PTH/PTHrelated protein receptor 1 (PTHR1), on stromal/osteoblastic cells (1). These cells support osteoclast formation via expression of M-CSF and receptor activator of NFB-ligand (RANKL) (2). Mice deficient in either are incapable of forming osteoclasts, and together these proteins are sufficient to stimulate osteoclast formation in vitro in the absence of stromal/osteoblastic cells (3)(4)(5). In vitro and in vivo studies have demonstrated that the magnitude of osteoclast formation is directly proportional to the level of RANKL expression, whereas M-CSF appears to play primarily a permissive role (4, 6, 7). RANKL binds to its receptor RANK on hematopoietic precursors of osteoclasts and stimulates their differentiation and survival (8). The action of RANKL is blocked by osteoprotegerin (OPG), which functions as a soluble decoy receptor (9). PTH simultaneously stimulates RANKL and inhibits OPG expression in primary cultures of stromal/osteoblastic cells and in bone tissue from parathyroidectomized rats infused with this hormone (10, 11). Based on this, it has been proposed that PTH influences osteoclast formation primarily by regulation of the RANKL/OPG ratio (2). However, the signaling pathways and molecular mechanisms utilized by PTH to regulate these genes remain largely unknown.Binding of PTH to PTHR1 on stromal/osteoblastic cells activates both the protein kinase A (PKA) and protein kinase C (PKC) pathways, and both pathways have been implicated in P...

Suppression of Osteoprotegerin Expression by Prostaglandin E2 Is Crucially Involved in Lipopolysaccharide-Induced Osteoclast Formation

“…The previous studies have shown that OPG production by osteoblasts is down-regulated by bone-resorbing factors such as 1,25(OH) 2 D 3 , PTH, and PGE 2 (38,(41)(42)(43)(44). Our results confirmed the previous finding that osteoclasts spontaneously form in the control cocultures containing OPG Ϫ/Ϫ osteoblasts.…”

“…6). Recently, Fu et al (38) reported that the activation of CREB by PTH is required for PTH-induced down-regulation of OPG expression. This suggests that the cAMP-PKA signals play a role in PGE 2 -induced suppression of OPG mRNA expression.…”

Circasemidian rather than circadian variation of circulating osteoprotegerin in clinical health

“…In particular, hormones such as PTH [10] and glucocorticoids [11,12] have been reported to decrease OPG concentrations while anabolic agents such as estrogens, transforming growth factor β, related bone morphogenic factor and thrombopoietin [13][14][15][16] have been shown to enhance the OPG production in the osteoblastic and bone stromal cells. Recent studies [17][18][19] have described a bimodal pattern for the variation along the 24-h scale of circulating PTH, with a major peak during the night and a secondary peak in the late afternoon.…”

Phosphodiesterase inhibitors stimulate osteoclast formation via TRANCE/RANKL expression in osteoblasts: possible involvement of ERK and p38 MAPK pathways

“…This pathway is known to induce TRANCE expression in osteoblasts following stimulation with PTH and PGE 2 [12][13][14]. Intracellular cAMP is generated by adenylate cyclase from adenosine triphosphate (ATP) as a substrate, whereas cAMP-specific phosphodiesterases (PDEs) degrade cAMP by hydrolysis [10,11].…”