The calcium-sensing receptor complements parathyroid hormone-induced bone turnover in discrete skeletal compartments in mice

Abstract

The calcium-sensing receptor complements parathyroid hormone-induced bone turnover in discrete skeletal compartments in mice. Am J Physiol Endocrinol Metab 302: E841-E851, 2012. First published January 24, 2012; doi:10.1152/ajpendo.00599.2011.-Although the calcium-sensing receptor (CaSR) and parathyroid hormone (PTH) may each exert skeletal effects, it is uncertain how CaSR and PTH interact at the level of bone in primary hyperparathyroidism (PHPT). Therefore, we simulated PHPT with 2 wk of continuous PTH infusion in adult mice with deletion of the PTH gene (Pth Ϫ/Ϫ mice) and with deletion of both PTH and CaSR genes (Pth Ϫ/Ϫ -Casr Ϫ/Ϫ mice) and compared skeletal phenotypes. PTH infusion in Pth Ϫ/Ϫ mice increased cortical bone turnover, augmented cortical porosity, and reduced cortical bone volume, femoral bone mineral density (BMD), and bone mineral content (BMC); these effects were markedly attenuated in PTH-infused Pth Ϫ/Ϫ -Casr Ϫ/Ϫ mice. In the absence of CaSR, the PTH-stimulated expression of receptor activator of nuclear factor-B ligand and tartrate-resistant acid phosphatase and PTH-stimulated osteoclastogenesis was also reduced. In trabecular bone, PTH-induced increases in bone turnover, trabecular bone volume, and trabecular number were lower in Pth Ϫ/Ϫ -Casr Ϫ/Ϫ mice than in Pth Ϫ/Ϫ mice. PTH-stimulated genetic markers of osteoblast activity were also lower. These results are consistent with a role for CaSR in modulating both PTH-induced bone resorption and PTH-induced bone formation in discrete skeletal compartments. (40) and their osteoblastic progeny (4, 34), resulting in increased bone turnover in both cortical and trabecular bone and producing either net bone loss or net bone gain. In the fetus (27) and neonate (52), endogenous PTH is required for normal trabecular bone formation, and in both neonates and adult animals, exogenous PTH, when administered intermittently, can have an anabolic effect predominantly by increasing trabecular bone formation, although it may also concomitantly produce a small reduction in cortical bone (9,29,48). Primary hyperparathyroidism (PHPT) and continuous rather than intermittent PTH treatment induce cortical bone loss at least in part by enhancing endosteal bone resorption and intracortical bone resorption (17,25,36). Although severe, persistent elevations of PTH levels may also lead to trabecular bone loss (17), both PHPT and continuous PTH treatment often produce a modest increase in trabecular bone (33,44,54). The catabolic effect of PTH has been shown to be mediated in part by enhanced production of the cytokine receptor activator of nuclear factor-B ligand (RANKL) and by decreased production of the RANKL decoy receptor osteoprotegerin (OPG) by BMSC and osteoblasts (26,49).CaSR has been reported to function in vitro in a variety of skeletal cells, including BMSC, osteoblasts, monocytes/macrophages, and osteoclasts (6). By in situ hybridization, CaSR transcripts have been found mainly in osteoblasts, osteocytes, and bone marrow cells but rarely in mature osteocl...

The calcium-sensing receptor in bone—mechanistic and therapeutic insights

“…61 These findings seem to be consistent with a role for CaSR in augmenting PTHinduced bone formation in adults as well as in neonates.…”

“…72 In addition, CaSR has been reported to modulate PTHstimulated bone turnover in 8weekold mouse models, in which PTH was infused in Pth -/-;Casr -/-mice. 61 In Pth -/-;Casr -/-mice, the increases in cortical porosity and decreases in cortical bone volume induced by PTH infusion were reduced in comparison with cortical bone changes induced by PTH in Pth -/-mice. 61 Additionally, PTH stimulated RANKL expression and osteoclastogenesis were lower in cultured bone marrow from Pth -/-;Casr -/-mice than PTH responses in cultured bone marrow from Pth -/-mice.…”

“…61 In Pth -/-;Casr -/-mice, the increases in cortical porosity and decreases in cortical bone volume induced by PTH infusion were reduced in comparison with cortical bone changes induced by PTH in Pth -/-mice. 61 Additionally, PTH stimulated RANKL expression and osteoclastogenesis were lower in cultured bone marrow from Pth -/-;Casr -/-mice than PTH responses in cultured bone marrow from Pth -/-mice. 61 Overall, these observations are consistent with a role for CaSR in augment ing PTHinduced corti cal bone loss.…”

The calcium-sensing receptor in bone metabolism: from bench to bedside and back

“…Indeed, the continuous infusion of PTH, which mimics a state of primary hyperparathyroidism, increases cortical bone turnover and cortical porosity in PTH−/− but to a much lower extent in CaSR−/ −PTH−/−, indicating that CaSR mediates, at least in part, the effects of PTH on bone turnover [65].…”

The dual face of parathyroid hormone and prostaglandins in the osteoimmune system

“…Silver et al (109) demonstrated that HSCs express the calcium-sensing receptor (CaR), which is indispensable for niche spatial localization, and more recently, Adams et al (1) revealed that CaR is indispensable for the proper engraftment of HSCs within the endosteal niche that is rich in extracellular calcium. In addition, genetic deletion of CaR reduced PTH-stimulated bone turnover in trabecular bone and tended to impede PTH-induced increases in trabecular bone (145). Under normal physiological conditions, PTH plays a watchdog role for calcium homeostasis, and, further, PTH treatment provides stromal support for primitive hematopoietic cells (17,49).…”