Parathyroid hormone inhibits Notch signaling in osteoblasts and osteocytes

Abstract

Parathyroid hormone (PTH) and Notch receptors regulate bone formation by governing the function of osteoblastic cells. To determine whether PTH interacts with Notch signaling as a way to control osteoblast function, we tested the effects of PTH on Notch activity in osteoblast- and osteocyte-enriched cultures. Notch signaling was activated in osteoblast-enriched cells from wild-type C57BL/6J mice following exposure to the Notch ligand Delta-like (Dll)1 or by the transient transfection of the Notch intracellular domain (NICD), the transcriptionally active fragment of Notch1. To induce Notch signaling in osteocyte-enriched cultures, a murine model of Notch2 gain-of-function was used. PTH opposed the stimulatory effects of Dll1 on Hey1, Hey2 and HeyL mRNA levels in osteoblast-enriched cells and suppressed the expression of selected Notch target genes in osteocyte-enriched cultures, either under basal conditions or in the context of Notch2 gain-of-function. Induction of Notch signaling in osteocytes did not alter the inhibitory effect of PTH on Sost expression, but reduced the stimulation of Tnfsf11 mRNA levels by PTH. In agreement with these in vitro observations, male mice administered with PTH displayed suppressed Hey1 and HeyL expression in parietal bones. Transactivation experiments with a Notch reporter construct and electrophoretic mobility shift assays in osteoblast-enriched cells suggest that PTH acts by decreasing the capacity of Rbpjκ to bind to DNA. In conclusion, downregulation of Notch in osteoblasts and osteocytes may represent a mechanism contributing to the anabolic effects of PTH in bone.

Notch in skeletal physiology and disease

“…Functional differences among Notch receptors are related to structural distinctions in the NICD, to differential interactions of each NICD with RBPJκ, to the temporal and cellular expression of each receptor, to variations in the affinity of the extracellular domain of Notch for its ligands and to NRR sequence differences conveying specificity to the activation of Notch [13, 14]. Notch1, 2 and 3 and low levels of Notch4 are detected in skeletal cells [1, 15]. Notch 1 and 2 are expressed by cells of the osteoblast and osteoclast lineage, whereas Notch3 is mostly expressed by osteoblasts and osteocytes, and not by cells of the osteoclast lineage.…”

“…Because Notch2 induces RANKL and enhances osteoclastogenesis, a consideration is the use of denosumab and the agent was used successfully in the treatment of a subject with osteoporosis and fractures [44]. Although PTH suppresses Notch signaling, the use of teriparatide in the treatment of HCS could pose risks [15]. There is evidence of Notch activation in osteosarcoma in humans and prolonged activation of Notch in mice can cause osteosarcoma, suggesting that Notch activation could be a risk factor for osteosarcoma [45, 46].…”

The lateral meningocele syndrome mutation causes marked osteopenia in mice

“…Since a possible explanation for the Notch3 tm1.1Ecan phenotype was an increase in osteoclastogenesis, and since osteocytes play a major role in the control of bone resorption, we examined for the expression of Tnfsf11 mRNA in enzymatically/EDTA digested osteocyte-rich femurs (14,35).…”

“…Notch1, 2 and 3, and low levels of Notch4 are expressed in skeletal cells (13,14). Although some functional overlap is possible between Notch receptors, each Notch receptor exhibits unique roles in physiology (15)(16)(17).…”

Ex Vivo Organ Cultures as Models to Study Bone Biology

“…As described above, we have also employed ex vivo organ cultures to identify a PTH‐induced new regulatory axis: Mmp 14‐sRankl . In a recent example, ex vivo bone organ cultures were used to study the interplay between PTH signaling and Notch signaling in bone . Ex vivo cultures established with bones from WT mice and from genetically modified mice with Notch 2 gain‐of‐function allowed the investigators to evaluate the contribution of Notch receptor 2 signaling to the skeletal effects of PTH .…”