Role of the Parathyroid Hormone Type 1 Receptor (PTH1R) as a Mechanosensor in Osteocyte Survival

Abstract

Osteocytes have a major role in the control of bone remodeling. Mechanical stimulation decreases osteocyte apoptosis and promotes bone accrual, whereas skeletal unloading is deleterious in both respects. PTH1R ablation or overexpression in osteocytes in mice produces trabecular bone loss or increases bone mass, respectively. The latter effect was related to a decreased osteocyte apoptosis. Here, the putative role of PTH1R activation in osteocyte protection conferred by mechanical stimulation was assessed. Osteocytic MLO-Y4 cells were subjected to mechanical stimuli represented by hypotonic shock (216 mOsm/kg) or pulsatile fluid flow (8 Hz, 10 dynes/cm 2 ) for a short pulse (10 min), with or without PTH1R antagonists or after transfection with specific PTHrP or PTH1R siRNA. These mechanical stimuli prevented cell death induced within 6 hours by etoposide (50 mM), related to PTHrP overexpression; and this effect was abolished by the calcium antagonist verapamil (1 mM), a phospholipase C (PLC) inhibitor (U73122; 10 mM), and a PKA activation inhibitor, Rp-cAMPS (25 mM), in these cells. Each mechanical stimulus also rapidly induced b-catenin stabilization and nuclear ERK translocation, which were inhibited by the PTH1R antagonist PTHrP(7-34) (1 mM), or PTH1R siRNA, and mimicked by PTHrP(1-36) (100 nM). Mechanical stretching by hypotonic shock did not affect cAMP production but rapidly (<1 min) stimulated Ca i 2þ transients in PTH1R-overexpressing HEK-293 cells and in MLO-Y4 cells, in which calcium signaling was unaffected by the presence of a PTHrP antiserum or PTHrP siRNA but inhibited by knocking down PTH1R. These novel findings indicate that PTH1R is an important component of mechanical signal transduction in osteocytic MLO-Y4 cells, and that PTH1R activation by PTHrPindependent and dependent mechanisms has a relevant role in the prosurvival action of mechanical stimulus in these cells.

Osteocytic signalling pathways as therapeutic targets for bone fragility

“…PTHrP expression is increased by mechanical stimulation in osteocytic and osteoblastic cells, 125, 126 and the anti-apoptotic effect of mechanical stimulation is mimicked by treatment with PTHrP and abolished by silencing PTHR1 in vitro . 126, 127 PTHrP, as PTH, decreases Sost expression in vitro in primary cultures of osteoblastic cells containing osteocytes. 13 Taken together, this evidence strongly suggests that local increases in PTHrP induced by mechanical loading could contribute to maintaining osteocyte survival and to the anabolic action of mechanical loading.…”

Modeling and remodeling effects of intermittent administration of teriparatide (parathyroid hormone 1-34) on bone morphogenetic protein-induced bone in a rat spinal fusion model

“…Osteocytes, the most abundant cells in bone, have recently emerged as important modulators of bone modeling (Maycas et al, 2015, Saini et al, 2013, Zhang et al, 2009). During normal physiological skeletal growth, bone modeling involves bone formation at the periosteum and bone resorption at the endosteum.…”

Parathyroid Hormone-Related Protein Analogs as Osteoporosis Therapies

“…Indeed, native PTH or PTHrP ligands of the PTH1R activate cAMP/PKA-and phosphoinositide 3-kinase (PI-3K)/Akt-dependent pathways to induce, with the concourse of transcription factor Runx2, pro-survival actions in various osteoblastic cell preparations [21][22][23]. Moreover, recent data indicate the relevant role of intracellular calcium as well as PKA and ERK pathways related to PTHrP/PTH1R system activation in the pro-survival effects triggered by mechanical stimulus in osteocytes [24]. Also of note, modified PTH/PTHrP agonists that predominantly activate ERK by a G protein-independent but b-arrestin-dependent mechanism may favor bone formation over bone resorption outcome [5,6,25].…”