An Akt-dependent Increase in Canonical Wnt Signaling and a Decrease in Sclerostin Protein Levels Are Involved in Strontium Ranelate-induced Osteogenic Effects in Human Osteoblasts

Abstract

Sclerostin is an important regulator of bone homeostasis and canonical Wnt signaling is a key regulator of osteogenesis. Strontium ranelate is a treatment for osteoporosis that has been shown to reduce fracture risk, in part, by increasing bone formation. Here we show that exposure of human osteoblasts in primary culture to strontium increased mineralization and decreased the expression of sclerostin, an osteocyte-specific secreted protein that acts as a negative regulator of bone formation by inhibiting canonical Wnt signaling. Strontium also activated, in an apparently separate process, an Akt-dependent signaling cascade via the calcium-sensing receptor that promoted the nuclear translocation of ␤-catenin. We propose that two discrete pathways linked to canonical Wnt signaling contribute to strontium-induced osteogenic effects in osteoblasts.Strontium ranelate is a treatment for osteoporosis that decreases the incidence of vertebral and femoral fracture risk in postmenopausal women (1-3). Strontium ranelate has been shown to modulate the physiological processes of bone formation and bone resorption (2), resulting in increased bone apposition rates (3) and bone mineral density (1, 2, 4 -6), while maintaining the quality of bone mineral (6).Sclerostin is exclusively expressed by osteocytes in adult life (7, 8) but is more widely expressed during development (7) and plays a physiological role as a negative regulator of bone formation by repressing bone morphogenic protein-induced osteogenesis (9 -11). The importance of sclerostin in bone-loss disorders has been described in several in vivo studies. Positional cloning studies identified loss of function mutations in the SOST gene that cause sclerostosis and van Buchem disease, bone dysplasias characterized by progressive skeletal overgrowth (12). In contrast, transgenic mice overexpressing sclerostin had significant reductions in bone mass and mineral apposition rate compared with wild type (7). A sclerostin knock-out mouse model was shown to have increased osteoblast activity and enhanced osteoblast/osteocyte viability and was resistant to mechanical unloading-induced bone loss. This phenotype was associated with an increase in canonical Wnt signaling when compared with wild-type mice (8).Sclerostin functions as an antagonist of canonical Wnt signaling, whereby GSK-3␤ 2 -stimulated, ubiquitin-mediated breakdown of ␤-catenin is alleviated, resulting in its nuclear translocation, and binding to transcription factors of the T-cell factor/lymphoid enhancer factor family, to induce the transcription of growth-associated genes (13). Non-canonical Wnt signaling does not involve ␤-catenin translocation to the nucleus (11). Sclerostin binds to the extracellular domains of the Wnt co-receptors LRP5, LRP6, and LRP4 and disrupts extracellular Wnt-induced Frizzled/LRP complex formation thus providing a molecular mechanism by which loss of sclerostin function may lead to conditions such as sclerostosis (12,14). In addition to Frizzled/LRP-mediated activation of canonical Wnt s...

Histological and Histomorphometric Study of the Effect of Strontium Ranelate on the Healing of One-Wall Intrabony Periodontal Defects in Dogs

“…Finally, both calcium and strontium were found to act on osteoclast precursor cells via the CaSR [34,36]. In addition, strontium, decreased the expression of sclerostin, an osteocyte specific secreted protein that acts as a negative regulator of bone formation by inhibiting canonical Wnt signaling, hence strentium expected to increase canonical Wnt signalling that stimulates osteoblastic proliferation, differentiation and function [37]. Moreover, Gulhan et al [38] showed that the treatment with strontium ranelate for 6 months results in increased serum IGF-1 levels, which may suggest that IGF-1 may be a mediator in the antiresorptive effect of SrRan.…”

Osteoblastic expansion induced by parathyroid hormone receptor signaling in murine osteocytes is not sufficient to increase hematopoietic stem cells

“…This disassociation has also been demonstrated in strontium ranelate treatment. 19 Interestingly, recent data in human osteoblastic cells have suggested that stimulation of Wnt signaling mediates the osteogenic effects of strontium through decreased Sclerostin levels, 20 which is strongly diminished in TG mice. 8 Therefore, these results suggest that potential Sclerostin-targeted therapies 21,22 would not achieve HSC expansion despite their osteogenic actions.…”

Role of transglutaminase 2 in PAC1 receptor mediated protection against hypoxia-induced cell death and neurite outgrowth in differentiating N2a neuroblastoma cells

“…Rp-cAMPs (10-100 µM; Werner et al, 2014;Mangiavacchi and Wolf, 2004;Wang et al, 2012;Pratt et al, 2016;Vallejo and Vallejo, 2002); Ro 31-8220 (1-10 µM; Lee et al, 2013;Montejo-López et al, 2016), PD 98059 (10-50 µM; Sutter et al, 2004;Kim et al, 2008), SP 600125 (20 µM; Hah et al, 2013;Kim et al, 2010) and Akt inhibitor XI (1 µM; Frampton et al,2012;Rybchyn et al, 2011 Inhibitor Cocktail 2). Cell lysates were scraped and clarified by centrifugation at 4°C for 10 min at 14000 x g prior to being assayed for TG activity using the biotin-labeled cadaverine incorporation assay (see below).…”