A bisphosphonate that does not affect osteoclasts prevents osteoblast and osteocyte apoptosis and the loss of bone strength induced by glucocorticoids in mice

Although a major effect of bisphosphonates on bone is inhibition of resorption resulting from their ability to interfere with osteoclast function, these agents also prevent osteoblast and osteocyte apoptosis in vitro and in vivo. However, the contribution of the latter property to the overall beneficial effects of the drugs on bone remains unknown. We compared herein the action on glucocorticoid-induced bone disease of the classical bisphosphonate alendronate with that of IG9402, a bisphosphonate analog that preserves osteoblast and osteocyte viability but does not induce osteoclast apoptosis in vitro. The bisphosphonates were injected daily (2.3 μmol/kg) to 5-month-old Swiss Webster mice (6–11 per group), starting three days prior to implantation of pellets releasing the glucocorticoid prednisolone (2.1 mg/kg/d). IG9402 did not affect levels of circulating C-telopeptide or osteocalcin, markers of resorption and formation, respectively, nor did it decrease mRNA levels of osteocalcin or collagen1A1 in bone. On the other hand, alendronate decreased all these parameters. Moreover, IG9402 did not reduce cancellous mineralizing surface, mineral apposition rate or bone formation rate, whereas alendronate induced a decrease in each of these bone formation measures. These findings demonstrate that in contrast to alendronate, IG9402 does not inhibit bone turnover. Both alendronate and IG9402, on the other hand, activated survival kinase signaling in vivo, as evidenced by induction of ERK phosphorylation in bone. Furthermore, both bisphosphonates prevented the increase in osteoblast and osteocyte apoptosis as well as the decrease in vertebral bone mass and strength induced by glucocorticoids. We conclude that a bisphosphonate that does not affect osteoclasts prevents osteoblast and osteocyte apoptosis and the loss of bone strength induced by glucocorticoids in mice

The importance of connexin 43 in enamel development and mineralization

During enamel development, formation of hydroxyapatite crystals and regulation of pH in the enamel matrix require massive transport of ions. Both ameloblasts and adjacent dental epithelial cells in the stellate reticulum co-express several transmembrane cotransporters/ion-exchangers for transport of ions across plasma membranes. Gap junctions (GJs) enable intercellular exchanges of ions between neighboring cells. This suggests that the ameloblasts and other cell layers of the enamel organ, form a functional unit. During the bell stage of tooth formation, the non-ameloblast dental epithelium highly expresses the Na-K-Cl cotransporter (Nkcc1). Nkcc1-null mice are associated with enamel hypomineralization and increased expression of GJ protein connexin 43 (Cx43), suggesting that reduced ion transport in the Nkcc1-null mouse is in part compensated by increased intercellular ion transport through GJs. To understand the role of GJs in ion transport and its effect on pH regulation, we examined in a mouse strain in which Cx43 was ablated selectively in DMP1 expressing cells (Cx43flox/flox mice crossed with DMP1-8kb-Cre mice), including ameloblasts. Micro-CT analysis showed that the mineral density at late maturation stage incisal enamel of the Cx43-null mice was 10% less than in controls, whereas that in dentin was unchanged. Maturation stage ameloblasts of mice lacking the pH regulating sodium/bicarbonate transporter NBCe1 (Nbce1-null), or chloride channel Cftr (Cftr-null) were found to have increased Cx43-immunostaining. These results support the possibility that GJs in the ameloblast-papillary complex at the maturation stage contribute to ion transport by enabling passage of ions directly from cells of the papillary layer into ameloblast layer. Increasing the number of GJs may partly compensate the reduction of ion-cotransporters and ion exchangers in dental epithelium

Nongenotropic, Sex-Nonspecific Signaling through the Estrogen or Androgen Receptors Dissociation from Transcriptional Activity

AbstractThe relationship of the classical receptors and their transcriptional activity to nongenotropic effects of steroid hormones is unknown. We demonstrate herein a novel paradigm of sex steroid action on osteoblasts, osteocytes, embryonic fibroblasts, and HeLa cells involving activation of a Src/Shc/ERK signaling pathway and attenuating apoptosis. This action is mediated by the ligand binding domain and eliminated by nuclear targeting of the receptor protein; ERα, ERβ, or AR can transmit it with similar efficiency irrespective of whether the ligand is an estrogen or an androgen. This antiapoptotic action can be dissociated from the transcriptional activity of the receptor with synthetic ligands, providing proof of principle for the development of function-specific—as opposed to tissue-selective—and gender-neutral pharmacotherapeutics