Modulation of growth factor incorporation into ECM of human osteoblast-like cells in vitro by 17 beta-estradiol

The mTORC2 Regulator Homer1 Modulates Protein Levels and Sub-Cellular Localization of the CaSR in Osteoblast-Lineage Cells

We recently found that, in human osteoblasts, Homer1 complexes to Calcium-sensing receptor (CaSR) and mediates AKT initiation via mechanistic target of rapamycin complex (mTOR) complex 2 (mTORC2) leading to beneficial effects in osteoblasts including β-catenin stabilization and mTOR complex 1 (mTORC1) activation. Herein we further investigated the relationship between Homer1 and CaSR and demonstrate a link between the protein levels of CaSR and Homer1 in human osteoblasts in primary culture. Thus, when siRNA was used to suppress the CaSR, we observed upregulated Homer1 levels, and when siRNA was used to suppress Homer1 we observed downregulated CaSR protein levels using immunofluorescence staining of cultured osteoblasts as well as Western blot analyses of cell protein extracts. This finding was confirmed in vivo as the bone cells from osteoblast specific CaSR-/- mice showed increased Homer1 expression compared to wild-type (wt). CaSR and Homer1 protein were both expressed in osteocytes embedded in the long bones of wt mice, and immunofluorescent studies of these cells revealed that Homer1 protein sub-cellular localization was markedly altered in the osteocytes of CaSR-/- mice compared to wt. The study identifies additional roles for Homer1 in the control of the protein level and subcellular localization of CaSR in cells of the osteoblast lineage, in addition to its established role of mTORC2 activation downstream of the receptor.

Glucose-loading reduces bone remodeling in women and osteoblast function in vitro

Aging is associated with a reduction in osteoblast life span and the volume of bone formed by each basic multicellular unit. Each time bone is resorbed, less is deposited producing microstructural deterioration. Aging is also associated with insulin resistance and hyperglycemia, either of which may cause, or be the result of, a decline in undercarboxylated osteocalcin (ucOC), a protein produced by osteoblasts that increases insulin sensitivity. We examined whether glucose‐loading reduces bone remodeling and ucOC in vivo and osteoblast function in vitro, and so compromises bone formation. We administered an oral glucose tolerance test (OGTT) to 18 pre and postmenopausal, nondiabetic women at rest and following exercise and measured serum levels of bone remodeling markers (BRMs) and ucOC. We also assessed whether increasing glucose concentrations with or without insulin reduced survival and activity of cultured human osteoblasts. Glucose‐loading at rest and following exercise reduced BRMs in pre and postmenopausal women and reduced ucOC in postmenopausal women. Higher glucose correlated negatively, whereas insulin correlated positively, with baseline BRMs and ucOC. The increase in serum glucose following resting OGTT was associated with the reduction in bone formation markers. D‐glucose (>10 mmol L⁻¹) increased osteoblast apoptosis, reduced cell activity and osteocalcin expression compared with 5 mmol L⁻¹. Insulin had a protective effect on these parameters. Collagen expression in vitro was not affected in this time course. In conclusion, glucose exposure reduces BRMs in women and exercise failed to attenuate this suppression effect. The suppressive effect of glucose on BRMs may be due to impaired osteoblast work and longevity. Whether glucose influences material composition and microstructure remains to be determined.

The interrelationship between bone and fat: from cellular see-saw to endocrine reciprocity

The number of mature osteoblasts and marrow adipocytes in bone is influenced by the differentiation of the common mesenchymal progenitor cell towards one phenotype and away from the other. Consequently, factors which promote adipogenesis not only lead to fatty marrow but also inhibit osteoblastogenesis, resulting in decreased osteoblast numbers, diminished bone formation and, potentially, inadequate bone mass and osteoporosis. In addition to osteoblast and bone adipocyte numbers being influenced by this skewing of progenitor cell differentiation towards one phenotype, mature osteoblasts and adipocytes secrete factors which may evoke changes in the cell fate and function of each other. This review examines the endogenous factors, such as PPAR-γ2, Wnt, IGF-1, GH, FGF-2, oestrogen, the GP130 signalling cytokines, vitamin D and glucocorticoids, which regulate the selection between osteoblastogenesis and adipogenesis and the interrelationship between fat and bone. The role of adipokines on bone, such as adiponectin and leptin, as well as adipose-derived oestrogen, is reviewed and the role of bone as an energy regulating endocrine organ is discussed.

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

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 β-catenin. We propose that two discrete pathways linked to canonical Wnt signaling contribute to strontium-induced osteogenic effects in osteoblasts.

Osteoblasts play key roles in the mechanisms of action of strontium ranelate

BACKGROUND AND PURPOSE

Strontium ranelate reduces fracture risk in postmenopausal women with osteoporosis. Evidence from non-clinical studies and analyses of bone markers in phase III trials indicate that this is due to an increase in osteoblast formation and a decrease of osteoclastic resorption. The aim of this work was to investigate, in human cells, the mechanisms by which strontium ranelate is able to influence the activities of osteoblasts and osteoclasts.

EXPERIMENTAL APPROACH

Human primary osteoblasts were used to examine effects of strontium ranelate on replication (thymidine incorporation), differentiation (Runx2 and alkaline phosphatase) and cell survival (cell counts and caspase activity). Osteoprotegerin (OPG) was measured by quantitative reverse transcription PCR (qRT-PCR) and elisa and receptor activator of NFkappaB ligand (RANKL) by qRT-PCR and Western blot. As strontium ranelate has been proposed as an agonist of the calcium-sensing receptor (CaSR), the involvement of CaSR in the effects of strontium ranelate on OPG and RANKL expression, and cell replication was examined using siRNA.

KEY RESULTS

Strontium ranelate increased mRNA and protein levels of OPG and suppressed those of RANKL. Strontium ranelate also stimulated osteoblast replication and differentiation and increased cell survival under stress. Knocking down CaSR suppressed strontium ranelate-induced stimulation of OPG mRNA, reduction of RANKL mRNA, and increase in replication, indicating the involvement of CaSR in these responses.

CONCLUSIONS AND IMPLICATIONS

Our results demonstrate that osteoblasts play a key role in the mechanism of action of the anti-fracture agent, strontium ranelate by mediating both its anabolic and anti-resorptive actions, at least in part, via activation of CaSR.

Functional alpha1- and beta2-adrenergic receptors in human osteoblasts

Central (hypothalamic) control of bone mass is proposed to be mediated through beta2-adrenergic receptors (beta2-ARs). While investigations in mouse bone cells suggest that epinephrine enhances both RANKL and OPG mRNA via both beta-ARs and alpha-ARs, whether alpha-ARs are expressed in human bone cells is controversial. The current study investigated the expression of alpha1-AR and beta2-AR mRNA and protein and the functional role of adrenergic stimulation in human osteoblasts (HOBs). Expression of alpha1B- and beta2-ARs was examined by RT-PCR, immunofluorescence microscopy and Western blot (for alpha1B-ARs). Proliferation in HOBs was assessed by (3)H-thymidine incorporation and expression of RANKL and OPG was determined by quantitative RT-PCR. RNA message for alpha1B- and beta2-ARs was expressed in HOBs and MG63 human osteosarcoma cells. alpha1B- and beta2-AR immunofluorescent localization in HOBs was shown for the first time by deconvolution microscopy. alpha1B-AR protein was identified in HOBs by Western blot. Both alpha1-agonists and propranolol (beta-blocker) increased HOB replication but fenoterol, a beta2-agonist, inhibited it. Fenoterol nearly doubled RANKL mRNA and this was inhibited by propranolol. The alpha1-agonist cirazoline increased OPG mRNA and this increase was abolished by siRNA knockdown of alpha1B-ARs in HOBs. These data indicate that both alpha1-ARs and beta2-ARs are present and functional in HOBs. In addition to beta2-ARs, alpha1-ARs in human bone cells may play a role in modulation of bone turnover by the sympathetic nervous system.

Use of human fetal tissue for biomedical research in Australia, 1994-2002

Human fetal tissue is a scarce resource that has been used in Australia for biomedical research since 1980. From 1994 to 2002, it has been used for research by 19 biomedical researchers at 12 separate Australian institutions (four universities, six major teaching hospitals and two research institutes). With an average of 265 samples distributed annually, researchers have conducted experiments in biomedical research with the approval of their Human Ethics Committees, and published 74 manuscripts in peer reviewed journals over the past decade. The tissue is obtained from therapeutic termination of pregnancies at 8-20 weeks', but mostly 14-18 weeks', gestation. The average number of fetuses obtained over the past 10 years was 108 per annum. Our understanding of the pathogenesis of human diseases such as diabetes, multiple sclerosis, retinopathy of prematurity and osteoporosis has been advanced because of such experiments, and better drug treatment of disorders such as osteoarthritis has been made possible with the use of human fetal tissue. The benefits of human fetal tissue research need greater recognition.

The purinergic calcium channels P2X1,2,5,7 are down-regulated while P2X3,4,6 are up-regulated during apoptosis in the ageing rat prostate

Subtype-specific antibodies were used to measure purinergic (P2X) receptor expression in the rat prostate. In mature Wistar rats, apoptosis and expression of P2X1, P2X2, P2X5 and P2X7 subtypes were all significantly decreased compared with the levels found in immature rat prostates. Accompanying this age-related reduction in purinergic calcium channel expression was a reduction in epithelial and stromal calcium as well as the calcium-regulating hormone stanniocalcin. In contrast, expression of P2X3, P2X4 and P2X6 increased with age. These results suggest that distinct changes in P2X subtype expression accompany apoptosis in the rat prostate.

Distributional changes of purinergic receptor subtypes (P2X 1-7) in uterine epithelial cells during early pregnancy

Expression of each of the purinergic receptor subtypes (P2X7) was studied by immunohistochemical localization in the apical, lateral and basal plasma membranes of rat uterine epithelial cells during early pregnancy to the time of implantation on Day 6. Labelling for each P2X subtype was seen in the apical, lateral and basal compartments on Days 1 and 3, except for P2X2 which was only observed in the basement membrane. The P2X5 signal was similar in temporal and spatial expression to the other subtypes, but with a greatly reduced intensity. At the time of implantation on Day 6, this pattern altered dramatically. Apical expression markedly increased for most subtypes while the lateral and basal signals were markedly reduced. The exceptions to this pattern were P2X2, which displayed both a strong basal and apical label, and P2X4 which became de-expressed in all areas. We propose that the changing spatial and temporal expression of the P2X receptors is a significant factor in the regulation of events during early pregnancy. They are expressed in the same location as remodelling. apoptosis, and protein activation events prior to implantation on Day 6. These observations suggest an up-regulation of calcium-mediated events, including cytoskeletal alterations, a decrease in luminal pH and transmembrane molecule activation.

Changes in growth factor expression in the ageing prostate may disrupt epithelial-stromal homeostasis

The alterations in expression of six growth factors known to be regulators of prostatic function have been examined in the ventral lobe of prostates from young adult (14 week) and ageing (1.5 year) Wistar rats. The selected growth factors were transforming growth factor beta (TGFbeta1), insulin-like growth factor I (IGF-I), insulin-like growth factor II (IGF-II), platelet derived growth factor (PDGF), basic fibroblast growth factor (FGF2) and epidermal growth factor (EGF). The extracellular matrix growth co-factor thrombospondin (TSP) was also examined. Our study demonstrated a 2.9-fold up-regulation of TGFbeta1 (p < 0.0001), a 2.0-fold increase in FGF2 (p < 0.0002), an 8.3-fold increase in IGF-II (p < 0.0007) and a 5.4-fold increase in EGF (p < 0.0001) in ageing compared to adult prostate tissue. Conversely, we observed a 2.7-fold down-regulation of IGF-I (p < 0.0005), a 1.7-fold decrease in PDGF (p < 0.0097) and a 5.8-fold decrease in TSP (p < 0.0079) in ageing rat prostate tissue. The observed alterations in growth factor expression in this study may be the result or cause of, an imbalance in the proliferative-apoptotic balance during ageing. This imbalance may explain the increase in epithelial proliferation that is characteristic of the normal ageing prostate. As in other systems it seems likely that these factors work synergistically rather than in isolation.

Multiple promoters direct the tissue-specific expression of novel N-terminal variant human vitamin D receptor gene transcripts

The effects of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] are mediated by the vitamin D receptor (VDR), a member of the nuclear receptor superfamily of transcriptional regulators. We have identified upstream exons of the human (h) VDR gene that are incorporated into variant transcripts, two of which encode N-terminal variant receptor proteins. Expression of the hVDR gene, which spans more than 60 kb and consists of at least 14 exons, is directed by two distinct promoters. A tissue-specific distal promoter generates unique transcripts in tissues involved in calcium regulation by 1, 25-(OH)2D3 and can direct the expression of a luciferase reporter gene in a cell line-specific manner. These major N-terminal differences in hVDR transcripts, potentially resulting in structural differences in the expressed receptor, may contribute to cellular responsiveness to 1,25-(OH)2D3 through tissue differences in the regulation of VDR expression.