Production of tumor necrosis factor by human osteoblasts is modulated by other cytokines, but not by osteotropic hormones

Human osteoblast cultures derived as out-growths from trabecular bone released tumor necrosis factor (TNF alpha) upon stimulation of the cells with human recombinant interleukin 1 (IL1; 10(-13)-10(-11) M), human recombinant granulocyte-macrophage colony-stimulating factor (100-1000 U/ml), and bacterial lipopolysaccharide (5-500 ng/ml). The osteotropic hormones 1,25-dihydroxyvitamin D3, PTH, and calcitonin had no effect on TNF production. The TNF released by the osteoblasts was identified as TNF alpha, using a specific anti-TNF alpha monoclonal antibody to neutralize its activity. Immunohistochemical staining of the cells using the same antibody revealed that all of the cells in the cultures were capable of producing TNF alpha, including those that also expressed alkaline phosphatase activity. Immunoreactive protein could be detected in the perinuclear region when cells were cultured in the presence of monensin, suggesting accumulation of newly synthesised protein in the Golgi apparatus. These results suggest that human osteoblasts, which have been shown previously to respond to TNF alpha, can synthesize and release TNF in response to IL1 and granulocyte-macrophage colony-stimulating factor. TNF may, therefore, not only have a pathological role in conditions of chronic inflammation, but also may act as a local paracrine or autocrine regulator of osteoblast function.     

Oestradiol inhibits the release of tumour necrosis factor but not interleukin 6 from adult human osteoblasts in vitro

Oestrogens may control bone remodelling by directly regulating the synthesis of cytokines in osteoblasts. We have investigated the effects of oestradiol on the release of two cytokines, IL-6 and TNF, known to be produced by normal human osteoblast-like cells. The effect of oestradiol on basal and stimulated IL-6 and TNF release was investigated. The concentration of IL-6 and TNF in 24 h bone cell-conditioned medium was determined using bio- and immunoradiometric assays. The results showed that 17-oestradiol (10^–10 and 10^–8 M) inhibited IL-1-stimulated TNF release in a dose dependent manner in 7 out of 9 patients. Maximal inhibition was observed with 10^–8 M17-oestradiol, producing an average 30% reduction in TNF release. In contrast 17-oestradiol (10^–12 – 10^–8 M) failed to consistently regulate basal or stimulated IL-6 release. IL-6 mRNA levels were also shown not to be modulated by 17-oestradiol (10^–9M) under stimulatory conditions. rhIL-l (10 U/ml) was a consistent and potent stimulator of IL-6 and TNF release, and the glucocorticoid hydrocortisone was found to be a powerful suppressor of both IL-6 and TNF release under basal or stimulatory conditions. In conclusion direct regulation of bone remodelling by oestradiol does not appear to be effected via the control of IL-6 production in osteoblasts. However, a suppression of osteoblastic TNF release could represent one facet of the control of bone formation and resorption by oestrogens.     

Characterization of Brca1 deficient mice

BRCA1 is a nuclear phosphoprotein that is expressed in a cell cycle regulated manner in virtually all normal dividing cells. Inheritance of a mutated copy of the BRCA1 gene increases a woman's risk for developing breast and ovarian cancer (1-3). Since the tumors that arise in these individuals consistently fail to express the wild-type allele, BRCA1 is believed to encode a tumor suppressor. Loss of the remaining functional BRCA1 allele, therefore, is one of the steps leading to neoplastic transformation of some types of epithelial cells. The isolation of the murine homologue of the human BRCA1 gene opened up the possibility of using a powerful genetic approach to study the role of this gene in both normal development and tumor formation. This genetic approach involves in vitro manipulation of the genome of embryonic stem (ES) cells, stable tissue culture cell lines derived from mouse blastocysts. After introducing mutations into the murine homologue of the BRCA1 gene Brca1 in these cell lines, four groups have generated mouse lines carrying the same mutations (4-7). Surprisingly, mice carrying a single mutant Brca1 allele do not display the increased risk for breast tumors seen in humans carrying similar mutations. However, while loss of BRCA1 appears to be a one of the many events involved in tumorgenesis in humans, these mouse lines demonstrate that gene expression is essential for development; as homozygosity for each of the Brca1 mutations results in postimplantation embryonic lethality. The survival of Brca1 deficient embryos is extended by one or two days in the absence of p53 and p21 (7,8).     

Potent and selective inhibition of human cathepsin K leads to inhibition of bone resorption in vivo in a nonhuman primate.

Cathepsin K is a cysteine protease that plays an essential role in osteoclast-mediated degradation of the organic matrix of bone. Knockout of the enzyme in mice, as well as lack of functional enzyme in the human condition pycnodysostosis, results in osteopetrosis. These results suggests that inhibition of the human enzyme may provide protection from bone loss in states of elevated bone turnover, such as postmenopausal osteoporosis. To test this theory, we have produced a small molecule inhibitor of human cathepsin K, SB-357114, that potently and selectively inhibits this enzyme (Ki = 0.16 nM). This compound potently inhibited cathepsin activity in situ, in human osteoclasts (inhibitor concentration [IC]50 = 70 nM) as well as bone resorption mediated by human osteoclasts in vitro (IC50 = 29 nM). Using SB-357114, we evaluated the effect of inhibition of cathepsin K on bone resorption in vivo using a nonhuman primate model of postmenopausal bone loss in which the active form of cathepsin K is identical to the human orthologue. A gonadotropin-releasing hormone agonist (GnRHa) was used to render cynomolgus monkeys estrogen deficient, which led to an increase in bone turnover. Treatment with SB-357114 (12 mg/kg subcutaneously) resulted in a significant reduction in serum markers of bone resorption relative to untreated controls. The effect was observed 1.5 h after the first dose and was maintained for 24 h. After 5 days of dosing, the reductions in N-terminal telopeptides (NTx) and C-terminal telopeptides (CTx) of type I collagen were 61% and 67%, respectively. A decrease in serum osteocalcin of 22% was also observed. These data show that inhibition of cathepsin K results in a significant reduction of bone resorption in vivo and provide further evidence that this may be a viable approach to the treatment of postmenopausal osteoporosis.     

Antagonism of the osteoclast vitronectin receptor with an orally active nonpeptide inhibitor prevents cancellous bone loss in the ovariectomized rat.

An orally active, nonpeptide Arg-Gly-Asp (RGD) mimetic alpha(v)beta3 antagonist, (S)-3-Oxo-8-[2-[6-(methylamino)-pyridin-2-yl]-1-ethoxy]-2-(2,2,2-trifluoroethyl)-2,3,4,5-tetrahydro-1H-2-benzazepine-4-acetic acid (compound 1), has been generated, which prevented net bone loss and inhibited cancellous bone turnover in vivo. The compound binds alpha(v)beta3 and the closely related integrin alpha(v)beta5 with low nanomolar affinity but binds only weakly to the related integrins alpha(IIb)beta3, and alpha5beta1. Compound 1 inhibited alpha(v)beta3-mediated cell adhesion with an IC50 = 3 nM. More importantly, the compound inhibited human osteoclast-mediated bone resorption in vitro with an IC50 = 11 nM. In vivo, compound 1 inhibited bone resorption in a dose-dependent fashion, in the acute thyroparathyroidectomized (TPTX) rat model of bone resorption with a circulating EC50 approximately 20 microM. When dosed orally at 30 mg/kg twice a day (b.i.d.) in the chronic ovariectomy (OVX)-induced rat model of osteopenia, compound 1 also prevented bone loss. At doses ranging from 3 to 30 mg/kg b.i.d., compound 1 partially prevented the OVX-induced increase in urinary deoxypyridinoline. In addition, the compound prevented the OVX-induced reduction in cancellous bone volume (BV), trabecular number (Tb.N), and trabecular thickness (Tb.Th), as assessed by quantitative microcomputerized tomography (microCT) and static histomorphometry. Furthermore, both the 10-mg/kg and 30-mg/kg doses of compound prevented the OVX-induced increase in bone turnover, as measured by percent osteoid perimeter (%O.Pm). Together, these data indicate that the alpha(v)beta3 antagonist compound 1 inhibits OVX-induced bone loss. Mechanistically, compound 1 prevents bone loss in vivo by inhibiting osteoclast-mediated bone resorption, ultimately preventing cancellous bone turnover.