M-CSF neutralization and egr-1 deficiency prevent ovariectomy-induced bone loss.

Increased stromal cell production of M-CSF, an event caused by enhanced phosphorylation of the nuclear protein Egr-1, is central to the mechanism by which estrogen (E2) deficiency upregulates osteoclast (OC) formation. However, the contribution of enhanced M-CSF production to the bone loss induced by E2 deficiency remains to be determined. We found that treatment with an Ab that neutralizes M-CSF in vivo completely prevents the rise in OC number, the increase in bone resorption, and the resulting bone loss induced by ovariectomy (ovx). We also found that adult, intact Egr-1-deficient mice, a strain characterized by maximally stimulated stromal cell production of M-CSF, exhibit increased bone resorption and decreased bone mass. In these mice, treatment with anti-M-CSF Ab restored normal levels of bone resorption, thus confirming that increased M-CSF production accounts for the remodeling abnormalities of Egr-1-deficient mice. Consistent with the failure of ovx to further increase M-CSF production in Egr-1-deficient mice, ovx neither increased bone resorption further, nor caused bone loss in these animals. In summary, the data demonstrate that E2 deficiency induces M-CSF production via an Egr-1-dependent mechanism that is central to the pathogenesis of ovx-induced bone loss. Thus, Egr-1 and M-CSF are critical mediators of the bone sparing effects of E2 in vivo.

Estrogen deficiency induces bone loss by enhancing T-cell production of TNF-alpha.

Estrogen deficiency induces bone loss by upregulating osteoclastogenesis by mechanisms not completely defined. We found that ovariectomy-enhanced T-cell production of TNF-alpha, which, acting through the TNF-alpha receptor p55, augments macrophage colony-stimulating factor-induced (M-CSF-induced) and RANKL-induced osteoclastogenesis. Ovariectomy failed to induce bone loss, stimulate bone resorption, or increase M-CSF- and RANKL-dependent osteoclastogenesis in T-cell deficient mice, establishing T cells as essential mediators of the bone-wasting effects of estrogen deficiency in vivo. These findings demonstrate that the ability of estrogen to target T cells, suppressing their production of TNF-alpha, is a key mechanism by which estrogen prevents osteoclastic bone resorption and bone loss.

Estrogen decreases TNF gene expression by blocking JNK activity and the resulting production of c-Jun and JunD.

Central to the bone-sparing effect of estrogen (E(2)) is its ability to block the monocytic production of the osteoclastogenic cytokine TNF-alpha (TNF). However, the mechanism by which E(2) downregulates TNF production is presently unknown. Transient transfection studies in HeLa cells, an E(2) receptor-negative line, suggest that E(2) inhibits TNF gene expression through an effect mediated by estrogen receptor beta (ERbeta). We also report that in RAW 264.7 cells, an E(2) receptor-positive murine monocytic line, E(2) downregulates cytokine-induced TNF gene expression by decreasing the activity of the Jun NH(2)-terminal kinase (JNK). The resulting diminished phosphorylation of c-Jun and JunD at their NH(2)-termini decreases the ability of these nuclear proteins to autostimulate the expression of the c-Jun and JunD genes, thus leading to lower production of c-Jun and JunD. The consequent decrease in the nuclear levels of c-Jun and JunD leads to diminished binding of c-Jun/c-Fos and JunD/c-Fos heterodimers to the AP-1 consensus sequence in the TNF promoter and, thus, to decreased transactivation of the TNF gene.

Estrogen blocks M-CSF gene expression and osteoclast formation by regulating phosphorylation of Egr-1 and its interaction with Sp-1.

Central to the pathogenesis of osteoporosis is the ability of estrogen deficiency to increase osteoclast formation by enhancing stromal cell production of the osteoclastogenic cytokine macrophage colony-stimulating factor (M-CSF). We report that stromal cells from ovariectomized mice exhibit increased casein kinase II-dependent phosphorylation of the nuclear protein Egr-1. Phosphorylated Egr-1 binds less avidly to the transcriptional activator Sp-1 and the resulting higher levels of free Sp-1 stimulate transactivation of the M-CSF gene. Estrogen replacement fails to block M-CSF mRNA expression and osteoclast formation in ovariectomized mice lacking Egr-1, confirming the critical role played by this transcription factor in mediating the antiosteoclastogenic effects of estrogen. Thus, by downregulating formation of a novel Egr-1/Sp-1 complex in stromal cells, estrogen deficiency results in enhanced levels of free Sp-1 and increased M-CSF gene expression and osteoclast formation.

Bone re/modeling is more dynamic in the endothelial nitric oxide synthase(-/-) mouse.

Nitric oxide is a ubiquitous estrogen-regulated signaling molecule that has been implicated in the regulation of bone maturation and remodeling. To better understand the role that bone-cell-secreted nitric oxide plays in ovariectomy-induced modifications of bone turnover, we examined the expression of endothelial NO synthase (eNOS) in bone cells and bone progenitor cells at regular intervals up to 10 wk after acute estrogen deprivation. Ovariectomy led to an anticipated initial decline in bone cell eNOS production, but surprisingly, 17 d after ovariectomy, eNOS expression by bone and marrow stromal cells dramatically rebounded and was maintained at high levels for at least 10 wk after surgery. We examined the long-term consequences of eNOS in the process of ovariectomy-induced bone loss by prospectively analyzing bone mineral density in wild-type and eNOS(-/-) mice for 10 wk after ovariectomy. Ovariectomized eNOS(-/-) mice were observed to undergo an exaggerated state of estrogen-deficiency-induced bone remodeling compared with wild-type controls, suggesting that eNOS may act to mitigate this process. Furthermore, we found that whereas bone formation in estrogen-replete wild-type mice slowed between 14 and 20 wk of age, eNOS knockout mice continued to accrue basal bone mass at a high rate and showed no sign of entering a remodeling stage. Our data suggest that eNOS may play an important role in limiting ovariectomy-induced bone remodeling as well as regulating the transition from basal modeling to remodeling.

T cell activation induces human osteoclast formation via receptor activator of nuclear factor kappaB ligand-dependent and -independent mechanisms.

In unstimulated conditions, osteoclast (OC) formation is regulated by stromal cell production of the key osteoclastogenic factors receptor activator of nuclear factor kappaB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). However, the mechanisms of accelerated osteoclastogenesis and bone loss characteristic of inflammatory conditions are poorly understood but appear to involve T cells. In addition, the mechanism by which OCs arise spontaneously in cultures of peripheral blood mononuclear cells in the absence of stromal cells or added cytokines remains unclear. Using a stromal cell free human osteoclast generating system, we investigated the ability of activated T cells to support osteoclastogenesis. We show that when activated by phytohemagglutinin-P (PHA), T cells (both CD4+ and CD8+) stimulate human OC formation in vitro. Although both soluble M-CSF and RANKL were detected in activated T cell supernatants, the presence of M-CSF was not essential for macrophage survival or RANKL-dependent osteoclast formation, suggesting that other soluble T cell-derived factors were capable of substituting for this cytokine. We also found that saturating concentrations of osteoprotegerin (OPG) failed to neutralize 30% of the observed OC formation and that T cell conditioned medium (CM) could superinduce osteoclastogenesis in cultures of purified monocytes maximally stimulated by RANKL and M-CSF. Together, these data suggest that activated T cells support osteoclastogenesis via RANKL-dependent and -independent mechanisms. Although not relevant for T cell-induced osteoclastogenesis, secretion of soluble M-CSF is a previously undescribed property of activated T cells.

Cloning of an antibody binding DNA sequence: pitfalls of DNA/protein immunoprecipitation reactions.

This paper presents evidence of cross-reactivity of a normal genomic DNA sequence to a number of antibodies raised against common proteins. This DNA fragment was inadvertently isolated during DNA-protein immunoprecipitation cloning experiments and was found to bind not only to the antibody used in the immunoprecipitation experiment, but to several different antibodies in gel-shift assays. This data demonstrates the ability of certain DNA sequences to bind non-specifically to antibodies and highlights the danger of DNA/antibody binding which can produce artifacts. Possible means of overcoming these reactions are discussed.

Failure of interleukin-1 to induce intracellular calcium fluxes in K562 cells.

The ability of interleukin-1 to induce calcium fluxes in K562 cells, was examined using flow cytometry and the calcium chelator Indo-1. Over a period of up to 40 minutes, interleukin-1 failed to cause any significant change in the concentration of intracellular calcium in K562 cells.

Suppression of NF-κB activation by gentian violet promotes osteoblastogenesis and suppresses osteoclastogenesis.

Skeletal mass is regulated by the coordinated action of bone forming osteoblasts and bone resorbing osteoclasts. Accelerated rates of bone resorption relative to bone formation lead to net bone loss and the development of osteoporosis, a devastating disease that predisposes the skeleton to fractures. Bone fractures are associated with significant morbidity and in the case of hip fractures, high mortality. Gentian violet (GV), a cationic triphenylmethane dye, has long been used as an antifungal and antibacterial agent and is presently under investigation as a potential chemotherapeutic and antiangiogenic agent. However, effects on bone cells have not been previously reported and the mechanisms of action of GV, are poorly understood. In this study we show that GV suppresses receptor activator of NF-κB ligand (RANKL)-induced differentiation of RAW264.7 osteoclast precursors into mature osteoclasts, but paradoxically stimulates the differentiation of MC3T3 cells into mineralizing osteoblasts. These actions stem from the capacity of GV to suppress activation of the nuclear factor kappa B (NF-κB) signal transduction pathway that is required for osteoclastogenesis, but inhibitory to osteoblast differentiation and activity. Our data reveal that GV is an inhibitor of NF-κB activation and may hold promise for modulation of bone turnover to promote a balance between bone formation and bone resorption, favorable to gain of bone mass.

B Cell Production of Both OPG and RANKL is Significantly Increased in Aged Mice.

Aging is a risk factor for osteoclastic bone loss and bone fracture. Receptor activator of NF-κB ligand (RANKL) is the key effector cytokine for osteoclastogenesis and bone resorption, and is moderated by its decoy receptor osteoprotegerin (OPG). The development of an inflammatory environment during aging leads to increased bone resorption and loss of bone mineral density (BMD). Interestingly, animal and clinical studies show that OPG is actually increased in aging but fails to fully compensate for endogenous RANKL. Osteoblast- and B-lineage cells are significant sources of physiological OPG, however osteoblast OPG production declines with age, suggesting that elevated OPG in aging may be a consequence of changes in B cell function. In this study we examined BMD and indices of trabecular bone structure during aging, and B cell production of both RANKL and OPG in young and aged mice. Our data reveal significant loss of BMD and trabecular structure with age commensurate with significantly elevated concentrations of both OPG and RANKL in aged mice, and a decline in B cell populations in aged animals. Taken together our data suggest that B cells may be responsible for the elevated concentrations of OPG during aging and are essential to counteract excessive age-associated bone resorption. Paradoxically, B cells themselves likely contribute RANKL in aging and the loss of B cells with age may further contribute to the imbalance in OPG relative to RANKL that predisposes age-associated bone loss.

The bone anabolic carotenoids p-hydroxycinnamic acid and ß-cryptoxanthin antagonize NF-κB activation in MC3T3 preosteoblasts.

The carotene p-hydroxycinnamic acid and the xanthophyll ß-cryptoxanthin are members of the carotenoid family of plant-derived pigments, which are endowed with anti-osteoporotic properties in vivo. p-Hydroxycinnamic acid and ß-cryptoxanthin have been demonstrated to stimulate osteoblastic bone formation while simultaneously repressing osteoclastic bone resorption in vitro. However, their mechanisms of action remain poorly elucidated. It is well established that the NF-;kgr;B signal transduction pathway plays a critical role in osteoclast differentiation. Moreover, we recently demonstrated that NF-κB activity potently antagonizes osteoblastic differentiation and mineralization in vitro. In this study, we used transient transfection assays of a NF-κB luciferase reporter to demonstrate that p-hydroxycinnamic acid and ß-cryptoxanthin antagonize NF-κB activation in MC3T3 preosteoblastic cells. The data obtained suggest that NF-κB may be a common molecular target by which several bone active agents, including carotenoids, promote osteoblastic bone formation.

Cyclic adenosine 3',5'-monophosphate, a second messenger in interleukin-1 mediated K562 cytostasis.

This study evaluates intracellular cAMP concentrations, in interleukin-1 (IL-1) mediated cytostasis of K562 cells. Dibutyryl-cAMP, forskolin, staurosporine and phorbol 12-myristate 13-acetate were all found to mimic the cytostatic action of IL-1 on K562 cells. IL-1 was found to elicit a dose-dependent rise in adenyl cyclase activity in isolated K562 membranes, as well as increasing cAMP concentrations in whole cells. cAMP concentrations were found to peak within 3 minutes of IL-1 stimulation. A 27-29% reduction in proliferative activity elicited by IL-1, forskolin and staurosporine, correlated with a 6 fold increase in adenyl cyclase activity, in each case.

Nuclear internalisation and DNA binding activities of interleukin-1, interleukin-1 receptor and interleukin-1/receptor complexes.

This paper presents evidence to suggest that interleukin-1 alpha as a complex with its receptor is able to express DNA binding activity. Both the interleukin-1/receptor complex and the interleukin-1 receptor appear to be able to bind to DNA, however interleukin-1 on its own showed no binding activity. Interleukin-1 was found to be internalised into the nuclei of all cells examined (EL4, MEL, HL-60, K562, THP-1 and Jurkat cells). The data suggest the possible modulation of genes by interaction of interleukin-1/receptor complexes with DNA structures.

DNA secondary structures and the evolution of hypervariable tandem arrays.

Tandem repeats are ubiquitous in nature and constitute a major source of genetic variability in populations. This variability is associated with a number of genetic disorders in humans including triplet expansion diseases such as Fragile X syndrome and Huntington's disease. The mechanism responsible for the variability/instability of these tandem arrays remains contentious. We show here that formation of secondary structures, in particular intrastrand tetraplexes, is an intrinsic property of some of the more unstable arrays. Tetraplexes block DNA polymerase progression and may promote instability of tandem arrays by increasing the likelihood of reiterative strand slippage. In the course of doing this work we have shown that some of these tetraplexes involve unusual base interactions. These interactions not only generate tetraplexes with novel properties but also lead us to conclude that the number of sequences that can form stable tetraplexes might be much larger than previously thought.

The mouse Ms6-hm hypervariable microsatellite forms a hairpin and two unusual tetraplexes.

The mouse Ms6-hm microsatellite consists of a tandem array of the pentamer d(CAGGG)n. This microsatellite is extremely hypervariable, showing a germ line mutation rate of 2.5%/gamete. The mechanism responsible for this instability is not known. The ability to form intrastrand structures is a conserved feature of many hypervariable sequences, and it has been suggested that the formation of such structures might account for instability by affecting DNA replication, repair, or recombination. Here we show that this microsatellite is able to form intrastrand structures as well. Under physiological conditions, the Ms6-hm microsatellite forms a hairpin as well as two different unusual intrastrand tetraplexes. The hairpin forms in the absence of monovalent cation and contains G.A, G.C, and G.G base pairs in a 1:1:1 ratio. In the presence of K+, a tetraplex is formed in which the adenines are unpaired and extrahelical, and the cytosines are involved in C.C pairs. In Na+, a tetraplex forms that contains C.C+ pairs, with the adenines being intrahelical and hydrogen-bonded to guanines. Tetraplex formation in the presence of Na+ requires both cytosines and adenines and might reflect the altered internal dimensions of this tetraplex, perhaps resulting from the ability of the C.C+ pairs to become intercalated in this sequence context. Our demonstration of the stabilization of tetraplexes by hydrogen bonding between adenines and guanines expands the hydrogen-bonding possibilities for tetraplexes and suggests that the category of sequences with tetraplex-forming potential may be larger than previously appreciated.

The development and use of a DNA polymerase arrest assay for the evaluation of parameters affecting intrastrand tetraplex formation.

We show here that a K+-dependent block to DNA synthesis is a sensitive and specific indicator of intrastrand tetraplex formation that can be used, both to identify sequences with tetraplex-forming potential and to examine parameters that affect tetraplex formation. We show that tetraplex formation is determined by a complex combination of factors including the size and base composition of its constituent loops and stems. In the process of carrying out this study we have found that the number of sequences with the ability to form tetraplexes is larger than previously thought, and that such sequences are ubiquitous in eukaryote genomes.

Estrogen decreases osteoclast formation by down-regulating receptor activator of NF-kappa B ligand (RANKL)-induced JNK activation.

The differentiation of cells of the monocytic lineage into mature osteoclasts (OC) is specifically induced by the tumor necrosis factor-related factor, RANKL (receptor activator of NF-kappaB ligand; also known as OPGL, ODF, or TRANCE). Because inhibition of osteoclastogenesis is one of the main mechanisms by which estrogen (E2) prevents bone loss, it is likely that E2 may regulate either the production of, or the target cell responsiveness to RANKL. We found that E2 decreases the differentiation into OC of both murine bone marrow monocytes and RAW 264.7 cells, a monocytic line, by down-regulating the activation of Jun N-terminal kinase 1 (JNK1). Diminished JNK1 activity results in decreased nuclear levels of the key osteoclastogenic transcription factors, c-Fos and c-Jun, and lower binding of these transcriptional inducers to DNA. Thus, one novel mechanism by which E2 down-regulates osteoclastogenesis is by decreasing the responsiveness of OC precursors to RANKL.

Interleukin-7 stimulates osteoclast formation by up-regulating the T-cell production of soluble osteoclastogenic cytokines.

In unstimulated conditions osteoclast renewal occurs as a result of the stromal cell production of the key osteoclastogenic factors, receptor activator of NFkB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). Inflammation is known to cause increased osteoclastogenesis; however, the mechanisms responsible for this phenomenon are poorly understood. We now show that interleukin-1 (IL-1) and tumor necrosis factor alpha (TNFalpha), cytokines typically produced in inflammatory conditions, increase the stromal cell production of IL-7. This factor, in turn, up-regulates production of osteoclastogenic cytokines by T cells leading to stimulation of osteoclast (OC) formation. Although T cells were found to produce soluble forms of both RANKL and M-CSF, saturating concentrations of osteoprotegerin failed to inhibit approximately 40% of the OC formation, suggesting that IL-7 acts via both RANKL-dependent and RANKL-independent pathways. Despite the identification of T-cell-secreted M-CSF, this cytokine was not essential for either RANKL-dependent or -independent OC formation, suggesting that T cells secrete other cytokines capable of substituting for M-CSF action. On the basis of our data, we propose a novel mechanism for inflammatory bone loss in which induction of IL-7 from stromal cells by IL-1 and TNFalpha leads to the production of soluble osteoclastogenic cytokines by T cells. Thus, the mechanism by which IL-7 causes bone resorption involves the activation of T cells and the T-cell-dependent augmentation of osteoclastogenesis. (Blood. 2000;96:1873-1878)

B lymphocytes inhibit human osteoclastogenesis by secretion of TGFbeta.

The role of B lymphocytes in osteoclast (OC) formation is controversial, because both stimulatory and inhibitory effects of B-lineage cells on osteoclastogenesis and life span have been reported. In this study, we have investigated the effects of mature B cells on human osteoclastogenesis using cultures of peripheral blood stem cells (PBSC), a system that generates functional OCs in the absence of stromal cells. We report that B cells inhibit the formation of OCs and shorten the life span of mature OCs by secreting transforming growth factor beta (TGFbeta), a factor that induces apoptosis in these cells. The antiosteoclastogenic effects of B cells are abolished by addition of anti-TGFbeta antibody to osteoclast cultures and mimicked by treatment of B cell-deprived PBSC cultures with recombinant TGFbeta, thus confirming TGFbeta as the B cell produced antiosteoclastogenic activity. Thus, the ability of B cells to downregulate osteoclastogenesis by secretion of the apoptotic cytokine TGFbeta provides new insights into the ability of immune cells to regulate OC formation under basal and inflammatory conditions.

The chicken beta-globin gene promoter forms a novel "cinched" tetrahelical structure.

We have previously shown that the G-rich sequence G16CG(GGT)2GG in the promoter region of the chicken beta-globin gene poses a formidable barrier to DNA synthesis in vitro (Woodford et al., 1994, J. Biol. Chem. 269, 27029-27035). The K+ requirement, template-strand specificity, template concentration independence, and involvement of Hoogsteen bonding suggested that the underlying basis of this new type of DNA synthesis arrest site might be an intrastrand tetrahelical structure. However, the arrest site lacks the four G-rich repeats that are a hallmark of previously described intramolecular tetraplexes and contains a number of noncanonical bases that would be expected to greatly destabilize such a structure. Here we report evidence for an unusual K+-dependent intrastrand "cinched" tetraplex. This structure has several unique features including the incorporation of bases other than guanine into the stem of the tetraplex, interaction between loop bases and bases in the flanking region, and base pairing between bases 3 and 5 of the tetrahelix-forming region to form a molecular "cinch." This finding extends the range of sequences capable of tetraplex formation as well as our appreciation of the conformational complexity of the chicken beta-globin promoter.