Osteoclast differentiation factor is a ligand for osteoprotegerin/osteoclastogenesis-inhibitory factor and is identical to TRANCE/RANKL

Osteoclasts, the multinucleated cells that resorb bone, develop from hematopoietic cells of monocyte/macrophage lineage. Osteoclast-like cells (OCLs) are formed by coculturing spleen cells with osteoblasts or bone marrow stromal cells in the presence of bone-resorbing factors. The cell-to-cell interaction between osteoblasts/stromal cells and osteoclast progenitors is essential for OCL formation. Recently, we purified and molecularly cloned osteoclastogenesis-inhibitory factor (OCIF), which was identical to osteoprotegerin (OPG). OPG/OCIF is a secreted member of the tumor necrosis factor receptor family and inhibits osteoclastogenesis by interrupting the cell-to-cell interaction. Here we report the expression cloning of a ligand for OPG/OCIF from a complementary DNA library of mouse stromal cells. The protein was found to be a member of the membrane-associated tumor necrosis factor ligand family and induced OCL formation from osteoclast progenitors. A genetically engineered soluble form containing the extracellular domain of the protein induced OCL formation from spleen cells in the absence of osteoblasts/stromal cells. OPG/OCIF abolished the OCL formation induced by the protein. Expression of its gene in osteoblasts/stromal cells was up-regulated by bone-resorbing factors. We conclude that the membrane-bound protein is osteoclast differentiation factor (ODF), a long-sought ligand mediating an essential signal to osteoclast progenitors for their differentiation into osteoclasts. ODF was found to be identical to TRANCE/RANKL, which enhances T-cell growth and dendritic-cell function. ODF seems to be an important regulator in not only osteoclastogenesis but also immune system.

Oncoprotein MDM2 is a ubiquitin ligase E3 for tumor suppressor p53

The tumor suppressor p53 is degraded by the ubiquitin-proteasome system. p53 was polyubiquitinated in the presence of E1, UbcH5 as E2 and MDM2 oncoprotein. A ubiquitin molecule bound MDM2 through sulfhydroxy bond which is characteristic of ubiquitin ligase (E3)-ubiquitin binding. The cysteine residue in the carboxyl terminus of MDM2 was essential for the activity. These data suggest that the MDM2 protein, which is induced by p53, functions as a ubiquitin ligase, E3, in human papillomavirus-uninfected cells which do not have E6 protein.

Identity of osteoclastogenesis inhibitory factor (OCIF) and osteoprotegerin (OPG): a mechanism by which OPG/OCIF inhibits osteoclastogenesis in vitro

The morphogenesis and remodeling of bone depends on the integrated activity of osteoblasts that form bone and osteoclasts that resorb bone. We previously reported the isolation of a new cytokine termed osteoclastogenesis inhibitory factor, OCIF, which specifically inhibits osteoclast development. Here we report the cloning of a complementary DNA of human OCIF. OCIF is identical to osteoprotegerin (OPG), a soluble member of the tumor-necrosis factor receptor family that inhibits osteoclastogenesis. Recombinant human OPG/OCIF specifically acts on bone tissues and increases bone mineral density and bone volume associated with a decrease of active osteoclast number in normal rats. Osteoblasts or bone marrow-derived stromal cells support osteoclastogenesis through cell-to-cell interactions. A single class of high affinity binding sites for OPG/OCIF appears on a mouse stromal cell line, ST2, in response to 1,25-dihydroxyvitamin D3. An anti-OPG/OCIF antibody that blocks the binding abolishes the biological activity of OPG/OCIF. When the sites are blocked with OPG/OCIF, ST2 cells fail to support osteoclastogenesis. These results suggest that the sites are involved in cell-to-cell signaling between stromal cells and osteoclast progenitors and that OPG/OCIF inhibits osteoclastogenesis by interrupting the signaling through the sites.

Regulation of AMPA receptor endocytosis by a signaling mechanism shared with LTD

The endocytosis of AMPA receptors is thought to be important in the expression of long-term depression (LTD) triggered by NMDA receptor activation. Although signaling pathways necessary for LTD induction have been identified, those responsible for the regulated internalization of AMPA receptors are unknown. Here we show that activation of NMDA receptors alone can trigger AMPA receptor endocytosis through calcium influx and activation of the calcium-dependent protein phosphatase calcineurin. A distinct signaling mechanism mediates the AMPA receptor endocytosis stimulated by insulin. These results demonstrate that although multiple signaling pathways can induce AMPA receptor internalization, NMDA receptor activation enhances AMPA receptor endocytosis via a signaling mechanism required for the induction of LTD.

Association of p19(ARF) with Mdm2 inhibits ubiquitin ligase activity of Mdm2 for tumor suppressor p53

We have demonstrated previously that the oncoprotein Mdm2 has a ubiquitin ligase activity for the tumor suppressor p53 protein. In the present study, we characterize this ubiquitin ligase activity of Mdm2. We first demonstrate the ubiquitination of several p53 point mutants and deletion mutants by Mdm2. The point mutants, which cannot bind to Mdm2, are not ubiquitinated by Mdm2. The ubiquitination of the C-terminal deletion mutants, which contain so-called Mdm2-binding sites, is markedly decreased, compared with that of wild-type p53. The binding of Mdm2 to p53 is essential for ubiquitination, but p53's tertiary structure and/or C-terminal region may also be important for this reaction. DNA-dependent protein kinase is known to phosphorylate p53 on Mdm2-binding sites, where DNA damage induces phosphorylation, and p53 phosphorylated by this kinase is not a good substrate for Mdm2. This suggests that DNA damage-induced phosphorylation stabilizes p53 by inhibiting its ubiquitination by Mdm2. We further investigated whether the tumor suppressor p19(ARF) affects the ubiquitin ligase activity of Mdm2 for p53. The activity of p19(ARF)-bound Mdm2 was found to be lower than that of free Mdm2, suggesting that p19(ARF) promotes the stabilization of p53 by inactivating Mdm2.

Severe osteoporosis in mice lacking osteoclastogenesis inhibitory factor/osteoprotegerin

Osteoclasts are multinucleated cells that resorb bone. Osteoclastogenesis inhibitory factor (OCIF), also called osteoprotegerin (OPG), acts as a naturally occurring decoy receptor for osteoclast differentiation factor, which mediates an essential signal to osteoclast progenitors for their differentiation into osteoclasts. Here we show that the OCIF/OPG knockout mice exhibited severe osteoporosis due to enhanced osteoclastogenesis when they grew to be adults. These mice were viable and fertile. They exhibited marked bone loss accompanied by destruction of growth plate and lack of trabecular bone in their femurs. The strength of their bones dramatically decreased. These results demonstrate that OCIF/OPG is a key factor acting as a negative regulator against osteoclastogenesis. The OCIF/OPG knockout mice provide the first animal model for osteoporosis without other obvious abnormalities.

RANK is the essential signaling receptor for osteoclast differentiation factor in osteoclastogenesis

Osteoclast differentiation factor (ODF) is a ligand for osteoclastogenesis-inhibitory factor/osteoprotegerin (OCIF/OPG), and mediates an essential signal for osteoclastogenesis. Soluble-form ODF binds directly to osteoclast progenitors, suggesting the presence of a membrane-bound receptor for ODF (ODFR) on the cells. To understand the ODF-mediated signal transduction mechanism in osteoclastogenesis, we molecularly cloned ODFR from a mouse macrophage-like osteoclast progenitor cell line, C7. Nucleotide sequence analysis revealed that ODFR is identical to RANK, a recently identified member of the tumor necrosis factor receptor (TNFR) family, which is involved in the regulation of dendritic cell function. A polyclonal antibody against the extracellular domain of RANK induced osteoclastogenesis in the presence of macrophage colony-stimulating factor (M-CSF). In contrast, both a genetically engineered soluble RANK and Fab fragment of the antibody blocked the binding of ODF to RANK and ODF-mediated osteoclastogenesis. These results indicate that RANK is the signaling receptor essential for ODF-mediated osteoclastogenesis.

Involvement of PIAS1 in the sumoylation of tumor suppressor p53

Sumoylation of p53 by the ubiquitin-like protein, SUMO-1/sentrin/PIC1, has been shown to stimulate its transcriptional activation activity. The SUMO E3 ligase, a key enzyme in the recognition of substrates to be sumoylated, has not yet been identified. We isolated PIAS1 (protein inhibitor of activated STAT1) as a SUMO-1 binding protein by yeast two-hybrid screening. In addition, PIAS1 bound p53 and Ubc9, the E2 for SUMO. PIAS1 that was mutated in the RING finger-like domain bound p53 and SUMO-1, but not Ubc9. PIAS1 catalyzed the sumoylation of p53 both in U2OS cells and in vitro in a domain-dependent manner. These data suggest that PIAS1 functions as a SUMO ligase, or possibly as a tightly bound regulator of it, toward p53.

Amelioration of accelerated diabetic mesangial expansion by treatment with a PKC beta inhibitor in diabetic db/db mice, a rodent model for type 2 diabetes

Activation of protein kinase C (PKC) is implicated as an important mechanism by which diabetes causes vascular complications. We have recently shown that a PKC beta inhibitor ameliorates not only early diabetes-induced glomerular dysfunction such as glomerular hyperfiltration and albuminuria, but also overexpression of glomerular mRNA for transforming growth factor beta1 (TGF-beta1) and extracellular matrix (ECM) proteins in streptozotocin-induced diabetic rats, a model for type 1 diabetes. In this study, we examined the long-term effects of a PKC beta inhibitor on glomerular histology as well as on biochemical and functional abnormalities in glomeruli of db/db mice, a model for type 2 diabetes. Administration of a PKC beta inhibitor reduced urinary albumin excretion rates and inhibited glomerular PKC activation in diabetic db/db mice. Administration of a PKC beta inhibitor also prevented the mesangial expansion observed in diabetic db/db mice, possibly through attenuation of glomerular expression of TGF-beta and ECM proteins such as fibronectin and type IV collagen. These findings provide the first in vivo evidence that the long-term inhibition of PKC activation in the renal glomeruli can ameliorate glomerular pathologies in diabetic state, and thus suggest that a PKC beta inhibitor might be an useful therapeutic strategy for the treatment of diabetic nephropathy.

Osteoblasts/stromal cells stimulate osteoclast activation through expression of osteoclast differentiation factor/RANKL but not macrophage colony-stimulating factor: receptor activator of NF-kappa B ligand

We previously reported that osteoblasts/stromal cells are essentially involved in the activation as well as differentiation of osteoclasts through a mechanism involving cell-to-cell contact between osteoblasts/stromal cells and osteoclast precursors/osteoclasts. Osteoclast differentiation factor (ODF, also called RANKL/OPGL/TRANCE) and macrophage colony-stimulating factor (M-CSF, also called CSF-1) are two essential factors produced by osteoblasts/stromal cells for osteoclastogenesis. In other words, osteoblasts/stromal cells were not necessary to generate osteoclasts from spleen cells in the presence of both ODF/RANKL and M-CSF. In the present study, we examined the precise roles of ODF/RANKL and M-CSF in the activation of osteoclasts induced by calvarial osteoblasts. Osteoclasts were formed in mouse bone marrow cultures on collagen gel-coated dishes in response to a soluble form of ODF/RANKL (sODF/sRANKL) and M-CSF, and recovered by collagenase digestion. When recovered osteoclasts were further cultured on plastic dishes, most of the osteoclasts spontaneously died within 24 h. Osteoclasts cultured for 24 h on dentine slices could not form resorption pits. Addition of sODF/sRANKL to the recovered osteoclasts markedly enhanced their survival and pit-forming activity. M-CSF similarly stimulated the survival of osteoclasts, but did not induce their pit-forming activity. When primary mouse osteoblasts were added to the recovered osteoclasts, resorption pits were formed on dentine slices. Bone-resorbing factors such as 1alpha,25-dihydroxyvitamin D3, parathyroid hormone, or prostaglandin E2 enhanced pit-forming activity of osteoclasts only in the presence of osteoblasts. M-CSF-deficient osteoblasts prepared from op/op mice similarly enhanced pit-forming activity of osteoclasts. The pit-forming activity of osteoclasts induced by sODF/sRANKL or osteoblasts was completely inhibited by simultaneous addition of osteoprotegerin/osteoclastogenesis inhibitory factor, a decoy receptor of ODF/RANKL. Primary osteoblasts constitutively expressed ODF/RANKL mRNA, and its level was upregulated by treatment with 1alpha,25-dihydroxyvitamin D3, parathyroid hormone, and prostaglandin E2. These results, obtained by using an assay system that unequivocally assesses osteoclast activation, suggest that ODF/RANKL but not M-CSF mediates osteoblast-induced pit-forming activity of osteoclasts, and that bone-resorbing factors stimulate osteoclast activation through upregulation of ODF/RANKL by osteoblasts/stromal cells.

Exosomal Fetuin-A identified by proteomics: a novel urinary biomarker for detecting acute kidney injury

Urinary exosomes containing apical membrane and intracellular fluid are normally secreted into the urine from all nephron segments, and may carry protein markers of renal dysfunction and structural injury. We aimed to discover biomarkers in urinary exosomes to detect acute kidney injury (AKI), which has a high mortality and morbidity. Animals were injected with cisplatin. Urinary exosomes were isolated by differential centrifugation. Protein changes were evaluated by two-dimensional difference in gel electrophoresis and changed proteins were identified by mass spectrometry. The identified candidate biomarkers were validated by Western blotting in individual urine samples from rats subjected to cisplatin injection; bilateral ischemia and reperfusion (I/R); volume depletion; and intensive care unit (ICU) patients with and without AKI. We identified 18 proteins that were increased and nine proteins that were decreased 8 h after cisplatin injection. Most of the candidates could not be validated by Western blotting. However, exosomal Fetuin-A increased 52.5-fold at day 2 (1 day before serum creatinine increase and tubule damage) and remained elevated 51.5-fold at day 5 (peak renal injury) after cisplatin injection. By immunoelectron microscopy and elution studies, Fetuin-A was located inside urinary exosomes. Urinary Fetuin-A was increased 31.6-fold in the early phase (2-8 h) of I/R, but not in prerenal azotemia. Urinary exosomal Fetuin-A also increased in three ICU patients with AKI compared to the patients without AKI. We conclude that (1) proteomic analysis of urinary exosomes can provide biomarker candidates for the diagnosis of AKI and (2) urinary Fetuin-A might be a predictive biomarker of structural renal injury.

Activity of MDM2, a ubiquitin ligase, toward p53 or itself is dependent on the RING finger domain of the ligase

We previously showed that oncoprotein MDM2 has ubiquitin ligase activity toward tumor suppressor p53. In that paper, we showed very weak homology in the carboxyl terminal portion between MDM2 and E6AP (HECT domain). We mutated the cysteine residue (C464) corresponding to the residue essential for the ubiquitin ligase activity of E6AP and this mutation diminished the ligase activity of MDM2. The cysteine residue described above is also one of the cysteine residues that form the RING finger domain of MDM2. We tried to find out whether the diminishing of the activity by the mutation is attributable to the disruption of the RING finger domain or not. When the ring finger domain of MDM2 was deleted, the truncation mutant did not have the ubiquitin ligase activity. When we mutated the seven cysteine residues of RING finger domain of MDM2 in the carboxyl terminus, the disruption of each residue in the RING finger completely diminished the ubiquitin ligase activity of MDM2 toward MDM2 itself and toward tumor suppressor p53. These data indicate that the RING finger domain in MDM2 is essential for its ubiquitin ligase activity toward p53 and itself.

NG-methyl-L-arginine, an inhibitor of L-arginine-derived nitric oxide synthesis, stimulates renal sympathetic nerve activity in vivo. A role for nitric oxide in the central regulation of sympathetic tone?

Continuous production of endothelium-derived nitric oxide (NO) in peripheral vessels has been shown to modulate vascular resistance and blood pressure. NO is also formed in the brain upon activation of glutamate receptors, which are thought to mediate central autonomic reflexes. In the present study we assessed whether NO plays a role in central autonomic regulation. For this, we have investigated the effects of NG-methyl-L-arginine (NMA), a selective inhibitor of NO synthesis from L-arginine, on sympathetic renal nerve activity (RNA), blood pressure, and heart rate in the anesthetized rat. NMA elicited a dose-dependent sustained increase in blood pressure (approximately 20 and 30 mm Hg, 5 minutes after 10 and 50 mumol/kg i.v., respectively). Heart rate and RNA decreased transiently (15 beats per minute and 40%, respectively); RNA subsequently increased (100%) while blood pressure remained elevated. Baroreceptor deafferentation markedly altered these responses to NMA; the transient decreases in heart rate and RNA were abolished, whereas the increases in RNA and blood pressure were significantly potentiated. After spinal C-1-C-2 transection, there was no increase in RNA, and blood pressure increased to a smaller extent. L-Arginine blocked the NMA-induced increases in blood pressure and RNA. Thus, in addition to modulating vascular resistance by a peripheral action, NO may also play a role in the central regulation of sympathetic tone.

Osteoprotegerin produced by osteoblasts is an important regulator in osteoclast development and function

Osteoprotegerin (OPG), a soluble decoy receptor for receptor activator of nuclear factor-kappaB ligand (RANKL)/osteoclast differentiation factor, inhibits both differentiation and function of osteoclasts. We previously reported that OPG-deficient mice exhibited severe osteoporosis caused by enhanced osteoclastic bone resorption. In the present study, potential roles of OPG in osteoclast differentiation were examined using a mouse coculture system of calvarial osteoblasts and bone marrow cells prepared from OPG-deficient mice. In the absence of bone-resorbing factors, no osteoclasts were formed in cocultures of wild-type (+/+) or heterozygous (+/-) mouse-derived osteoblasts with bone marrow cells prepared from homozygous (-/-) mice. In contrast, homozygous (-/-) mouse-derived osteoblasts strongly supported osteoclast formation in the cocultures with homozygous (-/-) bone marrow cells, even in the absence of bone-resorbing factors. Addition of OPG to the cocultures with osteoblasts and bone marrow cells derived from homozygous (-/-) mice completely inhibited spontaneously occurring osteoclast formation. Adding 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] to these cocultures significantly enhanced osteoclast differentiation. In addition, bone-resorbing activity in organ cultures of fetal long bones derived from homozygous (-/-) mice was markedly increased, irrespective of the presence and absence of bone-resorbing factors, in comparison with that from wild-type (+/+) mice. Osteoblasts prepared from homozygous (-/-), heterozygous (+/-), and wild-type (+/+) mice constitutively expressed similar levels of RANKL messenger RNA, which were equally increased by the treatment with 1alpha,25(OH)2D3. When homozygous (-/-) mouse-derived osteoblasts and hemopoietic cells were cocultured, but direct contact between them was prevented, no osteoclasts were formed, even in the presence of 1alpha,25(OH)2D3 and macrophage colony-stimulating factor. These findings suggest that OPG produced by osteoblasts/stromal cells is a physiologically important regulator in osteoclast differentiation and function and that RANKL expressed by osteoblasts functions as a membrane-associated form.

Osteoclast differentiation factor (ODF) induces osteoclast-like cell formation in human peripheral blood mononuclear cell cultures

We have reported that osteoclast differentiation factor (ODF) expressed on the plasma membrane of osteoblasts/ stromal cells is a ligand for osteoclastogenesis inhibitory factor (OCIF). A genetically engineered soluble form of ODF (sODF) induced osteoclast-like multinucleated cells (OCLs) in the presence of M-CSF in mouse spleen cell cultures. Osteoblasts/stromal cells were not required in this process. To elucidate the mechanism of human osteoclastogenesis, human peripheral blood mononuclear cells (PBMCs) were cultured for 7 days with sODF and human M-CSF in the presence or absence of dexamethasone. Treatment of human PBMCs with sODF together with M-CSF induced OCLs, which expressed tartrate-resistant acid phosphatase and vitronectin receptors, produced cAMP in response to calcitonin, and formed resorption pits on dentine slices. OCLs were also formed from the adherent cell population of human PBMCs. Dexamethasone was required for human OCL formation in culture of whole PBMCs but not in culture of the adherent cell population. OCL formation was strongly inhibited by OCIF simultaneously added. These results clearly indicate that like in mouse osteoclastogenesis, ODF is a critical factor for human osteoclastogenesis. The present study also indicates that OCIF acts as a naturally occurring decoy receptor for ODF in inhibiting signal transduction in human osteoclast formation.

Identity of osteoclastogenesis inhibitory factor (OCIF) and osteoprotegerin (OPG): a mechanism by which OPG/OCIF inhibits osteoclastogenesis in vitro

The morphogenesis and remodeling of bone depends on the integrated activity of osteoblasts that form bone and osteoclasts that resorb bone. We previously reported the isolation of a new cytokine termed osteoclastogenesis inhibitory factor, OCIF, which specifically inhibits osteoclast development. Here we report the cloning of a complementary DNA of human OCIF. OCIF is identical to osteoprotegerin (OPG), a soluble member of the tumor-necrosis factor receptor family that inhibits osteoclastogenesis. Recombinant human OPG/OCIF specifically acts on bone tissues and increases bone mineral density and bone volume associated with a decrease of active osteoclast number in normal rats. Osteoblasts or bone marrow-derived stromal cells support osteoclastogenesis through cell-to-cell interactions. A single class of high affinity binding sites for OPG/OCIF appears on a mouse stromal cell line, ST2, in response to 1,25-dihydroxyvitamin D3. An anti-OPG/OCIF antibody that blocks the binding abolishes the biological activity of OPG/OCIF. When the sites are blocked with OPG/OCIF, ST2 cells fail to support osteoclastogenesis. These results suggest that the sites are involved in cell-to-cell signaling between stromal cells and osteoclast progenitors and that OPG/OCIF inhibits osteoclastogenesis by interrupting the signaling through the sites.

Okadaic acid, a potent inhibitor of type 1 and type 2A protein phosphatases, activates cdc2/H1 kinase and transiently induces a premature mitosis-like state in BHK21 cells

When BHK21 cells synchronized in early S phase were exposed to okadaic acid (OA), an inhibitor of protein phosphatases 1 and 2A, mitosis specific events such as premature chromosome condensation, the production of MPM-2 antigens, dispersion of nuclear lamins and the appearance of mitotic asters were induced, and then disappeared upon further incubation. These mitosis specific events occurred even in the presence of cycloheximide. Within 1 h of exposure to OA, cdc2/histone H1 kinase activity rose 10-fold compared with untreated controls, but returned to the control level upon further incubation. Using antibodies against either p34cdc2 or cyclin B it was found that p34cdc2 complexed with cyclin B was dephosphorylated after OA treatment concomitant with the activation of cdc2 kinase, and that cyclin B was subsequently degraded concomitant with a decrease in cdc2 kinase activity, as in normal mitosis. In contrast, when cells in G1 phase were treated with OA no increase in cdc2 kinase activity was observed. Moreover when cells in pseudo-metaphase induced by nocodazole were treated with OA, cdc2 kinase was inactivated. These results suggest that OA sensitive protein phosphatases control both the activation and inactivation of the p34cdc2 kinase.

Antibodies against GM1 ganglioside affect K+ and Na+ currents in isolated rat myelinated nerve fibers

High titers of anti-GM1 ganglioside antibodies (anti-GM1 antibodies) may be implicated in lower motor neuron disease. We studied the pathogenic role of anti-GM1 antibody using the petroleum jelly-gap voltage clamp technique on isolated single myelinated rat nerve fibers. Anti-GM1 antisera were obtained from rabbits immunized with GM1 ganglioside. Extracellularly applied anti-GM1 antisera without complement activity increased both the rate of rise and the amplitude of the K+ current elicited by step depolarization, with little effect on Na+ current. In the presence of active complement, however, anti-GM1 antibodies decreased the Na+ current, and caused a progressive increase of nonspecific leakage current. Neither complement alone nor complement-supplemented antisera from which anti-GM1 antibodies were depleted by affinity chromatography had any effect on ionic current. These observations indicate that anti-GM1 antibodies themselves can uncover K+ channels in the paranodal region, while anti-GM1 antibodies bound to the nodal membrane in the presence of complement may form antibody-complement complexes that block Na+ channels and disrupt the membrane at the node of Ranvier.

Osteoclast differentiation factor mediates an essential signal for bone resorption induced by 1 alpha,25-dihydroxyvitamin D3, prostaglandin E2, or parathyroid hormone in the microenvironment of bone

Osteoclast differentiation factor (ODF), a ligand for osteoprotegerin (OPG)/osteoclastogenesis-inhibitory factor (OCIF), induces osteoclast-like cell formation in vitro. To elucidate the role of ODF in the microenvironment of bone, we examined effects of ODF, OPG/OCIF, and anti-ODF polyclonal antibody on bone resorption using a fetal mouse long bone culture system. A genetically engineered soluble-form ODF (sODF) elicited bone resorption in a concentration-dependent manner and OPG/OCIF blocked the bone resorption. Anti-ODF polyclonal antibody, which neutralizes ODF activity, negated bone resorption induced by 1 alpha,25-dihydroxyvitamin D3, parathyroid hormone, or prostaglandin E2. OPG/OCIF also abolished bone-resorbing activity elicited by these bone-resorbing agents. Interleukin 1 alpha-stimulated bone resorption was also significantly suppressed by anti-ODF polyclonal antibody and OPG/OCIF. Thus, we conclude that ODF plays a critical role in bone resorption in the microenvironment of bone.

A novel molecular mechanism modulating osteoclast differentiation and function

Osteoclasts, the multinucleated giant cells that resorb bone, develop from hematopoietic cells of the monocyte/ macrophage lineage. Osteoblasts, as well as bone marrow stromal cells, support osteoclast development through a mechanism of cell-to-cell interaction with osteoclast progenitors. We recently purified and molecularly cloned osteoclastogenesis inhibitory factor (OCIF), which was identical to osteoprotegerin (OPG). OPG/OCIF, a secreted member of the tumor necrosis factor (TNF) receptor family, inhibited differentiation and activation of osteoclasts. A single class of high-affinity binding sites for OPG/OCIF appeared on a mouse bone marrow stromal cell line, ST2, in response to 1alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] and dexamethasone (Dex). When the binding sites were occupied by OPG/OCIF, ST2 cells failed to support the osteoclast formation from spleen cells. To identify an OPG/OCIF ligand, we screened a cDNA expression library of ST2 cells treated with 1,25(OH)2D3 and Dex using OPG/OCIF as a probe. The cloned molecule was found to be a member of the membrane-associated TNF ligand family, and it induced osteoclast formation from mouse and human osteoclast progenitors in the presence of macrophage colony-stimulating factor (M-CSF) in vitro. Expression of its gene in osteoblasts/stromal cells was up-regulated by osteotropic factors, such as 1,25(OH)2D3, prostaglandin E2 (P(GE2), parathyroid hormone (PTH), and interleukin (IL)-11. A polyclonal antibody against this protein, as well as OPG/OCIF, negated not only the osteoclastogenesis induced by the protein, but also bone resorption elicited by various osteotropic factors in a fetal mouse long bone culture system. These findings led us to conclude that the protein is osteoclast differentiation factor (ODF), a long sought-after ligand that mediates an essential signal to osteoclast progenitors for their differentiation into active osteoclasts. Recent analyses of ODF receptor demonstrated that RANK, a member of the TNF receptor family, is the signaling receptor for ODF in osteoclastogenesis, and that OPG/OCIF acts as a decoy receptor for ODF to compete against RANK. The discovery of ODF, OPG/OCIF, and RANK opens a new era in the investigation of the regulation of osteoclast differentiation and function.

Transforming growth factor beta affects osteoclast differentiation via direct and indirect actions

Transforming growth factor beta (TGF-beta) is abundant in bone and has complex effects on osteolysis, with both positive and negative effects on osteoclast differentiation, suggesting that it acts via more than one mechanism. Osteoclastogenesis is determined primarily by osteoblast (OB) expression of the tumor necrosis factor (TNF)-related molecule receptor activator of NF-kappaB ligand (RANKL) and its decoy receptor osteoprotegerin (OPG), which are increased and decreased, respectively, by osteolytic factors. A RANKL-independent osteoclastogenic mechanism mediated by TNF-alpha has also been shown. Therefore, we investigated TGF-beta effects on osteoclast formation in culture systems in which osteoclastogenic stimulus is dependent on OBs and culture systems where it was provided by exogenously added RANKL or TNF-alpha. Both OPG and TGF-beta inhibited osteoclast formation in hemopoietic cell/OB cocultures, but the kinetics of their action differed. TGF-beta also inhibited osteoclastogenesis in cocultures of cells derived from OPG null (opg-/-) mice. TGF-beta strongly decreased RANKL messenger RNA (mRNA) expression in cultured osteoblasts, and addition of exogenous RANKL to TGFbeta-inhibited cocultures of opg-/- cells partially restored osteoclastogenesis. Combined, these data indicate that the inhibitory actions of TGF-beta were mediated mainly by decreased OB production of RANKL. In contrast, in the absence of OBs, TGF-beta greatly increased osteoclast formation in recombinant RANKL- or TNF-alpha-stimulated cultures of hemopoietic cells or RAW 264.7 macrophage-like cells to levels several-fold greater than attainable by maximal stimulation by RANKL or TNF-alpha. These data suggest that TGF-beta may increase osteoclast formation via action on osteoclast precursors. Therefore, although RANKL (or TNF-alpha) is essential for osteoclast formation, factors such as TGF-beta may powerfully modify these osteoclastogenic stimuli. Such actions may be critical to the control of physiological and pathophysiological osteolysis.

In vitro SUMO-1 modification requires two enzymatic steps, E1 and E2

The SUMO-1 has been identified as a protein that is highly similar to ubiquitin and shown to conjugate to RanGAP1, PML, Sp200 and I kappa B alpha. The conjugation steps are thought to be similar to those of ubiquitination; and human Ubc9, which is homologous to the E2 enzyme for the ubiquitin conjugation step, was identified and shown to be necessary for the conjugation of SUMO-1 to its target protein. Other essential enzymes involved in this modification, however, remain to be clarified. Here we cloned human Sua1 (SUMO-1 activating enzyme) and hUba2, which are human homologs of yeast Saccharomyces cerevisiae Aos1 and Uba2, respectively. The recombinant proteins, Sua1p and hUba2p, formed a complex. In this complex, hUba2 bound SUMO-1 and this complex had the activity of the SUMO-1 activating enzyme. Furthermore, in an in vitro system, RanGAP1 was modified by SUMO-1 in the presence of Sua1p/Uba2p and hUbc9p, showing that the modification of SUMO-1 could be catalyzed by two enzyme steps, although ubiquitination usually requires three enzyme steps.

Identity of a tumor cytotoxic factor from human fibroblasts and hepatocyte growth factor

Human embryonic lung diploid fibroblast, IMR-90 cells secreted a tumor cytotoxic factor. The fibroblast-derived tumor cytotoxic factor (F-TCF) has a cytotoxic activity to Sarcoma 180 and a cytostatic and degenerative activities to KB cells. F-TCF has been purified about 540,000-fold with 23.3% recovery from 75 liters of the conditioned medium containing 5% newborn calf serum. The purified F-TCF is a basic glycoprotein with isoelectric point values of 7.4 to 8.6. It was stable in the pH range from 6.0 to 9.0 and was stable at the heating temperature of 60 degrees C for 10 min, but completely inactivated by reducing it with 2-mercaptoethanol. F-TCF has molecular weight of 76 to 80 kD on SDS-PAGE under non-reducing conditions and is a heterodimer consisting of a large alpha subunit with 52 to 56 kD and a small beta subunit with 30 to 34 kD. F-TCF was identified as one of human hepatocyte growth factors by the physicochemical properties including N terminal and a few internal amino acid sequences. We have confirmed that F-TCF has an ability to dramatically stimulate DNA synthesis in adult rat hepatocytes in the low dose range of 1 to 10 ng/ml.