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Autocrine stimulation of tumor cells is an important mechanism for the growth of skeletal tumors. In tumors that are sensitive, growth factor inhibitors can dramatically reduce tumor growth. In this study, our aim was to investigate the effects of Secreted phosphoprotein 24 kD (Spp24) on the growth of osteosarcoma (OS) cells in the presence and absence of exogenous BMP‐2 both in vitro and in vivo. Our study demonstrated that Spp24 inhibited proliferation and promoted apoptosis of OS cells as confirmed by 3‐(4,5‐dimethylthiazolyl‐2)‐2,5‐diphenyltetrazolium bromide assay and immunohistochemical staining. We found that BMP‐2 increased the mobility and invasiveness of tumor cells in vitro whereas Spp24 inhibited both of these processes alone and in the presence of exogenous BMP‐2. Phosphorylation of Smad1/5/8 and Smad8 gene expression was enhanced by treatment with BMP‐2 but inhibited by treatment with Spp24. Subcutaneous and intratibial tumor models in nude mice demonstrated that BMP‐2 promoted OS growth in vivo, while Spp24 significantly inhibited tumor growth. We conclude that the BMP‐2/Smad signaling pathway is involved in the pathogenesis of OS growth and that Spp24 inhibits the growth of human OS induced by BMP‐2 both in vitro and in vivo. Interruption of Smad signaling and increased apoptosis appear to be the primary mechanisms involved. These results confirm the potential of Spp24 as a therapeutic agent for the treatment of OS and other skeletal tumors. [ABSTRACT FROM AUTHOR]

The correlation between BMP‐2 and osteosarcoma growth has gained increased interest in the recent years, however, there is still no consensus. In this study, we tested the effects of BMP‐2 on osteosarcoma cells through both in vitro and in vivo experiments. The effect of BMP‐2 on the proliferation, migration and invasion of osteosarcoma cells was tested in vitro. Subcutaneous and intratibial tumor models were used for the in vivo experiments in nude mice. The effects of BMP‐2 on EMT of osteosarcoma cells and the Wnt/β‐catenin signaling pathway were also tested using a variety of biochemical methods. In vitro tests did not show a significant effect of BMP‐2 on tumor cell proliferation. However, BMP‐2 increased the mobility of tumor cells and the invasion assay demonstrated that BMP‐2 promoted invasion of osteosarcoma cells in vitro. In vivo animal study showed that BMP‐2 dramatically enhanced tumor growth. We also found that BMP‐2 induced EMT of osteosarcoma cells. The expression levels of Axin2 and Dkk‐1 were both down regulated by BMP‐2 treatment, while β‐catenin, c‐myc and Cyclin‐D1 were all upregulated. The expression of Wnt3α and p‐GSK‐3β were also significantly upregulated indicating that the Wnt/β‐catenin signaling pathway was activated during the EMT of osteosarcoma driven by BMP‐2. From this study, we can conclude that BMP‐2 significantly promotes growth of osteosarcoma cells (143B, MG63), and enhances mobility and invasiveness of tumor cells as demonstrated in vitro. The underlying mechanism might be that BMP‐2 promotes EMT of osteosarcoma through the Wnt/β‐catenin signaling pathway. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1638–1648, 2019. [ABSTRACT FROM AUTHOR]

Bone morphogenetic proteins (BMPs) and transforming growth factor-beta (TGF-β) contribute to the growth of some skeletal metastases through autocrine stimulation. Secreted phosphoprotein 24 kDa (spp24) has been shown to bind to both BMP-2 and TGF-β and to markedly inhibit the osteogenic properties of rhBMP-2. We hypothesized that the addition of spp24 would sequester autocrine growth factors (especially BMP-2) and reduce tumor growth in a system (A549 human non-small cell lung cancer cell line) where autocrine stimulation by BMP-2 is known to be important. A549 cells were injected into two sites (subcutaneous and intraosseus) in SCID mice with and without the co-injection of BMP-2 and spp24. Tumor growth after 8 weeks was assessed through gross examination, radiological imaging, and histological analysis. Spp24 attenuated the tumor growth enhancing effects of rhBMP-2 and reduced the tumor growth when added to tumor cells that were not treated with BMP-2. We conclude that spp24 can reduce A549 cell tumor growth in both soft tissue and intraosseus environments. We hypothesize that the mechanism for this inhibition is interruption of autocrine stimulation through the sequestration of BMP-2. Spp24 can be developed into a therapeutic agent that can be employed in clinical situations where the inhibitions of BMPs and related proteins is advantageous. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 29:1712-1718, 2011 [ABSTRACT FROM AUTHOR]

Secreted phosphoprotein-24 kDa (spp24) is a bone morphogenetic protein (BMP)-binding protein isolated from bone. It exists in a number of size forms and is hypothesized to function as a BMP latency protein and/or a 'slow release' mechanism for BMPs involved in bone turnover and repair. We have examined the hypothesis that proteolytic modification of the C-terminus of spp24 affects its BMP-2-binding properties and bioactivity in the BMP-2-stimulated ectopic bone forming bioassay. Three different size forms of recombinant spp24 that correspond to predicted 18.1 kDa, 16.0 kDa, and 14.5 kDa proteolytic products were compared to full-length (fl) spp24. One of these forms (spp18.1) we hypothesize to be the protein which Urist initially, but apparently inaccurately, called 'BMP.' Only full-length spp24 completely inhibited BMP-2-induced bone formation. The 18.1 kDa truncated isoform of spp24 which we hypothesize to be Urist's protein did not. The inhibitory capacity of the proteins was correlated with their kinetic constants, assessed by surface plasmon resonance. At the highest, inhibitory, dose of spp24 and its derivatives, kd ('stability') best predicted the extent of ectopic bone formation whereas at the lowest dose, which was not inhibitory, ka ('recognition') best predicted the extent of ectopic bone formation. We conclude that proteolytic processing of spp24 affects the interaction of this protein with BMP-2 and this affects the function of the protein. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:1200-1207, 2010 [ABSTRACT FROM AUTHOR]

BMP-binding peptide (BBP) enhances the osteogenic activity of recombinant human bone morphogenetic protein-2 (rhBMP-2), but the mechanism underlying the enhancement remains unclear. We aimed to elucidate the potential enhanced efficacy of BBP using critical-sized segmental femoral bone defects in rats. Seventy defects in seven groups of rats were filled with various amounts (0, 2, 5, and 10 µg) of rhBMP-2 with or without 1000 µg BBP. Radiographs were obtained after 4 and 8 weeks. The animals were euthanized at 8 weeks, and femoral specimens were assessed manually, evaluated for bone volume using microcomputed tomography, and subjected to histological or biomechanical analysis. Although 10 µg rhBMP-2 yielded consistent results in terms of bone healing and quality of bone repair across the segmental defect, lower doses of rhBMP-2 failed to induce satisfactory bone healing. However, the combined administration of lower doses of rhBMP-2 and BBP induced the formation of significantly large amounts of bone. Our results suggest that the combined administration of rhBMP-2 and BBP facilitates bone healing and has potential clinical applications. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:258-264, 2010 [ABSTRACT FROM AUTHOR]

Secreted phosphoprotein 24 kDa (spp24) is a bone matrix protein. It contains a TGF-β receptor II homology 1 (TRH1) domain. A cyclic, synthetic 19 amino acid peptide (bone morphogenetic protein binding peptide or BBP) based on the sequence of the TRH1 domain enhances BMP-2 induced osteogenesis. Many observations suggest that different size forms of this protein have very different effects (inhibiting or enhancing) on BMP-2 induced osteogenesis. Using the stable recombinant Met(His)6-tagged secretory form of full-length (fl) bovine spp24 [Met(His)6-spp24 (residues 24-203)] and transgenic (TG) mice expressing fl bovine spp24 (residues 1-203), we have demonstrated that spp24 inhibits BMP-2 induced bone formation. The effects of Met(His)6-spp24 (24-203) were determined in the ectopic bone-forming bioassay in male mice. Implantation of 5 µg of BMP-2 stimulated bone formation, assessed densitometrically as bone area and mineral content. When Met(His)6-spp24 (24-203) was implanted with BMP-2, it elicited a dose-dependent decrease in BMP-2-medicated ectopic bone formation. When added at a 50-fold excess (w/w), Met(His)6-spp24 (24-203) completely ablated the effects of BMP-2, while addition of a 10-fold excess had no effect. Constitutive expression of fl bovine spp24 (1-203) under the control of the osteocalcin promoter in TG female mice reduced femoral and vertebral bone mineral density at 3 months of age and reduced femoral BMD at 8 months of age, but had no effects in male mice, which can exhibit less osteocalcin-promoter driven gene transcription than females. Histomorphometric analysis demonstrated that bone volume and trabecular thickness were lower in TG female mice at 3 months of age than in sex- and age-matched wild type (WT) controls. Thus, fl spp24 and its secretory isoform (Met(His)6-spp24 [24-203]), which contain a BMP-binding or TRH1 motif, inhibit ectopic bone formation in male mice and adversely affects BMD and histological parameters related to bone mass and formation in female mice expressing the human transgene. Under these conditions, fl spp24 acts as a BMP antagonist in vivo. © 2008 Orthopaedic Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:753-758, 2008 [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to an article by Benham and colleague about bone morphogenetic protein in a 2005 issue.

Abstract: A 97-kDa protein called valosin-containing protein (VCP) has been implicated in osteosarcoma metastasis and Paget’s disease of bone, two conditions that complicate the course and outcome of orthopaedic surgery. High VCP gene expression is associated with high metastatic potential in osteosarcoma cells, while loss-of-function VCP mutations cause inclusion body myopathy associated with Paget’s disease of bone and frontotemporal dementia (IBMPFD). VCP protein expression and regulation have not been examined in normal osteoblasts. The purpose of these studies was to characterize VCP protein expression in control and stressed untransformed osteoblasts. Proteins from confluent MC3T3-E1 mouse osteoblast-like cells were separated by 2D IEF/SDS-PAGE. An abundant spot with a M r of 94kDa and a pI of 5.4 was identified as VCP by MALDI/ToF and peptide mass fingerprint analysis. High constitutive VCP protein expression in subconfluent and confluent resting and mildly physiologically stressed MC3T3-E1 cells was confirmed by Western blotting. When assessed by indirect immunofluorescence in fixed cells or Western blotting of subcellular fractions, VCP was more abundant in the cytoplasm than in the nucleus. Induction of mild physiological stress sufficient to stimulate the ubiquitin-proteasome pathway, which is partially dependent on VCP-mediated targeting of polyubiquitinylated substrates, did not affect steady-state VCP levels or distribution. Thus, VCP is a constitutively abundant protein in untransformed osteoblastic cells under all conditions tested. Such high levels of VCP protein expression in untransformed osteoblastic cells argue against a major causative role for it in metastasis, while the occurrence of Paget’s disease in patients with missense VCP mutations supports a major role for VCP in normal osteoblast proliferation and regulation. [Copyright &y& Elsevier]

Forty years ago, Marshall Urist described a partially purified extract of demineralized bone matrix which induced the formation of ectopic bone. This substance, bone morphogenetic protein/non-collagenous protein (BMP/NCP), was never purified to homogeneity but other investigators used similar starting materials to clone a number of recombinant BMPs. Urist recognized that his material probably contained the BMPs which had been cloned by others but always contended that it contained another, more potent, bone inducing material which differed significantly in its physical and chemical properties from the known BMPs. We have used Urist''s protocol to isolate a protein that has the chemical and physical properties of Urist''s “BMP”. It is an 18.5 kD fragment of the bone matrix protein, SPP-24. This fragment contains the cystatin-like domain of SPP-24. We have located a 19 amino acid region which is similar to the TGF-β/BMP-binding region of fetuin, a member of the cystatin family of protease inhibitors. A cyclic peptide, which we call BMP binding peptide (BBP) was generated using this sequence. The peptide avidly bound rhBMP-2 with a KD of 3 × 10-5 M. When implanted alone in mouse muscle, the peptide frequently induced dystrophic calcification. When implanted with rhBMP-2, the peptide enhanced the osteogenic activity of the recombinant molecule. We hypothesize that Urist''s “BMP” was a fragment of SPP-24 which influenced bone induction by binding to bone morphogenetic proteins. BBP may be clinically useful because of its effects on other bone-inducing substances. [Copyright &y& Elsevier]

Bone is subjected to a variety of physiological, as well as cell-deforming biomechanical stresses, including hydrostatic compression and fluid flow. However, little is known about the molecular mechanisms that protect bone cells from mechanical, ischemic, or oxidative damage. Crystallins are 20 kD heat shock proteins that function as molecular chaperones. We tested the hypothesis that alpha B-crystallin (αB-crystallin), the most widely expressed vertebrate crystallin, is present in bone and osteoblast-like cells. Noncollagenous proteins (NCPs) were extracted from human demineralized bone matrix with 4 M guanidine HCl containing 0.5 M CaCl2 and protease inhibitors, defatted, dialyzed against 0.2% (v/v) Triton X-100 in 100 mM Tris-HCI (pH 7.2) and water, centrifuged, and lyophilized. The NCPs were separated by 2D IEF/SDS-PAGE. The two most abundant 20 kD spots, with apparent pIs of 7.85 and 7.42 in urea gels, were excised, subjected to matrix-assisted laser desorption ionization/time-of-flight mass spectrometry, and identified as αB-crystallins. Indirect immunofluorescence localized αB-crystallin to the interphase nucleus, cytoskeleton and cytoplasm of proliferating MC3T3-E1 mouse osteoblast-like cells, as well as the cytoskeleton and cytoplasm of confluent cells. In conclusion, αB-crystallin is present in bone and osteoblast-like cells. We hypothesize that αB-crystallin may play a role in protecting the osteoblast cytoskeleton from mechanical stress and may be important in modulating nuclear or cellular functions, such as transcription or apoptosis, as observed in other tissues. [Copyright &y& Elsevier]