Oligodeoxynucleotide targeted to the alphav gene inhibits alphav integrin synthesis, impairs osteoclast function, and activates intracellular signals to apoptosis

Positive and negative regulators of osteoclast apoptosis

Survival and apoptosis are of major importance in the osteoclast life cycle. As osteoclasts have short lifespan, any alteration that prolongs their viability may cause enhanced osteoclast activity. Hence, the regulation of OC apoptosis has been recognized as a critical factor in bone remodeling. An imbalance in bone remodeling due to increased osteoclast activity leads to most adult bone diseases such as osteoporosis, rheumatoid arthritis and multiple myeloma. Therefore, manipulating osteoclast death would be a viable therapeutic approach in ameliorating bone diseases, with accelerated resorption. Over the last few decades we have witnessed the unraveling of many of the intracellular mechanisms responsible for osteoclast apoptosis. Thus, an understanding of the underlying mechanisms by which osteoclasts undergo programmed cell death and the regulators that modulate that activity will undoubtedly provide an insight into the development of pharmacological agents to treat such pathological bone diseases.

Induction of apoptosis of human primary osteoclasts treated with extracts from the medicinal plant Emblica officinalis

Background

Osteoclasts (OCs) are involved in rheumatoid arthritis and in several pathologies associated with bone loss. Recent results support the concept that some medicinal plants and derived natural products are of great interest for developing therapeutic strategies against bone disorders, including rheumatoid arthritis and osteoporosis. In this study we determined whether extracts of Emblica officinalis fruits display activity of possible interest for the treatment of rheumatoid arthritis and osteoporosis by activating programmed cell death of human primary osteoclasts.

Methods

The effects of extracts from Emblica officinalis on differentiation and survival of human primary OCs cultures obtained from peripheral blood were determined by tartrate-acid resistant acid phosphatase (TRAP)-positivity and colorimetric MTT assay. The effects of Emblica officinalis extracts on induction of OCs apoptosis were studied using TUNEL and immunocytochemical analysis of FAS receptor expression. Finally, in vitro effects of Emblica officinalis extracts on NF-kB transcription factor activity were determined by gel shift experiments.

Results

Extracts of Emblica officinalis were able to induce programmed cell death of mature OCs, without altering, at the concentrations employed in our study, the process of osteoclastogenesis. Emblica officinalis increased the expression levels of Fas, a critical member of the apoptotic pathway. Gel shift experiments demonstrated that Emblica officinalis extracts act by interfering with NF-kB activity, a transcription factor involved in osteoclast biology. The data obtained demonstrate that Emblica officinalis extracts selectively compete with the binding of transcription factor NF-kB to its specific target DNA sequences. This effect might explain the observed effects of Emblica officinalis on the expression levels of interleukin-6, a NF-kB specific target gene.

Conclusion

Induction of apoptosis of osteoclasts could be an important strategy both in interfering with rheumatoid arthritis complications of the bone skeleton leading to joint destruction, and preventing and reducing osteoporosis. Accordingly, we suggest the application of Emblica officinalis extracts as an alternative tool for therapy applied to bone diseases.

Antiresorptive agents and osteoclast apoptosis

Antiresorptive agents have proven to be effective therapies for the treatment of bone diseases associated with excessive osteoclast activity. Decreased osteoclast formation, inhibition of osteoclast actions, and reduced osteoclast survival represent mechanisms by which antiresorptive agents could act. The goals of this article are to present the evidence that antiresorptive agents can decrease osteoclast survival through apoptosis, to review the mechanisms by which they are thought to activate the apoptotic process, and to consider whether the actions on apoptosis fully account for the antiresorptive effects. As background, the apoptotic process will be briefly summarized together with the evidence that factors that promote osteoclast survival affect steps in the process. Following this, therapeutic agents that are both antiresorptive and can stimulate osteoclast apoptosis will be discussed. Other bone therapeutic agents that are either antiresorptive or apoptotic, but not both, will be described. Finally, newer antiresorptive compounds that elicit apoptosis and could represent potential therapeutic agents will be noted.

Effects of antisense mediated inhibition of cathepsin K on human osteoclasts obtained from peripheral blood

Cathepsin K is a cystein protease that displays a proteolytic activity against Type I collagen and is abundantly and selectively expressed in osteoclasts where it plays a critical role in bone degradation. Its direct role in bone tissue has been defined by knock-out mice studies and inhibiting strategies in animals models. However, direct proof of cathepsin K function in human osteoclast model in vitro is lacking. The aim of this study is to analyze cathepsin K expression and localization in human osteoclasts obtained from peripheral blood and to examine cathepsin K function in these cells by antisense oligodeoxynucleotide (AS-ODN) strategy. AS-ODN was added to the culture of osteoclast precursors induced to differentiate by RANKL and M-CSF. AS-ODN treatment produced a significant down-regulation of cathepsin K mRNA (>80%) and protein expression, as verified respectively by Real-time PCR and by immunocytochemistry or Western blot. The cathepsin K inhibition caused an impairment of resorption activity as evaluated by a pit formation assay ( p = 0.045) and by electron microscopy, while the acidification process was unaffected. We demonstrated that antisense strategies against cathepsin K are selectively effective to inhibit resorption activity in human osteoclasts, like in animal models.

Induction of apoptosis of human primary osteoclasts treated with a transcription factor decoy mimicking a promoter region of estrogen receptor alpha

In this paper we investigated how the increase of human estrogen receptor alfa (ERalpha) gene expression may affect breast, osteoblast and osteoclast cells. Increase of ERalpha expression was obtained by interfering with the activity of a negative transcription factor and by removing it with a short and powerful decoy oligonucleotide (RA4-3') mimicking a region of distal promoter C of ERalpha gene. We provide evidence that this decoy was able to induce apoptosis in osteoclasts, but not in osteoblasts and in breast cancer cells, in an estrogen dependent manner. This effect was associated with increase of the levels of Caspase 3 and Fas receptor. Since ERalpha is important in the transcription of different genes and is involved in several pathological processes, including neoplastic and osteopenic diseases, our findings may be of relevance for a possible new therapeutical approach of such diseases.

N-Arylpiperazine modified analogues of the P2X7 receptor KN-62 antagonist are potent inducers of apoptosis of human primary osteoclasts

The P2X7 nucleotide receptor is an ATP-gated ion channel that plays an important role in bone cell function. Here, we investigated the effects of L: -tyrosine derivatives 1-3 as potent P2X7 antagonists on human primary osteoclasts. We found that the level of expression of P2X7 receptor increased after treatment with the derivatives 1-3, together with the induction of high levels of apoptosis. This effect is associated with activation of caspase-3 and inhibition of expression of IL-6. Interestingly, no pro-apoptotic effect of compounds 1-3 was found on human osteoblasts. Our results suggest that the development of specific P2X7 receptor antagonists may be considered a useful tool to modulate apoptosis of human osteoclasts. Since bone loss due to osteoclast-mediated resorption represents one of the major unsolved problem in osteopenic disorders, the identification of molecules able to induce apoptosis of osteoclasts is of great interest for the development of novel therapeutic strategies.

A novel protein kinase C α-dependent signal to ERK1/2 activated by αVβ3 integrin in osteoclasts and in Chinese hamster ovary (CHO) cells

We identified a novel protein kinase C (PKC)α-dependent signal to extracellular signal-regulated kinase (ERK)1/2 in mouse osteoclasts and Chinese hamster ovary (CHO) cells, specifically activated by the αVβ3 integrin. It involves translocation (i.e. activation) of PKCα from the cytosol to the membrane and/or the Triton X-100-insoluble subcellular fractions, with recruitment into a complex with αVβ3 integrin, growth factor receptor-bound protein (Grb2), focal adhesion kinase (FAK) in CHO cells and proline-rich tyrosine kinase (PYK2) in osteoclasts. Engagement of αvβ3 integrin triggered ERK1/2 phosphorylation, but the underlying molecular mechanism was surprisingly independent of the well known Shc/Ras/Raf-1 cascade, and of phosphorylated MAP/ERK kinase (MEK)1/2, so far the only recognized direct activator of ERK1/2. In contrast, PKCα was involved in ERK1/2 activation because inhibition of its activity prevented ERK1/2 phosphorylation. The tyrosine kinase c-Src also contributed to ERK1/2 activation, however, it did not interact with PKCα in the same molecular complex. The αVβ3/PKCα complex formation was fully dependent upon the intracellular calcium concentration ([Ca2+]i), and the use of the intracellular Ca2+ chelator 1,2-bis(o-amino-phenoxy)ethane-N,N,N′,N′-tetraaceticacidtetra (acetoxymethyl) ester (BAPTA-AM) also inhibited PKCα translocation and ERK1/2 phosphorylation. Functional studies showed that αVβ3 integrin-activated PKCα was involved in cell migration and osteoclast bone resorption, but had no effect on the ability of cells to attach to LM609, suggesting a role in events downstream of αVβ3 integrin engagement.

Biomimetic Collagen Scaffolds for Human Bone Cell Growth and Differentiation

Type I collagen provides a structural framework for connective tissues and plays a central role in the temporal cascade of events leading to the formation of new bone from progenitors. The aim of this study was to examine the ability of the cell-binding domain of type I collagen (P-15 peptide) to promote human bone marrow stromal cell adhesion, proliferation, and differentiation on three-dimensional scaffolds. Human bone marrow stromal cells were selected, expanded, and cultured on particulate microporous ABM ("pure" hydroxyapatite) phase adsorbed with or without P-15 under basal or osteogenic conditions. Immobilized P-15 increased alkaline phosphatase activity and bone morphogenetic protein 2 (BMP-2) gene expression after 1 and 5 days as determined by real-time polymerase chain reaction. P-15 promoted human bone marrow stromal cell attachment, spreading, and alignment on ABM as well as alkaline phosphatase-specific activity in basal and osteogenic cultures. The presence of mineralized bone matrix, extensive cell ingrowth, and cellular bridging between three-dimensional matrices adsorbed with P-15 was confirmed by confocal microscopy, scanning electron microscopy, and alizarin red staining. Negligible cell growth was observed on ABM alone. In vivo diffusion chamber studies using MF1-nu/nu mice showed bone matrix formation and organized collagen formation after 6 weeks. These studies indicate the potential of P-15 to generate appropriate biomimetic microenvironments for osteoblasts and demonstrate the potential for the exploitation of extracellular matrix cues for osteogenesis and, ultimately, bone regeneration.

Reduction of c-Src activity by substituted 5,7-diphenyl-pyrrolo[2,3-d]-pyrimidines induces osteoclast apoptosis in vivo and in vitro. Involvement of ERK1/2 pathway

We employed potent and selective c-Src inhibitors to investigate the functional and molecular consequences of inhibited c-Src tyrosine kinase activity in osteoclasts. These pyrrolopyrimidine derivatives reduced osteoclast numbers and induced osteoclast disruption in vivo. In vitro, they inhibited resorption pit formation and osteoclastogenesis, impaired adhesion ability and actin ring organization, and induced programmed cell death in mature osteoclasts. The cell death receptor Fas and p53 were insensitive to c-Src modulation. The expression of the cyclin-dependent kinase (CDK)-inhibitor p21WAF1/CIP1 was markedly reduced, but neither Bcl-2 nor Bcl-xL or Bax were modulated by c-Src inhibition. Caspase-9, and to a lesser extent caspase-3, but not caspase-8, were transiently cleaved (activated) by treatment with the c-Src inhibitors. c-Src inhibition stabilized p38 mitogen-activated protein kinase (MAPK), whereas the c-Jun N-terminal kinase (JNK) pathway did not appear to be modulated by our compounds. Most interestingly, transient extracellular signal regulated kinase (ERK1/2) dephosphorylation followed by sustained remarkable rephosphorylation overwhelming control levels was observed in response to c-Src inhibition. Blockade of ERK1/2 rephosphorylation by PD98059 reduced osteoclast nuclear disruption, suggesting the involvement of this pathway in apoptosis. Collectively, these data demonstrate that small pyrrolopyrimidine derivatives impair osteoclast function and induce cell damage suggestive of apoptosis in vivo and in vitro, with mechanisms presumably involving selective sustained ERK1/2 phosphorylation.

Decoy oligodeoxynucleotides targeting NF-kappaB transcription factors: induction of apoptosis in human primary osteoclasts

Proteins belonging to the nuclear factor kappaB (NF-kappaB) superfamily are involved in osteoclast formation, playing a very important role for both differentiation of osteoclast precursors and survival of mature osteoclasts. Several drugs used to fight bone loss in a variety of human pathologies, including osteoporosis, act by increasing the frequency of osteoclast apoptosis, since it was demonstrated that small changes in osteoclast apoptosis can result in large changes in bone formation. In this respect, targeting of NF-kappaB transcription factor could be of great interest. Among nonviral gene therapy strategies recently proposed to inhibit or even block NF-kappaB activity, the transcription factor decoy (TFD) should be taken in great consideration. The main issue of the present study was to examine the effects of decoy DNA/DNA molecules targeting NF-kappaB on apoptosis of human osteoclasts (OCs), with the aim to interfere with the pathway regulating osteoclast differentiation and programmed cell death. To this aim, we used a mixture of receptor activator of NF-kappaB ligand (RANKL), macrophage colony-stimulating factor (M-CSF) and parathyroid hormone (PTH) to prepare human OCs from peripheral blood cells. Then, transfection with the decoy molecules targeting NF-kappaB was performed. The results obtained demonstrate that in primary cells expressing typical osteoclast markers such as TRAP and MMP9, NF-kappaB decoy significantly stimulated apoptosis. Inhibition of IL-6 expression and induction of Caspase 3 were found in OCs treated with NF-kappaB DNA/DNA decoys. We consider these data as the basis for setting up experimental conditions allowing nonviral gene therapy of several bone disorders.

Inhibition of angiogenesis in vitro by alphav integrin-directed antisense oligonucleotides

The integrin alpha v beta 3 plays a central role in angiogenesis. In this study, we used antisense oligodeoxyribonucleotides (ONs) directed against the alpha v subunit of alpha v beta 3 to inhibit integrin expression. Ten ON sequences, which were selected by systematic alignment of computer-predicted secondary structures of alpha v mRNA, were transfected into human umbilical vein endothelial cells (HUVECs). Following stimulation by PMA, five antisense ONs significantly inhibited alpha v mRNA and protein expression in activated HUVEC at a concentration of 0.05 mciroM with complete prevention of PMA-induced alpha v up-regulation by the most potent antisense ON. Inhibition of alpha v expression was associated with significant inhibition of migration of HUVEC by 28% and had no effect on proliferation and apoptosis. Moreover, transfection of antisense ON inhibited the formation of tube-like structures of HUVEC in Matrigel by 44%. In a cell culture model of angiogenesis consisting of a co-culture of endothelial cells with fibroblasts, transfection of antisense ONs resulted in an inhibition of tube formation of 61%. In conclusion, alpha v antisense ONs are potent inhibitors of angiogenesis in vitro. They might, therefore, be a therapeutic alternative to antagonists, which directly bind to alpha v integrins, and might be useful for the treatment of malignant tumors and hematological malignancies.

Human osteoprogenitor growth and differentiation on synthetic biodegradable structures after surface modification

The ability to generate new bone for skeletal use is a major clinical need. Biomimetic scaffolds that interact and promote osteoblast differentiation and osteogenesis offer a promising approach to the generation of skeletal tissue to resolve this major health-care issue. In this study we examine the ability of surface-modified poly(lactic acid) (PLA) films and poly(lactic-co-/glycolic acid) (PLGA) (75:25) porous structures to promote human osteoprogenitor adhesion, spreading, growth, and differentiation. Cell spreading and adhesion were examined using Cell Tracker green fluorescence and confocal microscopy. Osteogenic differentiation was confirmed with alkaline phosphatase activity as well as immunocytochemistry for type I collagen, core binding factor-1 (Cbfa-1), and osteocalcin. Poor cell growth was observed on nonmodified PLA films and PLGA scaffolds. The polymers were then coupled with RGD peptides [using poly(L-lysine), or PLL] and physical adsorption as well as PLA films presenting adsorbed fibronectin (FN). Both modifications enhanced cell attachment and spreading. On PLA-FN and PLA-PLL-GRGDS films, the osteoblast response was dose dependent (20 pmol/L to 0.2 micromol/L FN and 30 nmol/L to 30 micromol/L PLL-GRGDS) and significant at concentrations as low as 2 nmol/L FN and 30 nmol/L PLL-GRGDS. With optimal concentrations of FN or RGD, adhesion and cell spreading were comparable to tissue culture plastic serum controls. In PLGA (75:25) biodegradable porous scaffolds, coated with FN, PLL-GRGDS, or fetal calf serum for 24 h in alpha MEM alone, prior to growth in dexamethasone and ascorbate-2-phosphate for 4-6 weeks, extensive osteoblast impregnation was observed by confocal and fluorescence microscopy. Cell viability in extended culture was maintained as analyzed by expression of Cell Tracker green and negligible ethidium homodimer-1 (a marker of cell necrosis) staining. Alkaline phosphatase activity, type I collagen, Cbfa-1, and osteocalcin expression were observed by immunocytochemistry. Mineralization of collagenous matrix took place after 4 weeks, which confirmed the expression of the mature osteogenic phenotype. These observations demonstrate successful adhesion and growth of human osteoprogenitors on protein- and peptide-coupled polymer films as well as migration, expansion, and differentiation on three-dimensional biodegradable PLGA scaffolds. The use of peptides/proteins and three-dimensional structures that provide positional and environmental information indicate the potential for biomimetic structures coupled with appropriate factors in the development of protocols for de novo bone formation.

Downregulation of small GTPase Rab7 impairs osteoclast polarization and bone resorption

During skeletal growth and remodeling the mineralized bone matrix is resorbed by osteoclasts through the constant secretion of protons and proteases to the bone surface. This relies on the formation of specialized plasma membrane domains, the sealing zone and the ruffled border, and vectorial transportation of intracellular vesicles in bone-resorbing osteoclasts. Here we show that Rab7, a small GTPase that is associated with late endosomes, is highly expressed and is predominantly localized at the ruffled border in bone-resorbing osteoclasts. The decreased expression of Rab7 in cultured osteoclasts by antisense oligodeoxynucleotides disrupted the polarization of the osteoclasts and the targeting of vesicles to the ruffled border. These impairments caused a significant inhibition of bone resorption in vitro. The results indicate that the late endocytotic pathway is involved in the osteoclast polarization and bone resorption and underscore the importance of Rab7 in osteoclast function.

Cell adhesion is a prerequisite for osteoclast survival

The present study demonstrates that loss of cell adhesion potently promotes apoptosis in osteoclasts, a process termed "anoikis." When purified mature rabbit osteoclasts were cultured on plastic for 18 h, about 25% of them were spontaneously committed to apoptosis. The death rate increased more than twofold, after osteoclasts were subjected to suspension culture in inverted Terasaki plates. The osteoclast anoikis was significantly prevented by bongkrekic acid, an inhibitor of mitochondrial permeability transition (PT), and z-VAD-FMK, a caspase inhibitor, suggesting involvement of mitochondrial PT and caspase activation in the death process. Colony-stimulating factor-1 (CSF-1), receptor activator of NF-kappaB ligand (RANKL), and calcitonin protected adherent osteoclasts, but not floating osteoclasts from anoikis. These data show that adhesion is a primary requirement for osteoclast survival.

An antisense oligonucleotide targeting the alphaV integrin gene inhibits adhesion and induces apoptosis in breast cancer cells

The aim of this study was to show the anti-adhesive potential of an antisense oligodeoxynucleotide (ODN) approach when designed to suppress the cellular function of the alphaV integrin subunit in breast cancer cells. The alphaV integrins play major roles in favouring breast cancer spreading. In this study, we inhibited alphaV subunit synthesis in the human breast carcinoma cell line, MDA-MB231, by a partially phosphorothioated antisense oligodeoxynucleotide (5543-ODN). The alphaV antisense 5543-ODN reduced alphaV, but not actin, mRNA transcription and protein expression by 55% and 65% respectively (1 microM, 72 h). Control sense and mismatch reagents were inactive. The antisense, but not the sense and mismatch, 5543-ODN induced dose- and time-dependent inhibition of MDA-MB231 adhesion to serum, vitronectin, fibrinogen and fibronectin substrates but was inactive on adhesion to laminin. Thus, the alphaV integrin was located in adhesion structures, which were disrupted by treatment with the alphaV antisense 5543-ODN. Antisense treated cells also showed evidence of programmed cell death with the appearance of apoptotic bodies. MDA-MB231 cells express a mutant form of the pro-apoptotic factor p53; however, no changes in the expression of p53 were observed by Western blotting. Immunofluorescence did reveal an increased nuclear translocation of p53 suggesting activation of the protein, but such a translocation did not lead to significant changes in either the expression of the cyclin dependent kinase inhibitor, p21(WAF1/CIP1) the cell survival factor Bcl-2 or the pro-apoptotic factor Bax.