The cellular uptake and metabolism of clodronate in RAW 264 macrophages

Clodronate Reduces ATP-Containing Microvesicle Releasing Induced by Nociceptive Stimuli in Human Keratinocytes

Clodronate (Clod), a first-generation bisphosphonate, acts as a natural analgesic inhibiting vesicular storage of the nociception mediator ATP by vesicular nucleotide transporter (VNUT). Epidermal keratinocytes participate in cutaneous nociception, accumulating ATP within vesicles, which are released following different stimulations. Under stress conditions, keratinocytes produce microvesicles (MVs) by shedding from plasma membrane evagination. MV secretion has been identified as a novel and universal mode of intercellular communication between cells. The aim of this project was to evaluate if two nociceptive stimuli, Capsaicin and Potassium Hydroxide (KOH), could stimulate MV shedding from human keratinocytes, if these MVs could contain ATP, and if Clod could inhibit this phenomenon. In our cellular model, the HaCaT keratinocyte monolayer, both Capsaicin and KOH stimulated MV release after 3 h incubation, and the released MVs contained ATP. Moreover, Clod (5 µM) was able to reduce Caps-induced MV release and abolish the one KOH induced, while the Dansylcadaverine, an endocytosis inhibitor of Clod uptake, partially failed to block the bisphosphonate activity. Based on these new data and given the role of the activation of ATP release by keratinocytes as a vehicle for nociception and pain, the "old" bisphosphonate Clodronate could provide the pharmacological basis to develop new local analgesic drugs.

Molecular mechanisms of action of bisphosphonates and new insights into their effects outside the skeleton

Bisphosphonates (BP) are a class of calcium-binding drug used to prevent bone resorption in skeletal disorders such as osteoporosis and metastatic bone disease. They act by selectively targeting bone-resorbing osteoclasts and can be grouped into two classes depending on their intracellular mechanisms of action. Simple BPs cause osteoclast apoptosis after cytoplasmic conversion into toxic ATP analogues. In contrast, nitrogen-containing BPs potently inhibit FPP synthase, an enzyme of the mevalonate (cholesterol biosynthesis) pathway. This results in production of a toxic metabolite (ApppI) and the loss of long-chain isoprenoid lipids required for protein prenylation, a process necessary for the function of small GTPase proteins essential for the survival and activity of osteoclasts. In this review we provide a state-of-the-art overview of these mechanisms of action and a historical perspective of how they were discovered. Finally, we challenge the long-held dogma that BPs act only in the skeleton and highlight recent studies that reveal insights into hitherto unknown effects on tumour-associated and tissue-resident macrophages.

Abnormal Cannabidiol Affects Production of Pro-Inflammatory Mediators and Astrocyte Wound Closure in Primary Astrocytic-Microglial Cocultures

Abnormal cannabidiol (abn-CBD) exerts neuroprotective effects in vivo and in vitro. In the present study, we investigated the impact of abn-CBD on the glial production of proinflammatory mediators and scar formation within in vitro models. Primary astrocytic-microglial cocultures and astrocytic cultures from neonatal C57BL/6 mice and CB2 receptor knockout mice were stimulated with lipopolysaccharide (LPS), and the concentrations of tumor necrosis factor α (TNFα), interleukin-6 (IL-6) and nitrite were determined. Furthermore, we performed a live cell microscopy-based scratch-wound assay. After LPS stimulation, TNFα, IL-6 and nitrite production was more strongly increased in cocultures than in isolated astrocytes. Abn-CBD treatment attenuated the LPS-induced production of TNFα and nitrite in cocultures, while IL-6 production remained unaltered. In isolated astrocytes, only LPS-induced TNFα production was reduced by abn-CBD. Similar effects were observed after abn-CBD application in cocultures of CB2 knockout mice. Interestingly, LPS-induced TNFα and nitrite levels were far lower in CB2 knockout cultures compared to wildtypes, while IL-6 levels did not differ. In the scratch-wound assay, treatment with abn-CBD decelerated wound closure when microglial cells were present. Our data shows a differential role of abn-CBD for modulation of glial inflammation and astrocytic scar formation. These findings provide new explanations for mechanisms behind the neuroprotective potential of abn-CBD.

Reducing macrophage numbers alleviates temporomandibular joint ankylosis

Temporomandibular joint (TMJ) ankylosis is a severe joint disease mainly caused by trauma that leads to a series of oral and maxillofacial function disorders and psychological problems. Our series of studies indicate that TMJ ankylosis development is similar to fracture healing and that severe trauma results in bony ankylosis instead of fibrous ankylosis. Macrophages are early infiltrating inflammatory cells in fracture and play a critical role in initiating fracture repair. We hypothesize that the large numbers of macrophages in the early phase of TMJ ankylosis trigger ankylosed bone mass formation and that the depletion of these macrophages in the early phase could inhibit the development of TMJ ankylosis. By analysing human TMJ ankylosis specimens, we found large numbers of infiltrated macrophages in the less-than-1-year ankylosis samples. A rabbit model of TMJ bony ankylosis was established and large numbers of infiltrated macrophages were found at 4 days post-operation. Local clodronate liposome (CLD-lip) injection, which depleted macrophages, alleviated the severity of ankylosis compared with local phosphate-buffered saline (PBS)-loaded liposome (PBS-lip) injection (macrophage number, PBS-lips vs. CLD-lips: 626.03 ± 164.53 vs. 341.4 ± 108.88 n/mm²; ankylosis calcification score, PBS-lips vs. CLD-lips: 2.11 ± 0.78 vs. 0.78 ± 0.66). Histological results showed that the cartilage area was reduced in the CLD-lip-treated side (PBS-lips vs. CLD-lips: 6.82 ± 4.42% vs. 2.71 ± 2.78%) and that the Wnt signalling regulating cartilage formation was disrupted (Wnt5a expression decreased 60% and Wnt4 expression decreased 73%). Thus, our study showed that large numbers of macrophages infiltrated during the early phase of ankylosis and that reducing macrophage numbers alleviated ankylosis development by reducing cartilage formation.

Preclinical evaluation of an innovative anti-TAM approach based on zoledronate-loaded erythrocytes

In tumor microenvironment, tumor-associated macrophages (TAMs) are implicated in cancer sustainment, metastasis, and drug resistance, raising a growing interest as targets in cancer therapy. Since the bisphosphonate zoledronate has proven to affect TAMs' functions, the anti-tumor effect of single or repeated administrations of red blood cells (RBCs) encapsulating zoledronate was evaluated in a mouse model of mammary carcinoma. The obtained results showed that loaded RBCs, but not free zoledronate, caused a significant (p < 0.01) and time-lasting reduction of TAMs' extent in tumor mass of Balb/C mice inoculated with murine mammary carcinoma T41 cells; in addition, a significant reduction (p < 0.05) of tumor growth rate has been obtained only following repeated administrations of zoledronate-loaded RBCs. The anti-tumor effect was secondary to the early depletion of spleen macrophages. Moreover, by assessing the IgG2a/IgG1 ratio, a prevalence of Th1 cytotoxic response in tumor-bearing mice receiving zoledronate by means of RBCs has been observed. These encouraging findings provide further evidence for the central role played by macrophages in tumor setting and highlight the suitability of zoledronate-loaded RBCs as pharmacological agents in depleting, even if indirectly, TAMs and, thus, their eligibility as part of a therapeutic strategy in cancer treatment.

Bisphosphonate-Generated ATP-Analogs Inhibit Cell Signaling Pathways

Bisphosphonates are a major class of drugs used to treat osteoporosis, Paget's disease, and cancer. They have been proposed to act by inhibiting one or more targets including protein prenylation, the epidermal growth factor receptor, or the adenine nucleotide translocase. Inhibition of the latter is due to formation in cells of analogs of ATP: the isopentenyl ester of ATP (ApppI) or an AppXp-type analog of ATP, such as AMP-clodronate (AppCCl2p). We screened both ApppI as well as AppCCl2p against a panel of 369 kinases finding potent inhibition of some tyrosine kinases by AppCCl2p, attributable to formation of a strong hydrogen bond between tyrosine and the terminal phosphonate. We then synthesized bisphosphonate preprodrugs that are converted in cells to other ATP-analogs, finding low nM kinase inhibitors that inhibited cell signaling pathways. These results help clarify our understanding of the mechanisms of action of bisphosphonates, potentially opening up new routes to the development of bone resorption, anticancer, and anti-inflammatory drug leads.

Macrophage depletion by free bisphosphonates and zoledronate-loaded red blood cells

Bisphosphonates, besides being important drugs for the treatment of various bone diseases, could also be used to induce apoptosis in macrophage-like and cancer cells. However, their activity in vivo is limited by a short plasma half-life and rapid uptake within bone. Therefore, several delivery systems have been proposed to modify their pharmacokinetic profile and biodistribution. Among these, red blood cells (RBCs) represent one of the most promising biological carriers. The aim of this study was to select the best performing compound among Clodronate, Pamidronate, Ibandronate and Zoledronate in killing macrophages and to investigate RBCs as innovative carrier system to selectively target bisphosphonates to macrophages. To this end, the encapsulation of the selected bisphosphonates in autologous RBCs as well as the effect on macrophages, both in vitro and in vivo were studied. This work shows that, among the tested bisphosphonates, Zoledronate has proven to be the most active molecule. Human and murine RBCs have been successfully loaded with Zoledronate by a procedure of hypotonic dialysis and isotonic resealing, obtaining a dose-dependent drug entrapment with a maximal loading of 7.96±2.03, 6.95±3.9 and 7.0±1.89 µmoles of Zoledronate/ml of packed RBCs for human, Swiss and Balb/C murine RBCs, respectively. Engineered RBCs were able to detach human and murine macrophages in vitro, leading to a detachment of 66±8%, 67±8% and 60.5±3.5% for human, Swiss and Balb/C RBCs, respectively. The in vivo efficacy of loaded RBCs was tested in Balb/C mice administering 59 µg/mouse of RBC-encapsulated Zoledronate. By a single administration, depletion of 29.0±16.38% hepatic macrophages and of 67.84±5.48% spleen macrophages was obtained, confirming the ability of encapsulated Zoledronate to deplete macrophages in vivo. In conclusion, RBCs loaded with Zoledronate should be considered a suitable system for targeted delivery to macrophages, both in vitro and in vivo.

Tumour macrophages as potential targets of bisphosphonates

Tumour cells communicate with the cells of their microenvironment via a series of molecular and cellular interactions to aid their progression to a malignant state and ultimately their metastatic spread. Of the cells in the microenvironment with a key role in cancer development, tumour associated macrophages (TAMs) are among the most notable. Tumour cells release a range of chemokines, cytokines and growth factors to attract macrophages, and these in turn release numerous factors (e.g. VEGF, MMP-9 and EGF) that are implicated in invasion-promoting processes such as tumour cell growth, flicking of the angiogenic switch and immunosuppression. TAM density has been shown to correlate with poor prognosis in breast cancer, suggesting that these cells may represent a potential therapeutic target. However, there are currently no agents that specifically target TAM's available for clinical use.

Bisphosphonates (BPs), such as zoledronic acid, are anti-resorptive agents approved for treatment of skeletal complication associated with metastatic breast cancer and prostate cancer. These agents act on osteoclasts, key cells in the bone microenvironment, to inhibit bone resorption. Over the past 30 years this has led to a great reduction in skeletal-related events (SRE's) in patients with advanced cancer and improved the morbidity associated with cancer-induced bone disease. However, there is now a growing body of evidence, both from in vitro and in vivo models, showing that zoledronic acid can also target tumour cells to increase apoptotic cell death and decrease proliferation, migration and invasion, and that this effect is significantly enhanced in combination with chemotherapy agents. Whether macrophages in the peripheral tumour microenvironment are exposed to sufficient levels of bisphosphonate to be affected is currently unknown. Macrophages belong to the same cell lineage as osteoclasts, the major target of BPs, and are highly phagocytic cells shown to be sensitive to bisphosphonates in model studies; In vitro, zoledronic acid causes increased apoptotic cell death; in vivo the drug has been shown to inhibit the production of pro-angiogenic factor MMP-9, as well as most recent evidence showing it can trigger the reversal of the TAMs phenotype from pro-tumoral M2 to tumoricidal M1. There is thus accumulating evidence supporting the hypothesis that effects on TAMs may contribute to the anti-tumour effect of bisphosphonates. This review will focus in detail on the role of tumour associated macrophages in breast cancer progression, the actions of bisphosphonates on macrophages in vitro and in tumour models in vivo and summarise the evidence supporting the potential for the targeting of tumour macrophages with bisphosphonates.

Biochemical and molecular mechanisms of action of bisphosphonates

This review describes the key discoveries over the last 15 years that have led to a clearer understanding of the molecular mechanisms by which bisphosphonate drugs inhibit bone resorption. Once released from bone mineral surfaces during bone resorption, these agents accumulate intracellularly in osteoclasts. Simple bisphosphonates such as clodronate are incorporated into non-hydrolysable analogues of adenosine triphosphate, which induce osteoclast apoptosis. The considerably more potent nitrogen-containing bisphosphonates are not metabolised but potently inhibit farnesyl pyrophosphate (FPP) synthase, a key enzyme of the mevalonate pathway. This prevents the synthesis of isoprenoid lipids necessary for the post-translational prenylation of small GTPases, thereby disrupting the subcellular localisation and normal function of these essential signalling proteins. Inhibition of FPP synthase also results in the accumulation of the upstream metabolite isopentenyl diphosphate, which is incorporated into the toxic nucleotide metabolite ApppI. Together, these properties explain the ability of bisphosphonate drugs to inhibit bone resorption by disrupting osteoclast function and survival. These discoveries are also giving insights into some of the adverse effects of bisphosphonates, such as the acute phase reaction that is triggered by inhibition of FPP synthase in peripheral blood monocytes.

Protein phosphatases: possible bisphosphonate binding sites mediating stimulation of osteoblast proliferation

We investigated the existence of a bisphosphonate (BP) target site in osteoblasts. Binding assays using [³H]-olpadronate ([³H]OPD) in whole cells showed the presence of specific, saturable and high affinity binding for OPD (K(d)=1.39 ± 0.33 μM) in osteoblasts. [³H]OPD was displaced from its binding site by micromolar concentrations of lidadronate, alendronate and etidronate (K(d)=1.42 ± 0.15 μM, 2.00 ± 0.2 μM and 2.4 ± 0.4 μM, respectively), and by millimolar concentrations of the non-permeant protein phosphatase (PP) substrates p-nitrophenylphosphate and α-naphtylphosphate. PP inhibitors orthovanadate, NaF or vpb(bipy) did not displace [³H]OPD. As expected, specific OPD binding was detected in the plasma membrane of ROS 17/2.8 cells, although significant BP binding was also found intracellularly. Moreover, OPD increased DNA synthesis in these cells with a temporal profile similar to the protein tyrosine phosphatase (PTP) inhibitors, Na₃VO₄ and vpb(bipy); but different from a general PP inhibitor (NaF). The stimulatory effect of OPD and PTP inhibitors on osteoblast proliferation was inhibited by the protein tyrosine kinase inhibitors genistein and geldanamycin. These results provide new evidence on the existence of a BP target in osteoblastic cells, presumably a PTP, which may be involved in the stimulatory action of BPs on osteoblast proliferation.

Pro-IL-1β accumulation in macrophages by alendronate and its prevention by clodronate

Nitrogen-containing bisphosphonates (NBPs), anti-bone-resorptive drugs, exhibit inflammatory side effects (fever, jaw osteomyelitis or osteonecrosis, etc.). We previously reported that in mice: (i) a single intraperitoneal injection of alendronate (an NBP, 40 μmol/kg or less) induces various inflammatory reactions, (ii) these effects, which are minimal in IL-1-deficient mice, can be prevented by co-administration of clodronate (a non-NBP, 40 μmol/kg or less), and (iii) alendronate increases IL-1β in tissues (liver, spleen, and lung), but strangely not in blood. Here, we found the following in mice. (a) The IL-1β in tissues is pro-IL-1β. (b) Unlike LPS, alendronate induces minimal activation of caspase-1 (pro-IL-1β-processing enzyme). (c) The tissue pro-IL-1β elevations are largely absent in macrophage-depleted mice. (d) In vitro, 100 μM alendronate directly stimulates RAW 264 cells (murine macrophage-like cells) to produce pro-IL-1β, and 1 μM clodronate inhibits this effect. These results suggest that in mice: (i) the major pro-IL-1β-producing cells in response to alendronate are macrophages, (ii) alendronate directly stimulates them to produce pro-IL-1β, but the release of mature IL-1β is below detectable levels due to insufficient activation of caspase-1, and (iii) clodronate inhibits the pro-IL-1β production by acting directly on macrophages, although the in vivo mechanism may differ from the in vitro one.

RANKL increases the level of Mcl-1 in osteoclasts and reduces bisphosphonate-induced osteoclast apoptosis in vitro

INTRODUCTION

Bisphosphonates are the most widely used class of drug for inhibiting osteoclast-mediated bone loss, but their effectiveness at preventing joint destruction in rheumatoid arthritis has generally been disappointing. We examined whether the ability of bisphosphonates to induce osteoclast apoptosis and inhibit bone resorption in vitro is influenced by the cytokine receptor activator of nuclear factor-kappa B ligand (RANKL), an important mediator of inflammation-induced bone loss.

METHODS

Rabbit osteoclasts were treated with the bisphosphonates clodronate or alendronate for up to 48 hours in the absence or presence of RANKL. Changes in cell morphology and induction of apoptosis were examined by scanning electron microscopy, whilst resorptive activity was determined by measuring the area of resorption cavities. Changes in the level of anti-apoptotic proteins, including Mcl-1, Bcl-2, and Bcl-x>L, were determined in rabbit osteoclasts and in cytokine-starved mouse osteoclasts by Western blotting.

RESULTS

RANKL significantly attenuated the ability of both clodronate and alendronate to induce osteoclast apoptosis and inhibit bone resorption. Treatment of rabbit osteoclasts with RANKL was associated with an increase in the anti-apoptotic protein Mcl-1 but not Bcl-2. A role for Mcl-1 in osteoclast survival was suggested using osteoclasts generated from mouse bone marrow macrophages in the presence of RANKL + macrophage colony-stimulating factor (M-CSF) since cytokine deprivation of mouse osteoclasts caused a rapid loss of Mcl-1 (but not Bcl-2 or Bcl-xL), which preceded the biochemical and morphological changes associated with apoptosis. Loss of Mcl-1 from mouse osteoclasts could be prevented by factors known to promote osteoclast survival (RANKL, M-CSF, tumour necrosis factor-alpha [TNF-alpha], or lipopolysaccharide [LPS]).

CONCLUSIONS

RANKL protects osteoclasts from the apoptosis-inducing and anti-resorptive effects of bisphosphonates in vitro. The ability of RANKL (and other pro-inflammatory factors such as TNF-alpha and LPS) to increase the level of Mcl-1 in osteoclasts may explain the lack of effectiveness of some bisphosphonates in preventing inflammation-induced bone loss.

Molecular mechanisms of inflammatory bone damage: emerging targets for therapy

Chronic inflammatory bone diseases, such as rheumatoid arthritis (RA), ankylosing spondylitis and periodontal disease, demonstrate the major impact of chronic inflammation on both bone metabolism and bone architecture. During the past decade, scientists have gained increasing insight into the link between inflammation and bone. As a result of new discoveries about the molecular mechanisms of inflammatory bone loss, several molecules have been identified that are attractive and novel targets for the treatment of inflammatory bone loss. These novel therapeutic approaches include anti-tumor necrosis factor (TNF)-alpha blocking agents, neutralizing antibodies against certain pro-inflammatory cytokines, such as interleukin (IL)-6 and IL-17, and a set of other promising targets that still require extensive research, such as the Wnt signaling network.

Molecular mechanisms of action of bisphosphonates: current status

PURPOSE

Bisphosphonates are currently the most important class of antiresorptive agents used in the treatment of metabolic bone diseases, including tumor-associated osteolysis and hypercalcemia. These compounds have high affinity for calcium ions and therefore target bone mineral, where they are internalized by bone-resorbing osteoclasts and inhibit osteoclast function.

EXPERIMENTAL DESIGN

This article reviews the pharmacology of bisphosphonates and the relationship between chemical structure and antiresorptive potency. We also describe new insights into their intracellular molecular mechanisms of action, methods for assessing the effects of bisphosphonates on protein prenylation, and their potential as direct antitumor agents.

RESULTS

Nitrogen-containing bisphosphonates act intracellularly by inhibiting farnesyl diphosphate synthase, an enzyme of the mevalonate pathway, thereby preventing prenylation of small GTPase signaling proteins required for normal cellular function. Inhibition of farnesyl diphosphate synthase also seems to account for their antitumor effects observed in vitro and for the activation of gamma,delta T cells, a feature of the acute-phase response to bisphosphonate treatment in humans. Bisphosphonates that lack a nitrogen in the chemical structure do not inhibit protein prenylation and have a different mode of action that seems to involve primarily the formation of cytotoxic metabolites in osteoclasts.

CONCLUSIONS

Bisphosphonates are highly effective inhibitors of bone resorption that selectively affect osteoclasts in vivo but could also have direct effects on other cell types, such as tumor cells. After >30 years of clinical use, their molecular mechanisms of action on osteoclasts are finally becoming clear but their exact antitumor properties remain to be clarified.

Comparative effects of five bisphosphonates on apoptosis of macrophage cells in vitro

Bisphosphonates (BPs) inhibits bone resorption by reducing osteoclastic activity; they induce osteoclast apoptosis. Pathophysiology of prostheses loosening is complex and implies an inflammatory reaction secondary to the phagocytosis of wear debris by macrophages with a secondary increased bone resorption by osteoclasts. BPs inhibit proliferation and cause cell death in macrophages by induction of apoptosis. We have used mouse macrophage-like J774.1 cells to evaluate the effects of five BPs. J774A.1 cells were cultured in a standard culture medium for 2-days. BPs (alendronate, pamidronate, etidronate, risedronate, zoledronic acid) were added in the medium at concentration of 10(-6) to 10(-4)M during 3 days. Cells were studied by fluorescence microscopy after staining with the fluorescent dye Hoescht H33342 and the percentage of apoptotic cells was determined on 300 nuclei. Cells were analyzed by flow cytofluorometry after staining with annexin V-FITC (for counting apoptotic cells) and propidium iodide (for necrotic/late-apoptotic cells) on 2000 cells. Etidronate did not cause significant apoptosis or necrosis, at any concentration. Alendronate and pamidronate caused apoptosis and death only at very high concentration [10(-4)M]. On the contrary, apoptotic and necrotic cells were evidenced with risedronate or zoledronic acid at lower concentrations. These effects were dose-dependant and occurred when concentration reached [10(-5)M]. The number of apoptotic cells was higher with zoledronic acid and then with risedronate. Cytofluorometry appeared superior to cytologic analysis in the investigation of macrophage apoptosis, since necrotic cells loose contact with the glass slides and are not identifiable in cytological counts. Some amino-BPs appear to induce apoptosis in macrophages.

A new endogenous ATP analog (ApppI) inhibits the mitochondrial adenine nucleotide translocase (ANT) and is responsible for the apoptosis induced by nitrogen-containing bisphosphonates

1. Bisphosphonates are currently the most important class of antiresorptive drugs used for the treatment of diseases with excess bone resorption. On the basis of their molecular mechanism of action, bisphosphonates can be divided into two pharmacological classes; nitrogen-containing (N-BPs) and non-nitrogen-containing bisphosphonates (non-N-BP). Both classes induce apoptosis but they evoke it differently; N-BPs by inhibiting the intracellular mevalonate pathway and protein isoprenylation, and non-N-BPs via cytotoxic ATP analog-type metabolites. N-BPs are not metabolized to ATP analogs, but we report here that these bisphosphonates can induce formation of a novel ATP analog (ApppI) as a consequence of the inhibition of the mevalonate pathway in cells. We also investigated whether ApppI is involved in the apoptosis induced by N-BPs. 2. Mass spectrometry and NMR were used to identify ApppI in N-BP treated osteoclasts, macrophages and glioma cells. The potency of different bisphosphonates to promote ApppI production was tested in J774 macrophages. The effects of ApppI on ADP/ATP translocase in isolated mitochondria and its capability to induce apoptosis in osteoclasts were also studied. 3. ApppI production correlated well with the capacity of N-BPs to inhibit mevalonate pathway. ApppI inhibited the mitochondrial ADP/ATP translocase and caused apoptosis in osteoclasts. 4. In conclusion, these findings provide the basis for a new mechanism of action for N-BPs. Some of these very potent bisphosphonates, such as zoledronic acid, represent a third class of bisphosphonates that can act both via the inhibition of the mevalonate pathway and by the blockade of mitochondrial ADP/ATP translocase, which is known to be involved in the induction of apoptosis.

In vitro Comparison of Clodronate, Pamidronate and Zoledronic Acid Effects on Rat Osteoclasts and Human Stem Cell-Derived Osteoblasts

In the present study we compared the first generation non-nitrogen-containing bisphosphonate, clodronate with second and third generation nitrogen-containing bisphosphonates, pamidronate and zoledronic acid in dynamic rat osteoclast resorption and apoptosis assays and in human mesenchymal stem cell-derived osteoblast assay. We found that due to high bisphosphonate-bone binding affinity, bone surface exposure to clodronate for 3 min. had maximal resorption inhibition. The mechanism of action of both clodronate and zoledronic acid involved osteoclast apoptosis, whereas pamidronate had only minor apoptotic effect at dosages, which readily inhibited resorption. Zoledronic acid was not metabolised into an intracellular ATP-analogue in vitro in contrast to clodronate. All bisphosphonates had a dose-dependent inhibitory effect on the human bone marrow mesenchymal stem cell (hMSC)-derived osteoblast calcium deposition. None of the compounds had inhibitory effect on hMSC differentiation. Zoledronic acid was the most potent of all three bisphosphonates in terms of both apoptosis induction and resorption inhibition. Zoledronic acid efficacy might thus use its capacity to trigger osteoclast apoptosis in an unknown, but similar manner to that of the non-nitrogen-containing bisphosphonates. It appears that zoledronic acid has properties of both bisphosphonate classes and could well be the first member of a new class of bisphosphonates, by definition.

From molds and macrophages to mevalonate: a decade of progress in understanding the molecular mode of action of bisphosphonates

Although bisphosphonates were first used as therapeutic agents to inhibit bone resorption in the early 1970s, their mode of action at the molecular level has only become fully clear within the last few years. One of the reasons for this lack of understanding was the difficulty in isolating large numbers of pure osteoclasts for biochemical studies. In the last decade, the identification of appropriate surrogate models that reflected the antiresorptive potencies of bisphosphonates, such as Dictyostelium slime molds and macrophages, helped overcome this problem and proved to be instrumental in elucidating the molecular pathways by which these compounds inhibit osteoclast-mediated bone resorption. This brief review summarizes our current understanding of these pathways.

Bone cancer pain: the effects of the bisphosphonate alendronate on pain, skeletal remodeling, tumor growth and tumor necrosis

Patients with metastatic breast, lung or prostate cancer frequently have significant bone cancer pain. In the present report we address, in a single in vivo mouse model, the effects the bisphosphonate alendronate has on bone cancer pain, bone remodeling and tumor growth and necrosis. Following injection and confinement of green fluorescent protein-transfected murine osteolytic tumor cells into the marrow space of the femur of male C3H/HeJ mice, alendronate was administered chronically from the time the tumor was established until the bone cancer pain became severe. Alendronate therapy reduced ongoing and movement-evoked bone cancer pain, bone destruction and the destruction of sensory nerve fibers that innervate the bone. Whereas, alendronate treatment did not change viable tumor burden, both tumor growth and tumor necrosis increased. These data emphasize that it is essential to utilize a model where pain, skeletal remodeling and tumor growth can be simultaneously assessed, as each of these can significantly impact patient quality of life and survival.

Crucial role of synovial lining macrophages in the promotion of transforming growth factor beta-mediated osteophyte formation

OBJECTIVE

To investigate in vivo and in vitro whether macrophages have an intermediate role in transforming growth factor beta (TGFbeta)-induced osteophyte formation.

METHODS

In vivo, synovial lining macrophages were selectively depleted by injection of clodronate-laden liposomes 7 days prior to injection of 20 ng or 200 ng of TGFbeta into murine knee joints 3 times, on alternate days. Total knee joint sections were obtained on day 7 after the last injection and stained with Safranin O. Production of bone morphogenetic protein 2 (BMP-2) and BMP-4 was determined by immunolocalization. The interaction between murine macrophages and mesenchymal cells (precursors with chondrogenic potential) was studied in vitro using a Transwell system in which RAW macrophages were cocultured with C3H10T1/2 mesenchymal cells. Spheroid neocartilage formation was quantified microscopically after staining with May-Grünwald-Giemsa.

RESULTS

Triple injections of 20 ng or 200 ng of TGFbeta into normal murine knee joints induced significant osteophyte formation at the lateral and medial sites of the patella and femur on day 7 after the last injection. Strikingly, removal of synovial lining macrophages prior to TGFbeta injection resulted in a drastic reduction of osteophyte formation (by 70% and 64% after injection of 20 ng and 200 ng of TGFbeta, respectively). Synovial lining cells produced BMP-2 and BMP-4 after TGFbeta stimulation, whereas BMP-2 and BMP-4 were absent in the synovial tissue after macrophage depletion. In vitro, clustering and spheroid formation of C3H10T1/2 was induced by TGFbeta concentrations of >1 ng/ml. However, in the Transwell system, in the presence of murine macrophages, 0.5 ng/ml of TGFbeta was very effective in generating large spheroids, suggestive of macrophage-derived (co)factors. In coculture supernatants, TGFbeta concentrations were not elevated in the presence of macrophages, indicating generation of other growth factors involved in spheroid formation.

CONCLUSION

These findings indicate that macrophages are crucial intermediate factors in osteophyte formation induced by TGFbeta, probably by inducing other chondrogenic signals.

Cellular uptake and metabolism of clodronate and its derivatives in Caco-2 cells: a possible correlation with bisphosphonate-induced gastrointestinal side-effects

PURPOSE

To investigate possible reasons for the low frequency of GI side-effects of clodronate, even though clodronate is known to be metabolised into a cytotoxic nucleotide analogue (AppCCl(2)p) by many cell types. The effects of some lipophilic prodrugs of clodronate were also studied.

METHODS

The effects of clodronate and its lipophilic derivatives on the proliferation and viability of Caco-2 cells were examined using an MTT assay. The intracellular uptake of 14C-clodronate and the accumulation of a clodronate metabolite (AppCCl(2)p) in Caco-2 cells were evaluated using ion-pairing HPLC-ESI-MS.

RESULTS

Clodronate had little effect on growth of proliferating, or the viability of confluent, Caco-2 cells. The uptake of clodronate by Caco-2 cells was only about 0.04% of total clodronate. The potentially cytotoxic clodronate metabolite, AppCCl(2)p, was detected in Caco-2 cell extracts after 3 h of exposure. Dianhydride- and triPOM-clodronate were metabolised to AppCCl(2)p more efficiently and also affected the viability of Caco-2 cells more than clodronate.

CONCLUSIONS

Clodronate appears to be metabolised into a cytotoxic ATP-analogue (AppCCl(2)p) by any cell type capable of internalising the drug. However, the cytotoxicity depends on the degree of uptake of clodronate. Due to the very low initial uptake of clodronate by epithelial Caco-2 cells, they do not accumulate sufficient intracellular concentrations of AppCCl(2)p to affect cell function. This explains the low frequency of gastrointestinal side-effects caused by oral clodronate therapy.

The bisphosphonate clodronate depletes microglial cells in excitotoxically injured organotypic hippocampal slice cultures

The bisphosphonate clodronate, clinically used in the treatment of osteoporosis, is known to deplete cells of the monocytic lineage. Using an in vitro model of excitotoxic damage in organotypic hippocampal slice cultures (OHSC), we investigated whether clodronate can also prevent microglial activation that occurs in CNS pathologies. Lesioning of OHSC was performed by application of 50 microM N-methyl-D-aspartate (NMDA) for 4 h after 6 days in vitro (div). Treatment of lesioned OHSC with clodronate (1000, 100, or 10 microg/ml) resulted in an almost complete abrogation of the microglial reaction after 3 further div: Confocal laser scanning microscopy showed that the number of Griffonia simplicifolia isolectin B(4)-labeled (IB4+) microglial cells in the dentate gyrus (DG) was reduced to 4.25% compared with OHSC treated with NMDA alone. Continuous treatment with clodronate (100 or 10 microg/ml) of lesioned OHSC for 9 days resulted in a further reduction in the number of microglial cells (reduction to 2.72%). The number of degenerating, propidium iodide-labeled (PI(+)) neurons in lesioned OHSC that received clodronate treatment between 6 and 9 div was not significantly different from OHSC treated with NMDA alone. However, the number of PI(+) neurons in lesioned OHSC that received continuous clodronate treatment for 9 div was significantly higher when compared to NMDA-lesioned OHSC. In summary, clodronate is able to reduce microglial activation induced by excitotoxic neuronal injury. Our results demonstrate that clodronate is a useful tool in the investigation of neuron-glia interactions because it induces an efficient depletion of microglial cells that are activated after excitotoxic CNS injury.

Further insight into mechanism of action of clodronate: inhibition of mitochondrial ADP/ATP translocase by a nonhydrolyzable, adenine-containing metabolite

Bisphosphonates are currently the most important class of antiresorptive drugs used for the treatment of diseases with excess bone resorption. Recent studies have shown that bisphosphonates can be divided into two groups with distinct molecular mechanisms of action depending on the nature of the R(2) side chain. Alendronate, like other nitrogen-containing bisphosphonates, inhibits bone resorption and causes apoptosis of osteoclasts and other cells in vitro by preventing post-translational modification of GTP-binding proteins with isoprenoid lipids. Clodronate, a bisphosphonate that lacks a nitrogen, does not inhibit protein isoprenylation but can be metabolized intracellularly to a beta-gamma-methylene (AppCp-type) analog of ATP, which is cytotoxic to macrophages in vitro. The detailed molecular basis for the cytotoxic effects of adenosine-5'-[beta,gamma-dichloromethylene]triphosphate (AppCCl(2)p) has not been determined yet. We addressed this question by studying the effects of alendronate, clodronate, and the clodronate metabolite AppCCl(2)p on isolated mitochondria, mitochondrial fractions, and mitochondrial membrane potential in isolated human osteoclasts. We found that AppCCl(2)p inhibits mitochondrial oxygen consumption by a mechanism that involves competitive inhibition of the ADP/ATP translocase. Alendronate or the native form of clodronate did not have any immediate effect on mitochondria. However, longer treatment with liposome-encapsulated clodronate caused collapse of the mitochondrial membrane potential, although prominent apoptosis was a late event. Hence, inhibition of the ADP/ATP translocase by the metabolite AppCCl(2)p is a likely route by which clodronate causes osteoclast apoptosis and inhibits bone resorption.