Bisphosphonates induce apoptosis in mouse macrophage-like cells in vitro by a nitric oxide-independent mechanism

Phosphate ions mediate chondrocyte apoptosis through a plasma membrane transporter mechanism

In a previous investigation we showed that phosphate ions (Pi) induced apoptosis of terminally differentiated hypertrophic chondrocytes. To explore the mechanism by which Pi induces cell death, we asked the following two questions. First, can we prevent Pi-induced apoptosis by inhibiting plasma membrane Na-Pi cotransport? Second, which specific Na-Pi transporters are expressed in chondrocytes and are they developmentally regulated? Terminally differentiated hypertrophic chondrocytes were isolated from chick tibial cartilage and cell death was measured in the presence of 3-7 mmol/L Pi. To ascertain whether apoptosis was linked to a rise in cellular Pi loading, we examined the effect of phosphonoformic acid (PFA), a competitive inhibitor of Na-Pi cotransport on Pi-induced apoptosis in chondrocytes. We found that 1 mmol/L PFA blocked anion-induced cell death and prevented an increase in the cell Pi content. In a parallel study, we determined that the bisphosphonate, alendronate, also protected chondrocytes from death, albeit at a lower concentration than PFA. Using a DNA end-labeling procedure, we showed that the Pi-treated cells were apoptotic and, as might be predicted, the presence of PFA blocked induction of the death sequence. Next, we examined the expression of two Pi transporters in relation to chondrocyte maturation and anion treatment. We noted that there was expression of the constitutive transporter, Glvr-1, and a type II cotransporter in chick growth plate cells. Although these transport systems are active in terminally differentiated cells, it is probable that the initiation of apoptosis may require the induction of other Pi-transport systems. It is concluded that, at the mineralization front, cell death is linked directly to the elevation in environmental anion concentration and the concomitant rise in intracellular Pi levels.

Adjuvant clodronate treatment does not reduce the frequency of skeletal metastases in node-positive breast cancer patients: 5-year results of a randomized controlled trial

PURPOSE

Bisphosphonates have effectively reduced the development and progression of bone metastases in advanced breast cancer. The aim of this study was to determine whether bone metastases could be prevented by adjuvant clodronate treatment in patients with primary breast cancer.

PATIENTS AND METHODS

Between 1990 and 1993, 299 women with primary node-positive breast cancer were randomized to clodronate (n = 149) or control groups (n = 150). Clodronate 1,600 mg daily was given orally for 3 years. All patients received adjuvant therapy: premenopausal six cycles of CMF chemotherapy and postmenopausal antiestrogens (randomized to tamoxifen 20 mg or toremifene 60 mg/d for 3 years). Seventeen patients were excluded from the analyses because of major protocol violations. The final population was 282 patients. Intent-to-treat analyses were also performed for all major end points. The follow-up time was 5 years for all patients.

RESULTS

Bone metastases were detected equally often in the clodronate and control groups: 29 patients (21%) versus 24 patients (17%) (P: = .27). The development of nonskeletal recurrence was significantly higher in the clodronate group compared with controls: 60 patients (43%) versus 36 patients (25%) (P: = .0007). The overall survival (OS) and disease-free survival (DFS) rates were also significantly lower in the clodronate group than in the controls (OS, 70% v 83%, P: = .009; DFS, 56% v 71%, P: = .007, respectively). In multivariate analyses, clodronate remained significantly associated with DFS (P: = .009).

CONCLUSION

Adjuvant clodronate treatment does not prevent the development of bone metastases in node-positive breast cancer patients. However, clodronate seems to have a negative effect on DFS by increasing the development of nonskeletal metastases.

Bisphosphonates and atherosclerosis

Bisphosphonates are used for the treatment of bone resorption, hypercalcemia, osteoporosis and Paget's disease. Etidronate, pamidronate and clodronate also inhibit the development of experimental atherosclerosis without altering serum lipid profile. Bisphosphonates inhibit the arterial calcification, lipid accumulation and fibrosis. They accumulate extensively in arterial walls and suppress macrophages in atheromatous lesions. In macrophage cultures, bisphosphonates inhibit the cellular accumulation and degradation of atherogenic LDL-cholesterol and foam cell formation. Further, they inhibit various enzymes involved in cell signal transduction and cholesterol biosynthesis. Recently, etidronate has been shown to inhibit the thickening of carotid arterial wall even in man.

Parathyroid hormone-related peptide is involved in protection against invasion of tooth germs by bone via promoting the differentiation of osteoclasts during tooth development

In order to elucidate the role of parathyroid hormone-related peptide (PTHrP) in tooth development, we treated tooth germ explants of mouse molars with antisense phosphorothioate-oligodeoxynucleotide (ODN) against PTHrP. Antisense ODN-treatment of the explants resulted in the invasion of the tooth germs by bone. The number of tartrate-resistant acid phosphatase (TRAP)-positive cells around the tooth germs in antisense ODN-treated explants was much lower than that of the control explants. Electron microscopic examination suggested that the antisense ODN-treatment inhibited differentiation of osteoclasts. Treatment of the explants with bisphosphonate or vitamin K2, inhibitors of the differentiation of osteoclasts, induced the invasion by bone into the tooth germs as observed in the antisense ODN-treated explants. The results obtained suggest that PTHrP is involved in the mechanism protecting tooth germs from bone invasion by promoting the differentiation of osteoclasts around them.

The potential role of bisphosphonates as adjuvant therapy in the prevention of bone metastases

BACKGROUND

Bisphosphonates (BPs) reduce bone resorption rates by inhibiting osteoclast function, although direct antineoplastic effects and poorly understood effects on bone pain also may occur. Within the family of BPs there are more similarities in pharmacologic effects than differences, although side effect profiles, rates of oral absorption, and potency do differ. Oral clodronate and intravenous pamidronate reduce skeletal complications in patients with bone metastases from breast carcinoma (as well as in myeloma). Uncontrolled trials of prostate carcinoma also suggest clinical benefit.

METHODS

Animal studies show that BPs can reduce the rate of development of bone metastases (for example, in Walker 256 carcinoma), but there is little evidence of an effect at nonosseous sites. The hypothesis that the growth of subclinical osseous metastases is augmented by products of bone resorption (a "vicious cycle") and may be diminished by a local reduction of these substances has led to trials of BPs involving patients with no clinical evidence of bone metastases. These trials are critically assessed in this review.

RESULTS

In patients with recurrent breast carcinoma but no overt bone metastases, oral clodronate reduced the number of diagnosed bone metastases; but the number of patients who had relapses in bone, though smaller, was not significantly different from the number among patients who took placebo. In a trial of oral pamidronate, no effect was seen, but compliance was a problem because of gastrointestinal side effects. Patients treated for operable breast carcinoma have four or five times the normal rate of vertebral fracture, and BPs do reduce the rate of bone loss. Three adjuvant clodronate trials have been reported. The first, an open-label controlled trial (Diel et al.), showed a reduction in osseous and nonosseous recurrences and an increase in disease free and overall survival with 2 years of clodronate. A second open-label trial (Saarto et al.) of similar size involving lymph node positive breast carcinoma patients showed no effect on the rate of bone metastasis relapse and a deleterious effect on relapse rates of nonosseous metastases with 3 years of clodronate. A third placebo-controlled trial involving 1079 patients reported, in an interim analysis, a reduction in osseous metastases during treatment with 2 years of clodronate, but no effect on nonosseous metastases or survival.

CONCLUSIONS

A confirmatory clinical trial is required for two interrelated reasons: 1) scientifically, it is important to demonstrate that an agent that has its dominant effect on a normal tissue cell, the osteoclast, can influence the growth of neoplastic cells; and 2) from the perspective of patient care, it must be unequivocally shown that a reduction in the rate of osseous recurrence translates into an improvement in disease free survival or an improvement in quality of life through reduction of adverse skeletal events. The National Surgical Adjuvant Breast Project has committed to conducting this study and including women with operable breast carcinoma.

Bisphosphonates in the adjuvant treatment of cancer: experimental evidence and first clinical results. International Bone and Cancer Study Group (IBCG)

Several animal models, as well as a number of cell culture experiments, indicate a prophylactic effect of bisphosphonates in respect of subsequent bone metastasis. Moreover, in preliminary clinical trials involving patients with advanced breast cancer and local or remote metastases, biophosphonates produced a reduction in new skeletal metastases. This overview summarizes and discusses the results of the latest investigations. It opens with a section on the pathophysiology of bone metastasis, which is followed by a report on animal models and first studies of bisphosphonate treatment as a new approach in systemic adjuvant therapy.

Treatment of metastatic bone disease in breast cancer: bisphosphonates

Like other metastases, bone metastases in breast cancer patients are not only a sign of the incurable nature of the underlying disease, but are also associated with specific complications. In particular, bone pain and pathological fractures impair the quality of life of those affected. Any treatment concept must, therefore, place the highest priority on preventing or reducing skeletal complications. There are two treatment options--local and systemic. Local therapy includes radiotherapy as well as surgical and orthopedic measures. The four pillars of systemic treatment are hormone therapy, chemotherapy, antiresorptive therapy with bisphosphonates, and treatment with centrally and/or peripherally acting analgesics. A precondition for successful treatment is close cooperation between medical/clinical oncologists, radiotherapists, surgeons/orthopedists, gynecologists, pain specialists, and endocrinologists (in the presence of a hypercalcemic syndrome). Patients with breast cancer associated solely with osseous metastasis may live for a number of years. It is, therefore, all the more important to start appropriate therapeutic measures early. Bisphosphonates play a particularly valuable role, since their main effect lies in the prevention of skeletal complications. Rather than replacing antineoplastic therapy, this class of substances supplements other treatments. Once started, bisphosphonate therapy should be given for the remainder of the patient's life, even in the event of osseous progression.

Compactin suppresses bone resorption by inhibiting the fusion of prefusion osteoclasts and disrupting the actin ring in osteoclasts

Compactin (mevastatin), which inhibits 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, and thus biosynthesis of cholesterol and the prenylation of proteins, inhibits osteoclastic bone resorption. Although it has been suggested that compactin inhibits bone resorption by inducing apoptosis of osteoclasts, the pathway by which compactin inhibits resorption has not been established. We investigated the effect of compactin on the differentiation of osteoclasts and the relationship between the morphological changes elicited by compactin and its inhibitory effect on bone resorption. Compactin inhibited the differentiation of osteoclasts, interfering with the fusion process by which prefusion osteoclasts (pOCs) develop into multinucleated osteoclast-like cells (OCLs), and also disrupted the actin ring of OCLs. The potency of compactin to inhibit fusion of pOCs and to disrupt the actin ring of OCLs corresponded to that of compactin to inhibit bone resorption. The effects of compactin were prevented by the addition of MVA lactone or its downstream products farnesylpyrophosphate (FPP) and geranylgeranyl-pyrophosphate (GGPP) but not by squalene. Apoptosis of OCLs was not induced by the concentration of compactin that inhibited fusion of pOCs and disrupted the actin ring. The normal process of pOC fusion and the integrity of the actin ring were restored by the withdrawal of compactin from the cultures after they had been treated with compactin for 24 h, but they were not restored by the addition of zVAD-fmk, a caspase inhibitor. Compactin also reversibly inhibited interleukin-1beta (IL-1beta)-, 1alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3)-, and parathyroid hormone (PTH)-stimulated 45Ca release in bone organ cultures. Our results indicate that the inhibitory effects of compactin on bone resorption result from the inhibition of fusion of pOCs into OCLs and disruption of actin ring in OCLs and that apoptosis of OCLs is not necessary for these inhibitory effects of compactin. These effects of compactin are likely to be a consequence of the inhibition of prenylation of proteins that play an important role in the fusion of pOCs and in maintaining actin ring integrity in OCLs.

Apoptosis: a two-edged sword in aging

Here we summarize briefly what is known about both the positive and negative impacts of apoptosis during aging in mammalian systems and also update an earlier review. It is important to understand both of these impacts to devise useful interventions. Such interventions include both physiological and molecular approaches, including transgenic interventions. The critical roles of the mitochondria in both generating reactive oxygen species, and in initiating apoptosis are recognized, suggesting that maintaining mitochondrial function could be an important therapeutic goal, especially in post-mitotic tissues. In contrast, the ability to eliminate unwanted, damaged and dysfunctional cells through apoptosis has anti-aging implications in mitotic tissues.

Antitumour effects of bisphosphonates: first evidence and possible mechanisms

Bisphosphonates have been used successfully for many years in the treatment of hypercalcaemia and to reduce skeletal complications of metastases. In the first years of bisphosphonate use the efficacy of these substances was thought to lie purely in the inhibition of osteoclasts. However, there is recent evidence to suggest that an antitumour effect may also play a role. As well as having an apoptotic and antiproliferative effect on osteoclasts, bisphosphonates may exert a similar influence on macrophages and tumour cells. Whether this effect (at low doses) also plays a role in vivo remains unclear and requires further investigation. Improvements in the survival time of certain subpopulations have been found in many phase III studies with bisphosphonates to date, both in the setting of metastatic breast cancer and in multiple myeloma. However, because survival time in subgroups of patients was neither a primary nor a secondary objective in these studies, these advantages could only be seen as important pointers for future studies. Some preclinical studies have shown that down-regulation of bone metabolism by bisphosphonates is associated with a lower incidence of bone metastases and destruction in animals, whereas activation is correlated with a higher number of metastases. However, varying results were found in animal experiments with regard to the effect of bisphosphonates on the incidence and growth pattern of non-osseous metastases. The results of 3 randomised studies in patients with primary breast cancer who received clodronate 1600 mg/day orally have now been evaluated and presented. All 3 studies arrived at different results. In the Heidelberg study there was a reduction in both osseous and non-osseous metastases, whereas in a much larger study performed in Great Britain, Canada and Scandinavia there was a reduction only in the incidence of skeletal metastases. A third study from Finland found no effect on bone metastases, but an increase in the number of visceral metastases and a deterioration in overall survival. Because the dosage was identical in all 3 studies, the differing results can only be either random or methodological (for example inclusion criteria or sample size). Overall, the results are very promising, but there is a need for further studies.

Evaluation of apoptosis and the glucocorticoid receptor in the cartilage growth plate and metaphyseal bone cells of rats after high-dose treatment with corticosterone

A connection has been suggested between glucocorticoid-induced osteopenia and an increase in the apoptosis of bone cells, and between the dimerization of the glucocorticoid receptor (GR) and the development of apoptosis. On this basis, a study has been carried out on the relationships between the occurrence of apoptotic cells and their detectable GR content, and between apoptosis frequency and changes in histomorphometric variables, in the growth plate and secondary spongiosa of rat long bones after the high-dose (10 mg/day) administration of corticosterone (CORT) and after recovery. The main results of the CORT treatment were: a significant increase in apoptotic osteoblasts, and a concomitant decrease in the histomorphometric variables of bone formation, with a reversal of both values during recovery; a nonsignificant increase in the apoptosis of osteoclasts, without changes in the histomorphometric variables of bone resorption; a significant increase in apoptotic terminal hypertrophic chondrocytes; the presence of GR in all types of skeletal cells in control rats, with different (cytoplasmic and/or nuclear) immunohistochemical detection in the same type of cell; a decrease in GR detection in proliferative chondrocytes and osteocytes in CORT and recovery groups, and in the maturative/hypertrophic chondrocytes of the recovery group; a fall in growth cartilage width, possibly due to the reduced proliferation of proliferative chondrocytes and increased apoptosis in terminal hypertrophic chondrocytes. In conclusion, pharmacological doses of CORT reduce bone formation by increasing osteoblast apoptosis; they reduce growth cartilage width, probably by inhibiting chondrocyte proliferation and increasing the apoptosis of terminal hypertrophic chondrocytes, and they reduce osteocyte GR. Although these effects appear to be mediated by the presence of GR in all skeletal cells, no precise correlation between GR immunohistochemical detection and apoptosis induction has been found.

Stimulation of bone formation in vitro and in rodents by statins

Osteoporosis and other diseases of bone loss are a major public health problem. Here it is shown that the statins, drugs widely used for lowering serum cholesterol, also enhance new bone formation in vitro and in rodents. This effect was associated with increased expression of the bone morphogenetic protein-2 (BMP-2) gene in bone cells. Lovastatin and simvastatin increased bone formation when injected subcutaneously over the calvaria of mice and increased cancellous bone volume when orally administered to rats. Thus, in appropriate doses, statins may have therapeutic applications for the treatment of osteoporosis.

Zoledronate is a potent inhibitor of myeloma cell growth and secretion of IL-6 and MMP-1 by the tumoral environment

Bisphosphonates have recently been introduced in the therapeutic armamentarium for the long-term treatment of patients with multiple myeloma (MM). These pyrophosphate analogs not only reduce the occurrence of skeletal-related events but also provide patients with a clinical benefit and improve the survival of some of them. We investigated the effects of two bisphosphonates, pamidronate and zoledronate, on both myeloma cells and bone marrow stromal cells (BMSCs). We show here that both bisphosphonates induce both myeloma cell and BMSC apoptosis. Furthermore, at lower concentrations, they induce a significant inhibition (40% and 60%, respectively) of the constitutive production of interleukin-6 (IL-6) by BMSCs. We have recently shown that BMSCs produce MMP-1, the major metalloproteinase involved in the initiation of bone resorption, production up-regulated by IL-1beta. Here, we demonstrate that zoledronate significantly inhibits MMP-1 production by BMSCs stimulated with IL-1beta more efficiently than pamidronate. However, zoledronate and to a lesser extent pamidronate are responsible for an up-regulation of MMP-2 secretion by BMSCs. MMP-2 is involved both in bone resorption and in the metastatic process. In conclusion, the apoptosis of myeloma cells and BMSCs and the inhibition of both IL-6 and MMP-1 production induced by bisphosphonates, mainly zoledronate, could have antitumoral effects in patients with MM. However, the up-regulation of MMP-2 secretion observed in vitro suggests a putative risk of tumor cell dissemination in vivo when using these new potent bisphosphonates. This potentially deleterious effect could be abolished by combining bisphosphonates with metalloproteinase inhibitors.

Trypanosoma cruzi contains major pyrophosphate stores, and its growth in vitro and in vivo is blocked by pyrophosphate analogs

High field (31)P nuclear magnetic resonance spectroscopy showed that inorganic pyrophosphate (P(2)O(7)(4-)) is more abundant than ATP in Trypanosoma cruzi, the causative agents of Chagas' disease. These results were confirmed by specific analytical assays, which showed that in epimastigotes, the concentrations of inorganic pyrophosphate and ATP were 194.7 +/- 25.9 and 37.6 +/- 5.5 nmol/mg of protein, respectively, and for the amastigote form, the corresponding concentrations were 358.0 +/- 17.0 and 36.0 +/- 1.9 nmol/mg of protein. High performance liquid chromatographic analysis of perchloric acid extracts of epimastigotes labeled for 3 h with (32)P-orthophosphate showed a significant incorporation of the precursor into inorganic pyrophosphate. Inorganic pyrophosphate was not uniformly distributed in T. cruzi but was shown by (31)P-NMR and chemical analysis to be particularly associated with acidocalcisomes, organelles shown previously to contain large amounts of phosphorus and various elements. Electron microscopy analysis of pyrophosphatase-treated permeabilized epimastigotes showed disappearance of the electron density of the acidocalcisomes. Nonmetabolizable analogs of pyrophosphate, currently used for the treatment of bone resorption disorders, selectively inhibited the proliferation of intracellular T. cruzi amastigotes and produced a profound suppression in the number of circulating trypomastigotes in mice with an acute infection of T. cruzi, offering a potentially new route to chemotherapy.

The pharmacology of bisphosphonates and new insights into their mechanisms of action

Bisphosphonates are chemically stable analogs of inorganic pyrophosphate, which are resistant to breakdown by enzymatic hydrolysis. The biological effects of bisphosphonates on calcium metabolism were originally ascribed to their physico-chemical effects on hydroxyapatite crystals. Although such effects may contribute to their overall action, their effects on cells are probably of greater importance, particularly for the more potent compounds. Remarkable progress has been made in increasing the potency of bisphosphonates as inhibitors of bone resorption, and the most potent compounds in current use are characterized by the presence of a nitrogen atom at critical positions in the side chain which, together with the bisphosphonate moiety itself, seems to be essential for maximal activity. As a class the bisphosphonates offer a very effective means of treating Paget's disease.

Bisphosphonates: from the laboratory to the clinic and back again

Bisphosphonates (BPs) used as inhibitors of bone resorption all contain two phosphonate groups attached to a single carbon atom, forming a "P-C-P" structure. The bisphosphonates are therefore stable analogues of naturally occuring pyrophosphate-containing compounds, which now helps to explain their intracellular as well as their extracellular modes of action. Bisphosphonates adsorb to bone mineral and inhibit bone resorption. The mode of action of bisphosphonates was originally ascribed to physico-chemical effects on hydroxyapatite crystals, but it has gradually become clear that cellular effects must also be involved. The marked structure-activity relationships observed among more complex compounds indicate that the pharmacophore required for maximal activity not only depends upon the bisphosphonate moiety but also on key features, e.g., nitrogen substitution in alkyl or heterocyclic side chains. Several bisphosphonates (e.g., etidronate, clodronate, pamidronate, alendronate, tiludronate, risedronate, and ibandronate) are established as effective treatments in clinical disorders such as Paget's disease of bone, myeloma, and bone metastases. Bisphosphonates are also now well established as successful antiresorptive agents for the prevention and treatment of osteoporosis. In particular, etidronate and alendronate are approved as therapies in many countries, and both can increase bone mass and produce a reduction in fracture rates to approximately half of control rates at the spine, hip, and other sites in postmenopausal women. In addition to inhibition of osteoclasts, the ability of bisphosphonates to reduce the activation frequency and birth rates of new bone remodeling units, and possibly to enhance osteon mineralisation, may also contribute to the reduction in fractures. The clinical pharmacology of bisphosphonates is characterized by low intestinal absorption, but highly selective localization and retention in bone. Significant side effects are minimal. Current issues with bisphosphonates include the introduction of new compounds, the choice of therapeutic regimen (e.g., the use of intermittent dosing rather than continuous), intravenous vs. oral therapy, the optimal duration of therapy, the combination with other drugs, and extension of their use to other conditions, including steroid-associated osteoporosis, male osteoporosis, arthritis, and osteopenic disorders in childhood. Bisphosphonates inhibit bone resorption by being selectively taken up and adsorbed to mineral surfaces in bone, where they interfere with the action of osteoclasts. It is likely that bisphosphonates are internalized by osteoclasts and interfere with specific biochemical processes and induce apoptosis. The molecular mechanisms by which these effects are brought about are becoming clearer. Recent studies show that bisphosphonates can be classified into at least two groups with different modes of action. Bisphosphonates that closely resemble pyrophosphate (such as clodronate and etidronate) can be metabolically incorporated into nonhydrolysable analogues of ATP that may inhibit ATP-dependent intracellular enzymes. The more potent, nitrogen-containing bisphosphonates (such as pamidronate, alendronate, risedronate, and ibandronate) are not metabolized in this way but can inhibit enzymes of the mevalonate pathway, thereby preventing the biosynthesis of isoprenoid compounds that are essential for the posttranslational modification of small GTPases. The inhibition of protein prenylation and the disruption of the function of these key regulatory proteins explains the loss of osteoclast activity and induction of apoptosis. These different modes of action might account for subtle differences between compounds in terms of their clinical effects. In conclusion, bisphosphonates are now established as an important class of drugs for the treatment of bone diseases, and their mode of action is being unravelled. As a result, their full therapeutic potential is gradual

Farnesol and geranylgeraniol prevent activation of caspases by aminobisphosphonates: biochemical evidence for two distinct pharmacological classes of bisphosphonate drugs

Recently, advances have been made in understanding the molecular mechanisms by which bisphosphonate drugs inhibit bone resorption. Studies with the macrophage-like cell line J774 have suggested that alendronate, an amino-containing bisphosphonate, causes apoptosis by preventing post-translational modification of GTP-binding proteins with isoprenoid lipids. However, clodronate, a nonaminobisphosphonate, does not inhibit protein isoprenylation but can be metabolized intracellularly to a cytotoxic, beta-gamma-methylene (AppCp-type) analog of ATP. These observations raise the possibility that bisphosphonates can be divided into two groups with distinct molecular mechanisms of action depending on the nature of the R2 side chain. We addressed this question by directly comparing the ability of three aminobisphosphonates (alendronate, ibandronate, and pamidronate) and three nonaminobisphosphonates (clodronate, etidronate, and tiludronate) to inhibit protein isoprenylation and activate caspase-3-like proteases or to be metabolized to AppCp-type nucleotides by J774 cells. All three aminobisphosphonates inhibited protein isoprenylation and activated caspase-3-like proteases. Apoptosis and caspase activation after 24-h treatment with the aminobisphosphonates could be prevented by addition of farnesol or geranylgeraniol, confirming that these bisphosphonates inhibit the metabolic mevalonate pathway. No AppCp-type metabolites of the aminobisphosphonates could be detected by mass spectrometry. The three nonaminobisphosphonates did not inhibit protein isoprenylation or cause activation of caspase-3-like proteases, but were incorporated into AppCp-type nucleotides. Taken together, these observations clearly demonstrate that bisphosphonate drugs can be divided into two pharmacological classes: the aminobisphosphonates, which act by inhibiting protein isoprenylation, and the less potent nonaminobisphosphonates, which act through the intracellular accumulation of AppCp-type metabolites.

Phosphate depletion in the rat: effect of bisphosphonates and the calcemic response to PTH

BACKGROUND

The removal of phosphate from the diet of the growing rat rapidly produces hypercalcemia, hypophosphatemia, hypercalciuria, and hypophosphaturia. Increased calcium efflux from bone has been shown to be the important cause of the hypercalcemia and hypercalciuria. It has been proposed that the increased calcium efflux from bone is osteoclast mediated. Because bisphosphonates have been shown to inhibit osteoclast-mediated bone resorption, this study was performed to determine whether bisphosphonate-induced inhibition of osteoclast function changed the biochemical and bone effects induced by phosphate depletion.

METHODS

Four groups of pair-fed rats were studied: (a) low-phosphate diet (LPD; phosphate less than 0.05%), (b) LPD plus the administration of the bisphosphonate Pamidronate (APD; LPD + APD), (c) normal diet (ND, 0.6% phosphate), and (d) ND + APD. All diets contained 0.6% calcium. A high dose of APD was administered subcutaneously (0.8 mg/kg) two days before the start of the study diet and on days 2, 6, and 9 during the 11 days of the study diet. On day 10, a 24-hour urine was collected, and on day 11, rats were either sacrificed or received an additional APD dose before a 48-hour parathyroid hormone (PTH) infusion (0.066 microgram/100 g/hr) via a subcutaneously implanted miniosmotic pump.

RESULTS

Serum and urinary calcium were greater in the LPD and LPD + APD groups than in the ND and ND + APD groups [serum, 11.12 +/- 0.34 and 11.57 +/- 0.45 vs. 9.49 +/- 0.17 and 9.48 +/- 0.15 mg/dl (mean +/- SE), P < 0.05; and urine, 8.78 +/- 2.74 and 16.30 +/- 4.68 vs. 0.32 +/- 0.09 and 0.67 +/- 0.28 mg/24 hr, P < 0.05]. Serum PTH and serum and urinary phosphorus were less in the LPD and LPD + APD than in the ND and ND + APD groups (P < 0.05). The calcemic response to PTH was less (P < 0.05) in the LPD and LPD + APD groups than in the ND group and was less (P = 0.05) in the LPD + APD than in the ND + APD group. Bone histology showed that phosphate depletion increased the osteoblast and osteoclast surface, and treatment with APD reduced the osteoblast surface (LPD vs. LPD + APD, 38 +/- 4 vs. 4 +/- 2%, P < 0.05, and ND vs. ND + APD, 20 +/- 2 vs. 5 +/- 2%, P < 0.05) and markedly altered osteoclast morphology by inducing cytoplasmic vacuoles.

CONCLUSIONS

(a) Phosphate depletion induced hypercalcemia and hypercalciuria that were not reduced by APD administration. (b) The calcemic response to PTH was reduced in phosphate-depleted rats and was unaffected by APD administration in normal and phosphate-depleted rats, and (c) APD administration markedly changed bone histology without affecting the biochemical changes induced by phosphate depletion.

Alendronate mechanism of action: geranylgeraniol, an intermediate in the mevalonate pathway, prevents inhibition of osteoclast formation, bone resorption, and kinase activation in vitro

Nitrogen-containing bisphosphonates were shown to cause macrophage apoptosis by inhibiting enzymes in the biosynthetic pathway leading from mevalonate to cholesterol. This study suggests that, in osteoclasts, geranylgeranyl diphosphate, the substrate for prenylation of most GTP binding proteins, is likely to be the crucial intermediate affected by these bisphosphonates. We report that murine osteoclast formation in culture is inhibited by both lovastatin, an inhibitor of hydroxymethylglutaryl CoA reductase, and alendronate. Lovastatin effects are blocked fully by mevalonate and less effectively by geranylgeraniol whereas alendronate effects are blocked partially by mevalonate and more effectively by geranylgeraniol. Alendronate inhibition of bone resorption in mouse calvaria also is blocked by mevalonate whereas clodronate inhibition is not. Furthermore, rabbit osteoclast formation and activity also are inhibited by lovastatin and alendronate. The lovastatin effects are prevented by mevalonate or geranylgeraniol, and alendronate effects are prevented by geranylgeraniol. Farnesol and squalene are without effect. Signaling studies show that lovastatin and alendronate activate in purified osteoclasts a 34-kDa kinase. Lovastatin-mediated activation is blocked by mevalonate and geranylgeraniol whereas alendronate activation is blocked by geranylgeraniol. Together, these findings support the hypothesis that alendronate, acting directly on osteoclasts, inhibits a rate-limiting step in the cholesterol biosynthesis pathway, essential for osteoclast function. This inhibition is prevented by exogenous geranylgeraniol, probably required for prenylation of GTP binding proteins that control cytoskeletal reorganization, vesicular fusion, and apoptosis, processes involved in osteoclast activation and survival.

In vivo incidence of apoptosis evaluated with the TdT FragEL DNA fragmentation detection kit in cartilage and bone cells of the rat tibia

Previous studies have shown the occurrence of cell death by apoptosis in cartilage and bone cells, and have suggested a functional relationship between bone growth and remodelling on one hand, and numbers of apoptotic cells on the other. At present, no in vivo studies are available on the frequency of the apoptotic process measured at one time and in one place using the cartilage and bone cells of single specimens. The aim of the present investigation was to measure the in vivo incidence of apoptosis in cartilage and bone cells of the upper epiphysis and secondary ossification metaphyseal bone of the tibia in normal young adult rats. Apoptotic cells were visualized with the terminal deoxynucleotidyl transferase (TdT) FragEL DNA fragmentation detection kit, which is analogous to the TdT-mediated nick end-labelling (TUNEL) method. In the growth cartilage, only a few TUNEL-positive terminal hypertrophic chondrocytes were found; they were 1.32 +/- 0.70% of the total hypertrophic chondrocytes counted along the chondro-osseous junction. There were only a few apoptotic osteoblastic cells and osteocytes (0.22 +/- 0.22% and 0.15 +/- 0.16% of total osteoblasts and osteocytes respectively). TUNEL-positive osteoclasts were 1.03 +/- 0.57% of the total of osteoclastic cells; they usually showed only one or two apoptotic nuclei. The total number of TUNEL-positive bone marrow cells were also counted (56.78 +/- 10.29/mm2 of bone marrow spaces). Our results confirm that apoptosis does occur in hypertrophic chondrocytes and bone cells, and show that its frequency is very low. However, chiefly because of its short lifespan, the frequency of apoptosis in cartilage and bone may be higher than that shown by the TUNEL method. The static estimate that can be obtained with this method might lead to misleading conclusions on the physiological significance of such a dynamic, rapid and asynchronous process, whose precise importance in bone growth and remodelling remains to be determined.

Anti-tumour activity of bisphosphonates in human myeloma cells

Multiple myeloma is a haematological malignancy characterized by an expansion of malignant plasma cells within the bone marrow and is frequently associated with bone disease involving the development of osteolytic bone lesions, pathological fractures, osteoporosis and hypercalcaemia. A class of anti-resorptive drugs known as bisphosphonates have been in use to treat osteoclast-mediated bone diseases for the past 3 decades, and are currently proving effective in the treatment of the bone disease associated with multiple myeloma. Recent studies have suggested that bisphosphonate treatment may also result in an improvement in survival in some patients with multiple myeloma. These effects on survival may reflect an indirect effect of the bisphosphonates on tumour growth, via inhibition of osteoclast activity and hence a reduction in the release of tumour growth factors. However, it is also possible that bisphosphonates may have a direct effect on myeloma cells. In support of this we have demonstrated that bisphosphonates can decrease cell proliferation and induce apoptosis in human myeloma cells in vitro, and this review discusses the possibility that bisphosphonates may have not only an anti-resorptive action, but may also have a direct anti-tumour activity.

Heterocycle-containing bisphosphonates cause apoptosis and inhibit bone resorption by preventing protein prenylation: evidence from structure-activity relationships in J774 macrophages

Recent evidence suggests that bisphosphonates (BPs) may inhibit bone resorption by mechanisms that lead to osteoclast apoptosis. We have previously shown that BPs also reduce cell viability and induce apoptosis in the macrophage-like cell line J774. To determine whether BPs inhibit osteoclast-mediated bone resorption and affect J774 macrophages by the same molecular mechanism, we examined the potency to reduce J774 cell viability of pairs of nitrogen-containing BPs that differ slightly in the structure of the heterocycle-containing side chain but that differ markedly in antiresorptive potency. In all cases, the most potent antiresorptive BP of each pair also caused the greatest loss of J774 viability, while the less potent antiresorptive BPs were also less potent at reducing J774 cell viability. Similarly, the bisphosphinate, phosphonoalkylphosphinate and monophosphonate analogs of BPs (in which one or both phosphonate groups are modified, giving rise to much less potent or inactive antiresorptive agents) were much less potent or inactive at reducing J774 cell viability. Thus, the structure-activity relationships of BPs for inhibiting bone resorption match those for causing loss of cell viability in J774 cells, indicating that BPs inhibit osteoclast-mediated bone resorption and reduce J774 macrophage viability by the same molecular mechanism. Loss of J774 cell viability after treatment with BPs was associated with a parallel increase in apoptotic cell death. We have recently proposed that nitrogen-containing BPs reduce cell viability and cause J774 apoptosis as a consequence of inhibition of enzymes of the mevalonate pathway and hence loss of prenylated proteins. In this study, the BPs that were potent inducers of J774 apoptosis and potent antiresorptive agents were also found to be effective inhibitors of protein prenylation in J774 macrophages, whereas the less potent BP analogs did not inhibit protein prenylation. This provides strong evidence that BPs with a heterocyclic, nitrogen-containing side chain, such as risedronate, inhibit osteoclast-mediated bone resorption and induce J774 apoptosis by preventing protein prenylation.

OxLDL-induced macrophage cytotoxicity is mediated by lysosomal rupture and modified by intralysosomal redox-active iron

Oxidized low density lipoprotein (oxLDL) is believed to play a central role in atherogenesis. LDL is oxidized in the arterial intima by mechanisms that are still only partially understood. OxLDL is then taken up by macrophages through scavenger receptor-mediated endocytosis, which then leads to cellular damage, including apoptosis. The complex mechanisms by which oxLDL induces cell injury are mostly unknown. This study has demonstrated that oxLDL-induced damage of macrophages is associated with iron-mediated intralysosomal oxidative reactions, which cause partial lysosomal rupture and ensuing apoptosis. This series of events can be prevented by pre-exposing cells to the iron-chelator, desferrioxamine (DFO), whereas it is augmented by pretreating the cells with a low molecular weight iron complex. Since both DFO and the iron complex would be taken up by endocytosis, and thus directed to the lysosomal compartment, the results suggest that the normal contents of lysosomal low molecular weight iron may play an important role in oxLDL-induced cell damage, presumably by catalyzing intralysosomal fragmentation of lipid peroxides and the formation of toxic aldehydes and oxygen-centered radicals.

Reduction in new metastases in breast cancer with adjuvant clodronate treatment

BACKGROUND

Bisphosphonates are effective against the increased bone resorption caused by certain diseases because they inhibit the activity of osteoclasts. In patients who have breast cancer and metastatic bone disease, the bisphosphonate clodronate (clodronic acid) reduces the frequency of skeletal complications. Experiments in animals and preliminary clinical observations indicate that early clodronate therapy reduces the incidence of new bony metastases in breast cancer. We investigated the effects of clodronate on the incidence and extent of new metastases in patients with breast cancer.

METHODS

Between 1990 and 1995, 302 patients with primary breast cancer and tumor cells in the bone marrow (the presence of which is a risk factor for the development of distant metastases) were randomly assigned to receive clodronate at a dose of 1600 mg per day orally for two years (157 patients) or standard follow-up (145 patients). The median length of observation was 36 months. All patients in both groups received standard surgical treatment and customary hormonal therapy or chemotherapy.

RESULTS

Distant metastases were detected in 21 patients in the clodronate group and in 42 patients in the control group (P<0.001). The incidence of both osseous and visceral metastases was significantly lower in the clodronate group than in the control group (P=0.003 for both osseous and visceral metastases). Six patients in the clodronate group died, as did 22 in the control group (P=0.001). The mean number of bony metastases per patient in the clodronate group was roughly half that in the control group (3.1 vs. 6.3).

CONCLUSIONS

Clodronate can reduce the incidence and number of new bony and visceral metastases in women with breast cancer who are at high risk for distant metastases.

Nitrogen-containing bisphosphonates inhibit the mevalonate pathway and prevent post-translational prenylation of GTP-binding proteins, including Ras

Bisphosphonates are currently the most important class of antiresorptive drugs used for the treatment of metabolic bone diseases. Although the molecular targets of bisphosphonates have not been identified, these compounds inhibit bone resorption by mechanisms that can lead to osteoclast apoptosis. Bisphosphonates also induce apoptosis in mouse J774 macrophages in vitro, probably by the same mechanisms that lead to osteoclast apoptosis. We have found that, in J774 macrophages, nitrogen-containing bisphosphonates (such as alendronate, ibandronate, and risedronate) inhibit post-translational modification (prenylation) of proteins, including the GTP-binding protein Ras, with farnesyl or geranylgeranyl isoprenoid groups. Clodronate did not inhibit protein prenylation. Mevastatin, an inhibitor of 3-hydroxy-3-methylglutatyl (HMG)-CoA reductase and hence the biosynthetic pathway required for the production of farnesyl pyrophosphate and geranylgeranyl pyrophosphate, also caused apoptosis in J774 macrophages and murine osteoclasts in vitro. Furthermore, alendronate-induced apoptosis, like mevastatin-induced apoptosis, could be suppressed in J774 cells by the addition of farnesyl pyrophosphate or geranylgeranyl pyrophosphate, while the effect of alendronate on osteoclast number and bone resorption in murine calvariae in vitro could be overcome by the addition of mevalonic acid. These observations suggest that nitrogen-containing bisphosphonate drugs cause apoptosis following inhibition of post-translational prenylation of proteins such as Ras. It is likely that these potent antiresorptive bisphosphonates also inhibit bone resorption by preventing protein prenylation in osteoclasts and that enzymes of the mevalonate pathway or prenyl protein transferases are the molecular targets of the nitrogen-containing bisphosphonates. Furthermore, the data support the view that clodronate acts by a different mechanism.

Effects of liposome-encapsulated bisphosphonates on acetylated LDL metabolism, lipid accumulation and viability of phagocyting cells

Bisphosphonates, the drugs used for the treatment of e.g. osteoporosis, inhibit the development of experimental atherosclerosis. When encapsulated in liposomes, they also inactivate macrophages, which have a key role in atherogenesis. We studied the effects of three clinically used bisphosphonates, i.e. clodronate, etidronate and pamidronate, on 1) the viability of mouse peritoneal macrophages and macrophage-like RAW 264 cells, 2) the degradation of 125I-labeled acetylated LDL by RAW 264 cells, and 3) the formation of LDL-derived foam cells in vitro. Liposome-encapsulated clodronate and pamidronate, but not etidronate, decreased the fraction of viable peritoneal macrophages in a concentration-dependent manner, whereas RAW 264 cells were much more resistant to the cytotoxic effects of bisphosphonates. Preincubation with liposomal clodronate and etidronate inhibited in a concentration-dependent manner the degradation of acetylated LDL in RAW 264 cells, but non-cytotoxic concentrations of liposomal pamidronate had only a weak inhibitory effect. The inhibition was more pronounced by liposomal clodronate than by liposomal etidronate. At high concentrations (500 microg protein/ml) of acetylated and aggregated LDL, RAW 264 cells transformed to foam cells. Preincubation with liposomal clodronate and etidronate reduced the cellular accumulation of acetylated LDL-derived lipids, but the drugs had no effect on the lipid accumulation caused by aggregated LDL. The results suggest that liposomal clodronate and etidronate inhibit the activity of phagocyting cells in internalizing and degrading atherogenic modified LDL.

Clodronate and liposome-encapsulated clodronate are metabolized to a toxic ATP analog, adenosine 5'-(beta, gamma-dichloromethylene) triphosphate, by mammalian cells in vitro

Clodronate, alendronate, and other bisphosphonates are widely used in the treatment of bone diseases characterized by excessive osteoclastic bone resorption. The exact mechanisms of action of bisphosphonates have not been identified but may involve a toxic effect on mature osteoclasts due to the induction of apoptosis. Clodronate encapsulated in liposomes is also toxic to macrophages in vivo and may therefore be of use in the treatment of inflammatory diseases. It is generally believed that bisphosphonates are not metabolized. However, we have found that mammalian cells in vitro (murine J774 macrophage-like cells and human MG63 osteosarcoma cells) can metabolize clodronate (dichloromethylenebisphosphonate) to a nonhydrolyzable adenosine triphosphate (ATP) analog, adenosine 5'-(beta, gamma-dichloromethylene) triphosphate, which could be detected in cell extracts by using fast protein liquid chromatography. J774 cells could also metabolize liposome-encapsulated clodronate to the same ATP analog. Liposome-encapsulated adenosine 5'-(beta, gamma-dichloromethylene) triphosphate was more potent than liposome-encapsulated clodronate at reducing the viability of cultures of J774 cells and caused both necrotic and apoptotic cell death. Neither alendronate nor liposome-encapsulated alendronate were metabolized. These results demonstrate that the toxic effect of clodronate on J774 macrophages, and probably on osteoclasts, is due to the metabolism of clodronate to a nonhydrolyzable ATP analog. Alendronate appears to act by a different mechanism.

What does cell death have to do with aging?

When Lockshin and Zakeri discussed the relevance of apoptosis to aging 7 years ago, the common view was that apoptosis would have primarily a negative impact on aging by destroying essential and often irreplaceable cells. That view has now changed to one that acknowledges that there are two general ways in which apoptosis can play a role in aging: (1) elimination of damaged and presumably dysfunctional cells (e.g., fibroblasts, hepatocytes), which can then be replaced by cell proliferation, thereby maintaining homeostasis, and (2) elimination of essential post-mitotic cells (e.g., neurons, cardiac myocytes), which cannot be replaced, thereby leading to pathology. Evidence exists in two systems (fibroblasts and thymocytes/lymphocytes) that there are age-related decreases in the potential for apoptosis, although the molecular bases for the decreases in these two systems appear to differ. Upon becoming senescent, fibroblasts lose the ability to down-regulate expression of the bcl-2 gene in response to an apoptotic signal; thus, apoptosis is blocked even though an initiating signal has been received. In contrast, thymocytes/lymphocytes lack the ability to initiate the signal because of down-regulation of the cell surface receptor Fas. There is limited information available for other tissue types, and nothing is known about why and how age-related changes occur. An interesting observation is that the frequency of up-regulation of the bcl-2 gene as a result of chromosome translocation in otherwise normal B cells increases with age; the functional consequences of this phenomenon during aging are not known. The role of apoptosis in regulating cell number is also a promising area of research. The studies on liver damage and neoplastic lesions suggest an extremely important role for apoptosis in controlling cancer. This may be particularly important in the prostate where hypertrophy and/or cancer are a virtual certainty with ever-increasing age. It is not known whether the ability to undergo apoptosis declines in the prostate with increasing age, but it appears possible that it may, thus explaining the loss of control over cell number in this tissue. A particularly important area of research is whether apoptosis plays a role in the changing balance between bone formation and resorption observed during osteoporosis. Monica Driscoll has already pointed out that, "regulation and execution of cell death is an absolutely critical process that interfaces with nearly every aspect of life. Future investigation of the links of cell death to cellular aging and the aging of organisms should be an exciting enterprise." The results currently available do suggest that apoptosis is a process that may be important in aging, at least in some tissues, and the mechanism of its regulation, in particular, needs to be understood. Several tumor suppressor gene and oncogene products are involved in signal transduction associated with apoptosis, but it remains to be shown which of these, if any, are actually involved in "on-off" switches for apoptosis. Where great progress has been made is in understanding the events occurring after binding of either Fas ligand or tumor necrosis factor to their respective receptors. However, one area about which little is known is the identity of the signals that initiate this process in response to intracellular damage. Through continuing research on cell death mechanisms, funded by the NIA, we hope to provide answers to such fundamental questions as: 1. Are there age-related changes in apoptosis, and what role, if any, do these have in the aging process? 2. If age-related changes in apoptosis do occur, what molecular mechanisms are altered to produce these changes? 3. Can approaches be developed to improve the detection and elimination of damaged cells in vivo in tissues where cell replacement is possible? 4. Can death of damaged cells be attenuated or delayed in nonrenewable tissues, and, if so, is it advantageous to the org