G. D. Roodman

Interleukin 6 - A potential autocrine/paracrine factor in Paget's disease of bone

Pagetic osteoclasts are greatly increased in number and size and have increased numbers of nuclei per cell compared to normal osteoclasts. The mechanisms responsible for enhanced osteoclast formation in Pagets disease are unknown. We have used our recently described model system for pagetic osteoclast formation to evaluate culture media conditioned by these atypical multinucleated cells (MNC) to determine if pagetic osteoclasts produce an autocrine or paracrine factor that enhances osteoclast formation. Conditioned media from long-term bone marrow cultures from patients with Pagets disease stimulated osteoclast-like MNC formation in normal marrow cultures. At least part of this activity could be ascribed to interleukin 6 (IL-6). In contrast, conditioned media from normal marrow cultures contained lower levels of IL-6 and did not stimulate formation of osteoclast-like MNC. 7 of 8 bone marrow plasma samples taken from involved bones and 18 of 27 peripheral blood serum samples from Pagets patients had high levels of IL-6. Normal marrow plasma and peripheral blood serum had no or very low levels of IL-6. These results suggest that IL-6 produced by marrow and/or bone cells in patients with Pagets disease may be an autocrine/paracrine factor for pagetic osteoclasts.

Interleukin-6 enhances hypercalcemia and bone resorption mediated by parathyroid hormone-related protein in vivo.

Tumors frequently induce the multifunctional cytokine IL-6, which has been linked to several paraneoplastic syndromes, most notably cachexia. IL-6 stimulates osteoclast formation, causes mild hypercalcemia, and is produced by bone cells in vitro upon exposure to systemic hormones. Since IL-6 is produced together with parathyroid hormone-related protein (PTH-rP) in some patients with cancer, we tested the hypothesis that production of IL-6 potentiates the effects of PTH-rP on Ca2+ homeostasis and osteoclastic bone resorption and examined potential mechanisms for these interactions in vivo. Chinese hamster ovarian (CHO) cells stably transfected with cDNAs for IL-6 (CHO/IL-6) and PTH-rP sense (CHO/PTH-rP) or antisense (CHO/PTH-rP AS) were inoculated intramuscularly into nude mice. Experimental groups included CHO/IL-6 plus CHO/PTH-rP; CHO/IL-6 plus CHO/PTH-rP AS; CHO/IL-6 alone; and CHO/PTH-rP alone. Blood ionized Ca2+ was measured on days 0, 7, 10, 12, and 13. Three different developmental stages in the osteoclast lineage were examined at day 13: the early multipotential precursor, granulocyte macrophage colony-forming units (CFU-GM); more mature mononuclear osteoclast precursors, assessed by their capacity to form tartrate-resistant acid phosphatase-positive multinucleated cells in marrow cultures; and mature osteoclasts, assessed by histomorphometry. IL-6 increased CFU-GM but not bone resorption or Ca2+. In contrast, PTH-rP induced hypercalcemia and bone resorption and increased multinucleated osteoclasts and more mature precursors cells, but not CFU-GM. However, mice treated with both IL-6 and PTH-rP had very marked hypercalcemia and osteoclastosis as well as an increase in the number of both CFU-GM and mature osteoclast precursors. These data demonstrate that IL-6 enhances PTH-rP-mediated hypercalcemia and bone resorption, most likely by increasing the pool of early osteoclast precursors that in turn can differentiate to mature osteoclasts. We conclude that IL-6 stimulatory effects on osteoclast precursors may enhance the effects of other bone resorption factors that act at later stages in the osteoclast lineage.

Intracellular Fragmentation of Bone Resorption Products by Reactive Oxygen Species Generated by Osteoclastic Tartrate-resistant Acid Phosphatase

Tartrate-resistant acid phosphatase (TRAP) is highly expressed in bone-resorbing osteoclasts and activated macrophages. It has been suggested that a redox-active iron in the binuclear iron center of TRAP could have the capacity to react with hydrogen peroxide to produce highly destructive reactive oxygen species (ROS). Here we show that TRAP can generate ROS in vitro and that cells over-expressing TRAP produce higher amounts of intracellular ROS than their parent cells. We further demonstrate that these ROS can be targeted to destroy collagen and other proteins. In resorbing osteoclasts, TRAP was found in transcytotic vesicles transporting matrix degradation products through the cell, suggesting that TRAP-facilitated fragmentation of endocytosed material takes place in a specific cellular compartment. These results suggest that bone matrix degradation occurs not only extracellularly in the resorption lacunae but also intracellularly in the transcytotic vesicles. We propose that proteins containing redox-active iron could represent a novel mechanism of physiological fragmentation of organic molecules. This mechanism could be important in tissue remodeling and as a defense mechanism of phagocytosing cells.

Recombinant human transforming growth factor-alpha stimulates the formation of osteoclast-like cells in long-term human marrow cultures.

Transforming growth factor-alpha (TGF-alpha) is synthesized by a variety of tumor cell lines and stimulates osteoclastic bone resorption in vitro. The mechanism by which TGF-alpha increases osteoclast activity is unknown. We used a human marrow culture system that forms osteoclast-like multinucleated cells (MNCs) to determine the effects of recombinant human TGF-alpha on MNC formation. Addition of 0.01 ng/ml TGF-alpha for the 1st week followed by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] for the subsequent 2 wk significantly increased MNCs. Treatment of these cultures with TGF-alpha without later addition of 1,25(OH)2D3 did not increase MNC formation. Autoradiographic studies revealed that TGF-alpha stimulated proliferation of precursors for MNCs, and 1,25(OH)2D3 increased their rate of fusion into MNCs. Addition of murine epidermal growth factor (EGF) (0.1 ng/ml) followed by 1,25(OH)2D3 also significantly stimulated MNC formation. These data suggest that TGF-alpha and EGF may stimulate bone resorption by increasing the proliferation of osteoclast precursors, which leads to increased numbers of osteoclasts.

Atypical multinucleated cells form in long-term marrow cultures from patients with Paget's disease.

Although Pagets disease is the most flagrant example of a primary osteoclast disorder, little is known of osteoclast biology in this disease. In this report we have studied the formation of cells with the osteoclast phenotype in long-term cultures of marrow mononuclear cells derived from patients with Pagets disease, and compared these with similar cells formed in long-term marrow cultures from normal individuals, and with osteoclasts present in pagetic bone. Osteoclasts formed in pagetic marrow cultures resembled osteoclasts present in pagetic bone, but were distinctly different from osteoclasts formed in normal marrow cultures. Osteoclast formation was 10-20-fold greater in pagetic marrow cultures than in normal cultures. The multinucleated cells formed in cultures of pagetic marrow were much larger in size, were hyperresponsive to 1,25(OH)2 vitamin D, had more nuclei per cell, had increased levels of tartrate-resistant acid phosphatase activity and had ultrastructural features which were not seen in multinucleated cells formed from normal marrow mononuclear cells. These pagetic marrow-derived multinucleated cells formed large resorption lacunae on calcified matrices and cross-reacted with monoclonal antibodies which preferentially bind to osteoclasts. The multinucleated cells formed from marrow obtained from uninvolved sites in Pagets patients also displayed these abnormal features.

Osteoclast-like cells form in long-term human bone marrow but not in peripheral blood cultures.

Transplantation studies have suggested that peripheral blood mononuclear cells contain precursors for osteoclasts. Thus we tested the capacity of peripheral blood monocytes to form osteoclasts in long-term culture. We have reported previously that mononuclear cells from feline, baboon, and human marrow form osteoclast-like cells in long term cultures. Further, the formation of these cells is increased in response to bone resorption stimulatory agents such as PTH, interleukin 1, and transforming growth factor alpha. We now report that these cells show characteristic cytoplasmic contraction with calcitonin and form resorption lacunae when cultured on sperm whale dentine. Thus, these bone marrow-derived multinucleated cells fulfill the functional criteria for osteoclasts. Although cultured peripheral blood monocytes can be induced to form multinucleated cells with 1,25-dihydroxyvitamin D3, these cells did not show similar responses to the osteotropic factors as multinucleated cells formed in the bone marrow cultures multinucleated cells. These results indicate that osteoclasts or cells closely related to osteoclasts form in long-term human bone marrow cultures. In contrast, few mononuclear cells in the peripheral blood appear capable of forming osteoclasts under the culture conditions used in these experiments.

Multinucleated cells formed in vitro from Paget's bone marrow express viral antigens

Pagets disease of bone is characterized by large numbers of osteoclasts that have viral-like nuclear and/or cytoplasmic inclusions. Pagetic osteoclasts express respiratory syncytial viral (RSV) and measles viral (MV) nucleocapsid antigens. The data suggest a possible viral etiology for Pagets disease. However, studies to characterize further the putative viral inclusions in Pagets osteoclasts have been severely hampered by the extreme difficulty in isolating large numbers of osteoclasts from pagetic bone. The recent demonstration that osteoclast-like multinucleated cells (MNC), that had certain characteristics of pagetic osteoclasts formed in marrow cultures from Pagets patients, may permit studies to describe this virus further. Therefore, we have cultured marrow samples from involved and uninvolved bones from Pagets patients and from normal subjects to determine if the MNC formed in these cultures express viral antigens. RSV and/or MV antigens were expressed in the mononuclear cells and/or the MNC formed in 12 of 12 marrow cultures from active lesions of patients with Pagets disease, with 40-50% of the cells expressing viral antigens. In contrast, less than 5% of cells isolated from cultures from normal subjects expressed RSV and/or MV. These results suggest that MNC formed in long-term marrow cultures from patients with Pagets disease frequently express paramyxoviral antigens and are very similar to pagetic osteoclasts. Thus, these marrow cultures may be useful for further characterizing the virus in Pagets disease.

Immortalization of osteoclast precursors by targeting Bcl -XL and Simian virus 40 large T antigen to the osteoclast lineage in transgenic mice.

Cellular and molecular characterization of osteoclasts (OCL) has been extremely difficult since OCL are rare cells, and are difficult to isolate in large numbers. We used the tartrate-resistant acid phosphatase promoter to target the bcl-XL and/or Simian Virus 40 large T antigen (Tag) genes to cells in the OCL lineage in transgenic mice as a means of immortalizing OCL precursors. Immunocytochemical studies confirmed that we had targeted Bcl-XL and/or Tag to OCL, and transformed and mitotic OCL were readily apparent in bones from both Tag and bcl-XL/Tag mice. OCL formation in primary bone marrow cultures from bcl-XL, Tag, or bcl-XL/Tag mice was twofold greater compared with that of nontransgenic littermates. Bone marrow cells from bcl-XL/Tag mice, but not from singly transgenic bcl-XL or Tag mice, have survived in continuous culture for more than a year. These cells form high numbers of bone-resorbing OCL when cultured using standard conditions for inducing OCL formation, with approximately 50% of the mononuclear cells incorporated into OCL. The OCL that form express calcitonin receptors and contract in response to calcitonin. Studies examining the proliferative capacity and the resistance of OCL precursors from these transgenic mice to apoptosis demonstrated that the increased numbers of OCL precursors in marrow from bcl-XL/Tag mice was due to their increased survival rather than an increased proliferative capacity compared with Tag, bcl-XL, or normal mice. Histomorphometric studies of bones from bcl-XL/Tag mice also confirmed that there were increased numbers of OCL precursors (TRAP + mononuclear cells) present in vivo. These data demonstrate that by targeting both bcl-XL and Tag to cells in the OCL lineage, we have immortalized OCL precursors that form bone-resorbing OCL with an efficiency that is 300-500 times greater than that of normal marrow.

Osteoclast function in Paget's disease and multiple myeloma

Pagets disease of bone and multiple myeloma are characterized by increased numbers of osteoclasts and markedly increased bone resorption at the sites of the disease. In Pagets disease the osteoclasts are abnormal morphologically and contain viral-like nuclear inclusions, but in multiple myeloma the osteoclasts are normal. The bone lesions in both Pagets disease and multiple myeloma appear to be due to local stimulation of osteoclast formation and bone resorption. In situ hybridization techniques, bone marrow cultures, and cytokine assays have been used to examine osteoclast function in Pagets disease and multiple myeloma. Interleukin-6 (IL-6) has been implicated as a potential mediator for the increased osteoclast activity in both diseases. In Pagets disease, IL-6 is produced by the osteoclasts, the osteoclasts express IL-6 receptors and IL-6 mRNA, and increased levels of IL-6 are present in the marrow plasma and serum of these patients. Similarly, increased levels of IL-6 have been detected in sera from some patients with multiple myeloma. Multiple myeloma cells do not produce IL-6 in vivo but marrow stromal cells or the osteoclasts may be the source of IL-6 in multiple myeloma. IL-6 is a growth factor for multiple myeloma cells, and treating patients with anti-IL-6 decreases the tumor burden in some patients. Thus, IL-6 may be an autocrine/paracrine factor in both Pagets disease and in multiple myeloma. Multiple myeloma cells also produce osteoclast activating factors (OAFs) that can stimulate osteoclast formation and activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Paget's disease and osteoclast biology.

Pagets disease is the most exaggerated example of bone remodeling in which abnormal osteoclastic bone resorption remains coupled to new bone formation. There are abnormalities in the stages of osteoclast development, and studies in Pagets disease have suggested a major role for IL-6 in human osteoclast activity. The pathophysiologic basis for these abnormalities is not clearly defined, except that the primary cellular abnormality resides in the osteoclast. Many important questions about the pathophysiology of Pagets disease still remain to be answered, including: (1) What is the identity of the virus in pagetic osteoclasts?; (2) Are Pagets patients in different geographical locales harboring a similar virus in their osteoclasts, or can different paramyxoviruses induce Pagets disease?; (3) How is the virus maintained and propagated for many years, so that it can be expressed in the osteoclast, a cell with a finite lifespan?; and (4) Since Pagets disease has a very high familial tendency, with up to 40% of patients having an affected relative, what is the genetic locus associated with Pagets disease, and does this genotype result in an increased propensity for hematopoietic cells such as the osteoclast to harbor paramyxoviruses? The application of the techniques of molecular and cell biology to Pagets disease should provide answers to some of these questions and give important insights into the normal bone remodeling process.