Interferon-gamma inhibits 1,25-dihydroxyvitamin D3-stimulated synthesis of bone GLA protein in rat osteosarcoma cells by a pretranslational mechanism

Costimulation of Murine Osteoblasts with Interferon-γ and Tumor Necrosis Factor-α Induces Apoptosis through Downregulation of Bcl-2 and Release of Cytochrome c from Mitochondria

During chronic inflammation from diseases, such as periodontal disease, the proinflammatory cytokines interferon-gamma (IFNγ) and tumor necrosis factor-α (TNFα) alter bone remodeling. To elucidate the underlying molecular mechanisms, we investigated the effect of IFNγ and TNFα on the proliferation and survival of clonal MC3T3-E1 mouse osteoblasts. We found that although IFNγ or TNFα alone affected cell growth and survival only marginally, costimulation with both synergistically inhibited cell growth and reduced cell viability. The diminished cell viability was due to apoptosis, as indicated by increased TUNEL staining and elevated caspase 3, 8, and 9 activities. Western blot also showed that costimulation with IFNγ and TNFα elicited cytochrome c release and downregulated B cell lymphoma 2 (Bcl-2) expression without affecting Bcl-2-associated X (Bax) protein expression. Furthermore, stable Bcl-2 overexpression significantly alleviated cell death following costimulation. Collectively, these results suggested that IFNγ and TNFα elicited osteoblast apoptosis via cytochrome c release from damaged mitochondria, caspase activation, and Bcl-2 downregulation.

Concurrent nematode infection and pregnancy induce physiological responses that impair linear growth in the murine foetus

This study examined concurrent stresses of nematode infection and pregnancy using pregnant and non-pregnant CD1 mice infected 3 times with 0, 50 or 100 Heligmosomoides bakeri larvae. Physiological, energetic, immunological and skeletal responses were measured in maternal and foetal compartments. Resting metabolic rate (RMR) was elevated by pregnancy, but not by the trickle infection. Energy demands during pregnancy were met through increased food intake and fat utilization whereas mice lowered their body temperature during infection. Both infection and pregnancy increased visceral organ mass and both altered regional bone area and mineralization. During pregnancy, lumbar mineralization was lower but femur area and mineralization were higher. On the other hand, infection lowered maternal femur bone area and this was associated with higher IFN-γ in maternal serum of heavily infected pregnant mice. Infection also reduced foetal crown-rump length which was associated with higher amniotic fluid IL-1β.

Response to mechanical strain in an immortalized pre-osteoblast cell is dependent on ERK1/2

Mechanical strain inhibits osteoclastogenesis by regulating osteoblast functions: We have shown that strain inhibits receptor activator of NF-kappaB ligand (RANKL) expression and increases endothelial nitric oxide synthase (eNOS) and nitric oxide levels through ERK1/2 signaling in primary bone stromal cells. The primary stromal culture system, while contributing greatly to understanding of how the microenvironment regulates bone remodeling is limited in use for biochemical assays and studies of other osteoprogenitor cell responses to mechanical strain: Stromal cells proliferate poorly and lose aspects of the strain response after a relatively short time in culture. In this study, we used the established mouse osteoblast cell line, conditionally immortalized murine calvarial (CIMC-4), harvested from mouse calvariae conditionally immortalized by insertion of the gene coding for a temperature-sensitive mutant of SV40 large T antigen (TAg) and support osteoclastogenesis. Mechanical strain (0.5-2%, 10 cycles per min, equibiaxial) caused magnitude-dependent decreases in RANKL expression to less than 50% those of unstrained cultures. Overnight strains of 2% also increased osterix (OSX) and RUNX2 expression by nearly twofold as measured by RT-PCR. Importantly, the ERK1/2 inhibitor, PD98059, completely abrogated the strain effects bringing RANKL, OSX, and RUNX2 gene expression completely back to control levels. These data indicate that the strain effects on CIMC-4 cells require activation of ERK1/2 pathway. Therefore, the CIMC-4 cell line is a useful alternative in vitro model which effectively recapitulates aspects of the primary stromal cells and adds an extended capacity to study osteoblast control of bone remodeling in a mechanically active environment.

Stat1-vitamin D receptor interactions antagonize 1,25-dihydroxyvitamin D transcriptional activity and enhance stat1-mediated transcription

The cytokine gamma interferon (IFN-gamma) and the calcitropic steroid hormone 1,25-dihydroxyvitamin D (1,25D) are activators of macrophage immune function. In sarcoidosis, tuberculosis, and several granulomatoses, IFN-gamma induces 1,25D synthesis by macrophages and inhibits 1,25D induction of 24-hydroxylase, a key enzyme in 1,25D inactivation, causing high levels of 1,25D in serum and hypercalcemia. This study delineates IFN-gamma-1,25D cross talk in human monocytes-macrophages. Nuclear accumulation of Stat1 and vitamin D receptor (VDR) by IFN-gamma and 1,25D promotes protein-protein interactions between Stat1 and the DNA binding domain of the VDR. This prevents VDR-retinoid X receptor (RXR) binding to the vitamin D-responsive element, thus diverting the VDR from its normal genomic target on the 24-hydroxylase promoter and antagonizing 1,25D-VDR transactivation of this gene. In contrast, 1,25D enhances IFN-gamma action. Stat1-VDR interactions, by preventing Stat1 deactivation by tyrosine dephosphorylation, cooperate with IFN-gamma/Stat1-induced transcription. This novel 1,25D-IFN-gamma cross talk explains the pathogenesis of abnormal 1,25D homeostasis in granulomatous processes and provides new insights into 1,25D immunomodulatory properties.

Effects of nerve growth factor and nicotine on the expression of nicotinic acetylcholine receptor subunits in PC12 cells

The neuroprotective effect of nicotine via nicotinic acetylcholine receptor (nAChR) has been reported in differentiated PC12 cells. We examined the effects of nerve growth factor (NGF) and nicotine on the expressions of the nAChR subunits alpha3, alpha5, alpha7, beta2, beta3 and beta4, in PC12 cells using Northern blot analysis. NGF increased the transcriptions of alpha5 (1,4- to 2,3-fold), alpha7 (1.7- to 2.5-fold) and beta4 (1.9- to 3.0-fold) subunits, but changes in alpha3 and beta2 subunit levels were smaller than alpha5, alpha7, or beta4. Nicotine also increased the levels of mRNA encoding alpha5 and beta2 subunits. The pattern of subunit mRNA changes was different between NGF and nicotine treatment. No signal for beta3 subunit was detected. The results suggest that NGF changes the expression of nAChR in a subtype-specific manner over the course of the differentiation, and the disproportionate subunit expressions might be related to the neuroprotective effect exerted by nicotine.

Immortalized cementoblasts and periodontal ligament cells in culture

Cementum, a mineralized tissue lining the surface of the tooth root, is required for formation of a functional periodontal ligament attachment during development. Additionally, during regeneration of tissues after disease, cementum is thought to play a critical role in the reparative process. Research efforts aimed toward understanding mechanisms involved in periodontal development and regeneration, and in particular the formation of root cementum, have been hampered by an inability to isolate and culture cells involved in cementum production, i.e., cementoblasts. Using classical techniques for osteoblast isolation, immortalized, heterogeneous cementoblast/periodontal ligament cell (CM/PDL) populations were established from cells lining the tooth root surface of: 1) CD-1 mice, where cells were immortalized using SV40, or 2) H-2KbtsA58 "immorto" mice, where cells containing an immortalizing transgene were removed and cultured. CM/PDL populations were derived from tissues adherent to developing tooth root surfaces, while tissues adherent to the surrounding alveolar bone were specifically excluded from the population. Immortalized CM/PDL cells were characterized to ensure their phenotype reflected that previously demonstrated in situ and in primary, nonimmortalized cultures. Proteins/mRNAs associated with bone/cementum and known to be expressed by root lining cementoblasts, but not by PDL cells, in situ, e.g., bone sialoprotein, osteopontin, and osteocalcin, were expressed by cells within the immortalized populations. Furthermore, CM/PDL cells, in vitro, attached to bone sialoprotein in an arginine-glycineaspartic acid (RGD)-dependent manner, promoted mineral nodule formation and exhibited a PTH/PTHrP-mediated cAMP response. These immortalized heterogeneous populations, containing both CM and PDL cells, provide a unique opportunity to study cells involved in cementogenesis and to enhance our knowledge of the mechanisms controlling development, maintenance, and regeneration of periodontal tissues.

Potential role of insulin-like growth factor binding protein-4 in the uncoupling of bone turnover in multiple myeloma

Decreased bone formation plays an important role in the development of lytic lesions during the late stage of multiple myeloma (MM). Release of insulin-like growth factor binding protein-4 (IGFBP4) by tumour cells adjacent to bone may inhibit IGF-I-stimulated osteoblast growth and contribute to decreased bone formation. The present study demonstrates that the human MM cell line, ARH-77, expresses IGFBP4 and, to a lesser extent, IGFBP6 mRNA and protein. IGFBP4 expression in myeloma cells may be modulated by cytokines released by stromal cells and T cells in the microenvironment. We tested the effect of recombinant interferon-gamma (INF) on IGFBP4 expression in ARH-77. INF increased IGFBP4 mRNA and protein levels at 12 h, with a decline to baseline by 24 h. In contrast, IGFBP4 was not regulated in response to IL-6, TNF-alpha, PDGF BB, bFGF, TGF-beta or the cAMP agonist, forskolin. In other systems. IGFBP4 may also be regulated post-transcriptionally by a protease that is activated by IGF-I or -II. Conditioned medium from ARH-77 cultures incubated with IGF-I or -II for up to 24 h failed to demonstrate proteolytic activity. Proteolysis was also not observed when conditioned medium containing exogenous rhIGFBP4 was incubated with IGF-I or -II under cell-free conditions. To determine if human myeloma tumours also express IGFBP4, total RNA was isolated from four tumour biopsies. All samples expressed detectable levels of IGFBP4 mRNA. These findings indicate that interferon-gamma may indirectly modulate bone formation via the the release of tumour-derived IGFBP4. suggesting that the immune system may influence bone turnover in MM. Failure of myeloma cells to release protease activity may promote IGFBP4 accumulation in the microenvironment during tumour growth.

LMP-1, a LIM-domain protein, mediates BMP-6 effects on bone formation

Glucocorticoids can promote osteoblast differentiation from fetal calvarial cells and bone marrow stromal cells. We recently reported that glucocorticoid specifically induced bone morphogenetic protein-6 (BMP-6), a glycoprotein signaling molecule that is a multifunctional regulator of vertebrate development. In the present study, we used fetal rat secondary calvarial cultures to determine genes induced during early osteoblast differentiation as initiated by glucocorticoid treatment. Glucocorticoid, and subsequently BMP-6, was found to induce a novel rat intracellular protein, LIM mineralization protein-1 (LMP-1), that in turn resulted in synthesis of one or more soluble factors that could induce de novo bone formation. Blocking expression of LMP-1 using antisense oligonucleotide prevented osteoblast differentiation in vitro. Overexpression of LMP-1 using a mammalian expression vector was sufficient to initiate de novo bone nodule formation in vitro and in sc implants in vivo. These data demonstrate that LMP-1 is an essential positive regulator of the osteoblast differentiation program as well as an important intermediate step in the BMP-6 signaling pathway.

The osteocalcin gene: a model for multiple parameters of skeletal-specific transcriptional control

Influences of promoter regulatory elements that are responsive to basal and tissue-restricted transactivation factors, steroid hormones, growth factors and other physiologic mediators has provided the basis for understanding regulatory mechanisms contributing to developmental expression of osteocalcin, tissue specificity and biological activity (reviewed in [1-3]). These regulatory elements and cognate transcription factors support postproliferative transcriptional activation and steroid hormone (e.g. vitamin D) enhancement at the onset of extracellular matrix mineralization during osteoblast differentiation. Three parameters of nuclear structure contribute to osteocalcin gene transcriptional control. The linear representation of promoter elements provides competency for physiological responsiveness within the contexts of developmental as well as phenotype-dependent regulation. Chromatin structure and nucleosome organization reduce distances between independent regulatory elements providing a basis for integrating components of transcriptional control. The nuclear matrix supports gene expression by imposing physical constraints on chromatin related to three dimensional genomic organization. In addition, the nuclear matrix facilitates gene localization as well as the concentration and targeting of transcription factors. Several lines of evidence are presented which are consistent with involvement of multiple levels of nuclear architecture in tissue-specific gene expression during differentiation. Growth factor and steroid hormone responsive modifications in chromatin structure, nucleosome organization and the nuclear matrix are considered which influence transcription of the bone tissue-specific osteocalcin gene during progressive expression of the osteoblast phenotype.

1,25(OH)2D3 and cAMP synergistically induce complement 5a receptor messenger RNA

Complement 5a receptor (C5aR) mediates both acute and chronic participation of monocytes in the immune response. In the human U937 monoblast, C5aR is maximally expressed 4 days after treatment with 1,25(OH)2D3 (or cycloheximide) and prostaglandin E2 combined. The authors asked whether these agents altered expression of C5aR messenger RNA (mRNA). Unstimulated U937 cells expressed neither C5aR mRNA nor C5a binding. Complement 5aR mRNA rose 3 hours after prostaglandin E2 application and fell to basal levels by 12 hours. This early rise in C5aR mRNA did not cause an acute rise in C5a binding, which gradually increased between 1 and 4 days. Neither 1,25(OH)2D3 nor cycloheximide induced expression of C5aR mRNA in the absence of prostaglandin E2 but did enhance prostaglandin E2-stimulated C5aR mRNA expression and C5a binding. The authors observed a late increase in C5aR mRNA at day 3 in treated cells. Inhibition of this late rise in mRNA with 5,6-dichlorobenzimidazole riboside attenuated C5a binding by 65%, indicating its importance in the generation of C5a binding sites. The expression of functional C5aR is, therefore, a complex process involving regulation at transcriptional and posttranscriptional levels.

Contributions of nuclear architecture to transcriptional control

Three parameters of nuclear structure contribute to transcriptional control. The linear representation of promoter elements provides competency for physiological responsiveness within the contexts of development as well as cycle- and phenotype-dependent regulation. Chromatin structure and nucleosome organization reduce distances between independent regulatory elements providing a basis for integrating components of transcriptional control. The nuclear matrix supports gene expression by imposing physical constraints on chromatin related to three-dimensional genomic organization. In addition, the nuclear matrix facilitates gene localization as well as the concentration and targeting of transcription factors. Several lines of evidence are presented that are consistent with involvement of multiple levels of nuclear architecture in cell growth and tissue-specific gene expression during differentiation. Growth factor and steroid hormone responsive modifications in chromatin structure, nucleosome organization, and the nuclear matrix that influence transcription of the cell cycle-regulated histone gene and the bone tissue-specific osteocalcin gene during progressive expression of the osteoblast phenotype are considered.

17 beta-estradiol increases the receptor number and modulates the action of 1,25-dihydroxyvitamin D3 in human osteosarcoma-derived osteoblast-like cells

It is well known that 17 beta-estradiol (E2) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) have important roles in bone metabolism. This study was undertaken to examine E2 regulation of 1,25(OH)2D3 receptor (VDR) expression and the biological action of 1,25(OH)2D3 in human osteoblast-like cells. When human osteosarcoma-derived osteoblast-like cells were treated with varying concentrations of E2, the VDR levels increased by up to 100% in a dose-dependent manner. VDR levels significantly increased at 10 nM E2 and this increase plateaued at 100 nM E2. E2-dependent increase of VDR was time dependent, plateauing at 24 hours and was maintained for at least 48 hours in human osteoblast-like cells. Scatchard analysis showed that E2 increased the number of VDR (12.3 +/- 0.4 versus 26.5 +/- 0.3 fmol/mg protein; mean +/- SE of three independent experiments) rather than the Kd (0.15 +/- 0.02 versus 0.16 +/- 0.01 nM; mean +/- SE of three independent experiments). Tamoxifen (50 nM), a specific competitor with E2, completely abolished the E2-induced increase of VDR. The levels of VDR mRNA (4.5 kb) from the cells increased in a dose-dependent manner after E2 treatment. With regard to the biological effects, E2 increased by 10-25% the inhibitory effect of 1,25(OH)2D3 on cell growth. However, E2 did not increase the stimulation of alkaline phosphatase activity by 1,25(OH)2D3. The present study suggests that E2 modulates the biological action of 1,25(OH)2D3 through VDR levels in bone cells.

Bone mineral metabolism in T lymphocyte-deficient and -replete strains of rat

The immune and skeletal systems are known to interact. We have repeatedly shown that in contrast to in vitro data, the administration of T lymphocyte immunosuppressants, such as cyclosporin A, leads to an increase in bone resorption and a high turnover osteopenia. The purpose of this study was to characterize the bone metabolism of the T lymphocyte deficient Rowett athymic homozygous (rnu/rnu) nude rat. We wished to determine whether these rats share the bone abnormalities of cyclosporin A-treated rats. Eleven 10-week-old Sprague-Dawley rats and 12 similarly aged nude rats were studied over a 4-week period. Metaphyseal cancellous bone histomorphometry was similar in the two groups of rats and only differed with regard to percentage eroded perimeter (lower in nude rats, p = 0.0008) and longitudinal growth rate (49% lower in nude rats, p < 0.001). The nude rats had less body mass (p < 0.001) but nevertheless gained the same percentage of their body weight over the study period. The athymic rats had lower levels of serum, 1,25-dihydroxyvitamin D (p < 0.014) and serum osteocalcin(p < 0.009), and at the age of 14 weeks the nude rats had lower concentrations of serum creatinine (p = 0.001) and blood ionized calcium (p = 0.0002), yet serum PTH was similar throughout. RNA isolated from the contralateral tibias revealed that the nude group had lower steady-state levels of osteocalcin mRNA despite similar rates of bone formation. In its entirety, the data suggest that T cell deficiency per se is not necessarily associated with high turnover osteopenia.

Comparison of the type-2 insulin-like growth factor receptor in normal osteoblasts and osteosarcoma-derived osteoblast-like cells

Insulin-like growth factor-II is known to stimulate the proliferation and differentiation of osteoblasts in part through activation of the type-2 insulin-like growth factor receptor. The present study examined the type-2 insulin-like growth factor receptors of three normal osteoblast-like cells and three osteosarcoma-derived osteoblast-like cells (OGA, SU, and IMAI) from humans. [125I]insulin-like growth factor-II was used for the binding studies. All of the cell types had high affinity binding sites for insulin-like growth factor-II (dissociation constants [Kd] < or = 1 nM). The concentration of these sites was 10 to 24-fold higher in normal osteoblasts than in the osteosarcoma cells studied. Unlabeled insulin-like growth factor-II inhibited the binding of [125I]insulin-like growth factor-II to the cells in a dose-dependent manner; however, unlabeled insulin-like growth factor-I and insulin were less effective. Covalent crosslinking of insulin-like growth factor-II binding sites gave molecular mass estimates of M(r) 250,000 in human osteoblast cells, 250,000 and 130,000 in OGA cells, 240,000 in SU cells, and 250,000 and 130,000 in IMAI cells. Unlabeled insulin-like growth factor-II inhibited all affinity labeling. In Northern blot analysis, the type-2 insulin-like growth factor receptor mRNA of normal osteoblasts was seen in greater abundance than it was in osteosarcoma cells. These results indicate that the numbers of type-2 insulin-like growth factor receptors differ between normal and transformed osteoblasts and that the differential expression of the receptor may be due to the differentiation of osteoblasts.

Comparative study of inhibitory effects by murine interferon gamma and a new bisphosphonate (alendronate) in hypercalcemic, nude mice bearing human tumor (LJC-1-JCK)

The inhibitory effect of murine interferon gamma (muIFN gamma) on humoral hypercalcemia in nude mice bearing lower-jaw cancer (LJC-1-JCK), in which parathyroid-hormone(PTH)-related protein is responsible for causing humoral hypercalcemia by activating bone resorption, was examined in comparison with that of a new bisphosphonate, 4-amino-1-hydroxybutylidene-1,1-bisphosphonate (alendronate). muIFN gamma was injected into tumor-bearing nude mice for 5 days before the establishment of hypercalcemia. The increase of plasma calcium concentration was delayed and this effect continued for more than 6 days even after the injection was stopped. Alendronate markedly suppressed hypercalcemia in tumor-bearing nude mice but this inhibitory effect continued for less than 6 days. Neither muIFN gamma nor alendronate affected the tumor volume or serum PTH-related protein concentration. Injection of muIFN gamma into mice for 3 days almost completely abolished the formation of multinucleated osteoclast-like cells from bone marrow cells in vitro, whereas injection of alendronate into mice had no effect. These findings suggested that muIFN gamma suppressed the formation of osteoclasts, resulting in the prolonged decrease of plasma calcium concentration in hypercalcemic tumor-bearing nude mice, whereas alendronate is cytotoxic to functionally mature osteoclasts and inhibited osteoclastic bone resorption, resulting in a marked decrease in the plasma calcium concentration in tumor-bearing hypercalcemic nude mice.

Tumor necrosis factor alpha decreases 1,25-dihydroxyvitamin D3 receptors in osteoblastic ROS 17/2.8 cells

Bone remodeling is a complex process regulated by systemic hormones, local cytokines, and growth factors. One cytokine, tumor necrosis factor alpha (TNF-alpha), is known to have potent inhibitory effects on osteoblast matrix protein production and to stimulate osteoclast recruitment. We have previously shown that TNF-alpha inhibits 1,25-(OH)2D3-stimulated synthesis of bone gla protein (BGP), an abundant and osteoblast-specific matrix constituent. We hypothesized that the mechanism of TNF-alpha action included inhibition of intracellular 1,25-(OH)2D3 receptor (VDR) number or function. To test this, the osteoblastic cell line ROS 17/2.8 was cultured in the presence or absence of TNF-alpha (100 ng/ml), and binding of [3H]1,25-(OH)2D3 to 0.3 M KCl extracts of cytosol was measured by equilibrium assay. Specific [3H]1,25-(OH)2D3 binding decreased 70%, 25 h after addition of TNF-alpha. The decrease in [3H]1,25-(OH)2D3 binding was seen by 18 h, was sustained throughout the 72 h culture period, and was greater in low-density cultures. Scatchard analysis confirmed that TNF-alpha (100 ng/ml for 24 h) caused a decrease in the number of binding sites without change in VDR affinity. Northern analysis with a VDR riboprobe revealed that the decrease in VDR occurred without a change in the 4.4 kb steady-state VDR mRNA [VDR/cyclophilin mRNA signal ratio: control, 2.25; TNF-alpha, 2.24 (24 h), 2.17 (40 h), n = 2 flasks/time point]. These results suggest that TNF-alpha action on osteoblastic cells includes an inhibitory effect on VDR number at a point distal to the synthesis of VDR mRNA.

Transcriptional control of vitamin D-regulated proteins

Vitamin D is a physiological regulator of gene transcription associated with control of a broad spectrum of biological processes that include but is not restricted to growth, differentiation and calcium-mediated homeostatic control. Transcriptional regulation is mediated by sequence-specific interactions of a 1,25(OH)2D3-vitamin D receptor-accessory factor complex with vitamin D responsive elements (VDRE) residing in the promoters of hormone responsive genes. Functioning primarily as a transcription enhancer, activity at the VDRE is controlled by diverse and integrated cellular signalling pathways acting synergistically and/or antagonistically with a series of basal regulatory elements and other hormone regulated sequences that are components of modularly organized vitamin D-responsive gene promoters. Molecular mechanisms that integrate the activities at promoter elements contributing to vitamin D-related transcriptional control include overlapping transcription factor binding domains within regulatory elements and cooperative activities at independent regulatory sequences that determine the level of vitamin D responsiveness.

Concepts of osteoblast growth and differentiation: basis for modulation of bone cell development and tissue formation

The combined application of molecular, biochemical, histochemical, and ultrastructural approaches has defined a temporal sequence of gene expression associated with development of the bone cell phenotype in primary osteoblast cultures. The peak levels of expressed genes reflect a developmental sequence of bone cell differentiation characterized by three principal periods: proliferation, extracellular matrix maturation and mineralization, and two restriction points to which the cells can progress but cannot pass without further signals. The regulation of cell growth and bone-specific gene expression has been examined during this developmental sequence and is discussed within the context of several unique concepts. These are (1) that oncogene expression in proliferating osteoblasts contributes to the suppression of genes expressed postproliferatively, (2) that hormone modulation of a gene is dependent upon the maturational state of the osteoblast, and (3) that chromatin structure and the presence of nucleosomes contribute to three-dimensional organization of gene promoters that support synergistic and/or antagonistic activities of physiologic mediators of bone cell growth and differentiation.

Osteocalcin: diagnostic methods and clinical applications

Osteocalcin is a small (Mr 5800), very interesting bone specific protein, synthesized by osteoblasts and measured in plasma as a biochemical indicator of bone formation. Many immunoassays for osteocalcin have been developed, including radio- and enzymoimmunoassays, with the use of monoclonal and polyclonal antibodies. These are used in many different clinical settings, including bone, kidney, and liver diseases. However, there is a wide range of published values for plasma osteocalcin concentrations in control and patient samples and this has hindered a more widespread adoption of osteocalcin measurement by clinicians. This review discusses how various immunoassays for osteocalcin may contribute to the wide variation of published values and suggests approaches for the development of standardized assays. For example, epitope specificity and immunoreactivity with multiple forms of osteocalcin and osteocalcin peptides in plasma are discussed. It also includes a recent update on interesting clinical applications of osteocalcin.