Estrogen prevents glucocorticoid-induced apoptosis in osteoblasts in vivo and in vitro

Role of Bcl2 in osteoclastogenesis and PTH anabolic actions in bone

INTRODUCTION

B-cell leukemia/lymphoma 2 (Bcl2) is a proto-oncogene best known for its ability to suppress cell death. However, the role of Bcl2 in the skeletal system is unknown. Bcl2 has been hypothesized to play an important anti-apoptotic role in osteoblasts during anabolic actions of PTH. Although rational, this has not been validated in vivo; hence, the impact of Bcl2 in bone remains unknown.

MATERIALS AND METHODS

The bone phenotype of Bcl2 homozygous mutant (Bcl2(-/-)) mice was analyzed with histomorphometry and muCT. Calvarial osteoblasts were isolated and evaluated for their cellular activity. Osteoclastogenesis was induced from bone marrow cells using RANKL and macrophage-colony stimulating factor (M-CSF), and their differentiation was analyzed. PTH(1-34) (50 microg/kg) or vehicle was administered daily to Bcl2(+/+) and Bcl2(-/-) mice (4 days old) for 9 days to clarify the influence of Bcl2 ablation on PTH anabolic actions. Western blotting and real-time PCR were performed to detect Bcl2 expression in calvarial osteoblasts in response to PTH ex vivo.

RESULTS

There were reduced numbers of osteoclasts in Bcl2(-/-) mice, with a resultant increase in bone mass. Bcl2(-/-) bone marrow-derived osteoclasts ex vivo were significantly larger in size and short-lived compared with wildtype, suggesting a pro-apoptotic nature of Bcl2(-/-) osteoclasts. In contrast, osteoblasts were entirely normal in their proliferation, differentiation, and mineralization. Intermittent administration of PTH increased bone mass similarly in Bcl2(+/+) and Bcl2(-/-) mice. Finally, Western blotting and real-time PCR showed that Bcl2 levels were not induced in response to PTH in calvarial osteoblasts.

CONCLUSIONS

Bcl2 is critical in osteoclasts but not osteoblasts. Osteoclast suppression is at least in part responsible for increased bone mass of Bcl2(-/-) mice, and Bcl2 is dispensable in PTH anabolic actions during bone growth.

Regulation of bim in glucocorticoid-mediated osteoblast apoptosis

Osteoblasts undergo apoptosis both in vitro and in vivo in response to high dose glucocorticoid (GC) treatment. However, the molecular mechanisms remain elusive, hindering the prevention and treatment of this side-effect. Apoptosis was induced by dexamethasone (Dex) in murine MBA-15.4 osteoblasts within 24-48 h of treatment. We found dose- and time-dependent upregulation of Bim protein, a pro-apoptotic Bcl-2 family member, with highest levels at 24-48 h for 1 microM Dex. This was also observed in primary human bone marrow stromal cells. Bim is subjected to stringent transcriptional and post-translational regulation in osteoblasts. Bim mRNA was upregulated in response to 1 microM Dex; both cycloheximide and the GC receptor antagonist, RU486, prevented Dex-induction of Bim protein, indicating transcriptional regulation involving the GC receptor. The proteasome inhibitor, MG132, potently increased Bim protein levels. Bim was also upregulated in osteoblasts undergoing apoptosis in response to serum deprivation and matrix detachment. Gene silencing experiments show that short interference RNA (siRNA) specific for Bim or the downstream effector Bax both reduced apoptosis induced by Dex in osteoblastic cells. These findings suggest that Bim is a novel regulator of osteoblast apoptosis and may be a therapeutic target.

l-Carnitine protects against apoptosis of murine MC3T3-E1 osteoblastic cells

L-carnitine (LC), an amino acid with a major role in cellular energy metabolism, has positive effects on bone metabolism. However, the effect of LC on apoptosis of osteoblast in vitro has not been reported. The aim of this study was to investigate the action of LC on apoptosis of mouse osteoblastic cell line MC3T3-E1. Cell apoptosis was measured by sandwich-enzyme-immunoassay. Release of cytochrome c from mitochondria into cytosol and Bcl-2, Bax protein levels were determined by Western blot analysis. The enzyme substrate was used to assess the activation of caspase-3 and caspase-9. LC inhibited MC3T3-E1 cell apoptosis induced by serum deprivation. Our study also shows that LC decreased cytochrome c release and caspase-3 and caspase-9 activation in serum-deprived MC3T3-E1 cells. Furthermore, LC protected against MC3T3-E1 cell apoptosis induced by the glucocorticoid (GC) dexamethasone (Dex).

Quantifying osteoblast and osteocyte apoptosis: challenges and rewards

Since the initial demonstration of the phenomenon in murine and human bone sections approximately 10 yr ago, appreciation of the biologic significance of osteoblast apoptosis has contributed greatly not only to understanding the regulation of osteoblast number during physiologic bone remodeling, but also the pathogenesis of metabolic bone diseases and the pharmacology of some of the drugs used for their treatment. It is now appreciated that all major regulators of bone metabolism including bone morphogenetic proteins (BMPs), Wnts, other growth factors and cytokines, integrins, estrogens, androgens, glucocorticoids, PTH and PTH-related protein (PTHrP), immobilization, and the oxidative stress associated with aging contribute to the regulation of osteoblast and osteocyte life span by modulating apoptosis. Moreover, osteocyte apoptosis has emerged as an important regulator of remodeling on the bone surface and a critical determinant of bone strength, independently of bone mass. The detection of apoptotic osteoblasts in bone sections remains challenging because apoptosis represents only a tiny fraction of the life span of osteoblasts, not unlike a 6-mo-long terminal illness in the life of a 75-yr-old human. Importantly, the phenomenon is 50 times less common in human bone biopsies because human osteoblasts live longer and are fewer in number. Be that as it may, well-controlled assays of apoptosis can yield accurate and reproducible estimates of the prevalence of the event, particularly in rodents where there is an abundance of osteoblasts for inspection. In this perspective, we focus on the biological significance of the phenomenon for understanding basic bone biology and the pathogenesis and treatment of metabolic bone diseases and discuss limitations of existing techniques for quantifying osteoblast apoptosis in human biopsies and their methodologic pitfalls.

Effect of raloxifene treatment on osteocyte apoptosis in postmenopausal women

Increased osteocyte apoptosis, as the result of estrogen deficiency, could play a role in the decrease of bone mass and bone strength seen in postmenopausal osteoporosis. We investigated whether treatment with raloxifene of postmenopausal women with osteoporosis affects osteocyte apoptosis. Transiliac bone biopsies were obtained from 26 osteoporotic women at baseline and after 2 years of treatment with placebo or raloxifene. Immunohistochemical detection of cleaved caspase-3 was performed on sections from nondecalcified bone biopsies to visualize apoptosis. In the trabecular bone total osteocytes, positively stained osteocytes and empty lacunae were counted and percent positive cells and percent empty lacunae determined. Statistical evaluation was performed by Wilcoxon's paired t-test and Spearman's rank correlations. There was no significant difference in percentage positive osteocytes between baseline and follow-up biopsies in both the placebo and the raloxifene groups. The percentage empty lacunae increased significantly in the placebo group (11.20 +/- 1.43 vs. 9.00 +/- 2.25, P = 0.014) but not in the raloxifene group. At baseline in both groups combined, there was a negative correlation between indices of bone remodeling and the percentage positive osteocytes (bone formation rate/bone volume r = -0.67, P = 0.001). We found no direct evidence for an effect of raloxifene treatment on osteocyte apoptosis, but small effects of raloxifene treatment cannot be excluded. The percent of apoptotic osteocytes was dependent on the level of bone remodeling in an individual.

Glucocorticoids induce osteocyte apoptosis by blocking focal adhesion kinase-mediated survival. Evidence for inside-out signaling leading to anoikis

Bone fragility induced by chronic glucocorticoid excess is due, at least in part, to induction of osteocyte apoptosis through direct actions on these cells. However, the molecular mechanism by which glucocorticoids shorten osteocyte life span has remained heretofore unknown. We report that apoptosis of osteocytic MLO-Y4 cells induced by the synthetic glucocorticoid dexamethasone is abolished by the glucocorticoid receptor antagonist RU486, but not by inhibition of protein or RNA synthesis. Dexamethasone-induced apoptosis is preceded by a decrease in the number of cytoplasmic processes, an indicator of cell detachment. In addition, the focal adhesion kinase FAK prevents dexamethasone-induced apoptosis, whereas the FAK-related kinase Pyk2 increases the basal levels of apoptosis. Dexamethasone-induced apoptosis is also prevented in cells expressing kinase-deficient or phosphorylation-defective (Y402F) dominant negative mutants of Pyk2. Consistent with the requirement of tyrosine 402, dexamethasone induces rapid Pyk2 phosphorylation in this residue. Moreover, knocking down Pyk2 expression abolishes apoptosis and cell detachment induced by dexamethasone, and transfection with human Pyk2 rescues both responses. Furthermore, induction of apoptosis as well as cell detachment by dexamethasone is abolished by inhibiting the activity of JNK, a recognized downstream target of Pyk2 activation. These results demonstrate that glucocorticoids promote osteocyte apoptosis via a receptor-mediated mechanism that does not require gene transcription and that is mediated by rapid activation of Pyk2 and JNK, followed by inside-out signaling that leads to cell detachment-induced apoptosis or anoikis.

Bone-targeted overexpression of Bcl-2 increases osteoblast adhesion and differentiation and inhibits mineralization in vitro

Apoptosis is a process important for the development and homeostasis of self-renewing tissues, including bone. However, little is known about the function of Bcl-2, a key player of apoptosis, in the regulation of osteoblast activity. Ex vivo cultures of osteoblasts from Col2.3Bcl-2 mice, in which human Bcl-2 was targeted to bone by the 2.3 kb fragment of the type I collagen promoter, were used to study the effect of Bcl-2 in osteoblasts. During 35 days of culture, hBcl-2 expression increased without any effect on endogenous mouse Bcl-2 and Bax expression. Adhesion of transgenic (TG) osteoblasts was twofold more than that of wild-type (WT) cells, with significantly higher expression of integrins alpha(1), alpha(2), and alpha(5) but similar levels of alpha(v) and beta(1) relative to WT cells. Proliferation of osteoblasts was not affected. Overexpression of hBcl-2 promoted the differentiation of osteoblasts, as shown by increased message levels of alkaline phosphatase, type I collagen, bone sialoprotein, and osteocalcin in the TG compared to WT cells throughout the culture period. The two transcription factors essential for osteoblast differentiation, core binding factor alpha 1 (Cbfa-1) and osterix, had significantly higher expression in TG than WT cells during the early culture period. ss-Catenin, a central player in the canonical Wnt pathway, also had higher expression in TG than WT cultures. Mineralization was significantly decreased in TG cultures, with less osteoblast apoptosis, compared to WT. Thus, Bcl-2 seems to have multiple roles in modulating osteoblast activities.

Osteogenic gene expression decreases in stromal cells of patients with osteonecrosis

Nontraumatic osteonecrosis is related to alcohol and glucocorticoid with unknown pathogenesis. Increased adipogenesis decreases bone morphogenetic protein 2 (BMP2) gene expression after glucocorticoid treatment. Lovastatin enhances BMP2 gene expression in rodents, reverses the effects of glucocorticoids on bone, and prevents glucocorticoid-induced osteonecrosis in chickens and humans. We hypothesized patients with osteonecrosis are more susceptible to glucocorticoid treatment than patients without osteonecrosis. Marrow stromal cell cultures from 14 patients with osteonecrosis, and 10 patients without osteonecrosis were treated with dexamethasone (0.1 micromol/L), lovastatin (1 micromol/L), or combined treatment. BMP2 and osteocalcin gene expression were evaluated by reverse-transcriptase polymerase chain reaction and real-time polymerase chain reaction. The suppression of BMP2 by dexamethasone was more pronounced and the enhancement by lovastatin was less pronounced in the osteonecrosis group. Dexamethasone suppressed osteocalcin in the osteonecrosis group. Among the subgroups of osteonecrosis, suppression of BMP2 and osteocalcin by dexamethasone occurred in glucocorticoid-induced osteonecrosis group. Our data suggest individuals who are more susceptible to a glucocorticoid-induced decreases in BMP2 and osteocalcin gene expression are more likely to have osteonecrosis, especially glucocorticoid-induced osteonecrosis.

Apelin suppresses apoptosis of human osteoblasts

OBJECTIVES

Apelin is a recently discovered peptide that is the endogenous ligand for the orphan G-protein-coupled receptor APJ. Adipocytes can express and secrete apelin. Osteoblast can express apelin and APJ. The aim of this study was to investigate the action of apelin on apoptosis of human osteoblasts.

RESULTS

Apelin inhibited human osteoblasts apoptosis induced by serum deprivation. Suppression of APJ with small-interfering RNA (siRNA) abolished the anti-apoptotic activity of apelin. Our study also showed an increased Bcl-2 protein expression and decreased Bax protein expression under the treatment of apelin. Apelin decreased cytochrome c release and caspase-3 activation in human osteoblasts. Apelin activated phosphatidylinositol-3 kinase (PI-3 kinase) and Akt. The apelin-induced activation of Akt was blocked by suppression of APJ with siRNA. LY294002 (a PI-3 kinase inhibitor) or 1L-6-hydroxymethyl-chiro-inositol 2-(R)-2-O-methyl-3-O-octadecylcarbonate (HIMO; an Akt inhibitor) abolished apelin induced activation of Akt, and, LY294002 or HIMO abolished the anti-apoptotic activity of apelin. Furthermore, apelin protects against apoptosis induced by the glucocorticoid dexamethasone.

CONCLUSIONS

Apelin suppresses serum deprivation-induced apoptosis of human osteoblasts and the anti-apoptotic action is mediated via the APJ/PI-3 kinase/Akt signaling pathway.

Osteogenic growth peptide effects on primary human osteoblast cultures: potential relevance for the treatment of glucocorticoid-induced osteoporosis

The osteogenic growth peptide (OGP) is a naturally occurring tetradecapeptide that has attracted considerable clinical interest as a bone anabolic agent and hematopoietic stimulator. In vivo studies on animals have demonstrated that the synthetic peptide OGP (10-14), reproducing the OGP C-terminal active portion [H-Tyr-Gly-Phe-Gly-Gly-OH] increases bone formation, trabecular bone density and fracture healing. In vitro studies performed on cellular systems based on osteoblastic-like cell lines or mouse stromal cells, have demonstrated that OGP (10-14) increases osteoblast proliferation, alkaline phosphatase (ALKP) activity and matrix synthesis and mineralization. In view of a potential application of OGP (10-14) in clinical therapy, we have tested different concentrations of OGP (10-14) on primary human osteoblast (hOB) cultures. We have observed significant increases of hOB proliferation (+35%), ALKP activity (+60%), osteocalcin secretion (+50%), and mineralized nodules formation (+49%). Our experimental model based on mature hOBs was used to investigate if OGP (10-14) could prevent the effects on bone loss induced by sustained glucocorticoid (GC) treatments. A strong decrease in bone formation has been attributed to the effects of GCs on osteoblastogenesis and osteocyte apoptosis, while an increase in bone resorption was due to a transient osteoblastic stimulation, mediated by the OPG/RANKL/RANK system, of osteoclasts recruitment and activation. Moreover, GCs act on hOBs decreasing the release of osteoprotegerin (OPG) a regulator of the RANKL/RANK interaction. Here, we provide evidences that OGP (10-14) inhibits hOB apoptosis induced by an excess of dexamethasone (-48% of apoptotic cells). Furthermore, we show that OGP (10-14) can increase OPG secretion (+20%) and can restore the altered expression of OPG induced by GCs to physiological levels. Our results support the employment of OGP (10-14) in clinical trials addressed to the treatment of different bone remodeling alterations including the GC-induced osteoporosis.

Impaired cortical bone acquisition and osteoblast differentiation in mice with osteoblast-targeted disruption of glucocorticoid signaling

To determine the role of endogenous glucocorticoids in bone, we previously developed transgenic mice in which a 2.3 kb fragment of the Col1a1 promoter drives 11beta-hydroxysteroid dehydrogenase 2 expression in mature osteoblasts. This transgene should inactivate glucocorticoids upstream of all receptor signaling pathways. In the present study, we show that femoral cortical bone area and thickness were approximately 10-15% lower in transgenic mice than in wild-type littermates. Femur length was unchanged, indicating that bone elongation was not affected in this model. Expression of osteocalcin mRNA, pOBCol2.3-GFP (a green fluorescent protein marker of mature osteoblasts), and the formation of mineralized nodules were impaired in ex vivo transgenic primary calvarial cultures. The extent of crystal violet staining in bone marrow cultures, indicative of the number of adherent stromal cells, was also decreased. These data suggest that endogenous glucocorticoids are required for cortical bone acquisition and full osteoblast differentiation. It appears that blocking glucocorticoid signaling in vivo leads to a decrease in the commitment and/or expansion of progenitors entering the osteoblast lineage.

High osteoblastic activity in C3H/HeJ mice compared to C57BL/6J mice is associated with low apoptosis in C3H/HeJ osteoblasts

This study sought to confirm that osteoblasts of C3H/HeJ (C3H) mice, which have higher differentiation status and bone-forming ability compared to C57BL/6J (B6) osteoblasts, also have a lower apoptosis level and to test whether the higher differentiation status and bone-forming ability of C3H osteoblasts were related to the lower apoptosis. C3H mice had 50% fewer (P < 0.01) apoptotic osteoblasts on the endocortical bone surface than B6 mice as determined by the TUNEL assay. Primary C3H osteoblasts in cultures also showed a 50% (P < 0.05) lower apoptosis level than B6 osteoblasts assayed by acridine orange/ethidium bromide staining of apoptotic osteoblasts. The lower apoptosis in C3H osteoblasts was accompanied by 22% (P < 0.05) and 56% (P < 0.001) reduction in the activity of total caspases and caspases 3/7, respectively. C3H osteoblasts also displayed greater alkaline phosphatase (ALP) activity (P < 0.001) and higher expression of Cbfa1, type-1 collagen, osteopontin, and osteocalcin genes (P < 0.05 for each). To assess if an association existed between population apoptosis and the differentiation status (ALP-specific activity) and/or bone-forming activity (insoluble collagen synthesis), C3H and B6 osteoblasts were treated with several apoptosis enhancers (tumor necrosis factor-alpha, dexamethasone, lipopolysaccharide, etoposide) and inhibitors (parathyroid hormone, insulin-like growth factor I, transforming growth factor beta1, estradiol). Both ALP (r = -0.61, P < 0.001) and insoluble collagen synthesis (r = -0.61, P < 0.001) were inversely correlated with apoptosis, suggesting that differentiation (maturation) and/or bone-forming activity of these mouse osteoblasts were inversely associated with apoptosis. In conclusion, these studies support the premise that higher bone density and bone formation rate in C3H mice could be due in part to lower apoptosis in C3H osteoblasts.

Proteome analysis for the identification ofin vivo estrogen-regulated proteins in bone

Estrogen deficiency results in a reduced bone mass, which can be prevented by treatment with estrogens. This study used a proteomic approach for the first time to obtain a global perspective of estrogens' effects on whole-bone proteins. Bone proteome profiles were examined in three groups of mice: (1) sham-operated with normal ovarian functions, (2) ovariectomised and (3) ovariectomised with estrogen replacement therapy. Bone proteins extracted from the humerus were separated by 2-DE and visualised by CBB colloidal staining. Spot detection and quantification was done by image analysis. Differentially expressed proteins were identified by MS and database search, using peptide mass fingerprint and peptide sequence analysis. Differential expression analysis in the three experimental groups showed significant changes for 14 proteins. These included proteins related to bone metabolism, cytoskeleton components and energy metabolic pathways. Our data suggest that some proteins related to cytoskeleton and to energy pathways, such as tropomyosins, aconitase 2 and enolase beta, might be new molecular targets responsive to the effects of estrogen. Differentially expressed proteins identified in this model may offer a useful starting point for elucidating novel aspects of the pleiotropic effects of estrogens on bone.

Estrogen protects primary osteocytes against glucocorticoid-induced apoptosis

Glucocorticoid-induced osteoporosis may be at least in part due to the increased apoptosis of osteocytes. To study the role of osteocyte apoptosis in glucocorticoid-induced osteoporosis, we isolated primary osteocytes from murine calvaria for the analysis of the effects of dexamethasone in in vitro culture. The cells were identified by morphology, cytochemical staining, immunocytochemical staining and mRNA expression of phosphate-regulating gene with homology to endopeptidases on the X chromosome (PHEX) and sclerosteosis/van Buchem disease gene (SOST). We found that dexamethasone induced osteocyte apoptosis in a dose-dependent manner. A glucocorticoid receptor antagonist, mifepristone (RU486), suppressed dexamethasone-induced osteocyte apoptosis, suggesting that it was mediated by glucocorticoid receptor. Immunocytochemical stainings showed that glucocorticoid receptors are present in primary osteocytes, and they were translocated to nuclei after the exposure to dexamethasone. Addition of estrogen prevented glucocorticoid receptor translocation into nuclei. Corresponding antiapoptotic effects in primary osteocytes were also seen after the pretreatment of primary osteocytes with a picomolar concentration of estrogen. The pure antiestrogen ICI 182,780 inhibited estrogen effect on apoptosis induced by dexamethasone. These data suggest that glucocorticoid receptors play an important role in glucocorticoid-induced osteocyte apoptosis. Most importantly, estrogen has a protective effect against osteocyte apoptosis. To conclude, the mechanism of glucocorticoid-induced osteoporosis may be due to the apoptosis of osteocytes, which can be opposed by estrogen.

Effect of osteoblast-targeted expression of bcl-2 in bone: differential response in male and female mice

Transgenic mice (Col2.3Bcl-2) with osteoblast-targeted human Bcl-2 expression were established. Phenotypically, these mice were smaller than their wildtype littermates and showed differential effects of the transgene on bone parameters and osteoblast activity dependent on sex. The net effect was an abrogation of sex differences normally observed in wildtype mice and an inhibition of bone loss with age. Ex vivo osteoblast cultures showed that the transgene had no effect on osteoblast proliferation, but decreased bone formation. Estrogen was shown to stimulate endogenous Bcl-2 message levels. These studies suggest a link between Bcl-2 and sex regulation of bone development and age-related bone loss.

INTRODUCTION

Whereas Bcl-2 has been shown to be an important regulator of apoptosis in development, differentiation, and disease, its role in bone homeostasis and development is not well understood. We have previously showed that the induction of glucocorticoid-induced apoptosis occurred through a dose-dependent decrease in Bcl-2. Estrogen prevented glucocorticoid-induced osteoblast apoptosis in vivo and in vitro by preventing the decrease in Bcl-2 in osteoblasts. Therefore, Bcl-2 may be an important regulator of bone growth through mechanisms that control osteoblast longevity and function.

MATERIALS AND METHODS

Col2.3Bcl-2 mice were developed carrying a 2.3-kb region of the type I collagen promoter driving 1.8 kb of human Bcl-2 (hBcl-2). Tissue specific expression of hBcl-2 in immunoassays validated the transgenic animal model. Histomorphometry and DXA were performed. Proliferation, mineralization, and glucocorticoid-induced apoptosis were examined in ex vivo cultures of osteoblasts. The effect of estrogen on mouse Bcl-2 in ex vivo osteoblast cultures was assayed by RT-PCR and Q-PCR.

RESULTS AND CONCLUSIONS

Two Col2.3Bcl-2 (tg/+) founder lines were established and appeared normal except that they were smaller than their nontransgenic wildtype (+/+) littermates at 1, 2, and 6 months of age, with the greatest differences at 2 months. Immunohistochemistry showed hBcl-2 in osteoblasts at the growth plate and cortical surfaces. Nontransgenic littermates were negative. Western blots revealed hBcl-2 only in type I collagen-expressing tissues. Histomorphometry of 2-month-old mice showed a significant decrease in tg/+ calvaria width with no significant differences in femoral trabecular area or cortical width compared with +/+. However, tg/+ males had significantly more trabecular bone than tg/+ females. Female +/+ mice showed increased bone turnover with elevated osteoblast and osteoclast parameters compared with +/+ males. Col2.3Bcl-2 mice did not show such significant differences between sexes. Male tg/+ mice had a 76.5 +/- 1.5% increase in ObS/BS with no significant differences in bone formation rate (BFR) or mineral apposition rate (MAR) compared with male +/+ mice. Transgenic females had a significant 48.4 +/- 0.1% and 20.1 +/- 5.8% decrease in BFR and MAR, respectively, compared with +/+ females. Osteoclast and osteocyte parameters were unchanged. By 6 months, femurs from female and male +/+ mice had lost a significant amount of their percent of trabecular bone compared with 2-month-old mice. There was little to no change in femoral bone in the tg/+ mice with age. Ex vivo cultures of osteoblasts from +/+ and Col2.3Bcl-2 mice showed a decrease in mineralization, no effect on proliferation, and an inhibition of glucocorticoid-induced apoptosis in Col2.3Bcl-2 cultures. Estrogen was shown to increase mouse Bcl-2 transcript levels in osteoblast cultures of wildtype mice, supporting a role for Bcl-2 in the sex-related differences in bone phenotype regulated by estrogen. Therefore, Bcl-2 differentially affected bone phenotype in male and female transgenic mice, altered bone cell activity associated with sex-related differences, and decreased bone formation, suggesting that apoptosis is necessary for mineralization. In addition, Bcl-2 targeted to mature osteoblasts seemed to delay bone development, producing a smaller transgenic mouse compared with wildtype littermates. These studies suggest that expression of Bcl-2 in osteoblasts is important in regulating bone mass in development and in the normal aging process of bone.

Regulation of apoptosis in osteoclasts and osteoblastic cells

In postnatal life, the skeleton undergoes continuous remodeling in which osteoclasts resorb aged or damaged bone, leaving space for osteoblasts to make new bone. The balance of proliferation, differentiation, and apoptosis of bone cells determines the size of osteoclast or osteoblast populations at any given time. Bone cells constantly receive signals from adjacent cells, hormones, and bone matrix that regulate their proliferation, activity, and survival. Thus, the amount of bone and its microarchitecture before and after the menopause or following therapeutic intervention with drugs, such as sex hormones, glucocorticoids, parathyroid hormone, and bisphosphonates, is determined in part by effects of these on survival of osteoclasts, osteoblasts, and osteocytes. Understanding the mechanisms and regulation of bone cell apoptosis will enhance our knowledge of bone cell function and help us to develop better therapeutics for the management of osteoporosis and other bone diseases.

Low-dose parathyroid hormone and estrogen reverse alkaline phosphatase activity suppressed by dexamethasone in mouse osteoblastic cells

Glucocorticoid (GC)-induced osteoporosis (GIO) is frequently seen in patients with excessive GC. Numerous questions remain to be clarified about the pathogenesis and treatment of GIO, and the mechanism of GC-inhibited bone formation is not well known. Several studies suggest that parathyroid hormone (PTH) and hormone replacement therapy are effective for GIO. We therefore investigated whether PTH and estrogen would affect cell proliferation and alkaline phosphatase (ALP) activity inhibited by dexamethasone (Dex) in mouse osteoblastic cell-line MC3T3-E1 cells. Low-dose (10(-11) M) PTH as well as 10(-8) M 17-beta-estradiol (17beta-E2) significantly attenuated Dex-inhibited ALP activity, although 10(-8) M PTH did not affect it. ICI 182780 (10(-8) M) antagonized the effects of 17beta-E(2) on Dex-suppressed ALP activity. Neutralizing anti-IGF-I antibody (3 microg/ml) blocked the reverse effects of 17beta-E2 on ALP activity suppressed by Dex. PTH (10(-11) M), but not 17beta-E2, significantly attenuated [3H]thymidine incorporation inhibited by Dex. On the other hand, PTH and estrogen did not affect the level of 11-beta-hydrosteroid dehydrogenase type I mRNA increased by Dex. In conclusion, the present study demonstrated that low-dose PTH and estrogen reversed Dex-inhibited ALP activity in the mouse osteoblastic cell-line.

Differential transcriptional effects of PTH and estrogen during anabolic bone formation

The aim of this study was to compare transcriptional regulation in vivo during anabolic bone formation induced by either estradiol (E2) treatment or intermittent parathyroid hormone[1-34] (PTH) therapy. We utilized an ovariectomized (OVX) mouse model of osteoporosis and transcriptional profiling to identify genes upregulated by either high-dose E2 or PTH. Five weeks post-OVX, the mice were administered either E2 and/or PTH, or vehicle for 4 weeks. Femoral bones were analyzed by microCT and histomorphometry to confirm the anabolic effect of each treatment. OVX vehicle-treated control mice lost metaphyseal trabecular bone, with significant decrease in trabecular number, thickness, and connectivity. Both E2 and PTH treatments increased trabecular and cortical bone indices above the level of the sham operated controls, fully restoring both bone volume and bone mineral density (BMD). Moreover, PTH/E2 combination treatment led to significantly greater increase in cancellous bone and BMD than would be expected from the additive effects of the separate treatments. To determine whether PTH and E2 treatments were stimulating similar bone anabolic mechanisms, or were activating distinct signaling pathways, we compared patterns of gene expression using transcriptional profiling after either E2 or PTH treatment. After 4, 11, and 24 days of treatment, total RNA was collected from both the distal femoral metaphysis and diaphysis. Transcriptional profiling was performed using Affymetrix GeneChip probe arrays, comprised of approximately 36,000 full-length mouse genes and EST clusters from the UniGene database. Several markers of osteoblast activity, including c-fos, RANKL, PHEX, and PTHR1, were consistently upregulated by PTH in both skeletal sites. PTH treatment also increased expression of Cathespin K, consistent with the predicted increase in osteoclast activity. E2 treatment upregulated a largely distinct set of genes, including TGFbeta3, and BMP1, as well as several genes critical for cell cycle control, including Cyclin D1 and CDK inhibitor 1A. Overall, comparison of transcriptional profiles suggest that anabolic responses in bone to PTH and high-dose E2 treatment after OVX-induced osteoporosis involve largely distinct patterns of gene regulation, each resulting in restoration of bone mass.

Prevention of glucocorticoid-induced apoptosis in osteocytes and osteoblasts by calbindin-D28k

This study show for the first time that calbindin-D28k can prevent glucocorticoid-induced bone cell death. The anti-apoptotic effect of calbindin-D28k involves inhibition of glucocorticoid induced caspase 3 activation as well as ERK activation.

INTRODUCTION

Recent studies have indicated that deleterious effects of glucocorticoids on bone involve increased apoptosis of osteocytes and osteoblasts. Because the calcium-binding protein calbindin-D28k has been reported to be anti-apoptotic in different cell types and in response to a variety of insults, we investigated whether calbindin-D28k could protect against glucocorticoid-induced cell death in bone cells.

MATERIALS AND METHODS

Apoptosis was induced by addition of dexamethasone (dex; 10-6 M) for 6 h to MLO-Y4 osteocytic cells as well as to osteoblastic cells. Apoptosis percentage was determined by examining the nuclear morphology of transfected cells. Caspase 3 activity was evaluated in bone cells and in vitro. SELDI mass spectrometry (MS) was used to examine calbindin-D28k-caspase 3 interaction. Phosphorylation of calbindin-D28k was examined by 32P incorporation as well as by MALDI-TOF MS. ERK activation was determined by Western blot.

RESULTS

The pro-apoptotic effect of dex in MLO-Y4 cells was completely inhibited in cells transfected with calbindin-D28k cDNA (5.6% apoptosis in calbindin-D28k transfected cells compared with 16.2% apoptosis in vector-transfected cells, p < 0.05). Similar results were observed in osteoblastic cells. We found that dex-induced apoptosis in bone cells was accompanied by an increase in caspase 3 activity. This increase in caspase 3 activity was inhibited in the presence of calbindin-D28k. In vitro assays indicated a concentration-dependent inhibition of caspase 3 by calbindin-D28k (Ki = 0.22 microM). Calbindin-D28k was found to inhibit caspase 3 specifically because the activity of other caspases was unaffected by calbindin-D28k. The anti-apoptotic effect of calbindin-D28k in response to dex was also reproducibly associated with an increase in the phosphorylation of ERK 1 and 2, suggesting that calbindin-D28k affects more than one signal in the glucocorticoid-induced apoptotic pathway.

CONCLUSION

Calbindin-D28k, a natural non-oncogenic protein, could be an important target in the therapeutic intervention of glucocorticoid-induced osteoporosis.

Immunocytochemical evidence on the effects of glucocorticoids on type I collagen synthesis in human osteoblastic cells

The effects of glucocorticoids on DNA synthesis and cellular function were assessed in cultures of human osteoblastic cells by using indirect immunoperoxidase staining with a type I antiprocollagen antibody and by measuring procollagen type I N and C propeptides (PINP, PICP) in the culture medium by radiometric methods. Likewise, we analyzed the correlation between intracellular immunostaining and procollagen propeptides released into the culture medium, as well as the correlation between PINP and PICP. Human osteoblasts were cultured with and without addition of dexamethasone (DEX) at two supraphysiological concentrations, 10(-6) M and 10(-7) M, for 24 and 48 h. Treatment with DEX at 10(-6) M was associated with a significant decrease in the percentage of cells showing intracellular type I procollagen immunoreactivity at 24 and 48 h ( P < 0.05). Similar effects were observed with 10(-7) M DEX. Dexamethasone 10(-6) M and 10(-7) M also induced significant decreases in PINP and PICP values after 24 and 48 h of treatment ( P < 0.05). The decrease in intracellular procollagen immunoreactivity and propeptide secretion was not associated with a reduction in DNA synthesis. A highly significant correlation was observed between the values of PINP and PICP in the culture medium as well as between the values of intracellular immunostaining and PINP and PICP ( P < 0.001). In conclusion, our results suggest that supraphysiological doses of glucocorticoids produce a direct inhibition on osteoblastic function through their effect on type I procollagen synthesis. Immunoperoxidase detection of type I intracellular procollagen as well as the quantification of PINP and PICP in the culture medium are reliable methods of assessing osteoblast function.

Transgenic expression of 11beta-hydroxysteroid dehydrogenase type 2 in osteoblasts reveals an anabolic role for endogenous glucocorticoids in bone

Glucocorticoid excess leads to bone loss, primarily by decreasing bone formation. However, a variety of in vitro models show that glucocorticoids can promote osteogenesis. To elucidate the role of endogenous glucocorticoids in bone metabolism, we developed transgenic (TG) mice in which a 2.3-kb Col1a1 promoter fragment drives 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) expression in mature osteoblasts. 11beta-HSD2 should metabolically inactivate endogenous glucocorticoids in the targeted cells, thereby reducing glucocorticoid signaling. The inhibitory effect of 300 nm hydrocortisone on percent collagen synthesis was blunted in TG calvariae, demonstrating that the transgene was active. Collagen synthesis rates were lower in TG calvarial organ cultures compared with wild-type. Trabecular bone parameters measured by microcomputed tomography were reduced in L3 vertebrae, but not femurs, of 7- and 24-wk-old TG females. These changes were also not seen in males. In addition, histomorphometry showed that osteoid surface was increased in TG female vertebrae, suggesting that mineralization may be impaired. Our data demonstrate that endogenous glucocorticoid signaling is required for normal vertebral trabecular bone volume and architecture in female mice.

The effects of patient age on human osteoblasts' response to Ti-6Al-4V implants in vitro

Osseointegrated implants are a common therapy for the elderly population as lifespan increases. Understanding the effects of age and sex on osseointegration is important for successful implant therapy. Therefore, the response of primary human osteoblasts (HOB) to implant materials was studied. HOBs were obtained by outgrowth of cells from bone from orthopaedic procedures and categorized as Young (Y), <15; Middle (M), 30-50; and Old (O), >60 years old. Initially the HOB phenotype was determined on tissue culture plastic. Alkaline phosphatase (ALP) staining and activity were significantly increased in HOBs from older patients. Message levels of type I collagen (COL), bone sialoprotein (BSP) and ALP were significantly higher (from 2.3- to 3.8-fold) in Y subjects compared to M and O patients at 2 weeks. Studies of the response of HOBs to implant materials were undertaken using Ti-6Al-4V disks prepared in a manner similar to orthopaedic implants. A 1.4-fold (p<0.05) increase in cell attachment was found in HOBs from Y compared with O in female subjects but not in male subjects. Cell proliferation at 24 h was not significantly different by age or sex, nor was DNA content different at 2 and 4 weeks. Mineralization in HOB-implant cultures was 2.3-fold higher in Y than in O, and 1.7-fold higher in Y compared to M HOBs from female but not male subjects at 4 weeks. Northern blot and RT-PCR analysis at 2 weeks of culture showed significantly higher levels (1.6-2.3-fold) of COL, BSP, and osteocalcin (OC) mRNAs in Y HOBs compared to M and O HOBs from female subjects. We conclude that human osteoblasts from older female patients have a decreased ability to form bone on implants.

Estrogen potentiates the combined effects of transforming growth factor-beta and tumor necrosis factor-alpha on adult human osteoblast-like cell prostaglandin E2 biosynthesis

Reports that estrogen treatment modulates arachidonic acid metabolism by bone and bone cells are found in the literature. However, conflicting indications of the relationship that exists between estrogen and arachidonic acid metabolism emerge from the analysis of those reports. The present studies were undertaken to determine if estrogen effected the production of prostaglandins (PG) in human osteoblast-like (hOB) cell cultures derived from adults, under basal or cytokine-stimulated conditions. A 48-hour estrogen pretreatment did not modify hOB cell PG biosynthesis on a qualitative basis, and PGE2 formation predominated under all tested conditions. Estrogen pretreatment did lead to increased PGE2 production in specimens stimulated conjointly with transforming growth factor-beta1 and tumor necrosis factor-alpha ( p < 0.001). No changes in PGE2 production were observed in estrogen pretreated specimens stimulated singly with either tested cytokine, nor in samples in which either TGFbeta or TNF was replaced by interleukin-1beta. Anti-estrogen (ICI 164,384) inclusion prevented the estrogen-dependent increase in PGE2 production in the TGFbeta plus TNF-stimulated samples. These results suggest that an estrogen effect on bone cell prostaglandin biosynthesis may be most evident and significant under conditions in which the cells are exposed to multiple osteotropic cytokines, a condition that applies during the bone remodeling process.

Runx2 integrates estrogen activity in osteoblasts

Steroids significantly effect skeletal integrity. For example, bone mass decreases with glucocorticoid excess or with estrogen depletion after menopause. Glucocorticoid suppresses gene expression by an essential skeletal tissue transcription factor, Runx2, in rat osteoblasts. We now report that estrogen enhances Runx2 activity in dose- and estrogen receptor-dependent ways independently of changes in Runx2 levels or its DNA binding potential. Estrogen receptor and Runx2 can be collected by co-immunoprecipitation. By two-hybrid gene expression analysis, high affinity complex formation involves portions of Runx2 outside of its own DNA binding domain and the DNA binding domain of the estrogen receptor. Consistent with this interaction, the stimulatory effect of estrogen on Runx2 activity is lost when the DNA binding domain of the estrogen receptor is eliminated. Unlike the stimulatory effect of estrogen and the inhibitory effect of glucocorticoid, androgen fails to increase Runx2 activity, whereas Runx2 potently suppresses gene expression induced by all three steroids. Finally, estrogen increases gene transcription by the transforming growth factor-beta type I receptor gene promoter, which contains several Runx binding sequences, and enhances Smad dependent gene expression by transforming growth factor-beta in osteoblasts. These results reveal that Runx2 can integrate complex effects on gene transcription in hormone-, growth factor-, and tissue-restricted ways.

Dexamethasone induces caspase activation in murine osteoblastic MC3T3-E1 cells

Glucocorticoids are widely used as anti-inflammatory and chemotherapeutic agents. However, prolonged use of glucocorticoids leads to osteoporosis. This study was designed to examine the mechanism of dexamethasone (DEX)-induced apoptosis in murine osteoblastic MC3T3-E1 cells. Total RNA was extracted from MC3T3-E1 cells treated with 10(-7) M DEX for 6 h. DEX exerted a variety of effects on apoptotic gene expression in osteoblasts. Ribonuclease protection assays (RPA) revealed that DEX upregulated mRNA levels of caspases-1, -3, -6, -8, -11, -12, and bcl-XL. Western blot analysis showed enhanced processing of these caspases, with the appearance of their activated enzymes 8 h after DEX treatment. In addition, DEX also induced the activation of caspase-9. DEX elevated the levels of cleaved poly(ADP-ribose) polymerase and lamin A, a caspase-3 and a caspase-6 substrate, respectively. Expression of bcl-XL protein level was upregulated by DEX. Cytochrome c release was detected in the cytosol of DEX-treated cells. Furthermore, caspase-3 enzyme activity was elevated by 2-fold after DEX treatment for 7 h. Finally, early apoptotic cells were detected in cells treated with DEX for 3 h. Our results demonstrate that DEX-induced apoptosis involves gene activation of a number of caspases.

The phospholipids sphingosine-1-phosphate and lysophosphatidic acid prevent apoptosis in osteoblastic cells via a signaling pathway involving G(i) proteins and phosphatidylinositol-3 kinase

The naturally occurring phospholipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have recently emerged as bioactive compounds that exert mitogenic effects in many cell types, including osteoblasts. In the current study, we examined the ability of each of these compounds to influence osteoblast survival. Using terminal deoxynucleotidyl transferase-mediated deoxyuridine 5'-triphosphate nick-end labeling and DNA fragmentation assays, we found that both LPA and S1P dose-dependently inhibited (by at least 50% and 40%, respectively) the apoptosis induced by serum withdrawal in cultures of primary calvarial rat osteoblasts and SaOS-2 cells. The antiapoptotic effects were inhibited by pertussis toxin, wortmannin, and LY294002, implicating G(i) proteins and phosphatidylinositol-3 kinase (PI-3 kinase) in the signaling pathway that mediates phospholipid-induced osteoblast survival. Specific inhibitors of p42/44 MAPK signaling did not block LPA- or S1P-induced osteoblast survival. LPA and S1P induced PI-3 kinase-dependent activation of p70 S6 kinase, but rapamycin, a specific inhibitor of p70 S6 kinase activation, did not prevent phospholipid-induced osteoblast survival. LPA and S1P also inhibited apoptosis in Swiss 3T3 fibroblastic cells in a G(i) protein-dependent fashion. In fibroblastic cells, however, the antiapoptotic effects of S1P were sensitive to inhibition of both PI-3 kinase and p42/44 MAPK signaling, whereas those of LPA were partially abrogated by inhibitors of p42/44 MAPK signaling but not by PI-3 kinase inhibitors. These data demonstrate that LPA and S1P potently promote osteoblast survival in vitro, and that cell-type specificity exists in the antiapoptotic signaling pathways activated by phospholipids.

Sex steroids and the construction and conservation of the adult skeleton

Here we review and extend a new unitary model for the pathophysiology of involutional osteoporosis that identifies estrogen (E) as the key hormone for maintaining bone mass and E deficiency as the major cause of age-related bone loss in both sexes. Also, both E and testosterone (T) are key regulators of skeletal growth and maturation, and E, together with GH and IGF-I, initiate a 3- to 4-yr pubertal growth spurt that doubles skeletal mass. Although E is required for the attainment of maximal peak bone mass in both sexes, the additional action of T on stimulating periosteal apposition accounts for the larger size and thicker cortices of the adult male skeleton. Aging women undergo two phases of bone loss, whereas aging men undergo only one. In women, the menopause initiates an accelerated phase of predominantly cancellous bone loss that declines rapidly over 4-8 yr to become asymptotic with a subsequent slow phase that continues indefinitely. The accelerated phase results from the loss of the direct restraining effects of E on bone turnover, an action mediated by E receptors in both osteoblasts and osteoclasts. In the ensuing slow phase, the rate of cancellous bone loss is reduced, but the rate of cortical bone loss is unchanged or increased. This phase is mediated largely by secondary hyperparathyroidism that results from the loss of E actions on extraskeletal calcium metabolism. The resultant external calcium losses increase the level of dietary calcium intake that is required to maintain bone balance. Impaired osteoblast function due to E deficiency, aging, or both also contributes to the slow phase of bone loss. Although both serum bioavailable (Bio) E and Bio T decline in aging men, Bio E is the major predictor of their bone loss. Thus, both sex steroids are important for developing peak bone mass, but E deficiency is the major determinant of age-related bone loss in both sexes.

Preservation of splenic immune functions by female sex hormones after trauma-hemorrhage

OBJECTIVES

Immune functions are markedly depressed in males but not in proestrus females after trauma-hemorrhage. Nonetheless, it is unclear what role sex steroids play in the maintenance of immune function in females after trauma-hemorrhage.

DESIGN

Prospective, controlled animal study.

SETTING

University research laboratory.

SUBJECTS

Eight-week-old female CBA/J mice.

INTERVENTIONS

Mice underwent sham-ovariectomy or ovariectomy. Two weeks thereafter, ovariectomized and proestrus sham-ovariectomized mice were subjected to laparotomy (i.e., soft tissue trauma) and hemorrhagic shock (35 +/- 5 mm Hg for 90 mins, resuscitated) or sham operation. Splenocyte proliferation and interleukin-2, interleukin-3, and interferon-gamma release were determined at 2 hrs after trauma-hemorrhage.

MEASUREMENTS AND MAIN RESULTS

These immune functional capacities were maintained in proestrus sham-ovariectomized mice after trauma-hemorrhage, whereas they were suppressed in ovariectomized mice subjected to trauma-hemorrhage. 17beta-Estradiol in vitro had no effect on splenocyte functions in proestrus sham-ovariectomized females; however, addition of 17beta-estradiol to splenocytes from ovariectomized females subjected to trauma-hemorrhage normalized immune functional capacities.

CONCLUSIONS

These findings suggest that elevated circulating 17beta-estradiol in proestrus females plays a direct role in the maintenance of immunocompetence after trauma-hemorrhage.

Connective tissue progenitors: practical concepts for clinical applications

Tissue engineering can be defined as any effort to create or induce the formation of a specific tissue in a specific location through the selection and manipulation of cells, matrices, and biologic stimuli. The biologic concepts and the biochemical and biophysical principles on which these efforts are based have become an exciting and rapidly evolving field of biomedical research. More importantly, tissue engineering is becoming a clinical reality in the practice of orthopaedic surgery, providing patients and physicians with an expanding set of practical tools for effective therapy. New and improved matrices and bioactive factors inevitably will play important roles in the evolution of orthopaedic tissue engineering. However, tissue engineering never can stray far from fundamental biologic principles, and one of these is that cells do all the work. No new tissue forms except through the activity of living cells. No bone graft, no matrix, no growth factor, no cytokine can contribute to the generation or integration of new tissue, except through the influence it has on the behavior of cells. The efficacy of all current clinical tools depends entirely on the cells in the grafted site, particularly the small subset of stem cells and progenitor cells that are capable of generating new tissue. The current authors review a series of key biologic concepts related to the rational design and selection of composites of cells and matrices in contemporary bone grafting and tissue engineering efforts. The functional paradigms of stem cell biology are reviewed, including self renewal, asymmetric and symmetric mitosis, and lineage restriction. Several potential sources for autogenous stem cells for connective tissues are discussed. Finally, a simple mathematical model is introduced as a tool for understanding the functional demands placed on stem cells and progenitors in a graft site and to provide a conceptual framework for the rational design of cell matrix composite grafts.

Female sex hormones regulate macrophage function after trauma-hemorrhage and prevent increased death rate from subsequent sepsis

OBJECTIVE

To determine whether reduction of circulating female sex hormones by ovariectomy causes suppression of macrophage (Mphi) function after trauma-hemorrhage and increases susceptibility to subsequent sepsis.

SUMMARY BACKGROUND DATA

Studies indicate that immune functions are markedly depressed in males but not in proestrus females after trauma-hemorrhage. Although male sex steroids are immunosuppressive, it remains unknown whether female sex hormones are immunoprotective after trauma-hemorrhage.

METHODS

Circulating female sex hormones were reduced by ovariectomy of 8-week-old female CBA/J mice. Two weeks afterward, ovariectomy and proestrus sham-ovariectomy mice were subjected to laparotomy (i.e., soft tissue trauma) and hemorrhagic shock (35 +/- 5 mm Hg for 90 minutes, then resuscitated) or sham operation. Two hours afterward, splenic and peritoneal Mphi and Kupffer cells were isolated and cytokine production was assessed. In a second series of experiments, animals were subjected to sepsis by cecal ligation and puncture at 24 hours after trauma-hemorrhage or sham operation, and survival was assessed.

RESULTS

Release of interleukin-1 and interleukin-6 by splenic and peritoneal Mphi from proestrus mice was maintained after trauma-hemorrhage, whereas release of interleukin-1 and interleukin-6 by Mphi from ovariectomized mice was depressed by approximately 50%. In contrast, trauma-hemorrhage resulted in a fourfold increase of Kupffer cell release of tumor necrosis factor-alpha in ovariectomized females and a fivefold increase in plasma concentrations of tumor necrosis factor-alpha. Release of tumor necrosis factor-alpha and plasma concentrations were unchanged in proestrus mice under such conditions. When proestrus and ovariectomized animals were subjected to sepsis by cecal ligation and puncture at 24 hours after trauma-hemorrhage or sham operation, ovariectomized mice had a significantly higher death rate than proestrus mice.

CONCLUSIONS

These findings suggest that female sex hormones play a critical role in maintaining immune responses after trauma-hemorrhage by suppressing the elaboration of tumor necrosis factor-alpha and prevent the increased lethality from subsequent sepsis. Thus, female sex hormones may be a useful adjunct in preventing trauma-induced immunodepression and increased susceptibility to subsequent sepsis.

17 beta-Estradiol normalizes immune responses in ovariectomized females after trauma-hemorrhage

Recent studies indicate that immune responses in proestrus females are maintained after trauma-hemorrhage but markedly depressed in ovariectomized females under such conditions. The current study tested the hypothesis that the decreased estrogen levels after ovariectomy are responsible for this immune depression. To study this hypothesis, ovariectomized female CBA/J mice were subjected to laparotomy (i.e., soft tissue trauma) and hemorrhagic shock (35 +/- 5 mmHg for 90 min, then resuscitated) or sham operation. The mice received either 17 beta-estradiol (E2; 100 microg/25 g body wt) or vehicle subcutaneously during resuscitation. Immune cells were isolated 24 h thereafter. Splenocyte proliferation and interferon-gamma, interleukin (IL)-2, and IL-3 release were significantly depressed after trauma-hemorrhage in vehicle-treated mice, whereas these functions were maintained in E2-treated mice. Peritoneal macrophage IL-1 beta and IL-6 release and splenic macrophage IL-6 and IL-12 release were also significantly depressed in vehicle-treated mice after trauma-hemorrhage, and release of these cytokines was restored by E2 treatment. In summary our findings indicate that the depressed splenic and peritoneal immune responses after trauma-hemorrhage can be normalized by a single dose of E2. Thus estrogen appears to be the causative factor in the maintenance of immunocompetence in females after trauma-hemorrhage, and its administration to ovariectomized or postmenopausal females should be helpful in preventing immune depression under such conditions.

Hepatic stellate cells contain the functional estrogen receptor beta but not the estrogen receptor alpha in male and female rats

In an earlier study, we showed that estradiol (E2) inhibits proliferation and transformation in cultured rat hepatic stellate cells (HSCs) and that the actions of E2 are mediated through estrogen receptors (ERs). This study reports on an investigation of the cellular localization of ER subtypes ERalpha and ERbeta using immunohistochemistry in experimental fibrotic liver rats and of each ER subtype expression in cultured rat HSCs by evaluating the produced mRNA and protein. The results indicate that high levels of ERbeta expression and low or no levels of ERalpha expression were observed in normal and fibrotic livers and in quiescent and activated HSCs from both males and females. The specificity of E2-mediated antiapoptotic induction through the ERbeta was shown by dose-dependent inhibition by the pure ER antagonist ICI 182,780 in HSCs which were undergoing early apoptosis. These findings demonstrate for the first time that rat HSCs possess functional Erbeta, but not Eralpha, to respond directly to E2 exposure.

Increased osteoblast apoptosis in apert craniosynostosis: role of protein kinase C and interleukin-1

Apert syndrome is an autosomal dominant disorder characterized by premature cranial ossification resulting from fibroblast growth factor receptor-2 (FGFR-2)-activating mutations. We have studied the effects of the prominent S252W FGFR-2 Apert mutation on apoptosis and the underlying mechanisms in human mutant osteoblasts. In vivo analysis of terminal deoxynucleotidyl transferase-mediated nick-end labeling revealed premature apoptosis of mature osteoblasts and osteocytes in the Apert suture compared to normal coronal suture. In vitro, mutant osteoblasts showed increased apoptosis, as demonstrated by terminal deoxynucleotidyl transferase-mediated nick-end labeling analysis, trypan blue staining, and DNA fragmentation. Mutant osteoblasts also showed increased activity of caspase-8 and effector caspases (-3, -6, -7) constitutively. This was related to protein kinase C activation because the selective protein kinase C inhibitor calphostin C inhibited caspase-8, effector caspases, and apoptosis in mutant osteoblasts. Apert osteoblasts also showed increased expression of interleukin (IL)-1alpha, IL-1beta, Fas, and Bax, and decreased Bcl-2 levels. Specific neutralizing anti-IL-1 antibody reduced Fas levels, Bax expression, effector caspases activity, and apoptosis in mutant cells. Thus, the Apert S252W FGFR-2 mutation promotes apoptosis in human osteoblasts through activation of protein kinase C, overexpression of IL-1 and Fas, activation of caspase-8, and increased Bax/Bcl-2 levels, leading to increased effector caspases and DNA fragmentation. This identifies a complex FGFR-2 signaling pathway involved in the premature apoptosis induced by the Apert S252W FGFR-2 mutation in human calvaria osteoblasts.

Apoptosis may determine the release of skeletal alkaline phosphatase activity from human osteoblast-line cells

Although quantitative measurement of skeletal alkaline phosphatase (sALP) activity in serum can provide an index of the rate of bone formation, the metabolic process that determines the release of sALP - from the surface of osteoblasts, into circulation-is unknown. The current studies were intended to examine the hypothesis that the release of sALP from human osteoblasts is a consequence of apoptotic cell death. We measured the release of sALP activity from human osteosarcoma (SaOS-2) cells and normal human bone cells, under basal conditions and in response to agents that increased apoptosis (TNF-a, okadiac acid) and agents that inhibit apoptosis (IGF-I, calpain, and caspase inhibitors). Apoptosis was determined by the presence of nucleosomes (histone-associated DNA) in the cytoplasm of the cells by using a commercial kit. The results of these studies showed that TNF-a and okadiac acid caused dose- and time-dependent increases in apoptosis in the SaOS-2 cells (r = 0.78 for doses of TNF-a and r = 0.93 for doses of okadiac acid, P <0.005 for each), with associated decreases in cell layer protein (P <0.05 for each) and concomitant increases in the release of sALP activity (e.g., r = 0.89 for TNF-a and r = 0.75 for okadiac acid, P <0.001 for each). In contrast, caspase and calpain inhibitors reduced apoptosis, increased cell layer protein, and decreased the release of sALP activity (P <0.05 for each). Exposure to IGF-I also decreased apoptosis, in a time- and dose-dependent manner (e.g., r = 0.93, P <0.001 for IGF-I doses), with associated proportional effects to increase cell layer protein (P <0.001) and decrease the release of sALP activity (P <0.001). IGF-I also inhibited the actions of TNF-a and okadiac acid to increase apoptosis and sALP release. The associations between apoptosis and sALP release were not unique to osteosarcoma (i.e., SaOS-2) cells, but also seen with osteoblast-line cells derived from normal human bone. Together, these data demonstrate that the release of sALP activity from human osteoblast-line cells in vitro is associated with, and may be a consequence of, apoptotic cell death. These findings are consistent with the general hypothesis that the appearance of sALP activity in serum may reflect the turnover of osteoblast-line cells.

Dexamethasone suppresses tumor necrosis factor-alpha-induced apoptosis in osteoblasts: possible role for ceramide

Ceramide has been proposed as a second messenger molecule implicated in a variety of biological processes, including apoptosis. Recently, it has been reported that tumor necrosis factor-alpha (TNF-alpha) activates the release of ceramide and that ceramide acts as a mediator for the TNF-alpha-induced stimulation of the binding affinity of nuclear factor-KB (NF-KB), a ubiquitous transcription factor of particular importance in immune and inflammatory responses. In this study we demonstrate that dexamethasone, which reduces the production of ceramide, significantly inhibits TNF-alpha-induced activation of NF-KB, c-Jun N-terminal kinase, also known as stress-activating protein kinase, caspase-3-like cysteine protease, redistribution of cytochrome c, and apoptosis in MC3T3E1 osteoblasts. Compared with TNF-alpha-induced JNK activation, ceramide elicits a more rapid activation of JNK within 30 min. C2-ceramide activates NF-KB and caspase-3 like protease to the same degree and with kinetics similar to those of TNF-alpha. This study provides evidence that the release of ceramide may be required as a second messenger in TNF-alpha-induced apoptosis. These results also suggest a regulatory role for dexamethasone in TNF-alpha-induced apoptosis via inhibition of ceramide release. Therefore, our in vitro results suggest that therapies targeted at the inhibition of ceramide release may abrogate inflammatory processes in TNF-alpha-related diseases, including rheumatoid arthritis and periodontitis.

Glucocorticoid-induced osteoporosis

Glucocorticoid drugs interact with bone metabolism at many levels, but their principal action is to reduce osteoblast number and bone matrix synthesis. Virtually all patients receiving glucocorticoids in doses above 5 mg per day lose bone, the amount lost being dependent on the cumulative steroid dose. The risk of fracture is also related to the individual's initial bone density, which in turn reflects race, sex, age, menopausal status, body weight, smoking and the nature of any underlying illness. Bone density measurement and personal fracture history are the best predictors of future fracture risk. Steroid-induced bone loss is reversible, so measures to minimize the systemic steroid dose or to withdraw these drugs altogether should be pursued no matter how long an individual has been using them. Increasing the calcium intake to 1.5 g per day, encouraging them to stop smoking and take more exercise, and treating any vitamin D deficiency are sensible measures in all patients. In those at high risk, bisphosphonates are the best documented interventions, although sex hormone replacement is also effective and can be used alone or in addition to bisphosphonates.