Time- and dose-related interactions between glucocorticoid and cyclic adenosine 3',5'-monophosphate on CCAAT/enhancer-binding protein-dependent insulin-like growth factor I expression by osteoblasts

Host Lipid Mediators in Leprosy: The Hypothesized Contributions to Pathogenesis

The spectrum of clinical forms observed in leprosy and its pathogenesis are dictated by the host's immune response against Mycobacterium leprae, the etiological agent of leprosy. Previous results, based on metabolomics studies, demonstrated a strong relationship between clinical manifestations of leprosy and alterations in the metabolism of ω3 and ω6 polyunsaturated fatty acids (PUFAs), and the diverse set of lipid mediators derived from PUFAs. PUFA-derived lipid mediators provide multiple functions during acute inflammation, and some lipid mediators are able to induce both pro- and anti-inflammatory responses as determined by the cell surface receptors being expressed, as well as the cell type expressing the receptors. However, little is known about how these compounds influence cellular immune activities during chronic granulomatous infectious diseases, such as leprosy. Current evidence suggests that specialized pro-resolving lipid mediators (SPMs) are involved in the down-modulation of the innate and adaptive immune response against M. leprae and that alteration in the homeostasis of pro-inflammatory lipid mediators versus SPMs is associated with dramatic shifts in the pathogenesis of leprosy. In this review, we discuss the possible consequences and present new hypotheses for the involvement of ω3 and ω6 PUFA metabolism in the pathogenesis of leprosy. A specific emphasis is placed on developing models of lipid mediator interactions with the innate and adaptive immune responses and the influence of these interactions on the outcome of leprosy.

Vitamin D Signaling Modulators in Cancer Therapy

The antiproliferative and pro-apoptotic effects of 1α,25-dihydroxycholecalciferol (1,25(OH)2D3, 1,25D3, calcitriol) have been demonstrated in various tumor model systems in vitro and in vivo. However, limited antitumor effects of 1,25D3 have been observed in clinical trials. This may be attributed to a variety of factors including overexpression of the primary 1,25D3 degrading enzyme, CYP24A1, in tumors, which would lead to rapid local inactivation of 1,25D3. An alternative strategy for improving the antitumor activity of 1,25D3 involves the combination with a selective CYP24A1 inhibitor. The validity of this approach is supported by numerous preclinical investigations, which demonstrate that CYP24A1 inhibitors suppress 1,25D3 catabolism in tumor cells and increase the effects of 1,25D3 on gene expression and cell growth. Studies are now required to determine whether selective CYP24A1 inhibitors+1,25D3 can be used safely and effectively in patients. CYP24A1 inhibitors plus 1,25D3 can cause dose-limiting toxicity of vitamin D (hypercalcemia) in some patients. Dexamethasone significantly reduces 1,25D3-mediated hypercalcemia and enhances the antitumor activity of 1,25D3, increases VDR-ligand binding, and increases VDR protein expression. Efforts to dissect the mechanisms responsible for CYP24A1 overexpression and combinational effect of 1,25D3/dexamethasone in tumors are underway. Understanding the cross talk between vitamin D receptor (VDR) and glucocorticoid receptor (GR) signaling axes is of crucial importance to the design of new therapies that include 1,25D3 and dexamethasone. Insights gained from these studies are expected to yield novel strategies to improve the efficacy of 1,25D3 treatment.

Prostaglandin dependent control of an endogenous estrogen receptor agonist by osteoblasts

Estrogen receptor (ER) activation has complex effects on bone cells, and loss of circulating estradiol adversely affects skeletal status in women. Hormone replacement therapy effectively circumvents bone loss after menopause, but enhances disease risk in other tissues. Here we show that prostaglandin E2 (PGE2) augments the activity of an osteoblast-derived selective ER modulator, ObSERM. The stimulatory effect of PGE2 is replicated in part by either the PG receptor EP3 agonist 17-phenyl trinor PGE2 or by the PG receptor FP agonist PGF2α⋅ Whereas activation of the various PG receptors induces multiple downstream signals, the response to PGE2 was mimicked by activators of protein kinase C, and suppressed by inhibition of protein kinase C but not by inhibition of protein kinase A. Moreover, inhibition of nitric oxide synthesis and activation of the PTH and Wnt pathways increases ObSERM activity. Our studies therefore reveal that ObSERM activity is controlled in distinct ways and revise our understanding of ER activation within bone by agents or events associated with PG expression. They also predict ways to sustain or improve bone formation, fracture repair, and surgical healing without adding the risk of disease in other tissues where ER activation also has important biological functions.

Stratified control of IGF-I expression by hypoxia and stress hormones in osteoblasts

Bone cells respond to the integrated effects of local and systemic regulation. Here we show that hypoxia and the stress hormones PGE2 and glucocorticoid interact in complex ways in osteoblasts, converging on insulin like growth factor I (IGF-I) expression. Whereas hypoxia alone rapidly increased transcription factor HIF activity, it suppressed DNA synthesis, had no significant effects on protein synthesis or alkaline phosphatase activity, and drove discrete changes in a panel of osteoblast mRNAs. Notably, hypoxia increased expression of the acute phase response transcription factor C/EBPδ which can induce IGF-I in response to PGE2, but conversely prevented the stimulatory effect of PGE2 on IGF-I mRNA. However, unlike its effect on C/EBPδ, hypoxia suppressed expression of the obligate osteoblast transcription factor Runx2, which can activate an upstream response element in the IGF-I gene promoter. Hypoxic inhibition of IGF-I and Runx2 were enforced by glucocorticoid, and continued with prolonged exposure. Our studies thus reveal that IGF-I expression is stratified by two critical transcriptional elements in osteoblasts, which are resolved by the individual and combined effects of hypoxic stress and stress hormones. In so doing, hypoxia suppresses Runx2, limits the enhancing influence of PGE2, and interacts with glucocorticoid to reduce IGF-I expression by osteoblasts.

Aldosterone stimulates fibronectin synthesis in renal fibroblasts through mineralocorticoid receptor-dependent and independent mechanisms

In addition to its role in regulation of salt transport in the kidney, the mineralocorticoid hormone aldosterone plays an independent role as a mediator of kidney injury and progression of chronic kidney disease. Studies in both animal models and patients have shown that aldosterone enhances the accumulation of extracellular matrix and progression of fibrosis in the kidney. However, the cellular mechanisms that lead to aldosterone-dependent fibrogenesis are poorly understood. In this study we find that aldosterone stimulates fibronectin synthesis through mineralocorticoid receptor (MCR) dependent activation of the c-Jun NH2-terminal protein kinase (JNK) and subsequent phosphorylation of the AP1 transcription factor c-jun, which forms a nuclear complex with the mineralocorticoid receptor in a kidney fibroblast cell line (NRK 49f). Furthermore, MCR-independent phosphorylation of Src family kinase induces IgF1 receptor phosphorylation, which leads to stimulation of the extracellular signal-regulated kinase (ERK1/2) to enhanced fibronectin synthesis. We further find that the IgF1-R-dependent signaling pathway activates fibronectin expression faster than the MCR-dependent pathway. We propose that the mechanisms described in this study are important to aldosterone-dependent progression of interstitial fibrosis in the kidney. Due to the duality of aldosterone-dependent activation of fibronectin synthesis in kidney fibroblasts, MCR-specific inhibitors may not entirely prevent the progression of fibrosis by aldosterone in the kidney.

Phenotypes of CCAAT/enhancer-binding protein beta deficiency: hyperdontia and elongated coronoid process

OBJECTIVES

This investigation aimed to conduct a case-control study of mandibular morphology and dental anomalies to propose a relationship between mandibular/dental phenotypes and deficiency of CCAAT/enhancer-binding protein beta (CEBPB).

MATERIALS AND METHODS

Skulls of CEBPB(-/-), CEBPB(+/-) and CEBPB(+/+) mice were inspected with micro-computed tomography. Mandibular morphology was assessed with a method of Euclidean distance matrix analysis.

RESULTS

Elongation of the coronoid process was identified in CEBPB(+/-) (P ≤ 0.046) and CEBPB(-/-) 12-month-olds (P ≤ 0.028) but not in 14-day-olds (P ≥ 0.217) and 0-day-olds (P ≥ 0.189) of either genotype. Formation of supernumerary teeth in CEBPB(-/-) adult mice was demonstrated (χ(2) = 6.00, df = 1, P = 0.014).

CONCLUSIONS

CEBPB deficiency was related to elongation of the coronoid process and formation of supernumerary teeth. The mandibular and dental phenotypes of CEBPB deficiency were unseen by the 14th day after birth. Future investigations into the influence of CEBPB on mandibular and dental development are needed.

Enhancement of lipid stability of broiler breast meat and meat products fed on alpha lipoic acid and alpha tocopherol acetate supplemented feed

This study was designed to investigate the effect of alpha lipoic acid (ALA) and alpha tocopherol acetate (ATA) on the antioxidant potential, lipid stability and the quality of the broiler breast meat and meat products. The treatment plan was as (T₁ = control feed, T₂ = 200 mg ATA + 25 mg ALA/kg feed, T₃ = 200 mg ATA + 75 mg ALA/kg feed, T₄ = 200 mg ATA + 150 mg ALA/kg feed, T₅ = Oxidized oil (4%), T₆ = 200 mg ATA + 150 mg ALA + Oxidized oil (4%)/kg feed). After two weeks of acclimatization the birds were fed with ALA and ATA enriched diet. The results revealed that maximum deposition of ALA took place in T₄ which contain maximum dose of ALA. The TBARS and DPPH values of the broiler breast meat were in T₄ (0.14 ± 0.01 MDA/kg of meat, 76.69 ± 0.14%) and in T₅ were (0.24 ± 0.15 MDA/Kg of meat, 44.98 ± 0.04%) accordingly. ATA concentration were also highest in T₄ (206.43 ± 0.22 mg/g of meat) and lowest in T₅ (79.09 ± 0.06 mg/g of meat). Sensory evaluation results showed that nuggets and patties made of T₅ containing oxidized oil were least liked and T₄ got highest score. In a nutshell, 150 mg/kg feed dietary supplementation of ALA with constant level of ATA can ameliorate the antioxidant potential, lipid stability and nutritional qualities of broiler breast meat and meat products.

Impact of metabolic regulators on the expression of the obesity associated genes FTO and NAMPT in human preadipocytes and adipocytes

Background

FTO and NAMPT/PBEF/visfatin are thought to play a role in obesity but their transcriptional regulation in adipocytes is not fully understood. In this study, we evaluated the transcriptional regulation of FTO and NAMPT in preadipocytes and adipocytes by metabolic regulators.

Methodology and Principal Findings

We assessed FTO mRNA expression during human adipocyte differentiation of Simpson-Golabi-Behmel syndrome (SGBS) cells and primary subcutaneous preadipocytes in vitro and evaluated the effect of the metabolic regulators glucose, insulin, dexamethasone, IGF-1 and isoproterenol on FTO and NAMPT mRNA expression in SGBS preadipocytes and adipocytes. FTO mRNA levels were not significantly modulated during adipocyte differentiation. Also, metabolic regulators had no impact on FTO expression in preadipocytes or adipocytes. In SGBS preadipocytes NAMPT expression was more than 3fold induced by dexamethasone and isoproterenol and 1.6fold by dexamethasone in adipocytes. Complete glucose restriction caused an increase in NAMPT mRNA expression by more than 5fold and 1.4fold in SGBS preadipocytes and adipocytes, respectively.

Conclusion

FTO mRNA expression is not significantly affected by differentiation or metabolic regulators in human adipocytes. The stimulation of NAMPT expression by dexamethasone, isoproterenol and complete glucose restriction may indicate a regulation of NAMPT by metabolic stress, which was more pronounced in preadipocytes compared to mature adipocytes.

Timing, rather than the concentration of cyclic AMP, correlates to osteogenic differentiation of human mesenchymal stem cells

Previously, we demonstrated that protein kinase A (PKA) activation using dibutyryl-cAMP in human mesenchymal stem cells (hMSCs) induces in vitro osteogenesis and bone formation in vivo. To further investigate the physiological role of PKA in hMSC osteogenesis, we tested a selection of G-protein-coupled receptor ligands which signal via intracellular cAMP production and PKA activation. Treatment of hMSCs with parathyroid hormone, parathyroid hormone-related peptide, melatonin, epinephrine, calcitonin or calcitonin gene-related peptide did not result in accumulation of cAMP or induction of alkaline phosphatase (ALP) expression. The only ligand that did induce cAMP, prostaglandin E2, even inhibited ALP expression and mineralization, suggesting that physiological levels of cAMP may inhibit osteogenesis. Furthermore, intermittent exposure of hMSCs to dibutyryl-cAMP inhibited ALP expression, whereas we did not observe an inhibitive effect at low dibutyryl-cAMP concentrations. Taken together, our results demonstrate that cAMP can either stimulate or inhibit osteogenesis in hMSCs, depending on the duration, rather than the strength, of the signal provided.

Novel regulation of 25-hydroxyvitamin D3 24-hydroxylase (24(OH)ase) transcription by glucocorticoids: cooperative effects of the glucocorticoid receptor, C/EBP beta, and the Vitamin D receptor in 24(OH)ase transcription

Glucocorticoid-induced bone loss has been proposed to involve direct effects on bone cells as well as alterations in calcium absorption and excretion. Since vitamin D is important for the maintenance of calcium homeostasis, in the present study the effects of glucocorticoids on vitamin D metabolism through the expression of 24(OH)ase, an enzyme involved in the catabolism of 1,25(OH)(2)D(3), were examined. Injection of vitamin D replete mice with dexamethasone (dex) resulted in a significant induction in 24(OH)ase mRNA in kidney, indicating a regulatory effect of glucocorticoids on vitamin D metabolism. Whether glucocorticoids can affect 24(OH)ase transcription is not known. Here we demonstrate for the first time a glucocorticoid receptor (GR) dependent enhancement of 1,25(OH)(2)D(3)-induced 24(OH)ase transcription. Dex treatment of GR and vitamin D receptor (VDR) transfected COS-7 cells and dex treatment of osteoblastic cells (in which VDR and GR are present endogenously) potentiated 1,25(OH)(2)D(3)-induced 24(OH)ase transcription. In addition, GR was found to cooperate with C/EBP beta to enhance VDR-mediated 24(OH)ase transcription. Using the rat 24(OH)ase promoter with the C/EBP site mutated, GR-mediated potentiation of 1,25(OH)(2)D(3)-induced 24(OH)ase transcription was inhibited. Immunoprecipitation indicated that that GR can interact with C/EBP beta and ChIP/re-ChIP analysis showed that C/EBP beta and GR bind simultaneously to the 24(OH)ase promoter. These findings indicate a novel mechanism whereby glucocorticoids can alter VDR-mediated 24(OH)ase transcription through functional cooperation between C/EBP beta and GR that results in an enhanced ability of C/EBP beta to cooperate with VDR in the regulation of 24(OH)ase.

Positive association between nuclear Runx2 and oestrogen-progesterone receptor gene expression characterises a biological subtype of breast cancer

PURPOSE

The runt-related transcription factor, Runx2 may have an oncogenic role in mediating metastatic events in breast cancer, but whether Runx2 has a role in the early phases of breast cancer development is not clear. We examined the expression of Runx2 and its relationship with oestrogen receptor (ER) and progesterone receptor (PR) in breast cancer cell lines and tissues.

METHODS

Two human breast cancer cell lines, MCF-7 and MDA-MB-231 were transiently transfected with vectors expressing either Runx2 or ER and the levels of both proteins and mRNA were examined by Western blot analysis and quantitative real-time PCR, respectively. Runx2 expression was also examined in tissue microarray sections of 123 breast cancer patients by immunohistochemistry and results were correlated with clinico-pathological characteristics.

RESULTS

Expression of Runx2 and ER was reciprocal in the breast cell culture models and Runx2 suppressed ERbeta but not ERalpha mRNA levels. In contrast, functional expression of Runx2 was evident in the nucleus in 28% of the breast cancer tissues and in both early and late stages of tumour growth. Importantly, Runx2 expression was significantly more frequent in Grade 2 compared to Grade 1 and Grade 3 tumours (48% versus 39% versus 13%) and the expression was significantly associated with ER (p=0.005), PR (p=0.008) expressions in Grade 2 & Grade 3 tumours than Grade 1 tumours.

CONCLUSION

We propose that Runx2, ER and PR triple positivity in Grades 2 and 3 defines a biological subtype in breast cancer.

CCAAT/enhancer-binding protein beta: its role in breast cancer and associations with receptor tyrosine kinases

The CCAAT/enhancer-binding proteins (C/EBPs) are a family of leucine-zipper transcription factors that regulate gene expression to control cellular proliferation, differentiation, inflammation and metabolism. Encoded by an intronless gene, C/EBPbeta is expressed as several distinct protein isoforms (LAP1, LAP2, LIP) whose expression is regulated by the differential use of several in-frame translation start sites. LAP1 and LAP2 are transcriptional activators and are associated with differentiation, whereas LIP is frequently elevated in proliferative tissue and acts as a dominant-negative inhibitor of transcription. However, emerging evidence suggests that LIP can serve as a transcriptional activator in some cellular contexts, and that LAP1 and LAP2 might also have unique actions. The LIP:LAP ratio is crucial for the maintenance of normal growth and development, and increases in this ratio lead to aggressive forms of breast cancer. This review discusses the regulation of C/EBPbeta activity by post-translational modification, the individual actions of LAP1, LAP2 and LIP, and the functions and downstream targets that are unique to each isoform. The role of the C/EBPbeta isoforms in breast cancer is discussed and emphasis is placed on their interactions with receptor tyrosine kinases.

Corticosteroids induce COX-2 expression in cardiomyocytes: role of glucocorticoid receptor and C/EBP-beta

Psychological stress increases the level of glucocorticoids in the circulating system. We found that dexamethasone administration in adult mice elevates the expression of COX-2 in the myocardium. With isolated neonatal cardiomyocytes, corticosterone (CT) at physiologically relevant doses (0.01-1 microM) induces the expression of COX-2 gene. The induction first appeared at 4 h and remained for at least 24 h with 1 microM CT treatment. This response is likely cardiomyocyte cell type specific since CT did not induce COX-2 expression in cardiac fibroblasts and glucocorticoids are known to suppress the expression of COX-2 in lymphocytes and several organs. Corticosteroids, but not estrogen or progesterone, induce COX-2 expression. The glucocorticoid receptor (GR) antagonist mifepristone (MF) prevented CT from inducing COX-2 gene, suggesting a GR-dependent induction in cardiomyocytes. COX-2 gene promoter deletion and mutation studies indicate a role of CCAAT/enhancer binding protein-beta (C/EBP-beta) in CT-induced COX-2 gene expression. Chromatin immunoprecipitation assays revealed that CT caused the binding of both GR and C/EBP-beta to COX-2 promoter, while MF pretreatment blocked such binding. Coimmunoprecipitation experiments demonstrated that CT treatment induced the interaction of GR with C/EBP-beta. Small interfering RNA against C/EBP-beta prevented CT from activating COX-2 promoter or elevating COX-2 protein. Our data suggest that the interaction between GR and C/EBP-beta contributes to elevated COX-2 gene transcription by CT in cardiomyocytes.

Expression of an estrogen receptor agonist in differentiating osteoblast cultures

Osteoblasts respond in direct and indirect ways to estrogens, and age-dependent changes in hormone levels and bone health can be limited by focused hormone replacement therapy. In this study, we report the release and isolation of an estrogen receptor agonist from osteoblast cultures. This entity reprises many aspects of estradiol activity in isolated osteoblasts, but differs from authentic estradiol by several biochemical and physical criteria. At levels that occur in conditioned medium from differentiating osteoblast cultures, the agonist directly drives gene expression through estrogen-sensitive response elements, activates the obligate osteoblast transcription factor Runx2, and potently enhances Smad-dependent gene expression in response to TGF-beta, but exhibits relatively lesser suppressive effects on gene expression through C/EBP and AP-1-binding protein transcription factors. Estrogen receptor agonist activity is resistant to heating at 100 degrees C and separable from the bulk of the remaining alcohol- and hexane-soluble molecules by C18 chromatography. MS and molecular fragmentation analyses predict a M(r) of 415.2 to 437.2. Therefore, in addition to earlier studies showing that osteoblasts readily respond to and metabolize various sex steroid-like substrates, we find that they also generate a potent estrogen receptor agonist during differentiation in vitro. Changes in the availability of a molecule like this within bone may relate to differences in skeletal integrity with aging or metabolic disease.

3-ketosteroid reductase activity and expression by fetal rat osteoblasts

In addition to reproductive tissue, sex hormones induce transcriptional events in many connective tissue cells, including osteoblasts. Some sex hormone receptor modulators with bone sparing effects selectively target estrogen or androgen receptors, whereas others appear more promiscuous, in part through enzymatic metabolism. Rat osteoblasts express significant oxidative 3alpha-hydroxysteroid dehydrogenase activity, which can convert precursor substrates to potent androgen receptor agonists. Here we show that they also express 3-ketosteroid reductase activity, exemplified by 7-methyl-17-ethynyl-19-norandrostan-5 (10)en-3-one (tibolone) conversion to potent estrogen receptor alpha agonists. Conversion was rapid and quantitative, with 3alpha-hydroxytibolone as the primary metabolite. Consistently, tibolone induced estrogen receptor alpha-dependent gene promoter activity through cis-acting estrogen response elements, increased the stimulatory effect of TGF-beta on Smad-dependent gene promoter activity, and enhanced prostaglandin E2-induced activity of transcription factor Runx2. Rat osteoblasts express the 3-ketosteroid reductase AKR1C9, an aldo-keto reductase gene family member. Exposure to prostaglandin E2 increased AKR1C9 gene promoter activity and mRNA expression. AKR1C9 promoter activity was also enhanced by overexpression of protein kinase A catalytic subunit or transcription factor C/EBPdelta, and the effect of PGE2 was reduced by dominant negative C/EBPdelta competition or C/EBPdelta antisense expression. Moreover, prostaglandin E2 increased the amount of functional endogenous nuclear C/EBPdelta that could bind specifically to a distinct domain approximately 1.8-kb upstream from the start site of AKR1C9 transcription. In summary, in addition to 3alpha-hydroxysteroid dehydrogenase, rat osteoblasts express significant and regulatable 3-ketosteroid reductase activity. Through these enzymes, they may selectively metabolize precursor compounds into potent steroid receptor agonists locally within bone.

Prostaglandin E2 increases transforming growth factor-beta type III receptor expression through CCAAT enhancer-binding protein delta in osteoblasts

Variations in individual TGF-beta receptors (TbetaRs) may modify TGF-beta activity and significantly alter its effects on connective tissue growth or repair. Differences in the amount of TbetaR type III (TbetaRIII) relative to signal transducing TbetaRI occur on bone cells during differentiation or in response to other growth regulators. Here we investigated prostaglandin (PG) E2, a potent effector during trauma, inflammation, or mechanical load, on TbetaR expression in primary osteoblast-enriched cultures. PGE2 rapidly increased TbetaRIII mRNA and protein expression and enhanced TbetaRIII gene promoter activity through a discrete region within 0.4 kb of the transcription start site. PGE2 alters osteoblast function through multiple signal-inducing pathways. In this regard, protein kinase A (PKA) activators, PGE1 and forskolin, also enhanced gene expression through the TbetaRIII gene promoter, whereas protein kinase C activators, PGF2alpha and phorbol myristate acetate, did not. The stimulatory effect of PGE2 on TbetaRIII promoter activity was suppressed by a dominant negative PKA-regulatory subunit, but not by dominant negative protein kinase C. PGE2 specifically increased nuclear factor CCAAT enhancer-binding protein delta (C/EBPdelta) binding to a half-binding site upstream of the basal TbetaRIII promoter region, and promoter activity was sensitive to C/EBPdelta overexpression and to dominant-negative C/EBPdelta competition. In parallel with their effect on TbetaRIII expression, activators of PKA decreased TGF-beta-induced activity. In summary, high levels of PGE2 that occur with inflammation or trauma may, through PKA-activated C/EBPdelta, preferentially increase TbetaRIII expression and in this way delay TGF-beta-dependent activation of osteoblasts during the early stabilization phase of bone repair.

Glucocorticosteroid-induced spinal osteoporosis: scientific update on pathophysiology and treatment

Glucocorticosteroid-induced spinal osteoporosis (GIOP) is the most frequent of all secondary types of osteoporosis. The understanding of the pathophysiology of glucocorticoid (GC) induced bone loss is of crucial importance for appropriate treatment and prevention of debilitating fractures that occur predominantly in the spine. GIOP results from depressed bone formation due to lower activity and higher death rate of osteoblasts on the one hand, and from increase bone resorption due to prolonged lifespan of osteoclasts on the other. In addition, calcium/phosphate metabolism may be disturbed through GC effects on gut, kidney, parathyroid glands and gonads. Therefore, therapeutic agents aim at restoring balanced bone cell activity by directly decreasing apoptosis rate of osteoblasts (e.g., cyclical parathyroid hormone) or by increasing apoptosis rate of osteoclasts (e.g., bisphosphonates). Other therapeutical efforts aim at maintaining/restoring calcium/phosphate homeostasis: improving intestinal calcium absorption (using calcium supplementation, vitamin D and derivates) and avoiding increased urinary calcium loss (using thiazides) prevent or counteract a secondary hyperparthyroidism. Bisphosphonates, particularly the aminobisphosphonates risedronate and alendronate, have been shown to protect patients on GCs from (further) bone loss to reduce vertebral fracture risk. Calcitonin may be of interest in situation where bisphosphonates are contraindicated or not applicable and in cases where acute pain due to vertebral fracture has to be manage. The intermittent administration of 1-34-parathormone may be an appealing treatment alternative, based on its documented anabolic effects on bone resulting from the reduction of osteoblastic apoptosis. Calcium and vitamin D should be a systematic adjunctive measure to any drug treatment for GIOP. Based on currently available evidence, fluoride, androgens, estrogens (opposed or unopposed) cannot be recommended for the prevention and treatment of GIOP. However, substitution of gonadal hormones may be indicated if GC-induced hypogonadism is present and leads to clinical symptoms. Data using the SERM raloxifene to treat or prevent GIOP are lacking, as are data using the promising bone anabolic agent strontium ranelate. Kyphoplasty performed in appropriately selected osteoporotic patients with painful vertebral fractures is a promising addition to current medical treatment.

Effect of systemic glucocorticoid therapy on bone metabolism: an update

Glucocorticoids are widely used for a range of inflammatory conditions. However, their use is complicated by significant side effects. The most important of these from a clinical point of view is on bone, where glucocorticoids substantially increase the risk of osteoporosis and fracture. This review will give an overview of the pathophysiologic basis and epidemiology of glucocorticoid-induced osteoporosis, examine diagnostic and therapeutic approaches currently available, and suggest the likely impact of the most recent scientific, clinical and pharmaceutical advances. Glucocorticoids impact on both bone formation and bone resorption, a combination that leads to rapid bone loss and increase in fracture risk. Epidemiologic studies indicate that these risks are substantial, especially at the spine, increase with age and independently of bone density, and are maintained during glucocorticoid use. The best available treatments are bisphosphonates that preserve bone density and reduce the risk of fracture at the spine. Future areas that need to be addressed are the relationship between inflammation and the action of glucocorticoids on the skeleton, and the development of anabolic therapies for glucocorticoid-induced osteoporosis.

Functional cooperation between CCAAT/enhancer-binding proteins and the vitamin D receptor in regulation of 25-hydroxyvitamin D3 24-hydroxylase

1,25-Dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] induces the synthesis of 25-hydroxyvitamin D(3) 24-hydroxylase [24(OH)ase], an enzyme involved in its catabolism, thereby regulating its own metabolism. Here we demonstrate that CCAAT enhancer binding protein beta (C/EBPbeta) is induced by 1,25(OH)(2)D(3) in kidney and in osteoblastic cells and is a potent enhancer of vitamin D receptor (VDR)-mediated 24(OH)ase transcription. Transfection studies indicate that 1,25(OH)(2)D(3) induction of 24(OH)ase transcription is enhanced a maximum of 10-fold by C/EBPbeta. Suppression of 1,25(OH)(2)D(3)-induced 24(OH)ase transcription was observed with dominant negative C/EBP or osteoblastic cells from C/EBPbeta(-/-) mice. A C/EBP site was identified at positions -395 to -388 (-395/-388) in the rat 24(OH)ase promoter. Mutation of this site inhibited C/EBPbeta binding and markedly attenuated the transcriptional response to C/EBPbeta. We also report the cooperation of CBP/p300 with C/EBPbeta in regulating VDR-mediated 24(OH)ase transcription. We found that not only 1,25(OH)(2)D(3) but also parathyroid hormone (PTH) can induce C/EBPbeta expression in osteoblastic cells. PTH potentiated the induction of C/EBPbeta and 24(OH)ase expression in response to 1,25(OH)(2)D(3) in osteoblastic cells. Data with the human VDR promoter (which contains two putative C/EBP sites) indicate a role for C/EBPbeta in the protein kinase A-mediated induction of VDR transcription. From this study a fundamental role has been established for the first time for cooperative effects and cross talk between the C/EBP family of transcription factors and VDR in 1,25(OH)(2)D(3)-induced transcription. These findings also indicate a novel role for C/EBPbeta in the cross talk between PTH and 1,25(OH)(2)D(3) that involves the regulation of VDR transcription.

Dexamethasone down-regulates cAMP-phosphodiesterase in human osteosarcoma cells

Cyclic adenosine monophosphate (cAMP) is an important second messenger in the hormonal regulation of bone metabolism. cAMP is inactivated by the cyclic nucleotide phosphodiesterases (PDEs), a superfamily of enzymes divided into 11 known families, designated PDE1-11. Interference with the cAMP signaling pathway has been suggested as one mechanism causing glucocorticoid induced osteoporosis. We speculated that glucocorticoids could affect the cAMP pathway by a down-regulation of PDE-mediated cAMP hydrolysis. The main cAMP hydrolysing enzyme families of human MG-63 and SaOS-2 osteosarcoma cells were identified as PDE1 and PDE4 by assaying the PDE activity of Q-sepharose fractions and cell homogenates with selective inhibitors. Treatment with the glucocorticoid dexamethasone (Dex) decreased cAMP-PDE activity by up to 50%, without affecting cGMP-PDE activity. Dex treatment reduced the sensitivity of the total cAMP-PDE activity towards the PDE4 selective PDE inhibitor rolipram. Forskolin stimulated cAMP accumulation was increased 30-60-fold in the presence of rolipram. Treatment with Dex did not affect the basal or forskolin stimulated cAMP accumulation, but treatment resulted in a reduced effect of rolipram on cAMP accumulation. Expression of the following cAMP-PDE subtypes were detected by reverse transcriptase PCR (RT-PCR): PDE1A, PDE1C, PDE2A, PDE3A, PDE4A, PDE4B, PDE4C, PDE4D, PDE7A, PDE7B, PDE8A, PDE10A and PDE11A. Using semi-quantitative RT-PCR, we detected a 50-70% decrease in the mRNA of PDE4A and PDE4B subtypes following Dex treatment. Further analysis revealed that Dex reduced the PDE4A4 and PDE4B1 isoforms. PDE4A1 PDE4A, PDE4A7, PDE4A10, PDE4B2 were also expressed, but Dex did not affect the transcription of these isoforms. We conclude that Dex treatment could affect the cAMP signaling pathway of human osteosarcoma cells by reducing type 4 cAMP-phosphodiesterase (PDE4).

Fos-related antigen 2 controls protein kinase A-induced CCAAT/enhancer-binding protein beta expression in osteoblasts

Transcription factor CCAAT/enhancer-binding protein beta (C/EBPbeta) plays an important role in hormone-dependent gene expression. In osteoblasts C/EBPbeta can increase insulin-like growth factor I (IGF-I) transcription following treatment with hormones that activate protein kinase A, but little is known as yet about the expression of C/EBPbeta itself in these cells. We initially showed that prostaglandin E2 (PGE2) rapidly enhances C/EBPbeta mRNA and protein expression, and in this study we identified a 3'-proximal region of the C/EBPbeta promoter containing a 541-bp upstream sequence that could account for this effect. PGE2-dependent activation of C/EBPbeta was blocked by expression of a mutated regulatory subunit of protein kinase A or by mutation of two previously identified cAMP-sensitive cis-acting regulatory elements within the promoter between bp -111 and -61. Nuclear protein binding to these elements was induced by PGE2, required new protein synthesis, and was sensitive to antibody to the transcription factor termed Fos-related antigen 2 (Fra-2). Fra-2 cDNA generated from rat osteoblasts by reverse transcriptase PCR was 95% homologous to human Fra-2, and PGE2 rapidly induced Fra-2 mRNA and protein expression. Consistent with these findings, over-expression of Fra-2 significantly increased C/EBPbeta promoter activity in PGE2-induced osteoblasts, whereas expression of Fra-2 lacking its activation domain had a dominant negative inhibitory effect. Together, these results reveal a significant, hormone-dependent role for Fra-2 in osteoblast function, both directly, through its ability to increase new C/EBPbeta gene expression, and indirectly, through downstream C/EBP sensitive genes.

Sensitivity of bone to glucocorticoids

Glucocorticoids are used widely in a range of medical specialities, but their main limitation is an adverse impact on bone. Although physicians are increasingly aware of these deleterious effects, the marked variation in susceptibility between individuals makes it difficult to predict who will develop skeletal complications with these drugs. Although the mechanisms underlying the adverse effects on bone remain unclear, the most important effect appears to be a rapid and substantial decrease in bone formation. This review will examine recent studies that quantify the risk of fracture with glucocorticoids, the mechanisms that underlie this increase in risk and the potential basis for differences in individual sensitivity. An important determinant of glucocorticoid sensitivity appears to be the presence of glucocorticoid-metabolizing enzymes within osteoblasts and this may enable improved estimates of risk and generate new approaches to the development of bone-sparing anti-inflammatory drugs.

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.

Activation domains of CCAAT enhancer binding protein delta: regions required for native activity and prostaglandin E2-dependent transactivation of insulin-like growth factor I gene expression in rat osteoblasts

In osteoblasts, hormones such as prostaglandin E2 that activate protein kinase A increase the translocation of transcription factor CCAAT/enhancer binding protein delta (C/EBPdelta) from the cytoplasm to the nucleus where it rapidly induces IGF-I gene expression. In this study, we identified activation and suppression domains in C/EBPdelta using native and heterologous gene promoter assay systems. We demonstrated functional interactions between C/EBPdelta and trans-gene-expressed cAMP response element binding protein-binding protein, and showed that the ability of C/EBPdelta to promote gene expression was suppressed by viral protein E1A, which blocks the activity of native cAMP response element binding protein-binding protein. Site-directed mutations at serines 62 or 191 within C/EBPdelta reduced its basal transcriptional activity, whereas mutation at serine 191 suppressed the stimulatory effect of prostaglandin E2 on C/EBPdelta function as well as its DNA binding potential. These results are consistent with the location of serine 191 in the DNA binding domain of C/EBPdelta. Our studies provide the first evidence for regions of C/EBPdelta that are important for basal and for hormone-induced transcriptional activity, and for its interactions with other enhancers and suppressers of gene expression.

The influence of culture conditions on extracellular matrix proteins synthesized by osteoblasts derived from rabbit bone marrow

The influence of culture conditions on the extracellular matrix (ECM) protein expressions of rabbit bone marrow stromal cells has been studied. The focus was on the effects of two kinds of sera, fetal calf serum (FCS) and Ultroser, on cells treated with dexamethasone. The induction of osteoblastic differentiation by dexamethasone addition is confirmed, particularly when cells are cultured in FCS. Bone marrow stromal cells produce alkaline phosphatase positive CFU-F and produce ECM with some mineralized nodules. Analysis by means of two-dimensional gel electrophoresis showed important changes in the composition of ECM proteins after dexamethasone treatment. Overexpression, underexpression, and new synthesized proteins were observed. The most significant modification was linked to the synthesis of four new proteins visible in the acidic area with a low molecular weight of around 17 kDa. These proteins did not correspond to those ECM proteins known to be induced by dexamethasone. Moreover, the effect of dexamethasone on osteoblastic differentiation induction appears very limited when cells are cultured in Ultroser compared to FCS. The protein pattern with Ultroser is different to that obtained with FCS. Cells cultured in Ultroser synthesized no new protein. The different behavior of cells according to the type of medium used is discussed in terms of the osteogenic factors present in the two different sera.

Distinct anabolic response of osteoblast to low-intensity pulsed ultrasound

Low-intensity pulsed ultrasound, a form of mechanical energy transmitted as high-frequency acoustical pressure waves, provides noninvasive therapeutic treatment for accelerating fracture repair and distraction osteogenesis. Relatively young osteoblasts respond to ultrasound by transiently upregulating message levels of immediate-early genes as well as that of osteocalcin and insulin-like growth factor I (IGF-I). Osteocytes derived from newborn rat tibia and calvaria responded to a lesser extent only in c-fos and cyclooxygenase-2 (COX-2) messages. Compared with the stretched osteocytes, which use stretch-activated and parathyroid hormone (PTH)-potentiated Ca2+ influx as an entry route to the protein kinase A (PKA) signal transduction pathways, there was no evidence of Ca2+ internalization by any of the cells tested on exposure to the ultrasound. On the other hand, inhibitors of p38 mitogen-activated protein kinase (MAPK) and upstream phosphoinositide 3-kinase (PI3K) blocked COX-2 and osteocalcin upregulation by the ultrasound-exposed ST2, murine bone marrow-derived cells. This is distinct from the aforementioned osteocytic response to low-frequency stretching and implies the involvement of integrins. Our findings suggested that accelerated fracture repair and distraction osteogenesis by the low-intensity pulsed ultrasound depend, at least in part, on the stimulation of osteoblastic cells at relatively early stages of osteogenic lineage. Bone is under control of multiple regulatory mechanisms so that diverse physical forces can be reflected to the microenvironment of each cell, in turn, to the entire bone.

Links among growth factors, hormones, and nuclear factors with essential roles in bone formation

Research performed during the last several years implicates important roles for a variety of growth factors that affect osteoblasts or their precursors during bone development, remodeling, or repair. Of these, three families of growth factors in particular-the transforming growth factor betas (TGF-betas), insulin-like growth factors (IGFs), and bone morphogenetic proteins (BMPs)-are considered to be principal local regulators of osteogenesis, although none is specific for cells of the osteoblast lineage. Therefore, mechanisms to induce skeletal tissue specificity might occur through interactions among these growth factors, with circulating hormones, or through specific intracellular mediators. In the latter case, even more recent studies point to two nuclear transcription factors, termed Core Binding Factor a1 (CBFa1) and CCAAT/Enhancer Binding Protein delta (C/EBPdelta), as significant regulators of the expression or activity of specific bone growth factors or their receptors. Perhaps more importantly, events that link these growth factors to nuclear proteins occur in response to glucocorticoids, sex steroids, parathyroid hormone (PTH), or prostaglandin E2 (PGE2), which themselves have well-known effects on bone biology. In this review, we discuss the situations and processes that initially suggested growth-factor- and hormone-specific interactions on cells within the osteoblast lineage, and present evidence for roles that CBFa1 and C/EBPdelta have on osteoblast function. Finally, we offer examples for how these factors integrate events that are associated with various aspects of bone formation.

Runt domain factor (Runx)-dependent effects on CCAAT/ enhancer-binding protein delta expression and activity in osteoblasts

Transcription factor CCAAT/enhancer-binding protein delta (C/EBPdelta) is normally associated with acute-phase gene expression. However, it is expressed constitutively in primary osteoblast cultures where it increases insulin-like growth factor I synthesis in a cAMP-dependent way. Here we show that the 3' proximal region of the C/EBPdelta gene promoter contains a binding sequence for Runt domain factor Runx2, which is essential for osteogenesis. This region of the C/EBPdelta promoter directed high reporter gene expression in osteoblasts, and specifically bound Runx2 in osteoblast-derived nuclear extract. C/EBPdelta gene promoter activity was reduced by mutating the Runx binding sequence or by co-transfecting with Runx2 antisense expression plasmid, and was enhanced by overexpression of Runx-2. Exposure to prostaglandin E(2) increased Runx-dependent gene transactivation independently of Runx2 binding to DNA. Runx2 bound directly to the carboxyl-terminal region of C/EBPdelta itself, and its ability to drive C/EBPdelta expression was suppressed when C/EBPdelta or its carboxyl-terminal fragment was increased by overexpression. Consistent effects also occurred on C/EBPdelta-dependent increases in gene expression driven by synthetic or insulin-like growth factor I gene promoter fragments. These interactions between Runx2 and C/EBPdelta, and their activation by prostaglandin E(2), provide new evidence for their importance during skeletal remodeling, inflammatory bone disease, or fracture repair.