Mitogen-activated protein kinase phosphatase 1/dual specificity phosphatase 1 mediates glucocorticoid inhibition of osteoblast proliferation

The combination of olive oil and Lepidium sativum improves the deleterious effects resulting from dexamethasone-induced osteoporosis in rats

Introduction

Osteoporosis is characterized by deterioration of bone microarchitecture and reduced bone mass and can increase the risk of fracture. To reduce this risk, the aim of this study was to compare the combination effects of olive oil and Lepidium sativum compared to the conventional drug therapy alendronate.

Methods

Osteoporosed-induced rat model was established by administration of dexamethasone in female adult albino rats. The serum level of Ca2+, P3+, and osteocalcin was assessed. In addition, histopathological changes and immunohistochemical expression of osteopontin within bone specimens were performed.

Results

Our results showed that a combination of olive oil and Lepidium sativum had a beneficial therapeutic effect in the treatment of osteoporosis as compared to alendronate therapy. This was demonstrated by increase of serum Ca2+, P3+, and osteocalcin levels in treated compared to control groups. Intriguingly, the highest effect was noticed in rats that received a combination of olive oil and Lepidium sativum compared to the individual treatment. This was reflected by an increase in the cortical bone thickness and a decrease in immunohistochemical expression of osteopontin compared to individual treated groups.

Conclusion

We concluded that the administration of a combination of olive oil and Lepidium sativum improves bone mineral health and intensity and reduces the risk of osteoporosis in a rat model.

Comparative study of holothurin A and echinoside A on inhibiting the high bone turnover via downregulating PI3K/AKT/β-catenin and OPG/RANKL/NF-κB signaling in ovariectomized mice

Holothurin A (HA) and Echinoside A (EA) are the most abundant monomers in sea cucumber saponins, exhibiting different structures only in the side chain at position 20. However, although sea cucumber saponins have been proved to have osteogenic activity, the effect and structure-activity relationship of sea cucumber saponins to improve osteoporosis remain unknown. In the current study, mice with ovariectomization-induced osteoporosis were orally administered with HA and EA for 90 days. The result showed that both HA and EA reduced the levels of serum osteogenesis markers ALP, collagen I, and OCN and bone resorption markers MMP-9, Cath-K and TRAP, and thus inhibited the high bone turnover induced by ovariectomy. Furthermore, we found that HA and EA increased the bone mineral density and apposition rate, reversed the loss of trabecular bone and bone marrow stroma, in which EA exhibited more effective effects. CB1 and MKP-1 are the targets of the glucocorticoid-like effect of saponins. PCR and western blot results indicated that HA and EA alleviated overactive osteogenesis via stimulating CB1 and MKP-1, downregulating the PI3K/AKT/β-catenin signal pathway. The OPG/RANKL/NF-κB pathway was identified as a critical regulator of bone resorption. Further investigation revealed that HA and EA significantly downregulate the expression of IKK, NF-κB and phosphorylated NF-κB p65, suppressing the expression of osteoclastogenesis transcription factors c-fos and NFATC1. To the best of our knowledge, this is the first report showing that both HA and EA improved osteoporosis, and these activities depend on the structure of saponins. These findings would provide guidance for the application of saponins in the management of osteoporosis.

Bad to the Bone: The Effects of Therapeutic Glucocorticoids on Osteoblasts and Osteocytes

Despite the continued development of specialized immunosuppressive therapies in the form of monoclonal antibodies, glucocorticoids remain a mainstay in the treatment of rheumatological and auto-inflammatory disorders. Therapeutic glucocorticoids are unmatched in the breadth of their immunosuppressive properties and deliver their anti-inflammatory effects at unparalleled speed. However, long-term exposure to therapeutic doses of glucocorticoids decreases bone mass and increases the risk of fractures - particularly in the spine - thus limiting their clinical use. Due to the abundant expression of glucocorticoid receptors across all skeletal cell populations and their respective progenitors, therapeutic glucocorticoids affect skeletal quality through a plethora of cellular targets and molecular mechanisms. However, recent evidence from rodent studies, supported by clinical data, highlights the considerable role of cells of the osteoblast lineage in the pathogenesis of glucocorticoid-induced osteoporosis: it is now appreciated that cells of the osteoblast lineage are key targets of therapeutic glucocorticoids and have an outsized role in mediating their undesirable skeletal effects. As part of this article, we review the molecular mechanisms underpinning the detrimental effects of supraphysiological levels of glucocorticoids on cells of the osteoblast lineage including osteocytes and highlight the clinical implications of recent discoveries in the field.

Role of Dual-Specificity Phosphatase 1 in Glucocorticoid-Driven Anti-inflammatory Responses

Glucocorticoids (GCs) potently inhibit pro-inflammatory responses and are widely used for the treatment of inflammatory diseases, such as allergies, autoimmune disorders, and asthma. Dual-specificity phosphatase 1 (DUSP1), also known as mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1), exerts its effects by dephosphorylation of MAPKs, i.e., extracellular-signal-regulated kinase (ERK), p38, and c-Jun N-terminal kinase (JNK). Endogenous DUSP1 expression is tightly regulated at multiple levels, involving both transcriptional and post-transcriptional mechanisms. DUSP1 has emerged as a central mediator in the resolution of inflammation, and upregulation of DUSP1 by GCs has been suggested to be a key mechanism of GC actions. In this review, we discuss the impact of DUSP1 on the efficacy of GC-mediated suppression of inflammation and address the underlying mechanisms.

Osteoblast autophagy in glucocorticoid-induced osteoporosis

Administration of glucocorticoids is an effective strategy for treating many inflammatory and autoimmune diseases. However, glucocorticoid treatment can have adverse effects on bone, leading to glucocorticoid-induced osteoporosis (GIO), the most common form of secondary osteoporosis. Although the pathogenesis of GIO has been studied for decades, over the past ten years the autophagy machinery has been implicated as a novel mechanism. Autophagy in osteoblasts, osteocytes, and osteoclasts plays a critical role in the maintenance of bone homeostasis. Herein, we specifically discuss how osteoblast autophagy responds to glucocorticoids and its role in the development of GIO.

Molecular mechanisms of glucocorticoids on skeleton and bone regeneration after fracture

Glucocorticoid hormones (GCs) have profound effects on bone metabolism. Via their nuclear hormone receptor - the GR - they act locally within bone cells and modulate their proliferation, differentiation, and cell death. Consequently, high glucocorticoid levels - as present during steroid therapy or stress - impair bone growth and integrity, leading to retarded growth and glucocorticoid-induced osteoporosis, respectively. Because of their profound impact on the immune system and bone cell differentiation, GCs also affect bone regeneration and fracture healing. The use of conditional-mutant mouse strains in recent research provided insights into the cell-type-specific actions of the GR. However, despite recent advances in system biology approaches addressing GR genomics in general, little is still known about the molecular mechanisms of GCs and GR in bone cells. Here, we review the most recent findings on the molecular mechanisms of the GR in general and the known cell-type-specific actions of the GR in mesenchymal cells and their derivatives as well as in osteoclasts during bone homeostasis, GC excess, bone regeneration and fracture healing.

MAPK signaling has stage-dependent osteogenic effects on human adipose-derived stem cells in vitro

OVERVIEW

The use of pro-osteogenic growth factors, such as BMP2, in human adipose-derived stem cell (ASC) osteogenesis is well described. Because these growth factors work via signal transduction pathways, such as the mitogen-activated protein kinase (MAPK) cascade, a study of the relationship between MAPK signaling and ASC osteogenesis was conducted.

MATERIALS AND METHODS

ERK, JNK, and p38MAPK activation were measured in ASCs osteo-induced using either dexamethasone or vitamin D3 and correlated with mineralization. Activation and mineralization were also measured without dexamethasone or using the glucocorticoid, cortisone. The expression of the MAPK phosphatase, MKP1, and its relationship to mineralization was also assessed. The effect of decreasing MAPK activation on mineralization through the use of exogenous inhibitors was examined along with siRNA-knockdown and adenoviral overexpression of ERK1/2. Finally, the effect of ERK1/2 overexpression on ASCs induced on PLGA scaffolds was assessed.

RESULTS

ASC mineralization in dexamethasone or vitamin D3-induced ASCs correlated with both increased ERK1/2 and JNK1/2 activation. ASCs induced without dexamethasone also mineralized, with JNK1/2 signaling possibly mediating this event. No link between cortisone induction and MAPK signaling could be ascertained. ASCs treated with ERK, JNK, or p38MAPK inhibitors showed decreased osteogenic gene expression and diminished mineralization. Mineralization levels were also affected by viruses designed to inhibit or augment ERK1/2 expression and activity. Finally, ASC mineralization appeared to be a balance between the MAPK kinase activity and MKP1.

CONCLUSIONS

It is likely that MAPK signaling plays a significant role in ASC osteogenesis, affecting differentiation in kinase- and stage-specific manners.

Impact of glucocorticoid on neurogenesis

Neurogenesis is currently an area of great interest in neuroscience. It is closely linked to brain diseases, including mental disorders and neurodevelopmental disease. Both embryonic and adult neurogeneses are influenced by glucocorticoids secreted from the adrenal glands in response to a variety of stressors. Moreover, proliferation/differentiation of the neural stem/progenitor cells (NSPCs) is affected by glucocorticoids through intracellular signaling pathways such as phosphoinositide 3-kinase (PI3K)/Akt, hedgehog, and Wnt. Our review presents recent evidence of the impact of glucocorticoids on NSPC behaviors and the underlying molecular mechanisms; this provides important information for understanding the pathological role of glucocorticoids on neurogenesis-associated brain diseases.

Understanding how long-acting β2 -adrenoceptor agonists enhance the clinical efficacy of inhaled corticosteroids in asthma - an update

In moderate-to-severe asthma, adding an inhaled long-acting β₂ -adenoceptor agonist (LABA) to an inhaled corticosteroid (ICS) provides better disease control than simply increasing the dose of ICS. Acting on the glucocorticoid receptor (GR, gene NR3C1), ICSs promote anti-inflammatory/anti-asthma gene expression. In vitro, LABAs synergistically enhance the maximal expression of many glucocorticoid-induced genes. Other genes, including dual-specificity phosphatase 1(DUSP1) in human airways smooth muscle (ASM) and epithelial cells, are up-regulated additively by both drug classes. Synergy may also occur for LABA-induced genes, as illustrated by the bronchoprotective gene, regulator of G-protein signalling 2 (RGS2) in ASM. Such effects cannot be produced by either drug alone and may explain the therapeutic efficacy of ICS/LABA combination therapies. While the molecular basis of synergy remains unclear, mechanistic interpretations must accommodate gene-specific regulation. We explore the concept that each glucocorticoid-induced gene is an independent signal transducer optimally activated by a specific, ligand-directed, GR conformation. In addition to explaining partial agonism, this realization provides opportunities to identify novel GR ligands that exhibit gene expression bias. Translating this into improved therapeutic ratios requires consideration of GR density in target tissues and further understanding of gene function. Similarly, the ability of a LABA to interact with a glucocorticoid may be suboptimal due to low β₂ -adrenoceptor density or biased β₂ -adrenoceptor signalling. Strategies to overcome these limitations include adding-on a phosphodiesterase inhibitor and using agonists of other Gs-coupled receptors. In all cases, the rational design of ICS/LABA, and derivative, combination therapies requires functional knowledge of induced (and repressed) genes for therapeutic benefit to be maximized.

Dicer ablation in osteoblasts by Runx2 driven cre-loxP recombination affects bone integrity, but not glucocorticoid-induced suppression of bone formation

Glucocorticoid-induced osteoporosis (GIO) is one of the major side effects of long-term glucocorticoid (GC) therapy mediated mainly via the suppression of bone formation and osteoblast differentiation independently of GC receptor (GR) dimerization. Since microRNAs play a critical role in osteoblast differentiation processes, we investigated the role of Dicer dependent microRNAs in the GC-induced suppression of osteoblast differentiation. MicroRNA sequencing of dexamethasone-treated wild-type and GR dimer-deficient mesenchymal stromal cells revealed GC-controlled miRNA expression in a GR dimer-dependent and GR dimer-independent manner. To determine the functional relevance of mature miRNAs in GC-induced osteoblast suppression, mice with an osteoblast-specific deletion of Dicer (Dicer^Runx2Cre) were exposed to glucocorticoids. In vitro generated Dicer-deficient osteoblasts were treated with dexamethasone and analyzed for proliferation, differentiation and mineralization capacity. In vivo, abrogation of Dicer-dependent miRNA biogenesis in osteoblasts led to growth retardation and impaired bone formation. However, subjecting these mice to GIO showed that bone formation was similar reduced in Dicer^Runx2Cre mice and littermate control mice upon GC treatment. In line, differentiation of Dicer deficient osteoblasts was suppressed to the same extent as wild type cells by GC treatment. Therefore, Dicer-dependent small RNA biogenesis in osteoblasts plays only a minor role in the pathogenesis of GC-induced inhibition of bone formation.

Molecular Actions of Glucocorticoids in Cartilage and Bone During Health, Disease, and Steroid Therapy

Cartilage and bone are severely affected by glucocorticoids (GCs), steroid hormones that are frequently used to treat inflammatory diseases. Major complications associated with long-term steroid therapy include impairment of cartilaginous bone growth and GC-induced osteoporosis. Particularly in arthritis, GC application can increase joint and bone damage. Contrarily, endogenous GC release supports cartilage and bone integrity. In the last decade, substantial progress in the understanding of the molecular mechanisms of GC action has been gained through genome-wide binding studies of the GC receptor. These genomic approaches have revolutionized our understanding of gene regulation by ligand-induced transcription factors in general. Furthermore, specific inactivation of GC signaling and the GC receptor in bone and cartilage cells of rodent models has enabled the cell-specific effects of GCs in normal tissue homeostasis, inflammatory bone diseases, and GC-induced osteoporosis to be dissected. In this review, we summarize the current view of GC action in cartilage and bone. We further discuss future research directions in the context of new concepts for optimized steroid therapies with less detrimental effects on bone.

Glucocorticoid-Induced Osteoporosis

Osteoporosis is among the most devastating side effects of glucocorticoid (GC) therapy for the management of inflammatory and auto-immune diseases. Evidence from both humans and mice indicate deleterious skeletal effects within weeks of pharmacological GC administration, both related and unrelated to a decrease in bone mineral density (BMD). Osteoclast numbers and bone resorption are also rapidly increased, and together with osteoblast inactivation and decreased bone formation, these changes lead the fastest loss in BMD during the initial disease phase. Bone resorption then decreases to sub-physiological levels, but persistent and severe inhibition of bone formation leads to further bone loss and progressively increased fracture risk, up to an order of magnitude higher than that observed in untreated individuals. Bone forming osteoblasts are thus considered the main culprits in GC-induced osteoporosis (GIO). Accordingly, we focus this review primarily on deleterious effects on osteoblasts: inhibition of cell replication and function and acceleration of apoptosis. Mediating these adverse effects, GCs target pivotal regulatory mechanisms that govern osteoblast growth, differentiation and survival. Specifically, GCs inhibit growth factor pathways, including Insulin Growth Factors, Growth Hormone, Hepatocyte Growth/Scatter Factor and IL6-type cytokines. They also inhibit downstream kinases, including PI3-kinase and the MAP kinase ERK, the latter attributable in part to direct transcriptional stimulation of MAP kinase phosphatase 1. Most importantly, however, GCs inhibit the Wnt signaling pathway, which plays a pivotal role in osteoblast replication, function and survival. They transcriptionally stimulate expression of Wnt inhibitors of both the Dkk and Sfrp families, and they induce reactive oxygen species (ROS), which result in loss of ß-catenin to ROS-activated FoxO transcription factors. Identification of dissociated GCs, which would suppress the immune system without causing osteoporosis, is proving more challenging than initially thought, and GIO is currently managed by co-treatment with bisphosphonates or PTH. These drugs, however, are not ideally suited for GIO. Future therapeutic approaches may aim at GC targets such as those mentioned above, or newly identified targets including the Notch pathway, the AP-1/Il11 axis and the osteoblast master regulator RUNX2.

The Role of MKP-1 in the Anti-Proliferative Effects of Glucocorticoids in Primary Rat Pre-Osteoblasts

Glucocorticoid (GC)-induced osteoporosis has been attributed to a GC-induced suppression of pre-osteoblast proliferation. Our previous work identified a critical role for mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) in mediating the anti-proliferative effects of GCs in immortalized pre-osteoblasts, but we subsequently found that MKP-1 null mice were not protected against the pathological effects of GCs on bone. In order to reconcile this discrepancy, we have assessed the effects of GCs on proliferation, activation of the MAPK ERK1/2 and MKP-1 expression in primary adipose-derived stromal cells (ADSCs) and ADSC-derived pre-osteoblasts (ADSC-OBs). ADSCs were isolated by means of collagenase digestion from adipose tissue biopsies harvested from adult male Wistar rats. ADSC-OBs were prepared by treating ADSCs with osteoblast differentiation media for 7 days. The effects of increasing concentrations of the GC dexamethasone on basal and mitogen-stimulated cell proliferation were quantified by tritiated thymidine incorporation. ERK1/2 activity was measured by Western blotting, while MKP-1 expression was quantified on both RNA and protein levels, using semi-quantitative real-time PCR and Western blotting, respectively. GCs were strongly anti-proliferative in both naïve ADSCs and ADSC-OBs, but had very little effect on mitogen-induced ERK1/2 activation and did not upregulate MKP-1 protein expression. These findings suggest that the anti-proliferative effects of GCs in primary ADSCs and ADSC-OBs in vitro do not require the inhibition of ERK1/2 activation by MKP-1, which is consistent with our in vivo findings in MKP-1 null mice.

Targeting glucocorticoid side effects: selective glucocorticoid receptor modulator or glucocorticoid-induced leucine zipper? A perspective

Glucocorticoids (GCs) are steroid hormones that are necessary for life and important in health and disease. They regulate crucial homeostatic functions, including metabolism, cell growth, and development. Although GCs are regulated by circadian rhythm, increased production is associated with stress. Synthetic GCs are a valuable resource for anti-inflammatory and immunosuppressive therapy. Natural and synthetic GCs transduce signals mainly through GC receptor (GR) activation. Extensive research has explored the downstream targets of the GR, and optimization of GC therapy has required collaborative efforts. One highly promising approach involves new dissociative GR modulators. Because transrepression and transactivation of GR genes induce beneficial and adverse effects, respectively, this approach favors transrepression. Another approach involves the use of GC-dependent genes to generate proteins to mediate therapeutic GC effects. In a third approach, drug discovery is used to identify agents that selectively target GR isoforms to obtain differential gene transcription and effects. In this review, we focus on mechanisms of GR function compatible with the use of dissociative drugs. We highlight GC-induced leucine zipper (GILZ), a gene cloned in our laboratory, as a mediator of GC anti-inflammatory and immunosuppressive effects, to outline our perspective on the future of GC therapy.

Nitric oxide affects ERK signaling through down-regulation of MAP kinase phosphatase levels during larval development of the ascidian Ciona intestinalis

In the ascidian Ciona intestinalis larval development and metamorphosis require a complex interplay of events, including nitric oxide (NO) production, MAP kinases (ERK, JNK) and caspase-3 activation. We have previously shown that NO levels affect the rate of metamorphosis, regulate caspase activity and promote an oxidative stress pathway, resulting in protein nitration. Here, we report that NO down-regulates MAP kinase phosphatases (mkps) expression affecting positively ERK signaling. By pharmacological approach, we observed that the reduction of endogenous NO levels caused a decrease of ERK phosphorylation, whereas increasing levels of NO induced ERK activation. We have also identified the ERK gene network affected by NO, including mpk1, mpk3 and some key developmental genes by quantitative gene expression analysis. We demonstrate that NO induces an ERK-independent down-regulation of mkp1 and mkp3, responsible for maintaining the ERK phosphorylation levels necessary for transcription of key metamorphic genes, such as the hormone receptor rev-erb and the van willebrand protein vwa1c. These results add new insights into the role played by NO during larval development and metamorphosis in Ciona, highlighting the cross-talk between different signaling pathways.

Glucocorticoid induced osteoblast apoptosis by increasing E4BP4 expression via up-regulation of Bim

It is well known that glucocorticoid (GC)-induced bone loss is caused primarily by hypofunction and apoptosis of osteoblasts. However, the precise molecular events underlying the effect of GC on osteoblast apoptosis are not fully understood. Recent studies implicated an important role of E4BP4 in the regulation of osteoblast apoptosis and differentiation. Furthermore, E4BP4 is a GC-regulated gene required for GC-induced apoptosis in many cells. Therefore, we hypothesize that E4BP4 may be implicated in the process of GC-induced osteoblast apoptosis. Western blot, reverse-transcription-PCR, flow cytometry, and Hoechst 33258 staining were employed to investigate the role of E4BP4 in dexamethasone (DEX)-induced osteoblast apoptosis. We found that the expression of E4BP4 is significantly up-regulated in osteoblasts exposed to DEX. Furthermore, the depletion of E4BP4 significantly decreased DEX-induced osteoblast apoptosis. In addition, E4BP4 plays a crucial role in GC-evoked apoptosis of osteoblasts by enabling induction of Bim. On the basis of these results above, we can draw the conclusion that E4BP4 may contribute to the process of DEX-induced osteoblast apoptosis.

Glucocorticoids and bone: local effects and systemic implications

Glucocorticoids (GCs) are highly effective in the treatment of inflammatory and autoimmune conditions but their therapeutic use is limited by numerous adverse effects. Recent insights into the mechanisms of action of both endogenous and exogenous GCs on bone cells have unlocked new approaches to the development of effective strategies for the prevention and treatment of GC-induced osteoporosis. Furthermore, topical studies in rodents indicate that the osteoblast-derived peptide, osteocalcin, plays a central role in the pathogenesis of GC-induced diabetes and obesity. These exciting findings mechanistically link the detrimental effects of GCs on bone and energy metabolism. In this article we review the physiology and pathophysiology of GC action on bone cells, and discuss current and emerging concepts regarding the molecular mechanisms underlying adverse effects of GCs such as osteoporosis and diabetes.

Molecular determinants of glucocorticoid actions in inflammatory joint diseases

Since their discovery in 1948, glucocorticoids have been widely used clinically to treat inflammatory disorders like rheumatoid arthritis. However, their usefulness, especially in rheumatoid arthritis therapy, is hampered by severe side effects on bone leading to glucocorticoid-induced osteoporosis. The molecular and cellular mechanisms mediating the beneficial and adverse effects remain poorly understood. Nevertheless, advanced molecular biological analyses and in vivo approaches using conditional mutant mice have helped to unravel in part the underlying mechanisms of immunosuppression and side effects of glucocorticoid therapy in arthritis, thereby contributing to an improved understanding of these therapeutically important hormones.

Mechanisms of bone anabolism regulated by statins

Osteoporosis is a common disease in the elderly population. The progress of this disease results in the reduction of bone mass and can increase the incidence of fractures. Drugs presently used clinically can block the aggravation of this disease. However, these drugs cannot increase the bone mass and may result in certain side effects. Statins, also known as HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) reductase inhibitors, have been widely prescribed for CVD (cardiovascular disease) for decades. Nonetheless, several studies have demonstrated that statins exert bone anabolic effect and may be helpful for the treatment of osteoporosis. Several experiments have analysed the mechanisms of bone anabolism regulated by statins. In the present paper, we review the mechanisms of promoting osteogenesis, suppressing osteoblast apoptosis and inhibiting osteoclastogenesis.

The multiple facets of glucocorticoid action in rheumatoid arthritis

Glucocorticoids have potent anti-inflammatory effects and have been used to treat patients with rheumatoid arthritis for more than 60 years. However, severe adverse effects of glucocorticoid treatment, including loss of bone mass and increased risk of fractures, are common. Data from studies of glucocorticoid-mediated gene regulation, which utilized conditional knockout mice in animal models of arthritis or glucocorticoid-induced osteoporosis, have substantially increased our understanding of the mechanisms by which glucocorticoids act via the glucocorticoid receptor. Following glucocorticoid binding, the receptor regulates gene expression either by interacting with DNA-bound transcription factors as a monomer or by binding directly to DNA as a dimer. In contrast to the old hypothesis that transrepression mechanisms involving monomeric glucocorticoid receptor actions were responsible for the anti-inflammatory effects of glucocorticoids, whereas dimeric glucocorticoid receptor binding resulted in adverse effects, data from animal models have shown that the anti-inflammatory and adverse effects of glucocorticoids are mediated by both monomeric and dimeric glucocorticoid receptor binding. This improved knowledge of the molecular mechanisms that underlie the beneficial and adverse effects of glucocorticoid therapy might lead to the development of rationales for novel glucocorticoid receptor ligands that could potentially have anti-inflammatory efficacy without adverse effects on bone.

Inflammatory regulation of glucocorticoid metabolism in mesenchymal stromal cells

Objective

Tissue glucocorticoid (GC) levels are regulated by the GC-activating enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1). This enzyme is expressed in cells and tissues arising from mesenchymal stromal cells. Proinflammatory cytokines dramatically increase expression of 11β-HSD1 in stromal cells, an effect that has been implicated in inflammatory arthritis, osteoporosis, obesity, and myopathy. Additionally, GCs act synergistically with proinflammatory cytokines to further increase enzyme expression. The present study was undertaken to investigate the mechanisms underlying this regulation.

Methods

Gene reporter analysis, rapid amplification of complementary DNA ends (RACE), chemical inhibition experiments, and genetic disruption of intracellular signaling pathways in mouse embryonic fibroblasts (MEFs) were used to define the molecular mechanisms underlying the regulation of 11β-HSD1 expression.

Results

Gene reporter, RACE, and chemical inhibitor studies demonstrated that the increase in 11β-HSD1 expression with tumor necrosis factor α (TNFα)/interleukin-1β (IL-1β) occurred via the proximal HSD11B1 gene promoter and depended on NF-κB signaling. These findings were confirmed using MEFs with targeted disruption of NF-κB signaling, in which RelA (p65) deletion prevented TNFα/IL-1β induction of 11β-HSD1. GC treatment did not prevent TNFα-induced NF-κB nuclear translocation. The synergistic enhancement of TNFα-induced 11β-HSD1 expression with GCs was reproduced by specific inhibitors of p38 MAPK. Inhibitor and gene deletion studies indicated that the effects of GCs on p38 MAPK activity occurred primarily through induction of dual-specificity phosphatase 1 expression.

Conclusion

The mechanism by which stromal cell expression of 11β-HSD1 is regulated is novel and distinct from that in other tissues. These findings open new opportunities for development of therapeutic interventions aimed at inhibiting or stimulating local GC levels in cells of mesenchymal stromal lineage during inflammation.

Mechanisms of the anti-inflammatory effects of glucocorticoids: genomic and nongenomic interference with MAPK signaling pathways

Glucocorticoids (GCs) are steroid hormones produced by the adrenal gland and regulated by the hypothalamus-pituitary-adrenal axis. GCs mediate effects that mostly result in transcriptional regulation of glucocorticoid receptor target genes. Mitogen-activated protein kinases (MAPKs) comprise a family of signaling proteins that convert extracellular stimuli into the activation of intracellular transduction pathways via phosphorylation of a cascade of substrates. They modulate a variety of physiological cell processes, such as proliferation, apoptosis, and development. However, when MAPKs are improperly activated by proinflammatory and/or extracellular stress stimuli, they contribute to the regulation of proinflammatory transcription factors, thus perpetuating activation of the inflammatory cascade. One of the mechanisms by which GCs exert their anti-inflammatory effects is negative interference with MAPK signaling pathways. Several functional interactions between GCs and MAPK signaling have been discovered and studied. Some of these interactions involve the GC-mediated up-regulation of proteins that in turn interfere with the activation of MAPK, such as glucocorticoid-induced-leucine zipper, MAPK phosphatase-1, and annexin-1. Other mechanisms include activated GR directly interacting with components of the MAPK pathway and negatively regulating their activation. The multiple interactions between GCs and MAPK pathways and their potential biological relevance in mediating the anti-inflammatory effects of GCs are reviewed.

Glucocorticoid receptor signaling in bone cells

Glucocorticoids are used for treating a wide range of diseases including inflammation and autoimmune disorders. However, there are drawbacks, primarily due to adverse effects on bone cells resulting in osteoporosis. Evidence indicates that the ratio of benefits to adverse effects depends greatly on glucocorticoid receptor (GR)-mediated mechanisms. Delineating GR-mediated signaling in bone cells will allow development of selective GR ligands/agonists (SEGRAs), which would dissociate the positive therapeutic (anti-inflammatory) effects from the negative effects on the skeleton. The present review provides an in-depth account of the current knowledge of GR-mediated transcriptional regulation of specific genes and proteins engaged in the proliferation, differentiation, and apoptosis of bone cells (osteoblasts, osteocytes, osteoclasts). We hope this knowledge will advance research in the development of SEGRAs with improved benefit/risk ratios.

Maps and legends: the quest for dissociated ligands of the glucocorticoid receptor

Glucocorticoids are steroid hormones that have pleiotropic effects on development, metabolism, cognitive function and other aspects of physiology. Since the demonstration more than sixty years ago of their capacity to suppress inflammation, synthetic glucocorticoids have been extremely widely used in the treatment of inflammatory diseases. However, their clinical use is limited by numerous, unpredictable and potentially serious side effects. Glucocorticoids regulate gene expression both positively and negatively. Both of these effects are mediated by the glucocorticoid receptor, a ligand-dependent transcription factor. It has become widely accepted that anti-inflammatory effects of glucocorticoids are mostly due to inhibition of transcription, whereas the activation of transcription by the glucocorticoid receptor accounts for the majority of side effects. This dogma (which we refer to as the "transrepression hypothesis") predicts the possibility of uncoupling therapeutic, anti-inflammatory effects from side effects by identifying novel, selective ligands of the glucocorticoid receptor, which preferentially mediate inhibition rather than activation of transcription. It is argued that such "dissociated" glucocorticoid receptor ligands should retain anti-inflammatory potency but cause fewer side effects. Here we critically re-examine the history and foundations of the transrepression hypothesis. We argue that it is incompatible with the complexity of gene regulation by glucocorticoids and poorly supported by experimental evidence; that it no longer aids clear thinking about the actions of the glucocorticoid receptor; and that it will not prove a fruitful basis for continued refinement and improvement of anti-inflammatory drugs that target the glucocorticoid receptor.

Cannabinoid receptor 1 regulates ERK and GSK-3β-dependent glucocorticoid inhibition of osteoblast differentiation in murine MC3T3-E1 cells

Supraphysiological glucocorticoid administration accelerates loss of survival and differentiation in osteoblastic cells, thereby increasing the risks of osteopenic or osteonecrotic disorders. Neuroendocrine component type 1 cannabinoid receptor (CB1) is found to regulate bone mass. This study characterized the biological role of CB1 in glucocorticoid-induced suppression of osteoblast differentiation. Murine MC3T3-E1 osteoblasts were incubated under osteogenic conditions in the presence or absence of 1 μM glucocorticoid, RNA interference, CB1 antagonist AM251, and agonist WIN55212-2. Cell survival was detected by formazan synthesis and TUNEL staining. Osteoblast differentiation was quantified by mineralized matrix accumulation and expression of the osteogenic factors Runx2 and osteocalcin. Expression of signaling molecules was assessed by immunoblotting. Glucocorticoid increased CB1 expression in association with decreased osteocalcin expression and mineralized nodule deposition. CB1 RNA interference and AM251 attenuated the deleterious actions of glucocorticoid treatment on survival and osteogenic activities, whereas activating CB1 by WIN55212-2 impaired osteoblast differentiation. CB1 signaling regulated JNK, ERK, GSK-3β, and Akt activation as well as Runx2 and IGF-I expression. Inhibition of GSK-3β by the kinase-inactive GSK-3β mutant or activation of ERK by the active MEK-1 mutant abrogated glucocorticoid-induced inhibition of osteoblast differentiation. Glucocorticoid-induced CB1 expression occurred via glucocorticoid receptor-dependent transcriptional and translational regulation. Gain of Runx2 function and loss of MKP-1 action attenuated glucocorticoid-induced enhancement of CB1 expression. Taken together, CB1 regulation of ERK and GSK-3β-dependent pathways participates in glucocorticoid inhibition of Runx2 signaling and osteoblast differentiation. Runx2 reciprocally regulates glucocorticoid-induced promotion of CB1 signaling. Our findings provide new insights into the role of the neuroendocrine component CB1 in glucocorticoid-induced osteoblast dysfunction.

Heat shock protein 60 protects skeletal tissue against glucocorticoid-induced bone mass loss by regulating osteoblast survival

Excessive glucocorticoid administration accelerates osteoblast apoptosis and skeletal deterioration. Heat shock proteins (HSPs) regulate metabolic activities in osteoblastic cells. This study characterized the biological significance of HSP60 in glucocorticoid-induced bone loss. Rats were treated with glucocorticoid, HSP60 antisense oligonucleotides, or adenovirus-mediated HSP60 gene transfer. Bone mineral density, metaphyseal trabecular micro-architecture, and fragility were analyzed by dual X-ray absorptiometry, micro-computed tomography, and material testing, respectively. Differential proteomic profiles of bone tissue extracts were detected by bi-dimensional electrophoresis and mass spectrometry. Survival and proapoptotic signal transduction were quantified by immunoblotting. Glucocorticoid-treated rats had low bone mineral density and metaphyseal trabecular microstructure in association with downregulation of collagen 1α1 and HSP60 expressions in bone tissue. Gain of HSP60 function by adenovirus-mediated HSP60 gene transfer abrogated the deleterious effects of glucocorticoid treatment on bone mass, trabecular microstructure, and mechanical strength. Enhancement of HSP60 signaling attenuated the glucocorticoid-induced loss of trabecular bone volume, mineral acquisition reactions and osteoblast surface. HSP60 gene transfer activated ERK and Akt and reduced Bax and cytochrome c release, as well as caspase-3 cleavage, which attenuated the inhibitory effects of glucocorticoid treatment on osteoblast survival. Loss of HSP60 function by HSP60 antisense oligonucleotides accelerated mitochondrial apoptotic programs and osteoblast apoptosis. Knockdown of HSP60 induced loss of bone mass, micro-architecture integrity, and mechanical property. Taken together, loss of HSP60 signaling contributes to the glucocorticoid-induced enhancement of pro-apoptotic reactions, thereby accelerating osteoblast apoptosis and bone mass loss. Enhancement of HSP60 function is beneficial for protecting bone tissue against the glucocorticoid-induced inhibition of bone cell viability and bone formation.

MKP-1 knockout does not prevent glucocorticoid-induced bone disease in mice

Glucocorticoid-induced osteoporosis (GCOP) is predominantly caused by inhibition of bone formation, resulting from a decrease in osteoblast numbers. Employing mouse (MBA-15.4) and human (MG-63) osteoblast cell lines, we previously found that the glucocorticoid (GC) dexamethasone (Dex) inhibits cellular proliferation as well as activation of the MAPK/ERK signaling pathway, essential for mitogenesis in these cells, and that both these effects could be reversed by the protein tyrosine phosphatase (PTP) inhibitor vanadate. In a rat model of GCOP, the GC-induced changes in bone formation, mass, and strength could be prevented by vanadate cotreatment, suggesting that the GC effects on bone were mediated by one or more PTPs. Employing phosphatase inhibitors, qRT-PCR, Western blotting, and overexpression/knockdown experiments, we concluded that MKP-1 was upregulated by Dex, that this correlated with the dephosphorylation of ERK, and that it largely mediated the in vitro effects of GCs on bone. To confirm the pivotal role of MKP-1 in vivo, we investigated the effects of the GC methylprednisolone on the quantitative bone histology of wild-type (WT) and MKP-1 homozygous knockout (MKP-1(-/-)) mice. In WT mice, static bone histology revealed that GC administration for 28 days decreased osteoid surfaces, volumes, and osteoblast numbers. Dynamic histology, following time-spaced tetracycline labeling, confirmed a significant GC-induced reduction in osteoblast appositional rate and bone formation rate. However, identical results were obtained in MKP-1 knockout mice, suggesting that in these animals upregulation of MKP-1 by GCs cannot be regarded as the sole mediator of the GC effects on bone.

MKP-1 mediates glucocorticoid-induced ERK1/2 dephosphorylation and reduction in pancreatic ß-cell proliferation in islets from early lactating mothers

Maternal pancreatic islets undergo a robust increase of mass and proliferation during pregnancy, which allows a compensation of gestational insulin resistance. Studies have described that this adaptation switches to a low proliferative status after the delivery. The mechanisms underlying this reversal are unknown, but the action of glucocorticoids (GCs) is believed to play an important role because GCs counteract the pregnancy-like effects of PRL on isolated pancreatic islets maintained in cell culture. Here, we demonstrate that ERK1/2 phosphorylation (phospho-ERK1/2) is increased in maternal rat islets isolated on the 19th day of pregnancy. Phospho-ERK1/2 status on the 3rd day after delivery (L3) rapidly turns to values lower than that found in virgin control rats (CTL). MKP-1, a protein phosphatase able to dephosphorylate ERK1/2, is increased in islets from L3 rats. Chromatin immunoprecipitation assay revealed that binding of glucocorticoid receptor (GR) to MKP-1 promoter is also increased in islets from L3 rats. In addition, dexamethasone (DEX) reduced phospho-ERK1/2 and increased MKP-1 expression in RINm5F and MIN-6 cells. Inhibition of transduction with cycloheximide and inhibition of phosphatases with orthovanadate efficiently blocked DEX-induced downregulation of phospho-ERK1/2. In addition, specific knockdown of MKP-1 with siRNA suppressed the downregulation of phospho-ERK1/2 and the reduction of proliferation induced by DEX. Altogether, our results indicate that downregulation of phospho-ERK1/2 is associated with reduction in proliferation found in islets of early lactating mothers. This mechanism is probably mediated by GC-induced MKP-1 expression.

Identification of NURR1 as a mediator of MIF signaling during chronic arthritis: effects on glucocorticoid-induced MKP1

Elucidation of factors regulating glucocorticoid (GC) sensitivity is required for the development of "steroid-sparing" therapies for chronic inflammatory diseases, including rheumatoid arthritis (RA). Accumulating evidence suggests that macrophage migration inhibitory factor (MIF) counterregulates the GC-induction of anti-inflammatory mediators, including mitogen-activated protein kinase phosphatase 1 (MKP1), a critical mitogen-activated protein kinase signaling inhibitor. This observation has yet to be extended to human disease; the molecular mechanisms remain unknown. We investigated NURR1, a GC-responsive transcription factor overexpressed in RA, as a MIF signaling target. We reveal abrogation by recombinant MIF (rMIF) of GC-induced MKP1 expression in RA fibroblast-like synoviocytes (FLS). rMIF enhanced NURR1 expression, artificial NBRE (orphan receptor DNA-binding site) reporter transactivation, and reversed GC-inhibition of NURR1. NURR1 expression was reduced during experimental arthritis in MIF-/- synovium, and silencing MIF reduced RA FLS NURR1 mRNA. Consistent with NBRE identification on the MKP1 gene, MKP1 mRNA was reduced in FLS that ectopically express NURR1, and silencing NURR1 enhanced MKP1 mRNA in RA FLS. rMIF enhanced NBRE binding on the MKP1 gene, and the absence of the NBRE prevented NURR1-repressive effects on basal and GC-induced MKP1 transactivation. This study defines NURR1 as a novel MIF target in chronic inflammation and demonstrates a role for NURR1 in regulating the anti-inflammatory mediator, MKP1. We propose a MIF-NURR1 signaling axis as a regulator of the GC sensitivity of MKP1.

Modulation of gonadotropin-releasing hormone-induced extracellular signal-regulated kinase activation by dual-specificity protein phosphatase 1 in LbetaT2 gonadotropes

As the regulator of pituitary reproductive hormone synthesis, the hypothalamic neuropeptide GnRH is the central regulator of reproduction. A hallmark of GnRH action is the differential control of gene expression in pituitary gonadotropes through varied pulsatile stimulation. Among other signaling events, GnRH activation of the ERK family of MAPKs plays a significant role in the transcriptional regulation of the luteinizing hormone β-subunit gene and regulation of cap-dependent translation. We evaluated the ERK response to different GnRH pulse amplitudes in the gonadotrope cell line LβT2. We found that low-amplitude stimulation with GnRH invokes a rapid and transient ERK activation, whereas high-amplitude stimulation invokes a prolonged activation specifically in the cytoplasm fraction of LβT2 cells. Nuclear and cytoplasmic targets of ERK, Ets-like gene 1, and eukaryotic initiation factor 4E, respectively, are similarly activated. Feedback control of ERK activation occurs mainly through the dual-specificity protein phosphatases (DUSPs). DUSP1 is localized to the nucleus in LβT2 cells but DUSP4, another member implicated in GnRH feedback, exists in both the nucleus and cytoplasm. Manipulation of nuclear DUSP activity through overexpression or knockdown of Dusp1 modulates the ERK response to low and high GnRH pulse amplitudes and activation of the Lhb promoter. Dusp1 overexpression abolishes sustained ERK activation and inhibits Lhb promoter activity induced by high amplitude pulses. Conversely, Dusp1 knockdown enhances ERK activation by low-amplitude stimulation and increases stimulation of Lhb promoter activity. We conclude that DUSP1 feedback activity modulates ERK activation and the transcriptional response to GnRH.

Runx2 transcriptome of prostate cancer cells: insights into invasiveness and bone metastasis

Background

Prostate cancer (PCa) cells preferentially metastasize to bone at least in part by acquiring osteomimetic properties. Runx2, an osteoblast master transcription factor, is aberrantly expressed in PCa cells, and promotes their metastatic phenotype. The transcriptional programs regulated by Runx2 have been extensively studied during osteoblastogenesis, where it activates or represses target genes in a context-dependent manner. However, little is known about the gene regulatory networks influenced by Runx2 in PCa cells. We therefore investigated genome wide mRNA expression changes in PCa cells in response to Runx2.

Results

We engineered a C4-2B PCa sub-line called C4-2B/Rx2^dox, in which Doxycycline (Dox) treatment stimulates Runx2 expression from very low to levels observed in other PCa cells. Transcriptome profiling using whole genome expression array followed by in silico analysis indicated that Runx2 upregulated a multitude of genes with prominent cancer associated functions. They included secreted factors (CSF2, SDF-1), proteolytic enzymes (MMP9, CST7), cytoskeleton modulators (SDC2, Twinfilin, SH3PXD2A), intracellular signaling molecules (DUSP1, SPHK1, RASD1) and transcription factors (Sox9, SNAI2, SMAD3) functioning in epithelium to mesenchyme transition (EMT), tissue invasion, as well as homing and attachment to bone. Consistent with the gene expression data, induction of Runx2 in C4-2B cells enhanced their invasiveness. It also promoted cellular quiescence by blocking the G1/S phase transition during cell cycle progression. Furthermore, the cell cycle block was reversed as Runx2 levels declined after Dox withdrawal.

Conclusions

The effects of Runx2 in C4-2B/Rx2^dox cells, as well as similar observations made by employing LNCaP, 22RV1 and PC3 cells, highlight multiple mechanisms by which Runx2 promotes the metastatic phenotype of PCa cells, including tissue invasion, homing to bone and induction of high bone turnover. Runx2 is therefore an attractive target for the development of novel diagnostic, prognostic and therapeutic approaches to PCa management. Targeting Runx2 may prove more effective than focusing on its individual downstream genes and pathways.

Protein kinase networks regulating glucocorticoid-induced apoptosis of hematopoietic cancer cells: fundamental aspects and practical considerations

Glucocorticoids (GCs) are integral components in the treatment protocols of acute lymphoblastic leukemia, multiple myeloma, and non-Hodgkin lymphoma owing to their ability to induce apoptosis of these malignant cells. Resistance to GC therapy is associated with poor prognosis. Although they have been used in clinics for decades, the signal transduction pathways involved in GC-induced apoptosis have only partly been resolved. Accumulating evidence shows that this cell death process is mediated by a communication between nuclear GR affecting gene transcription of pro-apoptotic genes such as Bim, mitochondrial GR affecting the physiology of the mitochondria, and the protein kinase glycogen synthase kinase-3 (GSK3), which interacts with Bim following exposure to GCs. Prevention of Bim up-regulation, mitochondrial GR translocation, and/or GSK3 activation are common causes leading to GC therapy failure. Various protein kinases positively regulating the pro-survival Src-PI3K-Akt-mTOR and Raf-Ras-MEK-ERK signal cascades have been shown to be activated in malignant leukemic cells and antagonize GC-induced apoptosis by inhibiting GSK3 activation and Bim expression. Targeting these protein kinases has proven effective in sensitizing GR-positive malignant lymphoid cells to GC-induced apoptosis. Thus, intervening with the pro-survival kinase network in GC-resistant cells should be a good means of improving GC therapy of hematopoietic malignancies.

Identification of NURR1 as a mediator of MIF signaling during chronic arthritis: effects on glucocorticoid-induced MKP1

Elucidation of factors regulating glucocorticoid (GC) sensitivity is required for the development of "steroid-sparing" therapies for chronic inflammatory diseases, including rheumatoid arthritis (RA). Accumulating evidence suggests that macrophage migration inhibitory factor (MIF) counterregulates the GC-induction of anti-inflammatory mediators, including mitogen-activated protein kinase phosphatase 1 (MKP1), a critical mitogen-activated protein kinase signaling inhibitor. This observation has yet to be extended to human disease; the molecular mechanisms remain unknown. We investigated NURR1, a GC-responsive transcription factor overexpressed in RA, as a MIF signaling target. We reveal abrogation by recombinant MIF (rMIF) of GC-induced MKP1 expression in RA fibroblast-like synoviocytes (FLS). rMIF enhanced NURR1 expression, artificial NBRE (orphan receptor DNA-binding site) reporter transactivation, and reversed GC-inhibition of NURR1. NURR1 expression was reduced during experimental arthritis in MIF-/- synovium, and silencing MIF reduced RA FLS NURR1 mRNA. Consistent with NBRE identification on the MKP1 gene, MKP1 mRNA was reduced in FLS that ectopically express NURR1, and silencing NURR1 enhanced MKP1 mRNA in RA FLS. rMIF enhanced NBRE binding on the MKP1 gene, and the absence of the NBRE prevented NURR1-repressive effects on basal and GC-induced MKP1 transactivation. This study defines NURR1 as a novel MIF target in chronic inflammation and demonstrates a role for NURR1 in regulating the anti-inflammatory mediator, MKP1. We propose a MIF-NURR1 signaling axis as a regulator of the GC sensitivity of MKP1.

Identification of NURR1 as a mediator of MIF signaling during chronic arthritis: effects on glucocorticoid-induced MKP1

Elucidation of factors regulating glucocorticoid (GC) sensitivity is required for the development of "steroid-sparing" therapies for chronic inflammatory diseases, including rheumatoid arthritis (RA). Accumulating evidence suggests that macrophage migration inhibitory factor (MIF) counterregulates the GC-induction of anti-inflammatory mediators, including mitogen-activated protein kinase phosphatase 1 (MKP1), a critical mitogen-activated protein kinase signaling inhibitor. This observation has yet to be extended to human disease; the molecular mechanisms remain unknown. We investigated NURR1, a GC-responsive transcription factor overexpressed in RA, as a MIF signaling target. We reveal abrogation by recombinant MIF (rMIF) of GC-induced MKP1 expression in RA fibroblast-like synoviocytes (FLS). rMIF enhanced NURR1 expression, artificial NBRE (orphan receptor DNA-binding site) reporter transactivation, and reversed GC-inhibition of NURR1. NURR1 expression was reduced during experimental arthritis in MIF-/- synovium, and silencing MIF reduced RA FLS NURR1 mRNA. Consistent with NBRE identification on the MKP1 gene, MKP1 mRNA was reduced in FLS that ectopically express NURR1, and silencing NURR1 enhanced MKP1 mRNA in RA FLS. rMIF enhanced NBRE binding on the MKP1 gene, and the absence of the NBRE prevented NURR1-repressive effects on basal and GC-induced MKP1 transactivation. This study defines NURR1 as a novel MIF target in chronic inflammation and demonstrates a role for NURR1 in regulating the anti-inflammatory mediator, MKP1. We propose a MIF-NURR1 signaling axis as a regulator of the GC sensitivity of MKP1.

Crosstalk between TNF and glucocorticoid receptor signaling pathways

TNF is a Janus-faced protein. It possesses impressive anti-tumor activities, but it is also one of the strongest known pro-inflammatory cytokines, which hampers its use as a systemic anti-cancer agent. TNF has been shown to play a detrimental role in inflammatory diseases such as rheumatoid arthritis and inflammatory bowel disease. Glucocorticoids are strongly anti-inflammatory and exert their therapeutic effects through binding to their receptor, the glucocorticoid receptor. Therefore, glucocorticoids have been used for over half a century for the treatment of inflammatory diseases. However, many patients are or become resistant to the therapeutic effects of glucocorticoids. Inflammatory cytokines have been suggested to play an important role in this steroid insensitivity or glucocorticoid resistance. This review aims to highlight the mechanisms of mutual inhibition between TNF and GR signaling pathways.

Systems-level comparison of host-responses elicited by avian H5N1 and seasonal H1N1 influenza viruses in primary human macrophages

Human disease caused by highly pathogenic avian influenza (HPAI) H5N1 can lead to a rapidly progressive viral pneumonia leading to acute respiratory distress syndrome. There is increasing evidence from clinical, animal models and in vitro data, which suggests a role for virus-induced cytokine dysregulation in contributing to the pathogenesis of human H5N1 disease. The key target cells for the virus in the lung are the alveolar epithelium and alveolar macrophages, and we have shown that, compared to seasonal human influenza viruses, equivalent infecting doses of H5N1 viruses markedly up-regulate pro-inflammatory cytokines in both primary cell types in vitro. Whether this H5N1-induced dysregulation of host responses is driven by qualitative (i.e activation of unique host pathways in response to H5N1) or quantitative differences between seasonal influenza viruses is unclear. Here we used microarrays to analyze and compare the gene expression profiles in primary human macrophages at 1, 3, and 6 h after infection with H5N1 virus or low-pathogenic seasonal influenza A (H1N1) virus. We found that host responses to both viruses are qualitatively similar with the activation of nearly identical biological processes and pathways. However, in comparison to seasonal H1N1 virus, H5N1 infection elicits a quantitatively stronger host inflammatory response including type I interferon (IFN) and tumor necrosis factor (TNF)-α genes. A network-based analysis suggests that the synergy between IFN-β and TNF-α results in an enhanced and sustained IFN and pro-inflammatory cytokine response at the early stage of viral infection that may contribute to the viral pathogenesis and this is of relevance to the design of novel therapeutic strategies for H5N1 induced respiratory disease.

Crosstalk in inflammation: the interplay of glucocorticoid receptor-based mechanisms and kinases and phosphatases

Glucocorticoids (GCs) are steroidal ligands for the GC receptor (GR), which can function as a ligand-activated transcription factor. These steroidal ligands and derivatives thereof are the first line of treatment in a vast array of inflammatory diseases. However, due to the general surge of side effects associated with long-term use of GCs and the potential problem of GC resistance in some patients, the scientific world continues to search for a better understanding of the GC-mediated antiinflammatory mechanisms. The reversible phosphomodification of various mediators in the inflammatory process plays a key role in modulating and fine-tuning the sensitivity, longevity, and intensity of the inflammatory response. As such, the antiinflammatory GCs can modulate the activity and/or expression of various kinases and phosphatases, thus affecting the signaling efficacy toward the propagation of proinflammatory gene expression and proinflammatory gene mRNA stability. Conversely, phosphorylation of GR can affect GR ligand- and DNA-binding affinity, mobility, and cofactor recruitment, culminating in altered transactivation and transrepression capabilities of GR, and consequently leading to a modified antiinflammatory potential. Recently, new roles for kinases and phosphatases have been described in GR-based antiinflammatory mechanisms. Moreover, kinase inhibitors have become increasingly important as antiinflammatory tools, not only for research but also for therapeutic purposes. In light of these developments, we aim to illuminate the integrated interplay between GR signaling and its correlating kinases and phosphatases in the context of the clinically important combat of inflammation, giving attention to implications on GC-mediated side effects and therapy resistance.

Pharmacological strategies for improving the efficacy and therapeutic ratio of glucocorticoids in inflammatory lung diseases

Glucocorticoids are widely used to treat various inflammatory lung diseases. Acting via the glucocorticoid receptor (GR), they exert clinical effects predominantly by modulating gene transcription. This may be to either induce (transactivate) or repress (transrepress) gene transcription. However, certain individuals, including those who smoke, have certain asthma phenotypes, chronic obstructive pulmonary disease (COPD) or some interstitial diseases may respond poorly to the beneficial effects of glucocorticoids. In these cases, high dose, often oral or parental, glucocorticoids are typically prescribed. This generally leads to adverse effects that compromise clinical utility. There is, therefore, a need to enhance the clinical efficacy of glucocorticoids while minimizing adverse effects. In this context, a long-acting beta(2)-adrenoceptor agonist (LABA) can enhance the clinical efficacy of an inhaled corticosteroid (ICS) in asthma and COPD. Furthermore, LABAs can augment glucocorticoid-dependent gene expression and this action may account for some of the benefits of LABA/ICS combination therapies when compared to ICS given as a monotherapy. In addition to metabolic genes and other adverse effects that are induced by glucocorticoids, there are many other glucocorticoid-inducible genes that have significant anti-inflammatory potential. We therefore advocate a move away from the search for ligands of GR that dissociate transactivation from transrepression. Instead, we submit that ligands should be functionally screened by virtue of their ability to induce or repress biologically-relevant genes in target tissues. In this review, we discuss pharmacological methods by which selective GR modulators and "add-on" therapies may be exploited to improve the clinical efficacy of glucocorticoids while reducing potential adverse effects.

Glucocorticoids and the risk of osteoporosis

BACKGROUND

Glucocorticoid-induced osteoporosis (GIO) refers to a clinical condition in which a class of corticosteroids increases the susceptibility of bones to fracture. Numerous recent studies have improved our understanding of the underlying biology of this condition, whereas data from randomized controlled trials have provided clinicians with more options for prevention of GIO.

OBJECTIVE

To review the pathophysiology and epidemiology of GIO, as well as current pharmacologic treatment and prevention modalities available. To review the state of healthcare provider concordance with GIO prevention guidelines.

METHODS

Representative examples of various cellular and molecular processes underlying GIO were included, with an emphasis towards more recent discoveries. The data used to describe the epidemiology of GIO were derived from both randomized controlled studies and observational studies, framed through a discussion of known osteoporosis risk factors.

RESULTS/CONCLUSION

Progress has been made in clarifying the pathophysiologic mechanisms that result in GIO. Although the options for preventions and treatment of GIO continue to expand, provider compliance with preventive measures remains suboptimal.

Differential regulation of RGS-2 by constant and oscillating PTH concentrations

PTH has diverse effects on bone metabolism: anabolic when given intermittently, catabolic when given continuously. The cellular mechanisms underlying the varying target cell response are not clear yet. PTH induces RGS-2, a member of the Regulator of G-protein Signaling protein family, via cAMP/PKA, and inactivates PKC-mediated signaling. To investigate intracellular signaling pathways with different PTH concentration-time patterns, we treated UMR 106-01 osteoblast-like cells in a perfusion system. PTH was administered intermittently (4 min/h, 10(-7) M) or continuously at an equivalent cumulative dose (6.6 x 10(-9) M). cAMP was measured using radioimmunoassay, mRNA levels using real-time rtPCR and ribonuclease protection assay, and protein levels using Western immunoblotting. A single PTH pulse transiently increased cAMP levels by 2000% +/- 1200%. In contrast to continuous PTH exposure, cAMP induction remained unchanged with intermittent PTH, ruling out desensitization of the PTH receptor. In continuously perfused cells, RGS-2 abundance was three to five times higher than in cells intermittently exposed to PTH for up to 12 h. MKP-1 and -3 were significantly less induced with pulsatile PTH; exposure-mode-dependent differences in MMP-13 and IGFBP-5 were small. Pulsatile but not continuous PTH administration prevents PTHrP receptor desensitization and accumulation of RGS-2 in osteoblasts, which should preserve PKC-dependent signaling.

Genistein aglycone reverses glucocorticoid-induced osteoporosis and increases bone breaking strength in rats: a comparative study with alendronate

BACKGROUND AND PURPOSE

Glucocorticoid-induced osteoporosis (GIO) is the leading cause of secondary osteoporosis. Clinical evidence suggests a role for genistein aglycone in the treatment of post-menopausal osteopenia although proof of efficacy in comparison with currently available treatments is still lacking. To clarify this issue, we investigated the effects of genistein on bone compared with alendronate in experimental GIO.

EXPERIMENTAL APPROACH

A total of 28 female Sprague-Dawley rats were used. GIO was induced by daily injections of methylprednisolone (MP; 30 mg x kg(-1) s.c.) for 60 days. Sham GIO animals (Sham-MP) were injected daily with the MP vehicle. At the end of the osteoporosis development period, MP rats were randomized to receive: vehicle (n= 7), genistein aglycone (5 mg x kg(-1) s.c.; n= 7) or alendronate (0.03 mg x kg(-1) s.c.; n= 7). Treatment lasted 60 days. Sham-MP animals were treated with vehicle for an additional 60 days. At the beginning and at the end of treatments, animals were examined for bone mineral density and bone mineral content. Bone-alkaline phosphatase and carboxy-terminal collagen cross links were determined; femurs were removed and tested for breaking strength and histology.

KEY RESULTS

Genistein aglycone showed a greater increase in bone mineral density, bone mineral content and in breaking strength than alendronate and significantly increased bone-alkaline phosphatase (bone formation marker), reduced carboxy-terminal collagen cross links (bone resorption marker), compared with alendronate. Both treatments improved bone histology and the histological score.

CONCLUSION AND IMPLICATIONS

The results strongly suggest that the genistein aglycone might be an alternative therapy for the management of secondary osteoporosis.

The anti-proliferation mechanism of glucocorticoid mediated by glucocorticoid receptor-regulating gene expression

Glucocorticoid (GC) hormones exert an antiproliferative effect on various cells. The effect is mainly mediated by glucocorticoid receptor (GR) which acts as a transcription factor. Ligand-bound GR translocates from the cytoplasm into the nucleus to modulate gene expression in a variety of ways. Although the framework of transcriptional regulation by the GC/GR has been described, the molecular mechanism of antiproliferative effect of GC is still largely unclear. In this article, we reviewed GC-induced changes in gene expression that are involved in GC-antiproliferative effect, and mainly focused on our recently identified glucocorticoid-responsive genes, TGF-beta receptor type II (TbetaRII) and small GTP binding protein RhoB. We found that expressions of TbetaRII and RhoB were up-regulated by ligand-bound GR at mRNA and protein levels. Blocking the effect of TbetaRII by TbetaRII neutralizing antibody or reduction of RhoB mRNA expression by RNAi diminished dexamethasone-inhibitory effect on cell proliferation, thus confirming that these genes are involved in GC anti-proliferation effect. Collectively, GC up-regulating the expressions of RhoB and TbetaRII play an important role in GC anti-proliferation effect.

Inhibition of extracellular signal-regulated kinase (ERK) signaling participates in repression of nuclear factor (NF)-kappaB activity by glucocorticoids

Glucocorticoid (GC) effects are mediated via the GC-receptor (GR), which either stimulates or represses gene expression. Repression of target genes often involves negative cross-talk between the GR and other transcription factors e.g. NF-kappaB, important for gene activation. Using HEK293 cells we here describe that repression of NF-kappaB requires functions of the GR that are dependent on the signaling pathways employed to activate NF-kappaB. While a GR mutant was able to repress NF-kappaB activity following activation by TNFalpha, it did not so following activation by the phorbol ester TPA. In these cells, TPA stimulation but not TNFalpha, activated extracellular signal-regulated kinase (ERK). We demonstrated that the ability of the dexamethasone activated GR mutant to repress TPA-induced NF-kappaB activity was restored in conjunction with ERK1/2 inhibition. Previous reports have shown GC-mediated inhibition of ERK1/2 phosphorylation to involve GC induction of MAPK phosphatase-1 (MKP-1). Here, we demonstrated that the GRR488Q mutant was incapable of inducing gene expression of endogenous MKP-1 following dexamethasone treatment, in contrast to the GRwt. However, TPA treatment alone resulted in much stronger MKP-1 expression in both GRwt and GRR488Q containing cells than that of dexamethasone suggesting that the inability of GRR488Q to inhibit TPA-induced NF-kappaB activity did not involve a lack of MKP-1 expression. In line with this, RNAi targeted towards MKP-1 did not abolish or inhibit the ability of the GRwt to repress NF-kappaB activity. Importantly, we observed no difference in activated ERK1/2 (phospho-ERK1/2) expression over time between GRwt and GRR488Q containing cells following co-treatment with TPA and dexamethasone. Based on these results we suggest that GRwt does not directly regulate ERK1/2 but rather alters ERK1/2-mediated effects allowing it to repress NF-kappaB activity, a capacity lacked by the GRR488Q mutant.

Human coronary artery smooth muscle cell response to a novel PLA textile/fibrin gel composite scaffold

Previous studies have demonstrated the potential of fibrin as a cell carrier for cardiovascular tissue engineering applications. Unfortunately, fibrin exhibits poor mechanical properties. One method of addressing this issue is to incorporate a textile in fibrin to provide structural support. However, it is first necessary to develop a deeper understanding of the effect of the textile on cell response. In this study, the cytotoxicity of a polylactic acid (PLA) warp-knit textile was assessed with human coronary artery smooth muscle cells (HCASMC). Subsequently, quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) was employed to examine the gene expression of HCASMC embedded in fibrin with and without the textile. Five genes were examined over a 3-week period: smooth muscle alpha-actin (SMalphaA), myosin heavy chain 11 smooth muscle (SM1/SM2), calponin, myosin heavy chain 10 non-muscle (SMemb) and collagen. Additionally, a microarray analysis was performed to examine a wider range of genes. The knitting process did not adversely affect the cell response; there was no dramatic change in cell number or metabolic rate compared to the negative control. After 3 weeks, there was no significant difference in gene expression, except for a slight decrease of 10% in SMemb in the fibrin with textile. After 3 weeks, there were no obvious cytotoxic effects observed as a result of the knitting process and the gene expression profile did not appear to be altered in the presence of the mesh in the fibrin gel.

Thyroid hormone-mediated activation of the ERK/dual specificity phosphatase 1 pathway augments the apoptosis of GH4C1 cells by down-regulating nuclear factor-kappaB activity

Thyroid hormone (T3) plays a crucial role in processes such as cell proliferation and differentiation, whereas its implication on cellular apoptosis has not been well documented. Here we examined the effect of T3 on the apoptosis of GH4C1 pituitary cells and the mechanisms underlying this effect. We show that T3 produced a significant increase in apoptosis in serum-depleted conditions. This effect was accompanied by a decrease in nuclear factor-kappaB (NF-kappaB)-dependent transcription, IkappaBalpha phosphorylation, translocation of p65/NF-kappaB to the nucleus, phosphorylation, and transactivation. Moreover, these effects were correlated with a T3-induced decrease in the expression of antiapoptotic gene products, such as members of the inhibitor of apoptosis protein and Bcl-2 families. On the other hand, ERK but not c-Jun N-terminal kinase or MAPK p38, was activated upon exposure to T3, and inhibition of ERK alone abrogated T3-mediated apoptosis. In addition, T3 increased the expression of the MAPK phosphatase, dual specificity phosphatase 1 (DUSP1), in an ERK-dependent manner. Interestingly, the suppression of DUSP1 expression abrogated T3-induced inhibition of NF-kappaB-dependent transcription and p65/NF-kappaB translocation to the nucleus, as well as T3-mediated apoptosis. Overall, our results indicate that T3 induces apoptosis in rat pituitary tumor cells by down-regulating NF-kappaB activity through a mechanism dependent on the ERK/DUSP1 pathway.