Tumor necrosis factor-alpha autoregulates interleukin-6 synthesis via activation of protein kinase C. Function of sphingosine 1-phosphate and phosphatidylcholine-specific phospholipase C

Osteogenic mechanism of chlorogenic acid and its application in clinical practice

Natural polyphenols may have a role in counteracting oxidative stress, which is associated with aging and several bone-related diseases. Chlorogenic acid (CGA) is a naturally occurring polyphenolic compound formed by the esterification of caffeic and quininic acids with osteogenic, antioxidant, and anti-inflammatory properties. This review discusses the potential of CGA to enhance osteogenesis by increasing the osteogenic capacity of mesenchymal stem cells (MSCs), osteoblast survival, proliferation, differentiation, and mineralization, as well as its ability to attenuate osteoclastogenesis by enhancing osteoclast apoptosis and impeding osteoclast regeneration. CGA can be involved in bone remodeling by acting directly on pro-osteoclasts/osteoblasts or indirectly on osteoclasts by activating the nuclear factor kB (RANK)/RANK ligand (RANKL)/acting osteoprotegerin (OPG) system. Finally, we provide perspectives for using CGA to treat bone diseases.

Oncostatin M attenuates tumor necrosis factor-α-induced synthesis of macrophage-colony stimulating factor via suppression of Akt in osteoblasts

BACKGROUND

Oncostatin M produced by osteal macrophages, a cytokine that belongs to the interleukin-6 family, is implicated in bone fracture healing. Macrophage colony-stimulating factor (M-CSF) secreted from osteoblasts plays an important role in osteoclastogenesis. We have previously reported that tumor necrosis factor-α (TNF-α), a potent bone resorptive agent, stimulates the activation of p44/p42 mitogen-activated protein (MAP) kinase, Akt, and p70 S6 kinase in osteoblast-like MC3T3-E1 cells, and induces the synthesis of M-CSF at least in part via Akt.

OBJECTIVE

In the present study, we investigated whether oncostatin M affects the TNF-α-induced M-CSF synthesis in MC3T3-E1 cells and the underlying mechanisms.

METHODS

Clonal osteoblast-like MC3T3-E1 cells were treated with oncostatin M or rapamycin and then stimulated with TNF-α. M-CSF release was assessed by ELISA. M-CSF mRNA expression level was assessed by real-time RT-PCR. Phosphorylation of Akt, p44/p42 MAP kinase, and p70 S6 kinase was detected by Western blot analysis.

RESULTS

Oncostatin M dose-dependently reduced the TNF-α-stimulated M-CSF release. The expression of M-CSF mRNA induced by TNF-α was significantly suppressed by oncostatin M. Rapamycin, an inhibitor of mTOR/p70 S6 kinase, had little effect on the M-CSF release by TNF-α. Oncostatin M significantly reduced the TNF-α-induced phosphorylation of Akt and p44/p42 MAP kinase. However, the p70 S6 kinase phosphorylation by TNF-α was not affected by oncostatin M.

CONCLUSION

These results strongly suggest that oncostatin M attenuates TNF-α-stimulated synthesis of M-CSF in osteoblasts, and the inhibitory effect is exerted at a point upstream of Akt and p44/p42 MAP kinase but not p70 S6 kinase.

Distinct Metabolic States Are Observed in Hypoglycemia Induced in Mice by Ricin Toxin or by Fasting

Hypoglycemia may be induced by a variety of physiologic and pathologic stimuli and can result in life-threatening consequences if untreated. However, hypoglycemia may also play a role in the purported health benefits of intermittent fasting and caloric restriction. Previously, we demonstrated that systemic administration of ricin toxin induced fatal hypoglycemia in mice. Here, we examine the metabolic landscape of the hypoglycemic state induced in the liver of mice by two different stimuli: systemic ricin administration and fasting. Each stimulus produced the same decrease in blood glucose and weight loss. The polar metabolome was studied using ¹H NMR, quantifying 59 specific metabolites, and untargeted LC-MS on approximately 5000 features. Results were analyzed by multivariate analyses, using both principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA), to identify global metabolic patterns, and by univariate analyses (ANOVA) to assess individual metabolites. The results demonstrated that while there were some similarities in the responses to the two stimuli including decreased glucose, ADP, and glutathione, they elicited distinct metabolic states. The metabolite showing the greatest difference was O-phosphocholine, elevated in ricin-treated animals and known to be affected by the pro-inflammatory cytokine TNF-α. Another difference was the alternative fuel source utilized, with fasting-induced hypoglycemia primarily ketotic, while the response to ricin-induced hypoglycemia involves protein and amino acid catabolism.

Olive polyphenols attenuate TNF-α-stimulated M-CSF and IL-6 synthesis in osteoblasts: Suppression of Akt and p44/p42 MAP kinase signaling pathways

BACKGROUND

Olive oil polyphenols, which possess cytoprotective activities like anti-oxidant and anti-inflammatory effects, could modulate osteoblast functions. The aim of this study is to elucidate the effects and the underlying mechanisms of hydroxytyrosol and oleuropein on the tumor necrosis factor-α (TNF-α)-induced macrophage colony-stimulating factor (M-CSF) and interleukin-6 (IL-6) synthesis in osteoblasts.

METHODS

Osteoblast-like MC3T3-E1 cells were pretreated with hydroxytyrosol, oleuropein, deguelin, PD98059 or wedelolactone, and then stimulated by TNF-α. The levels of M-CSF and IL-6 in the conditioned medium were determined with ELISA. The mRNA expression levels of M-CSF or IL-6 were determined with real-time RT-PCR. The phosphorylation levels of Akt, p44/p42 mitogen-activated protein (MAP) kinase or NF-κB in the cell lysates were determined with Western blot analysis.

RESULTS

Hydroxytyrosol and oleuropein attenuated the TNF-α-stimulated M-CSF release. Deguelin, an inhibitor of Akt, significantly suppressed the TNF-α-stimulated M-CSF release, which failed to be affected by the MEK1/2 inhibitor PD98059 or the IκB inhibitor wedelolactone. Hydroxytyrosol and oleuropein suppressed the TNF-α-induced phosphorylation of Akt and p44/p42 MAP kinase. Hydroxytyrosol and oleuropein attenuated the TNF-α-stimulated IL-6 release. Hydroxytyrosol suppressed the TNF-α-induced mRNA expressions of M-CSF and IL-6. Hydroxytyrosol or oleuropein failed to affect the cell viability.

CONCLUSION

Our present findings strongly suggest that olive oil polyphenols hydroxytyrosol and oleuropein down-regulates TNF-α signaling at the points upstream of Akt and p44/p42 MAP kinase in osteoblasts, leading to the attenuation of M-CSF and IL-6 synthesis.

Association of HSP22 with mTOR in osteoblasts: regulation of TNF-α-stimulated IL-6 synthesis

Heat shock protein 22 (HSP22) is ubiquitously expressed in various types of cells including in osteoblasts. We previously reported that tumor necrosis factor (TNF)-α stimulates interleukin (IL)-6 synthesis via p44/p42 MAPK in osteoblast-like MC3T3-E1 cells and that mTOR/p70 S6 kinase (p70 S6K) negatively regulates the IL-6 synthesis. In this study, we investigated the involvement of HSP22 in TNF-α-stimulated-IL-6 synthesis and the underlying mechanism in MC3T3-E1 cells. HSP22 knockdown reduces TNF-α-stimulated release of IL-6. In addition, HSP22 knockdown strengthens TNF-α-induced phosphorylation of p70 S6K but suppresses that of p44/p42 MAPK. HSP22 coimmunoprecipitates with mTOR. HSP22 knockdown increases the basal levels of phosphorylated mTOR. These results strongly suggest that HSP22 interacts with mTOR and regulates TNF-α-induced IL-6 synthesis in osteoblasts.

Drp1-dependent mitochondrial fission mediates osteogenic dysfunction in inflammation through elevated production of reactive oxygen species

Although previous studies have implicated pro-inflammatory cytokines, such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), to be detrimental for osteogenic activity, the related regulatory mechanisms are not yet fully validated. Since mitochondria host several essential metabolic processes and play a pivotal role in cellular functions, whether and how mitochondrial function contributes to inflammation-induced bone destruction needs further exploration. Our findings revealed that TNF-α impaired osteoblast function, including decreased mRNA levels of osteogenic markers, suppressed ALP expression and activity, and compromised cellular viability. Moreover, increased reactive oxygen species (ROS)-mediated oxidative stress in the TNF-α-treated group enhanced excessive mitochondrial fragmentation and disrupted mitochondrial function. However, treatment with antioxidant N-acetyl cysteine (NAC) or mitochondrial division inhibitor Mdivi-1 protected the cells from these adverse phenomena. These findings provide new insights into the role of the Drp1-dependent mitochondrial pathway in the osteogenic dysfunction during inflammation, indicating that this pathway may be a target for the development of new therapeutic approaches for the prevention and treatment of inflammation-induced bone destruction.

Incretins amplify TNF-α-stimulated IL-6 synthesis in osteoblasts: Suppression of the IκB/NF-κB pathway

Incretins including glucagon-like peptide-1 (GLP-1) and glucose‑dependent insulinotropic polypeptide (GIP) secreted from the small intestine after oral food ingestion are currently recognized to stimulate insulin secretion from pancreatic β cells. We previously reported that p70 S6 kinase limits the tumor necrosis factor‑α (TNF‑α)‑stimulated interleukin-6 (IL‑6) synthesis in osteoblast‑like MC3T3‑E1 cells. In the present study, we investigated the effects of incretins on the TNF‑α‑induced IL‑6 synthesis and the underlying mechanism in MC3T3‑E1 cells. GLP‑1 and GIP significantly upregulated both TNF‑α‑stimulated IL‑6 release and mRNA levels. Wedelolactone, an inhibitor of IκB kinase, amplified the TNF-α-induced IL‑6 release. GLP‑1 significantly attenuated the TNF‑α‑induced phosphorylation of IκB without affecting the phosphorylation of p70 S6 kinase. On the other hand, GLP‑1 markedly induced the phosphorylation of cAMP response element-binding protein (CREB). H‑89, an inhibitor of protein kinase A, significantly suppressed the enhancement by GLP-1 of TNF-α-stimulated IL‑6 release. Dibutyryl cAMP, a permeable analogue of cAMP, which suppressed the TNF-α-induced IκB phosphorylation, amplified the IL‑6 release. These results strongly suggest that incretins upregulate the TNF-α-stimulated IL‑6 synthesis in osteoblasts, and that the amplifying effect of incretin is exerted via reducing the IκB/NF‑κB pathway through the adenylyl cyclase-cAMP system.

Andrographolide attenuates LPS-stimulated up-regulation of C-C and C-X-C motif chemokines in rodent cortex and primary astrocytes

Background

Andrographolide is the major bioactive compound isolated from Andrographis paniculata, a native South Asian herb used medicinally for its anti-inflammatory properties. In this study, we aimed to assess andrographolide’s potential utility as an anti-neuroinflammatory therapeutic.

Methods

The effects of andrographolide on lipopolysaccharide (LPS)-induced chemokine up-regulation both in mouse cortex and in cultured primary astrocytes were measured, including cytokine profiling, gene expression, and, in cultured astrocytes, activation of putative signaling regulators.

Results

Orally administered andrographolide significantly attenuated mouse cortical chemokine levels from the C-C and C-X-C subfamilies. Similarly, andrographolide abrogated a range of LPS-induced chemokines as well as tumor necrosis factor (TNF)-α in astrocytes. In astrocytes, the inhibitory actions of andrographolide on chemokine and TNF-α up-regulation appeared to be mediated by nuclear factor-κB (NF-κB) or c-Jun N-terminal kinase (JNK) activation.

Conclusions

These results suggest that andrographolide may be useful as a therapeutic for neuroinflammatory diseases, especially those characterized by chemokine dysregulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0498-6) contains supplementary material, which is available to authorized users.

Amplification by (-)-epigallocatechin gallate and chlorogenic acid of TNF-α-stimulated interleukin-6 synthesis in osteoblasts

Polyphenolic compounds in foods and beverages have beneficial effects on human health. (-)-Epigallocatechin gallate (EGCG) and chlorogenic acid (CGA), a major flavonoid in green tea and a major phenolic acid in coffee, respectively, have potent properties, including antioxidative effects. Our previous study demonstrated that p70 S6 kinase acts as a negative regulator in tumor necrosis factor-α (TNF-α)-stimulated interleukin-6 synthesis in osteoblast-like MC3T3-E1 cells. In the present study, the effects of EGCG and CGA on the TNF-α-stimulated interleukin‑6 synthesis were investigated in MC3T3‑E1 cells. EGCG and CGA significantly enhanced TNF-α-stimulated interleukin-6 release. In addition, the interleukin-6 mRNA expression levels induced by TNF‑α were supported by EGCG, as well as CGA. EGCG markedly attenuated the TNF-α-induced phosphorylation of p70 S6 kinase whereas CGA failed to affect the phosphorylation. These results strongly suggest that EGCG and CGA enhance the TNF-α-stimulated interleukin-6 synthesis in osteoblasts, and that the amplifying effect of EGCG, but not CGA, is exerted via inhibiting p70 S6 kinase.

NMR metabolomics analysis of the effects of 5-lipoxygenase inhibitors on metabolism in glioblastomas

Changes across metabolic networks are emerging as an integral part of cancer development and progression. Increasing comprehension of the importance of metabolic processes as well as metabolites in cancer is stimulating exploration of novel, targeted treatment options. Arachidonic acid (AA) is a major component of phospholipids. Through the cascade catalyzed by cyclooxygenases and lipoxygenases, AA is also a precursor to cellular signaling molecules as well as molecules associated with a variety of diseases including cancer. 5-Lipoxygenase catalyzes the transformation of AA into leukotrienes (LT), important mediators of inflammation. High-throughput analysis of metabolic profiles was used to investigate the response of glioblastoma cell lines to treatment with 5-lipoxygenase inhibitors. Metabolic profiling of cells following drug treatment provides valuable information about the response and metabolic alterations induced by the drug action and give an indication of both on-target and off-target effects of drugs. Four different 5-lipoxygenase inhibitors and antioxidants were tested including zileuton, caffeic acid, and its analogues caffeic acid phenethyl ester and caffeic acid cyclohexethyl ester. A NMR approach identified metabolic signatures resulting from application of these compounds to glioblastoma cell lines, and metabolic data were used to develop a better understanding of the mode of action of these inhibitors.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Inhibition of renin angiotensin aldosterone system causes abrogation of obliterative airways disease through inhibition of tumor necrosis factor-α-dependant interleukin-17

BACKGROUND

Alloimmune-induced immune responses to self-antigens are involved in the development of chronic lung allograft rejection. Angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin II receptor blockers (ARBs) have been shown to modulate autoimmune diseases. This study investigated the effect of modulation of the renin angiotensin aldosterone system (RAAS) a murine model of obliterative airways disease (OAD).

METHODS

Major histocompatibility complex (MHC) class I antibodies were administered intrabronchially to C57Bl/6 mice on Days 1, 2, 3, and 6, and weekly thereafter. ACEI/ARB (10 mg/kg/day) were administered in water 5 days before antibody administration. Antibodies were analyzed by enzyme-linked immunosorbent assay, cytokines by Luminex, Th-frequency by enzyme-linked immunosorbent spot, and transcription factors by Western blotting and real-time polymerase chain reaction.

RESULTS

Significant decreases (50%-70%) in airway lesions and fibrous deposition were noted in lungs at Day 30 in the animals administered ACEI and ARB vs controls. Antibody concentrations to self-antigens also decreased from 14 ± 21 to 62 ± 18 μg/ml for collagen V and from 263 ± 43 to 84 ± 28 μg/ml for K-α1 tubulin. Th-precursor frequency and cytokine analysis showed increased interleukin (IL)-10 (3-fold increase) and decreased levels of IL-6 (3.4-fold) and IL-17 (4-fold decrease; p < 0.05) in ACEI and ARB groups. There was also messenger RNA level downregulation of tumor necrosis factor-α (8.6-fold) and p38/mitogen-activated protein (MAP)kinase (3.1-fold) in the treatment groups.

CONCLUSIONS

Our results demonstrate that modulation of RAAS leads to downregulation of IL-17 through tumor necrosis factor-α-dependant IL-6 through p38/MAPKinase pathway and thus abrogation of anti-MHC-induced OAD.

Regulation by heat shock protein 27 of osteocalcin synthesis in osteoblasts

We have previously reported that various stimuli, including sphingosine 1-phosphate, are able to induce heat shock protein (HSP) 27 in osteoblast-like MC3T3-E1 cells. However, the precise role of HSP27 in bone metabolism has not been satisfactory clarified. In this study, we investigated the effect of HSP27 on osteocalcin synthesis induced by bone morphogenetic protein (BMP)-4 or T₃ in these cells. In MC3T3-E1 cells, pretreatment with sphingosine 1-phosphate, sodium arsenite, or heat stress caused the attenuation of osteocalcin synthesis induced by BMP-4 or T₃ with concurrent HSP27 induction. To further investigate the effect of HSP27, we established stable HSP27-transfected cells. The osteocalcin synthesis was significantly reduced in the stable HSP27-transfected MC3T3-E1 cells and normal human osteoblasts compared with empty-vector transfected cells. On the other hand, anisomycin, a p38 MAPK activator, caused the phosphorylation of HSP27 in both sphingosine 1-phosphate-stimulated untransfected MC3T3-E1 cells and HSP27-transfected MC3T3-E1 cells. An immunofluorescence microscopy study showed that the phosphorylated HSP27 induced by anisomycin concentrated perinuclearly in these cells, in which it colocalized with the endoplasmic reticulum. We also established stable mutant-HSP27-transfected cells. Osteocalcin synthesis induced by either BMP-4 or T₃ was markedly suppressed in the nonphosphorylatable HSP27-overexpressing MC3T3-E1 cells compared with the phosphomimic HSP27-overexpressing cells. In contrast, the matrix mineralization was more obvious in nonphosphorylatable HSP27-overexpressing cells than that in phosphomimic HSP27-overexpressing cells. Taken together, these results strongly suggest that unphosphorylated HSP27 has an inhibitory effect on osteocalcin synthesis, but has a stimulatory effect on mineralization, in osteoblasts.

TNF-alpha promotes fracture repair by augmenting the recruitment and differentiation of muscle-derived stromal cells

With an aging population, skeletal fractures are increasing in incidence, including the typical closed and the less common open fractures in normal bone, as well as fragility fractures in patients with osteoporosis. For the older age group, there is an urgent unmet need to induce predictable bone formation as well as improve implant fixation in situations such as hip joint replacement. Using a murine model of slow-healing fractures, we have previously shown that coverage of the fracture with muscle accelerated fracture healing and increased union strength. Here, we show that cells from muscle harvested after 3 d of exposure to an adjacent fracture differentiate into osteoblasts and form bone nodules in vitro. The osteogenic potential of these cells exceeds that of adipose and skin-derived stromal cells and is equivalent to bone marrow stromal cells. Supernatants from human fractured tibial bone fragments promote osteogenesis and migration of muscle-derived stromal cells (MDSC) in vitro. The main factor responsible for this is TNF-α, which promotes first MDSC migration, then osteogenic differentiation at low concentrations. However, TNF-α is inhibitory at high concentrations. In our murine model, addition of TNF-α at 1 ng/mL at the fracture site accelerated healing. These data indicate that manipulating the local inflammatory environment to recruit, then differentiate adjacent MDSC, may be a simple yet effective way to enhance bone formation and accelerate fracture repair. Our findings are based on a combination of human specimens and an in vivo murine model and may, therefore, translate to clinical care.

Wnt3a regulates tumor necrosis factor-α-stimulated interleukin-6 release in osteoblasts

It is recognized that Wnt pathways regulate bone metabolism. We have previously shown that tumor necrosis factor-α (TNF-α) stimulates synthesis of interleukin-6 (IL-6), a potent bone resorptive agent, via p44/p42 mitogen-activated protein (MAP) kinase and phosphatidylinositol 3-kinase (PI3-kinase)/Akt in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the effect of Wnt3a on TNF-α-stimulated IL-6 synthesis in these cells. Wnt3a, which alone did not affect the IL-6 levels, significantly suppressed the TNF-α-stimulated IL-6 release. Lithium Chloride (LiCl), which is an inhibitor of GSK3β, markedly reduced the TNF-α-stimulated IL-6 release, similar to the results with Wnt3a. The suppression by Wnt3a or LiCl was also observed in the intracellular protein levels of IL-6 elicited by TNF-α. Wnt3a failed to affect the TNF-α-induced phosphorylation of p44/p42 MAP kinase, Akt, IκB or NFκB. Either Wnt3a or LiCl failed to reduce, rather increased the IL-6 mRNA expression stimulated by TNF-α. Lactacystin, a proteasome inhibitor, and bafilomycin A1, a lysosomal protease inhibitor, significantly restored the suppressive effect of Wnt3a on TNF-α-stimulated IL-6 release. Taken together, our results strongly suggest that Wnt3a regulates IL-6 release stimulated by TNF-α at post-transcriptional level in osteoblasts.

p70 S6 kinase limits tumor necrosis factor-alpha-induced interleukin-6 synthesis in osteoblast-like cells

Our previous study demonstrated that tumor necrosis factor-alpha (TNF-alpha) stimulates the synthesis of interleukin-6 (IL-6), a potent bone resorptive agent, via p44/p42 mitogen-activated protein (MAP) kinase and phosphatidylinositol 3-kinase/Akt in osteoblast-like MC3T3-E1 cells. In the present study, we investigated whether p70 S6 kinase is involved in TNF-alpha-stimulated IL-6 synthesis in MC3T3-E1 cells. TNF-alpha time dependently induced the phosphorylation of p70 S6 kinase. Rapamycin, an inhibitor of p70 S6 kinase, which attenuated the phosphorylation of p70 S6 kinase induced by TNF-alpha, significantly amplified the TNF-alpha-stimulated IL-6 synthesis. TNF-alpha-induced phosphorylations of both p44/p42 MAP kinase and Akt were markedly enhanced by rapamycin. The amplification by rapamycin of TNF-alpha-induced IL-6 synthesis was reduced by PD98059, a specific inhibitor of MEK1/2, or Akt inhibitor. Rapamycin enhanced the IL-6 synthesis and the phosphorylation of Akt induced by TNF-alpha also in human osteoblasts. Taken together, these results strongly suggest that p70 S6 kinase limits the TNF-alpha-stimulated IL-6 synthesis at a point upstream from p44/p42 MAP kinase and Akt in osteoblast-like cells.

Pharmacodynamic effects of the murine p75-Fc fusion protein in mice

Overproduction of inflammatory mediators, such as tumor necrosis factor (TNF), is key to the development and maintenance of inflammatory processes. Etanercept is a soluble TNF receptor fusion protein used in the treatment of various chronic inflammatory diseases, including rheumatoid arthritis and psoriasis. This study investigated the effects of murine p75-Fc, a soluble TNF receptor protein, on TNF-induced IL-6 production in mice. Six groups of mice received either murine p75-Fc (0.15, 0.50, 1.5, 5, and 15 mg/kg) or phosphate-buffered saline. Three days later, mice were injected intravenously with 10 microg of murine TNF and blood samples were taken after 3 hours. Serum IL-6 and TNF were measured by ELISA. Mice treated with 5 and 15 mg/kg murine p75-Fc demonstrated complete inhibition of TNF-induced IL-6 production. Murine p75-Fc (1.5 mg/kg) resulted in a partial but significant reduction of TNF-induced IL-6 production. No TNF was detected in 5 and 15 mg/kg murine p75-Fc-treated mice, except one in the 5 mg/kg dose group. In conclusion, murine p75-Fc completely inhibits TNF-induced IL-6 production in mice.

Interleukin-6 and IL-6 receptor cell expression in testis of rats with autoimmune orchitis

Experimental autoimmune orchitis (EAO) is an organ-specific model of autoimmunity characterized by an interstitial lymphomononuclear cell infiltrate as well as sloughing and apoptosis of germ cells. EAO was induced in adult male Sprague-Dawley rats by active immunization with testicular homogenate and adjuvants. Rats injected with saline solution and adjuvants were used as control group. The aim of this work was to study the expression of interleukin-6 (IL-6) and its receptor (IL-6R) in the testis of rats with EAO and analyze whether IL-6 could be involved in germ cell apoptosis. By immunohistochemistry, we detected IL-6 expression in testicular macrophages and Leydig cells of control and EAO rats. Sertoli cells showed IL-6 immunoreactivity in most of the seminiferous tubules of control rats, while a few IL-6+ Sertoli cells were found in the testis of rats with EAO. IL-6R immunoreactivity was observed in macrophages, Leydig and germ cells. A significant increase was noted in the number of IL-6R+ germ cells in rats with EAO compared to control rats. The content of IL-6 (ELISA) in the conditioned media obtained from testicular macrophages of rats with orchitis was significantly higher than in the control group. By immunofluorescence performed on isolated testicular macrophages, IL-6 was shown to be expressed by monocytes recently arrived from circulation (ED1+ cells), while resident macrophages (ED2+ cells) were negative. In vitro experiments (trypan blue and MTS assays) showed that IL-6 (50 ng/ml) reduced germ cell viability. We demonstrated also using the TUNEL technique that IL-6 added to cultures of seminiferous tubule segments induced apoptosis of germ cells. Our results suggest that IL-6 and IL-6R may be involved in the pathogenesis of autoimmune orchitis by promoting testicular inflammation and germ cell apoptosis.

The neuropeptide VIP potentiates IL-6 production induced by proinflammatory osteotropic cytokines in calvarial osteoblasts and the osteoblastic cell line MC3T3-E1

Skeletal turnover is orchestrated by a complex network of regulatory factors. Lately, regulation of bone metabolism through neuro-osteological interactions has been proposed. Here, we address the question whether IL-6 production can be affected by interactions between the neuropeptide VIP and proinflammatory, bone-resorbing cytokines. By using calvarial osteoblasts, we showed that IL-1beta increased IL-6 production time- and concentration-dependently, and that these effects were potentiated by VIP. Furthermore, IL-1beta stimulated IL-6 promoter activity in the osteoblastic cell line MC3T3-E1 stably transfected with a human IL-6 promoter/luciferase construct, and both VIP, and the related neuropeptide PACAP-38, increased the effect of IL-1beta in a synergistic manner. The IL-6 protein release from calvarial osteoblasts was also stimulated by the osteoclastogenic, proinflammatory cytokines IL-11, LIF, OSM, IL-17, TGF-beta, and TNF-alpha. All effects, except for that of TNF-alpha, were synergistically potentiated by VIP. These findings further support the role of neuropeptides, and the presence of neuro-immunological interactions, in bone metabolism.

Sphingosine 1-phosphate acts as a signal molecule in ceramide signal transduction of TNF-alpha-induced activator protein-1 in osteoblastic cell line MC3T3-E1 cells

We previously demonstrated that tumor necrosis factor (TNF)-alpha stimulated the production of activation protein (AP)-1, a transcriptional factor, in mouse osteoblastic MC3T3-E1 cells. Recent studies have shown the importance of ceramide and its metabolites as signal molecules for TNF-alpha-induced gene expression in several cell types. Therefore, our interest was to investigate whether sphingosine metabolites are involved in TNF-alpha-induced signaling in MC3T3-E1 cells. DL-threo-1-phenyl-2-hexadecanoyl-amino-3-pyrrolidino-1-propanol (PPPP), which causes accumulation of intracellular ceramide, stimulated the TNF-alpha-induced expression of the c-fos and c-jun genes. Gel shift assay clearly showed that PPPP increased the cytokine-induced specific binding of nuclear proteins to the 12-tetra-decanoyl phorbol 13-acetate-responsive element (TRE), a consensus sequence for AP-1. In addition, cell-permeable ceramide (N-acetylsphingosine, N-hexanoylsphingosine or N-octanoylsphingosine) stimulated expression of the c-fos and c-jun genes and nuclear protein binding to TRE. Interestingly, DL-threo-dihydrosphingosine (DHS), an inhibitor of sphingosine kinase, clearly blocked the ceramide analogue-induced stimulation. Sphingosine 1-phosphate (SPP) actually induced expression of these oncogenes and activated AP-1. Although TNF-alpha stimulated the AP-1-mediated expression of the monocyte chemoattractant JE/MCP-1, this stimulation was inhibited by DHS. SPP also stimulated JE/MCP-1 gene expression. The present study thus suggests that SPP acts as a signal molecule in ceramide-dependent signal transduction in TNF-alpha-induced AP-1 in osteoblastic MC3T3-E1 cells.

Interleukin (IL)-17 enhances tumor necrosis factor-alpha-stimulated IL-6 synthesis via p38 mitogen-activated protein kinase in osteoblasts

Inflammatory cytokines are well known to play crucial roles in the pathogenesis of rheumatoid arthritis. Among them, interleukin (IL)-17 is a cytokine that is mainly synthesized by activated T cells and its receptors are present in osteoblasts. The synthesis of IL-6, known to stimulate osteoclastic bone resorption, is reportedly responded to bone resorptive agents such as tumor necrosis factor-alpha (TNF-alpha) in osteoblasts. It has been reported that IL-17 enhances TNF-alpha-stimulated IL-6 synthesis in osteoblast-like MC3T3-E1 cells. We previously showed that sphingosine 1-phosphate (S1-P) mediates TNF-alpha-stimulated IL-6 synthesis in these cells. In the present study, we investigated the mechanism of IL-17 underlying enhancement of IL-6 synthesis in MC3T3-E1 cells. IL-17 induced phosphorylation of p38 mitogen-activated protein (MAP) kinase. SB203580 and PD169316, specific inhibitors of p38 MAP kinase, significantly reduced the enhancement by IL-17 of TNF-alpha-stimulated IL-6 synthesis. IL-17 also amplified S1-P-stimulated IL-6 synthesis, and the amplification by IL-17 was suppressed by SB203580. Anisomycin, an activator of p38 MAP kinase, which alone had no effect on IL-6 level, enhanced the IL-6 synthesis stimulated by TNF-alpha. SB203580 and PD169316 inhibited the amplification by anisomycin of the TNF-alpha-induced IL-6 synthesis. Taken together, our results strongly suggest that IL-17 enhances TNF-alpha-stimulated IL-6 synthesis via p38 MAP kinase activation in osteoblasts.

New developments in IL-6 dependent biology and therapy: where do we stand and what are the options?

Interleukin-6 (IL-6) is a four-helical protein which, on target cells, binds to a specific IL-6-receptor and two molecules of the promiscuous signal transducing protein gp130. Structure-function analysis defined three molecular contact sites between IL-6 and its receptor subunits. Using this information, competitive antagonistic proteins as well as hyperagonistic proteins were developed. Possible therapeutic applications of IL-6 antagonists are in IL-6 dependent haematological disorders (Castleman's disease, POEMS syndrome, multiple myeloma) and bone diseases (Paget's disease, osteoporosis). Designer IL-6 antagonists could suppress inflammatory activity in rheumatic and autoimmune diseases and could prevent secondary amyloidosis. IL-6 antagonists could also prove advantageous in myocardial infarction and unstable angina pectoris. IL-6 antagonists might slow down development of (mesangioproliferative) glomerulonephritis. On the other hand, hyperagonistic variants of IL-6 have a potential in ex vivo expansion of bone marrow stem cells and as thrombopoietic agents. They might also be developed into drugs to support liver regeneration in vivo and to treat stress-induced cardiac insufficiency.

Tumor necrosis factor-alpha in diabetic plasma increases the activity of core 2 GlcNAc-T and adherence of human leukocytes to retinal endothelial cells: significance of core 2 GlcNAc-T in diabetic retinopathy

A large body of evidence now implicates increased leukocyte-endothelial cell adhesion as a key early event in the development of diabetic retinopathy. We recently reported that raised activity of the glycosylating enzyme core 2 beta 1,6-N-acetylglucosaminyltransferase (GlcNAc-T) through protein kinase C (PKC)beta2-dependent phosphorylation plays a fundamental role in increased leukocyte-endothelial cell adhesion and capillary occlusion in retinopathy. In the present study, we demonstrate that following exposure to plasma from diabetic patients, the human promonocytic cell line U937 exhibits a significant elevation in core 2 GlcNAc-T activity and increased adherence to cultured retinal capillary endothelial cells. These effects of diabetic plasma on enzyme activity and cell adhesion, mediated by PKCbeta2-dependent phosphorylation of the core 2 GlcNAc-T protein, were found to be triggered by increased plasma levels of tumor necrosis factor (TNF)-alpha. Levels of enzyme activity in plasma-treated U937 cells were closely dependent on the severity of diabetic retinopathy, with the highest values observed upon treatment with plasma of patients affected by proliferative retinopathy. Furthermore, we noted much higher correlation, as compared with control subjects, between increased values of core 2 GlcNAc-T activity and cell adhesion properties. Based on the prominent role of TNF-alpha in the development of diabetic retinopathy, these observations further validate the significance of core 2 GlcNAc-T in the pathogenesis of capillary occlusion, thereby enhancing the therapeutic potential of specific enzyme inhibitors.

Proinflammatory mediator expression in a novel murine model of titanium-particle-induced intramedullary inflammation

Wear debris from total joint replacement prostheses is implicated in periprosthetic osteolysis and implant loosening. The pathophysiology of this biological process remains unclear. Animal models of particle-induced osteolysis have proven useful in the study of specific tissue responses to wear debris. However, existing in vivo murine models of particle-mediated inflammation do not permit analysis of cortical bone degradation. This study describes a murine model of particle disease using an intramedullary rod in the mouse femur to parallel the clinical situation. The model consists of placing a 10-mm-long Kirschner wire retrograde in both femurs of C57b1/6 male mice via a medial parapatellar arthrotomy. Phagocytosable titanium particles were also implanted unilaterally to replicate generation of wear debris. Mice were sacrificed at 2, 10, and 26 weeks and whole femurs were cultured for 72 h. Levels of interleukin-6, monocyte chemotactic protein-1, and macrophage colony stimulating factor were assayed by ELISA. Transverse histological sections, at the level of the implant, were taken and stained with hematoxylin and eosin (H&E). Results demonstrated increased expression of proinflammatory mediators at 2 weeks in femora with rod and particles compared to femora with rods alone. Destruction of the endosteum was evident at 2, 10, and 26 weeks in the femora with titanium. This novel murine model of particle-induced intramedullary inflammation may facilitate cost-effective genetic studies and offers investigators a simple, clinically relevant intramedullary model to readily examine the pathogenesis of particle-mediated periprosthetic osteolysis.

Role of diacylglycerol induced by hypoxia in the regulation of HIF-1alpha activity

Hypoxia-inducible factor 1 (HIF-1) is a critical transcription factor for the adaptation to lowered oxygen environments. We have previously reported that hypoxia induced phosphatidic acid (PA) accumulation through diacylglycerol kinase (DGK) activity and provided evidence that this PA production regulated HIF-1 expression. Here we report that hypoxia also produces a marked intracellular accumulation of diacylglycerol (DAG) in different cell types. The previously proposed inhibitor of phosphatidylcholine phospholipase C (PC-PLC)/sphingomyelin synthase (SMS) activities, D609, specifically abrogates both hypoxia-dependent DAG accumulation and hypoxia-induced HIF-1 expression. We show that DAG-dependent protein kinase C (PKC) isoforms do not play an essential role in the regulation of HIF-1 expression. D609 inhibits PA accumulation triggered by hypoxia, suggesting that DAG could act as substrate for its conversion into PA by DGK upon these conditions. Therefore, this work provides novel evidence for the existence of DAG/PA-dependent intracellular mechanisms involved in the regulation of HIF-1 expression.

Role of FAN in tumor necrosis factor-alpha and lipopolysaccharide-induced interleukin-6 secretion and lethality in D-galactosamine-sensitized mice

Tumor necrosis factor (TNF) alpha-induced neutral sphingomyelinase-mediated generation of ceramide, a bioactive lipid molecule, is transduced by the adaptor protein FAN, which binds to the intracellular region of the CD120a TNFalpha receptor. FAN-deficient mice do not exhibit any gross abnormality. To further explore the functions of FAN in vivo and because CD120a-deficient mice are resistant to endotoxin-induced liver failure and lethality, we investigated the susceptibility of FAN-deficient animals to lipopolysaccharide (LPS). We show that after d-galactosamine sensitization, FAN-deficient mice were partially resistant to LPS- and TNFalpha-induced lethality. Although LPS challenge resulted in a hepatic ceramide content lower in mutant mice than in control animals, it triggered similar histological alterations, caspase activation, and DNA fragmentation in the liver. Interestingly, LPS-induced elevation of IL-6 (but not TNFalpha) serum concentrations was attenuated in FAN-deficient mice. A less pronounced secretion of IL-6 was also observed after LPS or TNFalpha treatment of cultured peritoneal macrophages and embryonic fibroblasts isolated from FAN-deficient mice, as well as in human fibroblasts expressing a mutated FAN. Finally, we show that d-galactosamine-sensitized IL-6-deficient mice were partially resistant to endotoxin-induced liver apoptosis and lethality. These findings highlight the role of FAN and IL-6 in the inflammatory response initiated by endotoxin, implicating TNFalpha.

Effects of (+)-catechin on the function of osteoblastic cells

Tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) are candidate cytokines which are produced by osteoblastic linage cells and promote osteoblast apoptosis, osteoclastogenesis and bone resorption. Here, we examined the effect of (+)-catechin, one of the most common grape flavonols, on osteoblastic MC3T3-E1 cells. (+)-Catechin caused a significant elevation of cell survival at 10(-5) and 10(-4) M and alkaline phosphatase activity at 10(-5) M. Also, treatment with (+)-catechin (10(-5) M) decreased bone-resorbing cytokines (TNF-alpha and IL-6) production and apoptosis in osteoblasts. Our data indicate that the reduction of bone-resorbing cytokines and apoptosis in osteoblasts by (+)-catechin may result in the prevention and therapy for osteoporosis and inflammatory bone diseases.

Sphingomyelinase amplifies BMP-4-induced osteocalcin synthesis in osteoblasts: role of ceramide

We previously reported that extracellular sphingomyelinase induces sphingomyelin hydrolysis in osteoblast-like MC3T3-E1 cells and that mitogen-activated protein (MAP) kinases are involved in bone morphogenetic protein (BMP)-4-stimulated osteocalcin synthesis in these cells. In the present study, we investigated whether sphingomyelinase affects BMP-4-stimulated synthesis of osteocalcin in osteoblast-like MC3T3-E1 cells. Sphingomyelinase significantly enhanced the BMP-4-stimulated osteocalcin synthesis. Among sphingomyelin metabolites, C(2)-ceramide enhanced the BMP-4-stimulated osteocalcin synthesis while sphingosine and sphingosine 1-phosphate had little effect on the synthesis. D-erythro-MAPP, an inhibitor of ceramidase, amplified the sphingomyelinase-effect on the osteocalcin synthesis. C(2)-ceramide suppressed the BMP-4-induced phosphorylation of p44/p42 MAP kinase, while having little effect on the phosphorylation of Smad1 and p38 MAP kinase. Taken together, our results strongly suggest that extracellular sphingomyelinase enhances the BMP-stimulated osteocalcin synthesis via ceramide in osteoblasts and that the effect of ceramide is exerted at a point upstream from p44/p42 MAP kinase.

The role of transsignalling via the agonistic soluble IL-6 receptor in human diseases

The activation of cells that do not express the membrane bound interleukin-6 6 receptor (IL-6R) by IL-6 and the soluble IL-6 receptor (sIL-6R) is termed transsignalling. Transsignalling may be an pathogenetic factor in human diseases as diverse as multiple myeloma (MM), Castleman's disease, prostate carcinoma, Crohn's disease, systemic sclerosis, Still's disease, osteoporosis and cardiovascular diseases. IL-6 and sIL-6R may directly or indirectly enhance their own production on endothelial or bone marrow stromal cells. Positive feedback autocrine loops thus created in affected organs may either cause or maintain disease progression. In autoimmune or vasculitic disease, the ability of the IL-6/sIL-6R complex to inhibit apoptosis of autoreactive T-cells may be central to the development of tissue specific autoimmunity. The anti-apoptotic effect of the IL-6/sIL-6R complex may be involved in tumour genesis and resistance to chemotherapy. Only in rare cases, where counterregulation has failed, there is a notable systemic effect of IL-6/sIL-6R. Appropriate animal models are necessary to establish the pathogenetic role of the IL-6/sIL-6R complex. A specific treatment option for diseases influenced by the sIL-6R could be based on gp130-Fc, a soluble gp130 (sgp130) linked to the Fc-fragment of IgG1. gp130-Fc has shown efficacy in vivo in animal models of Crohn's disease.

TNF receptor subtype signalling: differences and cellular consequences

Tumour necrosis factor-alpha (TNF alpha) is a multifunctional cytokine belonging to a family of ligands with an associated family of receptor proteins. The pleiotropic actions of TNF range from proliferative responses such as cell growth and differentiation, to inflammatory effects and the mediation of immune responses, to destructive cellular outcomes such as apoptotic and necrotic cell death mechanisms. Activated TNF receptors mediate the association of distinct adaptor proteins that regulate a variety of signalling processes including kinase or phosphatase activation, lipase stimulation, and protease induction. Moreover, the cytokine regulates the activities of transcription factors, heterotrimeric or monomeric G-proteins and calcium ion homeostasis in order to orchestrate its cellular functions. This review addresses the structural basis of TNF signalling, the pathways employed with their cellular consequences, and focuses on the specific role played by each of the two TNF receptor isotypes, TNFR1 and TNFR2.

Involvement of PKC-beta in PTH, TNF-alpha, and IL-1 beta effects on IL-6 promoter in osteoblastic cells and on PTH-stimulated bone resorption

Protein kinase C (PKC) has been shown to be activated by parathyroid hormone (PTH) in osteoblasts. Prior evidence suggests that this activation mediates responses leading to bone resorption, including production of the osteoclastogenic cytokine interleukin-6 (IL-6). However, the importance of specific PKC isozymes in this process has not been investigated. A selective antagonist of PKC-beta, LY379196, was used to determine the role of the PKC-beta isozyme in the expression of IL-6 in UMR-106 rat osteoblastic cells and in bone resorption in fetal rat limb bone organ cultures. PTH, tumor necrosis factor-alpha (TNF-alpha), and interleukin-1 beta (IL-1 beta) induced translocation of PKC-alpha and -beta(I) to the plasma membrane in UMR-106 cells within 5 min. The stimulation of PKC-beta(I) translocation by PTH, TNF-alpha or IL-1 beta was inhibited by LY379196. In contrast, LY379196 did not affect PTH, TNF-alpha-, or IL-1 beta-stimulated translocation of PKC-alpha. PTH, TNF-alpha, and IL-1 beta increased luciferase expression in UMR-106 cells transiently transfected with a -224/+11 bp IL-6 promoter-driven reporter construct. The IL-6 responses were also attenuated by treatment with LY379196. Furthermore, LY379196 inhibited bone resorption elicited by PTH in fetal rat bone organ cultures. These results indicate that PKC-beta(I) is a component of the signaling pathway that mediates PTH-, TNF-alpha-, and IL-1 beta-stimulated IL-6 expression and PTH-stimulated bone resorption.

Stimulation of interleukin-6 promoter by parathyroid hormone, tumor necrosis factor alpha, and interleukin-1beta in UMR-106 osteoblastic cells is inhibited by protein kinase C antagonists

To investigate the level at which protein kinase C (PKC) regulates expression of interleukin-6 (IL-6) in osteoblastic cells, effects of several PKC antagonists and PKC down-regulation by phorbol ester were studied in UMR-106 osteoblastic cells that had been transiently transfected with a -224/+11-base pair (bp) IL-6 promoter coupled to a luciferase reporter. Parathyroid hormone (PTH) elicited a dose-dependent stimulation of the IL-6 promoter expression, with significant increases produced by 5 h of treatment with concentrations of PTH as low as 10(-14) M. The increase in IL-6 promoter expression was inhibited by the PKC antagonists GF109203X, 30 nM to 1 microM, and calphostin C, 250 nM. Prior down-regulation of PKC with 100 nM phorbol-12,13-dibutyrate (PDBU) for 48 h inhibited the PTH effect as well as the smaller stimulatory effects elicited by tumor necrosis factor alpha (TNF-alpha), 10(-9)-10(-8) M, and by IL-1beta, 1-10 ng/ml. In contrast to these findings, the stimulatory effects of PTH, TNF-alpha, and IL-1beta on the IL-6 promoter expression were enhanced by staurosporine. Treatment with GF109203X or down-regulation of PKC with PDBU prevented the stimulatory effects of staurosporine. PKC activity was increased by staurosporine. The findings with staurosporine are consistent with our earlier observations that this agent enhances the calcium signaling and bone resorption elicited by PTH. The studies support the role of PKC in the stimulatory effects of PTH, TNF-alpha, and IL-1beta on IL-6 expression.

NF-kappa B activation in tumor necrosis factor alpha-stimulated neutrophils is mediated by protein kinase Cdelta. Correlation to nuclear Ikappa Balpha

The transcription factor NF-kappaB is critical for the expression of multiple genes involved in inflammatory responses and apoptosis. However, the signal transduction pathways regulating NF-kappaB activation in human neutrophils in response to stimulation with tumor necrosis factor-alpha (TNFalpha) are undefined. Since recent studies implicated activation of NF-kappaB as well as protein kinase C-delta (PKCdelta) in neutrophil apoptosis, we investigated involvement of PKCdelta in the activation of NF-kappaB in TNFalpha-stimulated neutrophils. Specific inhibition of PKCdelta by rottlerin prevented IkappaBalpha degradation and NF-kappaB activation in TNFalpha-stimulated neutrophils. This regulation of NF-kappaB activation by PKCdelta was specific only for TNFalpha signaling, since lipopolysaccharide- or interleukin-1beta-induced NF-kappaB activation and IkappaBalpha degradation were not inhibited by rottlerin. In addition, we show that in human neutrophils, but not monocytes, IkappaBalpha localizes in significant amounts in the nucleus of unstimulated cells, and the amount of IkappaBalpha in the nucleus, as well as in the cytoplasm, correlates with the NF-kappaB DNA binding. These results suggest that in human neutrophils, the presence of IkappaBalpha in the nucleus may function as a safeguard against initiation of NF-kappaB dependent transcription of pro-inflammatory and anti-apoptotic genes, and represents a distinct and novel mechanism of NF-kappaB regulation.

Combined effect of titanium particles and TNF-alpha on the production of IL-6 by osteoblast-like cells

To clarify the role of tumor necrosis factor (TNF)-alpha on osteoblast functions in the presence of metal particles, two human osteoblast-like cell lines (MG-63 and SaOS-2) were cultured with TNF-alpha in the presence or absence of titanium particles in vitro. A combination of TNF-alpha and titanium particles showed additive effects on inhibition of cell proliferation and alkaline phosphatase production. On the other hand, production of interleukin-6, which is well known to induce osteoclastogenesis and to directly stimulate bone resorption, was additively stimulated by the combination of TNF-alpha and titanium particles. These results suggest that the association of TNF-alpha and titanium particles may play an important role in the pathogenesis of periprosthetic osteolysis through two different pathways: a reduced periprosthetic bone formation due to inhibition of osteoblast proliferation and alkaline phosphatase production, and osteoblast-mediated activation of osteoclastic bone resorption as suggested by the enhancement of interleukin-6 production.

Sphingosine 1-phosphate signalling via the endothelial differentiation gene family of G-protein-coupled receptors

Sphingosine 1-phosphate (S1P) is stored in and released from platelets in response to cell activation. However, recent studies show that it is also released from a number of cell types, where it can function as a paracrine/autocrine signal to regulate cell proliferation, differentiation, survival, and motility. This review discusses the role of S1P in cellular regulation, both at the molecular level and in terms of health and disease. The main biochemical routes for S1P synthesis (sphingosine kinase) and degradation (S1P lyase and S1P phosphatase) are described. The major focus is on the ability of S1P to bind to a novel family of G-protein-coupled receptors (endothelial differentiation gene [EDG]-1, -3, -5, -6, and -8) to elicit signal transduction (via G(q)-, G(i)-, G(12)-, G(13)-, and Rho-dependent routes). Effector pathways regulated by S1P are divergent, such as extracellular signal-regulated kinase, p38 mitogen-activated protein kinase, phospholipases C and D, adenylyl cyclase, and focal adhesion kinase, and occur in multiple cell types, such as immune cells, neurones, smooth muscle, etc. This provides a molecular basis for the ability of S1P to act as a pleiotropic bioactive lipid with an important role in cellular regulation. We also give an account of the expanding role for S1P in health and disease; in particular, with regard to its role in atherosclerosis, angiogenesis, cancer, and inflammation. Finally, we describe future directions for S1P research and novel approaches whereby S1P signalling can be manipulated for therapeutic intervention in disease.

Expression of a catalytically inactive sphingosine kinase mutant blocks agonist-induced sphingosine kinase activation. A dominant-negative sphingosine kinase

Sphingosine kinase (SK) catalyzes the formation of sphingosine 1-phosphate (S1P), a lipid messenger that plays an important role in a variety of mammalian cell processes, including inhibition of apoptosis and stimulation of cell proliferation. Basal levels of S1P in cells are generally low but can increase rapidly when cells are exposed to various agonists through rapid and transient activation of SK activity. To date, elucidation of the exact signaling pathways affected by these elevated S1P levels has relied on the use of SK inhibitors that are known to have direct effects on other enzymes in the cell. Furthermore, these inhibitors block basal SK activity, which is thought to have a housekeeping function in the cell. To produce a specific inhibitor of SK activation we sought to generate a catalytically inactive, dominant-negative SK. This was accomplished by site-directed mutagenesis of Gly(82) to Asp of the human SK, a residue identified through sequence similarity to the putative catalytic domain of diacylglycerol kinase. This mutant had no detectable SK activity when expressed at high levels in HEK293T cells. Activation of endogenous SK activity by tumor necrosis factor-alpha (TNFalpha), interleukin-1beta, and phorbol esters in HEK293T cells was blocked by expression of this inactive sphingosine kinase (hSK(G82D)). Basal SK activity was unaffected by expression of hSK(G82D). Expression of hSK(G82D) had no effect on TNFalpha-induced activation of protein kinase C and sphingomyelinase activities. Thus, hSK(G82D) acts as a specific dominant-negative SK to block SK activation. This discovery provides a powerful tool for the elucidation of the exact signaling pathways affected by elevated S1P levels following SK activation. To this end we have employed the dominant-negative SK to demonstrate that TNFalpha activation of extracellular signal-regulated kinases 1 and 2 (ERK1,2) is dependent on SK activation.

Selective stimulation by ceramide of the expression of the alpha isoform of retinoic acid and retinoid X receptors in osteoblastic cells. A role of sphingosine 1-phosphate-mediated AP-1 in the ligand-dependent transcriptional activity of these receptors

Recent studies have demonstrated that sphingosine 1-phosphate (SPP) plays a functional role as a signaling molecule in gene expression in several kinds of cells. The present study demonstrates selective expression by ceramide of retinoic acid receptor-alpha (RAR-alpha) and retinoic X receptor-alpha (RXR-alpha) in osteoblastic MC3T3-E1 cells and a functional role of SPP-mediated AP-1 in the signaling mechanism of ligand-dependent transcriptional activity of heterodimers of these receptors in the cells. C(2)- and C(6)-ceramides selectively stimulated the expression of RAR-alpha and RXR-alpha genes, but not that of beta and gamma isoform genes of RAR and RXR, in the cells. The C(2)-ceramide-induced stimulation was clearly inhibited by dl-threo-dihydrosphingosine, an inhibitor of sphingosine kinase. SPP also selectively stimulated the expression of both receptors and increased the specific binding of the nuclear proteins to direct repeat 5 (DR-5), a consensus sequence of RAR-RXR. In addition, SPP markedly stimulated transient chloramphenicol acetyltransferase (CAT) activity of retinoic acid-dependent transcriptional activity in the cells transfected with a DR-5-CAT reporter gene. The SPP stimulation was activation protein-1 (AP-1)-dependent, because the SPP stimulatory action toward these nuclear gene expressions and the transient CAT activity were inhibited by antisense c-fos and c-jun oligonucleotides. We observed that SPP actually stimulated AP-1 transcriptional activity in the cells. This study suggests an important role of SPP-mediated AP-1 in the selective expression of RAR-alpha and RXR-alpha in osteoblastic cells via the sphingosine pathway.

Sphingosine 1-phosphate signalling in mammalian cells

Sphingosine 1-phosphate is formed in cells in response to diverse stimuli, including growth factors, cytokines, G-protein-coupled receptor agonists, antigen, etc. Its production is catalysed by sphingosine kinase, while degradation is either via cleavage to produce palmitaldehyde and phosphoethanolamine or by dephosphorylation. In this review we discuss the most recent advances in our understanding of the role of the enzymes involved in metabolism of this lysolipid. Sphingosine 1-phosphate can also bind to members of the endothelial differentiation gene (EDG) G-protein-coupled receptor family [namely EDG1, EDG3, EDG5 (also known as H218 or AGR16), EDG6 and EDG8] to elicit biological responses. These receptors are coupled differentially via G(i), G(q), G(12/13) and Rho to multiple effector systems, including adenylate cyclase, phospholipases C and D, extracellular-signal-regulated kinase, c-Jun N-terminal kinase, p38 mitogen-activated protein kinase and non-receptor tyrosine kinases. These signalling pathways are linked to transcription factor activation, cytoskeletal proteins, adhesion molecule expression, caspase activities, etc. Therefore sphingosine 1-phosphate can affect diverse biological responses, including mitogenesis, differentiation, migration and apoptosis, via receptor-dependent mechanisms. Additionally, sphingosine 1-phosphate has been proposed to play an intracellular role, for example in Ca(2+) mobilization, activation of non-receptor tyrosine kinases, inhibition of caspases, etc. We review the evidence for both intracellular and extracellular actions, and extensively discuss future approaches that will ultimately resolve the question of dual action. Certainly, sphingosine 1-phosphate will prove to be unique if it elicits both extra- and intra-cellular actions. Finally, we review the evidence that implicates sphingosine 1-phosphate in pathophysiological disease states, such as cancer, angiogenesis and inflammation. Thus there is a need for the development of new therapeutic compounds, such as receptor antagonists. However, identification of the most suitable targets for drug intervention requires a full understanding of the signalling and action profile of this lysosphingolipid. This article describes where the research field is in relation to achieving this aim.

Sphingosine 1-phosphate amplifies phosphoinositide hydrolysis stimulated by prostaglandin f2 alpha in osteoblasts: involvement of p38MAP kinase

We previously showed that sphingosine 1-phosphate phosphorylates p42/p44 mitogen-activated protein (MAP) kinase and p38 MAP kinase in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the effect of sphingosine 1-phosphate on phospholipase C-catalyzing phosphoinositide hydrolysis induced by prostaglandin F2alpha (PGF2 alpha) in these cells. Sphingosine 1-phosphate significantly amplified the inositol phosphates formation by PGF2 alpha. Sphingosine 1-phosphate did not enhance the formation induced by NaF, a direct activator of heterotrimeric GTP-binding proteins. PD98059, an inhibitor of the kinase that activates p42/p44 MAP kinase, had little effect on the amplification by sphingosine 1-phosphate. SB203580, an inhibitor of p38 MAP kinase, reduced the effect of sphingosine 1-phosphate on the formation of inositol phosphates by PGF2 alpha. The phosphorylation of p42/p44 MAP kinase by PGF alpha was attenuated by PD98059. SB203580 suppressed the phosphorylation of p38 MAP kinase by PGF2 alpha. Tumor necrosis factor-alpha enhanced the PGF2 alpha-stimulated formation of inositol phosphates. These results strongly suggest that sphingosine 1-phosphate amplifies PGF2 alpha-induced phosphoinositide hydrolysis by phospholipase C through p38 MAP kinase in osteoblasts.

Protein kinase C involvement in interleukin-6 production by parathyroid hormone and tumor necrosis factor-alpha in UMR-106 osteoblastic cells

The cytokine interleukin-6 (IL-6) is increased in bone and bone cells by several resorptive stimuli, including parathyroid hormone (PTH), IL-1beta, and tumor necrosis factor-alpha (TNF-alpha). The current studies were designed to determine the contribution of the protein kinase C (PKC) signaling pathway to the effects of these three agents to increase IL-6 in UMR-106 rat osteoblastic cells. Cells were pretreated with vehicle (dimethylsulfoxide [DMSO]) or the phorbol ester, phorbol 12,13-dibutyrate (PDB; 300 nM) for 48 h to down-regulate phorbol-sensitive PKC isozymes. Either PTH (0.1-10 nM), IL-1beta (0.1-10 nM), or TNF-alpha (5 nM and 10 nM) was then added for 24 h in the continued presence of vehicle or PDB. PKC isozymes were visualized by Western immunoblotting and IL-6 was determined by bioassay. PDB pretreatment caused a partial down-regulation of the conventional alpha-PKC and betaI-PKC isozymes and complete down-regulation of the novel delta-isoenzyme and epsilon-isozymes but it had no effect on the atypical zeta-PKC isozyme. PDB pretreatment reduced IL-6 responses to 5 nM and 10 nM PTH by 61% and 33%, respectively, reduced IL-6 responses to 5nM and 10 nM TNF-a by 54% and 42%, respectively, and failed to inhibit the IL-6 responses to 0.1-10 nM IL-1beta. The PDB pretreatment protocol significantly enhanced PTH-stimulated cyclic adenosine monophosphate (cAMP) production. The PKC inhibitor calphostin C also decreased IL-6 responses to PTH. Thus, in this osteoblast cell line, the PKC pathway is an important component of the signaling pathway for the IL-6 production stimulated by PTH and TNF-alpha but not that from IL-1beta.

Tumor necrosis factor-alpha-mediated signal transduction in human neutrophils: involvement of sphingomyelin metabolites in the priming effect of TNF-alpha on the fMLP-stimulated superoxide production

We investigated the mechanism underlying the priming effect of TNF-alpha on fMLP-stimulated superoxide production in human neutrophils. TNF-alpha enhanced fMLP-stimulated superoxide production in a concentration-dependent manner. TNF-alpha also induced sphingomyelin (SM) hydrolysis and increased the formation of its metabolite, sphingosine-1-phosphate (SP-1-P). The treatment of neutrophils with sphingomyelinase also resulted in a similar priming effect. C2 ceramide produced a concentration-dependent inhibition of fMLP-stimulated superoxide production within the concentration range of 1-30 microM. Sphingosine had a dual effect on fMLP-stimulated superoxide generation, exhibiting a priming effect at lower concentrations (0.2-1 microM), but an inhibitory effect at higher concentrations (1-30 microM). SP-1-P (1-30 microM), showed a concentration-dependent enhancement of fMLP stimulated superoxide production. Furthermore, after treating neutrophils with DL-threo-dihydro-sphingosine, a competitive inhibitor of sphingosine kinase, TNF-alpha produced a similar dual effect as observed with sphingosine. These results strongly suggest that SM hydrolysis plays a key role in the intracellular signal transduction mediating the TNF-alpha-mediated priming effect.

Potentiation of lipopolysaccharide-induced IL-6 release by uridine triphosphate in macrophages: cross-interaction with cyclooxygenase-2-dependent prostaglandin E(2) production

Our previous study has demonstrated the potentiation by uridine triphosphate (UTP) of nitric oxide (NO) and prostaglandin E(2) (PGE(2)) production in lipopolysaccharide (LPS)-stimulated murine J774 macrophages. In this study, we found that the amount of interleukin-6 (IL-6) release in response to LPS stimulation was greatly enhanced in the presence of UTP. This enhancement exhibited concentration dependence and occurred after 8 h of treatment with LPS. RT-PCR analysis indicated that the steady-state level of IL-6 mRNA induced by LPS was apparently increased upon co-addition of UTP. The potentiation by UTP was inhibited by the treatment with U73122 (a phosphatidylinositol-phospholipase C inhibitor), BAPTA/AM (an intracellular Ca(2+) chelator), KN-93 (a selective inhibitor of calmodulin-dependent protein kinase) or PDTC (a nuclear factor kappaB inhibitor). To understand the cross-regulation among NO, PGE(2) and IL-6, all of which are dramatically induced after LPS stimulation, the effects of L-NAME (a nitric oxide synthase inhibitor), indomethacin (a cyclooxygenase inhibitor), NS-398 (a cycloxygenase-2 inhibitor) and IL-6 antibody were tested. The results revealed the positive regulation between PGE(2) and IL-6 synthesis because NS-398 and indomethacin inhibited LPS plus UTP-induced IL-6 release, and IL-6 antibody attenuated LPS plus UTP-induced PGE(2) release. Taken together these results reinforce the role of UTP as a regulatory element in inflamed sites by demonstrating the capacity of this nucleotide to potentiate LPS-induced release of inflammatory mediators.

Sphingosine 1-phosphate induces heat shock protein 27 via p38 mitogen-activated protein kinase activation in osteoblasts

We previously showed that sphingosine 1-phosphate acts as a second messenger for tumor necrosis factor alpha-induced interleukin-6 synthesis in osteoblast-like MC3T3-E1 cells and that the synthesis by sphingosine 1-phosphate is dependent on p42/p44 mitogen-activated protein (MAP) kinase activation. In the present study, we investigated the effect of sphingosine 1-phosphate on the induction of heat shock protein 27 (HSP27) in MC3T3-E1 cells. Not C2-ceramide, but sphingosine and sphingosine 1-phosphate significantly induced HSP27 accumulation dose dependently in the range between 1microM and 30 microM. DL-threo-dihydrosphingosine, an inhibitor of sphingosine kinase, markedly inhibited the sphingosine-induced HSP27 accumulation. Sphingosine 1-phosphate induced increase in the levels of the mRNA for HSP27. Sphingosine 1-phosphate stimulated the phosphorylation of p38 MAP kinase. The sphingosine 1-phosphate-induced HSP27 accumulation was dose dependently suppressed by SB203580, an inhibitor of p38 MAP kinase, but not PD98059, an inhibitor of the upstream kinase that activates p42/p44 MAP kinase. SB203580 reduced the sphingosine 1-phosphate-induced increase of mRNA for HSP27. These results strongly suggest that sphingosine 1-phosphate-stimulated HSP27 induction is mediated via p38 MAP kinase activation in osteoblasts.

Effect of ceramide on interleukin-6 synthesis in osteoblast-like cells

We previously showed that prostaglandin (PG) E1 stimulates the synthesis of interleukin-6 (IL-6) through activation of protein kinase (PK) A in osteoblast-like MC3T3-E1 cells and that PGF2alpha induces IL-6 synthesis through PKC activation. In other studies, we demonstrated that thrombin stimulates IL-6 synthesis, which depends on intracellular Ca2+ mobilisation in these cells and that tumour necrosis factor-alpha (TNF) induces IL-6 synthesis through sphingosine 1-phosphate, a product of sphingomyelin turnover. In the present study, among sphingomyelin metabolites, we examined the effect of ceramide on the IL-6 synthesis induced by various agonists in MC3T3-E1 cells. C2-ceramide, a cell-permeable ceramide analogue, suppressed the PGE1-induced IL-6 synthesis. C2-ceramide inhibited the IL-6 synthesis induced by PGF2alpha or 12-O-tetradecanoylphorbol-13-acetate, an activator of PKC. C2-ceramide reduced the IL-6 synthesis induced by cholera toxin, forskolin or dibutyryl cAMP. C2-ceramide inhibited the IL-6 synthesis induced by thrombin. The IL-6 synthesis stimulated by thapsigargin, which is known to stimulate Ca2+ mobilisation from intracellular Ca2+ stores, or A23187, a Ca-ionophore, was also inhibited by C2-ceramide. C2-ceramide did not affect the IL-6 synthesis induced by interleukin-1. On the contrary, C2-ceramide enhanced the TNF-induced IL-6 synthesis. D,L-threo-dihydrosphingosine, an inhibitor of sphingosine kinase, inhibited the enhancement by C2-ceramide as well as the TNF-effect. These results strongly suggest that ceramide modulates the IL-6 synthesis stimulated by various agonists in osteoblasts.

Extracellular sphingomyelinase induces interleukin-6 synthesis in osteoblasts

In osteoblast-like MC3T3-E1 cells, we have recently reported that sphingosine 1-phosphate among sphingomyelin metabolites acts as a second messenger for tumor necrosis factor-alpha (TNF)-induced interleukin-6 (IL-6) synthesis. In the present study, we investigated the effect of extracellular sphingomyelinase on IL-6 synthesis in MC3T3-E1 cells. Sphingomyelinase stimulated IL-6 synthesis in a time-dependent manner for up to 24 h. This stimulative effect was dose dependent in the range between 1 and 300 mU/ml. Calphostin C, a highly and potent inhibitor of protein kinase C, enhanced sphingomyelinase-induced IL-6 synthesis. DL-Threo-dihydrosphingosine, an inhibitor of sphingosine kinase, significantly inhibited the IL-6 synthesis induced by sphingomyelinase. Sphingomyelinase markedly elicited sphingomyelin hydrolysis. In addition, the effect of a combination of sphingomyelinase and TNF on IL-6 synthesis was synergistic. These results strongly suggest that extracellular sphingomyelinase induces sphingomyelin hydrolysis in osteoblasts, resulting in IL-6 synthesis, and that protein kinase C acts as a negative controller of the IL-6 synthesis.

Sphingosine modulates interleukin-6 synthesis in osteoblasts

We previously reported that prostaglandin (PG)E1 and PGF2alpha induce the synthesis of interleukin-6 (IL-6) via activation of protein kinase (PK)A and PKC, respectively, in osteoblast-like MC3T3-E1 cells. In addition, we have shown that basic fibroblast growth factor (bFGF) elicits IL-6 synthesis through intracellular Ca2+ mobilization in these cells and that tumor necrosis factor-alpha (TNF) induces IL-6 synthesis through sphingosine 1-phosphate produced by sphingomyelin hydrolysis. In the present study, among sphingomyelin metabolites, we examined the effect of sphingosine on IL-6 synthesis induced by various agonists in MC3T3-E1 cells. Sphingosine inhibited the IL-6 synthesis induced by PGF2alpha or 12-O-tetradecanoylphorbol-13-acetate, an activator of PKC. Sphingosine suppressed the PGE1-induced IL-6 synthesis. The IL-6 synthesis induced by cholera toxin, forskolin, or dibutyryl cAMP was inhibited by sphingosine. Sphingosine inhibited the IL-6 synthesis induced by bFGF or A23187. However, sphingosine did not affect the IL-6 synthesis induced by interleukin-1. On the contrary, sphingosine enhanced the TNF-induced IL-6 synthesis. DL-threo-Dihydrosphingosine, an inhibitor of sphingosine kinase, reduced the enhancement by sphingosine as well as the TNF-effect. These results indicate that sphingosine modulates the IL-6 synthesis stimulated by various agonists in osteoblasts.

Phorbol ester-induced production of prostaglandin E2 from phosphatidylcholine through the activation of phospholipase D in UMR-106 cells

To determine the effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) on phospholipase D (PLD) activity in osteoblast-like UMR-106 cells, we used cells prelabeled with [3H] myristic acid or [3H] arachidonic acid, which were preferentially incorporated to phosphatidylcholine. The treatment of [3H] myristate-labeled cells with TPA in the presence of 1% ethanol caused a dose-dependent formation of [3H] phosphatidylethanol (PEt), a product specific to PLD, suggesting an activation of this enzyme. Pretreatment of the cells with protein kinase C (PKC) inhibitors (GF109203X, staurosporine or H-7) abolished the TPA-dependent formation of PEt. The PEt formation in response to TPA treatment was not observed after the pretreatment of the cells with TPA to downregulate PKC. These results suggest the involvement of PKC in the TPA-induced activation of PLD. With [3H] arachidonate-labeled cells, TPA treatment in the absence of ethanol resulted in the liberation of [3H] arachidonic acid, which was gradually converted to prostaglandin E2 (PGE2), but the accumulations of [3H] phosphatidic acid (PA) and [3H] diacylglycerol (DAG) were very small and temporary. In contrast, PA was linearly accumulated following TPA treatment, when the cells were pretreated with an inhibitor of phosphatidate phosphohydrolase (PAP), propranolol, with no accumulation of either DAG or arachidonic acid. The TPA treatment of the cells pretreated with a DAG lipase inhibitor, RHC-80267, caused the generation of DAG after a lag period of approximately 5 min, with a very small and temporary accumulation of PA. The TPA treatment of cells pretreated with a cyclooxygenase (COX) inhibitor, indomethacin, blocked the PGE2 production. The TPA-induced PGE2 production was not affected by the pretreatment of cells with a phospholipase A2 inhibitor, p-bromophenacylbromide, or with a phospholipase C inhibitor, D-609. TPA also stimulated PGE2 production in osteoblastic cells that were enzymatically isolated from adult rat calvaria, and the experiments with lipid metabolizing enzyme inhibitors gave the same profile of inhibition of TPA-induced PGE2 production as was observed in UMR-106 cells. These results suggest that PA formed as a consequence of the activation of PLD by TPA is rapidly converted to arachidonic acid via a PAP/DAG lipase pathway, followed by a gradual conversion of arachidonic acid to PGE2 by COX in both UMR-106 cells and isolated adult osteoblastic cells, and that neither phospholipase A2 nor phospholipase C is involved in the TPA-induced PGE2 production. To the best of our knowledge, this is the first report that shows that the activation of PKC in osteoblastic cells leads to the production of PGE2 via a PLD/PAP/DAG lipase/COX pathway.