Mechanism of prostaglandin D(2)-stimulated heat shock protein 27 induction in osteoblasts

Oncostatin M stimulates prostaglandin D2-induced osteoprotegerin and interleukin-6 synthesis in osteoblasts

Oncostatin M produced by osteal macrophages plays a significant role in fracture healing. Osteoprotegerin (OPG) secreted by osteoblasts, binds to the receptor activator of nuclear factor-κB (RANK) ligand (RANKL) as a decoy receptor and prevents RANKL from binding to RANK, resulting in bone resorption suppression. Interleukin-6 (IL-6) is a pro-inflammatory cytokine and generally regulates bone resorption. However, accumulating evidence suggests that IL-6 plays pivotal roles in bone formation. We previously showed that prostaglandin D₂ (PGD₂) induces OPG synthesis by activating p38 mitogen-activated protein (MAP) kinase, stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK), and p44/p42 MAP kinase in osteoblast-like MC3T3-E1 cells. Furthermore, we demonstrated that PGD₂ stimulates IL-6 synthesis by activating p38 MAP kinase and p44/p42 MAP kinase in MC3T3-E1 cells. In the present study, we investigated whether oncostatin M affects PGD₂-stimulated OPG and IL-6 synthesis in MC3T3-E1 cells through MAP kinase activation. The osteoblast-like MC3T3-E1 cells and normal human osteoblasts were treated with oncostatin M and subsequently stimulated with PGD₂. Consequently, oncostatin M significantly increased the PGD₂-stimulated OPG and IL-6 release in both cells. Oncostatin M significantly enhanced mRNA expression levels of OPG and IL-6 induced by PGD₂ similarly in both cells. Regarding the signaling mechanism, oncostatin M did not affect the phosphorylation of p38 MAP kinase, SAPK/JNK, and p44/p42 MAP kinase. Our results suggest that oncostatin M upregulates the PGD₂-stimulated OPG and IL-6 synthesis in osteoblasts and therefore affects bone remodeling. However, OPG and IL-6 synthesis are not mediated through p38 MAP kinase, p44/p42 MAP kinase, or SAPK/JNK pathways.

Enhancement by HSP90 inhibitor of PGD2-stimulated HSP27 induction in osteoblasts: Suppression of SAPK/JNK and p38 MAP kinase

Heat shock protein (HSP) 90 that is ubiquitously expressed in various tissues is a major molecular chaperone. We have previously demonstrated that prostaglandin D₂ (PGD₂), a bone remodeling factor, elicits the expression of HSP27, a small HSP, through stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) and p38 mitogen-activated protein (MAP) kinase in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the involvement of HSP90 in the PGD₂-stimulated HSP27 induction and the underlying mechanism in MC3T3-E1 cells. Onalespib, an inhibitor of HSP90, significantly enhanced the PGD₂-stimulated HSP27 induction. In addition, geldanamycin, another HSP90 inhibitor, potentiated the HSP27 induction. Both onalespib and geldanamycin markedly amplified the PGD₂-induced phosphorylation of SAPK/JNK and p38 MAP kinase. SP600125, an inhibitor of SAPK/JNK, and SB203580, an inhibitor of p38 MAP kinase, suppressed the amplification by onalespib of the PGD₂-stimulated HSP27 induction. These results strongly suggest that HSP90 plays a negative role in the HSP27 induction stimulated by PGD₂ in osteoblasts, and that the inhibitory effect of HSP90 is mediated through the regulation of SAPK/JNK and p38 MAP kinase.

Role of the heat shock protein family in bone metabolism

Heat shock proteins (HSPs) are a family of proteins produced by cells in response to exposure to stressful conditions. In addition to their role as chaperones, they also play an important role in the cardiovascular, immune, and other systems. Normal bone tissue is maintained by bone metabolism, particularly by the balance between osteoblasts and osteoclasts, which are physiologically regulated by multiple hormones and cytokines. In recent years, studies have reported the vital role of HSPs in bone metabolism. However, the conclusions remain largely controversial, and the exact mechanisms are still unclear, so a review and analyses of previous studies are of importance. This article reviews the current understanding of the roles and effects of HSPs on bone cells (osteoblasts, osteoclasts, and osteocytes), in relation to bone metabolism.

Heat shock protein 27 (HSPB1) suppresses the PDGF-BB-induced migration of osteoblasts

Heat shock protein 27 (HSP27/HSPB1), one of the small heat shock proteins, is constitutively expressed in various tissues. HSP27 and its phosphorylation state participate in the regulation of multiple physiological and pathophysiological cell functions. However, the exact roles of HSP27 in osteoblasts remain unclear. In the present study, we investigated the role of HSP27 in the platelet-derived growth factor‑BB (PDGF‑BB)‑stimulated migration of osteoblast-like MC3T3-E1 cells. PDGF-BB by itself barely upregulated the expression of HSP27 protein, but stimulated the phosphorylation of HSP27 in these cells. The PDGF-BB‑induced cell migration was significantly downregulated by HSP27 overexpression. The PDGF-BB-induced migrated cell numbers of the wild‑type HSP27-overexpressing cells and the phospho‑mimic HSP27-overexpressing (3D) cells were less than those of the unphosphorylatable HSP27-overexpressing (3A) cells. PD98059, an inhibitor of MEK1/2, SB203580, an inhibitor of p38 mitogen-activated protein kinase, and SP600125, an inhibitor of stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) reduced the PDGF-BB-induced migration of these cells, whereas Akt inhibitor or rapamycin, an inhibitor of upstream kinase of p70 S6 kinase (mTOR), barely affected the migration. However, the PDGF-BB-induced phosphorylation of p44/p42 MAPΚ, p38 MAPK and SAPK/JNK was not affected by HSP27 overexpression. There were no significant differences in the phosphorylation of p44/p42 MAPΚ, p38 MAP kinase or SAPK/JNK between the 3D cells and the 3A cells. These results strongly suggest that HSP27 functions as a negative regulator in the PDGF-BB-stimulated migration of osteoblasts, and the suppressive effect is amplified by the phosphorylation state of HSP27.

Activation of the MAPK/Akt/Nrf2-Egr1/HO-1-GCLc axis protects MG-63 osteosarcoma cells against 15d-PGJ2-mediated cell death

Despite considerable efforts to improve treatment modalities for osteosarcoma (OS), patient survival remains poor mainly due to pro-survival pathways in OS cells. Among others, prostaglandins (PGs) are the potent regulators of bone homoeostasis and OS pathophysiology. Therefore, the present study aimed to elucidate the impact of 15-deoxy-Δ^12,14-PGJ₂ (15d-PGJ₂, a stable PGD₂ degradation product) on cell death/cell survival pathways in p53-deficient MG-63 OS cells. Our findings show that 15d-PGJ₂ induces generation of reactive oxygen species that promote p38 MAPK activation and subsequent Akt phosphorylation. This pathway induced nuclear expression of Nrf2 and Egr1, and increased transcription of haem oxygenase-1 (HO-1) and the catalytic subunit of glutamate cysteine ligase (GCLc), catalysing the first step in GSH synthesis. Silencing of Nrf2, Egr1 and HO-1 significantly elevated 15d-PGJ₂-mediated reduction of cellular metabolic activity. Activation of cell survival genes including HO-1 and GCLc inhibited 15d-PGJ₂-induced cleavage of pro-caspase-3 and PARP. Annexin V/propidium iodide staining showed an increase in early/late apoptotic cells in response to 15d-PGJ₂. The observed 15d-PGJ₂-mediated signalling events are independent of PGD₂ receptors (DP1 and DP2) and PPARγ. In addition, the electrophilic carbon atom C9 is a prerequisite for the observed activity of 15d-PGJ₂. The present data show that the intracellular redox imbalance acted as a node and triggered both death and survival pathways in response to 15d-PGJ₂. Pharmacological or genetic interference of the pro-survival pathway, the p38 MAPK/Akt/Nrf2-Egr1/HO-1-GCLc axis, sensitizes MG-63 cells towards 15d-PGJ₂-mediated apoptosis.

PGD2 stimulates osteoprotegerin synthesis via AMP-activated protein kinase in osteoblasts: Regulation of ERK and SAPK/JNK

AMP-activated protein kinase (AMPK), a key enzyme sensing cellular energy metabolism, is currently known to regulate multiple metabolic pathways. Osteoprotegerin plays a pivotal role in the regulation of bone metabolism by inhibiting osteoclast activation. We have previously reported that prostaglandin D2 (PGD2) stimulates the synthesis of osteoprotegerin through the activation of p38 mitogen-activated protein (MAP) kinase, p44/p42 MAP kinase and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) in osteoblast-like MC3T3-E1 cells. On the basis of these findings, we herein investigated the implication of AMPK in PGD2-stimulated osteoprotegerin synthesis in these cells. PGD2 induced the phosphorylation of AMPKα (Thr-172) and AMPKβ (Ser-108), and the phosphorylation of acetyl-coenzyme A carboxylase, a direct AMPK substrate. Compound C, an AMPK inhibitor, which suppressed the phosphorylation of acetyl-coenzyme A carboxylase, significantly attenuated both the release and the mRNA levels of osteoprotegerin stimulated by PGD2. The PGD2-induced phosphorylation of p44/p42 MAP kinase and SAPK/JNK but not p38 MAP kinase were markedly inhibited by compound C. These results strongly suggest that AMPK regulates the PGD2-stimulated osteoprotegerin synthesis at a point upstream of p44/p42 MAP kinase and SAPK/JNK in osteoblasts.

Suppression by resveratrol of prostaglandin D2-stimulated osteoprotegerin synthesis in osteoblasts

Resveratrol, a natural polyphenol with health-related properties mainly existing in grape skins and red wine, possesses beneficial effects on human being. We have previously reported that prostaglandin D2 (PGD2) stimulates heat shock protein 27 (HSP27) induction via activation of p44/p42 mitogen-activated protein (MAP) kinase, p38 MAP kinase and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the mechanism behind the effect of PGD2 on osteoprotegerin (OPG) synthesis and the effect of resveratrol on the OPG synthesis in MC3T3-E1 cells. PGD2 significantly stimulated both the OPG release and the expression levels of OPG mRNA. Resveratrol and SRT1720, an activator of SIRT1, markedly suppressed the PGD2-induced OPG release and the mRNA levels of OPG. PD98059, a specific MEK inhibitor, SB203580, a specific p38 MAP kinase inhibitor, and SP600125, a specific SAPK/JNK inhibitor suppressed the PGD2-stimulated OPG release. PGD2-induced phosphorylation of p38 MAP kinase and SAPK/JNK was attenuated by resveratrol or SRT1720. However, resveratrol or SRT1720 failed to affect the phosphorylation of myosin phosphatase-targeting subunit-1 (MYPT-1), a downstream substrate of Rho-kinase and p44/p42 MAP kinase. These results strongly suggest that resveratrol suppresses PGD2-stimulated OPG synthesis through inhibiting p38 MAP kinase and SAPK/JNK in osteoblasts, and that the suppressive effect is exerted at the point downstream of Rho-kinase but upstream of p38 MAP kinase or SAPK/JNK.

Mild heat stress enhances angiogenesis in a co-culture system consisting of primary human osteoblasts and outgrowth endothelial cells

The repair and regeneration of large bone defects, including the formation of functional vasculature, represents a highly challenging task for tissue engineering and regenerative medicine. Recent studies have shown that vascularization and ossification can be stimulated by mild heat stress (MHS), which would offer the option to enhance the bone regeneration process by relatively simple means. However, the mechanisms of MHS-enhanced angiogenesis and osteogenesis, as well as potential risks for the treated cells are unclear. We have investigated the direct effect of MHS on angiogenesis and osteogenesis in a co-culture system of human outgrowth endothelial cells (OECs) and primary osteoblasts (pOBs), and assessed cytotoxic effects, as well as the levels of various heat shock proteins (HSPs) synthesized under these conditions. Enhanced formation of microvessel-like structures was observed in co-cultures exposed to MHS (41°C, 1 h), twice per week, over a time period of 7-14 days. As shown by real-time polymerase chain reaction (PCR), the expression of vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1), angiopoietin-2 (Ang-2), and tumor necrosis factor-alpha was up-regulated in MHS-treated co-cultures 24 h post-treatment. At the protein level, significantly elevated VEGF and Ang-1 concentrations were observed in MHS-treated co-cultures and pOB mono-cultures compared with controls, indicating paracrine effects associated with MHS-induced angiogenesis. MHS-stimulated co-cultures and OEC mono-cultures released higher levels of Ang-2 than untreated cultures. On the other hand MHS treatment of co-cultures did not result in a clear effect regarding osteogenesis. Nevertheless, real-time PCR demonstrated that MHS increased the expression of mitogen-activated protein kinase, interleukin-6, and bone morphogenetic protein 2, known as HSP-related molecules in angiogenic and osteogenic regulation pathways. In agreement with these observations, the expression of some selected HSPs also increased at both the mRNA and protein levels in MHS-treated co-cultures.

Unphosphorylated heat shock protein 27 suppresses fibroblast growth factor‑2‑stimulated vascular endothelial growth factor release in osteoblasts

Heat shock protein 27 (HSP27) also known as heat shock protein β1 (HSPB1) is a member of the family of small heat shock proteins ubiquitously expressed in all tissues. It has previously been demonstrated that HSP27 regulated the synthesis of osteocalcin and interleukin‑6 in osteoblast‑like MC3T3‑E1 cells. In the present study, the effect of HSP27 on basic fibroblast growth factor (FGF‑2)‑stimulated vascular endothelial growth factor (VEGF) synthesis in MC3T3‑E1 cells, was observed. The levels of VEGF release stimulated by FGF‑2 in the HSP27‑overexpressing MC3T3‑E1 cells were significantly lower compared with those in the control cells. In addition, the levels of VEGF release stimulated by FGF-2 in the phosphomimic HSP27-overexpressing cells were significantly higher compared with those in the non‑phosphorylatable HSP27‑overexpressing cells. Furthermore, no significant differences were observed in the FGF‑2‑induced phosphorylation levels of p44/p42 mitogen‑activated protein (MAP) kinase, p38 MAP kinase, stress‑activated protein kinase/c‑Jun N‑terminal kinase (SAPK/JNK) or p70 S6 kinase among the four types of transfected cells. These results suggested that unphosphorylated HSP27 attenuated the FGF‑2‑stimulated VEGF synthesis in osteoblasts.

The heat shock protein response following eccentric exercise in human skeletal muscle is unaffected by local NSAID infusion

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely consumed in relation to pain and injuries in skeletal muscle, but may adversely affect muscle adaptation probably via inhibition of prostaglandin synthesis. Induction of heat shock proteins (HSP) represents an important adaptive response in muscle subjected to stress, and in several cell types including cardiac myocytes prostaglandins are important in induction of the HSP response. This study aimed to determine the influence of NSAIDs on the HSP response to eccentric exercise in human skeletal muscle. Healthy males performed 200 maximal eccentric contractions with each leg with intramuscular infusion of the NSAID indomethacin or placebo. Biopsies were obtained from m. vastus lateralis before and after (5, 28 hrs and 8 days) the exercise bout from both legs (NSAID vs unblocked leg) and analysed for expression of the HSPs HSP70, HSP27 and αB-crystallin (mRNA and protein). NSAID did not affect the mRNA expression of any of the HSPs. Compared to pre values, the mRNA expression of all HSPs was increased; αB-crystallin, 3.6- and 5.4-fold; HSP70, 26- and 3.4-fold; and HSP27: 4.8- and 6.5-fold at 5 and 28 hrs post-exercise, respectively (all p < 0.008). Immunohistochemical stainings for αB-crystallin and HSP70 revealed increased staining in some samples but with no differences between legs. Changes in force-generating capacity correlated with both αB-crystallin and HSP70 mRNA and immunohistochemisty data. Increased expression of HSPs was observed on mRNA and protein level following eccentric exercise; however, this response was unaffected by local intramuscular infusion of NSAIDs.

Response of preosteoblasts to thermal stress conditioning and osteoinductive growth factors

Conditioning protocols involving mechanical stress independently or with chemical cues such as growth factors (GFs) possess significant potential to enhance bone regeneration. However, utilization of thermal stress conditioning alone or with GFs for bone therapy has been under-investigated. In this study, a preosteoblast cell line (MC3T3-E1) was exposed to treatment with water bath heating (44°C, 4 and 8 min) and osteoinductive GFs (bone morphogenetic protein-2 and transforming growth factor-β1) individually or in combination to investigate whether these stimuli could promote induction of bone-related markers, an angiogenic factor, and heat shock proteins (HSPs). Cells remained viable when heating durations were less than 20 min at 40ºC, 16 min at 42ºC, and 10 min at 44ºC. Increasing heating duration at 44°C, promoted gene expression of HSPs, osteocalcin (OCN), and osteopontin (OPN) at 8 h post-heating (PH). Heating in combination with GFs caused the greatest gene induction of osteoprotegerin (OPG; 6.9- and 1.6-fold induction compared to sham-treated and GF only treated groups, respectively) and vascular endothelial growth factor (VEGF; 16.0- and 1.6-fold compared to sham and GF-only treated groups, respectively) at 8 h PH. Both heating and GFs independently suppressed the matrix metalloproteinase-9 (MMP-9) gene. GF treatment caused a more significant decrease in MMP-9 protein secretion to non-detectable levels compared to heating alone at 72 h PH. Secretion of OCN, OPN, and OPG increased with the addition of GFs but diminished with heating as measured by ELISA at 72 h PH. These results suggest that conditioning protocols utilizing heating and GFs individually or in combination can induce HSPs, bone-related proteins, and VEGF while also causing downregulation of osteoclastic activity, potentially providing a promising bone therapeutic strategy.

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.

Rho-kinase regulates prostaglandin D(2)-stimulated heat shock protein 27 induction in osteoblasts

We previously reported that prostaglandin D(2) (PGD(2)) stimulates heat shock protein 27 (HSP27) induction through p44/p42 mitogen-activated protein (MAP) kinase, p38 MAP kinase and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) in osteoblast-like MC3T3-E1 cells. In addition, we recently showed that PGD(2) activates Rho-kinase, resulting in the regulation of interleukin-6 synthesis via activation of p38 MAP kinase but not p44/p42 MAP kinase in these cells. In the present study, in order to investigate whether Rho-kinase is involved in the PGD(2)-stimulated HSP27 induction in MC3T3-E1 cells, we examined the effects of Rho-kinase inhibitors on HSP27 induction. Y27632 and fasudil, Rho-kinase inhibitors, markedly suppressed the HSP27 induction stimulated by PGD(2) in a dose-dependent manner without affecting levels of HSP70 in the presence of PGD(2). Immunofluorescence microscopy studies also revealed that Y27632 and fasudil markedly suppressed the induction of HSP27. Y27632 and fasudil attenuated the PGD(2)-induced phosphorylation levels of SAPK/JNK. In conclusion, Rho-kinase inhibitors regulate PGD(2)-stimulated HSP27 induction via activation of both SAPK/JNK and p38 MAP kinase in osteoblasts.

Prostaglandin D2 receptors control osteoclastogenesis and the activity of human osteoclasts

We recently showed that human osteoblasts synthesize prostaglandin D(2) (PGD(2)) and express both DP and CRTH2 receptors. Activation of the DP receptor decreased osteoprotegerin production, whereas activation of the CRTH2 receptor induced osteoblast chemotaxis and decreased RANKL expression. Our objectives in this study were to determine the presence, distribution, and action of these receptors in the functions of human osteoclasts and in osteoclastogenesis. Immunohistochemistry was used to detect the presence of DP and CRTH2 in in vitro-differentiated human osteoclasts in culture and in osteoclasts in situ. The effects of the activation of PGD(2) receptors on the cytoskeleton were determined by fluorescence microscopy. Specific agonists and antagonists allowed the study of the roles of these receptors on bone resorption and osteoclast differentiation. Our results show that in vitro-differentiated human osteoclasts and authentic fetal osteoclasts express both DP and CRTH2 receptors, as shown by immunocytochemistry. Similar results were obtained in osteoclasts from normal, osteoporotic, pagetic, and osteoarthritic adult bone tissues. Stimulation of osteoclasts with PGD(2) induced a robust reorganization of the cytoskeleton with a decrease in the number of cells presenting actin rings and an increase of lamellipodia, effects mediated by the DP and CRTH2 receptors, respectively. PGD(2) showed an inhibitory effect on bone resorption activity acting through the DP receptor. In vitro osteoclastogenesis from peripheral blood mononuclear cells cultured in the presence of RANKL and macrophage-colony stimulating factor was decreased by activation of either DP or CRTH2 receptors. These results suggest that PGD(2) receptors could be useful targets in certain bone diseases because their specific activation/inhibition leads to a decrease in osteoclastogenesis and to inhibition of bone resorption by osteoclasts.

Function of Rho-kinase in prostaglandin D2-induced interleukin-6 synthesis in osteoblasts

We have previously reported that prostaglandin D2 (PGD2) stimulates interleukin-6 (IL-6), a potent bone resorptive agent, in osteoblast-like MC3T3-E1 cells. In the present study, we investigated whether Rho-kinase is implicated in the PGD2-stimulated IL-6 synthesis in MC3T3-E1 cells. PGD2 time-dependently induced the phosphorylation of myosin phosphatase targeting subunit (MYPT-1), a Rho-kinase substrate. Y27632, a specific Rho-kinase inhibitor, significantly reduced the PGD2-stimulated IL-6 synthesis as well as the MYPT-1 phosphorylation. Fasudil, another inhibitor of Rho-kinase, suppressed the PGD2-stimulated IL-6 synthesis. The PGD2-stimulated IL-6 synthesis was reduced by PD98059, a MEK inhibitor, and SB203580, an inhibitor of p38 mitogen-activated protein (MAP) kinase, but not SP600125, an inhibitor of stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK). However, Y27632 and fasudil failed to affect the PGD2-induced phosphorylation of p44/p42 MAP kinase. On the other hand, Y27632 as well as fasudil markedly attenuated the PGD2-induced phosphorylation of p38 MAP kinase. In addition, PGD2 additively induced IL-6 synthesis in combination with endothelin-1 which induces IL-6 synthesis through p38 MAP kinase regulated by Rho-kinase. These results strongly suggest that Rho-kinase regulates PGD2-stimulated IL-6 synthesis via p38 MAP kinase activation in osteoblasts.

(-)-epigallocatechin gallate inhibits prostaglandin D2-stimulated HSP27 induction via suppression of the p44/p42 MAP kinase pathway in osteoblasts

We previously reported that prostaglandin D2 (PGD2) stimulates heat shock protein 27 (HSP27) induction through p38 mitogen-activated protein (MAP) kinase, stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK) and p44/p42 MAP kinase in osteoblast-like MC3T3-E1 cells. In the present study, we investigated whether (-)-epigallocatechin gallate (EGCG), the major polyphenol found in green tea, affects the induction of HSP27 in these cells and the mechanism. EGCG significantly reduced the HSP27 induction stimulated by PGD2 without affecting the levels of HSP70. The PGD2-induced phosphorylation of p38 MAP kinase or SAPK/JNK was not affected by EGCG. On the contrary, EGCG markedly suppressed the PGD2-induced phosphorylation of p44/p42 MAP kinase and MEK1/2. However, the PGD2-induced phosphorylation of Raf-1 was not inhibited by EGCG. These results strongly suggest that EGCG suppresses the PGD2-stimulated induction of HSP27 at the point between Raf-1 and MEK1/2 in osteoblasts.

Prostaglandin D2 induces the phosphorylation of HSP27 in osteoblasts: function of the MAP kinase superfamily

We previously reported that prostaglandin D(2) (PGD(2)) stimulates the induction of heat shock protein 27 (HSP27) in osteoblast-like MC3T3-E1 cells. In the present study, we investigated whether PGD(2) stimulates the phosphorylation of HSP27 in MC3T3-E1 cells exposed to heat shock. In the cultured MC3T3-E1 cells, PGD(2) markedly stimulated the phosphorylation of HSP27 at Ser-15 and Ser-85 in a time-dependent manner. Among the mitogen-activated protein (MAP) kinase superfamily, p44/p42 MAP kinase and p38 MAP kinase were phosphorylated by PGD(2) which had little effect on the phosphorylation of stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK). The PGD(2)-induced phosphorylation of HSP27 was attenuated by PD169316, an inhibitor of p38 MAP kinase or PD98059, a MEK inhibitor. SP600125, a SAPK/JNK inhibitor did not affect the HSP27 phosphorylation. In addition, PD169316 suppressed the PGD(2)-induced phosphorylation of MAPKAP kinase 2. These results strongly suggest that PGD(2) stimulates HSP27 phosphorylation via p44/p42 MAP kinase and p38 MAP kinase but not SAPK/JNK in osteoblasts.

Production of prostaglandin D(2) by human osteoblasts and modulation of osteoprotegerin, RANKL, and cellular migration by DP and CRTH2 receptors

Human osteoblasts produce PGD(2), which acts on the DP receptor to decrease osteoprotegerin production and on the CRTH2 receptor to decrease RANKL expression and to induce osteoblast chemotaxis. These results indicate that activation of CRTH2 may lead to an anabolic response in bone.

INTRODUCTION

Whereas the actions of prostaglandin (PG)E(2) as a modulator of bone and osteoblast function are relatively well characterized, little is known about PGD(2) and bone metabolism. The objectives of this study were to determine if human osteoblasts can produce PGD(2), which prostaglandin D(2) synthases are implicated in this synthesis, to identify the PGD(2) receptors (DP and CRTH2) on these cells and to characterize the biological effects resulting from their activation.

MATERIALS AND METHODS

RT-PCR analysis and immunohistochemistry were used to detect PGD(2) receptor and synthases in cultured human osteoblasts. Immunohistochemistry was used to identify the synthases and receptors in human bone tissue. Intracellular cAMP and calcium levels were determined to verify receptor activation. The cells were stimulated with PGD(2) or the specific agonists BW 245C (DP) and DK-PGD(2) (CRTH2), and the resulting effects on osteoprotegerin (OPG) secretion, RANKL expression, and chemotaxis were determined. Osteoblast production of PGD(2) was evaluated by measuring PGD(2) in the culture supernatants after stimulation with interleukin (IL)-1, TNF-alpha, PTH, vascular endothelial growth factor (VEGF), and insulin-like growth factor I (IGF-I).

RESULTS

Human osteoblasts in culture generated PGD(2) when stimulated. Both osteoblasts in culture and in situ present the lipocalin-type PGD(2) synthase only. Both DP and CRTH2 receptors were present in human osteoblasts in culture and in situ. Stimulation of DP resulted in an increase in cAMP, whereas CRTH2 increased the intracellular calcium level. OPG production was reduced by 60% after DP receptor stimulation, whereas CRTH2 receptor stimulation decreased RANKL expression on human osteoblasts. As reported for other cell types, CRTH2 was a potent inducer of chemotaxis for human osteoblasts in culture.

CONCLUSIONS

Human osteoblasts in culture produce PGD(2) under biologically relevant stimuli through the lipocalin-type PGD(2) synthase (L-PGDS) pathway. As an autacoid, PGD(2) can act on DP and CRTH2 receptors, both present on these cells. Specific activation of CRTH2 could lead directly and indirectly to an anabolic response in bone.

Methotrexate enhances prostaglandin D2-stimulated heat shock protein 27 induction in osteoblasts

As for the pathogenesis of rheumatoid arthritis (RA), prostaglandins (PGs) act as important mediators of inflammation and joint destruction. Among them, PGD2 is well recognized as a potent regulator of osteoblastic functions. We previously showed that PGD2 stimulates the induction of heat shock protein 27 (HSP27) via protein kinase C (PKC)-dependent p38 mitogen-activated protein (MAP) kinase and p44/p42 MAP kinase in osteoblast-like MC3T3-E1 cells. Therefore, it is a current topic to clarify how HSP27 plays a role for regulating osteoblastic functions in the lesion of RA. On the other hand, methotrexate (MTX) is one of the most effective medicines for the treatment of RA. Here, we examined the effect of MTX on PGD2-stimulated HSP27 induction in MC3T3-E1 cells. The cells were pretreated with various doses of MTX including therapeutic dosage for RA, and then stimulated by PGD2. MTX significantly enhanced the PGD2- increased levels of HSP27 in a dose-dependent manner, although MTX alone had no effect on the levels of HSP27. In addition, MTX amplified the PGD2-increased levels of HSP27 mRNA. On the contrary, MTX had little effect on PGD2-induced formation of inositol phosphates, PKC activation and phosphorylations of MAP kinases. Our results strongly suggest that MTX enhances PGD2-stimulated HSP27 induction at a point downstream from MAP kinases in osteoblasts.

Functional Role for Heat Shock Factors in the Transcriptional Regulation of Human RANK Ligand Gene Expression in Stromal/Osteoblast Cells

RANK Ligand (RANKL) is a critical osteoclastogenic factor that is expressed on stromal cells and osteoblasts. Most resorption stimuli induce osteoclast formation by modulating RANKL gene expression in marrow stromal/osteoblast cells. However, it is unclear how these stimuli modulate RANKL gene expression in the bone microenvironment. To characterize the transcriptional control of human RANKL gene expression in stromal/osteoblast cells, we PCR-amplified and cloned a 2-kb 5'-flanking sequence of the RANKL gene, using normal human osteoblast derived genomic DNA as a template. Sequence analysis identified the presence of several potential Heat Shock Factor (HSF) responsive elements (HSE) in the human RANKL gene promoter region. Co-expression of HSF-1 or HSF-2 with the RANKL gene promoter-luciferase reporter plasmid in human osteoblastic cells (NOBC) demonstrated a 2-fold and 4.5-fold increase in promoter activity, respectively. RT-PCR analysis for HSF-1 and 2 mRNA expression in human bone marrow-derived stromal cells (SAKA-T) and osteoblast cells detected only HSF-2 expression. As evident from EMSA analysis, in contrast to 1,25(OH)(2)D(3) SAKA-T cells treated with b-FGF demonstrated increased levels of HSF-2 binding to the HSE present in the RANKL gene promoter region. Immunocytochemical staining further confirmed nuclear localization of HSF-2 in both SAKA-T transformed stromal cells and human bone marrow derived primary stromal/preosteoblastic cells in response to b-FGF treatment. Furthermore, b-FGF treatment of SAKA-T cells transfected with the luciferase reporter plasmid containing the hRANKL HSE region (-2 kb to -1275 bp) upstream to a heterologous promoter showed increased levels of transactivation. Western blot analysis further demonstrated enhanced levels of RANKL expression and HSP-27 phosphorylation in SAKA-T cells treated with b-FGF. In addition, overexpression of HSF-2 in SAKA-T cells resulted in a 5-fold increase in the levels of RANKL expression in these cells. These data further suggest that HSF-2 is a downstream target of b-FGF to induce RANKL expression in stromal/osteoblast cells, and that HSF may play an important role in modulating RANKL gene expression in the bone microenvironment.

50 Hz magnetic fields activate mussel immunocyte p38 MAP kinase and induce HSP70 and 90

Fifty hertz magnetic fields (MFs) induced the expression of heat shock proteins (HSPs) 70 and 90 in immunocytes of the mussel Mytilus galloprovincialis. Animals exposed at 300 microT for three different times (30; 2 x 30; 3 x 30 min), did not show differences in the HSP densitometric values in comparison with non-exposed mussels. At 400 microT, exposed animals showed a time-dependent increase in HSP expression as revealed by Western blot. After exposure to 600 microT, the HSP densitometric values were significantly higher than controls but not related to exposure duration. The induction of HSPs is concomitant with the activation of p38 MAP kinase signalling pathway. The present findings suggest the possibility to modulate the expression of HSPs by an appropriate time-intensity magnetic field exposure.

Effects of 50-Hz magnetic fields on the signalling pathways of fMLP-induced shape changes in invertebrate immunocytes: the activation of an alternative "stress pathway"

N-formyl-Meth-Leu-Phe (fMLP)-induced immunocyte shape changes in the mussel Mytilus galloprovincialis through both the phosphatidylinositol and the cAMP pathways are studied. Fifteen- and thirty-minute exposures of mussels to 50-Hz magnetic fields (MFs) at intensities of 300 and 400 microT do not provoke permanent cell damage, since immunocytes maintain the capacity to respond to fMLP. This avoidance of external insult seems to be achieved through the activation of a "stress pathway" which is not functionally detectable in nonexposed animals and which involves mitogen activated protein (MAP) kinase members. This phenomenon is clearly evident at 400 microT. Contemporaneously, a different expression of Jun transcriptional regulatory proteins is also found.

[Heat shock protein 27 in osteoblasts]

Physiological stresses such as heat stress, chemical stress and mechanical stress induce the expression of heat shock protein (HSP) families in cells, which affects cell function. In the present review, we describe HSP27, a small HSP in osteoblasts, especially the regulatory mechanism of the induction of HSP27 stimulated by physiological bone agents. Chemical stress by sodium arsenite (arsenite) induces HSP27 coupled to the metabolic activity of the arachidonic acid cascade, and the HSP27 induction by arsenite is negatively regulated by activation of protein kinase C (PKC). On the contrary, physiological regulators of bone such as endothelin-1, prostaglandin F2 alpha (PGF2 alpha), PGD2, and basic fibroblast growth factor (bFGF) induce HSP 27 via protein kinase C (PKC) activation. In addition, the mitogen-activated protein (MAP) kinase super-family takes part in the HSP27 induction. Thus, not only stress but also physiological agonists induce HSP 27 in osteoblasts, and PKC or MAP kinases play important roles in the induction of HSP27.