Distinct regulation of mitogen-activated protein kinase activities is coupled with enhanced cardiac differentiation of human embryonic stem cells

Improving cardiac differentiation of human pluripotent stem cells is mandatory to provide functional heart muscle cells for novel therapies. Here, we have investigated the enhancing effect of the small molecule SB203580, a p38 MAPK inhibitor, on cardiomyogenesis in hESC by monitoring the phosphorylation patterns of the major MAPK pathway components p38, JNK and ERK by western immunoblotting. A remarkable drop in phosphorylation levels of all three MAPK pathways was induced after overnight embryoid body (EB) formation. Upon further differentiation, phosphorylation dynamics in EBs were specifically altered by distinct inhibitor concentrations. At 5μM of SB203580, cardiomyogenesis was most efficient and associated with the expected p38 pathway inhibition. In parallel, JNK activation was observed suggesting a regulatory interlink between these pathways in hESC ultimately supporting cardiac differentiation. In contrast, moderately elevated SB203580 concentrations (15-30μM) resulted in complete disruption of cardiomyogenesis which was associated with prominent inhibition of ERK and further elevated JNK activity. We propose that a tightly-balanced pattern in MAPK phosphorylation is important for early mesoderm and subsequent cardiomyocyte formation. Our data provide novel insights into molecular consequences of small molecule supplementation in hESC differentiation, emphasizing the role of MAPK-signaling.     

Vascular endothelial growth factor induces heat shock protein (HSP) 27 serine 82 phosphorylation and endothelial tubulogenesis via protein kinase D and independent of p38 kinase

Proteomic analysis identified HSP27 phosphorylation as a major change in protein phosphorylation stimulated by Vascular Endothelial Growth Factor (VEGF) in Human Umbilical Vein Endothelial Cells (HUVEC). VEGF-induced HSP27 phosphorylation at serines 15, 78 and 82, but whereas HSP27 phosphorylation induced by H2O2 and TNFalpha was completely blocked by the p38 kinase inhibitor, SB203580, VEGF-stimulated serine 82 phosphorylation was resistant to SB203580 and small interfering(si)RNA-mediated knockdown of p38 kinase and MAPKAPK2. The PKC inhibitor, GF109203X, partially reduced VEGF-induced HSP27 serine 82 phosphorylation, and SB203580 plus GF109203X abolished phosphorylation. VEGF activated Protein Kinase D (PKD) via PKC, and siRNAs targeted to PKD1 and PKD2 inhibited VEGF-induced HSP27 serine 82 phosphorylation. Furthermore recombinant PKD selectively phosphorylated HSP27 at serine 82 in vitro, and PKD2 activated by VEGF in HUVECs also phosphorylated HSP27 selectively at this site. Knockdown of HSP27 and PKDs markedly inhibited VEGF-induced HUVEC migration and tubulogenesis, whereas inhibition of the p38 kinase pathway using either SB203580 or siRNAs against p38alpha or MAPKAPK2, had no significant effect on the chemotactic response to VEGF. These findings identify a novel pathway for VEGF-induced HSP27 serine 82 phosphorylation via PKC-mediated PKD activation and direct phosphorylation of HSP27 by PKD, and show that PKDs and HSP27 play major roles in the angiogenic response to VEGF.     

Tyrosine hydroxylase phosphorylation in bovine adrenal chromaffin cells: the role of MAPKs after angiotensin II stimulation

Angiotensin II (AII, 100 nM) stimulation of bovine adrenal chromaffin cells (BACCs) produced angiotensin II receptor subtype 1 (AT1)-mediated increases in extracellular regulated protein kinase 1/2 (ERK1/2) and stress-activated p38MAPK (p38 kinase) phosphorylation over a period of 10 min. ERK1/2 and p38 kinase phosphorylation preceded Ser31 phosphorylation on tyrosine hydroxylase (TOH). The inhibitors of mitogen-activated protein kinase kinase 1/2 (MEK1/2) activation, PD98059 (0.1-50 microM) and UO126 (0.1-10 microM), dose-dependently inhibited both ERK2 and Ser31 phosphorylation on TOH in response to AII, suggesting MEK1/2 involvement. The p38 kinase inhibitor SB203580 (20 microM, 30 min) abolished Ser31 and Ser19 phosphorylation on TOH and partially inhibited ERK2 phosphorylation produced by AII. In contrast, 1 microM SB203580 did not affect AII-stimulated TOH phosphorylation, but fully inhibited heat shock protein 27 (HSP27) phosphorylation produced by AII. Also, 1 microM SB203580 fully inhibited Ser19 phosphorylation on TOH and HSP27 phosphorylation in response to anisomycin (30 min, 10 microg/mL). The results suggest that ERKs mediate Ser31 phosphorylation on TOH in response to AII, but p38 kinase is not involved. Previous studies suggesting a role for p38 kinase in the phosphorylation of Ser31 are explained by the non-specific effects of 20 microM SB203580 in BACCs. The p38 kinase pathway is able to phosphorylate Ser19 on TOH in response to anisomycin, but does not do so in response to AII.     

Enhanced cardiomyogenesis of human embryonic stem cells by a small molecular inhibitor of p38 MAPK

Abstract Human embryonic stem cells (hESC) can differentiate to cardiomyocytes in vitro but with generally poor efficiency. Here, we describe a novel method for the efficient generation of cardiomyocytes from hESC in a scalable suspension culture process. Differentiation in serum-free medium conditioned by the cell line END2 (END2-CM) readily resulted in differentiated cell populations with more than 10% cardiomyocytes without further enrichment. By screening candidate molecules, we have identified SB203580, a specific p38 MAP kinase inhibitor, as a potent promoter of hESC-cardiogenesis. SB203580 at concentrations <10 μM, induced more than 20% of differentiated cells to become cardiomyocytes and increased total cell numbers, so that the overall cardiomyocyte yield was approximately 2.5-fold higher than controls. Gene expression indicated that early mesoderm formation was favored in the presence of SB203580. Accordingly, transient addition of the inhibitor at the onset of differentiation only was sufficient to determine the hESC fate. Patch clamp electrophysiology showed that the distribution of cardiomyocyte phenotypes in the population was unchanged by the compound. Interestingly, cardiomyogenesis was strongly inhibited at SB203580 concentrations ≥15 μM. Thus, modulation of the p38MAP kinase pathway, in combination with factors released by END2 cells, plays an essential role in early lineage determination in hESC and the efficiency of cardiomyogenesis. Our findings contribute to transforming human cardiomyocyte generation from hESC into a robust and scalable process.     

Platelet-derived growth factor-BB phosphorylates heat shock protein 27 in cardiac myocytes

It is recognized that heat shock protein 27 (HSP27) is highly expressed in heart. In the present study, we investigated whether platelet-derived growth factor (PDGF) phosphorylates HSP27 in mouse myocytes, and the mechanism underlying the HSP27 phosphorylation. Administration of PDGF-BB induced the phosphorylation of HSP27 at Ser-15 and -85 in mouse cardiac muscle in vivo. In primary cultured myocytes, PDGF-BB time dependently phosphorylated HSP27 at Ser-15 and -85. PDGF-BB stimulated the phosphorylation of p44/p42 mitogen-activated protein (MAP) kinase, p38 MAP kinase, and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) among the MAP kinase superfamily. SB203580, a specific inhibitor of p38 MAP kinase, reduced the PDGF-BB-stimulated phosphorylation of HSP27 at both Ser-15 and -85, and phosphorylation of p38 MAP kinase. However, PD98059, a specific inhibitor of MEK, or SP600125, a specific inhibitor of SAPK/JNK, failed to affect the HSP27 phosphorylation. These results strongly suggest that PDGF-BB phosphorylates HSP27 at Ser-15 and -85 via p38 MAP kinase in cardiac myocytes.     

hhlim促进DMSO诱导的P19细胞向心肌分化

为了确定hhlim是否参与胚胎期的心肌分化和发育过程,用可表达hhlim蛋白和hhlim反义RNA的真核表达质粒转染P19胚胎干细胞,经G418筛选得到稳定表达hhlim和hhlim反义RNA的P19细胞克隆后,观察hhlim对P19细胞向心肌分化和发育的影响.结果显示,Nkx2.5和GATA-4在未被外源性hhlim基因转染的P19细胞中不表达.DMSO刺激细胞2天后,GATA-4开始表达,3天后Nkx2.5的表达活性显著升高.hhlim的过表达不但有利于P19细胞的存活和生长,而且还可以使Nkx2.5和GATA-4的表达比对照细胞提前1天.反义hhlim细胞株被DMSO诱导5天后,细胞仍呈集落化生长.同时,Nkx2.5和GATA-4开始表达的时间明显延滞.结果表明,hhlim能促进P19细胞向心肌细胞分化,其作用是通过促进转录因子GATA-4和Nkx2.5的表达而实现的.     

p38 mitogen-activated protein kinase (MAPK) first regulates filamentous actin at the 8-16-cell stage during preimplantation development

BACKGROUND INFORMATION: The MAPK (mitogen-activated protein kinase) superfamily of proteins consists of four separate signalling cascades: the c-Jun N-terminal kinase or stress-activated protein kinases (JNK/SAPK); the ERKs (extracellular-signal-regulated kinases); the ERK5 or big MAPK1; and the p38 MAPK group of protein kinases, all of which are highly conserved. To date, our studies have focused on defining the role of the p38 MAPK pathway during preimplantation development. p38 MAPK regulates actin filament formation through the downstream kinases MAPKAPK2/3 (MAPK-activated protein kinase 2/3) or MAPKAPK5 [PRAK (p38 regulated/activated kinase)] and subsequently through HSP25/27 (heat-shock protein 25/27). We recently reported that 2-cell-stage murine embryos treated with cytokine-suppressive anti-inflammatory drugs (CSAIDtrade mark; SB203580 and SB220025) display a reversible blockade of development at the 8-16-cell stage, indicating that p38 (MAPK) activity is required to complete murine preimplantation development. In the present study, we have investigated the stage-specific action and role of p38 MAPK in regulating filamentous actin during murine preimplantation development. RESULTS: Treatment of 8-cell-stage embryos with SB203580 and SB220025 (CSAIDtrade mark) resulted in a blockade of preimplantation development, loss of rhodamine phalloidin fluorescence, MK-p (phosphorylated MAPKAPK2/3), HSP-p (phosphorylated HSP25/27) and a redistribution of alpha-catenin immunofluorescence by 12 h of treatment. In contrast, treatment of 2- and 4-cell-stage embryos with CSAIDtrade mark drugs resulted in a loss of MK-p and HSP-p, but did not result in a loss of rhodamine phalloidin fluorescence. All these effects of p38 MAPK inhibition were reversed upon removal of the inhibitor, and development resumed in a delayed but normal manner to the blastocyst stage. Treatment of 8-cell embryos with PD098059 (ERK pathway inhibitor) did not affect development or fluorescence of MK-p, HSP-p or rhodamine phalloidin. CONCLUSION: Murine preimplantation development becomes dependent on p38 MAPK at the 8-16-cell stage, which corresponds to the stage when p38 MAPK first regulates filamentous actin during early development.     

Activation of the neutrophil NADPH oxidase is inhibited by SB 203580, a specific inhibitor of SAPK2/p38.

Activation of the neutrophil NADPH oxidase by either the bacterial peptide fMLP or phorbol myristate acetate (PMA) is partially suppressed by SB 203580, a specific inhibitor of the MAP kinase family member, SAPK2/p38. The concentration of SB 203580 that suppresses activation of NADPH oxidase is similar to that which inhibits SAPK2/p38 in vitro, and both fMLP and PMA induce an extremely rapid and potent activation of SAPK2/p38 in neutrophils. SB 203580 does not exert its effect by preventing the neutrophil priming reaction, by suppressing the phosphorylation of p47phax, or by preventing the translocation of p47phax/p67phax to the plasma membrane.     

Specific induction of heat shock protein 27 by glucocorticoid in osteoblasts

It is generally recognized that osteoporosis is a common complication of patients with glucocorticoid excess and that glucocorticoid receptor is associated with heat shock protein (HSP) 70 and HSP90 in a heterocomplex. In the present study, we investigated whether glucocorticoid induces HSP27, HSP70, and HSP90 in osteoblast-like MC3T3-E1 cells. Dexamethasone time-dependently increased the levels of HSP27, while having no effect on the levels of HSP70 or HSP90. The effect of dexamethasone was dose-dependent in the range between 0.1 nM and 0.1 microM. Dexamethasone induced an increase of the levels of mRNA for HSP27. Dexamethasone induced the phosphorylation of p38 mitogen-activated protein (MAP) kinase. SB203580 and PD169316, inhibitors of p38 MAP kinase, suppressed the HSP27 accumulation by dexamethasone. In addition, SB203580 reduced the dexamethasone-stimulated increase of the mRNA levels for HSP27. The dexamethasone-induced phosphorylation of p38 MAP kinase was reduced by SB203580. These results strongly suggest that glucocorticoid stimulates the induction of neither HSP70 nor HSP90, but HSP27 in osteoblasts, and that p38 MAP kinase is involved in the induction of HSP27.     

Phosphorylation of initiation factor 4E is resistant to SB203580 in cells expressing a drug-resistant mutant of stress-activated protein kinase 2a/p38

Previous work has shown that increased phosphorylation of eukaryotic initiation factor (eIF) 4E at Ser209 in the C-terminal loop of the protein is observed in response to cellular stress. SB203580, a cell permeable inhibitor of stress-activated protein kinase 2a (SAPK2a/p38), suppresses this response in a number of cell types. To validate the in vivo specificity of this inhibitor for the investigation of signalling pathways, which modulate the phosphorylation of eIF4E, we have used 293 cells which inducibly express either a wild-type form (WT-SAPK2a) or a drug-resistant mutant of SAPK2a (DR-SAPK2a). These data show that while the arsenite-induced increase in the phosphorylation of eIF4E and hsp25 was sensitive to SB203580 in cells expressing WT-SAPK2a, these responses to SB203580 were abrogated in cells expressing DR-SAPK2a. In addition, the phosphorylation of the eIF4E kinase, MAP kinase integrating kinase-1 (Mnk1), which is activated in response to growth factors or stress, was insensitive to SB203580 in DR-SAPK2a-expressing cells. However, a cell-permeable, specific inhibitor of Mnk1, CGP57380 and the phosphatidylinositol-3-kinase (PI3-K) inhibitor, LY294002, prevented eIF4E phosphorylation in 293 cells irrespective of SAPK2a expression. Therefore, this study validates the use of SB203580 for investigating signalling pathways modulating the phosphorylation of eIF4E in cultured cells.     

SAPK/JNK plays a role in transforming growth factor-beta-induced VEGF synthesis in osteoblasts.

We previously reported that transforming growth factor-beta (TGF-beta) activates p44/p42 mitogen-activated protein (MAP) kinase and p38 MAP kinase, resulting in the stimulation of vascular endothelial growth factor (VEGF) synthesis in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the involvement of stress-activated protein kinase/c- Jun N-terminal kinase (SAPK/JNK), another member of the MAP kinase superfamily, in TGF-beta-induced VEGF synthesis in these cells. TGF-beta markedly induced SAPK/JNK phosphorylation. SP600125, a specific inhibitor of SAPK/JNK, markedly reduced TGF-beta-induced VEGF synthesis. SP600125 suppressed TGF-beta-induced SAPK/JNK phosphorylation. PD98059, an inhibitor of upstream kinase of p44/p42 MAP kinase and SB203580, an inhibitor of p38 MAP kinase, each failed to reduce TGF-beta-induced SAPK/JNK phosphorylation. A combination of SP600125 and PD98059 or SP600125 and SB203580 suppressed TGF-beta-stimulated VEGF synthesis in an additive manner. These results strongly suggest that TGF-beta activates SAPK/JNK in osteoblasts, and that SAPK/JNK plays a role in addition to p42/p44 MAP kinase and p38 MAP kinase in TGF-beta-induced VEGF synthesis.     

SB203580 Modulates p38 MAPK Signaling and Dengue Virus-Induced Liver Injury by Reducing MAPKAPK2, HSP27, and ATF2 Phosphorylation

Dengue virus (DENV) infection causes organ injuries, and the liver is one of the most important sites of DENV infection, where viral replication generates a high viral load. The molecular mechanism of DENV-induced liver injury is still under investigation. The mitogen activated protein kinases (MAPKs), including p38 MAPK, have roles in the hepatic cell apoptosis induced by DENV. However, the in vivo role of p38 MAPK in DENV-induced liver injury is not fully understood. In this study, we investigated the role of SB203580, a p38 MAPK inhibitor, in a mouse model of DENV infection. Both the hematological parameters, leucopenia and thrombocytopenia, were improved by SB203580 treatment and liver transaminases and histopathology were also improved. We used a real-time PCR microarray to profile the expression of apoptosis-related genes. Tumor necrosis factor α, caspase 9, caspase 8, and caspase 3 proteins were significantly lower in the SB203580-treated DENV-infected mice than that in the infected control mice. Increased expressions of cytokines including TNF-α, IL-6 and IL-10, and chemokines including RANTES and IP-10 in DENV infection were reduced by SB203580 treatment. DENV infection induced the phosphorylation of p38MAPK, and its downstream signals including MAPKAPK2, HSP27 and ATF-2. SB203580 treatment did not decrease the phosphorylation of p38 MAPK, but it significantly reduced the phosphorylation of MAPKAPK2, HSP27, and ATF2. Therefore, SB203580 modulates the downstream signals to p38 MAPK and reduces DENV-induced liver injury.     

Possible involvement of heat shock protein 25 in the angiotensin II-induced glomerular mesangial cell contraction via p38 MAP kinase.

In the rat kidney, mesangial cells (MCs), especially those in the extraglomerular mesangium (EGM) region of the juxtagomerular apparatus, express high amounts of heat shock protein 25 (HSP25). Because MCs are contractile in vivo and HSP25 is known to modulate polymerization/depolymerization of F-actin and to be involved in smooth muscle contraction, it is possible that HSP25 participates in the contraction process of MCs. We analyzed a permanent mouse MC line using Northern and Western blot analyses, and observed that similar to the MCs in the glomerulus, these cells also express high amounts of HSP25 constitutively. Exposure of these cells to angiotensin II (ANG II: 2 x 10(-7) M) evoked contraction and a concomitant increase in HSP25 phosphorylation, while the cytoplasmic fraction of HSP25 was transiently reduced. Because phosphorylation of HSP25 is essential for its actin-modulating function, we suppressed the activity of p38 MAP kinase, the major upstream activator of HSP25 phosphorylation, with the specific inhibitor SB 203580. This maneuver reduced HSP25 phosphorylation dramatically, abolished cell contraction, and prevented the decrease of the cytoplasmic HSP25 content. This suggests that HSP25 might be a component of the contraction machinery in MCs and that this process depends on p38 MAP kinase-mediated HSP25 phosphorylation. The decrease of cytoplasmic HSP25 content observed after ANG II exposure is probably the result of a transient redistribution of HSP25 into a buffer-insoluble fraction, because the whole cell content of HSP25 did not change, a phenomenon known to be related to the actin-modulating activity of HSP25. The fact that this function requires phosphorylation of HSP25 would explain the observation that HSP25 does not redistribute in SB 203580-pretreated cells.     

Transendothelial migration of colon carcinoma cells requires expression of E-selectin by endothelial cells and activation of stress-activated protein kinase-2 (SAPK2/p38) in the tumor cells.

Adhesion and migration of tumor cells on and through the vascular endothelium are critical steps of the metastatic invasion. We investigated the roles of E-selectin and of stress-activated protein kinase-2 (SAPK2/p38) in modulating endothelial adhesion and transendothelial migration of HT-29 colon carcinoma cells. Tumor necrosis factor alpha (TNF alpha) strongly increased the expression of E-selectin in human umbilical vein endothelial cells (HUVEC). This effect was independent of the activation of SAPK2/p38 induced by TNF alpha. Adhesion of HT-29 cells on a monolayer of HUVEC pretreated with TNF alpha was dependent on E-selectin expression but was independent of SAPK2/p38 activity of both HUVEC and tumor cells. The adhesion of HT-29 cells to E-selectin-expressing HUVEC led to the activation of SAPK2/p38 in the tumor cells as reflected by the increased phosphorylation of the actin-polymerizing factor HSP27 by mitogen-activated protein kinase 2/3, a direct target of SAPK2/p38. Moreover, a recombinant E-selectin/Fc chimera quickly increased the activation of SAPK2/p38 in HT-29 cells. Blocking the increased activity of SAPK2/p38 of HT-29 cells by SB203580 or by expressing a dominant negative form of SAPK2/p38 inhibited their transendothelial migration. Similarly, HeLa cells stably expressing a kinase-inactive mutant of SAPK2/p38 showed a decreased capacity to cross a layer of HUVEC. Overall, our results suggest that the regulation of transendothelial migration of tumor cells involves two essential steps as follows: adhesion to the endothelium through adhesion molecules, such as E-selectin, and increased motogenic potential through adhesion-mediated activation of the SAPK2/p38 pathway.     

The protein kinase inhibitor SB203580 uncouples PMA-induced differentiation of HL-60 cells from phosphorylation of Hsp27

HL-60 cells are an attractive model for studies of human myeloid cell differentiation. Among the well-examined parameters correlated to differentiation of HL-60 cells are the expression and phosphorylation of the small heat shock protein Hsp27. Here we demonstrate that PMA treatment of HL-60 cells stimulates different MAP kinase cascades, leading to significant activation of ERK2 and p38 reactivating kinase (p38RK). Using the protein kinase inhibitor SB 203580, we specifically inhibited p38RK and, thereby, activation of its target MAP kinase-activated protein kinase 2(MAPKAP kinase 2), which is the major enzyme responsible for small Hsp phosphorylation. As a result, PMA-induced Hsp27 phosphorylation is inhibited in SB 203580-treated HL-60 cells indicating that p38RK and MAPKAP kinase 2 are components of the PMA-induced signal transduction pathway leading to Hsp27 phosphorylation. We further demonstrate that, although PMA-induced phosphorylation is inhibited, SB 203580-treated HL-60 cells are still able to differentiate to the macrophage-like phenotype as judged by decrease in cell proliferation, induction of expression of the cell surface antigen CD11b and changes in cell morphology. These results indicate that, although correlated, Hsp27 phosphorylation is not required for HL-60 cell differentiation. However, the results do not exclude that increased Hsp27 expression is involved in HL-60 cell differentiation.     

Receptor activator of nuclear factor-kappaB is induced by a rottlerin-sensitive and p38 MAP kinase-dependent pathway during monocyte differentiation

Receptor activator of nuclear factor-kappaB (RANK) plays a central role in the regulation of osteoclast differentiation and activation, but the mechanisms underlying its expression remain to be elucidated. In the present study we showed that expression of RANK was strongly induced by phorbol-12-myristate-13-acetate (PMA) during monocyte differentiation of U937 cells, and was enhanced by concomitant treatment with vitamin D3. Induction was dramatically inhibited by protein kinase C (PKC) inhibitors such as rottlerin and G?6983, but not by G?6976. Interestingly, rottlerin, a selective inhibitor of PKCdelta, reduced PMA-induced RANK expression while having no effect on CD11b expression. However overexpression of wild type PKCdelta, or a kinase-inactive mutant, did not affect PMA-induction of RANK, suggesting that rottlerin inhibits PMA-induced expression of RANK via a PKCdelta-independent mechanism. Rottlerin also inhibited PMA-induced phosphorylation of p38 mitogen-activated protein kinase (p38MAPK), and the p38 MAPK inhibitor SB203580 inhibited induction of RANK. Rottlerin and SB203580 also substantially reduced RANK mRNA expression in mouse BMM cells stimulated with macrophage colony stimulating factor (M-CSF). Together, these results demonstrate that expression of RANK is dependent upon a rottlerin-sensitive and p38MAPK-dependent pathway during monocyte differentiation.     

Mechanism of simvastatin on induction of heat shock protein in osteoblasts

It has recently been reported that 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) stimulate bone formation. However, the mechanism of stimulation of bone metabolism by statins is not precisely clarified. In this study, we investigated whether simvastatin induces heat shock protein (HSP) 27, HSP70, and HSP90 in osteoblast-like MC3T3-E1 cells. Simvastatin increased the levels of HSP27 while having little effect on the levels of HSP70 or HSP90. The effect of simvastatin on HSP27 accumulation was dose dependent. Cycloheximide reduced the accumulation. Simvastatin induced an increase in the levels of mRNA for HSP27. Actinomycin D suppressed the mRNA levels. Simvastatin induced the phosphorylation of p38 mitogen-activated protein (MAP) kinase among the MAP kinase superfamily. SB203580 and PD169316, inhibitors of p38 MAP kinase, suppressed the HSP27 accumulation by simvastatin while SB202474, a negative control of p38 MAP kinase inhibitor, had no effect. SB203580 reduced the simvastatin-increased mRNA levels for HSP27. Lovastatin, another statin, also induced the HSP27 accumulation and SB203580 suppressed the HSP27 accumulation. These results strongly suggest that statins such as simvastatin do not stimulate the induction of HSP70 and HSP90, but do stimulate the induction of HSP27 in osteoblasts and that p38 MAP kinase plays a role in this induction.     

Specific inhibitor of MEK-mediated cross-talk between ERK and p38 MAPK during differentiation of human osteosarcoma cells

Osteosarcoma is the most common primary malignant bone tumor, accounting for approximately 20% of all primary sarcomas in bone. Although treatment modalities have been improved over the past decades, it is still a tumor with a high mortality rate in children and young adults. Based on histological considerations, osteosarcoma arises from impaired differentiation of these immature cells into more mature types and that correction of this impairment may reduce malignancy and increase the efficiency of chemotherapy. The purpose of this study was to determine the effect of specific inhibitors of MAPK extracellular signaling-regulated kinase (ERK) kinase (MEK) and p38 on the differentiation of human osteosarcoma cell line SaOS-2 cells. We found that PD98059, a specific inhibitor of MEK, inhibited the serum-stimulated proliferation of SaOS-2 cells; whereas SB203580, a specific inhibitor of p38 MAPK, had little effect on it. SB203580 suppressed ALPase activity, gene expression of type I collagen, and expression of ALP and BMP-2 mRNAs; whereas PD98059 upregulated them dose dependently. In addition, immunoblot and immunostaining analysis revealed that phosphorylation of ERK was increased by treatment with SB203580; whereas PD98059 increased the phosphorylation of p38, which implies a seesaw-like balance between ERK and p38 phosphorylation. We suggest that osteosarcoma cell differentiation is regulated by the balance between the activities of the ERK and p38 pathways and that the MEK/ERK pathway negatively regulates osteosarcoma cell differentiation, whereas the p38 pathway does so positively. MEK inhibitor may thus be a good candidate for altering the expression of the osteosarcoma malignant phenotype.