SB203580 promotes EGF-stimulated early morphological differentiation in PC12 cell through activating ERK pathway

Dual Targeting of BRAF and mTOR Signaling in Melanoma Cells with Pyridinyl Imidazole Compounds

BRAF inhibitors can delay the progression of metastatic melanoma, but resistance usually emerges, leading to relapse. Drugs simultaneously targeting two or more pathways essential for cancer growth could slow or prevent the development of resistant clones. Here, we identified pyridinyl imidazole compounds SB202190, SB203580, and SB590885 as dual inhibitors of critical proliferative pathways in human melanoma cells bearing the V600E activating mutation of BRAF kinase. We found that the drugs simultaneously disrupt the BRAF V600E-driven extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) activity and the mechanistic target of rapamycin complex 1 (mTORC1) signaling in melanoma cells. Pyridinyl imidazole compounds directly inhibit BRAF V600E kinase. Moreover, they interfere with the endolysosomal compartment, promoting the accumulation of large acidic vacuole-like vesicles and dynamic changes in mTOR signaling. A transient increase in mTORC1 activity is followed by the enrichment of the Ragulator complex protein p18/LAMTOR1 at contact sites of large vesicles and delocalization of mTOR from the lysosomes. The induced disruption of the endolysosomal pathway not only disrupts mTORC1 signaling, but also renders melanoma cells sensitive to endoplasmic reticulum (ER) stress. Our findings identify new activities of pharmacologically relevant small molecule compounds and provide a biological rationale for the development of anti-melanoma therapeutics based on the pyridinyl imidazole core.

Dual Role For A MEK Inhibitor As A Modulator Of Inflammation And Host Defense Mechanisms With Potential Therapeutic Application In COPD

Background

Unlike p38 mitogen-activated protein Kinases (MAPK) that has been extensively studied in the context of lung-associated pathologies in COPD, the role of the dual-specificity mitogen-activated protein kinase kinase (MEK1/2) or its downstream signaling molecule extracellular signal-regulated kinases 1/2 (ERK1/2) in COPD is poorly understood.

Objectives

The aim of this study was to address whether MEK1/2 pathway activation is linked to COPD and that targeting this pathway can improve lung inflammation through decreased immune-mediated inflammatory responses without compromising bacterial clearance.

Methods

Association of MEK1/2 pathway activation to COPD was investigated by immunohistochemistry using lung tissue biopsies from COPD and healthy individuals and through analysis of sputum gene expression data from COPD patients. The anti-inflammatory effect of MEK1/2 inhibition was assessed on cytokine release from lipopolysaccharide-stimulated alveolar macrophages. The effect of MEK1/2 inhibition on bacterial clearance was assessed using Staphylococcus aureus killing assays with RAW 264.7 macrophage cell line and human neutrophils.

Results

We report here MEK1/2 pathway activation demonstrated by increased pERK1/2 staining in bronchial epithelium and by the presence of MEK gene activation signature in sputum samples from COPD patients. Inhibition of MEK1/2 resulted in a superior anti-inflammatory effect in human alveolar macrophages in comparison to a p38 inhibitor. Furthermore, MEK1/2 inhibition led to an increase in bacterial killing in human neutrophils and RAW 264.7 cells that was not observed with the p38 inhibitor.

Conclusion

Our data demonstrate the activation of MEK1/2 pathway in COPD and highlight a dual function of MEK1/2 inhibition in improving host defense responses whilst also controlling inflammation.

Erythropoietin Pathway: A Potential Target for the Treatment of Depression

During the past decade, accumulating evidence from both clinical and experimental studies has indicated that erythropoietin may have antidepressant effects. In addition to the kidney and liver, many organs have been identified as secretory tissues for erythropoietin, including the brain. Its receptor is expressed in cerebral and spinal cord neurons, the hypothalamus, hippocampus, neocortex, dorsal root ganglia, nerve axons, and Schwann cells. These findings may highlight new functions for erythropoietin, which was originally considered to play a crucial role in the progress of erythroid differentiation. Erythropoietin and its receptor signaling through JAK2 activate multiple downstream signaling pathways including STAT5, PI3K/Akt, NF-κB, and MAPK. These factors may play an important role in inflammation and neuroprogression in the nervous system. This is particularly true for the hippocampus, which is possibly related to learning, memory, neurocognitive deficits and mood alterations. Thus, the influence of erythropoietin on the downstream pathways known to be involved in the treatment of depression makes the erythropoietin-related pathway an attractive target for the development of new therapeutic approaches. Focusing on erythropoietin may help us understand the pathogenic mechanisms of depression and the molecular basis of its treatment.

c-Src mediated tyrosine phosphorylation of plakophilin 3 as a new mechanism to control desmosome composition in cells exposed to oxidative stress

Plakophilins (PKP1 to PKP3) are essential for the structure and function of desmosomal junctions as demonstrated by the severe skin defects observed as a result of loss-of-function mutations in mice and men. PKPs play additional roles in cell signaling processes, such as those controlling the cellular stress response and cell proliferation. A key post-translational process controlling PKP function is phosphorylation. We have discovered that reactive oxygen species (ROS) trigger the c-Src kinase-mediated tyrosine (Tyr)-195 phosphorylation of PKP3. This modification is associated with a change in the subcellular distribution of the protein. Specifically, PKP3 bearing phospho-Tyr-195 is released from the desmosomes, suggesting that phospho-Tyr-195 is relevant for the control of desmosome disassembly and function, at least in cells exposed to ROS. Tyr-195 phosphorylation is transient under normal physiological conditions and seems to be strictly regulated, as the activation of particular growth factor receptors results in a modification at this site only when tyrosine phosphatases are inactivated by pervanadate. We have identified Tyr-195 of PKP3 as a phosphorylation target of epidermal growth factor receptor signaling. Interestingly, this PKP3 phosphorylation also occurs in certain poorly differentiated adenocarcinomas of the prostate, suggesting a possible role in tumor progression. Our study thus identifies a new mechanism controlling PKP3 and hence desmosome function in epithelial cells.

Fibroblasts from long-lived species of mammals and birds show delayed, but prolonged, phosphorylation of ERK

Fibroblasts from long-lived mutant mice show diminished phosphorylation of the stress-activated protein kinases ERK1/2 after exposure to peroxide, cadmium, or paraquat. We have now evaluated the kinetics of ERK phosphorylation in fibroblasts from long-lived and short-lived species of mammals and birds in response to stress by cadmium or hydrogen peroxide. Fibroblasts from the shorter-lived species of rodents and birds showed rapid induction of ERK phosphorylation, with a decline to basal level within 60 min. In contrast, cells from longer-lived species showed slower and more prolonged activation of ERK phosphorylation. These results suggest that fibroblasts from long-lived species may be less susceptible to the early phases of damage from cadmium or peroxide and suggest that altered kinetics of ERK activity may contribute to their stress resistance properties.

Light-mediated kinetic control reveals the temporal effect of the Raf/MEK/ERK pathway in PC12 cell neurite outgrowth

It has been proposed that differential activation kinetics allows cells to use a common set of signaling pathways to specify distinct cellular outcomes. For example, nerve growth factor (NGF) and epidermal growth factor (EGF) induce different activation kinetics of the Raf/MEK/ERK signaling pathway and result in differentiation and proliferation, respectively. However, a direct and quantitative linkage between the temporal profile of Raf/MEK/ERK activation and the cellular outputs has not been established due to a lack of means to precisely perturb its signaling kinetics. Here, we construct a light-gated protein-protein interaction system to regulate the activation pattern of the Raf/MEK/ERK signaling pathway. Light-induced activation of the Raf/MEK/ERK cascade leads to significant neurite outgrowth in rat PC12 pheochromocytoma cell lines in the absence of growth factors. Compared with NGF stimulation, light stimulation induces longer but fewer neurites. Intermittent on/off illumination reveals that cells achieve maximum neurite outgrowth if the off-time duration per cycle is shorter than 45 min. Overall, light-mediated kinetic control enables precise dissection of the temporal dimension within the intracellular signal transduction network.

Protein arginine methyltransferase 5 regulates ERK1/2 signal transduction amplitude and cell fate through CRAF

The RAS to extracellular signal-regulated kinase (ERK) signal transduction cascade is crucial to cell proliferation, differentiation, and survival. Although numerous growth factors activate the RAS-ERK pathway, they can have different effects on the amplitude and duration of the ERK signal and, therefore, on the biological consequences. For instance, nerve growth factor, which elicits a larger and more sustained increase in ERK phosphorylation in PC12 cells than does epidermal growth factor (EGF), stimulates PC12 cell differentiation, whereas EGF stimulates PC12 cell proliferation. Here, we show that protein arginine methylation limits the ERK1/2 signal elicited by particular growth factors in different cell types from various species. We found that this restriction in ERK1/2 phosphorylation depended on methylation of RAF proteins by protein arginine methyltransferase 5 (PRMT5). PRMT5-dependent methylation enhanced the degradation of activated CRAF and BRAF, thereby reducing their catalytic activity. Inhibition of PRMT5 activity or expression of RAF mutants that could not be methylated not only affected the amplitude and duration of ERK phosphorylation in response to growth factors but also redirected the response of PC12 cells to EGF from proliferation to differentiation. This additional level of regulation within the RAS pathway may lead to the identification of new targets for therapeutic intervention.

V3 versican isoform alters the behavior of human melanoma cells by interfering with CD44/ErbB-dependent signaling

Versican is a hyaluronan-binding, extracellular chondroitin sulfate proteoglycan produced by several tumor types, including malignant melanoma, which exists as four different splice variants. The short V3 isoform contains the G1 and G3 terminal domains of versican that may potentially interact directly or indirectly with the hyaluronan receptor CD44 and the EGFR, respectively. We have previously described that overexpression of V3 in MeWo human melanoma cells markedly reduces tumor cell growth in vitro and in vivo. In this study we have investigated the signaling mechanism of V3 by silencing the expression of CD44 in control and V3-expressing melanoma cells. Suppression of CD44 had the same effects on cell proliferation and cell migration than those provoked by V3 expression, suggesting that V3 acts through a CD44-mediated mechanism. Furthermore, CD44-dependent hyaluronan internalization was blocked by V3 expression and CD44 silencing, leading to an accumulation of this glycosaminoglycan in the pericellular matrix and to changes in cell migration on hyaluronan. Furthermore, ERK1/2 and p38 activation after EGF treatment were decreased in V3-expressing cells suggesting that V3 may also interact with the EGFR through its G3 domain. The existence of a EGFR/ErbB2 receptor complex able to interact with CD44 was identified in MeWo melanoma cells. V3 overexpression resulted in a reduced interaction between EGFR/ErbB2 and CD44 in response to EGF treatment. Our results indicate that the V3 isoform of versican interferes with CD44 and the CD44-EGFR/ErbB2 interaction, altering the signaling pathways, such as ERK1/2 and p38 MAPK, that regulate cell proliferation and migration.

Cigarette smoke-related hydroquinone induces filamentous actin reorganization and heat shock protein 27 phosphorylation through p38 and extracellular signal-regulated kinase 1/2 in retinal pigment epithelium: implications for age-related macular degeneration

Retinal pigment epithelium (RPE)-derived membranous debris named blebs, may accumulate and contribute to sub-RPE deposit formation, which is the earliest sign of age-related macular degeneration (AMD). Oxidative injury to the RPE might play a significant role in AMD. However, the underlying mechanisms are unknown. We previously reported that hydroquinone (HQ), a major pro-oxidant in cigarette smoke, foodstuff, and atmospheric pollutants, induces actin rearrangement and membrane blebbing in RPE cells as well as sub-RPE deposits in mice. Here, we show for the first time that phosphorylated Heat shock protein 27 (Hsp27), a key regulator of actin filaments dynamics, is up-regulated in RPE from patients with AMD. Also, HQ-induced nonlethal oxidative injury led to Hsp27mRNA up-regulation, dimer formation, and Hsp27 phosphorylation in ARPE-19 cells. Furthermore, we found that a cross talk between p38 and extracellular signal-regulated kinase (ERK) mediates HQ-induced Hsp27 phosphorylation and actin aggregate formation, revealing ERK as a novel upstream mediator of Hsp27 phosphorylation. Finally, we demonstrated that Hsp25, p38, and ERK phosphorylation are increased in aging C57BL/6 mice chronically exposed to HQ, whereas Hsp25 expression is decreased. Our data suggest that phosphorylated Hsp27 might be a key mediator in AMD and HQ-induced oxidative injury to the RPE, which may provide helpful insights into the early cellular events associated with actin reorganization and bleb formation involved in sub-RPE deposits formation relevant to the pathogenesis of AMD.

Erythropoietin enhancement of rat pancreatic tumor cell proliferation requires the activation of ERK and JNK signals

Erythropoietin (EPO) regulates the proliferation and differentiation of erythroid cells by binding to its specific transmembrane receptor EPOR. Recent studies, however, have shown that the EPOR is additionally present in various cancer cells and EPO induces the proliferation of these cells, suggesting a different function for EPO other than erythropoiesis. Therefore, the purpose of the present study was to examine EPOR expression and the role of EPO in the proliferation and signaling cascades involved in this process, using the rat pancreatic tumor cell line AR42J. Our results showed that AR42J cells expressed EPOR, and EPO significantly enhanced their proliferation. Cell cycle analysis of EPO-treated cells indicated an increased percentage of cells in the S phase, whereas cell numbers in G0/G1 phase were significantly reduced. Phosphorylation of extracellular regulatory kinase 1/2 (ERK1/2) and c-Jun NH2terminal kinase 1/2 (JNK1/2) was rapidly stimulated and sustained after EPO addition. Treatment of cells with mitogen-activated protein/ERK kinase (MEK) inhibitor PD98059 or JNK inhibitor SP600125 significantly inhibited EPO-enhanced proliferation and also increased the fraction of cells in G0/G1 phase. Furthermore, the inhibition of JNK using small interference RNA (siRNA) suppressed EPO-enhanced proliferation of AR42J cells. Taken together, our results indicate that AR42J cells express EPOR and that the activation of both ERK1/2 and JNK1/2 by EPO is essential in regulating proliferation and the cell cycle. Thus both appear to play a key role in EPO-enhanced proliferation and suggest that the presence of both is required for EPO-mediated proliferation of AR42J cells.

A novel collagen-binding peptide promotes osteogenic differentiation via Ca2+/calmodulin-dependent protein kinase II/ERK/AP-1 signaling pathway in human bone marrow-derived mesenchymal stem cells

The intracellular signaling events controlling human mesenchymal stem cell (hMSC) differentiation into osteoblasts are poorly understood. Collagen-binding domain is considered an essential component of bone mineralization. In the present study, we investigated the regulatory mechanism of osteoblastic differentiation of hMSC by the peptide with a novel collagen-binding motif derived from osteopontin. The peptide induced influx of extracellular Ca2+ via calcium channels and increased intracellular Ca2+ concentration ([Ca2+]i) independent of both pertussis toxin and phospholipase C, and activated ERK, which was inhibited by Ca2+/calmodulin-dependent protein kinase (CaMKII) antagonist, KN93. The peptide-induced increase of [Ca2+]i is correlated with ERK activation in a various cell types. The peptide stimulated the migration of hMSC but suppressed cell proliferation. Furthermore, the peptide increased the phosphorylation of cAMP-response element-binding protein, leading to a significant increase in the transactivation of cAMP-response element and serum response element. Ultimately, the peptide increased AP-1 transactivation, c-jun expression, and bone mineralization, which are suppressed by KN93. Taken together, these results indicate that the novel collagen-binding peptide promotes osteogenic differentiation via Ca2+/CaMKII/ERK/AP-1 signaling pathway in hMSC, suggesting the potential application in cell therapy for bone regeneration.

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.

Modulation of autocrine TNF-alpha-stimulated matrix metalloproteinase 9 (MMP-9) expression by mitogen-activated protein kinases in THP-1 monocytic cells

Matrix metalloproteinase 9 (MMP-9) is implicated in various physiological processes by its ability to degrade the extracellular matrix (ECM) and process multiple regulatory proteins. Normally, MMP-9 expression is tightly controlled in cells. Sustained or enhanced MMP-9 secretion, however, has been demonstrated to contribute to the pathophysiology of numerous diseases, including arthritis and tumor progression, rendering this enzyme a major target for clinical interventions. Here we show that constitutive MMP-9 secretion was abrogated in THP-1 monocytic leukemia cells by addition of neutralizing antibodies against tumor necrosis factor alpha (TNF-alpha) or TNF receptor type 1 (TNF-R1), as well as by inhibition of TNF-alpha converting enzyme (TACE). This indicates that MMP-9 production in these cells is maintained by autocrine stimulation, with TNF-alpha acting via TNF-R1. To investigate the intracellular signaling routes involved in MMP-9 gene transcription, cells were treated with different inhibitors of major mitogen-activated protein kinase (MAPK) pathways. Interruption of the extracellular signal-regulated kinase pathway 1/2 (ERK1/2) using PD98059 significantly downregulated constitutive MMP-9 release. In contrast, blockage of p38 kinase activity by addition of SB203580 or SB202190, as well as inhibition of c-Jun N-terminal kinase (JNK) using L-JNK-I1, clearly augmented MMP-9 expression and secretion by an upregulation of ERK1/2 phosphorylation. Moreover, exogenously added TNF-alpha augmented MMP-9 synthesis and secretion in THP-1 cells via enhancement of ERK1/2 activity. Taken together, our results indicate that ERK1/2 activity plays a pivotal role in TNF-alpha-induced MMP-9 production and demonstrate its negative modulation by p38 and JNK activity. These findings suggest ERK1/2 rather than p38 and JNK as a reasonable target to specifically block MMP-9 expression using MAPK inhibitors in therapeutic applications.

Differential roles of MAP kinases in atorvastatin-induced VEGF release in cardiac myocytes

Statins, specific inhibitors of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, are now widely used for treatment of patients with hypercholesterolemia. In addition to the reduction of cholesterol biosynthesis, accumulating evidence indicates that statins have several pleiotropic effects especially on cardiovascular system. However, the exact role of statin in cardiac myocytes remains unclear. In the present study, we investigated whether atorvastatin induces vascular endothelial growth factor (VEGF) release in cardiac myocytes, and the underlying mechanism. We observed that atorvastatin significantly stimulated VEGF release in a dose-dependent manner. It induced the phosphorylation of p44/p42 mitogen-activated protein (MAP) kinase and p38 MAP kinase but not SAPK (stress-activated protein kinase)/JNK (c-Jun N-terminal kinase). The atorvastatin-induced VEGF release was enhanced by PD98059, which is a specific inhibitor of the upstream kinase that activates p44/p42 MAP kinase (MEK). Further, it was significantly reduced by SB203580, a specific inhibitor of p38 MAP kinase. Furthermore, the atorvastatin-induced phosphorylation of p38 MAP kinase was attenuated by SB203580, whereas it was enhanced by PD98059. Taken together, these results suggest that the atorvastatin-induced VEGF release in cardiac myocytes is positively regulated by p38 MAP kinase and negatively regulated byp44/p42 MAP kinase and that the atorvastatin-induced phosphorylation of p38 MAP kinase is regulated by p44/p42 MAP kinase in these cells.

ERK-mediated production of neurotrophic factors by astrocytes promotes neuronal stem cell differentiation by erythropoietin

Erythropoietin (EPO), a hematopoietic factor, is also required for normal brain development, and its receptor is localized in brain. Our previous study showed that EPO promotes differentiation of neuronal stem cells into astrocytes. Since astrocytes have influence on the neuronal function, we investigated whether EPO-activated astrocytes could stimulate differentiation of neuronal stem cells into neurons. EPO did not promote neuronal differentiation of neuronal stem cells isolated from 17 day embryos, however, neuronal differentiation was promoted when the neuronal stem cells were co-cultured with astrocyte isolated from post neonatal (Day 1) rat brain. Moreover, neuronal differentiation was further promoted when the neuronal stem cells were cultured with astrocyte culture medium treated by EPO (10U/ml) showing increase of morphological differentiation, and expression of neuronal differentiation marker proteins, neurofilament, and tyrosine hydroxylase. The promoting effect of EPO-treated astrocyte medium was also found in the differentiation of PC12 cells. EPO-promoted morphological differentiation of neuronal stem cells as well as astrocytes was dose dependently reduced by treatment with anti-EPO receptor antibodies in culture with astrocyte culture medium. To clarify whether EPO itself or via production of well-known neurotropic factor could promote neuronal cell differentiation, we determined the level of neurotropic factors in the EPO-treated astrocytes. Compared to untreated astrocytes, EPO-treated astrocytes increased about 2-fold in beta-NGF and 3-4-fold in BMP2, but did not increase BNDF and NT-3 levels. Since the previous study showed that extracellular signal-regulated kinase (ERK) is involved in activation of astrocytes by EPO, we determined whether generation of neurotrophic factor may also be involved with the ERK pathway. In the presence of ERK inhibitor, PD98059, the generation of beta-NGF was diminished in a dose dependent manner consistent with the inhibiting effect on neuronal differentiation. These data demonstrate that EPO promotes neuronal cell differentiation through increased release of beta-NGF and BMP2 from astrocytes, and this effect may be associated with ERK pathway signals.

Involvement of p38 MAP kinase-mediated cytochrome c release on sphingosine-1-phosphate (S1P)- and N-monomethyl-S1P-induced cell death of PC12 cells

d-erythro-Sphingosine-1-phosphate (S1P), a sphingolipid metabolite, affects various neuronal functions including cell fate. S1P appears to have contradictory effects in PC12 cells, a neuronal model cell line; neurite retraction and cell survival/differentiation. In the present study, we examined whether S1P induces cell death in undifferentiated PC12 cells. Culture with S1P at 20 microM for 4 h caused lactate dehydrogenase leakage 24 h later. The response was reduced by an inhibitor of caspases and accompanied by the release of cytochrome c and DNA fragmentation. S1P caused the phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK) within 10 min. An inhibitor of p38 MAPK (10 microM SB203580) inhibited both the release of cytochrome c and DNA fragmentation induced by S1P. Treatment with nerve growth factor or pertussis toxin (PTX) decreased S1P-induced phosphorylation of p38 MAPK and cell death. These findings suggest that S1P-activated p38 MAPK acts as a death signal upstream of the release of cytochrome c. N-Monomethyl-S1P (MM-S1P), a weak agonist in cells expressing S1P1 receptors, had marked effects (phosphorylation of p38 MAPK, release of cytochrome c and DNA fragmentation) at lower concentrations than S1P and in a PTX-sensitive manner. These findings show that the activation of S1P receptors by S1P and MM-S1P causes cell death accompanied by DNA fragmentation via the p38 MAPK pathway-mediated release of cytochrome c in PC12 cells. The potential of S1P and MM-S1P to act as agonists of S1P receptors and as intracellular messengers is discussed.

Decrease in cytosolic phospholipase A2α mRNA levels by reactive oxygen species via MAP kinase pathways in PC12 cells: effects of dopaminergic neurotoxins

Excess production of reactive oxygen species (ROS), including H2O2, leads to neuronal death in pathological conditions. Although ROS stimulates alpha-type cytosolic phospholipase A2 (cPLA2alpha) activity, their role in cPLA2alpha expression has not been elucidated. We investigated the effect of ROS on cPLA2alpha mRNA levels and signaling pathways in rat pheochromocytoma PC12 cells. Treatment with H2O2 and xanthine-xanthine oxidase (X/XO) for 4 h decreased cPLA2alpha mRNA levels without changing the mRNA levels of other tested proteins. H2O2 and X/XO caused cell toxicity not after 4 h but 24 h after their addition. The H2O2-induced decrease in cPLA2alpha mRNA levels was inhibited in cells treated with N-acetyl-cysteine and selective inhibitors of mitogen-activated protein kinase (MAPK) pathways (extracellular signal-regulated kinase and p38 MAPK). Treatment with dopaminergic neurotoxins, including 1,2,3,4-tetrahydroisoquinoline (TIQ)-inducing ROS formation, decreased cPLA2alpha mRNA levels. These findings suggest that ROS decreases cPLA2alpha mRNA levels via MAPK pathways in PC12 cells.

Rit contributes to nerve growth factor-induced neuronal differentiation via activation of B-Raf-extracellular signal-regulated kinase and p38 mitogen-activated protein kinase cascades

Rit is one of the original members of a novel Ras GTPase subfamily that uses distinct effector pathways to transform NIH 3T3 cells and induce pheochromocytoma cell (PC6) differentiation. In this study, we find that stimulation of PC6 cells by growth factors, including nerve growth factor (NGF), results in rapid and prolonged Rit activation. Ectopic expression of active Rit promotes PC6 neurite outgrowth that is morphologically distinct from that promoted by oncogenic Ras (evidenced by increased neurite branching) and stimulates activation of both the extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein (MAP) kinase signaling pathways. Furthermore, Rit-induced differentiation is dependent upon both MAP kinase cascades, since MEK inhibition blocked Rit-induced neurite outgrowth, while p38 blockade inhibited neurite elongation and branching but not neurite initiation. Surprisingly, while Rit was unable to stimulate ERK activity in NIH 3T3 cells, it potently activated ERK in PC6 cells. This cell type specificity is explained by the finding that Rit was unable to activate C-Raf, while it bound and stimulated the neuronal Raf isoform, B-Raf. Importantly, selective down-regulation of Rit gene expression in PC6 cells significantly altered NGF-dependent MAP kinase cascade responses, inhibiting both p38 and ERK kinase activation. Moreover, the ability of NGF to promote neuronal differentiation was attenuated by Rit knockdown. Thus, Rit is implicated in a novel pathway of neuronal development and regeneration by coupling specific trophic factor signals to sustained activation of the B-Raf/ERK and p38 MAP kinase cascades.

Novel signaling stimulated by arsenite increases cholesterol metabolism through increases in unphosphorylated steroidogenic acute regulatory (StAR) protein

Cholesterol metabolism to pregnenolone is dependent on the steroidogenic acute regulatory protein (StAR), which activates mitochondrial transfer of cholesterol to cytochrome CYP450scc. In mouse Y-1 adrenal cells and testis MA10 cells stimulation by 8-Bromo-cAMP (Br-cAMP) is augmented by a novel signaling initiated by low concentrations of arsenite (3-20 microM) and anisomycin (0.2 microM), a more selective stress agent. Each elevated StAR mRNA (three-fold after 6 h treatment) even with simultaneous stimulation by Br-cAMP. Arsenite produced parallel increases in StAR protein expression and cholesterol metabolism, but not for P450scc-mediated metabolism of 20alpha-hydroxycholesterol. Although arsenite and anisomycin each stimulated the phosphorylation of p38, the p38 inhibitor SB203580 (SB) produced additive increases in StAR expression. Cholesterol metabolism increased in parallel but without the increased StAR protein phosphorylation produced by Br-cAMP. Arsenite and anisomycin each elevated StAR mRNA but preferentially increased the 3.5 kb form relative to the 1.6 kb form. Arsenite and anisomycin each enhanced the stability of the more labile 3.5 kb mRNA which contains AU-rich elements that control mRNA stability. Although there were increases in both forms of StAR mRNA, arsenite did not stimulate a StAR promoter-reporter that exhibited a typical three-fold response to Br-cAMP. Arsenite and anisomycin may therefore activate a novel SB-independent MAP kinase which in part increases StAR expression through stabilizing the 3.5 kb mRNA but which may also activate a mechanism that by-passes transcription factors detected by the reporter. SB stimulation, which was completely blocked by a MEK inhibitor, was also selective towards the 3.5 kb StAR mRNA suggesting a second pathway for mRNA stabilization. These activations contrast with inhibition of StAR expression by arsenite at higher concentrations or longer incubation times.

Transient versus sustained phosphorylation and nuclear accumulation of ERKs underlie anti-versus pro-apoptotic effects of estrogens

Sex steroids exert anti-apoptotic effects on osteoblasts/osteocytes but exert pro-apoptotic effects on osteoclasts, in both cases requiring activation of the extracellular signal-regulated kinases (ERKs). To explain the mechanistic basis of this divergence, we searched for differences in the kinetics of phosphorylation and/or in the subcellular localization of ERKs in response to 17beta-estradiol in the two cell types. In contrast to its transient effect on ERK phosphorylation in osteocytic cells (return to base line by 30 min), 17beta-estradiol-induced ERK phosphorylation in osteoclasts was sustained for at least 24 h following exposure to the hormone. Conversion of sustained ERK phosphorylation to transient, by means of cholera toxin-induced activation of the adenylate cyclase/cAMP/protein kinase A pathway, abrogated the pro-apoptotic effect of 17beta-estradiol on osteoclasts. Conversely, prolongation of ERK activation in osteocytes, by means of leptomycin B-induced inhibition of ERK export from the nucleus or overexpression of a green fluorescent protein-ERK2 mutant that resides permanently in the nucleus, converted the anti-apoptotic effect of 17beta-estradiol to a pro-apoptotic one. These findings indicate that the kinetics of ERK phosphorylation and the length of time that phospho-ERKs are retained in the nucleus are responsible for pro-versus anti-apoptotic effects of estrogen on different cell types of bone and perhaps their many other target tissues.

EPO receptor-mediated ERK kinase and NF-kappaB activation in erythropoietin-promoted differentiation of astrocytes

Erythropoietin (EPO), a hematopoietic factor, is also required for normal brain development, and its receptor is localized in brain. Therefore, it is possible that EPO could act as a neurotropic factor inducing differentiation of neurons. In the present study, we investigated whether EPO can promote differentiation of neuronal stem cells into astrocytes. In primary culture of cortical neuronal stem cells isolated from post neonatal (Day 1) rat brain, EPO dose (0.1-10U/ml) dependently promoted initiation of morphological differentiation of astrocyte and expression of an astrocyte marker protein, glial fibrillary acidic protein (GFAP). Expression of EPO receptor was also increased during morphological differentiation of astrocytes. EPO-induced increased morphological differentiation of astrocytes and GFAP expression were reduced by treatment with anti-EPO and EPO receptor antibodies. Since our previous study showed that activation of MAPK family and transcription factors is differentially involved in neuronal cell differentiation, we further determined the activation of MAP kinase family and NF-kappaB during morphological differentiation of astrocytes. Concomitant with the progression of the morphological differentiation of astrocytes, ERK(2) but not JNK(1) and p38 MAPK as well as NF-kappaB were activated. However, in the presence of PD98,059, an inhibitor of ERK, and salicylic acid, an NF-kappaB inhibitor, the EPO-induced morphological differentiation of astrocytes and expression of FGAP and EPO receptor were reduced. Conversely, treatment with anti-EPO and EPO receptor antibodies also reduced EPO-induced ERK(2) and NF-kappaB activation. These data demonstrate that EPO can promote differentiation of neuronal stem cells into astrocytes in an EPO receptor dependent manner, and this effect may be associated with the activation of ERK kinase and NF-kappaB pathway.

Inhibition of p38 mitogen-activated protein kinase augments progression of remnant kidney model by activating the ERK pathway

p38, a mitogen-activated protein kinase, is a major intracellular signaling molecule involved in inflammation. To test the hypothesis that p38 mediates renal disease progression, we administered a novel p38 alpha inhibitor, NPC31169, to rats with remnant kidneys (RKs). RK rats showed increased p38 activation at 9 weeks (by p38 kinase assay), which was blocked by the inhibitor. In contrast to our expectation, treatment with the NPC31169 resulted in worse renal function, more proteinuria, and more severe glomerulosclerosis and tubulointerstitial injury. p38 inhibition resulted in marked cell proliferation in RK rats, with more proliferating tubular cells, myofibroblasts, and macrophages. In contrast, p38 suppression resulted in less tubular cell apoptosis. Interestingly, Western blot demonstrated increased ERK1/2 phosphorylation in p38-treated rats. No histological changes were observed in p38 inhibited sham-operated rats. Our findings indicate that, whereas blocking p38 usually shows benefit in inflammatory disease, in this model p38 inhibition resulted in accelerated renal progression. We conclude that blocking p38-dependent inflammation may have resulted in enhanced proliferation and increased ERK1/2 activation, and thereby explains the worse renal lesions observed.

Delayed but sustained induction of mitogen-activated protein kinase activity is associated with β-adrenergic receptor-mediated morphological differentiation of astrocytes

Astroglial beta-adrenergic receptors (beta-ARs) are functionally linked to regulate cellular morphology. In primary cultures, the beta-AR agonist isoproterenol (ISP) can transform flat polygonal astrocytes into process-bearing, mature stellate cells by 48 h, an effect that can be blocked by the beta-AR antagonist, propranolol. ISP induced immediate activation of protein kinase A (PKA) which persisted up to 2 h, with no visible change in cell morphology. However, activation of PKA was sufficient to drive the process of transformation to completion, suggesting the involvement of downstream regulators of PKA. In addition to PKA inhibitors, the mitogen-activated protein kinase (MAPK) kinase inhibitor PD098059 also blocked ISP-induced morphological transformation. ISP treatment resulted in a biphasic response of cellular phosphorylated MAPK (phosphorylated extracellular signal-regulated kinase; p-ERK) level: an initial decline in p-ERK level followed by a sustained induction at 12-24 h, both of which were blocked by PKA inhibitor. The induction in pERK level coincided with initiation of morphological differentiation of the astrocytes and nuclear translocation of p-ERK. A long-lasting activation of p-ERK activity by ISP, at a later stage, appears to be critical for the transformation of astrocytes.

Association of sustained ERK activity with integrin beta3 induction during receptor activator of nuclear factor kappaB ligand (RANKL)-directed osteoclast differentiation

Osteoclast differentiation is a multi-step process that involves cell proliferation, commitment, and fusion. Some adhesion molecules, including integrin alphavbeta3, have been shown to have roles in osteoclast fusion. In the course of studying with pharmacologic agents known to inhibit protein tyrosine kinases of the Src family, we found that radicicol increased cell fusion during receptor activator of nuclear factor kappaB ligand (RANKL)-driven differentiation of osteoclasts at concentrations far below the ones shown to inhibit its targets in previous studies. Treatments of low doses of radicicol to RAW 264.7 cells that undergo osteoclastic differentiation in the presence of RANKL enhanced the RANKL-induced gene expression of integrin beta3 without any effect on the expression of integrin alphav, which was constitutively high. The cell surface level of integrin alphavbeta3 complexes was consequently augmented by radicicol. In addition, sustained ERK and MEK activation was observed in cells treated with both radicicol and RANKL. More importantly, modulation of ERK activity by the MEK inhibitor U0126 or the gene transduction of a constitutively active form of MEK resulted in a suppression and increment, respectively, of integrin beta3 induction by RANKL. Our data indicate that sustained ERK activity is associated with integrin beta3 induction and subsequent cell surface expression of the alphavbeta3 integrin complex, which may contribute to cell fusion during RANKL-directed osteoclastogenesis.

p38 Mitogen-activated protein kinase regulation of JB6 Cl41 cell transformation promoted by epidermal growth factor

The relationship between cell transformation and p38 MAP kinase, a major mitogen-activated protein (MAP) kinase pathway converting signals of various extracellular stimuli into expression of specific target genes through activation of transcription factors, still remains unclear. The aim of the present study was to investigate the role of the p38 MAP kinase pathway in epidermal growth factor (EGF)-induced cell transformation in JB6 cells. Our data show that a dominant negative mutant of p38 MAP (DN-p38) kinase inhibits EGF-promoted JB6 Cl41 cell transformation and that SB202190, an inhibitor of p38 MAP kinase, also inhibits JB6 Cl41 cell transformation in a dose-dependent manner. Moreover, our results show that DN-p38 MAP kinase inhibits the phosphorylation of EGF-stimulated activating transcription factor-2 (ATF-2) and signal transducer and activator of transcription 1 (STAT1). Additionally, DN-p38 MAP kinase inhibits EGF-induced phosphorylation of c-Myc (Thr58/Ser62). Gel shift assays indicate that DN-p38 MAP kinase inhibits EGF-induced activator protein-1 (AP-1) DNA binding in a dose-dependent manner. These results show that p38 MAP kinase plays a key role in the regulation of EGF-induced cell transformation in JB6 cells through regulation of phosphorylation of p38 MAP kinase and activation of its target genes in phosphorylation, c-Myc cell transformation-related genes, and AP-1 binding ability.

AhR- and ERK-dependent pathways function synergistically to mediate 2,3,7,8-tetrachlorodibenzo-p-dioxin suppression of peroxisome proliferator-activated receptor-gamma1 expression and subsequent adipocyte differentiation

Activation of the aryl-hydrocarbon receptor (AhR) by pretreatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) blocks hormone (IDM/BRL)-induced adipocyte differentiation of C3H10T1/2 cells in proportion to the suppression of the elevation of the key mediator, peroxisome proliferator-activated receptor (PPARgamma1). Inhibition of MEK-induced ERK phosphorylation had no effect on adipogenesis but prevented this TCDD suppression. Initiation of MEK inhibition up to 6 h after IDM/BRL stimulation in combination with serum addition completely reversed the TCDD-mediated suppression but declined to ineffectiveness when delayed to 24 h after stimulation. This period occurs well after the decline of serum-induced ERK activation, at a time when ERK phosphorylation is low, and prior to the onset of IDM/BRL-stimulated PPARgamma1 expression. This temporal separation of ERK activation from the affected PPARgamma1 expression suggests that ERK does not act directly on either PPARgamma1 transcription or receptor function. Thus, ERK activation and TCDD/AhR stimulation work synergistically to inhibit adipocyte differentiation. Nonrenewal of serum at the time of IDM/BRL addition removed most of the ERK activation and also the TCDD-mediated suppressions of PPARgamma1 expression and adipocyte differentiation. Transfection of a vector expressing constitutively active MEK1 generated a constant, high level of phosphorylated ERK comparable to the peak serum-induced level and fully restored TCDD suppression without a TCDD-mediated effect on ERK phosphorylation. We conclude that low levels of activated MEK and ERK cooperate with AhR-induced factor(s) to generate a suppressor that prevents PPARgamma1 transcription and then differentiation.

Induction of HL-60 cell differentiation by the p38 mitogen-activated protein kinase inhibitor SB203580 is mediated through the extracellular signal-regulated kinase signaling pathway

The pyridinyl imidazole p38 kinase inhibitor, SB203580, was initially used to block inflammatory cytokine synthesis. Here we report that SB203580 by itself could induce human promyeloid leukemic HL-60 cells to differentiate mainly along the granulocytic lineage, as evidenced by cellular morphological changes, and the concurrent expression of cell surface markers CD11b and CD14. This differentiation induction was time and dose dependent. After 12 h exposure to 10 microM SB203580, 12.5% of the cells became CD11b as compared to only 2.6% in untreated control cells. By 96 h, CD11b cells increased to 72.3%, and among them, 26% were CD14. Morphologically, the cells were smaller in size with lower nuclear/cytoplasmic ratio. The nucleus was indented and nucleoli markedly reduced. However, 10 microM SB203580 had little effect on HL-60 cell growth and survival during the first 72 h, but by 96 h the percentage of cells in G1 phase was markedly increased. These effects of SB203580 were not attributable to its inhibition of p38 kinase activity. Instead, the essential kinases in the extracellular signal-regulated kinase (ERK) pathway such as phospho-Raf-1, phospho-MEK1/2, phospho-ERK1/2 and phospho-p90RSK were all elevated dramatically shortly after cells were exposed to SB203580 and lasted for 24 h before declining. Pre-incubation of cells with 20 microM of PD98059 1 h before addition of SB203580 could completely block the expression of differentiation markers. Our results suggest that SB203580-induced differentiation in HL-60 cells was mediated by activation of MEK/ERK signaling. In conclusion, our data have shown that SB203580 possessed biological activities other than inhibition of p38 and these activities could make it a potential candidate as an inducing agent for cell differentiation in the therapeutic treatment of leukemia.

U0126 and PD98059, specific inhibitors of MEK, accelerate differentiation of RAW264.7 cells into osteoclast-like cells

Osteoclasts are multinucleated cells that differentiate from hematopoietic cells and possess characteristics responsible for bone resorption. To study the involvement of mitogen-activated protein kinases (MAPKs) in osteoclastogenesis of the murine monocytic cell line RAW264.7, which can differentiate into osteoclast-like cells in the presence of the receptor activator of nuclear factor kappa B ligand (RANKL), we treated the cells with specific inhibitors of p38 MAPK, PD169316 and SB203580, and specific inhibitors of MAPK extracellular signaling-regulated kinase (ERK) kinase (MEK), U0126 and PD98059. Each inhibitor blocked differentiation into osteoclast-like cells when the cells were plated at the standard cell density (2000-4000 cells per well (96-well)). However, the effect of MEK inhibitors on osteoclastogenesis varied according to the initial cell density during culture, because cell growth was clearly inhibited by them. When the cells were plated at more than 8000 cells per well, marked enhancement and acceleration of the differentiation were observed. In addition, immunoblot analysis revealed that phosphorylation of ERK was increased by treatment with the p38 inhibitors, whereas the MEK inhibitors increased phosphorylation of p38, which implies a seesaw-like balance between ERK and p38 phosphorylation. We suggest that osteoclastogenesis is regulated under a balance between ERK and p38 pathways and that the MEK/ERK pathway negatively regulates osteoclastogenesis while the p38 pathway does so positively. This is the first report that an inhibitor of signal transduction enhanced osteoclastogenesis.

Regulation of growth factor receptors by gangliosides

Since their discovery in the 1940s, gangliosides have been associated with a number of biological processes, such as growth, differentiation, and toxin uptake. Hypotheses about regulation of these processes by gangliosides are based on indirect observations and lack a clear definition of their mechanisms within the cell. The first insights were provided when a reduction in cell proliferation in the presence of gangliosides was attributed to inhibition of the epidermal growth factor receptor (EGFR). Since that initial finding, most, if not all, growth factor receptors have been described as regulated by gangliosides. In this review, we describe the effects of gangliosides on growth factor receptors, beginning with a list of known effects of gangliosides on growth factor receptors; we then present three models based on fibroblast growth factor (FGFR), platelet-derived growth factor receptor (PDGFR), and EGFR. We focus first on ganglioside modulation of ligand binding; second, we discuss ganglioside regulation of receptor dimerization; and third, we describe a model that implicates gangliosides with receptor activation state and subcellular localization. The methodology used to develop the three models may be extended to all growth factor receptors, bearing in mind that the three models may not be mutually exclusive. We believe that gangliosides do not act independently of many well-established mechanisms of receptor regulation, such as clathrin-coated pit internalization and ubiquitination, but that gangliosides contribute to these functions and to signal transduction pathways. We hypothesize a role for the diverse structures of gangliosides in biology through the organization of the plasma membrane into lipid raft microdomains of unique ganglioside composition, which directly affect the signal duration and membrane localization of the growth factor receptor.

Dinoflagellate-cyanobacterium communication may determine the composition of phytoplankton assemblage in a mesotrophic lake

The reasons for annual variability in the composition of phytoplankton assemblages are poorly understood but may include competition for resources and allelopathic interactions. We show that domination by the patch-forming dinoflagellate, Peridinium gatunense, or, alternatively, a bloom of a toxic cyanobacterium, Microcystis sp., in the Sea of Galilee may be accounted for by mutual density-dependent allelopathic interactions. Over the last 11 years, the abundance of these species in the lake displayed strong negative correlation. Laboratory experiments showed reciprocal, density-dependent, but nutrient-independent, inhibition of growth. Application of spent P. gatunense medium induced sedimentation and, subsequently, massive lysis of Microcystis cells within 24 hr, and sedimentation and lysis were concomitant with a large rise in the level of McyB, which is involved in toxin biosynthesis by Microcystis. P. gatunense responded to the presence of Microcystis by a species-specific pathway that involved a biphasic oxidative burst and activation of certain protein kinases. Blocking this recognition by MAP-kinase inhibitors abolished the biphasic oxidative burst and affected the fate (death or cell division) of the P. gatunense cells. We propose that patchy growth habits may confer enhanced defense capabilities, providing ecological advantages that compensate for the aggravated limitation of resources in the patch. Cross-talk via allelochemicals may explain the phytoplankton assemblage in the Sea of Galilee.