Structure, mapping, and expression of erp, a growth factor-inducible gene encoding a nontransmembrane protein tyrosine phosphatase, and effect of ERP on cell growth

Knockdown of TRIM9 attenuates irinotecan‑induced intestinal mucositis in IEC‑6 cells by regulating DUSP6 expression via the P38 pathway

Intestinal mucositis is a common side effect of cancer chemotherapy and it limits the dose of chemotherapy given to a patient. Tripartite motif family (TRIM) proteins have been reported to be implicated in the regulation of cancer chemotherapy. The present study aimed to investigate the effect of TRIM9 on irinotecan‑induced intestinal mucositis in the rat intestinal epithelial cell line IEC‑6. The expression of several TRIMs, such as TRIM1, TRIM9, TRIM18, TRIM36, TRIM46 and TRIM67, was examined. After TRIM9 knockdown or overexpression by lentivirus infection, cell proliferation and apoptosis, epithelial barrier tight‑junction proteins, inflammatory cytokines, transepithelial electrical resistance (TEER) and FITC dextran were measured. Treatment with irinotecan significantly inhibited cell proliferation and induced cell apoptosis, TRIM9 expression, intestinal mucosal barrier impairment, the levels of inflammatory cytokines and P38 phosphorylation in IEC‑6 cells, while the expression levels of epithelial barrier tight‑junction protein ZO‑1 and Claudin‑4 were decreased. Knockdown of TRIM9 partly counteracted the effect of irinotecan treatment, and inhibition of P38 potently reversed the effect of TRIM9 overexpression in IEC‑6 cells. Moreover, co‑immunoprecipitation showed an interaction between TRIM9 and DUSP6 in IEC‑6 cells, and overexpression of DUSP6 notably counteracted the effect of TRIM9 overexpression. The results demonstrated that TRIM9 knockdown may benefit patients with intestinal mucositis by inhibiting inflammatory cytokine expression and repairing intestinal barrier functions, which was probably due to inhibition of the activation of the P38 pathway via targeting DUSP6.

Depletion of cyclic-GMP levels and inhibition of cGMP-dependent protein kinase activate p21Cip1 /p27Kip1 pathways and lead to renal fibrosis and dysfunction

Cell-cycle regulatory proteins (p21^Cip1 /p27^Kip1 ) inhibit cyclin and cyclin-dependent kinase (CDK) complex that promotes fibrosis and hypertrophy. The present study examined the role of CDK blockers, p21^Cip1 /p27^Kip1 in the progression of renal fibrosis and dysfunction using Npr1 (encoding guanylyl cyclase/natriuretic peptide receptor-A, GC-A/NPRA) gene-knockout (0-copy; Npr1^-/- ), 2-copy (Npr1^+/+ ), and 4-copy (Npr1^++/++ ) mice treated with GC inhibitor, A71915 and cGMP-dependent protein kinase (cGK) inhibitor, (Rp-8-Br-cGMPS). A significant decrease in renal cGMP levels and cGK activity was observed in 0-copy mice and A71915- and Rp-treated 2-copy and 4-copy mice compared with controls. An increased phosphorylation of Erk1/2, p38, p21^Cip1 , and p27^Kip1 occurred in 0-copy and A71915-treated 2-copy and 4-copy mice, while Rp treatment caused minimal changes than controls. Pro-inflammatory (TNF-α, IL-6) and pro-fibrotic (TGF-β1) cytokines were significantly increased in plasma and kidneys of 0-copy and A71915-treated 2-copy mice, but to lesser extent in 4-copy mice. Progressive renal pathologies, including fibrosis, mesangial matrix expansion, and tubular hypertrophy were observed in 0-copy and A71915-treated 2-copy and 4-copy mice, but minimally occurred in Rp-treated mice compared with controls. These results indicate that Npr1 has pivotal roles in inhibiting renal fibrosis and hypertrophy and exerts protective effects involving cGMP/cGK axis by repressing CDK blockers p21^Cip1 and p27^Kip1 .

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

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

Dual-specificity phosphatase 6 deletion protects the colonic epithelium against inflammation and promotes both proliferation and tumorigenesis

The Ras/mitogen‐activated protein kinase (MAPK) pathway controls fundamental cellular processes such as proliferation, differentiation, and apoptosis. The dual‐specificity phosphatase 6 (DUSP6) regulates cytoplasmic MAPK signaling by dephosphorylating and inactivating extracellular signal‐regulated kinase (ERK1/2) MAPK. To determine the role of DUSP6 in the maintenance of intestinal homeostasis, we characterized the intestinal epithelial phenotype of Dusp6 knockout (KO) mice under normal, oncogenic, and proinflammatory conditions. Our results show that loss of Dusp6 increased crypt depth and epithelial cell proliferation without altering colonic architecture. Crypt regeneration capacity was also enhanced, as revealed by ex vivo Dusp6 KO organoid cultures. Additionally, loss of Dusp6 induced goblet cell expansion without affecting enteroendocrine and absorptive cell differentiation. Our data also demonstrate that Dusp6 KO mice were protected from acute dextran sulfate sodium‐induced colitis, as opposed to wild‐type mice. In addition, Dusp6 gene deletion markedly enhanced tumor load in Apc^Min/+ mice. Decreased DUSP6 expression by RNA interference in HT29 colorectal cancer cells enhanced ERK1/2 activation levels and promoted both anchorage‐independent growth in soft agar as well as invasion through Matrigel. Finally, DUSP6 mRNA expression in human colorectal tumors was decreased in advanced stage tumors compared with paired normal tissues. These results demonstrate that DUSP6 phosphatase, by controlling ERK1/2 activation, regulates colonic inflammatory responses, and protects the intestinal epithelium against oncogenic stress.

Role and regulation of MKP-1 in airway inflammation

Mitogen-activated protein kinase (MAPK) phosphatase 1 (MKP-1) is a protein with anti-inflammatory properties and the archetypal member of the dual-specificity phosphatases (DUSPs) family that have emerged over the past decade as playing an instrumental role in the regulation of airway inflammation. Not only does MKP-1 serve a critical role as a negative feedback effector, controlling the extent and duration of pro-inflammatory MAPK signalling in airway cells, upregulation of this endogenous phosphatase has also emerged as being one of the key cellular mechanism responsible for the beneficial actions of clinically-used respiratory medicines, including β₂-agonists, phosphodiesterase inhibitors and corticosteroids. Herein, we review the role and regulation of MKP-1 in the context of airway inflammation. We initially outline the structure and biochemistry of MKP-1 and summarise the multi-layered molecular mechanisms responsible for MKP-1 production more generally. We then focus in on some of the key in vitro studies in cell types relevant to airway disease that explain how MKP-1 can be regulated in airway inflammation at the transcriptional, post-translation and post-translational level. And finally, we address some of the potential challenges with MKP-1 upregulation that need to be explored further to fully exploit the potential of MKP-1 to repress airway inflammation in chronic respiratory disease.

Mitogen-activated protein kinase phosphatase 1 (MKP-1) in macrophage biology and cardiovascular disease. A redox-regulated master controller of monocyte function and macrophage phenotype

MAPK pathways play a critical role in the activation of monocytes and macrophages by pathogens, signaling molecules and environmental cues and in the regulation of macrophage function and plasticity. MAPK phosphatase 1 (MKP-1) has emerged as the main counter-regulator of MAPK signaling in monocytes and macrophages. Loss of MKP-1 in monocytes and macrophages in response to metabolic stress leads to dysregulation of monocyte adhesion and migration, and gives rise to dysfunctional, proatherogenic monocyte-derived macrophages. Here we review the properties of this redox-regulated dual-specificity MAPK phosphatase and the role of MKP-1 in monocyte and macrophage biology and cardiovascular diseases.

The Role of the MAPK Signaling, Topoisomerase and Dietary Bioactives in Controlling Cancer Incidence

Reactive oxygen species (ROS) are common products of mitochondrial oxidative phosphorylation, xenobiotics metabolism and are generated in response to several environmental stress conditions. Some of them play important biochemical roles in cellular signal transduction and gene transcription. On the other hand, ROS are known to be involved in a wide range of human diseases, including cancer. The excessive production of such ROS together with disruption of homeostasis detoxifying mechanisms can mediate a series of cellular oxidative stresses. The oxidative stress of redundant free radicals production can lead to oxidative denaturation of cellular macromolecules including proteins, lipids and DNA. Moreover, oxidative damage is one of the major causes of DNA mutations, replication errors and genomic abnormalities which result in either inhibition or induction of transcription, and end with the disturbance of signal transduction pathways. Among affected signaling pathways are redox-sensitive kinases. The stimulation of these kinases induces several transcription factors through the phosphorylation of their module proteins. The activation of such pathways induces proliferation and cellular transformation. A diet rich in antioxidant compounds has potential health benefits, and there is a growing interest in the role of natural antioxidants in nutrition for prevention and cure of cancer diseases. A controversy has risen regarding the relation between antioxidants and the significant decrease in the risk of cancer incidence. In this review, we will focus on redox-sensitive kinases signaling pathways, highlighting the effects of dietary antioxidant on the prevention, incidence, prognosis or even treatment of human cancers. In addition, we will place emphasis on the chemical classes of pterocarpans as natural anti-oxidants/cancers as well as their underlying mechanisms of action, including their effects on MAPKs and topoisomerase activities.

Role for DUSP1 (dual-specificity protein phosphatase 1) in the regulation of autophagy

Accumulating evidence suggests that mitogen-activated protein kinases (MAPKs) regulate macroautophagy/autophagy. However, the involvement of dual-specificity protein phosphatases (DUSPs), endogenous inhibitors for MAPKs, in autophagy remains to be determined. Here we report that DUSP1/MKP-1, the founding member of the DUSP family, plays a critical role in regulating autophagy. Specifically, we demonstrate that DUSP1 knockdown by shRNA in human ovarian cancer CAOV3 cells and knockout in murine embryonic fibroblasts, increases both basal and rapamycin-increased autophagic flux. Overexpression of DUSP1 had the opposite effect. Importantly, knockout of Dusp1 promoted phosphorylation of ULK1 at Ser555, and BECN1/Beclin 1 at Ser15, and the association of PIK3C3/VPS34, ATG14, BECN1 and MAPK, leading to the activation of the autophagosome-initiating class III phosphatidylinositol 3-kinase (PtdIns3K) complex. Furthermore, knockdown and pharmacological inhibitor studies indicated that DUSP1-mediated suppression of autophagy reflected inactivation of the MAPK1-MAPK3 members of the MAPK family. Knockdown of DUSP1 sensitized CAOV3 cells to rapamycin-induced antigrowth activity. Moreover, CAOV3-CR cells, a line that had acquired cisplatin resistance, exhibited an elevated DUSP1 level and were refractory to rapamycin-induced autophagy and cytostatic effects. Knockdown of DUSP1 in CAOV3-CR cells restored sensitivity to rapamycin. Collectively, this work identifies a previously unrecognized role for DUSP1 in regulating autophagy and suggests that suppression of DUSP1 may enhance the therapeutic activity of rapamycin.

Anti-Inflammatory Effects of β2-Receptor Agonists Salbutamol and Terbutaline Are Mediated by MKP-1

Mitogen-activated protein kinase phosphatase 1 (MKP-1) expression is induced by inflammatory factors, and it is an endogenous suppressor of inflammatory response. MKP-1 expression is increased by PDE4 inhibitor rolipram suggesting that it is regulated by cAMP-enhancing compounds. Therefore, we investigated the effect of β₂-receptor agonists on MKP-1 expression and inflammatory response. We found that β₂-receptor agonists salbutamol and terbutaline, as well as 8-Br-cAMP, increased MKP-1 expression. Salbutamol and terbutaline also inhibited p38 MAPK phosphorylation and TNF production in J774 mouse macrophages. Interestingly, salbutamol suppressed carrageenan-induced paw inflammation in wild-type mice, but the effect was attenuated in MKP-1(-/-) mice. In conclusion, these data show that β₂-receptor agonists increase MKP-1 expression, which seems to mediate, at least partly, the observed anti-inflammatory effects of β₂-receptor agonists.

Regulation of angiopoietin-1/Tie-2 receptor signaling in endothelial cells by dual-specificity phosphatases 1, 4, and 5

Background

Angiopoietin‐1 (Ang‐1) promotes survival and migration of endothelial cells, in part through the activation of mitogen‐activated protein kinase (MAPK) pathways downstream of Tie‐2 receptors. Dual‐specificity phosphatases (DUSPs) dephosphorylate phosphotyrosine and phosphoserine/phosphothreonine residues on target MAPKs. The mechanisms by which DUSPs modulate MAPK activation in Ang‐1/Tie‐2 receptor signaling are unknown in endothelial cells.

Methods and Results

Expression of various DUSPs in human umbilical vein endothelial cells exposed to Ang‐1 was measured. The functional roles of DUSPs in Ang‐1‐induced regulation of MAPK activation, endothelial cell survival, migration, differentiation, and permeability were measured using selective siRNA oligos. Ang‐1 differentially induces DUSP1, DUSP4, and DUSP5 in human umbilical vein endothelial cells through activation of the PI‐3 kinase, ERK1/2, p38, and SAPK/JNK pathways. Lack‐of‐function siRNA screening revealed that DUSP1 preferentially dephosphorylates p38 protein and is involved in Ang‐1‐induced cell migration and differentiation. DUSP4 preferentially dephosphorylates ERK1/2, p38, and SAPK/JNK proteins and, under conditions of serum deprivation, is involved in Ang‐1‐induced cell migration, several antiapoptotic effects, and differentiation. DUSP5 preferentially dephosphorylates ERK1/2 proteins and is involved in cell survival and inhibition of permeability.

Conclusions

DUSP1, DUSP4, and DUSP5 differentially modulate MAPK signaling pathways downstream of Tie‐2 receptors, thus highlighting the importance of these phosphatases to Ang‐1‐induced angiogenesis.

Anti-inflammatory effects of mapracorat, a novel selective glucocorticoid receptor agonist, is partially mediated by MAP kinase phosphatase-1 (MKP-1)

Mapracorat is a novel selective glucocorticoid receptor agonist (SEGRA), structurally distinct from corticosteroids. In preclinical studies, mapracorat potently inhibits the production of a variety of inflammatory mediators including cytokines and prostaglandin E2 (PGE(2)), with limited side effects associated with traditional corticosteroids. The objective of this study was to delineate the mechanisms underlying the anti-inflammatory properties of mapracorat. We found that mapracorat potently inhibited the production of GM-CSF and TNF-α in LPS-stimulated Raw 264.7 macrophages. Mapracorat also substantially attenuated the expression of COX-2 and the production of PGE(2). The inhibition of mapracorat on the inflammatory response was dose-dependent, and substantially inhibitory effects were observed at concentrations in the 10-100 nm range. Examination of the activation kinetics of p38 and its downstream target MAPK-activated protein kinase-2 (MK-2) revealed a shortened activation course after LPS stimulation in cells pretreated with mapracorat. Supporting the notion that mapracorat augments a feedback control mechanism restraining the p38 pathway, we found that mapracorat enhanced the expression of MAPK phosphatase-1 (MKP-1), a critical negative regulator of MAPKs that drive the production of cytokines and other inflammatory mediators. While mapracorat alone did not stimulate MKP-1 expression, it markedly enhanced the expression of MKP-1 in cells stimulated by LPS, in a similar manner and potency to the augmenting effect of dexamethasone. Blocking MKP-1 expression by triptolide also abolished the accelerating effects of mapracorat on p38 and MK-2 deactivation, further supporting a role of MKP-1 in the anti-inflammatory mechanism of mapracorat. Taken together, these results indicate that mapracorat exerts its anti-inflammatory effects, at least in part, by augmenting MKP-1 expression.

Diversity and specificity of the mitogen-activated protein kinase phosphatase-1 functions

The balance of protein phosphorylation is achieved through the actions of a family of protein serine/threonine kinases called the mitogen-activated protein kinases (MAPKs). The propagation of MAPK signals is attenuated through the actions of the MAPK phosphatases (MKPs). The MKPs specifically inactivate the MAPKs by direct dephosphorylation. The archetypal MKP family member, MKP-1 has garnered much of the attention amongst its ten other MKP family members. Initially viewed to play a redundant role in the control of MAPK signaling, it is now clear that MKP-1 exerts profound regulatory functions on the immune, metabolic, musculoskeletal and nervous systems. This review focuses on the physiological functions of MKP-1 that have been revealed using mouse genetic approaches. The implications from studies using MKP-1-deficient mice to uncover the role of MKP-1 in disease will be discussed.

Mitogen-activated protein kinase phosphatase (MKP)-1 in immunology, physiology, and disease

Mitogen-activated protein kinases (MAPKs) are key regulators of cellular physiology and immune responses, and abnormalities in MAPKs are implicated in many diseases. MAPKs are activated by MAPK kinases through phosphorylation of the threonine and tyrosine residues in the conserved Thr-Xaa-Tyr domain, where Xaa represents amino acid residues characteristic of distinct MAPK subfamilies. Since MAPKs play a crucial role in a variety of cellular processes, a delicate regulatory network has evolved to control their activities. Over the past two decades, a group of dual specificity MAPK phosphatases (MKPs) has been identified that deactivates MAPKs. Since MAPKs can enhance MKP activities, MKPs are considered as an important feedback control mechanism that limits the MAPK cascades. This review outlines the role of MKP-1, a prototypical MKP family member, in physiology and disease. We will first discuss the basic biochemistry and regulation of MKP-1. Next, we will present the current consensus on the immunological and physiological functions of MKP-1 in infectious, inflammatory, metabolic, and nervous system diseases as revealed by studies using animal models. We will also discuss the emerging evidence implicating MKP-1 in human disorders. Finally, we will conclude with a discussion of the potential for pharmacomodulation of MKP-1 expression.

Mitogen-activated protein kinase phosphatase-1 - a potential therapeutic target in metabolic disease

IMPORTANCE OF THE FIELD

Metabolic disease, which is associated with obesity and cardiovascular disease, is a worldwide epidemic. There continues to be a tremendous effort towards the development of therapies to curtail obesity and its associated pathophysiological sequelae. MAPKs have been implicated in metabolic disease suggesting that these enzymes, and those that regulate them, can potentially serve as therapeutic targets to combat this disease. The MAPK phosphatase-1 (MKP-1) mediates the dephosphorylation and inactivation of MAPKs in insulin-responsive tissues. Therefore, the actions of MKP-1 may play an important role in the maintenance of metabolic homeostasis.

AREAS COVERED IN THIS REVIEW

The functional effects of MKP-1 in MAPK regulation with emphasis on its role in physiological and pathophysiological signaling functions that have been elucidated through the use of mouse genetics.

WHAT THE READER WILL GAIN

The reader will learn that MAPK inactivation through the effects of MKP-1 is essential for the maintenance of metabolic homeostasis. We will convey the idea that MKP-1 acts as a critical signaling node in MAPK-mediated regulation of cell signaling and metabolism.

TAKE HOME MESSAGE

Pharmacological inactivation of MKP-1 may be of therapeutic value in the treatment of obesity and possibly other metabolic disorders.

Role of RNA-Binding Proteins in MAPK Signal Transduction Pathway

Mitogen-activated protein kinases (MAPKs), which are found in all eukaryotes, are signal transducing enzymes playing a central role in diverse biological processes, such as cell proliferation, sexual differentiation, and apoptosis. The MAPK signaling pathway plays a key role in the regulation of gene expression through the phosphorylation of transcription factors. Recent studies have identified several RNA-binding proteins (RBPs) as regulators of MAPK signaling because these RBPs bind to the mRNAs encoding the components of the MAPK pathway and regulate the stability of their transcripts. Moreover, RBPs also serve as targets of MAPKs because MAPK phosphorylate and regulate the ability of RBPs to bind and stabilize target mRNAs, thus controlling various cellular functions. In this review, we present evidence for the significance of the MAPK signaling in the regulation of RBPs and their target mRNAs, which provides additional information about the regulatory mechanism underlying gene expression. We further present evidence for the clinical importance of the posttranscriptional regulation of mRNA stability and its implications for drug discovery.

Mechanism of glucocorticoid protection of airway smooth muscle from proasthmatic effects of long-acting beta2-adrenoceptor agonist exposure

BACKGROUND

Chronic use of long-acting beta2-adrenergic receptor agonists (LABAs), resulting in beta2-adrenergic receptor desensitization, has been associated with increased asthma morbidity. When LABAs are used in combination with inhaled glucocorticoids, however, asthma control is improved, raising the following question: Do glucocorticoids inhibit the proasthmatic mechanism that mediates altered contractility in LABA-exposed airway smooth muscle (ASM)?

OBJECTIVE

This study aimed to identify the potential protective role and mechanism of action of glucocorticoids in mitigating the effects of prolonged LABA exposure on ASM constrictor and relaxation responsiveness.

METHODS

Cultured human ASM cells and isolated rabbit ASM tissues were examined for induced changes in agonist-mediated cyclic adenosine monophosphate accumulation, constrictor and relaxation responsiveness, and expression of specific glucocorticoid-regulated molecules after 24-hour exposure to the LABA salmeterol in the absence and presence of dexamethasone.

RESULTS

Salmeterol-exposed ASM exhibited impaired cyclic adenosine monophosphate and relaxation responses to isoproterenol and increased acetylcholine-induced contractility. These proasthmatic effects of prolonged LABA exposure were attributed to upregulated phosphodiesterase 4 (PDE4) activity and were ablated by pretreatment with dexamethasone. Further studies demonstrated that (1) dexamethasone suppressed activation of the mitogen-activated protein kinases extracellular signal-regulated kinases 1 and 2 (ERK1/2), which upregulate PDE4 expression in salmeterol-exposed ASM; and (2) the inhibitory actions of dexamethasone on salmeterol-induced ERK1/2 activation and resultant PDE4-mediated changes in ASM responsiveness were prevented by gene silencing or pharmacologic inhibition of dexamethasone-induced expression of mitogen-activated protein kinase phosphatase 1, an endogenous deactivator of ERK1/2 signaling.

CONCLUSION

Glucocorticoids prevent the adverse proasthmatic effects of prolonged LABA exposure on airway responsiveness as a result of glucocorticoid-induced upregulation of mitogen-activated protein kinase phosphatase 1, which inhibits proasthmatic ERK1/2 signaling in the LABA-exposed ASM.

Distinct roles for mitogen-activated protein kinase phosphatase-1 (MKP-1) and ERK-MAPK in PTH1R signaling during osteoblast proliferation and differentiation

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) activate one single receptor (PTH1R) which mediates catabolic and anabolic actions in the bone. Activation of PTH1R modulates multiple intracellular signaling responses. We previously reported that PTH and PTHrP down-regulate pERK1/2 and cyclin D1 in differentiated osteoblasts. In this study we investigate the role of MAPK phosphatase-1 (MKP-1) in PTHrP regulation of ERK1/2 activity in relation to osteoblast proliferation, differentiation and bone formation. Here we show that PTHrP increases MKP-1 expression in differentiated osteoblastic MC3T3-E1 cells, primary cultures of differentiated bone marrow stromal cells (BMSCs) and calvarial osteoblasts. PTHrP had no effect on MKP-1 expression in proliferating osteoblastic cells. Overexpression of MKP-1 in MC-4 cells inhibited osteoblastic cell proliferation. Cell extracts from differentiated MC-4 cells treated with PTHrP inactivate/dephosphorylate pERK1/2 in vitro; immunodepletion of MKP-1 blocked the ability of the extract to dephosphorylate pERK1/2; these data indicate that MKP-1 is involved in PTHrP-induced pERK1/2 dephosphorylation in the differentiated osteoblastic cells. PTHrP regulation of MKP-1 expression is partially dependent on PKA and PKC pathways. Treatment of nude mice, bearing ectopic ossicles, with intermittent PTH for 3weeks, up-regulated MKP-1 and osteocalcin, a bone formation marker, with an increase in bone formation. These data indicate that PTH and PTHrP increase MKP-1 expression in differentiated osteoblasts; and that MKP-1 induces growth arrest of osteoblasts, via inactivating pERK1/2 and down-regulating cyclin D1; and identify MKP-1 as a possible mediator of the anabolic actions of PTH1R in mature osteoblasts.

Involvement of MKP-1 and Bcl-2 in acquired cisplatin resistance in ovarian cancer cells

Although cisplatin is a very effective anticancer agent against several types of cancer including ovarian cancer, the mechanisms of acquired resistance are not fully understood. By chronically exposing cisplatin to ovarian cancer cell lines, we established two cisplatin-resistant cell lines OV433 and TOV112D. Our results indicate that the mechanisms underlying their cisplatin resistance are distinct. In OV433 cells, cisplatin resistance is associated with increased expression of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1). By knocking down MKP-1 expression by siRNA or inhibiting MKP-1 expression by its pharmacological inhibitor triptolide, cisplatin-resistant OV433 cells became cisplatin-sensitive and subsequently increased cisplatin-induced apoptosis. In TOV112D cells, on the other hand, acquired cisplatin resistance is associated with increased levels of Bcl-2 protein. By inhibiting the activity of Bcl-2 protein with its pharmacological inhibitor gossypol or knocking down Bcl-2 expression by siRNA, cisplatin-resistant TOV112D cells became cisplatin-sensitive and subsequently increased cisplatin-induced apoptosis. Therefore, our data suggest that the mechanisms of acquired cisplatin resistance vary among ovarian cancer cells, which involve upregulation of molecules associated with the cell survival pathways.

MKP-1 is necessary for T cell activation and function

MAPKs are evolutionarily conserved immune regulators. MAPK phosphatases (MKPs) that negatively regulate MAPK activities have recently emerged as critical players in both innate and adaptive immune responses. MKP-1, also known as DUSP1, was previously shown to negatively regulate innate immunity by inhibiting pro-inflammatory cytokine production. Here, we found that MKP-1 is necessary in T cell activation and function. MKP-1 deficiency in T cells impaired the activation, proliferation, and function of T cells in vitro, associated with enhanced activation of JNK and reduced NFATc1 translocation into the nucleus. Consistently, MKP-1(-/-) mice were defective in anti-influenza immunity in vivo and resistant to experimental autoimmune encephalomyelitis. Our results thus demonstrate that MKP-1 is a critical positive regulator of T cell activation and function and may be targeted in treatment of autoimmune diseases.

Estrogen inhibits glucocorticoid action via protein phosphatase 5 (PP5)-mediated glucocorticoid receptor dephosphorylation

Although glucocorticoids suppress proliferation of many cell types and are used in the treatment of certain cancers, trials of glucocorticoid therapy in breast cancer have been a disappointment. Another suggestion that estrogens may affect glucocorticoid action is that the course of some inflammatory diseases tends to be more severe and less responsive to corticosteroid treatment in females. To date, the molecular mechanism of cross-talk between estrogens and glucocorticoids is poorly understood. Here we show that, in both MCF-7 and T47D breast cancer cells, estrogen inhibits glucocorticoid induction of the MKP-1 (mitogen-activated protein kinase phosphatase-1) and serum/glucocorticoid-regulated kinase genes. Estrogen did not affect glucocorticoid-induced glucocorticoid receptor (GR) nuclear translocation but reduced ligand-induced GR phosphorylation at Ser-211, which is associated with the active form of GR. We show that estrogen increases expression of protein phosphatase 5 (PP5), which mediates the dephosphorylation of GR at Ser-211. Gene knockdown of PP5 abolished the estrogen-mediated suppression of GR phosphorylation and induction of MKP-1 and serum/glucocorticoid-regulated kinase. More importantly, after PP5 knockdown estrogen-promoted cell proliferation was significantly suppressed by glucocorticoids. This study demonstrates cross-talk between estrogen-induced PP5 and GR action. It also reveals that PP5 inhibition may antagonize estrogen-promoted events in response to corticosteroid therapy.

H89, an inhibitor of PKA and MSK, inhibits cyclic-AMP response element binding protein-mediated MAPK phosphatase-1 induction by lipopolysaccharide

OBJECTIVE

Lipopolysaccharide (LPS) stimulates the production of inflammatory cytokines and the amplification of immune responses via MAPK pathways. MAPK phosphatases (MKPs) feedback-regulate the activities of MAPKs to prevent excessive immunological functions. H89 has been used as an inhibitor of the protein kinase A (PKA) and mitogen- and stress-activated protein kinase (MSK) pathways. In view of the potential roles of PKA and MSK for MKP-1 induction and the ability of H89 to inhibit these kinases, this study examined the effect of H89 on MKP-1 induction by LPS and the role of cyclic-AMP response element binding protein (CREB) in the MKP-1 induction.

RESULTS

H89 treatment inhibited increases in MKP-1 protein and mRNA levels, and gene transcription by LPS in Raw264.7 cells. Immunoblot, gel-shift, and chromatin-immunoprecipitation assays showed the activation of CREB by LPS, and the ability of H89 to inhibit it, suggesting that H89's inhibition of CREB may affect MKP-1 induction. In addition, H89 prevented the ability of LPS to induce other MKP genes (Dusp-2, 4, 8, and 16). Experiments using MAPK inhibitors showed that MAPKs are involved in CREB phosphorylation and MKP-1 induction, suggesting that CREB-mediated MKP-1 induction serves in part as a feedback-inhibitory loop of MAPKs.

CONCLUSION

Our results demonstrate that H89 inhibits the activation of CREB and the CREB-mediated MKP-1 induction by LPS, which may result from its inhibition of PKA and MSK.

A novel mitogen-activated protein kinase phosphatase-1 and glucocorticoid receptor (GR) interacting protein-1-dependent combinatorial mechanism of gene transrepression by GR

Glucocorticoids have major antiinflammatory effects. Because COX-2 is the rate-limiting enzyme for proinflammatory prostaglandins, this study investigated the combinatorial inhibitory role of glucocorticoid receptor (GR) in COX-2 gene induction in macrophages and sought to identify the molecular mechanisms for that inhibition. Glucocorticoid-activated GR repressed COX-2 gene induction by lipopolysaccharide (LPS). Activated GR inhibited LPS-induced activator protein 1 activity, which in turn decreased activating transcription factor 2/c-Jun phosphorylation. The inhibition of MAPK-dependent activating transcription factor 2/c-Jun phosphorylation by GR in COX-2 repression was a result of MAPK phosphatase-1 (MKP-1) induction. Although GR did not inhibit LPS-induced p65 phosphorylation or nuclear factor-kappaB DNA binding activity, deletion of the nuclear factor-kappaB binding site in the COX-2 gene suppressed the ability of glucocorticoid to attenuate COX-2 induction. Chromatin immunoprecipitation and transfection assays revealed that a p65 DNA complex involving GR-bound GR-interacting protein 1 (GRIP1) also contributed to COX-2 repression. Additional knockdown and transfection assays identified other inflammatory genes coordinately regulated by MKP-1 and GRIP1. In summary, activated GR was found to antagonize the LPS-dependent induction of the COX-2 gene via a novel combinatorial mechanism involving MKP-1-mediated activator protein 1 inhibition and GR/GRIP1 recruitment to the p65 DNA complex; moreover, this work facilitated the identification of other GR-responding MKP-1/GRIP1-regulated genes.

The glucocorticoid receptor signalling in breast cancer

Glucocorticoids (GCs) are provided as co-medication with chemotherapy in breast cancer, albeit several lines of evidence indicate that their use may have diverse effects and in fact may inhibit chemosensitivity. The molecular basis of GC-induced resistance to chemotherapy in breast cancer remains poorly defined. Recent researchers, in an attempt to clarify some aspects of the underlying pathways, provide convincing evidence that GCs induce effects that are dependent upon the glucocorticoid-receptor (GR)-mediated transcriptional regulation of specific genes known to play key roles in cellular/tissue functions, including growth, apoptosis, differentiation, metastasis and cell survival. In this review, we focus on how GC-induced chemoresistance in breast cancer is mediated by the GR, unravelling the molecular interplay of GR signalling with other signalling cascades prevalent in breast cancer. We also include a detailed description of GR structure and function, summarizing data gained during recent years into the mechanism(s) of the cross-talk between the GR and other signalling cascades and secondary messengers, via which GCs exert their pleiotropic effects.

Mitogen-activated protein kinase phosphatase-1 is required for cisplatin resistance

Mitogen-activated protein kinase (MAPK) phosphatase (MKP)-1 is a member of the MKP family that negatively regulates MAPK signaling. MKP-1 has been implicated in cell survival in response to stressful stimuli, including anticancer treatment, but its role in cisplatin resistance is not fully understood. Here, we show that cisplatin induces MKP-1 in several human cancer cell lines. Induction of MKP-1 by cisplatin was through the transcriptional mechanism regulated by extracellular signal-regulated kinase (ERK). Overexpression of MKP-1 rendered human lung cancer cells resistant to cisplatin. Conversely, down-regulation of MKP-1 by small interfering RNA silencing sensitized human lung cancer cells to cisplatin-induced cell death. Using primary mouse embryonic fibroblasts (MEF) from MKP-1 knockout mice, we show that induction of MKP-1 by cisplatin correlates with inactivation of c-Jun NH(2)-terminal kinase (JNK) but not ERK and p38. Furthermore, apoptosis induced by cisplatin was significant in MKP-1(-/-) MEFs, whereas such change was minimal in MKP-1(+/+) MEFs. More importantly, cisplatin-induced cell death is inhibited by blocking JNK but not ERK and p38 activities. Collectively, our results establish a critical role of JNK in cisplatin-induced apoptosis and suggest that MKP-1 is required for cisplatin resistance.

Specific activation of the glucocorticoid receptor and modulation of signal transduction pathways in human lens epithelial cells

PURPOSE

Prolonged use of glucocorticoids (GCs) can lead to cataract formation. Lens GC responses have been difficult to elucidate. A previous study showed the presence of the glucocorticoid receptor (GR) in immortalized and primary human lens epithelial cells (hLECs) and GC-induced changes in gene expression. This study demonstrates specific GR activation and identifies the biological effect of GC-induced changes in gene expression in hLECs.

METHODS

HLE B-3 (B-3) and primary cultures of hLECs were transfected with pGRE.Luc and treated with or without dexamethasone (Dex), RU-486, spironolactone, or vehicle. mRNA and protein expression were examined by real-time PCR and Western blot analysis, respectively. Cell proliferation and apoptosis were examined by WST-1 and flow cytometry, respectively.

RESULTS

Dex treatment of B-3 and primary cultures demonstrated specific GR, but not mineralocorticoid receptor (MR), activation and phosphorylation. Pathway analysis revealed GC-induced changes in expression of MAPK regulators. Increased expression of GILZ mRNA and MKP-1 mRNA and protein was observed in immortalized and donor hLECs. This corresponded with a decrease in the phosphorylated forms of RAF, ERK, p38, and AKT, but not in JNK. No net change in LEC proliferation or apoptosis was observed with Dex treatment.

CONCLUSIONS

GC treatment of hLECs activates the GR to modulate the expression of MAPK and PI3K/AKT regulators. This is the first demonstration of GC signaling in hLECs. GCs, MAPK, and PI3K/AKT are involved in cell processes implicated in steroid-induced cataractogenesis. The absence of a net change in cell activity with acute steroid treatment is consistent with the possibility that chronic treatment leads to prolonged modulation of these pathways and steroid-induced cataract.

The role of mitogen-activated protein kinase phosphatase-1 in oxidative damage-induced cell death

Mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) is a member of the MAPK phosphatase family that functions as a negative regulator of MAPK signaling. MKP-1 is induced by oxidative stress, but the role of its induction in cell death is not fully understood. Here, we show that hydrogen peroxide (H(2)O(2)) induces MKP-1 and activates MAPKs. Induction of MKP-1 by H(2)O(2) correlated with inactivation of p38 and c-Jun-NH(2)-kinase (JNK). Overexpression of MKP-1 increased cell resistance to H(2)O(2)-induced death. Furthermore, we show by small interfering RNA silencing that down-regulation of MKP-1 increases phosphorylated p38 and JNK and subsequent cell death induced by H(2)O(2). More importantly, primary embryonic fibroblasts from mice lacking MKP-1 had a higher level of phosphorylated p38 and JNK and were more sensitive to H(2)O(2)-induced cell death compared with corresponding cells with MKP-1, indicating that p38 and JNK pathways may play important roles in H(2)O(2)-mediated cell death. Thus, these results suggest that activation of MKP-1 is a survival mechanism against oxidative damage.

Protein kinase Czeta attenuates hypoxia-induced proliferation of fibroblasts by regulating MAP kinase phosphatase-1 expression

We have previously found that hypoxia stimulates proliferation of vascular fibroblasts through Galphai-mediated activation of ERK1/2. Here, we demonstrate that hypoxia also activates the atypical protein kinase Czeta (PKCzeta) isozyme and stimulates the expression of ERK1/2-specific phosphatase, MAP kinase phosphatase-1 (MKP-1), which attenuates ERK1/2-mediated proliferative signals. Replication repressor activity is unique to PKCzeta because the blockade of classical and novel PKC isozymes does not affect fibroblast proliferation. PKCzeta is phosphorylated upon prolonged (24 h) exposure to hypoxia, whereas ERK1/2, the downstream kinases, are maximally activated in fibroblasts exposed to acute (10 min) hypoxia. However, PKCzeta blockade results in persistent ERK1/2 phosphorylation and marked increase in hypoxia-induced replication. Similarly prolonged ERK1/2 phosphorylation and increase in hypoxia-stimulated proliferation are also observed upon blockade of MKP-1 activation. Because of the parallel suppressive actions of PKCzeta and MKP-1 on ERK1/2 phosphorylation and proliferation, the role of PKCzeta in the regulation of MKP-1 expression was evaluated. PKCzeta attenuation reduces MKP-1 expression, whereas PKCzeta overexpression increases MKP-1 levels. In conclusion, our results indicate for the first time that hypoxia activates PKCzeta, which acts as a terminator of ERK1/2 activation through the regulation of downstream target, MKP-1 expression and thus serves to limit hypoxia-induced proliferation of fibroblasts.

The noncatalytic amino terminus of mitogen-activated protein kinase phosphatase 1 directs nuclear targeting and serum response element transcriptional regulation

The mitogen-activated protein kinase (MAPK) phosphatase 1 (MKP-1) is an immediate-early gene comprised of a dual-specificity phosphatase domain and a noncatalytic NH(2) terminus. Here, we show that the NH(2) terminus of MKP-1, containing the cdc25 homology domains A (CH2A) and B (CH2B), mediates MKP-1 nuclear targeting and modulates MAPK-mediated gene expression. An LXXLL motif which is known to mediate protein-protein interactions with nuclear-targeted hormone receptors was identified proximal to the CH2A domain of MKP-1. The NH(2) terminus alone of MKP-1 containing this LXXLL motif was sufficient to direct nuclear targeting, and mutating this motif to LXXAA resulted in the exclusion of MKP-1 from the nucleus. We found that the LXXLL motif proximal to the CH2A domain was present in other nuclear-localized MKPs but was absent in MKPs that localized to the cytoplasm. These data suggest that this LXXLL motif confers nuclear targeting properties to the MKPs. The NH(2) terminus of MKP-1 was also found to inhibit the activation of the serum response element (SRE) by preventing MAPK-mediated phosphorylation of the regulatory serine 383 residue on Elk-1. Moreover, we show that MKP-1 plays a major role in the attenuation of serum-induced SRE activity, since MKP-1 null fibroblasts exhibited enhanced SRE activity in response to serum compared with wild-type fibroblasts. The NH(2) terminus of MKP-1, when reconstituted into MKP-1 null fibroblasts to levels similar to endogenous MKP-1 following serum stimulation, reduced serum-mediated SRE activity. Collectively, these data reveal novel roles for the NH(2) terminus of MKP-1 in nuclear targeting and transcriptional regulation.

Glucocorticoids induce rapid up-regulation of mitogen-activated protein kinase phosphatase-1 and dephosphorylation of extracellular signal-regulated kinase and impair proliferation in human and mouse osteoblast cell lines

A central feature of glucocorticoid (GC)-induced osteoporosis is decreased bone formation, secondary to decreased numbers of functional osteoblasts. We find that ERK activity is essential for serum-induced osteoblast proliferation in vitro because inhibition of MAPK/ERK kinase activity by U0126 completely abolished both serum-induced activation of ERK and proliferation of mouse (MBA-15.4) and human (MG-63) osteoblast cell lines. Dexamethasone (Dex) rapidly (<2 h) inhibits the sustained phase of ERK activation, required for nuclear shift and mitogenesis. This inhibition is reversed by cotreatment with the protein synthesis inhibitor, cycloheximide, and by the GC receptor antagonist, RU486, suggesting a classical transcriptional mechanism. Phosphatase activity was up-regulated by Dex treatment, and inhibition of ERK activity by Dex was also reversed by the protein tyrosine phosphatase inhibitor, vanadate. Coupled with the rapidity of Dex action, this indicates immediate-early gene phosphatase involvement, and we therefore used quantitative, real-time PCR to examine expression profiles of the dual-specificity MAPK phosphatases, MKP-1 and MKP-3. MKP-1, but not MKP-3, mRNA expression was 10-fold up-regulated in both mouse and human osteoblast cell lines within 30 min of Dex treatment and remained elevated for 24 h. MKP-1 protein was also markedly up-regulated following 1-8 h of Dex treatment, and this correlated precisely with dephosphorylation of ERK. Cell proliferation was impaired by Dex treatment, and this was reversed by both RU486 and vanadate. Therefore, MKP-1 up-regulation provides a novel and rapid mechanism, whereby GCs inhibit osteoblast proliferation.

CL100 expression is down-regulated in advanced epithelial ovarian cancer and its re-expression decreases its malignant potential

Although early stage ovarian cancer can be effectively treated with surgery and chemotherapy, the majority of cases present with advanced disease, which remains essentially incurable. Unfortunately, little is known about the genes important for the development and progression of this disease. In this study, the expression of 68 phosphatases was determined in immortalized ovarian epithelial cells (IOSE) and compared to ovarian cancer cell lines. CL100, a dual specificity phosphatase, displayed 10-25-fold higher expression in normal compared to malignant ovarian cell lines. Immunohistochemical staining of normal ovaries and 68 ovarian cancer specimens confirmed this differential expression. Re-expression of CL100 in ovarian cancer cells decreased adherent and non-adherent cell growth and induced phenotypic changes including loss of filopodia and lamellipodia with an associated decrease in cell motility. Induced expression of CL100 in ovarian cancer cells suppressed intraperitoneal tumor growth in nude mice. These results show for the first time that CL100 expression is altered in human ovarian cancer, that CL100 expression changes cell morphology and motility, and that it suppresses intraperitoneal growth of human ovarian epithelial cancer. These data suggest that down-regulation of CL100 may play a role in the progression of human ovarian cancer.

The carboxyl-terminal domains of MKP-1 and MKP-2 have inhibitory effects on their phosphatase activity

Both the mitogen-activated protein kinase (MAPK) phosphatases MKP-1 and MKP-2 exert important feedback control of MAPK-mediated signaling events. The function of MKP-1 and MKP-2 is regulated via complex mechanisms, ranging from increased transcription of the MKP-1 and MKP-2 genes to post-translational catalytic activation of MKP-1 and MKP-2 proteins upon binding to their substrate MAPKs. In addition, MKP-1 stability increases upon ERK-dependent phosphorylation of two serine residues in its C-terminus. The C-terminal regions of MKP-1 and MKP-2, but not those of other MKPs, are homologous. To investigate the role of this domain, we have deleted the C-terminal tails from MKP-1 and MKP-2 and examined the effect of these deletions on their enzymatic activity. C-terminally truncated MKP-1 and MKP-2 exhibited, both in vivo and in vitro, substantially greater phosphatase activity towards their substrate MAPKs than did the full-length counterparts. However, C-terminal truncations did not significantly change either their substrate affinity, or their substrate-mediated catalytic activation. Basal phosphatase activity of the truncated proteins was also significantly higher than that of the wild-type counterparts. Collectively, these results suggest that the C-terminal domain may potentially play a role in the regulation of MKP-1 and MKP-2.

MAP kinase phosphatases

Mitogen-activated protein MAP kinases are key signal-transducing enzymes that are activated by a wide range of extracellular stimuli. They are responsible for the induction of a number of cellular responses, such as changes in gene expression, proliferation, differentiation, cell cycle arrest and apoptosis. Although regulation of MAP kinases by a phosphorylation cascade has long been recognized as significant, their inactivation through the action of specific phosphatases has been less studied. An emerging family of structurally distinct dual-specificity serine, threonine and tyrosine phosphatases that act on MAP kinases consists of ten members in mammals, and members have been found in animals, plants and yeast. Three subgroups have been identified that differ in exon structure, sequence and substrate specificity.

Glucocorticoids inhibit MAP kinase via increased expression and decreased degradation of MKP-1

Glucocorticoids inhibit the proinflammatory activities of transcription factors such as AP-1 and NF-kappa B as well as that of diverse cellular signaling molecules. One of these signaling molecules is the extracellular signal-regulated kinase (Erk-1/2) that controls the release of allergic mediators and the induction of proinflammatory cytokine gene expression in mast cells. The mechanism of inhibition of Erk-1/2 activity by glucocorticoids is unknown. Here we report a novel dual action of glucocorticoids for this inhibition. Glucocorticoids increase the expression of the MAP kinase phosphatase-1 (MKP-1) gene at the promoter level, and attenuate proteasomal degradation of MKP-1, which we report to be triggered by activation of mast cells. Both induction of MKP-1 expression and inhibition of its degradation are necessary for glucocorticoid-mediated inhibition of Erk-1/2 activation. In NIH-3T3 fibroblasts, although glucocorticoids up-regulate the MKP-1 level, they do not attenuate the proteasomal degradation of this protein and consequently they are unable to inhibit Erk-1/2 activity. These results identify MKP-1 as essential for glucocorticoid-mediated control of Erk-1/2 activation and unravel a novel regulatory mechanism for this anti-inflammatory drug.

Transcriptional induction of MKP-1 in response to stress is associated with histone H3 phosphorylation-acetylation

Mitogen-activated protein (MAP) kinase phosphatase 1 (MKP-1) has been shown to play a critical role in mediating the feedback control of MAP kinase cascades in a variety of cellular processes, including proliferation and stress responsiveness. Although MKP-1 expression is induced by a broad array of extracellular stimuli, the mechanisms mediating its induction remain poorly understood. Here we show that MKP-1 mRNA was potently induced by arsenite and ultraviolet light and modestly increased by heat shock and hydrogen peroxide. Interestingly, arsenite also dramatically induces phosphorylation-acetylation of histone H3 at a global level which precedes the induction of MKP-1 mRNA. The transcriptional induction of MKP-1, histone H3 modification, and elevation in MKP-1 mRNA in response to arsenite are all partially prevented by the p38 MAP kinase inhibitor SB203580, suggesting that the p38 pathway is involved in these processes. Finally, analysis of the DNA brought down by chromatin immunoprecipitation (ChIP) reveals that arsenite induces phosphorylation-acetylation of histone H3 associated with the MKP-1 gene and enhances binding of RNA polymerase II to MKP-1 chromatin. ChIP assays following exposure to other stress agents reveal various degrees of histone H3 modification at the MKP-1 chromatin. The differential contribution of p38 and ERK MAP kinases in mediating MKP-1 induction by different stress agents further illustrates the complexity and versatility of stress-induced MKP-1 expression. Our results strongly suggest that chromatin remodeling after stress contributes to the transcriptional induction of MKP-1.

Insulin regulates MAP kinase phosphatase-1 induction in Hirc B cells via activation of both extracellular signal-regulated kinase (ERK) and c-Jun-N-terminal kinase (JNK)

Previously, we have reported that insulin induces the expression of the dual-specificity tyrosine phosphatase Mitogen-activated protein (MAP) kinase phosphatase-1 (MKP-1) and that this may represent a negative feedback mechanism to regulate insulin-stimulated MAP kinase activity. In this work, the mechanism of regulation of MKP-1 expression by insulin was examined, particularly the role of the MAP kinase superfamily. Inhibition of the ERK pathway attenuated insulin-stimulated MKP-1 mRNA expression. Expression of dominant negative molecules of the JNK pathway also abolished insulin-stimulated MKP-1 expression. However, inhibition of p38MAPK activity by SB202190 had no effect on insulin-stimulated MKP-1 induction. Simultaneous inhibition of the ERK and JNK pathways abolished the ability of insulin to stimulate MKP-1 expression, however, this combined inhibition was neither additive nor synergistic, suggesting these pathways converge to act on a common final effector. In conclusion, induction of MKP-1 mRNA expression in Hirc B cells by insulin requires activation of both the ERK and JNK pathways, but not p38MAPK.

CL100/MKP-1 modulates JNK activation and apoptosis in response to cisplatin

Treatment of cells with cisplatin induces a sustained activation of the stress activated protein kinase SAPK/JNK and the mitogen-activated protein kinase p38. Activation of JNK by cisplatin is necessary for the induction of apoptosis. Expression of the MAPK phosphatases CL100/MKP-1 and hVH-5 selectively prevents JNK/SAPK activation by cisplatin in a dose dependent fashion and results in protection against cisplatin-induced apoptosis. In contrast, expression of the ERK-specific phosphatase Pyst1 inhibits JNK/SAPK activity only when expressed at very high levels and does not confer protection against cisplatin. Furthermore, expression of a catalytically inactive mutant of CL100 in 293 cells decreases the IC50 for cisplatin and increases the toxicity of transplatin. This effect seems to be mediated by an increase in JNK activity since p38 activity is unaffected. These results suggest that dual-specificity MAPK phosphatases may be candidate drug targets in order to optimize cisplatin based therapeutic protocols.

Substitution of two insulin receptor carboxy-terminal tyrosines with phenylalanine impairs the expression of MAP kinase phosphatase-1 (MKP-1) mRNA

Cells expressing mutant insulin receptors (Y/F2), in which tyrosines 1316 and 1322 have been replaced with phenylalanine, exhibit enhanced insulin-induced MAP kinase activity and DNA synthesis in comparison with cells expressing wild type insulin receptors (Hirc B). To elucidate the mechanism of enhanced responsiveness, the expression of MAP kinase phosphatase-1 (MKP-1), a negative regulator of MAP kinase activity, was measured in Hirc B and Y/F2 cells incubated in the absence and presence of insulin for various periods of time, and over increasing concentrations of the ligand. Treatment of both cell lines with insulin induced a time and concentration-dependent relative increase in MKP-1 mRNA expression. However, in Y/F2 cells both basal and insulin-stimulated MKP-1 mRNA levels were more than 60% lower than that observed in cells transfected with the wildtype receptors. Cyclic AMP analog (8-Br-cAMP)/inducer (Forskoline) increased MKP-1 mRNA levels in both cell lines, and to a lesser extent in Y/F2 cells. In contrast to insulin the relative increase in MKP-1 mRNA expression induced by 8-Br-cAMP or forskoline was similar in Y/F2 and Hirc B cells. The overexpression of MKP-1 in Y/F2 cells inhibited insulin stimulated DNA synthesis. Transfection of wild type insulin receptors into Y/F2 cells increased basal levels of MKP-1. These results suggest that insulin receptor tyrosine residues 13/16 and 1322 play an important role in the regulation of MKP-1 expression both under basal and insulin stimulated conditions, and are not necessary for the induction of MKP-1 mRNA by cAMP. Furthermore, the enhanced insulin induced mitogenic signaling seen in Y/F2 cells is, at least in part, due to impaired MKP-1 expression.

MKP5, a new member of the MAP kinase phosphatase family, which selectively dephosphorylates stress-activated kinases

Dual-specificity protein tyrosine phosphatases are a burgeoning family of enzymes, some of which, the MKPs, are implicated in the regulation of mitogen-activated protein (MAP) kinases. MKPs have been shown to reverse the activation of the MAP kinases by hydrolyzing phosphothreonine and phosphotyrosine residues present in the substrates. Here we describe the characterization of a novel member of the MKP family, MKP5. The MKP5 gene, which maps to human chromosome 1q32, is expressed tissue-specifically as two transcripts of approximately 3.4 and 2.4 kb in human liver and skeletal muscle. When expressed in mammalian cells, MKP5 blocks the enzymatic activation of MAP kinases with the selectivity p38 approximately JNK/SAPK >> ERK. Immunoprecipitation of endogenous MAP kinases by the catalytically inactive transfected MKP5 demonstrates that it preferentially binds to the p38 and JNK/SAPK kinases. These findings suggest that the selectivity of this phosphatase may be determined at least in part at the level of substrate binding.

Macrophage Colony-Stimulating Factor Induces the Expression of Mitogen-Activated Protein Kinase Phosphatase-1 Through a Protein Kinase C-Dependent Pathway

M-CSF triggers the activation of extracellular signal-regulated protein kinases (ERK)-1/2. We show that inhibition of this pathway leads to the arrest of bone marrow macrophages at the G0/G1 phase of the cell cycle without inducing apoptosis. M-CSF induces the transient expression of mitogen-activated protein kinase phosphatase-1 (MKP-1), which correlates with the inactivation of ERK-1/2. Because the time course of ERK activation must be finely controlled to induce cell proliferation, we studied the mechanisms involved in the induction of MKP-1 by M-CSF. Activation of ERK-1/2 is not required for this event. Therefore, M-CSF activates ERK-1/2 and induces MKP-1 expression through different pathways. The use of two protein kinase C (PKC) inhibitors (GF109203X and calphostin C) revealed that M-CSF induces MKP-1 expression through a PKC-dependent pathway. We analyzed the expression of different PKC isoforms in bone marrow macrophages, and we only detected PKCβI, PKCε, and PKCζ. PKCζ is not inhibited by GF109203X/calphostin C. Of the other two isoforms, PKCε is the best candidate to mediate MKP-1 induction. Prolonged exposure to PMA slightly inhibits MKP-1 expression in response to M-CSF. In bone marrow macrophages, this treatment leads to a complete depletion of PKCβI, but only a partial down-regulation of PKCε. Moreover, no translocation of PKCβI or PKCζ from the cytosol to particulate fractions was detected in response to M-CSF, whereas PKCε was constitutively present at the membrane and underwent significant activation in M-CSF-stimulated macrophages. In conclusion, we remark the role of PKC, probably isoform ε, in the negative control of ERK-1/2 through the induction of their specific phosphatase.

Extracellular signal-regulated kinase (ERK) controls immediate early gene induction on corticostriatal stimulation

Activity-dependent changes in neuronal structure and synaptic remodeling depend critically on gene regulation. In an attempt to understand how glutamate receptor stimulation at the membrane leads to gene regulation in the nucleus, we traced intracellular signaling pathways targeting DNA regulatory elements of immediate early genes (IEGs). For this purpose we used an in vivo electrical stimulation of the glutamatergic corticostriatal pathway. We show that a transient activation of extracellular signal-regulated kinase (ERK) proteins (detected by immunocytochemistry with an anti-active antibody) is spatially coincident with the onset of IEG induction [c-fos, zif 268, and map kinase phosphatase-1 (MKP-1) detected by in situ hybridization] in the striatum, bilaterally. Both Elk-1 and CREB transcription factors (targeting SRE and CRE DNA regulatory elements, respectively) were hyperphosphorylated in register with ERK activation and IEG mRNA induction. However, their hyperphosphorylation occurred in different subcellular compartments: the cytoplasm and the nucleus for Elk-1 and the nucleus for CREB. The role of the ERK signaling cascade in gene regulation was confirmed after intrastriatal and unilateral injection of the specific ERK inhibitor PD 98059, which completely abolished c-fos, zif 268, and MKP-1 mRNA induction in the injected side. Of interest, both Elk-1 and CREB hyperphosphorylation also was impaired after PD 98059 injection. Thus two different ERK modules, one depending on the cytoplasmic activation of Elk-1 and the other one depending on the nuclear activation of CREB, control IEG transcriptional regulation in our model. Our findings provide significant insights into intracellular mechanisms underlying synaptic plasticity in the striatum.

Conditional expression of mitogen-activated protein kinase phosphatase-1, MKP-1, is cytoprotective against UV-induced apoptosis

UV irradiation induces apoptosis in U937 human leukemic cells that is accompanied by the activation of both the stress-activated protein kinase (SAPK) and p38 mitogen-activated protein kinase (MAPK) signal transduction pathways. The MAPK phosphatase, MKP-1, is capable of inactivating both SAPK and p38 MAPK in vivo. To determine whether MKP-1-mediated inhibition of SAPK and/or p38 MAPK activity provided cytoprotection against UV-induced apoptosis, a U937 cell line conditionally expressing MKP-1 from the human metallothionein IIa promoter was established. Conditional expression of MKP-1 was found to abolish UV-induced SAPK and p38 MAPK activity, and inhibit UV-induced apoptosis as judged by both morphological criteria and DNA fragmentation. MKP-1 was also found to inhibit other biochemical events associated with apoptosis, including activation of caspase-3 and the proteolytic cleavage of the caspase-3 substrate, poly(ADP ribose) polymerase. These findings demonstrate that MKP-1 acts at a site upstream of caspase activation within the apoptotic program. The cytoprotective properties of MKP-1 do not appear to be mediated by its ability to inhibit p38 MAPK because the p38 MAPK specific inhibitor SB203580 had no effect on UV-induced apoptosis in U937 cells. Furthermore, by titrating the level of MKP-1 expression it was found that MKP-1 inhibited UV-induced SAPK activity, DNA fragmentation, and caspase-3 activation in a similar dose-dependent manner. The dual-specificity phosphatase, PAC1, which does not inhibit UV-induced activation of SAPK, did not provide a similar cytoprotection against UV-induced apoptosis. These results are consistent with a model whereby MKP-1 provides cytoprotection against UV-induced apoptosis by inhibiting UV-induced SAPK activity.

puckered encodes a phosphatase that mediates a feedback loop regulating JNK activity during dorsal closure in Drosophila

The activation of MAPKs is controlled by the balance between MAPK kinase and MAPK phosphatase activities. The latter is mediated by a subset of phosphatases with dual specificity (VH-1 family). Here, we describe a new member of this family encoded by the puckered gene of Drosophila. Mutations in this gene lead to cytoskeletal defects that result in a failure in dorsal closure related to those associated with mutations in basket, the Drosophila JNK homolog. We show that puckered mutations result in the hyperactivation of DJNK, and that overexpression of puc mimics basket mutant phenotypes. We also show that puckered expression is itself a consequence of the activity of the JNK pathway and that during dorsal closure, JNK signaling has a dual role: to activate an effector, encoded by decapentaplegic, and an element of negative feedback regulation encoded by puckered.

FIN13, a novel growth factor-inducible serine-threonine phosphatase which can inhibit cell cycle progression

We have identified a novel type 2C serine-threonine phosphatase, FIN13, whose expression is induced by fibroblast growth factor 4 and serum in late G1 phase. The protein encoded by FIN13 cDNA includes N- and C-terminal domains with significant homologies to type 2C phosphatases, a domain homologous to collagen, and an acidic domain. FIN13 expression predominates in proliferating tissues. Bacterially expressed FIN13 and FIN13 expressed in mammalian cells exhibit serine-threonine phosphatase activity, which requires Mn2+ and is insensitive to inhibition by okadaic acid. FIN13 is localized in the nuclei of transiently transfected cells. Cotransfection of FIN13-expressing plasmids with a plasmid that expresses the neomycin resistance gene inhibits the growth of drug-resistant colonies in NIH 3T3, HeLa and Rat-1 cells. In transiently transfected cells, FIN13 inhibits DNA synthesis and results in the accumulation of cells in G1 and early S phases. Similarly, the induction of expression of FIN13 under the control of a tetracycline-regulated promoter in NIH 3T3 cells leads to growth inhibition, with accumulation of cells in G1 and early S phases. Thus, overexpression and/or unregulated expression of FIN13 inhibits cell cycle progression, indicating that the physiological role of this phosphatase may be that of regulating the orderly progression of cells through the mitotic cycle by dephosphorylating specific substrates which are important for cell proliferation.

Mitogen-activated protein kinase/extracellular signal-regulated kinase 2 regulates cytoskeletal organization and chemotaxis via catalytic and microtubule-specific interactions

The extracellular signal-regulated kinases (ERKs) 1 and 2 are mitogen-activated protein kinases that act as key components in a signaling cascade linking growth factor receptors to the cytoskeleton and the nucleus. ERK2 mutants have been used to alter cytoskeletal regulation in Chinese hamster ovary cells without affecting cell growth or feedback signaling. Mutation of the unique loop L6 (residues 91-95), which is in a portion of the molecule that is cryptic upon the binding of ERK2 to the microtubules (MTs), generated significant morphological alterations. Most notable phenotypes were observed after expression of a combined mutant incorporating changes to both L6 and the TEY phosphorylation lip, including a 70% increase in cell spreading. Actin stress fibers in these cells, which normally formed a single broad parallel array, were arranged in three or more orientations or in fan-like arrays. MTs, which ordinarily extend longitudinally from the centrosome, spread radially, covering a larger surface area. Single, but not the double, mutations of the Thr and Tyr residues of the TEY phosphorylation lip caused a ca. 25% increase in cell spreading, accompanied by a threefold increase in chemotactic cell migration. Mutation of Lys-52 triggered a 48% increase in cell spreading but no alteration to chemotaxis. These findings suggest that wild-type ERK2 inhibits the organization of the cytoskeleton, the spreading of the cell, and chemotactic migration. This involves control of the orientation of actin and MTs and the positioning of focal adhesions via regulatory interactions that may occur on the MTs.

Mitogen-activated protein kinase phosphatase-1 in rat arterial smooth muscle cell proliferation

Smooth muscle cell proliferation and migration is important in arteriosclerosis. In this process, cytokines and growth factors are upregulated and bind to their respective receptors, which in turn stimulate mitogen-activated protein (MAP) kinases. MAP kinases then relay signals to the nucleus that activate quiescent smooth muscle cells. Phosphatases downregulate MAP kinases. We investigated the role of a dual-specificity tyrosine phosphatase, MAP kinase phosphatase-1 (MKP-1), in smooth muscle cell proliferation. MKP-1 expression was high in arterial tissue by Northern analysis, and MKP-1 message was detected mainly in the arterial smooth muscle layer by in situ hybridization. After balloon injury of the rat carotid artery, expression of MKP-1 decreased greatly, whereas that of MAP kinases, especially p44 MAP kinase, increased. The time course of the reduction in MKP-1 message correlated with increased tyrosine phosphorylation and elevated p44 MAP kinase enzymatic activity. In rat arterial smooth muscle cells overexpressing MKP-1, growth was arrested in the G1 phase and entry into the S phase was blocked. A reduction in MKP-1 expression may contribute in part to proliferation of smooth muscle cells after vascular injury, possibly through a decrease in dephosphorylation of p44 MAP kinase.

Activation of the mitogen-activated protein kinase pathway induces transcription of the PAC-1 phosphatase gene

PAC-1, an early-response gene originally identified in activated T cells, encodes a dual-specificity mitogen-activated protein kinase phosphatase. Here we report on the regulation of PAC-1 expression in murine hemopoietic cells. PAC-1 mRNA levels rapidly increase in mitogen-stimulated lymphocytes, with the induced expression being transient in B cells but sustained in activated T cells. Transfection analysis of murine PAC-1 promoter-reporter constructs established that in T cells, sequences necessary for basal and induced transcription reside within a 200-bp region located immediately upstream of the transcription initiation sites. Basal transcription is regulated in part by an E-box element that binds a 53-kDa protein. PAC-1 transcription induced by phorbol myristate acetate stimulation and the expression of the v-ras or v-raf oncogene is mediated via the E-box motif and an AP-2-related site and coincides with increased binding activity of the constitutive 53-kDa E-box-binding protein and induced binding of AP-2. The ability of an interfering ERK-2 mutant to block phorbol myristate acetate and v-ras-dependent PAC-1 transcription indicates that mitogen-activated protein kinase activation is necessary for these stimuli to induce transcription of the PAC-1 gene in T cells.

Cyr61, a product of a growth factor-inducible immediate-early gene, promotes cell proliferation, migration, and adhesion

cyr61 was first identified as a growth factor-inducible immediate-early gene in mouse fibroblasts. The encoded Cyr61 protein is a secreted, cystein-rich heparin-binding protein that associates with the cell surface and the extracellular matrix, and in these aspects it resembles the Wnt-1 protein and a number of known growth factors. During embryogenesis, cyr61 is expressed most notably in mesenchymal cells that are differentiating into chondrocytes and in the vessel walls of the developing circulatory system. cyr61 is a member of an emerging gene family that encodes growth regulators, including the connective tissue growth factor and an avian proto-oncoprotein, Nov cyr61 also shares sequence similarities with two Drosophila genes, twisted gastrulation and short gastrulation, which interact with decapentaplegic to regulate dorsal-ventral patterning. In this report we describe the purification of the Cyr61 protein in a biologically active form, and we show that purified Cyr61 has the following activities: (i) it promotes the attachment and spreading of endothelial cells in a manner similar to that of fibronectin; (ii) it enhances the effects of basic fibroblast growth factor and platelet-derived growth factor on the rate of DNA synthesis of fibroblasts and vascular endothelial cells, although it has no detectable mitogenic activity by itself; and (iii) it acts as a chemotactic factor for fibroblasts. Taken together, these activities indicate that Cyr61 is likely to function as an extracellular matrix signaling molecule rather than as a classical growth factor and may regulate processes of cell proliferation, migration, adhesion, and differentiation during development.

Identification of a cytoplasmic, phorbol ester-inducible isoform of protein tyrosine phosphatase epsilon

The protein-tyrosine phosphatase epsilon (PTP epsilon) is a transmembranal, receptor-type protein that possesses two phosphatase catalytic domains characteristic of transmembranal phosphatases. Here we demonstrate the existence of a nontransmembranal isoform of PTP epsilon, PTP epsilon-cytoplasmic. PTP epsilon-cytoplasmic and the transmembranal isoform of PTP epsilon have separate, nonoverlapping expression patterns. Further, the data clearly indicate that control of which of the two isoforms is to be expressed is initiated at the transcriptional level, suggesting that they have distinct physiological roles. PTP epsilon-cytoplasmic mRNA is the product of a delayed early response gene in NIH 3T3 fibroblasts, and its transcription is regulated through a pathway that requires protein kinase C. The human homologue of PTP epsilon-cytoplasmic has also been cloned and is strongly up-regulated in the early stages of phorbol 12-tetradecanoate 13-acetate-induced differentiation of HL-60 cells. Sequence analysis indicates and cellular fractionation experiments confirm that this isoform is a cytoplasmic molecule. PTP epsilon-cytoplasmic is therefore the initial example to our knowledge of a nontransmembranal protein-tyrosine phosphatase that contains two tandem of catalytic domains.

Association of mitogen-activated protein kinase with the microtubule cytoskeleton

Using indirect immunofluorescence microscopy and biochemical techniques, we have determined that approximately one-third of the total mitogen-activated protein kinase (MAPK) is associated with the microtubule cytoskeleton in NIH 3T3 mouse fibroblasts. This population of enzyme can be separated from the soluble form that is found distributed throughout the cytosol and is also present in the nucleus after mitogen stimulation. The microtubule-associated enzyme pool constitutes half of all detectable MAPK activity after mitogenic stimulation. These findings extend the known in vivo associations of MAPK with microtubules to include the entire microtubule cytoskeleton of proliferating cells, and they suggest that a direct association of MAPK with microtubules may be in part responsible for the observed correlations between MAPK activities and cytoskeletal alteration.

A role for mitogen-activated protein kinase in mediating activation of the glycoprotein hormone alpha-subunit promoter by gonadotropin-releasing hormone

Gonadotropin-releasing hormone (GnRH) interacts with a G protein-coupled receptor and increases the transcription of the glycoprotein hormone alpha-subunit gene. We have explored the possibility that mitogen-activated protein kinase (MAPK) plays a role in mediating GnRH effects on transcription. Activation of the MAPK cascade by an expression vector for a constitutively active form of the Raf-1 kinase led to stimulation of the alpha-subunit promoter in a concentration-dependent manner. GnRH treatment was found to increase the phosphorylation of tyrosine residues of MAPK and to increase MAPK activity, as determined by an immune complex kinase assay. A reporter gene assay using the MAPK-responsive, carboxy-terminal domain of the Elk1 transcription factor was also consistent with GnRH-induced activation of MAPK. Interference with the MAPK pathway by expression vectors for kinase-defective MAPKs or vectors encoding MAPK phosphatases reduced the transcription-stimulating effects of GnRH. The DNA sequences which are required for responses to GnRH include an Ets factor-binding site. An expression vector for a dominant negative form of Ets-2 was able to reduce GnRH effects on expression of the alpha-subunit gene. These findings provide evidence that GnRH treatment leads to activation of the MAPK cascade in gonadotropes and that activation of MAPK contributes to stimulation of the alpha-subunit promoter. It is likely that an Ets factor serves as a downstream transcriptional effector of MAPK in this system.