Phosphorylation of paxillin via the ERK mitogen-activated protein kinase cascade in EL4 thymoma cells

Drug-Free Mesoporous Silica Nanoparticles Enable Suppression of Cancer Metastasis and Confer Survival Advantages to Mice with Tumor Xenografts

Despite advancements in nanomedicine for drug delivery, many drug-loaded nanoparticles reduce tumor sizes but often fail to prevent metastasis. Mesoporous silica nanoparticles (MSNs) have attracted attention as promising nanocarriers. Here, we demonstrated that MSN-PEG/TA 25, with proper surface modifications, exhibited unique antimetastatic properties. In vivo studies showed that overall tumor metastasis decreased in 4T1 xenografts mice treated with MSN-PEG/TA 25 with a notable reduction in lung tumor metastasis. In vitro assays, including wound-healing, Boyden chamber, tube-formation, and real-time cell analyses, showed that MSN-PEG/TA 25 could modulate cell migration of 4T1 breast cancer cells and interrupt tube formation by human umbilical vein endothelial cells (HUVECs), key factors in suppressing cancer metastasis. The synergistic effect of MSN-PEG/TA 25 combined with liposomal-encapsulated doxorubicin (Lipo-Dox) significantly boosted mouse survival rates, outperforming Lipo-Dox monotherapy. We attributed the improved survival to the antimetastatic capabilities of MSN-PEG/TA 25. Moreover, Dox-loaded MSN-PEG/TA 25 suppressed primary tumors while retaining the antimetastatic effect, thereby enhancing therapeutic outcomes and overall survival. Western blot and qPCR analyses revealed that MSN-PEG/TA 25 interfered with the phosphorylation of ERK, FAK, and paxillin, thus impacting focal adhesion turnover and inhibiting cell motility. Our findings suggest that drug-free MSN-PEG/TA 25 is highly efficient for cancer treatment via suppressing metastatic activity and angiogenesis.

DNA/RNA heteroduplex oligonucleotide technology for regulating lymphocytes in vivo

Manipulating lymphocyte functions with gene silencing approaches is promising for treating autoimmunity, inflammation, and cancer. Although oligonucleotide therapy has been proven to be successful in treating several conditions, efficient in vivo delivery of oligonucleotide to lymphocyte populations remains a challenge. Here, we demonstrate that intravenous injection of a heteroduplex oligonucleotide (HDO), comprised of an antisense oligonucleotide (ASO) and its complementary RNA conjugated to α-tocopherol, silences lymphocyte endogenous gene expression with higher potency, efficacy, and longer retention time than ASOs. Importantly, reduction of Itga4 by HDO ameliorates symptoms in both adoptive transfer and active experimental autoimmune encephalomyelitis models. Our findings reveal the advantages of HDO with enhanced gene knockdown effect and different delivery mechanisms compared with ASO. Thus, regulation of lymphocyte functions by HDO is a potential therapeutic option for immune-mediated diseases.

Paxillin family of focal adhesion adaptor proteins and regulation of cancer cell invasion

The paxillin family of proteins, including paxillin, Hic-5, and leupaxin, are focal adhesion adaptor/scaffolding proteins which localize to cell-matrix adhesions and are important in cell adhesion and migration of both normal and cancer cells. Historically, the role of these proteins in regulating the actin cytoskeleton through focal adhesion-mediated signaling has been well documented. However, studies in recent years have revealed additional functions in modulating the microtubule and intermediate filament cytoskeletons to affect diverse processes including cell polarization, vesicle trafficking and mechanosignaling. Expression of paxillin family proteins in stromal cells is also important in regulating tumor cell migration and invasion through non-cell autonomous effects on the extracellular matrix. Both paxillin and Hic-5 can also influence gene expression through a variety of mechanisms, while their own expression is frequently dysregulated in various cancers. Accordingly, these proteins may serve as valuable targets for novel diagnostic and treatment approaches in cancer.

The role of CCN4/WISP-1 in the cancerous phenotype

CCN proteins are secreted into the extracellular environment where they interact with both components of the extracellular matrix and with cell surface receptors to regulate cellular function. Through these interactions, CCNs act as extracellular ligands to activate intracellular signal transduction pathways. CCN4/WISP-1, like other CCNs, plays multiple physiologic roles in development and also participates in pathogenesis. CCN4 is of particular interest with respect to cancer, showing promise as a biomarker or prognostic factor as well as a potential therapeutic target. This review focuses on recent work addressing the role of CCN4 in cancer. While CCN4 has been identified as an oncogene in a number of cancers, where it enhances cell migration and promoting epithelial-mesenchymal transition, there are other cancers where CCN4 appears to play an inhibitory role. The mechanisms underlying these differences in cellular response have not yet been delineated, but are an active area of investigation. The expression and activities of CCN4 splice variants are likewise an emerging area for study. CCN4 acts as an autocrine factor that regulates the cancer cells from which it is secreted. However, CCN4 is also a paracrine factor that is secreted by stromal fibroblasts, and can affect the function of vascular endothelial cells. In summary, current evidence is abundant in regard to establishing potential roles for CCN4 in oncogenesis, but much remains to be learned about the functions of this fascinating protein as both an autocrine and paracrine regulator in the tumor microenvironment.

Paxillin: a crossroad in pathological cell migration

Paxilllin is a multifunctional and multidomain focal adhesion adapter protein which serves an important scaffolding role at focal adhesions by recruiting structural and signaling molecules involved in cell movement and migration, when phosphorylated on specific Tyr and Ser residues. Upon integrin engagement with extracellular matrix, paxillin is phosphorylated at Tyr31, Tyr118, Ser188, and Ser190, activating numerous signaling cascades which promote cell migration, indicating that the regulation of adhesion dynamics is under the control of a complex display of signaling mechanisms. Among them, paxillin disassembly from focal adhesions induced by extracellular regulated kinase (ERK)-mediated phosphorylation of serines 106, 231, and 290 as well as the binding of the phosphatase PEST to paxillin have been shown to play a key role in cell migration. Paxillin also coordinates the spatiotemporal activation of signaling molecules, including Cdc42, Rac1, and RhoA GTPases, by recruiting GEFs, GAPs, and GITs to focal adhesions. As a major participant in the regulation of cell movement, paxillin plays distinct roles in specific tissues and developmental stages and is involved in immune response, epithelial morphogenesis, and embryonic development. Importantly, paxillin is also an essential player in pathological conditions including oxidative stress, inflammation, endothelial cell barrier dysfunction, and cancer development and metastasis.

Focal adhesion kinase activity is required for actomyosin contractility-based invasion of cells into dense 3D matrices

The focal adhesion kinase (FAK) regulates the dynamics of integrin-based cell adhesions important for motility. FAK's activity regulation is involved in stress-sensing and focal-adhesion turnover. The effect of FAK on 3D migration and cellular mechanics is unclear. We analyzed FAK knock-out mouse embryonic fibroblasts and cells expressing a kinase-dead FAK mutant, R454-FAK, in comparison to FAK wild-type cells. FAK knock-out and FAKR454/R454 cells invade dense 3D matrices less efficiently. These results are supported by FAK knock-down in wild-type fibroblasts and MDA-MB-231 human breast cancer cells showing reduced invasiveness. Pharmacological interventions indicate that in 3D matrices, cells deficient in FAK or kinase-activity behave similarly to wild-type cells treated with inhibitors of Src-activity or actomyosin-contractility. Using magnetic tweezers experiments, FAKR454/R454 cells are shown to be softer and exhibit impaired adhesion to fibronectin and collagen, which is consistent with their reduced 3D invasiveness. In line with this, FAKR454/R454 cells cannot contract the matrix in contrast to FAK wild-type cells. Finally, our findings demonstrate that active FAK facilitates 3D matrix invasion through increased cellular stiffness and transmission of actomyosin-dependent contractile force in dense 3D extracellular matrices.

Discovering Functional ERK Substrates Regulating Caenorhabditis elegans Germline Development

The Rat Sarcoma (RAS) GTPAse-mediated extracellular signal-regulated kinase (ERK) pathway regulates multiple biological processes across metazoans. In particular during Caenorhabditis elegans oogenesis, ERK signaling has been shown to regulate over seven distinct biological processes in a temporal and sequential manner. To fully elucidate how ERK signaling cascade orchestrates these different biological processes in vivo, identification of the direct functional substrates of the pathway is critical. This chapter describes the methods that were used to identify ERK substrates in a global manner and study their functions in the germline. These approaches can also be generally applied to study ERK-dependent biological processes in other systems.

Differential Expression of Ccn4 and Other Genes Between Metastatic and Non-metastatic EL4 Mouse Lymphoma Cells

BACKGROUND

Previous work characterized variants of the EL4 murine lymphoma cell line. Some are non-metastatic, and others metastatic, in syngenic mice. In addition, metastatic EL4 cells were stably transfected with phospholipase D2 (PLD2), which further enhanced metastasis.

MATERIALS AND METHODS

Microarray analyses of mRNA expression was performed for non-metastatic, metastatic, and PLD2-expressing metastatic EL4 cells.

RESULTS

Many differences were observed between non-metastatic and metastatic cell lines. One of the most striking new findings was up-regulation of mRNA for the matricellular protein WNT1-inducible signaling pathway protein 1 (CCN4) in metastatic cells; increased protein expression was verified by immunoblotting and immunocytochemistry. Other differentially expressed genes included those for reproductive homeobox 5 (Rhox5; increased in metastatic) and cystatin 7 (Cst7; decreased in metastatic). Differences between PLD2-expressing and parental cell lines were limited but included the signaling proteins Ras guanyl releasing protein 1 (RGS18; increased with PLD2) and suppressor of cytokine signaling 2 (SOCS2; decreased with PLD2).

CONCLUSION

The results provide insights into signaling pathways potentially involved in conferring metastatic ability on lymphoma cells.

Molecular pathways: adaptive kinome reprogramming in response to targeted inhibition of the BRAF-MEK-ERK pathway in cancer

The central role of the BRAF-MEK-ERK pathway in controlling cell fate has made this pathway a primary target for deregulated activation in cancer. BRaf is activated by Ras proteins allowing Ras oncogenes to constitutively activate the pathway. Activating BRaf mutations are also frequent in several cancers, being the most common oncogenic mutation in thyroid carcinoma and melanoma. There are currently two inhibitors, vemurafenib and dabrafenib, approved for treatment of malignant melanoma having activating BRaf mutations. Concurrent administration of BRAF and MAP-ERK kinase (MEK) inhibitor (trametinib) is significantly more active in patients with BRAF-mutant melanoma than either single agent alone, but progression to resistance ultimately occurs by different mechanisms that increase the activation of extracellular signal-regulated kinase (ERK). Such adaptive changes in tumor cell signaling networks allow bypass of targeted oncoprotein inhibition. This is true with targeted inhibitors for BRaf and MEK as well as specific inhibitors for AKT, mTOR, and many receptor tyrosine kinases such as EGF receptor (EGFR) and HER2. It is this adaptive response to targeted kinase inhibitors that contributes to the failure of single-agent kinase inhibitors to have durable responses. This failure is seen in virtually all cancers treated with single-agent kinase inhibitors, most of which are not as dependent on a single signaling pathway such as BRaf-MEK-ERK in melanoma. Thus, understanding the breadth of adaptive reprogramming responses to specific targeted kinase inhibition will be critical to develop appropriate combination therapies for durable clinical responses.

Effects of lysophosphatidic acid on calpain-mediated proteolysis of focal adhesion kinase in human prostate cancer cells

BACKGROUND

Calcium-mediated proteolysis plays an important role in cell migration. Lysophosphatidic acid (LPA), a lipid mediator present in serum, enhances migration of carcinoma cells. The effects of LPA on calpain-mediated proteolysis were, therefore, examined in PC-3, a human prostate cancer cell line.

METHODS

Cultured PC-3 cells were used in studies utilizing pharmacologic interventions, immunoblotting, and confocal immunolocalization.

RESULTS

Focal adhesion kinase (FAK), a tyrosine kinase involved in cell adhesion, is rapidly proteolyzed in serum-starved PC-3 cells exposed to the calcium ionophore, ionomycin; Nck, p130CAS, PKCα, and Ras-GAP are also degraded. Thapsigargin, which causes more moderate increases in intracellular calcium, induces partial proteolysis of these proteins. Calpain inhibitors block the proteolytic responses to ionomycin and thapsigargin. Ionomycin does not induce proteolysis in cells maintained in serum, suggesting a protective role for growth factors contained in serum. LPA causes minor FAK proteolysis when added alone, but protects against ionomycin-induced proteolysis in a time-dependent manner. LPA also protects against the cell detachment that eventually follows ionomycin treatment. The response to LPA is blocked by an LPA receptor antagonist. A similar effect of LPA is observed in ionomycin-treated Rat-1 fibroblasts. In PC-3 cells, the protective effects of LPA and serum are correlated with phosphorylation and redistribution of paxillin, suggesting roles for phosphorylation-mediated protein-protein interactions.

CONCLUSIONS

The complex effects of LPA on calpain-mediated proteolysis of FAK and other adhesion proteins are likely to play a role in the ability of LPA to promote attachment, migration, and survival of prostate cancer cells.

Paxillin phosphorylation by JNK and p38 is required for NFAT activation

Paxillin is an adaptor protein associated with focal adhesion complex, and is activated by tyrosine phosphorylation through focal adhesion kinase (FAK) and Src kinase. Recent studies reveal that serine phosphorylation of paxillin by JNK and p38 MAPK is essential for cell migration or neurite extension, but their cellular targets remain unclear. In this study, we examined the requirement of paxillin phosphorylation by p38 MAPK or JNK in T-cell motility and activation using paxillin mutants at the respective phosphorylation sites, Ser85, and Ser178. (S85A)-paxillin, (S178A)-paxillin, or (S85A/S178A)-paxillin inhibited the motility of NIH/3T3 fibroblasts, but did not interfere with T-cell migration and integrin-mediated T-cell adhesion. In contrast, activation of T cells was effectively suppressed by (S85A/S178A)-paxillin. Transgenic (S85A/S178A)-paxillin expression inhibited T-cell proliferation and reduced the production of IL-2, IFN-γ, and IL-4. In searching for signals modulated by (S85A/S178A)-paxillin, we found that NFAT activation was specifically blocked by (S85A/S178A)-paxillin. This could be partly attributed to diminished stromal interaction molecule 1 (STIM1) expression and attenuated TCR-induced Ca(2+) influx. Our results demonstrate that dual phosphorylation of paxillin by JNK and p38 MAPK is essential for T-cell activation and suggest that NFAT is a functional target of the JNK/p38 phosphorylated paxillin.

Focal adhesion disassembly is regulated by a RIAM to MEK-1 pathway

Cell migration and invasion require regulated turnover of integrin-dependent adhesion complexes. Rap1-GTP-interacting adaptor molecule (RIAM) is an adaptor protein that mediates talin recruitment to the cell membrane, and whose depletion leads to defective melanoma cell migration and invasion. In this study, we investigated the potential involvement of RIAM in focal adhesion (FA) dynamics. RIAM-depleted melanoma and breast carcinoma cells displayed an increased number, size and stability of FAs, which accumulated centrally at the ventral cell surface, a phenotype caused by defective FA disassembly. Impairment in FA disassembly resulting from RIAM knockdown correlated with deficient integrin-dependent mitogen-activated protein kinase kinase (MEK)-Erk1/2 activation and, importantly, overexpression of constitutively active MEK resulted in rescue of FA disassembly and recovery of cell invasion. Furthermore, RIAM-promoted Ras homologue gene family, member A (RhoA) activation following integrin engagement was needed for subsequent Erk1/2 activation. In addition, RhoA overexpression partially rescued the FA phenotype in RIAM-depleted cells, also suggesting a functional role for RhoA downstream of RIAM, but upstream of Erk1/2. RIAM knockdown also led to enhanced phosphorylation of paxillin Tyr118 and Tyr31. However, expression of phosphomimetic and nonphosphorylatable mutants at these paxillin residues indicated that paxillin hyperphosphorylation is a subsequent consequence of the blockade of FA disassembly, but does not cause the FA phenotype. RIAM depletion also weakened the association between FA proteins, suggesting that it has important adaptor roles in the correct assembly of adhesion complexes. Our data suggest that integrin-triggered, RIAM-dependent MEK activation represents a key feedback event required for efficient FA disassembly, which could help explain the role of RIAM in cell migration and invasion.

Paxillin associates with the microtubule cytoskeleton and the immunological synapse of CTL through its leucine-aspartic acid domains and contributes to microtubule organizing center reorientation

The cytoskeletal adaptor protein paxillin localizes to the microtubule organizing center (MTOC) in T cells and, upon target cell binding, is recruited to the supramolecular activation complex (SMAC). We mapped the region of paxillin that associates with both the MTOC and SMAC to the leucine-aspartic acid (LD) domains and showed that a protein segment containing LD2-4 was sufficient for MTOC and SMAC recruitment. Examination of the localization of paxillin at the SMAC revealed that paxillin localizes to the peripheral area of the SMAC along with LFA-1, suggesting that LFA-1 may contribute to its recruitment. LFA-1 or CD3 engagement alone was insufficient for paxillin recruitment because there was no paxillin accumulation at the site of CTL contact with anti-LFA-1- or anti-CD3-coated beads. In contrast, paxillin accumulation was detected when beads coated with both anti-CD3 and anti-LFA-1 were bound to CTL, suggesting that signals from both the TCR and LFA-1 are required for paxillin accumulation. Paxillin was shown to be phosphorylated downstream of ERK, but when we generated a mutation (S83A/S130A) that abolished the mobility shift as a result of phosphorylation, we found that paxillin still bound to the MTOC and was recruited to the SMAC. Furthermore, ERK was not absolutely required for MTOC reorientation in CTL that require ERK for killing. Finally, expression of the LD2-4 region of paxillin substantially reduced MTOC reorientation. These studies demonstrated that paxillin is recruited, through its LD domains, to sites of integrin engagement and may contribute to MTOC reorientation required for directional degranulation.

Differential expression of FAK and Pyk2 in metastatic and non-metastatic EL4 lymphoma cell lines

The murine EL4 lymphoma cell line exists in variants that are either sensitive or resistant to phorbol 12-myristate 13-acetate (PMA). In sensitive cells, PMA causes Erk MAPK activation and Erk-mediated growth arrest. In resistant cells, PMA induces a low level of Erk activation, without growth arrest. A relatively unexplored aspect of the phenotypes is that resistant cells are more adherent to culture substrate than are sensitive cells. In this study, the roles of the protein tyrosine kinases FAK and Pyk2 in EL4 phenotype were examined, with a particular emphasis on the role of these proteins in metastasis. FAK is expressed only in PMA-resistant (or intermediate phenotype) EL4 cells, correlating with enhanced cell-substrate adherence, while Pyk2 is more highly expressed in non-adherent PMA-sensitive cells. PMA treatment causes modulation of mRNA for FAK (up-regulation) and Pyk2 (down-regulation) in PMA-sensitive but not PMA-resistant EL4 cells. The increase in Pyk2 mRNA is correlated with an increase in Pyk2 protein expression. The roles of FAK in cell phenotype were further explored using transfection and knockdown experiments. The results showed that FAK does not play a major role in modulating PMA-induced Erk activation in EL4 cells. However, the knockdown studies demonstrated that FAK expression is required for proliferation and migration of PMA-resistant cells. In an experimental metastasis model using syngeneic mice, only FAK-expressing (PMA-resistant) EL4 cells form liver tumors. Taken together, these studies suggest that FAK expression promotes metastasis of EL4 lymphoma cells.

Paxillin regulates androgen- and epidermal growth factor-induced MAPK signaling and cell proliferation in prostate cancer cells

Although transcriptional effects of androgens have been extensively studied, mechanisms regulating transcription-independent (nongenomic) androgen actions are poorly understood. Previously, we have shown that paxillin, a multidomain adaptor protein, is a critical regulator of testosterone-induced MAPK-signaling during Xenopus oocyte maturation. Here we examine the nongenomic effects of dihydrotestosterone (DHT) in prostate cancer cells, focusing on how paxillin mediates Erk signaling and downstream physiologic actions. We show that in LnCAP cells DHT functions as a growth factor that indirectly activates the EGF-receptor (EGFR) via androgen receptor binding and matrix metalloproteinase-mediated release of EGFR ligands. Interestingly, siRNA-mediated knockdown of paxillin expression in androgen-dependent LnCAP cells as well as in androgen-independent PC3 cells abrogates DHT- and/or EGF-induced Erk signaling. Furthermore, EGFR-induced Erk activation requires Src-mediated phosphorylation of paxillin on tyrosines 31/118. In contrast, paxillin is not required for PKC-induced Erk signaling. However, Erk-mediated phosphorylation of paxillin on serines 83/126/130 is still needed for both EGFR and PKC-mediated cellular proliferation. Thus, paxillin serves as a specific upstream regulator of Erk in response to receptor-tyrosine kinase signaling but as a general regulator of downstream Erk actions regardless of agonist. Importantly, Erk-mediated serine phosphorylation of paxillin is also required for DHT-induced prostate-specific antigen mRNA expression in LnCAP cells as well as EGF-induced cyclin D1 mRNA expression in PC3 cells, suggesting that paxillin may regulate prostate cancer proliferation by serving as a liaison between extra-nuclear kinase signaling and intra-nuclear transcriptional signals. Thus, paxillin may prove to be a novel diagnostic or therapeutic target in prostate cancer.

Phospholipase D2 Enhances Epidermal Growth Factor-Induced Akt Activation in EL4 Lymphoma Cells

Phospholipase D2 (PLD2) generates phosphatidic acid through hydrolysis of phosphatidylcholine. PLD2 has been shown to play a role in enhancing tumorigenesis. The epidermal growth factor receptor (EGFR) can both activate and interact with PLD2. Murine lymphoma EL4 cells lacking endogenous PLD2 present a unique model to elucidate the role of PLD2 in signal transduction. In the current study, we investigated effects of PLD2 on EGF response. Western blotting and RT-PCR were used to establish that both parental cells and PLD2 transfectants express endogenous EGFR. Levels of EGFR protein are increased in cells expressing active PLD2, as compared to parental cells or cells expressing inactive PLD2. EGF stimulates proliferation of EL4 cells transfected with active PLD2, but not parental cells or cells transfected with inactive PLD2. EGF-mediated proliferation in cells expressing active PLD2 is dependent on the activities of both the EGFR and the PI3K/Akt pathway, as demonstrated by studies using protein kinase inhibitors. EGF-induced invasion through a synthetic extracellular matrix is enhanced in cells expressing active PLD2, as compared to parental cells or cells expressing inactive PLD2. Taken together, the data suggest that PLD2 acts in concert with EGFR to enhance mitogenesis and invasion in lymphoma cells.

c-Fos accelerates hepatocyte conversion to a fibroblastoid phenotype through ERK-mediated upregulation of paxillin-Serine178 phosphorylation

Transforming growth factor beta (TGF-beta) exerts an important role in the late steps of carcinogenesis by cooperating with Ras to induce cell motility and tumor invasion. The transcription complex AP-1 has been implicated in the regulation of genes involved in motility and invasion, by mechanisms not yet delineated. We utilized a model of immortalized human hepatocytes (IHH) overexpressing c-Fos (IHH-Fos) or not (IHH-C) to investigate the role of c-Fos on cell motility in response to a prolonged treatment with TGF-beta, EGF or a combination of both. Cotreatment with EGF and TGF-beta, but neither cytokine alone, induced the conversion of hepatocytes to a fibroblastoid phenotype and increased their motility in Boyden chambers. EGF/TGF-beta cotreatment induced a higher effect on ERK phosphorylation compared to TGF-beta treatment alone. It also induced an increase in total and phosphorylated Ser(178) paxillin, a protein previously implicated in cell motility. This response was inhibited by two specific MEK inhibitors, indicating the involvement of the ERK pathway in paxillin activation. Overexpression of c-Fos correlated with increased cell scattering and motility, higher levels of ERK activation and phospho Ser(178) paxillin, increased levels of EGF receptor (EGF-R) mRNA and higher EGF-R phosphorylation levels following EGF/TGF-beta cotreatment. Conversely, siRNA-mediated invalidation of c-Fos delayed the appearance of fibroblastoid cells, decreased EGF-R mRNA and downregulated ERK and Ser(178) paxillin phosphorylations, indicating that c-Fos activates hepatocyte motility through an EGF-R/ERK/paxillin pathway. Since c-Fos is frequently overexpressed in hepatocarcinomas, this newly identified mechanism might be involved in the progression of hepatic tumors in vivo.

MAPK signaling pathways in the regulation of hematopoiesis

The MAPKs are a family of serine/threonine kinases that play an essential role in connecting cell-surface receptors to changes in transcriptional programs. MAPKs are part of a three-component kinase module consisting of a MAPK, an upstream MEK, and a MEKK that couples the signals from cell-surface receptors to trigger downstream pathways. Three major groups of MAPKs have been characterized in mammals, including ERKs, JNKs, and p38MAPKs. Over the last decade, extensive work has established that these proteins play critical roles in the regulation of a wide variety of cellular processes including cell growth, migration, proliferation, differentiation, and survival. It has been demonstrated that ERK, JNK, and p38MAPK activity can be regulated in response to a plethora of hematopoietic cytokines and growth factors that play critical roles in hematopoiesis. In this review, we summarize the current understanding of MAPK function in the regulation of hematopoiesis in general and myelopoiesis in particular. In addition, the consequences of aberrant MAPK activation in the pathogenesis of various myeloid malignancies will be discussed.

Integrated modulation of phorbol ester-induced Raf activation in EL4 lymphoma cells

The EL4 murine lymphoma cell line exists in variant phenotypes that differ with respect to responses to the tumor promoter phorbol 12-myristate 13-acetate (PMA1). Previous work showed that "PMA-sensitive" cells, characterized by a high magnitude of PMA-induced Erk activation, express RasGRP, a phorbol ester receptor that directly activates Ras. In "PMA-resistant" and "intermediate" EL4 cell lines, PMA induces Erk activation to lesser extents, but with a greater response in intermediate cells. In the current study, these cell lines were used to examine mechanisms of Raf-1 modulation. Phospho-specific antibodies were utilized to define patterns and kinetics of Raf-1 phosphorylation on several sites. Further studies showed that Akt is constitutively activated to a greater extent in PMA-resistant than in PMA-sensitive cells, and also to a greater extent in resistant than intermediate cells. Akt negatively regulates Raf-1 activation (Ser259), partially explaining the difference between resistant and intermediate cells. Erk activation exerts negative feedback on Raf-1 (Ser289/296/301), thus resulting in earlier termination of the signal in cells with a higher level of Erk activation. RKIP, a Raf inhibitory protein, is expressed at higher levels in resistant cells than in sensitive or intermediate cells. Knockdown of RKIP increases Erk activation and also negative feedback. In conclusion, this study delineates Raf-1 phosphorylation events occurring in response to PMA in cell lines with different extents of Erk activation. Variations in the levels of expression and activation of multiple signaling proteins work in an integrated fashion to modulate the extent and duration of Erk activation.

Multiple ERK substrates execute single biological processes in Caenorhabditis elegans germ-line development

RAS-extracellular signal regulated kinase (ERK) signaling governs multiple aspects of cell fate specification, cellular transitions, and growth by regulating downstream substrates through phosphorylation. Understanding how perturbations to the ERK signaling pathway lead to developmental disorders and cancer hinges critically on identification of the substrates. Yet, only a limited number of substrates have been identified that function in vivo to execute ERK-regulated processes. The Caenorhabditis elegans germ line utilizes the well-conserved RAS-ERK signaling pathway in multiple different contexts. Here, we present an integrated functional genomic approach that identified 30 ERK substrates, each of which functions to regulate one or more of seven distinct biological processes during C. elegans germ-line development. Our results provide evidence for three themes that underlie the robustness and specificity of biological outcomes controlled by ERK signaling in C. elegans that are likely relevant to ERK signaling in other organisms: (i) multiple diverse ERK substrates function to control each individual biological process; (ii) different combinations of substrates function to control distinct biological processes; and (iii) regulatory feedback loops between ERK and its substrates help reinforce or attenuate ERK activation. Substrates identified here have conserved orthologs in humans, suggesting that insights from these studies will contribute to our understanding of human diseases involving deregulated ERK activity.

Effects of active and inactive phospholipase D2 on signal transduction, adhesion, migration, invasion, and metastasis in EL4 lymphoma cells

The phosphatidylcholine-using phospholipase D (PLD) isoform PLD2 is widely expressed in mammalian cells and is activated in response to a variety of promitogenic agonists. In this study, active and inactive hemagglutinin-tagged human PLD2 (HA-PLD2) constructs were stably expressed in an EL4 cell line lacking detectable endogenous PLD1 or PLD2. The overall goal of the study was to examine the roles of PLD2 in cellular signal transduction and cell phenotype. HA-PLD2 confers PLD activity that is activated by phorbol ester, ionomycin, and okadaic acid. Proliferation and Erk activation are unchanged in cells transfected with active PLD2; proliferation rate is decreased in cells expressing inactive PLD2. Basal tyrosine phosphorylation of focal adhesion kinase (FAK) is increased in cells expressing active PLD2, as is phosphorylation of Akt; inactive PLD2 has no effect. Expression of active PLD2 is associated with increased spreading and elongation of cells on tissue culture plastic, whereas inactive PLD2 inhibits cell spreading. Inactive PLD2 also inhibits cell adhesion, migration, and serum-induced invasion. Cells expressing active PLD2 form metastases in syngeneic mice, as do the parental cells; cells expressing inactive PLD2 form fewer metastases than parental cells. In summary, active PLD2 enhances FAK phosphorylation, Akt activation, and cell invasion in EL4 lymphoma cells, whereas inactive PLD2 exerts inhibitory effects on adhesion, migration, invasion, and tumor formation. Overall, expression of active PLD2 enhances processes favorable to lymphoma cell metastasis, whereas expression of inactive PLD2 inhibits metastasis.

Paxillin comes of age

Paxillin is a multi-domain scaffold protein that localizes to the intracellular surface of sites of cell adhesion to the extracellular matrix. Through the interactions of its multiple protein-binding modules, many of which are regulated by phosphorylation, paxillin serves as a platform for the recruitment of numerous regulatory and structural proteins that together control the dynamic changes in cell adhesion, cytoskeletal reorganization and gene expression that are necessary for cell migration and survival. In particular, paxillin plays a central role in coordinating the spatial and temporal action of the Rho family of small GTPases, which regulate the actin cytoskeleton, by recruiting an array of GTPase activator, suppressor and effector proteins to cell adhesions. When paxillin was first described 18 years ago, the amazing complexity of cell-adhesion organization, dynamics and signaling was yet to be realized. Herein we highlight our current understanding of how the multiple protein interactions of paxillin contribute to the coordination of cell-adhesion function.

Low substratum rigidity of collagen gel promotes ERK phosphorylation via lipid raft to augment cell migration

Previous study demonstrated that low substratum rigidity down-regulates focal adhesion proteins. In this study we found that cells cultured on collagen gel exhibited higher migration capacity than those cultured on collagen gel-coated dishes. Low rigidity of collagen gel induced delayed but persistent phosphorylation of ERK1/2. Inhibition of collagen gel-induced ERK1/2 phosphorylation by MEK inhibitors and ERK2 kinase mutant induced a rounding up of the cells and prevented collagen gel-induced cell migration. Interestingly, phosphorylated ERK1/2 induced by low rigidity was present in focal adhesion sites and the lipid raft. MbetaCD (Methyl-beta-cyclodextrin), a lipid raft inhibitor, inhibited collagen gel-induced ERK1/2 phosphorylation, and cell migration. Overexpression of FAK C-terminal fragment (FRNK) in MDCK cells triggered ERK phosphorylation. Meanwhile, low substratum rigidity induced degradation of FAK into a 35 kDa C-terminal fragment. A calpain inhibitor that partially rescued FAK degradation also prevented low rigidity-induced ERK phosphorylation. However, MbetaCD did not prevent low rigidity-induced FAK degradation. Taken together, we demonstrate that the degradation product of FAK induced by collagen gel triggers activation of ERK1/2, which in turn facilitates cell spreading and migration through the lipid raft.

Potential protein partners for the human TIMAP revealed by bacterial two-hybrid screening

BacterioMatch Two-Hybrid System (Stratagene) was applied in order to identify potential human TIMAP interaction proteins in the lung. TIMAP highly expressed in endothelial cells and may be involved in endothelial cytoskeletal and barrier regulation. Seven TIMAP interacting partner proteins were identified. Four of identified proteins: cystein and glycine-rich protein 1, eukaryotic translation elongation factor 2, U5 snRNP-specific protein 116 kD, and solute carrier family 3 member 2 are involved in actin cytoskeleton organization, cell adhesion or translation and transcriptional regulation.

RasGRP1 confers the phorbol ester-sensitive phenotype to EL4 lymphoma cells

The murine EL4 lymphoma cell line exists in variants that are either sensitive or resistant to the tumor promoter phorbol 12-myristate 13-acetate (PMA). In sensitive EL4 cells, PMA causes robust Erk mitogen-activated protein kinase activation that results in growth arrest. In resistant cells, PMA induces minimal Erk activation, without growth arrest. PMA stimulates IL-2 production in sensitive, but not resistant, cells. The role of RasGRP1, a PMA-activated guanine nucleotide exchange factor for Ras, in EL4 phenotype was examined. Endogenous RasGRP1 protein is expressed at much higher levels in sensitive than in resistant cells. PMA-induced Ras activation is observed in sensitive cells but not in resistant cells lacking Ras-GRP1. PMA induces down-regulation of RasGRP1 protein in sensitive cells but increases RasGRP1 in resistant cells. Transfection of RasGRP1 into resistant cells enhances PMA-induced Erk activation. In the reverse experiment, introduction of small interfering RNA (siRNA) for RasGRP1 suppresses PMA-induced Ras and Erk activations in sensitive cells. Sensitive cells incubated with siRNA for RasGRP1 exhibit the PMA-resistant phenotype, in that they are able to proliferate in the presence of PMA and do not secrete IL-2 when stimulated with PMA. These studies indicate that the PMA-sensitive phenotype, as previously defined for the EL4 cell line, is conferred by endogenous expression of RasGRP1 protein.

The ERK cascade: a prototype of MAPK signaling

Sequential activation of protein kinases within the mitogen-activated protein kinase (MAPK) cascades is a common mechanism of signal transduction in many cellular processes. Four such cascades have been elucidated thus far, and named according to their MAPK tier component as the ERK1/2, JNK, p38MAPK, and ERK5 cascades. These cascades cooperate in transmitting various extracellular signals, and thus control cellular processes such as proliferation, differentiation, development, stress response, and apoptosis. Here we describe the classic ERK1/2 cascade, and concentrate mainly on the properties of MEK1/2 and ERK1/2, including their mode of regulation and their role in various cellular processes and in oncogenesis. This cascade may serve as a prototype of the other MAPK cascades, and the study of this cascade is likely to contribute to the understanding of mitogenic and other processes in many cell lines and tissues.

Glycogen synthase kinase 3- and extracellular signal-regulated kinase-dependent phosphorylation of paxillin regulates cytoskeletal rearrangement

Paxillin is a 68-kDa focal adhesion-associated protein that plays an important role in controlling cell spreading and migration. Phosphorylation of paxillin regulates its biological activity and thus has warranted investigation. Serine 126 and serine 130 were previously identified as two major extracellular signal-regulated kinase (ERK)-dependent phosphorylation sites in Raf-transformed fibroblasts. Here serine 126 is identified as a phosphorylation site induced by lipopolysaccharide (LPS) stimulation of RAW264.7 cells. A number of other stimuli, including adhesion and colony-stimulating factor, induce serine 126 phosphorylation in RAW264.7 cells, and nerve growth factor (NGF) treatment induces serine 126 phosphorylation in PC12 cells. The kinase responsible for phosphorylation of this site is identified as glycogen synthase kinase 3 (GSK-3). Interestingly, this GSK-3-dependent phosphorylation is regulated via an ERK-dependent priming mechanism, i.e., phosphorylation of serine 130. Phosphorylation of S126/S130 was required to promote spreading in paxillin null cells, and LPS-induced spreading of RAW264.7 cells was inhibited by expression of the paxillin S126A/S130A mutant. Furthermore, this mutant also retarded NGF-induced PC12 cell neurite outgrowth. Hence, phosphorylation of paxillin on serines 126 and 130, which is mediated by an ERK/GSK-3 dual-kinase mechanism, plays an important role in cytoskeletal rearrangement.

The extracellular signal-regulated kinase: multiple substrates regulate diverse cellular functions

The extracellular signal-regulated kinase (ERK) cascade is a central pathway that transmits signals from many extracellular agents to regulate cellular processes such as proliferation, differentiation and cell cycle progression. The signaling via the ERK cascade is mediated by sequential phosphorylation and activation of protein kinases in the different tiers of the cascade. Although the main core phosphorylation chain of the cascade includes Raf kinases, MEK1/2, ERK1/2 (ERKs) and RSKs, other alternatively spliced forms and distinct components exist in the different tiers, and participate in ERK signaling under specific conditions. These components enhance the complexity of the ERK cascade and thereby, enable the wide variety of functions that are regulated by it. Another factor that is important for the dissemination of ERKs' signals is the multiplicity of the cascade's substrates, which include transcription factors, protein kinases and phosphatases, cytoskeletal elements, regulators of apoptosis, and a variety of other signaling-related molecules. About 160 substrates have already been discovered for ERKs, and the list of these substrates, as well as the function and mechanism of activation of representative substrates, are described in the current review. Many of these substrates are localized in the nucleus, and seem to participate in the regulation of transcription upon stimulation. However, other substrates are found in the cytosol as well as in other cellular organelles, and those are responsible for processes such as translation, mitosis and apoptosis. Understanding of these processes may provide a full picture of the distinct, and even opposing cellular processes that are regulated by the ERK cascade.

Low-dose exposure of intestinal epithelial cells to formaldehyde results in MAP kinase activation and molecular alteration of the focal adhesion protein paxillin

We investigated the potential pathophysiological role of non-lethal formaldehyde concentrations on human intestinal epithelial HT-29 cells. Expression levels of actin, tubulin and detectable cytokeratin isoforms 5, 13, 18, 19 and 20 were not affected after 24h of exposure to 1mM formaldehyde. By contrast, cellular organization of cytoskeletal constituents was already changed after 60 min. Within 15 min, formaldehyde induced profound tyrosine phosphorylation of the focal adhesion protein paxillin and of proteins at about 120-130 kDa. Concomitantly, phosphorylation of ERK-1/2 and p38 MAP kinase occurred. Paxillin was not only tyrosine phosphorylated but underwent a sustained molecular weight shift representing serine/threonine phosphorylation that was independent of MAP kinase activity and EGF-R-mediated signalling. Our data show that exposure of intestinal epithelial cells to low-dose formaldehyde is followed by rapid and profound signalling events. The data suggest a modifier role of environmental or endogenous formaldehyde for epithelial cell functions.

A Role for Phosphatidylinositol 3-Kinase in TCR-Stimulated ERK Activation Leading to Paxillin Phosphorylation and CTL Degranulation

PI3K is an important regulator of a number of cellular processes. We examined the contribution of PI3K to mouse CTL signaling, leading to degranulation. We show that TCR-triggered, but not phorbol ester and calcium ionophore-induced, CTL degranulation is dependent on PI3K activity. Although PI3K activity is required for optimal LFA-1-mediated adhesion and cell spreading, this most likely does not account for its full contribution to degranulation. We demonstrate that PI3K is required for TCR-stimulated ERK activation in CTL, which we have shown previously to be required for CTL degranulation. We thus define a pathway through which PI3K most likely regulates degranulation and in which ERK appears to be a key signaling molecule. Furthermore, we identified the cytoskeletal adaptor paxillin as a target of ERK downstream of TCR stimulation. Consistent with a role in degranulation, we demonstrate that paxillin is localized to the microtubule organizing center in resting cells and upon target cell binding is recruited to the contact point with the target cell. These studies demonstrate that PI3K regulates ERK activity leading to CTL degranulation, and identify paxillin as a target of ERK downstream of the TCR. That paxillin is independently phosphorylated by both tyrosine kinase(s) and ERK downstream of the TCR and localized both at the microtubule organizing center and at the target cell contact point suggests an important role for paxillin in CTL-mediated killing.

Loss of SMEK, a novel, conserved protein, suppresses MEK1 null cell polarity, chemotaxis, and gene expression defects

MEK/extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase signaling is imperative for proper chemotaxis. Dictyostelium mek1(-) (MEK1 null) and erk1(-) cells exhibit severe defects in cell polarization and directional movement, but the molecules responsible for the mek1(-) and erk1(-) chemotaxis defects are unknown. Here, we describe a novel, evolutionarily conserved gene and protein (smkA and SMEK, respectively), whose loss partially suppresses the mek1(-) chemotaxis phenotypes. SMEK also has MEK1-independent functions: SMEK, but not MEK1, is required for proper cytokinesis during vegetative growth, timely exit from the mound stage during development, and myosin II assembly. SMEK localizes to the cell cortex through an EVH1 domain at its N terminus during vegetative growth. At the onset of development, SMEK translocates to the nucleus via a nuclear localization signal (NLS) at its C terminus. The importance of SMEK's nuclear localization is demonstrated by our findings that a mutant lacking the EVH1 domain complements SMEK deficiency, whereas a mutant lacking the NLS does not. Microarray analysis reveals that some genes are precociously expressed in mek1(-) and erk1(-) cells. The misexpression of some of these genes is suppressed in the smkA deletion. These data suggest that loss of MEK1/ERK1 signaling compromises gene expression and chemotaxis in a SMEK-dependent manner.

The novel triterpenoid CDDO-Me suppresses MAPK pathways and promotes p38 activation in acute myeloid leukemia cells

Development of novel therapeutic strategies is a continuing challenge for the treatment of acute myeloid leukemia (AML). The novel triterpenoid, C-28 methyl ester of 2-cyano-3,12-dioxoolen-1,9-dien-28-oic acid (CDDO-Me), induces apoptosis in myeloid leukemic cell lines and in primary AML samples. In this report, the effects of CDDO-Me on CD34(+) AML progenitor cells in vitro were examined. CDDO-Me induced apoptosis in all but one of ten AML samples. CDDO-Me is known to inhibit the activation of ERK1/2. In this series of primary AML samples, ERK was expressed and phosphorylated in all patient samples studied and CDDO-Me inhibited ERK phosphorylation in five of 10 samples. However, CDDO-Me induced apoptosis in four of five samples without decreasing pERK levels, suggesting that pERK is not the sole target of the compound. CDDO-Me induced phosphorylation of p38 in AML-derived U937 cells. Pretreatment of U937 cells with a p38 inhibitor protected cells from the cyto-toxic effects of CDDO-Me. These findings suggest a role for p38 in CDDO-Me-induced apoptosis. In preliminary studies, CDDO-Me induced p38 phosphorylation in seven of eight primary AML samples. These findings suggest that CDDO-Me treatment shifts cell signaling away from cyto-protective pathways and thus CDDO-Me may be effective for the treatment of AML.

p130cas but not paxillin is essential for Caco-2 intestinal epithelial cell spreading and migration on collagen IV

We have previously observed that collagen IV regulates Caco-2 intestinal epithelial cell spreading and migration via Src kinase and stimulates Src-dependent tyrosine phosphorylation of p130cas. We observed that collagen IV also stimulated Src-dependent phosphorylation of both paxillin Tyr31 and paxillin Tyr118. Caco-2 transfection with paxillin or p130cas siRNAs inhibited expression of these proteins by more than 90% for at least 5 days after transfection. Although p130cas siRNA inhibited cell spreading on collagen IV by 33%, three different paxillin siRNAs did not inhibit cell spreading. p130cas siRNA did not affect Src Tyr416 or Src Tyr527 phosphorylation, FAK Tyr397 phosphorylation, or Src-dependent phosphorylation of FAK Tyr925, suggesting that p130cas did not inhibit cell spreading by altering FAK or Src activity. Rat p130cas expression after siRNA knock-out of endogenous human p130cas in Caco-2 cells reduced cell spreading inhibition by 71%. In contrast, expression of rat p130cas from which the Src-phosphorylated substrate domain was deleted did not rescue siRNA inhibition of cell spreading. Combined treatment with siRNAs to Crk and CrkL, which bind to the p130cas substrate domain, inhibited cell spreading by 54%. Both p130cas siRNA and the combined Crk/CrkL siRNAs strongly inhibited (52 and 46% inhibition, respectively) Caco-2 sheet migration on collagen IV and noticeably inhibited lamellipodial extension, whereas paxillin siRNA only inhibited migration by 18% and did not noticeably affect lamellipodial extension. These results suggest that Src may regulate Caco-2 migration on collagen IV via both p130cas and paxillin but that Src phosphorylation of p130cas is more important for this process.

Intracellular protein phosphorylation in adherent U937 monocytes mediated by various culture conditions and fibronectin-derived surface ligands

Macrophages play a central role in the normal healing process after tissue injury and the host response to foreign objects such as biomaterials. The process leading to macrophage adhesion and activation on protein-adsorbed substrates is complex and unresolved. While the use of primary cells offers clinical relevancy, macrophage cell lines offer unique advantages such as availability and relatively homogeneous phenotype as models to probe the molecular mechanism of cell-surface interaction. Our goal was to better characterize the effect of the culture condition and surface-associated ligands on the extent of U937 adhesion. Tyrosine phosphorylation of intracellular proteins was surveyed as a basis to seek a greater understanding of the molecular mechanism involved in mediating U937 adhesion on various ligand-adsorbed surfaces. U937 viability and adhesion on tissue culture polystyrene (TCPS) increased with (i) increasing serum level, (ii) decreasing tyrosine phosphorylation inhibitor AG18 concentration, or (iii) increasing culture time. The adsorption of various adhesion proteins such as fibronectin and peptide ligands (i.e., RGD, PHSRN) on TCPS did not significantly increase the adherent density of U937 when compared with albumin and PBS ligand controls. However, ligand identity and the presence of phorbol myristate acetate dramatically affected the extent (i.e., increase or decrease) and the identity (i.e., molecular weight) of phosphotyrosine proteins in adherent U937 in a time-dependent manner. The extent and identity of phosphotyrosine proteins did not exhibit a clear AG18 dose dependency, rather the level of tyrosine phosphorylation for a distinct group of proteins was either increased or decreased for a given AG18 concentration.

CXCR4-mediated adhesion and MMP-9 secretion in head and neck squamous cell carcinoma

The chemokine CXCL12 (SDF-1) and its receptor, CXCR4, have been implicated in organ-specific metastases of several malignancies. Head and neck squamous cell carcinoma (HNSCC) predominantly metastasizes to lymph nodes, and recent evidence has shown that CXCL12 stimulates HNSCC migration. We explored the potential role of CXCR4 in mediating other metastatic processes in HNSCC cells. CXCR4 mRNA and cell-surface expression was assessed in HNSCC cell lines. CXCR4 mRNA expression was detected in five HNSCC cell lines. Cell-surface CXCR4 was also detected in each of the HNSCC cell lines and in resected HNSCC tissues. CXCL12 induced rapid intracellular calcium mobilization in a metastatic HNSCC cell line (HN), as well as rapid phosphorylation of ERK-1/2. HNSCC cell adhesion to fibronectin and collagen was increased by CXCL12 treatment, while the addition of an inhibitor of ERK-1/2 signaling, PD98059, reduced the effects of CXCL12. CXCL12 also increased the active matrix metalloproteinase (MMP)-9 secreted. Thus, HNSCC cells express functional CXCR4 receptors that induce rapid intracellular signaling upon binding to CXCL12. Such binding leads to increased HNSCC cell adhesion and MMP secretion, suggesting that CXCR4 may be a novel regulator of HNSCC metastatic processes.

Phosphorylation of paxillin by p38MAPK is involved in the neurite extension of PC-12 cells

Cell adhesions play an important role in neurite extension. Paxillin, a focal adhesion adaptor protein involved in focal adhesion dynamics, has been demonstrated to be required for neurite outgrowth. However, the molecular mechanism by which paxillin regulates neurite outgrowth is unknown. Here, we show that paxillin is phosphorylated by p38MAPK in vitro and in nerve growth factor (NGF)–induced PC-12 cells. Ser 85 (Ser 83 for endogenous paxillin) is identified as one of major phosphorylation sites by phosphopeptide mapping and mass spectrometry. Moreover, expression of the Ser 85 → Ala mutant of paxillin (pax_S85A) significantly inhibits NGF-induced neurite extension of PC-12 cells, whereas expression of wild-type (wt) paxillin does not influence neurite outgrowth. Further experiments indicate that cells expressing pax_S85A exhibit small, clustered focal adhesions which are not normally seen in cells expressing wt paxillin. Although wt paxillin and pax_S85A have the same ability to bind vinculin and focal adhesion kinase, wt paxillin more efficiently associates with Pyk2 than pax_S85A. Thus, phosphorylation of paxillin is involved in NGF-induced neurite extension of PC-12 cells, probably through regulating focal adhesion organization.

Mitogen-activated protein kinase feedback phosphorylation regulates MEK1 complex formation and activation during cellular adhesion

Cell adhesion and spreading depend on activation of mitogen-activated kinase, which in turn is regulated both by growth factor and integrin signaling. Growth factors, such as epidermal growth factor, are capable of activating Ras and Raf, but integrin signaling is required to couple Raf to MEK and MEK to extracellular signal-regulated protein kinase (ERK). It was previously shown that Rac-p21-activated kinase (PAK) signaling regulated the physical association of MEK1 with ERK2 through phosphorylation sites in the proline-rich sequence (PRS) of MEK1. It was also shown that activation of MEK1 and ERK by integrins depends on PAK phosphorylation of S298 in the PRS. Here we report a novel MEK1-specific regulatory feedback mechanism that provides a means by which activated ERK can terminate continued PAK phosphorylation of MEK1. Activated ERK can phosphorylate T292 in the PRS, and this blocks the ability of PAK to phosphorylate S298 and of Rac-PAK signaling to enhance MEK1-ERK complex formation. Preventing ERK feedback phosphorylation on T292 during cellular adhesion prolonged phosphorylation of S298 by PAK and phosphorylation of S218 and S222, the MEK1 activating sites. We propose that activation of ERK during adhesion creates a feedback system in which ERK phosphorylates MEK1 on T292, and this in turn blocks additional S298 phosphorylation in response to integrin signaling.

Cynaropicrin, a sesquiterpene lactone, as a new strong regulator of CD29 and CD98 functions

Cynaropicrin is a sesquiterpene lactone displaying immunomodulatory effects on the production of cytokine and nitric oxide from macrophages/monocytes. In this study we have examined inhibitory effect of cynaropicrin on activation of major adhesion molecules [CD29 (beta1 integrins), CD43, and CD98] on the cells assessed by U937 (promonocytic cells) homotypic aggregation. Cynaropicrin potently blocked CD29 (beta1 integrins)- and CD98-induced homotypic aggregation with IC(50) values of 3.46 and 2.98 microM, respectively, without displaying cytotoxicity. Similarly, flow cytometric analysis exhibited that cynaropicrin down-regulated strikingly surface level of CD29 and CD147, a functional regulator of CD98, but not CD43. More importantly, cynaropicrin inhibition was linked to blockade of extracellular signal-related kinase (ERK) activation and distinct from other enzyme inhibitors including rottlerin, propranolol, forskolin, and chloroquine, but not cytochalasin B. Therefore, our finding is the first demonstration that cynaropicrin may be a potent functional regulator of CD29 and CD98 via interrupting ERK activation which may be linked to cytoskeleton rearrangement, suggesting further application to CD29- and CD98-mediated diseases such as virus-induced chronic inflammation, and invasion, migration, and metastasis of leukocyte cancer cells.

T cell aggregation induced through CD43: intracellular signals and inhibition by the immunomodulatory drug leflunomide

The CD43 coreceptor molecule has been shown to participate in lymphocyte adhesion and activation. Leukocyte homotypic aggregation results from a cascade of intracellular signals delivered to the cells upon engagement of different cell-surface molecules with their natural ligands. This phenomenon requires an active metabolism, reorganization of the cytoskeleton, and relocalization of cell-surface molecules. The aim of this study was to identify some of the key members of the signaling cascade leading to T lymphocyte homotypic aggregation following CD43 engagement. CD43-mediated homotypic aggregation of T lymphocytes required the participation of Src kinases, phospholipase C-gamma2, protein kinase C, phosphatidylinositol-3 kinase, as well as extracellular-regulated kinase 1/2 and p38. Data shown here suggest that these signaling molecules play a central role in regulating actin cytoskeleton remodeling after CD43 ligation. We also evaluated the ability of immunomodulatory drugs such as leflunomide to block the CD43-mediated homotypic aggregation. Leflunomide blocked the recruitment of targets of the Src family kinases as well as actin polymerization, diminishing the ability of T lymphocytes to aggregate in response to CD43-specific signals, suggesting that this drug might control the migration and recruitment of lymphoid cells to inflamed tissues.

In vitro anti-inflammatory and pro-aggregative effects of a lipid compound, petrocortyne A, from marine sponges

(3 S,14 S)-Petrocortyne A, a lipid compound (a C(46) polyacetylenic alcohol), from marine sponges ( Petrosia sp.) is potently cytotoxic against several solid tumour cells. In this study, we investigated in vitro anti-inflammatory and pro-aggregative effects of petrocortyne A at non-cytotoxic concentrations on various cellular inflammatory phenomena using the macrophage and monocytic cell lines RAW264.7 and U937. Petrocortyne A blocked tumour necrosis factor-alpha (TNF-alpha) production strongly and concentration-dependently in lipopolysaccharide (LPS)-activated RAW264.7 cells and phorbol 12-myristate 13-acetate (PMA)/LPS-treated U937 cells. It also blocked NO production concentration-dependently in LPS- or interferon (IFN)-gamma-treated RAW264.7 cells. Among the migration factors tested, the compound selectively blocked the expression of hepatocyte growth factor/scatter factor (HGF/SF). On the other hand, as assessed by a cell-cell adhesion assay, petrocortyne A did not block the activation of adhesion molecules induced by aggregative antibodies to adhesion molecules, but suppressed PMA-induced cell-cell adhesion significantly. Intriguingly, petrocortyne A induced U937 homotypic aggregation following long exposure (2 and 3 days), accompanied by weak induction of pro-aggregative signals such as tyrosine phosphorylation of p132 and phosphorylation of extracellular signal-related kinase 1 and 2 (ERK 1/2). Petrocortyne A may thus inhibit cellular inflammatory processes and immune cell migration to inflamed tissue.

CD98-dependent homotypic aggregation is associated with translocation of protein kinase Cdelta and activation of mitogen-activated protein kinases

CD98 is a protein found on the surface of many activated cell types, and is implicated in the regulation of cellular differentiation, adhesion, growth, and apoptosis. Despite many studies addressing CD98 function, there is little information on the intracellular signalling pathways that mediate its activity. In this study, we examine protein kinase pathways that are activated following ligation by the CD98 antibody AHN-18, an antibody that induces U937 homotypic aggregation and inhibits antigen presenting activity and T-cell activation. Ligation by CD98 antibody AHN-18 induces tyrosine kinase activity, but inhibition of this activity does not affect U937 aggregation. Ligation also induces membrane translocation of the serine/threonine kinase novel PKCdelta, but not other members of the PKC family. Translocation is blocked by rottlerin, and this inhibitor also blocks aggregation. PKCdelta activation in turn mediates activation of ERK1/2 and p38, as well as tyrosine phosphorylation of multiple proteins, and MAPK activation is essential for cellular aggregation. One of the targets of CD98-induced tyrosine phosphorylation is itself PKCdelta, suggesting that this phosphorylation may act as a negative feedback to limit the overall activation of the CD98 pathway.

The protein tyrosine phosphatase HePTP regulates nuclear translocation of ERK2 and can modulate megakaryocytic differentiation of K562 cells

In response to PMA treatment K562 myelogenous leukemia cells undergo megakaryocytic differentiation, which is dependent on prolonged ERK activation and is characterized by growth arrest, upregulation of CD41 and IL-6, and, finally, by characteristic changes in cell morphology. The tyrosine phosphatase HePTP was recently demonstrated to regulate ERK activity and changes in HePTP expression have been associated with hematopoietic malignancies. Here, we have studied the function of HePTP during PMA-induced megakaryocytic differentiation of K562 cells. Overexpression of HePTP or inhibition of HePTP expression with antisense cDNA had no effect on PMA-induced cell cycle arrest or upregulation of cyclin D in K562 cells. The expression of megakaryocytic markers such as CD41 and IL6, however, were highly reduced in cells overexpressing HePTP, due to reduced ERK activation, and the cells were impaired in their ability to differentiate. Compared to control cells, HePTP antisense expressing cells did not show increased basal or PMA-induced ERK activity. However, antisense inhibition of HePTP enhanced nuclear translocation of ERK and the expression of the megakaryocytic markers CD41 and IL-6. Interestingly, like cells overexpressing HePTP, morphological differentiation was also impaired in HePTP antisense expressing cells. The results for the first time demonstrate that different aspects of megakaryocytic differentiation have distinct requirements for ERK activity. They further show that HePTP is involved in the regulation of nuclear translocation of ERK2 and that HePTP protein levels can modulate K562 cell differentiation.

Lysophosphatidic acid induces focal adhesion assembly through Rho/Rho-associated kinase pathway in human ovarian cancer cells

OBJECTIVE

The level of lysophosphatidic acid (LPA) is elevated in patients with ovarian cancer, and LPA has been reported to have a pivotal role in cancer dissemination. In the current study, the effect of LPA on the motility of ovarian cancer cells was investigated.

METHODS

We analyzed the effects of LPA on the migration activity, the focal adhesion formation, and the tyrosine phosphorylation of focal adhesion proteins in human ovarian cancer cell lines Caov-3 and OVCAR-3. Inhibitors of the small GTPase Rho, one of its downstream effectors (Rho-associated kinase (ROCK)), myosin light chain kinase (MLCK), and myosin light chain (MLC) phosphatase were used to examine the mechanism of LPA-induced cellular effects.

RESULTS

LPA enhanced the migration of ovarian cancer cells and facilitated their invasion. Rho and ROCK played essential roles in the migratory process, as evidenced by the inhibition of migration and focal adhesion formation of cancer cells by Clostridium botulinum C3 exoenzyme (C3), an inhibitor of Rho, or Y-27632, an inhibitor of ROCK. LPA also evoked the formation of focal adhesions and tyrosine phosphorylation of focal adhesion kinase and paxillin, all of which were inhibited by C3 or Y-27632.

CONCLUSION

These results suggest that LPA induced the migration of ovarian cancer cells, at least in part, through accelerated formation of focal adhesions mediated by Rho/ROCK-induced actomyosin contractility. This study may provide the basis for new therapies to control the metastasis of ovarian cancer.

Epidermal growth factor stimulates serine/threonine phosphorylation of the focal adhesion protein paxillin in a MEK-dependent manner in normal rat kidney cells

Epidermal growth factor (EGF)-stimulated proliferation of renal epithelial cells plays an important role in the recovery of kidney tubule epithelia following exposure to insult. Numerous studies have demonstrated that tyrosine phosphorylation of the focal adhesion protein paxillin mediates in part the effects of growth factors on cell growth, migration, and organization of the actin-based cytoskeleton. The experiments in this report were designed to determine the effect of EGF on paxillin phosphorylation in normal rat kidney (NRK) epithelial cells. Interestingly, treatment of NRK cells with EGF stimulated paxillin serine/threonine phosphorylation, which caused a reduction in the mobility of paxillin on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The EGF-stimulated mobility shift of paxillin was independent of an intact cytoskeleton, phosphatidylinositol 3-kinase (PI 3-kinase) activation, protein kinase C (PKC) activation, and cellular adhesion. However, inhibitors of the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase abrogated the EGF-stimulated change in paxillin mobility. In addition, the EGF-stimulated change in paxillin serine/threonine phosphorylation was not accompanied by a profound reorganization of the actin cytoskeleton. These results identify paxillin as a component EGF signaling in renal epithelial cells and implicate members of the MAP kinase pathway as critical regulators of paxillin serine/threonine phosphorylation.

v-Src's hold over actin and cell adhesions

The oncoprotein v-Src and its cellular homologue (c-Src) are tyrosine kinases that modulate the actin cytoskeleton and cell adhesions. Through the concerted action of their protein-interaction and kinase domains, they are targeted to cell matrix integrin adhesions or cadherin-dependent junctions between epithelial cells, where they phosphorylate substrates that induce adhesion turnover and actin re-modelling. Recent experiments have defined some of the key targets and effector pathways that mediate the pleiotropic oncogenic effects of v-Src.

Hepatocyte growth factor induces ERK-dependent paxillin phosphorylation and regulates paxillin-focal adhesion kinase association

Hepatocyte growth factor (HGF) modulates cell adhesion, migration, and branching morphogenesis in cultured epithelial cells, events that require regulation of cell-matrix interactions. Using mIMCD-3 epithelial cells, we studied the effect of HGF on the focal adhesion proteins, focal adhesion kinase (FAK) and paxillin and their association. HGF was found to increase the tyrosine phosphorylation of paxillin and to a lesser degree FAK. In addition, HGF induced association of paxillin and activated ERK, correlating with a gel retardation of paxillin that was prevented with the ERK inhibitor U0126. The ability of activated ERK to phosphorylate and induce gel retardation of paxillin was confirmed in vitro in both full-length and amino-terminal paxillin. Several potential ERK phosphorylation sites in paxillin flank the paxillin-FAK association domains, so the ability of HGF to regulate paxillin-FAK association was examined. HGF induced an increase in paxillin-FAK association that was inhibited by pretreatment with U0126 and reproduced by in vitro phosphorylation of paxillin with ERK. The prevention of the FAK-paxillin association with U0126 correlated with an inhibition of the HGF-mediated FAK tyrosine phosphorylation and inhibition of HGF-dependent cell spreading and adhesion. An examination of cellular localization of FAK and paxillin demonstrated that HGF caused a condensation of focal adhesion complexes at the leading edges of cell processes and FAK-paxillin co-localization in these large complexes. Thus, these data suggest that HGF can induce serine/threonine phosphorylation of paxillin most probably mediated directly by ERK, resulting in the recruitment and activation of FAK and subsequent enhancement of cell spreading and adhesion.

Noradrenaline-induced paxillin phosphorylation, ERK activation and MEK-regulated contraction in intact rat mesenteric arteries

In rat mesenteric arteries, noradrenaline (NA) induces a time-dependent increase in tyrosine phosphorylation of a number of proteins, one of which was identified as paxillin. NA-induced protein tyrosine phosphorylation was ablated by tyrosine kinase inhibition, virtually unaffected by protein kinase C (PKC) inhibition or PKC downregulation and was mimicked by KCl. NA also caused a time-dependent activation of the extracellular signal-regulated kinases (ERK)1 and ERK2. These responses were blocked by the ERK-activating kinase (MEK) inhibitor PD98059 and by tyrosine kinase inhibition but only modestly attenuated by PKC downregulation or inhibition. Pretreatment of cannulated mesenteric arteries (50 mm Hg internal pressure) with PD98059 significantly reduced the contractile responsiveness of the vessels to NA (1.56 +/- 0.14 microM, EC(50) control; 3.32 +/- 0.49 microM, EC(50) + PD98059, p < 0.01). Thus, NA induces time-dependent increases in protein-tyrosine phosphorylation and ERK activation in rat mesenteric arteries that could suggest a role for Ca(2+)-dependent non-receptor tyrosine kinases and ERKs in the response of small arteries to NA. In addition, the modulation of NA-induced mesenteric artery contraction by inhibition of the MEK/ERK pathway further implicates ERK in the regulation of, though perhaps not the mediation of NA-induced small artery contraction.

Paxillin: a focal adhesion-associated adaptor protein

Paxillin is a focal adhesion-associated, phosphotyrosine-containing protein that may play a role in several signaling pathways. Paxillin contains a number of motifs that mediate protein-protein interactions, including LD motifs, LIM domains, an SH3 domain-binding site and SH2 domain-binding sites. These motifs serve as docking sites for cytoskeletal proteins, tyrosine kinases, serine/threonine kinases, GTPase activating proteins and other adaptor proteins that recruit additional enzymes into complex with paxillin. Thus paxillin itself serves as a docking protein to recruit signaling molecules to a specific cellular compartment, the focal adhesions, and/or to recruit specific combinations of signaling molecules into a complex to coordinate downstream signaling. The biological function of paxillin coordinated signaling is likely to regulate cell spreading and motility.