Activation of a Ca2+-inhibitable protein kinase that phosphorylates microtubule-associated protein 2 in vitro by growth factors, phorbol esters, and serum in quiescent cultured human fibroblasts

Fibroblast growth factor signalling influences homologous recombination-mediated DNA damage repair to promote drug resistance in ovarian cancer

BACKGROUND

Ovarian cancer patients frequently develop chemotherapy resistance, limiting treatment options. We have previously shown that individuality in fibroblast growth factor 1 (FGF1) expression influences survival and chemotherapy response.

METHODS

We used MTT assays to assess chemosensitivity to cisplatin and carboplatin following shRNA-mediated knockdown or heterologous over-expression of FGF1 (quantified by qRT-PCR and immunoblot analysis), and in combination with the FGFR inhibitors AZD4547 and SU5402, the ATM inhibitor KU55933 and DNA-PK inhibitor NU7026. Immunofluorescence microscopy was used to quantify the FGF1-dependent timecourse of replication protein A (RPA) and γH2AX foci formation.

RESULTS

Pharmacological inhibition of FGF signalling reversed drug resistance in immortalised cell lines and in primary cell lines from drug-resistant ovarian cancer patients, while FGF1 over-expression induced resistance. Ataxia telangiectasia mutated (ATM) phosphorylation, but not DNA adduct formation was FGF1 dependent, following cisplatin or carboplatin challenge. Combining platinum drugs with the ATM inhibitor KU55933, but not with the DNA-PK inhibitor NU7026 re-sensitised resistant cells. FGF1 expression influenced the timecourse of damage-induced RPA and γH2AX nuclear foci formation.

CONCLUSION

Drug resistance arises from FGF1-mediated differential activation of high-fidelity homologous recombination DNA damage repair. FGFR and ATM inhibitors reverse platinum drug resistance, highlighting novel combination chemotherapy approaches for future clinical trial evaluation.

Stress Relief Techniques: p38 MAPK Determines the Balance of Cell Cycle and Apoptosis Pathways

Protein signaling networks are formed from diverse and inter-connected cell signaling pathways converging into webs of function and regulation. These signaling pathways both receive and conduct molecular messages, often by a series of post-translation modifications such as phosphorylation or through protein-protein interactions via intrinsic motifs. The mitogen activated protein kinases (MAPKs) are components of kinase cascades that transmit signals through phosphorylation. There are several MAPK subfamilies, and one subfamily is the stress-activated protein kinases, which in mammals is the p38 family. The p38 enzymes mediate a variety of cellular outcomes including DNA repair, cell survival/cell fate decisions, and cell cycle arrest. The cell cycle is itself a signaling system that precisely controls DNA replication, chromosome segregation, and cellular division. Another indispensable cell function influenced by the p38 stress response is programmed cell death (apoptosis). As the regulators of cell survival, the BCL2 family of proteins and their dynamics are exquisitely sensitive to cell stress. The BCL2 family forms a protein-protein interaction network divided into anti-apoptotic and pro-apoptotic members, and the balance of binding between these two sides determines cell survival. Here, we discuss the intersections among the p38 MAPK, cell cycle, and apoptosis signaling pathways.

ERK signalling: a master regulator of cell behaviour, life and fate

The proteins extracellular signal-regulated kinase 1 (ERK1) and ERK2 are the downstream components of a phosphorelay pathway that conveys growth and mitogenic signals largely channelled by the small RAS GTPases. By phosphorylating widely diverse substrates, ERK proteins govern a variety of evolutionarily conserved cellular processes in metazoans, the dysregulation of which contributes to the cause of distinct human diseases. The mechanisms underlying the regulation of ERK1 and ERK2, their mode of action and their impact on the development and homeostasis of various organisms have been the focus of much attention for nearly three decades. In this Review, we discuss the current understanding of this important class of kinases. We begin with a brief overview of the structure, regulation, substrate recognition and subcellular localization of ERK1 and ERK2. We then systematically discuss how ERK signalling regulates six fundamental cellular processes in response to extracellular cues. These processes are cell proliferation, cell survival, cell growth, cell metabolism, cell migration and cell differentiation.

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.

Nicotinic modulation of hippocampal cell signaling and associated effects on learning and memory

The hippocampus is a key brain structure involved in synaptic plasticity associated with long-term declarative memory formation. Importantly, nicotine and activation of nicotinic acetylcholine receptors (nAChRs) can alter hippocampal plasticity and these changes may occur through modulation of hippocampal kinases and transcription factors. Hippocampal kinases such as cAMP-dependent protein kinase (PKA), calcium/calmodulin-dependent protein kinases (CAMKs), extracellular signal-regulated kinases 1 and 2 (ERK1/2), and c-jun N-terminal kinase 1 (JNK1), and the transcription factor cAMP-response element-binding protein (CREB) that are activated either directly or indirectly by nicotine may modulate hippocampal plasticity and in parallel hippocampus-dependent learning and memory. Evidence suggests that nicotine may alter hippocampus-dependent learning by changing the time and magnitude of activation of kinases and transcription factors normally involved in learning and by recruiting additional cell signaling molecules. Understanding how nicotine alters learning and memory will advance basic understanding of the neural substrates of learning and aid in understanding mental disorders that involve cognitive and learning deficits.

Cellular and sub-cellular responses to UVA in relation to carcinogenesis

PURPOSE

UVA radiation (315-400 nm) contributes to skin aging and carcinogenesis. The aim of this review is to consider the mechanisms that underlie UVA-induced cellular damage, how this damage may be prevented or repaired and the signal transduction processes that are elicited in response to it.

RESULTS

Exposure to ultraviolet (UV) light is well-established as the causative factor in skin cancer. Until recently, most work on the mechanisms that underlie skin carcinogenesis focused on shorter wavelength UVB radiation (280-315 nm), however in recent years there has been increased interest in the contribution made by UVA. UVA is able to cause a range of damage to cellular biomolecules including lipid peroxidation, oxidized protein and DNA damage, such as 8-oxoguanine and cyclobutane pyrimidine dimers. Such damage is strongly implicated in both cell death and malignant transformation and cells have a number of mechanisms in place to mitigate the effects of UVA exposure, including antioxidants, DNA repair, and stress signalling pathways.

CONCLUSIONS

The past decade has seen a surge of interest in the biological effects of UVA exposure as its significance to the process of photo-carcinogenesis has become increasingly evident. However, unpicking the unique complexity of the cellular response to UVA, which is only now becoming apparent, will be a major challenge for the field of photobiology in the 21st century.

Growth factor-stimulated phosphorylation cascades: activation of growth factor-stimulated MAP kinase

Protein phosphorylation is an important mechanism in the response of cells to growth factors by which signals can be conveyed from cell surface receptors to intracellular targets. In addition to stimulation of protein tyrosine phosphorylation, activation of growth factor receptors having protein tyrosine kinase activity leads to dramatic alterations in the levels of protein serine/threonine phosphorylation. Several growth factor-stimulated serine/threonine-specific kinases have been identified as potential mediators of such signalling. MAP (microtubule-associated protein) kinase has emerged as a very interesting member of this group, because it activates a separate kinase, pp90rsk, which is also growth factor-stimulated. MAP kinase itself appears to be regulated by protein phosphorylation, because it can be inactivated by protein phosphatases. We have identified two 60 kDa proteins that promote the phosphorylation and full activation of MAP kinase in a manner paralleling its activation by growth factors in intact cells. These 'MAP kinase activators' are themselves stimulated by growth factors, suggesting that they function as intermediates between the MAP kinase and cell surface receptors in a growth factor-stimulated kinase cascade. Identification of the components of this protein kinase cascade reveals a mechanism by which at least some of the effects of receptor tyrosine kinases can be mediated through serine/threonine phosphorylation.

Mitogen-activated protein kinase/extracellular signal-regulated protein kinase activation of cultured human dental pulp cells by low-power gallium-aluminium-arsenic laser irradiation

AIM

To examine whether low-power laser irradiation (LPLI) promotes cellular proliferation of human dental pulp-derived fibroblast-like cells (dental pulp cells).

METHODOLOGY

Dental pulp cells were obtained by primary culture of human dental pulp tissues from extracted third molar teeth. The phosphorylation of the mitogen-activated protein kinase (MAPK) family after LPLI of these cells was investigated by Western blotting. By using a specific MAPK/ERK kinase (MEK) inhibitor (PD098059), the specific effect of LPLI on the MAPK pathway was also investigated by Western blotting as described above. The incorporation of [3H]thymidine into the cells after LPLI was determined, and statistical analysis was performed by Wilcoxon signed-ranks test.

RESULTS

Extracellular signal-regulated protein kinase (ERK) 1/2 was phosphorylated between 5 and 30 min after LPLI. Moreover, PD098059 inhibited LPLI-mediated ERK1/2 activation. LPLI did not affect p38 MAPK or c-Jun N-terminal kinase (JNK) phosphorylation. But LPLI did not stimulate [3H]thymidine incorporation into these cells.

CONCLUSIONS

These results indicated that LPLI activated MAPK/ERK, a signal for proliferation, differentiation and survival, but did not activate the stress signals p38 MAPK and JNK in human dental pulp cells.

Thymic myoid cells protect thymocytes from apoptosis and modulate their differentiation: implication of the ERK and Akt signaling pathways

Thymic myoid cells correspond to a muscle-like cell population present in the thymic medulla. They are well conserved throughout species evolution, but their biological role is not known. We demonstrated that myoid cells protected thymocytes from apoptosis as evidenced by a strong decrease of annexin-V-FITC positive thymocytes. This effect was (1) specific of myoid cells compared to thymic epithelial cells; (2) dependent on direct cell-to-cell contacts and (3) triggered rapidly after 2 h in cocultures. This protective phenomenon was due to the activation of prosurvival mechanisms. Indeed, myoid cells activated extracellular-regulated kinases (ERK1/2) and Akt in thymocytes. Myoid cells also influenced thymocyte maturation. We observed an increase in CD4(+) and a decrease in CD8(+) single positive (SP) thymocytes when cocultured with myoid cells, independently of a CD8(+)SP increased death or a CD4(+)SP overproliferation. Consequently, thymic myoid cells protect thymocytes from apoptosis and could also modulate their differentiation process.

p38 mitogen-activated protein kinase activation by ultraviolet A radiation in human dermal fibroblasts

UVA radiation penetrates deeply into the skin reaching both the epidermis and the dermis. We thus investigated the effects of naturally occurring doses of UVA radiation on mitogen-activated protein kinase (MAPK) activities in human dermal fibroblasts. We demonstrated that UVA selectively activates p38 MAPK with no effect on extracellular-regulated kinases (ERK1-ERK2) or JNK-SAPK (cJun NH2-terminal kinase-stress-activated protein kinase) activities. We then investigated the signaling pathway used by UVA to activate p38 MAPK. L-Histidine and sodium azide had an inhibitory effect on UVA activation of p38 MAPK, pointing to a role of singlet oxygen in transduction of the UVA effect. Afterward, using prolonged cell treatments with growth factors to desensitize their signaling pathways or suramin to block growth factor receptors, we demonstrated that UVA signaling pathways shared elements with growth factor signaling pathways. In addition, using emetine (a translation inhibitor altering ribosome functioning) we detected the involvement of ribotoxic stress in p38 MAPK activation by UVA. Our observations suggest that p38 activation by UVA in dermal fibroblasts involves singlet oxygen-dependent activation of ligand-receptor signaling pathways or ribotoxic stress mechanism (or both). Despite the activation of these two distinct signaling mechanisms, the selective activation of p38 MAPK suggests a critical role of this kinase in the effects of UVA radiation.

Role of osteoclast extracellular signal-regulated kinase (ERK) in cell survival and maintenance of cell polarity

Morphological changes of osteoclasts by a MEK1 inhibitor, PD98059, were investigated to clarify a role of ERK. PD98059 promoted apoptosis of osteoclasts and the loss of ruffled borders. This study supports the importance of ERK in survival and polarity of osteoclasts.

INTRODUCTION

Extracellular signal-regulated kinase (ERK) is a mitogen activated protein kinase (MAPK) that has been reported to play a role in the survival and apoptosis of osteoclasts. However, the precise signal transduction mechanism is not fully understood. The aim of this study was to clarify the role of ERK in osteoclasts by histological analysis.

MATERIALS AND METHODS

Using a rat calvarial organ culture system, the inhibition of ERK phosphorylation by PD98059, a MAPK/ERK kinase 1 (MEK1) inhibitor, was assayed by immunoblotting. Morphological changes in osteoclasts induced by PD98059 were elucidated by light and electron microscopy. The cellular localization of ERK was also determined by immunoelectron microscopy.

RESULTS

PD98059 inhibited phosphorylated ERK after a 1-h incubation. Ultrastructural study demonstrated that PD98059 induced the accumulation of vesicles and vacuoles in osteoclasts and the loss of ruffled border at 1 h. At 3 h, some osteoclasts showed apoptosis with nuclear condensation, and at 6 h after PD98059 treatment, many osteoclasts were detached from the bone surface and had lost their cell polarity. Electron microscopic immunohistochemistry revealed that ERK was mainly localized in the cytoplasm of clear zones in control osteoclasts, but apoptotic osteoclasts also showed immunoreactivity in clear zone-like structures in contact with osteoblast-lineage cells.

CONCLUSION

These findings indicate that ERK in osteoclasts is involved in their survival and may be involved in the formation of a ruffled border and the maintenance of cell polarity.

Transactivation of the epidermal growth factor receptor is involved in 12-O-tetradecanoylphorbol-13-acetate-induced signal transduction

The mechanism of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced tumor promotion is still not well understood even though it is thought to be related to the protein kinase C/mitogen-activated protein kinase/AP-1 pathway. Recently, TPA was also found to induce epidermal growth factor receptor (EGFR) activity. Here, we investigated whether the EGFR is a necessary component for TPA-induced signal transduction associated with tumor promotion. We demonstrated that potent inhibitors of the EGFR, PD153035 and AG1478, blocked TPA-induced phosphorylation of extracellular signal-regulated kinases (ERKs), AP-1 activity, and cell transformation. Egfr gene deficiency blocked TPA-induced ERK activity and AP-1 binding activity. The blocking of the ectodomain of the EGFR by a monoclonal antibody depressed TPA-induced ERK activity and AP-1 DNA binding activity. The use of a neutralizing antibody for heparin-binding EGF, one of the ligands of EGFR, blocked TPA-induced phosphorylation of ERKs. BB-94, a potent inhibitor of matrix metalloproteinases, which are activators of ectodomain shedding of EGFR ligands, also blocked TPA-induced ERK activity, AP-1 DNA binding, and cell transformation but had no effect on EGF-induced signal transduction. Anti-EGFR, anti-heparin-binding EGF, and BB-94 each blocked TPA-induced EGFR phosphorylation, but only anti-EGFR could block EGF-induced EGFR phosphorylation. Based on these results, we conclude that the EGFR is required for mediating TPA-induced signal transduction. EGFR transactivation induced by TPA is a mechanism by which the EGFR mediates TPA-induced tumor promotion-related signal transduction.

Transforming growth factor-beta signal transduction in epithelial cells

Transforming growth factor (TGF)-beta is a natural and potent growth inhibitor of a variety of cell types, including epithelial, endothelial, and hematopoietic cells. The ability of TGF-beta to potently inhibit the growth of many solid tumors of epithelial origin, including breast and colon carcinomas, is of particular interest. However, many solid tumor cells become refractory to the growth inhibitory effects of TGF-beta due to defects in TGF-beta signaling pathways. In addition, TGF-beta may stimulate the invasiveness of tumor cells via the paracrine effects of TGF-beta. Accordingly, in order to develop more effective anticancer therapeutics, it is necessary to determine the TGF-beta signal transduction pathways underlying the growth inhibitory effects and other cellular effects of TGF-beta in normal epithelial cells. Thus far, two primary signaling cascades downstream of the TGF-beta receptors have been elucidated, the Sma and mothers against decapentaplegic homologues and the Ras/mitogen-activated protein kinase pathways. The major objective of this review is to summarize TGF-beta signaling in epithelial cells, focusing on recent advances involving the Sma and mothers against decapentaplegic homologues and Ras/mitogen-activated protein kinase pathways. This review is particularly timely in that it provides a comprehensive summary of both signal transduction mechanisms and the cell cycle effects of TGF-beta.

Upregulation of p21(WAF1/CIP1) leads to morphologic changes and esterase activity in TPA-mediated differentiation of human prostate cancer cell line TSU-Pr1

We reported previously that human prostate cancer cell line TSU-Pr1 can differentiate into microglia-like cells by 12-O-tetra-decanoylphorbol-13-acetate (TPA) treatment. In this study, we identified a signal transduction pathway involved in TPA-induced TSU-Pr1 cell differentiation and investigated the mechanism of growth arrest that accompanies this differentiation. TPA-induced differentiation and growth arrest of TSU-Pr1 cells were inhibited by treatment with Protein kinase C (PKC) inhibitor GF109203X and mitogen-activated protein (MAP) kinase inhibitor PD98059. Treatment of TSU-Pr1 cells with TPA for 15 min or longer resulted in translocation of PKCalpha, PKCgamma, and PKCepsilon from cytosolic to membrane fraction. Our results suggest that TPA-induced TSU-Pr1 cell differentiation is associated with activation of MAP kinase and PKCalpha, PKCgamma, and PKCepsilon. The mechanism of growth arrest in TSU-Pr1 cells that underwent TPA-induced differentiation were examined for factors in the signaling pathway downstream of MAP kinase that control the cell cycle. Upregulation of p21(WAF1/CIP1) cyclin-dependent kinase inhibitor protein was observed in a manner dependent on PKC or MAP kinase. Moreover, adenovirus-mediated overexpression of recombinant p21(WAF1/CIP1) in TSU-Pr1 cells result in growth arrest, morphological change to microglia-like cells, and increased alpha-naphthyl acetate esterase activity, all of which are associated with cellular differentiation. Thus, our results indicate that p21(WAF1/CIP1) mediates TPA-induced growth arrest and differentiation of TSU-Pr1 cells.

The neuronal MAP kinase cascade: a biochemical signal integration system subserving synaptic plasticity and memory

The mitogen-activated protein kinase (MAP kinase, MAPK) cascade, as the name implies, was originally discovered as a critical regulator of cell division and differentiation. As further details of this signaling cascade were worked out, it became clear that the MAPK cascade is in fact a prototype for a family of signaling cascades that share the motif of three serially linked kinases regulating each other by sequential phosphorylation. Thus, a revised nomenclature arose that uses the term MAPK to refer to the entire superfamily of signaling cascades (comprising the erks, the JNKs and the p38 stress activated protein kinases), and specifies the prototype MAPK as the extracellular signal-regulated kinase (erk). The two erk MAPK isoforms, p44 MAPK and p42 MAPK, are referred to as erk1 and erk2, respectively. The erks are abundantly expressed in neurons in the mature central nervous system, raising the question of why the prototype molecular regulators of cell division and differentiation are present in these non-dividing, terminally differentiated neurons. This review will describe the beginnings of an answer to this question. Interestingly, the general model has begun to emerge that the erk signaling system has been co-opted in mature neurons to function in synaptic plasticity and memory. Moreover, recent insights have led to the intriguing prospect that these molecules serve as biochemical signal integrators and molecular coincidence detectors for coordinating responses to extracellular signals in neurons. In this review I will first outline the essential components of this signal transduction cascade, and briefly describe recent results implicating the erks in mammalian synaptic plasticity and learning. I will then proceed to outline recent results implicating the erks as molecular signal integrators and, potentially, coincidence detectors. Finally, I will speculate on what the critical downstream effectors of the erks are in neurons, and how they might provide a readout of the integrated signal.

Stimulation of hepatocyte growth factor production in human fibroblasts by the protein phosphatase inhibitor okadaic acid

In this study, we examined whether the production of hepatocyte growth factor (HGF) in fibroblasts is regulated by protein phosphatase(s). Inhibitors of the enzymes okadaic acid and calyculin A were used for this purpose. Both inhibitors markedly stimulated HGF production in human skin fibroblasts in a dose-dependent manner. The effects of okadaic acid and calyculin A were maximal at 25-37.5 and 1.25 nM, respectively. Highly active HGF production in MRC-5 human embryonic lung fibroblasts was also promoted by both inhibitors. The effect of okadaic acid was accompanied by an up-regulation of HGF gene expression. The stimulating effect of okadaic acid on HGF production was synergistic with that of phorbol 12-myristate 13-acetate (PMA) and epidermal growth factor (EGF), whereas it was additive to the effect of cholera toxin. The protein kinase C (PKC) inhibitor GF 109203X inhibited the effect of PMA, but not of okadaic acid and EGF. The effect of okadaic acid as well as EGF was not inhibited, but rather enhanced in human skin fibroblasts pretreated for 24 hr with a high dose of PMA to deplete PKC, as compared with its effect in untreated cells. PD 98059, an inhibitor of mitogen-activated protein (MAP) kinase kinase, suppressed the effects of okadaic acid and EGF, but not those of cholera toxin and 8-bromo-adenosine 3',5'-cyclic monophosphate (cAMP). These results suggest that HGF production in human skin fibroblasts is down-regulated by protein phosphatase(s) and that HGF production stimulated by okadaic acid is, at least in part, dependent on the activation of the MAP kinase cascade.

A role for the MEK-MAPK pathway in okadaic acid-induced meiotic resumption of incompetent growing mouse oocytes

Fully grown competent mouse oocytes spontaneously resume meiosis in vitro when released from their follicular environment, in contrast to growing incompetent oocytes, which remain blocked in prophase I. The cell cycle regulators, maturation promoting factor (MPF; [p34(cdc2)/cyclin B kinase]) and mitogen-activated protein (MAP) kinases (p42(MAPK) and p44(MAPK)), are implicated in meiotic competence acquisition. Incompetent oocytes contain levels of p42(MAPK), p44(MAPK), and cyclin B proteins that are comparable to those in competent oocytes, but their level of p34(cdc2) is markedly lower. Okadaic acid (OA), an inhibitor of phosphatases 1 and 2A, induces meiotic resumption of incompetent oocytes. The kinetics and the percentage of germinal vesicle breakdown depends on whether or not oocytes have been cultured before OA treatment. We show that the fast kinetics and the high percentage of germinal vesicle breakdown induced by OA following 2 days in culture is neither the result of an accumulation of p34(cdc2) protein, nor to the activation of MPF in incompetent oocytes, but rather by the premature activation of MAP kinases. Indeed, a specific inhibitor of MAPK kinase (MEK) activity, PD98059, inhibits activation of MAP kinases and meiotic resumption. Altogether, these results indicate that the MEK-MAPK pathway is implicated in OA-induced meiotic resumption of incompetent mouse oocytes, and that the MEK-MAPK pathway can induce meiotic resumption in the absence of MPF activation.

The topography and subcellular distribution of mitogen-activated protein kinase kinase1 (MEK1) in adult rat brain and differentiating PC12 cells

Mitogen-activated protein kinase signal transduction pathway involved in the regulation of proliferation and differentiation of various mammalian cells consists of a sequential activation of three protein kinases, Raf, mitogen-activated protein kinase kinase, and mitogen-activated protein kinase. These kinases are highly expressed in brain and play an important role in neuronal signalling. In this study, to further characterize mitogen-activated protein kinase signalling pathway in brain, we have elucidated the topography and subcellular distribution of mitogen-activated protein kinase kinasel in adult rat brain and differentiating PC12 cells. Our immunohistochemical data indicate that mitogen-activated protein kinase kinase1 is widely distributed throughout the brain and expressed prominently in cortex, hippocampus, brainstem, hypothalamus and cerebellum. In these areas of brain mitogen-activated protein kinase kinasel is exclusively neuronal in origin and is localized within perikarya and dendrites. Confocal microscopy data has determined that a portion of mitogen-activated protein kinase kinase1 in rat brain is co-localized with microtubules. This co-localization was observed only within neuritic shaft and cilia of ventricular ependymal cells. In nerve growth factor-induced differentiating PC12 cells, mitogen-activated protein kinase kinase1 displays co-localization with microtubules within proximal regions of neuritic shafts and their junctions with the cell somas. From bovine brain extract, mitogen-activated protein kinase kinasel co-purifies with microtubules. In vitro kinase assay detected mitogen-activated protein kinase kinase1 activity within purified microtubules. These observations indicate that mitogen-activated protein kinase kinase1 is associated with microtubules within some specialized compartments of the brain and microtubule-associated mitogen-activated protein kinase kinase1 is catalytically active.

Activation of stress-activated protein kinase/c-Jun NH2-terminal kinase and p38 kinase in calphostin C-induced apoptosis requires caspase-3-like proteases but is dispensable for cell death

Apoptosis was induced in human glioma cell lines by exposure to 100 nM calphostin C, a specific inhibitor of protein kinase C. Calphostin C-induced apoptosis was associated with synchronous down-regulation of Bcl-2 and Bcl-xL as well as activation of caspase-3 but not caspase-1. The exposure to calphostin C led to activation of stress-activated protein kinase/c-Jun NH2-terminal kinase (SAPK/JNK) and p38 kinase and concurrent inhibition of extracellular signal-regulated kinase (ERK). Upstream of ERK, Shc was shown to be activated, but its downstream Raf1 and ERK were inhibited. The pretreatment with acetyl-Tyr-Val-Ala-Asp-aldehyde, a relatively selective inhibitor of caspase-3, or benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD.fmk), a broad spectrum caspase inhibitor, similarly inhibited calphostin C-induced activation of SAPK/JNK and p38 kinase as well as apoptotic nuclear damages (chromatin condensation and DNA fragmentation) and cell shrinkage, suggesting that caspase-3 functions upstream of SAPK/JNK and p38 kinase, but did not block calphostin C-induced surface blebbing and cell death. On the other hand, the inhibition of SAPK/JNK by transfection of dominant negative SAPK/JNK and that of p38 kinase by SB203580 induced similar effects on the calphostin C-induced apoptotic phenotypes and cell death as did z-VAD.fmk and acetyl-Tyr-Val-Ala-Asp-aldehyde, but the calphostin C-induced PARP cleavage was not changed, suggesting that SAPK/JNK and p38 kinase are involved in the DNA fragmentation pathway downstream of caspase-3. The present findings suggest, therefore, that the activation of SAPK/JNK and p38 kinase is dispensable for calphostin C-mediated and z-VAD.fmk-resistant cell death.

In vivo and in vitro assessment of mitogen activated protein kinase involvement during quail secondary palate formation

Spatiotemporally regulated cell proliferation and differentiation are crucial for the successful completion of morphogenesis of the vertebrate secondary palate. An understanding of the mechanisms by which these cellular phenomena are regulated during palate development involves the identification of the various signal transduction pathways. In the present study, the presence and activation of mitogen-activated protein (MAP) kinases were investigated during the development of quail secondary palate. The palatal shelves were dissected on days 5-9 of incubation, homogenized, and centrifuged, after which the samples were separated by anion exchange fast protein liquid chromatography. The fractions were analyzed for myelin basic protein (MBP) phosphorylation. In addition, primary cultures of quail palate mesenchymal cells (QPMCs) were treated with epidermal growth factor (EGF) and prepared for MBP phosphorylation assays. A temporally regulated pattern of phosphotransferase activity, characterized by a three-fold increase in phosphotransferase activity toward MBP between days 5 and 8 of incubation, was observed during quail palate development. Western blotting, using MAP kinase antibodies, demonstrated the presence of a 42-kDa isoform between days 5 and 9 of incubation, during which the level of protein remained constant. Antityrosine immunoblotting with 4G10 also detected a 42-kDa protein. Phosphotransferase assays, using either a MAP kinase-specific substrate peptide (S5) or a protein kinase C inhibitor (R3), further confirmed the presence of a MAP kinase in the developing palate of quail. Because diverse biological processes occur concurrently during in vivo palate morphogenesis, the involvement of MAP kinase was explored further in primary cell culture. The data showed that EGF stimulated proliferation and activated 42-kDa MAP kinase in QPMCs. It is suggested that MAP kinase cascade may be involved in growth factor-regulated cell proliferation during morphogenesis of quail secondary palate.

MAP kinase-dependent pathways in cell cycle control

Mitogen-activated protein kinases such as Erk1 and Erk2 serve as a paradigm for a growing family of proline-directed protein kinases that mediate entry, progression and exit from the cell cycle in diverse eukaryotic cells. These enzymes function within highly conserved modules of sequentially activating protein kinases that transduce signals from diverse extracellular stimuli. In vertebrates, at least three distinct kinases modules have been characterized. Mitogens induce the sequential activation of the kinases Raf1-->Mek1-->Erk2-->Rsk via the G-protein Ras. Stress factors stimulate c-Jun activation through a related kinase pathway involving Mekk-->Sek-->SAPK c-Jun, and hsp27 phosphorylation via the MKK3-->Hog-->MAPKAPK-2 hsp27 route. Genetic and biochemical studies, for example from budding yeast, imply the existence of several related protein kinase modules that can operate in parallel or within integrated systems.

Activation of mitogen-activated protein kinases/extracellular signal-regulated kinases in human hepatocellular carcinoma

Mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) is a key molecule in intracellular signal transducing pathways that transport extracellular stimuli from cell surface to nuclei. MAPK/ERK has been revealed to be involved in the physiological proliferation of mammalian cells and also to potentiate them to transform. However, its role in the outgrowth of human hepatocellular carcinoma (HCC) has yet to be clarified. Therefore, in this study, we investigated the activation of MAPK/ERK and its associated gene expression in HCC. MAPK/ERK was activated in 15 of 26 cases of HCC we examined (58%), and its activity level was significantly higher in HCC than in the adjacent non-cancerous lesions. Besides, MAPK/ERK activation in HCC was positively correlated with protein expression of transcription factor c-Fos. Furthermore, in 25 of 26 cases of HCC which genomic DNA was available, 22 cases without genomic DNA amplification exhibited positive correlation, not only between protein expression of c-Fos and cyclin D1, but also between MAPK/ERK activation and cyclin D1 expression. Concerning the relationship between MAPK/ERK activation and the clinicohistopathological features of HCC, the tumor (HCC) versus non-tumor (non-cancerous counterpart) ratio (T/N) of MAPK/ERK activity was positively correlated with tumor size, but neither with the stage of HCC nor the degree of differentiation of HCC. In conclusion, these findings suggest that MAPK/ERK activation in human HCC may play an important role in multistep hepatocarcinogenesis, especially in the progression of HCC; at least in part, through cyclin D1 up-regulation primarily induced by MAPK/ERK via c-Fos.

The ORF3 protein of hepatitis E virus is a phosphoprotein that associates with the cytoskeleton

Hepatitis E virus (HEV) is a major human pathogen in the developing world. In the absence of an in vitro culture system, very little information exists on the basic biology of the virus. A small protein (approximately 13.5 kDa) of unknown function, pORF3, is encoded by the third open reading frame of HEV. We expressed pORF3 in transiently transfected COS-1 and Huh-7 cells and showed that it is a phosphoprotein which is modified at a serine residue(s). Deletion and site-directed mutants were created to establish Ser-80 as the phosphorylation site. This residue is present within a conserved primary sequence that showed consensus sites for phosphorylation by p34cdc2 kinase (cdc2K) and mitogen-activated protein kinase (MAPK). In vitro experiments with hexahistidine-tagged pORF3 expressed either in Escherichia coli or in COS-1 cells showed efficient phosphorylation with exogenously added MAPK. The pORF3 mutants also exhibited an in vitro phosphorylation profile with MAPK which was identical to that observed in vivo. In its primary sequence, pORF3 possesses two highly hydrophobic N-terminal domains. On subcellular fractionation, pORF3 was found to partition with the cytoskeletal fraction, and this association with the cytoskeleton was lost on deletion of hydrophobic domain I (amino acid residues 1 to 32). These results suggest that HEV pORF3 is a cytoskeleton-associated phosphoprotein and are discussed in terms of a possible function for pORF3 within the HEV replicative cycle.

Presence of ERK2 in rat retinal cells

PURPOSE

Extracellular signal-regulated kinase 2 (ERK2) participates in the phosphorylation cascade that is activated in the an early intracellular response to various hormones and growth factors. We examined the expression and distribution of the ERK2 protein and mRNA in the rat retina before and after light exposure.

METHODS

Rats were held on a 12 hr light/dark cycle and their retinas were removed and examined either just before or 2 or 30 min after light exposure. The tissue was processed for Western blotting to evaluate the presence of the protein for ERK2, and for in situ hybridization to evaluate the mRNA of ERK2.

RESULTS

The Western blotting method showed a strong specific staining of a 42 kDa protein band in the retinal samples. This band corresponded to the expected size of p42 MAP kinase (ERK2). In situ hybridization histochemistry showed an intense localization of ERK2 mRNA in the outer nuclear layer (ONL), the inner nuclear layer (INL), and the ganglion cell layer (GCL) of the retina. The intensity and distribution of these signals did not differ among the animals, regardless of exposure to light.

CONCLUSIONS

While ERK2 may be involved in the signal transduction system activated in retinal cells by light exposure, its precise role remains to be defined.

Regulation of the ERK subgroup of MAP kinase cascades through G protein-coupled receptors

The extracellularly-responsive kinase (ERK) subfamily of mitogen-activated protein kinases (MAPKs) has been implicated in the regulation of cell growth and differentiation. Activation of ERKs involves a two-step protein kinase cascade lying upstream from ERK, in which the Raf family are the MAPK kinase kinases and the MEK1/MEK2 isoforms are the MAPK kinases. The linear sequence of Raf --> MEK --> ERK constitutes the ERK cascade. Although the ERK cascade is activated through growth factor-regulated receptor protein tyrosine kinases, they are also modulated through G protein-coupled receptors (GPCRs). All four G protein subfamilies (Gq/11 Gi/o, Gs and G12/13) influence the activation state of ERKs. In this review, we describe the ERK cascade and characteristics of its activation through GPCRs. We also discuss the identity of the intervening steps that may couple agonist binding at GPCRs to activation of the ERK cascade.

Identification and characterization of an auto-activating MEK kinase from bovine brain: phosphorylation of serine-298 in the proline-rich domain of the mammalian MEKs

Mitogen-activated protein kinase kinases (MKKs or MEKs) are dual specificity tyrosine/threonine protein kinases that are activated by phosphorylation at two closely spaced serine residues (serines-218 and -222) by the c-mos and raf proto-oncogenes. This double phosphorylation is both necessary and sufficient for MEKs to activate the MAP kinase enzymes in vitro. The specificity or regulation of in vivo signaling to the mammalian MEKs (MEK1 and MEK2) was recently reported also to involve the differential phosphorylation of a proline-rich peptide located between the MEK kinase-subdomains IX and X. Here we report the purification and characterization of an auto-activating protein kinase from bovine brain that phosphorylates serine-298 of the MEK1 and MEK2 proline-rich insert peptides. The auto-activation of the MEK-S298 peptide kinase is the result of an intermolecular phosphorylation event that can be prevented by the peptide substrates. The inactive kinase migrates on gel filtration as a 90 kDa protein, and after activation as a 43 kDa phosphoprotein. Incorporation of 32P[phosphate] into 40-42 kDa proteins on SDS-PAGE parallels the activation of the enzyme, and dephosphorylation by protein phosphatase 2Ac reverses the activation. SDS-PAGE renaturation assays show that the 40 kDa protein has the capacity to autophosphorylate, and exhibits kinase activity towards myelin basic protein after activation. Phosphorylation of purified bovine brain MEK or recombinant MEK1 by the auto-activated kinase does not activate the enzyme, and does not interfere with the in vitro raf-mediated MEK activation. We conclude that still unknown kinases may control the MAP kinase pathway by targeting MEK.

Mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways are not sufficient for insulin-like growth factor I-induced mitogenesis and tumorigenesis

Insulin-like growth factor-I (IGF-I) and insulin are known to activate a signaling cascade involving ras --> kappa raf-1 --> mitogen-activated protein (MAP) kinase kinase (MEK) --> p42/p44 MAP kinase (Erk-1 and -2). Recent reports suggest that activation of this ras/MAP kinase pathway is involved in mitogenesis and c-fos transcription but is not required for insulin action on metabolic processes such as glycogen synthesis, lipogenesis, and GLUT-4-mediated glucose transport. Previously we and others have demonstrated that substitution of both tyrosines at positions 1250 and 1251 in the carboxy-terminal region of the human IGF-I receptor has relatively small effects on receptor and endogenous substrate phosphorylation but completely abrogated the ability of these cells to form tumors in nude mice or proliferate in response to IGF-I in culture. Replacement of the tyrosine at position 1316 also did not affect the kinase activity of the receptor with respect to autophosphorylation or phosphorylation of endogenous substrates but did reduce the ability of the receptor to mediate mitogenic or tumorigenic signals. To further characterize the role of these tyrosines in IGF-I receptor function, we have used three distinct approaches to examine the ras/MAP kinase pathway in IGF-I-induced mitogenesis and tumorigenesis in NIH-3T3 cells overexpressing wild-type and mutated IGF-I receptors: 1) tyrosine phosphorylation of the MAP kinases Erk-1 and -2; 2), mobility shifts indicative of MAP kinase phosphorylation; and 3) in vitro MAP kinase activation. We have also examined IGF-I-induced phosphatidylinositol (PI) 3-kinase activation in the same cell lines. By each method we show that the IGF-I-induced MAP kinase phosphorylation/activation and PI 3-kinase activation, are not different between cells overexpressing wild-type IGF-I receptors and cells carrying IGF-I receptors having tyrosine motifs replaced at positions 1250 and 1251. We conclude that mitogenic and tumorigenic signals involving tyrosine residues in the C-terminal domain of the IGF-I-receptor include pathways other than the MAP kinase and PI 3-kinase pathways.

The effect of cyclic adenosine monophosphate on the mitogen-activated protein kinase pathway depends on both the cell type and the type of tyrosine kinase-receptor

The mitogen-activated protein kinase (MAP kinase) is a key participant in growth factor-stimulated intracellular events such as proliferation and differentiation. We and others have previously described a cross-talk between the MAP kinase pathway and the cAMP pathway. Indeed, in several cell lines and, in particular in fibroblasts, an increase in the level of cAMP produced an inhibition of MAP kinase together with decreased cell proliferation. In contrast, in PC12 cells, cAMP induced an increase in the NGF-induced activation of MAP kinase concomitantly with augmented NGF-induced differentiation. Therefore, it has been proposed that the cellular context is important for the nature of the cAMP effects on growth factor-stimulated MAP kinase activity. Here we show that the type of tyrosine kinase receptor stimulated also participates in the nature of the cAMP effect. Thus, in NIH3T3 fibroblasts expressing NGF receptors (NIH3T3/trk cells) we found that cAMP potentiates NGF-stimulated ERK1 and MEK1 activities, whereas in NIH3T3 fibroblasts expressing insulin receptors (NIH3T3/IR cells) we saw no effect of cAMP on the activation of insulin-stimulated ERK1 and MEK1. In PC12 cells and in Rat1 fibroblasts expressing insulin receptors (PC12/IR and Rat1/IR cells) we observed, respectively, a potentiation and an inhibition of insulin-stimulated ERK1 activity. In addition, cAMP does not seem to modify the basal nor growth factor-stimulated She or IRS-1 tyrosine phosphorylation in the different cell lines studied. Finally, we observed that cAMP inhibited serum- and insulin-induced, but not NGF-induced, cell proliferation in NIH3T3 cells. However, cAMP potentiated insulin-stimulated cell differentiation in PC12/IR cells. These results led us to conclude that the cAMP effect on cell proliferation in NIH3T3 fibroblasts and PC12/IR cells appears to be correlated, in part, with the effect of cAMP on the MAP kinase pathway, but by itself this pathway cannot fully account for these observations.

The fumagillin analogue TNP-470 inhibits DNA synthesis of vascular smooth muscle cells stimulated by platelet-derived growth factor and insulin-like growth factor-I. Possible involvement of cyclin-dependent kinase 2

The effect of an angiogenesis inhibitor, TNP-470, on DNA synthesis and its underlying signaling cascades stimulated by platelet-derived growth factor (PDGF)-BB and insulin-like growth factor (IGF)-I were examined in bovine vascular smooth muscle cells (SMCs). PDGF-BB (10 ng/mL)- and IGF-I (100 ng/mL)-stimulated increase in DNA synthesis was completely abolished by simultaneous treatment with TNP-470 (1.0 ng/mL). TNP-470 had no effects on PDGF receptor autophosphorylation or early signal transduction, such as activation of mitogen-activated protein kinase and immediate early gene expression. PDGF-BB induced an increase in mRNA levels of cyclin D1, cyclin-dependent kinase (cdk) 4, and cdk2, as well as the activity of cdk2, which preceded the G1/S boundary, as estimated by the kinetics of DNA synthesis. The PDGF-BB-induced activation of cdk2 was inhibited by TNP-470, which was correlated with decreased cdk2 mRNA levels. In contrast, TNP-470 had no or less marked effect on cyclin D1 and cdk4 mRNA levels induced by PDGF-BB. TNP-470 also inhibited a much smaller increase in cdk2 mRNA levels and activation stimulated by IGF-I. In conclusion, TNP-470 potently inhibits DNA synthesis of SMCs, and this inhibition is associated with decreased levels of cdk2 mRNA and activity.

Protein kinase C activates the MEK-ERK pathway in a manner independent of Ras and dependent on Raf

Although the involvement of protein kinase C (PKC) in the activation of the mitogen-activated protein (MAP) kinase pathway has been implicated through experiments using 12-O-tetradecanoylphorbol-13-acetate (TPA), there has been no direct demonstration that PKC activates the MAP kinase pathway. A Raf-dependent intact cell assay system for monitoring the activation of MAPK/ERK kinase (MEK) and extracellular signal-related kinase (ERK) permitted us to evaluate the role of PKC isotypes in MAP kinase activation. Treatment of cells with TPA or epidermal growth factor resulted in the activation of MEK and ERK. The activation of the MAP kinase pathway triggered by epidermal growth factor was completely inhibited by dominant-negative Ras (RasN17), whereas the activation triggered by TPA was not, consistent with previous observations. The introduction of an activated point mutant of PKCdelta, but not PKCalpha or PKCepsilon, resulted in the activation of the MAP kinase pathway. The activation of MEK and ERK by an activated form of PKCdelta requires the presence of c-Raf and is independent of RasN17. These results demonstrate that activation of PKCdelta is sufficient for the activation of MEK and ERK and that the pathway operates in a manner dependent on c-Raf and independent of Ras.

Protein kinase C-independent activation of mitogen-activated protein kinase by epidermal growth factor in skin fibroblasts

In this study, we demonstrated that epidermal growth factor (EGF) stimulated the phosphorylation of myelin basic protein (MBP), a mitogen-activated protein kinase (MAPK) substrate, in crude extracts of human dermal fibroblasts. Moreover, using a selective protein kinase C inhibitor, GF 109203X (3-[1-[3-(dimethylamino)propyl]-1 H-indol-3-yl]-4 (1 H-indol-3-yl)-1 H-pyrrole-2,5-dione monohydrochloride), we observed that protein kinase C was partially involved in the total MBP phosphorylation. To determine the role of protein kinase C in the MBP phosphorylation, we separated, using fast protein liquid chromatography, the proteins present in the fibroblast crude extracts; we thus detected two distinct MBP kinase activities. The first one was stimulated by EGF and corresponded to p42mapk and p44mapk isoforms; this stimulation was not modified by GF 109203X. The second MBP kinase activity was not stimulated by EGF and was due to two protein kinase C isoforms reacting with an anti-protein kinase C zeta antibody. These results show that, in human dermal fibroblasts, EGF stimulates p42mapk and p44mapk isoforms in a protein kinase C-independent manner.

The cooperative interaction of two different signaling pathways in response to bufalin induces apoptosis in human leukemia U937 cells

Bufalin, an active principle of Chinese medicine, chan'su, induced typical apoptosis in human leukemia U937 cells. When U937 cells were treated with 10(-8) M bufalin in the absence of serum, mitogen-activated protein (MAP) kinase activity was markedly increased 6 h after the start of treatment and elevated so for 12 h. Prior to the activation of MAP kinase, increased activities of Ras, Raf-1, and MAP kinase kinase were found, but these enzymes were transiently activated by the treatment with bufalin. These results suggest that the signal was transmitted sequentially from Ras, Raf-1, and MAP kinase kinase to MAP kinase. In association with this signal transduction, the concentration of cAMP in the cells decreased markedly, suggesting that Raf-1 was also activated by a decrease in the extent of phosphorylation by protein kinase A. In fact, pretreatment of U937 cells with forskolin and 3-isobutyl-1-methylxanthine, which are known to increase the concentration of cAMP in the cells, and subsequent treatment with bufalin resulted in a decrease in both Raf-1 activity and DNA fragmentation. To confirm the participation of MAP kinase in the apoptotic process, antisense cDNA for MAP kinase kinase 1 was expressed in U937 cells. The transformants were significantly resistant to both DNA fragmentation and cell death in response to bufalin. Our findings suggest that a pathway with the persistent activation of MAP kinase in U937 cells in response to bufalin is at least one of the signal transduction pathways involved in the induction of apoptosis.

Developmental expression in the mouse nervous system of the p493F12 SAP kinase

Mitogen-activated protein (MAP) kinases are proline-directed, serine/threonine kinases that respond to a variety of extracellular signals. A subgroup of these kinases, stress-activated protein (SAP) kinases, phosphorylate c-jun in response to cellular stress. Using monoclonal antibody (MAb) 3F12, we have cloned and partially characterized p493F12 kinase, a mouse homologue of the rat SAP beta kinase and described its expression in the adult and developing mouse. Unlike previously reported MAP and SAP kinases, it is primarily expressed as a 2.7 kb transcript in neurons in the nervous system of the adult mouse. A 2.4 kb transcript is also expressed in the testis. Immunocytochemically, MAb 3F12 decorates a loop-like structure encircling the nucleus in the cytoplasm of neurons in the adult brain, and distinct perinuclear dots in the embryos. In situ hybridization first reveals expression in post-mitotic neurons, on embryonic day 11. The mRNA is also expressed in the Neuro-2A neuroblastoma cell line and is not upregulated in response to differentiating agents. The neuronal specificity of this kinase suggests the presence of a signal transduction cascade unique to neurons. As the amino acid sequence is highly conserved in the human and mouse, the latter may serve as a model for regulation and expression of this kinase.

Enhanced interaction between tubulin and microtubule-associated protein 2 via inhibition of MAP kinase and CDC2 kinase by paclitaxel

Paclitaxel, an anti-mitotic anti-cancer agent, is active against solid tumors. The inhibition of depolymerization and promotion of microtubular assembly are essential for the anti-tumor activity of paclitaxel. Microtubule-associated proteins (MAPs) co-polymerize with tubulin and play some roles in microtubular dynamics. We examined the effect of paclitaxel on the interaction between tubulin and MAPs. Human lung-cancer cells, PC-14, were synchronized to G1/S border by the thymidine-double-block technique. After release from exposure to thymidine, the cells were treated briefly with 2 nM paclitaxel and the levels of alpha and beta tubulins and MAPs were examined after various times. Immunoblot analysis of paclitaxel-treated cells showed no changes in the overall expression of alpha and beta tubulins, microtubule-associated protein 2 (MAP2) or MAPs in comparison with controls. The samples were immunoprecipitated with anti-alpha- and anti-beta-tubulin antibodies and reblotted with an anti-MAP2 antibody, which showed that the amount of co-immuno-precipitated MAP2 in the synchronized cells, were increased by the brief paclitaxel treatment. These results suggest that paclitaxel treatment enhances the interaction between alpha and beta tubulins and MAP2. Since the phosphorylation state of MAP2 regulates the affinity of MAP2 for tubulins, and mitogen-activated protein (MAP) kinase is considered to be one of the kinases responsible for MAP2 phosphorylation, the effect of paclitaxel treatment on the MAP-kinase activity of synchronized PC-14 cells was examined. Two bands with molecular masses of 42 and 44 kDa were detected by an "intra-gel" MAP-kinase assay using myelin basic protein as the substrate. Paclitaxel treatment inhibited the MAP-kinase activity of PC-14 cells and inhibition was maximal at the G2/M phase of the cell cycle. Similar, concentration-dependent inhibition by paclitaxel of cellular MAP kinase of human synchronized small-cell lung carcinoma, H69, was observed. No inhibition of the MAP kinase of the paclitaxel-resistant sub-line H69/Txl by paclitaxel was observed, suggesting that some change of the MAP-kinase cascade had occurred in these cells. No direct inhibition of MAP-kinase activity by paclitaxel was observed in the cell-free assay (in vitro), suggesting that paclitaxel did not inhibit MAP kinase directly. Since it has been speculated that p34cdc2 kinase is also a kinase that phosphorylates MAP2, the effect of paclitaxel treatment on the p34cdc2-kinase activity of synchronized PC-14 and PC-9 cells was examined. Paclitaxel inhibited p34cdc2-kinase activation at the G2/M phase. These results suggest that paclitaxel inhibited MAP kinase and p34cdc2 kinase in vivo indirectly. These actions of paclitaxel may be responsible for the increased affinity between MAP2 and tubulins that it induces.

Increased levels of mitogen activated protein kinase (MAP-K) detected in the injured adult mouse sciatic nerve

Adult mouse sciatic nerves (SNs) with attached dorsal root ganglia (DRG) were analysed for the presence of mitogen activated protein kinase (MAP-K) during normal and regenerative conditions. By immunohistochemistry, MAP-K was found to be present in the normal nerve at low levels in both Schwann cells and DRG nerve cell bodies, with a profoundly increased expression during regeneration. In axonal outgrowth assays, treatment with 2 mM 2-aminopurine (2-AP), a MAP-K antagonist, inhibited the regeneration of axons from the SN as well as from the cultured superior cervical ganglia. The reduced outgrowth was probably not due to toxic effects of the drug since the ganglionic protein synthesis was not inhibited. It is possible that 2-AP interferes with regeneration-related events by inhibition of MAP-K.

MMK2, a novel alfalfa MAP kinase, specifically complements the yeast MPK1 function

Mitogen-activated protein (MAP) kinases are serine/threonine protein kinases that are activated in response to a variety of stimuli. Here we report the isolation of an alfalfa cDNA encoding a functional MAP kinase, termed MMK2. The predicted amino acid sequence of MMK2 shares 65% identity with a previously identified alfalfa MAP kinase, termed MMK1. Both alfalfa cDNA clones encode functional kinases when expressed in bacteria, undergoing autophosphorylation and activation to phosphorylate myelin basic protein in vitro. However, only MMK2 was able to phosphorylate a 39 kDa protein from the detergent-resistant cytoskeleton of carrot cells. The distinctiveness of MMK2 was further shown by complementation analysis of three different MAP kinase-dependent yeast pathways; this revealed a highly specific replacement of the yeast MPK1(SLT2) kinase by MMK2, which was found to be dependent on activation by the upstream regulators of the pathway. These results establish the existence of MAP kinases with different characteristics in higher plants, suggesting the possibility that they could mediate different cellular responses.

A neuroprotective compound, aurin tricarboxylic acid, stimulates the tyrosine phosphorylation cascade in PC12 cells

Aurin tricarboxylic acid (ATA), a general nuclease inhibitor, was reported to prevent PC12 cells from cell death caused by serum starvation (1). In our study, ATA also protected PC12 cells, but not NIH3T3 cells, from serum-starved cell death. When we investigated the mechanism of action of ATA on these cells, ATA was found to increase tyrosine phosphorylation in PC12 cells, but not in NIH3T3 cells. Further investigation on tyrosine-phosphorylated proteins revealed that ATA, similar to nerve growth factor and epidermal growth factor, induced tyrosine phosphorylation of mitogen-activated protein kinases. Since the tyrosine phosphorylation of mitogen-activated protein kinases is thought to play an important role inn growth factor-dependent signal pathways, this finding suggests that the action of ATA on PC12 cells is mediated by tyrosine phosphorylation cascade, similar to growth factor signaling. In addition, we found that Shc proteins, phosphatidylinositol 3-kinase, and phospholipase C-gamma were also phosphorylated in ATA-treated PC12 cells. These key proteins in signal transduction pathways are known to associate with ligand-activated growth factor receptors and are phosphorylated on tyrosine. Thus, the phosphorylation of these three proteins by ATA stimulation supports the speculation that ATA activates a certain receptor tyrosine kinase.

Networking with proline-directed protein kinases implicated in tau phosphorylation

Proline-directed kinases such as the mitogen-activated protein (MAP) kinases, cyclin-dependent protein kinase 5 (CDK5) and glycogen synthase 3 (GSK3) have been implicated in the hyperphosphorylation of the tau protein associated with Alzheimer's disease. Such aberrant phosphorylation of tau appears to compromise on its ability to bind to and stabilize microtubules, and this may contribute to Alzheimer's disease pathology. In this review, the architecture of the intracellular signal transduction pathways that regulate proline-directed kinases is described. The MAP kinases serve as major intersection points in the flow of information from a plethora of extracellular stimuli and affect diverse cellular processes that are often important for cell proliferation. Although brain contains terminally differentiated neurons, many of the known components of MAP kinase-dependent lines of communication are highly expressed in the nervous system. Similar signalling pathways may also regulate CDK5 and GSK3. In mitotic cells, abnormal activation of the protein kinase network at multiple points can contribute to oncogenic transformation. It is proposed that Alzheimer's disease may also result from accumulated defects in the kinase network that governs the proline-directed kinases such that their inappropriate activation is sustained in the affected neurons. A detailed understanding of proline-directed kinase-dependent pathways may permit the identification of rational targets for the therapeutic intervention of Alzheimer's disease and other neurological disorders.

Secreted beta-APP stimulates MAP kinase and phosphorylation of tau in neurons

We have previously demonstrated that the secreted form of the beta-amyloid precursor protein (beta-APP) activates mitogen-activated protein (MAP) kinases in PC-12 pheochromocytoma cells. beta-APP as well as other treatments that activate MAP kinase also enhance phosphorylation of the microtubule-associated protein tau in these cells. In this study, we extended this analysis to neurons. Using dissociated cultures of cortical neurons, we found that exposure to beta-APP activated MAP kinase 4 and 7 days but not 1 day after plating. Phosphorylation of tau in neurons was measured by immunoreactivity with the AT8 antibody, which recognizes a phosphorylated epitope present in tau from paired helical filaments. We found that activation of MAP kinase in neurons was associated with increased amounts of AT8-reactive tau. These results support a role for MAP kinase in transducing the biological effects of secreted beta-APP on neurons and suggest possible mechanisms by which beta-APP might be involved in the pathogenesis of Alzheimer's disease.

The MAP kinase cascade: its role in Xenopus oocytes, eggs and embryos

Mitogen-activated protein kinase (MAPK) was originally identified as a serine/threonine kinase that is activated by mitogens. Now MAPK and its activator, MAPK kinase (MAPKK), are thought to function in a wide variety of intracellular signalling pathways from yeast to vertebrate. We describe here a brief summary of the dissection of the MAPK cascade and its possible functions, especially in Xenopus oocytes and embryos.

Rapid and gentle extraction, reconstitution and characterization of microfilament and glia filament from rat astrocytes

We developed gentle and rapid methods for depolymerization and extraction of both microfilament and glia filament separately from a crude cytoskeletal fraction of rat astrocytes. Electron microscopy revealed that the filament reconstituted from the microfilament extract closely resembled F-actin that was formed from G-actin of rabbit skeletal muscle. It was found by immunoblotting analysis that even the reconstituted microfilament-like filaments, which had been purified by affinity chromatography with heavy meromyosin subfragment 1 (S1)-conjugated Sepharose, contained vimentin and glia fibrillary acidic protein (GFAP) besides actin, inferring the interaction between microfilament and glia filament. The filaments (9-10 nm thick) reconstituted from the glia filament extract were composed of actin and other minor components in addition to vimentin and GFAP. Actin, GFAP, 101, 34, 32.5, 30.5, 29.5 and 28 kDa proteins found in the reconstituted glia filament-like filaments were suggested to be glia filament-associated proteins.

Rapid tyrosine phosphorylation of an 85,000 M(r) protein after phorbol ester stimulation of EL4 thymoma cells

Early signalling events between protein kinase C (PKC) activation and lymphokine transcription were compared between phorbol ester-sensitive and -resistant EL4 cell lines which do or do not respond with interleukin 2 (IL2) production, respectively. The earliest event detected in the sensitive cell line was a dramatic increase in the tyrosine phosphorylation of an 85,000 M(r) protein (p85; 30 s), followed by mobility shifts of raf-1, mitogen-activated protein kinase kinase (MEK), mitogen-activated protein (MAP) kinase, lck and ZAP-70 (within 5 min). In contrast, p85 was not detected in the resistant cell line and lck and raf-1 mobility shifts exhibited delayed kinetics. Both vanadate and okadaic acid blocked the phorbol ester-stimulated p85 tyrosine phosphorylation in the sensitive cell line, suggesting that a phosphatase activity downstream of PKC activation may be required for p85 tyrosine phosphorylation. Characterization of p85 and its regulation should help elucidate some of the earliest events in this PKC pathway.