The ERK Signaling Cascade Inhibits Gonadotropin-Stimulated Steroidogenesis

The response of granulosa cells to Luteinizing Hormone (LH) and Follicle- Stimulating Hormone (FSH) is mediated mainly by cAMP/protein kinase A (PKA) signaling. Notably, the activity of the extracellular signal-regulated kinase (ERK) signaling cascade is elevated in response to these stimuli as well. We studied the involvement of the ERK cascade in LH- and FSH-induced steroidogenesis in two granulosa-derived cell lines, rLHR-4 and rFSHR-17, respectively. We found that stimulation of these cells with the appropriate gonadotropin induced ERK activation as well as progesterone production downstream of PKA. Inhibition of ERK activity enhanced gonadotropin-stimulated progesterone production, which was correlated with increased expression of the Steroidogenic Acute Regulatory Protein (StAR), a key regulator of progesterone synthesis. Therefore, it is likely that gonadotropin-stimulated progesterone formation is regulated by a pathway that includes PKA and StAR, and this process is down-regulated by ERK, due to attenuation of StAR expression. Our results suggest that activation of PKA signaling by gonadotropins not only induces steroidogenesis but also activates down-regulation machinery involving the ERK cascade. The activation of ERK by gonadotropins as well as by other agents may be a key mechanism for the modulation of gonadotropin-induced steroidogenesis.

ERK1b, a 46-kDa ERK Isoform That Is Differentially Regulated by MEK

The extracellular signal‐regulated kinases (ERK) 1 and 2 (ERK1/2) are members of the mitogen‐activated protein kinase family. Using various stimulated rodent cells and kinase activation techniques, we identified a 46‐kDa ERK. The kinetics of activation of this ERK isoform was similar to that of ERK1 and ERK2 under most but not all circumstances. We purified this isoform from rat cells followed by its cloning. The sequence of this isoform revealed that it is an alternatively spliced version of the 44‐kDa ERK1 and therefore we termed it ERK1b. Interestingly, this isoform had a 26‐amino acid insertion between residues 340 and 341 of ERK1, which results from Intron 7 insertion to the sequence. Examining the expression pattern, we found that ERK1b is detected mainly in rat and particularly in Ras‐transformed Rat1 cells. In this cell line, ERK1b was more sensitive to extracellular stimulation than ERK1 and ERK2. Moreover, unlike ERK1 and ERK2, ERK1b had a very low binding affinity to MEK1. This low interaction led to nuclear localization of this isoform when expressed together with MEK1 under conditions in which ERK1 and ERK2 are retained in the cytoplasm. In addition, ERK1b was not coimmunoprecipitated with MEK1. We identified a new, 46‐kDa ERK alternatively spliced isoform. Our results indicate that this isoform is the major one to respond to exogenous stimulation in Ras‐transformed cells, probably due to its differential regulation by MAPK/ERK kinase and by phosphatases.

Intrinsically active MEK variants are differentially regulated by proteinases and phosphatases

MAPK/ERK kinase (MEK) 1/2 are central signaling proteins that serve as specificity determinants of the MAPK/ERK cascade. More than twenty activating mutations have been reported for MEK1/2, and many of them are known to cause diseases such as cancers, arteriovenous malformation and RASopathies. Changes in their intrinsic activity do not seem to correlate with the severity of the diseases. Here we studied four MEK1/2 mutations using biochemical and molecular dynamic methods. Although the studied mutants elevated the activating phosphorylation of MEK they had no effect on the stimulated ERK1/2 phosphorylation. Studying the regulatory mechanism that may explain this lack of effect, we found that one type of mutation affects MEK stability and two types of mutations demonstrate a reduced sensitivity to PP2A. Together, our results indicate that some MEK mutations exert their function not only by their elevated intrinsic activity, but also by modulation of regulatory elements such as protein stability or dephosphorylation.

The nuclear translocation of the kinases p38 and JNK promotes inflammation-induced cancer

The stimulated nuclear translocation of signaling proteins, such as MAPKs, is a necessity for the initiation and regulation of their physiological functions. Previously, we determined that nuclear translocation of the MAPKs p38 and JNK involves binding to heterodimers comprising importin 3 and either importin 7 or importin 9. Here, we identified the importin-binding region in p38 and JNK and developed a myristoylated peptide targeting this site that we called PERY. The PERY peptide specifically blocked the interaction of p38 and JNK with the importins, restricted their nuclear translocation, and inhibited phosphorylation of their nuclear (but not cytoplasmic) substrates. Through these effects, the PERY peptide reduced the proliferation of several (but not all) cancer cell lines in culture and inhibited the growth of a human breast cancer xenograft in mice. In addition, the PERY peptide substantially inhibited inflammation in mice, as manifested in models of colitis and colitis-associated colon cancer. The PERY peptide more effectively prevented colon cancer development than did a commercial p38 inhibitor. In vivo analysis further suggested that this effect was mediated by PERY peptide-induced prevention of the nuclear translocation of p38 in macrophages. Together, these results support the use of the nuclear translocation of p38 and JNK as a novel drug target to treat various cancers and inflammation-induced diseases.

Myotubularin-related protein 7 inhibits insulin signaling in colorectal cancer

Phosphoinositide (PIP) phosphatases such as myotubularins (MTMs) inhibit growth factor receptor signaling. However, the function of myotubularin-related protein 7 (MTMR7) in cancer is unknown. We show that MTMR7 protein was down-regulated with increasing tumor grade (G), size (T) and stage (UICC) in patients with colorectal cancer (CRC) (n=1786). The presence of MTMR7 in the stroma correlated with poor prognosis, whereas MTMR7 expression in the tumor was not predictive for patients' survival. Insulin reduced MTMR7 protein levels in human CRC cell lines, and CRC patients with type 2 diabetes mellitus (T2DM) or loss of imprinting (LOI) of insulin-like growth factor 2 (IGF2) had an increased risk for MTMR7 loss. Mechanistically, MTMR7 lowered PIPs and inhibited insulin-mediated AKT-ERK1/2 signaling and proliferation in human CRC cell lines. MTMR7 provides a novel link between growth factor signaling and cancer, and may thus constitute a potential marker or drug target for human CRC.

Activation of Signaling Cascades by Weak Extremely Low Frequency Electromagnetic Fields

Background/Aims: Results from recent studies suggest that extremely low frequency magnetic fields (ELF-MF) interfere with intracellular signaling pathways related to proliferative control. The mitogen-activated protein kinases (MAPKs), central signaling components that regulate essentially all stimulated cellular processes, include the extracellular signal-regulated kinases 1/2 (ERK1/2) that are extremely sensitive to extracellular cues. Anti-phospho-ERK antibodies serve as a readout for ERK1/2 activation and are able to detect minute changes in ERK stimulation. The objective of this study was to explore whether activation of ERK1/2 and other signaling cascades can be used as a readout for responses of a variety of cell types, both transformed and non-transformed, to ELF-MF. Methods: We applied ELF-MF at various field strengths and time periods to eight different cell types with an exposure system housed in a tissue culture incubator and followed the phosphorylation of MAPKs and Akt by western blotting. Results: We found that the phosphorylation of ERK1/2 is increased in response to ELF-MF. However, the phosphorylation of ERK1/2 is likely too low to induce ELF-MF-dependent proliferation or oncogenic transformation. The p38 MAPK was very slightly phosphorylated, but JNK or Akt were not. The effect on ERK1/2 was detected for exposures to ELF-MF strengths as low as 0.15 µT and was maximal at ∼10 µT. We also show that ERK1/2 phosphorylation is blocked by the flavoprotein inhibitor diphenyleneiodonium, indicating that the response to ELF-MF may be exerted via NADP oxidase similar to the phosphorylation of ERK1/2 in response to microwave radiation. Conclusions: Our results further indicate that cells are responsive to ELF-MF at field strengths much lower than previously suspected and that the effect may be mediated by NADP oxidase. However, the small increase in ERK1/2 phosphorylation is probably insufficient to affect proliferation and oncogenic transformation. Therefore, the results cannot be regarded as proof of the involvement of ELF-MF in cancer in general or childhood leukemia in particular.

Mitotic Golgi translocation of ERK1c is mediated by a PI4KIIIβ-14-3-3γ shuttling complex

Golgi fragmentation is a highly regulated process that allows division of the Golgi complex between the two daughter cells. The mitotic reorganization of the Golgi is accompanied by a temporary block in Golgi functioning, as protein transport in and out of the Golgi stops. Our group has previously demonstrated the involvement of the alternatively spliced variants ERK1c and MEK1b (ERK1 is also known as MAPK3, and MEK1 as MAP2K1) in mitotic Golgi fragmentation. We had also found that ERK1c translocates to the Golgi at the G2 to M phase transition, but the molecular mechanism underlying this recruitment remains unknown. In this study, we narrowed the translocation timing to prophase and prometaphase, and elucidated its molecular mechanism. We found that CDK1 phosphorylates Ser343 of ERK1c, thereby allowing the binding of phosphorylated ERK1c to a complex that consists of PI4KIIIβ (also known as PI4KB) and the 14-3-3γ dimer (encoded by YWHAB). The stability of the complex is regulated by protein kinase D (PKD)-mediated phosphorylation of PI4KIIIβ. The complex assembly induces the Golgi shuttling of ERK1c, where it is activated by MEK1b, and induces Golgi fragmentation. Our work shows that protein shuttling to the Golgi is not completely abolished at the G2 to M phase transition, thus integrating several independent Golgi-regulating processes into one coherent pathway.

Regulation of cell proliferation by ERK and signal-dependent nuclear translocation of ERK is dependent on Tm5NM1-containing actin filaments

Tropomyosin Tm5NM1 regulates cell proliferation and organ size. It mediates this effect by regulating the interaction of pERK and Imp7, leading to the regulation of pERK nuclear translocation. This demonstrates a role for a specific population of actin filaments in regulating a critical step in the MAPK/ERK signaling pathway.

ERK-regulated cell proliferation requires multiple phosphorylation events catalyzed first by MEK and then by casein kinase 2 (CK2), followed by interaction with importin7 and subsequent nuclear translocation of pERK. We report that genetic manipulation of a core component of the actin filaments of cancer cells, the tropomyosin Tm5NM1, regulates the proliferation of normal cells both in vitro and in vivo. Mouse embryo fibroblasts (MEFs) lacking Tm5NM1, which have reduced proliferative capacity, are insensitive to inhibition of ERK by peptide and small-molecule inhibitors, indicating that ERK is unable to regulate proliferation of these knockout (KO) cells. Treatment of wild-type MEFs with a CK2 inhibitor to block phosphorylation of the nuclear translocation signal in pERK resulted in greatly decreased cell proliferation and a significant reduction in the nuclear translocation of pERK. In contrast, Tm5NM1 KO MEFs, which show reduced nuclear translocation of pERK, were unaffected by inhibition of CK2. This suggested that it is nuclear translocation of CK2-phosphorylated pERK that regulates cell proliferation and this capacity is absent in Tm5NM1 KO cells. Proximity ligation assays confirmed a growth factor–stimulated interaction of pERK with Tm5NM1 and that the interaction of pERK with importin7 is greatly reduced in the Tm5NM1 KO cells.

Interaction with MEK causes nuclear export and downregulation of peroxisome proliferator-activated receptor gamma

The mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) cascade plays a central role in intracellular signaling by many extracellular stimuli. One target of the ERK cascade is peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor that promotes differentiation and apoptosis. It was previously demonstrated that PPARgamma activity is attenuated upon mitogenic stimulation due to phosphorylation of its Ser84 by ERKs. Here we show that stimulation by tetradecanoyl phorbol acetate (TPA) attenuates PPARgamma's activity in a MEK-dependent manner, even when Ser84 is mutated to Ala. To elucidate the mechanism of attenuation, we found that PPARgamma directly interacts with MEKs, which are the activators of ERKs, but not with ERKs themselves, both in vivo and in vitro. This interaction is facilitated by MEKs' phosphorylation and is mediated by the basic D domain of MEK1 and the AF2 domain of PPARgamma. Immunofluorescence microscopy and subcellular fractionation revealed that MEK1 exports PPARgamma from the nucleus, and this finding was supported by small interfering RNA knockdown of MEK1 and use of a cell-permeable interaction-blocking peptide, which prevented TPA-induced export of PPARgamma from the nucleus. Thus, we show here a novel mode of downregulation of PPARgamma by its MEK-dependent redistribution from the nucleus to the cytosol. This unanticipated role for the stimulation-induced nuclear shuttling of MEKs shows that MEKs can regulate additional signaling components besides the ERK cascade.

Infectious inflammation of the CNS involves activation of mitogen-activated protein kinase and AKT proteins in CSF in humans

The mitogen-activated protein kinases (MAPKs) and the AKT are interacting proteins that serve as transmitters of numerous extracellular signals to their intracellular targets, thereby regulating many cellular processes, such as proliferation, differentiation, development or stress responses. Whereas a large amount of information about the MAPKs/AKT participation in biological processes is available, less is known about their role in human diseases. We postulated that the MAPKs/AKT could be involved in inflammatory processes of the central nervous system (CNS) in humans and we investigated the CSF of 12 patients with viral infection of the CNS for the presence of the distinct components of these cascades. The cerebrospinal fluid (CSF) of 18 individuals who underwent a lumbar puncture for diagnostic purposes served as controls. Six patients with inflammatory disease of the CNS revealed the presence of activated ERK. In five patients p38MAPK was detected, in three in its activated form. The activity of AKT could be demonstrated in four patients. JNK was not found. None of the control patients showed the presence of MAPK enzymes. The mean CSF cellularity was higher in MAPK-positive than in MAPKnegative patients. There was no difference in mean age or gender between the patients and controls, or between the MAPK- and AKT-positive or -negative patients. Our work demonstrates that the MAPK and AKT cascades might participate in inflammatory processes of the CNS. As selective inhibitors of the MAPKs are available, their application in the future might reduce an inappropriate inflammatory response and thus limit brain damage in severe cases of meningoencephalitis.

ECTO- AND EXO-PROTEIN KINASES IN SCHISTOSOMA MANSONI: REGULATION OF SURFACE PHOSPHORYLATION BY ACETYLCHOLINE AND IDENTIFICATION OF THE ALPHA SUBUNIT OF CKII AS A MAJOR SECRETED PROTEIN KINASE

The Schistosoma mansoni parasite life cycle involves complex developmental processes that enable it to cause severe hepatic damage. Protein phosphorylation has previously been implicated in the transformation of cercariae to schistosomula of S. mansoni. Here, we studied the possible involvement of surface (ecto) and shed (exo) protein kinases (PKs) in this developmental process. We found that ecto-PKs are indeed located on the surface of the schistosomula and can phosphorylate up to 5 distinct proteins at this location. Surface phosphorylation was sensitive to acetylcholine, which increased phosphorylation of 3 proteins and reduced phosphorylation of the other 2. The ecto-PKs can be shed from the surface into the incubation medium during parasite differentiation. The main exo-PK is CKII, as concluded from the substrate specificity of the PK, its inhibition by heparin, activation by spermin, and recognition by antibody directed to the anti--alpha-subunit of CKII in the incubation medium of the schistosomula. In spite of its similarity to the ecto-PKs, the activity of the exo-PK is not affected by addition of acetylcholine. These results indicate that ecto- and exo-PKs could be involved in the parasite's development or host-parasite interactions.

Extracellular signal-regulated kinase 1c (ERK1c), a novel 42-kilodalton ERK, demonstrates unique modes of regulation, localization, and function

Extracellular signal-regulated kinases (ERKs) are signaling molecules that regulate many cellular processes. We have previously identified an alternatively spliced 46-kDa form of ERK1 that is expressed in rats and mice and named ERK1b. Here we report that the same splicing event in humans and monkeys causes, due to sequence differences in the inserted introns, the production of an ERK isoform that migrates together with the 42-kDa ERK2. Because of the differences of this isoform from ERK1b, we named it ERK1c. We found that its expression levels are about 10% of ERK1. ERK1c seems to be expressed in a wide variety of tissues and cells. Its activation by MEKs and inactivation by phosphatases are slower than those of ERK1, which is probably the reason for its differential regulation in response to extracellular stimuli. Unlike ERK1, ERK1c undergoes monoubiquitination, which is increased with elevated cell density concomitantly with accumulation of ERK1c in the Golgi apparatus. Elevated cell density also causes enhanced Golgi fragmentation, which is facilitated by overexpression of native ERK1c and is prevented by dominant-negative ERK1c, indicating that ERK1c mediates cell density-induced Golgi fragmentation. The differential regulation of ERK1c extends the signaling specificity of MEKs after stimulation by various extracellular stimuli.

Gonadotropin-releasing hormone induces apoptosis of prostate cancer cells: role of c-Jun NH2-terminal kinase, protein kinase B, and extracellular signal-regulated kinase pathways

A standard therapy used today for prostate cancer is androgen ablation by gonadotropin-releasing hormone analogs (GnRH-a). Although most patients respond to androgen ablation as an initial systemic therapy, nearly all cases will develop androgen resistance, the management of which is still a major challenge. Here, we report that GnRH-a can directly induce apoptosis of the androgen-independent prostate cancer-derived DU145 and PC3 cell lines. Using specific inhibitors, we found that the apoptotic effect of GnRH-a is mediated by c-Jun NH2-terminal kinase (JNK) and inhibited by the phosphatidylinositol 3'-kinase (PI3K)-protein kinase B (PKB) pathway. Indeed, in DU145 cells, GnRH-a activates the JNK cascade in a c-Src- and MLK3-dependent manner but does not involve protein kinase C and epidermal growth factor receptor. Concomitantly, GnRH-a reduces the activity of the PI3K-PKB pathway, which results in the dephosphorylation of PKB mainly in the nucleus. The reduction of PKB activity releases PKB-induced inhibition of MLK3 and thus further stimulates JNK activity and accelerates the apoptotic effect of GnRH-a. Interestingly, extracellular signal-regulated kinase is also activated by GnRH-a, and this occurs via a pathway that involves matrix metalloproteinases and epidermal growth factor receptor, but its activation does not affect JNK activation and the GnRH-a-induced apoptosis. Our results support a potential use of GnRH-a for the treatment of advanced prostate cancer and suggest that the outcome of this treatment can be amplified by using PI3K-PKB inhibitors.

Mechanisms of gonadotropin desensitization

The gonadotropic hormones, FSH and LH exert a major effect on ovarian and testicular function through interaction with specific seven-transmembrane domain glycoprotein receptors. Desensitization to the hormones, which can occur both in vivo and in vitro, is essential for prevention of overstimulation of the gonadal cells. The long-term process of desensitization to the gonadotropic hormones is probably mediated, in part, by extensive clustering and internalization of the hormone-receptor complex. Short-term desensitization may occur as a result of phosphorylation of serine or threonine residues on the receptor molecules, although a specific receptor kinase has not yet been identified. Recently, we have discovered a novel mechanism of gonadotropin desensitization, which is exerted by down-regulation of StAR expression and steroidogenesis mediated by MAPK activation as a result of hormone-receptor interaction, cAMP accumulation and PKA activation. Thus, PKA not only mediates gonadotropin-induced steroidogenesis, it also activates the down-regulation mechanism that can silence steroidogenesis under certain conditions. Moreover, our findings raise the possibility that activation or inhibition of ERK by other pathways could be an important mechanism for diminution or amplification of gonadotropin-stimulated steroidogenesis. This could contribute to functional luteolysis, a process in which luteinized granulosa cells show reduced sensitivity to LH despite maintenance of LH receptors, or to up-regulation of the steroidogenic machinery during luteinization of granulosa cells.

Altered regulation of ERK1b by MEK1 and PTP-SL and modified Elk1 phosphorylation by ERK1b are caused by abrogation of the regulatory C-terminal sequence of ERKs

ERK1b is an alternatively spliced form of ERK1, containing a 26-amino acid insertion between residues 340 and 341 of ERK1. Although under most circumstances the kinetics of ERK1b activation are similar to that of ERK1 and ERK2, we have previously found several conditions under which the activation of ERK1b by extracellular stimuli differs from that of other ERKs. We studied the molecular mechanisms that cause this differential regulation of ERK1b and found that ERK1b is altered in its ability to interact with MEK1 and this influenced its subcellular localization but not its kinetics of activation. ERK1b had a decreased ability to phosphorylate Elk1, but this did not change much the transcriptional activity of the latter. Importantly, the interaction of ERK1b with PTP-SL, which can act as a MAPK phosphatase, shortly after mitogenic stimulation, was significantly affected as well. Using mutants of ERK1b we found that the differential interaction of ERK1b with the three effectors is caused by the site of insertion that abrogates the cytosolic retention sequence/common docking motif of ERKs, and is not dependent on the actual sequence of the insert. Prolonged epidermal growth factor stimulation of Rat1 cells resulted in a differential inactivation and not activation of ERK1b as compared with ERK1 and ERK2. The reduced sensitivity to phosphatases without major differences in the kinetics of activation or activation of substrates, suggests that ERK1b plays a role in the transmission of extracellular signals under conditions of persistent stimulation, where ERK1b and MAPK phosphatases are induced, and the activity of ERK1 and ERK2 is suppressed.

Involvement of the activation loop of ERK in the detachment from cytosolic anchoring

The Extracellular signal-regulated kinases (ERKs) are translocated into the nucleus in response to mitogenic stimulation. The mechanism of translocation and the residues in ERKs that govern this process are not clear as yet. Here we studied the involvement of residues in the activation loop of ERK2 in determining its subcellular localization. Substitution of residues in the activation loop to alanines indicated that residues 173-181 do not play a significant role in the phosphorylation and activation of ERK2. However, residues 176-181 are responsible for the detachment of ERK2 from MEK1 upon mitogenic stimulation. This dissociation can be mimicked by substitution of residues 176-178 to alanines and is prevented by deletion of these residues or by substitution of residues 179-181 to alanines. On the other hand, residues 176-181, as well as residues essential for ERK2 dimerization, do not play a role in the shuttle of ERK2 through nuclear pores. Thus, phosphorylation-induced conformational rearrangement of residues in the activation loop of ERK2 plays a major role in the control of subcellular localization of this protein.

The ERK signaling cascade inhibits gonadotropin-stimulated steroidogenesis

The response of granulosa cells to luteinizing hormone (LH) and follicle-stimulating hormone (FSH) is mediated mainly by cAMP/protein kinase A (PKA) signaling. Notably, the activity of the extracellular signal-regulated kinase (ERK) signaling cascade is elevated in response to these stimuli as well. We studied the involvement of the ERK cascade in LH- and FSH-induced steroidogenesis in two granulosa-derived cell lines, rLHR-4 and rFSHR-17, respectively. We found that stimulation of these cells with the appropriate gonadotropin induced ERK activation as well as progesterone production downstream of PKA. Inhibition of ERK activity enhanced gonadotropin-stimulated progesterone production, which was correlated with increased expression of the steroidogenic acute regulatory protein (StAR), a key regulator of progesterone synthesis. Therefore, it is likely that gonadotropin-stimulated progesterone formation is regulated by a pathway that includes PKA and StAR, and this process is down-regulated by ERK, due to attenuation of StAR expression. Our results suggest that activation of PKA signaling by gonadotropins not only induces steroidogenesis but also activates down-regulation machinery involving the ERK cascade. The activation of ERK by gonadotropins as well as by other agents may be a key mechanism for the modulation of gonadotropin-induced steroidogenesis.

ERK1b, a 46-kDa ERK isoform that is differentially regulated by MEK

We identified a 46-kDa ERK, whose kinetics of activation was similar to that of ERK1 and ERK2 in most cell lines and conditions, but showed higher fold activation in response to osmotic shock and epidermal growth factor treatments of Ras-transformed cells. We purified and cloned this novel ERK (ERK1b), which is an alternatively spliced form of ERK1 with a 26-amino acid insertion between residues 340 and 341 of ERK1. When expressed in COS7 cells, ERK1b exhibited kinetics of activation and kinase activity similar to those of ERK1. Unlike the uniform pattern of expression of ERK1 and ERK2, ERK1b was detected only in some of the tissues examined and seems to be abundant in the rat and human heart. Interestingly, in Ras-transformed Rat1 cells, there was a 7-fold higher expression of ERK1b, which was also more responsive than ERK1 and ERK2 to various extracellular treatments. Unlike ERK1 and ERK2, ERK1b failed to interact with MEK1 as judged from its nuclear localization in resting cells overexpressing ERK1b together with MEK1 or by lack of coimmunoprecipitation of the two proteins. Thus, ERK1b is a novel 46-kDa ERK isoform, which seems to be the major ERK isoform that responds to exogenous stimulation in Ras-transformed cells probably due to its differential regulation by MEK.

Detection of partially phosphorylated forms of ERK by monoclonal antibodies reveals spatial regulation of ERK activity by phosphatases

When cells are stimulated by mitogens, extracellular signal-regulated kinase (ERK) is activated by phosphorylation of its regulatory threonine (Thr) and tyrosine (Tyr) residues. The inactivation of ERK may occur by phosphatase-mediated removal of the phosphates from these Tyr, Thr or both residues together. In this study, antibodies that selectively recognize all combinations of phosphorylation of the regulatory Thr and Tyr residues of ERK were developed, and used to study the inactivation of ERK upon mitogenic stimulation. We found that inactivation of ERK in the early stages of mitogenic stimulation involves separate Thr and Tyr phosphatases which operate differently in the nucleus and in the cytoplasm. Thus, ERK is differentially regulated in various subcellular compartments to secure proper length and strength of activation, which eventually determine the physiological outcome of many external signals.

Identification of a cytoplasmic-retention sequence in ERK2

A key step in the signaling mechanism of the mitogen-activated protein kinase/extracellular signal-responsive kinase (ERK) cascade is its translocation into the nucleus where it regulates transcription and other nuclear processes. In an attempt to characterize the subcellular localization of ERK2, we fused it to the 3'-end of the gene expressing green fluorescent protein (GFP), resulting in a GFP-ERK2 protein. The expression of this construct in CHO cells resulted in a nuclear localization of the GFP-ERK2 protein. However, coexpression of the GFP-ERK2 with its upstream activator, MEK1, resulted in a cytosolic retention of the GFP-ERK2, which was the result of its association with MEK1, and was reversed upon stimulation. We then examined the role of the C-terminal region of ERK2 in its subcellular localization. Substitution of residues 312-319 of GFP-ERK2 to alanine residues prevented the cytosolic retention of ERK2 as well as its association with MEK1, without affecting its activity. Most important for the cytosolic retention are three acidic amino acids at positions 316, 319, and 320 of ERK2. Substitution of residues 321-327 to alanines impaired the nuclear translocation of ERK2 upon mitogenic stimulation. Thus, we conclude that residues 312-320 of ERK2 are responsible for its cytosolic retention, and residues 321-327 play a role in the mechanism of ERK2 nuclear translocation.

Stimulation of Jun N-terminal kinase (JNK) by gonadotropin-releasing hormone in pituitary alpha T3-1 cell line is mediated by protein kinase C, c-Src, and CDC42

The signaling of ligands operating via heterotrimeric G proteins is mediated by a complex network that involves sequential phosphorylation events. Signaling by the G protein-coupled receptor GnRH was shown to include elevation of Ca2+ and activation of phospholipases, protein kinase C (PKC) and extra-cellular signal-regulated kinase (ERK). In this study, GnRH was shown to activate Jun N-Terminal Kinase (JNK)/SAPK in alpha T3-1 cells in a PKC- and tyrosine kinase-dependent manner. GnRH as well as tumor-promoting agent (TPA) also increased c-Src activity, which peaked at 2 min after GnRH stimulation and was sensitive both to PKC and to tyrosine kinase inhibitors. Coexpression of Csk, which serves as a Src-dominant interfering kinase, and constitutively active forms of Src, together with JNK, confirmed the involvement of c-Src downstream of PKC in the GnRH-JNK pathway. Coexpression of dominant negative and constitutively active forms of CDC42, Rac1, Ras, MEKK1, and MEK1 with JNK indicated that JNK activation by GnRH and TPA is mediated by CDC42 and MEKK1. Ras and MEK1, which are involved in a related mitogen-activated protein kinase (MAPK) pathway, did not affect JNK activation in alpha T3-1 cells. Taken together, our results suggest that GnRH stimulation of JNK activity is mediated by a unique pathway that includes sequential activation of PKC, c-Src, CDC42, and probably also MEKK1.

Hippocampal plasticity involves extensive gene induction and multiple cellular mechanisms

Long-term plasticity of the central nervous system (CNS) involves induction of a set of genes whose identity is incompletely characterized. To identify candidate plasticity-related genes (CPGs), we conducted an exhaustive screen for genes that undergo induction or downregulation in the hippocampus dentate gyrus (DG) following animal treatment with the potent glutamate analog, kainate. The screen yielded 362 upregulated CPGs and 41 downregulated transcripts (dCPGs). Of these, 66 CPGs and 5 dCPGs are known genes that encode for a variety of signal transduction proteins, transcription factors, and structural proteins. Seven novel CPGs predict the following putative functions: cpg2--a dystrophin-like cytoskeletal protein; cpg4--a heat-shock protein: cpg16--a protein kinase; cpg20--a transcription factor; cpg21--a dual-specificity MAP-kinase phosphatase; and cpg30 and cpg38--two new seven-transmembrane domain receptors. Experiments performed in vitro and with cultured hippocampal cells confirmed the ability of the cpg-21 product to inactivate the MAP-kinase. To test relevance to neural plasticity, 66 CPGs were tested for induction by stimuli producing long-term potentiation (LTP). Approximately one-fourth of the genes examined were upregulated by LTP. These results indicate that an extensive genetic response is induced in mammalian brain after glutamate receptor activation, and imply that a significant proportion of this activity is coinduced by LTP. Based on the identified CPGs, it is conceivable that multiple cellular mechanisms underlie long-term plasticity of the nervous system.

Detection of ERK activation by a novel monoclonal antibody

The mitogen-activated protein kinase, ERK is activated by a dual phosphorylation on threonine and tyrosine residues. Using a synthetic diphospho peptide, we have generated a monoclonal antibody directed to the active ERK. The antibody specifically identified the active doubly phosphorylated, but not the inactive mono- or non- phosphorylated forms of ERKs. A direct correlation was observed between ERK activity and the intensity in Western blot of mitogen-activated protein kinases from several species. The antibody was proven suitable for immunofluorescence staining, revealing a transient reactivity with ERKs that were translocated to the nucleus upon stimulation. In conclusion, the antibody can serve as a useful tool in the study of ERK signaling in a wide variety of organisms.

Nuclear translocation of mitogen-activated protein kinase kinase (MEK1) in response to mitogenic stimulation

Mitogen-activated protein kinase kinase (MEK) is a dual-specificity protein kinase that is located primarily in the cellular cytosol, both prior to and upon mitogenic stimulation. The existence of a nuclear export signal in the N-terminal domain of MEK [Fukuda, M., Gotoh, I., Gotoh, Y. & Nishida, E. (1996) J. Biol. Chem. 271, 20024-20028] suggests that there are circumstances under which MEK enters the nucleus and must be exported. Using mutants of MEK, we show that the deletion of the nuclear export signal sequence from constitutively active MEK caused constitutive localization of MEK in the nucleus of COS7 and HEK-293T cells. However, when the same region was deleted from a catalytically inactive MEK, cytoplasmic localization was observed in resting cells, which turned nuclear upon stimulation. Confocal microscopy of COS7 cells expressing the above mutants showed localization of the active MEK in the nuclear envelope and also in the cell periphery. The differences in cellular localization between the wild-type and mutant MEKs are not due to severe changes in specificity because the recombinant, constitutively active MEK that lacked its N-terminal region exhibited the same substrate specificity as the wild-type MEK, both in vitro and in intact cells. Taken together, our results indicate that upon mitogenic stimulation, MEK, like extracellular signal responsive kinase and p90(RSK), is massively translocated to the nucleus. Rapid export from the nucleus, which is mediated by the nuclear export signal, is probably the cause for the cytoplasmic distribution observed with wild-type MEK.

Differential activation of mitogen-activated protein kinase and S6 kinase signaling pathways by 12-O-tetradecanoylphorbol-13-acetate (TPA) and insulin. Evidence for involvement of a TPA-stimulated protein-tyrosine kinase

AG-18, an inhibitor of protein-tyrosine kinases, was employed to study the role of tyrosine-phosphorylated proteins in insulin- and phorbol ester-induced signaling cascades. When incubated with Chinese hamster ovary cells overexpressing the insulin receptor, AG-18 reversibly inhibited insulin-induced tyrosine phosphorylation of insulin receptor substate-1, with minimal effects either on receptor autophosphorylation or on phosphorylation of Shc64. Under these conditions, AG-18 inhibited insulin-stimulated phosphorylation of the ribosomal protein S6, while no inhibition of insulin-induced activation of mitogen-activated protein kinase (MAPK) kinase or MAPK was detected. In contrast, 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced activation of MAPK kinase and MAPK and phosphorylation of S6 were inhibited by AG-18. This correlated with inhibition of TPA-stimulated tyrosine phosphorylation of several proteins, the most prominent ones being pp114 and pp120. We conclude that Tyr-phosphorylated insulin receptor substrate-1 is the main upstream regulator of insulin-induced S6 phosphorylation by p70s6k, whereas MAPK signaling seems to be activated in these cells primarily through the adaptor molecule Shc. In contrast, TPA-induced S6 phosphorylation is mediated by the MAPK/p90rsk cascade. A key element of this TPA-stimulated signaling pathway is an AG-18-sensitive protein-tyrosine kinase.