Cleavage arrest of early frog embryos by the G protein-activated protein kinase PAK I

PAK2 activated by Cdc42 and caspase 3 mediates different cellular responses to oxidative stress-induced apoptosis

p21-activated protein kinase (PAK2) is a unique member of the PAK family kinases that plays important roles in stress signaling. It can be activated by binding to the small GTPase, Cdc42 and Rac1, or by caspase 3 cleavage. Cdc42-activated PAK2 mediates cytostasis, whereas caspase 3-cleaved PAK2 contributes to apoptosis. However, the relationship between these two states of PAK2 activation remains elusive. In this study, through protein biochemical analyses and various cell-based assays, we demonstrated that full-length PAK2 activated by Cdc42 was resistant to the cleavage by caspase 3 in vitro and within cells. When mammalian cells were treated by oxidative stress using hydrogen peroxide, PAK2 was highly activated through caspase 3 cleavage that led to apoptosis. However, when PAK2 was pre-activated by Cdc42 or by mild stress such as serum deprivation, it was no longer able to be cleaved by caspase 3 upon hydrogen peroxide treatment, and the subsequent apoptosis was also largely inhibited. Furthermore, cells expressing active mutants of full-length PAK2 became more resistant to hydrogen peroxide-induced apoptosis than inactive mutants. Taken together, this study identified two states of PAK2 activation, wherein Cdc42- and autophosphorylation-dependent activation inhibited the constitutive activation of PAK2 by caspase cleavage. The regulation between these two states of PAK2 activation provides a new molecular mechanism to support PAK2 as a molecular switch for controlling cytostasis and apoptosis in response to different types and levels of stress with broad physiological and pathological relevance.

The p21-activated kinase (PAK) family member PakD is required for chemorepulsion and proliferation inhibition by autocrine signals in Dictyostelium discoideum

In Dictyostelium discoideum, the secreted proteins AprA and CfaD function as reporters of cell density and regulate cell number by inhibiting proliferation at high cell densities. AprA also functions to disperse groups of cells at high density by acting as a chemorepellent. However, the signal transduction pathways associated with AprA and CfaD are not clear, and little is known about how AprA affects the cytoskeleton to regulate cell movement. We found that the p21-activated kinase (PAK) family member PakD is required for both the proliferation-inhibiting activity of AprA and CfaD and the chemorepellent activity of AprA. Similar to cells lacking AprA or CfaD, cells lacking PakD proliferate to a higher cell density than wild-type cells. Recombinant AprA and CfaD inhibit the proliferation of wild-type cells but not cells lacking PakD. Like AprA and CfaD, PakD affects proliferation but does not significantly affect growth (the accumulation of mass) on a per-nucleus basis. In contrast to wild-type cells, cells lacking PakD are not repelled from a source of AprA, and colonies of cells lacking PakD expand at a slower rate than wild-type cells, indicating that PakD is required for AprA-mediated chemorepulsion. A PakD-GFP fusion protein localizes to an intracellular punctum that is not the nucleus or centrosome, and PakD-GFP is also occasionally observed at the rear cortex of moving cells. Vegetative cells lacking PakD show excessive actin-based filopodia-like structures, suggesting that PakD affects actin dynamics, consistent with previously characterized roles of PAK proteins in actin regulation. Together, our results implicate PakD in AprA/CfaD signaling and show that a PAK protein is required for proper chemorepulsive cell movement in Dictyostelium.

Unique catalytic activities and scaffolding of p21 activated kinase-1 in cardiovascular signaling

P21 activated kinase-1 (Pak1) has diverse functions in mammalian cells. Although a large number of phosphoproteins have been designated as Pak1 substrates from in vitro studies, emerging evidence has indicated that Pak1 may function as a signaling molecule through a unique molecular mechanism – scaffolding. By scaffolding, Pak1 delivers signals through an auto-phosphorylation-induced conformational change without transfer of a phosphate group to its immediate downstream effector(s). Here we review evidence for this regulatory mechanism based on structural and functional studies of Pak1 in different cell types and research models as well as in vitro biochemical assays. We also discuss the implications of Pak1 scaffolding in disease-related signaling processes and the potential in cardiovascular drug development.

Photodynamic treatment induces an apoptotic pathway involving calcium, nitric oxide, p53, p21-activated kinase 2, and c-Jun N-terminal kinase and inactivates survival signal in human umbilical vein endothelial cells

Photodynamic treatment (PDT) elicits a diverse range of cellular responses, including apoptosis. Previously, we showed that PDT stimulates caspase-3 activity, and subsequent cleavage and activation of p21-activated kinase 2 (PAK2) in human epidermal carcinoma A431 cells. In the current study, pretreatment with nitric oxide (NO) scavengers inhibited PDT-induced mitochondrial membrane potential (MMP) changes, activation of caspase-9, caspase-3, p21-activated protein kinase 2 (PAK2) and c-Jun N-terminal kinase (JNK), and gene expression of p53 and p21 involved in apoptotic signaling. Moreover, PAK2 activity was required for PDT-induced JNK activation and apoptosis. Inhibition of p53 mRNA expression using small interfering RNA (siRNA) additionally blocked activation of PAK2 and apoptosis induced by PDT. Importantly, our data also show that PDT triggers cell death via inactivation of ERK-mediated anti-apoptotic pathway. PDT triggers cell death via inactivation of the HSP90/multi-chaperone complex and subsequent degradation of Ras, further inhibiting anti-apoptotic processes, such as the Ras→ERK signal transduction pathway. Furthermore, we did not observe two-stage JNK activation for regulation of PAK2 activity in the PDT-induced apoptotic pathway in HUVECs, which was reported earlier in A431 cells. Based on the collective results, we have proposed a model for the PDT-triggered inactivation of the survival signal and apoptotic signaling cascade with Rose Bengal (RB), which sequentially involves singlet oxygen, Ca²⁺, NO, p53, caspase-9, caspase-3, PAK2, and JNK.

Ginkgolide B induces apoptosis via activation of JNK and p21-activated protein kinase 2 in mouse embryonic stem cells

Ginkgolide B (GKB), the major active component of Ginkgo biloba extracts, can both stimulate and inhibit apoptotic signaling. We previously showed that ginkgolide treatment of mouse blastocysts induces apoptosis, decreases cell numbers, retards the proliferation and development of mouse embryonic stem cells and blastocysts in vitro, and causes developmental injury in vivo. However, the precise molecular mechanisms underlying its actions are currently unknown. Here, our study further revealed that GKB induced apoptotic biochemical changes, including activation of JNK, caspase-3, and p21-activated protein kinase 2 (PAK2), in ESC-B5 mouse embryonic stem cells. Treatment of ESC-B5 cells with a JNK-specific inhibitor (SP600125) reduced GKB-induced activation of both JNK and caspase-3, indicating that JNK activity is required for GKB-induced caspase activation. Experiments using caspase-3 inhibitors and antisense oligonucleotides against PAK2 showed that caspase-3 activation is required for PAK2 activation and both of these activations are required for GKB-induced apoptosis in ESC-B5 cells.

Activation of JNK and PAK2 is essential for citrinin-induced apoptosis in a human osteoblast cell line

The mycotoxin citrinin (CTN), a natural contaminant in foodstuffs and animal feeds, exerts cytotoxic and genotoxic effects on various mammalian cells. CTN causes cell injury, including apoptosis. Previous studies by our group showed that CTN triggers apoptosis in mouse embryonic stem cells, as well as embryonic developmental injury. Here, we investigated the precise mechanisms governing this apoptotic effect in osteoblasts. CTN induced apoptotic biochemical changes in a human osteoblast cell line, including activation of c-Jun N-terminal kinase (JNK), loss of mitochondrial membrane potential, and caspase-3 and p21-activated protein kinase 2 (PAK2) activation. Experiments using a JNK-specific inhibitor, SP600125, and antisense oligonucleotides against JNK reduced CTN-induced activation of both JNK and caspase-3 in osteoblasts, indicating that JNK is required for caspase activation in this apoptotic pathway. Experiments using caspase-3 inhibitors and antisense oligonucleotides against PAK2 revealed that active caspase-3 is essential for PAK2 activation. Moreover, both caspase-3 and PAK2 require activation for CTN-induced apoptosis of osteoblasts. Interestingly, CTN stimulates two-stage activation of JNK in human osteoblasts. Early-stage JNK activation is solely ROS-dependent, whereas late-stage activation is dependent on ROS-mediated caspase activity, and regulated by caspase-induced activation of PAK2. On the basis of these results, we propose a signaling cascade model for CTN-induced apoptosis in human osteoblasts involving ROS, JNK, caspases, and PAK2.

Elevated p21-activated kinase 2 activity results in anchorage-independent growth and resistance to anticancer drug-induced cell death

p21-activated kinase 2 (PAK-2) seems to be a regulatory switch between cell survival and cell death signaling. We have shown previously that activation of full-length PAK-2 by Rac or Cdc42 stimulates cell survival, whereas caspase activation of PAK-2 to the proapoptotic PAK-2p34 fragment is involved in the cell death response. In this study, we present a role of elevated activity of full-length PAK-2 in anchorage-independent growth and resistance to anticancer drug-induced apoptosis of cancer cells. Hs578T human breast cancer cells that have low levels of PAK-2 activity were more sensitive to anticancer drug-induced apoptosis and showed higher levels of caspase activation of PAK-2 than MDA-MB435 and MCF-7 human breast cancer cells that have high levels of PAK-2 activity. To examine the role of elevated PAK-2 activity in breast cancer, we have introduced a conditionally active PAK-2 into Hs578T human breast cells. Conditional activation of PAK-2 causes loss of contact inhibition and anchorage-independent growth of Hs578T cells. Furthermore, conditional activation of PAK-2 suppresses activation of caspase 3, caspase activation of PAK-2, and apoptosis of Hs578T cells in response to the anticancer drug cisplatin. Our data suggest a novel mechanism by which full-length PAK-2 activity controls the apoptotic response by regulating levels of activated caspase 3 and thereby its own cleavage to the proapoptotic PAK-2p34 fragment. As a result, elevated PAK-2 activity interrupts the apoptotic response and thereby causes anchorage-independent survival and growth and resistance to anticancer drug-induced apoptosis.

Novel DNA microarray system for analysis of nascent mRNAs

Transcriptional activation and repression are a key step in the regulation of all cellular activities. The development of comprehensive analysis methods such as DNA microarray has advanced our understanding of the correlation between the regulation of transcription and that of cellular mechanisms. However, DNA microarray analysis based on steady-state mRNA (total mRNA) does not always correspond to transcriptional activation or repression. To comprehend these transcriptional regulations, the detection of nascent RNAs is more informative. Although the nuclear run-on assay can detect nascent RNAs, it has not been fully applied to DNA microarray analysis. In this study, we have developed a highly efficient method for isolating bromouridine-labeled nascent RNAs that can be successfully applied to DNA microarray analysis. This method can linearly amplify small amounts of mRNAs with little bias. Furthermore, we have applied this method to DNA microarray analysis from mouse G₂-arrested cells and have identified several genes that exhibit novel expression profiles. This method will provide important information in the field of transcriptome analysis of various cellular processes.

Inhibition of cap-dependent translation via phosphorylation of eIF4G by protein kinase Pak2

Translation is downregulated in response to a variety of moderate stresses, including serum deprivation, hyperosmolarity and ionizing radiation. The cytostatic p21-activated protein kinase 2 (Pak2)/gamma-PAK is activated under the same stress conditions. Expression of wild-type Pak2 in cells and addition of Pak2 to reticulocyte lysate inhibit translation, while kinase-inactive mutants have no effect. Pak2 binds to and phosphorylates initiation factor (eIF)4G, which inhibits association of eIF4E with m(7)GTP, reducing initiation. The Pak2-binding site maps to the region on eIF4G that contains the eIF4E-binding site; Pak2 and eIF4E compete for binding to this site. Using an eIF4G-depleted reticulocyte lysate, reconstitution with mock-phosphorylated eIF4G fully restores translation, while phosphorylated eIF4G reduces translation to 37%. RNA interference releases Pak2-induced inhibition of translation in contact-inhibited cells by 2.7-fold. eIF4G mutants of the Pak2 site show that S896D inhibits translation, while S896A has no effect. Activation of Pak2 in response to hyperosmotic stress inhibits cap-dependent, but not IRES-driven, initiation. Thus, a novel pathway for mammalian cell stress signaling is identified, wherein activation of Pak2 leads to inhibition of cap-dependent translation through phosphorylation of eIF4G.

Cell Type-specific Regulation of RhoA Activity during Cytokinesis

Rho family GTPases play pivotal roles in cytokinesis. By using probes based on the principle of fluorescence resonance energy transfer (FRET), we have shown that in HeLa cells RhoA activity increases with the progression of cytokinesis. Here we show that in Rat1A cells RhoA activity remained suppressed during most of the cytokinesis. Consistent with this observation, the expression of C3 toxin inhibited cytokinesis in HeLa cells but not in Rat1A cells. Furthermore, the expression of a dominant negative mutant of Ect2, a Rho GEF, or Y-27632, an inhibitor of the Rho-dependent kinase ROCK, inhibited cytokinesis in HeLa cells but not in Rat1A cells. In contrast to the activity of RhoA, the activity of Rac1 was suppressed during cytokinesis and started increasing at the plasma membrane of polar sides before the abscission of the daughter cells in both HeLa and Rat1A cells. This type of Rac1 suppression was shown to be essential for cytokinesis because a constitutively active mutant of Rac1 induced a multinucleated phenotype in both HeLa and Rat1A cells. Moreover, the involvement of MgcRacGAP/CYK-4 in this suppression of Rac1 during cytokinesis was shown by the use of a dominant negative mutant. Because ML-7, an inhibitor of myosin light chain kinase, delayed the cytokinesis of Rat1A cells and because Pak, a Rac1 effector, is known to suppress myosin light chain kinase, the suppression of the Rac1-Pak pathway by MgcRacGAP may play a pivotal role in the cytokinesis of Rat1A cells.

A novel role for p21-activated protein kinase 2 in T cell activation

To identify novel components of the TCR signaling pathway, a large-scale retroviral-based functional screen was performed using CD69 expression as a marker for T cell activation. In addition to known regulators, two truncated forms of p21-activated kinase 2 (PAK2), PAK2DeltaL(1-224) and PAK2DeltaS(1-113), both lacking the kinase domain, were isolated in the T cell screen. The PAK2 truncation, PAK2DeltaL, blocked Ag receptor-induced NFAT activation and TCR-mediated calcium flux in Jurkat T cells. However, it had minimal effect on PMA/ionomycin-induced CD69 up-regulation in Jurkat cells, on anti-IgM-mediated CD69 up-regulation in B cells, or on the migratory responses of resting T cells to chemoattractants. We show that PAK2 kinase activity is increased in response to TCR stimulation. Furthermore, a full-length kinase-inactive form of PAK2 blocked both TCR-induced CD69 up-regulation and NFAT activity in Jurkat cells, demonstrating that kinase activity is required for PAK2 function downstream of the TCR. We also generated a GFP-fused PAK2 truncation lacking the Cdc42/Rac interactive binding region domain, GFP-PAK2(83-149). We show that this construct binds directly to the kinase domain of PAK2 and inhibits anti-TCR-stimulated T cell activation. Finally, we demonstrate that, in primary T cells, dominant-negative PAK2 prevented anti-CD3/CD28-induced IL-2 production, and TCR-induced CD40 ligand expression, both key functions of activated T cells. Taken together, these results suggest a novel role for PAK2 as a positive regulator of T cell activation.

Phosphorylation of Mnk1 by caspase-activated Pak2/gamma-PAK inhibits phosphorylation and interaction of eIF4G with Mnk

The mitogen-activated protein kinase-interacting kinase 1 (Mnk1) is phosphorylated by caspase-cleaved protein kinase Pak2/gamma-PAK but not by Cdc42-activated Pak2. Phosphorylation of Mnk1 is rapid, reaching 1 mol/mol within 15 min of incubation with Pak2. A kinetic analysis of the phosphorylation of Mnk1 by Pak2 yields a K(m) of 0.6 microm and a V(max) of 14.9 pmol of (32)P/min/microg of Pak2. Two-dimensional tryptic phosphopeptide mapping of Mnk1 phosphorylated by Pak2 yields two distinct phosphopeptides. Analysis of the phosphopeptides by automated microsequencing and manual Edman degradation identified the sites in Mnk1 as Thr(22) and Ser(27). Mnk1, activated by phosphorylation with Erk2, phosphorylates the eukaryotic initiation factor (eIF) 4E and the eIF4G components of eIF4F. Phosphorylation of Mnk1 by Pak2 does not activate Mnk1, as measured with either eIF4E or eIF4F as substrate. Phosphorylation of Erk2-activated Mnk1 by Pak2 has no effect on phosphorylation of eIF4E but reduces phosphorylation of eIF4G by Mnk1 by up to 50%. Phosphorylation of Mnk1 by Pak2 inhibits binding of eIF4G peptides containing the Mnk1 binding site by up to 80%. When 293T cells are subjected to apoptotic induction by hydrogen peroxide, Mnk1 is phosphorylated at both Thr(22) and Ser(27). These results indicate a role for Pak2 in the down-regulation of translation initiation in apoptosis by phosphorylation of Mnk1.

Negative control of the Myc protein by the stress-responsive kinase Pak2

Pak2 is a serine/threonine kinase that participates in the cellular response to stress. Among the potential substrates for Pak2 is the protein Myc, encoded by the proto-oncogene MYC. Here we demonstrate that Pak2 phosphorylates Myc at three sites (T358, S373, and T400) and affects Myc functions both in vitro and in vivo. Phosphorylation at all three residues reduces the binding of Myc to DNA, either by blocking the requisite dimerization with Max (through phosphorylation at S373 and T400) or by interfering directly with binding to DNA (through phosphorylation at T358). Phosphorylation by Pak2 inhibits the ability of Myc to activate transcription, to sustain cellular proliferation, to transform NIH 3T3 cells in culture, and to elicit apoptosis on serum withdrawal. These results indicate that Pak2 is a negative regulator of Myc, suggest that inhibition of Myc plays a role in the cellular response to stress, and raise the possibility that Pak2 may be the product of a tumor suppressor gene.

Activation of Syk protein tyrosine kinase in response to osmotic stress requires interaction with p21-activated protein kinase Pak2/gamma-PAK

The p21-activated serine/threonine protein kinase Pak2/gamma-PAK and the nonreceptor type of protein tyrosine kinase Syk are known to be activated when the cells are exposed to osmotic stress. The purpose of the present study was to examine whether Pak2 and Syk functionally cooperate in cellular signaling. Cotransfection studies revealed that Pak2 associates with Syk in COS cells. The constitutively active form of Cdc42 increases the association of Pak2 with Syk. Pak2 coexpressed with an inactive form of Cdc42 or kinase-inactive Pak2 interacts to a lesser extent with Syk, suggesting that Pak2-Syk association is enhanced by Pak2 activation. Interaction with Pak2 enhances the intrinsic kinase activity of Syk. This is supported by in vitro studies showing that Pak2 phosphorylates and activates Syk. Treatment of cells with sorbitol to induce hyperosmolarity results in the translocation of Pak2 and Syk to the region surrounding the nucleus and in dramatic enhancement of their association. Furthermore, cotransfection of Pak2 and Syk leads to the activation of c-Jun N-terminal kinase (JNK) under hyperosmotic conditions. Pak2 short interfering RNA suppresses sorbitol-mediated activation of endogenous Syk and JNK, thus identifying a novel pathway for JNK activation by Cdc42. These results demonstrate that Pak2 and Syk positively cooperate to regulate cellular responses to stress.

The catalytic domain of xPAK1 is sufficient to induce myosin II dependent in vivo cell fragmentation independently of other apoptotic events

During apoptosis, cells are fragmented into sealed packages for safe disposal by phagocytosis, a process requiring major reorganisation of the cytoskeleton. The small p21 GTPase-activated kinases (PAKs) have been implicated in regulating cytoskeletal dynamics and a subset are activated by caspase 3/7 cleavage. However, the functional importance of this activation in apoptosis remains unknown. Using early Xenopus embryos, we have dissected xPAK1 activation from other causative events in apoptosis. An apoptotic-like cell fragmentation was observed 30 min after expression of the xPAK1 catalytic domain and occurred in the absence of other markers of apoptosis. In vitro, activated xPAK1 phosphorylated the regulatory light chain (xMLC) of myosin II at threonine 18 and serine 19, events known to activate the actin-dependent ATPase of cytoskeletal myosin. In vivo, activated xPAK1 induced hyperphosphorylation of xMLC. BDM, a myosin inhibitor, and ML-7, a MLCK inhibitor, both abrogated cell fragmentation induced by activated xPAK1, and ML-7 also inhibited xPAK1 activity. Endogenous xPAK1 was cleaved during normal apoptosis and this was associated with xPAK1 activation and increased serine 19 phosphorylation of xMLC. The data show that PAK activation is sufficient for apoptotic body formation in vivo and strongly suggest that activation of myosin II is essential for this process.

Localization of p21-activated protein kinase gamma-PAK/Pak2 in the endoplasmic reticulum is required for induction of cytostasis

The intracellular localization and physiological functions of the p21-activated protein kinase gamma-PAK have been examined in human embryonic kidney 293T and COS-7 cells. At 1-4 days post-transfection, cell division is inhibited by the expression of wild type (WT) gamma-PAK and the mutant S490A, whereas cells expressing S490D and the inactive mutants K278R and T402A grow exponentially, indicating a role for gamma-PAK in the induction of cytostasis. WT gamma-PAK and S490A are localized in a region surrounding the nucleus identified as the endoplasmic reticulum (ER), as determined by immunofluorescence, whereas K278R, T402A, and S490D lack localization. As shown by sucrose density gradient centrifugation, WT gamma-PAK, S490A, and endogenous gamma-PAK are distributed among the high density (ER-associated), intermediate density, and low density fractions, whereas the mutants that do not inhibit cell division are present only as soluble enzyme. The amount of endogenous gamma-PAK associated with the particulate fractions is increased 4-fold when cell division is inhibited by ionizing radiation. gamma-PAK in the ER and intermediate density fractions has high specific activity and is active, whereas the soluble form of gamma-PAK has low activity and is activable. The importance of localization of gamma-PAK is supported by data with the C-terminal mutants S490D and Delta 488; these mutants have high levels of protein kinase activity but do not induce cytostasis and are not bound to the ER. A model for the induction of cytostasis by gamma-PAK through targeting of gamma-PAK to the ER is presented in which gamma-PAK activity and Ser-490 are implicated in the regulation of cytostasis.

Profiling substrate phosphorylation at the phosphopeptide level

The identification of substrates is a key aspect in the study of the biological function of protein kinases. The procedure here described is aimed at profiling substrate phosphorylation at the phosphopeptide level by sequentially involving (i). the assessment of the in vitro activity of individual protein kinases on a complex mix of immobilized proteins, (ii). the fractionation of the phosphopeptides being released upon proteolysis of substrates, and (iii). the final identification of the targeted sequences. In particular, the protein sample is spotted onto nitrocellulose membrane and then subjected to a solid-phase kinase assay in the presence of [32P]ATP, prior to solid-phase proteolytic digestion and two-dimensional phosphopeptide mapping. Radiolabeled phosphopeptides are subsequently isolated and sequenced to identify the substrates being targeted by the examined protein kinase. Using the gamma-isotype of p21-activated protein kinase (gamma-PAK) and its known in vitro substrates, I verified that both the specificity of substrate phosphorylation and its efficiency are similar upon solid- and liquid-phase conditions. To demonstrate the feasibility of the overall experimental system, I then employed a fairly crude cell extract as a source of candidate substrates and successfully identified the sequence of a putative substrate of gamma-PAK.

XPak3 promotes cell cycle withdrawal during primary neurogenesis in Xenopus laevis

We have isolated the Xenopus p21-activated kinase 3 (XPak3) by virtue of its expression in the territory of primary neurogenesis in the developing embryo. XPak3, but not the other Pak variants, responds positively to X-Ngnr-1 and negatively to X-Notch-1. A constitutively active form of XPak3, generated by fusing a myristylation signal to the N-terminus (XPak3-myr), induces early cell cycle arrest at high concentrations, while ectopic expression of low amounts induces premature neuronal differentiation. Conversely, XPak3 loss of function achieved by use of an antisense morpholino oligonucleotide increases cell proliferation and inhibits neuronal differentiation; this phenotype is rescued by co-injection of XPak3-myr. We conclude that XPak3 is a novel member of the proneural pathway, functioning downstream of neurogenin to withdraw neuronally programmed cells from the mitotic cell cycle, thus allowing for their differentiation.

p21-activated protein kinase gamma-PAK in pituitary secretory granules phosphorylates prolactin

p21-activated protein kinase gamma-PAK phosphorylates prolactin (PRL) in rat pituitary secretory granules on Ser-177 and on the equivalent site, Ser-179, in recombinant human PRL. This is shown by comparison of phosphopeptide maps with the human PRL mutant S179D. gamma-PAK is present in rat and bovine granules as identified by in-gel phosphorylation of histone H4, and by immunoblotting. Thus, phosphorylation of PRL by gamma-PAK in granules produces the PRL molecule that has been shown to antagonize the growth-promoting activity of unmodified PRL, and is consistent with the identified role of gamma-PAK in the induction and maintenance of cytostasis.

Cdc42-independent activation and translocation of the cytostatic p21-activated protein kinase gamma-PAK by sphingosine

Autophosphorylation of p21-activated protein kinase gamma-PAK is stimulated at 10 microM sphingosine in vitro and is maximal at 100 microM. Sites autophosphorylated on gamma-PAK in response to sphingosine are identical to those obtained with Cdc42(GTP). Autophosphorylation is paralleled by stimulation of gamma-PAK activity as measured with peptide and protein substrates. In 3T3-L1 cells, sphingosine stimulates the autophosphorylation and activity of gamma-PAK associated with the membrane-containing particulate fraction by 2.8-fold, but does not stimulate the activity of the soluble enzyme. Thus, gamma-PAK is activatable via a Cdc42-independent mechanism, suggesting sphingosine has a role in gamma-PAK activation under conditions of cell stress.

Cytostatic p21 G protein-activated protein kinase gamma-PAK

The p21-activated protein kinase gamma-PAK, also known as PAK2, has very different properties from the other two highly conserved isoforms of the PAK family, alpha-PAK (PAK1) and beta-PAK (PAK3). gamma-PAK has cytostatic activity, as shown by inhibition of cleavage of early frog embryos following microinjection of gamma-PAK and by inhibition of growth when expressed in mammalian cells. gamma-PAK is activated in response to a variety of stresses including radiation- and chemically-induced DNA damage, hyperosmolarity, addition of sphingosine, serum starvation, and contact inhibition. Activation occurs through at least two signaling pathways, depending on the type of stress, one of which requires phosphoinositide 3-kinase and/or tyrosine kinase activity. During apoptosis gamma-PAK is cleaved by caspase 3 and activated and appears to have a role in the apoptotic response. gamma-PAK is present in the cytosol, associated with the membrane and in secretory granules. A wide variety of substrates have been identified for gamma-PAK. We propose gamma-PAK may be involved in coordinating the stress response, possibly in conjunction with other stress response proteins.

Functional interaction between c-Abl and the p21-activated protein kinase gamma-PAK

A member of the p21-activated protein kinase (PAK) family, gamma-PAK has cytostatic properties and is activated by cellular stresses such as hyperosmolarity or DNA damage. We report herein that gamma-PAK is associated in vivo with the nonreceptor protein tyrosine kinase c-Abl. gamma-PAK phosphorylates c-Abl on sites located in the kinase domain, in a region that is implicated in protein-protein interactions and in subcellular localization. Activation of gamma-PAK in human embryonic kidney 293T cells by cotransfection with constitutively active Cdc42 induces activation of c-Abl, resulting in increased phosphotyrosine levels. Cotransfection of c-Abl and gamma-PAK elicits phosphorylation of gamma-PAK on tyrosine and down-regulation of gamma-PAK activity, promoting accumulation of inactive gamma-PAK. gamma-PAK is also phosphorylated in vitro by c-Abl. gamma-PAK activity is regulated by ubiquitination and proteolysis in vivo, as shown by immunoblotting with an anti-ubiquitin antibody in the presence of proteasome inhibitors. In summary, we describe a functional interaction between gamma-PAK and c-Abl in which gamma-PAK stimulates c-Abl tyrosine kinase activity and c-Abl phosphorylates and down-regulates gamma-PAK, suggesting the existence of a negative feedback loop between c-Abl and gamma-PAK.

Independent activation of endogenous p21-activated protein kinase-3 (PAK3) and JNK by thrombin in CCL39 fibroblasts

Thrombin, a potent mitogen for CCL39 hamster lung fibroblasts, activates the seven membrane-spanning receptor PAR1. To better understand the signaling pathways controlled by this receptor we analyzed a potential downstream effector, p21-activated protein kinase (PAK). Thrombin and PAR1 agonist peptide, as well as serum and lysophosphatidic acid, were found to stimulate HA-mPAK3 activity in CCL39 cells transfected with a plasmid encoding the epitope-tagged kinase. Similar results were obtained using antibodies developed against the endogenous kinase. PAK3 activation is sensitive to pertussis toxin, but insensitive to LY 294002, an inhibitor of phosphatidylinositol 3'-kinase. Thrombin and serum also activate c-jun amino terminal kinase (JNK). Similar to PAK3 activation, thrombin-stimulated JNK activity is inhibited by pertussis toxin, but not by LY 294002. In a CCL39-derived cell line expressing constitutively active mPAK3 in a tetracyline-dependent manner, induction of PAK activity does not lead to corresponding increases in JNK activity. Our findings indicate that PAK3 is responsive to thrombin and other G protein-coupled receptor systems. Furthermore, our data suggest that in CCL39 cells, JNK activation by thrombin occurs independently of PAK3.

Substrates enhance autophosphorylation and activation of p21-activated protein kinase gamma-PAK in the absence of activation loop phosphorylation

The p21-activated protein kinase gamma-PAK from rabbit, expressed in insect cells, is activated following binding of Cdc42(GTPgammaS). The rate of autophosphorylation is increased fivefold and the protein kinase activity 13-fold, as measured with the synthetic heptapeptide (AKRESAA). The mutant K278R, where the invariant lysine in the catalytic site is replaced by arginine, shows neither autophosphorylation nor activity. Replacement of the conserved threonine in the catalytic domain with alanine (T402A) reduces autophosphorylation and protein kinase activity to 1% that of the wild-type gamma-PAK, indicating autophosphorylation of Thr402 in the activation loop is essential for protein kinase activity. In contrast, certain protein substrates such as histone 2B, histone 4 and myelin basic protein, stimulate both autophosphorylation and protein kinase activity to levels similar to those observed with Cdc42(GTPgammaS). This substrate-level activation does not require autophosphorylation of Thr402 in the activation loop. As shown with T402A, the protein kinase activity with histone 4 is similar to that observed with recombinant wild-type gamma-PAK. Basic proteins or peptides which are not substrates of gamma-PAK, such as histone 1 and polylysine, do not stimulate autophosphorylation or activity. Other substrates such as the Rous sarcoma virus protein NC are phosphorylated by gamma-PAK following activation by Cdc42(GTPgammaS), but are not phosphorylated by T402A. The data suggest that some substrates can override the requirement for Cdc42(GTPgammaS), by activating gamma-PAK directly.

p21-activated protein kinase gamma-PAK is translocated and activated in response to hyperosmolarity. Implication of Cdc42 and phosphoinositide 3-kinase in a two-step mechanism for gamma-PAK activation

A member of the family of p21-activated protein kinases, gamma-PAK, has cytostatic properties and is activated during apoptosis and in response to DNA damage. To determine whether gamma-PAK is activated by other types of cell stress and to assess its mechanism of activation, the response of gamma-PAK to hyperosmotic stress was examined. In 3T3-L1 mouse fibroblasts, there are two pools of gamma-PAK: the majority of the protein kinase is soluble and has low specific activity, whereas gamma-PAK associated with the particulate fraction has significantly higher specific activity. Hyperosmolarity promotes translocation of gamma-PAK from the soluble to the particulate fraction; this parallels activation of the protein kinase. Activation but not translocation of gamma-PAK is wortmannin-sensitive, suggesting the involvement of a phosphoinositide 3-kinase-related activity. gamma-PAK translocation in response to hyperosmolarity parallels Cdc42 translocation to the particulate fraction in vivo and can be induced in vitro by guanosine 5'-3-O-(thio)triphosphate. Cotransfection of gamma-PAK with constitutively active Cdc42 induces gamma-PAK activation and translocation, whereas inactive Cdc42 inhibits both processes in response to hyperosmotic stress, suggesting that Cdc42 has a role in the translocation and activation of gamma-PAK. alpha-PAK is not activated in response to hyperosmolarity in 3T3-L1 cells. A two-step model of gamma-PAK activation is presented.

The amino-terminus and membrane-spanning domains of LMP-1 inhibit cell proliferation

The LMP-1 oncoprotein of EBV is required to maintain proliferation of infected B-cells and shares several features with CD40, TNF-R1, and related receptors. Members of this family can bind TRAF and TRADD molecules and activate NF-kappaB and AP-1, as can LMP-1. While CD40 and TNF-R1 are dependent on binding their ligands for their signaling, LMP-1 apparently is not. We have found that LMP-1 can act as a governor of cell proliferation and thereby limit its own activities. Its inhibition of proliferation is not mediated by apoptosis but results in cytostasis in four cell lines tested. The structural moiety of LMP-1 that distinguishes it from CD40 and TNF-R1, its amino-terminus and multiple membrane spanning segments, alone can mediate its cytostatic activity.

p21-activated protein kinase gamma-PAK is activated by ionizing radiation and other DNA-damaging agents. Similarities and differences to alpha-PAK

The p21-activated protein kinase gamma-PAK is activated 2-5-fold in response to ionizing radiation (IR) in 3T3-L1 fibroblasts and U937 leukemia cells. gamma-PAK is activated in a dose- and time-dependent manner. Doses from 1 to 100 Gy result in significant stimulation of activity at 30 min, whereas maximal stimulation is observed at 120 min after irradiation. UV (80 J/m(2)) and the DNA-damaging drugs cytosine beta-D-arabinofuranoside (AraC) and cis-platinum(II)diammine dichloride (cisplatin) also induce gamma-PAK activation. The activation of gamma-PAK in response to IR or AraC is dependent on tyrosine kinase and phosphoinositide 3-kinase activity, as demonstrated by use of the inhibitors genistein and wortmannin; in contrast activation of gamma-PAK by cisplatin and UV is not affected significantly by these inhibitors, suggesting that gamma-PAK can be activated by more than one pathway in response to different types of DNA damage. In contrast to gamma-PAK, alpha-PAK and JNK are activated only by cisplatin and UV in 3T3-L1 cells, suggesting differential regulation of the protein kinases. This is the first time that members of the Ste20/PAK family of protein kinases have been shown to be involved in the cellular response to IR and other DNA-damaging agents.

Multisite autophosphorylation of p21-activated protein kinase gamma-PAK as a function of activation

p21-activated protein kinase (PAK) is a family of serine/threonine kinases whose activity is stimulated by binding to small G-proteins such as Cdc42 and subsequent autophosphorylation. Focusing on the ubiquitous gamma-isoform of PAK in this study, baculovirus-infected insect cells were used to obtain recombinant gamma-PAK, while native gamma-PAK was isolated from rabbit reticulocytes. Two-dimensional gel electrophoresis of gamma-PAK followed by immunoblot analysis revealed a similar profile for native and recombinant gamma-PAK, both consisting of multiple protein spots. Following Cdc42-stimulated autophosphorylation, the two-dimensional profiles of native and recombinant gamma-PAK were characterized by a similar acidic shift, suggesting a common response to Cdc42. To understand the effect of differential phosphorylation on its activation status, gamma-PAK autophosphorylation was conducted in the presence or absence of activators such as Cdc42 and histone II-AS, followed by tryptic digestion and comparative two-dimensional phosphopeptide mapping. The major phosphopeptides were subjected to a combination of manual and automated amino acid sequencing. Overall, eight autophosphorylation sites were identified in Cdc42-activated gamma-PAK, six of which are in common with those previously reported in alpha-PAK, while Ser-19 and Ser-165 appear to be uniquely phosphorylated in the gamma-form. Further, the phosphorylation of Ser-141, Ser-165, and Thr-402 was found to correlate with gamma-PAK activation.

Cdc42: An essential Rho-type GTPase controlling eukaryotic cell polarity

Cdc42p is an essential GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. These proteins act as molecular switches by responding to exogenous and/or endogenous signals and relaying those signals to activate downstream components of a biological pathway. The 11 current members of the Cdc42p family display between 75 and 100% amino acid identity and are functional as well as structural homologs. Cdc42p transduces signals to the actin cytoskeleton to initiate and maintain polarized gorwth and to mitogen-activated protein morphogenesis. In the budding yeast Saccharomyces cerevisiae, Cdc42p plays an important role in multiple actin-dependent morphogenetic events such as bud emergence, mating-projection formation, and pseudohyphal growth. In mammalian cells, Cdc42p regulates a variety of actin-dependent events and induces the JNK/SAPK protein kinase cascade, which leads to the activation of transcription factors within the nucleus. Cdc42p mediates these processes through interactions with a myriad of downstream effectors, whose number and regulation we are just starting to understand. In addition, Cdc42p has been implicated in a number of human diseases through interactions with its regulators and downstream effectors. While much is known about Cdc42p structure and functional interactions, little is known about the mechanism(s) by which it transduces signals within the cell. Future research should focus on this question as well as on the detailed analysis of the interactions of Cdc42p with its regulators and downstream effectors.

Inhibition of small G proteins by clostridium sordellii lethal toxin activates cdc2 and MAP kinase in Xenopus oocytes

The lethal toxin (LT) from Clostridium sordellii is a glucosyltransferase that modifies and inhibits small G proteins of the Ras family, Ras and Rap, as well as Rac proteins. LT induces cdc2 kinase activation and germinal vesicle breakdown (GVBD) when microinjected into full-grown Xenopus oocytes. Toxin B from Clostridium difficile, that glucosylates and inactivates Rac proteins, does not induce cdc2 activation, indicating that proteins of the Ras family, Ras and/or Rap, negatively regulate cdc2 kinase activation in Xenopus oocyte. In oocyte extracts, LT catalyzes the incorporation of [14C]glucose into a group of proteins of 23 kDa and into one protein of 27 kDa. The 23-kDa proteins are recognized by anti-Rap1 and anti-Rap2 antibodies, whereas the 27-kDa protein is recognized by several anti-Ras antibodies and probably corresponds to K-Ras. Microinjection of LT into oocytes together with UDP-[14C]glucose results in a glucosylation pattern similar to the in vitro glucosylation, indicating that the 23- and 27-kDa proteins are in vivo substrates of LT. In vivo time-course analysis reveals that the 27-kDa protein glucosylation is completed within 2 h, well before cdc2 kinase activation, whereas the 23-kDa proteins are partially glucosylated at GVBD. This observation suggests that the 27-kDa Ras protein could be the in vivo target of LT allowing cdc2 kinase activation. Interestingly, inactivation of Ras proteins does not prevent the phosphorylation of c-Raf1 and the activation of MAP kinase that occurs normally around GVBD.

The protein kinase Pak3 positively regulates Raf-1 activity through phosphorylation of serine 338

The pathway involving the signalling protein p21Ras propagates a range of extracellular signals from receptors on the cell membrane to the cytoplasm and nucleus. The Ras proteins regulate many effectors, including members of the Raf family of protein kinases. Ras-dependent activation of Raf-1 at the plasma membrane involves phosphorylation events, protein-protein interactions and structural changes. Phosphorylation of serine residues 338 or 339 in the catalytic domain of Raf-1 regulates its activation in response to Ras, Src and epidermal growth factor. Here we show that the p21-activated protein kinase Pak3 phosphorylates Raf-1 on serine 338 in vitro and in vivo. The p21-activated protein kinases are regulated by the Rho-family GTPases Rac and Cdc42. Our results indicate that signal transduction through Raf-1 depends on both Ras and the activation of the Pak pathway. As guanine-nucleotide-exchange activity on Rac can be stimulated by a Ras-dependent phosphatidylinositol-3-OH kinase, a mechanism could exist through which one Ras effector pathway can be influenced by another.

Cleavage and activation of p21-activated protein kinase gamma-PAK by CPP32 (caspase 3). Effects of autophosphorylation on activity

p21-activated protein kinase gamma-PAK (Pak2, PAK I) is cleaved by CPP32 (caspase 3) during apoptosis and plays a key role in regulation of cell death. In vitro, CPP32 cleaves recombinant gamma-PAK into two peptides; 1-212 contains the majority of the regulatory domain whereas 213-524 contains 34 amino acids of the regulatory domain plus the entire catalytic domain. Following cleavage, both peptides become autophosphorylated with [gamma-32P]ATP. Peptide 1-212 migrates at 27,000 daltons (p27) upon SDS-polyacrylamide gel electrophoresis and at 32,000 daltons following autophosphorylation on serine (p27P); the catalytic subunit migrates at 34,000 daltons (p34) before and after autophosphorylation on threonine. Following caspase cleavage, a significant lag (approximately 5 min) is observed before autophosphorylation and activity are detected. When gamma-PAK is autophosphorylated with ATP(Mg) alone and then cleaved, only p27 contains phosphate, and the enzyme is inactive with exogenous substrate. After autophosphorylation of gamma-PAK in the presence of Cdc42(GTPgammaS) or histone 4, both cleavage products contain phosphate and gamma-PAK is catalytically active. Mutation of the conserved Thr-402 to alanine greatly reduces autophosphorylation and protein kinase activity following cleavage. Thus activation of gamma-PAK via cleavage by CPP32 is a two-step mechanism wherein autophosphorylation of the regulatory domain is a priming step, and activation coincides with autophosphorylation of the catalytic domain.

A method for hormonal induction of sperm release in anurans (eight species) and in vitro fertilization in lepidobatrachus species

Injections of synthetic human gonadotropin releasing hormone (GnRH) into the dorsal pelvic area were used in an attempt to stimulate sperm release in isolated males of eight anuran species including Xenopus laevis, Rana pipiens and Lepidobatrachus laevis. Sperm were obtained within 1-5 h post injection either by mechanical stimulation or by cloacal lavage. Sperm suspensions varied from 8 microL to 7 mL and the cell densities ranged from 4 x 10(5) to 4 x 10(7) sperm/mL. The sperm obtained from seven species using GnRH-induced release were viable based on light microscopic observations of motility. In addition, sperm preparations fertilized eggs in vitro and produced normal tadpoles in the case of L. laevis and L. Ilanensis. This hormonal method of anuran sperm collection will provide a convenient non-injurious way to obtain anuran sperm for basic studies of reproduction and development.

Regulation of phosphorylation pathways by p21 GTPases. The p21 Ras-related Rho subfamily and its role in phosphorylation signalling pathways

The oncogenic Ras p21 GTPases regulate phosphorylation pathways that underlie a wealth of activities, including growth and differentiation, in organisms ranging from yeast to human. In metazoa, growth factors trigger conversion of Ras from an inactive GDP-bound form to an active GTP-bound form. This activation of Ras leads to activation of Raf. Raf is one of the initial kinases in the cytoplasmic mitogen-activated protein kinase (MAPK) cascade, involving extracellular-signal-regulated kinases (ERK), which culminates in nuclear transcription. The Ras-related subfamily of Rho p21s, including Rho, Rac and Cdc42 are similarly active in their GTP-bound forms. These p21s mediate growth-factor-induced morphological changes involving actin-based cellular structures. For example, in mammalian fibroblasts, Rho mediates the formation of cytoskeletal stress fibres induced by lysophosphatidic acid, while Rac mediates the formation of membrane ruffles induced by platelet-derived growth factor, and Cdc42 mediates the formation of peripheral filopodia by bradykinin. In some cases, factor-induced Rac activation results in Rho activation, and factor-induced Cdc42 activation leads to Rac activation, as determined by specific morphological changes. Although separate Cdc42/Rac and Rac/Rho hierarchies exist, these might not extend into a linear form (i.e. Cdc42-->Rac-->Rho) since Cdc42 and Rho activities may be competitive or even antagonistic. Thus Cdc42-mediated formation of filopodia is accompanied by loss of stress fibres (whose formation is mediated by Rho). Recently, mammalian kinases that bind to the GTP-bound forms of Rho p21s have been isolated. These kinases include the p21-activated serine/threonine kinase (PAK), which is stimulated by binding to Cdc42 and Rac, and the Rho-binding serine/threonine kinase (ROK), which is not as strongly stimulated by binding. These kinases act as effectors for their p21 partners since they can directly affect the reorganization of the relevant actin-containing structures. ROK promotes the formation of Rho-induced actin-containing stress fibres and focal-adhesion complexes, to which the ends of the stress fibres attach. PAK stimulates the disassembly of stress fibres, which has been shown to accompany formation of Cdc42-induced peripheral-actin-containing structures, including filopodia, which with Rac-induced membrane ruffles play a role in cell movement. PAK also fosters loss of focal-adhesion complexes. Thus, there is cooperation between different Rho p21s as well as antagonism, with their associated kinases having a role in the integration of the reorganization of the actin cytoskeleton. The similarity of PAK to the Saccharomyces cerevisiae kinase Ste20p, which initiates the yeast mating/pheromone MAPK cascade, led to experiments showing that Cdc42 regulates Ste20p in this MAPK pathway. This similarity has also led to the demonstration that mammalian Cdc42 and Rac can signal to the nucleus through MAPK pathways. However, c-Jun N-terminal kinase (JNK, stress-activated protein kinase) rather than ERK, is involved. PAK have been implicated in the JNK pathway, but their exact roles are uncertain. Thus members of the Rho subfamily, and kinases that bind to these p21s are intimately involved in immediate morphological processes as well as long-term transcriptional events.