Activation of the small GTPase Rac1 by cGMP-dependent protein kinase

Cyclic-GMP-dependent protein kinase (PKG) is widely appreciated as having diverse roles in a variety of cell types. Many reports have indicated that PKG might regulate cell function by activating members of the mitogen-activated protein kinase (MAPK) family of signaling proteins. In this study, stimulation of HEK-293 cells with nitric oxide (NO) was found to induce a rapid accumulation of phosphorylated p38 MAPK. The involvement of PKG in this process was confirmed by cotransfection of a dominant negative PKG construct (G1alphaR-GFP), which was able to block cGMP-induced p38 MAPK activation. Transfection of cells to express dominant negative Rac1(T17N) was also able to dose-dependently block cGMP-stimulated activation of p38 MAPK, thus indicating the importance of this pathway downstream of PKG. GST-PDB affinity-precipitation experiments revealed that stimulation of HEK293 cells with either nitric oxide or 8-Br-cGMP resulted in a rapid and transient activation of Rac1 with similar kinetics to p38 MAPK phosphorylation. Moreover, using in vitro kinase assays it was found that cGMP also stimulated the activity of the Rac1 effector Pak1. The activation of both Rac1 and Pak1 by 8-Br-cGMP was completely abolished by transfection of the cells with G1alphaR-GFP. Expression of the Rac1(T17N) mutant inhibited PKG-dependent activation of PAK1 indicating that Rac1 functions upstream of PAK1 in this pathway. Immunofluorescence experiments demonstrated clear colocalization of PKG and Rac1 in membrane ruffles and dynamic membrane regions supporting a functional interaction. However, in vitro kinase assays demonstrated that Rac1 is not a substrate for PKG suggesting an indirect activation mechanism. Taken together these data demonstrate a novel PKG-dependent pathway by which the Rac1/Pak1 pathway is activated. Furthermore, we demonstrate that this pathway is central to the activation of p38 MAPK by PKG in these cells.     

Hydrogen peroxide signaling through tumor necrosis factor receptor 1 leads to selective activation of c-Jun N-terminal kinase

Binding of tumor necrosis factor-alpha (TNFalpha) to its receptor, TNF-R1, results in the activation of inhibitor of kappaB kinase (IKK) and c-Jun N-terminal kinase (JNK) pathways that are coordinately regulated and important in survival and death. We demonstrated previously that in response to hydrogen peroxide (H2O2), the ability of TNFalpha to activate IKK in mouse lung epithelial cells (C10) was inhibited and that H2O2 alone was sufficient to activate JNK and induce cell death. In the current study, we investigated the involvement of TNF-R1 in H2O2-induced JNK activation. In lung fibroblasts from TNF-R1-deficient mice the ability of H2O2 to activate JNK was inhibited compared with fibroblasts from control mice. Additionally, in C10 cells expressing a mutant form of TNF-R1, H2O2-induced JNK activation was also inhibited. Immunoprecipitation of TNF-R1 revealed that in response to H2O2, the adapter proteins, TRADD and TRAF2, and JNK were recruited to the receptor. However, expression of the adaptor protein RIP, which is essential for IKK activation by TNFalpha, was decreased in cells exposed to H2O2, and its chaperone Hsp90 was cleaved. Furthermore, data demonstrating that expression of TRAF2 was not affected by H2O2 and that overexpression of TRAF2 was sufficient to activate JNK provide an explanation for the inability of H2O2 to activate IKK and for the selective activation of JNK by H2O2. Our data demonstrate that oxidative stress interferes with IKK activation while promoting JNK signaling, creating a signaling imbalance that may favor apoptosis.     

Cell-type-specific activation of c-Jun N-terminal kinase by salicylates

Salicylates inhibit signaling by tumor necrosis factor (TNF), including TNF-induced activation of mitogen-activated protein kinases (MAPKs). On the other hand, we recently showed that in normal human diploid fibroblasts sodium salicylate (NaSal) elicits activation of p38 MAPK but not activation of c-Jun N-terminal kinase (JNK). Here we show that NaSal treatment of COS-1 or HT-29 cells produced a sustained c-Jun N-terminal kinase (JNK) activation. Activation of JNK or p38 MAPK by NaSal (or aspirin) was not due to a nonspecific hyperosmotic effect because much higher molar concentrations of sorbitol or NaCl were required to produce a similar activation. Three structurally unrelated nonsteroidal antiinflammatory drugs (ibuprofen, acetaminophen, and indomethacin) failed to induce significant activation of JNK or p38 MAPK, suggesting that cyclooxygenase inhibition is not the underlying mechanism whereby salicylates induce p38 MAPK and JNK activation. Activation of JNK and p38 MAPKs may be relevant for some antiinflammatory actions of salicylates.     

Abba promotes PDGF-mediated membrane ruffling through activation of the small GTPase Rac1

Abba is a member of the I-BAR-domain protein family that is characterized by a convex-shaped membrane-binding motif. Overexpression of GFP-tagged Abba in murine fibroblasts potentiated PDGF-mediated formation of membrane ruffles and lamellipodia. Immunofluorescent microscopy and pull-down analysis revealed that GFP-Abba colocalized with an active form of Rac1 in the membrane ruffles and enhanced the Rac GTPase activity in response to PDGF stimulation. Further immunoprecipitation assays demonstrated that GFP-Abba bound to both wild-type and constitutively active Rac1 and that the binding to either of the Rac1 forms was significantly enhanced upon PDGF stimulation. On the other hand, an Abba mutant deficient in Rac1 binding failed to promote membrane ruffling and Rac1 activation in response to PDGF. However, the cells overexpressing a truncated mutant carrying the I-BAR domain alone displayed numerous filopodia-like microspikes in a manner independent of growth factors. Also, the Rac-binding activity of the mutant was not affected by PDGF treatment. Our data indicates that the interaction between full-length Abba and Rac1 is implicated in membrane deformation and subjected to a growth factor-mediated regulation through the C-terminal sequence.     

c-Jun N-terminal protein kinase 1 (JNK1), but not JNK2, is essential for tumor necrosis factor alpha-induced c-Jun kinase activation and apoptosis

Two ubiquitously expressed isoforms of c-Jun N-terminal protein kinase (JNK), JNK1 and JNK2, have shared functions and different functions. However, the molecular mechanism is unknown. Here we report that JNK1, but not JNK2, is essential for tumor necrosis factor alpha (TNF-alpha)-induced c-Jun kinase activation, c-Jun expression, and apoptosis. Using mouse fibroblasts deficient in either Jnk1 or Jnk2, we found that JNK1 was activated by TNF-alpha, whereas JNK2 activation was negligible. In addition, JNK2 interfered with JNK1 activation via its "futile" phosphorylation by upstream kinases. Consequently, expression and activation of c-Jun, which depends on JNK activity, were impaired in Jnk1 null cells but enhanced in Jnk2 null cells. TNF-alpha-induced apoptosis was also suppressed in Jnk1 null fibroblasts but increased in Jnk2 null cells. Thus, our results provide a molecular mechanism underlying the different biological functions of JNK isoforms.     

Nck-interacting Ste20 kinase couples Eph receptors to c-Jun N-terminal kinase and integrin activation

The mammalian Ste20 kinase Nck-interacting kinase (NIK) specifically activates the c-Jun amino-terminal kinase (JNK) mitogen-activated protein kinase module. NIK also binds the SH3 domains of the SH2/SH3 adapter protein Nck. To determine whether Nck functions as an adapter to couple NIK to a receptor tyrosine kinase signaling pathway, we determined whether NIK is activated by Eph receptors (EphR). EphRs constitute the largest family of receptor tyrosine kinases (RTK), and members of this family play important roles in patterning of the nervous and vascular systems. In this report, we show that NIK kinase activity is specifically increased in cells stimulated by two EphRs, EphB1 and EphB2. EphB1 kinase activity and phosphorylation of a juxtamembrane tyrosine (Y594), conserved in all Eph receptors, are both critical for NIK activation by EphB1. Although pY594 in the EphB1R has previously been shown to bind the SH2 domain of Nck, we found that stimulation of EphB1 and EphB2 led predominantly to a complex between NIK/Nck, p62(dok), RasGAP, and an unidentified 145-kDa tyrosine-phosphorylated protein. Tyrosine-phosphorylated p62(dok) most probably binds directly to the SH2 domain of Nck and RasGAP and indirectly to NIK bound to the SH3 domain of Nck. We found that NIK activation is also critical for coupling EphB1R to biological responses that include the activation of integrins and JNK by EphB1. Taken together, these findings support a model in which the recruitment of the Ste20 kinase NIK to phosphotyrosine-containing proteins by Nck is an important proximal step in the signaling cascade downstream of EphRs.     

Implication of the small GTPase Rac1 in the apoptosis induced by UV in Rat-2 fibroblasts.

Exposure of mammalian cells to ultraviolet (UV) light elicits a cellular response and also lead to apoptotic cell death. However, the role of Rac, a member of Rho family GTPases, in the UV-induced apoptosis has never been examined. In UV-irradiated Rat-2 fibroblasts, nuclear fragmentation began to be observed within 2 h and the total viability of Rat-2 cells were only about 15% at 6 h following by UV irradiation, whereas the total viability in Rat2-Rac(N17) cells stably expressing RacN17, a dominant negative Rac1 mutant, was almost close to 67%. Pretreatment with SB203580, a specific inhibitor of p38 kinase, likewise attenuated UV-induced cell death, but PD98059, a MEK inhibitor, did not. Thus, Rac1 and p38 kinase appear to be components in the apoptotic signaling pathway induced by UV irradiation in Rat-2 fibroblasts. In addition, our results show that p38 kinase stimulation by UV is dramatically inhibited by RacN17, suggesting that p38 kinase is situated downstream of Rac1 in the UV signaling to apoptosis.     

Immunofluorescent detection of the activation of the small GTPase Rac1 in mouse skeletal muscle fibers

The small GTPase Rac1 acts as a molecular switch of intracellular signaling in mammals. For understanding the regulatory mechanism, it is important to identify subcellular locations in which Rac1 is activated following multiple extracellular stimuli. However, it is difficult to detect Rac1 activation in situ in animal tissues, and thus a novel method is highly desirable. Here, we report a simple method to visualize the activation of endogenous Rac1 in mouse skeletal muscle fibers. In this assay, specific interaction between activated Rac1 and a binding polypeptide is detected by immunofluorescent microscopy. This approach is readily applicable to other small GTPases.     

Direct involvement of the small GTPase Rac in activation of the superoxide-producing NADPH oxidase Nox1

Activation of the non-phagocytic superoxide-producing NADPH oxidase Nox1, complexed with p22(phox) at the membrane, requires its regulatory soluble proteins Noxo1 and Noxa1. However, the role of the small GTPase Rac remained to be clarified. Here we show that Rac directly participates in Nox1 activation via interacting with Noxa1. Electropermeabilized HeLa cells, ectopically expressing Nox1, Noxo1, and Noxa1, produce superoxide in a GTP-dependent manner, which is abrogated by expression of a mutant Noxa1(R103E), defective in Rac binding. Superoxide production in Nox1-expressing HeLa and Caco-2 cells is decreased by depletion or sequestration of Rac; on the other hand, it is enhanced by expression of the constitutively active Rac1(Q61L), but not by that of a mutant Rac1 with the A27K substitution, deficient in binding to Noxa1. We also demonstrate that Nox1 activation requires membrane recruitment of Noxa1, which is normally mediated via Noxa1 binding to Noxo1, a protein tethered to the Nox1 partner p22(phox): the Noxa1-Noxo1 and Noxo1-p22(phox) interactions are both essential for Nox1 activity. Rac likely facilitates the membrane localization of Noxa1: although Noxa1(W436R), defective in Noxo1 binding, neither associates with the membrane nor activates Nox1, the effects of the W436R substitution are restored by expression of Rac1(Q61L). The Rac-Noxa1 interaction also serves at a step different from the Noxa1 localization, because the binding-defective Noxa1(R103E), albeit targeted to the membrane, does not support superoxide production by Nox1. Furthermore, a mutant Noxa1 carrying the substitution of Ala for Val-205 in the activation domain, which is expected to undergo a conformational change upon Rac binding, fully localizes to the membrane but fails to activate Nox1.     

Sustained activation of AMP-activated protein kinase induces c-Jun N-terminal kinase activation and apoptosis in liver cells

We report the first Fourier transform infrared analysis of prion protein (PrP) repeats and the first study of PrP repeats of marsupial origin. Large changes in the secondary structure and an increase in hydrogen bonding within the peptide groups were evident from a red shift of the amide I band by >7 cm(-1) and an approximately five-fold reduction in amide hydrogen-deuterium exchange for peptide interacting with Cu(2+) ions. Changes in the tertiary structure upon copper binding were also evident from the appearance of a new band at 1564 cm(-1), which arises from the ring vibration of histidine. The copper-induced conformational change is pH dependent, and occurs at pH >7.     

Increased c-Jun N-terminal kinase activation in human endometriotic endothelial cells

Endometriosis is a common inflammatory gynecological disease characterized by the presence of endometrial tissue outside of the uterine cavity. The c-Jun N-terminal kinase (JNK) is a subfamily of the mitogen-activated protein kinases (MAPKs) involved in cellular processes ranging from cytokine expression to apoptosis, and is activated in response to inflammation and cellular stress. We hypothesized that inflammatory cytokines in the peritoneal microenvironment increase JNK MAPK activity in endometriotic endothelial cells, and that human endometrial endothelial cells (HEECs) may be involved in inflammatory pathogenesis of endometriosis. Thus, we evaluated the expression of the total- and phosphorylated-(phospho)-JNK in endometrial and endometriotic endothelial cells in vivo, and in HEECs treated with normal peritoneal fluid (NPF), endometriotic peritoneal fluid (EPF), and the inflammatory cytokines interleukin-1beta (IL-1β) and tumor necrosis factor-alpha (TNF-α) in vitro. Phospho-JNK immunoreactivity in HEECs in normal endometrium was significantly higher in the early proliferative and late secretory phases compared to other phases. Both eutopic and ectopic HEECs from the early secretory phase also revealed higher phospho-JNK immunoreactivity, compared to their respective cycle-matched normal HEECs. Moreover, HEECs treated with EPF showed significantly higher phospho-JNK levels compared to that in HEECs treated with NPF. In conclusion, our in vivo and in vitro findings suggest that increased phosphorylation of JNK in HEECs from women with endometriosis is likely due to high level of IL-1β and TNF-α in peritoneal fluid; this in turn may up-regulate inflammatory cytokine expression and thus play a role in the pathogenesis of endometriosis.     

Hematopoietic progenitor kinase 1 is a component of transforming growth factor beta-induced c-Jun N-terminal kinase signaling cascade

The c-Jun N-terminal kinase (JNK) signaling pathway is involved in transforming growth factor beta (TGF-beta) signaling in a variety of cell systems. We report here that hematopoietic progenitor kinase 1 (HPK1), a novel Ste20-like protein serine/threonine kinase, serves as an upstream mediator for the TGF-beta-activated JNK1 cascade in 293T cells. TGF-beta treatment resulted in a time-dependent activation of HPK1, which was accompanied by similar kinetics of JNK1 activation. The activation of JNK1 by TGF-beta was abrogated by a kinase-defective HPK1 mutant but not by a kinase-defective mutant of kinase homologous to Ste20/Sps1. This result indicates that HPK1 is specifically required for TGF-beta-induced activation of JNK1. We also found that TGF-beta-induced JNK1 activation was blocked by a kinase-defective mutant of TGF-beta-activated kinase 1 (TAK1). In addition, interaction between HPK1 and TAK1 was observed in transient transfection assays, and this interaction was enhanced by TGF-beta treatment. Both stress-activated protein kinase/extracellular signal-regulated kinase kinase (SEK) and mitogen-activated protein kinase kinase 7 (MKK7) are immediate upstream activators of JNK1. Although SEK and MKK7 acted downstream of TAK1, only a kinase-defective SEK mutant blocked TGF-beta-induced activation of JNK1, indicating that the TGF-beta signal is relayed solely through SEK, but not MKK7, in vivo. Furthermore, TGF-beta-induced activating protein 1 activation was blocked by a HPK1 mutant, as well as by TAK1 and SEK mutants. Taken together, these studies establish a potential cascade of TGF-beta-activated interacting kinases beginning with HPK1, a Ste20 homolog, and ending in JNK1 activation: HPK1 --> TAK1 --> SEK --> JNK1.     

Involvement of TRAF4 in oxidative activation of c-Jun N-terminal kinase

We previously found that the angiogenic factors TNFalpha and HIV-1 Tat activate an NAD(P)H oxidase in endothelial cells, which operates upstream of c-Jun N-terminal kinase (JNK), a MAPK involved in the determination of cell fate. To further understand oxidant-related signaling pathways, we screened lung and endothelial cell libraries for interaction partners of p47(phox) and recovered the orphan adapter TNF receptor-associated factor 4 (TRAF4). Domain analysis suggested a tail-to-tail interaction between the C terminus of p47(phox) and the conserved TRAF domain of TRAF4. In addition, TRAF4, like p47(phox), was recovered largely in the cytoskeleton/membrane fraction. Coexpression of p47(phox) and TRAF4 increased oxidant production and JNK activation, whereas each alone had minimal effect. In addition, a fusion between p47(phox) and the TRAF4 C terminus constitutively activated JNK, and this activation was decreased by the antioxidant N-acetyl cysteine. In contrast, overexpression of the p47(phox) binding domain of TRAF4 blocked endothelial cell JNK activation by TNFalpha and HIV-1 Tat, suggesting an uncoupling of p47(phox) from upstream signaling events. A secondary screen of endothelial cell proteins for TRAF4-interacting partners yielded a number of proteins known to control cell fate. We conclude that endothelial cell agonists such as TNFalpha and HIV-1 Tat initiate signals that enter basic signaling cassettes at the level of TRAF4 and an NAD(P)H oxidase. We speculate that endothelial cells may target endogenous oxidant production to specific sites critical to cytokine signaling as a mechanism for increasing signal specificity and decreasing toxicity of these reactive species.