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

Spleen Tyrosine Kinase phosphorylates VE-cadherin to cause endothelial barrier disruption in acute lung injury

Increased endothelial cell (EC) permeability is a cardinal feature of acute lung injury/acute respiratory distress syndrome (ALI/ARDS). Tyrosine phosphorylation of VE-cadherin is a key determinant of EC barrier disruption. However, the identity and role of tyrosine kinases in this context are incompletely understood. Here we report that Spleen Tyrosine Kinase (Syk) is a key mediator of EC barrier disruption and lung vascular leak in sepsis. Inhibition of Syk by pharmacological or genetic approaches, each reduced thrombin-induced EC permeability. Mechanistically, Syk associates with and phosphorylates VE-cadherin to cause EC permeability. To study the causal role of endothelial Syk in sepsis-induced ALI, we used a remarkably efficient and cost-effective approach based on gene transfer to generate EC-ablated Syk mice. These mice were protected against sepsis-induced loss of VE-cadherin and inflammatory lung injury. Notably, the administration of Syk inhibitor R788 (fostamatinib); currently in phase II clinical trial for the treatment of COVID-19, mitigated lung injury and mortality in mice with sepsis. These data identify Syk as a novel kinase for VE-cadherin and a druggable target against ALI in sepsis.

miR-128-3p inhibits high-glucose-induced peritoneal mesothelial cells fibrosis via PAK2/SyK/TGF-β1 axis

AIM

To elucidate the mechanism of miR-128-3p in peritoneal fibrosis (PF).

METHODS

Peritoneal mesothelial cells (PMCs) were dealt with high glucose (HG) for 3 days. The expressions of miR-128-3p, p21-activated kinase 2 (PAK2), spleen tyrosine kinase (SyK), and transforming growth factor-β1 (TGF-β1) were detected with quantitative real-time reverse transcription polymerase chain reaction. The levels of IL-1β, TNF-α, IL-6, and monocyte chemotactic protein-1 in supernatant were measured by ELISA. Proteins of TGF-β1, SyK, PAK2, α-SMA, collagen I, vimentin, ERK/AP-1, and IκBα/NF-κB pathway related proteins were measured by Western blot. The correlation between miR-128-3p and PAK2 was found by bioinformatics analysis and luciferase reporter gene analysis.

RESULTS

miR-128-3p was decreased while PAK2, SyK, and TGF-β1 were increased in HG-induced PMCs. Moreover, miR-128-3p inhibited HG-induced fibrosis and inflammation in PMCs by targeting PAK2. PAK2 activated SyK, which induced TGF-β1 expression through ERK/AP-1 and IκBα/NF-κB pathways to promote HG-induced fibrosis of PMCs.

CONCLUSION

miR-128-3p inhibited HG-induced PMCs fibrosis via PAK2/SyK/TGF-β1 axis.

Syk Inhibitors: New Computational Insights into Their Intraerythrocytic Action in Plasmodium falciparum Malaria

Resistance to antimalarial drugs has spread rapidly over the past few decades. The WHO recommends artemisinin-based combination therapies for the treatment of uncomplicated malaria, but unfortunately these approaches are losing their efficacy in large areas of Southeast Asia. In 2016, artemisinin resistance was confirmed in 5 countries of the Greater Mekong subregion. We focused our study on Syk inhibitors as antimalarial drugs. The Syk protein is present in human erythrocytes, and the membrane of protein band 3 is its major target following activation by oxidant stress. Tyr phosphorylation of band 3 occurs during P. falciparum growth, leading to the release of microparticles containing hemicromes and structural weakening of the host cell membrane, simplifying merozoite reinfection. Syk inhibitors block these events by interacting with the Syk protein’s catalytic site. We performed in vitro proteomics and in silico studies and compared the results. In vitro studies were based on treatment of the parasite’s cellular cultures with different concentrations of Syk inhibitors, while proteomics studies were focused on the Tyr phosphorylation of band 3 by Syk protein with the same concentrations of drugs. In silico studies were based on different molecular modeling approaches in order to analyze and optimize the ligand–protein interactions and obtain the highest efficacy in vitro. In the presence of Syk inhibitors, we observed a marked decrease of band 3 Tyr phosphorylation according to the increase of the drug’s concentration. Our studies could be useful for the structural optimization of these compounds and for the design of novel Syk inhibitors in the future.

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.

Identifying a kinase network regulating FGF14:Nav1.6 complex assembly using split-luciferase complementation

Kinases play fundamental roles in the brain. Through complex signaling pathways, kinases regulate the strength of protein:protein interactions (PPI) influencing cell cycle, signal transduction, and electrical activity of neurons. Changes induced by kinases on neuronal excitability, synaptic plasticity and brain connectivity are linked to complex brain disorders, but the molecular mechanisms underlying these cellular events remain for the most part elusive. To further our understanding of brain disease, new methods for rapidly surveying kinase pathways in the cellular context are needed. The bioluminescence-based luciferase complementation assay (LCA) is a powerful, versatile toolkit for the exploration of PPI. LCA relies on the complementation of two firefly luciferase protein fragments that are functionally reconstituted into the full luciferase enzyme by two interacting binding partners. Here, we applied LCA in live cells to assay 12 kinase pathways as regulators of the PPI complex formed by the voltage-gated sodium channel, Nav1.6, a transmembrane ion channel that elicits the action potential in neurons and mediates synaptic transmission, and its multivalent accessory protein, the fibroblast growth factor 14 (FGF14). Through extensive dose-dependent validations of structurally-diverse kinase inhibitors and hierarchical clustering, we identified the PI3K/Akt pathway, the cell-cycle regulator Wee1 kinase, and protein kinase C (PKC) as prospective regulatory nodes of neuronal excitability through modulation of the FGF14:Nav1.6 complex. Ingenuity Pathway Analysis shows convergence of these pathways on glycogen synthase kinase 3 (GSK3) and functional assays demonstrate that inhibition of GSK3 impairs excitability of hippocampal neurons. This combined approach provides a versatile toolkit for rapidly surveying PPI signaling, allowing the discovery of new modular pathways centered on GSK3 that might be the basis for functional alterations between the normal and diseased brain.

PAK signaling in cancer

Transformation of a normal cell to a cancer cell is caused by mutations in genes that regulate proliferation, apoptosis, and invasion. Small GTPases such as Ras, Rho, Rac and Cdc42 orchestrate many of the signals that are required for malignant transformation. The p21-activated kinases (PAKs) are effectors of Rac and Cdc42. PAKs are a family of serine/threonine protein kinases comprised of six isoforms (PAK1–6), and they play important roles in cytoskeletal dynamics, cell survival and proliferation. They act as key signal transducers in several cancer signaling pathways, including Ras, Raf, NFκB, Akt, Bad and p53. Although PAKs are not mutated in cancers, they are overexpressed, hyperactivated or amplified in several human tumors and their role in cell transformation make them attractive therapeutic targets. This review discusses the evidence that PAK is important for cell transformation and some key signaling pathways it regulates. This review primarily discusses Group I PAKs (PAK1, PAK2 and PAK3) as Group II PAKs (PAK4, PAK5 and PAK6) are discussed elsewhere in this issue (by Minden).

TAZ suppresses NFAT5 activity through tyrosine phosphorylation

Transcriptional coactivator with PDZ-binding motif (TAZ) physically interacts with a variety of transcription factors and modulates their activities involved in cell proliferation and mesenchymal stem cell differentiation. TAZ is highly expressed in the kidney, and a deficiency of this protein results in multiple renal cysts and urinary concentration defects; however, the molecular functions of TAZ in renal cells remain largely unknown. In this study, we examined the effects of osmotic stress on TAZ expression and activity in renal cells. We found that hyperosmotic stress selectively increased protein phosphorylation at tyrosine 316 of TAZ and that this was enhanced by c-Abl activation in response to hyperosmotic stress. Interestingly, phosphorylated TAZ physically interacted with nuclear factor of activated T cells 5 (NFAT5), a major osmoregulatory transcription factor, and subsequently suppressed DNA binding and transcriptional activity of NFAT5. Furthermore, TAZ deficiency elicited an increase in NFAT5 activity in vitro and in vivo, which then reverted to basal levels following restoration of wild-type TAZ but not mutant TAZ (Y316F). Collectively, the data suggest that TAZ modulates cellular responses to hyperosmotic stress through fine-tuning of NFAT5 activity via tyrosine phosphorylation.

Spleen tyrosine kinase mediates high glucose-induced transforming growth factor-β1 up-regulation in proximal tubular epithelial cells

The role of spleen tyrosine kinase (Syk) in high glucose-induced intracellular signal transduction has yet to be elucidated. We investigated whether Syk is implicated in high glucose-induced transforming growth factor-β1 (TGF-β1) up-regulation in cultured human proximal tubular epithelial cells (HK-2 cell). High glucose increased TGF-β1 gene expression through Syk, extracellular signal-regulated kinase (ERK), AP-1 and NF-κB. High glucose-induced AP-1 DNA binding activity was decreased by Syk inhibitors and U0126 (an ERK inhibitor). Syk inhibitors suppressed high glucose-induced ERK activation, whereas U0126 had no effect on Syk activation. High glucose-induced NF-κB DNA binding activity was also decreased by Syk inhibitors. High glucose increased nuclear translocation of p65 without serine phosphorylation of IκBα and without degradation of IκBα, but with an increase in tyrosine phosphorylation of IκBα that may account for the activation of NF-κB. Both Syk inhibitors and Syk-siRNA attenuated high glucose-induced IκBα tyrosine phosphorylation and p65 nuclear translocation. Depletion of p21-activated kinase 2 (Pak2) by transfection of Pak2-siRNA abolished high glucose-induced Syk activation. In summary, high glucose-induced TGF-β1 gene transcription occurred through Pak2, Syk and subsequent ERK/AP-1 and NF-κB pathways. This suggests that Syk might be implicated in the diabetic kidney disease.

Enhancement of B-cell receptor signaling by a point mutation of adaptor protein 3BP2 identified in human inherited disease cherubism

Tyrosine phosphorylation of adaptor protein c-Abl-Src homology 3 (SH3) domain-binding protein-2 (3BP2, also referred to SH3BP2) positively regulates the B-cell antigen receptor (BCR)-mediated signal transduction, leading to the activation of nuclear factor of activated T cells (NFAT). Here we showed the effect of the proline to arginine substitution of 3BP2 in which is the most common mutation in patients with cherubism (P418R) on B-cell receptor signaling. Comparing to the wild type, overexpression of the mutant form of 3BP2 (3BP2-P416R, corresponding to P418R in human protein) enhanced BCR-mediated activation of NFAT. 3BP2-P416R increased the signaling complex formation with Syk, phospholipase C-γ2 (PLC-γ2), and Vav1. In contrast, 3BP2-P416R could not change the association with the negative regulator 14-3-3. Loss of the association mutant that was incapable to associate with 14-3-3 could not mimic BCR-mediated NFAT activation in Syk-deficient cells. Moreover, BCR-mediated phosphorylation of extracellular signal regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) was not affected by P416R mutation. These results showed that P416R mutation of 3BP2 causes the gain of function in B cells by increasing the interaction with specific signaling molecules.

AMP-activated protein kinase induces actin cytoskeleton reorganization in epithelial cells

AMP-activated protein kinase (AMPK), a known regulator of cellular and systemic energy balance, is now recognized to control cell division, cell polarity and cell migration, all of which depend on the actin cytoskeleton. Here we report the effects of A769662, a pharmacological activator of AMPK, on cytoskeletal organization and signalling in epithelial Madin-Darby canine kidney (MDCK) cells. We show that AMPK activation induced shortening or radiation of stress fibers, uncoupling from paxillin and predominance of cortical F-actin. In parallel, Rho-kinase downstream targets, namely myosin regulatory light chain and cofilin, were phosphorylated. These effects resembled the morphological changes in MDCK cells exposed to hyperosmotic shock, which led to Ca(2+)-dependent AMPK activation via calmodulin-dependent protein kinase kinase-beta(CaMKKbeta), a known upstream kinase of AMPK. Indeed, hypertonicity-induced AMPK activation was markedly reduced by the STO-609 CaMKKbeta inhibitor, as was the increase in MLC and cofilin phosphorylation. We suggest that AMPK links osmotic stress to the reorganization of the actin cytoskeleton.

Tyrosine phosphorylation of 3BP2 regulates B cell receptor-mediated activation of NFAT

Adaptor protein c-Abl SH3 domain-binding protein-2 (3BP2, also referred to SH3BP2) regulates immune receptor-mediated signal transduction. In this report we focused on the molecular mechanism of 3BP2 function in B cell receptor (BCR) signaling. Engagement of BCR induces tyrosine phosphorylation of 3BP2. Genetic analysis demonstrated that Syk is critical for BCR-mediated tyrosine phosphorylation of 3BP2. Mutational analysis of 3BP2 revealed that both Tyr(183) and Src homology 2 (SH2) domain are necessary for 3BP2-mediated BCR-induced activation of nuclear factor of activated T cells (NFAT). Point mutation of Tyr(183) or Arg(486) in the SH2 domain of 3BP2 diminished BCR-mediated tyrosine phosphorylation of 3BP2. Endogenous 3BP2 forms a complex with tyrosine-phosphorylated cellular signaling molecules. Peptide binding experiments demonstrated that only phosphorylated Tyr(183) in 3BP2 could form a complex with the SH2 domain(s) of phospholipase Cgamma2 and Vav1 from B cell lysates. These interactions were represented by using bacterial glutathione S-transferase-phospholipase Cgamma2 or -Vav1 SH2 domain. Furthermore, pulldown and Far Western experiments showed that the 3BP2-SH2 domain directly binds to B cell linker protein (BLNK) after BCR stimulation. These results demonstrated that 3BP2 induces the protein complex with cellular signaling molecules through phosphorylation of Tyr(183) and SH2 domain leading to the activation of NFAT in B cells.

Pak protein kinases and their role in cancer

Some of the characteristics of cancer cells are high rates of cell proliferation, cell survival, and the ability to invade surrounding tissue. The cytoskeleton has an essential role in these processes. Dynamic changes in the cytoskeleton are necessary for cell motility and cancer cells are dependent on motility for invasion and metastasis. The signaling pathways behind the reshaping and migrating properties of the cytoskeleton in cancer cells involve a group of Ras-related small GTPases and their effectors, including the p21-activated kinases (Paks). Paks are a family of serine/threonine protein kinases comprised of six isoforms (Pak 1-6), all of which are direct targets of the small GTPases Rac and Cdc42. Besides their role in cytoskeletal dynamics, Paks have recently been shown to regulate various other cellular activities, including cell survival, mitosis, and transcription. Paks are overexpressed and/or hyperactivated in several human tumors and their role in cell transformation makes them attractive therapeutic targets. Pak-targeted therapeutics may efficiently inhibit certain types of tumors and efforts to identify selective Pak-inhibitors are underway.

Essential and unique roles of PIP5K-gamma and -alpha in Fcgamma receptor-mediated phagocytosis

The actin cytoskeleton is dynamically remodeled during Fcγ receptor (FcγR)-mediated phagocytosis in a phosphatidylinositol (4,5)-bisphosphate (PIP₂)-dependent manner. We investigated the role of type I phosphatidylinositol 4-phosphate 5-kinase (PIP5K) γ and α isoforms, which synthesize PIP₂, during phagocytosis. PIP5K-γ−/− bone marrow–derived macrophages (BMM) have a highly polymerized actin cytoskeleton and are defective in attachment to IgG-opsonized particles and FcγR clustering. Delivery of exogenous PIP₂ rescued these defects. PIP5K-γ knockout BMM also have more RhoA and less Rac1 activation, and pharmacological manipulations establish that they contribute to the abnormal phenotype. Likewise, depletion of PIP5K-γ by RNA interference inhibits particle attachment. In contrast, PIP5K-α knockout or silencing has no effect on attachment but inhibits ingestion by decreasing Wiskott-Aldrich syndrome protein activation, and hence actin polymerization, in the nascent phagocytic cup. In addition, PIP5K-γ but not PIP5K-α is transiently activated by spleen tyrosine kinase–mediated phosphorylation. We propose that PIP5K-γ acts upstream of Rac/Rho and that the differential regulation of PIP5K-γ and -α allows them to work in tandem to modulate the actin cytoskeleton during the attachment and ingestion phases of phagocytosis.

Analysis of conformational changes during activation of protein kinase Pak2 by amide hydrogen/deuterium exchange

During apoptotic stress, protein kinase Pak2 is cleaved by caspase 3 to form a heterotetramer that is constitutively activated following autophosphorylation. The active protein kinase migrates slightly slower than the inactive holoenzyme when analyzed by gel filtration, suggesting an expanded conformation. Activation of Pak2 comprises a series of structural changes resulting from caspase cleavage, ATP binding, and autophosphorylation of Pak2. Changes at each step were individually analyzed by amide hydrogen/deuterium exchange coupled with mass spectrometry and compared with inactive Pak2. The auto-inhibited form was shown to bind ATP in the active site, with minor changes in the glycine loop and the autoinhibitory domain (AID). Caspase cleavage produced significant changes in solvent accessibility in the AID and upper lobe of the catalytic domain. Cleavage of ATP-bound Pak2 relaxes the allosteric inhibition, as shown by increased solvent accessibility in the upper and lower lobes, including the G-helix, facilitating the autophosphorylation of two sites required for activation, Ser-141 in the regulatory domain and Thr-402 in the catalytic domain. Autophosphorylation increased the amide hydrogen/deuterium exchange solvent accessibility of the contact region between the AID and the G-helix, the E-F loop, and the N terminus. Thus, activation of Pak2 via caspase cleavage is associated with structural relaxation of Pak2 that allows for complete auto-phosphorylation, resulting in a more comprehensive solvent-exposed and conformationally dynamic enzyme.

PAK is regulated by PI3K, PIX, CDC42, and PP2Calpha and mediates focal adhesion turnover in the hyperosmotic stress-induced p38 pathway

Fractionation of brain extracts and functional biochemical assays identified PP2Calpha, a serine/threonine phosphatase, as the major biochemical activity inhibiting PAK1. PP2Calpha dephosphorylated PAK1 and p38, both of which were activated upon hyperosmotic shock with the same kinetics. In comparison to growth factors, hyperosmolality was a more potent activator of PAK1. Therefore we characterize the PAK signaling pathway in the hyperosmotic shock response. Endogenous PAKs were recruited to the p38 kinase complex in a phosphorylation-dependent manner. Overexpression of a PAK inhibitory peptide or dominant negative Cdc42 revealed that p38 activation was dependent on PAK and Cdc42 activities. PAK mutants deficient in binding to Cdc42 or PAK-interacting exchange factor were not activated. Using a panel of kinase inhibitors, we identified PI3K acting upstream of PAK, which correlated with PAK repression by pTEN overexpression. RNA interference knockdown of PAK expression reduced stress-induced p38 activation and conversely, PP2Calpha knockdown increased its activation. Hyperosmotic stress-induced PAK translocation away from focal adhesions to the perinuclear compartment and resulted in disassembly of focal adhesions, which are hallmarks of PAK activation. Inhibition of PAK by overexpression of PP2Calpha or the kinase inhibitory domain prevented sorbitol-induced focal adhesion dissolution. Inhibition of MAPK pathways showed that MEK-ERK signaling but not p38 is required for full PAK activation and focal adhesion turnover. We conclude that 1) PAK plays a required role in hyperosmotic signaling through the PI3K/pTEN/Cdc42/PP2Calpha/p38 pathway, and 2) PAK and PP2Calpha modulate the effects of this pathway on focal adhesion dynamics.

Hepatitis C virus NS5A protein interacts with and negatively regulates the non-receptor protein tyrosine kinase Syk

Hepatitis C virus (HCV) is the major causative agent of hepatocellular carcinoma. However, the precise mechanism underlying the carcinogenesis is yet to be elucidated. It has recently been reported that Syk, a non-receptor protein tyrosine kinase, functions as a potent tumour suppressor in human breast carcinoma. This study first examined the possible effect of HCV infection on expression of Syk in vivo. Immunohistochemical analysis revealed that endogenous Syk, which otherwise was expressed diffusely in the cytoplasm of normal hepatocytes, was localized near the cell membrane with a patchy pattern in HCV-infected hepatocytes. The possible interaction between HCV proteins and Syk in human hepatoma-derived Huh-7 cells was then examined. Immunoprecipitation analysis revealed that NS5A interacted strongly with Syk. Deletion-mutation analysis revealed that an N-terminal portion of NS5A (aa 1-175) was involved in the physical interaction with Syk. An in vitro kinase assay demonstrated that NS5A inhibited the enzymic activity of Syk and that, in addition to the N-terminal 175 residues, a central portion of NS5A (aa 237-302) was required for inhibition of Syk. Moreover, Syk-mediated phosphorylation of phospholipase C-gamma1 was downregulated by NS5A. An interaction of NS5A with Syk was also detected in Huh-7.5 cells harbouring an HCV RNA replicon or infected with HCV. In conclusion, these results demonstrated that NS5A interacts with Syk resulting in negative regulation of its kinase activity. The results indicate that NS5A may be involved in the carcinogenesis of hepatocytes through the suppression of Syk kinase activities.

Activation of Syk by protein kinase C-delta regulates thrombin-induced intercellular adhesion molecule-1 expression in endothelial cells via tyrosine phosphorylation of RelA/p65

Protein kinase C-delta (PKC-delta) plays a pivotal role in mediating thrombin-induced NF-kappaB activation and ICAM-1 expression in endothelial cells. However, the downstream mechanisms mediating its function are unclear. In this study, we show that PKC-delta-mediated activation of protein-tyrosine kinase Syk plays an important role in thrombin signaling of NF-kappaB activation and intercellular adhesion molecule-1 (ICAM-1) expression in endothelial cells. Stimulation of human vascular endothelial cells with thrombin resulted in a time-dependent phosphorylation of Syk on tyrosine 525 and 526, an indication of Syk activation. Inhibition of PKC-delta by pharmacological and genetic approaches prevented Syk activation by thrombin. These results place Syk downstream of PKC-delta in transmitting thrombin-activated signaling in endothelial cells. Consistent with this, thrombin-induced NF-kappaB activity and ICAM-1 expression were prevented by the expression of a kinase-defective mutant or RNA interference knockdown of Syk. Similarly, inhibiting Syk also impaired NF-kappaB activity and ICAM-1 expression induced by a constitutively active mutant of PKC-delta. Analysis of the NF-kappaB pathway showed that Syk contributes to thrombin-induced NF-kappaB activation by controlling its transactivation potential and that this response is associated with tyrosine phosphorylation of RelA/p65. Thus, these data unveil a novel pathway in which Syk signals downstream of PKC-delta to mediate thrombin induced ICAM-1 expression in endothelial cells by increasing transcriptional capacity of NF-kappaB via a mechanism that relies on tyrosine phosphorylation of RelA/p65.

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Mechanosensitive activation of K+ channel via phospholipase C-induced depletion of phosphatidylinositol 4,5-bisphosphate in B lymphocytes

In various types of cells mechanical stimulation of the plasma membrane activates phospholipase C (PLC). However, the regulation of ion channels via mechanosensitive degradation of phosphatidylinositol 4,5-bisphosphate (PIP(2)) is not known yet. The mouse B cells express large conductance background K(+) channels (LK(bg)) that are inhibited by PIP(2). In inside-out patch clamp studies, the application of MgATP (1 mm) also inhibited LK(bg) due to the generation of PIP(2) by phosphoinositide (PI)-kinases. In the presence of MgATP, membrane stretch induced by negative pipette pressure activated LK(bg), which was antagonized by PIP(2) (> 1 microm) or higher concentration of MgATP (5 mm). The inhibition by PIP(2) was partially reversible. However, the application of methyl-beta-cyclodextrin, a cholesterol scavenger disrupting lipid rafts, induced the full recovery of LK(bg) activity and facilitated the activation by stretch. In cell-attached patches, LK(bg) were activated by hypotonic swelling of B cells as well as by negative pressure. The mechano-activation of LK(bg) was blocked by U73122, a PLC inhibitor. Neither actin depolymerization nor the inhibition of lipid phosphatase blocked the mechanical effects. Direct stimulation of PLC by m-3M3FBS or by cross-linking IgM-type B cell receptors activated LK(bg). Western blot analysis and confocal microscopy showed that the hypotonic swelling of WEHI-231 induces tyrosine phosphorylation of PLCgamma2 and PIP(2) hydrolysis of plasma membrane. The time dependence of PIP(2) hydrolysis and LK(bg) activation were similar. The presence of LK(bg) and their stretch sensitivity were also proven in fresh isolated mice splenic B cells. From the above results, we propose a novel mechanism of stretch-dependent ion channel activation, namely, that the degradation of PIP(2) caused by stretch-activated PLC releases LK(bg) from the tonic inhibition by PIP(2).

The effect of estrogen on phosphorylation of prolactin in the mouse pituitary gland

Several studies have indicated that prolactin (PRL) assumes oligomeric, proteolytically cleaved, phosphorylated and glycosylated forms. Phosphorylated PRL (PPRL) is considered to be the most important posttranslationally modified form in the rat. In the present study, we examined whether or not PRL is present in the mouse pituitary gland in the phosphorylated form. Mouse pituitary PRL was digested with acid phosphatase, resolved by two-dimensional gel electrophoresis, stained with Coomassie brilliant blue, and then immunoblotted against the anti-PRL, anti-phosphoserine and anti-phosphothreonine antibodies. We also examined whether PRL is phosphorylated by protein kinases and semi-quantified the ratios of PPRL to PRL in the pituitary gland. The results indicated that three types of PRL are present in the pituitary glands of both male and female mice. One was non-phosphorylated (isoform 1), and the other two were immunoreactive to anti-phosphoserine (isoform 2) and/or anti-phosphothreonine (isoform 3) antibodies. The ratio between isoforms 2 and 1 of the 30-day-old female mice was higher than that of the 20-day-old female mice. However, the ratios among the three isoforms in the male pituitary glands did not differ with age. The ratio of PPRL to isoform 1 was obviously reduced after ovariectomy (OVX), and it recovered with estrogen replacement. These results suggest that estrogen influences PRL phosphorylation in female mice.

Ste20-type kinases: evolutionarily conserved regulators of ion transport and cell volume

Ste20 serine/threonine kinases regulate fundamental cellular processes including the cell cycle, apoptosis, and stress responses. Recent studies in Caenorhabditis elegans and mammals demonstrate that Ste20 kinases also function in cell volume sensing and Cl- transport regulation. Yeast Ste20 initiates a shrinkage activated MAPK cascade that regulates organic osmolyte accumulation. Ste20 kinases thus play evolutionarily conserved roles in cellular volume sensing and regulation. They may also function in systemic osmotic homeostasis and to link cell-cycle events with cell volume.

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.

Selective Inhibition of FcεRI-Mediated Mast Cell Activation by a Truncated Variant of Cbl-b Related to the Rat Model of Type 1 Diabetes Mellitus

Ubiquitin-protein ligase Cbl-b negatively regulates high affinity IgE receptor (FcepsilonRI)-mediated degranulation and cytokine gene transcription in mast cells. In this study, we have examined the role of a truncated variant of Cbl-b related to the rat model of type 1 diabetes mellitus using the mast cell signaling model. Overexpression of the truncated Cbl-b that lacks the C-terminal region did not suppress the activation of proximal and distal signaling molecules leading to degranulation. FcepsilonRI-mediated tyrosine phosphorylation of Syk, Gab2, and phospholipase C-gamma1, and activation of c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAP kinase), and inhibitor of nuclear factor kappaB kinase (IKK), and generation of Rac1 are unaffected in cells overexpressing the truncated Cbl-b in the lipid raft. On the other hand, FcepsilonRI-mediated transcriptional activation of nuclear factor of activated T cells (NFAT), and transcription of interleukin-3 (IL-3) and IL-4 mRNA are inhibited by overexpression of the truncated variant of Cbl-b. This suppression parallels the re-compartmentalization of specific effector molecules in the lipid raft. These structural and functional analyses reveal the mechanism underlying the selective inhibition of cellular signaling by the truncated variant of Cbl-b related to insulin-dependent diabetes mellitus.

MAP kinases and the adaptive response to hypertonicity: functional preservation from yeast to mammals

The adaptation to hypertonicity in mammalian cells is driven by multiple signaling pathways that include p38 kinase, Fyn, the catalytic subunit of PKA, ATM, and JNK2. In addition to the well-characterized tonicity enhancer (TonE)-TonE binding protein interaction, other transcription factors (and their respective cis elements) can potentially respond to hypertonicity. This review summarizes the current knowledge about the signaling pathways that regulate the adaptive response to osmotic stress and discusses new insights from yeast that could be relevant to the osmostress response in mammals.

Phosphorylation and Recruitment of Syk by Immunoreceptor Tyrosine-based Activation Motif-based Phosphorylation of Tamalin

Tamalin is a scaffold protein that forms a multiple protein assembly including metabotropic glutamate receptors (mGluRs) and several postsynaptic and protein-trafficking scaffold proteins in distinct mode of protein-protein association. In the present investigation, we report that tamalin possesses a typical immunoreceptor tyrosine-based activation motif (ITAM), which enables Syk kinase to be recruited and phosphorylated by the Src family kinases. Coimmunoprecipitation analysis of rat brain membrane fractions showed that tamalin is present in a multimolecular protein assembly comprising not only mGluR1 but also c-Src, Fyn, and a protein phosphatase, SHP-2. The protein association of both tamalin and c-Src, as determined by truncation analysis of mGluR1 in COS-7 cells, occurred at the carboxyl-terminal tail of mGluR1. Mutation analysis of tyrosine with phenylalanine in COS-7 cells revealed that paired tyrosines at the ITAM sequence of tamalin are phosphorylated preferentially by c-Src and Fyn, and this phosphorylation can recruit Syk kinase and enables it to be phosphorylated by the Src family kinases. The phosphorylated tyrosines at the ITAM sequence of tamalin were highly susceptible to dephosphorylation by protein-tyrosine phosphatases in COS-7 cells. Importantly, tamalin was endogenously phosphorylated and associated with Syk in retinoic acid-treated P19 embryonal carcinoma cells that undergo neuron-like differentiation. The present investigation demonstrates that tamalin is a novel signaling molecule that possesses a PDZ domain and a PDZ binding motif and mediates Syk signaling in an ITAM-based fashion.

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.