The use of okadaic acid to elucidate the intracellular role(s) of protein phosphatase 2A: Lessons from the mast cell model system

Interaction of the Hippo Pathway and Phosphatases in Tumorigenesis

The Hippo pathway is an emerging tumor suppressor signaling pathway involved in a wide range of cellular processes. Dysregulation of different components of the Hippo signaling pathway is associated with a number of diseases including cancer. Therefore, identification of the Hippo pathway regulators and the underlying mechanism of its regulation may be useful to uncover new therapeutics for cancer therapy. The Hippo signaling pathway includes a set of kinases that phosphorylate different proteins in order to phosphorylate and inactivate its main downstream effectors, YAP and TAZ. Thus, modulating phosphorylation and dephosphorylation of the Hippo components by kinases and phosphatases play critical roles in the regulation of the signaling pathway. While information regarding kinase regulation of the Hippo pathway is abundant, the role of phosphatases in regulating this pathway is just beginning to be understood. In this review, we summarize the most recent reports on the interaction of phosphatases and the Hippo pathway in tumorigenesis. We have also introduced challenges in clarifying the role of phosphatases in the Hippo pathway and future direction of crosstalk between phosphatases and the Hippo pathway.

Mitochondrial and calcium perturbations in rat CNS neurons induce calpain-cleavage of Parkin: Phosphatase inhibition stabilizes pSer65Parkin reducing its calpain-cleavage

Mitochondrial impairment and calcium (Ca⁺⁺) dyshomeostasis are associated with Parkinson's disease (PD). When intracellular ATP levels are lowered, Ca⁺⁺-ATPase pumps are impaired causing cytoplasmic Ca⁺⁺ to be elevated and calpain activation. Little is known about the effect of calpain activation on Parkin integrity. To address this gap, we examined the effects of mitochondrial inhibitors [oligomycin (Oligo), antimycin and rotenone] on endogenous Parkin integrity in rat midbrain and cerebral cortical cultures. All drugs induced calpain-cleavage of Parkin to ~36.9/43.6 kDa fragments. In contrast, treatment with the proinflammatory prostaglandin J2 (PGJ2) and the proteasome inhibitor epoxomicin induced caspase-cleavage of Parkin to fragments of a different size, previously shown by others to be triggered by apoptosis. Calpain-cleaved Parkin was enriched in neuronal mitochondrial fractions. Pre-treatment with the phosphatase inhibitor okadaic acid prior to Oligo-treatment, stabilized full-length Parkin phosphorylated at Ser⁶⁵, and reduced calpain-cleavage of Parkin. Treatment with the Ca⁺⁺ ionophore A23187, which facilitates Ca⁺⁺ transport across the plasma membrane, mimicked the effect of Oligo by inducing calpain-cleavage of Parkin. Removing extracellular Ca⁺⁺ from the media prevented oligomycin- and ionophore-induced calpain-cleavage of Parkin. Computational analysis predicted that calpain-cleavage of Parkin liberates its UbL domain. The phosphagen cyclocreatine moderately mitigated Parkin cleavage by calpain. Moreover, the pituitary adenylate cyclase activating peptide (PACAP27), which stimulates cAMP production, prevented caspase but not calpain-cleavage of Parkin. Overall, our data support a link between Parkin phosphorylation and its cleavage by calpain. This mechanism reflects the impact of mitochondrial impairment and Ca⁺⁺-dyshomeostasis on Parkin integrity and could influence PD pathogenesis.

Hyperglycemia triggers HIPK2 protein degradation

Homeodomain interacting protein kinase-2 (HIPK2) is an evolutionary conserved kinase that modulates several key molecular pathways to restrain tumor growth and induce p53-depending apoptotic cell-death in response to anticancer therapies. HIPK2 silencing in cancer cells leads to chemoresistance and cancer progression, in part due to p53 inhibition. Recently, hyperglycemia has been shown to reduce p53 phosphorylation at serine 46 (Ser46), the target residue of HIPK2, thus impairing p53 apoptotic function. Here we asked whether hyperglycemia could, upstream of p53, target HIPK2. We focused on the effect of high glucose (HG) on HIPK2 protein stability and the underlying mechanisms. We found that HG reduced HIPK2 protein levels, therefore impairing HIPK2-induced p53 apoptotic activity. HG-triggered HIPK2 protein downregulation was rescued by both proteasome inhibitor MG132 and by protein phosphatase inhibitors Calyculin A (CL-A) and Okadaic Acid (OA). Looking for the phosphatase involved, we found that protein phosphatase 2A (PP2A) induced HIPK2 degradation, as evidenced by directly activating PP2A with FTY720 or by silencing PP2A with siRNA in HG condition. The effect of PP2A on HIPK2 protein degradation could be in part due to hypoxia-inducible factor-1 (HIF-1) activity which has been previously shown to induce HIPK2 proteasomal degradation through several ubiquitin ligases. Validation analysed performed with HIF-1α dominant negative or with silencing of Siah2 ubiquitin ligase clearly showed rescue of HG-induced HIPK2 degradation. These findings demonstrate how hyperglycemia, through a complex protein cascade, induced HIPK2 downregulation and consequently impaired p53 apoptotic activity, revealing a novel link between diabetes/obesity and tumor resistance to therapies.

Role of Protein Phosphatase 2A in Osteoblast Differentiation and Function

The reversible phosphorylation of proteins plays hugely important roles in a variety of cellular processes, such as differentiation, proliferation, and apoptosis. These processes are strictly controlled by protein kinases (phosphorylation) and phosphatases (de-phosphorylation). Here we provide a brief history of the study of protein phosphorylation, including a summary of different types of protein kinases and phosphatases. One of the most physiologically important serine/threonine phosphatases is PP2A. This review provides a description of the phenotypes of various PP2A transgenic mice and further focuses on the known functions of PP2A in bone formation, including its role in osteoblast differentiation and function. A reduction in PP2A promotes bone formation and osteoblast differentiation through the regulation of bone-related transcription factors such as Osterix. Interestingly, downregulation of PP2A also stimulates adipocyte differentiation from undifferentiated mesenchymal cells under the appropriate adipogenic differentiation conditions. In osteoblasts, PP2A is also involved in the ability to control osteoclastogenesis as well as in the proliferation and metastasis of osteosarcoma cells. Thus, PP2A is considered to be a comprehensive factor in controlling the differentiation and function of cells derived from mesenchymal cells such as osteoblasts and adipocytes.

Inhibition of PP2A activity confers a TRAIL-sensitive phenotype during malignant transformation

TRAIL is a promising anticancer agent because it induces apoptosis in the majority of human cancer cells but spares the normal cells. To determine the mechanistic nature of how normal cells acquire a TRAIL-sensitive phenotype during the process of malignant transformation, an experimental cell system was developed by sequential introduction of human telomerase reverse transcriptase and SV40 T antigens (large and small) into normal human prostatic epithelial cells (PrEC). This model system demonstrated that inhibition of protein phosphatase 2A (PP2A), either by SV40 small T antigen, okadaic acid, Calyculin A, or PP2A catalytic subunit siRNA, sensitized normal human PrEC and immortalized cells to TRAIL-induced apoptosis. Moreover, sensitization occurred during the premalignant period of tumorigenesis and PP2A exerted its antiapoptotic activity by negatively regulating c-Fos/AP-1. In addition, low-dose okadaic acid treatment sensitized TRAIL-resistant cancer cells to TRAIL, suggesting that PP2A inhibitors could be used as an enhancer of apoptosis induced by TRAIL or TRAIL-like agents. These data indicate that downregulation of PP2A activity is a critical step for normal cells to acquire a TRAIL-sensitive phenotype during tumorigenesis and that the level of PP2A activity may foretell cellular sensitivity to TRAIL-induced apoptosis.

IMPLICATIONS

Inhibition of PP2A is a key determinant in acquiring TRAIL sensitivity during tumorigenesis, with c-Fos/AP-1 as an essential mediator.

Cytoskeletal architecture regulates cyclooxygenase-2 in human endothelial cells: autocrine modulation by prostacyclin

Endothelium is a highly dynamic tissue that controls vascular homeostasis. This requires constant rearrangements of the shape or function of endothelial cells that cannot set aside the role of the cytoskeleton. The aim of this study was to determine the mechanisms by means of which cytoskeletal alterations induce cyclooxygenase-2 (Cox-2) expression in human endothelial cells using compounds that interfere with microtubule or actin architecture. Microtubule disruption by nocodazole markedly increased Cox-2 expression and activity, and provoked paracellular gap formation, a cardinal feature of endothelial barrier dysfunction. The Cox-2 metabolite prostacyclin down-regulated Cox-2 through an autocrine receptor-mediated mechanism, and partially prevented the disassembly of endothelial monolayers. There was also an interaction between microtubules and actin filaments in nocodazole-induced Cox-2 expression. Nocodazole provoked the dissolution of the F-actin cortical ring and stress fiber formation, increased actin glutathionylation, and concomitantly lowered intracellular levels of reduced glutathione. The restoration of glutathione levels by N-acetylcysteine opposed Cox-2 expression and preserved the integrity of endothelial monolayers. Among the signaling pathways connecting microtubule disruption with Cox-2 up-regulation, crucial roles are played by Src family kinase activation, serine/threonine phosphatase 2A inhibition, and the phosphorylation of mitogen activated protein kinase p38. Our findings provide a mechanistic insight into the observation that Cox-2 is induced in endothelial cells under cytoskeleton-perturbing conditions such as those occurring in the presence of atherogenic/inflammatory stimuli and oxidative stress. In this scenario, Cox-2 up-regulation by endothelia exposed to noxious conditions can be considered protective of the vasodilatory and anti-thrombotic properties of the vessel wall.

Dietary L-arginine supplementation alleviates liver injury caused by Escherichia coli LPS in weaned pigs

This study was conducted to evaluate whether dietary supplementation with L-arginine (Arg) could attenuate Escherichia coli LPS-induced liver injury through the TLR4 signaling pathway in weaned pigs. Eighteen weaned pigs were allotted to three treatments: non-challenged control, LPS challenged control and LPS + 0.5% Arg. On d 18, pigs were injected with LPS at 100 µg/kg of body weight (BW) or sterile saline. Blood samples were obtained at 4 h post-injection. Pigs were then sacrificed for the collection of liver samples. Arg supplementation (0.5%) alleviated liver morphological impairment, including hepatocyte caryolysis, karyopycnosis and fibroblast proliferation induced by LPS challenge; it mitigated the increase of serum aspartate aminotransferase and alkaline phosphatase activities induced by LPS ( P < 0.05); it prevented the increase of hepatic TNF-α, malondialdehyde contents and mast cell number induced by LPS administration ( P < 0.05); and it attenuated the elevation of hepatic NF- κB and TLR4-positive cell percentages ( P < 0.05). These results indicate that Arg supplementation has beneficial effects in attenuating hepatic morphological and functional injury induced by LPS challenge in piglets. Additionally, it is possible that the protective effects of Arg on the liver are associated with a decreased release of liver pro-inflammatory cytokines and free radicals through inhibiting TLR4 signaling.

Morphological and biochemical changes associated with apoptosis induced by okadaic acid in human amniotic FL cells

The marine toxin okadaic acid (OA) is an apoptosis inducer and a tumor promoter. During recent years, extensive studies have demonstrated that OA can induce apoptosis in a wide variety of cell types. In contrast to the relatively longer incubation time or higher treatment concentrations of OA in apoptosis shown previously, relatively lower concentrations (<or=100 nM) and shorter time (4 h) were designed in the current study to observe the toxic effects of OA in human amniotic cells (FL cells). The present study was undertaken to determine the morphological and biochemical changes of FL cells induced by OA. Results indicated that externalization of phosphatidylserine, cytoskeletal disruption, DNA strand breaks and decrease of Bcl-2 protein expression levels as well as increase of PP2A-A subunit protein were all involved in the apoptosis of FL cells induced by OA. This work not only provided further evidence of apoptosis induced by OA but also suggested that PP2A might play a pivotal role in apoptosis induced by protein phosphatases inhibitors.

Cyclooxygenase-2 mediates hydrogen peroxide-induced wound repair in human endothelial cells

Cyclooxygenase-2 (Cox-2) metabolites produced by endothelial cells, particularly prostacyclin and prostaglandin E(2), profoundly affect vascular tone, regional blood flow, and angiogenesis. We have previously shown that reactive oxygen species induce Cox-2 expression in human endothelial cells (HUVEC), either on their own or as components of the signaling pathway triggered by TNFalpha, the prototypical inflammatory cytokine. Here we investigated the role of Cox-2 induced by hydrogen peroxide (H(2)O(2)), either exogenous or endogenously generated by TNFalpha, in the repair of a mechanically wounded HUVEC monolayer and probed the sources of H(2)O(2) that are involved in TNFalpha signaling and the pathways through which H(2)O(2) modulates Cox-2 expression. Results indicate that H(2)O(2)-induced Cox-2 activity participates in the repair of wounded monolayers. Both NADPH oxidase and the mitochondrial electron transport chain are involved in H(2)O(2) generation. Signaling triggered by H(2)O(2) for Cox-2 induction acts by increasing the protein tyrosine kinase phosphorylation that follows inhibition of protein phosphatase activity. The activation of p38 MAPK and its interaction in the inhibition of serine/threonine phosphatase activity are both critical steps in this event. We conclude that Cox-2 induced by H(2)O(2) plays an important role in promoting endothelial wound repair after injury, so that the cardioprotective effect of Cox-2 is due at least in part to its power of healing damaged endothelium.

Vasoinhibins prevent retinal vasopermeability associated with diabetic retinopathy in rats via protein phosphatase 2A-dependent eNOS inactivation

Increased retinal vasopermeability contributes to diabetic retinopathy, the leading cause of blindness in working-age adults. Despite clinical progress, effective therapy remains a major need. Vasoinhibins, a family of peptides derived from the protein hormone prolactin (and inclusive of the 16-kDa fragment of prolactin), antagonize the proangiogenic effects of VEGF, a primary mediator of retinal vasopermeability. Here, we demonstrate what we believe to be a novel function of vasoinhibins as inhibitors of the increased retinal vasopermeability associated with diabetic retinopathy. Vasoinhibins inhibited VEGF-induced vasopermeability in bovine aortic and rat retinal capillary endothelial cells in vitro. In vivo, vasoinhibins blocked retinal vasopermeability in diabetic rats and in response to intravitreous injection of VEGF or of vitreous from patients with diabetic retinopathy. Inhibition by vasoinhibins was similar to that achieved following immunodepletion of VEGF from human diabetic retinopathy vitreous or blockage of NO synthesis, suggesting that vasoinhibins inhibit VEGF-induced NOS activation. We further showed that vasoinhibins activate protein phosphatase 2A (PP2A), leading to eNOS dephosphorylation at Ser1179 and, thereby, eNOS inactivation. Moreover, intravitreous injection of okadaic acid, a PP2A inhibitor, blocked the vasoinhibin effect on endothelial cell permeability and retinal vasopermeability. These results suggest that vasoinhibins have the potential to be developed as new therapeutic agents to control the excessive retinal vasopermeability observed in diabetic retinopathy and other vasoproliferative retinopathies.

Phosphatases regulate histamine synthesis in rat brain

Cyclic AMP-dependent protein kinase (PKA) and Ca(2+)-calmodulin dependent protein kinase II (CaMKII)-mediated phosphorylation activate histamine synthesis in nerve endings, but the phosphatases deactivating it had not been studied. In this work we show that the protein phosphatase 2A (PP2A)/protein phosphatase 1 (PP1) inhibitor okadaic acid increases histamine synthesis up to twofold in rat cortical miniprisms containing histaminergic nerve endings. This effect was mimicked by the PP2A/PP1 inhibitor calyculin, but not by the inactive analog 1-norokadaone. Other phosphatase inhibitors like endothall (PP2A), cypermethrin and cyclosporin A (protein phosphatase 2B, PP2B) had much lower effects. The effects of okadaic acid appeared to be mediated by an activation of the histamine synthesizing enzyme, histidine decarboxylase. PKA-mediated activation of histamine synthesis decreased the EC(50) and maximal effects of okadaic acid. On the other hand, CaMKII-mediated activation of histamine synthesis decreased okadaic acid maximal effects, but it increased its EC(50). In conclusion, our results indicate that brain histamine synthesis is subjected to regulation by phosphatases PP2A and PP1, and perhaps also PP2B as well as by protein kinases.

Differential expression of genes associated with cell proliferation and apoptosis induced by okadaic acid during the transformation process of BALB/c 3T3 cells

Okadaic acid (OA) is a tumor promoter in two-stage carcinogenesis experiments. Nevertheless, the effects of OA on cell transformation, cell proliferation and apoptosis vary widely, and the molecular events underlying these effects of OA are not well understood. In the present study, we examined the promoting activity and the associated effects on cell growth and apoptosis mediated by OA in BALB/c 3T3 cells, and evaluated alterations of gene transcriptional expression by microarray analysis. The promoting activity of OA was estimated by a two-stage transformation assay, in which cells were treated first with a low dose of the initiator N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and then with OA for 14 days. It showed that OA, at concentrations of 7.8-31.3 ng/ml, enhanced the transformation of MNNG-treated cells. In the promotion phase, cells exposed to OA (7.8 ng/ml) grew slowly for the first 2 days and subsequently died. As determined by Hoechst 33342 fluorescent dye and Annexin-V/PI dual-colored flow cytometry, OA induced morphologically apoptotic cells and increased the percentage of early apoptotic cells. The gene expression profile induced by OA at five time points in the promotion phase was determined by use of a specific mouse toxicological microarray containing 1796 clones, and a total of 177 differentially expressed genes were identified. By gene ontology analysis, 31 of these were determined to be functionally involved with cell growth and/or maintenance. In this group, numerous genes associated with the cell proliferation and cell cycle progression were down-regulated at early and/or middle time points. Among these was a subset of genes associated with apoptosis, in which Bnip3, Cycs, Casp3 and Bag1 genes are involved in the mitochondrial pathway of apoptosis. Ier3, Mdm2 and Bnip3 genes may be p53 targets. Furthermore, real-time PCR confirmed the expression changes of five genes selected at random from the differentially expressed genes. We conclude that OA induces cell growth inhibition and apoptosis in the two-stage, MNNG-initiated transformation of BALB/c 3T3 cells. The results of gene expression profile analysis imply that multiple molecular pathways are involved in OA-induced proliferation inhibition and apoptosis. Mitochondrial and p53-associated apoptotic pathways also may contribute to OA-induced apoptosis.

Negative regulation of Pim-1 protein kinase levels by the B56beta subunit of PP2A

The Pim protein kinases are serine threonine protein kinases that regulate important cellular signaling pathway molecules, and enhance the ability of c-Myc to induce lymphomas. We demonstrate that a cascade of events controls the cellular levels of Pim. We find that overexpression of the protein phosphatase (PP) 2A catalytic subunit decreases the activity and protein levels of Pim-1. This effect is reversed by the application of okadaic acid, an inhibitor of PP2A, and is blocked by SV40 small T antigen that is known to disrupt B subunit binding to PP2A A and C subunits. Pim-1 can coimmunoprecipitate with the PP2A regulatory B subunit, B56beta, but not B56alpha, gamma, delta, epsilon or B55alpha. Using short hairpin RNA targeted at B56beta, we demonstrate that decreasing the level of B56beta increases the half-life of Pim-1 from 0.7 to 2.8 h, and decreases the ubiquitinylation level of Pim-1. We also find that Pin1, a prolyl-isomerase, is capable of binding Pim-1 and leads to a decrease in the protein level of Pim-1. On the basis of these observations, we hypothesize that phosphorylated Pim-1 binds Pin1 allowing the interaction of PP2A through B56beta. Dephosphorylation of Pim-1 then allows for ubiquitinylation and protein degradation of Pim-1.

Mechanisms of the HRSL3 tumor suppressor function in ovarian carcinoma cells

HRSL3 (also known as H-REV107-1) belongs to a class II tumor suppressor gene family and is downregulated in several human tumors including ovarian carcinomas. To unravel the mechanism of HRSL3 tumor suppressor action, we performed a yeast two-hybrid screen and identified the alpha-isoform of the regulatory subunit A of protein phosphatase 2A (PR65alpha) as a new interaction partner of HRSL3. Interaction between HRSL3 and PR65alpha was confirmed in vitro and by co-immunoprecipitation in mammalian cells. We demonstrate that HRSL3 binds to the endogenous PR65alpha, thereby partially sequestering the catalytic subunit PR36 from the PR65 protein complex, and inhibiting PP2A catalytic activity. Furthermore, binding of HRSL3 to PR65 induces apoptosis in ovarian carcinoma cells in a caspase-dependent manner. Using several mutant HRSL3 constructs, we identified the N-terminal proline-rich region within the HRSL3 protein as the domain that is relevant for both binding of PR65alpha and induction of programmed cell death. This suggests that the negative impact of HRSL3 onto PP2A activity is important for the HRSL3 pro-apoptotic function and indicates a role of PP2A in survival of human ovarian carcinomas. The analysis of distinct PP2A target molecules revealed PKCzeta as being involved in HRSL3 action. These data implicate HRSL3 as a signaling regulatory molecule, which is functionally involved in the oncogenic network mediating growth and survival of ovarian cancer cells.

Apoptosis induced by protein phosphatase 2A (PP2A) inhibition in T leukemia cells is negatively regulated by PP2A-associated p38 mitogen-activated protein kinase

Serine/threonine phosphatase regulation of phosphorylation-mediated intracellular signaling controls a number of important processes in mammalian cells. In this study, we show that constitutively active protein phosphatase 2A (PP2A), which is a serine/threonine phosphatase, is essential for T leukemia cell survival. Jurkat and CCRF-CEM T leukemia cells treated with the PP2A-selective inhibitor okadaic acid (OA) showed a dose- and time-dependent induction of apoptosis, as indicated by loss of mitochondrial transmembrane potential (delta psi(m)), cleavage-induced activation of caspase-3, -8, and -9, and DNA fragmentation. In addition, caspase-8 or caspase-9 inhibition with z-IETD-fmk or z-LEHD-fmk, respectively, largely prevented OA-induced apoptosis. Although OA treatment did not affect constitutive Bcl-2 expression, overexpression of Bcl-2 prevented both OA-induced DNA fragmentation and dissipation of delta psi(m). Furthermore, inhibition of caspase-3, -8, or -9 partially protected against OA-induced loss of delta psi(m). In addition, caspase-9 and caspase-3 inhibition largely prevented procaspase-3 and procaspase-8 cleavage, respectively, while caspase-8 inhibition partially interfered with procaspase-9 cleavage in OA-treated T leukemia cells. Thus, PP2A inhibition triggered the intrinsic pathway of apoptosis, which was enhanced by a mitochondrial feedback amplification loop. PP2A has also been implicated in the regulation of p38 mitogen-activated protein kinase (MAPK). Co-immunoprecipitation analysis revealed a physical association between the catalytic subunit of PP2A and p38 MAPK in T leukemia cells. Moreover, OA treatment caused p38 MAPK to be phosphorylated in a dose- and time-dependent fashion, indicating that PP2A prevented p38 MAPK activation. Although p38 MAPK activation usually promotes apoptosis, pharmacologic inhibition of p38 MAPK exacerbated OA-induced DNA fragmentation and loss of delta psi(m) in T leukemia cells, suggesting that, in this instance, the p38 MAPK signaling pathway promoted cell survival. Collectively, these findings indicate that PP2A and p38 MAPK have coordinate effects on signaling pathways that regulate the survival of T leukemia cells.

Balancing co-stimulation and inhibition with BTLA and HVEM

The interaction between B- and T-lymphocyte attenuator (BTLA), an inhibitory receptor whose extracellular domain belongs to the immunoglobulin superfamily, and herpesvirus-entry mediator (HVEM), a co-stimulatory tumour-necrosis factor receptor, is unique in that it is the only receptor-ligand interaction that directly bridges these two families of receptors. This interaction has raised many questions about how receptors from two different families could interact and what downstream signalling events might occur as a result of receptor ligation. As we discuss, recent studies show that engagement of HVEM with its endogenous ligand (LIGHT) from the tumour-necrosis factor family induces a powerful immune response, whereas HVEM interactions with BTLA negatively regulate T-cell responses.

Role for Wee1 in inhibition of G2-to-M transition through the cooperation of distinct human papillomavirus type 1 E4 proteins

The infectious cycle of human papillomavirus type 1 (HPV1) is accompanied by abundant expression of the full-length E1;E4 protein (17-kDa) and smaller E4 polypeptides (16-, 11-, and 10-kDa) that arise by sequential loss of N-terminal E1;E4 sequences. HPV1 E4 inhibits G(2)-to-M transition of the cell cycle. Here, we show that HPV1 E4 proteins mediate inhibition of cell division by more than one mechanism. Cells arrested by coexpression of E1;E4 (E4-17K) and a truncated protein equivalent to the 16-kDa species (E4-16K) contain inactive cyclin B1-cdk1 complexes. Inactivation of cdk1 is through inhibitory Tyr(15) phosphorylation, with cells containing elevated levels of Wee1, the kinase responsible for inhibitory cdk1 phosphorylation. Consistent with these findings, overexpression of Wee1 enhanced the extent to which E4-17K/16K-expressing cells arrest in G(2), indicating that maintenance of Wee1 activity is necessary for inhibition of cell division induced by coexpression of the two E4 proteins. Moreover, we have determined that depletion of Wee1 by small interfering RNA (siRNA) alleviates the G(2) block imposed by E4-17K/16K. In contrast however, maintenance of Wee1 activity is not necessary for G(2)-to-M inhibition mediated by E4-16K alone, as overexpression or depletion of Wee1 does not influence the G(2) arrest function of E4-16K. Cells arrested by E4-16K expression contain low levels of active cyclin B1-cdk1 complexes. We hypothesize that differential expression of HPV1 E4 proteins during the viral life cycle determines the host cell cycle status. Different mechanisms of inhibition of G(2)-to-M transition reinforce the supposition that distinct E4 functions are important for HPV replication.

Mast cell function: regulation of degranulation by serine/threonine phosphatases

Mast cells play both effector and modulatory roles in a range of allergic and immune responses. The principal function of these cells is the release of inflammatory mediators from mast cells by degranulation, which involves a complex interplay of signalling molecules. Understanding the molecular architecture underlying mast cell signalling has attracted renewed interest as the capacity for therapeutic intervention through controlling mast cell degranulation is now accepted as a viable proposition. The dynamic regulation of signalling by protein phosphorylation is a well-established phenomenon and many of the early events involved in mast cell activation are well understood. Less well understood however are the events further downstream of receptor activation that allow movement of granules through the cytoskeletal barrier and docking and fusion of granules with the plasma membrane. Whilst a potential role for the protein phosphatase family of signalling enzymes in mast cell function has been accepted for some time, the evidence has largely been derived from the use of broad specificity pharmacological inhibitors and results often depend upon the experimental conditions, leading to conflicting views. In this review, we present and discuss the pharmacological and recent molecular evidence that protein phosphatases, and in particular the protein phosphatase serine/threonine phosphatase type 2A (PP2A), have major regulatory roles to play and may be potential targets for the design of new therapeutic agents.

Responses of protein phosphatases and cAMP-dependent protein kinase in a freeze-avoiding insect, Epiblema scudderiana

Larvae of the goldenrod gall moth, Epiblema scudderiana, use the freeze avoidance strategy of winter cold hardiness and show multiple metabolic adaptations for subzero survival including accumulation of large amounts of glycerol as a colligative antifreeze. Induction and regulation of cold hardiness adaptations requires the intermediary action of signal transduction enzymes. Changes in the activities of several signaling enzymes including cAMP-dependent protein kinase (PKA), protein phosphatases 1 (PP1), 2A, 2C, and protein tyrosine phosphatases (PTPs) were monitored over the winter and during experimental exposures of larvae to subzero temperatures (-4 degrees C, a temperature that triggers rapid glycerol synthesis, or -20 degrees C, a common midwinter ambient temperature) or anoxia. A strong increase in the amount of active PP1 in the latter part of the winter may be responsible for shutting off glycogenolysis once glycerol levels are maximized. There appears to be a limited role for PKA in overwintering but PP2A and PP2C activities rose when larvae were exposed to -20 degrees C and PTP activities rose significantly over the winter months and also in response to laboratory subzero (-20 degrees C) and anoxia exposures. The strong responses by PTPs suggest that these may be involved in cell cycle and growth arrest during winter diapause.

BTLA and HVEM Cross Talk Regulates Inhibition and Costimulation

Recently a new inhibitory immunoglobulin domain-containing lymphocyte receptor was identified on the basis of its T helper 1 (T(H)1)-selective expression in murine T cell lines, which was named B and T lymphocyte attenuator (BTLA). Several groups have confirmed the initial characterization of BTLA as an inhibitory receptor, which was initially inferred from the mild increases in several parameters of BTLA-deficient mice. The initial expectation that BTLA would interact with a B7 family ligand, such as the B7x protein, was surprisingly overturned with the functional cloning of the actual BTLA ligand as herpesvirus entry mediator (HVEM). This was unexpected largely due to the fact that this interaction represents the convergence of two very different, although each quite extensive, families of receptors and ligands. The interaction of BTLA, which belongs to the CD28 family of the immunoglobulin superfamily, and HVEM, a costimulatory tumor-necrosis factor (TNF) receptor (TNFR), is quite unique in that it is the only receptor-ligand interaction that directly bridges these two families of receptors. This interaction has raised many questions about how receptors from two different families could interact and which are the signaling events downstream of receptor ligation. As we discuss here and recently demonstrated, HVEM interaction with BTLA serves to negatively regulate T cell responses, in contrast to the strong activation observed when HVEM engages its endogenous ligand from the TNF family. Finally, as studies of BTLA are just now beginning to extend beyond the initial characterizations, it is becoming clear that there are many complex issues remaining to be resolved, particularly potential polymorphisms that may engender disease susceptibility in the human.