Factor VIII is a positive regulator of platelet function

FVIII is an important cofactor in the tenase coagulation factor complex, lack of FVIII causes severe bleeding, whereas high FVIII levels seem to be associated with venous and arterial thromboembolism. Resting platelets do not bind FVIII, but activated platelets bind unactivated FVIII if vWF is not present. We investigated a possible influence of platelet bound FVIII on platelet function itself as it is unclear if there is a direct effect of FVIII on platelet function. The influence of FVIII on platelet function was investigated by flow cytometric analysis of P-selectin expression (CD62P) and PAC-1 binding before and after submaximal stimulation with TRAP-6 (5 microM final concentration), by confocal microscopy and by platelet aggregometry. For flow cytometry and confocal microscopy, washed platelets were incubated with human recombinant FVIII for 5 min at 37 degrees C. Analysis of platelet surface area was measured by computerized image analysis. Treatment with FVIII only caused no changes in P-selectin expression or PAC-1 binding, respectively. Stimulation of platelets with TRAP-6 increased the expression of P-selectin (445%) and PAC-1 binding (934%) as expected. These effects were further increased when platelets were stimulated with TRAP-6 and FVIII (P-selectin 499%, difference not significant; PAC-1 1626%, P < 0.05. Values were expressed in%, related to unstimulated, buffer treated platelets). Platelet spreading on fibrinogen was significantly increased when platelets were treated with FVIII and TRAP-6 compared to TRAP-6 alone (368 vs. 307 average pixel/platelet, P<0.05). In addition platelet aggregation was enhanced when platelets were stimulated with FVIII and TRAP-6 compared to TRAP-6 alone. FVIII can act as a positive regulator of platelet function in TRAP-co-stimulated platelets. We hypothesize that FVIII induced increase in platelet activation might contribute to venous and even arterial thrombus formation in patients with high FVIII levels.

The formation of platelet–leukocyte aggregates varies during the menstrual cycle

Platelet-leukocyte aggregates are considered to play a significant role in blood coagulation and inflammatory processes. We hypothesized that hormonal changes during the menstrual cycle affect the formation of heterotypic aggregates and therefore may constitute cycle-dependent variations of the susceptibility for thromboembolic events and inflammatory disease. We therefore measured platelet-leukocyte interaction by the determination of platelet-leukocyte aggregates (PLA), platelet P-Selectin expression, and platelet fibrinogen receptor activation by PAC-1 binding in 20 healthy women during their menstrual cycle by flow cytometry. The number of platelet-granulocyte aggregates (PGA) and platelet-monocyte aggregates (PMA) was higher at ovulation compared to any other time-point of the menstrual cycle (p = 0.005, p = 0.022, respectively). Likewise, P-Selectin expression peaked on day 14 (p = 0.040). The course of PLA formation during the menstrual cycle followed the course of estrogen levels, strongly suggesting direct effects of estrogen on platelet-leukocyte interaction. The susceptibility to form platelet-leukocyte aggregates that are inducible in vitro by a suboptimal concentration of thrombin receptor activating peptide-6 decreased slightly during the transition from day 1 to 14 (p = 0.040). These data indicate that platelet function varies during particular phases of the normal menstrual cycle.

Gi protein-mediated translocation of serine/threonine phosphatase to the plasma membrane and apoptosis of ovarian cancer cell in response to gonadotropin-releasing hormone antagonist cetrorelix

Serine/threonine protein phosphatase 2A (PP2A), a crucial enzyme in apoptosis control, has been demonstrated within the plasma membrane as well as in the soluble fraction. This study aimed to examine hormonal translocation of PP2A to the plasma membrane in gonadotropin-releasing hormone (GnRH)-responsive ovarian cancer cells. Apoptosis of ovarian cancer cell lines Caov-3 and SK-Ov-3 was quantified by nuclear morphology after staining with Hoechst 33342 dye. PP2A protein and activity in plasma membrane were assessed by immunohistochemical staining with PP2A-specific antibodies and by the measurement of the dephosphorylation of phosphopeptide highly selective for the PP2A, respectively. Incubation for 48 h with a GnRH antagonist cetrorelix caused parallel increases in the percentage of cells undergoing apoptosis and the membrane-associated PP2A activity; half-maximal effects occurred with 5 nmol/l cetrorelix. PP2A protein was also localised to the plasma membrane when the cell lines were exposed to cetrorelix. Pretreatment of the cells with pertussis toxin, but not cholera toxin, completely inhibited cetrorelix-stimulated apoptotic cell death and PP2A redistribution. These findings demonstrate that translocation of PP2A to plasma membrane is closely coupled to the onset of apoptosis in ovarian cancer cells exposed to GnRH antagonist. These GnRH-induced cellular events may be mediated through pertussis toxin-sensitive Gi protein-linked GnRH receptor.

Normal function of the yeast TOR pathway requires the type 2C protein phosphatase Ptc1

Yeast ptc1 mutants are rapamycin and caffeine sensitive, suggesting a functional connection between Ptc1 and the TOR pathway that is not shared by most members of the type 2C phosphatase family. Genome-wide profiling revealed that the ptc1 mutation largely attenuates the transcriptional response to rapamycin. The lack of Ptc1 significantly prevents the nuclear translocation of Gln3 and Msn2 transcription factors to the nucleus, as well as the dephosphorylation of the Npr1 kinase, in response to rapamycin. This could explain the observed decrease in both the basal and rapamycin-induced expression of several genes subjected to nitrogen catabolite repression (GAT1, MEP1, and GLN1) and stress response element (STRE)-driven promoters. Interestingly, this decrease is abolished in the absence of the Sit4 phosphatase. Epitasis analysis indicates that the mutation of SIT4 or TIP41, encoding a Tap42-interacting protein, abolishes the sensitivity of the ptc1 strain to rapamycin and caffeine. All of these results suggest that Ptc1 is required for normal TOR signaling, possibly by regulating a step upstream of Sit4 function. According to this hypothesis, we observe that the mutation of PTC1 drastically diminishes the rapamycin-induced interaction between Tap42 and Tip41, and this can be explained by lower-than-normal levels of Tip41 in ptc1 cells. Ptc1 is not necessary for the normal expression of the TIP41 gene; instead, its absence dramatically affects the stability of Tip41. The lack of Ptc1 partially abolishes the rapamycin-induced dephosphorylation of Tip41, which may further decrease Tap42 binding. Reduced Tip41 levels contribute to the ptc1 phenotypes, although additional Ptc1 targets must exist. All of these results provide the first evidence showing that a type 2C protein phosphatase is required for the normal functioning of the TOR pathway.

PTC1 is required for vacuole inheritance and promotes the association of the myosin-V vacuole-specific receptor complex

Organelle inheritance occurs during cell division. In Saccharomyces cerevisiae, inheritance of the vacuole, and the distribution of mitochondria and cortical endoplasmic reticulum are regulated by Ptc1p, a type 2C protein phosphatase. Here we show that PTC1/VAC10 controls the distribution of additional cargoes moved by a myosin-V motor. These include peroxisomes, secretory vesicles, cargoes of Myo2p, and ASH1 mRNA, a cargo of Myo4p. We find that Ptc1p is required for the proper distribution of both Myo2p and Myo4p. Surprisingly, PTC1 is also required to maintain the steady-state levels of organelle-specific receptors, including Vac17p, Inp2p, and Mmr1p, which attach Myo2p to the vacuole, peroxisomes, and mitochondria, respectively. Furthermore, Vac17p fused to the cargo-binding domain of Myo2p suppressed the vacuole inheritance defect in ptc1Delta cells. These findings suggest that PTC1 promotes the association of myosin-V with its organelle-specific adaptor proteins. Moreover, these observations suggest that despite the existence of organelle-specific receptors, there is a higher order regulation that coordinates the movement of diverse cellular components.

Generation of active protein phosphatase 2A is coupled to holoenzyme assembly

Protein phosphatase 2A (PP2A) is a prime example of the multisubunit architecture of protein serine/threonine phosphatases. Until substrate-specific PP2A holoenzymes assemble, a constitutively active, but nonspecific, catalytic C subunit would constitute a risk to the cell. While it has been assumed that the severe proliferation impairment of yeast lacking the structural PP2A subunit, TPD3, is due to the unrestricted activity of the C subunit, we recently obtained evidence for the existence of the C subunit in a low-activity conformation that requires the RRD/PTPA proteins for the switch into the active conformation. To study whether and how maturation of the C subunit is coupled with holoenzyme assembly, we analyzed PP2A biogenesis in yeast. Here we show that the generation of the catalytically active C subunit depends on the physical and functional interaction between RRD2 and the structural subunit, TPD3. The phenotype of the tpd3Δ strain is therefore caused by impaired, rather than increased, PP2A activity. TPD3/RRD2-dependent C subunit maturation is under the surveillance of the PP2A methylesterase, PPE1, which upon malfunction of PP2A biogenesis, prevents premature generation of the active C subunit and holoenzyme assembly by counteracting the untimely methylation of the C subunit. We propose a novel model of PP2A biogenesis in which a tightly controlled activation cascade protects cells from untargeted activity of the free catalytic PP2A subunit.

Multisubunit enzymes, such as protein phosphatase 2A, consist of a catalytic subunit and one of several regulatory subunits that are responsible for substrate specificity. Whereas this molecular architecture enables the assembly of a few components into many different substrate-specific enzymes, it possesses an inherent danger in the form of the uncomplexed catalytic subunit with its unspecific phosphatase activity. Until substrate-specific complexes assemble, the catalytic subunit would constitute a risk to the cell if no control mechanisms existed. We recently obtained evidence for the existence of the catalytic subunit in a low-activity conformation that requires an activator for the switch into the active conformation. This requirement suggested that the existing model of protein phosphatase 2A biogenesis was incomplete, because it could not explain how the activity of the catalytic subunit is kept in check until it is assembled with the substrate-targeting subunits. In this study, we provide evidence that the generation of the active catalytic subunit is coupled with and regulated by holoenzyme assembly. We propose a novel model of protein phosphatase biogenesis in which a tightly controlled activation cascade protects cells from the potential risk of unspecific dephosphorylation events.

Analysis of protein phosphatase 2A (PP2A) biogenesis in yeast suggests that a tightly controlled activation cascade, involving an interaction between the protein RRD2 and the structural subunit TPD3, protects cells from untargeted activity of the free catalytic PP2A subunit.

IFN-gamma-induced IDO and WRS expression in microglia is differentially regulated by IL-4

Indoleamine 2,3-dioxygenase (IDO), a tryptophan catabolizing enzyme, has been implicated in the pathogenesis of various neurological disorders. IDO expression is induced by IFN-gamma and leads to neurotoxicity by generating quinolinic acid. Additionally, it inhibits the immune response through both tryptophan depletion and generating other tryptophan catabolites. IL-4 and IL-13 have been shown to control IDO expression by antagonizing the effects of IFN-gamma in different cell types. Here, we investigated the effects of these cytokines on IDO expression in microglia. Interestingly, we observed that both IL-4 and IL-13 greatly enhanced IFN-gamma-induced IDO expression. However, tryptophanyl-tRNA synthetase (WRS), which is coinduced with IDO by IFN-gamma, is downregulated by IL-4 and IL-13. The effect of IL-4 and IL-13 was independent of STAT-6. Modulation of IDO but not WRS was eliminated by inhibition of protein phosphatase 2A (PP2A) activity. The phosphatidylinositol 3-kinase (PI3K) pathway further differentiated the regulation of these two enzymes, as inhibiting the PI3K pathway eliminated IFN-gamma induction of IDO, whereas such inhibition greatly enhanced WRS expression. These findings show discordance between modulations of expression of two distinct enzymes utilizing tryptophan as a common substrate, and raise the possibility of their involvement in regulating immune responses in various neurological disorders.

Clonal divergence in lung cancer development is associated with allelic loss on chromosome 4

Patients who receive curative treatment for lung cancer can develop additional lung tumors that may or may not be related to the original tumor and thus require different clinical management. If a subsequent tumor has a pattern of allele loss, revealed by allelotype analysis, overlapping that of the first tumor, it is believed to be a local recurrence or metastasis. In this case history, we present loss of heterozygosity analyses of the original primary tumor, and two second primary tumors occurring in the ipsilateral and the contra-lateral lungs. The allelotyping suggests that these tumors are all clonally related but concordance is not complete. Our interpretation is that the original primary tumor and the two new primary tumors have developed to full malignancy independently, but are clonally related, possibly via a clone of motile progenitor cells. Deletion mapping of DNA from biopsies of this patient delineated a region in 4p16 that we had previously shown to be lost in the transition from carcinoma in situ to invasive tumor. We identified a minimally deleted region encompassing six genes including two candidate tumor suppressor genes, CRMP1 a lung cancer metastasis-suppressing gene and PPP2R2C a gene for a regulatory subunit of the PP2 complex known to suppress tumorigenesis, particularly viral induced transformation.

FTY720, a new alternative for treating blast crisis chronic myelogenous leukemia and Philadelphia chromosome-positive acute lymphocytic leukemia

Blast crisis chronic myelogenous leukemia (CML-BC) and Philadelphia chromosome-positive (Ph1-positive) acute lymphocytic leukemia (ALL) are 2 fatal BCR/ABL-driven leukemias against which Abl kinase inhibitors fail to induce a long-term response. We recently reported that functional loss of protein phosphatase 2A (PP2A) activity is important for CML blastic transformation. We assessed the therapeutic potential of the PP2A activator FTY720 (2-amino-2-[2-(4-octylphenyl)ethyl]-1,3-propanediol hydrochloride), an immunomodulator in Phase III trials for patients with multiple sclerosis or undergoing organ transplantation, in CML-BC and Ph1 ALL patient cells and in in vitro and in vivo models of these BCR/ABL+ leukemias. Our data indicate that FTY720 induces apoptosis and impairs clonogenicity of imatinib/dasatinib-sensitive and -resistant p210/p190(BCR/ABL) myeloid and lymphoid cell lines and CML-BC(CD34+) and Ph1 ALL(CD34+/CD19+) progenitors but not of normal CD34+ and CD34+/CD19+ bone marrow cells. Furthermore, pharmacologic doses of FTY720 remarkably suppress in vivo p210/p190(BCR/ABL)-driven [including p210/p190(BCR/ABL)(T315I)] leukemogenesis without exerting any toxicity. Altogether, these results highlight the therapeutic relevance of rescuing PP2A tumor suppressor activity in Ph1 leukemias and strongly support the introduction of the PP2A activator FTY720 in the treatment of CML-BC and Ph1 ALL patients.

Inhibition of endothelial barrier dysfunction by P21-activated kinase-1

We investigated the activity of P21-activated kinase-1 (Pak1) on myosin light chain phosphorylation and on thrombin-induced barrier dysfunction in human endothelial cells (HMEC). HMEC were infected with recombinant adenoviruses that express constitutively active Pak1, LacZ, wild-type, and a mutant myosin regulatory light chain, mMLC20 (Thr18Ala, Ser19Ala). Expression of the recombinant Pak1 mediated by adenovirus in HMEC was regulated. Active Pak1 induced dephosphorylation of MLC20 in HMEC, but not in smooth muscle cells. Active Pak1 significantly inhibited thrombin-induced endothelial barrier dysfunction. Expression of the unphosphorylatable MLC20 also inhibited thrombin-induced endothelial barrier dysfunction. Constitutively active Pak1 associated with phosphatase 2A and induced a post-translational modification of the phosphatase. Our data provide novel evidence indicating that Pak1 regulates endothelial barrier function through activation of phosphatase 2A.

Mediating ERK 1/2 signaling rescues congenital heart defects in a mouse model of Noonan syndrome

Noonan syndrome (NS) is an autosomal dominant disorder characterized by a wide spectrum of defects, which most frequently include proportionate short stature, craniofacial anomalies, and congenital heart disease (CHD). NS is the most common nonchromosomal cause of CHD, and 80%-90% of NS patients have cardiac involvement. Mutations within the protein tyrosine phosphatase Src homology region 2, phosphatase 2 (SHP2) are responsible for approximately 50% of the cases of NS with cardiac involvement. To understand the developmental stage- and cell type-specific consequences of the NS SHP2 gain-of-function mutation, Q79R, we generated transgenic mice in which the mutated protein was expressed during gestation or following birth in cardiomyocytes. Q79R SHP2 embryonic hearts showed altered cardiomyocyte cell cycling, ventricular noncompaction, and ventricular septal defects, while, in the postnatal cardiomyocyte, Q79R SHP2 expression was completely benign. Fetal expression of Q79R led to the specific activation of the ERK1/2 pathway, and breeding of the Q79R transgenics into ERK1/2-null backgrounds confirmed the pathway's necessity and sufficiency in mediating mutant SHP2's effects. Our data establish the developmental stage-specific effects of Q79R cardiac expression in NS; show that ablation of subsequent ERK1/2 activation prevents the development of cardiac abnormalities; and suggest that ERK1/2 modulation could have important implications for developing therapeutic strategies in CHD.

Nucleolar release of Hand1 acts as a molecular switch to determine cell fate

The bHLH transcription factor Hand1 is essential for placentation and cardiac morphogenesis in the developing embryo. Here we implicate Hand1 as a molecular switch that determines whether a trophoblast stem cell continues to proliferate or commits to differentiation. We identify a novel interaction of Hand1 with a protein that contains an I-mfa (inhibitor of myogenic factor) domain that anchors Hand1 in the nucleolus where it negatively regulates Hand1 activity. In the trophoblast stem-cell line Rcho-1, nucleolar sequestration of Hand1 accompanies sustained cell proliferation and renewal, whereas release of Hand1 into the nucleus leads to its activation, thus committing cells to a differentiated giant-cell fate. Site-specific phosphorylation is required for nucleolar release of Hand1, for its dimerization and biological function, and this is mediated by the non-canonical polo-like kinase Plk4 (Sak). Sak is co-expressed in Rcho-1 cells, localizes to the nucleolus during G2 and phosphorylates Hand1 as a requirement for trophoblast stem-cell commitment to a giant-cell fate. This study defines a novel cellular mechanism for regulating Hand1 that is a crucial step in the stem-cell differentiation pathway.

14-3-3 proteins and protein phosphatases are not reduced in tau-deficient mice

Tau is an axonal microtubule-associated protein, whose dysfunction causes neurodegenerative diseases such as Alzheimer's disease and other tauopathies. Earlier studies have shown the interactions of tau with glycogen synthase kinase-3beta, 14-3-3zeta, protein phosphatase 1 and protein phosphatase 2A. In this study, we compared the amounts of these tau-interacting proteins in brain microtubule-enriched fractions from wild-type and tau-deficient mice. Contrary to our expectation, we detected no difference in the amount of these proteins between wild-type and tau-deficient mice. Our findings indicate that only a small portion of tau-interacting proteins are bound to tau in vivo, and suggest the existence of other scaffolding proteins. We propose that tau-deficient mice are an ideal system for confirming the function of tau-interacting proteins.

An endogenous serine/threonine protein phosphatase inhibitor, G-substrate, reduces vulnerability in models of Parkinson's disease

Relative neuronal vulnerability is a universal yet poorly understood feature of neurodegenerative diseases. In Parkinson's disease, dopaminergic (DA) neurons in the substantia nigra (SN) (A9) are particularly vulnerable, whereas adjacent DA neurons within the ventral tegmental area (A10) are essentially spared. Our previous laser capture microdissection and microarray study (Chung et al., 2005) demonstrated that molecular differences between these DA neurons may underlie their differential vulnerability. Here we show that G-substrate, an endogenous inhibitor of Ser/Thr protein phosphatases, exhibits higher expression in A10 compared with A9 DA neurons in both rodent and human midbrain. Overexpression of G-substrate protected dopaminergic BE(2)-M17 cells against toxins, including 6-OHDA and MG-132 (carbobenzoxy-L-leucyl- L-leucyl-L-leucinal), whereas RNA interference (RNAi)-mediated knockdown of endogenous G-substrate increased their vulnerability to these toxins. G-substrate reduced 6-OHDA-mediated protein phosphatase 2A (PP2A) activation in vitro and increased phosphorylated levels of PP2A targets including Akt, glycogen synthase kinase 3beta, and extracellular signal-regulated kinase 2 but not p38. RNAi to Akt diminished the protective effect of G-substrate against 6-OHDA. In vivo, lentiviral delivery of G-substrate to the rat SN increased baseline levels of phosphorylated Akt and protected A9 DA neurons from 6-OHDA-induced toxicity. These results suggest that inherent differences in the levels of G-substrate contribute to the differential vulnerability of DA neurons and that enhancing G-substrate levels may be a neuroprotective strategy for the vulnerable A9 (SN) DA neurons in Parkinson's disease.

Identification of the PP2A-interacting region of heat shock transcription factor 2

Previous work in our laboratory demonstrated the existence of an association between heat shock transcription factor 2 (HSF2) and the serine/threonine phosphatase 2A, which is mediated by interaction between HSF2 and the A subunit (also called PR65) of this protein phosphatase. In light of the importance of HSF2-PP2A association for HSF2 cellular function, in this study, we have sought to dissect the sequences within HSF2 that are important for interaction with the A subunit of PP2A. The results of these experiments indicate that the HSF2 region comprising amino acids 343-363 is important for A subunit interaction. This region includes part of the C-terminal leucine zipper motif of HSF2 called heptad repeat C (HR-C). The results of transfection/immunoprecipitation experiments also show that deletion of the 6 amino acids from 343 to 348 from HSF2 (HSF2 (delta343-348)), is sufficient to prevent HSF2 from interacting with PP2A. These data provide insight into a new functional domain of HSF2, the PP2A A subunit-interacting region.

ER stress triggers apoptosis by activating BH3-only protein Bim

Endoplasmic reticulum (ER) stress caused by misfolded proteins or cytotoxic drugs can kill cells and although activation of this pathway has been implicated in the etiology of certain degenerative disorders its mechanism remains unresolved. Bim, a proapoptotic BH3-only member of the Bcl-2 family is required for initiation of apoptosis induced by cytokine deprivation or certain stress stimuli. Its proapoptotic activity can be regulated by several transcriptional or posttranslational mechanisms, such as ERK-mediated phosphorylation, promoting its ubiquitination and proteasomal degradation. We found that Bim is essential for ER stress-induced apoptosis in a diverse range of cell types both in culture and within the whole animal. ER stress activates Bim through two novel pathways, involving protein phosphatase 2A-mediated dephosphorylation, which prevents its ubiquitination and proteasomal degradation and CHOP-C/EBPalpha-mediated direct transcriptional induction. These results define the molecular mechanisms of ER stress-induced apoptosis and identify targets for therapeutic intervention in ER stress-related diseases.

Activation of endoplasmic reticulum stress response by hepatitis viruses up-regulates protein phosphatase 2A

The up-regulation of protein phosphatase 2 A (PP2A) is an important factor leading to an inhibition of IFNalpha signaling caused by viral protein expression. Here, we describe the molecular mechanism involved in PP2Ac up-regulation by HCV and HBV. HCV and HBV protein expression in cells induces an ER stress response leading to calcium release from the ER. HCV protein expression induces CREB activation, probably through calcium/calmodulin-dependent protein kinase. CREB binds to a CRE element in the promoter of PP2Ac and induces its transcriptional up-regulation. Because PP2Ac is involved in many important cellular processes including cell-cycle regulation, apoptosis, cell morphology, development, signal transduction and translation, its up-regulation during ER stress has potentially important implications.

Protein Phosphatase 2A and Separase Form a Complex Regulated by Separase Autocleavage

The onset of anaphase is triggered by the activation of a site-specific protease called separase. Separase cleaves the chromosomal cohesins holding the duplicated sister chromatids together, allowing sisters to simultaneously separate and segregate to opposite ends of the cell before division. Activated separase cleaves not only cohesin, but also itself; however, the biological significance of separase self-cleavage has remained elusive. Before anaphase, separase is inhibited by at least two mechanisms. The first involves the binding of securin, whereas the second requires the phosphorylation-dependent binding of cyclin-dependent kinase 1 (Cdk1)/cyclin B1. Because securin and Cdk1/cyclin B1 interact with separase in a mutually exclusive manner, the degradation of both these inhibitors plays an important role in activating separase at anaphase. Here we identify a new separase interacting partner, a specific subtype of the heterotrimeric protein phosphatase 2A (PP2A). PP2A associates with separase through the B' (B56) regulatory subunit and does so independently of securin and cyclin B1 binding. The association of PP2A with separase requires a 55-amino acid domain closely juxtaposed to separase autocleavage sites. Strikingly, mutation of these cleavage sites increases PP2A binding, suggesting that separase cleavage disrupts the interaction of PP2A with separase. Furthermore, expression of a non-cleavable separase, but not a non-cleavable mutant that cannot bind PP2A, causes a premature loss of centromeric cohesion. Together these observations provide a new mechanistic insight into a physiological function for separase self-cleavage.

Proteomic Studies of PP2A-B56γ1 Phosphatase Complexes Reveal Phosphorylation-Regulated Partners in Cardiac Local Signaling

Defects of kinase-phosphatase signaling in cardiac myocytes contribute to human heart disease. The activity of one phosphatase, PP2A, is governed by B targeting subunits, including B56gamma1, expressed in heart cells. As the role of PP2A/B56gamma1 on the heart function remains largely unknown, this study sought to identify protein partners through unbiased, affinity purification-based proteomics combined with the functional validation. The results reveal multiple interactors that are localized in strategic cardiac sites to participate in Ca2+ homeostasis and gene expression, exemplified by the Ca pump, SERCA2a, and the splicing factor ASF/SF2. These results are corroborated by confocal imaging where adenovirally overexpressed B56gamma1 is found in z-line/t-tubule region and nuclear speckles. Importantly, overexpression of B56gamma1 in cultured myocytes dramatically impairs cell contractility. These results provide a global view of B56gamma1-regulated local signaling and heart function.

Augmented Protein Kinase C-α–Induced Myofilament Protein Phosphorylation Contributes to Myofilament Dysfunction in Experimental Congestive Heart Failure

It is becoming clear that upregulated protein kinase C (PKC) signaling plays a role in reduced ventricular myofilament contractility observed in congestive heart failure. However, data are scant regarding which PKC isozymes are involved. There is evidence that PKC-alpha may be of particular importance. Here, we examined PKC-alpha quantity, activity, and signaling to myofilaments in chronically remodeled myocytes obtained from rats in either early heart failure or end-stage congestive heart failure. Immunoblotting revealed that PKC-alpha expression and activation was unaltered in early heart failure but increased in end-stage congestive heart failure. Left ventricular myocytes were isolated by mechanical homogenization, Triton-skinned, and attached to micropipettes that projected from a force transducer and motor. Myofilament function was characterized by an active force-[Ca(2+)] relation to obtain Ca(2+)-saturated maximal force (F(max)) and myofilament Ca(2+) sensitivity (indexed by EC(50)) before and after incubation with PKC-alpha, protein phosphatase type 1 (PP1), or PP2a. PKC-alpha treatment induced a 30% decline in F(max) and 55% increase in the EC(50) in control cells but had no impact on myofilament function in failing cells. PP1-mediated dephosphorylation increased F(max) (15%) and decreased EC(50) ( approximately 20%) in failing myofilaments but had no effect in control cells. PP2a-dependent dephosphorylation had no effect on myofilament function in either group. Lastly, PP1 dephosphorylation restored myofilament function in control cells hyperphosphorylated with PKC-alpha. Collectively, our results suggest that in end-stage congestive heart failure, the myofilament proteins exist in a hyperphosphorylated state attributable, in part, to increased activity and signaling of PKC-alpha.

The tumor suppressor PP2A Abeta regulates the RalA GTPase

The serine-threonine protein phosphatase 2A (PP2A) is a heterotrimeric enzyme family that regulates numerous signaling pathways. Biallelic mutations of the structural PP2A Abeta subunit occur in several types of human tumors; however, the functional consequences of these cancer-associated PP2A Abeta mutations in cell transformation remain undefined. Here we show that suppression of PP2A Abeta expression permits immortalized human cells to achieve a tumorigenic state. Cancer-associated Abeta mutants fail to reverse tumorigenic phenotype induced by PP2A Abeta suppression, indicating that these mutants function as null alleles. Wild-type PP2A Abeta but not cancer-derived Abeta mutants form a complex with the small GTPase RalA. PP2A Abeta-containing complexes dephosphorylate RalA at Ser183 and Ser194, inactivating RalA and abolishing its transforming function. These observations identify PP2A Abeta as a tumor suppressor gene that transforms immortalized human cells by regulating the function of RalA.

Selection of protein phosphatase 2A regulatory subunits is mediated by the C terminus of the catalytic Subunit

Protein phosphatase 2A (PP2A) is a family of multifunctional serine/threonine phosphatases all composed of a catalytic C, a structural A, and a regulatory B subunit. Assembly of the complex with the appropriate B subunit forms the key to the functional specificity and regulation of PP2A. Emerging evidence suggests a crucial role for methylation and phosphorylation of the PP2A C subunit in this process. In this study, we show that PP2A C subunit methylation was not absolutely required for binding the PR61/B' and PR72/B'' subunit families, whereas binding of the PR55/B subunit family was determined by methylation and the nature of the C-terminal amino acid side chain. Moreover mutation of the phosphorylatable Tyr(307) or Thr(304) residues differentially affected binding of distinct B subunit family members. Down-regulation of the PP2A methyltransferase LCMT1 by RNA interference gradually reduced the cellular amount of methylated C subunit and induced a dynamic redistribution of the remaining methylated PP2A(C) between different PP2A trimers consistent with their methylation requirements. Persistent knockdown of LCMT1 eventually resulted in specific degradation of the PR55/B subunit and apoptotic cell death. Together these results establish a crucial foundation for understanding PP2A regulatory subunit selection.

An in vivo assay to quantify stable protein phosphatase 2A (PP2A) heterotrimeric species

Protein phosphatase 2A (PP2A) regulates a broad spectrum of cellular processes. The enzyme is, in fact, largely a collection of varied heterotrimeric species composed of a catalytic (C) subunit and regulatory (B-type) subunit bound together by a structural (A) subunit. One important feature of the C subunit is that its carboxy-terminus can be modified by phosphorylation and methylation. The mechanisms that trigger such posttranslational modifications, as well as their consequences, are still under investigation. However, data collected thus far indicate that these modifications alter the binding to B subunits for an AC dimer, thereby affecting the makeup of the PP2A species in the cell. In this chapter, we describe an in vivo assay for assessing stable PP2A heterotrimer formation that is based on specific subcellular localizations of PP2A heterotrimers. This assay can be used to study the impact of a wide variety of alterations (such as mutations and covalent modifications) on PP2A heterotrimer formation. We specifically describe the use of this assay to quantify the effects of methylation on the stable formation of PP2ARts1p and PP2ACdc55p heterotrimers.

Mutagenesis and expression of the scaffolding Aalpha and Abeta subunits of PP2A: assays for measuring defects in binding of cancer-related Aalpha and Abeta mutants to the regulatory B and catalytic C subunits

Protein phosphatase 2A (PP2A) holoenzymes are composed of three subunits: one scaffolding A subunit, one regulatory B subunit, and one catalytic C subunit. The A subunit exists as two isoforms: Aalpha and Abeta. The C subunit also exists as two isoforms (Calpha and Cbeta) and B subunits fall into three families (B, B', and B") comprising over 15 members. The Aalpha and Abeta subunits consist of 15 nonidentical repeats, which are composed of two amphipathic alpha helices that are connected by a loop (intrarepeat loop). These loops are instrumental in binding regulatory B and catalytic C subunits. The genes encoding the Aalpha and Abeta subunits are relatively frequent targets for mutation in human cancer. The mutations often affect the intrarepeat loops and cause defects in the binding of specific B subunits or of B and C subunits. Here, we describe in vitro and in vivo binding assays for measuring these defects. Knowing which B subunits are affected in binding to the mutant A subunits sheds light on which holoenzymes might be involved in growth control and cancer.