Protein phosphatase 2A inactivates the mitogen-stimulated S6 kinase from Swiss mouse 3T3 cells

Mapping the substrate landscape of protein phosphatase 2A catalytic subunit PPP2CA

Protein phosphatase 2A (PP2A) is an essential Ser/Thr phosphatase. The PP2A holoenzyme complex comprises a scaffolding (A), regulatory (B), and catalytic (C) subunit, with PPP2CA being the principal catalytic subunit. The full scope of PP2A substrates in cells remains to be defined. To address this, we employed dTAG proteolysis-targeting chimeras to efficiently and selectively degrade dTAG-PPP2CA in homozygous knock-in HEK293 cells. Unbiased global phospho-proteomics identified 2,204 proteins with significantly increased phosphorylation upon dTAG-PPP2CA degradation, implicating them as potential PPP2CA substrates. A vast majority of these are novel. Bioinformatic analyses revealed involvement of the potential PPP2CA substrates in spliceosome function, cell cycle, RNA transport, and ubiquitin-mediated proteolysis. We identify a pSP/pTP motif as a predominant target for PPP2CA and confirm some of our phospho-proteomic data with immunoblotting. We provide an in-depth atlas of potential PPP2CA substrates and establish targeted degradation as a robust tool to unveil phosphatase substrates in cells.

Valproic acid decreases vascular smooth muscle cell proliferation via protein phosphatase 2A-mediated p70 S6 kinase inhibition

Valproic acid (VPA) has been used to treat epilepsy and bipolar disorder. Although the abnormal proliferation of vascular smooth muscle cells (VSMCs) is a well-established contributor to the development of various vascular diseases including atherosclerosis, the effect of VPA on VSMC proliferation and its mechanism of action have not been fully revealed. Herein, we investigated the molecular mechanism by which VPA inhibits rat VSMC proliferation. VPA dose-dependently decreased VSMC proliferation, which was accompanied by the dose-dependent decrease in phosphorylation of p70 S6 kinase (p70S6K) at Thr389 (p-p70S6K-Thr³⁸⁹), and overexpression of the p70S6K-T389E mutant gene significantly reversed VPA-inhibited VSMC proliferation. Co-treatment with okadaic acid, a specific protein phosphatase 2A (PP2A) inhibitor, significantly restored p-p70S6K-Thr³⁸⁹. Furthermore, knockdown of PP2Ac gene expression by siRNA significantly reversed VPA-inhibited p-p70S6K-Thr³⁸⁹ and VSMC proliferation. Confocal microscopic analyses and co-immunoprecipitation results clearly showed that the physical binding of p70S6K and PP2Ac was promoted by VPA. Valpromide, a VPA's structural derivative with no histone deacetylase (HDAC) inhibition activity, as well as VPA and sodium butyrate, an HDAC inhibitor similar to VPA, decreased VSMC proliferation and p-p70S6K-Thr³⁸⁹, indicating that HDAC is not involved in VPA-inhibited VSMC proliferation. Finally, the inhibitory effects of VPA on p-p70S6K-Thr³⁸⁹ and VSMC proliferation were reiterated in a platelet-derived growth factor (PDGF)-induced in vitro atherosclerosis model. In conclusion, our results demonstrate that VPA decreased cell proliferation via PP2A-mediated inhibition of p-p70S6K-Thr³⁸⁹ in basal and PDGF-stimulated VSMCs. The results suggest that VPA could be used in the treatment and prevention of atherosclerosis and in-stent restenosis.

Hypophosphorylation of ribosomal protein S6 is a molecular mechanism underlying ischemic tolerance induced by either hibernation or preconditioning

Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) have an extraordinary capacity to withstand prolonged and profound reductions in blood flow and oxygen delivery to the brain without incurring any cellular damage. As such, the hibernation torpor of I. tridecemlineatus provides a valuable model of tolerance to ischemic stress. Herein, we report that during hibernation torpor, a marked reduction in the phosphorylation of the ribosomal protein S6 (rpS6) occurs within the brains of I. tridecemlineatus. Of note, rpS6 phosphorylation was shown to increase in the brains of rats that underwent an occlusion of the middle cerebral artery. However, such an increase was attenuated after the implementation of an ischemic preconditioning paradigm. In addition, cultured cortical neurons treated with the rpS6 kinase (S6K) inhibitors, D-glucosamine or PF4708671, displayed a decrease in rpS6 phosphorylation and a subsequent increase in tolerance to oxygen/glucose deprivation, an in vitro model of ischemic stroke. Collectively, such evidence suggests that the down-regulation of rpS6 signal transduction may account for a substantial part of the observed increase in cellular tolerance to brain ischemia that occurs during hibernation torpor and after ischemic preconditioning. Further identification and characterization of the mechanisms used by hibernating species to increase ischemic tolerance may eventually clarify how the loss of homeostatic control that occurs during and after cerebral ischemia in the clinic can ultimately be minimized and/or prevented. Mammalian hibernation provides a valuable model of tolerance to ischemic stress. Herein, we demonstrate that marked reductions in the phosphorylation of ribosomal protein S6 (rpS6), extracellular signal-regulated kinase family of mitogen-activated protein (MAP) kinase p44/42 (p44/42MAPK) and ribosomal protein S6 kinase (S6K) occur within the brains of both hibernating squirrels and rats, which have undergone an ischemic preconditioning paradigm. We therefore propose that the down-regulation of rpS6 signal transduction may account for a substantial part of the observed increase in cellular tolerance to brain ischemia that occurs during hibernation torpor and after ischemic preconditioning, via a suppression of protein synthesis and/or energy consumption.

Ciliary transport regulates PDGF-AA/αα signaling via elevated mammalian target of rapamycin signaling and diminished PP2A activity

Primary cilia are built and maintained by intraflagellar transport (IFT), whereby the two IFT complexes, IFTA and IFTB, carry cargo via kinesin and dynein motors for anterograde and retrograde transport, respectively. Many signaling pathways, including platelet- derived growth factor (PDGF)-AA/αα, are linked to primary cilia. Active PDGF-AA/αα signaling results in phosphorylation of Akt at two residues: P-Akt(T308) and P-Akt(S473), and previous work showed decreased P-Akt(S473) in response to PDGF-AA upon anterograde transport disruption. In this study, we investigated PDGF-AA/αα signaling via P-Akt(T308) and P-Akt(S473) in distinct ciliary transport mutants. We found increased Akt phosphorylation in the absence of PDGF-AA stimulation, which we show is due to impaired dephosphorylation resulting from diminished PP2A activity toward P-Akt(T308). Anterograde transport mutants display low platelet-derived growth factor receptor (PDGFR)α levels, whereas retrograde mutants exhibit normal PDGFRα levels. Despite this, neither shows an increase in P-Akt(S473) or P-Akt(T308) upon PDGF-AA stimulation. Because mammalian target of rapamycin complex 1 (mTORC1) signaling is increased in ciliary transport mutant cells and mTOR signaling inhibits PDGFRα levels, we demonstrate that inhibition of mTORC1 rescues PDGFRα levels as well as PDGF-AA-dependent phosphorylation of Akt(S473) and Akt(T308) in ciliary transport mutant MEFs. Taken together, our data indicate that the regulation of mTORC1 signaling and PP2A activity by ciliary transport plays key roles in PDGF-AA/αα signaling.

An RCT on the effects of topical CGP on surgical wound appearance and residual scarring in bilateral total-knee arthroplasty patients

OBJECTIVE

To test the hypothesis that topically applied calcium glycerophosphate (CGP) would improve the appearance of the wound following bilateral knee replacement.

METHOD

Healthy patients, aged 45-75 years, scheduled for bilateral total-knee replacement surgery were recruited into the study. One knee was randomly assigned to the treatment group, while the contralateral knee was designated the control (standard care). Subjects were instructed to apply a preparation of 10% CGP in an aqueous lotion to the treated knee once daily for 42 days, starting at the third postoperative day. Functional sealing and cosmetic appearance of the incision were evaluated by two surgeons by direct examination of the patient and then by two experienced assessors from photographs. The investigators qualitatively scored the intensity and extent of erythema along the incision and over the entire knee, the appearance of visible oedema along the incision and over the knee, and the overall clinical impression of wound healing. All four assessors were blinded to the subjects' allocation and the latter two assessors to the initial investigators' assessments. Subjects were also followed up for an additional 46 weeks, giving a total study duration of 12 months.

RESULTS

Twenty patients completed the study. Statistical analysis showed that both the area and intensity of erythema along the incision were significantly reduced in the treated vs untreated knee over the entire study period. The analysis further showed that treatment significantly reduced oedema, both along the incision and across the entire knee. The differences were most marked at the seventh postoperative day and diminished with time. No adverse effects were observed for any patient, in either treated or untreated knees.

CONCLUSION

These data demonstrate that postoperative application of 10% CGP could improve the appearance of the wound following total knee arthroplasty.

PP2A regulatory subunit PP2A-B' counteracts S6K phosphorylation

The insulin/TOR signaling pathway plays a crucial role in animal homeostasis, sensing nutrient status to regulate organismal growth and metabolism. We identify here the Drosophila B' regulatory subunit of PP2A (PP2A-B') as a novel, conserved component of the insulin pathway that specifically targets the PP2A holoenzyme to dephosphorylate S6K. PP2A-B' knockout flies have elevated S6K phosphorylation and exhibit phenotypes typical of elevated insulin signaling such as reduced total body triglycerides and reduced longevity. We show that PP2A-B' interacts with S6K both physically and genetically. The human homolog of PP2A-B', PPP2R5C, also counteracts S6K1 phosphorylation, indicating a conserved mechanism in mammals. Since S6K affects development of cancer and metabolic disease, our data identify PPP2R5C as a novel factor of potential medical relevance.

The adenovirus E4orf4 protein induces G2/M arrest and cell death by blocking protein phosphatase 2A activity regulated by the B55 subunit

Human adenovirus E4orf4 protein is toxic in human tumor cells. Its interaction with the B alpha subunit of protein phosphatase 2A (PP2A) is critical for cell killing; however, the effect of E4orf4 binding is not known. B alpha is one of several mammalian B-type regulatory subunits that form PP2A holoenzymes with A and C subunits. Here we show that E4orf4 protein interacts uniquely with B55 family subunits and that cell killing increases with the level of E4orf4 expression. Evidence suggesting that B alpha-specific PP2A activity, measured in vitro against phosphoprotein substrates, is reduced by E4orf4 binding was obtained, and two potential B55-specific PP2A substrates, 4E-BP1 and p70(S6K), were seen to be hypophosphorylated in vivo following expression of E4orf4. Furthermore, treatment of cells with low levels of the phosphatase inhibitor okadaic acid or coexpression of the PP2A inhibitor I(1)(PP2A) enhanced E4orf4-induced cell killing and G(2)/M arrest significantly. These results suggested that E4orf4 toxicity results from the inhibition of B55-specific PP2A holoenzymes, an idea that was strengthened by an observed growth arrest resulting from treatment of H1299 cells with B alpha-specific RNA interference. We believe that E4orf4 induces growth arrest resulting in cell death by reducing the global level of B55-specific PP2A activity, thus preventing the dephosphorylation of B55-specific PP2A substrates, including those involved in cell cycle progression.

Signaling logic of activity-triggered dendritic protein synthesis: an mTOR gate but not a feedback switch

Changes in synaptic efficacy are believed to form the cellular basis for memory. Protein synthesis in dendrites is needed to consolidate long-term synaptic changes. Many signals converge to regulate dendritic protein synthesis, including synaptic and cellular activity, and growth factors. The coordination of these multiple inputs is especially intriguing because the synthetic and control pathways themselves are among the synthesized proteins. We have modeled this system to study its molecular logic and to understand how runaway feedback is avoided. We show that growth factors such as brain-derived neurotrophic factor (BDNF) gate activity-triggered protein synthesis via mammalian target of rapamycin (mTOR). We also show that bistability is unlikely to arise from the major protein synthesis pathways in our model, even though these include several positive feedback loops. We propose that these gating and stability properties may serve to suppress runaway activation of the pathway, while preserving the key role of responsiveness to multiple sources of input.

Memory formation involves the controlled production of new proteins close to the site of input stimuli on nerve cells. Strong inputs, in combination with growth factors, stimulate the synthesis of several kinds of synaptic proteins. These new proteins are believed to participate in remodeling the contacts between cells. This gives rise to a potentially unstable situation of a self-modifying cellular machine, because the new proteins rebuild their own inputs and their own production machinery. We have analyzed these interactions by modeling multiple inputs and the process of self-modifying feedback. We find that runaway modifications are prevented in two ways: first, a molecule called mTOR acts as a gate to suppress synthesis except under very tightly regulated conditions. Second, the feedback processes operate in a range where it is very unlikely that they can give rise to runaway buildup. Thus, the system avoids instability even though it is capable of modifying itself in response to many kinds of inputs.

Growth factor-stimulated phosphorylation cascades: activation of growth factor-stimulated MAP kinase

Protein phosphorylation is an important mechanism in the response of cells to growth factors by which signals can be conveyed from cell surface receptors to intracellular targets. In addition to stimulation of protein tyrosine phosphorylation, activation of growth factor receptors having protein tyrosine kinase activity leads to dramatic alterations in the levels of protein serine/threonine phosphorylation. Several growth factor-stimulated serine/threonine-specific kinases have been identified as potential mediators of such signalling. MAP (microtubule-associated protein) kinase has emerged as a very interesting member of this group, because it activates a separate kinase, pp90rsk, which is also growth factor-stimulated. MAP kinase itself appears to be regulated by protein phosphorylation, because it can be inactivated by protein phosphatases. We have identified two 60 kDa proteins that promote the phosphorylation and full activation of MAP kinase in a manner paralleling its activation by growth factors in intact cells. These 'MAP kinase activators' are themselves stimulated by growth factors, suggesting that they function as intermediates between the MAP kinase and cell surface receptors in a growth factor-stimulated kinase cascade. Identification of the components of this protein kinase cascade reveals a mechanism by which at least some of the effects of receptor tyrosine kinases can be mediated through serine/threonine phosphorylation.

Involvement of PP2A in viral and cellular transformation

Although the small DNA tumor virus SV40 (simian virus 40) fails to replicate in human cells, understanding how SV40 transforms human and murine cells has and continues to provide important insights into cancer initiation and maintenance. The early region of SV40 encodes two oncoproteins: the large T (LT) and small t (ST) antigens. SV40 LT contributes to murine and human cell transformation in part by inactivating the p53 and retinoblastoma protein tumor suppressor proteins. SV40 ST inhibits the activity of the protein phosphatase 2A (PP2A) family of serine-threonine phosphatases, and this interaction is required for SV40-mediated transformation of human cells. PP2A regulates multiple signaling pathways, suggesting many possible targets important for viral replication and cell transformation. Genetic manipulation of particular PP2A subunits has confirmed a role for specific complexes in transformation, and recent work implicates the perturbation of the phosphatidylinositol 3-kinase/Akt pathway and c-Myc stability in transformation by ST and PP2A. Mutations in PP2A subunits occur at low frequency in human tumors, suggesting that alterations of PP2A signaling play a role in both experimentally induced and spontaneously arising cancers. Unraveling the complexity of PP2A signaling will not only provide further insights into cancer development but may identify novel targets with promise for therapeutic manipulation.

mTOR-dependent Suppression of Protein Phosphatase 2A Is Critical for Phospholipase D Survival Signals in Human Breast Cancer Cells

A critical aspect of tumor progression is the generation of survival signals that overcome default apoptotic programs. Recent studies have revealed that elevated phospholipase D activity generates survival signals in breast and perhaps other human cancers. We report here that the elevated phospholipase D activity in the human breast cancer cell line MDA-MB-231 suppresses the activity of the putative tumor suppressor protein phosphatase 2A in a mammalian target of rapamycin (mTOR)-dependent manner. Increasing the phospholipase D activity in MCF7 cells also suppressed protein phosphatase 2A activity. Elevated phospholipase D activity suppressed association of protein phosphatase 2A with both ribosomal subunit S6-kinase and eukaryotic initiation factor 4E-binding protein 1. Suppression of protein phosphatase 2A by SV40 small t-antigen has been reported to be critical for the transformation of human cells with SV40 early region genes. Consistent with a critical role for protein phosphatase 2A in phospholipase D survival signals, either SV40 small t-antigen or pharmacological suppression of protein phosphatase 2A restored survival signals lost by the suppression of either phospholipase D or mTOR. Blocking phospholipase D signals also led to reduced phosphorylation of the pro-apoptotic protein BAD at the protein phosphatase 2A dephosphorylation site at Ser-112. The ability of phospholipase D to suppress protein phosphatase 2A identifies a critical target of an emerging phospholipase D/mTOR survival pathway in the transformation of human cells.

Okadaic-acid-induced inhibition of protein phosphatase 2A produces activation of mitogen-activated protein kinases ERK1/2, MEK1/2, and p70 S6, similar to that in Alzheimer's disease

In Alzheimer's disease (AD) brain the activity of protein phosphatase (PP)-2A is compromised and that of the extracellular signal-regulated protein kinase (ERK1/2) of the mitogen-activated protein kinase (MAPK) family, which can phosphorylate tau, is up-regulated. We investigated whether a decrease in PP-2A activity could underlie the activation of these kinases and the abnormal hyperphosphorylation of tau. Rat brain slices, 400-microm-thick, kept under metabolically active conditions in oxygenated (95% O(2), 5% CO(2)) artificial CSF were treated with 1.0 micromol/L okadaic acid (OA) for 1 hour at 33 degrees C. Under this condition, PP-2A activity was decreased to approximately 35% of the vehicle-treated control slices, and activities of PP-1 and PP-2B were not affected. In the OA-treated slices, we observed a dramatic increase in the phosphorylation/activation of ERK1/2, MEK1/2, and p70 S6 kinase both immunohistochemically and by Western blots using phosphorylation-dependent antibodies against these kinases. Treatment of 6-microm sections of the OA-treated slices with purified PP-2A reversed the phosphorylation/activation of these kinases. Hyperphosphorylation of tau at several abnormal hyperphosphorylation sites was also observed, as seen in AD brain. These results suggest 1) that PP-2A down-regulates ERK1/2, MEK1/2, and p70 S6 kinase activities through dephosphorylation at the serine/threonine residues of these kinases, and 2) that in AD brain the decrease in PP-2A activity could have caused the activation of ERK1/2, MEK1/2, and p70 S6 kinase, and the abnormal hyperphosphorylation of tau both via an increase in its phosphorylation and a decrease in its dephosphorylation.

Regulated phosphorylation of 40S ribosomal protein S6 in root tips of maize

Ribosomal protein S6 (RPS6) is located in the mRNA binding site of the 40S subunit of cytosolic ribosomes. Two maize (Zea mays) rps6 genes were identified that encode polypeptides (30 kD, 11.4 pI) with strong primary amino acid sequence and predicted secondary structure similarity to RPS6 of other eukaryotes. Maize RPS6 was analyzed by the use of two-dimensional gel electrophoresis systems, in vivo labeling with [(32)P]P(i) and immunological detection. Nine RPS6 isoforms were resolved in a two-dimensional basic-urea/sodium dodecyl sulfate-polyacrylamide gel electrophoresis system. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry performed on trypsin-digested isoforms identified four serine (Ser) and one threonine (Thr) residue in the carboxy-terminal region as phosphorylation sites (RRS(238)KLS(241)AAAKAS(247)AAT(250)S(251)A-COOH). Heterogeneity in RPS6 phosphorylation was a consequence of the presence of zero to five phosphorylated residues. Phosphorylated isoforms fell into two groups characterized by (a) sequential phosphorylation of Ser-238 and Ser-241 and (b) the absence of phospho-Ser-238 and presence of phospho-Ser-241. The accumulation of hyper-phosphorylated isoforms with phospho-Ser-238 was reduced in response to oxygen deprivation and heat shock, whereas accumulation of these isoforms was elevated by cold stress. Salt and osmotic stress had no reproducible effect on RPS6 phosphorylation. The reduction in hyper-phosphorylated isoforms under oxygen deprivation was blocked by okadaic acid, a Ser/Thr phosphatase inhibitor. By contrast, the recovery of hyper-phosphorylated isoforms upon re-oxygenation was blocked by LY-294002, an inhibitor of phosphatidylinositol 3-kinases. Thus, differential activity of phosphatase(s) and kinase(s) determine complex heterogeneity in RPS6 phosphorylation.

Tumour suppressors hamartin and tuberin: intracellular signalling

Tumour suppressors hamartin and tuberin, encoded by tuberous sclerosis complex 1(TSC1) and TSC2 genes, respectively, are critical regulators of cell growth and proliferation. Mutations in TSC1 and TSC2 genes are the cause of an autosomal dominant disorder known as tuberous sclerosis complex (TSC). Another genetic disorder, lymphangioleiomyomatosis (LAM), is also associated with mutations in the TSC2 gene. Hamartin and tuberin control cell growth by negatively regulating S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1), potentially through their upstream modulator mammalian target of rapamycin (mTOR). Growth factors and insulin promote Akt/PKB-dependent phosphorylation of tuberin, which in turn, releases S6K1 from negative regulation by tuberin and results in the activation of S6K1. Although much has been written regarding the molecular genetics of TSC and LAM, which is associated with either the loss of or mutation in the TSC1 and TSC2 genes, few reviews have addressed the intracellular signalling pathways regulated by hamartin and tuberin. The current review will fill the gap in our understanding of their role in cellular signalling networks, and by improving this understanding, an integrated picture regarding the normal function of tuberin and hamartin is beginning to emerge.

Rapamycin-insensitive regulation of 4e-BP1 in regenerating rat liver

In cultured cells, growth factor-induced phosphorylation of two translation modulators, p70 S6 kinase and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), is blocked by nanomolar concentrations of the immunosuppressant rapamycin. Rapamycin also attenuates liver regeneration after partial hepatectomy, but it is not known if this growth-suppressive effect is due to dephosphorylation of p70 S6 kinase and/or 4E-BP1. We found that partial hepatectomy induced a transient increase in liver p70 S6 kinase activity and 4E-BP1 phosphorylation as compared with sham-operated rats. The amount of p70 S6 kinase protein in regenerating liver did not increase, but active kinase from partially hepatectomized animals was highly phosphorylated. Phosphorylated 4E-BP1 from regenerating liver was unable to form an inhibitory complex with initiation factor 4E. Rapamycin blocked the activation of p70 S6 kinase in response to partial hepatectomy in a dose-dependent manner, but 4E-BP1 phosphorylation was not inhibited. By contrast, functional phosphorylation of 4E-BP1 induced by injection of cycloheximide or growth factors was partially reversed by the drug. The mammalian target of rapamycin (mTOR) has been proposed to directly phosphorylate 4E-BP1. Western blot analysis using phospho-specific antibodies showed that phosphorylation of Thr-36/45 and Ser-64 increased in response to partial hepatectomy in a rapamycin-resistant manner. Thus, rapamycin inhibits p70 S6 kinase activation and liver regeneration, but not functional phosphorylation of 4E-BP1, in response to partial hepatectomy. These results indicate that the effect of rapamycin on 4E-BP1 function in vivo can be significantly different from its effect in cultured cells.

TGF-beta inhibits p70 S6 kinase via protein phosphatase 2A to induce G(1) arrest

On TGF-beta binding, the TGF-beta receptor directly phosphorylates and activates the transcription factors Smad2/3, leading to G(1) arrest. Here, we present evidence for a second, parallel, TGF-beta-dependent pathway for cell cycle arrest, achieved via inhibition of p70(s6k). TGF-beta induces association of its receptor with protein phosphatase-2A (PP2A)-Balpha. Concomitantly, three PP2A-subunits, Balpha, Abeta, and Calpha, associate with p70(s6k), leading to its dephosphorylation and inactivation. Although either pathway is sufficient to induce G(1) arrest, abrogation of both, the inhibition of p70(s6k), and transcription through Smad proteins is required for release of epithelial cells from TGF-beta-induced G(1) arrest. TGF-beta thereby modulates the translational and posttranscriptional control of cell cycle progression.

Synthesis of the translational apparatus is regulated at the translational level

The synthesis of many mammalian proteins associated with the translational apparatus is selectively regulated by mitogenic and nutritional stimuli, at the translational level. The apparent advantages of the regulation of gene expression at the translational level are the speed and the readily reversible nature of the response to altering physiological conditions. These two features enable cells to rapidly repress the biosynthesis of the translational machinery upon shortage of amino acids or growth arrest, thus rapidly blocking unnecessary energy wastage. Likewise, when amino acids are replenished or mitogenic stimulation is applied, then cells can rapidly respond in resuming the costly biosynthesis of the translational apparatus. A structural hallmark, common to mRNAs encoding many components of the translational machinery, is the presence of a 5' terminal oligopyrimidine tract (5'TOP), referred to as TOP mRNAs. This structural motif comprises the core of the translational cis-regulatory element of these mRNAs. The present review focuses on the mechanism underlying the translational control of TOP mRNAs upon growth and nutritional stimuli. A special emphasis is put on the pivotal role played by ribosomal protein S6 kinase (S6K) in this mode of regulation, and the upstream regulatory pathways, which might be engaged in transducing external signals into activation of S6K. Finally, the possible involvement of pyrimidine-binding proteins in the translational control of TOP mRNAs is discussed.

Role of S6 phosphorylation and S6 kinase in cell growth

This article reviews our current knowledge of the role of ribosomal protein S6 phosphorylation and the S6 kinase (S6K) signaling pathway in the regulation of cell growth and proliferation. Although 40S ribosomal protein S6 phosphorylation was first described 25 years ago, it only recently has been implicated in the translational up-regulation of mRNAs coding for the components of protein synthetic apparatus. These mRNAs contain an oligopyrimidine tract at their 5' transcriptional start site, termed a 5'TOP, which has been shown to be essential for their regulation at the translational level. In parallel, a great deal of information has accumulated concerning the identification of the signaling pathway and the regulatory phosphorylation sites involved in controlling S6K activation. Despite this knowledge we are only beginning to identify the direct upstream elements involved in growth factor-induced kinase activation. Use of the immunosupressant rapamycin, a bacterial macrolide, in conjunction with dominant interfering and activated forms of S6K1 has helped to establish the role of this signaling cascade in the regulation of growth and proliferation. In addition, current studies employing the mouse as well as Drosophila melanogaster have provided new insights into physiological function of S6K in the animal. Deletion of the S6K1 gene in mouse cells led to an animal of reduced size and the identification of the S6K1 homolog, S6K2, whereas loss of dS6K function in Drosophila demonstrated its paramount importance in development and growth control.

Protein ligands to HuR modulate its interaction with target mRNAs in vivo

AU-rich elements (AREs) present in the 3' untranslated regions of many protooncogene, cytokine, and lymphokine messages target them for rapid degradation. HuR, a ubiquitously expressed member of the ELAV (embryonic lethal abnormal vision) family of RNA binding proteins, selectively binds AREs and stabilizes ARE-containing mRNAs in transiently transfected cells. Here, we identify four mammalian proteins that bind regions of HuR known to be essential for its ability to shuttle between the nucleus and the cytoplasm and to stabilize mRNA: SETalpha, SETbeta, pp32, and acidic protein rich in leucine (APRIL). Three have been reported to be protein phosphatase 2A inhibitors. All four ligands contain long, acidic COOH-terminal tails, while pp32 and APRIL share a second motif: rev-like leucine-rich repeats in their NH(2)-terminal regions. We show that pp32 and APRIL are nucleocytoplasmic shuttling proteins that interact with the nuclear export factor CRM1 (chromosomal region maintenance protein 1). The inhibition of CRM1 by leptomycin B leads to the nuclear retention of pp32 and APRIL, their increased association with HuR, and an increase in HuR's association with nuclear poly(A)+ RNA. Furthermore, transcripts from the ARE-containing c-fos gene are selectively retained in the nucleus, while the cytoplasmic distribution of total poly(A)+ RNA is not altered. These data provide evidence that interaction of its ligands with HuR modulate HuR's ability to bind its target mRNAs in vivo and suggest that CRM1 is instrumental in the export of at least some cellular mRNAs under certain conditions. We discuss the possible role of these ligands upstream of HuR in pathways that govern the stability of ARE-containing mRNAs.

Treatment of mice with EGF and orthovanadate activates cytoplasmic and nuclear MAPK, p70S6k, and p90rsk in the liver

BACKGROUND/AIMS

Although signal transduction pathways activated by EGF have been extensively studied in cultured cells, few such studies have been done in whole animals. In this study, activation of hepatic kinases, phosphatases, and DNA-binding activity of AP-1 was examined after intraperitoneal injections of either EGF or sodium orthovanadate into mice.

METHODS

Cytoplasmic and nuclear proteins, extracted from isolated hepatocytes or whole liver tissue, were immunoprecipitated with either anti-ERK1/2, anti-70S6k, or anti-p90rsk antibodies and kinase activities were measured using specific substrates. Kinase protein levels was evaluated by Western blot analysis. AP-1 DNA binding activity was measured by electrophoretic mobility shift assay.

RESULTS

Systemic administration of EGF induced simultaneous increase in the activities of cytoplasmic and nuclear MAPK, p70S6k, and p90rsk. MAPK and p70S6k were more potently activated in the cytosol while p90rsk activation was more pronounced in the nucleus. Orthovanadate also activated these kinases but to a much lesser degree than EGF. In vitro phosphatase assays showed that neither EGF nor orthovanadate induced measurable changes in phosphatase activities. EGF, but not orthovanadate, activated nuclear AP-1 DNA-binding activity in intact liver, indicating that activation of MAPK, p70S6k, and p90rsk by orthovanadate is not sufficient to activate this transcription factor.

CONCLUSION

These observations provide groundwork for future studies to examine the role of EGF-induced kinase cascades and transcription factors in liver regeneration and other growth factor-mediated hepatic processes.

Ribosomal S6 kinase signaling and the control of translation

The highly homologous 40S ribosomal protein S6 kinases (S6K1 and S6K2) play a key role in the regulation of cell growth by controlling the biosynthesis of translational components which make up the protein synthetic apparatus, most notably ribosomal proteins. In the case of S6K1, at least eight phosphorylation sites are believed to mediate kinase activation in a hierarchical fashion. Activation is initiated by phosphatidylinositide-3OH kinase (PI3K)-mediated phosphorylation of key residues in the carboxy-terminus of the kinase, allowing phosphorylation of a critical residue residing in the activation loop of the catalytic domain by phosphoinositide-dependent kinase 1 (PDK1). The kinases responsible for phosphorylating the carboxy-terminal sites have yet to be identified. Additionally, S6 kinases are under the control of the PI3K relative, mammalian Target Of Rapamycin (mTOR), which may serve an additional function as a checkpoint for amino acid availability. In this review we set out to discuss the present state of knowledge regarding upstream signaling components which have been implicated in the control of S6K1 activation and the role of the kinase in controlling cell growth through regulating ribosome biogenesis at the translational level.

Regulation of protein kinase cascades by protein phosphatase 2A

Many protein kinases themselves are regulated by reversible phosphorylation. Upon cell stimulation, specific kinases are transiently phosphorylated and activated. Several of these protein kinases are substrates for protein phosphatase 2A (PP2A), and PP2A appears to be the major kinase phosphatase in eukaryotic cells that downregulates activated protein kinases. This idea is substantiated by the observation that some viral proteins and naturally occurring toxins target PP2A and modulate its activity. There is increasing evidence that PP2A activity is regulated by extracellular signals and during the cell cycle. Thus, PP2A is likely to play an important role in determining the activation kinetics of protein kinase cascades.

Identification of kinase-phosphatase signaling modules composed of p70 S6 kinase-protein phosphatase 2A (PP2A) and p21-activated kinase-PP2A

A growing body of evidence indicates that regulation of protein-serine/threonine phosphatase 2A (PP2A) involves its association with other cellular and viral proteins in multiprotein complexes. PP2A-containing protein complexes may exist that contribute to PP2A's important regulatory role in many cellular processes. To identify such protein complexes, PP2A was partially purified from rat brain soluble extracts following treatment with a reversible cross-linker to stabilize large molecular size forms of PP2A. Compared with native (uncross-linked) PP2A, cross-linked PP2A revealed an enrichment of p70 S6 kinase and two p21-activated kinases (PAK1 and PAK3) in the PP2A complex, indicating these kinases may associate with PP2A. The existence of protein kinase-PP2A complexes in rat brain soluble extracts was further substantiated by the following results: 1) independent immunoprecipitation of the kinases revealed that PP2A co-precipitated with p70 S6 kinase and the two PAK isoforms; 2) glutathione S-transferase fusion proteins of p70 S6 kinase and PAK3 each isolated PP2A; and 3) PAK3 and p70 S6 kinase bound to microcystin-Sepharose (an affinity resin for PP2A-PP1). Cumulatively, these findings provide evidence for association of PP2A with p70 S6 kinase, PAK1, and PAK3 in the context of the cellular environment. Moreover, together with the recent reports describing associations of PP2A with Ca2+/calmodulin-dependent protein kinase IV (Westphal, R. S., Anderson, K. A., Means, A. R., and Wadzinski, B. E. (1998) Science 280, 1258-1261) and casein kinase IIalpha (Heriche, J. K., Lebrin, F., Rabilloud, T., Leroy, D., Chambaz, E. M., and Goldberg, Y. (1997) Science 276, 952-955), the present data provide compelling evidence for the existence of protein kinase-PP2A signaling modules as a new paradigm for the control of various intracellular signaling cascades.

Cloning and characterization of p70(S6K beta) defines a novel family of p70 S6 kinases

The human cDNA encoding a novel protein serine/threonine kinase most closely related to p70 S6 kinase was isolated from the human erythroleukemia cDNA library and termed p70(S6Kbeta). p70(S6Kbeta) has 67% amino acid identity in overall sequence with human p70(S6K), and the potential phosphorylation sites of p70(S6K) are conserved in p70(S6Kbeta). Northern blot analysis identified two major transcripts of p70(S6Kbeta) that are ubiquitously expressed in human adult tissues. Similar to p70(S6K), p70(S6Kbeta) was activated by serum stimulation, and the serum-induced activation was inhibited by wortmannin and rapamycin. These findings suggest that p70(S6Kbeta) is an isoform of p70(S6K) with similar regulatory mechanisms.

Regulation of p70s6k/p85s6k and its role in the cell cycle

Two to three-fold increases in the rate of protein synthesis are required both to enter the G1 phase of the cell cycle from G0 and to proceed to S phase in response to growth factors and mitogens. This increase is in part regulated via multiple phosphorylation of the 40S ribosomal protein S6 by the mitogen-stimulated p70s6k/p85s6k. At the protein synthesis level this event appears to be involved in specifically increasing the efficiency of translation of a family of essential mRNAs containing a polypyrimidine tract at their 5' transcriptional start site. The activation of p70s6k/p85s6k and maintenance of its activity throughout G1 is controlled via multiple phosphorylation events mediated by a complex signalling network acting on distinct sets of phosphorylation sites.

Stimulation of p70S6 kinase via a growth hormone-controlled phosphatidylinositol 3-kinase pathway leads to the activation of a PDE4A cyclic AMP-specific phosphodiesterase in 3T3-F442A preadipocytes

The challenge of 3T3-F442A fibroblasts with growth hormone led to both a decrease in the mobility on SDS/PAGE and activation of the PDE4A cyclic AMP-specific phosphodiesterase isoform PDE4A5. Activation was mediated by a JAK-2-dependent pathway coupled to the activation of phosphatidylinositol 3-kinase and p70S6 kinase. Activation was not dependent on the ability of growth hormone to stimulate ERK2 or protein kinase C or any effect on transcription. Blockade of activation of murine PDE4A5 ablated the ability of growth hormone to decrease intracellular cAMP levels. Antisense depletion of murine PDE4A5 mimicked the ability of rolipram to enhance the growth hormone-stimulated differentiation of 3T3-F442A cells to adipocytes. It is suggested that activation of PDE4A5 by growth hormone serves as a brake on the differentiation processes.

Effect of rapamycin on prolactin-stimulated S6 kinase activity and milk product formation in mouse mammary explants

The in vitro effects of rapamycin on prolactin (PRL)-stimulated S6 kinase activity and milk product synthesis were investigated in cultured mouse mammary tissues. Mouse mammary gland explants were initially incubated for 24 h in M199 media containing 1 microgram/ml insulin and 10(-7) M cortisol. A subsequent treatment of the tissues with 1 microgram/ml PRL for 12 h caused a 98% increase in S6 kinase activity in the cytosolic fraction; effects were not observed at earlier times. PRL at or above 500 ng/ml was needed to elicit a maximum stimulation of S6 kinase activity, and this response was specific for lactogenic hormones (PRL, hPL, hGH). Rapamycin, an inhibitor of 70 K S6 kinase, was employed to assess the possible physiological role of S6 kinase in the PRL stimulation of milk product formation. Rapamycin (25-100 ng/ml) impeded, in a dose-dependent fashion, the PRL stimulation of casein, lipid and lactose synthesis in concert with its inhibition of cytoplasmic S6 kinase activity. These results suggest a possible role for the activation of 70 K kinase in the signalling pathway for the PRL regulation of milk product synthesis in the mammary gland.

Induction of down-regulation of the kinase activities of Mek, p42Erk, p90RSK, and p63SAMK in chicken embryo fibroblast at the late stage of src-induced cellular transformation

Two distinct stages in regulation of protein kinases are detectable upon cellular transformation of CEF induced by pp60v-src. Upon cellular transformation induced by ts v-src mutants, the kinase activities of Mek and p42Erk are rapidly induced at the early stage and significantly decreased at the late stage of cellular transformation. In contrast, a novel p63SAMK is partially activated at the early stage and is fully activated at the late stage of cellular transformation. However, p90RSK is activated through the entire course of cellular transformation. In this study, I detect a transient down-regulation of p90RSK activity that is inducible in cultures at the late stage of the src-induced cellular transformation by an increase of extracellular pH value from 7 to 8 and unidentified components in DMEM, but not in cultures which are at the early stage. Concomitant with down-regulation of p90RSK activity, the kinase activities of Mek, p42Erk, and p63SAMK are also down-regulated. Blockage of down-regulation of p90RSK activity by pretreatment of cells with different phosphatase inhibitors correlates with blockage of the down-regulation of either p42Erk or p63SAMK activity. Multiple pathways appear to involve in regulation of p90RSK activity. The discrepancy in regulation of protein kinase activity between the early and late stages of cellular transformation induced by pp60src may indicate a change in signaling cascades during the progress of cellular transformation. The induction of the down-regulation event in this study may provide a new approach to investigate the regulation not only of protein kinases but also phosphatases in transformed cells.

Cell cycle regulation of p70 S6 kinase and p42/p44 mitogen-activated protein kinases in Swiss mouse 3T3 fibroblasts

We show here using synchronized Swiss mouse 3T3 fibroblasts that p70 S6 kinase (p70S6k) and mitogen-activated protein kinases (p42mapk/p44mapk) are not only activated at the G0/G1 boundary, but also in cells progressing from M into G1. p70S6k activity increases 20-fold in G1 cells released from G0. Throughout G1, S, and G2 it decreases constantly, so that during M phase low kinase activity is measured. The kinase is reactivated 10-fold when cells released from a nocodazole-induced metaphase block enter G1 of the next cell cycle. p42mapk/p44mapk in G0 cells are activated transiently early in G1 and are reactivated late in mitosis after nocodazole release. p70S6k activity is dependent on permanent signaling from growth factors at all stages of the cell cycle. Immunofluorescence studies showed that p70S6k and its isoform p85S6k become concentrated in localized spots in the nucleus at certain stages in the cell cycle. Cell cycle-dependent changes in p70S6k activity are associated with alterations in the phosphorylation state of the protein. However, examination of the regulation of a p70S6k mutant in which the four carboxyl-terminal phosphorylation sites are changed to acidic amino acids suggests that a mechanism independent of these phosphorylation sites controls the activity of the enzyme during the cell cycle.

Protein phosphatase activity is required for prothoracicotropic hormone-stimulated ecdysteroidogenesis in the prothoracic glands of the tobacco hornworm, Manduca sexta

The multiple phosphorylation of ribosomal protein S6 appears to be required for prothoracicotropic hormone (PTTH)-stimulated protein synthesis and ecdysteroidogenesis by the prothoracic glands of Manduca sexta. The present study investigated the role of protein phosphatase in these phenomena by analyzing the effects of pretreatment of prothoracic glands with the phosphatase inhibitors okadaic acid and calyculin A in both basal and PTTH-stimulated glands. Okadaic acid or calyculin A treatment enhanced ribosomal S6 phosphorylation in control glands to a level similar to that observed with PTTH-stimulated glands. This treatment also prevented S6 dephosphorylation but had no apparent synergistic effect on S6 phosphorylation in PTTH-stimulated glands. Most importantly, okadaic acid or calyculin A treatment inhibited, rather than augmented, ecdysteroidogenesis in both PTTH-stimulated and non-stimulated glands. The composite data suggest that protein phosphatase activity sensitive to okadaic acid or calyculin A is required for PTTH-stimulated ecdysteroidogenesis.

Activation of p70 S6 kinase and erk-encoded mitogen-activated protein kinases is resistant to high cyclic nucleotide levels in Swiss 3T3 fibroblasts

Treatment of Swiss mouse 3T3 fibroblasts with certain cyclic nucleotide phosphodiesterase inhibitors (theophylline, SQ 20,006, and MY-5445) prevents the activation of the M(r) 70,000 S6 kinase (p70S6k) induced by a variety of external stimuli. Concentrations giving half-maximal inhibition were 800, 50, and 25 microM, respectively. Western blot analysis and immunocomplex kinase assays showed that these compounds inhibit the phosphorylation and activation of p70S6k without affecting the erk-encoded mitogen-activated protein (MAP) kinases or the rsk-encoded S6 kinase (p90rsk). A distinct collection of cAMP and cGMP agonists and analogues did not suppress p70S6k activation, indicating that 1) high intracellular cyclic nucleotide concentrations do not antagonize the p70S6k pathway and 2) phosphodiesterase inhibitors block p70S6k activation by a mechanism that is independent of cAMP or cGMP production. The effect of theophylline and SQ 20,006, but not MY-5445, on p70S6k signaling may be due in part to the inhibition of a phosphatidylinositol 3-kinase that acts upstream of p70S6k. Finally, in contrast to many other cell types, cAMP and cGMP were also found to have no inhibitory effect on the MAP kinase/p90rsk signaling pathway in Swiss 3T3 fibroblasts.

Rapamycin, wortmannin, and the methylxanthine SQ20006 inactivate p70s6k by inducing dephosphorylation of the same subset of sites

Activation of p70s6k in cells stimulated with serum correlates with the phosphorylation of seven sites. Pretreatment of Swiss 3T3 cells with the immunosuppressant rapamycin blocks phosphorylation of four of these sites (Thr229, Thr389, Ser404, and Ser411), whereas phosphorylation proceeds in the remaining three sites (Ser418, Thr421, and Ser424). If rapamycin is added postserum stimulation, the pattern of phosphorylation is qualitatively similar except that Ser411 is still highly phosphorylated. The inhibitory effect of rapamycin on serum-induced p70s6k activation and the phosphorylation of Thr229, Thr389, Ser404, and Ser411 is rescued by FK506, providing further evidence that the inhibitory effect is exerted through a complex of rapamycin-FKBP12. Wortmannin treatment pre- or post-serum stimulation inhibits phosphorylation of the same set of sites as rapamycin, supporting the argument that both agents act on the same pathway. Likewise, methylxanthine phosphodiesterase inhibitors block p70s6k activation and phosphorylation of the same set of sites as wortmannin and rapamycin. However, other agents that raise intracellular cAMP levels have no inhibitory effect, leading to the hypothesis that the inhibitory actions of methylxanthines on p70s6k activity are not through activating protein kinase A but through inhibition of an upstream kinase. Together the results indicate that there are two kinase signaling pathways that must converge to activate p70s6k and that only one of these pathways is sensitive to rapamycin, wortmannin, and methylxanthine inhibition.

Purification and characterization of multiple S6 phosphatases from the rat parotid gland

S6 phosphatase activities, which dephosphorylate the phosphorylated S6 synthetic peptide, RRLSSLRASTSKSESSQK, were purified to near homogeneity from the membrane and cytosolic fractions of the rat parotid gland. Multiple S6 phosphatases were fractionated on Mono Q and gel filtration columns. In the cytosolic fraction, at least three forms of S6 phosphatase, termed peaks I, II, and III, were differentially resolved. The three forms had different sizes and protein compositions. The peak I enzyme, which had an approximately Mr of 68 kDa on gel filtration, appears to represent a dimeric form of the 39 kDa protein. This S6 phosphatase showed the high activity in the presence of EGTA and was completely inhibited by nanomolar concentrations of either okadaic acid or inhibitor 2. The peak II S6 phosphatase enzyme, with an Mr of 35 kDa, was activated by Mn2+. This form could be a proteolytic product of the catalytic subunit of type 1 phosphatase, due to its sensitivities to okadaic acid and inhibitor 2. The peak III enzyme, with an Mr of 55 kDa, is a Mn(2+)-dependent S6 phosphatase. This S6 phosphatase can be classified as a type 1 phosphatase, due to its sensitivity to okadaic acid, since the IC50 of okadaic acid is 4 nM. However, the molecular mass of this S6 phosphatase differs from that of the type 1 catalytic subunit (37 kDa) and showed less sensitivity to inhibitor 2. On the other hand, the membrane fraction contained one form of the S6 phosphatases, termed peak V (Mr 34 and 28 kDa), which could be classified as a type 1 phosphatase. This S6 phosphatase activity was greatly stimulated by Mn2+.

Selective inhibition of p70 S6 kinase activation by phosphatidylinositol 3-kinase inhibitors

Treatment of fibroblasts with wortmannin or demethoxyviridin, two potent inhibitors of phosphatidylinositol 3-kinase, prevents the activation of ribosomal protein S6 kinase, which is induced by a variety of external stimuli. Concentrations giving 50% inhibition of 45 nM (wortmannin) and 400 nM (demethoxyviridin) were obtained when epidermal growth factor was used as an S6 kinase activator; with platelet-derived growth factor, the concentrations giving 50% inhibition were about three-times higher. Western-blot analysis and immunocomplex kinase assays showed that wortmannin and demethoxyviridin specifically block the phosphorylation and activation of p70 S6 kinase without affecting the M(r) 90,000 ribosomal S6 kinase (p90rsk) or mitogen-activated protein kinases. Consistent with the irreversible nature of the inhibition of phosphatidylinositol 3-kinase by these compounds, treatment of cells with wortmannin, followed by washing out of the inhibitor, still led to inhibition of p70 S6 kinase activation. Several S6 kinase agonists not previously known to activate phosphatidylinositol 3-kinase (A23187, bombesin and phorbol 12-myristate 13-acetate) were found to increase the production of phosphatidylinositol 3,4,5-trisphosphate in a wortmannin-sensitive manner. These results support a model in which phosphatidylinositol 3-kinase acts upstream of p70 S6 kinase in a mitogenic signalling pathway; the existence of a phosphatidylinositol 3-kinase-independent pathway is also evident.

Role of p70 S6 protein kinase in angiotensin II-induced protein synthesis in vascular smooth muscle cells

Angiotensin II (AII) is a growth factor which induces cellular hypertrophy in cultured vascular smooth muscle cells (SMC). To understand the molecular basis of this action, we have examined the role of the 70-kDa S6 kinases (p70S6K) in the hypertrophic response to AII in aortic SMC. AII potently stimulated the phosphotransferase activity of p70S6K, which reached a maximal value at 15 min and persisted for at least 4 h. This response was completely abolished when the cells were incubated in the presence of the AT1-selective receptor antagonist losartan. The enzymatic activation of p70S6K was associated with increased phosphorylation of the enzyme on serine and threonine residues. The immunosuppressant drug rapamycin was found to selectively inhibit the activation of p70S6K by AII, but not the activation of mitogen-activated protein kinase or the induction of c-fos mRNA expression. Treatment of aortic SMC with rapamycin also potently inhibited AII-stimulated protein synthesis with a half-maximal concentration similar to that required for inhibition of p70S6K. These results provide strong evidence that p70S6K plays a critical role in the signaling pathways by which AII induces hypertrophy of vascular SMC.

S6 phosphorylation and the p70s6k/p85s6k

Activation of cell growth leads to the multiple phosphorylation of 40S ribosomal protein S6. The kinase responsible for controling this event is termed p70s6k/p85s6k. Both isoforms of the kinase are derived from a common gene activated by a complex set of phosphorylation events; each resides in a unique cellular compartment: the p70s6k in the cytoplasm and the p85s6k in the nucleus. Although p70s6k/p85s6k represent the first mitogen-activated serine/threonine kinase described, the signaling pathway leading to activation of both isoforms remains obscure. Recent studies have shown that this pathway is distinct from that of p21ras and the p42mapk/p44mapk, and that bifurcation of these pathways takes place at the level of the receptor. Experiments with point mutants of the PDGF receptor and inhibitors of phosphatidyl-inositol-3-OH kinase have implicated the latter molecule in this signaling event, but more recent findings suggest an alternative route may be employed. The p70s6k signaling pathway can also be ablated by the immunosuppressant rapamycin, which blocks p70s6k activation and S6 phosphorylation without affecting the other kinases whose activation is triggered by mitogen treatment. In parallel, rapamycin suppresses the translation of a family of mRNAs that contain a polypyrimidine tract at their 5' transcriptional start site. The implication is that this event is mediated by the phosphorylated form of S6 that may either (1) directly interact with the polypyrimidine tract or (2) alter the affinity of the 40S ribosome mRNA binding site for polypyrimidine tract mRNAs, or (3) recognize proteins that directly bind to the polypyrimidine tract.

Serine/threonine protein phosphatases in the control of cell function

Reversible protein phosphorylation is a fundamental mechanism by which many biological functions are regulated. Achievement of such control requires the coordinated action of the interconverting enzymes, the protein kinases and protein phosphatases. By comparison with protein kinases, a limited number of protein phosphatase catalytic subunits are present in the cell, which raises the question of how such a small number of dephosphorylating enzymes can counterbalance the action of the more numerous protein kinases. In mammalian cells, four major classes of Ser/Thr-specific phosphatase catalytic subunits have been identified, comprising two distinct gene families. The high degree of homology among members of the same family, PP1, PP2A and PP2B, and the high degree of evolutionary conservation between organisms as divergent as mammals and yeast, implies that these enzymes are involved in fundamental cell functions. Type 1 enzymes appear to acquire specificity by association with targeting regulatory subunits which direct the enzymes to specific cellular compartments, confer substrate specificity and control enzyme activity. In spite of the progress made in determining the structure of the PP2A subunits, very little is known about the control of this activity and about substrate selection. Recent studies have unravelled a significant number of regulatory subunits. The potential existence of five distinct B or B-related polypeptides, some of which are present in multiple isoforms, two A and two C subunit isoforms, raises the possibility that a combinatorial association could generate a large number of specific PP2A forms with different substrate specificity and/or cellular localization. Moreover, biochemical, biological and genetic studies all concur in suggesting that the regulatory subunits may play an important role in determining the properties of the Ser/Thr protein phosphatases and hence their physiological functions.

On the importance of protein phosphorylation in cell cycle control

This chapter is written as a contribution to a volume commemorating the work of Krebs and Fischer that led to awarding of the Nobel Prize in 1992. This award was made because of their fundamental discovery in the mid-1950s that protein phosphorylation was the underlying mechanism that accounted for the reversible modification of activity of glycogen phosphorylase in mammalian skeletal muscle. Although it could not be anticipated at the time that phosphorylation would turn out to be such a ubiquitous regulator of cellular functions, it is now evident that phosphorylation controls virtually every important reaction in cells and provides the basis for understanding how integrated cellular behavior is regulated by both extracellular signals and internal control mechanisms. This chapter relates the historical development in biochemical terms of protein phosphorylation as a regulator of the cell cycle in Xenopus oocytes and eggs.

The MAP kinase cascade. Discovery of a new signal transduction pathway

Using biochemical techniques similar to those used by Krebs and Fischer in elucidating the cAMP kinase cascade, a protein kinase cascade has been found that represents a new pathway for signal transduction. This pathway is activated in almost all cells that have been examined by many different growth and differentiation factors, suggesting control of different cell responses. At this writing, four tiers of growth factor regulated kinases, each tier represented by more than one enzyme, have been reconstituted in vitro to form the MAP kinase cascade. Preliminary findings suggesting multiple feedback or feedforward regulation of several components in the cascade predict higher complexity than a simple linear pathway.

The regional distribution of extracellularly regulated kinase-1 and -2 messenger RNA in the adult rat central nervous system

It has previously been shown that an intracellular serine/threonine kinase known as extracellularly signal-regulated kinase, also known as microtubule-associated protein kinase, is phosphorylated and activated in response to a range of hormones, growth factors (e.g. nerve growth factor) and neurotransmitters (e.g. N-methyl-D-aspartate) in a variety of cells including neurons. Extracellularly regulated kinases phosphorylate transcription factors, cytoskeletal proteins and enzyme targets. As such they are believed to function in neuronal signal transduction. In situ hybridization histochemistry using synthetic oligonucleotide probes has been used to identify cells in the adult rat central nervous system containing messenger RNAs coding for two isoforms of extracellularly regulated kinase. Extracellularly regulated kinase-2 messenger RNA was observed in many regions including the cerebral cortex, olfactory bulb, hippocampus, amygdala, basal ganglia (except the globus pallidus and endopeduncular nucleus), basal nucleus, thalamus, hypothalamus, brain stem nuclei, cerebellum and neurons in the spinal cord. Extracellularly regulated kinase-1 messenger RNA was confined to fewer regions than extracellularly regulated kinase-2 messenger RNA. Hybridization signals for extracellularly regulated kinase-1 were seen in the olfactory bulb, cortex, regions of the hippocampus, amygdala, nucleus basalis of Maynert, substantia nigra, some hypothalamic and brainstem nuclei and cerebellum, as well as neurons of the spinal cord. Of particular interest, extracellularly regulated kinase-1 messenger RNA was absent from all regions of the basal ganglia and thalamus. Furthermore, extracellularly regulated kinase-1 was almost absent from the CA1 region, whereas extracellularly regulated kinase-2 was present in all neurons of the hippocampus. There were no CNS regions that expressed extracellularly regulated kinase-1 but not extracellularly regulated kinase-2; however, neurons of the dorsal root ganglia showed extracellularly regulated kinase-1 but not extracellularly regulated kinase-2 messenger RNA. Although extracellularly regulated kinase-1 and extracellularly regulated kinase-2 expression was selectively neuronal in the brain, extracellularly regulated kinase-1 messenger RNA was localized to glia in the spinal cord. The distinct cellular distribution of individual extracellularly regulated kinases in the adult rat CNS suggests that they play unique signalling roles.

Co-translational protein import into mitochondria: an alternative view

Recent in vitro and in vivo experiments suggest that the synthesis and import of mitochondrial proteins are very tightly coupled and that a co-translational import reaction may be mandatory for some proteins. These results are entirely consistent with early experiments which suggested that import occurs co-translationally and that cytosolic polysomes synthesizing mitochondrial proteins are bound to protein import sites on isolated mitochondria. This article discusses and seemingly contradictory reports concerning the involvement of co-translational and post-translational mechanisms in the import process and examines the impact of recent developments in the field.

Interleukin-3 inhibits cycloheximide induction of C-jun mRNA in human monocytes: possible role for a serine/threonine phosphatase

Cycloheximide is a strong inducer of the c-jun protooncogene mRNA at concentrations (< or = 50 ng/ml) that do not inhibit protein synthesis in human monocytes. This induction is transient lasting 30-60 min in contrast to the sustained induction obtained with concentrations that inhibit protein synthesis. The pluripotent colony stimulating factor interleukin-3 (IL-3) (10 ng/ml) is also a modest inducer of the c-jun gene in these cells; however, in combination with cycloheximide, IL-3 dramatically reduces the c-jun induction below levels induced by cycloheximide alone. This is a true inhibition and is not due to a change in temporal kinetics of induction because the suppression in the presence of IL-3 is observed at both 30 and 60 min after simultaneous addition of both IL-3 and cycloheximide. Preincubation of monocytes with 12.5 nM okadaic acid (a potent inhibitor of protein phosphatases 1 and 2A) and cycloheximide prior to addition of IL-3 restored the level of c-jun induction to that mediated by cycloheximide alone. This concentration of okadaic acid inhibited almost 70% of the phosphorylase phosphatase activity in monocyte lysates. These observations suggest that activation of protein serine/threonine phosphatase(s) underlies the ability of IL-3 to inhibit cycloheximide induction of c-jun in monocytes.