PDGF induces an early and a late wave of PI 3-kinase activity, and only the late wave is required for progression through G1

The effect of cilostazol on the platelet-derived growth factor-beta/beta isoform reduction on venous hyperplasia in an experimental balloon-induced injury model

BACKGROUND

Intimal hyperplasia is the response to endothelial injury. Platelet-derived growth factor is released early and favors the formation of intimal hyperplasia. Although multiple treatments, from open surgery to endovascular techniques, have been used they remain controversial. There is currently interest in developing pharmacological strategies to address this pathology. Local vascular inflammation induced by vessel barotrauma generates intimal hyperplasia due to mechanical stress over the venous endothelium. Cilostazol is a selective phosphodiesterase type 3 (PDE3) selective inhibitor with a regulatory effect over intimal hyperplasia. The objective was to investigate cilostazol's role in inhibiting smooth muscle cell proliferation due to changes in the expression and release of PDGF-BB isoform and the effect on developing IH using an experimental model of vascular barotrauma (balloon-induced injury model).

METHODS

We included 12 New Zealand rabbits. The balloon-induced injury model (BIIM) and experimental group cilostazol (20 mg/kg/day) included 6 rabbits each. Contralateral veins from 6 rabbits used in BIIM model has been taken as control group. We measured and compared the expression of PDGF-BB and the development of IH. A pathologist board chooses a PDGFRα antibody to localized its expression by immunohistochemistry analysis. Subsequently, using an automated immunohistochemical staining machine, the PDGFR expression was evaluated using a Zeiss Primo Star 4 light microscope.

RESULTS

The measurement obtained in the intimal layer was: 126.12 μm2 in the CG, 232 μm2 in the BIIM group, and 178 μm2 in the EG. A statistically significant difference was observed. Baseline serum concentrations of PDGF-BB in the BIIM group were 0.22 pg/mL. At 12 h 0.42 pg/mL, and 0.17 pg/mL at seven days. In the experimental group, the basal levels were 0.33 pg/mL. With the use of cilostazol, a lower peak was obtained at 12 h (0.08 pg/mL). This difference was statistically significant.

CONCLUSIONS

Cilostazol induced a significant reduction of IH caused by barotrauma in the venous endothelium, which correlates with decrease in the PDGF-BB in serum. This could be attributed to the pharmacologic effect on PDGFR expression.

Relating individual cell division events to single-cell ERK and Akt activity time courses

Biochemical correlates of stochastic single-cell fates have been elusive, even for the well-studied mammalian cell cycle. We monitored single-cell dynamics of the ERK and Akt pathways, critical cell cycle progression hubs and anti-cancer drug targets, and paired them to division events in the same single cells using the non-transformed MCF10A epithelial line. Following growth factor treatment, in cells that divide both ERK and Akt activities are significantly higher within the S-G2 time window (~ 8.5-40 h). Such differences were much smaller in the pre-S-phase, restriction point window which is traditionally associated with ERK and Akt activity dependence, suggesting unappreciated roles for ERK and Akt in S through G2. Simple metrics of central tendency in this time window are associated with subsequent cell division fates. ERK activity was more strongly associated with division fates than Akt activity, suggesting Akt activity dynamics may contribute less to the decision driving cell division in this context. We also find that ERK and Akt activities are less correlated with each other in cells that divide. Network reconstruction experiments demonstrated that this correlation behavior was likely not due to crosstalk, as ERK and Akt do not interact in this context, in contrast to other transformed cell types. Overall, our findings support roles for ERK and Akt activity throughout the cell cycle as opposed to just before the restriction point, and suggest ERK activity dynamics may be more important than Akt activity dynamics for driving cell division in this non-transformed context.

Regulation of Cell Cycle Progression by Growth Factor-Induced Cell Signaling

The cell cycle is the series of events that take place in a cell, which drives it to divide and produce two new daughter cells. The typical cell cycle in eukaryotes is composed of the following phases: G1, S, G2, and M phase. Cell cycle progression is mediated by cyclin-dependent kinases (Cdks) and their regulatory cyclin subunits. However, the driving force of cell cycle progression is growth factor-initiated signaling pathways that control the activity of various Cdk–cyclin complexes. While the mechanism underlying the role of growth factor signaling in G1 phase of cell cycle progression has been largely revealed due to early extensive research, little is known regarding the function and mechanism of growth factor signaling in regulating other phases of the cell cycle, including S, G2, and M phase. In this review, we briefly discuss the process of cell cycle progression through various phases, and we focus on the role of signaling pathways activated by growth factors and their receptor (mostly receptor tyrosine kinases) in regulating cell cycle progression through various phases.

The Importance of Being PI3K in the RAS Signaling Network

Ras proteins are essential mediators of a multitude of cellular processes, and its deregulation is frequently associated with cancer appearance, progression, and metastasis. Ras-driven cancers are usually aggressive and difficult to treat. Although the recent Food and Drug Administration (FDA) approval of the first Ras G12C inhibitor is an important milestone, only a small percentage of patients will benefit from it. A better understanding of the context in which Ras operates in different tumor types and the outcomes mediated by each effector pathway may help to identify additional strategies and targets to treat Ras-driven tumors. Evidence emerging in recent years suggests that both oncogenic Ras signaling in tumor cells and non-oncogenic Ras signaling in stromal cells play an essential role in cancer. PI3K is one of the main Ras effectors, regulating important cellular processes such as cell viability or resistance to therapy or angiogenesis upon oncogenic Ras activation. In this review, we will summarize recent advances in the understanding of Ras-dependent activation of PI3K both in physiological conditions and cancer, with a focus on how this signaling pathway contributes to the formation of a tumor stroma that promotes tumor cell proliferation, migration, and spread.

An endogenous PI3K interactome promoting astrocyte-mediated neuroprotection identifies a novel association with RNA-binding protein ZC3H14

Astrocytes can support neuronal survival through a range of secreted signals that protect against neurotoxicity, oxidative stress, and apoptotic cascades. Thus, analyzing the effects of the astrocyte secretome may provide valuable insight into these neuroprotective mechanisms. Previously, we characterized a potent neuroprotective activity mediated by retinal astrocyte conditioned media (ACM) on retinal and cortical neurons in metabolic stress models. However, the molecular mechanism underlying this complex activity in neuronal cells has remained unclear. Here, a chemical genetics screen of kinase inhibitors revealed phosphoinositide 3-kinase (PI3K) as a central player transducing ACM-mediated neuroprotection. To identify additional proteins contributing to the protective cascade, endogenous PI3K was immunoprecipitated from neuronal cells exposed to ACM or control media, followed by MS/MS proteomic analyses. These data pointed toward a relatively small number of proteins that coimmunoprecipitated with PI3K, and surprisingly only five were regulated by the ACM signal. These hits included expected PI3K interactors, such as the platelet-derived growth factor receptor A (PDGFRA), as well as novel RNA-binding protein interactors ZC3H14 (zinc finger CCCH-type containing 14) and THOC1 (THO complex protein 1). In particular, ZC3H14 has recently emerged as an important RNA-binding protein with multiple roles in posttranscriptional regulation. In validation studies, we show that PI3K recruitment of ZC3H14 is necessary for PDGF-induced neuroprotection and that this interaction is present in primary retinal ganglion cells. Thus, we identified a novel non-cell autonomous neuroprotective signaling cascade mediated through PI3K that requires recruitment of ZC3H14 and may present a promising strategy to promote astrocyte-secreted prosurvival signals.

Inhibitory signaling in mammalian olfactory transduction potentially mediated by Gαo

Olfactory GPCRs (ORs) in mammalian olfactory receptor neurons (ORNs) mediate excitation through the Gα_s family member Gα_olf. Here we tentatively associate a second G protein, Gα_o, with inhibitory signaling in mammalian olfactory transduction by first showing that odor evoked phosphoinositide 3-kinase (PI3K)-dependent inhibition of signal transduction is absent in the native ORNs of mice carrying a conditional OMP-Cre based knockout of Gα_o. We then identify an OR from native rat ORNs that are activated by octanol through cyclic nucleotide signaling and inhibited by citral in a PI3K-dependent manner. We show that the OR activates cyclic nucleotide signaling and PI3K signaling in a manner that reflects its functionality in native ORNs. Our findings lay the groundwork to explore the interesting possibility that ORs can interact with two different G proteins in a functionally identified, ligand-dependent manner to mediate opponent signaling in mature mammalian ORNs.

The (+)-Brevipolide H Displays Anticancer Activity against Human Castration-Resistant Prostate Cancer: The Role of Oxidative Stress and Akt/mTOR/p70S6K-Dependent Pathways in G1 Checkpoint Arrest and Apoptosis

Because conventional chemotherapy is not sufficiently effective against prostate cancer, various examinations have been performed to identify anticancer activity of naturally occurring components and their mechanisms of action. The (+)-brevipolide H, an α-pyrone-based natural compound, induced potent and long-term anticancer effects in human castration-resistant prostate cancer (CRPC) PC-3 cells. Flow cytofluorometric analysis with propidium iodide staining showed (+)-brevipolide H-induced G1 arrest of cell cycle and subsequent apoptosis through induction of caspase cascades. Since Akt/mTOR pathway has been well substantiated in participating in cell cycle progression in G1 phase, its signaling and downstream regulators were examined. Consequently, (+)-brevipolide H inhibited the signaling pathway of Akt/mTOR/p70S6K. The c-Myc inhibition and downregulation of G1 phase cyclins were also attributed to (+)-brevipolide H action. Overexpression of myristoylated Akt significantly rescued mTOR/p70S6K and downstream signaling under (+)-brevipolide H treatment. ROS and Ca²⁺, two key mediators in regulating intracellular signaling, were determined, showing that (+)-brevipolide H interactively induced ROS production and an increase of intracellular Ca²⁺ levels. The (+)-Brevipolide H also induced the downregulation of anti-apoptotic Bcl-2 family proteins (Bcl-2 and Bcl-xL) and loss of mitochondrial membrane potential, indicating the contribution of mitochondrial dysfunction to apoptosis. In conclusion, the data suggest that (+)-brevipolide H displays anticancer activity through crosstalk between ROS production and intracellular Ca²⁺ mobilization. In addition, suppression of Akt/mTOR/p70S6K pathway associated with downregulation of G1 phase cyclins contributes to (+)-brevipolide H-mediated anticancer activity, which ultimately causes mitochondrial dysfunction and cell apoptosis. The data also support the biological significance and, possibly, clinically important development of natural product-based anticancer approaches.

Cracking the context-specific PI3K signaling code

Specificity in signal transduction is determined by the ability of cells to "encode" and subsequently "decode" different environmental signals. Akin to computer software, this "signaling code" governs context-dependent execution of cellular programs through modulation of signaling dynamics and can be corrupted by disease-causing mutations. Class IA phosphoinositide 3-kinase (PI3K) signaling is critical for normal growth and development and is dysregulated in human disorders such as benign overgrowth syndromes, cancer, primary immune deficiency, and metabolic syndrome. Despite decades of PI3K research, understanding of context-dependent regulation of the PI3K pathway and of the underlying signaling code remains rudimentary. Here, we review current knowledge on context-specific PI3K signaling and how technological advances now make it possible to move from a qualitative to quantitative understanding of this pathway. Insight into how cellular PI3K signaling is encoded or decoded may open new avenues for rational pharmacological targeting of PI3K-associated diseases. The principles of PI3K context-dependent signal encoding and decoding described here are likely applicable to most, if not all, major cell signaling pathways.

Perspective: Potential Impact and Therapeutic Implications of Oncogenic PI3K Activation on Chromosomal Instability

Genetic activation of the class I PI3K pathway is very common in cancer. This mostly results from oncogenic mutations in PIK3CA, the gene encoding the ubiquitously expressed PI3Kα catalytic subunit, or from inactivation of the PTEN tumour suppressor, a lipid phosphatase that opposes class I PI3K signalling. The clinical impact of PI3K inhibitors in solid tumours, aimed at dampening cancer-cell-intrinsic PI3K activity, has thus far been limited. Challenges include poor drug tolerance, incomplete pathway inhibition and pre-existing or inhibitor-induced resistance. The principle of pharmacologically targeting cancer-cell-intrinsic PI3K activity also assumes that all cancer-promoting effects of PI3K activation are reversible, which might not be the case. Emerging evidence suggests that genetic PI3K pathway activation can induce and/or allow cells to tolerate chromosomal instability, which-even if occurring in a low fraction of the cell population-might help to facilitate and/or drive tumour evolution. While it is clear that such genomic events cannot be reverted pharmacologically, a role for PI3K in the regulation of chromosomal instability could be exploited by using PI3K pathway inhibitors to prevent those genomic events from happening and/or reduce the pace at which they are occurring, thereby dampening cancer development or progression. Such an impact might be most effective in tumours with clonal PI3K activation and achievable at lower drug doses than the maximum-tolerated doses of PI3K inhibitors currently used in the clinic.

Ras signals principally via Erk in G1 but cooperates with PI3K/Akt for Cyclin D induction and S-phase entry

Numerous studies exploring oncogenic Ras or manipulating physiological Ras signalling have established an irrefutable role for Ras as driver of cell cycle progression. Despite this wealth of information the precise signalling timeline and effectors engaged by Ras, particularly during G1, remain obscure as approaches for Ras inhibition are slow-acting and ill-suited for charting discrete Ras signalling episodes along the cell cycle. We have developed an approach based on the inducible recruitment of a Ras-GAP that enforces endogenous Ras inhibition within minutes. Applying this strategy to inhibit Ras stepwise in synchronous cell populations revealed that Ras signaling was required well into G1 for Cyclin D induction, pocket protein phosphorylation and S-phase entry, irrespective of whether cells emerged from quiescence or G2/M. Unexpectedly, Erk, and not PI3K/Akt or Ral was activated by Ras at mid-G1, albeit PI3K/Akt signalling was a necessary companion of Ras/Erk for sustaining cyclin-D levels and G1/S transition. Our findings chart mitogenic signaling by endogenous Ras during G1 and identify limited effector engagement restricted to Raf/MEK/Erk as a cogent distinction from oncogenic Ras signalling.

Biphasic activation of extracellular signal-regulated kinase (ERK) 1/2 in epidermal growth factor (EGF)-stimulated SW480 colorectal cancer cells

Cancer cells have different characteristics due to the genetic differences where these unique features may strongly influence the effectiveness of therapeutic interventions. Here, we show that the spontaneous reactivation of extracellular signalregulated kinase (ERK), distinct from conventional ERK activation, represents a potent mechanism for cancer cell survival. We studied ERK1/2 activation in vitro in SW480 colorectal cancer cells. Although ERK signaling tends to be transiently activated, we observed the delayed reactivation of ERK1/2 in epidermal growth factor (EGF)-stimulated SW480 cells. This effect was observed even after EGF withdrawal. While phosphorylated ERK1/2 translocated into the nucleus following its primary activation, it remained in the cytoplasm during late-phase activation. The inhibition of primary ERK1/2 activation or protein trafficking, blocked reactivation and concurrently increased caspase 3 activity. Our results suggest that the biphasic activation of ERK1/2 plays a role in cancer cell survival; thus, regulation of ERK1/2 activation may improve the efficacy of cancer therapies that target ERK signaling. [BMB Reports 2016; 49(4): 220-225]

Departure from optimal O2 level for mouse trophoblast stem cell proliferation and potency leads to most rapid AMPK activation

Previous studies showed that cultured mouse trophoblast stem cells (mTSCs) have the most rapid proliferation, normal maintenance of stemness/potency, the least spontaneous differentiation, and the lowest level of stress-activated protein kinase (SAPK) when incubated at 2% O₂ rather than at the traditional 20% O₂ or hypoxic (0.5% and 0% O₂) conditions. Switching from 2% O₂ induced fast SAPK responses. Here we tested the dose response of AMP-activated protein kinase (AMPK) in its active form (pAMPK Thr172P) at O₂ levels from 20–0%, and also tested whether pAMPK levels show similar rapid changes when mTSC cultures were switched from the optimal 2% O₂ to other O₂ conditions. There was a delayed increase in pAMPK levels ~6–8 h after switching conditions from 20% to 2%, 0.5%, or 0% O₂. Altering O₂ conditions from 2% to either 20%, 0.5%, or 0% led to rapid increase in pAMPK levels within 1 h, similar to the previously reported SAPK response in mTSC cells removed from 2% O₂. Twelve hours of 0.5% O₂ exposure led to cell program changes in terms of potency loss and suppressed biosynthesis, as indicated by levels of phosphorylated inactive acetyl CoA carboxylase (pACC). Phosphorylation of ACC was inhibited by the AMPK inhibitor Compound C. However, unlike other stressors, AMPK does not mediate hypoxia-induced potency loss in mTSCs. These results suggest an important aspect of stem cell biology, which demands rapid stress enzyme activation to cope with sudden changes in external environment, e.g., from least stressful (2% O₂) to more stressful conditions.

Phosphoinositide 3-kinases-a historical perspective

The phosphoinositide 3-kinase (PI 3-K) signal relay pathway represents arguably one of the most intensely studied mechanisms by which extracellular signals elicit cellular responses through the generation of second messengers that are associated with cell growth and transformation. This chapter reviews the many landmark discoveries in the PI 3-K signaling pathway in biology and disease, from the identification of a novel phosphoinositide kinase activity associated with transforming oncogenes in the 1980s, to the identification of oncogenic mutations in the catalytic subunit of PI 3-K in the mid 2000s. Two and a half decades of intense research have provided clear evidence that the PI 3-K pathway controls virtually all aspects of normal cellular physiology, and that deregulation of one or more proteins that regulate or transduce the PI 3-K signal ultimately leads to human pathology. The most recent efforts have focused on the development of specific PI 3-K inhibitors that are currently being evaluated in clinical trials for a range of disease states.This chapter is devoted to a historical review of the landmark findings in the PI 3-K from its relatively humble beginnings in the early to mid 1980s up until the present day. When considering the key findings in the history of PI 3-K, it is essential to recognize the landmark studies by Lowell and Mabel Hokin in the 1950s who were the first to describe that extracellular agonists such as acetylcholine could stimulate the incorporation of radiolabeled phosphate into phospholipids (Hokin and Hokin 1953). Their work initiated an entirely new field of lipid signaling, and subsequent studies in the 1970s by Michell and Lapetina who linked phosphoinositide turnover to membrane-associated receptors that initiate intracellular calcium mobilization (Lapetina and Michell 1973). Later studies revealed that the phospholipase-mediated breakdown of the same minor membrane phospholipids such as PtdIns-4,5-P(2) (phosphatidylinositol-4,5-bisphosphate) is responsible for the release of two additional key second messengers, diacylglycerol (DG) and IP(3) (inositol-1,4,5-trisphosphate) (Kirk et al. 1981; Berridge 1983; Berridge et al. 1983). Berridge, Irvine and Schulz then revealed that one of the byproducts of this lipid signal relay pathway is the release of calcium from intracellular stores such as the endoplasmic reticulum (Streb et al. 1983). Finally, pioneering studies by Nishizuka in the late 1970s identified PKC (protein kinase C) as a phospholipid and diacylglycerol-activated serine/threonine protein kinase (Inoue et al. 1977; Takai et al. 1977). At this point, it probably seemed to most at the time that the story was complete, such that hydrolysis of phosphoinositides such as PtdIns-4,5-P(2) and PtdIns-4-P would account for the major mechanisms of agonist-stimulated lipid signaling leading to physiological responses. On the contrary, the story was far from complete and was about to become a lot more complex.

Autophagy is needed for the growth of pancreatic adenocarcinoma and has a cytoprotective effect against anticancer drugs

BACKGROUND AND AIM

Autophagy is a regulated process of degradation and recycling of cellular constituents. The role of autophagy in pancreatic cancer is still not clear. Some studies indicate that in pancreatic cancer autophagy exerts cytoprotective effects, whereas others suggest that autophagy positively contributes to cell death by enhancing cytotoxicity of anticancer drugs. The aim of this study was to investigate the role of autophagy in pancreatic cancer, and to provide insights into new strategies for treatment.

MATERIALS AND METHODS

Pancreatic cancer cell lines PANC-1 and BxPC-3 were treated with anticancer drugs (5-fluorouracil or gemcitabine) alone and in combination with autophagy inhibitors (chloroquine or wortmannin). Biopsy samples were retrieved from patients from pancreatic normal tissue and adenocarcinoma. Western blot of microtubule-associated protein 1 light chain 3 (LC3)-II was performed to investigate the degree of autophagy and cell proliferation was assessed by a crystal violet assay.

RESULTS

Autophagy was active in PANC-1 cells under basal conditions. Autophagy was significantly induced in pancreatic ductal adenocarcinoma compared to healthy pancreatic tissue in patients. Inhibition of autophagy by chloroquine suppressed the growth of PANC-1 and BxPC-3. Autophagy was markedly increased after treatment with 5-fluorouracil or gemcitabine. Inhibition of autophagy by chloroquine potentiated the inhibition of cell proliferation of PANC-1 and BxPC-3 by 5-fluorouracil and gemcitabine.

CONCLUSIONS

Our results with pancreatic cancer cell lines and human pancreatic adenocarcinoma suggest that autophagy contributes to pancreatic cancer cell growth. Autophagy has a cytoprotective effect against 5-fluorouracil and gemcitabine in pancreatic cancer cells. Combination therapy of these anticancer drugs and chloroquine should be investigated.

Resolvins E1 and D1 inhibit interstitial fibrosis in the obstructed kidney via inhibition of local fibroblast proliferation

Resolvin E1 (RvE1) is a naturally occurring lipid-derived mediator generated during the resolution of inflammation. The anti-inflammatory effects of RvE1 have been demonstrated in a variety of disease settings; however, it is not known whether RvE1 may also exert direct anti-fibrotic effects. We examined the potential anti-fibrotic actions of RvE1 in the mouse obstructed kidney-a model in which tissue fibrosis is driven by unilateral ureteric obstruction (UUO), an irreversible, non-immune insult. Administration of RvE1 (300 ng/day) to mice significantly reduced accumulation of α-smooth muscle actin (SMA)(+) myofibroblasts and the deposition of collagen IV on day 6 after UUO. This protective effect was associated with a marked reduction of myofibroblast proliferation on days 2, 4 and 6 after UUO. RvE1 treatment also inhibited production of the major fibroblast mitogen, platelet-derived growth factor-BB (PDGF-BB), in the obstructed kidney. Acute resolvin treatment over days 2-4 after UUO also had a profound inhibitory effect upon myofibroblast proliferation without affecting the PDGF expression, suggesting a direct effect upon fibroblast proliferation. In vitro studies established that RvE1 can directly inhibit PDGF-BB-induced proliferation in primary mouse fibroblasts. RvE1 induced transient, but not sustained, activation of the pro-proliferative ERK and AKT signalling pathways. Of note, RvE1 inhibited the sustained activation of ERK and AKT pathways seen in response to PDGF stimulation, thereby preventing up-regulation of molecules required for progression through the cell cycle (c-Myc, cyclin D) and down-regulation of inhibitors of cell cycle progression (p21, cip1). Finally, siRNA-based knock-down studies showed that the RvE1 receptor, ChemR23, is required for the anti-proliferative actions of RvE1 in cultured fibroblasts. In conclusion, this study demonstrates that RvE1 can inhibit fibroblast proliferation in vivo and in vitro, identifying RvE1 as a novel anti-fibrotic therapy.

Functional proteomic analysis of long-term growth factor stimulation and receptor tyrosine kinase coactivation in Swiss 3T3 fibroblasts

In Swiss 3T3 fibroblasts, long-term stimulation with PDGF, but not insulin-like growth factor 1 (IGF-1) or EGF, results in the establishment of an elongated migratory phenotype, characterized by the formation of retractile dendritic protrusions and absence of actin stress fibers and focal adhesion complexes. To identify receptor tyrosine kinase-specific reorganization of the Swiss 3T3 proteome during phenotypic differentiation, we compared changes in the pattern of protein synthesis and phosphorylation during long-term exposure to PDGF, IGF-1, EGF, and their combinations using 2DE-based proteomics after (35)S- and (33)P-metabolic labeling. One hundred and five differentially regulated proteins were identified by mass spectrometry and some of these extensively validated. PDGF stimulation produced the highest overall rate of protein synthesis at any given time and induced the most sustained phospho-signaling. Simultaneous activation with two or three of the growth factors revealed both synergistic and antagonistic effects on protein synthesis and expression levels with PDGF showing dominance over both IGF-1 and EGF in generating distinct proteome compositions. Using signaling pathway inhibitors, PI3K was identified as an early site for signal diversification, with sustained activity of the PI3K/AKT pathway critical for regulating late protein synthesis and phosphorylation of target proteins and required for maintaining the PDGF-dependent motile phenotype. Several proteins were identified with novel PI3K/Akt-dependent synthesis and phosphorylations including eEF2, PRS7, RACK-1, acidic calponin, NAP1L1, Hsp73, and fascin. The data also reveal induction/suppression of key F-actin and actomyosin regulators and chaperonins that enable PDGFR to direct the assembly of a motile cytoskeleton, despite simultaneous antagonistic signaling activities. Together, the study demonstrates that long-term exposure to different growth factors results in receptor tyrosine kinase-specific regulation of relatively small subproteomes, and implies that the strength and longevity of receptor tyrosine kinase-specific signals are critical in defining the composition and functional activity of the resulting proteome.

Consistency test of the cell cycle: roles for p53 and EGR1

Mammalian cells are constantly exposed to multiple mitogens and, hence, have developed machineries that help them ignore fortuitous signals. In a recent report in Molecular Cell, we highlighted the molecular details of such a noise-reduction filter, including roles for EGR1, AKT, and p53. Brief exposure to a mitogen drives formation of inhibitory p53-chromatin complexes, which are disabled only if the growth factor is still present several hours later. We propose that this "consistency test" prevents repeated division cycles of normal cells but might become defective in most cancer cells.

Prevention of the wortmannin-induced inhibition of phosphoinositide 3-kinase by sulfhydryl reducing agents

The effects of the sulfhydryl reducing agents 2-mercaptoethanol and dithiothreitol on wortmannin-induced inhibition of phosphoinositide 3-kinase (PI3K) were studied in order to examine whether the sulfhydryl reducing agents directly affect the wortmannin inhibition of PI3K. These reducing agents are commonly used to stabilize enzyme structures by maintaining protein sulfhydryl groups in the reduced state. Preincubation of wortmannin with millimolar levels of 2-mercaptoethanol, a sulfhydryl derivative of ethanol, markedly prevented subsequent wortmannin-induced inhibition of PI3K. In contrast, ethanol, 2-mercaptoethanol lacking sulfhydryl group, and 2-(methylthio)ethanol, a methyl derivative of the sulfhydryl group of 2-mercaptoethanol, had little effect on the wortmannin-induced inhibition of PI3K, which suggests that the prevention of wortmannin-induced inhibition by 2-mercaptoethanol occurs through the sulfhydryl group of this agent. Moreover, dithiothreitol, a second sulfhydryl reducing agent, also markedly prevented wortmannin-induced inhibition of PI3K. These results indicate that the wortmannin-induced inhibition of PI3K is markedly prevented by millimolar concentrations of sulfhydryl reducing agents such as 2-mercaptoethanol and dithiothreitol in the medium, presumably by the binding of wortmannin to the agents.

RAS Interaction with PI3K: More Than Just Another Effector Pathway

RAS PROTEINS ARE SMALL GTPASES KNOWN FOR THEIR INVOLVEMENT IN ONCOGENESIS: around 25% of human tumors present mutations in a member of this family. RAS operates in a complex signaling network with multiple activators and effectors, which allows them to regulate many cellular functions such as cell proliferation, differentiation, apoptosis, and senescence. Phosphatidylinositol 3-kinase (PI3K) is one of the main effector pathways of RAS, regulating cell growth, cell cycle entry, cell survival, cytoskeleton reorganization, and metabolism. However, it is the involvement of this pathway in human tumors that has attracted most attention. PI3K has proven to be necessary for RAS-induced transformation in vitro, and more importantly, mice with mutations in the PI3K catalytic subunit p110α that block its ability to interact with RAS are highly resistant to endogenous oncogenic KRAS-induced lung tumorigenesis and HRAS-induced skin carcinogenesis. These animals also have a delayed development of the lymphatic vasculature. Many PI3K inhibitors have been developed that are now in clinical trials. However, it is a complex pathway with many feedback loops, and interactions with other pathways make the results of its inhibition hard to predict. Combined therapy with another RAS-regulated pathway such as RAF/MEK/ERK may be the most effective way to treat cancer, at least in animal models mimicking the human disease. In this review, we will summarize current knowledge about how RAS regulates one of its best-known effectors, PI3K.

Involvement of 14-3-3 proteins in the second epidermal growth factor-induced wave of Rac1 activation in the process of cell migration

Immense previous efforts have elucidated the core machinery in cell migration, actin remodeling regulated by Rho family small GTPases including RhoA, Cdc42, and Rac1; however, the spatiotemporal regulation of these molecules remains largely unknown. Here, we report that EGF induces biphasic Rac1 activation in the process of cell migration, and UTKO1, a cell migration inhibitor, inhibits the second EGF-induced wave of Rac1 activation but not the first wave. To address the regulation mechanism and role of the second wave of Rac1 activation, we identified 14-3-3ζ as a target protein of UTKO1 and also showed that UTKO1 abrogated the binding of 14-3-3ζ to Tiam1 that was responsible for the second wave of Rac1 activation, suggesting that the interaction of 14-3-3ζ with Tiam1 is involved in this event. To our knowledge, this is the first report to use a chemical genetic approach to demonstrate the mechanism of temporal activation of Rac1.

Two phases of mitogenic signaling unveil roles for p53 and EGR1 in elimination of inconsistent growth signals

Normal cells require continuous exposure to growth factors in order to cross a restriction point and commit to cell-cycle progression. This can be replaced by two short, appropriately spaced pulses of growth factors, where the first pulse primes a process, which is completed by the second pulse, and enables restriction point crossing. Through integration of comprehensive proteomic and transcriptomic analyses of each pulse, we identified three processes that regulate restriction point crossing: (1) The first pulse induces essential metabolic enzymes and activates p53-dependent restraining processes. (2) The second pulse eliminates, via the PI3K/AKT pathway, the suppressive action of p53, as well as (3) sets an ERK-EGR1 threshold mechanism, which digitizes graded external signals into an all-or-none decision obligatory for S phase entry. Together, our findings uncover two gating mechanisms, which ensure that cells ignore fortuitous growth factors and undergo proliferation only in response to consistent mitogenic signals.

"Competence" progress

Zwang et al. (2011) have identified outputs of two EGF pulses that commit cells to cycle. The first induces components for lipid biosynthesis and sets up an inhibitory latch through p53. The second works through ERK to EGR1 and releases the latch to promote restriction point traverse.

Phosphoinositide 3 kinase signalling may affect multiple steps during herpes simplex virus type-1 entry

Early interactions of herpes simplex virus type-1 (HSV-1) with cells lead to cytoskeletal changes facilitating filopodia formation and membrane fusion. Here, we demonstrate that phosphoinositide 3 kinase (PI3K) signalling may affect multiple steps during HSV-1 entry. An inhibitor of PI3K (LY294002) blocked HSV-1 entry and the blockage was cell-type- and gD receptor-independent. Entry inhibition was also observed with primary cultures of the human corneal fibroblasts and unrelated β- and γ-herpesviruses. Immunofluorescence analysis demonstrated that LY294002 negatively affected HSV-1-induced filopodia formation. Similar effects of the inhibitor were seen on HSV-1 glycoprotein-induced cell-to-cell fusion. Cells expressing HSV-1 glycoproteins (gB, gD, gH and gL) showed significantly less fusion with target cells in the presence of the inhibitor. Expression of a dominant-negative PI3K mutant negatively affected both entry and fusion. We also show that inhibition of PI3K signalling also affected RhoA activation required for HSV-1 entry into certain cell types.

A role for MAPK and PI-3K signaling pathways in brain-derived neurotrophic factor modification of conditioned taste aversion retention

Brain-derived neurotrophic factor (BDNF) has emerged as an important molecular mediator of synaptic plasticity. Our previous studies on the insular cortex (IC), a region of the temporal cortex implicated in the acquisition and storage of conditioned taste aversion (CTA), have demonstrated that the intracortical microinfusion of BDNF induces a lasting potentiation of synaptic efficacy in the projection from the basolateral nucleus of the amygdala (Bla) to the IC of adult rats in vivo. Recently, we have found that intracortical microinfusion of BDNF previous to CTA training modifies the retention of this task. In this work, we present experimental data showing that BDNF effects on CTA retention are dependent on both the activation of mitogen-activated protein kinases (MAPK) and phosphatidylinositol-3-kinase (PI-3K) at the insular cortex. Our results are evidence of the crucial role of both pathways in the modification of the CTA trace of memory caused by BDNF at a neocortical area.

Timing-dependent actions of NGF required for cell differentiation

Background

Continuous NGF stimulation induces PC12 cell differentiation. However, why continuous NGF stimulation is required for differentiation is unclear. In this study, we investigated the underlying mechanisms of the timing-dependent requirement of NGF action for cell differentiation.

Methodology/Principal Findings

To address the timing-dependency of the NGF action, we performed a discontinuous stimulation assay consisting of a first transient stimulation followed by an interval and then a second sustained stimulation and quantified the neurite extension level. Consequently, we observed a timing-dependent action of NGF on cell differentiation, and discontinuous NGF stimulation similarly induced differentiation. The first stimulation did not induce neurite extension, whereas the second stimulation induced fast neurite extension; therefore, the first stimulation is likely required as a prerequisite condition. These observations indicate that the action of NGF can be divided into two processes: an initial stimulation-driven latent process and a second stimulation-driven extension process. The latent process appears to require the activities of ERK and transcription, but not PI3K, whereas the extension-process requires the activities of ERK and PI3K, but not transcription. We also found that during the first stimulation, the activity of NGF can be replaced by PACAP, but not by insulin, EGF, bFGF or forskolin; during the second stimulation, however, the activity of NGF cannot be replaced by any of these stimulants. These findings allowed us to identify potential genes specifically involved in the latent process, rather than in other processes, using a microarray.

Conclusions/Significance

These results demonstrate that NGF induces the differentiation of PC12 cells via mechanically distinct processes: an ERK-driven and transcription-dependent latent process, and an ERK- and PI3K-driven and transcription-independent extension process.

Gq-coupled purinergic receptors inhibit insulin-like growth factor-I/phosphoinositide 3-kinase pathway-dependent keratinocyte migration

After skin wound, released growth factors and extracellular nucleotides regulate the different phases of healing, including re-epithelialization. Here, we show that, in keratinocytes, purinergic P2Y2 receptors inhibit the motogenic IGF-I/PI3K pathway. Therefore, extracellular nucleotides may play key roles during skin remodelling after wound.

Insulin-like growth factor-I (IGF-I) activation of phosphoinositol 3-kinase (PI3K) is an essential pathway for keratinocyte migration that is required for epidermis wound healing. We have previously reported that activation of Gα_(q/11)-coupled-P2Y₂ purinergic receptors by extracellular nucleotides delays keratinocyte wound closure. Here, we report that activation of P2Y₂ receptors by extracellular UTP inhibits the IGF-I–induced p110α-PI3K activation. Using siRNA and pharmacological inhibitors, we demonstrate that the UTP antagonistic effects on PI3K pathway are mediated by Gα_(q/11)—and not G_(i/o)—independently of phospholipase Cβ. Purinergic signaling does not affect the formation of the IGF-I receptor/insulin receptor substrate-I/p85 complex, but blocks the activity of a membrane-targeted active p110α mutant, indicating that UTP acts downstream of PI3K membrane recruitment. UTP was also found to efficiently attenuate, within few minutes, the IGF-I–induced PI3K-controlled translocation of the actin-nucleating protein cortactin to the plasma membrane. This supports the UTP ability to alter later migratory events. Indeed, UTP inhibits keratinocyte spreading and migration promoted by either IGF-I or a membrane-targeted active p110α mutant, in a Gα(q/11)-dependent manner both. These findings provide new insight into the signaling cross-talk between receptor tyrosine kinase and Gα_(q/11)-coupled receptors, which mediate opposite effects on p110α-PI3K activity and keratinocyte migration.

Role of RAS in the regulation of PI 3-kinase

Ras proteins are key regulators of signalling cascades, controlling many processes such as proliferation, differentiation and apoptosis. Mutations in these proteins or in their effectors, activators and regulators are associated with pathological conditions, particularly the development of various forms of human cancer. RAS proteins signal through direct interaction with a number of effector enzymes, one of the best characterized being type I phosphatidylinositol (PI) 3-kinases. Although the ability of RAS to control PI 3-kinase has long been well established in cultured cells, evidence for a role of the interaction of endogenous RAS with PI 3-kinase in normal and malignant cell growth in vivo has only been obtained recently. Mice with mutations in the PI 3-kinase catalytic p110a isoform that block its ability to interact with RAS are highly resistant to endogenous KRAS oncogene induced lung tumourigenesis and HRAS oncogene induced skin carcinogenesis. Cells from these mice show proliferative defects and selective disruption of signalling from certain growth factors to PI 3-kinase, while the mice also display delayed development of the lymphatic vasculature. The interaction of RAS with p110a is thus required in vivo for some normal growth factor signalling and also for RAS-driven tumour formation. RAS family members were among the first oncogenes identified over 40 years ago. In the late 1960s, the rat-derived Harvey and Kirsten murine sarcoma retroviruses were discovered and subsequently shown to promote cancer formation through related oncogenes, termed RAS (from rat sarcoma virus). The central role of RAS proteins in human cancer is highlighted by the large number of tumours in which they are activated by mutation: approximately 20% of human cancers carry a mutation in RAS proteins. Because of the complex signalling network in which RAS operates, with multiple activators and effectors, each with a different pattern of tissue-specific expression and a distinct set of intracellular functions, one of the critical issues concerns the specific role of each effector in RAS-driven oncogenesis. In this chapter, we summarize current knowledge about how RAS regulates one of its best-known effectors, phosphoinositide 3-kinase (PI3K).

Differential regulation of Akt/protein kinase B isoforms during cell cycle progression

Phosphatidylinositol 3-kinase pathways play key regulatory roles in cell cycle progression into S phase. In this study, we demonstrated that Akt1/PKBalpha isoform plays an essential role in G(1)/S transition and proliferation. Cells lacking Akt1/PKBalpha showed an attenuated proliferation as well as G(1)/S transition, whereas cells lacking Akt2/PKBbeta showed normal proliferation and G(1)/S transition. The effect of Akt1/PKBalpha on cell proliferation and G(1)/S transition was completely abolished by swapping pleckstrin homology (PH) domain with that of Akt2/PKBbeta. Finally, full activation of Akt/PKB and cyclin D expression was achieved by the Akt1/PKBalpha or chimeric proteins containing the PH domain of Akt1/PKBalpha indicating that the PH domain of Akt1/PKBalpha provides full kinase activity and is necessary for the G(1)/S transition.

Nuclear phosphoinositide signaling regulates messenger RNA export

Messenger RNA export from the nucleus to the cytoplasm plays an essential role in linking transcription to translation and consequently regulation of protein expression. mRNA export requires a series of events: pre-mRNA processing, ribonucleoprotein targeting to the NPC (nuclear pore complexes), and translocation through nuclear pores to the cytoplasm. Interestingly, the conventional nuclear export machinery, exportins and the Ran GTPase, is not required for mRNA export. Instead, a protein complex consisting of a number of RNA binding proteins is essential for this event including the Aly/REF protein. Phosphoinositide signaling regulates a variety of cellular functions including pre-mRNA splicing and mRNA export. In fact, a phospholipase C-dependent inositol polyphosphate kinase pathway is required for efficient mRNA export. Recently, we showed that Aly is a physiological target of nuclear phosphoinositide-3-kinase (PI3K) signaling, which regulates Aly localization as well as Aly function in cell proliferation and mRNA export through nuclear Akt-mediated phosphorylation and phosphoinositide association. Hence, water-soluble inositol polyphosphates and phosphatidylinositol lipids play pivotal roles in modulating mRNA export.

Akt phosphorylation and nuclear phosphoinositide association mediate mRNA export and cell proliferation activities by ALY

Nuclear PI3K and its downstream effectors play essential roles in a variety of cellular activities including cell proliferation, survival, differentiation, and pre-mRNA splicing. Aly is a nuclear speckle protein implicated in mRNA export. Here we show that Aly is a physiological target of nuclear PI3K signaling, which regulates its subnuclear residency, cell proliferation, and mRNA export activities through nuclear Akt phosphorylation and phosphoinositide association. Nuclear Akt phosphorylates Aly on threonine-219, which is required for its interaction with Akt. Aly binds phosphoinositides, and this action is regulated by Akt-mediated phosphorylation. Phosphoinositide binding but not Akt phosphorylation dictates Aly's nuclear speckle residency. Depletion of Aly results in cell growth suppression and mRNA export reduction. Inhibition of Aly phosphorylation substantially decreases cell proliferation and mRNA export. Furthermore, disruption of phosphoinositide association with Aly also significantly reduces these activities. Thus, nuclear PI3K signaling mediates both cell proliferation and mRNA export functions of Aly.

Sensing and integration of Erk and PI3K signals by Myc

The transcription factor Myc plays a central role in regulating cell-fate decisions, including proliferation, growth, and apoptosis. To maintain a normal cell physiology, it is critical that the control of Myc dynamics is precisely orchestrated. Recent studies suggest that such control of Myc can be achieved at the post-translational level via protein stability modulation. Myc is regulated by two Ras effector pathways: the extracellular signal-regulated kinase (Erk) and phosphatidylinositol 3-kinase (PI3K) pathways. To gain quantitative insight into Myc dynamics, we have developed a mathematical model to analyze post-translational regulation of Myc via sequential phosphorylation by Erk and PI3K. Our results suggest that Myc integrates Erk and PI3K signals to result in various cellular responses by differential stability control of Myc protein isoforms. Such signal integration confers a flexible dynamic range for the system output, governed by stability change. In addition, signal integration may require saturation of the input signals, leading to sensitive signal integration to the temporal features of the input signals, insensitive response to their amplitudes, and resistance to input fluctuations. We further propose that these characteristics of the protein stability control module in Myc may be commonly utilized in various cell types and classes of proteins.

Phosphoinositide 3-kinases p110alpha and p110beta regulate cell cycle entry, exhibiting distinct activation kinetics in G1 phase

Phosphoinositide 3-kinase (PI3K) is an early signaling molecule that regulates cell growth and cell cycle entry. PI3K is activated immediately after growth factor receptor stimulation (at the G(0)/G(1) transition) and again in late G(1). The two ubiquitous PI3K isoforms (p110alpha and p110beta) are essential during embryonic development and are thought to control cell division. Nonetheless, it is presently unknown at which point each is activated during the cell cycle and whether or not they both control S-phase entry. We found that p110alpha was activated first in G(0)/G(1), followed by a minor p110beta activity peak. In late G(1), p110alpha activation preceded that of p110beta, which showed the maximum activity at this time. p110beta activation required Ras activity, whereas p110alpha was first activated by tyrosine kinases and then further induced by active Ras. Interference with p110alpha and -beta activity diminished the activation of downstream effectors with different kinetics, with a selective action of p110alpha in blocking early G(1) events. We show that inhibition of either p110alpha or p110beta reduced cell cycle entry. These results reveal that PI3Kalpha and -beta present distinct activation requirements and kinetics in G(1) phase, with a selective action of PI3Kalpha at the G(0)/G(1) phase transition. Nevertheless, PI3Kalpha and -beta both regulate S-phase entry.

Enhanced sensitivity to IGF-II signaling links loss of imprinting of IGF2 to increased cell proliferation and tumor risk

Loss of imprinting (LOI) of the insulin-like growth factor-II gene (IGF2), leading to abnormal activation of the normally silent maternal allele, is a common human epigenetic population variant associated with a 5-fold increased frequency of colorectal neoplasia. Here, we show first that LOI leads specifically to increased expression of proliferation-related genes in mouse intestinal crypts. Surprisingly, LOI(+) mice also have enhanced sensitivity to IGF-II signaling, not simply increased IGF-II levels, because in vivo blockade with NVP-AEW541, a specific inhibitor of the IGF-II signaling receptor, showed reduction of proliferation-related gene expression to levels half that seen in LOI(-) mice. Signal transduction assays in microfluidic chips confirmed this enhanced sensitivity with marked augmentation of Akt/PKB signaling in LOI(+) cells at low doses of IGF-II, which was reduced in the presence of the inhibitor to levels below those found in LOI(-) cells, and was associated with increased expression of the IGF1 and insulin receptor genes. We exploited this increased IGF-II sensitivity to develop an in vivo chemopreventive strategy using the azoxymethane (AOM) mutagenesis model. LOI(+) mice treated with AOM showed a 60% increase in premalignant aberrant crypt foci (ACF) formation over LOI(-) mice. In vivo IGF-II blockade with NVP-AEW541 abrogated this effect, reducing ACF to a level 30% lower even than found in exposed LOI(-) mice. Thus, LOI increases cancer risk in a counterintuitive way, by increasing the sensitivity of the IGF-II signaling pathway itself, providing a previously undescribed epigenetic chemoprevention strategy in which cells with LOI are "IGF-II addicted" and undergo reduced tumorigenesis in the colon upon IGF-II pathway blockade.

STAT5 Is Essential for Akt/p70S6 Kinase Activity during IL-2-Induced Lymphocyte Proliferation

IL-2R activates two distinct signaling pathways mediated by the adaptor protein Shc and the transcription factor STAT5. Prior mutagenesis studies of the IL-2R have indicated that the Shc and STAT5 pathways are redundant in the ability to induce lymphocyte proliferation. Yet paradoxically, T cells from STAT5-deficient mice fail to proliferate in response to IL-2, suggesting that the Shc pathway is unable to promote mitogenesis in the genetic absence of STAT5. Here we show in the murine lymphocyte cell line Ba/F3 that low levels of STAT5 activity are essential for Shc signaling. In the absence of STAT5 activity, Shc was unable to sustain activation of the Akt/p70S6 kinase pathway or promote lymphocyte proliferation and viability. Restoring STAT5 activity via a heterologous receptor rescued Shc-induced Akt/p70S6 kinase activity and cell proliferation with kinetics consistent with a transcriptional mechanism. Thus, STAT5 appears to regulate the expression of one or more unidentified components of the Akt pathway. Our results not only explain the severe proliferative defect in STAT5-deficient T cells but also provide mechanistic insight into the oncogenic properties of STAT5 in various leukemias and lymphomas.

Phosphoinositide phosphatases in a network of signalling reactions

Phosphoinositide phosphatases dephosphorylate the three positions (D-3, 4 and 5) of the inositol ring of the poly-phosphoinositides. They belong to different families of enzymes. The PtdIns(3,4)P(2) 4-phosphatase family, the tumour suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN), SAC1 domain phosphatases and myotubularins belong to the tyrosine protein phosphatases superfamily. They share the presence of a conserved cysteine residue in the consensus CX(5)RT/S. Another family consists of the inositol polyphosphate 5-phosphatase isoenzymes. The importance of these phosphoinositide phosphatases in cell regulation is illustrated by multiple examples of their implications in human diseases such as Lowe syndrome, X-linked myotubular myopathy, cancer, diabetes or bacterial infection.

Molecular mechanisms of hepatic fibrogenesis

Liver fibrosis, a wound-healing response to a variety of chronic stimuli, is characterized by excessive deposition of extracellular matrix (ECM) proteins, of which type I collagen predominates. This alters the structure of the liver leading to organ dysfunction. The activated hepatic stellate cell (HSC) is primarily responsible for excess collagen deposition during liver fibrosis. Two important aspects are involved in mediating the fibrogenic response: first the HSC becomes directly fibrogenic by synthesizing ECM proteins; second, the activated HSC proliferates, effectively amplifying the fibrogenic response. Although the precise mechanisms responsible for HSC activation remain elusive, substantial insight is being gained into the molecular mechanisms responsible for ECM production and cell proliferation in the HSC. The activated HSC becomes responsive to both proliferative (platelet-derived growth factor) and fibrogenic (transforming growth factor-beta[TGF-beta]) cytokines. It is becoming clear that these cytokines activate both mitogen-activated protein kinase (MAPK) signaling, involving p38, and focal adhesion kinase-phosphatidylinositol 3-kinase-Akt-p70 S6 kinase (FAK-PI3K-Akt-p70(S6K)) signaling cascades. Together, these regulate the proliferative response, activating cell cycle progression as well as collagen gene expression. In addition, signaling by both TGF-beta, mediated by Smad proteins, and p38 MAPK influence collagen gene expression. Smad and p38 MAPK signaling have been found to independently and additively regulate alpha1(I) collagen gene expression by transcriptional activation while p38 MAPK, but not Smad signaling, increases alpha1(I) collagen mRNA stability, leading to increased synthesis and deposition of type I collagen. It is anticipated that by understanding the molecular mechanisms responsible for HSC proliferation and excess ECM production new therapeutic targets will be identified for the treatment of liver fibrosis.

Two waves of the nuclear phospholipase C activity in serum-stimulated HL-60 cells during G(1) phase of the cell cycle

Phosphatidylinositol-specific phospholipase C (PI-PLC) is activated in cell nuclei during the cell cycle progression. We have previously demonstrated two peaks of an increase in the nuclear PI-PLC activities in nocodazole-synchronized HL-60 cells. In this study, the activity of nuclear PI-PLC was investigated in serum-stimulated HL-60 cells. In serum-starved HL-60 cells, two peaks of the activity of nuclear PI-PLC were detected at 30 min and 11 h after the re-addition of serum with no parallel increase in PLC activity in cytosol, postnuclear membranes or total cell lysates. An increase in the serine phosphorylation of b splicing variant of PI-PLCbeta(1) was detected with no change in the amount of PI-PLCbeta(1b) in nuclei isolated at 30 min and 11 h after the addition of serum. PI-PLC inhibitor ET-18-OCH(3) and MEK inhibitor PD 98059 completely abolished serum-mediated increase at both time-points. The addition of inhibitors either immediately or 6 h after the addition of serum had inhibitory effects on the number of cells entering S phase. These results demonstrate that two waves of nuclear PI-PLCbeta(1b) activity occur in serum-stimulated cells during G(1) phase of the cell cycle and that the later increase in the PLC activity is equally important for the progression into the S phase.

Phosphoinositide 3-kinase activation in late G1 is required for c-Myc stabilization and S phase entry

Phosphoinositide 3-kinase (PI3K) is one of the early-signaling molecules induced by growth factor (GF) receptor stimulation that are necessary for cell growth and cell cycle entry. PI3K activation occurs at two distinct time points during G(1) phase. The first peak is observed immediately following GF addition and the second in late G(1), before S phase entry. This second activity peak is essential for transition from G(1) to S phase; nonetheless, the mechanism by which this peak is induced and regulates S phase entry is poorly understood. Here, we show that activation of Ras and Tyr kinases is required for late-G(1) PI3K activation. Inhibition of late-G(1) PI3K activity results in low c-Myc and cyclin A expression, impaired Cdk2 activity, and reduced loading of MCM2 (minichromosome maintenance protein) onto chromatin. The primary consequence of inhibiting late-G(1) PI3K was c-Myc destabilization, as conditional activation of c-Myc in advanced G(1) as well as expression of a stable c-Myc mutant rescued all of these defects, restoring S phase entry. These results show that Tyr kinases and Ras cooperate to induce the second PI3K activity peak in G(1), which mediates initiation of DNA synthesis by inducing c-Myc stabilization.

Gas6/Axl signaling activates the phosphatidylinositol 3-kinase/Akt1 survival pathway to protect oligodendrocytes from tumor necrosis factor alpha-induced apoptosis

Growth arrest-specific protein 6 (gas6) activity is mediated through the receptor tyrosine kinase family members Axl, Rse, and Mer, all of which are expressed in human oligodendrocytes. In this study, we examined whether recombinant human (rh) gas6 protects oligodendrocytes from growth factor (insulin) withdrawal or tumor necrosis factor-alpha (TNFalpha) cytotoxicity. In addition, we examined whether the effect was caspase-dependent, which receptor mediated the protective effect, and whether survival required Akt1 activation. Oligodendrocyte viability was assessed by O4 staining and terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling. Addition of rhgas6 to insulin-depleted cultures resulted in a significant increase in oligodendrocyte viability. Rhgas6 and caspase inhibitors also reduced active caspase-3 immunoreactivity relative to TNFalpha-only-treated cultures. In cultures treated with TNFalpha (100 ng/ml), the oligodendrocyte survival rate was 18% compared with cultures treated with TNFalpha and rhgas6 (64%) or the caspase inhibitors IETD-fmk [z-Ile-Glu(OMe)-Thr-Asp(OMe)-fluoromethyl ketone] (65%) and zVAD-fmk (N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone) (63%). Increased phosphoAkt (Ser473) immunoreactivity was detected 15 min after administration of gas6 and TNFalpha to oligodendrocyte cultures but not in TNFalpha-treated cultures. The gas6 protective effect was abrogated by the Axl decoy receptor Axl-Fc, by the phosphatidylinositol 3 (PI3) kinase inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one], and in Akt1(-/-) oligodendrocytes. Oligodendrocyte cultures established from wild-type and Rse(-/-) mice, but not from Axl(-/-) mice, were also protected from TNFalpha-induced cell death when maintained in rhgas6. We conclude that gas6 signaling through the Axl receptor and the PI3 kinase/Akt1 survival pathway protects oligodendrocytes from growth factor withdrawal and TNFalpha-mediated cell death.

Phosphoinositide-specific phospholipase C (PI-PLC) β1 and nuclear lipid-dependent signaling

Over the last years, evidence has suggested that phosphoinositides, which are involved in the regulation of a large variety of cellular processes both in the cytoplasm and in the plasma membrane, are present also within the nucleus. A number of advances has resulted in the discovery that phosphoinositide-specific phospholipase C signalling in the nucleus is involved in cell growth and differentiation. Remarkably, the nuclear inositide metabolism is regulated independently from that present elsewhere in the cell. Even though nuclear inositol lipids hydrolysis generates second messengers such as diacylglycerol and inositol 1,4,5-trisphosphate, it is becoming increasingly clear that in the nucleus polyphosphoinositides may act by themselves to influence pre-mRNA splicing and chromatin structure. Among phosphoinositide-specific phospholipase C, the beta(1) isoform appears to be one of the key players of the nuclear lipid signaling. This review aims at highlighting the most significant and up-dated findings about phosphoinositide-specific phospholipase C beta(1) in the nucleus.

Zn2+ inhibits mitochondrial movement in neurons by phosphatidylinositol 3-kinase activation

Mitochondria have been identified as targets of the neurotoxic actions of zinc, possibly through decreased mitochondrial energy production and increased reactive oxygen species accumulation. It has been hypothesized that impairment of mitochondrial trafficking may be a mechanism of neuronal injury. Here, we report that elevated intraneuronal zinc impairs mitochondrial trafficking. At concentrations just sufficient to cause injury, zinc rapidly inhibited mitochondrial movement without altering morphology. Zinc chelation initially restored movement, but the actions of zinc became insensitive to chelator in <10 min. A search for downstream signaling events revealed that inhibitors of phosphatidylinositol (PI) 3-kinase prevented this zinc effect on movement. Moreover, transient inhibition of PI 3-kinase afforded neuroprotection against zinc-mediated toxicity. These data illustrate a novel mechanism that regulates mitochondrial trafficking in neurons and also suggest that mitochondrial trafficking may be closely coupled to neuronal viability.

Prolonged TCR/CD28 Engagement Drives IL-2-Independent T Cell Clonal Expansion through Signaling Mediated by the Mammalian Target of Rapamycin

Proliferation of Ag-specific T cells is central to the development of protective immunity. The concomitant stimulation of the TCR and CD28 programs resting T cells to IL-2-driven clonal expansion. We report that a prolonged occupancy of the TCR and CD28 bypasses the need for autocrine IL-2 secretion and sustains IL-2-independent lymphocyte proliferation. In contrast, a short engagement of the TCR and CD28 only drives the expansion of cells capable of IL-2 production. TCR/CD28- and IL-2-driven proliferation revealed a different requirement for PI3K and for the mammalian target of rapamycin (mTOR). Thus, both PI3K and mTOR activities were needed for T cells to proliferate to TCR/CD28-initiated stimuli and for optimal cyclin E expression. In contrast, either PI3K or mTOR were sufficient for IL-2-driven cell proliferation as they independently mediated cyclin E induction. Interestingly, rapamycin delayed cell cycle entry of IL-2-sufficient T cells, but did not prevent their expansion. Together, our findings indicate that the TCR, CD28, and IL-2 independently control T cell proliferation via distinct signaling pathways involving PI3K and mTOR. These data suggest that Ag persistence and the availability of costimulatory signals and of autocrine and paracrine growth factors individually shape T lymphocyte expansion in vivo.

A role for PKCzeta in potentiation of the topoisomerase II activity and etoposide cytotoxicity by wortmannin

Enhanced cytotoxicity of etoposide by wortmannin, an inhibitor of enzymes holding a phosphatidylinositol 3-kinase domain, was investigated in eight cell lines proficient or deficient for DNA double-strand break repair. Wortmannin stimulated the decatenating activity of topoisomerase II, promoted etoposide-induced accumulation of DNA double-strand breaks, shifted the specificity for cell killing by etoposide from the S to G1 phase of the cell cycle, and potentiated the cytotoxicity of etoposide through two mechanisms. (a) Sensitization to high, micromolar amounts of etoposide required integrity of the nonhomologous end-joining repair pathway. (b) Wortmannin dramatically increased the susceptibility to low, submicromolar amounts of etoposide in a large fraction of the cell population irrespective of the status of ATM, Ku86, and DNA-PKCS. It is shown that this process correlates depression of phosphatidylinositol 3-kinase-dependent phosphorylation of the atypical, zeta isoform of protein kinase C (PKCzeta). Stable expression of a dominant-negative, kinase-dead mutant of PKCzeta in a tumor cell line reproduced the hypersensitivity pattern induced by wortmannin. The results are consistent with up-regulation of the topoisomerase II activity in relation to inactivation of PKCzeta and indicate that PKCzeta may be a useful target to improve the efficiency of topoisomerase II poisons at low concentration.

Alternative signaling pathways: when, where and why?

Alternative cell signal transduction pathways have been demonstrated in some experimental systems. The importance of their existence has not been completely appreciated. In this review we present the cases of alternative pathways resulted from a survey of the available experimental data. The alternative pathways could show different relationships, i.e., synergistic, redundant, additive, opposite and competitive effects. They could have distinct time courses and cell, organ, sex or species specification. Further, they could happen during physiological or pathological situations, and display differentiated sensitivity. These case studies together imply that alternative signal pathways could be involved in the regulation of cell functions at the pathway level. In-depth understanding of the importance of the alternative pathways will rely on building and exploration of mathematical models.

The priming/completion paradigm to explain growth factor-dependent cell cycle progression

Approximately 50 years ago, researchers established conditions to maintain cells in tissue culture: Likely et al. (1952), Scherer et al. (1953), Eagle (1955). This simple model system set the stage for discovery of growth factors and the signaling systems that they engage to mediate cellular responses such as proliferation. The purpose of this review is to present the original view of how growth factors regulate cell cycle progression and an updated (priming/completion) version of how growth factors advance resting cells through the cell cycle.

Divergent roles of c-Src in controlling platelet-derived growth factor-dependent signaling in fibroblasts

The vast complexity of platelet-derived growth factor (PDGF)-induced downstream signaling pathways is well known, but the precise roles of critical players still elude us due to our lack of specific and temporal control over their activities. Accordingly, although Src family members are some of the better characterized effectors of PDGFbeta signaling, considerable controversy still surrounds their precise functions. To address these questions and limitations, we applied a chemical-genetic approach to study the role of c-Src at the cellular level, in defined signaling cascades; we also uncovered novel phosphorylation targets and defined its influence on transcriptional events. The spectacular control of c-Src on actin reorganization and chemotaxis was delineated by global substrate labeling and transcriptional analysis, revealing multiple cytoskeletal proteins and chemotaxis promoting genes to be under c-Src control. Additionally, this tool revealed the contrasting roles of c-Src in controlling DNA synthesis, where it transmits conflicting inputs via the phosphatidylinositol 3 kinase and Ras pathways. Finally, this study reveals a mechanism by which Src family kinases may control PDGF-mediated responses both at transcriptional and translational levels.

Lithium activates the Wnt and phosphatidylinositol 3-kinase Akt signaling pathways to promote cell survival in the absence of soluble survival factors

Mouse proximal tubular cells (BUMPT), when cultured in the absence of growth factors, activate a default apoptotic pathway. Although Wnt signaling antagonizes the effect of proapoptotic triggers, its role in regulating the default pathway of apoptosis is less well defined. The present study examines the hypothesis that lithium (Li+) and (2′Z,3′E)-6-bromoindirubin-3′-oxime (BIO), two glycogen synthase kinase-3β (GSK3β) inhibitors, promote survival of growth factor-deprived renal epithelial cells by activating the Wnt pathway. These studies demonstrate that Li+and BIO activate Wnt signaling as indicated by the following changes: phosphorylation (inhibition) of GSK3β; decreased phosphorylation of β-catenin (a GSK3β substrate); nuclear translocation of β-catenin; specific transcriptional activation of Tcf/catenin-responsive pTopflash constructs; and an increase in the expression of cyclin D1 (indicative of a promitogenic cell response). In addition, Li+or BIO significantly increases the phosphorylation (activation) of Akt, an anti-apoptotic protein, and inhibits apoptosis (decreases both annexin-V staining and caspase-3 activation), during serum deprivation. Inhibition of phosphatidylinositol 3-kinase (responsible for Akt activation) either by wortmanin or LY-294002 prevented Li+- or BIO-induced Akt phosphorylation and reduces cell survival without altering the phosphorylation state of GSK3β. Li+or BIO also increases the expression of insulin-like growth factor-II (IGF-II), a potent proliferative signaling protein. Li+or BIO-free conditioned medium harvested from Li+- or BIO-exposed cells also induced Akt phosphorylation, mimicking the protective effect of the two GSK3β inhibitors on serum-starved cells. Furthermore, the effect of conditioned medium on Akt phosphorylation could be inhibited by either LY-294002 or IGF-binding protein. BIO, a specific GSK3β inhibitor, replicated the protective effect of Li+on cell viability, suggesting that GSK3β activation is important for initiating the apoptotic pathway. Taken together, these data suggest that Li+or BIO promotes renal epithelial cell survival by inhibiting apoptosis through GSK3β-dependent activation of the Wnt pathway and subsequent release of IGF-II. Extracellular IGF-II serves as an autocrine survival factor that is responsible, in part, for activating the anti-apoptotic phosphatidylinositol-3-kinase-Akt pathway during serum deprivation.

Nuclear phospholipase C-β1b activation during G2/M and late G1 phase in nocodazole-synchronized HL-60 cells

In this study, the activity of nuclear phosphatidylinositol-specific phosholipase C (PI-PLC) was investigated in HL-60 cells blocked at G(2)/M phase by the addition of nocodazole, and released into medium as synchronously progressing cells. Two peaks of an increase in the nuclear PI-PLC activities were detected; an early peak reached a maximum at 1 h after release from the nocodazole block, and a second increase was detected at 8.5 h after the release. Immunoprecipitation studies indicated that the increase in the activity was due to the activation of the nuclear PI-PLC-beta(1). Western blot analysis demonstrated no changes in the level of both a and b splicing variants of PI-PLC-beta(1) in the nuclei of cells isolated at either 1 h or 8.5 h after the block. However, an increase in the serine-phosphorylation of PI-PLC-beta(1b) was detected in the nuclei of HL-60 cells isolated at 1 and 8.5 h after the block, and the presence of MEK-inhibitor PD98059 completely inhibited both the serine phosphorylation and the increase in the PI-PLC activities in vitro. The presence of PI-PLC inhibitor prevented the progression of HL-60 cells through the G(1) into S phase of the cell cycle. These results demonstrate that two peaks of nuclear PI-PLC activities, which are due to a PD98059-sensitive phosphorylation of nuclear PLC-beta(1b) on serine, occur at the G(2)/M and late G(1) phase and are necessary for the progression of the cells through the cell cycle.

Platelet-Derived Growth Factor-BB–Induced Human Smooth Muscle Cell Proliferation Depends on Basic FGF Release and FGFR-1 Activation

We have shown that the G protein–coupled receptor (GPCR) agonists, thrombin and Factor Xa, stimulate smooth muscle cell (SMC) proliferation through transactivation of the EGF receptor (EGFR) or the FGF receptor (FGFR), both of which are tyrosine kinase receptors. In the present study, we investigated whether platelet-derived growth factor (PDGF), a tyrosine kinase receptor agonist, might transactivate another tyrosine kinase receptor to induce SMC proliferation. Because heparin inhibits PDGF-mediated proliferation in human SMCs, we investigated whether the heparin-binding growth factor basic fibroblast growth factor (bFGF) and one of its receptors, FGFR-1, play a role in the response of human arterial SMCs to PDGF-BB. PDGF-BB induced the release of bFGF and sustained phosphorylation of FGFR-1 (30 minutes to 6 hours). A bFGF-neutralizing antibody inhibited PDGF-BB–mediated phosphorylation of FGFR-1, DNA synthesis, and cell proliferation. In the presence of bFGF antibody, PDGF-BB–induced early activation of ERK (0 to 60 minutes) was not affected, whereas late ERK activation (2 to 4 hours) was reduced. When FGFR-1 expression was suppressed using small interfering RNA (siRNA), ERK activation was reduced at late, but not early, time points after PDGF-BB stimulation. Addition of bFGF antibody to cells treated with siRNA to FGFR-1 had no further effect on ERK activation. Our results provide support for a novel mechanism by which PDGF-BB induces the release of bFGF and activation of FGFR-1 followed by the sustained activation of ERK and proliferation of human SMCs.