Drosophila PI3 kinase and Akt involved in insulin-stimulated proliferation and ERK pathway activation in Schneider cells

Effect of Insulin Receptor on Juvenile Hormone Signal and Fecundity in Spodoptera litura (F.)

Simple Summary

The tobacco cutworm, Spodoptera litura (F.), exemplifies strong reproductive capacities and damages many agricultural crops. The insulin signaling pathway is known as a key determinant of female reproduction in insects. However, the detailed molecular mechanisms in these processes are poorly studied. Here, we injected bovine insulin into the newly emerged moth, resulting in gene expression changes in the insulin pathway, while knockdown of SlInR caused an inverse gene expression change involved in the insulin pathway. Further studies indicated that the content of JH-III, Vg, total proteins and triacylgycerol could be suppressed by SlInR dsRNA injection. Furthermore, stunted ovaries and lower fecundity were observed by RNAi. Our studies indicated that SlInR plays a key role in JH-III synthesis and the ovarian development in S. litura.

Abstract

Insulin signaling can regulate various physiological functions, such as energy metabolism and reproduction and so on, in many insects, including mosquito and locust. However, the molecular mechanism of this physiological process remains elusive. The tobacco cutworm, Spodoptera litura, is one of the most important pests of agricultural crops around the world. In this study, phosphoinositide 3-kinase (SlPI3K), protein kinase B (SlAKT), target of rapamycin (SlTOR), ribosomal protein S6 kinase (SlS6K) and transcription factor cAMP-response element binding protein (SlCREB) genes, except transcription factor forkhead box class O (SlFoxO), can be activated by bovine insulin injection. Then, we studied the influence of the insulin receptor gene (SlInR) on the reproduction of S. litura using RNA interference technology. qRT-PCR analysis revealed that SlInR was most abundant in the head. The SlPI3K, SlAKT, SlTOR, SlS6K and SlCREB genes were decreased, except SlFoxO, after the SlInR gene knockdown. Further studies revealed that the expression of vitellogenin mRNA and protein, Methoprene-tolerant gene (SlMet), could be down-regulated by the injection of dsRNA of SlInR significantly. Furthermore, a depletion in the insulin receptor by RNAi significantly decreased the content of juvenile hormone III (JH-III), total proteins and triacylgycerol. These changes indicated that a lack of SlInR could impair ovarian development and decrease fecundity in S. litura. Our studies contribute to a comprehensive insight into reproduction, regulated by insulin and the juvenile hormone signaling pathway through nutrition, and a provide theoretical basis for the reproduction process in pest insects.

Activity-induced synaptic structural modifications by Akt

The activity-dependent regulation of synaptic structures plays a key role in synaptic development and plasticity; however, the signaling mechanisms involved remain largely unknown. The serine/threonine protein kinase Akt, a downstream effector of phosphoinositide 3-kinase (PI3K), plays a pivotal role in a wide range of physiological functions. We focused on the importance of Akt in rapid synaptic structural changes after stimulation at the Drosophila neuromuscular junction, a well-studied model synapse. Compared with wild-type larvae, akt mutants showed significantly reduced muscle size and an increased number of boutons per area, suggesting that Akt is required for proper pre- and postsynaptic growth. In addition, the level of cysteine string protein (CSP) was significantly increased, and its distribution was different in akt mutants. After high K⁺ single stimulation, the CSP level of akt mutant NMJs increased dramatically compared with that of wild-type NMJs. Interestingly, ghost boutons without postsynaptic specialization were found in akt mutant NMJs, and the number of these boutons was significantly increased by patterned stimulation. In contrast, the postsynaptic change in the subsynaptic reticulum (SSR) in the akt mutant occurred independent of stimulation. These results suggest that Akt functions in both pre- and postsynaptic growth and differentiation, and in particular, presynaptic action occurs in an activity-dependent manner.

Integrated transcriptomics and proteomics provide new insights into the cadmium-induced ovarian toxicity on Pardosa pseudoannulata

Cadmium (Cd) pollution is intractable heavy metal pollution in the farmland ecosystem, posing a life-threatening challenge to the paddy field organisms. Spiders are riveting animal biomarkers for evaluating Cd-induced toxicity, yet the effects of long-term Cd toxicity on spider reproductive function and its underlying mechanism remain unclear. In the present study, we found that Cd exposure impaired the antioxidant enzyme system in the wolf spider Pardosa pseudoannulata and decreased the concentration of four antioxidant enzymes (catalase, glutathione peroxidase, superoxide dismutase, and peroxidase) (p < 0.05). The content of vitellogenin and the number of hatched spiderlings were also dramatically reduced under Cd stress (p < 0.05), indicating that Cd stress could vitiate the fecundity of P. pseudoannulata. Moreover, a total of 10,511 differentially expressed genes (DEGs) and 391 proteins (DEPs) were yielded from the ovarian transcriptome and proteome, and a mass of genes and proteins involved in protein processing in endoplasmic reticulum (ER) were significantly down-regulated. DEGs and DEPs directly encoding the antioxidant enzyme system and/or vitellogenesis were also distinctively down-regulated. In addition, we illustrated that the PI3K-AKT signaling pathway might play a crucial role in regulating protein synthesis, cell cycle, growth, differentiation and survival in P. pseudoannulata. The effects of protein processing in ER and PI3K-AKT pathways could further trigger transcriptional factor Forkhead shackling the protein synthesis and cell growth process. Collectively, this integrated analysis identified the Cd-induced reproductive toxicity on P. pseudoannulata and provided multifaceted insights to investigate the molecular mechanisms of spiders to Cd pollution.

Two insulin receptors coordinate oogenesis and oviposition via two pathways in the green lacewing, Chrysopa pallens

Insulin signalling in insects, as in mammals, regulates various physiological functions, such as reproduction. However, the molecular mechanism by which insulin signals orchestrate ovarian stem cell proliferation, vitellogenesis, and oviposition remains elusive. Here, we investigate the functions of the phosphoinositide 3-kinase (PI3K)-serine/threonine kinase (Akt) pathway, GTPase Ras/mitogen-activated protein kinase (MAPK) pathway, and their downstream messengers in a natural predator, Chrysopa pallens, by the RNAi method. When C. pallens vitellogenin gene 1 (CpVg1) expression was knocked down, the follicle maturation was arrested and total fecundity was reduced. Silencing C. pallens insulin receptor 1 (CpInR1) suppressed Vg transcription and reduced egg mass and hatching rate. Depletion of C. pallens insulin receptor 2 (CpInR2) transcripts lowered Vg transcript level, hampered ovarian development and decreased reproductive output. Knockdown of C. pallens Akt (CpAkt) and C. pallens extracellular-signal-regulated kinase (Cperk) caused phenotypes similar to those caused by knockdown of CpInR2. Disruption of C. pallens transcription factor forkhead box O (CpFoxO) expression caused no significant effects on ovarian development, but sharply impaired total fecundity. Interference with the expression of C. pallens target of rapamycin (CpTor) gene and C. pallens cAMP-response element binding protein (CpCreb) gene led to a down-regulation of Vg transcription, blocking of ovariole growth, and decrease in egg quality. These results suggested the two CpInRs orchestrate oogenesis and oviposition via two signalling pathways to guarantee natural reproduction in the green lacewing, C. pallens.

Microbiota-Dependent Priming of Antiviral Intestinal Immunity in Drosophila

Enteric pathogens must overcome intestinal defenses to establish infection. In Drosophila, the ERK signaling pathway inhibits enteric virus infection. The intestinal microflora also impacts immunity but its role in enteric viral infection is unknown. Here we show that two signals are required to activate antiviral ERK signaling in the intestinal epithelium. One signal depends on recognition of peptidoglycan from the microbiota, particularly from the commensal Acetobacter pomorum, which primes the NF-kB-dependent induction of a secreted factor, Pvf2. However, the microbiota is not sufficient to induce this pathway; a second virus-initiated signaling event involving release of transcriptional paused genes mediated by the kinase Cdk9 is also required for Pvf2 production. Pvf2 stimulates antiviral immunity by binding to the receptor tyrosine kinase PVR, which is necessary and sufficient for intestinal ERK responses. These findings demonstrate that sensing of specific commensals primes inflammatory signaling required for epithelial responses that restrict enteric viral infections.

Role of insulin receptor and insulin signaling on αPS2CβPS integrins' lateral diffusion

Integrins are ubiquitous transmembrane receptors with adhesion and signaling properties. The influence of insulin receptor and insulin signaling on αPS2CβPS integrins' lateral diffusion was studied using single particle tracking in S2 cells before and after reducing the insulin receptor expression or insulin stimulation. Insulin signaling was monitored by Western blotting for phospho-Akt expression. The expression of the insulin receptor was reduced using RNA interference (RNAi). After insulin receptor RNAi, four significant changes were measured in integrin diffusion properties: (1) there was a 24% increase in the mobile integrin population, (2) 14% of the increase was represented by integrins with Brownian diffusion, (3) for integrins that reside in confined zones of diffusion, there was a 45% increase in the diameter of the confined zone, and (4) there was a 29% increase in the duration integrins spend in confined zones of diffusion. In contrast to reduced expression of the insulin receptor, which alters integrin diffusion properties, insulin stimulation alone or insulin stimulation under conditions of reduced insulin receptor expression have minimal effects on altering the measured integrin diffusion properties. The differences in integrin diffusion measured after insulin receptor RNAi in the presence or absence of insulin stimulation may be the result of other insulin signaling pathways that are activated at reduced insulin receptor conditions. No change in the average integrin diffusion coefficient was measured for any conditions included in this study.

Erythropoietin-mediated protection of insect brain neurons involves JAK and STAT but not PI3K transduction pathways

The cytokine erythropoietin (Epo) initiates adaptive cellular responses to both moderate environmental challenges and tissue damaging insults in various non-hematopoietic mammalian tissues including the nervous system. Neuroprotective and neuroregenerative functions of Epo in mammals are mediated through receptor-associated Janus kinase 2 and intracellular signaling cascades that modify the transcription of Epo-regulated genes. Signal transducers and activators of transcription (STAT) and phosphoinositol-3-kinase (PI3K) represent key components of two important Epo-induced transduction pathways. Our previous study on insects revealed neuroprotective and regenerative functions of recombinant human Epo (rhEpo) similar to those in mammalian nervous tissues. Here we demonstrate that rhEpo effectively rescues primary cultured locust brain neurons from apoptotic cell death induced by hypoxia or the chemical compound H-7. The Janus kinase inhibitor AG-490 and the STAT inhibitor sc-355797 abolished protective effects of rhEpo on locust brain neurons. In contrast, inhibition of PI3K with LY294002 had no effect on rhEpo-mediated neuroprotection. The results indicate that rhEpo mediates the protection of locust brain neurons through interference with apoptotic pathways by the activation of a Janus kinase-associated receptor and STAT transcription factor(s). The involvement of similar transduction pathways in mammals and insects for the mediation of neuroprotection and support of neural regeneration by Epo indicates that an Epo/Epo receptor-like signaling system with high structural and functional similarity exists in both groups of animals. Epo-like signaling involved in tissue protection appears to be an ancient beneficial function shared by vertebrates and invertebrates.

Coordination of organ growth: principles and outstanding questions from the world of insects

In animal species undergoing determinate growth, the making of a full-size adult body requires a series of coordinated growth events culminating in the cessation of growth that precedes sexual maturation. The merger between physiology and genetics now coming to pass in the Drosophila model allows us to decipher these growth events with an unsurpassed level of sophistication. Here, we review several coordination mechanisms that represent fundamental aspects of growth control: adaptation of growth to environmental cues, interorgan coordination, and the coordination of growth with developmental transitions. The view is emerging of an integrated process where organ-autonomous growth is coordinated with both developmental and environmental cues to define final body size.

Ecdysone control of developmental transitions: lessons from Drosophila research

The steroid hormone ecdysone is the central regulator of insect developmental transitions. Recent new advances in our understanding of ecdysone action have relied heavily on the application of Drosophila melanogaster molecular genetic tools to study insect metamorphosis. In this review, we focus on three major aspects of Drosophila ecdysone biology: (a) factors that regulate the timing of ecdysone release, (b) molecular basis of stage- and tissue-specific responses to ecdysone, and (c) feedback regulation and coordination of ecdysone signaling.

13-Methyltetradecanoic acid induces mitochondrial-mediated apoptosis in human bladder cancer cells

OBJECTIVE

13-Methyltetradecanoic acid (13-MTD), a saturated branched-chain fatty acid purified from soy fermentation products, is known to induce apoptosis in many types of human cancer cells. This study was designed to investigate the molecular mechanisms involved in 13-MTD-induced apoptosis in human bladder cancer cells.

METHODS AND MATERIALS

MTT assay was used to investigate the potential effects of 13-MTD on the growth and viability of human bladder cancer cells. To find out whether anti-proliferation and cell death were associated with apoptosis, we used flow cytometry to quantify the extent of apoptosis and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay to measures DNA degradation of apoptotic cells. The proteins involved in the 13-MTD induced apoptosis were examined using Western blot.

RESULTS

We show that 13-MTD inhibits cellular proliferation and viability in human bladder cancer cells, which has been attributed to apoptosis. 13-MTD down-regulates Bcl-2 and up-regulates Bax. This promotes mitochondrial dysfunction, leading to the release of cytochrome c from the mitochondria to the cytoplasm, as well as the proteolytic activation of caspases. Moreover, 13-MTD down-regulates AKT phosphorylation and activates phosphorylation of p38 and c-Jun N-terminal kinase (JNK). Up-regulating AKT phosphorylation and down-regulating JNK and P38 phosphorylation could attenuate the13-MTD-induced apoptosis.

CONCLUSION

Taken together, these data indicate that 13-MTD induces mitochondrial-mediated apoptosis through regulation of the AKT and MAPK pathways, suggesting 13-MTD is a potential candidate for treatment of human bladder cancer.

Insulin signaling mediates sexual attractiveness in Drosophila

Sexually attractive characteristics are often thought to reflect an individual's condition or reproductive potential, but the underlying molecular mechanisms through which they do so are generally unknown. Insulin/insulin-like growth factor signaling (IIS) is known to modulate aging, reproduction, and stress resistance in several species and to contribute to variability of these traits in natural populations. Here we show that IIS determines sexual attractiveness in Drosophila through transcriptional regulation of genes involved in the production of cuticular hydrocarbons (CHC), many of which function as pheromones. Using traditional gas chromatography/mass spectrometry (GC/MS) together with newly introduced laser desorption/ionization orthogonal time-of-flight mass spectrometry (LDI-MS) we establish that CHC profiles are significantly affected by genetic manipulations that target IIS. Manipulations that reduce IIS also reduce attractiveness, while females with increased IIS are significantly more attractive than wild-type animals. IIS effects on attractiveness are mediated by changes in CHC profiles. Insulin signaling influences CHC through pathways that are likely independent of dFOXO and that may involve the nutrient-sensing Target of Rapamycin (TOR) pathway. These results suggest that the activity of conserved molecular regulators of longevity and reproductive output may manifest in different species as external characteristics that are perceived as honest indicators of fitness potential.

In nature, a myriad of specialized traits have evolved that are used for intraspecific communication and mate choice. We postulated that certain traits may have evolved to be attractive by virtue of their accurate representation of molecular pathways that are critical for determining evolutionary fitness. Insulin signaling (IIS) is one such pathway. It has been shown to modulate aging, reproduction, and stress resistance in several species and to contribute to variability of these traits in natural populations. We therefore asked whether IIS affected key sexual characteristics and overall attractiveness in the fruit fly Drosophila melanogaster. We found that IIS regulates cuticular hydrocarbons (the key pheromones in flies), that reduced IIS also reduced attractiveness, and that flies with increased IIS were significantly more attractive than wild-type animals. Further experiments revealed that these effects may also be influenced by a second conserved nutrient-sensitive pathway, the TOR pathway. We suggest that natural selection may have favored a plethora of species-specific sexual characteristics because they accurately represent a small number of influential pathways that determine longevity and reproductive output across taxa. In other words, it may be that, whether fly or human, beauty is more than skin-deep.

ERK7 is a negative regulator of protein secretion in response to amino-acid starvation by modulating Sec16 membrane association

RNAi screening for kinases regulating the functional organization of the early secretory pathway in Drosophila S2 cells has identified the atypical Mitotic-Associated Protein Kinase (MAPK) Extracellularly regulated kinase 7 (ERK7) as a new modulator. We found that ERK7 negatively regulates secretion in response to serum and amino-acid starvation, in both Drosophila and human cells. Under these conditions, ERK7 turnover through the proteasome is inhibited, and the resulting higher levels of this kinase lead to a modification in a site within the C-terminus of Sec16, a key ER exit site component. This post-translational modification elicits the cytoplasmic dispersion of Sec16 and the consequent disassembly of the ER exit sites, which in turn results in protein secretion inhibition. We found that ER exit site disassembly upon starvation is TOR complex 1 (TORC1) independent, showing that under nutrient stress conditions, cell growth is not only inhibited at the transcriptional and translational levels, but also independently at the level of secretion by inhibiting the membrane flow through the early secretory pathway. These results reveal the existence of new signalling circuits participating in the complex regulation of cell growth.

SLOB, a SLOWPOKE channel binding protein, regulates insulin pathway signaling and metabolism in Drosophila

There is ample evidence that ion channel modulation by accessory proteins within a macromolecular complex can regulate channel activity and thereby impact neuronal excitability. However, the downstream consequences of ion channel modulation remain largely undetermined. The Drosophila melanogaster large conductance calcium-activated potassium channel SLOWPOKE (SLO) undergoes modulation via its binding partner SLO-binding protein (SLOB). Regulation of SLO by SLOB influences the voltage dependence of SLO activation and modulates synaptic transmission. SLO and SLOB are expressed especially prominently in median neurosecretory cells (mNSCs) in the pars intercerebralis (PI) region of the brain; these cells also express and secrete Drosophila insulin like peptides (dILPs). Previously, we found that flies lacking SLOB exhibit increased resistance to starvation, and we reasoned that SLOB may regulate aspects of insulin signaling and metabolism. Here we investigate the role of SLOB in metabolism and find that slob null flies exhibit changes in energy storage and insulin pathway signaling. In addition, slob null flies have decreased levels of dilp3 and increased levels of takeout, a gene known to be involved in feeding and metabolism. Targeted expression of SLOB to mNSCs rescues these alterations in gene expression, as well as the metabolic phenotypes. Analysis of fly lines mutant for both slob and slo indicate that the effect of SLOB on metabolism and gene expression is via SLO. We propose that modulation of SLO by SLOB regulates neurotransmission in mNSCs, influencing downstream insulin pathway signaling and metabolism.

Involvement of PI3K/Akt signaling in PTTH-stimulated ecdysteroidogenesis by prothoracic glands of the silkworm, Bombyx mori

The prothoracicotropic hormone (PTTH) stimulates ecdysteroidogenesis by prothoracic gland in larval insects. Previous studies showed that Ca(2+), cAMP, extracellular signal-regulated kinase (ERK), and tyrosine kinase are involved in PTTH-stimulated ecdysteroidogenesis by the prothoracic glands of both Bombyx mori and Manduca sexta. In the present study, the involvement of phosphoinositide 3-kinase (PI3K)/Akt signaling in PTTH-stimulated ecdysteroidogenesis by B. mori prothoracic glands was further investigated. The results showed that PTTH-stimulated ecdysteroidogenesis was partially blocked by LY294002 and wortmannin, indicating that PI3K is involved in PTTH-stimulated ecdysteroidogenesis. Akt phosphorylation in the prothoracic glands appeared to be moderately stimulated by PTTH in vitro. PTTH-stimulated Akt phosphorylation was inhibited by LY294002. An in vivo PTTH injection into day 6 last instar larvae also increased Akt phosphorylation of the prothoracic glands. In addition, PTTH-stimulated ERK phosphorylation of the prothoracic glands was not inhibited by either LY294002 or wortmannin, indicating that PI3K is not involved in PTTH-stimulated ERK signaling. A23187 and thapsigargin, which stimulated B. mori prothoracic gland ERK phosphorylation and ecdysteroidogenesis, could not activate Akt phosphorylation. PTTH-stimulated ecdysteroidogenesis was not further activated by insulin, indicating the absence of an additive action of insulin and PTTH on the prothoracic glands. The present study, together with the previous demonstration that insulin stimulates B. mori ecdysteroidogenesis through PI3K/Akt signaling, suggests that crosstalk exists in B. mori prothoracic glands between insulin and PTTH signaling, which may play a critical role in precisely regulated ecdysteroidogenesis during development.

Two insulin-like peptide family members from the mosquito Aedes aegypti exhibit differential biological and receptor binding activities

Insects encode multiple ILPs but only one homolog of the vertebrate IR that activates the insulin-signaling pathway. However, it remains unclear whether all insect ILPs are high affinity ligands for the IR or have similar biological functions. The yellow fever mosquito, Aedes aegypti, encodes eight ILPs with prior studies strongly implicating ILPs from the brain in regulating metabolism and the maturation of eggs following blood feeding. Here we addressed whether two ILP family members expressed in the brain, ILP4 and ILP3, have overlapping functional and receptor binding activities. Our results indicated that ILP3 exhibits strong insulin-like activity by elevating carbohydrate and lipid storage in sugar-fed adult females, whereas ILP4 does not. In contrast, both ILPs exhibited dose-dependent gonadotropic activity in blood-fed females as measured by the stimulation of ovaries to produce ecdysteroids and the uptake of yolk by primary oocytes. Binding studies using ovary membranes indicated that ILP4 and ILP3 do not cross compete; a finding further corroborated by cross-linking and immunoblotting experiments showing that ILP3 binds the MIR while ILP4 binds an unknown 55kDa membrane protein. In contrast, each ILP activated the insulin-signaling pathway in ovaries as measured by enhanced phosphorylation of Akt. RNAi and inhibitor studies further indicated that the gonadotropic activity of ILP4 and ILP3 requires the MIR and a functional insulin-signaling pathway. Taken together, our results indicate that two members of the Ae. aegypti ILP family exhibit partially overlapping biological activity and different binding interactions with the MIR.

Molecular mechanisms of metabolic regulation by insulin in Drosophila

The insulin signalling pathway is highly conserved from mammals to Drosophila. Insulin signalling in the fly, as in mammals, regulates a number of physiological functions, including carbohydrate and lipid metabolism, tissue growth and longevity. In the present review, I discuss the molecular mechanisms by which insulin signalling regulates metabolism in Drosophila, comparing and contrasting with the mammalian system. I discuss both the intracellular signalling network, as well as the communication between organs in the fly.

Arrestin development: emerging roles for beta-arrestins in developmental signaling pathways

Arrestins were identified as mediators of G protein-coupled receptor (GPCR) desensitization and endocytosis. However, it is now clear that they scaffold many intracellular signaling networks to modulate the strength and duration of signaling by diverse types of receptors--including those relevant to the Hedgehog, Wnt, Notch, and TGFbeta pathways--and downstream kinases such as the MAPK and Akt/PI3K cascades. The involvement of arrestins in many discrete developmental signaling events suggests an indispensable role for these multifaceted molecular scaffolds.

Insulin delays the progression of Drosophila cells through G2/M by activating the dTOR/dRaptor complex

In Drosophila and mammals, insulin signalling can increase growth, progression through G1/S, cell size and tissue size. Here, we analyse the way insulin affects cell size and cell-cycle progression in two haemocyte-derived Drosophila cell lines. Surprisingly, we find that although insulin increases cell size, it slows the rate at which these cells increase in number. By using BrdU pulse-chase to label S-phase cells and follow their progression through the cell cycle, we show that insulin delays progression through G2/M, thereby slowing cell division. The ability of insulin to slow progression through G2/M is independent of its ability to stimulate progression through G1/S, so is not a consequence of feedback by the cell-cycle machinery to maintain cell-cycle length. Insulin's effects on progression through G2/M are mediated by dTOR/dRaptor signalling. Partially inhibiting dTOR/dRaptor signalling by dsRNAi or mild rapamycin treatment can increase cell number in cultured haemocytes and the Drosophila wing, respectively. Thus, insulin signalling can influence cell number depending on a balance between its ability to accelerate progression through G1/S and delay progression through G2/M.

Regulation of the RAP1/RAF-1/extracellularly regulated kinase-1/2 cascade and prolactin release by the phosphoinositide 3-kinase/AKT pathway in pituitary cells

In pituitary cells, prolactin (PRL) synthesis and release are controlled by multiple transduction pathways. In the GH4C1 somatolactotroph cell line, we previously reported that MAPK ERK-1/2 are a point of convergence between the pathways involved in the PRL gene regulation. In the present study, we focused on the involvement of the phosphoinositide 3-kinase (PI3K)/Akt pathway in the MAPK ERK-1/2 regulation and PRL secretion in pituitary cells. Either specific pharmacological PI3K and Akt inhibitors (LY294002, Akt I, and phosphoinositide analog-6) or Akt dominant-negative mutant (K179M) enhanced ERK-1/2 phosphorylation in unstimulated GH4C1 cells. Under the same conditions, PI3K and Akt inhibition also both increased Raf-1 kinase activity and the levels of GTP-bound (active form) monomeric G protein Rap1, which suggests that a down-regulation of the ERK-1/2 cascade is induced by the PI3K/Akt signaling pathway in unstimulated cells. On the contrary, ERK-1/2 phosphorylation, Raf-1 activity, and Rap1 activation were almost completely blocked in IGF-I-stimulated cells previously subjected to PI3K or Akt inhibition. Although the PRL promoter was not affected by either PI3K/Akt inhibition or activation, PRL release increased in response to the pharmacological PI3K/Akt inhibitors in unstimulated GH4C1 and rat pituitary primary cells. The IGF-I-stimulated PRL secretion was diminished, on the contrary, by the pharmacological PI3K/Akt inhibitors. Taken together, these findings indicate that the PI3K/Akt pathway exerts dual regulatory effects on both the Rap1/Raf-1/ERK-1/2 cascade and PRL release in pituitary cells, i.e. negative effects in unstimulated cells and positive ones in IGF-I-stimulated cells.

Phagocytosis by Lymnaea stagnalis haemocytes: A potential role for phosphatidylinositol 3-kinase but not protein kinase A

The molecular events that regulate phagocytosis, an important innate immune response, in invertebrate defence cells (haemocytes) are poorly understood. Lymnaea stagnalis haemocytes were used as a model to elucidate the role of cell signalling pathways in phagocytosis by molluscan defence cells. The phosphatidylinositol 3-kinase (PI3-K) inhibitor, LY294002, significantly impaired haemocyte phagocytic activity in a dose-responsive manner with 10 microM LY294002 reducing internalization of fluorescent-conjugated Escherichia coli by 62% (P < or = 0.001). In contrast, the protein kinase A (PKA) inhibitor KT5720 was without effect. Therefore, PI3-K, but not PKA, appears to control phagocytosis by haemocytes in these gastropod molluscs.