The effects of empagliflozin, dietary energy restriction, or both on appetite-regulatory gut peptides in individuals with type 2 diabetes and overweight or obesity: The SEESAW randomized, double-blind, placebo-controlled trial.

AIM: To assess the impact of the sodium-glucose co-transporter-2 (SGLT2) inhibitor empagliflozin (25 mg once-daily), dietary energy restriction, or both combined, on circulating appetite-regulatory peptides in people with type 2 diabetes (T2D) and overweight or obesity. MATERIALS AND METHODS: In a double-blind, placebo-controlled trial, 68 adults (aged 30-75 years) with T2D (drug naïve or on metformin monotherapy; HbA1c 6.0%-10.0% [42-86 mmol/mol]) and body mass index of 25 kg/m2 or higher were randomized to (a) placebo only, (b) placebo plus diet, (c) empagliflozin only or (d) empagliflozin plus diet for 24 weeks. Dietary energy restriction matched the estimated energy deficit elicited by SGLT2 inhibitor therapy through urinary glucose excretion (~360 kcal/day). The primary outcome was change in postprandial circulating total peptide-YY (PYY) during a 3-hour mixed-meal tolerance test from baseline to 24 weeks. Postprandial total glucagon-like peptide-1 (GLP-1), acylated ghrelin and subjective appetite perceptions formed secondary outcomes, along with other key components of energy balance. RESULTS: The mean weight loss in each group at 24 weeks was 0.44, 1.91, 2.22 and 5.74 kg, respectively. The change from baseline to 24 weeks in postprandial total PYY was similar between experimental groups and placebo only (mean difference [95% CI]: -8.6 [-28.6 to 11.4], 13.4 [-6.1 to 33.0] and 1.0 [-18.0 to 19.9] pg/ml in placebo-plus diet, empagliflozin-only and empagliflozin-plus-diet groups, respectively [all P ≥ .18]). Similarly, there was no consistent pattern of difference between groups for postprandial total GLP-1, acylated ghrelin and subjective appetite perceptions. CONCLUSIONS: In people with T2D and overweight or obesity, changes in postprandial appetite-regulatory gut peptides may not underpin the less than predicted weight loss observed with empagliflozin therapy. CLINICAL TRIALS REGISTRATION: NCT02798744, www. CLINICALTRIALS: gov; 2015-001594-40, www.EudraCT.ema.europa.eu; ISRCTN82062639, www.ISRCTN.org.

Relationships between immune gene expression and circulating cytokine levels in wild house mice.

Quantitative PCR (qPCR) has been commonly used to measure gene expression in a number of research contexts, but the measured RNA concentrations do not always represent the concentrations of active proteins which they encode. This can be due to transcriptional regulation or post-translational modifications, or localization of immune environments, as can occur during infection. However, in studies using free-living non-model species, such as in ecoimmunological research, qPCR may be the only available option to measure a parameter of interest, and so understanding the quantitative link between gene expression and associated effector protein levels is vital.Here, we use qPCR to measure concentrations of RNA from mesenteric lymph node (MLN) and spleen tissue, and multiplex ELISA of blood serum to measure circulating cytokine concentrations in a wild population of a model species, Mus musculus domesticus.Few significant correlations were found between gene expression levels and circulating cytokines of the same immune genes or proteins, or related functional groups. Where significant correlations were observed, these were most frequently within the measured tissue (i.e., the expression levels of genes measured from spleen tissue were more likely to correlate with each other rather than with genes measured from MLN tissue, or with cytokine concentrations measured from blood).Potential reasons for discrepancies between measures including differences in decay rates and transcriptional regulation networks are discussed. We highlight the relative usefulness of different measures under different research questions and consider what might be inferred from immune assays.

Immune state is associated with natural dietary variation in wild mice Mus musculus domesticus.

The ability, propensity and need to mount an immune response vary both among individuals and within a single individual over time.A wide array of parameters has been found to influence immune state in carefully controlled experiments, but we understand much less about which of these parameters are important in determining immune state in wild populations.Diet can influence immune responses, for example when nutrient availability is limited. We therefore predict that natural dietary variation will play a role in modulating immune state, but this has never been tested.We measured carbon and nitrogen stable isotope ratios in an island population of house mice Mus musculus domesticus as an indication of dietary variation, and the expression of a range of immune-related genes to represent immune state.After accounting for potential confounding influences such as age, sex and helminth load, we found a significant association between carbon isotope ratio and levels of immune activity in the mesenteric lymph nodes, particularly in relation to the inflammatory response.This association demonstrates the important interplay between diet and an animal's response to immune challenges, and therefore potentially its susceptibility to disease. A plain language summary is available for this article.

Geographical location influences the composition of the gut microbiota in wild house mice (Mus musculus domesticus) at a fine spatial scale.

The composition of the mammalian gut microbiota can be influenced by a multitude of environmental variables such as diet and infections. Studies investigating the effect of these variables on gut microbiota composition often sample across multiple separate populations and habitat types. In this study we explore how variation in the gut microbiota of the house mouse (Mus musculus domesticus) on the Isle of May, a small island off the east coast of Scotland, is associated with environmental and biological factors. Our study focuses on the effects of environmental variables, specifically trapping location and surrounding vegetation, as well as the host variables sex, age, body weight and endoparasite infection, on the gut microbiota composition across a fine spatial scale in a freely interbreeding population. We found that differences in gut microbiota composition were significantly associated with the trapping location of the host, even across this small spatial scale. Sex of the host showed a weak association with microbiota composition. Whilst sex and location could be identified as playing an important role in the compositional variation of the gut microbiota, 75% of the variation remains unexplained. Whereas other rodent studies have found associations between gut microbiota composition and age of the host or parasite infections, the present study could not clearly establish these associations. We conclude that fine spatial scales are important when considering gut microbiota composition and investigating differences among individuals.

The repeat region of the circumsporozoite protein is critical for sporozoite formation and maturation in Plasmodium.

The circumsporozoite protein (CSP) is the major surface protein of the sporozoite stage of malaria parasites and has multiple functions as the parasite develops and then migrates from the mosquito midgut to the mammalian liver. The overall structure of CSP is conserved among Plasmodium species, consisting of a species-specific central tandem repeat region flanked by two conserved domains: the NH2-terminus and the thrombospondin repeat (TSR) at the COOH-terminus. Although the central repeat region is an immunodominant B-cell epitope and the basis of the only candidate malaria vaccine in Phase III clinical trials, little is known about its functional role(s). We used the rodent malaria model Plasmodium berghei to investigate the role of the CSP tandem repeat region during sporozoite development. Here we describe two mutant parasite lines, one lacking the tandem repeat region (ΔRep) and the other lacking the NH2-terminus as well as the repeat region (ΔNΔRep). We show that in both mutant lines oocyst formation is unaffected but sporozoite development is defective.

Genome-wide functional analysis of Plasmodium protein phosphatases reveals key regulators of parasite development and differentiation.

Reversible protein phosphorylation regulated by kinases and phosphatases controls many cellular processes. Although essential functions for the malaria parasite kinome have been reported, the roles of most protein phosphatases (PPs) during Plasmodium development are unknown. We report a functional analysis of the Plasmodium berghei protein phosphatome, which exhibits high conservation with the P. falciparum phosphatome and comprises 30 predicted PPs with differential and distinct expression patterns during various stages of the life cycle. Gene disruption analysis of P. berghei PPs reveals that half of the genes are likely essential for asexual blood stage development, whereas six are required for sexual development/sporogony in mosquitoes. Phenotypic screening coupled with transcriptome sequencing unveiled morphological changes and altered gene expression in deletion mutants of two N-myristoylated PPs. These findings provide systematic functional analyses of PPs in Plasmodium, identify how phosphatases regulate parasite development and differentiation, and can inform the identification of drug targets for malaria.

Development of a transgenic Plasmodium berghei line (Pb pfpkg) expressing the P. falciparum cGMP-dependent protein kinase, a novel antimalarial drug target.

With the inevitable selection of resistance to antimalarial drugs in treated populations, there is a need for new medicines to enter the clinic and new targets to progress through the drug discovery pipeline. In this study we set out to develop a transgenic rodent model for testing inhibitors of the Plasmodium falciparum cyclic GMP-dependent kinase in vivo. A model was needed that would allow us to investigate whether differences in amino acid sequence of this enzyme between species influences in vivo efficacy. Here we report the successful development of a transgenic P. berghei line in which the cyclic GMP-dependent protein kinase (PKG) was replaced by the P. falciparum orthologue. We demonstrate that the P. falciparum orthologue was able to functionally complement the endogenous P. berghei pkg gene throughout blood stage development and early sexual development. However, subsequent development in the mosquito was severely compromised. We show that this is due to a defect in the female lineage of the transgenic by using genetic crosses with both male and female deficient P. berghei lines. This defect could be due to expression of a female-specific target in the mosquito stages of P. berghei that cannot be phosphorylated by the P. falciparum kinase. Using a previously reported anti-coccidial inhibitor of the cyclic GMP-dependent protein kinase, we show no difference in in vivo efficacy between the transgenic and control P. berghei lines. This in vivo model will be useful for screening future generations of cyclic GMP-dependent protein kinase inhibitors and allowing us to overcome any species-specific differences in the enzyme primary sequence that would influence in vivo efficacy in the rodent model. The approach will also be applicable to in vivo testing of other antimalarial compounds where the target is known.

Unique apicomplexan IMC sub-compartment proteins are early markers for apical polarity in the malaria parasite.

The phylum Apicomplexa comprises over 5000 intracellular protozoan parasites, including Plasmodium and Toxoplasma, that are clinically important pathogens affecting humans and livestock. Malaria parasites belonging to the genus Plasmodium possess a pellicle comprised of a plasmalemma and inner membrane complex (IMC), which is implicated in parasite motility and invasion. Using live cell imaging and reverse genetics in the rodent malaria model P. berghei, we localise two unique IMC sub-compartment proteins (ISPs) and examine their role in defining apical polarity during zygote (ookinete) development. We show that these proteins localise to the anterior apical end of the parasite where IMC organisation is initiated, and are expressed at all developmental stages, especially those that are invasive. Both ISP proteins are N-myristoylated, phosphorylated and membrane-bound. Gene disruption studies suggest that ISP1 is likely essential for parasite development, whereas ISP3 is not. However, an absence of ISP3 alters the apical localisation of ISP1 in all invasive stages including ookinetes and sporozoites, suggesting a coordinated function for these proteins in the organisation of apical polarity in the parasite.

The Plasmodium berghei Ca(2+)/H(+) exchanger, PbCAX, is essential for tolerance to environmental Ca(2+) during sexual development.

Ca(2+) contributes to a myriad of important cellular processes in all organisms, including the apicomplexans, Plasmodium and Toxoplasma. Due to its varied and essential roles, free Ca(2+) is tightly regulated by complex mechanisms. These mechanisms are therefore of interest as putative drug targets. One pathway in Ca(2+) homeostatic control in apicomplexans uses a Ca(2+)/H(+) exchanger (a member of the cation exchanger family, CAX). The P. falciparum CAX (PfCAX) has recently been characterised in asexual blood stage parasites. To determine the physiological importance of apicomplexan CAXs, tagging and knock-out strategies were undertaken in the genetically tractable T. gondii and P. berghei parasites. In addition, a yeast heterologous expression system was used to study the function of apicomplexan CAXs. Tagging of T. gondii and P. berghei CAXs (TgCAX and PbCAX) under control of their endogenous promoters could not demonstrate measureable expression of either CAX in tachyzoites and asexual blood stages, respectively. These results were consistent with the ability of parasites to tolerate knock-outs of the genes for TgCAX and PbCAX at these developmental stages. In contrast, PbCAX expression was detectable during sexual stages of development in female gametocytes/gametes, zygotes and ookinetes, where it was dispersed in membranous networks within the cytosol (with minimal mitochondrial localisation). Furthermore, genetically disrupted parasites failed to develop further from "round" form zygotes, suggesting that PbCAX is essential for ookinete development and differentiation. This impeded phenotype could be rescued by removal of extracellular Ca(2+). Therefore, PbCAX provides a mechanism for free living parasites to multiply within the ionic microenvironment of the mosquito midgut. Ca(2+) homeostasis mediated by PbCAX is critical and suggests plasmodial CAXs may be targeted in approaches designed to block parasite transmission.

An ancient protein phosphatase, SHLP1, is critical to microneme development in Plasmodium ookinetes and parasite transmission.

Signaling pathways controlled by reversible protein phosphorylation (catalyzed by kinases and phosphatases) in the malaria parasite Plasmodium are of great interest, for both increased understanding of parasite biology and identification of novel drug targets. Here, we report a functional analysis in Plasmodium of an ancient bacterial Shewanella-like protein phosphatase (SHLP1) found only in bacteria, fungi, protists, and plants. SHLP1 is abundant in asexual blood stages and expressed at all stages of the parasite life cycle. shlp1 deletion results in a reduction in ookinete (zygote) development, microneme formation, and complete ablation of oocyst formation, thereby blocking parasite transmission. This defect is carried by the female gamete and can be rescued by direct injection of mutant ookinetes into the mosquito hemocoel, where oocysts develop. This study emphasizes the varied functions of SHLP1 in Plasmodium ookinete biology and suggests that it could be a novel drug target for blocking parasite transmission.

A unique protein phosphatase with kelch-like domains (PPKL) in Plasmodium modulates ookinete differentiation, motility and invasion.

Protein phosphorylation and dephosphorylation (catalysed by kinases and phosphatases, respectively) are post-translational modifications that play key roles in many eukaryotic signalling pathways, and are often deregulated in a number of pathological conditions in humans. In the malaria parasite Plasmodium, functional insights into its kinome have only recently been achieved, with over half being essential for blood stage development and another 14 kinases being essential for sexual development and mosquito transmission. However, functions for any of the plasmodial protein phosphatases are unknown. Here, we use reverse genetics in the rodent malaria model, Plasmodium berghei, to examine the role of a unique protein phosphatase containing kelch-like domains (termed PPKL) from a family related to Arabidopsis BSU1. Phylogenetic analysis confirmed that the family of BSU1-like proteins including PPKL is encoded in the genomes of land plants, green algae and alveolates, but not in other eukaryotic lineages. Furthermore, PPKL was observed in a distinct family, separate to the most closely-related phosphatase family, PP1. In our genetic approach, C-terminal GFP fusion with PPKL showed an active protein phosphatase preferentially expressed in female gametocytes and ookinetes. Deletion of the endogenous ppkl gene caused abnormal ookinete development and differentiation, and dissociated apical microtubules from the inner-membrane complex, generating an immotile phenotype and failure to invade the mosquito mid-gut epithelium. These observations were substantiated by changes in localisation of cytoskeletal tubulin and actin, and the micronemal protein CTRP in the knockout mutant as assessed by indirect immunofluorescence. Finally, increased mRNA expression of dozi, a RNA helicase vital to zygote development was observed in ppkl(-) mutants, with global phosphorylation studies of ookinete differentiation from 1.5-24 h post-fertilisation indicating major changes in the first hours of zygote development. Our work demonstrates a stage-specific essentiality of the unique PPKL enzyme, which modulates parasite differentiation, motility and transmission.

A putative homologue of CDC20/CDH1 in the malaria parasite is essential for male gamete development.

Cell-cycle progression is governed by a series of essential regulatory proteins. Two major regulators are cell-division cycle protein 20 (CDC20) and its homologue, CDC20 homologue 1 (CDH1), which activate the anaphase-promoting complex/cyclosome (APC/C) in mitosis, and facilitate degradation of mitotic APC/C substrates. The malaria parasite, Plasmodium, is a haploid organism which, during its life-cycle undergoes two stages of mitosis; one associated with asexual multiplication and the other with male gametogenesis. Cell-cycle regulation and DNA replication in Plasmodium was recently shown to be dependent on the activity of a number of protein kinases. However, the function of cell division cycle proteins that are also involved in this process, such as CDC20 and CDH1 is totally unknown. Here we examine the role of a putative CDC20/CDH1 in the rodent malaria Plasmodium berghei (Pb) using reverse genetics. Phylogenetic analysis identified a single putative Plasmodium CDC20/CDH1 homologue (termed CDC20 for simplicity) suggesting that Plasmodium APC/C has only one regulator. In our genetic approach to delete the endogenous cdc20 gene of P. berghei, we demonstrate that PbCDC20 plays a vital role in male gametogenesis, but is not essential for mitosis in the asexual blood stage. Furthermore, qRT-PCR analysis in parasite lines with deletions of two kinase genes involved in male sexual development (map2 and cdpk4), showed a significant increase in cdc20 transcription in activated gametocytes. DNA replication and ultra structural analyses of cdc20 and map2 mutants showed similar blockage of nuclear division at the nuclear spindle/kinetochore stage. CDC20 was phosphorylated in asexual and sexual stages, but the level of modification was higher in activated gametocytes and ookinetes. Changes in global protein phosphorylation patterns in the Δcdc20 mutant parasites were largely different from those observed in the Δmap2 mutant. This suggests that CDC20 and MAP2 are both likely to play independent but vital roles in male gametogenesis.

The M3-muscarinic receptor regulates learning and memory in a receptor phosphorylation/arrestin-dependent manner.

Degeneration of the cholinergic system is considered to be the underlying pathology that results in the cognitive deficit in Alzheimer's disease. This pathology is thought to be linked to a loss of signaling through the cholinergic M(1)-muscarinic receptor subtype. However, recent studies have cast doubt on whether this is the primary receptor mediating cholinergic-hippocampal learning and memory. The current study offers an alternative mechanism involving the M(3)-muscarinic receptor that is expressed in numerous brain regions including the hippocampus. We demonstrate here that M(3)-muscarinic receptor knockout mice show a deficit in fear conditioning learning and memory. The mechanism used by the M(3)-muscarinic receptor in this process involves receptor phosphorylation because a knockin mouse strain expressing a phosphorylation-deficient receptor mutant also shows a deficit in fear conditioning. Consistent with a role for receptor phosphorylation, we demonstrate that the M(3)-muscarinic receptor is phosphorylated in the hippocampus following agonist treatment and following fear conditioning training. Importantly, the phosphorylation-deficient M(3)-muscarinic receptor was coupled normally to G(q/11)-signaling but was uncoupled from phosphorylation-dependent processes such as receptor internalization and arrestin recruitment. It can, therefore, be concluded that M(3)-muscarinic receptor-dependent learning and memory depends, at least in part, on receptor phosphorylation/arrestin signaling. This study opens the potential for biased M(3)-muscarinic receptor ligands that direct phosphorylation/arrestin-dependent (non-G protein) signaling as being beneficial in cognitive disorders.

Ubiquitination as a priming process of PKC alpha and PKC epsilon degradation in the alphaT3-1 gonadotrope cell line.

BACKGROUND/AIM: Protein kinase C (PKC) is a family of isoenzymes playing a key role in the regulation of gonadotrope cell functions. Specific PKC isoforms are activated and downregulated differentially by gonadotropin-releasing hormone (GnRH) and the phorbol ester TPA. In the present study, focusing mainly on PKC epsilon, the mechanisms underlying the proteasome-dependent downregulation of GnRH-activated PKC epsilon and TPA-sensitive PKC alpha and epsilon isoenzymes were investigated in alphaT3-1 gonadotrope cells. METHODS/RESULTS: In pull-down assays involving the use of glutathione-agarose affinity beads conjugated with a GST-fusion protein containing ubiquitin-associated domains of Rad23 that bind very likely to K48-linked polyubiquitinated proteins, TPA induced rapid (within 15 min) and sustained (up to 4 h) PKC alpha and PKC epsilon polyubiquitination. However, GnRH selectively elicited receptor-dependent polyubiquitination of PKC epsilon, but not that of PKC alpha. The GnRH-evoked PKC epsilon polyubiquitination was a strong, fast process (taking place as early as 10 min) which decreased progressively with time (but was still detectable after 4 h of treatment). In addition, no apparent association between PKC epsilon and the lysosomal compartment was observed upon performing double-labeling immunofluorescence and confocal microscopy, after either 10 min or 1 hour of stimulation by GnRH or the phorbol ester. CONCLUSION: In alphaT3-1 gonadotrope cells, polyubiquitination is therefore the event triggering GnRH-evoked PKC epsilon desensitization as well as TPA-induced PKC alpha and PKC epsilon downregulations; it precedes the respective isoenzyme's degradation by the proteasome complex.

Phosphorylation and regulation of a G protein-coupled receptor by protein kinase CK2.

We demonstrate a role for protein kinase casein kinase 2 (CK2) in the phosphorylation and regulation of the M3-muscarinic receptor in transfected cells and cerebellar granule neurons. On agonist occupation, specific subsets of receptor phosphoacceptor sites (which include the SASSDEED motif in the third intracellular loop) are phosphorylated by CK2. Receptor phosphorylation mediated by CK2 specifically regulates receptor coupling to the Jun-kinase pathway. Importantly, other phosphorylation-dependent receptor processes are regulated by kinases distinct from CK2. We conclude that G protein-coupled receptors (GPCRs) can be phosphorylated in an agonist-dependent fashion by protein kinases from a diverse range of kinase families, not just the GPCR kinases, and that receptor phosphorylation by a defined kinase determines a specific signalling outcome. Furthermore, we demonstrate that the M3-muscarinic receptor can be differentially phosphorylated in different cell types, indicating that phosphorylation is a flexible regulatory process where the sites that are phosphorylated, and hence the signalling outcome, are dependent on the cell type in which the receptor is expressed.

Intramolecular interaction between phosphorylated tyrosine-783 and the C-terminal Src homology 2 domain activates phospholipase C-gamma1.

Phospholipase C-gamma1 (PLC-gamma1) contains two tandem Src homology 2 (SH2) domains. The NH(2)-terminal SH2 domain has been known to mediate the binding of PLC-gamma1 to receptor protein tyrosine kinases, which then activate PLC-gamma1 via phosphorylation at Y783. We now show that the phosphorylated Y783 residue (pY783) associates with the COOH-terminal SH2 domain [SH2(C)] within the same molecule of PLC-gamma1. The specificity of this intramolecular interaction is demonstrated in several ways. The mutation of SH2(C), but not of the NH(2)-terminal SH2 domain, exposes pY783 and makes it available for binding by anti-pY783 antibodies, for intermolecular association with a GST fusion protein containing the tandem SH2 domains of PLC-gamma1 and for dephosphorylation by phosphatases. The intramolecular interaction between pY783 and SH2(C) induces a rearrangement of surface charge such that PLC-gamma1 molecules phosphorylated at Y783 are retained more strongly by heparin resins than are unphosphorylated molecules. Finally, the intramolecular interaction of pY783 with SH2(C) results in activation of phospholipase activity. Our results thus clarify the molecular mechanism of PLC-gamma1 activation, revealing the specific function of pY783 and the distinct roles of the two SH2 domains in this process.

Mechanism of B-cell receptor-induced phosphorylation and activation of phospholipase C-gamma2.

Phospholipase C-gamma2 (PLC-gamma2) plays an important role in B-cell signaling. Phosphorylation of various tyrosine residues of PLC-gamma2 has been implicated in regulation of its lipase activity. With the use of antibodies specific for each of the putative phosphorylation sites, we have now shown that PLC-gamma2 is phosphorylated on Y753, Y759, and Y1217 in response to engagement of the B-cell receptor in Ramos cells, as well as in murine splenic B cells. In cells stimulated maximally via this receptor, the extent of phosphorylation of Y1217 was three times that of Y753 or of Y759. Stimulation of Jurkat T cells or platelets via their immunoreceptors also elicited phosphorylation of Y753 and Y759 but not that of Y1217. A basal level of phosphorylation of Y753 was apparent in unstimulated lymphocytes. The extent of phosphorylation of Y753 and Y759, but not that of Y1217, correlated with the lipase activity of PLC-gamma2. Examination of the effects of various pharmacological inhibitors and of RNA interference in Ramos cells suggested that Btk is largely, but not completely, responsible for phosphorylation of Y753 and Y759, whereas phosphorylation of Y1217 is independent of Btk. Finally, phosphorylation of Y1217 and that of Y753 and Y759 occurred on different PLC-gamma2 molecules.

Mechanism of tyrosine phosphorylation and activation of phospholipase C-gamma 1. Tyrosine 783 phosphorylation is not sufficient for lipase activation.

Phospholipase C-gamma 1 (PLC-gamma 1) is phosphorylated on three tyrosine residues: Tyr-771, Tyr-783, and Tyr-1253. With the use of antibodies specific for each of these phosphorylation sites, we have now determined the kinetics and magnitude of phosphorylation at each site. Phosphorylation of Tyr-783, which is essential for lipase activation, was observed in all stimulated cell types examined. The extent of phosphorylation of Tyr-1253 was approximately 50 to 70% of that of Tyr-783 in cells stimulated with platelet-derived growth factor (PDGF) or epidermal growth factor (EGF), but Tyr-1253 phosphorylation was not detected in B or T cell lines stimulated through B- and T-cell antigen receptors, respectively. Tyr-771 was phosphorylated only at a low level in all cells studied. In cells stimulated with PDGF, phosphorylation and dephosphorylation of Tyr-783 and of Tyr-1253 occurred with similar kinetics; the receptor kinase appeared to phosphorylate both sites, albeit with Tyr-783 favored over Tyr-1253, before the bound PLC-gamma 1 was released, and phosphorylation at the two sites occurred independently. PDGF and EGF induced similar levels of phosphorylation of Tyr-783 and of Tyr-1253 in a cell line that expressed receptors for both growth factors. However, only PDGF, not EGF, elicited substantial PLC activity, suggesting that Tyr-783 phosphorylation was not sufficient for enzyme activation. Finally, concurrent production of phosphatidylinositol 3,4,5-trisphosphate was found to contribute to the activation of phosphorylated PLC-gamma 1.

Protein kinase Cdelta as gonadotropin-releasing hormone target isoenzyme in the alphaT3-1 gonadotrope cell line.

We investigated the kinetics of gonadotropin-releasing hormone (GnRH)-induced activation of the protein kinase C (PKC) delta isoform in alphaT3-1 gonadotrope cells. Results were evaluated in subcellular fractions and whole-cell lysates using specific antibodies recognizing either non- or (trans- and auto-)phosphorylated forms of the kinase at Thr505 and Ser643 residues modulating stability and/or activation of the enzyme. Under basal conditions, and in contrast to PKC epsilon, PKC delta was mainly associated with the membrane compartment. GnRH (10(-7)M) elicited further and rapid membrane translocation and time-dependent phosphorylation at both sites of PKC delta. The neuropeptide's effects did not show a refractory period after short but successive GnRH stimulation and were abolished by the GnRH antagonist, antide. Sustained GnRH stimulation (2-6 h) provoked rapid down-regulation of PKC delta. Antide, by inhibiting the initial processes (translocation, phosphorylation), counteracted the degradation of the enzyme. Proteolytic processing of PKC delta was shown to mainly involve proteasome activity. Indeed, specific proteasome inhibitors prevented GnRH-elicited kinase depletion and induced membrane accumulation of the enzyme in a phosphorylated (Thr505, Ser643) form. Thus, GnRH may regulate time-dependent cell responses by modulating the phosphorylation/activation state of its signal transduction effector proteins, and by maintaining their appropriate expression balance via proteolytic processes involving the proteasome system.

Specific beta1 integrin site selectively regulates Akt/protein kinase B signaling via local activation of protein phosphatase 2A.

Integrin transmembrane receptors generate multiple signals, but how they mediate specific signaling is not clear. Here we test the hypothesis that particular sequences along the beta(1) integrin cytoplasmic domain may exist that are intimately related to specific integrin-mediated signaling pathways. Using systematic alanine mutagenesis of amino acids conserved between different beta integrin cytoplasmic domains, we identified the tryptophan residue at position 775 of human beta(1) integrin as specific and necessary for integrin-mediated protein kinase B/Akt survival signaling. Stable expression of a beta(1) integrin mutated at this amino acid in GD25 beta(1)-null cells resulted in reduction of Akt phosphorylation at both Ser(473) and Thr(308) activation sites. As a consequence, the cells were substantially more sensitive to serum starvation-induced apoptosis when compared with cells expressing wild type beta(1) integrin. This inactivation of Akt resulted from increased dephosphorylation by a localized active population of protein phosphatase 2A. Both Akt and protein phosphatase 2A were present in beta(1) integrin-organized cytoplasmic complexes, but the activity of this phosphatase was 2.5 times higher in the complexes organized by the mutant integrin. The mutation of Trp(775) specifically affected Akt signaling, without effects on other integrin-activated pathways including phosphoinositide 3-kinase, MAPK, JNK, and p38 nor did it influence activation of the integrin-responsive kinases focal adhesion kinase and Src. The identification of Trp(775) as a specific site for integrin-mediated Akt signaling supports the concept of specificity of signaling along the integrin cytoplasmic domain.