GPR55 signalling promotes proliferation of pancreatic cancer cells and tumour growth in mice, and its inhibition increases effects of gemcitabine

The life expectancy for pancreatic cancer patients has seen no substantial changes in the last 40 years as very few and mostly just palliative treatments are available. As the five years survival rate remains around 5%, the identification of novel pharmacological targets and development of new therapeutic strategies are urgently needed. Here we demonstrate that inhibition of the G protein-coupled receptor GPR55, using genetic and pharmacological approaches, reduces pancreatic cancer cell growth in vitro and in vivo and we propose that this may represent a novel strategy to inhibit pancreatic ductal adenocarcinoma (PDAC) progression. Specifically, we show that genetic ablation of Gpr55 in the KRASWT/G12D/TP53WT/R172H/Pdx1-Cre+/+ (KPC) mouse model of PDAC significantly prolonged survival. Importantly, KPC mice treated with a combination of the GPR55 antagonist Cannabidiol (CBD) and gemcitabine (GEM, one of the most used drugs to treat PDAC), survived nearly three times longer compared to mice treated with vehicle or GEM alone. Mechanistically, knockdown or pharmacologic inhibition of GPR55 reduced anchorage-dependent and independent growth, cell cycle progression, activation of mitogen-activated protein kinase (MAPK) signalling and protein levels of ribonucleotide reductases in PDAC cells. Consistent with this, genetic ablation of Gpr55 reduced proliferation of tumour cells, MAPK signalling and ribonucleotide reductase M1 levels in KPC mice. Combination of CBD and GEM inhibited tumour cell proliferation in KPC mice and it opposed mechanisms involved in development of resistance to GEM in vitro and in vivo. Finally, we demonstrate that the tumour suppressor p53 regulates GPR55 protein expression through modulation of the microRNA miR34b-3p. Our results demonstrate the important role played by GPR55 downstream of p53 in PDAC progression. Moreover our data indicate that combination of CBD and GEM, both currently approved for medical use, might be tested in clinical trials as a novel promising treatment to improve PDAC patients' outcome.

The proline-rich tyrosine kinase Pyk2 regulates platelet integrin αIIbβ3 outside-in signaling

BACKGROUND

The proline-rich tyrosine kinase Pyk2 is a focal adhesion kinase expressed in blood platelets, and is activated downstream of G-protein coupled receptors as well as integrin α2β1.

OBJECTIVE

In this study we have investigated the involvement of Pyk2 in integrin αIIbβ3 outside-in signaling in human and murine platelets.

METHODS

We analyzed the stimulation of intracellular signaling pathways in platelets from Pyk2 knockout mice adherent to immobilized fibrinogen.

RESULTS

Pyk2 was rapidly phosphorylated and activated in human and murine platelets adherent to fibrinogen through integrin αIIbβ3. Activation of Pyk2 was Src-dependent, but did not require phospholipase Cγ2 activity. Platelets from Pyk2 knockout mice showed a defective ability to adhere and spread on fibrinogen, in association with a dramatic reduction of phosphatidylinositol 3-kinase (PI3K) activation and Akt phosphorylation. Pharmacological and genetic analysis demonstrated that integrin αIIbβ3 engagement selectively stimulated the β-isoform of PI3K (PI3Kβ), and that, as for Pyk2, PI3Kβ activation required Src family kinases activity, but not phospholipase Cγ2. In fibrinogen-adherent platelets, both Pyk2 and PI3Kβ were necessary for stimulation of the small GTPase Rap1b, a regulator of cell adhesion and spreading. Integrin αIIbβ3 engagement triggered the association of the PI3Kβ regulatory subunit p85 with the adaptor protein c-Cbl, which was mediated by the p85 SH3 domain, and was independent of c-Cbl tyrosine phosphorylation. However, p85-associated c-Cbl was tyrosine phosphorylated by activated Pyk2 in fibrinogen adherent platelets.

CONCLUSIONS

These results identify a novel pathway of integrin αIIbβ3 outside-in signaling and recognize the tyrosine kinase Pyk2 as a major regulator of platelet adhesion and spreading on fibrinogen.

Targeting PDK1 in cancer

Abnormal activation of phosphoinositide 3-kinase (PI3K) signalling is very common in cancer, leading to deregulation of several intracellular processes normally controlled by this enzyme, including cell survival, growth, proliferation and migration. Mutations in the gene encoding the tumour suppressor phosphatase and tensin homologue deleted on chromosome 10 (PTEN), which leads to uncontrolled activation of the PI3K pathway, are reported in different cancers. Among the downstream effectors of PI3Ks, 3- phosphoinositide-dependent protein kinase 1 (PDK1) and protein kinase B (PKB)/Akt have a key role in several cancer types. More recent data indicate that alteration of PDK1 is a critical component of oncogenic PI3K signalling in breast cancer, suggesting that inhibition of PDK1 can inhibit breast cancer progression. PDK1 has an essential role in regulating cell migration especially in the context of PTEN deficiency. Downregulation of PDK1 levels inhibits migration and experimental metastasis of human breast cancer cells. PDK1 activates a large number of proteins, including Akt, some PKC isoforms, S6K and SGK. Data also reveal that PDK1 is oncogenic and this is dependent on PI3K pathway. Therefore, accumulating evidence demonstrates that PDK1 is a valid therapeutic target and suggests that PDK1 inhibitors may be useful to prevent cancer progression and abnormal tissue dissemination. This review will focus on published data on the role of PDK1 in cancer and approaches used to inhibit PDK1.

A novel inhibitor of the PI3K/Akt pathway based on the structure of inositol 1,3,4,5,6-pentakisphosphate

BACKGROUND

Owing to its role in cancer, the phosphoinositide 3-kinase (PI3K)/Akt pathway is an attractive target for therapeutic intervention. We previously reported that the inhibition of Akt by inositol 1,3,4,5,6-pentakisphosphate (InsP(5)) results in anti-tumour properties. To further develop this compound we modified its structure to obtain more potent inhibitors of the PI3K/Akt pathway.

METHODS

Cell proliferation/survival was determined by cell counting, sulphorhodamine or acridine orange/ethidium bromide assay; Akt activation was determined by western blot analysis. In vivo effect of compounds was tested on PC3 xenografts, whereas in vitro activity on kinases was determined by SelectScreen Kinase Profiling Service.

RESULTS

The derivative 2-O-benzyl-myo-inositol 1,3,4,5,6-pentakisphosphate (2-O-Bn-InsP(5)) is active towards cancer types resistant to InsP(5) in vitro and in vivo. 2-O-Bn-InsP(5) possesses higher pro-apoptotic activity than InsP(5) in sensitive cells and enhances the effect of anti-cancer compounds. 2-O-Bn-InsP(5) specifically inhibits 3-phosphoinositide-dependent protein kinase 1 (PDK1) in vitro (IC(50) in the low nanomolar range) and the PDK1-dependent phosphorylation of Akt in cell lines and excised tumours. It is interesting to note that 2-O-Bn-InsP(5) also inhibits the mammalian target of rapamycin (mTOR) in vitro.

CONCLUSIONS

InsP(5) and 2-O-Bn-InsP(5) may represent lead compounds to develop novel inhibitors of the PI3K/Akt pathway (including potential dual PDK1/mTOR inhibitors) and novel potential anti-cancer drugs.

Role of class II phosphoinositide 3-kinase in cell signalling

Although it is now well established that PI3K (phosphoinositide 3-kinase) is a key enzyme in several intracellular processes, there are still relatively few reports that precisely identify the specific isoforms of PI3K actually involved in such events. The lack of specific inhibitors has made it particularly difficult to address the physiological roles of some isoforms, such as the members of class II. As a consequence, there is still relatively little understanding of the role of these enzymes and the question about the intracellular role of these isoforms still waits for more answers.

Phosphoinositide 3-kinase-dependent regulation of phospholipase Cγ

Activation of the enzyme PLC (phospholipase C) leads to the formation of second messengers Ins(1,4,5)P3 and diacylglycerol. RTKs (receptor tyrosine kinases) activate this reaction through PLCγ isoenzymes. It has been shown that PI3K (phosphoinositide 3-kinase) may regulate PLCγ activity through the interaction of PI3K product PtdIns(3,4,5)P3 and the PLCγ PH domain (pleckstrin homology domain). Here, we analyse the potential functional roles of the PI3K/PLC pathway.

Pyk2 regulates multiple signaling events crucial for macrophage morphology and migration

The biological role of the protein tyrosine kinase, Pyk2, was explored by targeting the Pyk2 gene by homologous recombination. Pyk2-/- mice are viable and fertile, without overt impairment in development or behavior. However, the morphology and behavior of Pyk2-/- macrophages were impaired. Macrophages isolated from mutant mice failed to become polarized, to undergo membrane ruffling, and to migrate in response to chemokine stimulation. Moreover, the contractile activity in the lamellipodia of Pyk2-/- macrophages was impaired, as revealed by measuring the rearward movement toward the nucleus of fibronectin-coated beads on the lamellipodia in opposition to an immobilizing force generated by optical tweezers. Consistently, the infiltration of macrophages into a carageenan-induced inflammatory region was strongly inhibited in Pyk2-/- mice. In addition, chemokine stimulation of inositol (1, 4, 5) triphosphate production and Ca2+ release, as well as integrin-induced activation of Rho and phosphatidyl inositol 3 kinase, were compromised in Pyk2-/- macrophages. These experiments reveal a role for Pyk2 in cell signaling in macrophages essential for cell migration and function.

Specificity in pleckstrin homology (PH) domain membrane targeting: a role for a phosphoinositide-protein co-operative mechanism

Pleckstrin homology (PH) domains are protein modules found in proteins involved in many cellular processes. The majority of PH domain-containing proteins require membrane association for their function. It has been shown that most PH domains interact directly with the cell membrane by binding to phosphoinositides with a broad range of specificity and affinity. While a highly specific binding of the PH domain to a phosphoinositide can be necessary and sufficient for the correct recruitment of the host protein to the membrane, a weaker and less specific interaction may be necessary but not sufficient, thus probably requiring alternative, co-operative mechanisms.

Modulation of oncogenic DBL activity by phosphoinositol phosphate binding to pleckstrin homology domain

The Dbl family guanine nucleotide exchange factors (GEFs) contain a region of sequence similarity consisting of a catalytic Dbl homology (DH) domain in tandem with a pleckstrin homology (PH) domain. PH domains are involved in the regulated targeting of signaling molecules to plasma membranes by protein-protein and/or protein-lipid interactions. Here we show that Dbl PH domain binding to phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-triphosphate results in the inhibition of Dbl GEF activity on Rho family GTPase Cdc42. Phosphatidylinositol 4,5-bisphosphate binding to the PH domain significantly inhibits the Cdc42 interactive activity of the DH domain suggesting that the DH domain is subjected to the PH domain modulation under the influence of phosphoinositides (PIPs). We generated Dbl mutants unable to interact with PIPs. These mutants retained GEF activity on Cdc42 in the presence of PIPs and showed a markedly enhanced activating potential for both Cdc42 and RhoA in vivo while displaying decreased cellular transforming activity. Immunofluorescence analysis of NIH3T3 transfectants revealed that whereas the PH domain localizes to actin stress fibers and plasma membrane, the PH mutants are no longer detectable on the plasma membrane. These results suggest that modulation of PIPs in both the GEF catalytic activity and the targeting to plasma membrane determines the outcome of the biologic activity of Dbl.

Patterns within protein/polyphosphoinositide interactions provide specific targets for therapeutic intervention

Signaling pathways involving the inositol polyphosphates and the polyphosphoinositides have become intricately linked with a number of disease states. More recently, this has principally involved the 3-phosphorylated products of phosphoinositide 3-kinase, an enzyme that itself shows oncogenic activity and has hence become of interest in the design of antitumorigenic drugs. The downstream effectors of phosphoinositide 3-kinase are involved in different aspects of cellular signaling and cytoskeleton and trafficking events that are linked to specific polyphosphoinositide binding properties of specific protein domains, which themselves have emerging roles in specific disease states. Our recent findings have demonstrated that there is a selectivity of the intracellular effects of extracellularly applied inositol polyphosphates in their abilities to inhibit a range of growth-related in vivo assay conditions, and that these can themselves be linked to the inhibition of the membrane localization of a green fluorescent protein (GFP) -tagged PH domain. We propose that GFP fusions of the polyphosphoinositides binding domains of specific proteins of interest can be used in high-throughput investigations of the therapeutic value of specific inositol polyphosphates analogs. Inhibition of in vivo membrane targeting of these domains from proteins involved in cell growth and tumorigenesis can thus be used in the search for new anticancer drugs.

Different subcellular localization and phosphoinositides binding of insulin receptor substrate protein pleckstrin homology domains

Insulin evokes diverse biological effects through receptor-mediated tyrosine phosphorylation of the insulin receptor substrate (IRS) proteins. Here, we show that, in vitro, the IRS-1, -2 and -3 pleckstrin homology (PH) domains bind with different specificities to the 3-phosphorylated phosphoinositides. In fact, the IRS-1 PH domain binds preferentially to phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P3), the IRS-2 PH domain to phosphatidylinositol 3,4-bisphosphate (PtdIns-3,4-P2), and the IRS-3 PH domain to phosphatidylinositol 3-phosphate. When expressed in NIH-IR fibroblasts and L6 myocytes, the IRS-1 and -2 PH domains tagged with green fluorescent protein (GFP) are localized exclusively in the cytoplasm. Stimulation with insulin causes a translocation of the GFP-IRS-1 and -2 PH domains to the plasma membrane within 3-5 min. This translocation is blocked by the phosphatidylinositol 3-kinase (PI 3-K) inhibitors, wortmannin and LY294002, suggesting that this event is PI 3-K dependent. Interestingly, platelet-derived growth factor (PDGF) did not induce translocation of the IRS-1 and -2 PH domains to the plasma membrane, indicating the existence of specificity for insulin. In contrast, the GFP-IRS-3 PH domain is constitutively localized to the plasma membrane. These results reveal a differential regulation of the IRS PH domains and a novel positive feedback loop in which PI 3-K functions as both an upstream regulator and a downstream effector of IRS-1 and -2 signaling.

Novel functional PI 3-kinase antagonists inhibit cell growth and tumorigenicity in human cancer cell lines

New efforts in cancer therapy are being focused at various levels of signaling pathways. With phosphoinositide 3-kinase (PI3-K) potentially being necessary for a range of cancer-related functions, we have investigated the influence of selected inositol tris- to hexakisphosphates on cell growth and tumorigenicity. We show that micromolar concentrations of inositol 1,3,4,5,6-pentakisphosphate and inositol 1,4,5,6-tetrakisphosphate [Ins(1,4,5,6)P(4)] inhibit IGF-1-induced [(3)H]-thymidine incorporation in human breast cancer (MCF-7) cells and the ability to grow in liquid medium and form colonies in agarose semisolid medium by small cell lung cancer (SCLC) cells, a human cancer cell line containing a constitutively active PI3-K. In an ovarian cancer cell line that also contains a constitutively active PI3-K (SKOV-3 cells), Ins(1,4,5,6)P(4) again inhibited liquid medium growth. Furthermore, when applied extracellularly, inositol 1,3,4,5-tetrakisphosphate was shown indeed to enter SCLC cells. These effects appeared specifically related to PH domains known to bind to phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P(2)] and phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)], indicating involvement of the PI3-K downstream target protein kinase B (PKB/Akt). This was further supported by inhibition of PKB/Akt PH domain membrane targeting in COS-7 cells by Ins(1,4,5,6)P(4). Thus, we propose that specific inositol polyphosphates inhibit PI3-K by competing with PtdIns(3,4, 5)P(3)-binding PH domains and that this occurs mainly at the level of the downstream PI3-K target, PKB/Akt.

The role of the pleckstrin homology domain in membrane targeting and activation of phospholipase Cbeta(1)

Current studies involve an investigation of the role of the pleckstrin homology (PH) domain in membrane targeting and activation of phospholipase Cbeta(1) (PLCbeta(1)). Here we report studies on the membrane localization of the isolated PH domain from the amino terminus of PLCbeta(1) (PLCbeta(1)-PH) using fluorescence microscopy of a green fluorescent protein fusion protein. Whereas PLCbeta(1)-PH does not localize to the plasma membrane in serum-starved cells, it undergoes a rapid but transient migration to the plasma membrane upon stimulation of cells with serum or lysophosphatidic acid (LPA). Regulation of the plasma membrane localization of PLCbeta(1)-PH by phosphoinositides was also investigated. PLCbeta(1)-PH was found to bind phosphatidylinositol 3-phosphate most strongly, whereas other phosphoinositides were bound with lower affinity. The plasma membrane localization of PLCbeta(1)-PH induced by serum and LPA was blocked by wortmannin pretreatment and by LY294002. In parallel, activation of PLCbeta by LPA was inhibited by wortmannin, by LY294002, or by the overexpression of PLCbeta(1)-PH. Microinjection of betagamma subunits of G proteins in serum-starved cells induced the translocation of PLCbeta(1)-PH to the plasma membrane. These results demonstrate that a cooperative mechanism involving phosphatidylinositol 3-phosphate and the Gbetagamma subunit regulates the plasma membrane localization and activation of PLCbeta(1)-PH.

A novel positive feedback loop mediated by the docking protein Gab1 and phosphatidylinositol 3-kinase in epidermal growth factor receptor signaling

The Gab1 protein is tyrosine phosphorylated in response to various growth factors and serves as a docking protein that recruits a number of downstream signaling proteins, including phosphatidylinositol 3-kinase (PI-3 kinase). To determine the role of Gab1 in signaling via the epidermal growth factor (EGF) receptor (EGFR) we tested the ability of Gab1 to associate with and modulate signaling by this receptor. We show that Gab1 associates with the EGFR in vivo and in vitro via pTyr sites 1068 and 1086 in the carboxy-terminal tail of the receptor and that overexpression of Gab1 potentiates EGF-induced activation of the mitogen-activated protein kinase and Jun kinase signaling pathways. A mutant of Gab1 unable to bind the p85 subunit of PI-3 kinase is defective in potentiating EGFR signaling, confirming a role for PI-3 kinase as a downstream effector of Gab1. Inhibition of PI-3 kinase by a dominant-interfering mutant of p85 or by Wortmannin treatment similarly impairs Gab1-induced enhancement of signaling via the EGFR. The PH domain of Gab1 was shown to bind specifically to phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P3], a product of PI-3 kinase, and is required for activation of Gab1-mediated enhancement of EGFR signaling. Moreover, the PH domain mediates Gab1 translocation to the plasma membrane in response to EGF and is required for efficient tyrosine phosphorylation of Gab1 upon EGF stimulation. In addition, overexpression of Gab1 PH domain blocks Gab1 potentiation of EGFR signaling. Finally, expression of the gene for the lipid phosphatase PTEN, which dephosphorylates PtdIns(3,4, 5)P3, inhibits EGF signaling and translocation of Gab1 to the plasma membrane. These results reveal a novel positive feedback loop, modulated by PTEN, in which PI-3 kinase functions as both an upstream regulator and a downstream effector of Gab1 in signaling via the EGFR.

Specificity and promiscuity in phosphoinositide binding by pleckstrin homology domains

Pleckstrin homology (PH) domains are small protein modules involved in recruitment of signaling molecules to cellular membranes, in some cases by binding specific phosphoinositides. We describe use of a convenient "dot-blot" approach to screen 10 different PH domains for those that recognize particular phosphoinositides. Each PH domain bound phosphoinositides in the assay, but only two (from phospholipase C-delta1 and Grp1) showed clear specificity for a single species. Using soluble inositol phosphates, we show that the Grp1 PH domain (originally cloned on the basis of its phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) binding) binds specifically to D-myo-inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) (the PtdIns(3,4,5)P3 headgroup) with KD = 27.3 nM, but binds D-myo-inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) or D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) over 80-fold more weakly. We show that this specificity allows localization of the Grp1 PH domain to the plasma membrane of mammalian cells only when phosphatidylinositol 3-kinase (PI 3-K) is activated. The presence of three adjacent equatorial phosphate groups was critical for inositol phosphate binding by the Grp1 PH domain. By contrast, another PH domain capable of PI 3-K-dependent membrane recruitment (encoded by EST684797) does not distinguish Ins(1,3,4)P3 from Ins(1,3,4,5)P3 (binding both with very high affinity), despite selecting strongly against Ins(1,4,5)P3. The remaining PH domains tested appear significantly less specific for particular phosphoinositides. Together with data presented in the literature, our results suggest that many PH domains bind similarly to multiple phosphoinositides (and in some cases phosphatidylserine), and are likely to be regulated in vivo by the most abundant species to which they bind. Thus, using the same simple approach to study several PH domains simultaneously, our studies suggest that highly specific phosphoinositide binding is a characteristic of relatively few cases.

Activation of the c-fos serum response element by phosphatidyl inositol 3-kinase and rho pathways in HeLa cells

Many growth factors rapidly induce transcription of the c-fos proto-oncogene. We have investigated the pathways for induction of the c-fos promoter by serum and epidermal growth factor (EGF) in HeLa cells. Induction of the serum response element (SRE) of the c-fos promoter could be split into two parts, one involving the serum response factor-associated ternary complex factor (TCF) factors and the second mediated by core SRE sequences. Serum induction was mediated primarily by the core SRE, whereas EGF used both the TCF and core SRE pathways. Using activated and inhibitory signaling proteins, we found that phosphatidyl inositol 3-kinase (PI3K) and rho family members could mediate activation by serum. Activation by PI3K was mediated by core SRE sequences and was dependent upon rac and rho, suggesting a PI3K-to-rac-to-rho pathway for core SRE activation. The PI3K target Akt was also capable of activating the SRE but functioned through the TCF pathway, suggesting that Akt does not mediate the primary PI3K pathway to the SRE and that Akt is capable of activating TCF family members. Serum and EGF induction of the core SRE was partially inhibited by rho and PI3K inhibitors. The use of these inhibitors demonstrates the complexity of signaling pathways to the SRE and suggests that serum activates rho by PI3K-dependent and -independent pathways.

Release of the mitogen lysophosphatidylinositol from H-Ras-transformed fibroblasts; a possible mechanism of autocrine control of cell proliferation

Lysophosphatidylinositol (LysoPtdIns) is formed by a constitutively-active phosphoinositide-specific phospholipase A2 in Ras-transformed cells and can stimulate cell proliferation. To evaluate whether LysoPtdIns could function as an autocrine modulator of cell growth, we examined whether LysoPtdIns can be released in the medium of Ras-transformed FRT-Fibro fibroblasts and thyroid cells. Here, we report that LysoPtdIns accumulates in the extracellular space of these lines and reaches levels up to tenfold higher than in the case of normal cells. Moreover, the ionophore A23187 increased the levels of the lysolipid in the extracellular medium. Extracellular LysoPtdIns was rapidly hydrolyzed to inositol 1:2-cyclic phosphate. LysoPtdIns induced thymidine incorporation in FRT-Fibro Ha-Ras fibroblasts, whereas inositol cyclic 1:2-cyclic phosphate did not affect cell growth per se, nor did it interfere with the LysoPtdIns mitogenic activity. We hypothesize that in Ras-transformed fibroblasts the formation and release of LysoPtdIns may function as an autocrine mechanism that participates in the Ras-dependent stimulation of cell growth.

Activation of phospholipase C gamma by PI 3-kinase-induced PH domain-mediated membrane targeting

Signaling via growth factor receptors frequently results in the concomitant activation of phospholipase C gamma (PLC gamma) and phosphatidylinositol (PI) 3-kinase. While it is well established that tyrosine phosphorylation of PLC gamma is necessary for its activation, we show here that PLC gamma is regulated additionally by the lipid products of PI 3-kinase. We demonstrate that the pleckstrin homology (PH) domain of PLC gamma binds to phosphatidylinositol 3,4,5-trisphosphate [PdtIns(3,4,5)P3], and is targeted to the membrane in response to growth factor stimulation, while a mutated version of this PH domain that does not bind PdtIns(3,4,5)P3 is not membrane targeted. Consistent with these observations, activation of PI 3-kinase causes PLC gamma PH domain-mediated membrane targeting and PLC gamma activation. By contrast, either the inhibition of PI 3-kinase by overexpression of a dominant-negative mutant or the prevention of PLC gamma membrane targeting by overexpression of the PLC gamma PH domain prevents growth factor-induced PLC gamma activation. These experiments reveal a novel mechanism for cross-talk and mutual regulation of activity between two enzymes that participate in the control of phosphoinositide metabolism.

Phosphatidylinositol 3-kinase mediates epidermal growth factor-induced activation of the c-Jun N-terminal kinase signaling pathway

The signaling events which mediate activation of c-Jun N-terminal kinase (JNK) are not yet well characterized. To broaden our understanding of upstream mediators which link extracellular signals to the JNK pathway, we investigated the role of phosphatidylinositol (PI) 3-kinase in epidermal growth factor (EGF)-mediated JNK activation. In this report we demonstrate that a dominant negative form of PI 3-kinase as well as the inhibitor wortmannin blocks EGF-induced JNK activation dramatically. However, wortmannin does not have an effect on JNK activation induced by UV irradiation or osmotic shock. In addition, a membrane-targeted, constitutively active PI 3-kinase (p110beta) was shown to produce in vivo products and to activate JNK, while a kinase-mutated form of this protein showed no activation. On the basis of these experiments, we propose that PI 3-kinase activity plays a role in EGF-induced JNK activation in these cells.

Fast receptor-induced formation of glycerophosphoinositol-4-phosphate, a putative novel intracellular messenger in the Ras pathway

Glycerophosphoinositols are phosphoinositide metabolites whose levels are constitutively elevated in Ras-transformed cells. Here, we show that one of these compounds, glycerophosphoinositol-4-phosphate (GroPIns-4-P) responds acutely to the stimulation of the epidermal growth factor receptor, with a fast, massive and transient increase. The mechanism leading to GroPIns-4-P formation involves the activation of phosphoinositide-3 kinase and the small GTP-binding protein Rac, since GroPIns-4-P was neither formed in cells expressing the dominant negative form of Rac nor in cells treated with the phosphoinositide-3 kinase inhibitor wortmannin. GroPIns-4-P has been previously shown to inhibit adenylyl cyclase. Accordingly, epidermal growth factor also decreased the basal, cholera toxin-stimulated, and forskolin-stimulated cyclic AMP levels with kinetics similar to those of GroPIns-4-P formation, suggesting that GroPIns-4-P mediates this inhibitory effect. The hormone-induced formation of GroPIns-4-P was detected in several cell lines of various origin, suggesting that GroPIns-4-P is a novel intracellular messenger of the Ras pathway, possibly able to convey information from tyrosine kinase receptors to the cyclic AMP cascade.

Changes in the levels of glycerophosphoinositols during differentiation of hepatic and neuronal cells

Glycerophosphoinositols are metabolites formed by a phosholipase A2 and a lysolipase specifically acting on membrane phosphoinositol lipids. High levels of these compounds characterize epithelial cells and fibroblasts transformed by ras and other cellular oncogenes. Here we have analyzed the glycerophosphoinositol levels in cells that are considered models of cell differentiation. Using rat hepatocytes at different stages of liver development we have shown that the glycerophosphoinositol basal levels of fetal cells were up to fourfold higher than in adult hepatocytes. No changes in glycerophosphoinositol were observed in regenerating rat liver, a model of differentiated cells proliferating in a synchronous manner, where only glycerophosphoinositol 4-phosphate increased by 80%. Similarly to fetal hepatocytes, a modest but significant increase (30%) in the levels of glycerophosphoinositols was observed in undifferentiated NG-108-15 cells as compared to the same cells induced to differentiate by cAMP. In a different neuronal cell line, PC12 cells, increased glycerophosphoinositol levels characterized the differentiated cells. Based on these observations we suggest that high glycerophosphoinositol levels characterize cellular phenomena associated with the activation of ras/mitogen-activated protein kinase pathways.

Glycerophosphoinositols as potential markers of ras-induced transformation and novel second messengers

Glycerophosphoinositols are metabolites formed by the sequential action of a phospholipase A2 specific for membrane phosphoinositides, and a lysolipase. Increased levels of these compounds characterize cell lines of different origin transformed by ras and other oncogenes. Thus, glycerophosphoinositols have been proposed as markers of cell transformation. These compounds are also biologically active. In particular, glycerophosphoinositol-4-phosphate (GroPIns4P) is an inhibitor of adenylyl cyclase that acts on the Gs protein. Since GroPIns4P is formed physiologically upon hormone stimulation in several cell lines including fibroblasts, thyrocytes, neuronal and leukemia cells, we propose that GroPIns4P could act as a second messenger able to mediate a cross-talk between the ras cascade and adenylyl cyclase.

Signalling pathways involved in the mitogenic action of lysophosphatidylinositol

Lysophosphatidylinositol has been previously shown to stimulate cell proliferation in differentiated and in K-ras transformed thyroid cells. Increased levels of lysophosphatidylinositol, but not lysophosphatidylcholine or lysophosphatidylethanolamine, are present in thyroid as well as in other ras-transformed cell lines. We have now investigated the mechanism of action of this lysolipid by analysing its effects in a differentiated thyroid cell line. Lysophosphatidylinositol did not increase the levels of cAMP, the main stimulator of cell proliferation in the thyroid, whereas it stimulated phosphoinositide breakdown, mobilization of cytosolic Ca2+ and arachidonic acid release, suggesting that it activates both phospholipases C and A2. None of the effects of lysophosphatidylinositol were prevented by pretreatment of cells with pertussis toxin. Instead, the tyrosine kinase inhibitors, tyrphostins AG18 and AG561, completely blocked its mitogenic action. The effects of lysophosphatidylinositol were distinguishable from those of the well known mitogen lysophosphatidic acid, which affected differently the signalling pathways analysed and was not mitogenic in ras-transformed cells. These results suggest that the mitogenic activity of lysophosphatidylinositol is associated with the activation of phospholipase C and phospholipase A2 and is relatively specific for ras-transformed cells.

Physiological concentrations of thyrotropin increase cytosolic calcium levels in primary cultures of human thyroid cells

The activity of TSH, the main regulator of growth and differentiation in the thyroid, has been mainly related to the activation of the adenylyl cyclase cascade. TSH also activates phospholipase-C and -A2; these effects, however, have been reported to require concentrations of the hormone up to 1000-fold higher than those effective on adenylyl cyclase, suggesting that the main physiological mechanism involved in the action of TSH is the activation of this enzyme. Using primary cultures of human thyroids, we here show that physiological concentrations of TSH (0.01-10 mU/L) are also able to increase intracellular Ca2+ levels. Cells were loaded with the fluorescent Ca2+ probe fura-2 and analyzed by single cell Ca2+ recording. The basal Ca2+ level was 105 +/- 30 nmol/L, and physiological concentrations of TSH increased it by 2- to 7-fold. The Ca2+ increase was transient and lasted up to 10 min. It is also shown that the TSH-dependent Ca2+ increase involves both the activation of phospholipase-C and the entry of extracellular Ca2+. TSH (100-10000 mU/L) increased cAMP levels by up to 20-fold in parallel experiments performed on the same cell preparations. These data demonstrate that physiological concentrations of TSH are able to increase cytosolic Ca2+ levels, indicating that this second messenger might directly mediate the action of this hormone in the thyroid.

Elevated levels and mitogenic activity of lysophosphatidylinositol in k-ras-transformed epithelial cells

In cell lines stably (KiKi) or reversibly (Ts) transformed by the k-ras oncogene originated from a differentiated rat thyroid line (FRTL5 cells), k-ras-induced transformation has been associated with an increased phospholipase A2 activity. Here we provide evidence that this enzymic activity is phosphoinositide specific and leads to the formation of lysophosphatidylinositol. The levels of this lysolipid increased by 2-3-fold in ras-transformed cells (KiKi cells and Ts cells at the permissive temperature of 33 degrees C) as compared to differentiated cells (FRTL5) or to Ts cells maintained at 39 degrees C, i.e. at the temperature where ras-p21, the product of the ras oncogene, is inactive. Since another lysoderivative, lysophosphatidic acid, has been shown to be a mitogen, we have tested whether lysophosphatidylinositol could have a similar activity on thyroid cells. Lysophosphatidylinositol (10-100 microM) induced a 5-10-fold increase in [3H]thymidine incorporation in both FRTL5 and KiKi cells, whereas lysophosphatidic acid was active only in differentiated cells. Lysophosphatidylinositol (approximately 25 microM) and lysophosphatidic acid (50-100 microM) acted synergistically with insulin in increasing [3H]thymidine incorporation. Moreover, lysophosphatidylinositol at concentrations three-fold higher than those found to be mitogenic, inhibited the activity of the GTPase-activating protein. We conclude that lysophosphatidylinositol is a mitogen that might play a role in the modulation of k-ras transformed cell proliferation.

Glycerophosphoinositol 4-phosphate, a putative endogenous inhibitor of adenylylcyclase

In a continuous line of rat thyroid cells transformed by the k-ras oncogene (KiKi), the expression of ras-p21 correlates with an increased activity of a phosphoinositide-specific phospholipase A2, which leads to elevated levels of glycerophosphoinositols. In this study we have characterized the biological activities of these compounds. Growth and differentiation in thyroid cells are mainly regulated by the activation of adenylylcyclase. Therefore, we have studied the effects of glycerophosphoinositols on the activity of this enzyme using a normal thyroid cell line (FRTL5). Micromolar concentrations of glycerophosphoinositol 4-phosphate (GroPIns-4-P) caused a approximately 50% inhibition of the adenylylcyclase activity in FRTL5 membranes stimulated by the GTP-binding protein activator fluoroaluminate. Similar concentrations of GroPIns-4-P were detected in KiKi cells but not in the normal FRTL5 line. Micromolar GroPIns-4-P was found to be taken up by intact FRTL5 cells and to induce nearly 50% inhibition of the thyrotropin- and cholera toxin-induced increase in cAMP levels. Similar results were also observed in other cell lines (smooth muscle, pituitary cells, and pneumocytes). GroPIns-4-P inhibited cAMP-dependent cellular functions such as iodide uptake and thymidine incorporation in FRTL5 cells when stimulated by thyrotropin and cholera toxin but not when induced by forskolin. These results are consistent with GroPIns-4-P exerting an inhibitory effect on the GTP-binding protein that stimulates adenylycyclase. We propose that GroPIns-4-P might mediate a mechanism of cross-talk between adenylylcyclase and phospholipase A2 in thyroid as well as in other cell systems.

Bretylium-induced voltage-gated sodium current in human lymphocytes

Using the whole-cell variation of the patch-clamp technique it has been determined that 0.25-3 mM bretylium tosylate (BT) exerts a repolarizing effect on partially depolarized human lymphocytes. The repolarizing effect was ouabain (40 microM)-sensitive, and was inhibited by the removal of external Na+ or by the Na(+)-channel-blocker amiloride (10-44 microM), but K(+)-channel-blockers 4-aminopyridine (0.1-5 mM) and quinine (100 microM) had no effect. The drug induced a sodium dependent, amiloride-sensitive transient inward current reaching its maximum value approx. 20-30 s after the administration of BT and lasting for 6-10 min. This current was activated by depolarization within 25 ms at around -42 mV, its inactivation took about 2 s and its reversal potential was +24 +/- 5 mV. An increase in the intracellular sodium concentration (1.8-3.2 mM) has been observed upon the addition of BT by monitoring the SBFI fluorescence of the dye-loaded cells. It has been shown that whole-cell K+ currents are significantly decreased by BT. The existence of voltage and ligand (BT)-gated sodium channels has been postulated in human lymphocytes. These channels are thought to participate in the initiation of membrane repolarization in human lymphocytes, and thereby influence mitogenic or antigen-induced cell-activation processes.

Adenosine deaminase from Saccharomyces cerevisiae: kinetics and interaction with transition and ground state inhibitors

Several adenosine analogs, such as coformycin, 2'-deoxycoformycin and erythro-9-(3-nonyl-p-aminobenzyl)adenine (EHNA), which are strong inhibitors of mammalian adenosine deaminase, are much weaker inhibitors of the Saccharomyces cerevisiae enzyme. The specificity of the yeast enzyme is more restricted than that of mammalian adenosine deaminase, particularly towards the ribose moiety and around position 6 and 1 of the substrate. The sulphydryl group appears to be more masked in the yeast than in the mammalian enzyme. The kinetic effects of pH with adenosine substrate and with the inhibitor purine riboside are reported. The findings on specificity and pH kinetic effects can be interpreted in a model involving proton transfer from the -SH group of the enzyme to the N-1 atom of the substrate.

Lipid peroxidation causes an increase of lipid order and a decrease of 5'-nucleotidase activity in the liver plasma membrane

The effect of peroxidation on 5'-nucleotidase activity as well as on membrane microviscosity has been investigated in liver plasma membranes from Wistar rats. The peroxidation was performed with 100 microM H2O2 and 200 microM FeSO4 and/or with 5 mM t-butylhydroperoxide. Treatment of the membranes with these oxidizing agents resulted in an elevation of the transition temperatures of the polarization of the lipid fluorescent probes 1,6 diphenyl-1,3,5 hexatriene (DPH), 3-p-(6-phenyl) 1,3,5 hexatriene phenylpropionic acid (PA-DPH) as well as of the fluorescent thiol reagent N-(1-pyrene) maleimide (1-PM). The peroxidation resulted in a decrease of the activity of 5'nucleotidase. Our data support that the increase of membrane microviscosity of the lipid domain regulates the activity of 5'-nucleotidase.

Cholesterol-rich rabbit serum modulates beta-adrenergic receptor density of human lymphocytes. A possible role of LDL-cholesterol

The effect of in vitro treatment of human lymphocytes with rabbit cholesterol-rich serum (RCS) on the membrane microviscosity as well as on the beta-adrenergic receptor density has been investigated. RCS treatment of cells resulted in a 30% decrease of receptor density without any effect on membrane microviscosity. A complete recovery was observed incubating the RCS cells either with the "Active Lipids" (AL) or with heparin. The AL are a mixture of neutral lipids, phosphatidylcholine and phosphatidylethanolamine from hen egg yolk known to fluidify the cell membrane. The AL modified membrane microviscosity of control lymphocytes without altering their beta-receptor number. These observations support the proposition that beta-receptor density of human lymphocytes is not regulated by membrane microviscosity and suggest that probably low density lipoprotein-cholesterol complex is involved in such a regulation.

A sodium channel opener inhibits stimulation of human peripheral blood mononuclear cells

The role of membrane potential changes in T cell activation was studied on human peripheral blood lymphocytes stimulated with phytohemagglutinin. Addition of bretylium tosylate, a sodium channels opener, to PHA treated lymphocytes modified the membrane potential and consequently blocked cell activation in a dose-dependent fashion. BT was non-toxic even in long-term (72 hr) incubations. It was reversibly removable, and the removal restored the stimulatory effect of PHA. 3H-thymidine incorporation was blocked if BT was present during the first 20-24 hr of the mitogenic activation. The later BT was added after PHA, the less inhibition of proliferation was observed. BT hyperpolarized the lymphocytes also in the presence of PHA. BT hindered the depolarizing effect of high extracellular potassium concns. The sustained polarized state of the lymphocytes did not influence the intracellular calcium increase upon PHA treatment. IL-2 and transferrin receptor expression was not hindered by BT during PHA stimulation of lymphocytes. Addition of rIL-2 did not abolish the inhibitory effect of BT. According to cell-cycle analysis BT arrested the majority of the cells in G1 phase. It is suggested that cell activation demands the flexible maintenance of a relatively narrow membrane potential "window". Any sustained and significant hyper-, or depolarization, may dramatically decrease the effectivity of transmembrane signalling.

Aging impairs membrane potential responsiveness as well as opening of voltage and ligand gated Na+ channels in human lymphocytes

Depolarizing effects of increasing concentrations of extracellular K(+), as well as the repolarizing effect of bretylium tosylate (BT) were evaluated in human lymphocytes from young and elderly volunteers. Cells from elderly volunteers were less responsive to depolarization induced by increased extracellular potassium concentrations than those from young volunteers. Upon a near complete depolarization induced by 140 mM K(+) in the extracellular space, a significant amount of non-responding cells were found in samples from elderly volunteers. BT, which opens the otherwise silent Na(+) channels of partially depolarized cells, with subsequent activation of the Na(+)-K(+) pump (Pieri et al., 1989). repolarized both young and old lymphocytes. However, the degree of the repolarization was only 40% in the case of lymphocytes from elderly volunteers than from that of the young. It is suggested that an increase of membrane microviscosity, characteristic of old cells, may be at least partially responsible for the decreased responsiveness of plasma membrane functions which were observed.

Food restriction in female Wistar rats: V. Lipid peroxidation and antioxidant enzymes in the liver

The activities of antioxidant enzymes as well as the levels of basal and enzyme induced peroxidation have been investigated in liver of female Wistar undernourished rats. Food restriction was applied starting from the age of 3.5 months by feeding the animals on every-other-day schedule (EOD). Diet restriction prevented the age-dependent increase of basal and enzyme induced lipid peroxidation in both mitochondrial and microsomal liver membrane preparations. The activities of antioxidant enzyme, i.e. superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) of liver decreased during aging in ad libitum fed rats. In the diet conditioned animals, a small increase of SOD and a complete recovery of CAT activities were observed. Present data support that food restriction improved the protection against peroxidation, and this may be in close relationship with the life prolonging effect of such a treatment.

Influence of membrane fluidity of 5'-nucleotidase activity in isolated hepatocyte plasma membrane

The influence of membrane microviscosity on 5'-nucleotidase activity has been investigated on liver plasma membrane preparations from rats during aging and following diet restriction. In addition the microviscosity of membranes from old rats was changed in vitro by the Active Lipids. During aging the membrane microviscosity increased progressively and in parallel the activity of 5'-nucleotidase decreased. Diet restriction was able to slow down the modification of both parameters. The experiment performed with the Active Lipids further supports that membrane microviscosity modulated the enzyme activity.

Food restriction in female Wistar rats, IV. Morphometric parameters of cerebellar synapses

The effect of food restriction on morphometric parameters of cerebellar synapses has been evaluated. A decrease in the number and surface density of synapses has been observed comparing 6 and 27-28 month old rats. Food restriction prevented the loss of the number and attenuated the reduction of the surface density of synaptic contacts occurring during aging. Present data support the idea that food restriction delays the appearance of age-related modifications of synaptic structures and may explain the improvement of motoric coordination and performance found in dietary restricted old animals.

Diet restriction, body temperature and physicochemical properties of cell membranes

This paper summarizes some recent results of the effect of diet restriction upon body temperature and membrane microviscosity of lymphocytes, hepatocytes and cerebellar cells of diet-restricted female Wistar rats. The treated animals were fed on an every-other-day schedule starting from the age of 3.5 months. It is suggested that a decrease in average body temperature (-1 degree C) of diet-restricted animals as compared to the ad libitum fed ones may stimulate the cells to synthesize more fluid membranes. Together with these the maintenance of the activity of protective enzymes is another key event which helps to prevent the age-dependent deterioration of cell membrane functions.

Parameters to monitor aging with a possible perspective for intervention — an immunological approach

Reliable aging markers are very rare, which are better than the chronological age or those symptoms which have such great individual variability that their scientific value is questionable. The effect of aging on immunological behavior of human (and animal) individuals is reasonably well established. In this communication an attempt is made to find an immunological marker of aging at the level of cell surface phenomena. It was observed that ion-channel activities, having a complex regulation, loose their flexible responsiveness in lymphocyte membranes during aging. A recently discovered voltage regulation of the calcium-activated potassium channels showed a distinct change with aging of human lymphocytes. A possibility to find a better marker system in complex regulatory processes is also discussed.

Antioxidant enzymes in erythrocytes from old and diet restricted old rats

Diet restriction, prolonging the lifespan of rodents, represents an interesting model for gerontological studies. We analyzed the activity of antioxidant enzymes, Superoxide Dismutase, Catalase and Glutathione Peroxidase in erythrocytes from young, old and old food restricted Wistar rats. Diet restriction was applied feeding the animals on every-other-day schedule starting from the age of 3.5 months. The age-dependent decrease of Catalase and Glutathione Peroxidase activities was prevented by food restriction, whereas Superoxide Dismutase activity was not influenced either by aging and dietary intervention. Present results support the hypothesis that diet restriction increases the protection of cell structure against the peroxidative damage, preserving the activity of antioxidant enzymes.

Food restriction in female Wistar rats. I. survival characteristics, membrane microviscosity and proliferative response in lymphocytes

The effect of food restriction on the survival characteristics, membrane microviscosity and proliferative response in lymphocytes of female Wistar undernourished rats has been evaluated. Diet restriction was applied starting from the age of 3.5 months by feeding the animals on an every-other-day schedule (EOD). Diet restricted animals showed an increase of both mean, median and maximal life span as compared to the rats fed ad libitum (AL). Analyzing the survival curves by a parametric model, it emerged that undernutrition increased the individual resistance to environmental insults. In particular, it could be speculated that the positive influence was more pronounced in individuals with the lowest physiological capacities. The membrane microviscosity of lymphocytes was lower in EOD animals as compared to the AL ones even if one assumes a decrease in body temperature of 1-2 degrees C in EOD groups. The improvement of membrane microviscosity due to diet restriction may in part explain the improvement of proliferative response of lymphocytes from EOD groups.

Food restriction in female Wistar rats. III. Thermotropic transition of membrane lipid and 5'-nucleotidase activity in hepatocytes

The effect of diet restriction was measured on the anisotropy parameter of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 5'-nucleotidase enzyme activity in liver plasma membrane preparates. Diet restriction was applied to rats 3.5 months old on an every-other-day schedule (EOD) and the rats were killed at the age of 28-29 months. Six months and 24 months rats, fed ad libitum (AL), were used as controls. The Arrhenius plots of anisotropy parameter of liver membranes from young, old AL and old EOD animals exhibited well defined breakpoints at 16.3 degrees C, 19.5 degrees C and 16.7 degrees C, respectively. The breakpoint temperature of 5'-nucleotidase activity was lower in samples from young rats as compared to those from old AL rats, whereas no difference was observed comparing young and EOD fed rats. Present results support the hypothesis that diet restriction modifies lipid composition of liver plasma membranes in such a way that the appearance of age-dependent alterations is delayed.

Food restriction in female Wistar rats. II. β-adrenoceptor density in the cerebellum and in the splenic lymphocytes

The effect of diet restriction, applied on an every-other-day schedule from 3.5 months of age on, has been investigated on the beta-adrenoceptor density in the cerebellum and in the splenic lymphocytes of old female Wistar rats. Comparing animals 6 months and 24 months old fed ad libitum, a 75% age-dependent reduction in specific binding of the agonist dihydroalprenolol was observed in cerebellar membrane preparations, while the beta-adrenoceptor density of lymphocytes remained unaltered. Diet restriction induced a partial recovery of the age-related decrease of this parameter in the cerebellum without affecting the receptor density of lymphocytes. Present results suggest that undernutrition delays the appearance of those alterations related to aging.