Circulating CD138 enhances disease progression by augmenting autoreactive antibody production in a mouse model of systemic lupus erythematosus.

Systemic lupus erythematosus (SLE) is a progressive autoimmune disease characterized by high levels of antibodies directed against nuclear antigens. Elevated serum CD138, a heparan sulfate-bearing proteoglycan, correlates with increased disease activity in patients with SLE, but the contribution of CD138 to lupus disease is not known. Corroborating patient data, we detected an increase in serum CD138 in MRL/MpJ-Faslpr/J (MRL/Lpr) mice (a model for SLE disease) parallel to disease activity. Although T-cell receptor ß (TCRß)+CD138+ T cells typically expand in MRL/Lpr mice as the disease progresses, surprisingly, TCRß+CD138- cells were the primary source of circulating CD138, as the transfer of TCRß+CD138- cells, but not TCRß+CD138+ cells, to young MRL/Lpr mice resulted in higher serum CD138 in the recipients. We found that trypsin was able to cleave CD138 from TCRß+CD138+ cells, and that trypsin was highly expressed in TCRß+CD138- cells. Moreover, trypsin inhibitors, the "defined trypsin inhibitor" and leupeptin, increased CD138 expression on TCRß+CD138- cells, suggesting a contribution of cleaved CD138 to the increase in blood CD138 levels. Furthermore, soluble CD138 was able to bind "a proliferation-inducing ligand" (APRIL) and enhance APRIL-mediated plasma cell generation and autoreactive antibody production through the phosphorylation of extracellular signal-regulated kinase in B cells. The APRIL receptor "transmembrane activator, calcium modulator, and cyclophilin ligand interactor" was involved in the enhancement of APRIL activity by CD138, as the synergistic effect of APRIL and CD138 was ablated in transmembrane activator, calcium modulator, and cyclophilin ligand interactor-deficient B cells. These findings indicate a regulatory role for soluble CD138 in B-cell differentiation and autoreactive antibody production in SLE disease.

Disease Stage-Specific Pathogenicity of CD138 (Syndecan 1)-Expressing T Cells in Systemic Lupus Erythematosus.

CD138 (syndecan 1), a member of the heparan-sulfate proteoglycan family, regulates diverse biological responses by interacting with chemokines, cytokines, growth factors, and adhesion molecules. Expression of CD138 has been detected on T cells from both healthy and sick mice mimicking systemic lupus erythematosus (SLE) disease. However, the characteristics and the role of CD138+ T cells in SLE pathogenesis remain largely unknown. We analyzed the lupus-prone MRL/Lpr mice and the control MRL/MpJ strain as well as the common laboratory strains Balb/c, and C57BL/6 for CD138-expression and found that only the MRL/Lpr strain harbored TCRß+CD138+ cells in various organs. The frequency of TCRß+CD138+ cells progressively expanded in MRL/Lpr mice with age and correlated with disease severity. Majority of the TCRß+CD138+ cells were CD4 and CD8 double-negative and 20% were CD4. At least a portion of TCRß+CD138+ cells originated from CD4+ cells because substantial number of CD4+TCRß+CD138- cells expressed CD138 after in vitro cultivation. Compared to TCRß+CD138- cells, TCRß+CD138+ cells exhibited central memory (Tcm) phenotype with reduced ability to proliferate and produce the cytokines IFNγ and IL-17. When co-cultured with B cells, the ability of TCRß+CD138+ cells to promote plasma cell formation and autoreactive antibody production was dependent on the presence of autoantigen, CD4 co-receptor expression and cell-to-cell contact. Surprisingly, adoptively transferred TCRß+CD138+ T cells slowed down disease progression in young recipient MRL/Lpr mice but had the opposite effect when DNA was co-administered with TCRß+CD138+ T cells or when TCRß+CD138+ cells were transferred to older MRL/Lpr mice with established disease. Thus, CD138-expressing T cells with Tcm phenotype enhance disease progression in SLE by rapidly activating autoreactive B cells when self-antigens are exposed to the immune system.

TACI-Deficient Macrophages Protect Mice Against Metaflammation and Obesity-Induced Dysregulation of Glucose Homeostasis.

Transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) is a receptor for the TNF superfamily cytokines, B cell-activating factor (BAFF), and A proliferation-inducing ligand (APRIL). Here, we demonstrate that TACI-deficient mice subjected to high-fat diet (HFD) are protected from weight gain and dysregulated glucose homeostasis. Resistance to HFD-induced metabolic changes in TACI-deficient mice does not involve TACI-mediated adipogenesis. Instead, accumulation of M2 macrophages (Mϕs), eosinophils, and type 2 innate lymphoid cells in visceral adipose tissue (VAT) is implicated in the protection from obesity-induced assaults. In support of this hypothesis, adoptively transferred TACI-deficient peritoneal or adipose tissue Mϕs, but not B cells, can improve glucose metabolism in the obese host. Interestingly, the transferred TACI-deficient Mϕs not only home to host VAT but also trigger the accumulation of host M2 Mϕs and eosinophils in VAT. The increase in host M2 Mϕs in VAT is likely a result of eosinophil recruitment in response to eotaxin-2 produced by TACI-deficient Mϕs. Insulin signaling experiments revealed that IL-10 secreted by TACI-deficient Mϕs is responsible for maintaining adipocyte insulin sensitivity. Thus, the adoptive transfer experiments offer a model where TACI-deficient Mϕs accumulate in VAT and protect against metaflammation and obesity-associated dysregulation of glucose metabolism.

Delayed onset of autoreactive antibody production and M2-skewed macrophages contribute to improved survival of TACI deficient MRL-Fas/Lpr mouse.

Anti-B cell activating factor belonging to TNF-family (BAFF) antibody therapy is indicated for the treatment of patients with active systemic lupus erythematosus (SLE). We hypothesized that the BAFF receptor, transmembrane activator and calcium-modulator and cyclophilin interactor (TACI) may be responsible for the generation of antibody secreting plasma cells in SLE. To test this hypothesis, we generated TACI deficient MRL-Fas/Lpr (LPR-TACI-/-) mouse. TACI deficiency resulted in improved survival of MRL-Fas/Lpr mice and delayed production of anti-dsDNA and anti-SAM/RNP antibodies. There was also a delay in the onset of proteinuria and the accumulation of IgG and inflammatory macrophages (Mϕs) in the glomeruli of young LPR-TACI-/- mice compared to wild-type mice. Underscoring the role of TACI in influencing Mϕ phenotype, the transfer of Mϕs from 12-week-old LPR-TACI-/- mice to age-matched sick wild-type animals led to a decrease in proteinuria and improvement in kidney pathology. The fact that, in LPR-TACI-/- mouse a more pronounced delay was in IgM and IgG3 autoreactive antibody isotypes and the kinetics of follicular helper T (Tfh) cell-development was comparable between the littermates suggest a role for TACI in T cell-independent autoantibody production in MRL-Fas/Lpr mouse prior to the onset of T cell-dependent antibody production.

MRL Strains Have a BAFFR Mutation without Functional Consequence.

It has been shown that B cell activating factor receptor (BAFFR) is critical for B cell development and survival. In this study, we sought to evaluate the expression and function of BAFFR across multiple stains of mice that vary in their potential to develop systemic autoimmune disease. The inability of a commercial antibody to bind to BAFFR in the autoimmune prone mouse strains, MRL and MRL/Lpr led to the discovery of a mutation in TNFRSF13C gene (encoding BAFFR) that resulted in a Pro44Ser substitution in the N-terminus near the BAFF binding site in these strains. To define the biological consequences of mutant BAFFR, we compared the expression and activity of BAFFR in MRL and MRL/Lpr mice to BALB/c, which express the consensus version of TNFRSF13C. B cells from MRL and MRL/Lpr mice expressed mutant BAFFR on surface and were capable of responding to BAFF as exhibited by BAFF-mediated reduction in apoptosis and NF-κB2 activation. Signaling through MAPK ERK1/2 was not significantly induced by BAFF in MRL/Lpr mice; however, MAPK ERK1/2 signaling was intact in MRL mice. The inability of MRL/Lpr B cells to significantly activate ERK1/2 in response to BAFF was due to the high basal activity of the signaling pathway in these cells. In fact, basal activity of ERK1/2 in B cells correlated with the degree of autoimmune susceptibility exhibited by each strain. In addition, aged MRL/Lpr mice with severe autoimmune disease had high BAFF levels, low surface BAFFR, and high basal NF-κB2 activation, a pattern which is attributed to the high frequency of antibody secreting cells. We conclude that P44S BAFFR mutation does not hinder BAFFR function or enhance B cell activity in MRL/Lpr and MRL mice and that other susceptibility loci on the MRL background contributed to the hyperactivity of these cells.

A High-Throughput, Multi-Cell Phenotype Assay for the Identification of Novel Inhibitors of Chemotaxis/Migration.

Chemotaxis and cell migration are fundamental, universal eukaryotic processes essential for biological functions such as embryogenesis, immunity, cell renewal, and wound healing, as well as for pathogenesis of many diseases including cancer metastasis and chronic inflammation. To identify novel chemotaxis inhibitors as probes for mechanistic studies and leads for development of new therapeutics, we developed a unique, unbiased phenotypic chemotaxis-dependent Dictyostelium aggregation assay for high-throughput screening using rapid, laser-scanning cytometry. Under defined conditions, individual Dictyostelium secrete chemoattractants, migrate, and aggregate. Chemotaxis is quantified by laser-scanning cytometry with a GFP marker expressed only in cells after chemotaxis/multi-cell aggregation. We applied the assay to screen 1,280 known compounds in a 1536-well plate format and identified two chemotaxis inhibitors. The chemotaxis inhibitory activities of both compounds were confirmed in both Dictyostelium and in human neutrophils in a directed EZ-TAXIscan chemotaxis assay. The compounds were also shown to inhibit migration of two human cancer cell lines in monolayer scratch assays. This test screen demonstrated that the miniaturized assay is extremely suited for high-throughput screening of very large libraries of small molecules to identify novel classes of chemotaxis/migratory inhibitors for drug development and research tools for targeting chemotactic pathways universal to humans and other systems.

Glia Maturation Factor-γ Regulates Monocyte Migration through Modulation of ß1-Integrin.

Monocyte migration requires the dynamic redistribution of integrins through a regulated endo-exocytosis cycle, but the complex molecular mechanisms underlying this process have not been fully elucidated. Glia maturation factor-γ (GMFG), a novel regulator of the Arp2/3 complex, has been shown to regulate directional migration of neutrophils and T-lymphocytes. In this study, we explored the important role of GMFG in monocyte chemotaxis, adhesion, and ß1-integrin turnover. We found that knockdown of GMFG in monocytes resulted in impaired chemotactic migration toward formyl-Met-Leu-Phe (fMLP) and stromal cell-derived factor 1α (SDF-1α) as well as decreased α5ß1-integrin-mediated chemoattractant-stimulated adhesion. These GMFG knockdown impaired effects could be reversed by cotransfection of GFP-tagged full-length GMFG. GMFG knockdown cells reduced the cell surface and total protein levels of α5ß1-integrin and increased its degradation. Importantly, we demonstrate that GMFG mediates the ubiquitination of ß1-integrin through knockdown or overexpression of GMFG. Moreover, GMFG knockdown retarded the efficient recycling of ß1-integrin back to the plasma membrane following normal endocytosis of α5ß1-integrin, suggesting that the involvement of GMFG in maintaining α5ß1-integrin stability may occur in part by preventing ubiquitin-mediated degradation and promoting ß1-integrin recycling. Furthermore, we observed that GMFG interacted with syntaxin 4 (STX4) and syntaxin-binding protein 4 (STXBP4); however, only knockdown of STXBP4, but not STX4, reduced monocyte migration and decreased ß1-integrin cell surface expression. Knockdown of STXBP4 also substantially inhibited ß1-integrin recycling in human monocytes. These results indicate that the effects of GMFG on monocyte migration and adhesion probably occur through preventing ubiquitin-mediated proteasome degradation of α5ß1-integrin and facilitating effective ß1-integrin recycling back to the plasma membrane.

Gliadin Induces Neutrophil Migration via Engagement of the Formyl Peptide Receptor, FPR1.

BACKGROUND: Gliadin, the immunogenic component within gluten and trigger of celiac disease, is known to induce the production of Interleukin-8, a potent neutrophil-activating and chemoattractant chemokine. We sought to study the involvement of neutrophils in the early immunological changes following gliadin exposure. METHODS: Utilizing immunofluorescence microscopy and flow cytometry, the redistribution of major tight junction protein, Zonula occludens (ZO)-1, and neutrophil recruitment were assessed in duodenal tissues of gliadin-gavaged C57BL/6 wild-type and Lys-GFP reporter mice, respectively. Intravital microscopy with Lys-GFP mice allowed monitoring of neutrophil recruitment in response to luminal gliadin exposure in real time. In vitro chemotaxis assays were used to study murine and human neutrophil chemotaxis to gliadin, synthetic alpha-gliadin peptides and the neutrophil chemoattractant, fMet-Leu-Phe, in the presence or absence of a specific inhibitor of the fMet-Leu-Phe receptor-1 (FPR1), cyclosporine H. An irrelevant protein, zein, served as a control. RESULTS: Redistribution of ZO-1 and an influx of CD11b+Lys6G+ cells in the lamina propria of the small intestine were observed upon oral gavage of gliadin. In vivo intravital microscopy revealed a slowing down of GFP+ cells within the vessels and influx in the mucosal tissue within 2 hours after challenge. In vitro chemotaxis assays showed that gliadin strongly induced neutrophil migration, similar to fMet-Leu-Phe. We identified thirteen synthetic gliadin peptide motifs that induced cell migration. Blocking of FPR1 completely abrogated the fMet-Leu-Phe-, gliadin- and synthetic peptide-induced migration. CONCLUSIONS: Gliadin possesses neutrophil chemoattractant properties similar to the classical neutrophil chemoattractant, fMet-Leu-Phe, and likewise uses FPR1 in the process.

TACI deficiency leads to alternatively activated macrophage phenotype and susceptibility to Leishmania infection.

The TNF family member, transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), is a key molecule for plasma cell maintenance and is required in infections where protection depends on antibody response. Here, we report that compared with WT mouse, TACI KO Μϕs expressed lower levels of Toll-like receptors (TLRs), CD14, myeloid differentiation primary response protein 88, and adaptor protein Toll/IL-1 receptor domain-containing adapter-inducing IFN-ß and responded poorly to TLR agonists. Analysis of Μϕ phenotype revealed that, in the absence of TACI, Μϕs adapt the alternatively activated (M2) phenotype. Steady-state expression levels for M2 markers IL-4Rα, CD206, CCL22, IL-10, Arg1, IL1RN, and FIZZ1 were significantly higher in TACI KO Μϕ than in WT cells. Confirming their M2 phenotype, TACI-KO Mϕs were unable to control Leishmania major infection in vitro, and intradermal inoculation of Leishmania resulted in a more severe manifestation of disease than in the resistant C57BL/6 strain. Transfer of WT Μϕs to TACI KO mice was sufficient to significantly reduce disease severity. TACI is likely to influence Mϕ phenotype by mediating B cell-activating factor belonging to the TNF family (BAFF) and a proliferation inducing ligand (APRIL) signals because both these ligands down-regulated M2 markers in WT but not in TACI-deficient Μϕs. Moreover, treatment of Μϕs with BAFF or APRIL enhanced the clearance of Leishmania from cells only when TACI is expressed. These findings may have implications for understanding the shortcomings of host response in newborns where TACI expression is reduced and in combined variable immunodeficiency patients where TACI signaling is ablated.

PKCßII acts downstream of chemoattractant receptors and mTORC2 to regulate cAMP production and myosin II activity in neutrophils.

Chemotaxis is a process by which cells polarize and move up a chemical gradient through the spatiotemporal regulation of actin assembly and actomyosin contractility, which ultimately control front protrusions and back retractions. We previously demonstrated that in neutrophils, mammalian target of rapamycin complex 2 (mTORC2) is required for chemoattractant-mediated activation of adenylyl cyclase 9 (AC9), which converts ATP into cAMP and regulates back contraction through MyoII phosphorylation. Here we study the mechanism by which mTORC2 regulates neutrophil chemotaxis and AC9 activity. We show that inhibition of protein kinase CßII (PKCßII) by CPG53353 or short hairpin RNA knockdown severely inhibits chemoattractant-induced cAMP synthesis and chemotaxis in neutrophils. Remarkably, PKCßII-inhibited cells exhibit specific and severe tail retraction defects. In response to chemoattractant stimulation, phosphorylated PKCßII, but not PKCα, is transiently translocated to the plasma membrane, where it phosphorylates and activates AC9. mTORC2-mediated PKCßII phosphorylation on its turn motif, but not its hydrophobic motif, is required for membrane translocation of PKCßII. Inhibition of mTORC2 activity by Rictor knockdown not only dramatically decreases PKCßII activity, but it also strongly inhibits membrane translocation of PKCßII. Together our findings show that PKCßII is specifically required for mTORC2-dependent AC9 activation and back retraction during neutrophil chemotaxis.

Radil controls neutrophil adhesion and motility through ß2-integrin activation.

Integrin activation is required to facilitate multiple adhesion-dependent functions of neutrophils, such as chemotaxis, which is critical for inflammatory responses to injury and pathogens. However, little is known about the mechanisms that mediate integrin activation in neutrophils. We show that Radil, a novel Rap1 effector, regulates ß1- and ß2-integrin activation and controls neutrophil chemotaxis. On activation and chemotactic migration of neutrophils, Radil quickly translocates from the cytoplasm to the plasma membrane in a Rap1a-GTP-dependent manner. Cells overexpressing Radil show a substantial increase in cell adhesion, as well as in integrin/focal adhesion kinase (FAK) activation, and exhibit an elongated morphology, with severe tail retraction defects. This phenotype is effectively rescued by treatment with either ß2-integrin inhibitory antibodies or FAK inhibitors. Conversely, knockdown of Radil causes severe inhibition of cell adhesion, ß2-integrin activation, and chemotaxis. Furthermore, we found that inhibition of Rap activity by RapGAP coexpression inhibits Radil-mediated integrin and FAK activation, decreases cell adhesion, and abrogates the long-tail phenotype of Radil cells. Overall, these studies establish that Radil regulates neutrophil adhesion and motility by linking Rap1 to ß2-integrin activation.

A Gßγ effector, ElmoE, transduces GPCR signaling to the actin network during chemotaxis.

Activation of G protein-coupled receptors (GPCRs) leads to the dissociation of heterotrimeric G-proteins into Gα and Gßγ subunits, which go on to regulate various effectors involved in a panoply of cellular responses. During chemotaxis, Gßγ subunits regulate actin assembly and migration, but the protein(s) linking Gßγ to the actin cytoskeleton remains unknown. Here, we identified a Gßγ effector, ElmoE in Dictyostelium, and demonstrated that it is required for GPCR-mediated chemotaxis. Remarkably, ElmoE associates with Gßγ and Dock-like proteins to activate the small GTPase Rac, in a GPCR-dependent manner, and also associates with Arp2/3 complex and F-actin. Thus, ElmoE serves as a link between chemoattractant GPCRs, G-proteins and the actin cytoskeleton. The pathway, consisting of GPCR, Gßγ, Elmo/Dock, Rac, and Arp2/3, spatially guides the growth of dendritic actin networks in pseudopods of eukaryotic cells during chemotaxis.

Review series: TOR kinase complexes and cell migration.

Cell migration is a fundamental process in a wide array of biological and pathological responses. It is regulated by complex signal transduction pathways in response to external cues that couple to growth factor and chemokine receptors. In recent years, the target of rapamycin (TOR) kinase, as part of either TOR complex 1 (TORC1) or TOR complex 2 (TORC2), has been shown to be an important signaling component linking external signals to the cytoskeletal machinery in a variety of cell types and organisms. Thus, these complexes have emerged as key regulators of cell migration and chemotaxis.

Glia maturation factor-γ mediates neutrophil chemotaxis.

Chemotaxis is fundamental to the directional migration of neutrophils toward endogenous and exogenous chemoattractants. Recent studies have demonstrated that ADF/cofilin superfamily members play important roles in reorganizing the actin cytoskeleton by disassembling actin filaments. GMFG, a novel ADF/cofilin superfamily protein that is expressed in inflammatory cells, has been implicated in regulating actin reorganization in microendothelial cells, but its function in neutrophils remains unclear. Here, we show that GMFG is an important regulator for cell migration and polarity in neutrophils. Knockdown of endogenous GMFG impaired fMLF- and IL-8 (CXCL8)-induced chemotaxis in dHL-60 cells. GMFG knockdown attenuated the formation of lamellipodia at the leading edge of cells exposed to fMLF or CXCL8, as well as the phosphorylation of p38 and PAK1/2 in response to fMLF or CXCL8. Live cell imaging revealed that GMFG was recruited to the leading edge of cells in response to fMLF, as well as CXCL8. Overexpression of GMFG enhanced phosphorylation of p38 but not of PAK1/2 in dHL-60 cells. In addition, we found that GMFG is associated with WAVE2. Taken together, our findings suggest that GMFG is a novel factor in regulating neutrophil chemotaxis by modulating actin cytoskeleton reorganization.

mTORC2 regulates neutrophil chemotaxis in a cAMP- and RhoA-dependent fashion.

We studied the role of the target of rapamycin complex 2 (mTORC2) during neutrophil chemotaxis, a process that is mediated through the polarization of actin and myosin filament networks. We show that inhibition of mTORC2 activity, achieved via knock down (KD) of Rictor, severely inhibits neutrophil polarization and directed migration induced by chemoattractants, independently of Akt. Rictor KD also abolishes the ability of chemoattractants to induce cAMP production, a process mediated through the activation of the adenylyl cyclase 9 (AC9). Cells with either reduced or higher AC9 levels also exhibit specific and severe tail retraction defects that are mediated through RhoA. We further show that cAMP is excluded from extending pseudopods and remains restricted to the cell body of migrating neutrophils. We propose that the mTORC2-dependent regulation of MyoII occurs through a cAMP/RhoA-signaling axis, independently of actin reorganization during neutrophil chemotaxis.

Biphasic regulation of extracellular signal-regulated kinases by scalaradial, a secretory phospholipase A(2) inhibitor.

The marine natural product scalaradial (SLD) is a potent inhibitor of secretory phospholipase A(2) (sPLA(2)). Our previous work has demonstrated that SLD inhibits epidermal growth factor receptor-mediated Akt phosphorylation, and this effect is independent of sPLA(2). Here we report the role of SLD in extracellular signal-regulated kinase (ERK)1/2 activation. SLD inhibited ERK1/2 phosphorylation within the first 15 min (early inhibition), then stimulated ERK1/2 phosphorylation after 15 min of SLD treatment (late stimulation) in BEL-7402 cells, displaying biphasic regulatory features. Other PLA(2) inhibitors such as the cytosolic and Ca(2+)-independent PLA(2) inhibitor methyl arachidonyl fluorophosphonate, and another sPLA(2) inhibitor, thioetheramide-phosphatidylcholine, only transiently inhibited ERK1/2 phosphorylation and did not display the stimulatory effect. The early inhibition of ERK1/2 phosphorylation by SLD was reversed by the PLA(2) metabolite arachidonic acid, while the late stimulation was abrogated by constitutively active myristolated-Akt. Furthermore, SLD dose- and time-dependently inhibited the phosphorylation of Raf-1 on Ser 259, which is an established event by which Akt inhibits ERK1/2 activation. Taken together, these data demonstrate a biphasic regulation of ERK1/2 phosphorylation by SLD in a time-dependent manner, i.e., early inhibition and late stimulation. The early inhibition of ERK1/2 phosphorylation is mediated by sPLA(2), at least in part, and the late stimulation is effected through SLD inhibition of Akt. These findings provide further insight into the mechanisms underlying the pharmacological effect of SLD.

PI3K is required for insulin-stimulated but not EGF-stimulated ERK1/2 activation.

The Ras/Raf/extracellular signal-regulated kinase 1 and 2 (ERK1/2) signaling pathway is known to cross-talk with other signaling pathways, including phosphatidylinositol 3-kinase (PI3K)/Akt pathway. However, the role of PI3K in ERK-1/2 activation induced by tyrosine kinase receptors was not fully understood. Here, we report that two structurally distinct PI3K inhibitors, wortmannin and LY294002, inhibited insulin-induced activation of ERK1/2 but had no effect on EGF-induced activation of ERK1/2 in hepatocellular carcinoma BEL-7402 and SMMC-7721 cells, breast cancer MCF-7 cells, and prostate cancer LNCaP cells. Although protein kinase C could act as a mediator between PI3K and ERK1/2, protein kinase C inhibitor chelerythrine chloride did not inhibit insulin-induced ERK1/2 activation. Both insulin- and EGF-induced ERK1/2 activation are strictly dependent on Ras activation, however, wortmannin only inhibited insulin-induced, but not EGF-induced Ras activation. These results indicate that PI3K plays different roles in the activation of Ras/ERK1/2 signaling by insulin and EGF, and that insulin-stimulated, but not EGF-stimulated, ERK1/2 and Akt signalings diverge at PI3K.

Cyclic AMP inhibition of proliferation of hepatocellular carcinoma cells is mediated by Akt.

Cyclic AMP (cAMP), one of the most important intracellular second messengers, has been reported to inhibit proliferation of human hepatocellular carcinoma (HCC) cells via negatively regulating p42/44 mitogen-activated protein kinase. Here, we reported that cAMP inhibited the proliferation of HCC BEL-7402 cells via a novel mechanism. Forskolin, an activator of adenylate cyclase, inhibited fetal bovine serum (FBS)-stimulated BEL-7402 cell proliferation in a dose- and time-dependent manner, along with the inhibition of FBS-stimulated serine/threoine protein kinase Akt (also known as PKB) phosphorylation which is required for Akt activation and this effect was mimicked by 8-Br cAMP. Forskolin also inhibited Akt phosphorylation stimulated by other growth factors such as IGF-1, epidermal growth factor, and insulin. These inhibitions were found not only in BEL-7402 cells, but also in another HCC cell line SMMC-7721 cells. Myr-Akt (myristolated-Akt), a constitutively active Akt which was relatively resistant to cAMP inhibition, conferred BEL-7402 cells resistance to cAMP treatment. However, overexpression of Myr-Akt alone was not sufficient to stimulate BEL-7402 cell proliferation. cAMP inhibited FBS-stimulated Akt phosphorylation in a cAMP-dependent protein kinase-dependent manner. Further studies demonstrated that cAMP inhibited FBS-induced membrane localization of 3-phosphoinositide-dependent kinase 1 (PDK-1) which is a required process for PDK-1 to phosphorylate Akt, but had no significant effect on phosphoinositide 3-kinase activity. These results indicate that cAMP inhibition of proliferation of HCC cells is mediated by Akt and cAMP inhibits Akt activation via blocking membrane localization of PDK-1.

Scalaradial inhibition of epidermal growth factor receptor-mediated Akt phosphorylation is independent of secretory phospholipase A2.

The marine natural product 12-epi-scalaradial (SLD) is a specific secretory phospholipase A(2) (sPLA(2)) inhibitor. However, little is known about whether this compound has other pharmacological effects. Here, we revealed a novel effect of SLD on epidermal growth factor receptor (EGFR)-mediated Akt phosphorylation. SLD dose- and time-dependently inhibited epidermal growth factor (EGF)-stimulated Akt phosphorylation, which is required for Akt activation. SLD also blocked the EGF-stimulated membrane translocation of 3-phosphoinositide-dependent protein kinase 1 and inhibited phosphatidylinositol 3-kinase activity. This inhibition is specific for SLD because other phospholipase inhibitors, including sPLA(2) inhibitor thioetheramide-phosphatidylcholine, cytosolic PLA(2) inhibitor arachidonyl trifluoromethyl ketone, cytosolic PLA(2) and Ca(2+)-independent PLA(2) inhibitor methyl arachidonyl fluorophosphonate, phospholipase C inhibitor U73122, and cyclooxygenases inhibitor indomethacin, failed to inhibit EGF-stimulated Akt phosphorylation. Furthermore, arachidonic acid, the main sPLA(2)-catalyzed metabolite, was not able to rescue SLD inhibition of EGF-stimulated Akt phosphorylation. Overexpression of group IIA or group X sPLA(2) did not reverse the inhibitory effect of SLD on Akt phosphorylation, either. Our results demonstrate that SLD inhibits EGFR-mediated Akt phosphorylation, and this novel effect of SLD is independent of sPLA(2).