Inhibitory potential of flavonoids on PtdIns(3,4,5)P3 binding with the phosphoinositide-dependent kinase 1 pleckstrin homology domain

Many membrane-associated proteins are involved in various signaling pathways, including the phosphoinositide 3-kinase (PI3K) pathway, which has key roles in diverse cellular processes. Disruption of the activities of these proteins is involved in the development of disease in humans, making these proteins promising targets for drug development. In most cases, the catalytic domain is targeted; however, it is also possible to target membrane associations in order to regulate protein activity. In this study, we established a novel method to study protein-lipid interactions and screened for flavonoid-derived antagonists of PtdIns(3,4,5)P₃ binding with the phosphoinositide-dependent kinase 1 (PDK1) pleckstrin homology (PH) domain. Using an enhanced green fluorescent protein (eGFP)-tagged PDK1 PH domain and 50% sucrose-loaded liposomes, the protein-lipid interaction could be efficiently evaluated using liposome pull-down assays coupled with fluorescence spectrophotometry, and a total of 32 flavonoids were screened as antagonists for PtdIns(3,4,5)P₃ binding with the PDK1 PH domain. From this analysis, we found that two adjunct hydroxyl groups in the C ring were responsible for the inhibitory effects of the flavonoids. Because the flavonoids shared structural similarities, the results were then subjected to quantitative structure-activity relationship (QSAR) analysis. The results were then further confirmed by in silico docking experiments. Taken together, our strategy presented herein to screen antagonists targeting lipid-protein interactions could be an alternative method for identification and characterization of drug candidates.     

The in vivo role of PtdIns(3,4,5)P3 binding to PDK1 PH domain defined by knockin mutation

We generated homozygous knockin ES cells expressing a form of 3-phosphoinositide-dependent protein kinase-1 (PDK1) with a mutation in its pleckstrin homology (PH) domain that abolishes phosphatidylinositol 3,4,5-tris-phosphate (PtdIns(3,4,5)P3) binding, without affecting catalytic activity. In the knockin cells, protein kinase B (PKB) was not activated by IGF1, whereas ribosomal S6 kinase (RSK) was activated normally, indicating that PtdIns(3,4,5)P3 binding to PDK1 is required for PKB but not RSK activation. Interestingly, amino acids and Rheb, but not IGF1, activated S6K in the knockin cells, supporting the idea that PtdIns(3,4,5)P3 stimulates S6K through PKB-mediated activation of Rheb. Employing PDK1 knockin cells in which either the PtdIns(3,4,5)P3 binding or substrate-docking 'PIF pocket' was disrupted, we established the roles that these domains play in regulating phosphorylation and stabilisation of protein kinase C isoforms. Moreover, mouse PDK1 knockin embryos in which either the PH domain or PIF pocket was disrupted died displaying differing phenotypes between E10.5 and E11.5. Although PDK1 plays roles in regulating cell size, cells derived from PH domain or PIF pocket knockin embryos were of normal size. These experiments establish the roles of the PDK1 regulatory domains and illustrate the power of knockin technology to probe the physiological function of protein-lipid and protein-protein interactions.     

Elucidation of different inhibition mechanism of small chemicals on PtdInsP-binding domains using in silico docking experiments

Phosphatidylinositides, most negatively charged lipids in cellular membranes, regulate diverse effector proteins through the interaction with their lipid binding domains. We have previously reported inhibitory effect of small chemicals on the interaction between PtdIns(3,4,5)P₃ and Btk PH domain. Here, we report that the inhibitory effects of same sets of chemicals on Grp1 PH domain and epsin1 ENTH domain to elucidate diversity of inhibitory mechanisms upon different lipid binding domains. Among the chemicals, chemical 8 showed best inhibition in vitro assay for Grp1 PH domain and epsin1 ENTH domain, and then the interaction between small chemicals and lipid binding domains was further investigated by in silico docking experiments. As a result, it was concluded that the diverse inhibitory effects on different lipid binding domains were dependent on not only the number of interactions between small chemical and domain, but also additional interaction with positively charged surfaces as the secondary binding sites. This finding will help to develop lipid binding inhibitors as antagonists for lipid–protein interactions, and these inhibitors would be novel therapeutic drug candidates via regulating effector proteins involved in severe human diseases.     

Chromenylchalcones with inhibitory effects on monoamine oxidase B

Structure–activity relationship (SAR) calculations were used to find monoamine oxidase-B (MAO-B) inhibitors by identifying pharmacophores exhibiting high inhibitory activities. Several such chromenylchalcones were designed and synthesized accordingly. Their inhibitory effects on MAO-B were determined using an HPLC-based method and an MAO-B enzyme assay kit. (E)-3-(6-Methoxy-2H-chromen-3-yl)-1-(2-methoxyphenyl)prop-2-en-1-one exhibited a half-maximal inhibitory concentration of 320 nM. Its molecular-level binding mode with the three-dimensional structure of MAO-B was elucidated using an in silico docking study. The chromenylchalcone scaffold, which is derived from natural products including isoflavonoids and chalcones, had not been previously reported as an MAO-B inhibitor.     

Interaction of the brain-specific protein p42IP4/centaurin-alpha1 with the peptidase nardilysin is regulated by the cognate ligands of p42IP4, PtdIns(3,4,5)P3 and Ins(1,3,4,5)P4, with stereospecificity

The brain-specific protein p42IP4, also called centaurin-alpha1, specifically binds phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. Here, we investigate the interaction of p42IP4/centaurin-alpha1 with nardilysin (NRDc), a member of the M16 family of zinc metalloendopeptidases. Members of this peptidase family exhibit enzymatic activity and also act as receptors for other proteins. We found that p42IP4/centaurin-alpha1 binds specifically to NRDc from rat brain. We further detected that centaurin-alpha2, a protein that is highly homologous to p42IP4/centaurin-alpha1 and expressed ubiquitously, also binds to NRDc. In vivo interaction was demonstrated by co-immunoprecipitation of p42IP4/centaurin-alpha1 with NRDc from rat brain. The acidic domain of NRDc (NRDc-AD), which does not participate in catalysis, is sufficient for the protein interaction with p42IP4. Interestingly, preincubation of p42IP4 with its cognate ligands D-Ins(1,3,4,5)P4 and the lipid diC8PtdIns(3,4,5)P3 negatively modulates the interaction between the two proteins. D-Ins(1,3,4,5)P4 and diC8PtdIns(3,4,5)P3 suppress the interaction with virtually identical concentration dependencies. This inhibition is highly ligand specific. The enantiomer L-Ins(1,3,4,5)P4 is not effective. Similarly, the phosphoinositides diC8PtdIns(3,4)P2, diC8PtdIns(3,5)P2 and diC8PtdIns(4,5)P2 all have no influence on the interaction. Further experiments revealed that endogenous p42IP4 from rat brain binds to glutathione-S-transferase (GST)-NRDc-AD. The proteins dissociate from each other when incubated with D-Ins(1,3,4,5)P4, but not with inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. In summary, we demonstrate that p42IP4 binds to NRDc via the NRDc-AD, and that this interaction is controlled by the cognate cellular ligands of p42IP4/centaurin-alpha1. Thus, specific ligands of p42IP4 can modulate the recruitment of proteins, which are docked to p42IP4, to specific cellular compartments.     

Stability and peptide binding specificity of Btk SH2 domain: molecular basis for X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is caused by mutations in the Bruton's tyrosine kinase (Btk). The absence of functional Btk leads to failure of B-cell development that incapacitates antibody production in XLA patients leading to recurrent bacterial infections. Btk SH2 domain is essential for phospholipase C-gamma phosphorylation, and mutations in this domain were shown to cause XLA. Recently, the B-cell linker protein (BLNK) was found to interact with the SH2 domain of Btk, and this association is required for the activation of phospholipase C-gamma. However, the molecular basis for the interaction between the Btk SH2 domain and BLNK and the cause of XLA remain unclear. To understand the role of Btk in B-cell development, we have determined the stability and peptide binding affinity of the Btk SH2 domain. Our results indicate that both the structure and stability of Btk SH2 domain closely resemble with other SH2 domains, and it binds with phosphopeptides in the order pYEEI > pYDEP > pYMEM > pYLDL > pYIIP. We expressed the R288Q, R288W, L295P, R307G, R307T, Y334S, Y361C, L369F, and 1370M mutants of the Btk SH2 domain identified from XLA patients and measured their binding affinity with the phosphopeptides. Our studies revealed that mutation of R288 and R307 located in the phosphotyrosine binding site resulted in a more than 200-fold decrease in the peptide binding compared to L295, Y334, Y361, L369, and 1370 mutations in the pY + 3 hydrophobic binding pocket (approximately 3- to 17-folds). Furthermore, mutation of the Tyr residue at the betaD5 position reverses the binding order of Btk SH2 domain to pYIIP > pYLDL > pYDEP > pYMEM > pYEEI. This altered binding behavior of mutant Btk SH2 domain likely leads to XLA.     

PtdIns(3,4,5)P3 binding is necessary for WAVE2-induced formation of lamellipodia

Polarized cell movement is triggered by the development of a PtdIns(3,4,5)P(3) gradient at the membrane, which is followed by rearrangement of the actin cytoskeleton. The WASP family verprolin homologous protein (WAVE) is essential for lamellipodium formation at the leading edge by activating the Arp2/3 complex downstream of Rac GTPase. Here, we report that WAVE2 binds to PtdIns(3,4,5)P(3) through its basic domain. The amino-terminal portion of WAVE2, which includes the PtdIns(3,4,5)P(3)-binding sequence, was localized at the leading edge of lamellipodia induced by an active form of Rac (RacDA) or by treatment with platelet-derived growth factor (PDGF). Production of PtdIns(3,4,5)P(3) at the cell membrane by myristoylated phosphatidylinositol-3-OH kinase (PI(3)K) is sufficient to recruit WAVE2 in the presence of dominant-negative Rac and latrunculin, demonstrating that PtdIns(3,4,5)P(3) alone is able to recruit WAVE2. Expression of a full-length mutant of WAVE2 that lacks the lipid-binding activity inhibited proper formation of lamellipodia induced by RacDA. These results suggest that one of the products of PI(3)K, PtdIns(3,4,5)P(3), recruits WAVE2 to the polarized membrane and that this recruitment is essential for lamellipodium formation at the leading edge.     

Molecular control of PtdIns(3,4,5)P3 signaling in neutrophils

Neutrophils play critical roles in innate immunity and host defense. However, excessive neutrophil accumulation or hyper-responsiveness of neutrophils can be detrimental to the host system. Thus, the response of neutrophils to inflammatory stimuli needs to be tightly controlled. Many cellular processes in neutrophils are mediated by localized formation of an inositol phospholipid, phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3), at the plasma membrane. The PtdIns(3,4,5)P3 signaling pathway is negatively regulated by lipid phosphatases and inositol phosphates, which consequently play a critical role in controlling neutrophil function and would be expected to act as ideal therapeutic targets for enhancing or suppressing innate immune responses. Here, we comprehensively review current understanding about the action of lipid phosphatases and inositol phosphates in the control of neutrophil function in infection and inflammation.     

Use of the GRP1 PH domain as a tool to measure the relative levels of PtdIns(3,4,5)P3 through a protein-lipid overlay approach

We describe a novel approach to the relative quantification of phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)] and its application to measure, in neutrophils, the activation of phosphoinositide 3-kinase (PI3K). This protein-lipid overlay-based assay allowed us to confirm and extend the observations, first, that N-formyl-methionyl-leucyl-phenylalanine (fMLP) stimulation of primed human neutrophils leads to a transient and biphasic increase in PtdIns(3,4,5)P(3) levels and, second, that the ability of fMLP to stimulate PtdIns(3,4,5)P(3) accumulation in neutrophils isolated from mice carrying a Ras-insensitive ('DASAA') knock-in of PI3Kgamma (p110gamma(DASAA/DASAA)) is substantially dependent on the Ras binding domain of PI3Kgamma.     

A novel and evolutionarily conserved PtdIns(3,4,5)P3-binding domain is necessary for DOCK180 signalling

The evolutionarily conserved DOCK180 protein has an indispensable role in cell migration by functioning as an exchange factor for Rac GTPase via its DOCK homology region (DHR)-2 domain. We report here that the conserved DHR-1 domain also has an important signalling role. A form of DOCK180 that lacks DHR-1 fails to promote cell migration, although it is capable of inducing Rac GTP-loading. The DHR-1 domain interacts with PtdIns(3,4,5)P(3) in vitro and in vivo, and mediates the DOCK180 signalling complex localization at sites of PtdIns(3,4,5)P(3) accumulation in the cell's leading edge. A form of DOCK180 in which the DHR-1 domain has been replaced by a canonical PtdIns(3,4,5)P(3)-binding pleckstrin homology domain is fully functional at inducing cell elongation and migration, suggesting that the main function of DHR-1 is to bind PtdIns(3,4,5)P(3). These results demonstrate that DOCK180, via its DHR-1 and DHR-2 domains, couples PtdIns(3,4,5)P(3) signalling to Rac GTP-loading, which is essential for directional cell movement.     

PtdIns(4,5)P2 and PtdIns(3,4,5)P3 dynamics during focal adhesions assembly and disassembly in a cancer cell line

Focal adhesions (FAs) are large assemblies of proteins that mediate intracellular signals between the cytoskeleton and the extracellular matrix (ECM). The turnover of FA proteins plays a critical regulatory role in cancer cell migration. Plasma membrane lipids locally generated or broken down by different inositide kinases and phosphatase enzymes to activate and recruit proteins to specific regions in the plasma membrane. Presently, little attention has been given to the use of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and Phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) fluorescent biosensors in order to determine the spatiotemporal organisation of PtdIns(4,5)P2 and PtdIns(3,4,5)P3 within and around or during assembly and disassembly of FAs. In this study, specific biosensors were used to detect PtdIns(4,5)P2, PtdIns(3,4,5)P3, and FAs proteins conjugated to RFP/GFP in order to monitor changes of PtdIns(4,5)P2 and PtdIns(3,4,5)P3 levels within FAs. We demonstrated that the localisation of PtdIns(4,5)P2 and PtdIns(3,4,5)P3 were moderately correlated with that of FA proteins. Furthermore, we demonstrate that local levels of PtdIns(4,5)P2 increased within FA assembly and declined within FA disassembly. However, PtdIns(3,4,5)P3 levels remained constant within FAs assembly and disassembly. In conclusion, this study shows that PtdIns(4,5)P2 and PtdIns(3,4,5)P3 localised in FAs may be regulated differently during FA assembly and disassembly.     

Benzochalcones bearing pyrazoline moieties show anti-colorectal cancer activities and selective inhibitory effects on aurora kinases

Colorectal cancer is the third and fourth leading cause of cancer in males and females, respectively. Flavonoids, including chalcones, are secondary metabolites in plants that exhibit diverse biological activities, including antibacterial, antimalarial, and antitumor activities. In order to find potent and novel chemotherapy drugs for colorectal cancer, a series of benzochalcone derivatives, in which an α,β-unsaturated carbonyl group was replaced with a pyrazoline, was designed and synthesized. A clonogenic survival assay was performed with each derivative to evaluate antitumor activity. 1-(5-(2,4-Dimethoxyphenyl)-4,5-dihydro-1H-pyrazol-3-yl)naphthalen-2-ol (derivative 7) had the most potent inhibitory effect on the long-term clonogenicity of HCT116 human colorectal cancer cells (IC₅₀ = 2.4 μM). The results of Western blot and flow cytometric analyses suggested that derivative 7 could inhibit the proliferation of colorectal cancer cells through inhibition of cell cycle progression and induction of apoptosis. To elucidate its molecular mechanism, in vitro kinase binding assays were carried out, which demonstrated that derivative 7 inhibited aurora kinases A and B selectively. The binding modes between the compound and aurora kinases were interpreted using in silico docking experiments to explain the selective inhibitory effects on aurora kinases A and B. These findings will facilitate the design of potent novel benzochalcones as anticancer agents.     

Phosphoinositides in insulin action on GLUT4 dynamics: not just PtdIns(3,4,5)P3

Accumulated evidence over the last several years indicates that insulin regulates multiple steps in the overall translocation of GLUT4 vesicles to the fat/muscle cell surface, including formation of an intracellular storage pool of GLUT4 vesicles, its movement to the proximity of the cell surface, and the subsequent docking/fusion with the plasma membrane. Insulin-stimulated formation of phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3); and in some cases, of its catabolite PtdIns(3,4)P(2)] plays a pivotal role in this process. PtdIns(3,4,5)P(3) is synthesized by the activated wortmannin-sensitive class IA phosphoinositide (PI) 3-kinase and controls the rate-limiting cell surface terminal stages of the GLUT4 journey. However, recent research is consistent with the conclusion that signals by each of the remaining five PIs, i.e., PtdIns(3)P, PtdIns(4)P, PtdIns(5)P, PtdIns(3,5)P(2), and PtdIns(4,5)P(2), may act in concert with that of PtdIns(3,4,5)P(3) in integrating the insulin receptor-issued signals with GLUT4 surface translocation and glucose transport activation. This review summarizes the experimental evidence supporting the complementary function of these PIs in insulin responsiveness of fat and muscle cells, with particular reference to mechanistic insights and functional significance in the regulation of overall GLUT4 vesicle dynamics.     

PtdIns(4,5)P2 interacts with CaM binding domains on TRPM3 N-terminus

TRPM3 has been reported to play an important role in Ca²⁺ homeostasis, but its gating mechanisms and regulation via Ca²⁺ are unknown. Ca²⁺ binding proteins such as calmodulin (CaM) could be probable modulators of this ion channel. We have shown that this protein binds to two independent domains, A35-K124 and H291-G382 on the TRPM3 N-terminus, which contain conserved hydrophobic as well as positively charged residues in specific positions, and that these residues have a crucial impact on its binding. We also showed that another Ca²⁺ binding protein, S100A1, is able to bind to these regions and that CaM and S100A1 compete for these binding sites on the TRPM3 N-terminus. Moreover, our results suggest that another very important TRP channel activity modulator, PtdIns(4,5)P₂, interacts with the CaM/S100A1 binding sites on the TRPM3 N-terminus with high affinity.     

A PtdIns(3,4,5)P3 dispersal switch engages cell ratcheting at specific cell surfaces

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Control of cell polarity and motility by the PtdIns(3,4,5)P3 phosphatase SHIP1

Proper neutrophil migration into inflammatory sites ensures host defense without tissue damage. Phosphoinositide 3-kinase (PI(3)K) and its lipid product phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) regulate cell migration, but the role of PtdIns(3,4,5)P(3)-degrading enzymes in this process is poorly understood. Here, we show that Src homology 2 (SH2) domain-containing inositol-5-phosphatase 1 (SHIP1), a PtdIns(3,4,5)P(3) phosphatase, is a key regulator of neutrophil migration. Genetic inactivation of SHIP1 led to severe defects in neutrophil polarization and motility. In contrast, loss of the PtdIns(3,4,5)P(3) phosphatase PTEN had no impact on neutrophil chemotaxis. To study PtdIns(3,4,5)P(3) metabolism in living primary cells, we generated a novel transgenic mouse (AktPH-GFP Tg) expressing a bioprobe for PtdIns(3,4,5)P(3.) Time-lapse footage showed rapid, localized binding of AktPH-GFP to the leading edge membrane of chemotaxing ship1(+/+)AktPH-GFP Tg neutrophils, but only diffuse localization in ship1(-/-)AktPH-GFP Tg neutrophils. By directing where PtdIns(3,4,5)P(3) accumulates, SHIP1 governs the formation of the leading edge and polarization required for chemotaxis.     

Constitutive PtdIns(3,4,5)P3 synthesis promotes the development and survival of early mammalian embryos

Mammalian preimplantation embryos develop in the oviduct as individual entities, and can develop and survive in vitro, in defined culture media lacking exogenous growth factors or serum. Therefore, early embryos must generate intrinsic signals that promote their development and survival. In other cells, activation of class I phosphoinositide 3-kinase (PI3K) is a universal mechanism to promote cell proliferation and survival. Here, we examined whether PI3K is intrinsically activated during preimplantation development. Using GFP-tagged pleckstrin homology domains to monitor PtdIns(3,4,5)P(3) synthesis, we show that PI3K is constitutively activated in mouse preimplantation embryos. E-cadherin ligation promotes PtdIns(3,4,5)P(3) synthesis at sites of blastomere adhesion at all cleavage stages. In addition, in culture conditions that promote autocrine signalling, a second pool of PtdIns(3,4,5)P(3) is generated in the apical membrane of early stage blastomeres. We show that constitutive PtdIns(3,4,5)P(3) synthesis is necessary for optimal development to blastocyst and to prevent large-scale apoptosis at the time of cavitation.     

PtdIns(3,4,5)P(3)-dependent and -independent roles for PTEN in the control of cell migration

BACKGROUND: Phosphatase and tensin homolog (PTEN) mediates many of its effects on proliferation, growth, survival, and migration through its PtdIns(3,4,5)P(3) lipid phosphatase activity, suppressing phosphoinositide 3-kinase (PI3K)-dependent signaling pathways. PTEN also possesses a protein phosphatase activity, the role of which is less well characterized. RESULTS: We have investigated the role of PTEN in the control of cell migration of mesoderm cells ingressing through the primitive streak in the chick embryo. Overexpression of PTEN strongly inhibits the epithelial-to-mesenchymal transition (EMT) of mesoderm cells ingressing through the anterior and middle primitive streak, but it does not affect EMT of cells located in the posterior streak. The inhibitory activity on EMT is completely dependent on targeting PTEN through its C-terminal PDZ binding site, but can be achieved by a PTEN mutant (PTEN G129E) with only protein phosphatase activity. Expression either of PTEN lacking the PDZ binding site or of the PTEN C2 domain, or inhibition of PI3K through specific inhibitors, does not inhibit EMT, but results in a loss of both cell polarity and directional migration of mesoderm cells. The PTEN-related protein TPTE, which normally lacks any detectable lipid and protein phosphatase activity, can be reactivated through mutation, and only this reactivated mutant leads to nondirectional migration of these cells in vivo. CONCLUSIONS: PTEN modulates cell migration of mesoderm cells in the chick embryo through at least two distinct mechanisms: controlling EMT, which involves its protein phosphatase activity; and controlling the directional motility of mesoderm cells, through its lipid phosphatase activity.