PDK1 regulates growth through Akt and S6K in Drosophila

The insulin/insulin-like growth factor-1 signaling pathway promotes growth in invertebrates and vertebrates by increasing the levels of phosphatidylinositol 3,4,5-triphosphate through the activation of p110 phosphatidylinositol 3-kinase. Two key effectors of this pathway are the phosphoinositide-dependent protein kinase 1 (PDK1) and Akt/PKB. Although genetic analysis in Caenorhabditis elegans has implicated Akt as the only relevant PDK1 substrate, cell culture studies have suggested that PDK1 has additional targets. Here we show that, in Drosophila, dPDK1 controls cellular and organism growth by activating dAkt and S6 kinase, dS6K. Furthermore, dPDK1 genetically interacts with dRSK but not with dPKN, encoding two substrates of PDK1 in vitro. Thus, the results suggest that dPDK1 is required for dRSK but not dPKN activation and that it regulates insulin-mediated growth through two main effector branches, dAkt and dS6K.     

A mouse orthologue of puromycin-insensitive leucyl-specific aminopeptidase is expressed in endothelial cells and plays an important role in angiogenesis

Using polymerase chain reaction-coupled subtractive hybridization, we have isolated genes expressed during the in vitro differentiation of murine embryonic stem cells into endothelial cells (ECs). Among the genes obtained, we identified one gene that was inducible by vascular endothelial growth factor (VEGF) in the murine EC line MSS31. Analysis of the nucleotide and deduced amino acid sequences revealed that the protein was composed of 930 amino acids, including an HEXXH(X)18E consensus sequence of the M1 aminopeptidase family, and is thought to be a mouse orthologue of puromycin-insensitive leucyl-specific aminopeptidase (mPILSAP). The recombinant protein hydrolyzed N-terminal leucyl and methionyl residues from synthetic substrates. Immunohistochemical analysis revealed that mPILSAP was expressed in ECs during postnatal angiogenesis. Specific elimination of mPILSAP expression by antisense oligodeoxynucleotide (AS-ODN) attenuated VEGF-stimulated proliferation, migration, and network formation of ECs in vitro. Moreover, AS-ODN to mPILSAP inhibited angiogenesis in vivo. These results suggest a novel function of mPILSAP, which is expressed in ECs and plays an important role in angiogenesis.     

An antagonistic vascular endothelial growth factor (VEGF) variant inhibits VEGF-stimulated receptor autophosphorylation and proliferation of human endothelial cells

Vascular endothelial growth factor (VEGF) is a potent mitogen with a unique specificity for endothelial cells and a key mediator of aberrant endothelial cell proliferation and vascular permeability in a variety of human pathological situations, such as tumor angiogenesis, diabetic retinopathy, rheumatoid arthritis, or psoriasis. VEGF is a symmetric homodimeric molecule with two receptor binding interfaces lying on each pole of the molecule. Herein we report on the construction and recombinant expression of an asymmetric heterodimeric VEGF variant with an intact receptor binding interface at one pole and a mutant receptor binding interface at the second pole of the dimer. This VEGF variant binds to VEGF receptors but fails to induce receptor activation. In competition experiments, the heterodimeric VEGF variant antagonizes VEGF-stimulated receptor autophosphorylation and proliferation of endothelial cells. A 15-fold excess of the heterodimer was sufficient to inhibit VEGF-stimulated endothelial cell proliferation by 50%, and a 100-fold excess resulted in an almost complete inhibition. By using a rational approach that is based on the structure of VEGF, we have shown the feasibility to construct a VEGF variant that acts as an VEGF antagonist.     

Glycogen synthase kinase (GSK)-3 promotes p70 ribosomal protein S6 kinase (p70S6K) activity and cell proliferation

The p70 ribosomal protein S6 kinase 1 (S6K1) plays a key role in cell growth and proliferation by regulating insulin sensitivity, metabolism, protein synthesis, and cell cycle. Thus, deregulation of S6K contributes to the progression of type 2 diabetes, obesity, aging, and cancer. Considering the biological and clinical importance of S6K1, a complete understanding of its regulation is critical. One of the key motifs in the activation of S6K1 is a turn motif, but its regulation is not well understood. Here we provide evidence for two mechanisms of modulating turn motif phosphorylation and S6K1 activity. First, mammalian target of rapamycin regulates turn motif phosphorylation by inhibiting its dephosphorylation. Second, we unexpectedly found that glycogen synthase kinase (GSK)-3 promotes turn motif phosphorylation. Our studies show that mammalian target of rapamycin and GSK-3 cooperate to control the activity of S6K1, an important regulator of cell proliferation and growth. Our unexpected results provide a clear rationale for the development and use of drugs targeting GSK-3 to treat diseases such as diabetes, cancer, and age-related diseases that are linked to improper regulation of S6K1.     

Activation of the planar cell polarity formin DAAM1 leads to inhibition of endothelial cell proliferation,migration, and angiogenesis

The Wnt/planar cell polarity (PCP) pathway regulates directed cell movement during development and was recently found to play a critical role in endothelial cell proliferation and angiogenesis [Zhang Y, et al. (2006) Chem Biol 13:1001–1009; Masckauchan TN, et al. (2006) Mol Biol Cell 17:5163–5172]. However, the mechanisms by which PCP signaling components regulate angiogenesis remain unknown. We report that expression of a constitutively active C-terminal domain of Dishevelled-associated activator of morphogenesis 1 (DAAM1) selectively inhibited endothelial cell proliferation. Moreover, this activated construct suppressed endothelial cell migration and the ability to form coordinated networks in vivo and in vitro. Although constitutively active DAAM1 (CDAAM1) induced both actin polymerization and microtubule (MT) stabilization, the stabilization of MTs alone was sufficient to inhibit endothelial cell growth selectively. Inhibition of actin polymerization alone by jasplakinolide treatment failed to reproduce the inhibitory effects of CDAAM1. These results indicate that DAAM1 regulates endothelial cell growth through MT stabilization in a cell type-selective manner and suggest that PCP signaling plays a pivotal role in angiogenesis by regulating MT stabilization.     

S6K1 and mTOR regulate Rac1-driven platelet activation and aggregation

Platelet activation and thrombus formation are under the control of signaling systems that integrate cellular homeostasis with cytoskeletal dynamics. Here, we identify a role for the ribosome protein S6 kinase (S6K1) and its upstream regulator mTOR in the control of platelet activation and aggregate formation under shear flow. Platelet engagement of fibrinogen initiated a signaling cascade that triggered the activation of S6K1 and Rac1. Fibrinogen-induced S6K1 activation was abolished by inhibitors of Src kinases, but not Rac1 inhibitors, demonstrating that S6K1 acts upstream of Rac1. S6K1 and Rac1 interacted in a protein complex with the Rac1 GEF TIAM1 and colocalized with actin at the platelet lamellipodial edge, suggesting that S6K1 and Rac1 work together to drive platelet spreading. Pharmacologic inhibitors of mTOR and S6K1 blocked Rac1 activation and prevented platelet spreading on fibrinogen, but had no effect on Src or FAK kinase activation. mTOR inhibitors dramatically reduced collagen-induced platelet aggregation and promoted the destabilization of platelet aggregates formed under shear flow conditions. Together, these results reveal novel roles for S6K1 and mTOR in the regulation of Rac1 activity and provide insights into the relationship between the pharmacology of the mTOR system and the molecular mechanisms of platelet activation.     

Activation of sphingosine kinase-1 mediates induction of endothelial cell proliferation and angiogenesis by epoxyeicosatrienoic acids

AIMS: Recent evidence suggests that the epoxyeicosatrienoic acids (EETs), which are products of cytochrome P450 (CYP) epoxygenases, possess mitogenic and angiogenic effects in vascular endothelial cells. However, the mechanisms underlying these effects are not fully elucidated. Because sphingosine kinase (SK) and its product S1P play essential roles in cell growth, survival and migration, we hypothesized that SK activation by EETs may mediate some of its angiogenic effects. METHODS AND RESULTS: We studied the effects of EETs on SK activity in human umbilical vein endothelial cells (HUVECs). Treatment with EETs, particularly 11,12-EET, markedly augmented SK activity in HUVECs. At the concentration of 1 micromol/L, 11,12-EET increased SK activity by 110% and the maximal effect on SK activation was observed at 20 min after 11,12-EET addition. Furthermore, inhibition of SK by a specific inhibitor, SKI-II, markedly attenuated 11,12-EET-induced EC proliferation. Importantly, 11,12-EET-induced activation of Akt kinase and transactivation of the epidermal growth factor (EGF) receptor was also inhibited by SKI-II. To investigate the isoform-specific role of SK in EET-induced angiogenesis, inhibition of SK1 by expression of dominant-negative SK1(G82D) substantially attenuated 11,12-EET-induced EC proliferation, migration, and tube formation in vitro and Matrigel plug angiogenesis in vivo. Furthermore, knockdown of SK1 expression by specific siRNA also inhibited 11,12-EET-induced EC proliferation and migration, whereas SK2 siRNA knockdown was without effect. CONCLUSION: These results suggest that SK1 is an important mediator of the 11,12-EET-induced angiogenic effects in human ECs. Thus, SK1 may represent a novel therapeutic modality for the treatment of angiogenesis-related diseases such as cancer and ischaemia.     

Domain 5 of high molecular weight kininogen (kininostatin) down-regulates endothelial cell proliferation and migration and inhibits angiogenesis

We have demonstrated that high molecular weight kininogen (HK) binds specifically on endothelial cells to domain 2/3 of the urokinase receptor (uPAR). Inhibition by vitronectin suggests that kallikrein-cleaved HK (HKa) is antiadhesive. Plasma kallikrein bound to HK cleaves prourokinase to urokinase, initiating cell-associated fibrinolysis. We postulated that HK cell binding domains would inhibit angiogenesis. We found that recombinant domain 5 (D5) inhibited endothelial cell migration toward vitronectin 85% at 0.27 microM with an IC(50) (concentration to yield 50% inhibition) = 0.12 microM. A D5 peptide, G486-K502, showed an IC(50) = 0.2 microM, but a 25-mer peptide from a D3 cell binding domain only inhibited migration 10% at 139 microM (IC(50) > 50 microM). D6 exhibited weaker inhibitory activity (IC(50) = 0.50 microM). D5 also potently inhibited endothelial cell proliferation with an IC(50) = 30 nM, while D3 and D6 were inactive. Using deletion mutants of D5, we localized the smallest region for full activity to H441-D474. To further map the active region, we created a molecular homology model of D5 and designed a series of peptides displaying surface loops. Peptide 440-455 was the most potent (IC(50) = 100 nM) in inhibiting proliferation but did not inhibit migration. D5 inhibited angiogenesis stimulated by fibroblast growth factor FGF2 (97%) in a chicken chorioallantoic membrane assay at 270 nM, and peptide 400-455 was also inhibitory (79%). HK D5 (for which we suggest the designation, "kininostatin") is a potent inhibitor of endothelial cell migration and proliferation in vitro and of angiogenesis in vivo. (Blood. 2000;95:543-550)     

Activated protein C induces endothelial cell proliferation by mitogen-activated protein kinase activation in vitro and angiogenesis in vivo

Activated protein C (APC), a natural anticoagulant, has recently been demonstrated to activate the mitogen-activated protein kinase (MAPK) pathway in endothelial cells in vitro. Because the MAPK pathway is implicated in endothelial cell proliferation, it is possible that APC induces endothelial cell proliferation, thereby causing angiogenesis. We examined this possibility in the present study. APC activated the MAPK pathway, increased DNA synthesis, and induced proliferation in cultured human umbilical vein endothelial cells dependent on its serine protease activity. Antibody against the endothelial protein C receptor (EPCR) inhibited these events. Early activation of the MAPK pathway was inhibited by an antibody against protease-activated receptor-1, whereas neither late and complete activation of the MAPK pathway nor endothelial cell proliferation were inhibited by this antibody. APC activated endothelial nitric oxide synthase (eNOS) via phosphatidylinositol 3-kinase-dependent phosphorylation, followed by activation of protein kinase G, suggesting that APC bound to EPCR might activate the endothelial MAPK pathway by a mechanism similar to that of VEGF. APC induced morphogenetic changes resembling tube-like structures of endothelial cells, whereas DIP-APC did not. When applied topically to the mouse cornea, APC clearly induced angiogenesis in wild-type mice, but not in eNOS knockout mice. These in vitro events induced by APC might at least partly explain the angiogenic activity in vivo. This angiogenic activity of APC might contribute to maintain proper microcirculation in addition to its antithrombotic activity.     

Binding and displacement of vascular endothelial growth factor (VEGF) by thrombospondin: Effect on human microvascular endothelial cell proliferation and angiogenesis

Vascular endothelial growth factor (VEGF) is a specific angiogenic factor, and thrombospondin (TSP), is a potent inhibitor of angiogenesis. To better understand the role of TSP as an anti-angiogenic agent, we have identified its specific domains that participate in its anti-angiogenic activity and examined the mechanism of its inhibitory effect on VEGF₁₆₅ induced angiogenesis. Exogenously added TSP inhibited VEGF₁₆₅ induced angiogenesis (proliferation and tube formation of human dermal microvascular endothelial cells {HDMEC} and neovascular outgrowth from human arterial rings). Although both VEGF₁₆₅ and TSP are heparin binding proteins, TSP had a higher affinity for ¹²⁵I-heparin than VEGF₁₆₅ (K _d1 4 nM and K _d2 14 nM for TSP; K _d 91 nM for VEGF₁₆₅). TSP displaced 36% of ¹²⁵I-VEGF₁₆₅ from HDMEC and this was comparable to the 27% reduction in ¹²⁵I-VEGF₁₆₅ binding to HDMEC upon cleavage of cell surface heparan sulfate (HS). About 35% of the mitogenic activity of VEGF₁₆₅ was attributable to its heparin binding region. These results indicate that a proportion of the mitogenic activity of VEGF₁₆₅ is inhibited by TSP via competition for cell surface HS. Further, ¹²⁵I-VEGF₁₆₅ bound directly to TSP in a saturable, concentration dependent manner, and heparin modulated this binding. The mAbs to the heparin binding domain to the type 1 and type 3 repeats of TSP inhibited the binding of VEGF₁₆₅ to TSP, and also blocked the inhibitory effect of TSP on VEGF₁₆₅ induced HDMEC proliferation. We conclude that (i) the anti-angiogenic activity of TSP is localized in its heparin binding domain and type 1 and type 3 repeats (ii) TSP inhibits angiogenesis by at least two separate mechanisms, (a) displacement of VEGF₁₆₅ from endothelial cell HS and (b) direct binding to VEGF₁₆₅. This revised version was published online in June 2006 with corrections to the Cover Date.     

Continuous endothelial cell activation increases angiogenesis: evidence for the direct role of endothelium linking angiogenesis and inflammation

There is increasing evidence that chronic inflammation is tightly linked to diseases associated with endothelial dysfunction, including the induction of aberrant angiogenesis. While leukocytes have been described as mediators of inflammation-associated angiogenesis, the effects of direct chronic endothelial activation have not been addressed in this context. Using an uncleavable mutant of the transmembrane form of tumor necrosis factor-alpha (TNF-alpha), we have established models of stable TNF-alpha expression in endothelial cells in vitro and in transgenic mice in vivo. In the in vitro model, continuous endothelial activation leads to increased leukocyte cellular adhesion molecule expression and intracellular reactive oxygen species, hallmarks of a proinflammatory and dysfunctional endothelium. In addition, stable expression of TNF-alpha in endothelial cells increased angiogenic sprout formation in the presence but also in the absence of angiogenic growth factors. The partial neutralization of this effect by TNF-alpha antibodies and the inability of conditioned media from stable TNF-alpha-expressing endothelial cells to induce angiogenic activities in control endothelial cells suggest that this effect does not require expression of additional autocrine factors, but is an autonomous effect of the transmembrane TNF on the endothelial cells. Furthermore, using the Matrigel plug assay in vivo, increased angiogenesis was observed in endothelial TNF-alpha-expressing transgenic versus control mice. In conclusion, chronic inflammatory changes mediated by TNF-alpha can induce angiogenesis in vitro and in vivo, suggesting endothelial cell activation as a direct link between inflammation and angiogenesis.     

High-density lipoprotein (HDL) promotes angiogenesis via S1P3-dependent VEGFR2 activation

High-density lipoprotein (HDL) has previously been shown to promote angiogenesis. However, the mechanisms by which HDL enhances the formation of blood vessels remain to be defined. To address this, the effects of HDL on the proliferation, transwell migration and tube formation of human umbilical vein endothelial cells were investigated. By examining the abundance and phosphorylation (i.e., activation) of the vascular endothelial growth factor receptor VEGFR2 and modulating the activity of the sphingosine-1 phosphate receptors S1P1–3 and VEGFR2, we characterized mechanisms controlling angiogenic responses in response to HDL exposure. Here, we report that HDL dose-dependently increased endothelial proliferation, migration and tube formation. These events were in association with increased VEGFR2 abundance and rapid VEGFR2 phosphorylation at Tyr1054/Tyr1059 and Tyr1175 residues in response to HDL. Blockade of VEGFR2 activation by the VEGFR2 inhibitor SU1498 markedly abrogated the pro-angiogenic capacity of HDL. Moreover, the S1P3 inhibitor suramin prevented VEGFR2 expression and abolished endothelial migration and tube formation, while the S1P1 agonist CYM-5442 and the S1P2 inhibitor JTE-013 had no effect. Last, the role of S1P3 was further confirmed in regulation of S1P-induced endothelial proliferation, migration and tube formation via up-regulation and activation of VEGFR2. Together, these findings argue that HDL promotes angiogenesis via S1P3-dependent up-regulation and activation of VEGFR2 and also suggest that the S1P–S1P3–VEGFR2 signaling cascades as a novel target for HDL-modulating therapy implicated in vascular remodeling and functional recovery in atherosclerotic diseases such as myocardial infarction and ischemic stroke.     

Pharmacological inhibition of S6K1 facilitates platelet activation by enhancing Akt phosphorylation

Platelet activation and thrombus formation is a delicate process involving a series of crosstalk between different pathways. P70 ribosomal S6 kinase1 (S6K1) is a member of serine/threonine kinases and can be phosphorylated by 3-phosphoinositide-dependent protein kinase 1 (PDK1). S6K1 is widely reported to play important roles in cancers and metabolic diseases, but the role of S6K1 and the importance of phosphorylation on Thr229 in platelet activation have not been defined. PF-4708671 is a recently synthesized highly specific inhibitor of S6K1. In this study, we tested PF-4708671 to assess the role of S6K1 in platelet. PF-4708671 facilitated mouse and human platelet aggregation and ATP secretion induced by collagen, thrombin, and adenosine diphosphate through enhanced Akt and Gsk3β phosphorylation. PF-4708671 also accelerated integrin αIIbβ3-mediated clot retraction and spreading. Intravenously given PF-4708671 shortened the occlusion time in arterial thrombosis model. Further results demonstrated that S6K1 was phosphorylated by PDK1 on Thr229 in the resting platelets and dephosphorylated in response to agonist stimulation. PDK1-deficient mice showed higher aggregation when PI3K–Akt–Gsk3β signaling was blocked by the Gsk3β-inhibitor SB216763. Thus, S6K1 Thr229 phosphorylation might function as a regulator that prevents platelets from activation. S6K1 inhibition may yield potential pro-thrombotic effects and should be used cautiously when considered as a therapy.