A Novel Mechanism of G Protein-dependent Phosphorylation of Vasodilator-stimulated Phosphoprotein

Endothelial Natriuretic Peptide Receptor 1 Play Crucial Role for Acute and Chronic Blood Pressure Regulation by Atrial Natriuretic Peptide

BACKGROUND

ANP (atrial natriuretic peptide), acting through NPR1 (natriuretic peptide receptor 1), provokes hypotension. Such hypotension is thought to be due to ANP inducing vasodilation via NPR1 in the vasculature; however, the underlying mechanism remains unclear. Here, we investigated the mechanisms of acute and chronic blood pressure regulation by ANP.

METHODS AND RESULTS

Immunohistochemical analysis of rat tissues revealed that NPR1 was abundantly expressed in endothelial cells and smooth muscle cells of small arteries and arterioles. Intravenous infusion of ANP significantly lowered systolic blood pressure in wild-type mice. ANP also significantly lowered systolic blood pressure in smooth muscle cell-specific Npr1-knockout mice but not in endothelial cell-specific Npr1-knockout mice. Moreover, ANP significantly lowered systolic blood pressure in Nos3-knockout mice. In human umbilical vein endothelial cells, treatment with ANP did not influence nitric oxide production or intracellular Ca²⁺ concentration, but it did hyperpolarize the cells. ANP-induced hyperpolarization of human umbilical vein endothelial cells was inhibited by several potassium channel blockers and was also abolished under knockdown of RGS2 (regulator of G-protein signaling 2), an GTPase activating protein in G-protein α-subunit. ANP increased Rgs2 mRNA expression in human umbilical vein endothelial cells but failed to lower systolic blood pressure in Rgs2-knockout mice. Endothelial cell-specific Npr1-overexpressing mice exhibited lower blood pressure than did wild-type mice independent of RGS2, and showed dilation of arterial vessels on synchrotron radiation microangiography.

CONCLUSIONS

Together, these results indicate that vascular endothelial NPR1 plays a crucial role in ANP-mediated blood pressure regulation, presumably by a mechanism that is RGS2-dependent in the acute phase and RGS2-independent in the chronic phase.

Endothelial PKA activity regulates angiogenesis by limiting autophagy through phosphorylation of ATG16L1

The cAMP-dependent protein kinase A (PKA) regulates various cellular functions in health and disease. In endothelial cells PKA activity promotes vessel maturation and limits tip cell formation. Here, we used a chemical genetic screen to identify endothelial-specific direct substrates of PKA in human umbilical vein endothelial cells (HUVEC) that may mediate these effects. Amongst several candidates, we identified ATG16L1, a regulator of autophagy, as novel target of PKA. Biochemical validation, mass spectrometry and peptide spot arrays revealed that PKA phosphorylates ATG16L1α at Ser268 and ATG16L1β at Ser269, driving phosphorylation-dependent degradation of ATG16L1 protein. Reducing PKA activity increased ATG16L1 protein levels and endothelial autophagy. Mouse in vivo genetics and pharmacological experiments demonstrated that autophagy inhibition partially rescues vascular hypersprouting caused by PKA deficiency. Together these results indicate that endothelial PKA activity mediates a critical switch from active sprouting to quiescence in part through phosphorylation of ATG16L1, which in turn reduces endothelial autophagy.

Small GTPase Rap1A/B Is Required for Lymphatic Development and Adrenomedullin-Induced Stabilization of Lymphatic Endothelial Junctions

Objective- Maintenance of lymphatic permeability is essential for normal lymphatic function during adulthood, but the precise signaling pathways that control lymphatic junctions during development are not fully elucidated. The G_s-coupled AM (adrenomedullin) signaling pathway is required for embryonic lymphangiogenesis and the maintenance of lymphatic junctions during adulthood. Thus, we sought to elucidate the downstream effectors mediating junctional stabilization in lymphatic endothelial cells. Approach and Results- We knocked-down both Rap1A and Rap1B isoforms in human neonatal dermal lymphatic cells (human lymphatic endothelial cells) and genetically deleted the mRap1 gene in lymphatic endothelial cells by producing 2 independent, conditional Rap1a/b knockout mouse lines. Rap1A/B knockdown caused disrupted junctional formation with hyperpermeability and impaired AM-induced lymphatic junctional tightening, as well as rescue of histamine-induced junctional disruption. Less than 60% of lymphatic- Rap1a/b knockout embryos survived to E13.5 exhibiting interstitial edema, blood-filled lymphatics, disrupted lymphovenous valves, and defective lymphangiogenesis. Consistently, inducible lymphatic- Rap1a/b deletion in adult animals prevented AM-rescue of histamine-induced lymphatic leakage and dilation. Conclusions- Rap1 (Ras-related protein) serves as the dominant effector downstream of AM to stabilize lymphatic junctions. Rap1 is required for maintaining lymphatic permeability and driving normal lymphatic development.

Vasodilator-Stimulated Phosphoprotein Activity Is Required for Coxiella burnetii Growth in Human Macrophages

Coxiella burnetii is an intracellular bacterial pathogen that causes human Q fever, an acute flu-like illness that can progress to chronic endocarditis and liver and bone infections. Humans are typically infected by aerosol-mediated transmission, and C. burnetii initially targets alveolar macrophages wherein the pathogen replicates in a phagolysosome-like niche known as the parasitophorous vacuole (PV). C. burnetii manipulates host cAMP-dependent protein kinase (PKA) signaling to promote PV formation, cell survival, and bacterial replication. In this study, we identified the actin regulatory protein vasodilator-stimulated phosphoprotein (VASP) as a PKA substrate that is increasingly phosphorylated at S157 and S239 during C. burnetii infection. Avirulent and virulent C. burnetii triggered increased levels of phosphorylated VASP in macrophage-like THP-1 cells and primary human alveolar macrophages, and this event required the Cα subunit of PKA. VASP phosphorylation also required bacterial protein synthesis and secretion of effector proteins via a type IV secretion system, indicating the pathogen actively triggers prolonged VASP phosphorylation. Optimal PV formation and intracellular bacterial replication required VASP activity, as siRNA-mediated depletion of VASP reduced PV size and bacterial growth. Interestingly, ectopic expression of a phospho-mimetic VASP (S239E) mutant protein prevented optimal PV formation, whereas VASP (S157E) mutant expression had no effect. VASP (S239E) expression also prevented trafficking of bead-containing phagosomes to the PV, indicating proper VASP activity is critical for heterotypic fusion events that control PV expansion in macrophages. Finally, expression of dominant negative VASP (S157A) in C. burnetii-infected cells impaired PV formation, confirming importance of the protein for proper infection. This study provides the first evidence of VASP manipulation by an intravacuolar bacterial pathogen via activation of PKA in human macrophages.

Q fever, caused by the intracellular bacterial pathogen Coxiella burnetii, is an aerosol-transmitted infection that can develop into life-threatening chronic infections such as endocarditis. The pathogen preferentially grows within alveolar macrophages in a phagolysosome-like compartment termed the parasitophorous vacuole (PV). C. burnetii actively manipulates host cAMP-dependent protein kinase (PKA) signaling to promote PV formation and cell survival. Identification of bacterial effector proteins that manipulate PKA and downstream target proteins is critical to fully understand pathogen-mediated signaling circuits and develop new therapeutic strategies. Here, we found that PKA controls vasodilator-stimulated phosphoprotein (VASP) activity to promote PV formation and bacterial replication. VASP regulates actin-based motility used by a subset of intracellular bacteria for propulsion through the host cell cytosol and into bystander cells. However, C. burnetii does not use actin-based motility and replicates throughout its life cycle within a membrane bound vacuole. Thus, this study provides the first evidence of VASP manipulation by an intravacuolar bacterial pathogen. Characterization of VASP function in PV formation and identification of additional PKA substrates that promote infection will provide new insight into host-pathogen interactions during Q fever.

Rho-kinase activation contributes to Lps-induced impairment of endothelial nitric oxide synthase activation by endothelin-1 in cultured hepatic sinusoidal endothelial cells

The purpose of this study is to understand the role of rho-kinase (ROCK-2) in the regulation of liver microcirculation after inflammatory stress. Endothelin-1 (ET-1)-induced nitric oxide (NO) is essential in the regulation of blood flow in hepatic sinusoids. Lipopolysaccharide (LPS) inhibits this ET-1-induced NO production and disrupts liver microcirculation; however, the exact molecular mechanism is unknown. Liver sinusoidal endothelial cells were isolated, pretreated with 10 ng/mL LPS for 6 h, and treated with 10 μM Y27632 (ROCK-2 inhibitor) for 30 min and 10 nM ET-1 for 30 min. Lipopolysaccharide induced RhoA membrane translocation that was attenuated by methyl-β-cyclodextrin (cholesterol sequester) or targeted mutation of caveolin-1. Lipopolysaccharide increased ROCK-2 expressions (+60%) and ROCK-2 activity (+36%). Endothelin-1 increased endothelial NO synthase (eNOS) activity (+70%), but LPS inhibited this ET-1-mediated eNOS response. Treatment with Y27632 restored ET-1-mediated eNOS activity (+61%) and stimulated NO production in the perinuclear region after LPS pretreatment. This treatment reduced cofilin-Ser3 phosphorylation (-73%), increased vasodilator-stimulated phosphoprotein-Ser239 phosphorylation (+88%), and stimulated globular actin/eNOS association. Lipopolysaccharide induces Rho/ROCKs signaling pathway to disrupt the ET-1-mediated eNOS activation in liver sinusoidal endothelial cells. Rho-kinase ROCK-2 inhibition restores ET-1-mediated NO production after the LPS pretreatment, in part, through an increase in actin depolymerization.

VASP activation via the Gα13/RhoA/PKA pathway mediates cucurbitacin-B-induced actin aggregation and cofilin-actin rod formation

Cucurbitacin B (CuB), a potent antineoplastic agent of cucurbitacin triterpenoids, induces rapid disruption of actin cytoskeleton and aberrant cell cycle inhibiting carcinogenesis. However, the underlying molecular mechanism of such anticancer effects remains incompletely understood. In this study, we showed that CuB treatment rapidly induced vasodilator-stimulated phosphoprotein (VASP) phosphorylation (i.e. activation) at the Ser157 residue and generated VASP clumps which were co-localized with amorphous actin aggregates prior to the formation of highly-ordered cofilin-actin rods in melanoma cells. Knockdown of VASP or inhibition of VASP activation using PKA-specific inhibitor H89 suppressed CuB-induced VASP activation, actin aggregation and cofilin-actin rod formation. The VASP activation was mediated by cAMP-independent PKA activation as CuB decreased the levels of cAMP while MDL12330A, an inhibitor of adenylyl cyclase, had weak effect on VASP activation. Knockdown of either Gα13 or RhoA not only suppressed VASP activation, but also ameliorated CuB-induced actin aggregation and abrogated cofilin-actin rod formation. Collectively, our studies highlighted that the CuB-induced actin aggregation and cofilin-actin rod formation was mediated via the Gα13/RhoA/PKA/VASP pathway.

Distinctive G Protein-Dependent Signaling by Protease-Activated Receptor 2 (PAR2) in Smooth Muscle: Feedback Inhibition of RhoA by cAMP-Independent PKA

We examined expression of protease-activated receptors 2 (PAR2) and characterized their signaling pathways in rabbit gastric muscle cells. The PAR2 activating peptide SLIGRL (PAR2-AP) stimulated G_q, G₁₃, G_i1, PI hydrolysis, and Rho kinase activity, and inhibited cAMP formation. Stimulation of PI hydrolysis was partly inhibited in cells expressing PAR2 siRNA, Ga_q or Ga_i minigene and in cells treated with pertussis toxin, and augmented by expression of dominant negative regulator of G protein signaling (RGS4(N88S)). Stimulation of Rho kinase activity was abolished by PAR-2 or Ga₁₃ siRNA, and by Ga₁₃ minigene. PAR2-AP induced a biphasic contraction; initial contraction was selectively blocked by the inhibitor of PI hydrolysis (U73122) or MLC kinase (ML-9), whereas sustained contraction was selectively blocked by the Rho kinase inhibitor (Y27632). PAR2-AP induced phosphorylation of MLC₂₀, MYPT1 but not CPI-17. PAR2-AP also caused a decrease in the association of NF-kB and PKA catalytic subunit: the effect of PAR2-AP was blocked by PAR2 siRNA or phosphorylation-deficient RhoA (RhoA(S188A)). PAR2-AP-induced degradation of IkBa and activation of NF-kB were abolished by the blockade of RhoA activity by Clostridium botulinum C3 exoenzyme suggesting RhoA-dependent activation of NF-kB. PAR2-AP-stimulated Rho kinase activity was significantly augmented by the inhibitors of PKA (myristoylated PKI), IKK2 (IKKIV) or NF-kB (MG132), and in cells expressing dominant negative mutants of IKK (IKK(K44A), IkBa (IkBa (S32A/S36A)) or RhoA(S188A), suggesting feedback inhibition of Rho kinase activity via PKA derived from NF-kB pathway. PAR2-AP induced phosphorylation of RhoA and the phosphorylation was attenuated in cells expressing phosphorylation-deficient RhoA(S188A). Our results identified signaling pathways activated by PAR2 to mediate smooth muscle contraction and a novel pathway for feedback inhibition of PAR2-stimulated RhoA. The pathway involves activation of the NF-kB to release catalytic subunit of PKA from its binding to IkBa and phosphorylation of RhoA at Ser¹⁸⁸.

G13 controls angiogenesis through regulation of VEGFR-2 expression

At sites of angiogenesis, the expression of the key angiogenesis regulator vascular endothelial growth factor (VEGF) and its main receptor, VEGF receptor 2 (VEGFR-2), are strongly upregulated. Whereas the processes controlling VEGF expression are well described, the mechanisms underlying VEGFR-2 upregulation have remained unclear. We found that endothelial VEGFR-2 expression is strongly reduced in the absence of the G protein G13, resulting in an impaired responsiveness to VEGF-A, a phenotype that can be rescued by normalization of VEGFR-2 levels. G13-mediated VEGFR-2 expression involved activation of the small GTPase RhoA and transcription factor NF-κB, the latter acting via a specific binding site at position -84 of the VEGFR-2 promoter. Mice with endothelial cell-specific loss of G13 showed reduced VEGFR-2 expression at sites of angiogenesis and attenuated VEGF effects, resulting in impaired retinal angiogenesis and tumor vascularization. Taken together, we identified G-protein-mediated signaling via G13 as a critical regulator of VEGFR-2 expression during angiogenesis.

A novel regulator of angiogenesis in endothelial cells: 5-hydroxytriptamine 4 receptor

The 5-hydroxytryptamine type 4 receptor (5-HT(4)R) regulates many physiological processes, including learning and memory, cognition, and gastrointestinal motility. Little is known about its role in angiogenesis. Using mouse hindlimb ischemia model of angiogenesis, we observed a significant reduction of limb blood flow recovery 14 days after ischemia and a decrease in density of CD31-positive vessels in adductor muscles in 5-HT(4)R(-/-) mice compared to wild type littermates. Our in vitro data indicated that 5-HT(4)R endogenously expressed in endothelial cells (ECs) may promote angiogenesis. Inhibition of the receptor with 5-HT(4)R antagonist RS 39604 reduced EC capillary tube formation in the reconstituted basement membrane. Using Boyden chamber migration assay and wound healing "scratch" assay, we demonstrated that RS 39604 treatment significantly suppressed EC migration. Transendothelial resistance measurement and immunofluorescence analysis showed that a 5-HT(4)R agonist RS 67333 led to an increase in endothelial permeability, actin stress fiber and interendothelial gap formation. Importantly, we provided the evidence that 5-HT(4)R-regulated EC migration may be mediated by Gα13 and RhoA. Our results suggest a prominent role of 5-HT(4)R in promoting angiogenesis and identify 5-HT(4)R as a potential therapeutic target for modulating angiogenesis under pathological conditions.

A novel role of sesamol in inhibiting NF-κB-mediated signaling in platelet activation

Background

Platelet activation is relevant to a variety of coronary heart diseases. Our previous studies revealed that sesamol possesses potent antiplatelet activity through increasing cyclic AMP formation. Although platelets are anucleated cells, they also express the transcription factor, NF-κB, that may exert non-genomic functions in platelet activation. Therefore, we further investigated the inhibitory roles of sesamol in NF-κB-mediated platelet function.

Methods

Platelet aggregation, Fura 2-AM fluorescence, and immunoblotting analysis were used in this study.

Results

NF-κB signaling events, including IKKβ phosphorylation, IκBα degradation, and p65 phosphorylation, were markedly activated by collagen (1 μg/ml) in washed human platelets, and these signaling events were attenuated by sesamol (2.5~25 μM). Furthermore, SQ22536 and ODQ, inhibitors of adenylate cyclase and guanylate cyclase, respectively, strongly reversed the sesamol (25 μM)-mediated inhibitory effects of IKKβ phosphorylation, IκBα degradation, and p65 phosphorylation stimulated by collagen. The protein kinase A (PKA) inhibitor, H89, also reversed sesamol-mediated inhibition of IκBα degradation. Moreover, BAY11-7082, an NF-κB inhibitor, abolished IκBα degradation, phospholipase C (PLC)γ2 phosphorylation, protein kinase C (PKC) activation, [Ca²⁺]i mobilization, and platelet aggregation stimulated by collagen. Preincubation of platelets with the inhibitors, SQ22536 and H89, both strongly reversed sesamol-mediated inhibition of platelet aggregation and [Ca²⁺]i mobilization.

Conclusions

Sesamol activates cAMP-PKA signaling, followed by inhibition of the NF-κB-PLC-PKC cascade, thereby leading to inhibition of [Ca²⁺]i mobilization and platelet aggregation. Because platelet activation is not only linked to hemostasis, but also has a relevant role in inflammation and metastasis, our data demonstrating that inhibition of NF-κB interferes with platelet function may have a great impact when these types of drugs are considered for the treatment of cancer and various inflammatory diseases.

Cyclic AMP: a selective modulator of NF-κB action

It has been known for several decades that cyclic AMP (cAMP), a prototypical second messenger, transducing the action of a variety of G-protein-coupled receptor ligands, has potent immunosuppressive and anti-inflammatory actions. These actions have been attributed in part to the ability of cAMP-induced signals to interfere with the function of the proinflammatory transcription factor Nuclear Factor-kappaB (NF-κB). NF-κB plays a crucial role in switching on the gene expression of a plethora of inflammatory and immune mediators, and as such is one of the master regulators of the immune response and a key target for anti-inflammatory drug design. A number of fundamental molecular mechanisms, contributing to the overall inhibitory actions of cAMP on NF-κB function, are well established. Paradoxically, recent reports indicate that cAMP, via its main effector, the protein kinase A (PKA), also promotes NF-κB activity. Indeed, cAMP actions appear to be highly cell type- and context-dependent. Importantly, several novel players in the cAMP/NF-κB connection, which selectively direct cAMP action, have been recently identified. These findings not only open up exciting new research avenues but also reveal novel opportunities for the design of more selective, NF-κB-targeting, anti-inflammatory drugs.

The soluble guanylate cyclase stimulator BAY 41-2272 inhibits vascular smooth muscle growth through the cAMP-dependent protein kinase and cGMP-dependent protein kinase pathways

Vascular smooth muscle (VSM) proliferation and migration are key components in vessel remodeling. Cyclic nucleotide signaling is protective and has long-served as a therapeutic target against undesired VSM growth. The present work analyzed the effects of the soluble guanylate cyclase (sGC) stimulator 3-(4-amino-5-cyclopropylpyrimidine-2-yl)-1-(2-fluorobenzyl)-1H-pyrazolo[3,4-b]pyridine [BAY 41-2272 (BAY)] on VSM growth, and we hypothesize that BAY has the capacity to reduce proliferation and migration via cyclic nucleotide-driven kinase signaling. Perivascular BAY postballoon injury reduced neointimal growth by ∼ 40% compared with vehicle controls after 2 weeks. In VSM cells, BAY (10 μM) reduced proliferation by ∼ 40% after 72 h and migration by ∼ 40% after 6 h and ∼ 60% after 18 h without deleterious effects on cell viability. cGMP content peaked (248 ×) 20 min after BAY treatment and remained elevated (140 ×) through 60 min; however, BAY did not affect cAMP levels compared with controls. Conventional and In-Cell Western analyses showed increases in vasodilator-stimulated phosphoprotein (VASP) phosphorylation (pVASP) at serines 239 (3 ×) and 157 (2 ×), respective markers of cGMP- and cAMP-directed protein kinases (PKG and PKA, respectively). The PKG inhibitor YGRKKRRQRRRPPLRKKKKKH peptide (DT-2) completely reversed BAY-mediated increases in pVASPSer(239) and BAY-mediated inhibition of migration. In comparison, the PKA inhibitor peptide PKI further potentiated BAY-stimulated pVASPSer(157) and pVASPSer(239) and partially reversed the antiproliferative effects of BAY. This is the first report demonstrating the effectiveness of BAY in reducing neointimal growth with direct evidence for PKG-specific antimigratory and PKA-specific antiproliferative mechanisms. Conclusively, the sGC stimulator BAY reduces VSM growth through cGMP-dependent PKG and PKA processes, providing support for continued evaluation of its clinical utility.

Involvement of vasodilator-stimulated phosphoprotein in UDP-induced microglial actin aggregation via PKC- and Rho-dependent pathways

Microglia are major immunocompetent cells in the central nervous system and retain highly dynamic motility. The processes which allow these cells to move, such as chemotaxis and phagocytosis, are considered part of their functions and are closely related to purinergic signaling. Previously, we reported that the activation of the P2Y(6) receptor by UDP stimulation in microglia evoked dynamic cell motility which enhanced their phagocytic capacity, as reported by Koizumi et al. (Nature 446(7139):1091-1095, 2007). These responses require actin cytoskeletal rearrangement, which is seen after UDP stimulation. However, the intracellular signaling pathway has not been defined. In this study, we found that UDP in rat primary microglia rapidly induced the transient phosphorylation at Ser157 of vasodilator-stimulated phosphoprotein (VASP). VASP, one of actin binding protein, accumulated at the plasma membrane where filamentous (F)-actin aggregated in a time-dependent manner. The phosphorylation of VASP was suppressed by inhibition of PKC. UDP-induced local actin aggregations were also abrogated by PKC inhibitors. The Rho inhibitor CT04 and the expression of p115-RGS, which suppresses G(12/13) signaling, attenuated UDP-induced phosphorylation of VASP and actin aggregation. These results indicate that PKC- and Rho-dependent phosphorylation of VASP is involved in UDP-induced actin aggregation of microglia.

Vasodilator-stimulated phosphoprotein deficiency potentiates PAR-1-induced increase in endothelial permeability in mouse lungs

Vasodilator-stimulated phosphoprotein (VASP) is implicated in the protection of the endothelial barrier in vitro and in vivo. The function of VASP in thrombin signaling in the endothelial cells (ECs) is not known. For the first time we studied the effects of VASP deficiency on EC permeability and pulmonary vascular permeability in response to thrombin receptor stimulation. We provided the evidence that VASP deficiency potentiates the increase in endothelial permeability induced by activation of thrombin receptor in cultured human umbilical vein endothelial cells (HUVECs) and isolated mouse lungs. Using transendothelial resistance measurement, we showed that siRNA-mediated VASP downregulation in HUVECs leads to a potentiation of thrombin- and protease-activated receptor 1 (PAR-1) agonist-induced increase in endothelial permeability. Compared to control cells, VASP-deficient HUVECs had delayed endothelial junctional reassembly and abrogated VE-cadherin cytoskeletal anchoring in the recovery phase after thrombin stimulation, as demonstrated by immunofluorescence studies and cell fractionation analysis, respectively. Measurement of the capillary filtration coefficient in isolated mouse lungs demonstrated that VASP(-/-) mice have increased microvascular permeability in response to infusion with PAR-1 agonist compared to wild type mice. Lack of VASP led to decreased Rac1 activation both in VASP-deficient HUVECs after thrombin stimulation and VASP(-/-) mouse lungs after PAR-1 agonist infusion, indicating that VASP effects on thrombin signaling may be correlated with changes in Rac1 activity. This study demonstrates that VASP may play critical and complex role in the regulation of thrombin-dependent disruption of the endothelial barrier function.

Functional roles of VASP phosphorylation in the regulation of chemotaxis and osmotic stress response

Vasodilator-stimulated phosphoprotein (VASP) plays crucial roles in controlling F-actin-driven processes and growing evidence indicates that VASP function is modulated by phosphorylation at multiple sites. However, the complexity of mammalian system prevents the clear understanding of the role of VASP phosphorylation. In this study, we took advantage of Dictyostelium which possesses only one member of the Ena/VASP family to investigate the functional roles of VASP phosphorylation. Our results demonstrated that hyperosmotic stress and cAMP stimulation cause VASP phosphorylation. VASP phosphorylation plays a negative role for the early steps of filopodia/microspikes formation. VASP phosphorylation appears to modulate VASP localization at the membrane cortex and its interactions with WASP and WIPa. Analysis of chemotaxis of cells expressing VASP mutants showed that VASP phosphorylation is required for the establishment of cell polarity under a cAMP gradient.

Nuclear emancipation: a platelet tour de force

Mammalian platelets are anucleate cells produced by the polyploid megakaryocyte. Platelets are more than just key players in hemostasis (blood clotting in response to injury); they also have important roles in inflammation, immunity, tumor progression, and thrombosis. Complex systems of homeostasis have been described for platelets, including posttranscriptional and translational mechanisms to regulate platelet function. Platelets contain transcription factors, and these proteins have essential roles in regulating nongenomic processes. A study provides evidence for a previously unknown negative feedback pathway for limiting platelet activation that occurs through the nuclear factor κB transcription factor family. This pathway is mediated by an adenosine 3',5'-monophosphate-independent protein kinase A activity in response to platelet stimulation. Our appreciation of the role of transcription factors in mammalian platelet biology is nascent but holds great promise for both understanding platelet function and translation into clinical uses.

Proteomic profiling of lipopolysaccharide-activated macrophages by isotope coded affinity tagging

Lipopolysaccharide (LPS), a glycolipid component of the outer membranes of Gram-negative bacteria, initiates proinflammatory, proapoptotic, and antiapoptotic pathways upon binding to macrophage TLR4. Macrophages that are exposed to LPS become activated and exhibit altered morphology and response to infection. We performed isotope coded affinity tagging (ICAT), multidimensional liquid chromatography, and mass spectrometry to identify proteins that are differently expressed between naive and LPS-activated macrophages. We performed replicate ICAT analyses on RAW 264.7 cultured mouse macrophages as well as C57BL/6 bone marrow derived mouse macrophages. We identified and obtained relative abundances for 1064 proteins, of which we identified 36 as having significantly different expression levels upon activation by LPS. We also compared our results with a two color microarray gene expression assay performed by the Institute for Systems Biology and observed approximately 75% agreement between mRNA transcription and protein expression regarding up- or down-regulation of gene products. We used Western blot analysis to confirm the findings of ICAT and mRNA for one protein, sequestosome 1, the cellular concentration of which was observed to increase upon activation by LPS.

Thrombin and collagen induce a feedback inhibitory signaling pathway in platelets involving dissociation of the catalytic subunit of protein kinase A from an NFkappaB-IkappaB complex

Protein kinase A (PKA) activation by cAMP phosphorylates multiple target proteins in numerous platelet inhibitory pathways that have a very important role in maintaining circulating platelets in a resting state. Here we show that in thrombin- and collagen-stimulated platelets, PKA is activated by cAMP-independent mechanisms involving dissociation of the catalytic subunit of PKA (PKAc) from an NFkappaB-IkappaBalpha-PKAc complex. We demonstrate mRNA and protein expression for most of the NFkappaB family members in platelets. From resting platelets, PKAc was co-immunoprecipitated with IkappaBalpha, and conversely, IkappaBalpha was also co-immunoprecipitated with PKAc. This interaction was significantly reduced in thrombin- and collagen-stimulated platelets. Stimulation of platelets with thrombin- or collagen-activated IKK, at least partly by PI3 kinase-dependent pathways, leading to phosphorylation of IkappaBalpha, disruption of an IkappaBalpha-PKAc complex, and release of free, active PKAc, which phosphorylated VASP and other PKA substrates. IKK inhibitor inhibited thrombin-stimulated IkBalpha phosphorylation, PKA-IkBalpha dissociation, and VASP phosphorylation, and potentiated integrin alphaIIbbeta3 activation and the early phase of platelet aggregation. We conclude that thrombin and collagen not only cause platelet activation but also appear to fine-tune this response by initiating downstream NFkappaB-dependent PKAc activation, as a novel feedback inhibitory signaling mechanism for preventing undesired platelet activation.

Enhanced antiproliferative and proapoptotic effects on prostate cancer cells by simultaneously inhibiting androgen receptor and cAMP-dependent protein kinase A

The androgen-signaling pathway with the androgen receptor (AR) as its key molecule is widely understood to influence prostate tumor growth significantly even after androgen ablation. Under androgen-deprived conditions, the AR may be activated inappropriately through interaction with other molecules, including cyclic AMP-dependent protein kinase A (PKA). In a previous study, we have shown that knocking down the AR significantly inhibits prostate tumor growth. In this study, we show that combined inhibition of the AR and the regulatory subunit I alpha of PKA (RIalpha) with small interference RNAs significantly increased the growth-inhibitory and proapoptotic effects of AR knockdown. This treatment strategy was effective in androgen-sensitive and in androgen ablation-resistant prostate cancer cells. In addition, we report that downregulating PKA RIalpha was sufficient to inhibit PKA signaling and interestingly also impaired AR expression and activation. Vice versa, AR knockdown induced a decline in PKA RIalpha, associated with reduced PKA activity. This mutual influence on expression level was specific, because siRNAs against the AR did not affect expression of PKA RIalpha in AR negative DU-145 cells and a siRNA control did not affect protein expression. Another important finding of our study was that depletion of PKA RIalpha also potentiated the antiproliferative effect of the antiandrogen bicalutamide in androgen-sensitive LNCaP. We therefore concluded that combined inhibition of PKA RIalpha and AR may be a promising new therapeutic option for prostate cancer patients and might be superior to solely preventing AR expression.

Differential VASP phosphorylation controls remodeling of the actin cytoskeleton

Proteins of the Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) family link signal transduction pathways to actin cytoskeleton dynamics. VASP is substrate of cAMP-dependent, cGMP-dependent and AMP-activated protein kinases that primarily phosphorylate the sites S157, S239 and T278, respectively. Here, we systematically analyzed functions of VASP phosphorylation patterns for actin assembly and subcellular targeting in vivo and compared the phosphorylation effects of Ena/VASP family members. Methods used were the reconstitution of VASP-null cells with ;locked' phosphomimetic VASP mutants, actin polymerization of VASP mutants in vitro and in living cells, site-specific kinase-mediated VASP phosphorylation, and analysis of the endogenous protein with phosphorylation-status-specific antibodies. Phosphorylation at S157 influenced VASP localization, but had a minor impact on F-actin assembly. Phosphorylation of the S157-equivalent site in the Ena/VASP family members Mena and EVL had no effect on the ratio of cellular F-actin to G-actin. By contrast, VASP phosphorylation at S239 (and the equivalent site in Mena) or T278 impaired VASP-driven actin filament formation. The data show that VASP functions are precisely regulated by differential phosphorylation and provide new insights into cytoskeletal control by serine/threonine kinase-dependent signaling pathways.

Lysophosphatidic acid (LPA)-induced vasodilator-stimulated phosphoprotein mediates lamellipodia formation to initiate motility in PC-3 prostate cancer cells

Prostate cancer remains the most frequently diagnosed malignancy and the second leading cause of cancer mortality among men in the United States. Hormone refractory, metastatic disease has no molecular therapeutics to date and survival is poor. Lysophosphatidic acid (LPA) is a bioactive lipid exhibiting motility, invasive, growth, proliferative and survival effects in multiple cancer cell lineages. Cells express different combinations of LPA-specific G protein-coupled receptors, LPA(1), LPA(2) LPA(3), and LPA(4) as well as other LPA receptors, which bind LPA and thereby regulate lipid signaling. The role of specific LPA receptors in functional outcomes of lysolipid signaling remains to be fully elucidated in prostate cancer. We hypothesized that LPA can initiate cell migration through specific LPA receptors by activating actin-associating proteins involved in motility, including the vasodilator-stimulated phosphoprotein (VASP). In the present study, we demonstrate that LPA-induced lamellipodia formation in cells is dependent on LPA receptor-mediated phosphorylation of VASP, demonstrating a previously unknown regulation by LPA. LPA induces phosphorylation of VASP at Ser(157), through protein kinase A (PKA) since the stimulation was abrogated by PKA inhibition. In addition, we found the effects of LPA-induced lamellipodia formation and migration were reduced by knockdown of either VASP or LPA receptor expression, suggesting that LPA receptor-induced VASP phosphorylation is a critical mediator of migration initiation. Thus the LPA(2) and LPA(3) receptors, in addition to the previously implicated LPA(1) receptor, play a role in cellular motility potentially contributing to invasion and metastases. Emerging drugs targeting the LPA pathway may be beneficial for the treatment of metastatic progression in prostate cancer.

Galpha13 regulates MEF2-dependent gene transcription in endothelial cells: role in angiogenesis

The alpha subunit of heterotrimeric G13 protein is required for the embryonic angiogenesis (Offermanns et al., Science 275:533-536, 1997). However, the molecular mechanism of Galpha13-dependent angiogenesis is not understood. Here, we show that myocyte-specific enhancer factor-2 (MEF2) mediates Galpha13-dependent angiogenesis. Our data showed that constitutively activated Galpha13Q226L stimulated MEF2-dependent gene transcription. In addition, downregulation of endogenous Galpha13 inhibited thrombin-stimulated MEF2-dependent gene transcription in endothelial cells. Both Ca(2+)/calmodulin-dependent kinase IV (CaMKIV) and histone deacetylase 5 (HDAC5) were involved in Galpha13-mediated MEF2-dependent gene transcription. Galpha13Q226L also increased Ca(2+)/calmodulin-independent CaMKIV activity, while dominant negative mutant of CaMKIV inhibited MEF2-dependent gene transcription induced by Galpha13Q226L. Furthermore, Galpha13Q226L was able to derepress HDAC5-mediated repression of gene transcription and induce the translocation of HDAC5 from nucleus to cytoplasm. Finally, downregulation of endogenous Galpha13 and MEF2 proteins in endothelial cells reduced cell proliferation and capillary tube formation. Decrease of endothelial cell proliferation that was caused by the Galpha13 downregulation was partially restored by the constitutively active MEF2-VP16. Our studies suggest that MEF2 proteins are an important component in Galpha13-mediated angiogenesis.

Calpain inhibition delays neutrophil apoptosis via cyclic AMP-independent activation of protein kinase A and protein kinase A-mediated stabilization of Mcl-1 and X-linked inhibitor of apoptosis (XIAP)

Human neutrophils underwent spontaneous apoptosis, which was accompanied with proteasome-mediated degradation of Mcl-1 and X-linked inhibitor of apoptosis (XIAP). Calpain inhibitors (PD150606 and N-acetyl-Leu-Leu-Nle-CHO) prevented spontaneous neutrophil apoptosis and degradation of Mcl-1 and XIAP, and the effects of calpain inhibitors on neutrophils were resistant to cycloheximide. Calpain inhibitors induced protein kinase A (PKA) activation, which was unaccompanied with an increase in intracellular cyclic AMP. Calpain inhibition-mediated delayed neutrophil apoptosis, stabilization of Mcl-1 and XIAP, and phosphorylation of PKA substrates were suppressed by H-89 (specific PKA inhibitor). These findings suggest that calpain inhibition delays neutrophil apoptosis via cyclic AMP-independent activation of PKA and PKA-mediated stabilization of Mcl-1 and XIAP.

G(q)-dependent signalling by the lysophosphatidic acid receptor LPA(3) in gastric smooth muscle: reciprocal regulation of MYPT1 phosphorylation by Rho kinase and cAMP-independent PKA

The present study characterized the signalling pathways initiated by the bioactive lipid, LPA (lysophosphatidic acid) in smooth muscle. Expression of LPA(3) receptors, but not LPA(1) and LPA(2), receptors was demonstrated by Western blot analysis. LPA stimulated phosphoinositide hydrolysis, PKC (protein kinase C) and Rho kinase (Rho-associated kinase) activities: stimulation of all three enzymes was inhibited by expression of the G(alphaq), but not the G(alphai), minigene. Initial contraction and MLC(20) (20 kDa regulatory light chain of myosin II) phosphorylation induced by LPA were abolished by inhibitors of PLC (phospholipase C)-beta (U73122) or MLCK (myosin light-chain kinase; ML-9), but were not affected by inhibitors of PKC (bisindolylmaleimide) or Rho kinase (Y27632). In contrast, sustained contraction, and phosphorylation of MLC(20) and CPI-17 (PKC-potentiated inhibitor 17 kDa protein) induced by LPA were abolished selectively by bisindolylmaleimide. LPA-induced activation of IKK2 {IkappaB [inhibitor of NF-kappaB (nuclear factor kappaB)] kinase 2} and PKA (protein kinase A; cAMP-dependent protein kinase), and degradation of IkappaBalpha were blocked by the RhoA inhibitor (C3 exoenzyme) and in cells expressing dominant-negative mutants of IKK2(K44A) or RhoA(N19RhoA). Phosphorylation by Rho kinase of MYPT1 (myosin phosphatase targeting subunit 1) at Thr(696) was masked by phosphorylation of MYPT1 at Ser(695) by PKA derived from IkappaB degradation via RhoA, but unmasked in the presence of PKI (PKA inhibitor) or C3 exoenzyme and in cells expressing IKK2(K44A). We conclude that LPA induces initial contraction which involves activation of PLC-beta and MLCK and phosphorylation of MLC(20), and sustained contraction which involves activation of PKC and phosphorylation of CPI-17 and MLC(20). Although Rho kinase was activated, phosphorylation of MYPT1 at Thr(696) by Rho kinase was masked by phosphorylation of MYPT1 at Ser(695) via cAMP-independent PKA derived from the NF-kappaB pathway.

The role of VASP in regulation of cAMP- and Rac 1-mediated endothelial barrier stabilization

Regulation of actin dynamics is critical for endothelial barrier functions. We provide evidence that the actin-binding protein vasodilator-stimulated phosphoprotein (VASP) is required for endothelial barrier maintenance. Baseline permeability was significantly increased in VASP-deficient (VASP(-/-)) microvascular myocardial endothelial cells (MyEnd) in the absence of discernible alterations of immunostaining for adherens and tight junctions. We tested whether VASP is involved in the endothelium-stabilizing effects of cAMP or Rac 1. Forskolin and rolipram (F/R) to increase cAMP and cytotoxic necrotizing factor 1 (CNF-1) to activate Rac 1 were equally efficient to stabilize barrier functions in VASP(-/-) and wild-type (wt) cells. In wt cells, VASP was phosphorylated in response to F/R but did not localize to intercellular junctions. In contrast, CNF-1 and expression of constitutively active Rac 1 induced translocation of VASP to cell borders in wt cells, where it colocalized with active Rac 1. In VASP(-/-) cells, Rac 1 activity was reduced to 0.4 of wt levels in controls and increased approximately 20-fold in response to CNF-1 compared with 7-fold activation in wt cells. Moreover, inactivation of Rac 1 by lethal toxin led to a greater increase of permeability compared with wt cells. All these data suggest that VASP is involved in the regulation of Rac 1 activity. Taking these findings together, our study indicates that VASP at least in part stabilizes endothelial barrier functions by control of Rho-family GTPases.

CTL-associated antigen-4 ligation induces rapid T cell polarization that depends on phosphatidylinositol 3-kinase, Vav-1, Cdc42, and myosin light chain kinase

CTLA-4 can negatively regulate cytokine production and proliferation, increase motility, and override the TCR-induced stop-signal needed for stable T cell-APC conjugation. Despite this, little is known regarding whether CTLA-4 can alter T cell morphology and the nature of the signaling events that could account for this event. In this study, we demonstrate that anti-CTLA-4 and CD3/CTLA-4 induce rapid T cell polarization (i.e., within 15-30 min) with increases in lamellipodia, filopodia, and uropod formation. This was observed with anti-CTLA-4 and CD80-Ig ligation of CTLA-4, but not with anti-CD3 alone, or anti-CD3/CD28 coligation. Polarization required PI3K, the guanine nucleotide exchange factor Vav1, the GTP-binding protein Cdc42, as well as myosin L chain kinase. By contrast, a key downstream target of PI3K, protein kinase B, as well as Rho kinase and RhoA, were not needed. Our results demonstrate that CTLA-4 is a potent activator T cell polarization needed for motility, and this process involves specific set of signaling proteins that might contribute to coreceptor regulation of T cell function.

Biologic functions of the G12 subfamily of heterotrimeric g proteins: growth, migration, and metastasis

The G12 subfamily of heterotrimeric G proteins has been the subject of intense scientific interest for more than 15 years. During this period, studies have revealed more than 20 potential G12-interacting proteins and numerous signaling axes emanating from the G12 proteins, Galpha12 and Galpha13. In addition, more recent studies have begun to illuminate the various and sundry functions that the G12 subfamily plays in biology. In this review, we summarize the diverse range of proteins that have been identified as Galpha12 and/or Galpha13 interactors and describe ongoing studies designed to dissect the biological roles of specific Galpha-effector protein interactions. Further, we describe and discuss the expanding role of G12 proteins in the biology of cells, focusing on the distinct properties of this subfamily in regulating cell proliferation, cell migration, and metastatic invasion.

Role of PKA in the anti-Thy-1 antibody-induced neurite outgrowth of dorsal root ganglionic neurons

Thy-1 is highly expressed in the mammalian nervous system. Our previous study showed that addition of anti-Thy-1 antibody to cultured dorsal root ganglionic (DRG) neurons promotes neurite outgrowth. In this study, we identified a novel signaling pathway mediating this event. Treatment with function-blocking anti-Thy-1 antibodies enhanced neurite outgrowth of DRG neurons in terms of total neurite length, longest neurite length, and total neurite branching points. To elucidate the possible signal transduction pathway involved, activation of kinases was evaluated by Western blotting. Transient phosphorylation of protein kinase A (PKA) and mitogen-activated kinase kinase (MEK) was induced after 15 min of anti-Thy-1 antibody treatment. Pretreatment with a PKA inhibitor (PKI) or an MEK inhibitor, PD98059, significantly decreased the neurite outgrowth response triggered by anti-Thy-1 antibody, indicating the involvement of both kinases. In addition, anti-Thy-1 antibody treatment also induced transient phosphorylation of cyclic AMP-response element-binding protein (CREB) and this effect was also blocked by a PKI or PD98059. Furthermore, the fact that PKI abolished anti-Thy-1 antibody-induced MEK phosphorylation showed that PKA acts upstream of the MEK-CREB cascade. In summary, the PKA-MEK-CREB pathway is a new pathway involved in the neurite outgrowth-promoting effect of anti-Thy-1 antibody.

PKCδ regulates collagen-induced platelet aggregation through inhibition of VASP-mediated filopodia formation

Protein kinase Cδ (PKCδ) has been shown by pharmacologic approaches to negatively regulate collagen-induced platelet aggregation. Here we addressed the molecular and cellular mechanisms underlying this negative regulation. Using PKCδ–/– platelets, we show that the mechanism did not involve altered inside-out signaling to integrin αIIbβ3 and did not affect early signaling events downstream of GPVI, because there was no difference in tyrosine phosphorylation of PLCγ2 between wild-type and PKCδ–/– platelets. There was also no increase in secretion of dense granule content, in contrast to studies using rottlerin where secretion was enhanced. Importantly, however, there was marked enhancement of filopodia generation in PKCδ–/– platelets upon adhesion to collagen compared with wild-type platelets. Filopodia play an essential role regulating adhesive events leading to platelet aggregation by increasing platelet-platelet contact. We show that the critical effector for PKCδ is vasodilator-stimulated phosphoprotein (VASP), a major regulator of actin cytoskeleton dynamics. PKCδ physically interacts with VASP constitutively and regulates its phosphorylation on Ser157. In VASP–/– platelets, the enhancement of filopodia generation, actin polymerization, and platelet aggregation by rottlerin is ablated. PKCδ is therefore a critical negative regulator of filopodia, and hence platelet aggregation, through a functional interaction with the actin organizer VASP.

Role of Scavenger Receptor Class B Type I and Sphingosine 1-Phosphate Receptors in High Density Lipoprotein-induced Inhibition of Adhesion Molecule Expression in Endothelial Cells

We characterized the molecular mechanisms by which high density lipoprotein (HDL) inhibits the expression of adhesion molecules, including vascular cell adhesion molecule-1 and intercellular adhesion molecule-1, induced by sphingosine 1-phosphate (S1P) and tumor necrosis factor (TNF) alpha in endothelial cells. HDL inhibited S1P-induced nuclear factor kappaB activation and adhesion molecule expression in human umbilical vein endothelial cells. The inhibitory HDL actions were associated with nitric-oxide synthase (NOS) activation and were reversed by inhibitors for phosphatidylinositol 3-kinase and NOS. The HDL-induced inhibitory actions were also attenuated by the down-regulation of scavenger receptor class B type I (SR-BI) and its associated protein PDZK1. When TNFalpha was used as a stimulant, the HDL-induced NOS activation and the inhibitory action on adhesion molecule expression were, in part, attenuated by the down-regulation of the expression of S1P receptors, especially S1P(1), in addition to SR-BI. Reconstituted HDL composed mainly of apolipoprotein A-I and phosphatidylcholine mimicked the SR-BI-sensitive part of HDL-induced actions. Down-regulation of S1P(3) receptors severely suppressed the stimulatory actions of S1P. Although G(i/o) proteins may play roles in either stimulatory or inhibitory S1P actions, as judged from pertussis toxin sensitivity, the coupling of S1P(3) receptors to G(12/13) proteins may be critical to distinguish the stimulatory pathways from the inhibitory ones. In conclusion, even though S1P alone stimulates adhesion molecule expression, HDL overcomes S1P(3) receptor-mediated stimulatory actions through SR-BI/PDZK1-mediated signaling pathways involving phosphatidylinositol 3-kinase and NOS. In addition, the S1P component of HDL plays a role in the inhibition of TNFalpha-induced actions through S1P receptors, especially S1P(1).

AMP-activated protein kinase impairs endothelial actin cytoskeleton assembly by phosphorylating vasodilator-stimulated phosphoprotein

Vasodilator-stimulated phosphoprotein (VASP) is an actin regulatory protein that links signaling pathways to remodeling of the cytoskeleton. VASP functions are modulated by protein kinases, which phosphorylate the sites Ser-157, Ser-239, and Thr-278. The kinase responsible for Thr-278 phosphorylation, biological functions of the phosphorylation, and association with disease states have remained enigmatic. Using VASP phosphorylation status-specific antibodies, we identified AMP-activated protein kinase (AMPK), a serine-threonine kinase and fundamental sensor of energy homeostasis, in a screen for kinases that phosphorylate the Thr-278 site of VASP in endothelial cells. Pharmacological AMPK inhibitors and activators and AMPK mutants revealed that the kinase specifically targets residue Thr-278 but not Ser-157 or Ser-239. Quantitative fluorescence-activated cell sorter analysis and serum response factor transcriptional reporter assays, which quantify the cellular F-/G-actin equilibrium, indicated that AMPK-mediated VASP phosphorylation impaired actin stress fiber formation and altered cell morphology. In the Zucker Diabetic Fatty (ZDF) rat model for type II diabetes, AMPK activity and Thr-278 phosphorylation were substantially reduced in arterial vessel walls. These findings suggest that VASP is a new AMPK substrate, that VASP Thr-278 phosphorylation translates metabolic signals into actin cytoskeleton rearrangements, and that this signaling system becomes down-regulated in diabetic vessels.

Recent evidence for an involvement of rho-kinase in cerebral vascular disease

BACKGROUND AND PURPOSE

The small G protein rhoA and its downstream effector rho-kinase are both expressed in vascular cells and are involved in several cellular processes. One of these processes is the regulation of the phosphorylation state of myosin light chain in vascular muscle and thus, the development of force. Recently, considerable evidence for increased activity of this pathway in cerebral and noncerebral vessels has been reported in several cardiovascular diseases associated with increased vascular tone.

SUMMARY OF REVIEW

The main aim of this brief review is to summarize current evidence for the involvement of rhoA/rho-kinase signaling in dysfunction of the cerebral circulation in disease states, such as cerebral vasospasm, hypertension, diabetes, and ischemic brain injury. We will also briefly consider the novel hypothesis that augmented activity of endothelial rho-kinase decreases nitric oxide production and contributes to increased vascular tone in disease and the possibility of this action being a key therapeutic target of statins (inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase) in cerebral and noncerebral arteries.

CONCLUSIONS

Considerable evidence indicates that rhoA/rho-kinase activity is commonly increased in cerebral vascular disease, not only in vascular muscle, but also in the endothelium and possibly in inflammatory cells and neurons.