Protein kinase D1-mediated phosphorylations regulate vasodilator-stimulated phosphoprotein (VASP) localization and cell migration

The role of PKC and PKD in CXCL12 and CXCL13 directed malignant melanoma and acute monocytic leukemic cancer cell migration

Cancer metastasis is the leading cause of cancer related mortality. Chemokine receptors and proteins in their downstream signalling axis represent desirable therapeutic targets for the prevention of metastasis. Despite this, current therapeutics have experienced limited success in clinical trials due to a lack of insight into the downstream signalling pathway of specific chemokine receptor cascades in different tumours. In this study, we investigated the role of protein kinase C (PKC) and protein kinase D (PKD) in CXCL12 and CXCL13 stimulated SK-MEL-28 (malignant melanoma) and THP-1 (acute monocytic leukaemia) cell migration. While PKC and PKD had no active role in CXCL12 or CXCL13 stimulated THP-1 cell migration, PKC and PKD inhibition reduced CXCL12 stimulated migration and caused profound effects upon the cytoskeleton of SK-MEL-28 cells. Furthermore, only PKC and not PKD inhibition reduced CXCL13 stimulated migration in SK-MEL-28 cells however PKC inhibition failed to stimulate any changes to the actin cytoskeleton. These findings indicate that PKC inhibitors would be a useful therapeutic for the prevention of both CXCL12 and CXCL13 stimulated migration and PKD inhibitors for CXCL12 stimulated migration in malignant melanoma.

PKCζ phosphorylates VASP to mediate chemotaxis in breast cancer cells

Breast carcinoma (BC) is one of the most common malignant cancers in females, and metastasis remains the leading cause of death in these patients. Chemotaxis plays an important role in cancer cell metastasis and the mechanism of breast cancer chemotaxis has become a central issue in contemporary research. PKCζ, a member of the atypical PKC family, has been reported to be an essential component of the EGF-stimulated chemotactic signaling pathway. However, the molecular mechanism through which PKCζ regulates chemotaxis remains unclear. Here, we used a proteomic approach to identify PKCζ-interacting proteins in breast cancer cells and identified VASP as a potential binding partner. Intriguingly, stimulation with EGF enhanced this interaction and induced the translocalization of PKCζ and VASP to the cell membrane. Further experiments showed that PKCζ catalyzes the phosphorylation of VASP at Ser157, which is critical for the biological function of VASP in regulating chemotaxis and actin polymerization in breast cancer cells. Furthermore, in PKCζ knockdown BC cells, the enrichment of VASP at the leading edge was reduced, and its interaction with profilin1 was attenuated, thereby reducing the chemotaxis and overall motility of breast cancer cells after EGF treatment. In functional assays, PKCζ promoted chemotaxis and motility of BC cells through VASP. Our findings demonstrate that PKCζ, a new kinase of VASP, plays an important role in promoting breast cancer metastasis and provides a theoretical basis for expanding new approaches to tumor biotherapy.

Oncogenic BRAF noncanonically promotes tumor metastasis by mediating VASP phosphorylation and filopodia formation

BRAF is frequently mutated in various cancer types and contributes to tumorigenesis and metastasis. As an important switch in RAS signaling pathway, BRAF typically enables the activation of MEK and ERK, and its mutation significantly promotes metastasis. However, whether BRAF could stimulate metastasis via a distinct manner is still unknown. Herein, we found that a portion of the BRAF protein localized at the plasma membrane and that the BRAFV600E mutation led to abundant formation of filopodia, which is a hallmark of invasive cancer cells. Mechanistically, BRAF physically interacts with the pseudopod formation-related protein Vasodilator-stimulated phosphoprotein (VASP), and BRAF specifically catalyzes VASP phosphorylation at Ser157. VASP depletion or disruption of Ser157 phosphorylation preferentially reduced the motility, invasion and metastasis of tumor cells harboring oncogenic BRAF or KRAS. Moreover, in clinical cancer tissues, BRAFV600E was positively correlated with the extent of invasion, and tissues with BRAFV600E expression exhibited elevated levels of VASP Ser157 phosphorylation. Our study therefor reveals a noncanonical mechanism by which oncogenic BRAF or KRAS promotes metastasis, suggests that VASP Ser157 phosphorylation might serve as a valuable therapeutic target in BRAF or KRAS mutant cancers.

Cardiovascular Functions of Ena/VASP Proteins: Past, Present and Beyond

Actin binding proteins are of crucial importance for the spatiotemporal regulation of actin cytoskeletal dynamics, thereby mediating a tremendous range of cellular processes. Since their initial discovery more than 30 years ago, the enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) family has evolved as one of the most fascinating and versatile family of actin regulating proteins. The proteins directly enhance actin filament assembly, but they also organize higher order actin networks and link kinase signaling pathways to actin filament assembly. Thereby, Ena/VASP proteins regulate dynamic cellular processes ranging from membrane protrusions and trafficking, and cell-cell and cell-matrix adhesions, to the generation of mechanical tension and contractile force. Important insights have been gained into the physiological functions of Ena/VASP proteins in platelets, leukocytes, endothelial cells, smooth muscle cells and cardiomyocytes. In this review, we summarize the unique and redundant functions of Ena/VASP proteins in cardiovascular cells and discuss the underlying molecular mechanisms.

Fucoidan from Sargassum hemiphyllum inhibits the stemness of cancer stem cells and epithelial-mesenchymal transitions in bladder cancer cells

A variety of anticancer activities have been established for fucoidan from brown algae, whereas whether cancer stem cells (CSCs) are inhibited by sulfated polysaccharides is unexplored. In this study, fucoidan extracted from Sargassum hemiphyllum was showed heat stable and might tolerate 140 °C treatment. Fucoidan did not exhibit cytotoxicity in 5637 and T24 bladder cancer cells. After fucoidan treatment, the stress fibers were aggregated into thick and abundant underneath the plasma membrane and getting around the cells, and the structure of F-actin showed a remarkable change in the filopodial protrusion in T24 and 5637 cells. Using culture inserts, transwell assays and time lapse recordings showed that fucoidan inhibited cell migration. In the epithelial-mesenchymal transition (EMT), fucoidan downregulated the expression of vimentin, a mesenchymal marker, and upregulated the expression of E-cadherin, an epithelial marker. Additionally, the transcription levels of Snail, Slug, Twist1, Twist2, MMP2 and MMP9 were significantly decreased by fucoidan, indicating EMT suppression. CSCs are implicated in tumor initiation, metastatic spread, drug resistance and tumor recurrence. Our results showed that fucoidan inhibited stemness gene expression and sphere formation in bladder CSCs. For the first time, our findings demonstrated that fucoidan inhibits CSC formation and provides evidence as potential anticancer therapy.

Polymerization power: effectors of actin polymerization as regulators of T lymphocyte migration through complex environments

During their life span, T cells are tasked with patrolling the body for potential pathogens. To do so, T cells migrate through numerous distinct anatomical sites and tissue environments with different biophysical characteristics. To migrate through these different environments, T cells use various motility strategies that rely on actin network remodeling to generate shape changes and mechanical forces. In this review, we initially discuss the migratory journey of T cells and then cover the actin polymerization effectors at play in T cells, and finally, we focus on the function of these effectors of actin cytoskeleton remodeling in mediating T-cell migration through diverse tissue environments. Specifically, we will discuss the current state of the field pertaining to our understanding of the roles in T-cell migration played by members of the three main families of actin polymerization machinery: the Arp2/3 complex; formin proteins; and Ena/VASP proteins.

Migration and homeostasis of regulatory T cells in rheumatoid arthritis

Regulatory T (T_reg) cells are garnering increased attention in research related to autoimmune diseases, including rheumatoid arthritis (RA). They play an essential role in the maintenance of immune homeostasis by restricting effector T cell activity. Reduced functions and frequencies of T_reg cells contribute to the pathogenesis of RA, a common autoimmune disease which leads to systemic inflammation and erosive joint destruction. T_reg cells from patients with RA are characterized by impaired functions and by an altered phenotype. They show increased plasticity towards Th17 cells and a reduced suppressive capacity. Besides the suppressive function of T_reg cells, their effectiveness is determined by their ability to migrate into inflamed tissues. In the past years, new mechanisms involved in T_reg cell migration have been identified. One example of such a mechanism is the phosphorylation of vasodilator-stimulated phosphoprotein (VASP). Efficient migration of T_reg cells requires the presence of VASP. IL-6, a cytokine which is abundantly present in the peripheral blood and in the synovial tissue of RA patients, induces posttranslational modifications of VASP. Recently, it has been shown in mice with collagen-induced arthritis (CIA) that this IL-6 mediated posttranslational modification leads to reduced T_reg cell trafficking. Another protein which facilitates T_reg cell migration is G-protein-signaling modulator 2 (GPSM2). It modulates G-protein coupled receptor functioning, thereby altering the cellular activity initiated by cell surface receptors in response to extracellular signals. The almost complete lack of GPSM2 in T_reg cells from RA patients contributes to their reduced ability to migrate towards inflammatory sites. In this review article, we highlight the newly identified mechanisms of T_reg cell migration and review the current knowledge about impaired T_reg cell homeostasis in RA.

Protein kinase A Mediated Effects of Protein kinase C Partial Agonist HMI-1a3 in Colorectal Cancer Cells

Colorectal cancer is the third most commonly occurring cancer in men and the second in women. The global burden of colorectal cancer is projected to increase to over 2 million new cases with over 1 million deaths within the next 10 years and there is a great need for new compounds with novel mechanisms of action. Our group has developed PKC modulating isophthalic acid derivatives that induce cytotoxicity towards human cervical and prostate cancer cell lines. In this study, we investigated the effects of 5-(hydroxymethyl)isophthalate 1a3 (HMI-1a3) on colorectal cancer cell lines (Caco2, Colo205 and HT29). HMI-1a3 inhibited cell proliferation, decreased cell viability and induced an apoptotic response in all studied cell lines. These effects, however, were independent of PKC. Using serine/threonine kinome profiling and pharmacological kinase inhibitors we identified activation of the cAMP/PKA pathway as a new mechanism-of-action for HMI-1a3-induced anti-cancer activity in colorectal cancer cell lines. Our current results strengthen the hypothesis for HMI-1a3 as a potential anti-cancer agent against various malignancies. Significance Statement Colorectal cancer (CRC) is a common solid organ malignancy. Here, we demonstrate that the protein kinase C (PKC) C1 domain-targeted isophthalatic acid derivative HMI-1a3 has anti-cancer activity on CRC cell lines independently of PKC. We identified protein kinase A (PKA) activation as a mechanism of HMI-1a3 induced anti-cancer effects. Our results reveal a new anti-cancer mechanism of action for the partial PKC agonist HMI-1a3 and thus provide new insights for the development of PKC and PKA modulators for cancer therapy.

Structural, biochemical, and functional properties of the Rap1-Interacting Adaptor Molecule (RIAM)

Leukocytes, the leading players of immune system, are involved in innate and adaptive immune responses. Leukocyte adhesion to endothelial cells during transmigration or to antigen presenting cells during T cell activation, requires integrin activation through a process termed inside-out integrin signaling. In hematopoietic cells, Rap1 and its downstream effector RIAM (Rap1-interacting adaptor molecule) form a cornerstone for inside-out integrin activation. The Rap1/RIAM pathway is involved in signal integration for activation, actin remodeling and cytoskeletal reorganization in T cells, as well as in myeloid cell differentiation and function. RIAM is instrumental for phagocytosis, a process requiring particle recognition, cytoskeletal remodeling and membrane protrusion for engulfment and digestion. In the present review, we discuss the structural and molecular properties of RIAM and the recent discoveries regarding the functional role of the Rap1/RIAM module in hematopoietic cells.

Multifaceted Functions of Protein Kinase D in Pathological Processes and Human Diseases

Protein kinase D (PKD) is a family of serine/threonine protein kinases operating in the signaling network of the second messenger diacylglycerol. The three family members, PKD1, PKD2, and PKD3, are activated by a variety of extracellular stimuli and transduce cell signals affecting many aspects of basic cell functions including secretion, migration, proliferation, survival, angiogenesis, and immune response. Dysregulation of PKD in expression and activity has been detected in many human diseases. Further loss- or gain-of-function studies at cellular levels and in animal models provide strong support for crucial roles of PKD in many pathological conditions, including cancer, metabolic disorders, cardiac diseases, central nervous system disorders, inflammatory diseases, and immune dysregulation. Complexity in enzymatic regulation and function is evident as PKD isoforms may act differently in different biological systems and disease models, and understanding the molecular mechanisms underlying these differences and their biological significance in vivo is essential for the development of safer and more effective PKD-targeted therapies. In this review, to provide a global understanding of PKD function, we present an overview of the PKD family in several major human diseases with more focus on cancer-associated biological processes.

Protein kinase D1 phosphorylation of KAT7 enhances its protein stability and promotes replication licensing and cell proliferation

The histone acetyltransferase (HAT) KAT7/HBO1/MYST2 plays a crucial role in the pre-replication complex (pre-RC) formation, DNA replication and cell proliferation via acetylation of histone H4 and H3. In a search for protein kinase D1 (PKD1)-interacting proteins, we have identified KAT7 as a potential PKD1 substrate. We show that PKD1 directly interacts and phosphorylates KAT7 at Thr97 and Thr331 in vitro and in vivo. PKD1-mediated phosphorylation of KAT7 enhances its expression levels and stability by reducing its ubiquitination-mediated degradation. Significantly, the phospho-defective mutant KAT7-Thr97/331A attenuates histone H4 acetylation levels, MCM2/6 loading on the chromatin, DNA replication and cell proliferation. Similarly, PKD1 knockdown decreases, whereas the constitutive active mutant PKD1-CA increases histone H4 acetylation levels and MCM2/6 loading on the chromatin. Overall, these results suggest that PKD1-mediated phosphorylation of KAT7 may be required for pre-RC formation and DNA replication.

LymphoAtlas: a dynamic and integrated phosphoproteomic resource of TCR signaling in primary T cells reveals ITSN2 as a regulator of effector functions

T-cell receptor (TCR) ligation-mediated protein phosphorylation regulates the activation, cellular responses, and fates of T cells. Here, we used time-resolved high-resolution phosphoproteomics to identify, quantify, and characterize the phosphorylation dynamics of thousands of phosphorylation sites in primary T cells during the first 10 min after TCR stimulation. Bioinformatic analysis of the data revealed a coherent orchestration of biological processes underlying T-cell activation. In particular, functional modules associated with cytoskeletal remodeling, transcription, translation, and metabolic processes were mobilized within seconds after TCR engagement. Among proteins whose phosphorylation was regulated by TCR stimulation, we demonstrated, using a fast-track gene inactivation approach in primary lymphocytes, that the ITSN2 adaptor protein regulated T-cell effector functions. This resource, called LymphoAtlas, represents an integrated pipeline to further decipher the organization of the signaling network encoding T-cell activation. LymphoAtlas is accessible to the community at: https://bmm-lab.github.io/LymphoAtlas.

RIAM-VASP Module Relays Integrin Complement Receptors in Outside-In Signaling Driving Particle Engulfment

The phagocytic integrins and complement receptors α_Mβ₂/CR3 and α_Xβ₂/CR4 are classically associated with the phagocytosis of iC3b-opsonized particles. The activation of this receptor is dependent on signals derived from other receptors (inside-out signaling) with the crucial involvement of the Rap1-RIAM-Talin-1 pathway. Here, we analyze the implication of RIAM and its binding partner VASP in the signaling events occurring downstream of β₂ integrins (outside-in) during complement-mediated phagocytosis. To this end, we used HL-60 promyelocytic cell lines deficient in RIAM or VASP or overexpressing EGFP-tagged VASP to determine VASP dynamics at phagocytic cups. Our results indicate that RIAM-deficient HL-60 cells presented impaired particle internalization and altered integrin downstream signaling during complement-dependent phagocytosis. Similarly, VASP deficiency completely blocked phagocytosis, while VASP overexpression increased the random movement of phagocytic particles at the cell surface, with reduced internalization. Moreover, the recruitment of VASP to particle contact sites, amount of pSer157-VASP and formation of actin-rich phagocytic cups were dependent on RIAM expression. Our results suggested that RIAM worked as a relay for integrin complement receptors in outside-in signaling, coordinating integrin activation and cytoskeletal rearrangements via its interaction with VASP.

The phosphorylation status of PIP5K1C at serine 448 can be predictive for invasive ductal carcinoma of the breast

Phosphatidylinositol-4-phosphate 5-kinase type-1C (PIP5K1C) is a lipid kinase that regulates focal adhesion dynamics and cell attachment through site-specific formation of phosphatidylinositol-4,5-bisphosphate (PI4,5P₂). By comparing normal breast tissue to carcinoma in situ and invasive ductal carcinoma subtypes, we here show that the phosphorylation status of PIP5K1C at serine residue 448 (S448) can be predictive for breast cancer progression to an aggressive phenotype, while PIP5K1C expression levels are not indicative for this event. PIP5K1C phosphorylation at S448 is downregulated in invasive ductal carcinoma, and similarly, the expression levels of PKD1, the kinase that phosphorylates PIP5K1C at this site, are decreased. Overall, since PKD1 is a negative regulator of cell migration and invasion in breast cancer, the phosphorylation status of this residue may serve as an indicator of aggressiveness of breast tumors.

Vasodilator-Stimulated Phosphoprotein: Regulators of Adipokines Resistin and Phenotype Conversion of Epicardial Adipocytes

Background

Endothelial dysfunction plays a central part in the pathogenesis of coronary atherosclerosis. The adipokine resistin is one of the key players in endothelial cell dysfunction. In addition, the role of epicardial fat in coronary artery endothelial dysfunction is also emphasized.

We investigated whether vasodilator-stimulated phosphoprotein (VASP) is involved in resistin-related endothelial dysfunction and the phenotype conversion of epicardial adipocytes.

Material/Methods

Cell proliferation and migration were evaluated by MTT and Transwell chamber assay, respectively. Next, we took epicardial fat samples from patients with valvular heart disease and non-coronary artery disease. Gene expression was determined by reverse transcription-quantitative polymerase chain reaction and relative abundance of the protein by Western blotting.

Results

Resistin induced endothelial proliferation and migration in a dose-dependent manner. Both resistin-induced cell proliferation and migration were effectively blocked by ablation of VASP. The brown adipose tissue-specific genes for uncoupling protein 1 (UCP-1) and PR-domain-missing16 (PRDM16) decreased, but the white adipose tissue-specific genes for resistin and RIP140 increased in VASP-deficient adipocytes compared with the LV-sicntr group. However, disruption of the Ras homolog gene family member A (RhoA) /Rho-associated kinase (ROCK) in VASP-deficient adipocytes with specific inhibitors inverted the adipocyte phenotype existing in VASP-deficient adipocytes. Furthermore, the expressions of proinflammatory cytokines interleukin-6 (IL-6), interleukin-8 (IL-8), and monocyte chemoattractantprotein-1 (MCP-1) in VASP-deficient adipocytes were markedly upregulated compared with the LV-sicntr group.

Conclusions

These results suggest a physiological role for VASP in coronary atherosclerosis through regulating adipokine resistin and phenotype conversion of epicardial adipose tissue.

The Role of Mst1 in Lymphocyte Homeostasis and Function

The Hippo pathway is an evolutionarily conserved pathway crucial for regulating tissue size and for limiting cancer development. However, recent work has also uncovered key roles for the mammalian Hippo kinases, Mst1/2, in driving appropriate immune responses by directing T cell migration, morphology, survival, differentiation, and activation. In this review, we discuss the classical signaling pathways orchestrated by the Hippo signaling pathway, and describe how Mst1/2 direct T cell function by mechanisms not seeming to involve the classical Hippo pathway. We also discuss why Mst1/2 might have different functions within organ systems and the immune system. Overall, understanding how Mst1/2 transmit signals to discrete biological processes in different cell types might allow for the development of better drug therapies for the treatments of cancers and immune-related diseases.

Role of Corneal Stromal Cells on Epithelial Cell Function during Wound Healing

Following injury, corneal stromal keratocytes transform into repair-phenotype of activated stromal fibroblasts (SFs) and participate in wound repair. Simultaneously, ongoing bi-directional communications between corneal stromal-epithelial cells also play a vital role in mediating the process of wound healing. Factors produced by stromal cells are known to induce proliferation, differentiation, and motility of corneal epithelial cells, which are also subsequently the main processes that occur during wound healing. In this context, the present study aims to investigate the effect of SFs conditioned medium (SFCM) on corneal epithelial cell function along with substance P (SP). Antibody microarrays were employed to profile differentially expressed cell surface markers and cytokines in the presence of SFCM and SP. Antibody microarray data revealed enhanced expression of the ITGB1 in corneal epithelial cells following stimulation with SP whereas SFCM induced abundant expression of IL-8, ITGB1, PD1L1, PECA1, IL-15, BDNF, ICAM1, CD8A, CD44 and NTF4. All these proteins have either direct or indirect roles in epithelial cell growth, movement and adhesion related signaling cascades during tissue regeneration. We also observed activation of MAPK signaling pathway along with increased expression of focal adhesion kinase (FAK), paxillin, vimentin, β-catenin and vasodilator-stimulated phosphoprotein (VASP) phosphorylation. Additionally, epithelial-to-mesenchymal transition (EMT) regulating transcription factors Slug and ZEB1 expression were enhanced in the presence of SFCM. SP enriched the expression of integrin subunits α4, α5, αV, β1 and β3 whereas SFCM increased α4, α5, αV, β1 and β5 integrin subunits. We also observed increased expression of Serpin E1 following SP and SFCM treatment. Wound healing scratch assay revealed enhanced migration of epithelial cells following the addition of SFCM. Taken together, we conclude that SFCM-mediated sustained activation of ZEB1, Slug in combination with upregulated migration-associated integrins and ERK (Extracellular signal-regulated kinase)-FAK-paxillin axis, may lead to induce type 2 EMT-like changes during corneal epithelial wound healing.

New cGMP analogues restrain proliferation and migration of melanoma cells

Melanoma is one of the most aggressive cancers and displays high resistance to conventional chemotherapy underlining the need for new therapeutic strategies. The cGMP/PKG signaling pathway was detected in melanoma cells and shown to reduce migration, proliferation and to increase apoptosis in different cancer types. In this study, we evaluated the effects on cell viability, cell death, proliferation and migration of novel dimeric cGMP analogues in two melanoma cell lines (MNT1 and SkMel28). These new dimeric cGMP analogues, by activating PKG with limited effects on PKA, significantly reduced proliferation, migration and increased cell death. No decrease in cell viability was observed in non-tumor cells suggesting a tumor-specific effect. These effects observed in melanoma are possibly mediated by PKG2 activation based on the decreased toxic effects in tumor cell lines not expressing PKG2. Finally, PKG-associated phosphorylation of vasodilator-stimulated-phosphoprotein (VASP), linked to cell death, proliferation and migration was found increased and with a change of subcellular localization. Increased phosphorylation of RhoA induced by activation of PKG may also contribute to reduced migration ability of the SkMel28 melanoma cell line when treated with cGMP analogues. These findings suggest that the cGMP/PKG pathway can be envisaged as a therapeutic target of novel dimeric cGMP analogues for the treatment of melanoma.

Lysophosphatidic acid via LPA-receptor 5/protein kinase D-dependent pathways induces a motile and pro-inflammatory microglial phenotype

BACKGROUND

Extracellular lysophosphatidic acid (LPA) species transmit signals via six different G protein-coupled receptors (LPAR1-6) and are indispensible for brain development and function of the nervous system. However, under neuroinflammatory conditions or brain damage, LPA levels increase, thereby inducing signaling cascades that counteract brain function. We describe a critical role for 1-oleyl-2-hydroxy-sn-glycero-3-phosphate (termed "LPA" throughout our study) in mediating a motile and pro-inflammatory microglial phenotype via LPAR5 that couples to protein kinase D (PKD)-mediated pathways.

METHODS

Using the xCELLigence system and time-lapse microscopy, we investigated the migrational response of microglial cells. Different M1 and M2 markers were analyzed by confocal microscopy, flow cytometry, and immunoblotting. Using qPCR and ELISA, we studied the expression of migratory genes and quantitated the secretion of pro-inflammatory cytokines and chemokines, respectively. Different transcription factors that promote the regulation of pro-inflammatory genes were analyzed by western blot. Reactive oxygen species (ROS) and nitric oxide (NO) production, phagocytosis, and microglial cytotoxicity were determined using commercially available assay kits.

RESULTS

LPA induces MAPK family and AKT activation and pro-inflammatory transcription factors' phosphorylation (NF-κB, c-Jun, STAT1, and STAT3) that were inhibited by both LPAR5 and PKD family antagonists. LPA increases migratory capacity, induces secretion of pro-inflammatory cytokines and chemokines and expression of M1 markers, enhances production of ROS and NO by microglia, and augments cytotoxicity of microglial cell-conditioned medium towards neurons. The PKD family inhibitor blunted all of these effects. We propose that interference with this signaling axis could aid in the development of new therapeutic approaches to control neuroinflammation under conditions of overshooting LPA production.

CONCLUSIONS

In the present study, we show that inflammatory LPA levels increased the migratory response of microglia and promoted a pro-inflammatory phenotype via the LPAR5/PKD axis. Interference with this signaling axis reduced microglial migration, blunted microglial cytotoxicity, and abrogated the expression and secretion of pro-inflammatory mediators.

Novel protein kinase targets in vascular smooth muscle therapeutics

Many signaling factors have been identified over the years that serve as mechanistic foundations for the pathogenesis and/or maintenance of cardiovascular disease (CVD). Of these, cyclic nucleotide-driven protein kinases in vascular smooth muscle (VSM) are of essential importance. Comprised primarily of cyclic AMP-dependent and cyclic GMP-dependent protein kinases, these ubiquitous signaling molecules have capacity to operate through numerous downstream effectors including vasodilator-stimulated phosphoprotein (VASP) to control aberrant VSM growth elemental to CVD. As more information is gathered regarding genetic, biochemical, molecular and cellular makeup of CVD including VSM cyclic nucleotide-dependent protein kinases and VASP, advances will be made in precision medicine by identifying more precise therapeutic targets to enhance clinical decision making.

Mechanisms of Yersinia YopO kinase substrate specificity

Yersinia bacteria cause a range of human diseases, including yersiniosis, Far East scarlet-like fever and the plague. Yersiniae modulate and evade host immune defences through injection of Yersinia outer proteins (Yops) into phagocytic cells. One of the Yops, YopO (also known as YpkA) obstructs phagocytosis through disrupting actin filament regulation processes - inhibiting polymerization-promoting signaling through sequestration of Rac/Rho family GTPases and by using monomeric actin as bait to recruit and phosphorylate host actin-regulating proteins. Here we set out to identify mechanisms of specificity in protein phosphorylation by YopO that would clarify its effects on cytoskeleton disruption. We report the MgADP structure of Yersinia enterocolitica YopO in complex with actin, which reveals its active site architecture. Using a proteome-wide kinase-interacting substrate screening (KISS) method, we identified that YopO phosphorylates a wide range of actin-modulating proteins and located their phosphorylation sites by mass spectrometry. Using artificial substrates we clarified YopO's substrate length requirements and its phosphorylation consensus sequence. These findings provide fresh insight into the mechanism of the YopO kinase and demonstrate that YopO executes a specific strategy targeting actin-modulating proteins, across multiple functionalities, to compete for control of their native phospho-signaling, thus hampering the cytoskeletal processes required for macrophage phagocytosis.

Protein Kinase D1 regulates focal adhesion dynamics and cell adhesion through Phosphatidylinositol-4-phosphate 5-kinase type-l γ

Focal adhesions (FAs) are highly dynamic structures that are assembled and disassembled on a continuous basis. The balance between the two processes mediates various aspects of cell behavior, ranging from cell adhesion and spreading to directed cell migration. The turnover of FAs is regulated at multiple levels and involves a variety of signaling molecules and adaptor proteins. In the present study, we show that in response to integrin engagement, a subcellular pool of Protein Kinase D1 (PKD1) localizes to the FAs. PKD1 affects FAs by decreasing turnover and promoting maturation, resulting in enhanced cell adhesion. The effects of PKD1 are mediated through direct phosphorylation of FA-localized phosphatidylinositol-4-phosphate 5-kinase type-l γ (PIP5Klγ) at serine residue 448. This phosphorylation occurs in response to Fibronectin-RhoA signaling and leads to a decrease in PIP5Klγs’ lipid kinase activity and binding affinity for Talin. Our data reveal a novel function for PKD1 as a regulator of FA dynamics and by identifying PIP5Klγ as a novel PKD1 substrate provide mechanistic insight into this process.

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.

The phosphorylation status of VASP at serine 322 can be predictive for aggressiveness of invasive ductal carcinoma

Vasodilator-stimulated phosphoprotein (VASP) signaling is critical for dynamic actin reorganization processes that define the motile phenotype of cells. Here we show that VASP is generally highly expressed in normal breast tissue and breast cancer. We also show that the phosphorylation status of VASP at S322 can be predictive for breast cancer progression to an aggressive phenotype. Our data indicate that phosphorylation at S322 is gradually decreased from normal breast to DCIS, luminal/ER+, HER2+ and basal-like/TN phenotypes. Similarly, the expression levels of PKD2, the kinase that phosphorylates VASP at this site, are decreased in invasive ductal carcinoma samples of all three groups. Overall, the phosphorylation status of this residue may serve as an indicator of aggressiveness of breast tumors.

Protein Kinase G facilitates EGFR-mediated cell death in MDA-MB-468 cells

The Epidermal Growth Factor Receptor (EGFR) is a transmembrane receptor tyrosine kinase with critical implications in cell proliferation, migration, wound healing and the regulation of apoptosis. However, the EGFR has been shown to be hyper-expressed in a number of human malignancies. The MDA-MB-468 metastatic breast cell line is one example of this. This particular cell line hyper-expresses the EGFR and undergoes EGFR-mediated apoptosis in response to EGF ligand. The goal of this study was to identify the kinases that could be potential intermediates for the EGFR-mediated induction of apoptosis intracellularly. After identifying Cyclic GMP-dependent Protein Kinase G (PKG) as a plausible intermediate, we wanted to determine the temporal relationship of these two proteins in the induction of apoptosis. We observed a dose-dependent decrease in MDA-MB-468 cell viability, which was co-incident with increased PKG activity as measured by VASPSer239 phosphorylation. In addition, we observed a dose dependent decrease in cell viability, as well as an increase in apoptosis, in response to two different PKG agonists, 8-Bromo-cGMP and 8-pCPT-cGMP. MDA-MB-468 cells with reduced PKG activity had attenuated EGFR-mediated apoptosis. These findings indicate that PKG does not induce cell death via transphosphorylation of the EGFR. Instead, PKG activity occurs following EGFR activation. Together, these data indicate PKG as an intermediary in EGFR-mediated cell death, likely via apoptotic pathway.

Soluble guanylyl cyclase-activated cyclic GMP-dependent protein kinase inhibits arterial smooth muscle cell migration independent of VASP-serine 239 phosphorylation

Coronary artery disease (CAD) accounts for over half of all cardiovascular disease-related deaths. Uncontrolled arterial smooth muscle (ASM) cell migration is a major component of CAD pathogenesis and efforts aimed at attenuating its progression are clinically essential. Cyclic nucleotide signaling has long been studied for its growth-mitigating properties in the setting of CAD and other vascular disorders. Heme-containing soluble guanylyl cyclase (sGC) synthesizes cyclic guanosine monophosphate (cGMP) and maintains vascular homeostasis predominantly through cGMP-dependent protein kinase (PKG) signaling. Considering that reactive oxygen species (ROS) can interfere with appropriate sGC signaling by oxidizing the cyclase heme moiety and so are associated with several CVD pathologies, the current study was designed to test the hypothesis that heme-independent sGC activation by BAY 60-2770 (BAY60) maintains cGMP levels despite heme oxidation and inhibits ASM cell migration through phosphorylation of the PKG target and actin-binding vasodilator-stimulated phosphoprotein (VASP). First, using the heme oxidant ODQ, cGMP content was potentiated in the presence of BAY60. Using a rat model of arterial growth, BAY60 significantly reduced neointima formation and luminal narrowing compared to vehicle (VEH)-treated controls. In rat ASM cells BAY60 significantly attenuated cell migration, reduced G:F actin, and increased PKG activity and VASP Ser239 phosphorylation (pVASP·S239) compared to VEH controls. Site-directed mutagenesis was then used to generate overexpressing full-length wild type VASP (FL-VASP/WT), VASP Ser239 phosphorylation-mimetic (FL-VASP/239D) and VASP Ser239 phosphorylation-resistant (FL-VASP/239A) ASM cell mutants. Surprisingly, FL-VASP/239D negated the inhibitory effects of FL-VASP/WT and FL-VASP/239A cells on migration. Furthermore, when FL-VASP mutants were treated with BAY60, only the FL-VASP/239D group showed reduced migration compared to its VEH controls. Intriguingly, FL-VASP/239D abrogated the stimulatory effects of FL-VASP/WT and FL-VASP/239A cells on PKG activity. In turn, pharmacologic blockade of PKG in the presence of BAY60 reversed the inhibitory effect of BAY60 on naïve ASM cell migration. Taken together, we demonstrate for the first time that BAY60 inhibits ASM cell migration through cGMP/PKG/VASP signaling yet through mechanisms independent of pVASP·S239 and that FL-VASP overexpression regulates PKG activity in rat ASM cells. These findings implicate BAY60 as a potential pharmacotherapeutic agent against aberrant ASM growth disorders such as CAD and also establish a unique mechanism through which VASP controls PKG activity.

Regulation of Cancer Cell Behavior by the Small GTPase Rab13

The members of the Rab family of GTPases are master regulators of cellular membrane trafficking. With ∼70 members in humans, Rabs have been implicated in all steps of membrane trafficking ranging from vesicle formation and transport to vesicle docking/tethering and fusion. Vesicle trafficking controls the localization and levels of a myriad of proteins, thus regulating cellular functions including proliferation, metabolism, cell-cell adhesion, and cell migration. It is therefore not surprising that impairment of Rab pathways is associated with diseases including cancer. In this review, we highlight evidence supporting the role of Rab13 as a potent driver of cancer progression.

Vasodilator-stimulated phosphoprotein restricts cell-to-cell spread of Shigella flexneri at the cell periphery

Shigella spp. are intracellular bacterial pathogens that cause diarrhoeal disease in humans. Shigella utilize the host actin cytoskeleton to enter cells, move through the cytoplasm of cells and pass into adjacent cells. Ena/VASP family proteins are highly conserved proteins that participate in actin-dependent dynamic cellular processes. We tested whether Ena/VASP family members VASP (vasodilator-stimulated phosphoprotein), Mena (mammalian-enabled) or EVL (Ena-VASP-like) contribute to Shigella flexneri spread through cell monolayers. VASP and EVL restricted cell-to-cell spread without significantly altering actin-based motility, whereas Mena had no effect on these processes. Phosphorylation of VASP on Ser153, Ser235 and Thr274 regulated its subcellular distribution and function. VASP derivatives that lack the Ena/VASP homology 1 (EVH1) domain or contain a phosphoablative mutation of Ser153 were defective in restricting S. flexneri spread, indicating that the EVH1 domain and phosphorylation on Ser153 are required for this process. The EVH1 domain and Ser153 of VASP were required for VASP localization to focal adhesions, and localization of VASP to focal adhesions and/or the leading edge was required for restriction of spread. The contribution of the EVH1 domain was from both the donor and the recipient cell, whereas the contribution of Ser153 phosphorylation was only from the donor cell. Thus, unlike host proteins characterized in Shigella pathogenesis that promote bacterial spread, VASP and EVL function to limit it. The ability of VASP and EVL to limit spread highlights the critical role of focal adhesion complexes and/or the leading edge in bacterial passage between cells.

Rab13 acts downstream of the kinase Mst1 to deliver the integrin LFA-1 to the cell surface for lymphocyte trafficking

In lymphocytes, the kinase Mst1 is required for the proper organization of integrins in the plasma membrane at the leading edge of migrating cells, which is critical for lymphocyte trafficking. We found a functional link between the small G protein Rab13 and Mst1 in lymphocyte adhesion and migration. In response to stimulation of T lymphocytes with chemokine, Mst1 promoted phosphorylation of the guanine nucleotide exchange factor DENND1C (differentially expressed in normal and neoplastic cells domain 1C), which activated Rab13. Active Rab13 associated with Mst1 to facilitate the delivery of the integrin LFA-1 (lymphocyte function-associated antigen 1) to the leading edge of lymphocytes. Delivery of LFA-1 involved the recruitment of myosin Va along actin filaments, which extended as a result of the localization of the actin regulatory protein VASP to the cell periphery through phosphorylation of VASP at Ser(157) by Mst1. Inhibition of Rab13 function reduced the adhesion and migration of lymphocytes on ICAM-1 (intercellular adhesion molecule-1), the ligand for LFA-1, and inhibited the formation of a ring-like arrangement of LFA-1 at the contact sites between T cells and antigen-presenting cells. The lymphoid tissues of Rab13-deficient mice had reduced numbers of lymphocytes because of the defective trafficking capability of these cells. These results suggest that Rab13 acts with Mst1 to regulate the spatial distribution of LFA-1 and the motility and trafficking of lymphocytes.

Protein kinase d isoforms differentially modulate cofilin-driven directed cell migration

Background

Protein kinase D (PKD) enzymes regulate cofilin-driven actin reorganization and directed cell migration through both p21-activated kinase 4 (PAK4) and the phosphatase slingshot 1L (SSH1L). The relative contributions of different endogenous PKD isoforms to both signaling pathways have not been elucidated, sufficiently.

Methodology/Principal Findings

We here analyzed two cell lines (HeLa and MDA-MB-468) that express the subtypes protein kinase D2 (PKD2) and protein kinase D3 (PKD3). We show that under normal growth conditions both isoforms can form a complex, in which PKD3 is basally-active and PKD2 is inactive. Basal activity of PKD3 mediates PAK4 activity and downstream signaling, but does not significantly inhibit SSH1L. This signaling constellation was required for facilitating directed cell migration. Activation of PKD2 and further increase of PKD3 activity leads to additional phosphorylation and inhibition of endogenous SSH1L. Net effect is a dramatic increase in phospho-cofilin and a decrease in cell migration, since now both PAK4 and SSH1L are regulated by the active PKD2/PKD3 complex.

Conclusions/Significance

Our data suggest that PKD complexes provide an interface for both cofilin regulatory pathways. Dependent on the activity of involved PKD enzymes signaling can be balanced to guarantee a functional cofilin activity cycle and increase cell migration, or imbalanced to decrease cell migration. Our data also provide an explanation of how PKD isoforms mediate different effects on directed cell migration.

Regulation of VASP by phosphorylation: consequences for cell migration

Phosphorylations control all aspects of vasodilator-stimulated phospho-protein (VASP) function. Mapped phosphorylation sites include Y39, S157, S239, T278, and S322, and multiple kinases have been shown to mediate their phosphorylation. Recently, Protein Kinase D1 (PKD1) as a direct kinase for S157 and S322 joined this group. While S157 phosphorylation generally seems to serve as a signal for membrane localization, phosphorylations at S322 or at S239 and T278 have opposite effects on F-actin accumulation. In migrating cells, S322 phosphorylation increases filopodia numbers and length, while S239/T278 phosphorylations decrease these and also disrupt formation of focal adhesions. Therefore, the kinases mediating these phosphorylations can be seen as switches needed to facilitate cell motility.