Inhibition of PKA blocks fibroblast migration in response to growth factors

Primary ciliary protein kinase A activity in the prefrontal cortex modulates stress in mice

Primary cilia are cellular antennae emanating from vertebrate cell surfaces to sense and transduce extracellular signals intracellularly to regulate cell behavior and function. However, their signal sensing and physiological functions in neocortical neurons remain largely unclear. Here, we show that, in response to various animal stressors, primary cilia in the mouse prefrontal cortex (PFC) exhibit consistent axonemal elongation. Selective removal of excitatory neuron primary cilia in the prefrontal but not sensory cortex leads to a reduction in animal stress sensing and response. Treatment with corticosterone, the major stress hormone, elicits an increase in primary ciliary cyclic adenosine 3',5'-monphosphate (cAMP) level in PFC excitatory neurons and a decrease in neuronal excitability dependent on primary cilia. Suppression of primary ciliary protein kinase A (PKA) activity in PFC excitatory neurons reduces animal stress. These results suggest that excitatory neurons in the PFC are involved in sensing and regulating animal stress via primary ciliary cAMP/PKA signaling.

Proteomic and phosphoproteomic characterisation of primary mouse embryonic fibroblasts

Fibroblasts are the most common cell type in stroma and function in the support and repair of most tissues. Mouse embryonic fibroblasts (MEFs) are amenable to isolation and rapid growth in culture. MEFs are therefore widely used as a standard model for functional characterisation of gene knockouts, and can also be used in co-cultures, commonly to support embryonic stem cell cultures. To facilitate their use as a research tool, we have performed a comprehensive proteomic and phosphoproteomic characterisation of wild-type primary MEFs from C57BL/6 mice. EIF2/4 and MTOR signalling pathways were abundant in both the proteome and phosphoproteome, along with extracellular matrix (ECM) and cytoskeleton associated pathways. Consistent with this, kinase enrichment analysis identified activation of P38A, P90RSK, P70S6K, and MTOR. Cell surface markers and matrisome proteins were also annotated. Data are available via ProteomeXchange with identifier PXD043244. This provides a comprehensive catalogue of the wild-type MEF proteome and phosphoproteome which can be utilised by the field to guide future work.

cAMP-PKA/EPAC signaling and cancer: the interplay in tumor microenvironment

Cancer is a complex disease resulting from abnormal cell growth that is induced by a number of genetic and environmental factors. The tumor microenvironment (TME), which involves extracellular matrix, cancer-associated fibroblasts (CAF), tumor-infiltrating immune cells and angiogenesis, plays a critical role in tumor progression. Cyclic adenosine monophosphate (cAMP) is a second messenger that has pleiotropic effects on the TME. The downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), exchange protein activated by cAMP (EPAC) and ion channels. While cAMP can activate PKA or EPAC and promote cancer cell growth, it can also inhibit cell proliferation and survival in context- and cancer type-dependent manner. Tumor-associated stromal cells, such as CAF and immune cells, can release cytokines and growth factors that either stimulate or inhibit cAMP production within the TME. Recent studies have shown that targeting cAMP signaling in the TME has therapeutic benefits in cancer. Small-molecule agents that inhibit adenylate cyclase and PKA have been shown to inhibit tumor growth. In addition, cAMP-elevating agents, such as forskolin, can not only induce cancer cell death, but also directly inhibit cell proliferation in some cancer types. In this review, we summarize current understanding of cAMP signaling in cancer biology and immunology and discuss the basis for its context-dependent dual role in oncogenesis. Understanding the precise mechanisms by which cAMP and the TME interact in cancer will be critical for the development of effective therapies. Future studies aimed at investigating the cAMP-cancer axis and its regulation in the TME may provide new insights into the underlying mechanisms of tumorigenesis and lead to the development of novel therapeutic strategies.

Protein Kinase A in cellular migration-Niche signaling of a ubiquitous kinase

Cell migration requires establishment and maintenance of directional polarity, which in turn requires spatial heterogeneity in the regulation of protrusion, retraction, and adhesion. Thus, the signaling proteins that regulate these various structural processes must also be distinctly regulated in subcellular space. Protein Kinase A (PKA) is a ubiquitous serine/threonine kinase involved in innumerable cellular processes. In the context of cell migration, it has a paradoxical role in that global inhibition or activation of PKA inhibits migration. It follows, then, that the subcellular regulation of PKA is key to bringing its proper permissive and restrictive functions to the correct parts of the cell. Proper subcellular regulation of PKA controls not only when and where it is active but also specifies the targets for that activity, allowing the cell to use a single, promiscuous kinase to exert distinct functions within different subcellular niches to facilitate cell movement. In this way, understanding PKA signaling in migration is a study in context and in the elegant coordination of distinct functions of a single protein in a complex cellular process.

The VASP-profilin1 (Pfn1) interaction is critical for efficient cell migration and is regulated by cell-substrate adhesion in a PKA-dependent manner

Dynamic regulation of the actin cytoskeleton is an essential feature of cell motility. Action of Enabled (Ena)/vasodilator-stimulated phosphoprotein (VASP), a family of conserved actin-elongating proteins, is an important aspect of regulation of the actin cytoskeletal architecture at the leading edge that controls membrane protrusion and cell motility. In this study, we performed mutagenesis experiments in overexpression and knockdown-rescue settings to provide, for the first time, direct evidence of the role of the actin-binding protein profilin1 (Pfn1) in VASP-mediated regulation of cell motility. We found that VASP's interaction with Pfn1 is promoted by cell-substrate adhesion and requires down-regulation of PKA activity. Our experimental data further suggest that PKA-mediated Ser¹³⁷ phosphorylation of Pfn1 potentially negatively regulates the Pfn1-VASP interaction. Finally, Pfn1's ability to be phosphorylated on Ser¹³⁷ was partly responsible for the anti-migratory action elicited by exposing cells to a cAMP/PKA agonist. On the basis of these findings, we propose a mechanism of adhesion-protrusion coupling in cell motility that involves dynamic regulation of Pfn1 by PKA activity.

cAMP Promotes Cell Migration Through Cell Junctional Complex Dynamics and Actin Cytoskeleton Remodeling: Implications in Skin Wound Healing

Stem cells have attracted great interest for their therapeutic capacity in tissue regeneration. Cyclic adenosine 3',5'-monophosphate (cAMP), existing in high concentration at wound sites, mediated various signaling pathways such as cytoskeleton dynamics, cell adhesion, and cell migration in stem cells, which suggest the critical roles of cAMP in the wound healing process through functional regulation of stem cells. However, the mechanisms behind the effect of cAMP on mouse embryonic stem cell (mESC) motility and its roles on skin wound healing remain to be fully elucidated. In the present study, 8-Bromo cAMP-treated mESCs showed significant wound closure and improved neovascularization. Moreover, 8-Bromo cAMP stimulated mESC migration into the wound bed. 8-Bromo cAMP also increased ESC motility in in vitro migration assay. 8-Bromo cAMP induced myosin light chain phosphorylation through Rac1 and Cdc42 signaling, which were involved in 8-Bromo cAMP-induced decrease in expression of junction proteins (connexin 43, E-cadherin, and occludin) at the plasma membrane. Subsequently, 8-Bromo cAMP induced the disruption of cell junctions (including gap junctions, adherens junctions, and tight junctions), which reduced the function of the gap junctions and cell adhesion. In addition, 8-Bromo cAMP-induced Rac1 and Cdc42 activation increased Arp3, TOCA, PAK, and N-WASP expression, but decreased cofilin phosphorylation level, which elicited actin cytoskeleton remodeling. In contrast to the control, 8-Bromo cAMP evoked a substantial migration of cells into the denuded area, which was blocked by the small interfering RNAs of the signaling pathway-related molecules or by inhibitors. In conclusion, cAMP enhanced the migration of mESCs through effective coordination of junctional disruption and actin cytoskeleton remodeling, which increased the wound healing capacity of ESCs.

HOXB13 mediates tamoxifen resistance and invasiveness in human breast cancer by suppressing ERα and inducing IL-6 expression

Most breast cancers expressing the estrogen receptor α (ERα) are treated successfully with the receptor antagonist tamoxifen (TAM), but many of these tumors recur. Elevated expression of the homeodomain transcription factor HOXB13 correlates with TAM-resistance in ERα-positive (ER+) breast cancer, but little is known regarding the underlying mechanism. Our comprehensive evaluation of HOX gene expression using tiling microarrays, with validation, showed that distant metastases from TAM-resistant patients also displayed high HOXB13 expression, suggesting a role for HOXB13 in tumor dissemination and survival. Here we show that HOXB13 confers TAM resistance by directly downregulating ERα transcription and protein expression. HOXB13 elevation promoted cell proliferation in vitro and growth of tumor xenografts in vivo. Mechanistic investigations showed that HOXB13 transcriptionally upregulated interleukin (IL)-6, activating the mTOR pathway via STAT3 phosphorylation to promote cell proliferation and fibroblast recruitment. Accordingly, mTOR inhibition suppressed fibroblast recruitment and proliferation of HOXB13-expressing ER+ breast cancer cells and tumor xenografts, alone or in combination with TAM. Taken together, our results establish a function for HOXB13 in TAM resistance through direct suppression of ERα and they identify the IL-6 pathways as mediator of disease progression and recurrence.

Osteosarcoma Phenotype Is Inhibited by 3,4-Methylenedioxy-β-nitrostyrene

β-nitrostyrene compounds, such as 3,4-methylenedioxy-β-nitrostyrene (MNS), inhibit growth and induce apoptosis in tumor cells, but no reports have investigated their role in osteosarcoma. In this study, human osteosarcoma cell families with cell lines of varying tumorigenic and metastatic potential were utilized. Scrape motility assays, colony formation assays, and colony survival assays were performed with osteosarcoma cell lines, both in the presence and absence of MNS. Effects of MNS on human osteoblasts and airway epithelial cells were assessed in monolayer cultures. MNS decreased metastatic cell line motility by 72–76% and colony formation by 95–100%. MNS consistently disrupted preformed colonies in a time-dependent and dose-dependent manner. MNS had similar effects on human osteoblasts but little effect on airway epithelial cells. An inactive analog of MNS had no detectable effects, demonstrating specificity. MNS decreases motility and colony formation of osteosarcoma cells and disrupts preformed cell colonies, while producing little effect on pulmonary epithelial cells.

β2-adrenergic receptor agonists modulate human airway smooth muscle cell migration via vasodilator-stimulated phosphoprotein

Severe asthma manifests as airway remodeling and irreversible airway obstruction, in part because of the proliferation and migration of human airway smooth muscle (HASM) cells. We previously reported that cyclic adenosine monophosphate-mobilizing agents, including β(2)-adrenergic receptor (β(2)AR) agonists, which are mainstay of asthma therapy, and prostaglandin E2 (PGE2), inhibit the migration of HASM cells, although the mechanism for this migration remains unknown. Vasodilator-stimulated phosphoprotein (VASP), an anticapping protein, modulates the formation of actin stress fibers during cell motility, and is negatively regulated by protein kinase A (PKA)-specific inhibitory phosphorylation at serine 157 (Ser157). Here, we show that treatment with β(2)AR agonists and PGE2 induces the PKA-dependent phosphorylation of VASP and inhibits the migration of HASM cells. The stable expression of PKA inhibitory peptide and the small interfering (si) RNA-induced depletion of VASP abolish the inhibitory effects of albuterol and PGE2 on the migration of HASM cells. Importantly, prolonged treatment with albuterol prevents the agonist-induced phosphorylation of VASP at Ser157, and reverses the inhibitory effects of albuterol and formoterol, but not PGE2, on the basal and PDGF-induced migration of HASM cells. Collectively, our data demonstrate that β(2)AR agonists selectively inhibit the migration of HASM cells via a β(2)AR/PKA/VASP signaling pathway, and that prolonged treatment with albuterol abolishes the inhibitory effect of β-agonists on the phosphorylation of VASP and migration of HASM cells because of β(2)AR desensitization.

Cell migration: PKA and RhoA set the pace

A new study shows that protein kinase A (PKA) activity establishes a signaling loop that governs protrusion-retraction cycles in migrating cells. PKA activity near the leading edge of protrusions phosphorylates RhoA and inhibits its activity via increased association with RhoGDI.

Protein kinase A activity and anchoring are required for ovarian cancer cell migration and invasion

Epithelial ovarian cancer (EOC) is the deadliest of the gynecological malignancies, due in part to its clinically occult metastasis. Therefore, understanding the mechanisms governing EOC dissemination and invasion may provide new targets for antimetastatic therapies or new methods for detection of metastatic disease. The cAMP-dependent protein kinase (PKA) is often dysregulated in EOC. Furthermore, PKA activity and subcellular localization by A-kinase anchoring proteins (AKAPs) are important regulators of cytoskeletal dynamics and cell migration. Thus, we sought to study the role of PKA and AKAP function in both EOC cell migration and invasion. Using the plasma membrane-directed PKA biosensor, pmAKAR3, and an improved migration/invasion assay, we show that PKA is activated at the leading edge of migrating SKOV-3 EOC cells, and that inhibition of PKA activity blocks SKOV-3 cell migration. Furthermore, we show that while the PKA activity within the leading edge of these cells is mediated by anchoring of type-II regulatory PKA subunits (RII), inhibition of anchoring of either RI or RII PKA subunits blocks cell migration. Importantly, we also show--for the first time--that PKA activity is up-regulated at the leading edge of SKOV-3 cells during invasion of a three-dimensional extracellular matrix and, as seen for migration, inhibition of either PKA activity or AKAP-mediated PKA anchoring blocks matrix invasion. These data are the first to demonstrate that the invasion of extracellular matrix by cancer cells elicits activation of PKA within the invasive leading edge and that both PKA activity and anchoring are required for matrix invasion. These observations suggest a role for PKA and AKAP activity in EOC metastasis.

Protein kinase A governs a RhoA-RhoGDI protrusion-retraction pacemaker in migrating cells

The cyclical protrusion and retraction of the leading edge is a hallmark of many migrating cells involved in processes such as development, inflammation, and tumorigenesis. The molecular identity of signaling mechanisms that control these cycles has remained unknown. Here, we used live cell imaging of biosensors to monitor spontaneous morphodynamic and signaling activities, and employed correlative image analysis to examine the role of cAMP-activated Protein Kinase A (PKA) in protrusion regulation. PKA activity at the leading edge is closely synchronized with rapid protrusion and with the activity of RhoA. Ensuing PKA phosphorylation of RhoA and the resulting increased interaction between RhoA and RhoGDI establishes a negative feedback that controls the cycling of RhoA activity at the leading edge. Thus, cooperation between PKA, RhoA, and a RhoGDI forms a pacemaker that governs the morphodynamic behavior of migrating cells.

WAVE2 forms a complex with PKA and is involved in PKA enhancement of membrane protrusions

PKA contributes to many physiological processes, including glucose homeostasis and cell migration. The substrate specificity of PKA is low compared with other kinases; thus, complex formation with A-kinase-anchoring proteins is important for the localization of PKA in specific subcellular regions and the phosphorylation of specific substrates. Here, we show that PKA forms a complex with WAVE2 (Wiskott-Aldrich syndrome protein family verprolin-homologous protein 2) in MDA-MB-231 breast cancer cells and mouse brain extracts. Two separate regions of WAVE2 are involved in WAVE2-PKA complex formation. This complex localizes to the leading edge of MDA-MB-231 cells. PKA activation results in enlargement of the membrane protrusion. WAVE2 depletion impairs PKA localization at membrane protrusions and the enlargement of membrane protrusion induced by PKA activation. Together, these results suggest that WAVE2 works as an A-kinase-anchoring protein that recruits PKA at membrane protrusions and plays a role in the enlargement of membrane protrusions induced by PKA activation.

Increased cAMP levels modulate transforming growth factor-beta/Smad-induced expression of extracellular matrix components and other key fibroblast effector functions

cAMP is a key messenger of many hormones and neuropeptides, some of which modulate the composition of extracellular matrix. Treatment of human dermal fibroblasts with dibutyryl cyclic AMP and forskolin antagonized the inductive effects of transforming growth factor-beta (TGF-beta) on the expression of collagen, connective tissue growth factor, tissue inhibitor of matrix metalloproteinase-1, and plasminogen activator inhibitor type I, four prototypical TGF-beta-responsive genes. Increased intracellular cAMP prevented TGF-beta-induced Smad-specific gene transactivation, although TGF-beta-mediated Smad phosphorylation and nuclear translocation remained unaffected. However, increased cAMP levels abolished TGF-beta-induced interaction of Smad3 with its transcriptional co-activator cAMP-response element-binding protein (CREB)-binding protein (CBP)/p300. Overexpression of the transcriptional co-activator CBP/p300 rescued Smad-specific gene transcription in the presence of cAMP suggesting that sequestration of limited amounts of CBP/p300 by the activated cAMP/CREB pathway is the molecular basis of this inhibitory effect. These findings were extended by two functional assays. Increased intracellular cAMP levels suppressed the inductive activity of TGF-beta to contract mechanically unloaded collagen lattices and resulted in an attenuation of fibroblast migration of mechanically induced cell layer wounds. Of note, cAMP and TGF-beta synergistically induced hyaluronan synthase 2 (HAS2) expression and hyaluronan secretion, presumably via putative CREB-binding sites adjacent to Smad-binding sites within the HAS2 promoter. Our findings identify the cAMP pathway as a potent but differential and promoter-specific regulator of TGF-beta-mediated effects involved in extracellular matrix homeostasis.

Role of VASP phosphorylation for the regulation of microglia chemotaxis via the regulation of focal adhesion formation/maturation

Microglia activation and migration are known to play crucial roles for the response to brain injuries. Extracellular ADP was reported to induce microglia chemotaxis and membrane ruffles through P2Y12 receptor. In this study, we examined the role of VASP phosphorylation in ADP-induced microglia chemotaxis and membrane ruffle formation. ADP stimulation transiently increased intracellular cAMP level, VASP phosphorylation at Ser153, membrane ruffle formation, and chemotaxis. PKA inhibitor effectively inhibited VASP phosphorylation and chemotaxis, indicating that P2Y12-mediated activation of PKA and subsequent VASP phosphorylation are involved in the regulation of microglia chemotaxis. Forskolin and okadaic acid induced sustained VASP phosphorylation at a high level, causing a significant reduction of the retraction of membrane ruffles and chemotaxis. In forskolin- or okadaic acid-treated cells, phosphorylated VASP remained at the membrane cortex, and size and number of mature focal adhesions were not increased, indicating that prolonged phosphorylation of VASP could inhibit transformation of focal complexes into focal adhesions. VASP knockdown cells showed markedly reduced frequency and distance of membrane ruffling upon ADP stimulation, reinforcing the idea that VASP is required for the ruffle formation. Cells expressing GFP-VASP(S153A) also showed a significant reduction of protrusion distance during ruffle formation, but the frequency and the distance of retraction were not affected by FSK at all. This result suggests that dephosphorylation of VASP might be required for the growth of adhesion strength during membrane retraction. Our results suggest that VASP phosphorylation by PKA plays an important role in membrane ruffle formation and chemotaxis via the regulation of focal adhesion formation/maturation.

Cooperation of Mtmr8 with PI3K regulates actin filament modeling and muscle development in zebrafish

BACKGROUND

It has been shown that mutations in at least four myotubularin family genes (MTM1, MTMR1, 2 and 13) are causative for human neuromuscular disorders. However, the pathway and regulative mechanism remain unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we reported a new role for Mtmr8 in neuromuscular development of zebrafish. Firstly, we cloned and characterized zebrafish Mtmr8, and revealed the expression pattern predominantly in the eye field and somites during early somitogenesis. Using morpholino knockdown, then, we observed that loss-of-function of Mtmr8 led to defects in somitogenesis. Subsequently, the possible underlying mechanism and signal pathway were examined. We first checked the Akt phosphorylation, and observed an increase of Akt phosphorylation in the morphant embryos. Furthermore, we studied the PH/G domain function within Mtmr8. Although the PH/G domain deletion by itself did not result in embryonic defect, addition of PI3K inhibitor LY294002 did give a defective phenotype in the PH/G deletion morphants, indicating that the PH/G domain was essential for Mtmr8's function. Moreover, we investigated the cooperation of Mtmr8 with PI3K in actin filament modeling and muscle development, and found that both Mtmr8-MO1 and Mtmr8-MO2+LY294002 led to the disorganization of the actin cytoskeleton. In addition, we revealed a possible participation of Mtmr8 in the Hedgehog pathway, and cell transplantation experiments showed that Mtmr8 worked in a non-cell autonomous manner in actin modeling.

CONCLUSION/SIGNIFICANCE

The above data indicate that a conserved functional cooperation of Mtmr8 with PI3K regulates actin filament modeling and muscle development in zebrafish, and reveal a possible participation of Mtmr8 in the Hedgehog pathway. Therefore, this work provides a new clue to study the physiological function of MTM family members.

Distinct phosphodiesterase-4D variants integrate into protein kinase A-based signaling complexes in cardiac and vascular myocytes

Numerous cAMP-elevating agents regulate events required for efficient migration of arterial vascular smooth muscle cells (VSMCs). Interestingly, when the impact of cAMP-elevating agents on individual migration-related events is studied, these agents have been shown to have distinct, and sometimes unexpected, effects. For example, although cAMP-elevating agents inhibit overall migration, they promote VSMC adhesion to extracellular matrix proteins and the formation of membrane extensions, which are both events that are essential for and promote migration. Herein, we extend previous observations that identified phosphodiesterase-4D3 (PDE4D3) as an integral component of a PKA/A kinase-anchoring protein (AKAP) complex in cultured/hypertrophied rat cardiac myocytes to the case for nonhypertrophied cardiac myocytes. Moreover, we show that while rat aortic VSMCs also express PDE4D3, this protein is not detected in PKA/AKAP complexes isolated from these cells. In contrast, we show that another PDE4D splice variant expressed in arterial vascular myocytes, namely, PDE4D8, integrates into PKA/AKAP-based signaling complexes in VSMCs. Consistent with the idea that a PDE4D8/PKA/AKAP complex regulates specific VSMC functions, PKA and PDE4D8 were each recruited to leading-edge structures in migrating VSMCs, and inhibition of PDE4D8 recruitment to pseudopodia of migrating cells caused localized changes in actin dynamics. Our data are presented in the context that cardiac myocytes and arterial VSMCs may use distinct PDE4D variants to regulate selected pools of targeted PKA activity and that disruption of this complex may allow selective regulation of cAMP-dependent events between these two cardiovascular cell types.

Protein kinase A regulates 3-phosphatidylinositide dynamics during platelet-derived growth factor-induced membrane ruffling and chemotaxis

Spatial regulation of the cAMP-dependent protein kinase (PKA) is required for chemotaxis in fibroblasts; however, the mechanism(s) by which PKA regulates the cell migration machinery remain largely unknown. Here we report that one function of PKA during platelet-derived growth factor (PDGF)-induced chemotaxis was to promote membrane ruffling by regulating phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) dynamics. Inhibition of PKA activity dramatically altered membrane dynamics and attenuated formation of peripheral membrane ruffles in response to PDGF. PKA inhibition also significantly decreased the number and size of PIP(3)-rich membrane ruffles in response to uniform stimulation and to gradients of PDGF. This ruffling defect was quantified using a newly developed method, based on computer vision edge-detection algorithms. PKA inhibition caused a marked attenuation in the bulk accumulation of PIP(3) following PDGF stimulation, without effects on PI3-kinase (PI3K) activity. The deficits in PIP(3) dynamics correlated with a significant inhibition of growth factor-induced membrane recruitment of endogenous Akt and Rac activation in PKA-inhibited cells. Simultaneous inhibition of PKA and Rac had an additive inhibitory effect on growth factor-induced ruffling dynamics. Conversely, the expression of a constitutively active Rac allele was able to rescue the defect in membrane ruffling and restore the localization of a fluorescent PIP(3) marker to membrane ruffles in PKA-inhibited cells, even in the absence of PI3K activity. These data demonstrate that, like Rac, PKA contributes to PIP(3) and membrane dynamics independently of direct regulation of PI3K activity and suggest that modulation of PIP(3)/3-phosphatidylinositol (3-PI) lipids represents a major target for PKA in the regulation of PDGF-induced chemotactic events.

Specific tyrosine kinase inhibitors regulate human osteosarcoma cells in vitro

Inhibitors of specific tyrosine kinases are attractive lead compounds for development of targeted chemotherapies for many tumors, including osteosarcoma. We asked whether inhibition of specific tyrosine kinases would decrease the motility, colony formation, and/or invasiveness by human osteosarcoma cell lines (TE85, MNNG, 143B, SAOS-2, LM-7). An EGF-R inhibitor reduced motility of all five cell lines by 50% to 80%. In contrast, an IGF-1R inhibitor preferentially reduced motility by 42% in LM-7 cells and a met inhibitor preferentially reduced motility by 80% in MNNG cells. The inhibitors of EGF-R, IGF-1R, and met reduced colony formation by more than 80% in all tested cell lines (TE85, MNNG, 143B). The EGF-R inhibitor reduced invasiveness by 62% in 143B cells. The JAK inhibitor increased motility of SAOS-2 and LM7 cells without affecting colony formation or invasiveness. Inhibitors of HER-2, NGF-R, and PDGF-Rs did not affect motility, invasiveness, or colony formation. These results support the hypothesis that specific tyrosine kinases regulate tumorigenesis and/or metastasis in osteosarcoma.

K+ channels and the cAMP-PKA pathway modulate TGF-beta1-induced migration of rat vascular myofibroblasts

Our previous studies have indicated that TGF-beta1 exerts its effect on the expression of A-type potassium channels (I(A)) in rat vascular myofibroblasts by activation of protein kinase C during the phenotypic transformation of vascular fibroblasts to myofibroblasts. In the present study, patch-clamp whole-cell recording and transwell-migration assays were used to examine the effects of TGF-beta1- and phorbol 12-myristate 13-acetate (PMA)-induced expression of I(A) channels on myofibroblast migration and its modulation by the protein kinase A (PKA) pathway. Our results reveal that incubation of fibroblasts with TGF-beta1 or PMA up-regulates the expression of I(A) channels and increases myofibroblast migration. Blocking I(A) channel expression by 4-aminopyridine (4-AP) significantly inhibits TGF-beta1- and PMA-induced myofibroblast migration. Incubation of fibroblasts with forskolin does not result in increased expression of I(A) channels but does cause a slight increase in fibroblast migration at higher concentrations. In addition, forskolin increases the TGF-beta1- and PMA-induced myofibroblast migration but inhibits TGF-beta1- and PMA-induced the expression of I(A) channels. Whole-cell current recordings showed that forskolin augments the delayed rectifier outward K(+) (I(K)) current amplitude of fibroblasts, but not the I(A) of myofibroblasts. Our results also indicate that TGF-beta1- and PMA-induced expression of I(A) channels might be related to increase TGF-beta1- or PMA-induced myofibroblast migration. Promoting fibroblast and myofibroblast migration via the PKA pathway does not seem to involve the expression of I(A) channels, but the modulation of I(K) and I(A) channels might be implicated.

PKA and Epac1 regulate endothelial integrity and migration through parallel and independent pathways

The vascular endothelium provides a semi-permeable barrier, which restricts the passage of fluid, macromolecules and cells to the surrounding tissues. Cyclic AMP promotes endothelial barrier function and protects the endothelium against pro-inflammatory mediators. This study analyzed the relative contribution of two cAMP targets, PKA and Epac1, to the control of endothelial barrier function and endothelial cell migration. Real-time recording of transendothelial electrical resistance showed that activation of either PKA or Epac1 with specific cAMP analogues increases endothelial barrier function and promotes endothelial cell migration. In addition, reduction of Epac1 expression showed that Epac1 and PKA control endothelial integrity and cell motility by two independent and complementary signaling pathways. We demonstrate that integrin-mediated adhesion is required for PKA, but not Epac1-Rap1-driven stimulation of endothelial barrier function. In contrast, both PKA- and Epac1-stimulated endothelial cell migration requires integrin function. These data show that activation of Epac1 and PKA by cAMP results in the stimulation of two parallel, independent signaling pathways that positively regulate endothelial integrity and cell migration, which is important for recovery after endothelial damage and for restoration of compromised endothelial barrier function.

cAMP-induced Epac-Rap activation inhibits epithelial cell migration by modulating focal adhesion and leading edge dynamics

Epithelial cell migration is a complex process crucial for embryonic development, wound healing and tumor metastasis. It depends on alterations in cell-cell adhesion and integrin-extracellular matrix interactions and on actomyosin-driven, polarized leading edge protrusion. The small GTPase Rap is a known regulator of integrins and cadherins that has also been implicated in the regulation of actin and myosin, but a direct role in cell migration has not been investigated. Here, we report that activation of endogenous Rap by cAMP results in an inhibition of HGF- and TGFbeta-induced epithelial cell migration in several model systems, irrespective of the presence of E-cadherin adhesion. We show that Rap activation slows the dynamics of focal adhesions and inhibits polarized membrane protrusion. Importantly, forced integrin activation by antibodies does not mimic these effects of Rap on cell motility, even though it does mimic Rap effects in short-term cell adhesion assays. From these results, we conclude that Rap inhibits epithelial cell migration, by modulating focal adhesion dynamics and leading edge activity. This extends beyond the effect of integrin affinity modulation and argues for an additional function of Rap in controlling the migration machinery of epithelial cells.

Loss-of-function screening by randomized intracellular antibodies: identification of hnRNP-K as a potential target for metastasis

We have developed a loss-of-function screening system on the basis of intracellular expression of single domain antibodies. We demonstrate its use in identification of potential targets of metastasis of human cancerous cells. Randomized intracellular antibodies were expressed in highly metastatic cells, and a derivative pool of cells with loss of migration phenotype in chemotaxis assay was isolated. Isolation of antibodies from cells with loss of migration phenotype and identification of their target proteins revealed the involvement of the heterogeneous nuclear ribonucleoprotein K (hnRNP-K), a multifunctional signaling protein, in metastasis. Furthermore, we found that the cytoplasmic accumulation of hnRNP-K is crucial for its role in metastasis. The results demonstrate (i) the advantages of our functional interference screening over the gene-knockouts and gene-silencing, (ii) hnRNP-K as a potential target of metastasis, and (iii) a potential anti-metastasis peptide validated in in vitro cell migration assays.

Dopaminergic modulation of striatal plateau depolarizations in corticostriatal organotypic cocultures

RATIONALE

It has been proposed that dopamine (DA) sustains up states in striatal medium spiny neurons (MSN). Testing this hypothesis requires an in vitro preparation, but up states are typically only observed in vivo.

OBJECTIVES

In this study, we used corticostriatal organotypic cocultures, a preparation in which up states have been previously observed, to test the DA control of cortically-driven plateau depolarizations.

RESULTS

After 7-21 days in vitro in serum-free conditions, plateau depolarizations resembling up states were only observed in cultures with a critical extent of striatal DA innervation. These plateaus were completely blocked by the non-NMDA antagonist CNQX and significantly shortened by the NMDA antagonist APV or the D(1) antagonist SCH23390. Intracellular interruption of Ca(++) or protein-kinase A (PKA) signaling also eliminated the plateaus. The D(2) antagonist eticlopride failed to disrupt the plateaus, but significantly increased MSN excitability.

CONCLUSIONS

These results suggest that coincident activation of corticostriatal glutamatergic and mesostriatal DA transmission may set ensembles of MSN into prolonged depolarizations through a D(1) enhancement of striatal NMDA function in a Ca(++) and PKA-dependent manner.

Prostaglandin E2 differentially modulates human fetal and adult dermal fibroblast migration and contraction: implication for wound healing

Cyclooxygenase-2 is up-regulated shortly after dermal injury and it has been shown to have important activity during the repair process. Its main product in the skin, prostaglandin E2 (PGE2), modulates both inflammatory and fibrotic processes during wound healing and partially dictates the overall outcome of wound healing. PGE2 signaling has been shown to be altered during fetal wound healing. This study was designed to examine the mechanism(s) by which PGE2 regulates fibroblast migration and contraction and to determine whether these mechanisms are conserved in fetal-derived dermal fibroblasts. Fetal and adult dermal fibroblasts express all four PGE2 receptors. PGE2 inhibits fetal and adult fibroblast migration in a dose-dependent manner through the EP2/EP4-cAMP-protein kinase A pathway. However, fetal fibroblasts appear to be refractory to this effect, requiring a 10-fold higher concentration of PGE2 to achieve a similar degree of inhibition as adult fibroblasts. Inhibition of adult fibroblast migration correlated with disruption of the actin cytoskeleton. In contrast, PGE2 or a cAMP analog did not disrupt the actin cytoskeleton of fetal dermal fibroblasts. These findings were extended using a modified free-floating, fibroblast-populated collagen lattice (FPCL) contraction assay designed to measure fibroblast contraction. PGE2-inhibited FPCL contraction by adult fibroblasts, but fetal fibroblasts exhibited higher rates of FPCL contraction and a blunted response to exogenous modulation by PGE2 or a cyclase activator (forskolin). These findings indicate that fetal dermal fibroblasts are partially refractory to the effects of PGE2, a major inflammatory mediator associated with dermal wound healing. This effect may have significant and specific relevance to the scarless fetal wound-healing phenotype.

Ghrelin inhibits angiotensin II-induced migration of human aortic endothelial cells

Ghrelin, the endogenous ligand for the GH secretagogue receptor, is produced by the oxyntic cells of the stomach and is involved in the regulation of energy balance. However, an increasing number of direct ghrelin cardiovascular effects, and, among them, high ghrelin binding in atherosclerotic coronary arteries, are being reported. We investigated whether ghrelin affects migration of human aorta endothelial cells (HAEC). HAEC bound ghrelin in specific, saturable manner. Ghrelin, as such, did not affect HAEC migration, however it inhibited the angiotensin II-induced migration, and this effect was inhibited by the antagonist (D-Lys(3))-GHRP-6. In HAEC, ghrelin elicited increased intracellular concentration of cAMP that was involved in its effect on AngII-induced HAEC migration, as the AMP cyclase inhibitor SQ22.536 and PKA inhibitor KT5720, respectively, inhibited and blunted it. These findings suggest a role of ghrelin in the control of endothelial cell migration and its possible involvement in vascular changes present in disorders characterized by low plasma ghrelin.

CUTL1 is phosphorylated by protein kinase A, modulating its effects on cell proliferation and motility

CUTL1, also known as CDP (CCAAT Displacement Protein), Cut, or Cux-1, is a homeodomain transcription factor known to play an essential role in development and cell cycle progression. Previously, we identified CUTL1 as modulator of cell motility and invasiveness. Here we report that protein kinase A (PKA), known to inhibit tumor progression in various tumor types, directly phosphorylates CUTL1 at serine 1215 in NIH3T3 fibroblasts. The PKA-induced phosphorylation results in decreased DNA binding affinity of CUTL1 and diminished CUTL1-mediated cell cycle progression and cell motility. Furthermore, the expression of several CUTL1 target genes involved in proliferation and migration, such as DNA polymerase A and DKK2, was modulated by PKA-induced phosphorylation. These data identify CUTL1 as a novel target of PKA through which this protein kinase can modulate tumor cell motility and tumor progression.

Involvement of beta1 integrin in microglial chemotaxis and proliferation on fibronectin: different regulations by ADP through PKA

Microglia are immune cells in the brain; their activation, migration, and proliferation have pivotal roles in brain injuries and diseases. Microglia are known to attach firmly to fibronectin, the upregulation of which is associated with several pathological conditions in the CNS, through beta1 integrin and become activated. Extracellular nucleotides can serve as potent signaling molecules. Recently, ATP and ADP were revealed to possess chemoattractive properties to microglia via Gi-coupled P2Y receptors. In the present study, we report that the ADP-induced chemotaxis of microglia is mediated by P2Y12/13 receptors and is beta1 integrin-dependent in the presence of fibronectin. Signals from P2Y12/13 receptors also cause beta1 integrin translocation to the membrane ruffle regions, but this redistribution was lost when the intracellular cyclic AMP (cAMP) was increased by forskolin or dibutyryl cAMP. This inhibitory effect of cAMP-elevating agents did not appear when microglia were co-incubated with a protein kinase A (PKA) inhibitor, KT-5720, suggesting that PKA is a negative regulator of the beta1 integrin translocation. We also show that the engagement of beta1 integrin enhanced microglial proliferation. Signals from P2Y12/13 receptors attenuated the proliferation, whereas ADP itself had no effect on microglial growth. Furthermore, beta1 integrin-induced proliferation is positively regulated by the cAMP-dependent PKA. Together, these results indicate the involvement of beta1 integrin in microglial proliferation and chemotaxis, both of which have clinical importance. The data also suggest that PKA is inversely involved in these two cellular functions.

The beta-agonist isoproterenol attenuates EGF-stimulated wound closure in human airway epithelial cells

Asthma is a disease characterized by reversible airway obstruction. An additional hallmark of chronic asthma is altered wound healing that leads to airway remodeling. Although beta-agonists are effective in treating the bronchospasm associated with asthma, their effects on airway wound healing, which are related to airway remodeling, are unknown. It has been demonstrated that beta-agonists can alter the signaling of epidermal growth factor (EGF) receptors, which are important in timely wound healing. Therefore, we hypothesized that the beta-agonist isoproterenol would affect wound healing. Using an in vitro scrape wound assay, we demonstrated that isoproterenol attenuates EGF-stimulated wound healing in 16HBE airway epithelial cell cultures. Through experiments with forskolin and cells overexpressing beta2-adrenergic receptor-yellow fluorescent protein, we show that attenuation is due to the accumulation of cAMP and the involvement of at least one additional pathway. Furthermore, attenuation is not due to a direct effect on the EGF receptor or to an alteration of the ERK/MAPK signaling cascade. Based on these results, we propose that isoproterenol may exert its effects through other MAPK signaling pathways (JNK and/or p38) or through parallel mechanisms. These results also demonstrate a problem of potential therapeutic relevance in which a commonly prescribed medication may alter wound healing and contribute to the remodeling of asthmatic airways.

Spatial regulation of the cAMP-dependent protein kinase during chemotactic cell migration

Historically, the cAMP-dependent protein kinase (PKA) has a paradoxical role in cell motility, having been shown to both facilitate and inhibit actin cytoskeletal dynamics and cell migration. In an effort to understand this dichotomy, we show here that PKA is regulated in subcellular space during cell migration. Immunofluorescence microscopy and biochemical enrichment of pseudopodia showed that type II regulatory subunits of PKA and PKA activity are enriched in protrusive cellular structures formed during chemotaxis. This enrichment correlates with increased phosphorylation of key cytoskeletal substrates for PKA, including the vasodilator-stimulated phosphoprotein (VASP) and the protein tyrosine phosphatase containing a PEST motif. Importantly, inhibition of PKA activity or its ability to interact with A kinase anchoring proteins inhibited the activity of the Rac GTPase within pseudopodia. This effect correlated with both decreased guanine nucleotide exchange factor activity and increased GTPase activating protein activity. Finally, inhibition of PKA anchoring, like inhibition of total PKA activity, inhibited pseudopod formation and chemotactic cell migration. These data demonstrate that spatial regulation of PKA via anchoring is an important facet of normal chemotactic cell movement.

Controlling cell behavior electrically: current views and future potential

Direct-current (DC) electric fields are present in all developing and regenerating animal tissues, yet their existence and potential impact on tissue repair and development are largely ignored. This is primarily due to ignorance of the phenomenon by most researchers, some technically poor early studies of the effects of applied fields on cells, and widespread misunderstanding of the fundamental concepts that underlie bioelectricity. This review aims to resolve these issues by describing: 1) the historical context of bioelectricity, 2) the fundamental principles of physics and physiology responsible for DC electric fields within cells and tissues, 3) the cellular mechanisms for the effects of small electric fields on cell behavior, and 4) the clinical potential for electric field treatment of damaged tissues such as epithelia and the nervous system.

Trapidil inhibits platelet-derived growth factor-induced migration via protein kinase A and RhoA/Rho-associated kinase in rat vascular smooth muscle cells

Trapidil suppresses platelet-derived growth factor (PDGF)-induced vascular smooth muscle cell (VSMC) proliferation by inhibiting Raf-1/extracellular signal-regulated kinase (ERK) via cAMP/protein kinase A (PKA). We examined whether trapidil inhibits PDGF-induced VSMC migration and investigated its mechanisms of action. VSMC migration was inhibited to a similar extent by trapidil and forskolin. A PKA inhibitor N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide (H89) blocked the inhibition by forskolin to a greater degree than that by trapidil. Trapidil but not forskolin suppressed PDGF-stimulated RhoA activation. In the presence of both H89 and (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl)cyclohexanecarboxamide dihydrochloride monohydrate, an inhibitor of Rho-associated kinase (ROCK), trapidil and forskolin inhibited migration to a similar extent. Thus, in addition to cAMP/PKA activation, trapidil inhibits RhoA/ROCK activation, which may be important in trapidil's inhibitory effect on migration.

Regulation of actin-based cell migration by cAMP/PKA

A wide variety of soluble signaling substances utilize the cyclic AMP-dependent protein kinase (PKA) pathway to regulate cellular behaviors including intermediary metabolism, ion channel conductivity, and transcription. A growing literature suggests that integrin-mediated cell adhesion may also utilize PKA to modulate adhesion-associated events such as actin cytoskeletal dynamics and migration. PKA is dynamically regulated by integrin-mediated cell adhesion to extracellular matrix (ECM). Furthermore, while some hallmarks of cell migration and cytoskeletal organization require PKA activity (e.g. activation of Rac and Cdc42; actin filament assembly), others are inhibited by it (e.g. activation of Rho and PAK; interaction of VASP with the c-Abl tyrosine kinase). Also, cell migration and invasion can be impeded by either inhibition or hyper-activation of PKA. Finally, a number of A-kinase anchoring proteins (AKAPs) serve to associate PKA with various components of the actin cytoskeleton, thereby enhancing and/or specifying cAMP/PKA signaling in those regions. This review discusses the growing literature that supports the hypothesis that PKA plays a central role in cytoskeletal regulation and cell migration.

Biphasic role of TGF-beta1 in signal transduction and crosstalk

TGF-beta1 induces cell cycle activation in mouse embryonic fibroblasts by down regulation of p27(Kip1) but it can also induce delay of EGF-induced cell cycle activation in these cells under similar conditions. In an attempt to determine the basis for these responses, the study of early TGF-beta1-induced signal transduction pathways in the presence and absence of EGF was undertaken. It is proposed that a likely target for the inhibition by TGF-beta1 of the early EGF-induced p42/p44 MAPK is at the c-Raf locus. The finding that the catalytic subunits of PKA are associated with Raf-1 within minutes following application of TGF-beta1 but not EGF in fibroblasts arrested in early G1 is suggestive of a role of PKA-Raf-1 interaction in TGF-beta1 induced delay of EGF-induced cell cycle kinetics. A model for TGF-beta1 induced translocation to the plasma membrane-associated Raf-1 is proposed. Reports that Rho-like GTPase activity is critical for the activation of TGF-beta1 downstream pathways raises the question as to whether Rho proteins are involved in these observed TGF-beta1-induced responses. Post-receptor signaling mechanisms for TGF-beta1 and cross-talk with PKA-mediated pathways are examined in an effort to explain the modulation by TGF-beta1 of mitogen-induced cell proliferation in mesenchymal cells.

p38 and ERK1/2 coordinate cellular migration and proliferation in epithelial wound healing: evidence of cross-talk activation between MAP kinase cascades

One important action of growth factors is their participation in tissue repair; however, the signaling pathways involved are poorly understood. In a model of corneal wound healing, we found that two paracrine growth factors, hepatocyte growth factor (HGF) and keratinocyte growth factor (KGF), induced rapid and marked activation and prompt nuclear accumulation of phospho-p38 (p-p38) and -ERK1/2 (p-ERK1/2), but not of JNK (p-JNK1/2), in corneal epithelial cells. Interruption of p38 and ERK1/2 signaling pathways by pretreatment with inhibitors SB203580 and PD98059 and subsequent stimulation with HGF or KGF abolished the activation and nuclear localization. Inhibition of either one of these mitogen-activated protein kinases, p38 or ERK1/2, induced a robust cross-activation of the other. In immunofluorescence studies of wounded cornea, p-p38, unlike p-ERK1/2, was immediately detectable in epithelium after injury. Inhibition of p38 by SB203580 blocked migration of epithelial cells almost completely. In contrast, PD98059 seemed to slightly increase the migration, through concomitant activation of p38. Unlike ERK1/2, p38 did not significantly contribute to proliferation of epithelial cells. Inhibition of either the ERK1/2 or p38 pathway resulted in delayed corneal epithelial wound healing. Interruption of both signaling cascades additively inhibited the wound-healing process. These findings demonstrate that both p38 and ERK1/2 coordinate the dynamics of wound healing: while growth factor-stimulated p38 induces epithelial migration, ERK1/2 activation induces proliferation. The cross-talk between these two signal cascades and the selective action of p38 in migration appear to be important to corneal wound healing, and possibly wound healing in general, and may offer novel drug targets for tissue repair.

Identification of genes involved in cell invasion by using a library of randomized hybrid ribozymes

Libraries of RNA helicase-coupled randomized ribozymes are a powerful tool for the identification of functional genes. We have demonstrated the usefulness of this functional gene-discovery system by identifying genes involved in tumor invasion, a process that is an essential feature of tumor metastasis: the spread of cancer cells from the original tumor to other sites in the body that imposes serious problems in the prognosis and treatment of cancer. Using a filter-based invasion assay in vitro, we isolated ribozymes that enhanced the invasive properties of NIH 3T3 fibroblasts. Sequence analysis of selected clones and a database search revealed that genes such as the gene for Gem GTPase and uncharacterized genes that resemble genes for myosin phosphatase and protein-tyrosine-phosphatase are involved in cell invasion. Our system for gene identification by using ribozymes and the functional analysis of target genes should help to clarify the complex mechanisms of invasion and metastasis and might provide information that is relevant to cancer therapy.

Regulation of vasodilator-stimulated phosphoprotein phosphorylation and interaction with Abl by protein kinase A and cell adhesion

Members of the vasodilator-stimulated phosphoprotein (VASP) family are important regulators of actin cytoskeletal dynamics whose functions and protein-protein interactions are regulated by phosphorylation by the cAMP-dependent protein kinase (PKA). Herein, we show that phosphorylation of VASP is dynamically regulated by cellular adhesion to extracellular matrix. Detachment of cells stimulated PKA activity and induced PKA-dependent phosphorylation of VASP and the related murine-Enabled (Mena) protein. VASP and Mena were rapidly dephosphorylated immediately following reattachment but showed an intermediate level of phosphorylation during active cell spreading. This pattern correlated closely with adhesion-dependent changes in PKA activity. The in vivo interaction of VASP with the Abl tyrosine kinase, shown here for the first time, was readily apparent in adherent cells, lost following cellular detachment, and induced upon reattachment to matrix. Importantly, inhibition of PKA activity prevented phosphorylation of VASP and dissociation of VASP-Abl complexes after cellular detachment, whereas activation of PKA completely eliminated the co-immunoprecipitation of Abl activity with VASP. These data establish a new biochemical link between cell adhesion and regulation of VASP proteins and provide the first demonstration of a regulated interaction between VASP and Abl in mammalian cells.

Prostacyclin analogs inhibit fibroblast migration

The controlled accumulation of fibroblasts to sites of inflammation is crucial to effective tissue repair after injury. Either inadequate or excessive accumulation of fibroblasts could result in abnormal tissue function. Prostacyclin (PGI(2)) is a potent mediator in the coagulation and inflammatory processes. The aim of this study was to investigate the effect of PGI(2) on chemotaxis of human fetal lung fibroblasts (HFL-1). Using the blind well chamber technique, we found that the PGI(2) analog carbaprostacyclin (10(-6) M) inhibited HFL-1 chemotaxis to human plasma fibronectin (20 microg/ml) 58.0 +/- 13.2% (P < 0.05) and to platelet-derived growth factor (PDGF)-BB (10 ng/ml) 48.7 +/- 4.6% (P < 0.05). Checkerboard analysis demonstrated that carbaprostacyclin inhibits both directed and undirected migration. The inhibitory effect of the carbaprostacyclin was concentration dependent and blocked by the cAMP-dependent protein kinase (PKA) inhibitor KT-5720, suggesting that a cAMP-PKA pathway may be involved in the process. Two other PGI(2) analogs, ciprostene and dehydro-15-cyclohexyl carbaprostacyclin (both 10(-6) M), significantly inhibited fibroblast migration to fibronectin. In summary, PGI(2) appears to inhibit fibroblast chemotaxis to fibronectin and PDGF-BB. Such an effect may contribute to the regulation of fibroblasts in wound healing and could contribute to the pathogenesis of diseases characterized by abnormal tissue repair remodeling.