Activation of protein kinase D by signaling through Rho and the alpha subunit of the heterotrimeric G protein G13

Derepression of pathological cardiac genes by members of the CaM kinase superfamily

In response to pathologic stresses such as hypertension or myocardial infarction, the heart undergoes a remodeling process that is characterized by myocyte hypertrophy, myocyte death and fibrosis, resulting in impaired cardiac function and heart failure. Cardiac remodeling is associated with derepression of genes that contribute to disease progression. This review focuses on evidence linking members of the Ca(2+)/calmodulin-dependent protein kinase (CaMK) superfamily, specifically CaMKII, protein kinase D (PKD) and microtubule associated kinase (MARK), to stress-induced derepression of pathological cardiac gene expression through their effects on class IIa histone deacetylases (HDACs).

Differential requirement for RhoA GTPase depending on the cellular localization of protein kinase D

This study explores the links between the GTPase RhoA and the serine kinase protein kinase D (PKD) during thymocyte development. The rationale is that RhoA and PKD regulate common biological responses during T cell development, but there is nothing known about their interdependence. In fibroblasts, Rho function is required for activation of PKD catalytic activity. However, the data show that activation of Rho is neither sufficient nor essential for PKD activation in T cells. One alternative explanation for the apparent convergence of PKD and Rho signaling in T cells is that PKD responses might be Rho-dependent. To address this latter possibility, we probed the Rho requirements for the actions of constitutively active PKD mutants in pre-T cells of transgenic mice. Active PKD can localize to either the plasma membrane or the cytosol, and we therefore compared the Rho requirements for the actions of membrane- or cytosol-localized PKD. Here we show that membrane-localized PKD regulation of pre-T cell differentiation is Rho-dependent, but the actions of cytosol-localized PKD are not. These studies demonstrate that a Rho requirement for PKD activation is not ubiquitous. Moreover, links between PKD and Rho are determined by the cellular location of PKD.

Regulation of cardiac stress signaling by protein kinase d1

In response to pathological stresses such as hypertension or myocardial infarction, the heart undergoes a remodeling process that is associated with myocyte hypertrophy, myocyte death, and fibrosis. Histone deacetylase 5 (HDAC5) is a transcriptional repressor of cardiac remodeling that is subject to phosphorylation-dependent neutralization in response to stress signaling. Recent studies have suggested a role for protein kinase C (PKC) and its downstream effector, protein kinase D1 (PKD1), in the control of HDAC5 phosphorylation. While PKCs are well-documented regulators of cardiac signaling, the function of PKD1 in heart muscle remains unclear. Here, we demonstrate that PKD1 catalytic activity is stimulated in cardiac myocytes by diverse hypertrophic agonists that signal through G protein-coupled receptors (GPCRs) and Rho GTPases. PKD1 activation in cardiomyocytes occurs through PKC-dependent and -independent mechanisms. In vivo, cardiac PKD1 is activated in multiple rodent models of pathological cardiac remodeling. PKD1 activation correlates with phosphorylation-dependent nuclear export of HDAC5, and reduction of endogenous PKD1 expression with small interfering RNA suppresses HDAC5 shuttling and associated cardiomyocyte growth. Conversely, ectopic overexpression of constitutively active PKD1 in mouse heart leads to dilated cardiomyopathy. These findings support a role for PKD1 in the control of pathological remodeling of the heart via its ability to phosphorylate and neutralize HDAC5.

Neuropeptide-stimulated cell migration in prostate cancer cells is mediated by RhoA kinase signaling and inhibited by neutral endopeptidase

The neuropeptides bombesin and endothelin-1 stimulate prostate cancer (PC) cell migration and invasion (J Clin Invest, 2000; 106: 1399-1407). The intracellular signaling pathways that direct this cell movement are not well delineated. The monomeric GTPase RhoA is required for migration in several cell types including neutrophils, monocytes and fibroblasts. We demonstrate that bombesin-stimulated PC cell migration occurs via the heterotrimeric G-protein-coupled receptors (G-protein) G alpha 13 subunit leading to activation of RhoA, and Rho-associated coiled-coil forming protein kinase (ROCK). Using siRNA to suppress expression of the three known G-protein alpha-subunit-associated RhoA guanine nucleotide exchange factors (GEFs), we also show that two of these RhoA GEFs, PDZ-RhoGEF and leukemia-associated RhoGEF (LARG), link bombesin receptors to RhoA in a non-redundant manner in PC cells. We next show that focal adhesion kinase, which activates PDZ-RhoGEF and LARG, is required for bombesin-stimulated RhoA activation. Neutral endopeptidase (NEP) is expressed on normal prostate epithelium whereas loss of NEP expression contributes to PC progression. We also demonstrate that NEP inhibits neuropeptide activation of RhoA. Together, these results establish a contiguous signaling pathway from the bombesin receptor to ROCK in PC cells, and they implicate NEP as a major regulator of neuropeptide-stimulated RhoA in these cells. This work also identifies members of this signaling pathway as potential targets for rational pharmacologic manipulation of neuropeptide-stimulated migration of PC cells.

Anchored cAMP signaling: onward and upward - a short history of compartmentalized cAMP signal transduction

Intracellular signal transduction pathways require a high degree of spatial and temporal resolution in order to deliver the appropriate outputs. Specific signaling mediated by the ubiquitous second messenger cAMP and its effector, the cAMP-dependent protein kinase (PKA), is governed by the spatial organization of different pathway components by A-kinase anchoring proteins (AKAPs). This review discusses the history and future of anchored cAMP signaling pathways.

Protein kinase D protects against oxidative stress-induced intestinal epithelial cell injury via Rho/ROK/PKC-delta pathway activation

Protein kinase D (PKD) is a novel protein serine kinase that has recently been implicated in diverse cellular functions, including apoptosis and cell proliferation. The purpose of our present study was 1) to define the activation of PKD in intestinal epithelial cells treated with H2O2, an agent that induces oxidative stress, and 2) to delineate the upstream signaling mechanisms mediating the activation of PKD. We found that the activation of PKD is induced by H2O2 in both a dose- and time-dependent fashion. PKD phosphorylation was attenuated by rottlerin, a selective PKC-delta inhibitor, and by small interfering RNA (siRNA) directed against PKC-delta, suggesting the regulation of PKD activity by upstream PKC-delta. Activation of PKD was also blocked by a Rho kinase (ROK)-specific inhibitor, Y-27632, as well as by C3, a Rho protein inhibitor, demonstrating that the Rho/ROK pathway also mediates PKD activity in intestinal cells. In addition, H2O2-induced PKC-delta phosphorylation was inhibited by C3 treatment, further suggesting that PKC-delta is downstream of Rho/ROK. Interestingly, H2O2-induced intestinal cell apoptosis was enhanced by PKD siRNA. Together, these results clearly demonstrate that oxidative stress induces PKD activation in intestinal epithelial cells and that this activation is regulated by upstream PKC-delta and Rho/ROK pathways. Importantly, our findings suggest that PKD activation protects intestinal epithelial cells from oxidative stress-induced apoptosis. These findings have potential clinical implications for intestinal injury associated with oxidative stress (e.g., necrotizing enterocolitis in infants).

The G12Family of G Proteins as a Reporter of Thromboxane A2Receptor Activity

Despite advances in the understanding of pathways regulated by the G12 family of heterotrimeric G proteins, much regarding the engagement of this family by receptors remains unclear. We explore here, using the thromboxane A2 receptor TPalpha, the ability of G12 and G13 to report differences in the potency and efficacy of receptor ligands. We were interested especially in the potential of the isoprostane 8-iso-prostaglandin F (8-iso-PGF2alpha), among other ligands examined, to activate G12 and G13 through TPalpha explicitly. We were also interested in the functionality of TPalpha-Galpha fusion proteins germane to G12 and G13. Using fusion proteins in Spodoptera frugiperda (Sf9) cells and independently expressed proteins in human embryonic kidney 293 cells, and using guanosine 5'-O-(3-[35S]thio)triphosphate binding to evaluate Galpha activation directly, we found for Galpha that no ligand tested, including 8-iso-prostaglandin F (8-iso-PGF2alpha and a purported antagonist (pinane thromboxane A2), was silent. The activity of agonists was especially pronounced when evaluated for TPalpha-Galpha13 and in the context of receptor reserve. Agonist activity for 8-iso-PGF2 was diminished and that for pinane thromboxane A nonexistent when Galpha12 was the reporter. These data establish that G12 and G13 can report differentially potency and efficacy and underscore the relevance of receptor and G protein context.

Activation of protein kinase D3 by signaling through Rac and the alpha subunits of the heterotrimeric G proteins G12 and G13

PKD is the founding member of a novel protein kinase family that also includes PKD2 and PKD3. PKD has been the focus of most studies up to date, but little is known about the mechanisms that mediate PKD3 activation. Here, we show that addition of aluminum fluoride to COS-7 cells cotransfected with PKD3 and Galpha13 or Galpha12 induced PKD3 activation, which was associated with a transient plasma membrane translocation of cytosolic PKD3. Treatment with Clostridium difficile toxin B blocked PKD3 activation induced by either bombesin or by aluminum fluoride-stimulated Galpha12/13 but did not affect Galphaq-induced PKD3 activation. Furthermore, PKD3 immunoprecipitated from cells cotransfected with a constitutively active Rac (RacV12) exhibited a marked increase in PKD3 basal catalytic activity. In contrast, cotransfection with active Rho (RhoQ63L), Cdc42 (Cdc42Q61L), or Ras (RasV12) did not promote PKD3 activation. Expression of either COOH-terminal dominant-negative fragment of Galpha13 or dominant negative Rac (Rac N17) attenuated bombesin-induced PKD3 activation. Treatment with protein kinase C (PKC) inhibitors prevented the increase in PKD3 activity induced by RacV12 and aluminum fluoride-stimulated Galpha12/13. The catalytic activation of PKD3 in response to RacV12, alpha12/13 signaling or bombesin correlated with Ser-731/Ser-735 phosphorylation in the activation loop of this enzyme. Our results indicate that Galpha12/13 and Rac are important components in the signal transduction pathways that mediate bombesin receptor-induced PKD3 activation.

Protein kinase D3 activation and phosphorylation by signaling through Gαq

PKD is the founding member of a novel protein kinase family that also includes PKD2 and PKD3. PKD has been the focus of most studies up to date, but little is known about the mechanisms that mediate PKD3 activation. Here, we demonstrate that PKD3 immunoprecipitated from COS-7 cells transfected with a constitutively active G alpha q subunit (alpha(q)Q209L) exhibited a marked increase in basal activity. Addition of aluminum fluoride to cells co-transfected with PKD3 and wild type G alpha(q) also induced PKD3 activation. G alpha(q)-mediated PKD3 activation is associated with persistent translocation of PKD3 from both cytosol and nucleus to plasma membrane. Expression of a COOH-terminal fragment of G alpha q that acts in a dominant-negative fashion attenuated PKD3 activation in response to bombesin receptor stimulation. Our results indicate that G alpha q activation is sufficient to stimulate sustained PKD3 activation and show that the endogenous G alpha q is a major component in the signaling pathway that mediates bombesin-induced PKD3 activation.

Receptors coupled to heterotrimeric G proteins of the G12 family

Much regarding the engagement of the G(12) family of heterotrimeric G proteins (G(12) and G(13)) by agonist-activated receptors remains unclear. For example, the identity of receptors that couple unequivocally to G(12) and G(13) and how signals are allocated among these and other G proteins remain open questions. Part of the problem in understanding signaling through G(12) and G(13) is that the activation of these G proteins is rarely demonstrated directly and is instead presumed usually from far removed downstream events. Furthermore, receptors that couple to G(12) and G(13) invariably couple to additional G proteins, and thus few events can be linked unambiguously to one G protein or another. In this article, we document receptors that reportedly couple to G(12), G(13) or both G(12) and G(13), evaluate the methodology used to understand the coupling of these receptors, and discuss the ability of these receptors to couple also to G(q).

Amino acid-stimulated Ca2+ oscillations produced by the Ca2+-sensing receptor are mediated by a phospholipase C/inositol 1,4,5-trisphosphate-independent pathway that requires G12, Rho, filamin-A, and the actin cytoskeleton

The G protein-coupled Ca(2+)-sensing receptor (CaR) is an allosteric protein that responds to two different agonists, Ca(2+) and aromatic amino acids, with the production of sinusoidal or transient oscillations in intracellular Ca(2+) concentration ([Ca(2+)](i)). Here, we examined whether these differing patterns of [Ca(2+)](i) oscillations produced by the CaR are mediated by separate signal transduction pathways. Using real time imaging of changes in phosphatidylinositol 4,5-biphosphate hydrolysis and generation of inositol 1,4,5-trisphosphate in single cells, we found that stimulation of CaR by an increase in the extracellular Ca(2+) concentration ([Ca(2+)](o)) leads to periodic synthesis of inositol 1,4,5-trisphosphate, whereas l-phenylalanine stimulation of the CaR does not induce any detectable change in the level this second messenger. Furthermore, we identified a novel pathway that mediates transient [Ca(2+)](i) oscillations produced by the CaR in response to l-phenylalanine, which requires the organization of the actin cytoskeleton and involves the small GTPase Rho, heterotrimeric proteins of the G(12) subfamily, the C-terminal region of the CaR, and the scaffolding protein filamin-A. Our model envisages that Ca(2+) or amino acids stabilize unique CaR conformations that favor coupling to different G proteins and subsequent activation of distinct downstream signaling pathways.

Small GTP-binding protein Rho-mediated signaling promotes proliferation of rheumatoid synovial fibroblasts

Rho is a major small GTP-binding protein that is involved in the regulation of various cell functions, including proliferation and cell migration, through activation of multiple signaling molecules in various types of cells. We studied its roles in synovial fibroblasts (SFs) in patients with rheumatoid arthritis (RA) and clarified its relevance to RA synovitis, with the following results. 1)We found that the thrombin receptor was overexpressed on RA synovial fibroblasts (RA SFs) and that thrombin induced a marked proliferation and progression of the cell cycle to the S phase in these cells. 2)We also found that thrombin efficiently activated Rho. 3)Rho activation and proliferation and the progression of the cell cycle to the S phase were completely blocked by p115RGS (an N-terminal regulator of the G-protein signaling domain of p115RhoGEF) and by the C-terminal fragments of Gα13 (an inhibitor of the interaction of receptors with G13). 4)Thrombin induced the secretion of IL-6 by RA SFs, but this action was blocked by p115RGS or Gα13. Our findings show that the actions of thrombin on the proliferation of RA SFs, cell-cycle progression to the S phase, and IL-6 secretion were mainly mediated by the G13 and RhoGEF pathways. These results suggest that p115RGS and Gα13 could be potent inhibitors of such functions. A rational design of future therapeutic strategies for RA synovitis could perhaps include the exploitation of the Rho pathway to directly reduce the growth of synovial cells.

Bombesin and nutrients independently and additively regulate hormone release from GIP/Ins cells

Glucose-dependent insulinotropic polypeptide (GIP) regulates glucose homeostasis and high-fat diet-induced obesity and insulin resistance. Therefore, elucidating the mechanisms that regulate GIP release is important. GIP is produced by K cells, a specific subtype of small intestinal enteroendocrine (EE) cell. Bombesin-like peptides produced by enteric neurons and luminal nutrients stimulate GIP release in vivo. We previously showed that PMA, bombesin, meat hydrolysate, glyceraldehyde, and methylpyruvate increase hormone release from a GIP-producing EE cell line (GIP/Ins cells). Here we demonstrate that bombesin and nutrients additively stimulate hormone release from GIP/Ins cells. In various cell systems, bombesin and PMA regulate cell physiology by activating PKD signaling in a PKC-dependent fashion, whereas nutrients regulate cell physiology by inhibiting AMPK signaling. Western blot analyses of GIP/Ins cells using antibodies specific for activated and/or phosphorylated forms of PKD and AMPK and one substrate for each kinase revealed that bombesin and PMA, but not nutrients, activated PKC, but not PKD. Conversely, nutrients, but not bombesin or PMA, inhibited AMPK activity. Pharmacological studies showed that PKC inhibition blocked bombesin- and PMA-stimulated hormone release, but AMPK activation failed to suppress nutrient-stimulated hormone secretion. Forced expression of constitutively active vs. dominant negative PKDs or AMPKs failed to perturb bombesin- or nutrient-stimulated hormone release. Thus, in GIP/Ins cells, PKC regulates bombesin-stimulated hormone release, whereas nutrients may control hormone release by regulating the activity of AMPK-related kinases, rather than AMPK itself. These results strongly suggest that K cells in vivo independently respond to neuronal vs. nutritional stimuli via two distinct signaling pathways.

Gα13 Stimulates Cell Migration through Cortactin-interacting Protein Hax-1

Galpha13, the alpha-subunit of the heterotrimeric G protein G13, has been shown to stimulate cell migration in addition to inducing oncogenic transformation. Cta, a Drosophila ortholog of G13, has been shown to be critical for cell migration leading to the ventral furrow formation in Drosophila embryos. Loss of Galpha13 has been shown to disrupt cell migration associated with angiogenesis in developing mouse embryos. Whereas these observations point to the vital role of G13-orthologs in regulating cell migration, widely across the species barrier, the mechanism by which Galpha13 couples to cytoskeleton and cell migration is largely unknown. Here we show that Galpha13 physically interacts with Hax-1, a cytoskeleton-associated, cortactin-interacting intracellular protein, and this interaction is required for Galpha13-stimulated cell migration. Hax-1 interaction is specific to Galpha13, and this interaction is more pronounced with the mutationally or functionally activated form of Galpha13 as compared with the wild-type Galpha13. Expression of Hax-1 reduces the formation of actin stress fibers and focal adhesion complexes in Galpha13-expressing NIH3T3 cells. Coexpression of Hax-1 also attenuates Galpha(13)-stimulated activity of Rho while potentiating Galpha13-stimulated activity of Rac. The presence of a quadnary complex consisting of Galpha13, Hax-1, Rac, and cortactin indicates the role of Hax-1 in tethering Galpha13 to the cytoskeletal component(s) involved in cell movement. Whereas the expression of Hax-1 potentiates Galpha13-mediated cell movement, silencing of endogenous Hax-1 with Hax-1-specific small interfering RNAs drastically reduces Galpha13-mediated cell migration. These findings, along with the observation that Hax-1 is overexpressed in metastatic tumors and tumor cell lines, suggest a novel role for the association of oncogenic Galpha13 and Hax-1 in tumor metastasis.

Lbc proto-oncogene product binds to and could be negatively regulated by metastasis suppressor nm23-H2

Lbc was identified as transforming gene from human leukemic cells and encodes Rho type guanine nucleotide exchange factor with 47kDa molecular weight. We isolated overlapping cDNAs of Lbc from human lung tissue. Full-length Lbc cDNA encodes 309kDa huge protein with Ht31 PKA anchoring motif, Dof domain, C1 domain, and coiled-coil structure. In order to analyze the regulatory mechanism of its activity, we searched for binding proteins. By yeast two-hybrid screening, we identified metastasis suppressor nm23-H2 as binding protein, which interacts with amino-terminal region of Lbc containing Dof domain. nm23 gene family encodes nucleoside diphosphate kinase, however, the binding of nm23-H2 to Lbc was independent of kinase activity. nm23-H1, which binds to Rac-specific GEF Tiam1, could not bind to Lbc suggesting nm23-H2 would be specific regulator for Lbc. Expression of nm23-H2 in cells leads to decrease the amount of GTP-bound Rho and suppress stress fiber formation stimulated by expression of Lbc. Our data suggest that metastasis suppressor nm23-H2 could regulate Lbc negatively by binding to amino-terminal region of Lbc proto-oncogene product.

AKAP-Lbc Nucleates a Protein Kinase D Activation Scaffold

The transmission of cellular signals often proceeds through multiprotein complexes where enzymes are positioned in proximity to their upstream activators and downstream substrates. In this report we demonstrate that the A-kinase anchoring protein AKAP-Lbc assembles an activation complex for the lipid-dependent enzyme protein kinase D (PKD). Using a combination of biochemical, enzymatic, and immunofluorescence techniques, we show that the anchoring protein contributes to PKD activation in two ways: it recruits an upstream kinase PKCeta and coordinates PKA phosphorylation events that release activated protein kinase D. Thus, AKAP-Lbc synchronizes PKA and PKC activities in a manner that leads to the activation of a third kinase. This configuration illustrates the utility of kinase anchoring as a mechanism to constrain the action of broad-spectrum enzymes.

The Role of Protein Kinase D in Neurotensin Secretion Mediated by Protein Kinase C-α/-δ and Rho/Rho Kinase

Neurotensin (NT) is a gut peptide that plays an important role in gastrointestinal (GI) secretion, motility, and growth as well as the proliferation of NT receptor positive cancers. Secretion of NT is regulated by phorbol ester-sensitive protein kinase C (PKC) isoforms-alpha and -delta and may involve protein kinase D (PKD). The purpose of our present study was: (i) to define the role of PKD in NT release from BON endocrine cells and (ii) to delineate the upstream signaling mechanisms mediating this effect. Here, we demonstrate that small interfering RNA (siRNA) targeted against PKD dramatically inhibited both basal and PMA-stimulated NT secretion; NT release is significantly increased by overexpression of PKD. PKC-alpha and -delta siRNA attenuated PKD activity, whereas overexpression of PKC-alpha and -delta enhanced PKD activity. Rho kinase (ROK) siRNA significantly inhibited NT secretion, whereas overexpression of ROKalpha effectively increased NT release. Rho protein inhibitor C3 dramatically inhibited both NT secretion and PKD activity. In conclusion, our results demonstrate that PKD activation plays a central role in NT peptide secretion; upstream regulators of PKD include PKC-alpha and -delta and Rho/ROK. Importantly, our results identify novel signaling pathways, which culminate in gut peptide release.

Gβγ signaling and Ca2+ mobilization co-operate synergistically in a Sos and Rac-dependent manner in the activation of JNK by Gq-coupled receptors

The mechanism by which G(q)-coupled receptors stimulate the c-Jun N-terminal kinase (JNK) activity has not been fully delineated. Here, we showed that stimulation of endogenous G(q)-coupled receptors in human hepatocarcinoma HepG2 cells resulted in an Src family kinase- and Ca(2+)-dependent JNK activation. Cos-7 cells transfected with HA-tagged JNK and various G(q)-coupled receptors also exhibited similar characteristics and provided further evidence for the involvement of Gbetagamma, an upstream intermediate for Src family kinases. The Ca(2+) and Gbetagamma signals operate in a high degree of independence. Transient expression of Gbetagamma subunits and elevation of cytoplasmic Ca(2+) level by thapsigargin activated JNK in a synergistic fashion. JNK activities triggered by G(q)-coupled receptors, Gbetagamma and thapsigargin were all suppressed by dominant negative (DN) mutants of Son of sevenless (Sos) and Rac. We propose that the co-operative effect between Gbetagamma-mediated signaling and the increased intracellular Ca(2+) level represents a robust mechanism for the stimulation of JNK by G(q)-coupled receptors.

G protein-coupled receptor-mediated phosphorylation of the activation loop of protein kinase D: dependence on plasma membrane translocation and protein kinase Cepsilon

Protein kinase D (PKD) is a serine/threonine protein kinase activated by G protein-coupled receptor (GPCR) agonists through an incompletely characterized mechanism that includes its reversible plasma membrane translocation and activation loop phosphorylation via a protein kinase C (PKC)-dependent pathway. To gain a better understanding of the mechanism regulating the activation of PKD in response to GPCR stimulation, we investigated the role of its rapid plasma membrane translocation on its activation loop phosphorylation and identified the endogenous PKC isozyme that mediates that event in vivo. We had found that the activation loop of a PKD mutant, with reduced affinity for diacylglycerol and phorbol esters, was only phosphorylated upon its plasma membrane association. We also found that the activation loop phosphorylation and rapid plasma membrane dissociation of PKD were inhibited either by preventing the plasma membrane translocation of PKCepsilon, through abolition of its interaction with receptor for activated C kinase, or by suppressing the expression of PKCepsilon via specific small interfering RNAs. Thus, this study demonstrates that the plasma membrane translocation of PKD, in response to GPCR stimulation, is necessary for the PKCepsilon-mediated phosphorylation of the activation loop of PKD and that this event requires the translocation of both kinases to the plasma membrane. Based on these and previous results, we propose a model of GPCR-mediated PKD regulation that integrates its changes in distribution, catalytic activity, and multisite phosphorylation.

Role of phospholipase D in the activation of protein kinase D by lysophosphatidic acid

Protein kinase D was auto-phosphorylated at Ser916 and trans-phosphorylated at Ser744/Ser748 in Rat-2 fibroblasts treated with lysophosphatidic acid. Both phosphorylations were inhibited by 1-butanol, which blocks phosphatidic acid formation by phospholipase D. The phosphorylations were also reduced in Rat-2 clones with decreased phospholipase D activity. Platelet-derived growth factor-induced protein kinase D phosphorylation showed a similar requirement for phospholipase D, but that induced by 4beta-phorbol 12 myristate 13-acetate did not. Propranolol an inhibitor of diacylglycerol formation from phosphatidic acid blocked the phosphorylation of protein kinase D, whereas dioctanoylglycerol induced it. The temporal pattern of auto-phosphorylation of protein kinase D closely resembled that of phospholipase D activation and preceded the trans-phosphorylation by protein kinase C. These results suggest that protein kinase D is activated by lysophosphatidic acid through sequential phosphorylation and that diacylglycerol produced by PLD via phosphatidic acid is required for the autophosphorylation that occurs prior to protein kinase C-mediated phosphorylation.

Protein kinase D potentiates DNA synthesis induced by Gq-coupled receptors by increasing the duration of ERK signaling in swiss 3T3 cells

Protein kinase D (PKD) potentiates cellular DNA synthesis in response to G protein-coupled receptor (GPCR) agonists but the mechanism(s) involved has not been elucidated. Here, we examined whether PKD overexpression in Swiss 3T3 cells regulates the activation/inactivation kinetics of the extracellular-regulated protein kinase (ERK) in response to the mitogenic GPCR agonists bombesin and vasopressin. Addition of bombesin or vasopressin to Swiss 3T3 cells overexpressing PKD induced a striking increase in the duration of MEK/ERK/RSK activation as compared with cultures of either control Swiss 3T3 cells or Swiss 3T3 cells expressing a kinase-inactive PKD mutant. In contrast, the duration of ERK activation in response to epidermal growth factor, which acts via protein kinase C/PKD-independent pathways, was not increased. Furthermore, bombesin or vasopressin promoted a striking increase in phosphorylation (at Ser-374) and accumulation of c-Fos (the c-fos proto-oncogene product) in Swiss 3T3 cells overexpressing wild-type (but not kinase-inactive) PKD. Inhibition of the sustained phase of ERK/RSK activation abrogated the increase in c-Fos accumulation and DNA synthesis induced by bombesin or vasopressin in PKD-overexpressing cells. Our results demonstrate that PKD selectively potentiates mitogenesis induced by bombesin or vasopressin in Swiss 3T3 cells by increasing the duration of MEK/ERK/RSK signaling.

Mechanism of persistent protein kinase D1 translocation and activation

The specificity of many signal transduction pathways relies on the spatiotemporal features of each signaling step. G protein-coupled receptor-mediated activation of protein kinases leads to diverse cellular effects. Upon receptor activation, PKD1 and several C-type protein kinases (PKCs), translocate to the plasma membrane and become catalytically active. Here we show that, unlike PKCs, PKD1 remains active at the membrane for hours. The two DAG binding C1 domains of PKD1 have distinct functional roles in targeting and maintaining PKD1 at the plasma membrane. C1A achieves fast, maximal, and reversible translocation, while C1B translocates partially, but persistently, to the plasma membrane. The persistent localization requires the C1B domain of PKD1, which binds Galphaq. We incorporate the kinetics of PKD1 translocation into a three-state model that suggests how PKD1 binding to DAG and Galphaq uniquely encodes frequency-dependent PKD1 signaling.

Differential requirement of G alpha12, G alpha13, G alphaq, and G beta gamma for endothelin-1-induced c-Jun NH2-terminal kinase and extracellular signal-regulated kinase activation

In the present study, we examined the roles of G(12), G(13), G(q), and G(i) in endothelin-1-induced hypertrophic responses. Endothelin-1 stimulation activated extracellular signal-regulated kinase (ERK) and c-Jun NH(2)-terminal kinase (JNK) in cultured rat neonatal myocytes. The activation of JNK, but not ERK, was inhibited by the expression of carboxyl terminal regions of G alpha(12) and G alpha(13). JNK activation was also inhibited by expression of the G alpha(12)/G alpha(13)-specific inhibitor regulator of G protein signaling (RGS) domain of p115RhoGEF and the G alpha(q)-specific inhibitor RGS domain of the G protein-coupled receptor kinase 2 (GRK2-RGS). JNK activation was not, however, inhibited by expression of the carboxyl terminal region of G protein-coupled receptor kinase 2 (GRK2-ct), which is a G beta gamma-sequestering polypeptide. Additionally, JNK activation but not ERK activation was inhibited by the expression of C3 exoenzyme that inactivates small GTPase Rho. These results suggest that JNK activation by G alpha(12), G alpha(13), and G alpha(q) is involved in Rho. On the other hand, ERK activation was inhibited by pertussis toxin treatment, the receptor-G(i) uncoupler, and GRK2-ct. Thus, ERK was activated by G alpha(i)- and G beta gamma-dependent pathways. These results clearly demonstrate that differential pathways activate JNK and ERK.

Cooperation of Gq, Gi, and G12/13 in protein kinase D activation and phosphorylation induced by lysophosphatidic acid

To examine the contribution of different G-protein pathways to lysophosphatidic acid (LPA)-induced protein kinase D (PKD) activation, we tested the effect of LPA on PKD activity in murine embryonic cell lines deficient in Galpha(q/11) (Galpha(q/11) KO cells) or Galpha(12/13) (Galpha(12/13) KO cells) and used cells lacking rhodopsin kinase (RK cells) as a control. In RK and Galpha(12/13) KO cells, LPA induced PKD activation through a phospholipase C/protein kinase C pathway in a concentration-dependent fashion with maximal stimulation (6-fold for RK cells and 4-fold for Galpha(12/13) KO cells in autophosphorylation activity) achieved at 3 microm. In contrast, LPA did not induce any significant increase in PKD activity in Galpha(q/11) KO cells. However, LPA induced a significantly increased PKD activity when Galpha(q/11) KO cells were transfected with Galpha(q). LPA-induced PKD activation was modestly attenuated by prior exposure of RK cells to pertussis toxin (PTx) but abolished by the combination treatments of PTx and Clostridium difficile toxin B. Surprisingly, PTx alone strikingly inhibited LPA-induced PKD activation in a concentration-dependent fashion in Galpha(12/13) KO cells. Similar results were obtained when activation loop phosphorylation at Ser-744 was determined using an antibody that detects the phosphorylated state of this residue. Our results indicate that G(q) is necessary but not sufficient to mediate LPA-induced PKD activation. In addition to G(q), LPA requires additional G-protein pathways to elicit a maximal response with G(i) playing a critical role in Galpha(12/13) KO cells. We conclude that LPA induces PKD activation through G(q), G(i), and G(12) and propose that PKD activation is a point of convergence in the action of multiple G-protein pathways.

Protein kinase A-mediated phosphorylation of the Galpha13 switch I region alters the Galphabetagamma13-G protein-coupled receptor complex and inhibits Rho activation

The present studies mapped the protein kinase A (PKA) phosphorylation site of Galpha(13) and studied the consequences of its phosphorylation. Initial experiments using purified human Galpha(13) and the PKA catalytic subunit established that PKA directly phosphorylates Galpha(13). The location of this phosphorylation site was next investigated with a new synthetic peptide (G(13)SRI(pep)) containing the PKA consensus sequence (Arg-Arg-Pro-Thr(203)) within the switch I region of Galpha(13). G(13)SRI(pep) produced a dose-dependent inhibition of PKA-mediated Galpha(13) phosphorylation. On the other hand, the Thr-phosphorylated derivative of G(13)SRI(pep) possessed no inhibitory activity, suggesting that Galpha(13) Thr(203) may represent the phosphorylation site. Confirmation of this notion was obtained by showing that the Galpha(13)-T203A mutant (in COS-7 cells) could not be phosphorylated by PKA. Additional studies using co-elution affinity chromatography and co-immunoprecipitation demonstrated that Galpha(13) phosphorylation stabilized coupling of Galpha(13) with platelet thromboxane A(2) receptors but destabilized coupling of Galpha(13) to its betagamma subunits. In order to determine the functional consequences of this phosphorylation on Galpha(13) signaling, activation of the Rho pathway was investigated. Specifically, Chinese hamster ovary cells overexpressing human Galpha(13) wild type (Galpha(13)-WT) or Galpha(13)-T203A mutant were generated and assayed for Rho activation. It was found that 8-bromo-cyclic AMP caused a significant decrease (50%; p < 0.002) of Rho activation in Galpha(13) wild type cells but produced no change of basal Rho activation levels in the mutant (p > 0.4). These results therefore suggest that PKA blocks Rho activation by phosphorylation of Galpha(13) Thr(203).

Protein kinase C phosphorylates protein kinase D activation loop Ser744 and Ser748 and releases autoinhibition by the pleckstrin homology domain

Persistent activation of protein kinase D (PKD) via protein kinase C (PKC)-mediated signal transduction is accompanied by phosphorylation at Ser(744) and Ser(748) located in the catalytic domain activation loop, but whether PKC isoforms directly phosphorylate these residues, induce PKD autophosphorylation, or recruit intermediate upstream kinase(s) is unclear. Here, we explore the mechanism whereby PKC activates PKD in response to cellular stimuli. We first assessed in vitro PKC-PKD transphosphorylation and PKD activation. A PKD738-753 activation loop peptide was well phosphorylated by immunoprecipitated PKC isoforms, consistent with similarities between the loop and their known substrate specificities. A similar peptide with glutamic acid replacing Ser(748) was preferentially phosphorylated by PKCepsilon, suggesting that PKD containing phosphate at Ser(748) is rapidly targeted by this isoform at Ser(744). When incubated in the presence of phosphatidylserine, phorbol 12,13-dibutyrate and ATP, intact PKD slowly autophosphorylated in the activation loop but only at Ser(748). In contrast, addition of purified PKCepsilon to the incubation mixture induced rapid Ser(744) and Ser(748) phosphorylation, concomitant with persistent 2-3-fold increases in PKD activity, measured using reimmunoprecipitated PKD to phosphorylate an exogenous peptide, syntide-2. We also further examined pleckstrin homology domain-mediated PKD regulation to determine its relationship with activation loop phosphorylation. The high constitutive activity of the pleckstrin homology (PH) domain deletion mutant PKD-deltaPH was not abrogated by mutation of Ser(744) and Ser(748) to alanines, suggesting that one function of activation loop phosphorylation in the PKD activation mechanism is to relieve autoinhibition by the PH domain. These studies provide evidence of a direct PKCepsilon-PKD phosphorylation cascade and provide additional insight into the activation mechanism.

Galpha(12/13) mediates alpha(1)-adrenergic receptor-induced cardiac hypertrophy

In neonatal cardiomyocytes, activation of the G(q)-coupled alpha(1)-adrenergic receptor (alpha(1)AR) induces hypertrophy by activating mitogen-activated protein kinases, including c-Jun NH(2)-terminal kinase (JNK). Here, we show that JNK activation is essential for alpha(1)AR-induced hypertrophy, in that alpha(1)AR-induced hypertrophic responses, such as reorganization of the actin cytoskeleton and increased protein synthesis, could be blocked by expressing the JNK-binding domain of JNK-interacting protein-1, a specific inhibitor of JNK. We also identified the classes and subunits of G proteins that mediate alpha(1)AR-induced JNK activation and hypertrophic responses by generating several recombinant adenoviruses that express polypeptides capable of inhibiting the function of specific G-protein subunits. alpha(1)AR-induced JNK activation was inhibited by the expression of carboxyl terminal regions of Galpha(q), Galpha(12), and Galpha(13). JNK activation was also inhibited by the Galpha(q/11)- or Galpha(12/13)-specific regulator of G-protein signaling (RGS) domains and by C3 toxin but was not affected by treatment with pertussis toxin or by expression of the carboxyl terminal region of G protein-coupled receptor kinase 2, a polypeptide that sequesters Gbetagamma. alpha(1)AR-induced hypertrophic responses were inhibited by Galpha(q/11)- and Galpha(12/13)-specific RGS domains, C3 toxin, and the carboxyl terminal region of G protein-coupled receptor kinase 2 but not by pertussis toxin. Activation of Rho was inhibited by carboxyl terminal regions of Galpha(12) and Galpha(13) but not by Galpha(q). Our findings suggest that alpha(1)AR-induced hypertrophic responses are mediated in part by a Galpha(12/13)-Rho-JNK pathway, in part by a G(q/11)-JNK pathway that is Rho independent, and in part by a Gbetagamma pathway that is JNK independent.

Protein kinase D is a downstream target of protein kinase Ctheta

Protein kinase D (PKD/PKCmu immunoprecipitated from either COS-7 cells or Jurkat T lymphocytes transiently transfected with a constitutively active mutant of PKCtheta AE (PKCthetaAE) exhibited a marked increase in basal activity. In contrast, coexpression of constitutively active mutant of PKCzeta does not induce PKD activation in both types of cells. PKCthetaAE does not induce kinase activity in immunocomplexes of PKD kinase-deficient mutants PKDK618N or PKDD733A. PKD activation in response to PKCthetaAE signaling was completely prevented by treatment with the protein kinase C (PKC) inhibitors, GF I or Ro 31-8220, or by mutation of Ser-744 and Ser-748 to Ala in the kinase activation loop of PKD. Our results show that PKD is a downstream target of the theta isoform of PKC in both COS-7 cells and lymphocytes. The regulation of PKD by PKCtheta reveals a new pathway in the signaling network existing between multiple members of the PKC superfamily and PKD.