Protein kinase C (PKC)eta-mediated PKC mu activation modulates ERK and JNK signal pathways

Cortactin: A major cellular target of viral, protozoal, and fungal pathogens

Many viral, protozoal, and fungal pathogens represent major human and animal health problems due to their great potential of causing infectious diseases. Research on these pathogens has contributed substantially to our current understanding of both microbial virulence determinants and host key factors during infection. Countless studies have also shed light on the molecular mechanisms of host-pathogen interactions that are employed by these microbes. For example, actin cytoskeletal dynamics play critical roles in effective adhesion, host cell entry, and intracellular movements of intruding pathogens. Cortactin is an eminent host cell protein that stimulates actin polymerization and signal transduction, and recently emerged as fundamental player during host-pathogen crosstalk. Here we review the important role of cortactin as major target for various prominent viral, protozoal and fungal pathogens in humans, and its role in human disease development and cancer progression. Most if not all of these important classes of pathogens have been reported to hijack cortactin during infection through mediating up- or downregulation of cortactin mRNA and protein expression as well as signaling. In particular, pathogen-induced changes in tyrosine and serine phosphorylation status of cortactin at its major phospho-sites (Y-421, Y-470, Y-486, S-113, S-298, S-405, and S-418) are addressed. As has been reported for various Gram-negative and Gram-positive bacteria, many pathogenic viruses, protozoa, and fungi also control these regulatory phospho-sites, for example, by activating kinases such as Src, PAK, ERK1/2, and PKD, which are known to phosphorylate cortactin. In addition, the recruitment of cortactin and its interaction partners, like the Arp2/3 complex and F-actin, to the contact sites between pathogens and host cells is highlighted, as this plays an important role in the infection process and internalization of several pathogens. However, there are also other ways in which the pathogens can exploit the function of cortactin for their needs, as the cortactin-mediated regulation of cellular processes is complex and involves numerous different interaction partners. Here, the current state of knowledge is summarized.

PKCμ promotes keratinocyte cell migration through Cx43 phosphorylation-mediated suppression of intercellular communication

Downregulation of intercellular communication through suppression of gap junctional conductance is necessary during wound healing. Connexin 43 (Cx43), a prominent gap junction protein in skin, is downregulated following wounding to restrict communication between keratinocytes. Previous studies found that PKCμ, a novel PKC isozyme, regulates efficient cutaneous wound healing. However, the molecular mechanism by which PKCμ regulates wound healing remains unknown. We have identified that PKCμ suppresses intercellular communication and enhances cell migration in an in vitro wound healing model by regulating Cx43 containing gap junctions. PKCμ can directly interact with and phosphorylate Cx43 at S368, which leads to Cx43 internalization and downregulation. Finally, utilizing phosphomimetic and non-phosphorylatable S368 substitutions and gap junction inhibitors, we confirmed that PKCμ regulates intercellular communication and in vitro wound healing by controlling Cx43-S368 phosphorylation. These results define PKCμ as a critical regulator of Cx43 phosphorylation to control cell migration and wound healing in keratinocytes.

Protein kinase D1 - A targetable mediator of pancreatic cancer development

Members of the Protein kinase D (PKD) kinase family each play important cell-specific roles in the regulation of normal pancreas functions. In pancreatic diseases PKD1 is the most widely characterized isoform with roles in pancreatitis and in induction of pancreatic cancer and its progression. PKD1 expression and activation increases in pancreatic acinar cells through macrophage secreted factors, Kirsten rat sarcoma viral oncogene homolog (KRAS) signaling, and reactive oxygen species (ROS), driving the formation of precancerous lesions. In precancerous lesions PKD1 regulates cell survival, growth, senescence, and generation of doublecortin like kinase 1 (DCLK1)-positive cancer stem cells (CSCs). Within tumors, regulation by PKD1 includes chemoresistance, apoptosis, proliferation, CSC features, and the Warburg effect. Thus, PKD1 plays a critical role throughout pancreatic disease initiation and progression.

Structural insights into regulation of the PEAK3 pseudokinase scaffold by 14-3-3

PEAK pseudokinases are molecular scaffolds which dimerize to regulate cell migration, morphology, and proliferation, as well as cancer progression. The mechanistic role dimerization plays in PEAK scaffolding remains unclear, as there are no structures of PEAKs in complex with their interactors. Here, we report the cryo-EM structure of dimeric PEAK3 in complex with an endogenous 14-3-3 heterodimer. Our structure reveals an asymmetric binding mode between PEAK3 and 14-3-3 stabilized by one pseudokinase domain and the SHED domain of the PEAK3 dimer. The binding interface contains a canonical phosphosite-dependent primary interaction and a unique secondary interaction not observed in previous structures of 14-3-3/client complexes. Additionally, we show that PKD regulates PEAK3/14-3-3 binding, which when prevented leads to PEAK3 nuclear enrichment and distinct protein-protein interactions. Altogether, our data demonstrate that PEAK3 dimerization forms an unusual secondary interface for 14-3-3 binding, facilitating 14-3-3 regulation of PEAK3 localization and interactome diversity.

Downregulation of LKB1/AMPK Signaling in Blood Mononuclear Cells Is Associated with the Severity of Guillain-Barre Syndrome

AMP-activated protein kinase (AMPK) is an intracellular energy sensor that regulates metabolic and immune functions mainly through the inhibition of the mechanistic target of rapamycin (mTOR)-dependent anabolic pathways and the activation of catabolic processes such as autophagy. The AMPK/mTOR signaling pathway and autophagy markers were analyzed by immunoblotting in blood mononuclear cells of 20 healthy control subjects and 23 patients with an acute demyelinating form of Guillain–Barré syndrome (GBS). The activation of the liver kinase B1 (LKB1)/AMPK/Raptor signaling axis was significantly reduced in GBS compared to control subjects. In contrast, the phosphorylated forms of mTOR activator AKT and mTOR substrate 4EBP1, as well as the levels of autophagy markers LC3-II, beclin-1, ATG5, p62/sequestosome 1, and NBR1 were similar between the two groups. The downregulation of LKB1/AMPK signaling, but not the activation status of the AKT/mTOR/4EBP1 pathway or the levels of autophagy markers, correlated with higher clinical activity and worse outcomes of GBS. A retrospective study in a diabetic cohort of GBS patients demonstrated that treatment with AMPK activator metformin was associated with milder GBS compared to insulin/sulphonylurea therapy. In conclusion, the impairment of the LKB1/AMPK pathway might contribute to the development/progression of GBS, thus representing a potential therapeutic target in this immune-mediated peripheral polyneuropathy.

ANXA10 promotes melanoma metastasis by suppressing E3 ligase TRIM41-directed PKD1 degradation

Melanoma is a highly metastatic cancer that requires effective and targeted curative therapy. Annexin A10 (ANXA10), a member of the annexin family, is a calcium- and phospholipid-binding protein. Considerable evidence indicates that ANXA10 is involved in tumour progression, but little is known about its role in melanoma development. In this study, we find that ANXA10 expression is significantly upregulated, and correlates with melanoma progression. ANXA10 knockout profoundly reduces cell migration and the metastatic activity of melanoma. In addition, ANXA10 knockout induces the N- to E-cadherin switch by upregulating SMAD6, an inhibitory SMAD in the TGF-β/SMAD pathway. The negative regulation of SMAD6 by ANXA10 is dependent on PKD1. ANXA10 interacts with PKD1 and inhibits E3 ligase TRIM41-targeted PKD1 degradation. In B16F10 melanoma cells, protein levels of ANXA10 and PKD1 are inversely correlated with SMAD6 level, but correlated with cell migration. Interestingly, ANXA10 and SMAD6 levels are inversely correlated in clinical samples of melanoma progression. Our findings suggest that the ANXA10-PKD1-SMAD6 axis is a new target for therapeutic strategies against melanoma metastasis.

A stochastic oscillator model simulates the entrainment of vertebrate cellular clocks by light

The circadian clock is a cellular mechanism that synchronizes various biological processes with respect to the time of the day. While much progress has been made characterizing the molecular mechanisms underlying this clock, it is less clear how external light cues influence the dynamics of the core clock mechanism and thereby entrain it with the light-dark cycle. Zebrafish-derived cell cultures possess clocks that are directly light-entrainable, thus providing an attractive laboratory model for circadian entrainment. Here, we have developed a stochastic oscillator model of the zebrafish circadian clock, which accounts for the core clock negative feedback loop, light input, and the proliferation of single-cell oscillator noise into population-level luminescence recordings. The model accurately predicts the entrainment dynamics observed in bioluminescent clock reporter assays upon exposure to a wide range of lighting conditions. Furthermore, we have applied the model to obtain refitted parameter sets for cell cultures exposed to a variety of pharmacological treatments and predict changes in single-cell oscillator parameters. Our work paves the way for model-based, large-scale screens for genetic or pharmacologically-induced modifications to the entrainment of circadian clock function.

In Vivo Induction of Regulatory T Cells Via CTLA-4 Signaling Peptide to Control Autoimmune Encephalomyelitis and Prevent Disease Relapse

Regulatory T cells play a key role in immune tolerance to self-antigens, thereby preventing autoimmune diseases. However, no drugs targeting Treg cells have been approved for clinical trials yet. Here, a chimeric peptide is generated by conjugation of the cytoplasmic domain of CTLA-4 (ctCTLA-4) with dNP2 for intracellular delivery, dNP2-ctCTLA-4, and evaluated Foxp3 expression during Th0, Th1, Treg, and Th17 differentiation dependent on TGF-β. The lysine motif of ctCTLA-4, not tyrosine motif, is required for Foxp3 expression for Treg induction and amelioration of experimental autoimmune encephalomyelitis (EAE). Transcriptome analysis reveals that dNP2-ctCTLA-4-treated T cells express Treg transcriptomic patterns with properties of suppressive functions. In addition, the molecular interaction between the lysine motif of ctCTLA-4 and PKC-η is critical for Foxp3 expression. Although both CTLA-4-Ig and dNP2-ctCTLA-4 treatment in vivo ameliorated EAE progression, only dNP2-ctCTLA-4 requires Treg cells for inhibition of disease progression and prevention of relapse. Furthermore, the CTLA-4 signaling peptide is able to induce human Tregs in vitro and in vivo as well as from peripheral blood mononuclear cells (PBMCs) of multiple sclerosis patients. These results collectively suggest that the chimeric CTLA-4 signaling peptide can be used for successful induction of regulatory T cells in vivo to control autoimmune diseases, such as multiple sclerosis.

The μ-opioid receptor induces miR-21 expression and is ERK/PKCμ-dependent

Micro RNA-21 (miR-21) is believed to perform an important role in the transition from inflammation to resolution in the innate immune response. The biochemical basis for the induction of miR-21 remains uncertain. However, the activation of the μ-opioid receptor (MOR) induces the expression of miR-21. Our results show that human monocytes treated with μ-opioid agonists exhibit a significant increase in miR-21 expression. We found that MOR-induction of miR-21 requires the activation of the Ras-Raf-MEK-ERK signaling cascade, and to our surprise, the activation of PKCμ (PKD1). These results are significant given the role of miR-21 in the sensitivity to pain.

Plasma membrane wound repair is characterized by extensive membrane lipid and protein rearrangements in vascular endothelial cells

Vascular endothelial cells are subject to mechanical stress resulting from blood flow and interactions with leukocytes. Stress occurs at the apical, vessel-facing cell surface and leads to membrane ruptures that have to be resealed to ensure cell survival. To mimic this process, we developed a laser ablation protocol selectively inducing wounds in the apical plasma membrane of endothelial cells. We show that Ca²⁺-dependent membrane resealing is initiated following this wounding protocol and that the process is accompanied by substantial membrane lipid dynamics at the wound site. Specifically, phosphatidylinositol (4,5)-bisphosphate, phosphatidylserine and phosphatidic acid rapidly accumulate at membrane wounds forming potential interaction platforms for Ca²⁺/phospholipid binding proteins of the annexin (Anx) family that are also recruited within seconds after wounding. Depletion of one annexin, AnxA2, and its putative binding partner S100A11 interferes with membrane resealing suggesting that Ca²⁺-dependent annexin-phospholipid interactions are required for efficient membrane wound repair in endothelial cells.

Lipin1 mediates cognitive impairment in fld mice via PKD-ERK pathway

Lipin1 is important in lipid synthesis because of its phosphatidate phosphatase activity, and it also functions as transcriptional coactivators to regulate the expression of genes involved in lipid metabolism. We found that fld mice exhibit cognitive impairment, and it is related to the DAG-PKD-ERK pathway. We used fld mice to explore the relationship between lipin1 and cognitive function. Our results confirmed the presence of cognitive impairment in the hippocampus of lipin1-deficient mice. As shown in behavioral test, the spatial learning and memory ability of fld mice was much worse than that of wild-type mice. Electron microscopy results showed that the number of synapses in hippocampus of fld mice was significantly reduced. BDNF,SYP, PSD95 were significantly reduced. These results suggest that lipin1 impairs synaptic plasticity. Hence,a deficiency of lipin1 leads to decreased DAG levels and inhibits PKD activation, thereby affecting the phosphorylation of ERK and the CREB.

Regulation of Osteoclast Differentiation at Multiple Stages by Protein Kinase D Family Kinases

Balanced osteoclast and osteoblast activity is necessary for skeletal health, whereas unbalanced osteoclast activity causes bone loss in many skeletal conditions. A better understanding of pathways that regulate osteoclast differentiation and activity is necessary for the development of new therapies to better manage bone resorption. The roles of Protein Kinase D (PKD) family of serine/threonine kinases in osteoclasts have not been well characterized. In this study we use immunofluorescence analysis to reveal that PKD2 and PKD3, the isoforms expressed in osteoclasts, are found in the nucleus and cytoplasm, the mitotic spindle and midbody, and in association with the actin belt. We show that PKD inhibitors CRT0066101 and CID755673 inhibit several distinct aspects of osteoclast formation. Treating bone marrow macrophages with lower doses of the PKD inhibitors had little effect on M-CSF + RANKL-dependent induction into committed osteoclast precursors, but inhibited their motility and subsequent differentiation into multinucleated mature osteoclasts, whereas higher doses of the PKD inhibitors induced apoptosis of the preosteoclasts. Treating post-fusion multinucleated osteoclasts with the inhibitors disrupted the osteoclast actin belts and impaired their resorptive activity. In conclusion, these data implicate PKD kinases as positive regulators of osteoclasts, which are essential for multiple distinct processes throughout their formation and function.

It Takes Two to Tango: Activation of Protein Kinase D by Dimerization

The recent discovery and structure determination of a novel ubiquitin-like dimerization domain in protein kinase D (PKD) has significant implications for its activation. PKD is a serine/threonine kinase activated by the lipid second messenger diacylglycerol (DAG). It is an essential and highly conserved protein that is implicated in plasma membrane directed trafficking processes from the trans-Golgi network. However, many open questions surround its mechanism of activation, its localization, and its role in the biogenesis of cargo transport carriers. In reviewing this field, the focus is primarily on the mechanisms that control the activation of PKD at precise locations in the cell. In light of the new structural findings, the understanding of the mechanisms underlying PKD activation is critically evaluated, with particular emphasis on the role of dimerization in PKD autophosphorylation, and the provenance and recognition of the DAG that activates PKD.

Lipin1 is required for skeletal muscle development by regulating MEF2c and MyoD expression

KEY POINTS

Lipin1 is critical for skeletal muscle development. Lipin1 regulates MyoD and myocyte-specific enhancer factor 2C (MEF2c) expression via the protein kinase C (PKC)/histone deacetylase 5-mediated pathway. Inhibition of PKCμ activity suppresses myoblast differentiation by inhibiting MyoD and MEF2c expression.

ABSTRACT

Our previous characterization of global lipin1-deficient (fld) mice demonstrated that lipin1 played a novel role in skeletal muscle (SM) regeneration. The present study using cell type-specific Myf5-cre;Lipin1^fl/fl conditional knockout mice (Lipin1^Myf5cKO ) shows that lipin1 is a major determinant of SM development. Lipin1 deficiency induced reduced muscle mass and myopathy. Our results from lipin1-deficient myoblasts suggested that lipin1 regulates myoblast differentiation via the protein kinase Cμ (PKCμ)/histone deacetylase 5 (HDAC5)/myocyte-specific enhancer factor 2C (MEF2c):MyoD-mediated pathway. Lipin1 deficiency leads to the suppression of PKC isoform activities, as well as inhibition of the downstream target of PKCμ, class II deacetylase HDAC5 nuclear export, and, consequently, inhibition of MEF2c and MyoD expression in the SM of lipin1^Myf5cKO mice. Restoration of diacylglycerol-mediated signalling in lipin1 deficient myoblasts by phorbol 12-myristate 13-acetate transiently activated PKC and HDAC5, and upregulated MEF2c expression. Our findings provide insights into the signalling circuitry that regulates SM development, and have important implications for developing intervention aimed at treating muscular dystrophy.

Superhero Rictor promotes cellular differentiation of mouse embryonic stem cells

Embryonic stem cells (ESCs) hold great promise for regenerative medicine. To harness the full therapeutic potential of ESCs, better understanding of the molecular mechanisms underlying the maintenance and differentiation of ESCs is required. Mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that integrates growth factor receptor signaling with cellular growth and proliferation. Dysregulation of mTOR signaling has been linked to various human diseases including cancer and metabolic syndromes. However, little is known regarding the function of mTOR signaling in the regulation of ES cell differentiation. Here we report that Rictor, a key component of mTORC2, functions as a novel ES cell differentiation promoting factor. Mechanistically, Rictor is able to interact with Prkch and facilitate Prkch phosphorylation at Ser-642. Upon phosphorylation, Prkch promotes Klf4 phosphorylation and inhibits Klf4-dependent E-cadherin expression, thereafter leading to the ES cell differentiation. These findings reveal a novel Rictor-Prkch-Klf4 pathway that plays an important role in the regulation of ES cell differentiation.

Amastigote Synapse: The Tricks of Trypanosoma cruzi Extracellular Amastigotes

To complete its life cycle within the mammalian host, Trypanosoma cruzi, the agent of Chagas’ disease, must enter cells. Trypomastigotes originating from the insect vector (metacyclic) or from infected cells (bloodstream/tissue culture-derived) are the classical infective forms of the parasite and enter mammalian cells in an actin-independent manner. By contrast, amastigotes originating from the premature rupture of infected cells or transformed from swimming trypomastigotes (designated extracellular amastigotes, EAs) require functional intact microfilaments to invade non-phagocytic host cells. Earlier work disclosed the key features of EA-HeLa cell interplay: actin-rich protrusions called ‘cups’ are formed at EA invasion sites on the host cell membrane that are also enriched in actin-binding proteins, integrins and extracellular matrix elements. In the past decades we described the participation of membrane components and secreted factors from EAs as well as the actin-regulating proteins of host cells involved in what we propose to be a phagocytic-like mechanism of parasite uptake. Thus, regarding this new perspective herein we present previously described EA-induced ‘cups’ as parasitic synapse since they can play a role beyond its architecture function. In this review, we focus on recent findings that shed light on the intricate interaction between extracellular amastigotes and non-phagocytic HeLa cells.

The tumor promoter-activated protein kinase Cs are a system for regulating filopodia

Different protein kinase C (PKC) isoforms have distinct roles in regulating cell functions. The conventional (α, β, γ) and novel (δ, ɛ, η, θ) classes are targets of phorbol ester tumor promoters, which are surrogates of endogenous second messenger, diacylglycerol. The promoter-stimulated disappearance of filopodia was investigated by use of blocking peptides (BPs) that inhibit PKC maturation and/or docking. Filopodia were partially rescued by a peptide representing PKC ɛ hydrophobic sequence, but also by a myristoylated PKC α/β pseudosubstrate sequence, and an inhibitor of T-cell protein tyrosine phosphatase (TC-PTP). The ability to turn over filopodia was widely distributed among PKC isoforms. PKC α and η hydrophobic sequences enhanced filopodia in cells in the absence of tumor promoter treatment. With transcriptional knockdown of PKC α, the content of PKC ɛ predominated over other isoforms. PKC ɛ could decrease filopodia significantly in promoter-treated cells, and this was attributed to ruffling. The presence of PKC α counteracted the PKC ɛ-mediated enhancement of ruffling. The results showed that there were two mechanisms of filopodia downregulation. One operated in the steady-state and relied on PKC α and η. The other was stimulated by tumor promoters and relied on PKC ɛ. Cycles of protrusion and retraction are characteristic of filopodia and are essential for the cell to orient itself during chemotaxis and haptotaxis. By suppressing filopodia, PKC ɛ can create a long-term "memory" of an environmental signal that may act in nature as a mnemonic device to mark the direction of a repulsive signal.

Pleiotropic effects of sphingosine-1-phosphate signaling to control human chorionic mesenchymal stem cell physiology

Chorionic stem cells represent a promising opportunity for regenerative medicine. A deeper understanding of the stimuli that regulate their physiology, could lead to innovative clinical approaches. We revealed the presence of multiple sphingosine-1-phosphate (S1P) receptor isoforms in chorion-derived mesenchymal stem cells (CMSCs). Their activation simultaneously propagated from the plasma membrane through Gi and other heterotrimeric G proteins and further diverged toward extracellular-signal-regulated kinase 1/2 (ERK1/2), p38 and protein kinase D 1. At a functional level, S1P signaling inhibited CMSC migration, while promoting proliferation. Instead, a reduction of cell density was obtained when S1P was combined to treatments that increased cAMP intracellular concentration. Such surprising reduction of cell viability was relatively specific as it was not observed with stromal stem cells from bone marrow. Neither it was observed by activating analogous G proteins with bradykinin nor by inducing cell death via a cAMP-independent pathway. S1P could thus reveal novel keys to improve CMSC differentiation programs acting on cAMP concentration. Furthermore, S1P receptor agonists/antagonists could become instrumental in favoring CMSC engraftment by controlling cell motility.

Protein kinase D signaling in cancer: A friend or foe?

Protein kinase D is a family of evolutionarily conserved serine/threonine kinases that belongs to the Ca⁺⁺/Calmodulin-dependent kinase superfamily. Signal transduction pathways mediated by PKD can be triggered by a variety of stimuli including G protein-coupled receptor agonists, growth factors, hormones, and cellular stresses. The regulatory mechanisms and physiological roles of PKD have been well documented including cell proliferation, survival, migration, angiogenesis, regulation of gene expression, and protein/membrane trafficking. However, its precise roles in disease progression, especially in cancer, remain elusive. A plethora of studies documented the cell- and tissue-specific expressions and functions of PKD in various cancer-associated biological processes, while the causes of the differential effects of PKD have not been thoroughly investigated. In this review, we have discussed the structural-functional properties, activation mechanisms, signaling pathways and physiological functions of PKD in the context of human cancer. Additionally, we have provided a comprehensive review of the reported tumor promoting or tumor suppressive functions of PKD in several major cancer types and discussed the discrepancies that have been raised on PKD as a major regulator of malignant transformation.

Emergency Spatiotemporal Shift: The Response of Protein Kinase D to Stress Signals in the Cardiovascular System

Protein Kinase D isoforms (PKD 1-3) are key mediators of neurohormonal, oxidative, and metabolic stress signals. PKDs impact a wide variety of signaling pathways and cellular functions including actin dynamics, vesicle trafficking, cell motility, survival, contractility, energy substrate utilization, and gene transcription. PKD activity is also increasingly linked to cancer, immune regulation, pain modulation, memory, angiogenesis, and cardiovascular disease. This increasing complexity and diversity of PKD function, highlights the importance of tight spatiotemporal control of the kinase via protein–protein interactions, post-translational modifications or targeting via scaffolding proteins. In this review, we focus on the spatiotemporal regulation and effects of PKD signaling in response to neurohormonal, oxidant and metabolic signals that have implications for myocardial disease. Precise targeting of these mechanisms will be crucial in the design of PKD-based therapeutic strategies.

PKD3 deficiency causes alterations in microtubule dynamics during the cell cycle

Protein kinase D 3 (PKD3) is a member of the PKD family that has been linked to many intracellular signaling pathways. However, defined statements regarding isoform specificity and in vivo functions are rare. Here, we use mouse embryonic fibroblast cells that are genetically depleted of PKD3 to identify isoform-specific functions. We show that PKD3 is involved in the regulation of the cell cycle by modulating microtubule nucleation and dynamics. In addition we also show that PKD1 partially can compensate for PKD3 function. Taken together our data provide new insights of a specific PKD3 signaling pathway by identifying a new function, which has not been identified before.

Subtilase cytotoxin produced by locus of enterocyte effacement-negative Shiga-toxigenic Escherichia coli induces stress granule formation

Subtilase cytotoxin (SubAB) is mainly produced by locus of enterocyte effacement (LEE)-negative strains of Shiga-toxigenic Escherichia coli (STEC). SubAB cleaves an endoplasmic reticulum (ER) chaperone, BiP/Grp78, leading to induction of ER stress. This stress causes activation of ER stress sensor proteins and induction of caspase-dependent apoptosis. We found that SubAB induces stress granules (SG) in various cells. Aim of this study was to explore the mechanism by which SubAB induced SG formation. Here, we show that SubAB-induced SG formation is regulated by activation of double-stranded RNA-activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK). The culture supernatant of STEC O113:H21 dramatically induced SG in Caco2 cells, although subAB knockout STEC O113:H21 culture supernatant did not. Treatment with phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, and lysosomal inhibitors, NH4 Cl and chloroquine, suppressed SubAB-induced SG formation, which was enhanced by PKC and PKD inhibitors. SubAB attenuated the level of PKD1 phosphorylation. Depletion of PKCδ and PKD1 by siRNA promoted SG formation in response to SubAB. Furthermore, death-associated protein 1 (DAP1) knockdown increased basal phospho-PKD1(S916) and suppressed SG formation by SubAB. However, SG formation by an ER stress inducer, Thapsigargin, was not inhibited in PMA-treated cells. Our findings show that SubAB-induced SG formation is regulated by the PERK/DAP1 signalling pathway, which may be modulated by PKCδ/PKD1, and different from the signal transduction pathway that results in Thapsigargin-induced SG formation.

miR-335 correlates with senescence/aging in human mesenchymal stem cells and inhibits their therapeutic actions through inhibition of AP-1 activity

MicroRNAs, small noncoding RNAs, regulate gene expression primarily at the posttranscriptional level. We previously found that miR-335 is critically involved in the regulation and differentiation capacity of human mesenchymal stem cells (hMSCs) in vitro. In this study, we investigated the significance of miR-335 for the therapeutic potential of hMSCs. Analysis of hMSCs in ex vivo culture demonstrated a significant and progressive increase in miR-335 that is prevented by telomerase. Expression levels of miR-335 were also positively correlated with donor age of hMSCs, and were increased by stimuli that induce cell senescence, such as γ-irradiation and standard O2 concentration. Forced expression of miR-335 resulted in early senescence-like alterations in hMSCs, including: increased SA-β-gal activity and cell size, reduced cell proliferation capacity, augmented levels of p16 protein, and the development of a senescence-associated secretory phenotype. Furthermore, overexpression of miR-335 abolished the in vivo chondro-osseous potential of hMSCs, and disabled their immunomodulatory capacity in a murine experimental model of lethal endotoxemia. These effects were accompanied by a severely reduced capacity for cell migration in response to proinflammatory signals and a marked reduction in Protein Kinase D1 phosphorylation, resulting in a pronounced decrease of AP-1 activity. Our results demonstrate that miR-335 plays a key role in the regulation of reparative activities of hMSCs and suggests that it might be considered a marker for the therapeutic potency of these cells in clinical applications.

The protein kinase D1 COOH terminus: marker or regulator of enzyme activity?

Protein kinase D1 (PKD1) is a Ser/Thr kinase implicated in a wide variety of cellular responses. PKD1 activation is generally attributed to a PKC-dependent pathway that leads to phosphorylation of the activation loop at Ser(744)/Ser(748). This modification increases catalytic activity, including that toward an autophosphorylation site (Ser(916)) in a postsynaptic density-95/disks large/zonula occludens-1 (PDZ)-binding motif at the extreme COOH terminus. However, there is growing evidence that PKD1 activation can also result from a PKC-independent autocatalytic reaction at Ser(744)/Ser(748) and that certain stimuli increase in PKD1 phosphorylation at Ser(744)/S(748) without an increase in autophosphorylation at Ser(916). This study exposes a mechanism that results in a discrepancy between PKD1 COOH-terminal autocatalytic activity and activity toward other substrates. We show that PKD1 constructs harboring COOH-terminal epitope tags display high levels of in vitro activation loop autocatalytic activity and activity toward syntide-2 (a peptide substrate), but no Ser(916) autocatalytic activity. Cell-based studies show that the COOH-terminal tag, adjacent to PKD1's PDZ1-binding motif, does not grossly influence PKD1 partitioning between soluble and particulate fractions in resting cells or PKD1 translocation to the particulate fraction following treatment with PMA. However, a COOH-terminal tag that confers a high level of activation loop autocatalytic activity decreases the PKC requirement for agonist-dependent PKD1 activation in cells. The recognition that COOH-terminal tags alter PKD1's pharmacological profile is important from a technical standpoint. The altered dynamics and activation mechanisms for COOH-terminal-tagged PKD1 enzymes also could model the signaling properties of localized pools of enzyme anchored through the COOH terminus to PDZ domain-containing scaffolding proteins.

Protein kinase D1 (PKD1) phosphorylation promotes dopaminergic neuronal survival during 6-OHDA-induced oxidative stress

Oxidative stress is a major pathophysiological mediator of degenerative processes in many neurodegenerative diseases including Parkinson’s disease (PD). Aberrant cell signaling governed by protein phosphorylation has been linked to oxidative damage of dopaminergic neurons in PD. Although several studies have associated activation of certain protein kinases with apoptotic cell death in PD, very little is known about protein kinase regulation of cell survival and protection against oxidative damage and degeneration in dopaminergic neurons. Here, we characterized the PKD1-mediated protective pathway against oxidative damage in cell culture models of PD. Dopaminergic neurotoxicant 6-hydroxy dopamine (6-OHDA) was used to induce oxidative stress in the N27 dopaminergic cell model and in primary mesencephalic neurons. Our results indicated that 6-OHDA induced the PKD1 activation loop (PKD1S744/S748) phosphorylation during early stages of oxidative stress and that PKD1 activation preceded cell death. We also found that 6-OHDA rapidly increased phosphorylation of the C-terminal S916 in PKD1, which is required for PKD1 activation loop (PKD1S744/748) phosphorylation. Interestingly, negative modulation of PKD1 activation by RNAi knockdown or by the pharmacological inhibition of PKD1 by kbNB-14270 augmented 6-OHDA-induced apoptosis, while positive modulation of PKD1 by the overexpression of full length PKD1 (PKD1^WT) or constitutively active PKD1 (PKD1^S744E/S748E) attenuated 6-OHDA-induced apoptosis, suggesting an anti-apoptotic role for PKD1 during oxidative neuronal injury. Collectively, our results demonstrate that PKD1 signaling plays a cell survival role during early stages of oxidative stress in dopaminergic neurons and therefore, positive modulation of the PKD1-mediated signal transduction pathway can provide a novel neuroprotective strategy against PD.

Protein kinase C (PKC) participates in acetaminophen hepatotoxicity through c-jun-N-terminal kinase (JNK)-dependent and -independent signaling pathways

This study examines the role of protein kinase C (PKC) and AMP-activated kinase (AMPK) in acetaminophen (APAP) hepatotoxicity. Treatment of primary mouse hepatocytes with broad-spectrum PKC inhibitors (Ro-31-8245, Go6983), protected against APAP cytotoxicity despite sustained c-jun-N-terminal kinase (JNK) activation. Broad-spectrum PKC inhibitor treatment enhanced p-AMPK levels and AMPK regulated survival-energy pathways including autophagy. AMPK inhibition by compound C or activation using an AMPK activator oppositely modulated APAP cytotoxicity, suggesting that p-AMPK and AMPK regulated energy survival pathways, particularly autophagy, play a critical role in APAP cytotoxicity. Ro-31-8245 treatment in mice up-regulated p-AMPK levels, increased autophagy (i.e., increased LC3-II formation, p62 degradation), and protected against APAP-induced liver injury, even in the presence of sustained JNK activation and translocation to mitochondria. In contrast, treatment of hepatocytes with a classical PKC inhibitor (Go6976) protected against APAP by inhibiting JNK activation. Knockdown of PKC-α using antisense (ASO) in mice also protected against APAP-induced liver injury by inhibiting JNK activation. APAP treatment resulted in PKC-α translocation to mitochondria and phosphorylation of mitochondrial PKC substrates. JNK 1 and 2 silencing in vivo decreased APAP-induced PKC-α translocation to mitochondria, suggesting PKC-α and JNK interplay in a feed-forward mechanism to mediate APAP-induced liver injury.

CONCLUSION

PKC-α and other PKC(s) regulate death (JNK) and survival (AMPK) proteins, to modulate APAP-induced liver injury.

Protein kinase D: a new player among the signaling proteins that regulate functions in the nervous system

Protein kinase D (PKD) is an evolutionarily-conserved family of protein kinases. It has structural, regulatory, and enzymatic properties quite different from the PKC family. Many stimuli induce PKD signaling, including G-protein-coupled receptor agonists and growth factors. PKD1 is the most studied member of the family. It functions during cell proliferation, differentiation, secretion, cardiac hypertrophy, immune regulation, angiogenesis, and cancer. Previously, we found that PKD1 is also critically involved in pain modulation. Since then, a series of studies performed in our lab and by other groups have shown that PKDs also participate in other processes in the nervous system including neuronal polarity establishment, neuroprotection, and learning. Here, we discuss the connections between PKD structure, enzyme function, and localization, and summarize the recent findings on the roles of PKD-mediated signaling in the nervous system.

Time-varying causal inference from phosphoproteomic measurements in macrophage cells

Cellular signaling circuitry in eukaryotes can be studied by analyzing the regulation of protein phosphorylation and its impact on downstream mechanisms leading to a phenotype. A primary role of phosphorylation is to act as a switch to turn "on" or "off" a protein activity or a cellular pathway. Specifically, protein phosphorylation is a major leit motif for transducing molecular signals inside the cell. Errors in transferring cellular information can alter the normal function and may lead to diseases such as cancer; an accurate reconstruction of the "true" signaling network is essential for understanding the molecular machinery involved in normal and pathological function. In this study, we have developed a novel framework for time-dependent reconstruction of signaling networks involved in the activation of macrophage cells leading to an inflammatory response. Several signaling pathways have been identified in macrophage cells, but the time-varying causal relationship that can produce a dynamic directed graph of these molecules has not been explored in detail. Here, we use the notion of Granger causality, and apply a vector autoregressive model to phosphoprotein time-course data in RAW 264.7 macrophage cells. Through the reconstruction of the phosphoprotein network, we were able to estimate the directionality and the dynamics of information flow. Significant interactions were selected through statistical hypothesis testing ( t-test) of the coefficients of a linear model and were used to reconstruct the phosphoprotein signaling network. Our approach results in a three-stage phosphoprotein network that represents the evolution of the causal interactions in the intracellular signaling pathways.

Different regulation of physiological and tumor angiogenesis in zebrafish by protein kinase D1 (PKD1)

Protein kinase D isoenzymes (PKDs, Prkds) are serine threonine kinases that belong to the CAMK superfamily. PKD1 is expressed in endothelial cells and is a major mediator of biological responses downstream of the VEGFRs that are relevant for angiogenesis such as endothelial cell migration, proliferation and tubulogenesis in vitro. PKDs also play a critical role in tumor development and progression, including tumor angiogenesis. However, given the plethora of signaling modules that drive angiogenesis, the precise role of PKD1 in both physiological and tumor angiogenesis in vivo has not been worked out so far. This study aimed at dissecting the contribution of PKD1 to physiological blood vessel formation, PKD1 was found to be widely expressed during zebrafish development. As far as physiological angiogenesis was concerned, morpholino-based silencing of PKD1 expression moderately reduced the formation of the intersomitic vessels and the dorsal longitudinal anastomotic vessel in tg(fli1:EGFP) zebrafish. In addition, silencing of PKD1 resulted in reduced formation of the parachordal lymphangioblasts that serves as a precursor for the developing thoracic duct. Interestingly, tumor angiogenesis was completely abolished in PKD1 morphants using the zebrafish/tumor xenograft angiogenesis assay. Our data in zebrafish demonstrate that PKD1 contributes to the regulation of physiological angiogenesis and lymphangiogenesis during zebrafish development and is essential for tumor angiogenesis.

In vitro cytotoxicity, pharmacokinetics, tissue distribution, and metabolism of small-molecule protein kinase D inhibitors, kb-NB142-70 and kb-NB165-09, in mice bearing human cancer xenografts

PURPOSE

Protein kinase D (PKD) mediates diverse biological responses including cell growth and survival. Therefore, PKD inhibitors may have therapeutic potential. We evaluated the in vitro cytotoxicity of two PKD inhibitors, kb-NB142-70 and its methoxy analogue, kb-NB165-09, and examined their in vivo efficacy and pharmacokinetics.

METHODS

The in vitro cytotoxicities of kb-NB142-70 and kb-NB165-09 were evaluated by MTT assay against PC-3, androgen-independent prostate cancer cells, and CFPAC-1 and PANC-1, pancreatic cancer cells. Efficacy studies were conducted in mice bearing either PC-3 or CPFAC-1 xenografts. Tumor-bearing mice were euthanized between 5 and 1,440 min after iv dosing, and plasma and tissue concentrations were measured by HPLC-UV. Metabolites were characterized by LC-MS/MS.

RESULTS

kb-NB142-70 and kb-NB165-09 inhibited cellular growth in the low-mid μM range. The compounds were inactive when administered to tumor-bearing mice. In mice treated with kb-NB142-70, the plasma C (max) was 36.9 nmol/mL, and the PC-3 tumor C (max) was 11.8 nmol/g. In mice dosed with kb-NB165-09, the plasma C (max) was 61.9 nmol/mL, while the PANC-1 tumor C (max) was 8.0 nmol/g. The plasma half-lives of kb-NB142-70 and kb-NB165-09 were 6 and 14 min, respectively. Both compounds underwent oxidation and glucuronidation.

CONCLUSIONS

kb-NB142-70 and kb-NB165-09 were rapidly metabolized, and concentrations in tumor were lower than those required for in vitro cytotoxicity. Replacement of the phenolic hydroxyl group with a methoxy group increased the plasma half-life of kb-NB165-09 2.3-fold over that of kb-NB142-70. Rapid metabolism in mice suggests that next-generation compounds will require further structural modifications to increase potency and/or metabolic stability.

Rapid protein kinase D1 signaling promotes migration of intestinal epithelial cells

We have examined the role of protein kinase D1 (PKD1) signaling in intestinal epithelial cell migration. Wounding monolayer cultures of intestinal epithelial cell line IEC-18 or IEC-6 induced rapid PKD1 activation in the cells immediately adjacent to the wound edge, as judged by immunofluorescence microscopy with an antibody that detects the phosphorylated state of PKD1 at Ser(916), an autophosphorylation site. An increase in PKD1 phosphorylation at Ser(916) was evident as early as 45 s after wounding, reached a maximum after 3 min, and persisted for ≥15 min. PKD1 autophosphorylation at Ser(916) was prevented by the PKD family inhibitors kb NB 142-70 and CRT0066101. A kb NB 142-70-sensitive increase in PKD autophosphorylation was also elicited by wounding IEC-6 cells. Using in vitro kinase assays after PKD1 immunoprecipitation, we corroborated that wounding IEC-18 cells induced rapid PKD1 catalytic activation. Further results indicate that PKD1 signaling is required to promote migration of intestinal epithelial cells into the denuded area of the wound. Specifically, treatment with kb NB 142-70 or small interfering RNAs targeting PKD1 markedly reduced wound-induced migration in IEC-18 cells. To test whether PKD1 promotes migration of intestinal epithelial cells in vivo, we used transgenic mice that express elevated PKD1 protein in the small intestinal epithelium. Enterocyte migration was markedly increased in the PKD1 transgenic mice. These results demonstrate that PKD1 activation is one of the early events initiated by wounding a monolayer of intestinal epithelial cells and indicate that PKD1 signaling promotes the migration of these cells in vitro and in vivo.

Protein kinase D1 mediates anchorage-dependent and -independent growth of tumor cells via the zinc finger transcription factor Snail1

We here identify protein kinase D1 (PKD1) as a major regulator of anchorage-dependent and -independent growth of cancer cells controlled via the transcription factor Snail1. Using FRET, we demonstrate that PKD1, but not PKD2, efficiently interacts with Snail1 in nuclei. PKD1 phosphorylates Snail1 at Ser-11. There was no change in the nucleocytoplasmic distribution of Snail1 using wild type Snail1 and Ser-11 phosphosite mutants in different tumor cells. Regardless of its phosphorylation status or following co-expression of constitutively active PKD, Snail1 was predominantly localized to cell nuclei. We also identify a novel mechanism of PKD1-mediated regulation of Snail1 transcriptional activity in tumor cells. The interaction of the co-repressors histone deacetylases 1 and 2 as well as lysyl oxidase-like protein 3 with Snail1 was impaired when Snail1 was not phosphorylated at Ser-11, which led to reduced Snail1-associated histone deacetylase activity. Additionally, lysyl oxidase-like protein 3 expression was up-regulated by ectopic PKD1 expression, implying a synergistic regulation of Snail1-driven transcription. Ectopic expression of PKD1 also up-regulated proliferation markers such as Cyclin D1 and Ajuba. Accordingly, Snail1 and its phosphorylation at Ser-11 were required and sufficient to control PKD1-mediated anchorage-independent growth and anchorage-dependent proliferation of different tumor cells. In conclusion, our data show that PKD1 is crucial to support growth of tumor cells via Snail1.

Dopamine transporter phosphorylation site threonine 53 regulates substrate reuptake and amphetamine-stimulated efflux

In the central nervous system, levels of extraneuronal dopamine are controlled primarily by the action of the dopamine transporter (DAT). Multiple signaling pathways regulate transport activity, substrate efflux, and other DAT functions through currently unknown mechanisms. DAT is phosphorylated by protein kinase C within a serine cluster at the distal end of the cytoplasmic N terminus, whereas recent work in model cells revealed proline-directed phosphorylation of rat DAT at membrane-proximal residue Thr(53). In this report, we use mass spectrometry and a newly developed phospho-specific antibody to positively identify DAT phosphorylation at Thr(53) in rodent striatal tissue and heterologous expression systems. Basal phosphorylation of Thr(53) occurred with a stoichiometry of ~50% and was strongly increased by phorbol esters and protein phosphatase inhibitors, demonstrating modulation of the site by signaling pathways that impact DAT activity. Mutations of Thr(53) to prevent phosphorylation led to reduced dopamine transport V(max) and total apparent loss of amphetamine-stimulated substrate efflux, supporting a major role for this residue in the transport kinetic mechanism.

Regulation of protein kinase D1 activity

Protein kinase D1 (PKD1) is a stress-activated serine/threonine kinase that plays a vital role in various physiologically important biological processes, including cell growth, apoptosis, adhesion, motility, and angiogenesis. Dysregulated PKD1 expression also contributes to the pathogenesis of certain cancers and cardiovascular disorders. Studies to date have focused primarily on the canonical membrane-delimited pathway for PKD1 activation by G protein-coupled receptors or peptide growth factors. Here, agonist-dependent increases in diacylglycerol accumulation lead to the activation of protein kinase C (PKC) and PKC-dependent phosphorylation of PKD1 at two highly conserved serine residues in the activation loop; this modification increases PKD1 catalytic activity, as assessed by PKD1 autophosphorylation at a consensus phosphorylation motif at the extreme C terminus. However, recent studies expose additional controls and consequences for PKD1 activation loop and C-terminal phosphorylation as well as additional autophosphorylation reactions and trans-phosphorylations (by PKC and other cellular enzymes) that contribute to the spatiotemporal control of PKD1 signaling in cells. This review focuses on the multisite phosphorylations that are known or predicted to influence PKD1 catalytic activity and may also influence docking interactions with cellular scaffolds and trafficking to signaling microdomains in various subcellular compartments. These modifications represent novel targets for the development of PKD1-directed pharmaceuticals for the treatment of cancers and cardiovascular disorders.

Tyrosine phosphorylation of protein kinase D2 mediates ligand-inducible elimination of the Type 1 interferon receptor

Type 1 interferons (including IFNα/β) activate their cell surface receptor to induce the intracellular signal transduction pathways that play an important role in host defenses against infectious agents and tumors. The extent of cellular responses to IFNα is limited by several important mechanisms including the ligand-stimulated and specific serine phosphorylation-dependent degradation of the IFNAR1 chain of Type 1 IFN receptor. Previous studies revealed that acceleration of IFNAR1 degradation upon IFN stimulation requires activities of tyrosine kinase TYK2 and serine/threonine protein kinase D2 (PKD2), whose recruitment to IFNAR1 is also induced by the ligand. Here we report that activation of PKD2 by IFNα (but not its recruitment to the receptor) depends on TYK2 catalytic activity. PKD2 undergoes IFNα-inducible tyrosine phosphorylation on specific phospho-acceptor site (Tyr-438) within the plekstrin homology domain. Activated TYK2 is capable of facilitating this phosphorylation in vitro. Tyrosine phosphorylation of PKD2 is required for IFNα-stimulated activation of this kinase as well as for efficient serine phosphorylation and degradation of IFNAR1 and ensuing restriction of the extent of cellular responses to IFNα.

Protein kinase Cδ mediates the activation of protein kinase D2 in platelets

Protein kinase D (PKD) is a subfamily of serine/threonine specific family of kinases, comprised of PKD1, PKD2 and PKD3 (PKCμ, PKD2 and PKCv in humans). It is known that PKCs activate PKD, but the relative expression of isoforms of PKD or the specific PKC isoform/s responsible for its activation in platelets is not known. This study is aimed at investigating the pathway involved in activation of PKD in platelets. We show that PKD2 is the major isoform of PKD that is expressed in human as well as murine platelets but not PKD1 or PKD3. PKD2 activation induced by AYPGKF was abolished with a G(q) inhibitor YM-254890, but was not affected by Y-27632, a RhoA/p160ROCK inhibitor, indicating that PKD2 activation is G(q)-, but not G₁₂/₁₃-mediated Rho-kinase dependent. Calcium-mediated signals are also required for activation of PKD2 as dimethyl BAPTA inhibited its phosphorylation. GF109203X, a pan PKC inhibitor abolished PKD2 phosphorylation but Go6976, a classical PKC inhibitor had no effect suggesting that novel PKC isoforms are involved in PKD2 activation. Importantly, Rottlerin, a non-selective PKCδ inhibitor, inhibited AYPGKF-induced PKD2 activation in human platelets. Similarly, AYPGKF- and Convulxin-induced PKD2 phosphorylation was dramatically inhibited in PKCδ-deficient platelets, but not in PKCθ- or PKCɛ-deficient murine platelets compared to that of wild type platelets. Hence, we conclude that PKD2 is a common signaling target downstream of various agonist receptors in platelets and G(q)-mediated signals along with calcium and novel PKC isoforms, in particular, PKCδ activate PKD2 in platelets.

Protein kinase D1 (PKD1) activation mediates a compensatory protective response during early stages of oxidative stress-induced neuronal degeneration

Background

Oxidative stress is a key pathophysiological mechanism contributing to degenerative processes in many neurodegenerative diseases and therefore, unraveling molecular mechanisms underlying various stages of oxidative neuronal damage is critical to better understanding the diseases and developing new treatment modalities. We previously showed that protein kinase C delta (PKCδ) proteolytic activation during the late stages of oxidative stress is a key proapoptotic signaling mechanism that contributes to oxidative damage in Parkinson's disease (PD) models. The time course studies revealed that PKCδ activation precedes apoptotic cell death and that cells resisted early insults of oxidative damage, suggesting that some intrinsic compensatory response protects neurons from early oxidative insult. Therefore, the purpose of the present study was to characterize protective signaling pathways in dopaminergic neurons during early stages of oxidative stress.

Results

Herein, we identify that protein kinase D1 (PKD1) functions as a key anti-apoptotic kinase to protect neuronal cells against early stages of oxidative stress. Exposure of dopaminergic neuronal cells to H₂O₂ or 6-OHDA induced PKD1 activation loop (PKD1S744/748) phosphorylation long before induction of neuronal cell death. Blockade of PKCδ cleavage, PKCδ knockdown or overexpression of a cleavage-resistant PKCδ mutant effectively attenuated PKD1 activation, indicating that PKCδ proteolytic activation regulates PKD1 phosphorylation. Furthermore, the PKCδ catalytic fragment, but not the regulatory fragment, increased PKD1 activation, confirming PKCδ activity modulates PKD1 activation. We also identified that phosphorylation of S916 at the C-terminal is a preceding event required for PKD1 activation loop phosphorylation. Importantly, negative modulation of PKD1 by the RNAi knockdown or overexpression of PKD1^S916A phospho-defective mutants augmented oxidative stress-induced apoptosis, while positive modulation of PKD1 by the overexpression of full length PKD1 or constitutively active PKD1 plasmids attenuated oxidative stress-induced apoptosis, suggesting an anti-apoptotic role for PKD1 during oxidative neuronal injury.

Conclusion

Collectively, our results demonstrate that PKCδ-dependent activation of PKD1 represents a novel intrinsic protective response in counteracting early stage oxidative damage in neuronal cells. Our results suggest that positive modulation of the PKD1-mediated compensatory protective mechanism against oxidative damage in dopaminergic neurons may provide novel neuroprotective strategies for treatment of PD.

PKCη promotes a proliferation to differentiation switch in keratinocytes via upregulation of p27Kip1 mRNA through suppression of JNK/c-Jun signaling under stress conditions

To maintain epidermal homeostasis, the balance between keratinocyte proliferation and differentiation is tightly controlled. However, the molecular mechanisms underlying this balance remain unclear. In 3D organotypic coculture with mouse keratinocytes and fibroblasts, the thickness of stratified cell layers was prolonged, and growth arrest and terminal differentiation were delayed when PKCη-null keratinocytes were used. Re-expression of PKCη in PKCη-null keratinocytes restored stratified cell layer thickness, growth arrest and terminal differentiation. We show that in 3D cocultured PKCη-null keratinocytes, p27^Kip1 mRNA was downregulated, whereas JNK/c-Jun signaling was enhanced. Furthermore, inhibition of JNK/c-Jun signaling in PKCη-null keratinocytes led to upregulation of p27^Kip1 mRNA, and to thinner stratified cell layers. Collectively, our findings indicate that PKCη upregulates p27^Kip1 mRNA through suppression of JNK/c-Jun signaling. This results in promoting a proliferation to differentiation switch in keratinocytes.

The involvement of specific PKC isoenzymes in phorbol ester-mediated regulation of steroidogenic acute regulatory protein expression and steroid synthesis in mouse Leydig cells

Protein kinase C (PKC) is a multigene family of serine/threonine kinases. PKC is involved in regulating adrenal and gonadal steroidogenesis; however, the functional relevance of the different PKC isoenzymes remains obscure. In this study, we demonstrate that MA-10 mouse Leydig tumor cells express several PKC isoforms to varying levels and that the activation of PKC signaling, by phorbol 12-myristate 13-acetate (PMA) elevated the expression and phosphorylation of PKCα, -δ, -ε, and -μ/protein kinase D (PKD). These responses coincided with the expression of the steroidogenic acute regulatory (StAR) protein and progesterone synthesis. Targeted silencing of PKCα, δ, and ε and PKD, using small interfering RNAs, resulted in deceases in basal and PMA-mediated StAR and steroid levels and demonstrated the importance of PKD in steroidogenesis. PKD was capable of controlling PMA and cAMP/PKA-mediated synergism involved in the steroidogenic response. Further studies pointed out that the regulatory events effected by PKD are associated with cAMP response element-binding protein (CREB) and c-Jun/c-Fos-mediated transcription of the StAR gene. Chromatin immunoprecipitation studies revealed that the activation of phosphorylated CREB, c-Jun, and c-Fos by PMA was correlated with in vivo protein-DNA interactions and the recruitment of CREB-binding protein, whereas knockdown of PKD suppressed the association of these factors with the StAR promoter. Ectopic expression of CREB-binding protein enhanced the trans-activation potential of CREB and c-Jun/c-Fos in StAR gene expression. Using EMSA, a -83/-67-bp region of the StAR promoter was shown to bind PKD-transfected MA-10 nuclear extract in a PMA-responsive manner, targeting CREB and c-Jun/c-Fos proteins. These findings provide evidence for the presence of multiple PKC isoforms and demonstrate the molecular events by which selective isozymes, especially PKD, influence PMA/PKC signaling involved in the regulation of the steroidogenic machinery in mouse Leydig cells.

Protein kinase D1 mediates stimulation of DNA synthesis and proliferation in intestinal epithelial IEC-18 cells and in mouse intestinal crypts

We examined whether protein kinase D1 (PKD1), the founding member of a new protein kinase family, plays a critical role in intestinal epithelial cell proliferation. Our results demonstrate that PKD1 activation is sustained, whereas that of PKD2 is transient in intestinal epithelial IEC-18 stimulated with the G(q)-coupled receptor agonists angiotensin II or vasopressin. PKD1 gene silencing utilizing small interfering RNAs dramatically reduced DNA synthesis and cell proliferation in IEC-18 cells stimulated with G(q)-coupled receptor agonists. To clarify the role of PKD1 in intestinal epithelial cell proliferation in vivo, we generated transgenic mice that express elevated PKD1 protein in the intestinal epithelium. Transgenic PKD1 exhibited constitutive catalytic activity and phosphorylation at the activation loop residues Ser(744) and Ser(748) and on the autophosphorylation site, Ser(916). To examine whether PKD1 expression stimulates intestinal cell proliferation, we determined the rate of crypt cell DNA synthesis by detection of 5-bromo-2-deoxyuridine incorporated into the nuclei of crypt cells of the ileum. Our results demonstrate a significant increase (p < 0.005) in DNA-synthesizing cells in the crypts of two independent lines of PKD1 transgenic mice as compared with non-transgenic littermates. Morphometric analysis showed a significant increase in the length and in the total number of cells per crypt in the transgenic PKD1 mice as compared with the non-transgenic littermates (p < 0.01). Thus, transgenic PKD1 signaling increases the number of cells per crypt by stimulating the rate of crypt cell proliferation. Collectively, our results indicate that PKD1 plays a role in promoting cell proliferation in intestinal epithelial cells both in vitro and in vivo.

Role of estrogen receptors, PKC and Src in ERK2 and p38 MAPK signaling triggered by 17β-estradiol in skeletal muscle cells

We have previously reported in C2C12 murine skeletal muscle cells that 10(-8)M 17β-estradiol promotes MAPKs stimulation which in turn mediates the activation of CREB and Elk-1 transcription factors. In this work, we demonstrated that the hormone induces ERK2 phosphorylation (without affecting ERK1 activation) and also stimulates p38 MAPK, both in a dose-dependent manner. Moreover, estrogen receptors involvement in MAPKs activation by the estrogen was studied. The use of ICI182780 (1 μM), an antagonist of ERs, and specific siRNAs to block ERα and ERβ expression, demonstrated that ERα mediates ERK2 activation but not p38 MAPK phosphorylation by 17β-estradiol, and that ERβ isoform is not implicated in MAPKs activation by the hormone. Furthermore, Src and PKC contribution in estrogen stimulation of the MAPKs was investigated. Compounds PP2 and Ro318220, Src and PKC family inhibitors, respectively abrogated ERK2 and p38 MAPK phosphorylation by 17β-estradiol. Of interest, the hormone was able to induce Src and PKCδ activation. In addition, Ro318220 decreased estrogen-dependent Src modulation implicating PKC in hormone upregulation of Src. Accordingly, PP2 and Ro318220 suppressed CREB and Elk-1 phosphorylation as well as c-Fos and c-Jun oncoprotein levels induced by 17β-estradiol. Altogether, these data indicate that 17β-estradiol activates ERK2 through ERα and p38 MAPK in an ERα/β-independent manner and that PKC and Src proteins are key upstream components on MAPKs activation in C2C12 skeletal muscle cells.

Protein kinase D-mediated phosphorylation of polycystin-2 (TRPP2) is essential for its effects on cell growth and calcium channel activity

PKD2 is mutated in 15% of patients with autosomal dominant polycystic kidney disease. The PKD2 protein, polycystin-2 or TRPP2, is a nonselective Ca2+-permeable cation channel that has been shown to function at several locations, including primary cilia, basolateral membrane, and at the endoplasmic reticulum (ER). Nevertheless, the factors that regulate the channel activity of polycystin-2 are not well understood. Polycystin-2 has been shown previously to be regulated by phosphorylation at two serine residues (Ser812 and Ser76) with distinct functional consequences. Here, we report the identification of a previously unrecognized phosphorylation site within the polycystin-2 C terminus (Ser801), and we demonstrate that it is phosphorylated by protein kinase D. Phosphorylation at this site was significantly increased in response to serum and epidermal growth factor stimulation. In nonciliated Madin-Darby canine kidney I cells, inducible expression of polycystin-2 inhibited cell proliferation compared with wild-type cells. Mutagenesis at Ser801 abolished these effects and reduced ATP-stimulated Ca2+ release from ER stores. Finally, we show that a pathogenic mutation (S804N) within the consensus kinase recognition sequence abolished Ser801 phosphorylation. These results suggest that growth factor-stimulated, protein kinase D-mediated phosphorylation of polycystin-2 is essential for its ER channel function and links extracellular stimuli to its effects on cell growth and intracellular calcium regulation.

Trop2 expression contributes to tumor pathogenesis by activating the ERK MAPK pathway

Background

Trop2 is a cell-surface glycoprotein overexpressed by a variety of epithelial carcinomas with reported low to restricted expression in normal tissues. Expression of Trop2 has been associated with increased tumor aggressiveness, metastasis and decreased patient survival, but the signaling mechanisms mediated by Trop2 are still unknown. Here, we studied the effects murine Trop2 (mTrop2) exerted on tumor cellular functions and some of the signaling mechanisms activated by this oncogene.

Results

mTrop2 expression significantly increased tumor cell proliferation at low serum concentration, migration, foci formation and anchorage-independent growth. These in vitro characteristics translated to increased tumor growth in both subcutaneous and orthotopic pancreatic cancer murine models and also led to increased liver metastasis. mTrop2 expression also increased the levels of phosphorylated ERK1/2 mediating cell cycle progression by increasing the levels of cyclin D1 and cyclin E as well as downregulating p27. The activation of ERK was also observed in human pancreatic ductal epithelial cells and colorectal adenocarcinoma cells overexpressing human Trop2.

Conclusions

These findings demonstrate some of the pathogenic effects mediated by mTrop2 expression on cancer cells and the importance of targeting this cell surface glycoprotein. This study also provides the first indication of a molecular signaling pathway activated by Trop2 which has important implications for cancer cell growth and survival.

Role of protein kinase D signaling in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancers with dismal survival rates. Its intransigence to conventional therapy renders PDAC an aggressive disease with early metastatic potential. Thus, novel targets for PDAC therapy are urgently needed. Multiple signal transduction pathways are implicated in progression of PDAC. These pathways stimulate production of intracellular messengers in their target cells to modify their behavior, including the lipid-derived diacylglycerol (DAG). One of the prominent intracellular targets of DAG is the protein kinase C (PKC) family. However, the mechanisms by which PKC-mediated signals are decoded by the cell remain incompletely understood. Protein kinase D1 (PKD or PKD1, initially called atypical PKCμ), is the founding member of a novel protein kinase family that includes two additional protein kinases that share extensive overall homology with PKD, termed PKD2, and PKD3. The PKD family occupies a unique position in the signal transduction pathways initiated by DAG and PKC. PKD lies downstream of PKCs in a novel signal transduction pathway implicated in the regulation of multiple fundamental biological processes. We and others have shown that PKD-mediated signaling pathways promote mitogenesis and angiogenesis in PDAC. Our recent observations demonstrate that PKD also potentiates chemoresistance and invasive potential of PDAC cells. This review will briefly highlight diverse biological roles of PKD family in multiple neoplasias including PDAC. Further, this review will underscore our latest advancement with the development of a potent PKD family inhibitor and its effect both in vitro and in vivo in PDAC.

Induced overexpression of protein kinase D1 stimulates mitogenic signaling in human pancreatic carcinoma PANC-1 cells

Neurotensin (NT) stimulates protein kinase D1 (PKD1), extracellular signal regulated kinase (ERK), c-Jun N-terminal Kinase (JNK), and DNA synthesis in the human pancreatic adenocarcinoma cell line PANC-1. To determine the effect of PKD1 overexpression on these biological responses, we generated inducible stable PANC-1 clones that express wild-type (WT) or kinase-dead (K618N) forms of PKD1 in response to the ecdysone analog ponasterone-A (PonA). NT potently stimulated c-Jun Ser(63) phosphorylation in both wild type and clonal derivatives of PANC-1 cells. PonA-induced expression of WT, but not K618N PKD1, rapidly blocked NT-mediated c-Jun Ser(63) phosphorylation either at the level of or upstream of MKK4, a dual-specificity kinase that leads to JNK activation. This is the first demonstration that PKD1 suppresses NT-induced JNK/cJun activation in PANC-1 cells. In contrast, PKD1 overexpression markedly increased the duration of NT-induced ERK activation in these cells. The reciprocal influence of PKD1 signaling on pro-mitogenicERK and pro-apopotic JNK/c-Jun pathways prompted us to examine whether PKD1 overexpression promotes DNA synthesis and proliferation of PANC-1 cells. Our results show that PKD1 overexpression increased DNA synthesis and cell numbers of PANC-1 cells cultured in regular dishes or in polyhydroxyethylmethacrylate [Poly-(HEMA)]-coated dishes to eliminate cell adhesion (anchorage-independent growth). Furthermore, PKD1 overexpression markedly enhanced DNA synthesis induced by NT (1-10 nM). These results indicate that PKD1 mediates mitogenic signaling in PANC-1 and suggests that this enzyme could be a novel target for the development of therapeutic drugs that restrict the proliferation of these cells.

Adhesion of renal carcinoma cells to endothelial cells depends on PKCmu

Background

The formation of metastases includes the separation of tumor cells from the primary tumor, cell migration into subendothelial tissue and cell proliferation in secondary organ. In this process, cell adhesion of tumor cells to the endothelium is an essential requirement for formation of metastases. Protein kinase C (PKC) regulates adhesion and proliferation. To identify a relation between PKC isoforms and tumor progression in renal cell carcinoma (RCC), the influence of PKC isoforms on cell adhesion and proliferation, and possible influences of integrins were analyzed in RCC cells.

Methods

The experiments were performed in the RCC cell lines CCF-RC1 and CCF-RC2 after pre-incubation (16 h) with the PKC inhibitors GF109203X (inhibits PKCα, βI, βII, γ, δ and ε), GÖ6976 (inhibits PKCα, βI and μ), RO31-8220 (inhibits PKCα, βI, βII, γ and ε) and rottlerin (inhibits PKCδ). Cell adhesion was assessed through adherence of RCC cells to an endothelial monolayer. Cell proliferation was analyzed by a BrdU incorporation assay. The expression of β1 integrins was analyzed by flow cytometry.

Results

In CCF-RC1 cells, cell adhesion was significantly reduced by GÖ6976 to 55% and by RO31-8220 to 45% of control. In CCF-RC2 cells, only GÖ6976 induced a significant reduction of cell adhesion to 50% of control levels. Proliferation of both cell lines was reduced by rottlerin to 39% and 45% of control, respectively. The β1 integrin expression on the cell surface of CCF-RC1 and CCR-RC2 cells was decreased by RO31-8220 to 8% and 7% of control, respectively. β2 and β3 integrins were undetectable in both cell lines.

Conclusions

The combination of the PKC inhibitors leads to the assumption that PKCμ influences cell adhesion in CCF-RC1 and CCF-RC2 cells, whereas in CCF-RC1 cells PKCε also seems to be involved in this process. The expression of β1 integrins appears to be regulated in particular by PKCε. Cell proliferation was inhibited by rottlerin, so that PKCδ might be involved in cell proliferation in these cells.

MAPK scaffolding by BIT1 in the Golgi complex modulates stress resistance

The endoplasmic reticulum (ER) is an essential organelle whose major functions are to ensure proper secretory protein folding and trafficking. These mechanisms involve the activation of specific ER-resident molecular machines, which might be regulated by their membranous environments. Based on this observation, we aimed to characterize the proteome of ER-membrane microdomains to identify new components of the ER that have a role in secretory pathway-associated functions. Using this approach with dog pancreatic rough microsomes, we found that mitochondrial Bcl-2 inhibitor of transcription (BIT1) localized in the early secretory pathway and accumulated in the Golgi complex. Using both a chimeric protein of the luminal and transmembrane domains of ER-resident TRAPalpha and the cytosolic domain of BIT1, and silencing of BIT1 expression, we perturbed endogenous BIT1 oligomerization and localization to the Golgi. This led to enhanced ERK signaling from the Golgi complex, which resulted in improved stress resistance. This work provides the first evidence for the existence of ER microdomains that are involved in the regulation of BIT1 structure and trafficking, and identifies BIT1 as a negative regulator of the ERK-MAPK signaling pathway in the Golgi.

Identification of crosstalk between phosphoprotein signaling pathways in RAW 264.7 macrophage cells

Signaling pathways mediate the effect of external stimuli on gene expression in cells. The signaling proteins in these pathways interact with each other and their phosphorylation levels often serve as indicators for the activity of signaling pathways. Several signaling pathways have been identified in mammalian cells but the crosstalk between them is not well understood. Alliance for Cellular Signaling (AfCS) has measured time-course data in RAW 264.7 macrophage cells on important phosphoproteins, such as the mitogen-activated protein kinases (MAPKs) and signal transducer and activator of transcription (STATs), in single- and double-ligand stimulation experiments for 22 ligands. In the present work, we have used a data-driven approach to analyze the AfCS data to decipher the interactions and crosstalk between signaling pathways in stimulated macrophage cells. We have used dynamic mapping to develop a predictive model using a partial least squares approach. Significant interactions were selected through statistical hypothesis testing and were used to reconstruct the phosphoprotein signaling network. The proposed data-driven approach is able to identify most of the known signaling interactions such as protein kinase B (Akt) --> glycogen synthase kinase 3alpha/beta (GSKalpha/beta) etc., and predicts potential novel interactions such as P38 --> RSK and GSK --> ezrin/radixin/moesin. We have also shown that the model has good predictive power for extrapolation. Our novel approach captures the temporal causality and directionality in intracellular signaling pathways. Further, case specific analysis of the phosphoproteins in the network has led us to propose hypothesis about inhibition (phosphorylation) of GSKalpha/beta via P38.

Protein kinase D stabilizes aldosterone-induced ERK1/2 MAP kinase activation in M1 renal cortical collecting duct cells to promote cell proliferation

Aldosterone elicits transcriptional responses in target tissues and also rapidly stimulates the activation of protein kinase signalling cascades independently of de novo protein synthesis. Here we investigated aldosterone-induced cell proliferation and extra-cellular regulated kinase 1 and 2 (ERK1/2) mitogen activated protein (MAP) kinase signalling in the M1 cortical collecting duct cell line (M1-CCD). Aldosterone promoted the proliferative growth of M1-CCD cells, an effect that was protein kinase D1 (PKD1), PKCdelta and ERK1/2-dependent. Aldosterone induced the rapid activation of ERK1/2 with peaks of activation at 2 and 10 to 30 min after hormone treatment followed by sustained activation lasting beyond 120 min. M1-CCD cells suppressed in PKD1 expression exhibited only the early, transient peaks in ERK1/2 activation without the sustained phase. Aldosterone stimulated the physical association of PKD1 with ERK1/2 within 2 min of treatment. The mineralocorticoid receptor (MR) antagonist RU28318 inhibited the early and late phases of aldosterone-induced ERK1/2 activation, and also aldosterone-induced proliferative cell growth. Aldosterone induced the sub-cellular redistribution of ERK1/2 to the nuclei at 2 min and to cytoplasmic sites, proximal to the nuclei after 30 min. This sub-cellular distribution of ERK1/2 was inhibited in cells suppressed in the expression of PKD1.