Protein kinase C signaling "in" and "to" the nucleus: Master kinases in transcriptional regulation

PKC is a multifunctional family of Ser-Thr kinases widely implicated in the regulation of fundamental cellular functions, including proliferation, polarity, motility, and differentiation. Notwithstanding their primary cytoplasmic localization and stringent activation by cell surface receptors, PKC isozymes impel prominent nuclear signaling ultimately impacting gene expression. While transcriptional regulation may be wielded by nuclear PKCs, it most often relies on cytoplasmic phosphorylation events that result in nuclear shuttling of PKC downstream effectors, including transcription factors. As expected from the unique coupling of PKC isozymes to signaling effector pathways, glaring disparities in gene activation/repression are observed upon targeting individual PKC family members. Notably, specific PKCs control the expression and activation of transcription factors implicated in cell cycle/mitogenesis, epithelial-to-mesenchymal transition and immune function. Additionally, PKCs isozymes tightly regulate transcription factors involved in stepwise differentiation of pluripotent stem cells toward specific epithelial, mesenchymal, and hematopoietic cell lineages. Aberrant PKC expression and/or activation in pathological conditions, such as in cancer, leads to profound alterations in gene expression, leading to an extensive rewiring of transcriptional networks associated with mitogenesis, invasiveness, stemness, and tumor microenvironment dysregulation. In this review, we outline the current understanding of PKC signaling "in" and "to" the nucleus, with significant focus on established paradigms of PKC-mediated transcriptional control. Dissecting these complexities would allow the identification of relevant molecular targets implicated in a wide spectrum of diseases.

IFNγ receptor down-regulation facilitates Legionella survival in alveolar macrophages

Legionella pneumophila is an opportunistic human pathogen and causative agent of the acute pneumonia known as Legionnaire's disease. Upon inhalation, the bacteria replicate in alveolar macrophages (AM), within an intracellular vacuole termed the Legionella-containing vacuole. We recently found that, in vivo, IFNγ was required for optimal clearance of intracellular L. pneumophila by monocyte-derived cells (MC), but the cytokine did not appear to influence clearance by AM. Here, we report that during L. pneumophila lung infection, expression of the IFNγ receptor subunit 1 (IFNGR1) is down-regulated in AM and neutrophils, but not MC, offering a possible explanation for why AM are unable to effectively restrict L. pneumophila replication in vivo. To test this, we used mice that constitutively express IFNGR1 in AM and found that prevention of IFNGR1 down-regulation enhanced the ability of AM to restrict L. pneumophila intracellular replication. IFNGR1 down-regulation was independent of the type IV Dot/Icm secretion system of L. pneumophila indicating that bacterial effector proteins were not involved. In contrast to previous work, we found that signaling via type I IFN receptors was not required for IFNGR1 down-regulation in macrophages but rather that MyD88- or Trif- mediated NF-κB activation was required. This work has uncovered an alternative signaling pathway responsible for IFNGR1 down-regulation in macrophages during bacterial infection.

MicroRNA-181a regulates the activation of the NLRP3 inflammatory pathway by targeting MEK1 in THP-1 macrophages stimulated by ox-LDL

Atherosclerosis (AS) is a chronic inflammatory disease that is characterized by the deposition of lipids in the vascular wall and the formation of foam cells. Macrophages play a critical role in the development of this chronic inflammation. An increasing amount of research shows that microRNAs affect many steps of inflammation. The goal of our study was to investigate the regulatory effect of miR-181a on the NLRP3 inflammasome pathway and explore its possible mechanism. Compared with the control group, the expression of miR-181a was downregulated in the carotid tissue of AS group mice, while the expression of MEK1 and NLRP3-related proteins was upregulated significantly. In vitro, when THP-1 macrophages were stimulated with oxidized low-density lipoprotein (ox-LDL), the expression of miR-181a was decreased, the MEK/ERK/NF-κB inflammatory pathways were activated and the expression of NLRP3 inflammasome-related proteins was upregulated. Exogenous overexpression of miR-181a downregulated the activation of the MEK/ERK/NF-κB pathway and decreased the expression of NLRP3 inflammasome-related proteins (such as NLRP3, caspase-1, interleukin-18 [IL-18], IL-1β, etc). Exogenous miR-181a knockdown showed the opposite results to those of overexpression group. A luciferase reporter assay proved that miR-181a inhibited the expression of MEK1 by binding to its 3'-untranslated region. When we knocked down miR-181a and then treated cells with U0126 before ox-LDL stimulation, we found that U0126 reversed the increased activation of the MEK/ERK/NF-κB pathway and upregulation of NLRP3 inflammasome-related proteins (NLRP3, caspase-1, IL-18, IL-1β) that resulted from miR-181a knockdown. Our study suggests that miR-181a regulates the activation of the NLRP3 inflammatory pathway by altering the activity of the MEK/ERK/NF-κB pathway via targeting of MEK1.

Insulin Promotes Wound Healing by Inactivating NFkβP50/P65 and Activating Protein and Lipid Biosynthesis and alternating Pro/Anti-inflammatory Cytokines Dynamics

Four hundred and twenty-two million people have diabetes due to excess free body glucose in their body fluids. Diabetes leads to various problems including retinopathy, neuropathy, arthritis, damage blood vessels etc; it also causes a delay in wound healing. Insufficiency of insulin is the main reason for diabetes-I and systemic insulin treatment is a remedy. The perspective of the potential use of insulin/insulin based drugs to treat chronic wounds in diabetic conditions is focused on in this review. At the site of the wound, TNF-ɑ, IFN-ϒ, IL-1β and IL-6 pro-inflammatory cytokines cause the generation of free radicals, leading to inflammation which becomes persistent in diabetes. Insulin induces expression of IL-4/IL-13, IL-10 anti-inflammatory cytokines etc which further down-regulates NFkβP50/P65 assembly. Insulin shifts the equilibrium towards NFkβP50/P50 which leads to down-regulation of inflammatory cytokines such as IL-6, IL-10 etc through STAT6, STAT3 and c-Maf activation causing nullification of an inflammatory condition. Insulin also promotes protein and lipid biosynthesis which indeed promotes wound recovery. Here, in this article, the contributions of insulin in controlling wound tissue microenvironments and remodulation of tissue have been summarised, which may be helpful to develop novel insulin-based formulation(s) for effective treatment of wounds in diabetic conditions.

Loss in PKC Epsilon Causes Downregulation of MnSOD and BDNF Expression in Neurons of Alzheimer's Disease Hippocampus

Oxidative stress and amyloid-β (Aβ) oligomers have been implicated in Alzheimer's disease (AD). The growth and maintenance of neuronal networks are influenced by brain derived neurotrophic factor (BDNF) expression, which is promoted by protein kinase C epsilon (PKCɛ). We investigated the reciprocal interaction among oxidative stress, Aβ, and PKCɛ levels and subsequent PKCɛ-dependent MnSOD and BDNF expression in hippocampal pyramidal neurons. Reduced levels of PKCɛ, MnSOD, and BDNF and an increased level of Aβ were also found in hippocampal neurons from autopsy-confirmed AD patients. In cultured human primary hippocampal neurons, spherical aggregation of Aβ (amylospheroids) decreased PKCɛ and MnSOD. Treatment with t-butyl hydroperoxide (TBHP) increased superoxide, the oxidative DNA/RNA damage marker, 8-OHG, and Aβ levels, but reduced PKCɛ, MnSOD, BDNF, and cultured neuron density. These changes were reversed with the PKCɛ activators, bryostatin and DCPLA-ME. PKCɛ knockdown suppressed PKCɛ, MnSOD, and BDNF but increased Aβ. In cultured neurons, the increase in reactive oxygen species (ROS) associated with reduced PKCɛ during neurodegeneration was inhibited by the SOD mimetic MnTMPyP and the ROS scavenger NAc, indicating that strong oxidative stress suppresses PKCɛ level. Reduction of PKCɛ and MnSOD was prevented with the PKCɛ activator bryostatin in 5-6-month-old Tg2576 AD transgenic mice. In conclusion, oxidative stress and Aβ decrease PKCɛ expression. Reciprocally, a depression of PKCɛ reduces BDNF and MnSOD, resulting in oxidative stress. These changes can be prevented with the PKCɛ-specific activators.

Cytoprotective pathways in the vascular endothelium. Do they represent a viable therapeutic target?

The vascular endothelium is a critical interface, which separates the organs from the blood and its contents. The endothelium has a wide variety of functions and maintenance of endothelial homeostasis is a multi-dimensional active process, disruption of which has potentially deleterious consequences if not reversed. Vascular injury predisposes to endothelial apoptosis, dysfunction and development of atherosclerosis. Endothelial dysfunction is an end-point, a central feature of which is increased ROS generation, a reduction in endothelial nitric oxide synthase and increased nitric oxide consumption. A dysfunctional endothelium is a common feature of diseases including rheumatoid arthritis, systemic lupus erythematosus, diabetes mellitus and chronic renal impairment. The endothelium is endowed with a variety of constitutive and inducible mechanisms that act to minimise injury and facilitate repair. Endothelial cytoprotection can be enhanced by exogenous factors such as vascular endothelial growth factor, prostacyclin and laminar shear stress. Target genes include endothelial nitric oxide synthase, heme oxygenase-1, A20 and anti-apoptotic members of the B cell lymphoma protein-2 family. In light of the importance of endothelial function, and the link between its disruption and the risk of atherothrombosis, interest has focused on therapeutic conditioning and reversal of endothelial dysfunction. A detailed understanding of cytoprotective signalling pathways, their regulation and target genes is now required to identify novel therapeutic targets. The ultimate aim is to add vasculoprotection to current therapeutic strategies for systemic inflammatory diseases, in an attempt to reduce vascular injury and prevent or retard atherogenesis.

Cardiac inflammation involving in PKCε or ERK1/2-activated NF-κB signalling pathway in mice following exposure to titanium dioxide nanoparticles

The evaluation of toxicological effects of nanoparticles (NPs) is increasingly important due to their growing occupational use and presence as compounds in consumer products. Recent researches have demonstrated that long-term exposure to air particulate matter can induce cardiovascular events, but whether cardiovascular disease, such as cardiac damage, is induced by NP exposure and its toxic mechanisms is rarely evaluated. In the present study, when mice were continuously exposed to TiO2 NPs at 2.5, 5 or 10mg/kg BW by intragastric administration for 90days, obvious histopathological changes, and great alterations of NF-κB and its inhibitor I-κB, as well as TNF-α, IL-1β, IL-6 and IFN-α expression were induced. The NPs significantly decreased Ca(2+)-ATPase, Ca(2+)/Mg(2+)-ATPase and Na(+)/K(+)-ATPase activities and enhanced NCX-1 content. The NPs also considerably increased CAMK II and α1/β1-AR expression and up-regulated p-PKCε and p-ERK1/2 in a dose-dependent manner in the mouse heart. These data suggest that low-dose and long-term exposure to TiO2 NPs may cause cardiac damage such as cardiac fragmentation or disordered myocardial fibre arrangement, tissue necrosis, myocardial haemorrhage, swelling or cardiomyocyte hypertrophy, and the inflammatory response was potentially mediated by NF-κB activation via the PKCε or ERK1/2 signalling cascades in mice.

PKCε-CREB-Nrf2 signalling induces HO-1 in the vascular endothelium and enhances resistance to inflammation and apoptosis

AIMS

Vascular injury leading to endothelial dysfunction is a characteristic feature of chronic renal disease, diabetes mellitus, and systemic inflammatory conditions, and predisposes to apoptosis and atherogenesis. Thus, endothelial dysfunction represents a potential therapeutic target for atherosclerosis prevention. The observation that activity of either protein kinase C epsilon (PKCε) or haem oxygenase-1 (HO-1) enhances endothelial cell (EC) resistance to inflammation and apoptosis led us to test the hypothesis that HO-1 is a downstream target of PKCε.

METHODS AND RESULTS

Expression of constitutively active PKCε in human EC significantly increased HO-1 mRNA and protein, whereas conversely aortas or cardiac EC from PKCε-deficient mice exhibited reduced HO-1 when compared with wild-type littermates. Angiotensin II activated PKCε and induced HO-1 via a PKCε-dependent pathway. PKCε activation significantly attenuated TNFα-induced intercellular adhesion molecule-1, and increased resistance to serum starvation-induced apoptosis. These responses were reversed by the HO antagonist zinc protoporphyrin IX. Phosphokinase antibody array analysis identified CREB1((Ser133)) phosphorylation as a PKCε signalling intermediary, and cAMP response element-binding protein 1 (CREB1) siRNA abrogated PKCε-induced HO-1 up-regulation. Likewise, nuclear factor (erythroid-derived 2)-like 2 (Nrf2) was identified as a PKCε target using nuclear translocation and DNA-binding assays, and Nrf2 siRNA prevented PKCε-mediated HO-1 induction. Moreover, depletion of CREB1 inhibited PKCε-induced Nrf2 DNA binding, suggestive of transcriptional co-operation between CREB1 and Nrf2.

CONCLUSIONS

PKCε activity in the vascular endothelium regulates HO-1 via a pathway requiring CREB1 and Nrf2. Given the potent protective actions of HO-1, we propose that this mechanism is an important contributor to the emerging role of PKCε in the maintenance of endothelial homeostasis and resistance to injury.

Death receptor 3 mediates TNFSF15- and TNFα-induced endothelial cell apoptosis

Tumor necrosis factor superfamily 15 (TNFSF15) suppresses angiogenesis by specifically inducing apoptosis in proliferating endothelial cells. Death receptor 3 (DR3), a member of the TNF receptor superfamily (TNFRSF25), has been identified as a receptor for TNFSF15 to activate T cells. It is unclear, however, whether DR3 mediates TNFSF15 activity on endothelial cells. Here we show that siRNA-mediated knockdown of DR3 in an in vivo Matrigel angiogenesis assay, or in adult bovine aortic endothelial (ABAE) cell cultures, leads to resistance of endothelial cells to TNFSF15-induced apoptosis. Interestingly, DR3-depleted cells also exhibited markedly diminished responsiveness to TNFα cytotoxicity, even though DR3 is not a receptor for TNFα. Treatment of the cells with either TNFSF15 siRNA or a TNFSF15-neutralizing antibody, 4-3H, also results in a significant inhibition of TNFα-induced apoptosis. Mechanistically, DR3 siRNA treatment gives rise to an increase of ERK1/2 MAPK activity, and up-regulation of the anti-apoptotic proteins c-FLIP and Bcl-2, thus strengthening apoptosis-resisting potential in the cells. These findings indicate that DR3 mediates TNFSF15-induced endothelial cell apoptosis, and that up-regulation of TNFSF15 expression stimulated by TNFα is partly but significantly responsible for TNFα-induced apoptosis in endothelial cells.

The "memory kinases": roles of PKC isoforms in signal processing and memory formation

The protein kinase C (PKC) isoforms, which play an essential role in transmembrane signal conduction, can be viewed as a family of "memory kinases." Evidence is emerging that they are critically involved in memory acquisition and maintenance, in addition to their involvement in other functions of cells. Deficits in PKC signal cascades in neurons are one of the earliest abnormalities in the brains of patients suffering from Alzheimer's disease. Their dysfunction is also involved in several other types of memory impairments, including those related to emotion, mental retardation, brain injury, and vascular dementia/ischemic stroke. Inhibition of PKC activity leads to a reduced capacity of many types of learning and memory, but may have therapeutic values in treating substance abuse or aversive memories. PKC activators, on the other hand, have been shown to possess memory-enhancing and antidementia actions. PKC pharmacology may, therefore, represent an attractive area for developing effective cognitive drugs for the treatment of many types of memory disorders and dementias.

Protein kinase Cε-calcineurin cosignaling downstream of toll-like receptor 4 downregulates fibrosis and induces wound healing gene expression in cardiac myofibroblasts

The pathways which regulate resolution of inflammation and contribute to positive remodeling of the myocardium following injury are poorly understood. Here we show that protein kinase C epsilon (PKCε) cooperates with the phosphatase calcineurin (CN) to potentiate induction of cardioprotective gene expression while suppressing expression of fibrosis markers. This was achieved by detailed analysis of the regulation of cyclooxygenase 2 (COX-2) expression as a marker gene and by using gene expression profiling to identify genes regulated by coexpression of CN-Aα/PKCε in adult rat cardiac myofibroblasts (ARVFs) on a larger scale. GeneChip analysis of CN-Aα/PKCε-coexpressing ARVFs showed that COX-2 provides a signature for wound healing and is associated with downregulation of fibrosis markers, including connective tissue growth factor (CTGF), fibronectin, and collagens Col1a1, Col3a1, Col6a3, Col11a1, Col12a1, and Col14a1, with concomitant upregulation of cardioprotection markers, including COX-2 itself, lipocalin 2 (LCN2), tissue inhibitor of metalloproteinase 1 (TIMP-1), interleukin-6 (IL-6), and inducible nitric oxide synthase (iNOS). In primary rat cardiomyocyte cultures Toll-like receptor 4 (TLR4) agonist- or PKCε/CN-dependent COX-2 induction occurred in coresident fibroblasts and was blocked by selective inhibition of CN or PKC α/ε or elimination of fibroblasts. Furthermore, ectopic expression of PKCε and CN in ARVFs showed that the effects on COX-2 expression are mediated by specific NFAT sites within the COX-2 promoter as confirmed by site-directed mutagenesis and chromatin immunoprecipitation (ChIP). Therefore, PKCε may negatively regulate adverse myocardial remodeling by cooperating with CN to downregulate fibrosis and induce transcription of cardioprotective wound healing genes, including COX-2.

Amelioration of compound 4,4'-diphenylmethane-bis(methyl)carbamate on high mobility group box1-mediated inflammation and oxidant stress responses in human umbilical vein endothelial cells via RAGE/ERK1/2/NF-κB pathway

High mobility group box-1 (HMGB1), a secreted nuclear protein, acts as an inflammatory mediator and has been implicated in pathophysiological damage of diabetic vascular complications. A compound 4,4'-diphenylmethane-bis(methyl) carbamate (CM1) has a protective activity on advanced glycation end products (AGEs)-induced endothelial dysfunction in our previous study. The aim of this study was to investigate whether CM1 could attenuate HMGB1-induced endothelial dysfunction in human umbilical vein endothelial cells (HUVECs), and also elucidate the possible underlying mechanism. The pre-treatment of CM1 (10(-9)M) could inhibit significantly the migration of macrophages in co-incubation with HUVECs system. HMGB1 stimulated intercellular adhesion molecule-1 (ICAM-1), transforming growth factor-beta1 (TGF-β1) and receptor for advanced glycation end products (RAGE) protein expression in HUVECs, which were inhibited by pretreatment with CM1. Furthermore, it also reduced significantly reactive oxygen species (ROS) generation and inflammatory cytokine interleukin-6 (IL-6) level in co-incubation system. Immunofluorescence and Western blotting assays showed that CM1 could attenuate HMGB1-induced intracellular ERK1/2 and NF-kB activation in HUVECs. Our findings indicated that CM1 attenuated HMGB1-mediated endothelial activation by ameliorating inflammation and oxidant stress responses via RAGE/ERK1/2/NF-κB pathway.