RESUMO
Protease activated receptors (PARs) are among the first receptors shown to transactivate other receptors: noticeably, these interactions are not limited to members of the same family, but involve receptors as diverse as receptor kinases, prostanoid receptors, purinergic receptors and ionic channels among others. In this review, we will focus on the evidence for PAR interactions with members of their own family, as well as with other types of receptors. We will discuss recent evidence as well as what we consider as emerging areas to explore; from the signalling pathways triggered, to the physiological and pathological relevance of these interactions, since this additional level of molecular cross-talk between receptors and signaling pathways is only beginning to be explored and represents a novel mechanism providing diversity to receptor function and play important roles in physiology and disease.
Assuntos
Receptores Ativados por Proteinase , Transdução de Sinais , Receptores Ativados por Proteinase/metabolismo , Transdução de Sinais/fisiologiaRESUMO
AIMS: The retinal pigment epithelium (RPE) is a highly specialized cell monolayer, that plays a key role in the maintenance of photoreceptor function and the blood-retina barrier (BRB). In this study, we found that a myristoylated pseudosubstrate of PKC-ζ (PKCζ PS), considered as a PKC-ζ inhibitor, plays a distinct role in RPE. MAIN METHODS: We demonstrated that PKCζ PS stimulates the release of Glutamate (Glu) using in vitro3H-Glutamate release experiments. By western blot, kinase assays, and Fluoresence Ca+2 Concentration Measurements, we determined the cellular mechanisms involved in such release. KEY FINDINGS: Surprisingly, PKCζ PS has no effect on either phosphorylation of T560, essential for catalytic activity, nor it has an effect on kinase activity. It induces the dose-dependent release of Glu by increasing intracellular Ca+2 levels. Interestingly, this release was not observed upon stimulation by other non-competitive PKC-ζ inhibitors. We here demonstrated that the PKCζ PS stimulates the release of Glutamate from RPE by activating the Ca2+-dependent Cl channel Bestrophin 1 (Best1). SIGNIFICANCE: These results question PKCζ PS specificity as an inhibitor of this enzyme. Furthermore, the present results underline the relevance of clarifying the molecular mechanisms involved in glutamate release from the retina under conditions derived from excitotoxic stimuli.
Assuntos
Bestrofinas/metabolismo , Ácido Glutâmico/metabolismo , Peptídeos/farmacologia , Proteína Quinase C/antagonistas & inibidores , Epitélio Pigmentado da Retina/metabolismo , Animais , Células Cultivadas , Relação Dose-Resposta a Droga , Peptídeos/administração & dosagem , Ratos , Ratos Long-Evans , Epitélio Pigmentado da Retina/citologiaRESUMO
PAR1 activation by thrombin promotes intracellular signaling leading to RPE cell transformation, proliferation, and migration, characteristic of fibroproliferative eye diseases. Due to the cleavage of PAR1 N-terminal domain, carried by thrombin, the arrest of PAR1 signaling is achieved by transport into lysosomes and degradation. Recent findings suggest that the GTPase Rab11a in conjunction with its effector RCP may direct PAR1 to lysosomes. Hereby we demonstrate that thrombin-induced PAR1 internalization and lysosomal targeting requires the disassembly of the Rab11a/RCP complex, and that this process depends on thrombin-induced intracellular calcium increase and calpain activation. These findings unveil a novel mechanism that regulates thrombin activated PAR1 internalization and degradation.
Assuntos
Proteínas Adaptadoras de Transdução de Sinal/metabolismo , Células Epiteliais/metabolismo , Proteínas de Membrana/metabolismo , Receptor PAR-1/metabolismo , Retina/metabolismo , Proteínas rab de Ligação ao GTP/metabolismo , Linhagem Celular , Proliferação de Células , Células Epiteliais/citologia , Humanos , Retina/citologiaRESUMO
Purpose: We analyzed the molecular mechanisms leading to glutamate release from rat primary cultures of RPE cells, under isosmotic conditions. Thrombin has been shown to stimulate glutamate release from astrocytes and retinal glia; however, the effect of thrombin on glutamate release from RPE cells has not been examined. Our previous work showed that upon the alteration of the blood-retina barrier, the serine protease thrombin could contribute to the transformation, proliferation, and migration of RPE cells. In this condition, elevated extracellular glutamate causes neuronal loss in many retinal disorders, including glaucoma, ischemia, diabetic retinopathy, and inherited photoreceptor degeneration. Methods: Primary cultures of rat RPE cells were preloaded with 1 µCi/ml 3H-glutamate in Krebs Ringer Bicarbonate (KRB) buffer for 30 min at 37 °C. Cells were rinsed and super-perfused with 1 ml/min KRB for 15 min. Stable release was reached at the 7th minute, and on the 8th minute, fresh KRB containing stimuli was added. Results: This study showed for the first time that thrombin promotes specific, dose-dependent glutamate release from RPE cells, induced by the activation of protease-activated receptor 1 (PAR-1). This effect was found to depend on the Ca2+ increase mediated by the phospholipase C-ß (PLC-ß) and protein kinase C (PKC) pathways, as well as by the reverse activity of the Na+/Ca2+ exchanger. Conclusions: Given the intimate contact of the RPE with the photoreceptor outer segments, diffusion of RPE-released glutamate could contribute to the excitotoxic death of retinal neurons, and the development of thrombin-induced eye pathologies.
Assuntos
Cálcio/metabolismo , Ácido Glutâmico/metabolismo , Proteína Quinase C/metabolismo , Epitélio Pigmentado da Retina/citologia , Trocador de Sódio e Cálcio/metabolismo , Trombina/farmacologia , Fosfolipases Tipo C/metabolismo , Animais , Forma Celular/efeitos dos fármacos , Transportador 1 de Aminoácido Excitatório/metabolismo , Fragmentos de Peptídeos/farmacologia , Transporte Proteico/efeitos dos fármacos , Ratos Long-Evans , Receptor PAR-1/metabolismo , Transdução de Sinais/efeitos dos fármacos , Trítio/metabolismoRESUMO
The serine protease thrombin activates Protease-Activated Receptors (PARs), a family of G-protein-coupled receptors (GPCRs) activated by the proteolytic cleavage of their extracellular N-terminal domain. Four members of this family have been identified: PAR1-4. The activation of Protease-Activated Receptor 1(PAR1), the prototype of this receptor family, leads to an increase in intracellular Ca+2 concentration ([Ca+2]i) mediated by Gq11α coupling and phospholipase C (PLC) activation. We have previously shown that the stimulation of PAR1 by thrombin promotes intracellular signaling leading to RPE cell transformation, proliferation, and migration which characterize fibroproliferative eye diseases leading to blindness. Within this context, the elucidation of the mechanisms involved in PAR1 inactivation is of utmost importance. Due to the irreversible nature of PAR1 activation, its inactivation must be efficiently regulated in order to terminate signaling. Using ARPE-19 human RPE cell line, we characterized thrombin-induced [Ca+2]i increase and demonstrated the calcium-dependent activation of µ-calpain mediated by PAR1. Calpains are a family of calcium-activated cysteine proteases involved in multiple cellular processes including the internalization of membrane proteins through clathrin-coated vesicles. We demonstrated that PAR1-induced calpain activation results in the degradation of α-spectrin by calpain, essential for receptor endocytosis, and the consequent decrease in PAR1 membrane expression. Collectively, the present results identify a novel µ-calpain-dependent mechanism for PAR1 inactivation following exposure to thrombin.
RESUMO
Paxilllin is a multifunctional and multidomain focal adhesion adapter protein which serves an important scaffolding role at focal adhesions by recruiting structural and signaling molecules involved in cell movement and migration, when phosphorylated on specific Tyr and Ser residues. Upon integrin engagement with extracellular matrix, paxillin is phosphorylated at Tyr31, Tyr118, Ser188, and Ser190, activating numerous signaling cascades which promote cell migration, indicating that the regulation of adhesion dynamics is under the control of a complex display of signaling mechanisms. Among them, paxillin disassembly from focal adhesions induced by extracellular regulated kinase (ERK)-mediated phosphorylation of serines 106, 231, and 290 as well as the binding of the phosphatase PEST to paxillin have been shown to play a key role in cell migration. Paxillin also coordinates the spatiotemporal activation of signaling molecules, including Cdc42, Rac1, and RhoA GTPases, by recruiting GEFs, GAPs, and GITs to focal adhesions. As a major participant in the regulation of cell movement, paxillin plays distinct roles in specific tissues and developmental stages and is involved in immune response, epithelial morphogenesis, and embryonic development. Importantly, paxillin is also an essential player in pathological conditions including oxidative stress, inflammation, endothelial cell barrier dysfunction, and cancer development and metastasis.
Assuntos
Movimento Celular , Paxilina/metabolismo , Animais , Adesões Focais/metabolismo , Humanos , Patologia Molecular , Fosforilação , Transdução de SinaisRESUMO
Glutamate, the main excitatory amino acid in the vertebrate retina, is a well know activator of numerous signal transduction pathways, and has been critically involved in long-term synaptic changes acting through ionotropic and metabotropic glutamate receptors. However, recent findings underlining the importance of intensity and duration of glutamate stimuli for specific neuronal responses, including excitotoxicity, suggest a crucial role for Na(+)-dependent glutamate transporters, responsible for the removal of this neurotransmitter from the synaptic cleft, in the regulation of glutamate-induced signaling. Transporter proteins are expressed in neurons and glia cells, albeit most of glutamate uptake occurs in the glial compartment. Within the retina, Müller glia cells are in close proximity to glutamatergic synapses and participate in the recycling of glutamate through the glutamate/glutamine shuttle. In this context, we decided to investigate a plausible role of glutamate as a regulatory signal for its own transport in human retinal glia cells. To this end, we determined [(3)H]-D-aspartate uptake in cultures of spontaneously immortalized human Müller cells (MIO-M1) exposed to distinct glutamatergic ligands. A time and dose-dependent increase in the transporter activity was detected. This effect was dependent on the activation of the N-methyl D-aspartate subtype of glutamate receptors, due to a dual effect: an increase in affinity and an augmented expression of the transporter at the plasma membrane, as established via biotinylation experiments. Furthermore, a NMDA-dependent association of glutamate transporters with the cystoskeletal proteins ezrin and glial fibrillary acidic protein was also found. These results add a novel mediator of the glutamate transporter modulation and further strengthen the notion of the critical involvement of glia cells in synaptic function.
Assuntos
Células Ependimogliais/metabolismo , Ácido Glutâmico/metabolismo , Neuroglia/metabolismo , Receptores de Glutamato/metabolismo , Regulação para Cima/fisiologia , Ácido Aspártico/metabolismo , Ácido Aspártico/farmacologia , Células Cultivadas , Células Ependimogliais/efeitos dos fármacos , Agonistas de Aminoácidos Excitatórios/farmacologia , Humanos , Neuroglia/efeitos dos fármacos , Regulação para Cima/efeitos dos fármacosRESUMO
The breakdown of the blood-retina barrier exposes retinal pigment epithelium (RPE) to serum components, thrombin among them. In addition to coagulation, thrombin acting through Protease-Activated Receptors (PARs 1-4) participates in a number of processes including cell proliferation, transformation, and migration. The purpose of this study was to identify interacting signaling pathways by which the activation of PAR1 by thrombin triggers cyclin D1 gene (Ccnd1) expression and the proliferation of RPE cells, characteristic of proliferative vitreoretinopathy (PVR). Our results demonstrate that thrombin induces the expression of the c-fos gene (c-fos), the activation of the (fos/jun) AP-1 site and the expression of Ccnd1, in precise correlation with the activation of CREB. Although the expression of both, c-fos and Ccnd1 requires the activation of conventional PKC isoforms and PI3K, downstream signaling from PI3K differs for both genes. Whereas the expression of c-fos requires PI3K-induced PDK1/Akt activity, that of Ccnd1 is mediated by PDK1-independent PKCζ signaling. Additionally, CREB activation may contribute to the induction of Ccnd1 expression through binding to the Ca/CRE element in the Ccnd1 gene promoter. Since cyclin D1 is a key regulator of cell cycle G1/S phase progression essential for proliferation, these findings further strengthen the critical involvement of thrombin in the development of proliferative retinopathies and may provide pharmacologic targets for the prevention or treatment of these diseases.
Assuntos
Ciclina D1/genética , RNA Mensageiro/genética , Epitélio Pigmentado da Retina/metabolismo , Regulação para Cima , Vitreorretinopatia Proliferativa/genética , Barreira Hematorretiniana/efeitos dos fármacos , Western Blotting , Proliferação de Células , Células Cultivadas , Ciclina D1/biossíntese , Hemostáticos/farmacologia , Humanos , Reação em Cadeia da Polimerase , Epitélio Pigmentado da Retina/patologia , Transdução de Sinais , Trombina/farmacologia , Vitreorretinopatia Proliferativa/tratamento farmacológico , Vitreorretinopatia Proliferativa/metabolismoRESUMO
Epithelial-mesenchymal transition (EMT), proliferation and migration of RPE cells characterize the development of proliferative vitreoretinopathy (PVR) and other fibro-proliferative eye diseases leading to blindness. A common event in these pathologies is the alteration of the BRB which allows the interaction of RPE cells with thrombin, a pro-inflammatory protease contained in serum. Thrombin promotion of cytoskeletal reorganization, proliferation, and migration has been reported in different cell types, although the molecular mechanisms involved in these processes remain poorly understood. Our previous work demonstrated that thrombin promotes RPE cell proliferation, cytoskeletal remodeling and migration, hallmark processes in the development of PVR. Thrombin induction of RPE cell proliferation requires PI3K, PDK1, and Akt/PKB (Akt) signaling leading to cyclin D1 gene expression. Since Akt functions as an upstream activator of mechanistic target of rapamycin complex 1 (mTORC1) and is also a downstream target for mTORC2, the aim of this work was to determine whether mTOR is involved in thrombin-induced RPE cell proliferation by regulating cyclin D1 expression in immortalized rat RPE-J cell line. Results demonstrate that thrombin-induced cyclin D1 expression and cell proliferation require Akt-independent phosphorylation/activation of mTOR at Ser 2448 mediated by PI3K/PKC-ζ/ERK1/2 signaling, concomitant to Akt-dependent activation of p70S6K carried by mTORC1.
Assuntos
Proteína Quinase 1 Ativada por Mitógeno/metabolismo , Proteína Quinase 3 Ativada por Mitógeno/metabolismo , Complexos Multiproteicos/metabolismo , Fosfatidilinositol 3-Quinases/metabolismo , Transdução de Sinais/efeitos dos fármacos , Serina-Treonina Quinases TOR/metabolismo , Trombina/farmacologia , Animais , Proteínas de Transporte/antagonistas & inibidores , Proteínas de Transporte/genética , Proteínas de Transporte/metabolismo , Linhagem Celular , Proliferação de Células/efeitos dos fármacos , Ciclina D1/antagonistas & inibidores , Ciclina D1/genética , Ciclina D1/metabolismo , Peptídeos e Proteínas de Sinalização Intracelular/antagonistas & inibidores , Peptídeos e Proteínas de Sinalização Intracelular/genética , Peptídeos e Proteínas de Sinalização Intracelular/metabolismo , Alvo Mecanístico do Complexo 1 de Rapamicina , Alvo Mecanístico do Complexo 2 de Rapamicina , Fosfoproteínas/antagonistas & inibidores , Fosfoproteínas/genética , Fosfoproteínas/metabolismo , Fosforilação/efeitos dos fármacos , Proteínas Serina-Treonina Quinases/metabolismo , Proteínas Proto-Oncogênicas c-akt/metabolismo , Piruvato Desidrogenase Quinase de Transferência de Acetil , Interferência de RNA , RNA Interferente Pequeno , Proteína Companheira de mTOR Insensível à Rapamicina , Ratos , Proteína Regulatória Associada a mTOR , Epitélio Pigmentado da Retina/citologia , Epitélio Pigmentado da Retina/efeitos dos fármacos , Epitélio Pigmentado da Retina/metabolismo , Proteínas Quinases S6 Ribossômicas 70-kDa/metabolismoRESUMO
Most retinal proliferative diseases involve blood-retinal barrier (BRB) breakdown, exposing the retinal pigment epithelium (RPE) to thrombin, which triggers cell transformation, proliferation and migration through the activation of PAR-1. These processes require the assembly of contractile stress fibers containing actin and non-muscle myosin II, which allow cell movement upon phosphorylation of the myosin light chains (MLCs). PKC family of kinases promotes agonist-mediated contraction in smooth muscle and endothelial cells through the activation of its downstream target, the PKC-potentiated inhibitory protein of 17 kDa (CPI-17), which specifically inhibits MLC phosphatase. Although the participation of PKC in RPE cell transdifferentiation has been suggested, the role of PKC/CPI-17 signaling has not been investigated. The purpose of this study was to analyze the involvement of specific PKC isoenzymes and their effector protein CPI-17 in thrombin-induced MLC phosphorylation and actin stress fiber assembly in RPE cells. Rat RPE cells in primary culture were shown to respond to thrombin stimulation by activation of conventional, novel and atypical PKC isoforms and the downstream phosphorylation of CPI-17 and MLC, which in turn promoted actin stress fiber assembly. These effects were prevented by the pharmacological inhibition of conventional PKC isoenzymes (Ro-32-0432) and novel PKCδ (rottlerin and δV1-1 antagonist peptide), as well as by myristoylated pseudosubstrates specifically directed to conventional and atypical PKC isoforms. Thrombin effects were mimicked by phorbol 12-myristate 13-acetate (PMA), further confirming the involvement of diacylglycerol (DAG)-sensitive classical and novel PKC isoforms in thrombin-induced actin cytoskeleton modification. The present work shows, for the first time, the functional expression of the oncoprotein CPI-17 in RPE cells and suggests that PKC/CPI-17 signaling is involved in the control of actin cytoskeletal remodeling leading to cell motility in RPE cells exposed to thrombin, and hence could contribute to the development of proliferative eye diseases.