Trafficking, ubiquitination, and down-regulation of the human platelet-activating factor receptor

A Plasmodium falciparum ubiquitin-specific protease (PfUSP) is essential for parasite survival and its disruption enhances artemisinin efficacy

Proteins associated with ubiquitin-proteasome system (UPS) are potential drug targets in the malaria parasite. The ubiquitination and deubiquitination are key regulatory processes for the functioning of UPS. In this study, we have characterized the biochemical and functional role of a novel ubiquitin-specific protease (USP) domain-containing protein of the human malaria parasite Plasmodium falciparum (PfUSP). We have shown that the PfUSP is an active deubiquitinase associated with parasite endoplasmic reticulum (ER). Selection linked integration (SLI) method for C-terminal tagging and GlmS-ribozyme mediated inducible knock-down (iKD) of PfUSP was utilized to assess its functional role. Inducible knockdown of PfUSP resulted in a remarkable reduction in parasite growth and multiplication; specifically, PfUSP-iKD disrupted ER morphology and development, blocked the development of healthy schizonts, and hindered proper merozoite development. PfUSP-iKD caused increased ubiquitylation of specific proteins, disrupted organelle homeostasis and reduced parasite survival. Since the mode of action of artemisinin and the artemisinin-resistance are shown to be associated with the proteasome machinery, we analyzed the effect of dihydroartemisinin (DHA) on PfUSP-iKD parasites. Importantly, the PfUSP-knocked-down parasite showed increased sensitivity to dihydroartemisinin (DHA), whereas no change in chloroquine sensitivity was observed, suggesting a role of PfUSP in combating artemisinin-induced cellular stress. Together, the results show that Plasmodium PfUSP is an essential protease for parasite survival, and its inhibition increases the efficacy of artemisinin-based drugs. Therefore, PfUSP can be targeted to develop novel scaffolds for developing new antimalarials to combat artemisinin resistance.

Membrane compartmentalisation of the ubiquitin system

We now have a comprehensive inventory of ubiquitin system components. Understanding of any system also needs an appreciation of how components are organised together. Quantitative proteomics has provided us with a census of their relative populations in several model cell types. Here, by examining large scale unbiased data sets, we seek to identify and map those components, which principally reside on the major organelles of the endomembrane system. We present the consensus distribution of > 50 ubiquitin modifying enzymes, E2s, E3s and DUBs, that possess transmembrane domains. This analysis reveals that the ER and endosomal compartments have a diverse cast of resident E3s, whilst the Golgi and mitochondria operate with a more restricted palette. We describe key functions of ubiquitylation that are specific to each compartment and relate this to their signature complement of ubiquitin modifying components.

Repurposing the Pathogen Box compounds for identification of potent anti-malarials against blood stages of Plasmodium falciparum with PfUCHL3 inhibitory activity

Malaria has endured as a global epidemic since ages and its eradication poses an immense challenge due to the complex life cycle of the causative pathogen and its tolerance to a myriad of therapeutics. PfUCHL3, a member of the ubiquitin C-terminal hydrolase (UCH) family of deubiquitinases (DUBs) is cardinal for parasite survival and emerges as a promising therapeutic target. In this quest, we employed a combination of computational and experimental approaches to identify PfUCHL3 inhibitors as novel anti-malarials. The Pathogen Box library was screened against the crystal structure of PfUCHL3 (PDB ID: 2WE6) and its human ortholog (PDB ID: 1XD3). Fifty molecules with better comparative score, bioavailability and druglikeliness were subjected to in-vitro enzyme inhibition assay and among them only two compounds effectively inhibited PfUCHL3 activity at micro molar concentrations. Both MMV676603 and MMV688704 exhibited anti-plasmodial activity by altering the parasite phenotype at late stages of the asexual life cycle and inducing the accumulation of polyubiquitinated substrates. In addition, both the compounds were non-toxic and portrayed high selectivity window for the parasite over mammalian cells. This is the first comprehensive study to demonstrate the anti-malarial efficacy of PfUCHL3 inhibitors and opens new avenues to exploit UCH family of DUBs as a promising target for the development of next generation anti-malaria therapy.

The Role of the Proteasome in Platelet Function

Platelets are megakaryocyte-derived acellular fragments prepped to maintain primary hemostasis and thrombosis by preserving vascular integrity. Although they lack nuclei, platelets harbor functional genomic mediators that bolster platelet activity in a signal-specific manner by performing limited de novo protein synthesis. Furthermore, despite their limited protein synthesis, platelets are equipped with multiple protein degradation mechanisms, such as the proteasome. In nucleated cells, the functions of the proteasome are well established and primarily include proteostasis among a myriad of other signaling processes. However, the role of proteasome-mediated protein degradation in platelets remains elusive. In this review article, we recapitulate the developing literature on the functions of the proteasome in platelets, discussing its emerging regulatory role in platelet viability and function and highlighting how its functional coupling with the transcription factor NF-κB constitutes a novel potential therapeutic target in atherothrombotic diseases.

Down-regulation of platelet adhesion receptors is a controlling mechanism of thrombosis, while also affecting post-transfusion efficacy of stored platelets

Physiologically, upon platelet activation, uncontrolled propagation of thrombosis is prevented by regulating mechanisms which affect the expression and function of either platelet adhesion receptors or integrins. Receptor ectodomain shedding is an elective mechanism which is mainly involved in down-regulation of adhesion receptors GPIbα and GPVI. Platelet integrin α_IIbβ₃ can also be modulated with a calpain-dependent proteolytic cleavage. In addition, activating signals may induce the internalization of expressed receptors to selectively down-regulate their intensity. Alternatively, further activation of platelets is associated with microvesiculation as a none-selective mechanism which leads to the loss of membrane- bearing receptors. In a non-physiological condition, the storage of therapeutic platelets has also shown to be associated with the unwilling activation of platelets which triggers receptors down-regulation via aforementioned different mechanisms. Notably, herein the changes are time-dependent and not controllable. While the expression and shedding of pro-inflammatory molecules can induce post-transfusion adverse effects, stored-dependent loss of adhesion receptors by ectodomain shedding or microvesiculation may attenuate post-transfusion adhesive functions of platelets causing their premature clearance from circulation. In its first part, the review presented here aims to describe the mechanisms involved in down-regulation of platelet adhesion receptors. It then highlights the crucial role of ectodomain shedding and microvesiculation in the propagation of "platelet storage lesion" which may affect the post-transfusion efficacy of platelet components.

CK19 stabilizes CFTR at the cell surface by limiting its endocytic pathway degradation

Protein interactions that stabilize the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) at the apical membranes of epithelial cells have not yet been fully elucidated. We identified keratin 19 (CK19 or K19) as a novel CFTR-interacting protein. CK19 overexpression stabilized both wild-type (WT)-CFTR and Lumacaftor (VX-809)-rescued F508del-CFTR (where F508del is the deletion of the phenylalanine residue at position 508) at the plasma membrane (PM), promoting Cl^- secretion across human bronchial epithelial (HBE) cells. CK19 prevention of Rab7A-mediated lysosomal degradation was a key mechanism in apical CFTR stabilization. Unexpectedly, CK19 expression was decreased by ∼40% in primary HBE cells from homogenous F508del patients with CF relative to non-CF controls. CK19 also positively regulated multidrug resistance-associated protein 4 expression at the PM, suggesting that this keratin may regulate the apical expression of other ATP-binding cassette proteins as well as CFTR.-Hou, X., Wu, Q., Rajagopalan, C., Zhang, C., Bouhamdan, M., Wei, H., Chen, X., Zaman, K., Li, C., Sun, X., Chen, S., Frizzell, R. A., Sun, F. CK19 stabilizes CFTR at the cell surface by limiting its endocytic pathway degradation.

S1P1 receptor phosphorylation, internalization, and interaction with Rab proteins: effects of sphingosine 1-phosphate, FTY720-P, phorbol esters, and paroxetine

Sphingosine 1-phosphate (S1P) and FTY720-phosphate (FTYp) increased intracellular calcium in cells expressing S1P₁ mCherry-tagged receptors; the synthetic agonist was considerably less potent. Activation of protein kinase C by phorbol myristate acetate (PMA) blocked these effects. The three agents induced receptor phosphorylation and internalization, with the action of FTYp being more intense. S1P₁ receptor–Rab protein (GFP-tagged) interaction was studied using FRET. The three agents were able to induce S1P₁ receptor–Rab5 interaction, although with different time courses. S1P₁ receptor–Rab9 interaction was mainly increased by the phorbol ester, whereas S1P₁ receptor–Rab7 interaction was only increased by FTYp and after a 30-min incubation. These actions were not observed using dominant negative (GDP-bound) Rab protein mutants. The data suggested that the three agents induce interaction with early endosomes, but that the natural agonist induced rapid receptor recycling, whereas activation of protein kinase C favored interaction with late endosome and slow recycling and FTYp triggered receptor interaction with vesicles associated with proteasomal/lysosomal degradation. The ability of bisindolylmaleimide I and paroxetine to block some of these actions suggested the activation of protein kinase C was associated mainly with the action of PMA, whereas G protein-coupled receptor kinase (GRK) 2 (GRK2) was involved in the action of the three agents.

Site-specific polyubiquitination differentially regulates parathyroid hormone receptor-initiated MAPK signaling and cell proliferation

G protein-coupled receptor (GPCR) signaling and trafficking are essential for cellular function and regulated by phosphorylation, β-arrestin, and ubiquitination. The GPCR parathyroid hormone receptor (PTHR) exhibits time-dependent reversible ubiquitination. The exact ubiquitination sites in PTHR are unknown, but they extend upstream of its intracellular tail. Here, using tandem MS, we identified Lys³⁸⁸ in the third loop and Lys⁴⁸⁴ in the C-terminal tail as primary ubiquitination sites in PTHR. We found that PTHR ubiquitination requires β-arrestin and does not display a preference for β-arrestin1 or -2. PTH stimulated PTHR phosphorylation at Thr³⁸⁷/Thr³⁹² and within the Ser⁴⁸⁹-Ser⁴⁹³ region. Such phosphorylation events may recruit β-arrestin, and we observed that chemically or genetically blocking PTHR phosphorylation inhibits its ubiquitination. Specifically, Ala replacement at Thr³⁸⁷/Thr³⁹² suppressed β-arrestin binding and inhibited PTHR ubiquitination, suggesting that PTHR phosphorylation and ubiquitination are interdependent. Of note, Lys-deficient PTHR mutants promoted normal cAMP formation, but exhibited differential mitogen-activated protein kinase (MAPK) signaling. Lys-deficient PTHR triggered early onset and delayed ERK1/2 signaling compared with wildtype PTHR. Moreover, ubiquitination of Lys³⁸⁸ and Lys⁴⁸⁴ in wildtype PTHR strongly decreased p38 signaling, whereas Lys-deficient PTHR retained signaling comparable to unstimulated wildtype PTHR. Lys-deficient, ubiquitination-refractory PTHR reduced cell proliferation and increased apoptosis. However, elimination of all 11 Lys residues in PTHR did not affect its internalization and recycling. These results pinpoint the ubiquitinated Lys residues in PTHR controlling MAPK signaling and cell proliferation and survival. Our findings suggest new opportunities for targeting PTHR ubiquitination to regulate MAPK signaling or manage PTHR-related disorders.

Modulation of Tumor-Associated Macrophages (TAM) Phenotype by Platelet-Activating Factor (PAF) Receptor

Platelet-activating factor (PAF) plays an important role in the pathogenesis of several types of tumors. The biological effects of PAF are mediated by the PAF receptor (PAFR), which can be expressed by tumor cells and host cells that infiltrate the tumor microenvironment. In the present study, we investigated the role of PAFR expressed by leukocytes that infiltrate two types of tumors, one that expresses PAFR (TC-1 carcinoma) and another that does not express the receptor (B16F10 melanoma) implanted in mice that express the receptor or not (PAFR KO). It was found that both tumors grew significantly less in PAFR KO than in wild-type (WT) mice. Analysis of the leukocyte infiltration shown in PAFR KO increased the frequency of neutrophils (Gr1⁺) and of CD8⁺ lymphocytes in B16F10 tumors and of CD4⁺ lymphocytes in TC-1 tumors. PAFR KO also had a higher frequency of M1-like (CD11c⁺) and lower M2-like (CD206⁺) macrophages infiltrated in both tumors. This was confirmed in macrophages isolated from the tumors that showed higher iNOS, lower arginase activity, and lower IL10 expression in PAFR KO tumors than WT mice. These data suggest that in the tumor microenvironment, endogenous PAF-like activity molecules bind PAFR in macrophages which acquire an M2-like profile and this promotes tumor growth.

Characterization of the Phospholipid Platelet-Activating Factor As a Mediator of Inflammation in Chickens

Lipid mediators are known to play important roles in the onset and resolution phases of the inflammatory response in mammals. The phospholipid platelet-activating factor (PAF) is a pro-inflammatory lipid mediator which participates in vascular- and innate immunity-associated processes by increasing vascular permeability, by facilitating leukocyte adhesion to the endothelium, and by contributing to phagocyte activation. PAF exerts its function upon binding to its specific receptor, PAF receptor (PAFR), which is abundantly expressed in leukocytes and endothelial cells (ECs). In chickens, lipid mediators and their functions are still poorly characterized, and the role of PAF as an inflammatory mediator has not yet been investigated. In the present study we demonstrate that primary chicken macrophages express PAFR and lysophosphatidylcholine acyltransferase 2 (LPCAT2), the latter being essential to PAF biosynthesis during inflammation. Also, exogenous PAF treatment induces intracellular calcium increase, reactive oxygen species release, and increased phagocytosis by primary chicken macrophages in a PAFR-dependent manner. We also show that PAF contributes to the Escherichia coli lipopolysaccharide (LPS)-induced pro-inflammatory response and boosts the macrophage response to E. coli LPS via phosphatidylinositol 3-kinase/Akt- and calmodulin kinase II-mediated intracellular signaling pathways. Exogenous PAF treatment also increases avian pathogenic E. coli intracellular killing by chicken macrophages, and PAFR and LPCAT2 are upregulated in chicken lungs and liver during experimental pulmonary colibacillosis. Finally, exogenous PAF treatment increases cell permeability and upregulates the expression of genes coding for proteins involved in leukocyte adhesion to the endothelium in primary chicken endothelial cells (chAEC). In addition to these vascular phenomena, PAF boosts the chAEC inflammatory response to bacteria-associated molecular patterns in a PAFR-dependent manner. In conclusion, we identified PAF as an inflammation amplifier in chicken macrophages and ECs, which suggests that PAF could play important roles in the endothelium-innate immunity interface in birds during major bacterial infectious diseases such as colibacillosis.

Regulation of G Protein-Coupled Receptors by Ubiquitination

G protein-coupled receptors (GPCRs) comprise the largest family of membrane receptors that control many cellular processes and consequently often serve as drug targets. These receptors undergo a strict regulation by mechanisms such as internalization and desensitization, which are strongly influenced by posttranslational modifications. Ubiquitination is a posttranslational modification with a broad range of functions that is currently gaining increased appreciation as a regulator of GPCR activity. The role of ubiquitination in directing GPCRs for lysosomal degradation has already been well-established. Furthermore, this modification can also play a role in targeting membrane and endoplasmic reticulum-associated receptors to the proteasome. Most recently, ubiquitination was also shown to be involved in GPCR signaling. In this review, we present current knowledge on the molecular basis of GPCR regulation by ubiquitination, and highlight the importance of E3 ubiquitin ligases, deubiquitinating enzymes and β-arrestins. Finally, we discuss classical and newly-discovered functions of ubiquitination in controlling GPCR activity.

Nuclear localization of platelet-activating factor receptor controls retinal neovascularization

Platelet-activating factor (PAF) is a pleiotropic phospholipid with proinflammatory, procoagulant and angiogenic actions on the vasculature. We and others have reported the presence of PAF receptor (Ptafr) at intracellular sites such as the nucleus. However, mechanisms of localization and physiologic functions of intracellular Ptafr remain poorly understood. We hereby identify the importance of C-terminal motif of the receptor and uncover novel roles of Rab11a GTPase and importin-5 in nuclear translocation of Ptafr in primary human retinal microvascular endothelial cells. Nuclear localization of Ptafr is independent of exogenous PAF stimulation as well as intracellular PAF biosynthesis. Moreover, nuclear Ptafr is responsible for the upregulation of unique set of growth factors, including vascular endothelial growth factor, in vitro and ex vivo. We further corroborate the intracrine PAF signaling, resulting in angiogenesis in vivo, using Ptafr antagonists with distinct plasma membrane permeability. Collectively, our findings show that nuclear Ptafr translocates in an agonist-independent manner, and distinctive functions of Ptafr based on its cellular localization point to another dimension needed for pharmacologic selectivity of drugs.

Influence of platelet-activating factor receptor (PAFR) on Brucella abortus infection: implications for manipulating the phagocytic strategy of B. abortus

BACKGROUND

Brucella abortus is an intracellular pathogen which can infect and persist in host cells through multiple interactions. Above all, its interaction to host cell receptor is important to understand the pathogenic mechanisms of B. abortus. Accordingly, we demonstrated that platelet-activating factor receptor (PAFR) affects host cell response against B. abortus infection.

RESULTS

First of all, B. abortus infection to macrophage induces secretion of platelet-activating factor (PAF), which is a PAFR agonist. The stimulation of PAFR by PAF remarkably increases B. abortus uptake into macrophages. It induces Janus kinase 2 (JAK2) and p38α phosphorylation, indicating that PAFR-mediated activation of JAK2 signaling leads to enhanced uptake of B. abortus. Moreover, the dynamics of F-actin polymerization revealed that PAFR-mediated B. abortus uptake is related with the reorganization of F-actin and JAK2. Upon B. abortus phagocytosis, reduced PAFR in the membrane and subsequently increased levels of PAFR colocalization with endosomes were observed which indicate that B. abortus uptake into macrophages allowed PAFR trafficking to endosomes.

CONCLUSIONS

This study demonstrated that PAFR has a compelling involvement in B. abortus uptake as a promoter of phagocytosis, which is associated with JAK2 activation. Thus, our findings establish a novel insight into a receptor-related phagocytic mechanism of B. abortus.

Chapter One - Ubiquitination and Deubiquitination of G Protein-Coupled Receptors

The seven-transmembrane containing G protein-coupled receptors (GPCRs) constitute the largest family of cell-surface receptors. Transmembrane signaling by GPCRs is fundamental to many aspects of physiology including vision, olfaction, cardiovascular, and reproductive functions as well as pain, behavior and psychomotor responses. The duration and magnitude of signal transduction is tightly controlled by a series of coordinated trafficking events that regulate the cell-surface expression of GPCRs at the plasma membrane. Moreover, the intracellular trafficking profiles of GPCRs can correlate with the signaling efficacy and efficiency triggered by the extracellular stimuli that activate GPCRs. Of the various molecular mechanisms that impart selectivity, sensitivity and strength of transmembrane signaling, ubiquitination of the receptor protein plays an important role because it defines both trafficking and signaling properties of the activated GPCR. Ubiquitination of proteins was originally discovered in the context of lysosome-independent degradation of cytosolic proteins by the 26S proteasome; however a large body of work suggests that ubiquitination also orchestrates the downregulation of membrane proteins in the lysosomes. In the case of GPCRs, such ubiquitin-mediated lysosomal degradation engenders long-term desensitization of transmembrane signaling. To date about 40 GPCRs are known to be ubiquitinated. For many GPCRs, ubiquitination plays a major role in postendocytic trafficking and sorting to the lysosomes. This chapter will focus on the patterns and functional roles of GPCR ubiquitination, and will describe various molecular mechanisms involved in GPCR ubiquitination.

SIRT1 attenuates PAF-induced MMP-2 production via down-regulation of PAF receptor expression in vascular smooth muscle cells

Silent mating type information regulation 2 homolog 1 (SIRT1) is known as a key regulator in the protection of various vascular disorders, however, no direct evidences have been reported in the progression of atherosclerosis. Considering the pivotal role of matrix metalloproteinase-2 (MMP-2) in plaque destabilization, this study investigated the role of SIRT1 on MMP-2 production in vascular smooth muscle cells (VSMCs) induced by platelet activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine). In VSMCs stimulated with resveratrol, SIRT1 activator, PAF receptor (PAFR) was internalized and then its protein levels were diminished. It was attenuated in cells pretreated with proteasome or lysosome inhibitor. Also, the degradation of PAFR in SIRT1-stimulated cells was significantly attenuated by β-arrestin2 depletion. In cells treated with nicotinamide, SIRT1 deacetylase inhibitor, PAFR internalization by resveratrol or reSIRT1 was inhibited, demonstrating that deacetylation of SIRT1 is an important step in SIRT1-induced PAFR down-regulation. Moreover, PAF-induced MMP-2 production in VSMCs and aorta was attenuated by resveratrol. In the aorta of SIRT1 transgenic mice, the PAF-induced MMP-2 expression was prominently attenuated compared to that in wild type mice. Taken together, it was suggested that SIRT1 down-regulated PAFR in VSMCs via β-arrestin2-mediated internalization and degradation, leading to an inhibition of PAF-induced MMP-2 production.

Fluid shear stress increases neutrophil activation via platelet-activating factor

Leukocyte exposure to hemodynamic shear forces is critical for physiological functions including initial adhesion to the endothelium, the formation of pseudopods, and migration into tissues. G-protein coupled receptors on neutrophils, which bind to chemoattractants and play a role in neutrophil chemotaxis, have been implicated as fluid shear stress sensors that control neutrophil activation. Recently, exposure to physiological fluid shear stresses observed in the microvasculature was shown to reduce neutrophil activation in the presence of the chemoattractant formyl-methionyl-leucyl-phenylalanine. Here, however, human neutrophil preexposure to uniform shear stress (0.1-2.75 dyn/cm(2)) in a cone-and-plate viscometer for 1-120 min was shown to increase, rather than decrease, neutrophil activation in the presence of platelet activating factor (PAF). Fluid shear stress exposure increased PAF-induced neutrophil activation in terms of L-selectin shedding, αMβ2 integrin activation, and morphological changes. Neutrophil activation via PAF was found to correlate with fluid shear stress exposure, as neutrophil activation increased in a shear stress magnitude- and time-dependent manner. These results indicate that fluid shear stress exposure increases neutrophil activation by PAF, and, taken together with previous observations, differentially controls how neutrophils respond to chemoattractants.

Topical application of a platelet activating factor receptor agonist suppresses phorbol ester-induced acute and chronic inflammation and has cancer chemopreventive activity in mouse skin

Platelet activating factor (PAF) has long been associated with acute edema and inflammatory responses. PAF acts by binding to a specific G-protein coupled receptor (PAF-R, Ptafr). However, the role of chronic PAF-R activation on sustained inflammatory responses has been largely ignored. We recently demonstrated that mice lacking the PAF-R (Ptafr-/- mice) exhibit increased cutaneous tumorigenesis in response to a two-stage chemical carcinogenesis protocol. Ptafr-/- mice also exhibited increased chronic inflammation in response to phorbol ester application. In this present study, we demonstrate that topical application of the non-hydrolysable PAF mimetic (carbamoyl-PAF (CPAF)), exerts a potent, dose-dependent, and short-lived edema response in WT mice, but not Ptafr -/- mice or mice deficient in c-Kit (c-Kit^W-sh/W-sh mice). Using an ear inflammation model, co-administration of topical CPAF treatment resulted in a paradoxical decrease in both acute ear thickness changes associated with a single PMA application, as well as the sustained inflammation associated with chronic repetitive PMA applications. Moreover, mice treated topically with CPAF also exhibited a significant reduction in chemical carcinogenesis. The ability of CPAF to suppress acute and chronic inflammatory changes in response to PMA application(s) was PAF-R dependent, as CPAF had no effect on basal or PMA-induced inflammation in Ptafr-/- mice. Moreover, c-Kit appears to be necessary for the anti-inflammatory effects of CPAF, as CPAF had no observable effect in c-Kit^W-sh/W-sh mice. These data provide additional evidence that PAF-R activation exerts complex immunomodulatory effects in a model of chronic inflammation that is relevant to neoplastic development.

The ubiquitin system: an essential component to unlocking the secrets of malaria parasite biology

Exploration of the ubiquitin system in eukaryotes has shown that the chemical modification of proteins by ubiquitin, known as ubiquitylation, is an incredibly important post-translational event that is crucial to numerous cellular processes. Ubiquitylation is carried out by a series of enzymes that specifically target proteins to either change their activity or their location or earmark them for degradation. Using a wide range of genome-wide approaches, the ubiquitin system has been shown to be of particular importance in the survival and propagation of the human malaria parasites. In this review, we highlight our current understanding of the ubiquitin system in Plasmodium, and discuss its possible role in the development of drug resistant malaria strains.

Dimers of G-protein coupled receptors as versatile storage and response units

The status and use of transmembrane, extracellular and intracellular domains in oligomerization of heptahelical G-protein coupled receptors (GPCRs) are reviewed and for transmembrane assemblies also supplemented by new experimental evidence. The transmembrane-linked GPCR oligomers typically have as the minimal unit an asymmetric ~180 kDa pentamer consisting of receptor homodimer or heterodimer and a G-protein αβγ subunit heterotrimer. With neuropeptide Y (NPY) receptors, this assembly is converted to ~90 kDa receptor monomer-Gα complex by receptor and Gα agonists, and dimers/heteropentamers are depleted by neutralization of Gαi subunits by pertussis toxin. Employing gradient centrifugation, quantification and other characterization of GPCR dimers at the level of physically isolated and identified heteropentamers is feasible with labeled agonists that do not dissociate upon solubilization. This is demonstrated with three neuropeptide Y (NPY) receptors and could apply to many receptors that use large peptidic agonists.

Paf receptor expression in the marsupial embryo and endometrium during embryonic diapause

The control of reactivation from embryonic diapause in the tammar wallaby (Macropus eugenii) involves sequential activation of the corpus luteum, secretion of progesterone that stimulates endometrial secretion and subsequent changes in the uterine environment that activate the embryo. However, the precise signals between the endometrium and the blastocyst are currently unknown. In eutherians, both the phospholipid Paf and its receptor, platelet-activating factor receptor (PTAFR), are present in the embryo and the endometrium. In the tammar, endometrial Paf releasein vitroincreases around the time of the early progesterone pulse that occurs around the time of reactivation, but whether Paf can reactivate the blastocyst is unknown. We cloned and characterised the expression of PTAFR in the tammar embryo and endometrium at entry into embryonic diapause, during its maintenance and after reactivation. Tammar PTAFR sequence and protein were highly conserved with mammalian orthologues. In the endometrium, PTAFR was expressed at a constant level in the glandular epithelium across all stages and in the luminal epithelium during both diapause and reactivation. Thus, the presence of the receptor appears not to be a limiting factor for Paf actions in the endometrium. However, the low levels of PTAFR in the embryo during diapause, together with its up-regulation and subsequent internalisation at reactivation, supports earlier results suggesting that endometrial Paf could be involved in reactivation of the tammar blastocyst from embryonic diapause.

The subcellular localization of the receptor for platelet-activating factor in neutrophils affects signaling and activation characteristics

The localization in neutrophils, of the receptor for platelet-activating factor (PAFR), has been determined using subcellular fractionation and a receptor mobilization protocol. We show that the PAFR is expressed primarily in the plasma membrane. Although activation of neutrophils by PAF induces responses typical also of agonists that bind the formyl peptide receptors (FPR), known to be stored in mobilizable organelles, some quantitative as well as qualitative differences were observed when neutrophils were activated through these receptors. PAF is equipotent to fMLF (high affinity agonist for FPR1) to cleave off L-selectin and to induce granule/vesicle secretion but is more potent than fMLF to induce a rise in intracellular Ca²⁺. Similar to fMLF, PAF induced also a robust release of reactive oxygen species, but with higher EC₅₀ value and was less sensitive to a PI3K inhibitor compared to the fMLF response. Despite the lack of a granule localized storage pool of receptors, the PAF-induced superoxide production could be primed; receptor mobilization was, thus, not required for priming of the PAF response. The desensitized PAFR could not be reactivated, suggesting that distinct signaling pathways are utilized for termination of the responses triggered through FPR1 and PAFR.

Minireview: ubiquitination-regulated G protein-coupled receptor signaling and trafficking

G protein-coupled receptors (GPCRs) are the largest and most diverse superfamily of membrane proteins and mediate most cellular responses to hormones and neurotransmitters. Posttranslational modifications are considered the main regulators of all GPCRs. In addition to phosphorylation, glycosylation, and palmitoylation, increasing evidence as reviewed here reveals that ubiquitination also regulates the magnitude and temporospatial aspects of GPCR signaling. Posttranslational protein modification by ubiquitin is a key molecular mechanism governing proteins degradation. Ubiquitination mediates the covalent conjugation of ubiquitin, a highly conserved polypeptide of 76 amino acids, to protein substrates. This process is catalyzed by 3 enzymes acting in tandem: an E1, ubiquitin-activating enzyme; an E2, ubiquitin-carrying enzyme; and an E3, ubiquitin ligase. Ubiquitination is counteracted by deubiquitinating enzymes that deconjugate ubiquitin-modified proteins and rescue the substrate from proteasomal degradation. Although ubiquitination is known to target many GPCRs for lysosomal or proteasomal degradation, emerging findings define novel roles for the basal status of ubiquitination and for rapid deubiquitination and transubiquitination controlling cell surface expression and cellular responsiveness of some GPCRs. In this review, we highlight the classical and novel roles of ubiquitin in the regulation of GPCR function, signaling, and trafficking.

Heterologous downregulation of vasopressin type 2 receptor is induced by transferrin

Vasopressin (VP) binds to the vasopressin type 2 receptor (V2R) to trigger physiological effects including body fluid homeostasis and blood pressure regulation. Signaling is terminated by receptor downregulation involving clathrin-mediated endocytosis and V2R degradation. We report here that both native and epitope-tagged V2R are internalized from the plasma membrane of LLC-PK1 kidney epithelial cells in the presence of another ligand, transferrin (Tf). The presence of iron-saturated Tf (holo-Tf; 4 h) reduced V2R binding sites at the cell surface by up to 33% while iron-free (apo-Tf) had no effect. However, no change in green fluorescent protein-tagged V2R distribution was observed in the presence of bovine serum albumin, atrial natriuretic peptide, or ANG II. Conversely, holo-Tf did not induce the internalization of another G protein-coupled receptor, the parathyroid hormone receptor. In contrast to the effect of VP, Tf did not increase intracellular cAMP or modify aquaporin-2 distribution in these cells, although addition of VP and Tf together augmented VP-induced V2R internalization. Tf receptor coimmunoprecipitated with V2R, suggesting that they interact closely, which may explain the additive effect of VP and Tf on V2R endocytosis. Furthermore, Tf-induced V2R internalization was abolished in cells expressing a dominant negative dynamin (K44A) mutant, indicating the involvement of clathrin-coated pits. We conclude that Tf can induce heterologous downregulation of the V2R and this might desensitize VP target cells without activating downstream V2R signaling events. It also provides new insights into urine-concentrating defects observed in rat models of hemochromatosis.

Mammalian α arrestins link activated seven transmembrane receptors to Nedd4 family e3 ubiquitin ligases and interact with β arrestins

The complement of fungal cell surface proteins is widely regulated by ubiquitination of membrane proteins, which results in their endocytosis and vacuolar degradation. For diverse fungal transporters, the specificity of ubiquitination is conferred by alpha arrestin adaptors, which recruit the Nedd4 family E3 ubiquitin ligase Rsp5. A recent study showed that one mammalian alpha arrestin also mediates ubiquitination and lysosomal trafficking of an activated plasma membrane receptor. Here we first screen all five widely-expressed human alpha arrestins for subcellular localization in ligand-stimulated and -unstimulated cells overexpressing the seven transmembrane receptor vasopressin 2. We then characterize the effects of alpha arrestins ARRDC3 and ARRDC4 upon activation of the seven transmembrane receptors vasopressin 2 and beta adrenergic 2. Using biochemical and imaging approaches, we show that ligand-activated receptors interact with alpha arrestins, and this results in recruitment of Nedd4 family E3 ubiquitin ligases and receptor ubiquitination - which are known to result in lysosomal trafficking. Our time course studies show these effects occur in the first 1-5 minutes after ligand activation, the same time that beta arrestins are known to have roles in receptor endocytic trafficking and kinase signaling. We tested the possibility that alpha and beta arrestins function coordinately and found co-immunoprecipitation and colocalization evidence to support this. Others recently reported that Arrdc3 knockout mice are lean and resistant to obesity. In the course of breeding our own Arrdc3-deficient mice, we observed two novel phenotypes in homozygotes: skin abnormalities, and embryonic lethality on normal chow diet, but not on high fat diet. Our findings suggest that alpha and beta arrestins function coordinately to maintain the optimal complement and function of cell surface proteins according to cellular physiological context and external signals. We discuss the implications of the alpha arrestin functions in fungi having evolved into coordinated alpha/beta arrestin functions in animals.

The ubiquitin-proteasome pathway regulates claudin 5 degradation

The tight junctions (TJs) form continuous intracellular contacts, which help create selective barriers in epithelial and endothelial cell layers. The structures created by the TJs are very dynamic and can be rapidly remodeled in response to physiological and pathological signals. Claudin 5 is a membranal TJ protein which plays a critical role in determining the permeability of endothelial barriers. We describe the regulation of claudin 5 degradation by the ubiquitin-proteasome system (UPS). Our results indicate that claudin 5 has a relatively short half-life and can be polyubiquitinated on lysine 199. This ubiquitination appears to trigger the proteasome-dependent degradation of claudin 5. Other mechanisms also seem to be involved in the post-translational regulation of claudin 5, including a ubiquitin-independent and probably indirect lysosomal-dependent pathway. These findings provide evidence for the involvement of the UPS in the regulation of claudin 5 levels, and set the stage for further research to determine the involvement of this pathway in the modulation of the properties of TJs and cell-layer barriers.

Protein damage, repair and proteolysis

Proteins are continuously affected by various intrinsic and extrinsic factors. Damaged proteins influence several intracellular pathways and result in different disorders and diseases. Aggregation of damaged proteins depends on the balance between their generation and their reversal or elimination by protein repair systems and degradation, respectively. With regard to protein repair, only few repair mechanisms have been evidenced including the reduction of methionine sulfoxide residues by the methionine sulfoxide reductases, the conversion of isoaspartyl residues to L-aspartate by L-isoaspartate methyl transferase and deglycation by phosphorylation of protein-bound fructosamine by fructosamine-3-kinase. Protein degradation is orchestrated by two major proteolytic systems, namely the lysosome and the proteasome. Alteration of the function for both systems has been involved in all aspects of cellular metabolic networks linked to either normal or pathological processes. Given the importance of protein repair and degradation, great effort has recently been made regarding the modulation of these systems in various physiological conditions such as aging, as well as in diseases. Genetic modulation has produced promising results in the area of protein repair enzymes but there are not yet any identified potent inhibitors, and, to our knowledge, only one activating compound has been reported so far. In contrast, different drugs as well as natural compounds that interfere with proteolysis have been identified and/or developed resulting in homeostatic maintenance and/or the delay of disease progression.

Rescue of internalization-defective platelet-activating factor receptor function by EBP50/NHERF1

Platelet-activating factor (PAF) is a potent phospholipid mediator involved in specific disease states such as allergic asthma, atherosclerosis and psoriasis. The human PAF receptor (PAFR) is a member of the G protein-coupled receptor (GPCR) family. Following PAF stimulation, cells become rapidly desensitized; this refractory state can be maintained for hours and is dependent on PAFR phosphorylation, internalization and trafficking. EBP50/NHERF1 has been found to interact with a variety of proteins and these interactions are involved in a growing range of functions including the assembly of signalling complexes, receptor recycling and transport of proteins to the cell surface. Crucial roles of EBP50 in GPCR physiology include its involvement in internalization, recycling, and downregulation. We were interested in identifying the role of EBP50 in PAFR trafficking. Our results showed that EBP50 binds the PAFR in its basal state, while stimulation decreased the ratio of interaction between the two proteins. We also demonstrated that EBP50 could bind PAFR via its PDZ 2 domain. In addition, we studied the role of EBP50 in various functions of the PAFR such as PAF-induced inositol phosphate accumulation and receptor internalization: EBP50 decreased the WT PAFR response and rescued the function of internalization-deficient mutant receptors, as previously described for the arrestins and the GRKs. These results suggest new roles for EBP50, some of which could help understanding the complex formation after receptor activation.

Balanced ubiquitination determines cellular responsiveness to extracellular stimuli

Signal strength evoked by ligand stimulation is crucial for cellular responses such as fate decision, cell survival/death, secretion, and migration. For example, morphogens are secreted signaling molecules that form concentration gradients within tissues and induce distinct cell fates in a signal strength-dependent manner. In addition to extracellular ligand abundance, the sensitivity of signal-receiving cells to ligands also influences signal strength. Cell sensitivity to ligands is controlled at various levels: receptor presentation at the cell surface, positive/negative regulation of signal transduction, and target gene activation/repression. While the regulation of signal transduction and gene transcription is well studied, receptor presentation is still not fully understood. Recently, it was reported that cellular sensitivity to the Wingless (Wg)/Wnt morphogen is regulated by balanced ubiquitination and deubiquitination of its receptor Frizzled (Fz). In this review, we review how ubiquitination regulates receptor presentation at the cell surface for the detection of extracellular signal strength.

Ubiquitylation of the chemokine receptor CCR7 enables efficient receptor recycling and cell migration

The chemokine receptor CCR7 is essential for lymphocyte and dendritic cell homing to secondary lymphoid organs. Owing to the ability to induce directional migration, CCR7 and its ligands CCL19 and CCL21 are pivotal for the regulation of the immune system. Here, we identify a novel function for receptor ubiquitylation in the regulation of the trafficking process of this G-protein-coupled seven transmembrane receptor. We discovered that CCR7 is ubiquitylated in a constitutive, ligand-independent manner and that receptor ubiquitylation regulates the basal trafficking of CCR7 in the absence of chemokine. Upon CCL19 binding, we show that internalized CCR7 recycles back to the plasma membrane via the trans-Golgi network. An ubiquitylation-deficient CCR7 mutant internalized normally after ligand binding, but inefficiently recycled in immune cells and was transiently retarded in the trans-Golgi network compartment of HEK293 transfectants. Finally, we demonstrate that the lack of CCR7 ubiquitylation profoundly impairs immune cell migration. Our results provide evidence for a novel function of receptor ubiquitylation in the regulation of CCR7 recycling and immune cell migration.

Ubiquitination of G protein-coupled receptors: functional implications and drug discovery

G protein-coupled receptors (GPCRs) comprise the largest and most diverse family of signaling receptors and control a vast array of physiological responses. Modulating the signaling responses of GPCRs therapeutically is important for the treatment of various diseases, and discovering new aspects of GPCR signal regulation is critical for future drug development. Post-translational modifications are integral to the regulation of GPCR function. In addition to phosphorylation, many GPCRs are reversibly modified with ubiquitin. Ubiquitin is covalently attached to lysine residues within the cytoplasmic domains of GPCRs by ubiquitin ligases and removed by ubiquitin-specific proteases. In many cases, ubiquitin functions as a sorting signal that facilitates trafficking of mammalian GPCRs from endosomes to lysosomes for degradation, but not all GPCRs use this pathway. Moreover, there are distinct types of ubiquitin conjugations that are known to serve diverse functions in controlling a wide range of cellular processes, suggesting broad roles for GPCR ubiquitination. In this review, we highlight recent studies that illustrate various roles for ubiquitin in regulation of GPCR function. Ubiquitination is known to target many GPCRs for lysosomal degradation, and current studies now indicate that basal ubiquitination, deubiquitination, and transubiquitination of certain GPCRs are important for controlling cell surface expression and cellular responsiveness. In addition, novel functions for ubiquitin in regulation of GPCR dimers and in mediating differential GPCR regulation induced by biased agonists have been reported. We will discuss the implications of these new discoveries for ubiquitin regulation of GPCR function in the context of drug development.

Ubiquitination and proteolysis in acute lung injury

Ubiquitination is a posttranslational modification that regulates a variety of cellular functions depending on timing, subcellular localization, and type of tagging, as well as modulators of ubiquitin binding leading to proteasomal or lysosomal degradation or nonproteolytic modifications. Ubiquitination plays an important role in the pathogenesis of acute lung injury (ALI) and other lung diseases with pathologies secondary to inflammation, mechanical ventilation, and decreased physical mobility. Particularly, ubiquitination has been shown to affect alveolar epithelial barrier function and alveolar edema clearance by targeting the Na,K-ATPase and epithelial Na(+) channels upon lung injury. Notably, the proteasomal system also exhibits distinct functions in the extracellular space, which may contribute to the pathogenesis of ALI and other pulmonary diseases. Better understanding of these mechanisms may ultimately lead to novel therapeutic modalities by targeting elements of the ubiquitination pathway.

Role of ubiquitination in endocytic trafficking of G-protein-coupled receptors

Lysyl ubiquitination has long been known to target cytoplasmic proteins for proteasomal degradation, and there is now extensive evidence that ubiquitination functions in vacuolar/lysosomal targeting of membrane proteins from both the biosynthetic and endocytic pathways. G-protein-coupled receptors (GPCRs) represent the largest and most diverse family of membrane proteins, whose function is of fundamental importance both physiologically and therapeutically. In this review, we discuss the role of ubiquitination in the vacuolar/lysosomal downregulation of GPCRs through the endocytic pathway, with a primary focus on lysosomal trafficking in mammalian cells. We will summarize evidence indicating that mammalian GPCRs are regulated by ubiquitin-dependent mechanisms conserved in budding yeast, and then consider evidence for additional ubiquitin-dependent and -independent regulation that may be specific to animal cells.

Sent to destroy: the ubiquitin proteasome system regulates cell signaling and protein quality control in cardiovascular development and disease

The ubiquitin proteasome system (UPS) plays a crucial role in biological processes integral to the development of the cardiovascular system and cardiovascular diseases. The UPS prototypically recognizes specific protein substrates and places polyubiquitin chains on them for subsequent destruction by the proteasome. This system is in place to degrade not only misfolded and damaged proteins, but is essential also in regulating a host of cell signaling pathways involved in proliferation, adaptation to stress, regulation of cell size, and cell death. During the development of the cardiovascular system, the UPS regulates cell signaling by modifying transcription factors, receptors, and structural proteins. Later, in the event of cardiovascular diseases as diverse as atherosclerosis, cardiac hypertrophy, and ischemia/reperfusion injury, ubiquitin ligases and the proteasome are implicated in protecting and exacerbating clinical outcomes. However, when misfolded and damaged proteins are ubiquitinated by the UPS, their destruction by the proteasome is not always possible because of their aggregated confirmations. Recent studies have discovered how these ubiquitinated misfolded proteins can be destroyed by alternative "specific" mechanisms. The cytosolic receptors p62, NBR, and histone deacetylase 6 recognize aggregated ubiquitinated proteins and target them for autophagy in the process of "selective autophagy." Even the ubiquitination of multiple proteins within whole organelles that drive the more general macro-autophagy may be due, in part, to similar ubiquitin-driven mechanisms. In summary, the crosstalk between the UPS and autophagy highlight the pivotal and diverse roles the UPS plays in maintaining protein quality control and regulating cardiovascular development and disease.

Dopamine 5 receptor mediates Ang II type 1 receptor degradation via a ubiquitin-proteasome pathway in mice and human cells

Hypertension is a multigenic disorder in which abnormal counterregulation between dopamine and Ang II plays a role. Recent studies suggest that this counterregulation results, at least in part, from regulation of the expression of both the antihypertensive dopamine 5 receptor (D5R) and the prohypertensive Ang II type 1 receptor (AT1R). In this report, we investigated the in vivo and in vitro interaction between these GPCRs. Disruption of the gene encoding D5R in mice increased both blood pressure and AT1R protein expression, and the increase in blood pressure was reversed by AT1R blockade. Activation of D5R increased the degradation of glycosylated AT1R in proteasomes in HEK cells and human renal proximal tubule cells heterologously and endogenously expressing human AT1R and D5R. Confocal microscopy, Förster/fluorescence resonance energy transfer microscopy, and fluorescence lifetime imaging microscopy revealed that activation of D5R initiated ubiquitination of the glycosylated AT1R at the plasma membrane. The regulated degradation of AT1R via a ubiquitin/proteasome pathway by activation of D5R provides what we believe to be a novel mechanism whereby blood pressure can be regulated by the interaction of 2 counterregulatory GPCRs. Our results therefore suggest that treatments for hypertension might be optimized by designing compounds that can target the AT1R and the D5R.

G protein-coupled receptor sorting to endosomes and lysosomes

The heptahelical G protein-coupled receptors (GPCRs) belong to the largest family of cell surface signaling receptors encoded in the human genome. GPCRs signal to diverse extracellular stimuli and control a vast number of physiological responses, making this receptor class the target of nearly half the drugs currently in use. In addition to rapid desensitization, receptor trafficking is crucial for the temporal and spatial control of GPCR signaling. Sorting signals present in the intracytosolic domains of GPCRs regulate trafficking through the endosomal-lysosomal system. GPCR internalization is mediated by serine and threonine phosphorylation and arrestin binding. Short, linear peptide sequences including tyrosine- and dileucine-based motifs, and PDZ ligands that are recognized by distinct endocytic adaptor proteins also mediate internalization and endosomal sorting of GPCRs. We present new data from bioinformatic searches that reveal the presence of these types of sorting signals in the cytoplasmic tails of many known GPCRs. Several recent studies also indicate that the covalent modification of GPCRs with ubiquitin serves as a signal for internalization and lysosomal sorting, expanding the diversity of mechanisms that control trafficking of mammalian GPCRs.

Agonist-promoted Lys63-linked polyubiquitination of the human kappa-opioid receptor is involved in receptor down-regulation

Ubiquitination of the human kappa opioid receptor (hKOR) expressed in Chinese hamster ovary (CHO) cells was observed in the presence of the proteasomal inhibitor N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal (MG132) and enhanced by the agonists (-)(trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidiny) cyclohexyl] benzeneacetamide (U50,488H) and dynorphin A (Dyn A). The dominant-negative (DN) mutants GRK2-K220R and beta-arrestin (319-418), but not dynamin I-K44A, reduced Dyn A-stimulated hKOR ubiquitination, and a phosphorylation-defective hKOR mutant (hKOR-S358N) did not undergo Dyn A-stimulated ubiquitination, indicating that hKOR ubiquitination is enhanced by receptor phosphorylation but not by receptor internalization. A hKOR mutant (hKOR-10 KR) in which all 10 intracellular Lys residues were changed to Arg showed greatly reduced basal and agonist-promoted receptor ubiquitination and substantially decreased Dyn A-induced receptor down-regulation, without changing ligand binding affinity, receptor-G protein coupling, or receptor internalization or desensitization. The ubiquitination sites were further determined to be the three Lys residues in the C-terminal domain. The K63R ubiquitin mutant decreased Dyn A-induced hKOR ubiquitination and down-regulation, but the K48R mutant did not. Expression of HN-CYLD, a DN mutant of deubiquitination enzyme cylindromatosis tumor suppressor gene (CYLD) that breaks Lys63-linked polyubiquitin chain, increased Dyn A-induced hKOR ubiquitination and down-regulation. These results indicate that ubiquitinated hKOR after agonist treatment contains predominantly Lys63-linked polyubiquitin chains and ubiquitination of the hKOR involved in agonist-induced down-regulation.

Regulation of GPCRs by endocytic membrane trafficking and its potential implications

The endocytic pathway tightly controls the activity of G protein-coupled receptors (GPCRs). Ligand-induced endocytosis can drive receptors into divergent lysosomal and recycling pathways, producing essentially opposite effects on the strength and duration of cellular signaling via heterotrimeric G proteins, and may also promote distinct signaling events from intracellular membranes. This chapter reviews recent developments toward understanding the molecular machinery and functional implications of GPCR sorting in the endocytic pathway, focusing on mammalian GPCRs whose ligand-induced endocytosis is mediated primarily by clathrin-coated pits. Lysosomal sorting of a number of GPCRs occurs via a highly conserved mechanism requiring covalent tagging of receptors with ubiquitin. There is increasing evidence that additional, noncovalent mechanisms control the sorting of endocytosed GPCRs to lysosomes in mammalian cells. Recycling of several GPCRs to the plasma membrane is also specifically sorted, via a mechanism requiring both receptor-specific and shared sorting proteins. The current data reveal an unprecedented degree of specificity and plasticity in the cellular regulation of mammalian GPCRs by endocytic membrane trafficking. These developments have fundamental implications for GPCR pharmacology, and suggest new mechanisms that could be exploited in GPCR-directed pharmacotherapy.

Inflammation and melanoma growth and metastasis: the role of platelet-activating factor (PAF) and its receptor

An inflammatory tumor microenvironment fosters tumor growth, angiogenesis and metastatic progression. Platelet-activating factor (PAF) is an inflammatory biolipid produced from membrane glycerophospholipids. Through the activity of its G-protein coupled receptor, PAF triggers a variety of pathological reactions including tumor neo-angiogenesis. Several groups have demonstrated that inhibiting PAF-PAF receptor pathway at the level of a ligand or receptor results in an effective inhibition of experimental tumor growth and metastasis. In particular, our group has recently demonstrated that PAF receptor antagonists can effectively inhibit the metastatic potential of human melanoma cells in nude mice. Furthermore, we showed that PAF stimulated the phosphorylation of CREB and ATF-1 in metastatic melanoma cells, which resulted in overexpression of MMP-2 and MT1-MMP. Our data indicate that PAF acts as a promoter of melanoma metastasis in vivo. Since only metastatic melanoma cells overexpress CREB/ATF-1, we propose that these cells are better equipped to respond to PAF within the tumor microenvironment when compared to their non-metastatic counterparts.

Ligand binding induces Cbl-dependent EphB1 receptor degradation through the lysosomal pathway

Eph receptor tyrosine kinases play a critical role in embryonic patterning and angiogenesis. In the adult, they are involved in carcinogenesis and pathological neovascularization. However, the mechanisms underlying their role in tumor formation and metastasis remain to be defined. Here, we demonstrated that stimulation of EphB1 with ephrinB1/Fc led to a marked downregulation of EphB1 protein, a process blocked by the lysosomal inhibitor bafilomycin. Following ephrinB1 stimulation, the ubiquitin ligase Cbl was recruited by EphB1 and then phosphorylated. Both Cbl phosphorylation and EphB1 ubiquitination were blocked by the Src inhibitor PP2. Overexpression of wild-type Cbl, but not of 70Z mutant lacking ligase activity, enhanced EphB1 ubiquitination and degradation. This negative regulation required the tyrosine kinase activity of EphB1 as kinase-dead EphB1-K652R was resistant to Cbl. Glutathione S-transferase binding experiments showed that Cbl bound to EphB1 through its tyrosine kinase-binding domain. In aggregate, we demonstrated that Cbl induces the ubiquitination and lysosomal degradation of activated EphB1, a process requiring EphB1 and Src kinase activity. To our knowledge, this is the first study dissecting the molecular mechanisms leading to EphB1 downregulation, thus paving the way to new means of modulating their angiogenic and tumorigenic properties.

The lysophosphatidic acid 2 receptor mediates down-regulation of Siva-1 to promote cell survival

Lysophosphatidic acid (LPA) promotes cell survival through the activation of G protein-coupled LPA receptors. However, whether different LPA receptors activate distinct anti-apoptotic signaling pathways is not yet clear. Here we report a novel mechanism by which the LPA(2) receptor targets the proapoptotic Siva-1 protein for LPA-dependent degradation, thereby attenuating Siva-1 function in DNA damage response. The carboxyl-terminal tail of the LPA(2) receptor, but not LPA(1) or LPA(3) receptor, specifically associates with the carboxyl cysteine-rich domain of Siva-1. Prolonged LPA stimulation promotes the association of Siva-1 with the LPA(2) receptor and targets both proteins for ubiquitination and degradation. As a result, adriamycin-induced Siva-1 protein stabilization is attenuated by LPA in an LPA(2)-dependent manner, and the function of Siva-1 in promoting DNA damage-induced apoptosis is inhibited by LPA pretreatment. Consistent with this result, inhibition of the LPA(2) receptor expression increases Siva-1 protein levels and augments adriamycin-induced caspase-3 cleavage and apoptosis. Together, these findings reveal a critical and specific role for the LPA(2) receptor through which LPA directly inactivates a critical component of the death machinery to promote cell survival.

Down-regulation of platelet-activating factor receptor gene expression during focal reversible cerebral ischemia in rats

Platelet-activating factor (PAF) is an endogenous potent phospholipid mediator in stroke and related to the post-ischemic brain damage. The aim of this study was to investigate the regulation and mechanisms of PAF receptor gene expression in the perifocal regions of cerebral infarction after middle cerebral artery occlusion/reperfusion. Sixty mature Wistar rats were randomly divided into 12 groups: sham-operated control group, simple ischemia 90 min group, 6, 12, 18 h, 1 day (1 d), 2, 3, 4, 5, 6, 7 d reperfusion groups. After the right middle cerebral artery occluded, the rats were suffered from ischemia for 90 min, and then reperfusion was allowed for different time courses. Reverse transcription-polymerase chain reaction (RT-PCR) and radioimmunoassay were applied to evaluate the PAF receptor messenger RNA (mRNA) expression and PAF levels in the perifocal regions of cerebral infarction respectively. RT-PCR analysis revealed that PAF receptor mRNA was 0.95 +/- 0.15 in control group. However, following ischemia-reperfusion, the levels of PAF receptor mRNA progressively decreased until 2 d of reperfusion (0.54 +/- 0.10), then returned to control group's levels gradually. Compared with the control group's (582 +/- 72 pg/g wet weight), the PAF concentrations of simple ischemic and 6, 12, 18 h, 1, 2 d reperfusion group were significantly higher than that of any other groups. These results indicate that PAF receptor gene expression may be subject to down-regulation in the perifocal regions of cerebral infarction after cerebral ischemia-reperfusion and relative to the increase of endogenous PAF concentrations.

Inverse agonist-induced signaling and down-regulation of the platelet-activating factor receptor

Platelet-activating factor (PAF) is a potent phospholipid mediator involved in several diseases such as allergic asthma, atherosclerosis and psoriasis. The human PAF receptor (PAFR) is a member of the G-protein-coupled receptor family. Following stimulation, PAFR becomes rapidly desensitized; this refractory state is dependent on PAFR phosphorylation, internalization and down-regulation. In this report, we show that the PAFR inverse agonist, WEB2086, can induce phosphorylation and down-regulation of PAFR. Using selective inhibitors, we determined that the agonist, PAF, and WEB2086 could induce phosphorylation of PAFR by PKC. Moreover, dominant-negative (DN) mutant of PKC isoforms beta inhibited WEB2086-stimulated PAFR phosphorylation, whereas PAF-stimulated phosphorylation was inhibited by DN PKCalpha and delta. WEB2086 also induced PAFR down-regulation which could be blocked by PKC inhibitors and by DN PKCbeta. WEB2086-induced down-regulation was dynamin-dependent but arrestin-independent. Unlike PAF, WEB2086-stimulated intracellular trafficking of PAFR was independent of Rab5. Specific inhibitors of lysosomal proteases and of proteasomes were both effective in reducing WEB2086-induced PAFR down-regulation, indicating the importance of receptor targeting to both lysosomes and proteasomes in long-term cell desensitization to WEB2086. These results indicate that although both agonists and inverse agonists induce receptor PAFR down-regulation, this may be accomplished through different signal transduction and trafficking pathways.

Ubiquitination differentially regulates clathrin-dependent internalization of protease-activated receptor-1

Protease-activated receptor-1 (PAR1), a G protein–coupled receptor (GPCR) for thrombin, is irreversibly activated by proteolysis. Consequently, PAR1 trafficking is critical for the fidelity of thrombin signaling. PAR1 displays constitutive and agonist-induced internalization, which are clathrin and dynamin dependent but are independent of arrestins. The clathrin adaptor AP2 (adaptor protein complex-2) is critical for constitutive but not for activated PAR1 internalization. In this study, we show that ubiquitination negatively regulates PAR1 constitutive internalization and specifies a distinct clathrin adaptor requirement for activated receptor internalization. PAR1 is basally ubiquitinated and deubiquitinated after activation. A PAR1 lysineless mutant signaled normally but was not ubiquitinated. Constitutive internalization of ubiquitin (Ub)-deficient PAR1 was markedly increased and inhibited by the fusion of Ub to the cytoplasmic tail. Ub-deficient PAR1 constitutive internalization was AP2 dependent like the wild-type receptor. However, unlike wild-type PAR1, AP2 was required for the internalization of activated Ub-deficient receptor, suggesting that the internalization of ubiquitinated PAR1 requires different endocytic machinery. These studies reveal a novel function for ubiquitination in the regulation of GPCR internalization.

Seven-transmembrane receptors and ubiquitination

Regulation of protein function by posttranslational modification plays an important role in many biological pathways. The most well known among such modifications is protein phosphorylation performed by highly specific protein kinases. In the past decade, however, covalent linkage of the low-molecular-weight protein ubiquitin to substrate proteins (protein ubiquitination) has proven to be yet another widely used mechanism of protein regulation playing a crucial role in virtually all aspects of cellular functions. This review highlights some of the recently discovered and provocative roles for ubiquitination in the regulation of the life cycle and signal transduction properties of 7-transmembrane receptors that serve to integrate many biological functions and play fundamental roles in cardiovascular homeostasis.

Understanding the regulation mechanisms of PAF receptor by agonists and antagonists: Molecular modeling and molecular dynamics simulation studies

Platelet-activating factor receptor (PAFR) is a member of G-protein coupled receptor (GPCR) superfamily. Understanding the regulation mechanisms of PAFR by its agonists and antagonists at the atomic level is essential for designing PAFR antagonists as drug candidates for treating PAF-mediated diseases. In this study, a 3D model of PAFR was constructed by a hierarchical approach integrating homology modeling, molecular docking and molecular dynamics (MD) simulations. Based on the 3D model, regulation mechanisms of PAFR by agonists and antagonists were investigated via three 8-ns MD simulations on the systems of apo-PAFR, PAFR-PAF and PAFR-GB. The simulations revealed that binding of PAF to PAFR triggers the straightening process of the kinked helix VI, leading to its activated state. In contrast, binding of GB to PAFR locks PAFR in its inactive state.

Protein degradation by the 26S proteasome system in the normal and stressed myocardium

The 26S proteasome is a multicatalytic threonine protease complex responsible for degradation of the majority of proteins in eukaryotic cells. In the last two decades, the ubiquitin proteasome system (UPS) has been increasingly recognized as an integral component in numerous biologic processes including cell proliferation, adaptation to stress, and cell death. The turnover of intracellular proteins inevitably affects the contributions of these molecules to cellular networks and pathways in any given tissue or organ, including the myocardium. Perturbations in the protein-degradation process have been shown to affect protein turnover and thereby affect the cardiac cell functions that these molecules are designated to carry out, engendering diseased cardiac phenotypes. Recent studies have implicated the role of proteasomes in stressed cardiac phenotypes including postischemia-reperfusion injury and cardiac remodeling (e.g., heart failure). The 26S proteasomes also appear to be susceptible to modulation by stresses (e.g., reactive oxygen species). This review focuses on roles of the 26S proteasome system in protein degradation; it provides an overview of the progress made in cardiac proteasome research as well as a discussion of recent controversies regarding the UPS system in diseased cardiac phenotypes.