Molecular Mechanisms Linking Diabetes with Increased Risk of Thrombosis

This review will provide an overview of what is currently known about mechanisms linking poor glycaemic control with increased thrombotic risk. The leading causes of death in people with diabetes are strokes and cardiovascular disease. Significant morbidity is associated with an increased risk of thrombosis, resulting in myocardial infarction, ischaemic stroke, and peripheral vascular disease, along with the sequelae of these events, including loss of functional ability, heart failure, and amputations. While the increased platelet activity, pro-coagulability, and endothelial dysfunction directly impact this risk, the molecular mechanisms linking poor glycaemic control with increased thrombotic risk remain unclear. This review highlights the complex mechanisms underlying thrombosis prevalence in individuals with diabetes and hyperglycaemia. Post-translational modifications, such as O-GlcNAcylation, play a crucial role in controlling protein function in diabetes. However, the role of O-GlcNAcylation remains poorly understood due to its intricate regulation and the potential involvement of multiple variables. Further research is needed to determine the precise impact of O-GlcNAcylation on specific disease processes.

Detection of microplastics in human saphenous vein tissue using μFTIR: A pilot study

Microplastics (MPs) are ubiquitous in the environment, in the human food chain, and have been recently detected in blood and lung tissues. To undertake a pilot analysis of MP contamination in human vein tissue samples with respect to their presence (if any), levels, and characteristics of any particles identified. This study analysed digested human saphenous vein tissue samples (n = 5) using μFTIR spectroscopy (size limitation of 5 μm) to detect and characterise any MPs present. In total, 20 MP particles consisting of five MP polymer types were identified within 4 of the 5 vein tissue samples with an unadjusted average of 29.28 ± 34.88 MP/g of tissue (expressed as 14.99 ± 17.18 MP/g after background subtraction adjustments). Of the MPs detected in vein samples, five polymer types were identified, of irregular shape (90%), with alkyd resin (45%), poly (vinyl propionate/acetate, PVAc (20%) and nylon-ethylene-vinyl acetate, nylon-EVA, tie layer (20%) the most abundant. While the MP levels within tissue samples were not significantly different than those identified within procedural blanks (which represent airborne contamination at time of sampling), they were comprised of different plastic polymer types. The blanks comprised n = 13 MP particles of four MP polymer types with the most abundant being polytetrafluoroethylene (PTFE), then polypropylene (PP), polyethylene terephthalate (PET) and polyfumaronitrile:styrene (FNS), with a mean ± SD of 10.4 ± 9.21, p = 0.293. This study reports the highest level of contamination control and reports unadjusted values alongside different contamination adjustment techniques. This is the first evidence of MP contamination of human vascular tissues. These results support the phenomenon of transport of MPs within human tissues, specifically blood vessels, and this characterisation of types and levels can now inform realistic conditions for laboratory exposure experiments, with the aim of determining vascular health impacts.

Targeting Protein O-GlcNAcylation, a Link between Type 2 Diabetes Mellitus and Inflammatory Disease

Unresolved hyperglycaemia, a hallmark of type 2 diabetes mellitus (T2DM), is a well characterised manifestation of altered fuel homeostasis and our understanding of its role in the pathologic activation of the inflammatory system continues to grow. Metabolic disorders like T2DM trigger changes in the regulation of key cellular processes such as cell trafficking and proliferation, and manifest as chronic inflammatory disorders with severe long-term consequences. Activation of inflammatory pathways has recently emerged as a critical link between T2DM and inflammation. A substantial body of evidence has suggested that this is due in part to increased flux through the hexosamine biosynthetic pathway (HBP). The HBP, a unique nutrient-sensing metabolic pathway, produces the activated amino sugar UDP-GlcNAc which is a critical substrate for protein O-GlcNAcylation, a dynamic, reversible post-translational glycosylation of serine and threonine residues in target proteins. Protein O-GlcNAcylation impacts a range of cellular processes, including inflammation, metabolism, trafficking, and cytoskeletal organisation. As increased HBP flux culminates in increased protein O-GlcNAcylation, we propose that targeting O-GlcNAcylation may be a viable therapeutic strategy for the prevention and management of glucose-dependent pathologies with inflammatory components.

Nutrient regulation of inflammatory signalling in obesity and vascular disease

Despite obesity and diabetes markedly increasing the risk of developing cardiovascular diseases, the molecular and cellular mechanisms that underlie this association remain poorly characterised. In the last 20 years it has become apparent that chronic, low-grade inflammation in obese adipose tissue may contribute to the risk of developing insulin resistance and type 2 diabetes. Furthermore, increased vascular pro-inflammatory signalling is a key event in the development of cardiovascular diseases. Overnutrition exacerbates pro-inflammatory signalling in vascular and adipose tissues, with several mechanisms proposed to mediate this. In this article, we review the molecular and cellular mechanisms by which nutrients are proposed to regulate pro-inflammatory signalling in adipose and vascular tissues. In addition, we examine the potential therapeutic opportunities that these mechanisms provide for suppression of inappropriate inflammation in obesity and vascular disease.

Revascularisation of type 2 diabetics with coronary artery disease: Insights and therapeutic targeting of O-GlcNAcylation

AIM

Coronary artery bypass graft (CABG) using autologous saphenous vein continues to be a gold standard procedure to restore the supply of oxygen-rich blood to the heart muscles in coronary artery disease (CAD) patients with or without type 2 diabetes mellitus (T2DM). However, CAD patients with T2DM are at higher risk of graft failure. While failure rates have been reduced through improvements in procedure-related factors, much less is known about the molecular and cellular mechanisms by which T2DM initiates vein graft failure. This review gives novel insights into these cellular and molecular mechanisms and identifies potential therapeutic targets for development of new medicines to improve vein graft patency.

DATA SYNTHESIS

One important cellular process that has been implicated in the pathogenesis of T2DM is protein O-GlcNAcylation, a dynamic, reversible post-translational modification of serine and threonine residues on target proteins that is controlled by two enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Protein O-GlcNAcylation impacts a range of cellular processes, including trafficking, metabolism, inflammation and cytoskeletal organisation. Altered O-GlcNAcylation homeostasis have, therefore, been linked to a range of human pathologies with a metabolic component, including T2DM.

CONCLUSION

We propose that protein O-GlcNAcylation alters vascular smooth muscle and endothelial cell function through modification of specific protein targets which contribute to the vascular re-modelling responsible for saphenous vein graft failure in T2DM.

Emerging roles of protein O-GlcNAcylation in cardiovascular diseases: Insights and novel therapeutic targets

Cardiovascular diseases (CVDs) are the leading cause of death globally. While the major focus of pharmacological and non-pharmacological interventions has been on targeting disease pathophysiology and limiting predisposing factors, our understanding of the cellular and molecular mechanisms underlying the pathogenesis of CVDs remains incomplete. One mechanism that has recently emerged is protein O-GlcNAcylation. This is a dynamic, site-specific reversible post-translational modification of serine and threonine residues on target proteins and is controlled by two enzymes: O-linked β-N-acetylglucosamine transferase (OGT) and O-linked β-N-acetylglucosaminidase (OGA). Protein O-GlcNAcylation alters the cellular functions of these target proteins which play vital roles in pathways that modulate vascular homeostasis and cardiac function. Through this review, we aim to give insights on the role of protein O-GlcNAcylation in cardiovascular diseases and identify potential therapeutic targets in this pathway for development of more effective medicines to improve patient outcomes.

Investigation of Novel Cavin-1/Suppressor of Cytokine Signaling 3 (SOCS3) Interactions by Coimmunoprecipitation, Peptide Pull-Down, and Peptide Array Overlay Approaches

The ability of inducible regulator suppressor of cytokine signaling 3 (SOCS3) to inhibit Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling requires interaction with specific cytokine receptors, JAKs, and components of the cellular ubiquitylation machinery. However, it is now clear that additional protein interactions are essential for effective inhibition of JAK-STAT signaling that have also identified new roles for SOCS3. For example, we have demonstrated that SOCS3 interaction with cavin-1, a core component of caveolae essential for their formation, is required for effective inhibition of interleukin (IL)-6 signaling and maintenance of cellular levels of caveolae. This is achieved through cavin-1 interaction with a discrete motif within the SOCS3 SH2 domain. Here, we describe in detail three methods (coimmunoprecipitation; peptide pull-down; peptide array overlay) we have used to validate and characterize cavin-1/SOCS3 interactions in vitro.

Identification of myeloid cells in the human enthesis as the main source of local IL-23 production

OBJECTIVE

We investigated whether the normal human spinal enthesis contained resident myeloid cell populations, capable of producing pivotal proinflammatory cytokines including tumour necrosis factor (TNF) and interleukin (IL)-23 and determined whether these could be modified by PDE4 inhibition.

METHODS

Normal human enthesis soft tissue (ST) and adjacent perientheseal bone (PEB) (n=15) were evaluated using immunohistochemistry (IHC), digested for myeloid cell phenotyping, sorted and stimulated with different adjuvants (lipopolysaccharide and mannan). Stimulated enthesis fractions were analysed for inducible production of spondyloarthropathy disease-relevant mediators (IL-23 full protein, TNF, IL-1β and CCL20). Myeloid populations were also compared with matched blood populations for further mRNA analysis and the effect of PDE4 inhibition was assessed.

RESULTS

A myeloid cell population (CD45+ HLADR+ CD14+ CD11c+) phenotype was isolated from both the ST and adjacent PEB and termed 'CD14+ myeloid cells' with tissue localisation confirmed by CD14+ IHC. The CD14- fraction contained a CD123+ HLADR+ CD11c- cell population (plasmacytoid dendritic cells). The CD14+ population was the dominant entheseal producer of IL-23, IL-1β, TNF and CCL20. IL-23 and TNF from the CD14+ population could be downregulated by a PDE4I and other agents (histamine and 8-Bromo-cAMP) which elevate cAMP. Entheseal CD14+ cells had a broadly similar gene expression profile to the corresponding CD14+ population from matched blood but showed significantly lower CCR2 gene expression.

CONCLUSIONS

The human enthesis contains a CD14+ myeloid population that produces most of the inducible IL-23, IL-1β, TNF and CCL20. This population has similar gene expression profile to the matched blood CD14+ population.

Canagliflozin inhibits interleukin-1β-stimulated cytokine and chemokine secretion in vascular endothelial cells by AMP-activated protein kinase-dependent and -independent mechanisms

Recent clinical trials of the hypoglycaemic sodium-glucose co-transporter-2 (SGLT2) inhibitors, which inhibit renal glucose reabsorption, have reported beneficial cardiovascular outcomes. Whether SGLT2 inhibitors directly affect cardiovascular tissues, however, remains unclear. We have previously reported that the SGLT2 inhibitor canagliflozin activates AMP-activated protein kinase (AMPK) in immortalised cell lines and murine hepatocytes. As AMPK has anti-inflammatory actions in vascular cells, we examined whether SGLT2 inhibitors attenuated inflammatory signalling in cultured human endothelial cells. Incubation with clinically-relevant concentrations of canagliflozin, but not empagliflozin or dapagliflozin activated AMPK and inhibited IL-1β-stimulated adhesion of pro-monocytic U937 cells and secretion of IL-6 and monocyte chemoattractant protein-1 (MCP-1). Inhibition of MCP-1 secretion was attenuated by expression of dominant-negative AMPK and was mimicked by the direct AMPK activator, A769662. Stimulation of cells with either canagliflozin or A769662 had no effect on IL-1β-stimulated cell surface levels of adhesion molecules or nuclear factor-κB signalling. Despite these identical effects of canagliflozin and A769662, IL-1β-stimulated IL-6/MCP-1 mRNA was inhibited by canagliflozin, but not A769662, whereas IL-1β-stimulated c-jun N-terminal kinase phosphorylation was inhibited by A769662, but not canagliflozin. These data indicate that clinically-relevant canagliflozin concentrations directly inhibit endothelial pro-inflammatory chemokine/cytokine secretion by AMPK-dependent and -independent mechanisms without affecting early IL-1β signalling.

Interaction of suppressor of cytokine signalling 3 with cavin-1 links SOCS3 function and cavin-1 stability

Effective suppression of JAK-STAT signalling by the inducible inhibitor "suppressor of cytokine signalling 3" (SOCS3) is essential for limiting signalling from cytokine receptors. Here we show that cavin-1, a component of caveolae, is a functionally significant SOCS3-interacting protein. Biochemical and confocal imaging demonstrate that SOCS3 localisation to the plasma membrane requires cavin-1. SOCS3 is also critical for cavin-1 stabilisation, such that deletion of SOCS3 reduces the expression of cavin-1 and caveolin-1 proteins, thereby reducing caveola abundance in endothelial cells. Moreover, the interaction of cavin-1 and SOCS3 is essential for SOCS3 function, as loss of cavin-1 enhances cytokine-stimulated STAT3 phosphorylation and abolishes SOCS3-dependent inhibition of IL-6 signalling by cyclic AMP. Together, these findings reveal a new functionally important mechanism linking SOCS3-mediated inhibition of cytokine signalling to localisation at the plasma membrane via interaction with and stabilisation of cavin-1.

Linking energy sensing to suppression of JAK-STAT signalling: A potential route for repurposing AMPK activators?

Exaggerated Janus kinase-signal transducer and activator of transcription (JAK-STAT) signalling is key to the pathogenesis of pro-inflammatory disorders, such as rheumatoid arthritis and cardiovascular diseases. Mutational activation of JAKs is also responsible for several haematological malignancies, including myeloproliferative neoplasms and acute lymphoblastic leukaemia. Accumulating evidence links adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK), an energy sensor and regulator of organismal and cellular metabolism, with the suppression of immune and inflammatory processes. Recent studies have shown that activation of AMPK can limit JAK-STAT-dependent signalling pathways via several mechanisms. These novel findings support AMPK activation as a strategy for management of an array of disorders characterised by hyper-activation of the JAK-STAT pathway. This review discusses the pivotal role of JAK-STAT signalling in a range of disorders and how both established clinically used and novel AMPK activators might be used to treat these conditions.

Taller height as a risk factor for venous thromboembolism: a Mendelian randomization meta-analysis

Essentials Observational data suggest taller people have a higher risk of venous thromboembolism (VTE). We used Mendelian randomization techniques to further explore this association in three studies. Risk of VTE increased by 30-40% for each 10 cm increment in height. Height was more strongly associated with deep vein thrombosis than with pulmonary embolism.

SUMMARY

Background Taller height is associated with a greater risk of venous thromboembolism (VTE). Objectives To use instrumental variable (IV) techniques (Mendelian randomization) to further explore this relationship. Methods Participants of European ancestry were included from two cohort studies (Atherosclerosis Risk in Communities [ARIC] study and Cardiovascular Health Study [CHS]) and one case-control study (Mayo Clinic VTE Study [Mayo]). We created two weighted genetic risk scores (GRSs) for height; the full GRS included 668 single-nucleotide polymorphisms (SNPs) from a previously published meta-analysis, and the restricted GRS included a subset of 362 SNPs not associated with weight independently of height. Standard logistic regression and IV models were used to estimate odds ratios (ORs) for VTE per 10-cm increment in height. ORs were pooled across the three studies by the use of inverse variance-weighted random effects meta-analysis. Results Among 9143 ARIC and 3180 CHS participants free of VTE at baseline, there were 367 and 109 incident VTE events. There were 1143 VTE cases and 1292 controls included from Mayo. The pooled ORs from non-IV models and models using the full and restricted GRSs as IVs were 1.27 (95% confidence interval [CI] 1.11-1.46), 1.34 (95% CI 1.04-1.73) and 1.45 (95% CI 1.04-2.01) per 10-cm greater height, respectively. Conclusions Taller height is associated with an increased risk of VTE in adults of European ancestry. Possible explanations for this association, including taller people having a greater venous surface area, a higher number of venous valves, or greater hydrostatic pressure, need to be explored further.

Activation of AMP-activated protein kinase rapidly suppresses multiple pro-inflammatory pathways in adipocytes including IL-1 receptor-associated kinase-4 phosphorylation

Inflammation of adipose tissue in obesity is associated with increased IL-1β, IL-6 and TNF-α secretion and proposed to contribute to insulin resistance. AMP-activated protein kinase (AMPK) regulates nutrient metabolism and is reported to have anti-inflammatory actions in adipose tissue, yet the mechanisms underlying this remain poorly characterised. The effect of AMPK activation on cytokine-stimulated proinflammatory signalling was therefore assessed in cultured adipocytes. AMPK activation inhibited IL-1β-stimulated CXCL10 secretion, associated with reduced interleukin-1 receptor associated kinase-4 (IRAK4) phosphorylation and downregulated MKK4/JNK and IKK/IκB/NFκB signalling. AMPK activation inhibited TNF-α-stimulated IKK/IκB/NFκB signalling but had no effect on JNK phosphorylation. The JAK/STAT3 pathway was also suppressed by AMPK after IL-6 stimulation and during adipogenesis. Adipose tissue from AMPKα1^-/- mice exhibited increased JNK and STAT3 phosphorylation, supporting suppression of these distinct proinflammatory pathways by AMPK in vivo. The inhibition of multiple pro-inflammatory signalling pathways by AMPK may underlie the reported beneficial effects of AMPK activation in adipose tissue.

The future of EPAC-targeted therapies: agonism versus antagonism

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Although tractable to drug development, targeting of cAMP signalling has side effects.

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Selectively targeting EPAC1 and EPAC2 cAMP sensor enzymes may limit some of these off-target effects.

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EPAC agonists could be used to treat vascular inflammation (EPAC1) or type 2 diabetes (EPAC2).

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EPAC1 and EPAC2 antagonists could be used to treat heart disease.

Pharmaceutical manipulation of cAMP levels exerts beneficial effects through the regulation of the exchange protein activated by cAMP (EPAC) and protein kinase A (PKA) signalling routes. Recent attention has turned to the specific regulation of EPAC isoforms (EPAC1 and EPAC2) as a more targeted approach to cAMP-based therapies. For example, EPAC2-selective agonists could promote insulin secretion from pancreatic β cells, whereas EPAC1-selective agonists may be useful in the treatment of vascular inflammation. By contrast, EPAC1 and EPAC2 antagonists could both be useful in the treatment of heart failure. Here we discuss whether the best way forward is to design EPAC-selective agonists or antagonists and the current strategies being used to develop isoform-selective, small-molecule regulators of EPAC1 and EPAC2 activity.

Role of Ubiquitylation in Controlling Suppressor of Cytokine Signalling 3 (SOCS3) Function and Expression

The realisation that unregulated activation of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway is a key driver of a wide range of diseases has identified its components as targets for therapeutic intervention by small molecule inhibitors and biologicals. In this review, we discuss JAK-STAT signalling pathway inhibition by the inducible inhibitor "suppressor of cytokine signaling 3 (SOCS3), its role in diseases such as myeloproliferative disorders, and its function as part of a multi-subunit E3 ubiquitin ligase complex. In addition, we highlight potential applications of these insights into SOCS3-based therapeutic strategies for management of conditions such as vascular re-stenosis associated with acute vascular injury, where there is strong evidence that multiple processes involved in disease progression could be attenuated by localized potentiation of SOCS3 expression levels.

β1-adrenergic receptor and sphingosine-1-phosphate receptor 1 (S1PR1) reciprocal downregulation influences cardiac hypertrophic response and progression to heart failure: protective role of S1PR1 cardiac gene therapy

BACKGROUND

The sphingosine-1-phosphate receptor 1 (S1PR1) and β1-adrenergic receptor (β1AR) are G-protein-coupled receptors expressed in the heart. These 2 receptors have opposing actions on adenylyl cyclase because of differential G-protein coupling. Importantly, both of these receptors can be regulated by the actions of G-protein-coupled receptor kinase-2, which triggers desensitization and downregulation processes. Although classic signaling paradigms suggest that simultaneous activation of β1ARs and S1PR1s in a myocyte would simply result in opposing action on cAMP production, in this report we have uncovered a direct interaction between these 2 receptors, with regulatory involvement of G-protein-coupled receptor kinase-2.

METHODS AND RESULTS

In HEK (human embryonic kidney) 293 cells overexpressing both β1AR and S1PR1, we demonstrated that β1AR downregulation can occur after stimulation with sphingosine-1-phosphate (an S1PR1 agonist), whereas S1PR1 downregulation can be triggered by isoproterenol (a β-adrenergic receptor agonist) treatment. This cross talk between these 2 distinct G-protein-coupled receptors appears to have physiological significance, because they interact and show reciprocal regulation in mouse hearts undergoing chronic β-adrenergic receptor stimulation and in a rat model of postischemic heart failure.

CONCLUSIONS

We demonstrate that restoration of cardiac plasma membrane levels of S1PR1 produces beneficial effects that counterbalance the deleterious β1AR overstimulation in heart failure.

Extracellular adenosine sensing-a metabolic cell death priming mechanism downstream of p53

Tumor cells undergo changes in metabolism to meet their energetic and anabolic needs. It is conceivable that mechanisms exist to sense these changes and link them to pathways that eradicate cells primed for cancer development. We report that the tumor suppressor p53 activates a cell death priming mechanism that senses extracellular adenosine. Adenosine, the backbone of ATP, accumulates under conditions of cellular stress or altered metabolism. We show that its receptor, A2B, is upregulated by p53. A2B expression has little effect on cell viability, but ligand engagement activates a caspase- and Puma-dependent apoptotic response involving downregulation of antiapoptotic Bcl-2 proteins. Stimulation of A2B also significantly enhances cell death mediated by p53 and upon accumulation of endogenous adenosine following chemotherapeutic drug treatment and exposure to hypoxia. Since extracellular adenosine also accumulates within many solid tumors, this distinct p53 function links programmed cell death to both a cancer- and therapy-associated metabolic change.

Regulation of the inflammatory response of vascular endothelial cells by EPAC1

Life-threatening diseases of the cardiovascular system, like atherosclerosis, are exacerbated by unwanted inflammation within the structures of large blood vessels. This inflammation involves increased permeability of the vascular endothelial cells (VECs) that form the lining of blood vessels, leading to exaggerated extravasation of blood components and accumulation of fluid in the extravascular space. This results in tissue dysfunction and increased secretion of chemokines that attract leukocytes and monocytes to the inflamed endothelium. Cyclic AMP is synthesized in VECs in response to endogenous Gs-coupled receptors and is known to limit cytokine action and reduce endothelial hyperpermeability induced by multiple pro-inflammatory stimuli. The mechanisms underlying this anti-inflammatory action of cyclic AMP are now being elucidated and it is becoming clear that the cyclic AMP sensor, exchange protein activated by cyclic AMP (EPAC1), appears to play a key role in suppressing unwanted inflammation. EPAC1 mediates at least three anti-inflammatory pathways in VECs by down-regulating inflammatory signalling through the induction of the suppressors of cytokine signalling 3 (SOCS-3) gene, limiting integrin-dependent vascular permeability and enhancing endothelial barrier function through the stabilization of VE-cadherin junctions. Given that manipulation of cellular cyclic AMP levels currently forms the basis of many effective pharmaceuticals and that EPAC1 is involved in multiple anti-inflammatory protective processes in VECs, does this make EPAC1 an attractive target for the development of activators capable of eliciting a coordinated programme of 'protection' against the development of endothelial dysfunction? Here we discuss whether EPAC1 represents an attractive therapeutic target for limiting endothelial dysfunction associated with cardiovascular diseases like atherosclerosis. LINKED ARTICLES This article is part of a themed section on Novel cAMP Signalling Paradigms. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-2.

Protein kinase A-mediated phosphorylation of RhoA on serine 188 triggers the rapid induction of a neuroendocrine-like phenotype in prostate cancer epithelial cells

Whilst androgen ablation therapy is used to treat locally advanced or metastatic forms of prostate cancer, side-effects can include the emergence of an androgen-independent neuroendocrine cell population which is associated with poor prognosis. Here we have examined how cyclic AMP elevation regulates early events in the neuroendocrine differentiation process. We demonstrate that selective activation of protein kinase A is necessary and sufficient for cyclic AMP (cAMP) elevation to rapidly promote a neuroendocrine phenotype in LNCaP cells independent of de novo protein synthesis. Furthermore, the effects of cAMP could be recapitulated by inhibition of RhoA signalling or pharmacological inhibition of Rho kinase. Conversely, expression of constitutively active Gln63Leu-mutated RhoA acted as a dominant-negative inhibitor of cAMP-mediated NE phenotype formation. Consistent with these observations, cAMP elevation triggered the PKA-dependent phosphorylation of RhoA on serine 188, and a non-phosphorylatable Ser188Ala RhoA mutant functioned as a dominant-negative inhibitor of cAMP-mediated neuroendocrine phenotype formation. These results suggest that PKA-mediated inhibition of RhoA via its phosphorylation on serine 188 and the subsequent inhibition of ROCK activity plays a key role in determining initial changes in cellular morphology during LNCaP cell differentiation to a neuroendocrine phenotype. It also raises the possibility that targeted suppression of this pathway to inhibit neuroendocrine cell expansion might be a useful adjuvant to conventional prostate cancer therapy.

Exchange protein directly activated by cyclic AMP-1-regulated recruitment of CCAAT/enhancer-binding proteins to the suppressor of cytokine signaling-3 promoter

The ability of prototypical second messenger cyclic AMP (cAMP) to positively control transcription of the somatostatin gene was pivotal to the original identification of the transcription factor cAMP response element-binding protein. However, it is now clear that alternative intracellular cAMP sensors, of which the exchange protein directly activated by cAMP (Epac) proteins have been studied most intensively, also initiate transcription of key genes in response to cAMP elevation. For example, we have demonstrated in vascular endothelial cells that activation of Epac1 is necessary for cAMP-mobilizing agents to trigger the induction of the gene-encoding suppressor of cytokine signaling-3 (SOCS-3), a potent inhibitor of interleukin (IL)-6 signaling. This is achieved through the recruitment of CCAAT/enhancer-binding protein (C/EBP) transcription factors to the SOCS-3 promoter. Here, we describe in detail how to identify and measure cAMP-mediated recruitment of a specific C/EBP isoform to a candidate regulator region of the SOCS-3 promoter in vascular endothelial cells in vitro. We also describe the RNA interference strategies with which we identified a role for Epac1 and SOCS-3 in being responsible for mediating the inhibitory effect of cAMP elevation on IL-6 signaling.

Anti-inflammatory and immunosuppressive effects of the A2A adenosine receptor

The production of adenosine represents a critical endogenous mechanism for regulating immune and inflammatory responses during conditions of stress, injury, or infection. Adenosine exerts predominantly protective effects through activation of four 7-transmembrane receptor subtypes termed A₁, A_2A, A_2B, and A₃, of which the A_2A adenosine receptor (A_2AAR) is recognised as a major mediator of anti-inflammatory responses. The A_2AAR is widely expressed on cells of the immune system and numerous in vitro studies have identified its role in suppressing key stages of the inflammatory process, including leukocyte recruitment, phagocytosis, cytokine production, and immune cell proliferation. The majority of actions produced by A_2AAR activation appear to be mediated by cAMP, but downstream events have not yet been well characterised. In this article, we review the current evidence for the anti-inflammatory effects of the A_2AAR in different cell types and discuss possible molecular mechanisms mediating these effects, including the potential for generalised suppression of inflammatory gene expression through inhibition of the NF-κB and JAK/STAT proinflammatory signalling pathways. We also evaluate findings from in vivo studies investigating the role of the A_2AAR in different tissues in animal models of inflammatory disease and briefly discuss the potential for development of selective A_2AAR agonists for use in the clinic to treat specific inflammatory conditions.

Deletion of the distal COOH-terminus of the A2B adenosine receptor switches internalization to an arrestin- and clathrin-independent pathway and inhibits recycling

BACKGROUND AND PURPOSE

We have investigated the effect of deletions of a postsynaptic density, disc large and zo-1 protein (PDZ) motif at the end of the COOH-terminus of the rat A(2B) adenosine receptor on intracellular trafficking following long-term exposure to the agonist 5'-(N-ethylcarboxamido)-adenosine.

EXPERIMENTAL APPROACH

The trafficking of the wild type A(2B) adenosine receptor and deletion mutants expressed in Chinese hamster ovary cells was studied using an enzyme-linked immunosorbent assay in combination with immunofluorescence microscopy.

KEY RESULTS

The wild type A(2B) adenosine receptor and deletion mutants were all extensively internalized following prolonged treatment with NECA. The intracellular compartment through which the Gln(325)-stop receptor mutant, which lacks the Type II PDZ motif found in the wild type receptor initially trafficked was not the same as the wild type receptor. Expression of dominant negative mutants of arrestin-2, dynamin or Eps-15 inhibited internalization of wild type and Leu(330)-stop receptors, whereas only dominant negative mutant dynamin inhibited agonist-induced internalization of Gln(325)-stop, Ser(326)-stop and Phe(328)-stop receptors. Following internalization, the wild type A(2B) adenosine receptor recycled rapidly to the cell surface, whereas the Gln(325)-stop receptor did not recycle.

CONCLUSIONS AND IMPLICATIONS

Deletion of the COOH-terminus of the A(2B) adenosine receptor beyond Leu(330) switches internalization from an arrestin- and clathrin-dependent pathway to one that is dynamin dependent but arrestin and clathrin independent. The presence of a Type II PDZ motif appears to be essential for arrestin- and clathrin-dependent internalization, as well as recycling of the A(2B) adenosine receptor following prolonged agonist addition.

Activation of protein kinase Calpha by EPAC1 is required for the ERK- and CCAAT/enhancer-binding protein beta-dependent induction of the SOCS-3 gene by cyclic AMP in COS1 cells

We recently found that induction of the anti-inflammatory SOCS-3 gene by cyclic AMP occurs through novel cyclic AMP-dependent protein kinase-independent mechanisms involving activation of CCAAT/enhancer-binding protein (C/EBP) transcription factors, notably C/EBPbeta, by the cyclic AMP GEF EPAC1 and the Rap1 GTPase. In this study we show that down-regulation of phospholipase (PL) Cepsilon with small interfering RNA or blockade of PLC activity with chemical inhibitors ablates exchange protein directly activated by cyclic AMP (EPAC)-dependent induction of SOCS-3 in COS1 cells. Consistent with this, stimulation of cells with 1-oleoyl-2-acetyl-sn-glycerol and phorbol 12-myristate 13-acetate, both cell-permeable analogues of the PLC product diacylglycerol, are sufficient to induce SOCS-3 expression in a Ca2+-dependent manner. Moreover, the diacylglycerol- and Ca2+-dependent protein kinase C (PKC) isoform PKCalpha becomes activated following cyclic AMP elevation or EPAC stimulation. Conversely, down-regulation of PKC activity with chemical inhibitors or small interfering RNA-mediated depletion of PKCalpha or -delta blocks EPAC-dependent SOCS-3 induction. Using the MEK inhibitor U0126, we found that activation of ERK MAPKs is essential for SOCS-3 induction by either cyclic AMP or PKC. C/EBPbeta is known to be phosphorylated and activated by ERK. Accordingly, we found ERK activation to be essential for cyclic AMP-dependent C/EBP activation and C/EBPbeta-dependent SOCS-3 induction by cyclic AMP and PKC. Moreover, overexpression of a mutant form of C/EBPbeta (T235A), which lacks the ERK phosphorylation site, blocks SOCS-3 induction by cyclic AMP and PKC in a dominant-negative manner. Together, these results indicate that EPAC mediates novel regulatory cross-talk between the cyclic AMP and PKC signaling pathways leading to ERK- and C/EBPbeta-dependent induction of the SOCS-3 gene.

Novel interactions between the 5-HT transporter, 5-HT1B receptors and Rho kinase in vivo and in pulmonary fibroblasts

BACKGROUND AND PURPOSE

While the 5-HT and Rho-kinase (ROCK) pathways have been implicated in the development of pulmonary arterial hypertension (PAH), the nature of any interactions between them remain unclear. This study investigated a role for ROCK in 5-HT-regulated proliferative responses in lung fibroblasts in vivo and in vitro.

EXPERIMENTAL APPROACH

PAH was examined in mice over-expressing human 5-HT transporters (SERT+), from which pulmonary artery fibroblasts (PFs) were isolated to assess ROCK expression. In vitro analysis of 5-HT signalling employed CCL39 hamster lung fibroblasts.

KEY RESULTS

ROCK inhibition ablated increased pulmonary remodelling and hypertension observed in SERT+ mice, and ROCK1/2 protein levels were elevated in SERT+ PFs. ROCK inhibition also reduced 5-HT-stimulated proliferation by suppressing MEK-stimulated ERK phosphorylation. While optimal 5-HT-stimulated proliferation required 5-HT(1B) and 5-HT(2A) receptors and SERT, receptor sensitivity to Y27632 was restricted to the 5-HT(1B) receptor. Also, while hypoxia-induced pulmonary vascular remodelling and hypertension were sensitive to Y27632 in WT and SERT+ animals, the proportions sensitive to ROCK inhibition were increased by SERT over-expression.

CONCLUSIONS AND IMPLICATIONS

SERT over-expression increased ROCK-dependent pulmonary remodelling in normoxia and hypoxia and SERT over-expression was associated with elevated ROCK1/2 levels. ROCK also potentiated 5-HT(1B) receptor-stimulated ERK activation and proliferation in vitro by facilitating MEK-ERK interaction.

Identification of CCAAT/enhancer-binding proteins as exchange protein activated by cAMP-activated transcription factors that mediate the induction of the SOCS-3 gene

The prototypical second messenger cAMP is a key regulator of immune and inflammatory responses. Its ability to inhibit interleukin (IL)-6 responses is due to induction of suppressor of cytokine signaling-3 (SOCS-3), a negative regulator of IL-6 receptor signaling. We have determined previously that SOCS-3 induction by cAMP occurs independently of cAMP-dependent protein kinase, instead requiring the recently identified cAMP sensor exchange protein activated by cAMP 1 (EPAC1). Here we present evidence to suggest that the C/EBP family of transcription factors link EPAC1 activation to SOCS-3 induction. Firstly, selective activation of EPAC in human umbilical vein endothelial cells increased C/EBP DNA binding activity and recruitment of C/EBPbeta to the SOCS-3 promoter. Secondly, knockdown of C/EBPbeta and -delta isoforms abolished both SOCS-3 induction and inhibition of IL-6 signaling in response to cAMP. Thirdly, overexpression of C/EBPalpha, -beta, or -delta potentiated EPAC-mediated accumulation of SOCS-3. Finally, these effects were not restricted to human umbilical vein endothelial cells, because similar phenomena were observed in murine embryonic fibroblasts in which C/EBPbeta or delta had been deleted. In summary, our findings constitute the first description of an EPAC-C/EBP pathway that can control cAMP-mediated changes in gene expression independently of protein kinase A.

Suppression of inflammatory and immune responses by the A(2A) adenosine receptor: an introduction

The purine nucleoside adenosine has been described as a 'retaliatory metabolite' by virtue of its ability to function in an autocrine manner to modify the activity of a range of cell types following its extracellular accumulation during cell stress or injury. These effects are largely protective and are triggered by the binding of adenosine to any of four G-protein-coupled adenosine receptors. Most of the anti-inflammatory effects of adenosine have been assigned to the adenosine A(2A) receptor subtype, which is expressed in many immune and inflammatory cells. In this brief article, we will outline the growing evidence to support the hypothesis that the development of agonists selective for the A(2A) receptor is an effective strategy for suppressing the exaggerated inflammatory responses associated with many diseases by virtue of the receptor's ability to inhibit multiple pro-inflammatory signalling cascades.

Regulated Overexpression of the A 1 -Adenosine Receptor in Mice Results in Adverse but Reversible Changes in Cardiac Morphology and Function

Background— Both the A 1 - and A 3 -adenosine receptors (ARs) have been implicated in mediating the cardioprotective effects of adenosine. Paradoxically, overexpression of both A 1 -AR and A 3 -AR is associated with changes in the cardiac phenotype. To evaluate the temporal relationship between AR signaling and cardiac remodeling, we studied the effects of controlled overexpression of the A 1 -AR using a cardiac-specific and tetracycline-transactivating factor–regulated promoter.

Methods and Results— Constitutive A 1 -AR overexpression caused the development of cardiac dilatation and death within 6 to 12 weeks. These mice developed diminished ventricular function and decreased heart rate. In contrast, when A 1 -AR expression was delayed until 3 weeks of age, mice remained phenotypically normal at 6 weeks, and >90% of the mice survived at 30 weeks. However, late induction of A 1 -AR still caused mild cardiomyopathy at older ages (20 weeks) and accelerated cardiac hypertrophy and the development of dilatation after pressure overload. These changes were accompanied by gene expression changes associated with cardiomyopathy and fibrosis and by decreased Akt phosphorylation. Discontinuation of A 1 -AR induction mitigated cardiac dysfunction and significantly improved survival rate.

Conclusions— These data suggest that robust constitutive myocardial A 1 -AR overexpression induces a dilated cardiomyopathy, whereas delaying A 1 -AR expression until adulthood ameliorated but did not eliminate the development of cardiac pathology. Thus, the inducible A 1 -AR transgenic mouse model provides novel insights into the role of adenosine signaling in heart failure and illustrates the potentially deleterious consequences of selective versus nonselective activation of adenosine-signaling pathways in the heart.

Exchange protein activated by cyclic AMP (Epac)-mediated induction of suppressor of cytokine signaling 3 (SOCS-3) in vascular endothelial cells

Here, we demonstrate that elevation of intracellular cyclic AMP (cAMP) in vascular endothelial cells (ECs) by either a direct activator of adenylyl cyclase or endogenous cAMP-mobilizing G protein-coupled receptors inhibited the tyrosine phosphorylation of STAT proteins by an interleukin 6 (IL-6) receptor trans-signaling complex (soluble IL-6Ralpha/IL-6). This was associated with the induction of suppressor of cytokine signaling 3 (SOCS-3), a bona fide inhibitor in vivo of gp130, the signal-transducing component of the IL-6 receptor complex. Attenuation of SOCS-3 induction in either ECs or SOCS-3-null murine embryonic fibroblasts abolished the inhibitory effect of cAMP, whereas inhibition of SHP-2, another negative regulator of gp130, was without effect. Interestingly, the inhibition of STAT phosphorylation and SOCS-3 induction did not require cAMP-dependent protein kinase activity but could be recapitulated upon selective activation of the alternative cAMP sensor Epac, a guanine nucleotide exchange factor for Rap1. Consistent with this hypothesis, small interfering RNA-mediated knockdown of Epac1 was sufficient to attenuate both cAMP-mediated SOCS-3 induction and inhibition of STAT phosphorylation, suggesting that Epac activation is both necessary and sufficient to observe these effects. Together, these data argue for the existence of a novel cAMP/Epac/Rap1/SOCS-3 pathway for limiting IL-6 receptor signaling in ECs and illuminate a new mechanism by which cAMP may mediate its potent anti-inflammatory effects.

Inhibition of pro-inflammatory cytokine receptor signalling by cAMP in vascular endothelial cells

The anti-inflammatory effects of the prototypical second messenger cAMP have been extensively documented in multiple cell types. However, in many instances, the molecular mechanisms by which cAMP elevation disrupts specific pro-inflammatory signalling cascades are unknown. In this review, we will describe the importance of the JAK-STAT (where JAK stands for Janus kinase and STAT for signal transducer and activator of transcription) signalling pathway in vascular endothelial cell function, outline key inhibitory processes that serve to reduce cytokine-stimulated tyrosine phosphorylation and activation of STAT proteins, and discuss possible mechanisms by which intracellular cAMP sensors could interact with these inhibitory processes to diminish cytokine receptor-mediated pro-inflammatory signalling.

Adenosine receptors and the control of endothelial cell function in inflammatory disease

The nucleoside adenosine accumulates in many tissues following the onset of ischaemia and inflammation. This initiates a series of protective mechanisms in target cells upon binding and activation of a family of four G-protein-coupled cell surface adenosine receptor (AR) proteins. The magnitude and duration of adenosine's effects are dictated by the identity and expression levels of each receptor subtype on individual cell types within the hypoxic microenvironment. Given the key role of endothelial cells (ECs) in the development of inflammatory diseases, such as sepsis, rheumatoid arthritis (RA) and atherosclerosis, ARs represent attractive targets for therapeutic intervention in these conditions. In this review, we examine several critical aspects of endothelial function in vivo, assess the role of individual AR subtypes in these events and, where known, discuss the molecular mechanisms by which specific ARs exert their effects.

Phosphorylation-independent internalisation and desensitisation of the human sphingosine-1-phosphate receptor S1P3

Here we demonstrate that phosphorylation of the sphingosine-1-phosphate (S1P) receptor S1P(3) is increased specifically in response to S1P. Truncation of the receptor's carboxyl-terminal domain revealed that the presence of a serine-rich stretch of residues between Leu332 and Val352 was essential to observe this effect. Although agonist-occupied wild-type (WT) S1P(3) could be phosphorylated in vitro by G-protein-coupled receptor kinase 2 (GRK2), a role of S1P(3) phosphorylation in controlling S1P(3)-G(q/11) coupling was excluded since A) a phosphorylation-resistant S1P(3) mutant desensitised in a manner indistinguishable from the WT receptor and was phosphorylated to a greater extent than the WT receptor by GRK2 in vitro, and B) co-expression with GRK2 or GRK3 failed to potentiate S1P(3) phosphorylation. S1P(3) phosphorylation was also not required for receptor sequestration away from the cell surface. Together, these data suggest that S1P(3) function is not subject to conventional regulation by GRK phosphorylation and that novel aspects of S1P(3) function distinct from classical G-protein coupling and receptor internalisation may be controlled its carboxyl-terminal domain.

Specific inhibition of nuclear factor-kappaB-dependent inflammatory responses by cell type-specific mechanisms upon A2A adenosine receptor gene transfer

Adenosine is a potent inhibitor of inflammatory processes, and the A(2A) adenosine receptor (A(2A)AR) plays a key nonredundant role as a suppresser of inflammatory responses in vivo. In this study, we demonstrate that increasing A(2A)AR gene expression suppressed multiple inflammatory responses in both human umbilical vein endothelial cells (HUVECs) and rat C6 glioma cells in vitro. In particular, the induction of the adhesion molecule E-selectin by either tumor necrosis factor alpha (TNFalpha) or Escherichia coli lipopolysaccharide (LPS) was reduced by more than 70% in HUVECs, whereas inducible nitric-oxide synthase (iNOS) induction was abolished in C6 cells after exposure to interferon-gamma in combination with LPS and TNFalpha, suggesting that the receptor inhibited a common step in the induction of each of these pro-inflammatory genes. Consistent with this hypothesis, A(2A)AR expression inhibited the activation of NF-kappaB, a key transcription factor whose proper function was essential for optimal iNOS and E-selectin induction. However, although NF-kappaB binding to target DNA was severely compromised in both cell types, the mechanisms by which this occurred were distinct. In C6 cells, A(2A)AR expression blocked IkappaBalpha degradation by inhibiting stimulus-induced phosphorylation, whereas in HUVECs, A(2A)AR expression inhibited NF-kappaB translocation to the nucleus independently of any effect on IkappaBalpha degradation. Together, these observations suggest that A(2A)AR-mediated inhibition NF-kappaB activation is a critical aspect of its anti-inflammatory signaling properties and that the molecular basis of this inhibition varies in a cell type-specific manner.

In-office vital tooth bleaching--what do lights add?

Aqueous hydrogen peroxide (H2O2) has been used clinically at 30% to 35% levels to lighten teeth for many years, but the process has required multiple visits. Heat and light have been used empirically in attempts to catalyze H2O2 decomposition and speed tooth lightening. The contribution of bleaching lights (LumaArch, Optilux 500, and Zoom!) to act as catalysts for lightening teeth was studied in 83 pairs of contralateral anterior maxillary and mandibular teeth on 15 human subjects. Split-arch design using centrals, laterals, and canines on one side treated with bleach plus light, were compared with contralateral teeth using bleach alone. Three researchers trained in the use of the Vitapan 3D-Master Shade Guide took shades with independently agreement within 0.5 value-chroma sum 89% of the time throughout the study Laboratory tests determined bleach gel chemistry, bleach light output, and effects on the bleaches of light alone and heat alone. Results showed that the three test lights did not lighten teeth more than their bleach gels alone. All teeth lightened to nearly the same degree (1.7 color increments), but LumaArch required 60% less time and Zoom! used 1/3 lower H2O2 concentration. Laboratory tests indicated that the proprietary chemicals mixed into each bleach gel just before use acted as catalysts and were probably responsible for more rapid lightening produced by LumaArch gel, and need for less H2O2 in Zoom! gel. Neither the heat produced by the accessory lights, nor the light output itself were responsible for catalytic activity with any of the three systems tested. Collectively, the data demonstrate positive effects from chemical catalysts added to bleaching gels. No output from any of the lights resulted in heat or light that catalyzed the gels.

Subtype-specific regulation of receptor internalization and recycling by the carboxyl-terminal domains of the human A1 and rat A3 adenosine receptors: consequences for agonist-stimulated translocation of arrestin3

In this study, we have characterized the differential effects on inhibitory adenosine receptor (AR) trafficking of disrupting predicted sites for palmitoylation and phosphorylation within each receptor's carboxyl terminus. While a Cys(302,305)Ala-mutated rat A(3)AR mutant internalizes significantly faster than the wild-type (WT) receptor in response to agonist exposure, analogous mutation of the human A(1)AR (Cys(309)Ala) had no effect on receptor internalization. Moreover, unlike the WT A(3)AR, the entire pool of internalized mutant A(3)AR is able to recycle back to the plasma membrane following agonist removal. These properties do not reflect utilization of an alternative trafficking pathway, as internalized WT and mutant A(3)ARs both accumulate into transferrin receptor-positive endosomal compartments. However, receptor accumulation into endosomes is dependent upon prior G-protein-coupled receptor kinase (GRK)-mediated phosphorylation of the receptor's carboxyl terminus, as replacement of the carboxyl-terminal domain of the human A(1)AR with the 14 GRK-phosphorylated amino acids of the rat A(3)AR confers rapid agonist-mediated endosomal accumulation of the resulting chimeric A(1)CT3AR. Sensitivity to GRK-mediated phosphorylation also dictates the distinct redistribution of arrestin3 observed upon agonist exposure. Thus, while the nonphosphorylated A(1)AR redistributes arrestin3 from the cytoplasm to punctate clusters at the plasma membrane, GRK-phosphorylated WT and Cys(302,305)Ala-mutated A(3)ARs, as well as the A(1)CT3AR chimera, each induce the redistribution of arrestin3 into punctate accumulations both at the plasma membrane and within the cytoplasm. Neither the human A(1)AR nor the rat A(3)AR colocalized with arrestin3 under basal or agonist-stimulated conditions. Together, these results demonstrate that inhibitory AR-mediated changes in arrestin3 distribution are subtype-specific, with specificity correlating with the sensitivity of the receptor's carboxyl-terminal domain to GRK phosphorylation. In the case of the rat A(3)AR, sensitivity to GRK-mediated internalization appears to be regulated in part by the integrity of putative palmitate attachment sites upstream of its GRK phosphoacceptor sites.

Resin polymerization problems--are they caused by resin curing lights, resin formulations, or both?

Negative effects of rapid, high-intensity resin curing have been predicted for both argon lasers and plasma-arc curing lights. To address these questions, six different resin restorative materials were cured with 14 different resin curing lights representing differences in intensities ranging from 400 mW/cm2 to 1,900 mW/cm2; delivery modes using constant, ramped, and stepped methods; cure times ranging from 1 second to 40 seconds; and spot sizes of 6.7 mm to 10.9 mm. Two lasers, five plasma-arc lights, and seven halogen lights were used. Shrinkage, modulus, heat generation, strain, and physical changes on the teeth and resins during strain testing were documented. Results showed effects associated with lights were not statistically significant, but resin formulation was highly significant. Microfill resins had the least shrinkage and the lowest modulus. An autocure resin had shrinkage and modulus as high as or higher than the light-cured hybrid resins. Lasers and plasma-arc lights produced the highest heat increases on the surface (up to 21 degrees C) and within the resin restorations (up to 14 degrees C), and the halogen lights produced the most heat within the pulp chamber (up to 2 degrees C). Strain within the tooth was least with Heliomolar and greatest with Z100 Restorative and BISFIL II autocure resin. Clinical effects of strain relief were evident as white lines at the tooth-resin interface and cracks in enamel adjacent to the margins. This work implicates resin formulation, rather than light type or curing mode, as the important factor in polymerization problems. Lower light intensity and use of ramped and stepped curing modes did not provide significant lowering of shrinkage, modulus, or strain, and did not prevent enamel cracking adjacent to margins and formation of "white line" defects at the margins. Until materials with lower shrinkage and modulus are available, use of low-viscosity surface sealants as a final step in resin placement is suggested to seal defects.

Dual regulation of EDG1/S1P(1) receptor phosphorylation and internalization by protein kinase C and G-protein-coupled receptor kinase 2

Here we demonstrate that phosphorylation of the sphingosine 1-phosphate (SSP) receptor "endothelial differentiation gene 1" (EDG1 or S1P(1)) receptor is increased in response to either SSP or phorbol 12-myristate 13-acetate (PMA) exposure but not lysophosphatidic acid. Phosphoamino acid analysis demonstrated that SSP stimulated the accumulation of phosphoserine and phosphothreonine but not phosphotyrosine. An inhibitor of PMA-stimulated EDG1 phosphorylation failed to block SSP-stimulated phosphorylation. Additionally, removal of 12 amino acids from the carboxyl terminus of EDG1 specifically reduced SSP- but not PMA-stimulated phosphorylation, suggesting that SSP and PMA increase EDG1 phosphorylation via distinct mechanisms. In vitro assays revealed that G-protein-coupled receptor kinase 2 may be at least partially responsible for SSP-stimulated EDG1 phosphorylation observed in intact cells. In addition, phosphorylation by PMA and SSP were associated with a loss of EDG1 from the cell surface by distinct mechanisms. Removal of 12 residues from the carboxyl terminus of EDG1 completely inhibited SSP-mediated internalization, suggesting that this domain dictates susceptibility to receptor internalization while retaining sensitivity to SSP-stimulated phosphorylation. Thus, we conclude that (a) EDG1 phosphorylation and internalization are controlled via independent mechanisms by agonist occupation of the receptor and protein kinase C activation, and (b) although determinants within the receptor's carboxyl-terminal tail conferring EDG1 sensitivity to agonist-mediated internalization and G-protein-coupled receptor kinase phosphorylation exhibit a degree of overlap, the two phenomena are separable.

The role of pit-and-fissure discoloration in caries assessment

Occlusal dental caries often is not apparent using traditional diagnostic techniques. The clinical significance of pit-and-fissure discoloration in the absence of dietary and habit substances was examined. The study included 462 extracted teeth (216 with obvious pit-and-fissure discoloration, 216 with subtle discoloration, and 30 with no discoloration); 130 clinical teeth with varying degrees of pit-and-fissure discoloration; and 159 teeth in young adults, which had sealants placed > or = 10 years. All teeth studied were excavated very conservatively using air abrasion and/or the uniquely small H1 004 carbide bur. Presence of caries and its depth and extent were recorded, photographed, and measured with a custom made calibrated probe. Of the 721 teeth with discolored pits and fissures studied (432 extracted teeth and 289 clinical teeth), 660 (92%) had two or more of the four clinical criteria used to define dental caries in this study. Sixty-four percent of the lesions were > 2 mm in depth and 27% were > 3 mm in depth. Of the 159 teeth sealed for > or = 10 years, 47 (92%) were carious, and 26% had large, deep carious lesions penetrating > 3 mm. These data indicate: more effective methods are needed to diagnose pit-and-fissure caries and presence of pit-and-fissure discoloration in the absence of substances causing extrinsic staining should be a strong warning for clinicians to examine carefully for dental caries.

Subtype-specific kinetics of inhibitory adenosine receptor internalization are determined by sensitivity to phosphorylation by G protein-coupled receptor kinases

Despite coupling to the same class of inhibitory G proteins and binding the same physiological ligand, the human A(1) and rat A(3) adenosine receptors (ARs) desensitize at different rates in response to sustained agonist exposure. This is due to the ability of the A(3)AR, but not the A(1)AR, to serve as a substrate for rapid phosphorylation and desensitization by members of the G protein-coupled receptor kinase (GRK) family. The aim of this study was to investigate whether these differences were also manifested in their abilities to undergo agonist-dependent receptor internalization. For the first time, we report that A(3)ARs internalize profoundly in response to short-term exposure to agonist but not activators of second messenger-regulated kinases. The A(3)AR-selective antagonist MRS1523 blocked both A(3)AR phosphorylation and internalization. Moreover, in contrast to the A(1)AR, which internalized quite slowly (t(1/2) = 90 min), A(3)ARs internalized rapidly (t(1/2) = 10 min) over a time frame that followed the onset of receptor phosphorylation. A nonphosphorylated A(3)AR mutant failed to internalize over a 60-min time course, suggesting that receptor phosphorylation was essential for rapid A(3)AR internalization to occur. In addition, fusion onto the A(1)AR of the A(3)AR C-terminal domain containing the sites for phosphorylation by GRKs conferred rapid agonist-induced internalization kinetics (t(1/2) = 10 min) on the resulting chimeric AR. In conclusion, these data suggest that GRK-stimulated phosphorylation of threonine residues within the C-terminal domain of the A(3)AR is obligatory to observe rapid agonist-mediated internalization of the receptor.

Identification of threonine residues controlling the agonist-dependent phosphorylation and desensitization of the rat A(3) adenosine receptor

Activation of the A(3) adenosine receptor (A(3)AR) contributes to the cardioprotective, bronchoconstrictive, and hypotensive effects of adenosine. Agonist occupation of the A(3)AR results in a rapid desensitization of receptor function, which is associated with the phosphorylation of the receptor protein by one or more members of the G protein-coupled receptor kinase family of protein kinases. Although we demonstrated previously that phosphorylation of the C-terminal 14 amino acids of the rat A(3)AR is crucial for rapid desensitization to occur, the identity of the critical phosphorylation sites has remained unknown. Here, we demonstrate that the simultaneous mutation of Thr(307), Thr(318), and Thr(319) to Ala residues dramatically reduces agonist-stimulated phosphorylation and rapid desensitization of the rat A(3)AR. Individual mutation of each residue demonstrated that Thr(318) and Thr(319) are the major sites of phosphorylation. Phosphorylation at Thr(318) appeared to be necessary to observe phosphorylation at Thr(319), but not vice versa. However, the replacement of Thr(318) with a glutamate residue demonstrated that the simple addition of negative charge at position 318 was not sufficient to rescue phosphorylation at position 319. In addition, the mutation of two predicted palmitoylation-site cysteine residues proximal to the regulatory domain resulted in the appearance of an agonist-independent basal phosphorylation. Therefore, G protein-coupled receptor kinase-mediated phosphorylation of the C-terminal tail of the A(3)AR in situ appears to follow a sequential mechanism, perhaps involving receptor depalmitoylation, with phosphorylation at Thr(318) being particularly important.

Stimulation of A(2A) adenosine receptor phosphorylation by protein kinase C activation: evidence for regulation by multiple protein kinase C isoforms

Activation of the A(2A) adenosine receptor (A(2A)AR) contributes to the neuromodulatory and neuroprotective effects of adenosine in the central nervous system. Here we demonstrate that, in rat C6 glioma cells stably expressing an epitope-tagged canine A(2A)AR, receptor phosphorylation on serine and threonine residues can be increased by pretreatment with either the synthetic protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) or endothelin 1, which increases PKC activity via binding to endogenous endothelin(A) receptors. Under conditions in which PMA was maximally effective, activation of other second messenger-regulated kinases was without effect. While basal and PMA-stimulated phosphorylation were unaffected by the A(2A)AR-selective antagonist ZM241385, they were both blocked by GF109203X (a selective inhibitor of conventional and novel PKC isoforms) and rottlerin (a PKCdelta-selective inhibitor) but not Go6976 (selective for conventional PKC isoforms). However, coexpression of the A(2A)AR with each of the alpha, betaI, and betaII isoforms of PKC increased basal and PMA-stimulated phosphorylation. Mutation of the three consensus PKC phosphorylation sites within the receptor (Thr298, Ser320, and Ser335) to Ala failed to inhibit either basal or PMA-stimulated phosphorylation. In addition, phosphorylation of the receptor was not associated with detectable changes in either its signaling capacity or cell surface expression. These observations suggest that multiple PKC isoforms can stimulate A(2A)AR phosphorylation via activation of one or more downstream kinases which then phosphorylate the receptor directly. In addition, it is likely that phosphorylation controls interactions with regulatory proteins distinct from those involved in the classical cAMP signaling pathway utilized by this receptor.

Induction of multiple effects on adenylyl cyclase regulation by chronic activation of the human A3 adenosine receptor

The A3 adenosine receptor (A3AR) contributes to several cardiovascular effects of adenosine, including antihypertensive and cardioprotective effects. Although several studies have detailed the mechanisms underlying agonist-mediated desensitization of the rat A3AR, the regulation of the human A3AR, which displays only a 70% amino acid identity with the rat homologue, has not been addressed. Using a Chinese hamster ovary cell line stably expressing a recombinant human A3AR, we demonstrated that prolonged treatment with the AR agonist 5'-N-ethylcarboxamidoadenosine induces uncoupling of the A3AR from G proteins and functional desensitization. In addition to A3AR desensitization, a 1.5-2.5-fold increase was noted in the adenylyl cyclase (AC) activity achieved in the presence of GTP with or without forskolin. This sensitization of AC activity was not a consequence of the down-regulation of Gi proteins induced by NECA treatment and was not associated with sustained or transient increases in the expression of Gs. Time course experiments revealed that the onset of sensitization was half-maximal between 2 and 3 hr but was not due to the synthesis of new proteins because cycloheximide treatment failed to inhibit sensitization. The inability of the sensitization process to alter the AC activity obtained in the presence of manganese chloride suggests that prolonged A3AR activation increases the coupling efficiency between Gs and AC catalytic units. This phenomenon has implications for long term cellular adaptation to agonist because in agonist-treated cells, the extent to which a suboptimal concentration of forskolin could increase phosphorylation of the cAMP-responsive element binding protein was elevated compared with vehicle-treated controls.