Epac-Rap signaling reduces oxidative stress in the tubular epithelium

Transcriptomics reveals dynamic changes in the "gene profiles" of rat supraspinatus tendon at three different time points after diabetes induction

OBJECTIVE

There is increasing evidence that type 2 diabetes mellitus (T2DM) is an independent risk factor for the occur of tendinopathy. Therefore, this study is the first to explore the dynamic changes of the "gene profile" of supraspinatus tendon in rats at different time points after T2DM induction through transcriptomics, providing potential molecular markers for exploring the pathogenesis of diabetic tendinopathy.

METHODS

A total of 40 Sprague-Dawley rats were randomly divided into normal (NG, n = 10) and T2DM groups (T2DM, n = 30) and subdivided into three groups according to the duration of diabetes: T2DM-4w, T2DM-8w, and T2DM-12w groups; the duration was calculated from the time point of T2DM rat model establishment. The three comparison groups were set up in this study, T2DM-4w group vs. NG, T2DM-8w group vs. NG, and T2DM-12w group vs. NG. Differentially expressed genes (DEGs) in 3 comparison groups were screened. The intersection of the three comparison groups' DEGs was defined as key genes that changed consistently in the supraspinatus tendon after diabetes induction. Cluster analysis, gene ontology (GO) functional annotation analysis and Kyoto encyclopedia of genes and genomes (KEGG) functional annotation and enrichment analysis were performed for DEGs.

RESULTS

T2DM-4w group vs. NG, T2DM-8w group vs. NG, and T2DM-12w group vs. NG detected 519 (251 up-regulated and 268 down-regulated), 459 (342 up-regulated and 117 down-regulated) and 328 (255 up-regulated and 73 down-regulated) DEGs, respectively. 103 key genes of sustained changes in the supraspinatus tendon following induction of diabetes, which are the first identified biomarkers of the supraspinatus tendon as it progresses through the course of diabetes.The GO analysis results showed that the most significant enrichment in biological processes was calcium ion transmembrane import into cytosol (3 DEGs). The most significant enrichment in cellular component was extracellular matrix (9 DEGs). The most significant enrichment in molecular function was glutamate-gated calcium ion channel activity (3 DEGs). The results of KEGG pathway enrichment analysis showed that there were 17 major pathways (p < 0.05) that diabetes affected supratinusculus tendinopathy, including cAMP signaling pathway and Calcium signaling pathway.

CONCLUSIONS

Transcriptomics reveals dynamic changes in the"gene profiles"of rat supraspinatus tendon at three different time points after diabetes induction. The 103 DEGs identified in this study may provide potential molecular markers for exploring the pathogenesis of diabetic tendinopathy, and the 17 major pathways enriched in KEGG may provide new ideas for exploring the pathogenesis of diabetic tendinopathy.

Effect of Poria cocos Terpenes: Verifying Modes of Action Using Molecular Docking, Drug-Induced Transcriptomes, and Diffusion Network Analyses

We characterized the therapeutic biological modes of action of several terpenes in Poria cocos F.A Wolf (PC) and proposed a broad therapeutic mode of action for PC. Molecular docking and drug-induced transcriptome analysis were performed to confirm the pharmacological mechanism of PC terpene, and a new analysis method, namely diffusion network analysis, was proposed to verify the mechanism of action against Alzheimer's disease. We confirmed that the compound that exists only in PC has a unique mechanism through statistical-based docking analysis. Also, docking and transcriptomic analysis results could reflect results in clinical practice when used complementarily. The detailed pharmacological mechanism of PC was confirmed by constructing and analyzing the Alzheimer's disease diffusion network, and the antioxidant activity based on microglial cells was verified. In this study, we used two bioinformatics approaches to reveal PC's broad mode of action while also using diffusion networks to identify its detailed pharmacological mechanisms of action. The results of this study provide evidence that future pharmacological mechanism analysis should simultaneously consider complementary docking and transcriptomics and suggest diffusion network analysis, a new method to derive pharmacological mechanisms based on natural complex compounds.

Elucidating Fibroblast Growth Factor-Induced Kinome Dynamics Using Targeted Mass Spectrometry and Dynamic Modeling

Fibroblast growth factors (FGFs) are paracrine or endocrine signaling proteins that, activated by their ligands, elicit a wide range of health and disease-related processes, such as cell proliferation and the epithelial-to-mesenchymal transition. The detailed molecular pathway dynamics that coordinate these responses have remained to be determined. To elucidate these, we stimulated MCF-7 breast cancer cells with either FGF2, FGF3, FGF4, FGF10, or FGF19. Following activation of the receptor, we quantified the kinase activity dynamics of 44 kinases using a targeted mass spectrometry assay. Our system-wide kinase activity data, supplemented with (phospho)proteomics data, reveal ligand-dependent distinct pathway dynamics, elucidate the involvement of not earlier reported kinases such as MARK, and revise some of the pathway effects on biological outcomes. In addition, logic-based dynamic modeling of the kinome dynamics further verifies the biological goodness-of-fit of the predicted models and reveals BRAF-driven activation upon FGF2 treatment and ARAF-driven activation upon FGF4 treatment.

The old second messenger cAMP teams up with novel cell death mechanisms: potential translational therapeutical benefit for Alzheimer's disease and Parkinson's disease

Alzheimer's disease (AD) and Parkinson's disease (PD) represent the most prevalent neurodegenerative disorders severely impacting life expectancy and quality of life of millions of people worldwide. AD and PD exhibit both a very distinct pathophysiological disease pattern. Intriguingly, recent researches, however, implicate that overlapping mechanisms may underlie AD and PD. In AD and PD, novel cell death mechanisms, encompassing parthanatos, netosis, lysosome-dependent cell death, senescence and ferroptosis, apparently rely on the production of reactive oxygen species, and seem to be modulated by the well-known, "old" second messenger cAMP. Signaling of cAMP via PKA and Epac promotes parthanatos and induces lysosomal cell death, while signaling of cAMP via PKA inhibits netosis and cellular senescence. Additionally, PKA protects against ferroptosis, whereas Epac1 promotes ferroptosis. Here we review the most recent insights into the overlapping mechanisms between AD and PD, with a special focus on cAMP signaling and the pharmacology of cAMP signaling pathways.

EPAC Regulates Melanoma Growth by Stimulating mTORC1 Signaling and Loss of EPAC Signaling Dependence Correlates with Melanoma Progression

Exchange proteins directly activated by cAMP (EPAC) belong to a family of RAP guanine nucleotide exchange factors (RAPGEF). EPAC1/2 (RAPGEF3/4) activates RAP1 and the alternative cAMP signaling pathway. We previously showed that the differential growth response of primary and metastatic melanoma cells to cAMP is mediated by EPAC. However, the mechanisms responsible for this differential response to EPAC signaling are not understood. In this study, we show that pharmacologic inhibition or siRNA-mediated knockdown of EPAC selectively inhibits the growth and survival of primary melanoma cells by downregulation of cell-cycle proteins and inhibiting the cell-cycle progression independent of ERK1/2 phosphorylation. EPAC inhibition results in upregulation of AKT phosphorylation but a downregulation of mTORC1 activity and its downstream effectors. We also show that EPAC regulates both glycolysis and oxidative phosphorylation, and production of mitochondrial reactive oxygen species, preferentially in primary melanoma cells. Employing a series of genetically matched primary and lymph node metastatic (LNM) melanoma cells, and distant organ metastatic melanoma cells, we show that the LNM and metastatic melanoma cells become progressively less responsive and refractory to EPAC inhibition suggesting loss of dependency on EPAC signaling correlates with melanoma progression. Analysis of The Cancer Genome Atlas dataset showed that lower RAPGEF3, RAPGEF4 mRNA expression in primary tumor is a predictor of better disease-free survival of patients diagnosed with primary melanoma suggesting that EPAC signaling facilitates tumor progression and EPAC is a useful prognostic marker. These data highlight EPAC signaling as a potential target for prevention of melanoma progression.

IMPLICATIONS

This study establishes loss of dependency on EPAC-mTORC1 signaling as hallmark of primary melanoma evolution and targeting this escape mechanism is a promising strategy for metastatic melanoma.

Epac: A Promising Therapeutic Target for Vascular Diseases: A Review

Vascular diseases affect the circulatory system and comprise most human diseases. They cause severe symptoms and affect the quality of life of patients. Recently, since their identification, exchange proteins directly activated by cAMP (Epac) have attracted increasing scientific interest, because of their role in cyclic adenosine monophosphate (cAMP) signaling, a well-known signal transduction pathway. The role of Epac in cardiovascular disease and cancer is extensively studied, whereas their role in kidney disease has not been comprehensively explored yet. In this study, we aimed to review recent studies on the regulatory effects of Epac on various vascular diseases, such as cardiovascular disease, cerebrovascular disease, and cancer. Accumulating evidence has shown that both Epac1 and Epac2 play important roles in vascular diseases under both physiological and pathological conditions. Additionally, there has been an increasing focus on Epac pharmacological modulators. Therefore, we speculated that Epac could serve as a novel therapeutic target for the treatment of vascular diseases.

Puerarin attenuates diabetic kidney injury through interaction with Guanidine nucleotide-binding protein Gi subunit alpha-1 (Gnai1) subunit

Radix puerariae, a traditional Chinese herbal medication, has been used to treat patients with diabetic kidney disease (DKD). Our previous studies demonstrated that puerarin, the active compound of radix puerariae, improves podocyte injury in type 1 DKD mice. However, the direct molecular target of puerarin and its underlying mechanisms in DKD remain unknown. In this study, we confirmed that puerarin also improved DKD in type 2 diabetic db/db mice. Through RNA-sequencing odf isolated glomeruli, we found that differentially expressed genes (DEGs) that were altered in the glomeruli of these diabetic mice but reversed by puerarin treatment were involved mostly in oxidative stress, inflammatory and fibrosis. Further analysis of these reversed DEGs revealed protein kinase A (PKA) was among the top pathways. By utilizing the drug affinity responsive target stability method combined with mass spectrometry analysis, we identified guanine nucleotide-binding protein Gi alpha-1 (Gnai1) as the direct binding partner of puerarin. Gnai1 is an inhibitor of cAMP production which is known to have protection against podocyte injury. In vitro, we showed that puerarin not only interacted with Gnai1 but also increased cAMP production in human podocytes and mouse diabetic kidney in vivo. Puerarin also enhanced CREB phosphorylation, a downstream transcription factor of cAMP/PKA. Overexpression of CREB reduced high glucose-induced podocyte apoptosis. Inhibition of PKA by Rp-cAMP also diminished the effects of puerarin on high glucose-induced podocyte apoptosis. We conclude that the renal protective effects of puerarin are likely through inhibiting Gnai1 to activate cAMP/PKA/CREB pathway in podocytes.

Epac activation ameliorates tubulointerstitial inflammation in diabetic nephropathy

Tubulointerstitial inflammation plays an important role in the progression of diabetic nephropathy (DN), and tubular epithelial cells (TECs) are crucial promoters of the inflammatory cascade. Exchange protein activated by cAMP (Epac) has been shown to suppress the angiotensin II (Ang-II)-induced release of inflammatory cytokines in tubular cells. However, the role of Epac in TEC-mediated tubulointerstitial inflammation in DN remains unknown. We found that administering the Epac agonist 8-pCPT-2'-O-Me-cAMP (8-O-cAMP) to db/db mice inhibited tubulointerstitial inflammation characterized by macrophage infiltration and increased inflammatory cytokine release and consequently alleviated tubulointerstitial fibrosis in the kidney. Furthermore, 8-O-cAMP administration restored CCAAT/enhancer binding protein β (C/EBP-β) expression and further upregulated the expression of Suppressor of cytokine signaling 3 (SOCS3), while inhibiting p-STAT3, MCP-1, IL-6, and TNF-α expression in the kidney cortex in db/db mice. And in vitro study showed that macrophage migration and MCP-1 expression induced by high glucose (HG, 30 mM) were notably reduced by 8-O-cAMP in human renal proximal tubule epithelial (HK-2) cells. In addition, 8-O-cAMP treatment restored C/EBP-β expression in HK-2 cells and promoted C/EBP-β translocation to the nucleus, where it transcriptionally upregulated SOCS3 expression, subsequently inhibiting STAT3 phosphorylation. Under HG conditions, siRNA-mediated knockdown of C/EBP-β or SOCS3 in HK-2 cells partially blocked the inhibitory effect of Epac activation on the release of MCP-1. In contrast, SOCS3 overexpression inhibited HG-induced activation of STAT3 and MCP-1 expression in HK-2 cells. These findings indicate that Epac activation via 8-O-cAMP ameliorates tubulointerstitial inflammation in DN through the C/EBP-β/SOCS3/STAT3 pathway.

Epac1 and PKA agonists inhibit ROS to reduce NLRP3 inflammasome proteins in retinal endothelial cells

Purpose

Reactive oxygen species (ROS) activate inflammatory pathways in several organs, including the retina. More recent work has shown that ROS activate the NOD-like receptor protein 3 (NLRP3) inflammasome pathway proteins. We recently showed that the exchange protein activated by cAMP 1 (Epac1) and protein kinase A (PKA) regulates NLRP3 proteins in the retina. Our goal was to determine whether Epac1 and PKA reduce ROS and NLRP3 inflammasome proteins.

Methods

We used human primary retinal endothelial cells (RECs) grown in normal glucose (5 mM) and stimulated in normal glucose with hydrogen peroxide (H₂O₂) to induce ROS and measured NLRP3 pathway proteins. In some groups, we treated cells with an Epac1 or a PKA agonist in addition to H₂O₂ treatment to determine whether Epac1 and PKA reduced ROS and induced NLRP3 pathway proteins.

Results

The data showed that 500 µM H₂O₂ was the optimal dose to increase ROS in RECs. In RECs treated with H₂O₂, NLRP3 pathway proteins were increased, which were significantly reduced by cotreatment with PKA or Epac1 agonists. H₂O₂ significantly increased NIMA-related kinase 7 (Nek7) and purinergic 2X7 receptor 7 (P2X7) levels, which were blocked by Epac1 and PKA agonists.

Conclusions

Taken together, these data suggest that Epac1 and PKA reduce retinal inflammation through the reduced ROS-induced activation of NLRP3 pathway proteins.

Dietary Bioactive Ingredients Modulating the cAMP Signaling in Diabetes Treatment

As the prevalence of diabetes increases progressively, research to develop new therapeutic approaches and the search for more bioactive compounds are attracting more attention. Over the past decades, studies have suggested that cyclic adenosine monophosphate (cAMP), the important intracellular second messenger, is a key regulator of metabolism and glucose homeostasis in diverse physiopathological states in multiple organs including the pancreas, liver, gut, skeletal muscle, adipose tissues, brain, and kidney. The multiple characteristics of dietary compounds and their favorable influence on diabetes pathogenesis, as well as their intersections with the cAMP signaling pathway, indicate that these compounds have a beneficial effect on the regulation of glucose homeostasis. In this review, we outline the current understanding of the diverse functions of cAMP in different organs involved in glucose homeostasis and show that a diversity of bioactive ingredients from foods activate or inhibit cAMP signaling, resulting in the improvement of the diabetic pathophysiological process. It aims to highlight the diabetes-preventative or -therapeutic potential of dietary bioactive ingredients targeting cAMP signaling.

Patent highlights April-May 2020

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.

The Role of Epac in Cancer Progression

Cancer continues to be a prime contributor to global mortality. Despite tremendous research efforts and major advances in cancer therapy, much remains to be learned about the underlying molecular mechanisms of this debilitating disease. A better understanding of the key signaling events driving the malignant phenotype of cancer cells may help identify new pharmaco-targets. Cyclic adenosine 3',5'-monophosphate (cAMP) modulates a plethora of biological processes, including those that are characteristic of malignant cells. Over the years, most cAMP-mediated actions were attributed to the activity of its effector protein kinase A (PKA). However, studies have revealed an important role for the exchange protein activated by cAMP (Epac) as another effector mediating the actions of cAMP. In cancer, Epac appears to have a dual role in regulating cellular processes that are essential for carcinogenesis. In addition, the development of Epac modulators offered new routes to further explore the role of this cAMP effector and its downstream pathways in cancer. In this review, the potentials of Epac as an attractive target in the fight against cancer are depicted. Additionally, the role of Epac in cancer progression, namely its effect on cancer cell proliferation, migration/metastasis, and apoptosis, with the possible interaction of reactive oxygen species (ROS) in these phenomena, is discussed with emphasis on the underlying mechanisms and pathways.

Stimulation of GLP-1 Receptor Inhibits Methylglyoxal-Induced Mitochondrial Dysfunctions in H9c2 Cardiomyoblasts: Potential Role of Epac/PI3K/Akt Pathway

Accumulation of methylglyoxal (MG) contributes to oxidative stress, apoptosis, and mitochondrial dysfunction, leading to the development of type 2 diabetes and cardiovascular diseases. Inhibition of mitochondrial abnormalities induced by MG in the heart may improve and delay the progression of heart failure. Although glucagon-like peptide-1 receptor (GLP-1R) agonists have been used as anti-diabetic drugs and GLP-1R has been detected in the heart, the cardioprotective effects of GLP-1R agonists on the inhibition of MG-induced oxidative stress and mitochondrial abnormalities have not been elucidated. Stimulation of GLP-1Rs leads to cAMP elevation and subsequently activates PKA- and/or Epac-dependent signaling pathway. However, the signaling pathway involved in the prevention of MG-induced mitochondrial dysfunctions in the heart has not been clarified so far. In the present study, we demonstrated that stimulation of GLP-1Rs with exendin-4 inhibited MG-induced intracellular and mitochondrial reactive oxygen species (ROS) production and apoptosis in H9c2 cardiomyoblasts. GLP-1R stimulation also improved the alterations of mitochondrial membrane potential (MMP) and expressions of genes related to mitochondrial functions and dynamics induced by MG. In addition, stimulation of GLP-1R exhibits antioxidant and antiapoptotic effects as well as the improvement of mitochondrial functions through cAMP/Epac/PI3K/Akt signaling pathway in H9c2 cells. Our study is the first work demonstrating a novel signaling pathway for cardioprotective effects of GLP-1R agonist on inhibition of oxidative stress and prevention of mitochondrial dysfunction. Thus, GLP-1R agonist represents a potential therapeutic target for inhibition of oxidative stress and modulation of mitochondrial functions in the heart.

Angiotensin-(1-7) Attenuates Protein O-GlcNAcylation in the Retina by EPAC/Rap1-Dependent Inhibition of O-GlcNAc Transferase

Purpose

O-GlcNAcylation of cellular proteins contributes to the pathophysiology of diabetes and evidence supports a role for augmented O-GlcNAcylation in diabetic retinopathy. The aim of this study was to investigate the impact of the renin-angiotensin system on retinal protein O-GlcNAcylation.

Methods

Mice fed a high-fat diet were treated chronically with the angiotensin-converting enzyme inhibitor captopril or captopril plus the angiotensin-(1-7) Mas receptor antagonist A779. Western blotting and quantitative polymerase chain reaction were used to analyze retinal homogenates. Similar analyses were performed on lysates from human MIO-M1 retinal Müller cell cultures exposed to media supplemented with angiotensin-(1-7). Culture conditions were manipulated to influence the hexosamine biosynthetic pathway and/or signaling downstream of the Mas receptor.

Results

In the retina of mice fed a high-fat diet, captopril attenuated protein O-GlcNAcylation in a manner dependent on Mas receptor activation. In MIO-M1 cells, angiotensin-(1-7) or adenylate cyclase activation were sufficient to enhance cyclic AMP (cAMP) levels and inhibit O-GlcNAcylation. The repressive effect of cAMP on O-GlcNAcylation was dependent on exchange protein activated by cAMP (EPAC), but not protein kinase A, and was recapitulated by a constitutively active variant of the small GTPase Rap1. We provide evidence that cAMP and angiotensin-(1-7) act to suppress O-GlcNAcylation by inhibition of O-GlcNAc transferase (OGT) activity. In cells exposed to an O-GlcNAcase inhibitor or hyperglycemic culture conditions, mitochondrial superoxide levels were elevated; however, angiotensin-(1-7) signaling prevented the effect.

Conclusions

Angiotensin-(1-7) inhibits retinal protein O-GlcNAcylation via an EPAC/Rap1/OGT signaling axis.

Selective small-molecule EPAC activators

The cellular signalling enzymes, EPAC1 and EPAC2, have emerged as key intracellular sensors of the secondary messenger cyclic 3',5'-adenosine monophosphate (cyclic adenosine monophosphate) alongside protein kinase A. Interest has been galvanised in recent years thanks to the emergence of these species as potential targets for new cardiovascular disease therapies, including vascular inflammation and insulin resistance in vascular endothelial cells. We herein summarise the current state-of-the-art in small-molecule EPAC activity modulators, including cyclic nucleotides, sulphonylureas, and N-acylsulphonamides.

A peek into Epac physiology in the kidney

cAMP is a critical second messenger of numerous endocrine signals affecting water-electrolyte transport in the renal tubule. Exchange protein directly activated by cAMP (Epac) is a relatively recently discovered downstream effector of cAMP, having the same affinity to the second messenger as protein kinase A, the classical cAMP target. Two Epac isoforms, Epac1 and Epac2, are abundantly expressed in the renal epithelium, where they are thought to regulate water and electrolyte transport, particularly in the proximal tubule and collecting duct. Recent characterization of renal phenotype in mice lacking Epac1 and Epac2 revealed a critical role of the Epac signaling cascade in urinary concentration as well as in Na⁺ and urea excretion. In this review, we aim to critically summarize current knowledge of Epac relevance for renal function and to discuss the applicability of Epac-based strategies in the regulation of systemic water-electrolyte homeostasis.

Antiproliferative activities of the second-generation antipsychotic drug sertindole against breast cancers with a potential application for treatment of breast-to-brain metastases

Epidemiological observations have shown that schizophrenia patients after long-term drug treatment exhibited reduced tumor incidences. The potential anticancer effects of antipsychotic drugs are subsequently demonstrated. These drugs are of great interest as agents against untreatable brain metastases because of their ability to traverse the blood-brain barrier (BBB). Most drugs tested thus far are the first-generation antipsychotics (FGAs). But their clinical application may be limited due to high risks of deaths in elderly patients. There is an urgent need to find additional BBB-traversing anticancer agents with lower risks of deaths. In this work, we investigated antitumor activities of eight second-generation-antipsychotic (SGA) drugs, since they exhibit lower mortality rates than FGAs. We discovered that sertindole showed broad antiproliferative activities against seven cancer types including 29 cell-lines and exhibited potent effects toward breast cancer cell-lines, with half maximal concentration to inhibit proliferation by 50% (IC₅₀) as low as 800 nM. We further found that sertindole caused cell death through autophagy-associated apoptosis and its directly-binding inhibition of 5-HT6 involved in this process. In xenotransplant mice, sertindole administration approaching maximal therapeutic dose attenuated breast-tumor growth by 22.7%. Therefore, our study reveals promising anticancer potentials of sertindole against breast cancers, with probable applications for breast-to-brain metastases.

PDE/cAMP/Epac/C/EBP-β Signaling Cascade Regulates Mitochondria Biogenesis of Tubular Epithelial Cells in Renal Fibrosis

AIMS

Cyclic adenosine 3'5'-monophosphate (cAMP) is a universal second messenger that plays an important role in intracellular signal transduction. cAMP is synthesized by adenylate cyclases from adenosine triphosphate and terminated by the phosphodiesterases (PDEs). In the present study, we investigated the role of the cAMP pathway in tubular epithelial cell mitochondrial biogenesis in the pathogenesis of renal fibrosis.

RESULTS

We found that the cAMP levels were decreased in fibrotic kidney tissues, and replenishing cAMP could ameliorate tubular atrophy and extracellular matrix deposition. The downregulation of cAMP was mainly attributed to the increased PDE4 expression in tubular epithelial cells. The inhibition of PDE4 by PDE4 siRNA or the specific inhibitor, rolipram, attenuated unilateral ureteral obstruction-induced renal interstitial fibrosis and transforming growth factor (TGF)-β1-stimulated primary tubular epithelial cell (PTC) damage. The Epac1/Rap1 pathway contributed to the main effect of cAMP on renal fibrosis. Rolipram could restore C/EBP-β and PGC-1α expression and protect the mitochondrial function and structure of PTCs under TGF-β1 stimulation. The antifibrotic role of rolipram in renal fibrosis relies on C/EBP-β and PGC-1α expression in tubular epithelial cells. Innovation and Conclusion: The results of the present study indicate that cAMP signaling regulates the mitochondrial biogenesis of tubular epithelial cells in renal fibrosis. Restoring cAMP by the PDE4 inhibitor rolipram may ameliorate renal fibrosis by targeting C/EBP-β/PGC1-α and mitochondrial biogenesis. Antioxid. Redox Signal. 29, 637-652.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

C-X-C Chemokine Receptor Type 4 Plays a Crucial Role in Mediating Oxidative Stress-Induced Podocyte Injury

AIMS

Oxidative stress plays a role in mediating podocyte injury and proteinuria. However, the underlying mechanism remains poorly understood. In this study, we investigated the potential role of C-X-C chemokine receptor type 4 (CXCR4), the receptor for stromal cell-derived factor 1α (SDF-1α), in mediating oxidative stress-induced podocyte injury.

RESULTS

In mouse model of adriamycin nephropathy (ADR), CXCR4 expression was significantly induced in podocytes as early as 3 days. This was accompanied by an increased upregulation of oxidative stress in podocyte, as demonstrated by malondialdehyde assay, nitrotyrosine staining and secretion of 8-hydroxy-2'-deoxyguanosine in urine, and induction of NOX2 and NOX4, major subunits of NADPH oxidase. CXCR4 was also induced in human kidney biopsies with proteinuric kidney diseases and colocalized with advanced oxidation protein products (AOPPs), an established oxidative stress trigger. Using cultured podocytes and mouse model, we found that AOPPs induced significant loss of podocyte marker Wilms tumor 1 (WT1), nephrin, and podocalyxin, accompanied by upregulation of desmin both in vitro and in vivo. Furthermore, AOPPs worsened proteinuria and aggravated glomerulosclerosis in ADR. These effects were associated with marked activation of SDF-1α/CXCR4 axis in podocytes. Administration of AMD3100, a specific inhibitor of CXCR4, reduced proteinuria and ameliorated podocyte dysfunction and renal fibrosis triggered by AOPPs in mice. In glomerular miniorgan culture, AOPPs also induced CXCR4 expression and downregulated nephrin and WT1. Innovation and Conclusion: These results suggest that chemokine receptor CXCR4 plays a crucial role in mediating oxidative stress-induced podocyte injury, proteinuria, and renal fibrosis. CXCR4 could be a new target for mitigating podocyte injury, proteinuria, and glomerular sclerosis in proteinuric chronic kidney disease. Antioxid. Redox Signal. 27, 345-362.

Epac2-Rap1 Signaling Regulates Reactive Oxygen Species Production and Susceptibility to Cardiac Arrhythmias

AIMS

In the heart, β1-adrenergic signaling involves cyclic adenosine monophosphate (cAMP) acting via both protein kinase-A (PKA) and exchange protein directly activated by cAMP (Epac): a guanine nucleotide exchange factor for the small GTPase Rap1. Inhibition of Epac-Rap1 signaling has been proposed as a therapeutic strategy for both cancer and cardiovascular disease. However, previous work suggests that impaired Rap1 signaling may have detrimental effects on cardiac function. The aim of the present study was to investigate the influence of Epac2-Rap1 signaling on the heart using both in vivo and in vitro approaches.

RESULTS

Inhibition of Epac2 signaling induced early afterdepolarization arrhythmias in ventricular myocytes. The underlying mechanism involved an increase in mitochondrial reactive oxygen species (ROS) and activation of the late sodium current (INalate). Arrhythmias were blocked by inhibition of INalate or the mitochondria-targeted antioxidant, mitoTEMPO. In vivo, inhibition of Epac2 caused ventricular tachycardia, torsades de pointes, and sudden death. The in vitro and in vivo effects of Epac2 inhibition were mimicked by inhibition of geranylgeranyltransferase-1, which blocks interaction of Rap1 with downstream targets.

INNOVATION

Our findings show for the first time that Rap1 acts as a negative regulator of mitochondrial ROS production in the heart and that impaired Epac2-Rap1 signaling causes arrhythmias due to ROS-dependent activation of INalate. This has implications for the use of chemotherapeutics that target Epac2-Rap1 signaling. However, selective inhibition of INalate provides a promising strategy to prevent arrhythmias caused by impaired Epac2-Rap1 signaling.

CONCLUSION

Epac2-Rap1 signaling attenuates mitochondrial ROS production and reduces myocardial arrhythmia susceptibility. Antioxid. Redox Signal. 27, 117-132.

NLRX1 dampens oxidative stress and apoptosis in tissue injury via control of mitochondrial activity

NLRX1 is a mitochondrial innate immune receptor involved in viral immunity. Stokman et al. found that loss of NLRX1 increased cellular mitochondrial activity, production of reactive oxygen species, and apoptosis during oxidative stress in kidney injury.

Mitochondrial dysfunction is the most prominent source of oxidative stress in acute and chronic kidney disease. NLRX1 is a receptor of the innate immune system that is ubiquitously expressed and localized in mitochondria. We investigated whether NLRX1 may act at the interface of metabolism and innate immunity in a model of oxidative stress. Using a chimeric mouse model for renal ischemia-reperfusion injury, we found that NLRX1 protects against mortality, mitochondrial damage, and epithelial cell apoptosis in an oxidative stress–dependent fashion. We found that NLRX1 regulates oxidative phosphorylation and cell integrity, whereas loss of NLRX1 results in increased oxygen consumption, oxidative stress, and subsequently apoptosis in epithelial cells during ischemia-reperfusion injury. In line, we found that NLRX1 expression in human kidneys decreased during acute renal ischemic injury and acute cellular rejection. Although first implicated in immune regulation, we propose that NLRX1 function extends to the control of mitochondrial activity and prevention of oxidative stress and apoptosis in tissue injury.

Multifunctional Mitochondrial Epac1 Controls Myocardial Cell Death

Rationale:

Although the second messenger cyclic AMP (cAMP) is physiologically beneficial in the heart, it largely contributes to cardiac disease progression when dysregulated. Current evidence suggests that cAMP is produced within mitochondria. However, mitochondrial cAMP signaling and its involvement in cardiac pathophysiology are far from being understood.

Objective:

To investigate the role of MitEpac1 (mitochondrial exchange protein directly activated by cAMP 1) in ischemia/reperfusion injury.

Methods and Results:

We show that Epac1 (exchange protein directly activated by cAMP 1) genetic ablation ( Epac1 −/− ) protects against experimental myocardial ischemia/reperfusion injury with reduced infarct size and cardiomyocyte apoptosis. As observed in vivo, Epac1 inhibition prevents hypoxia/reoxygenation–induced adult cardiomyocyte apoptosis. Interestingly, a deleted form of Epac1 in its mitochondrial-targeting sequence protects against hypoxia/reoxygenation–induced cell death. Mechanistically, Epac1 favors Ca 2+ exchange between the endoplasmic reticulum and the mitochondrion, by increasing interaction with a macromolecular complex composed of the VDAC1 (voltage-dependent anion channel 1), the GRP75 (chaperone glucose-regulated protein 75), and the IP3R1 (inositol-1,4,5-triphosphate receptor 1), leading to mitochondrial Ca 2+ overload and opening of the mitochondrial permeability transition pore. In addition, our findings demonstrate that MitEpac1 inhibits isocitrate dehydrogenase 2 via the mitochondrial recruitment of CaMKII (Ca 2+ /calmodulin-dependent protein kinase II), which decreases nicotinamide adenine dinucleotide phosphate hydrogen synthesis, thereby, reducing the antioxidant capabilities of the cardiomyocyte.

Conclusions:

Our results reveal the existence, within mitochondria, of different cAMP–Epac1 microdomains that control myocardial cell death. In addition, our findings suggest Epac1 as a promising target for the treatment of ischemia-induced myocardial damage.

Inhibition of Neutrophil Extracellular Trap Formation after Stem Cell Transplant by Prostaglandin E2

RATIONALE

Autologous and allogeneic hematopoietic stem cell transplant (HSCT) patients are susceptible to pulmonary infections, including bacterial pathogens, even after hematopoietic reconstitution. We previously reported that murine bone marrow transplant (BMT) neutrophils overexpress cyclooxygenase-2, overproduce prostaglandin E2 (PGE2), and exhibit defective intracellular bacterial killing. Neutrophil extracellular traps (NETs) are DNA structures that capture and kill extracellular bacteria and other pathogens.

OBJECTIVES

To determine whether NETosis was defective after transplant and if so, whether this was regulated by PGE2 signaling.

METHODS

Neutrophils isolated from mice and humans (both control and HSCT subjects) were analyzed for NETosis in response to various stimuli in the presence or absence of PGE2 signaling modifiers.

MEASUREMENTS AND MAIN RESULTS

NETs were visualized by immunofluorescence or quantified by Sytox Green fluorescence. Treatment of BMT or HSCT neutrophils with phorbol 12-myristate 13-acetate or rapamycin resulted in reduced NET formation relative to control cells. NET formation after BMT was rescued both in vitro and in vivo with cyclooxygenase inhibitors. Additionally, the EP2 receptor antagonist (PF-04418948) or the EP4 antagonist (AE3-208) restored NET formation in neutrophils isolated from BMT mice or HSCT patients. Exogenous PGE2 treatment limited NETosis of neutrophils collected from normal human volunteers and naive mice in an exchange protein activated by cAMP- and protein kinase A-dependent manner.

CONCLUSIONS

Our results suggest blockade of the PGE2-EP2 or EP4 signaling pathway restores NETosis after transplantation. Furthermore, these data provide the first description of a physiologic inhibitor of NETosis.

Involvement of intracellular cAMP in epirubicin-induced vascular endothelial cell injury

We investigated the involvement of intracellular cAMP in endothelial cell injury induced by epirubicin. Epirubicin-induced decrease in cell viability and increase in caspase-3/7 activity were reversed by a cAMP analog dibutyryl cAMP (DBcAMP) or an activator of adenylate cyclase forskolin concomitant with a phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine. Moreover, epirubicin-induced elevation of lipid peroxide levels was attenuated by DBcAMP. Interestingly, the exposure of epirubicin decreased intracellular cAMP levels before the onset of epirubicin-induced production of lipid peroxidation. These results suggest that intracellular cAMP plays an important role in epirubicin-induced endothelial cell injury.

Designing drugs that combat kidney damage

INTRODUCTION

Kidney disease remains one of the last worldwide frontiers in the field of non-communicable human disease. From 1990 to 2013, chronic kidney disease (CKD) was the top non-communicable cause of death with a greatest increase in global years of life lost while mortality of acute kidney injury (AKI) still hovers around 50%. This reflects the paucity (for CKD) or lack of (for AKI) therapeutic approaches beyond replacing renal function. Understanding what the barriers are and what potential pathways may facilitate the design of new drugs to combat kidney disease is a key public health priority.

AREAS COVERED

The authors discuss the hurdles and opportunities for future drug development for kidney disease in light of experience accumulated with drugs that made it to clinical trials.

EXPERT OPINION

Inflammation, cell death and fibrosis are key therapeutic targets to combat kidney damage. While the specific targeting of drugs to kidney cells would be desirable, the technology is only working at the preclinical stage and with mixed success. Nanomedicines hold promise in this respect. Most drugs undergoing clinical trials for kidney disease have been repurposed from other indications. Currently, the chemokine receptor inhibitor CCX140 holds promise for CKD and the p53 inhibitor QPI-1002 for AKI.

Cyclic nucleotide signalling in kidney fibrosis

Kidney fibrosis is an important factor for the progression of kidney diseases, e.g., diabetes mellitus induced kidney failure, glomerulosclerosis and nephritis resulting in chronic kidney disease or end-stage renal disease. Cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) were implicated to suppress several of the above mentioned renal diseases. In this review article, identified effects and mechanisms of cGMP and cAMP regarding renal fibrosis are summarized. These mechanisms include several signalling pathways of nitric oxide/ANP/guanylyl cyclases/cGMP-dependent protein kinase and cAMP/Epac/adenylyl cyclases/cAMP-dependent protein kinase. Furthermore, diverse possible drugs activating these pathways are discussed. From these diverse mechanisms it is expected that new pharmacological treatments will evolve for the therapy or even prevention of kidney failure.

Diverse origins of the myofibroblast—implications for kidney fibrosis

Fibrosis is the common end point of chronic kidney disease. The persistent production of inflammatory cytokines and growth factors leads to an ongoing process of extracellular matrix production that eventually disrupts the normal functioning of the organ. During fibrosis, the myofibroblast is commonly regarded as the predominant effector cell. Accumulating evidence has demonstrated a diverse origin of myofibroblasts in kidney fibrosis. Proposed major contributors of myofibroblasts include bone marrow-derived fibroblasts, tubular epithelial cells, endothelial cells, pericytes and interstitial fibroblasts; the published data, however, have not yet clearly defined the relative contribution of these different cellular sources. Myofibroblasts have been reported to originate from various sources, irrespective of the nature of the initial damage responsible for the induction of kidney fibrosis. Here, we review the possible relevance of the diversity of myofibroblast progenitors in kidney fibrosis and the implications for the development of novel therapeutic approaches. Specifically, we discuss the current status of preclinical and clinical antifibrotic therapy and describe targeting strategies that might help support resident and circulating cells to maintain or regain their original functional differentiation state. Such strategies might help these cells resist their transition to a myofibroblast phenotype to prevent, or even reverse, the fibrotic state.