A novel player in cellular hypertrophy: Giβγ/PI3K-dependent activation of the RacGEF TIAM-1 is required for α₁-adrenoceptor induced hypertrophy in neonatal rat cardiomyocytes

Using Omics to Identify Novel Therapeutic Targets in Heart Failure

Omics refers to the measurement and analysis of the totality of molecules or biological processes involved within an organism. Examples of omics data include genomics, transcriptomics, epigenomics, proteomics, metabolomics, and more. In this review, we present the available literature reporting omics data on heart failure that can inform the development of novel treatments or innovative treatment strategies for this disease. This includes polygenic risk scores to improve prediction of genomic data and the potential of multiomics to more efficiently identify potential treatment targets for further study. We also discuss the limitations of omic analyses and the barriers that must be overcome to maximize the utility of these types of studies. Finally, we address the current state of the field and future opportunities for using multiomics to better personalize heart failure treatment strategies.

TIAM-1 regulates polarized protrusions during dorsal intercalation in the Caenorhabditis elegans embryo through both its GEF and N-terminal domains

Mediolateral cell intercalation is a morphogenetic strategy used throughout animal development to reshape tissues. Dorsal intercalation in the Caenorhabditis elegans embryo involves the mediolateral intercalation of two rows of dorsal epidermal cells to create a single row that straddles the dorsal midline, and thus is a simple model to study cell intercalation. Polarized protrusive activity during dorsal intercalation requires the C. elegans Rac and RhoG orthologs CED-10 and MIG-2, but how these GTPases are regulated during intercalation has not been thoroughly investigated. In this study, we characterized the role of the Rac-specific guanine nucleotide exchange factor (GEF) TIAM-1 in regulating actin-based protrusive dynamics during dorsal intercalation. We found that TIAM-1 can promote formation of the main medial lamellipodial protrusion extended by intercalating cells through its canonical GEF function, whereas its N-terminal domains function to negatively regulate the generation of ectopic filiform protrusions around the periphery of intercalating cells. We also show that the guidance receptor UNC-5 inhibits these ectopic filiform protrusions in dorsal epidermal cells and that this effect is in part mediated via TIAM-1. These results expand the network of proteins that regulate basolateral protrusive activity during directed rearrangement of epithelial cells in animal embryos.

TIAM-1 regulates polarized protrusions during dorsal intercalation in the C. elegans embryo through both its GEF and N-terminal domains

Mediolateral cell intercalation is a morphogenetic strategy used throughout animal development to reshape tissues. Dorsal intercalation in the C. elegans embryo involves the mediolateral intercalation of two rows of dorsal epidermal cells to create a single row that straddles the dorsal midline, and so is a simple model to study cell intercalation. Polarized protrusive activity during dorsal intercalation requires the C. elegans Rac and RhoG orthologs CED-10 and MIG-2, but how these GTPases are regulated during intercalation has not been thoroughly investigated. In this study, we characterize the role of the Rac-specific guanine nucleotide exchange factor (GEF), TIAM-1, in regulating actin-based protrusive dynamics during dorsal intercalation. We find that TIAM-1 can promote protrusion formation through its canonical GEF function, while its N-terminal domains function to negatively regulate this activity, preventing the generation of ectopic protrusions in intercalating cells. We also show that the guidance receptor UNC-5 inhibits ectopic protrusive activity in dorsal epidermal cells, and that this effect is in part mediated via TIAM-1. These results expand the network of proteins that regulate basolateral protrusive activity during directed cell rearrangement.

Summary statement

TIAM-1 activates the Rac pathway to promote protrusion formation via its GEF domain, while its N-terminal domains suppress ectopic protrusions during dorsal intercalation in the C. elegans embryo.

Pharmacological Inhibition of P-Rex1/Rac1 Axis Blocked Angiotensin II-Induced Cardiac Fibrosis

PURPOSE

Phosphatidylinositol-3,4,5-trisphosphate-dependent Rac exchange factor-1 (P-Rex1), as one of the members of Rac-GEFs, has been proven to play a critical role in cancer progression and metastasis. Nonetheless, its role in cardiac fibrosis remains elusive. In the present study, we aimed to investigate whether and how the P-Rex1 mediates AngII-induced cardiac fibrosis.

METHOD

A cardiac fibrosis mouse model was established by chronic AngII perfusion. The heart structure, function, pathological changes of myocardial tissues, oxidative stress, and cardiac fibrotic protein expression were determined in an AngII induced mouse model. To provide a molecular mechanism for P-Rex1 involvement in cardiac fibrosis, a specific inhibitor or siRNA was used to block P-Rex1, and target the relationship between Rac1-GTPase and its downstream effector.

RESULTS

Blocking P-Rex1 showed down-regulation of its downstream effectors such as the profibrotic transcriptional regulator Paks, ERK1/2, and ROS generation. Intervention treatment with P-Rex1 inhibitor 1A-116 ameliorated AngII-induced abnormalities in heart structure and function. Moreover, pharmacological inhibition of the P-Rex1/Rac1 axis showed a protective effect in AngII-induced cardiac fibrosis through the down-regulation of collagen1, CTGF, and α-SMA expression.

CONCLUSION

Our findings demonstrated for the first time that P-Rex1 was an essential signaling mediator in CFs activation and subsequent cardiac fibrosis, and 1A-116 could be a potential pharmacological development drug.

Increased length-dependent activation of human engineered heart tissue after chronic α1A-adrenergic agonist treatment: testing a novel heart failure therapy

Chronic stimulation of cardiac α1A-adrenergic receptors (α1A-ARs) improves symptoms in multiple preclinical models of heart failure. However, the translational significance remains unclear. Human engineered heart tissues (EHTs) provide a means of quantifying the effects of chronic α1A-AR stimulation on human cardiomyocyte physiology. EHTs were created from thin slices of decellularized pig myocardium seeded with human induced pluripotent stem cell (iPSC)-derived cardiomyocytes and fibroblasts. With a paired experimental design, EHTs were cultured for 3 wk, mechanically tested, cultured again for 2 wk with α1A-AR agonist A61603 (10 nM) or vehicle control, and retested after drug washout for 24 h. Separate control experiments determined the effects of EHT age (3-5 wk) or repeat mechanical testing. We found that chronic A61603 treatment caused a 25% increase of length-dependent activation (LDA) of contraction compared with vehicle treatment (n = 7/group, P = 0.035). EHT force was not increased after chronic A61603 treatment. However, after vehicle treatment, EHT force was increased by 35% relative to baseline testing (n = 7/group, P = 0.022), suggesting EHT maturation. Control experiments suggested that increased EHT force resulted from repeat mechanical testing, not from EHT aging. RNA-seq analysis confirmed that the α1A-AR is expressed in human EHTs and found chronic A61603 treatment affected gene expression in biological pathways known to be activated by α1A-ARs, including the MAP kinase signaling pathway. In conclusion, increased LDA in human EHT after chronic A61603 treatment raises the possibility that chronic stimulation of the α1A-AR might be beneficial for increasing LDA in human myocardium and might be beneficial for treating human heart failure by restoring LDA.NEW & NOTEWORTHY Chronic stimulation of α1A-adrenergic receptors (α1A-ARs) is known to mediate therapeutic effects in animal heart failure models. To investigate the effects of chronic α1A-AR stimulation in human cardiomyocytes, we tested engineered heart tissue (EHT) created with iPSC-derived cardiomyocytes. RNA-seq analysis confirmed human EHT expressed α1A-ARs. Chronic (2 wk) α1A-AR stimulation with A61603 (10 nM) increased length-dependent activation (LDA) of contraction. Chronic α1A-AR stimulation might be beneficial for treating human heart failure by restoring LDA.

Identification of Rare Variants in Right Ventricular Outflow Tract Obstruction Congenital Heart Disease by Whole-Exome Sequencing

Background

Pulmonary atresia (PA) is a kind of congenital heart disease characterized by right ventricular outflow tract obstruction. It is divided into PA with intact ventricular septum (PA/IVS) whose favorable form is pulmonary valvular stenosis (PS), and PA with ventricular septal defect (PA/VSD) whose favorable form is tetralogy of Fallot (TOF). Due to limitations in genetics etiology, whole-exome sequencing (WES) was utilized to identify new variants associated with the diseases.

Methods

The data from PS-PA/IVS (n = 74), TOF-PA/VSD (n = 100), and 100 controls were obtained. The common sites between PS and PA/IVS, PA/VSD and TOF, were compared. The novel rare damage variants, and candidate genes were identified by gene-based burden analysis. Finally, the enrichment analysis of differential genes was conducted between case and control groups.

Results

Seventeen rare damage variants located in seven genes were predicted to be associated with the PS through burden analysis. Enrichment analysis identified that the Wnt and cadherin signaling pathways were relevant to PS-PA/IVS.

Conclusion

This study put forth seven candidate genes (APC, PPP1R12A, PCK2, SOS2, TNR, MED13, and TIAM1), resulting in PS-PA/IVS. The Wnt and cadherin signaling pathways were identified to be related to PS-PA/IVS by enrichment analysis. This study provides new evidence for exploring the genetic mechanism of PS-PA/IVS.

MicroRNA-183 as a Novel Regulator Protects Against Cardiomyocytes Hypertrophy via Targeting TIAM1

BACKGROUND

MicroRNAs serve as important regulators of the pathogenesis of cardiac hypertrophy. Among them, miR-183 is well documented as a novel tumor suppressor in previous studies, whereas it exhibits a downregulated expression in cardiac hypertrophy recently. The present study was aimed to examine the effect of miR-183 on cardiomyocytes hypertrophy.

METHODS

Angiotensin II (Ang II) was used for establishment of cardiac hypertrophy model in vitro. Neonatal rat ventricular cardiomyocytes transfected with miR-183 mimic or negative control were further utilized for the phenotype analysis. Moreover, the bioinformatics analysis and luciferase reporter assays were used for exploring the potential target of miR-183 in cardiomyocytes.

RESULTS

We observed a significant decreased expression of miR-183 in hypertrophic cardiomyocytes. Overexpression of miR-183 significantly attenuated the cardiomyocytes size morphologically and prohypertrophic genes expression. Moreover, we demonstrated that TIAM1 was a direct target gene of miR-183 verified by bioinformatics analysis and luciferase reporter assays, which showed a decreased mRNA and protein expression in the cardiomyocytes transfected with miR-183 upon Ang II stimulation. Additionally, the downregulated TIAM1 expression was required for the attenuated effect of miR-183 on cardiomyocytes hypertrophy.

CONCLUSIONS

Taken together, these evidences indicated that miR-183 acted as a cardioprotective regulator for the development of cardiomyocytes hypertrophy via directly regulation of TIAM1.

RGS3L allows for an M2 muscarinic receptor-mediated RhoA-dependent inotropy in cardiomyocytes

The role and outcome of the muscarinic M₂ acetylcholine receptor (M₂R) signaling in healthy and diseased cardiomyocytes is still a matter of debate. Here, we report that the long isoform of the regulator of G protein signaling 3 (RGS3L) functions as a switch in the muscarinic signaling, most likely of the M₂R, in primary cardiomyocytes. High levels of RGS3L, as found in heart failure, redirect the G_i-mediated Rac1 activation into a G_i-mediated RhoA/ROCK activation. Functionally, this switch resulted in a reduced production of reactive oxygen species (- 50%) in cardiomyocytes and an inotropic response (+ 18%) in transduced engineered heart tissues. Importantly, we could show that an adeno-associated virus 9-mediated overexpression of RGS3L in rats in vivo, increased the contractility of ventricular strips by maximally about twofold. Mechanistically, we demonstrate that this switch is mediated by a complex formation of RGS3L with the GTPase-activating protein p190RhoGAP, which balances the activity of RhoA and Rac1 by altering its substrate preference in cardiomyocytes. Enhancement of this complex formation could open new possibilities in the regulation of the contractility of the diseased heart.

Pak2 as a Novel Therapeutic Target for Cardioprotective Endoplasmic Reticulum Stress Response

Supplemental Digital Content is available in the text.

Rationale:

Secreted and membrane-bound proteins, which account for 1/3 of all proteins, play critical roles in heart health and disease. The endoplasmic reticulum (ER) is the site for synthesis, folding, and quality control of these proteins. Loss of ER homeostasis and function underlies the pathogenesis of many forms of heart disease.

Objective:

To investigate mechanisms responsible for regulating cardiac ER function, and to explore therapeutic potentials of strengthening ER function to treat heart disease.

Methods and Results:

Screening a range of signaling molecules led to the discovery that Pak (p21-activated kinase)2 is a stress-responsive kinase localized in close proximity to the ER membrane in cardiomyocytes. We found that Pak2 cardiac deleted mice (Pak2-CKO) under tunicamycin stress or pressure overload manifested a defective ER response, cardiac dysfunction, and profound cell death. Small chemical chaperone tauroursodeoxycholic acid treatment of Pak2-CKO mice substantiated that Pak2 loss-induced cardiac damage is an ER-dependent pathology. Gene array analysis prompted a detailed mechanistic study, which revealed that Pak2 regulation of protective ER function was via the IRE (inositol-requiring enzyme)-1/XBP (X-box–binding protein)-1–dependent pathway. We further discovered that this regulation was conferred by Pak2 inhibition of PP2A (protein phosphatase 2A) activity. Moreover, IRE-1 activator, Quercetin, and adeno-associated virus serotype-9–delivered XBP-1s were able to relieve ER dysfunction in Pak2-CKO hearts. This provides functional evidence, which supports the mechanism underlying Pak2 regulation of IRE-1/XBP-1s signaling. Therapeutically, inducing Pak2 activation by genetic overexpression or adeno-associated virus serotype-9–based gene delivery was capable of strengthening ER function, improving cardiac performance, and diminishing apoptosis, thus protecting the heart from failure.

Conclusions:

Our findings uncover a new cardioprotective mechanism, which promotes a protective ER stress response via the modulation of Pak2. This novel therapeutic strategy may present as a promising option for treating cardiac disease and heart failure.

Inflammation leads through PGE/EP3 signaling to HDAC5/MEF2-dependent transcription in cardiac myocytes

The myocyte enhancer factor 2 (MEF2) regulates transcription in cardiac myocytes and adverse remodeling of adult hearts. Activators of G protein-coupled receptors (GPCRs) have been reported to activate MEF2, but a comprehensive analysis of GPCR activators that regulate MEF2 has to our knowledge not been performed. Here, we tested several GPCR agonists regarding their ability to activate a MEF2 reporter in neonatal rat ventricular myocytes. The inflammatory mediator prostaglandin E₂ (PGE₂) strongly activated MEF2. Using pharmacological and protein-based inhibitors, we demonstrated that PGE₂ regulates MEF2 via the EP₃ receptor, the βγ subunit of G_i/o protein and two concomitantly activated downstream pathways. The first consists of Tiam1, Rac1, and its effector p21-activated kinase 2, the second of protein kinase D. Both pathways converge on and inactivate histone deacetylase 5 (HDAC5) and thereby de-repress MEF2. In vivo, endotoxemia in MEF2-reporter mice induced upregulation of PGE₂ and MEF2 activation. Our findings provide an unexpected new link between inflammation and cardiac remodeling by de-repression of MEF2 through HDAC5 inactivation, which has potential implications for new strategies to treat inflammatory cardiomyopathies.

Pannexin-1 channels and their emerging functions in cardiovascular diseases

Pannexin-1, Pannexin-2, and Pannexin-3 are three members of the Pannexin family of channel-forming glycoprotein. Their primary function is defined by their ability to form single-membrane channels. Pannexin-1 ubiquitously exists in many cells and organs throughout the body and is specially distributed in the circulatory system, while the expressions of Pannexin-2 and Pannexin-3 are mostly restricted to organs and tissues. Pannexin-1 oligomers have been shown to be functional single membrane channels that connect intracellular and extracellular compartments and are not intercellular channels in appositional membranes. The physiological functions of Pannexin-1 are to link to the adenosine triphosphate efflux that acts as a paracrine signal, and regulate cellular inflammasomes in a variety of cell types under physiological and pathophysiological conditions. However, there are still many functions to be explored. This review summarizes recent reports and discusses the role of Pannexin-1 in cardiovascular diseases, including ischemia, arrhythmia, cardiac fibrosis, and hypertension. Pannexin-1 has been suggested as an exciting, clinically relevant target in cardiovascular diseases.

Inhibition of prostate smooth muscle contraction and prostate stromal cell growth by the inhibitors of Rac, NSC23766 and EHT1864

BACKGROUND AND PURPOSE

Medical therapy of lower urinary tract symptoms (LUTS) suggestive of benign prostatic hyperplasia (BPH) targets smooth muscle contraction in the prostate, or prostate growth. However, current therapeutic options are insufficient. Here, we investigated the role of Rac in the control of smooth muscle tone in human prostates and growth of prostate stromal cells.

EXPERIMENTAL APPROACH

Experiments were performed using human prostate tissues from radical prostatectomy and cultured stromal cells (WPMY-1). Expression of Rac was examined by Western blot and fluorescence staining. Effects of Rac inhibitors (NSC23766 and EHT1864) on contractility were assessed in the organ bath. The effects of Rac inhibitors were assessed by pull-down, cytotoxicity using a cell counting kit, cytoskeletal organization by phalloidin staining and cell growth using an 5-ethynyl-2'-deoxyuridine assay.

KEY RESULTS

Expression of Rac1-3 was observed in prostate samples from each patient. Immunoreactivity for Rac1-3 was observed in the stroma, where it colocalized with the smooth muscle marker, calponin. NSC23766 and EHT1864 significantly reduced contractions of prostate strips induced by noradrenaline, phenylephrine or electrical field stimulation. NSC23766 and EHT1864 inhibited Rac activity in WPMY-1 cells. Survival of WPMY-1 cells ranged between 64 and 81% after incubation with NSC23766 (50 or 100 μM) or EHT1864 (25 μM) for 24 h. NSC23766 and EHT1864 induced cytoskeletal disorganization in WPMY-1 cells. Both inhibitors impaired the growth of WPMY-1 cells.

CONCLUSIONS AND IMPLICATIONS

Rac may be a link connecting the control of prostate smooth muscle tone with proliferation of smooth muscle cells. Improvements in LUTS suggestive of BPH by Rac inhibitors appears possible.

A systemic Pasteurella multocida toxin aggravates cardiac hypertrophy and fibrosis in mice

Pasteurella multocida toxin (PMT) persistently activates heterotrimeric G proteins of the Gαq/11 , Gα12/13 and Gαi family without interaction with G protein-coupled receptors (GPCRs). We show that PMT acts on heart tissue in vivo and on cardiomyocytes and cardiac fibroblasts in vitro by deamidation of heterotrimeric G proteins. Increased normalized ventricle weights and fibrosis were detected after intraperitoneal administration of PMT in combination with the GPCR agonist phenylephrine. In neonatal rat cardiomyocytes, PMT stimulated the mitogen-activated protein kinase pathway, which is crucial for the development of cellular hypertrophy. The toxin induced phosphorylation of the canonical phosphorylation sites of the extracellular-regulated kinase 1/2 and, additionally, caused phosphorylation of the recently recognized autophosphorylation site, which appears to be important for the development of cellular hypertrophy. Moreover, PMT stimulated the small GTPases Rac1 and RhoA. Both switch proteins are involved in cardiomyocyte hypertrophy. In addition, PMT stimulated RhoA and Rac1 in neonatal rat cardiac fibroblasts. RhoA and Rac1 have been implicated in the regulation of connective tissue growth factor (CTGF) secretion and expression. Accordingly, we show that PMT treatment increased secretion and expression of CTGF in cardiac fibroblasts. Altogether, the data indicate that PMT is an inducer of pathological remodelling of cardiac cells and identifies the toxin as a promising tool for studying heterotrimeric G protein-dependent signalling in cardiac cells.

A molecular signature in the pannexin1 intracellular loop confers channel activation by the α1 adrenoreceptor in smooth muscle cells

Both purinergic signaling through nucleotides such as ATP (adenosine 5'-triphosphate) and noradrenergic signaling through molecules such as norepinephrine regulate vascular tone and blood pressure. Pannexin1 (Panx1), which forms large-pore, ATP-releasing channels, is present in vascular smooth muscle cells in peripheral blood vessels and participates in noradrenergic responses. Using pharmacological approaches and mice conditionally lacking Panx1 in smooth muscle cells, we found that Panx1 contributed to vasoconstriction mediated by the α1 adrenoreceptor (α1AR), whereas vasoconstriction in response to serotonin or endothelin-1 was independent of Panx1. Analysis of the Panx1-deficient mice showed that Panx1 contributed to blood pressure regulation especially during the night cycle when sympathetic nervous activity is highest. Using mimetic peptides and site-directed mutagenesis, we identified a specific amino acid sequence in the Panx1 intracellular loop that is essential for activation by α1AR signaling. Collectively, these data describe a specific link between noradrenergic and purinergic signaling in blood pressure homeostasis.

Hydrogen sulfide improves glucose metabolism and prevents hypertrophy in cardiomyocytes

INTRODUCTION

Hydrogen sulfide (H2S) has been reported to inhibit myocardial hypertrophy in a cell model of cardiomyocyte hypertrophy. Our previous study also shows an H2S-induced increase in glucose metabolism in insulin-targeting cells. The present study aims to examine the hypothesis that H2S attenuates myocardial hypertrophy and promotes glucose utilization in cardiomyocytes.

METHODS

The cell model of cardiomyocyte hypertrophy was induced by application of phenylephrine and cardiomyocyte hypertrophy was examined using leucine incorporation assay. Protein levels were measured using Western blot analysis. The activity of related enzymes was measured with enzyme-linked immunosorbent assay (ELISA).

RESULTS

NaHS (an H2S donor) treatment increased the activity of cultured cardiomyocytes and reduced hypertrophy in cultured cardiomyocytes at concentrations ranging from 25 to 200 µmol/L. NaHS treatment increased glucose uptake and the efficiency of glycolysis and the citric acid cycle. The key enzymes in these reactions, including lactate dehydrogenase and pyruvate kinase and succinate dehydrogenase, were activated by NaHS treatment (100 µmol/L). Some intermediates of glycolysis and the citric acid cycle, including lactic acid, cyclohexylammonium, oxaloacetic acid, succinate, L-dimalate, sodium citrate, cis-aconitic acid, ketoglutarate and DL-isocitric acid trisodium also showed anti-hypertrophic effects in cardiomyocytes.

CONCLUSIONS

H2S improves glucose utilization and inhibits cardiomyocyte hypertrophy.

Pannexin-1 as a potentiator of ligand-gated receptor signaling

Pannexins are a class of plasma membrane spanning proteins that presumably form a hexameric, non-selective ion channel. Although similar in secondary structure to the connexins, pannexins notably do not form endogenous gap junctions and act as bona fide ion channels. The pannexins have been primarily studied as ATP-release channels, but the overall diversity of their functions is still being elucidated. There is an intriguing theme with pannexins that has begun to develop. In this review we analyze several recent reports that converge on the idea that pannexin channels (namely Panx1) can potentiate ligand-gated receptor signaling. Although the literature remains sparse, this emerging concept appears consistent between both ionotropic and metabotropic receptors of several ligand families.

Regulation of PP2AC carboxylmethylation and cellular localisation by inhibitory class G-protein coupled receptors in cardiomyocytes

The enzymatic activity of the type 2A protein phosphatase (PP2A) holoenzyme, a major serine/threonine phosphatase in the heart, is conferred by its catalytic subunit (PP2A_C). PP2A_C activity and subcellular localisation can be regulated by reversible carboxylmethylation of its C-terminal leucine309 (leu309) residue. Previous studies have shown that the stimulation of adenosine type 1 receptors (A1.Rs) induces PP2A_C carboxylmethylation and altered subcellular distribution in adult rat ventricular myocytes (ARVM). In the current study, we show that the enzymatic components that regulate the carboxylmethylation status of PP2A_C, leucine carboxylmethyltransferase-1 (LCMT-1) and phosphatase methylesterase-1 (PME-1) are abundantly expressed in, and almost entirely localised in the cytoplasm of ARVM. The stimulation of G_i-coupled A1.Rs with N⁶-cyclopentyladenosine (CPA), and of other G_i-coupled receptors such as muscarinic M₂ receptors (stimulated with carbachol) and angiotensin II AT₂ receptors (stimulated with CGP42112) in ARVM, induced PP2A_C carboxylmethylation at leu309 in a concentration-dependent manner. Exposure of ARVM to 10 µM CPA increased the cellular association between PP2A_C and its methyltransferase LCMT-1, but not its esterase PME-1. Stimulation of A1.Rs with 10 µM CPA increased the phosphorylation of protein kinase B at ser473, which was abolished by the PI3K inhibitor LY294002 (20 µM), thereby confirming that PI3K activity is upregulated in response to A1.R stimulation by CPA in ARVM. A1.R-induced PP2A_C translocation to the particulate fraction was abrogated by adenoviral expression of the alpha subunit (Gα_t1) coupled to the transducin G-protein coupled receptor. A similar inhibitory effect on A1.R-induced PP2A_C translocation was also seen with LY294002 (20 µM). These data suggest that in ARVM, A1.R-induced PP2A_C translocation to the particulate fraction occurs through a G_iPCR-Gβγ-PI3K mediated intracellular signalling pathway, which may involve elevated PP2A_C carboxylmethylation at leu309.

Cardiac alpha1-adrenergic receptors: novel aspects of expression, signaling mechanisms, physiologic function, and clinical importance

Adrenergic receptors (AR) are G-protein-coupled receptors (GPCRs) that have a crucial role in cardiac physiology in health and disease. Alpha1-ARs signal through Gαq, and signaling through Gq, for example, by endothelin and angiotensin receptors, is thought to be detrimental to the heart. In contrast, cardiac alpha1-ARs mediate important protective and adaptive functions in the heart, although alpha1-ARs are only a minor fraction of total cardiac ARs. Cardiac alpha1-ARs activate pleiotropic downstream signaling to prevent pathologic remodeling in heart failure. Mechanisms defined in animal and cell models include activation of adaptive hypertrophy, prevention of cardiac myocyte death, augmentation of contractility, and induction of ischemic preconditioning. Surprisingly, at the molecular level, alpha1-ARs localize to and signal at the nucleus in cardiac myocytes, and, unlike most GPCRs, activate "inside-out" signaling to cause cardioprotection. Contrary to past opinion, human cardiac alpha1-AR expression is similar to that in the mouse, where alpha1-AR effects are seen most convincingly in knockout models. Human clinical studies show that alpha1-blockade worsens heart failure in hypertension and does not improve outcomes in heart failure, implying a cardioprotective role for human alpha1-ARs. In summary, these findings identify novel functional and mechanistic aspects of cardiac alpha1-AR function and suggest that activation of cardiac alpha1-AR might be a viable therapeutic strategy in heart failure.

The activation of RhoC in vascular endothelial cells is required for the S1P receptor type 2-induced inhibition of angiogenesis

Sphingosine-1-phosphate (S1P) is a multifunctional phospholipid inducing a variety of cellular responses in endothelial cells (EC). S1P responses are mediated by five G protein coupled receptors of which three types (S1P1R-S1P3R) have been described to be of importance in vascular endothelial cells (EC). Whereas the S1P1R regulates endothelial barrier function by coupling to Gαi and the monomeric GTPase Rac1, the signaling pathways involved in the S1P-induced regulation of angiogenesis are ill defined. We therefore studied the sprouting of human umbilical vein EC (HUVEC) in vitro and analyzed the activation of the RhoGTPases RhoA and RhoC. Physiological relevant concentrations of S1P (100-300nM) induce a moderate activation of RhoA and RhoC. Inhibition or siRNA-mediated depletion of the S1P2R preferentially decreased the activation of RhoC. Both manipulations caused an increase of sprouting in a spheroid based in vitro sprouting assay. Interestingly, a similar increase in sprouting was detected after effective siRNA-mediated knockdown of RhoC. In contrast, the depletion of RhoA had no influence on sprouting. Furthermore, suppression of the activity of G proteins of the Gα12/13 subfamily by adenoviral overexpression of the regulator of G protein signaling domain of LSC as well as siRNA-mediated knockdown of the Rho specific guanine nucleotide exchange factor leukemia associated RhoGEF (LARG) inhibited the S1P-induced activation of RhoC and concomitantly increased sprouting of HUVEC with similar efficacy. We conclude that the angiogenic sprouting of EC is suppressed via the S1P2R subtype. Thus, the increase in basal sprouting can be attributed to blocking of the inhibitory action of autocrine S1P stimulating the S1P2R. This inhibitory pathway involves the activation of RhoC via Gα12/13 and LARG, while the simultaneously occurring activation of RhoA is apparently dispensable here.

Rho guanine exchange factors in blood vessels: fine-tuners of angiogenesis and vascular function

The angiogenic cascade is a multi-step process essential for embryogenesis and other physiological and pathological processes. Rho family GTPases are binary molecular switches and serve as master regulators of various basic cellular processes. Rho GTPases are known to exert important functions in angiogenesis and vascular physiology. These functions demand a tight and context-specific control of cellular processes requiring superordinate control by a multitude of guanine nucleotide exchange factors (GEFs). GEFs display various features enabling them to fine-tune the actions of Rho GTPases in the vasculature: (1) GEFs regulate specific steps of the angiogenic cascade; (2) GEFs show a spatio-temporally specific expression pattern; (3) GEFs differentially regulate endothelial function depending on their subcellular location; (4) GEFs mediate crosstalk between complex signaling cascades and (5) GEFs themselves are regulated by another layer of interacting proteins. The aim of this review is to provide an overview about the role of GEFs in regulating angiogenesis and vascular function and to point out current limitations as well as clinical perspectives.

Dysregulation of RNA polymerase I transcription during disease

Transcription of the ribosomal RNA genes by the dedicated RNA polymerase I enzyme and subsequent processing of the ribosomal RNA are fundamental control steps in the synthesis of functional ribosomes. Dysregulation of Pol I transcription and ribosome biogenesis is linked to the etiology of a broad range of human diseases. Diseases caused by loss of function mutations in the molecular constituents of the ribosome, or factors intimately associated with RNA polymerase I transcription and processing are collectively termed ribosomopathies. Ribosomopathies are generally rare and treatment options are extremely limited tending to be more palliative than curative. Other more common diseases are associated with profound changes in cellular growth such as cardiac hypertrophy, atrophy or cancer. In contrast to ribosomopathies, altered RNA polymerase I transcriptional activity in these diseases largely results from dysregulated upstream oncogenic pathways or by direct modulation by oncogenes or tumor suppressors at the level of the RNA polymerase I transcription apparatus itself. Ribosomopathies associated with mutations in ribosomal proteins and ribosomal RNA processing or assembly factors have been covered by recent excellent reviews. In contrast, here we review our current knowledge of human diseases specifically associated with dysregulation of RNA polymerase I transcription and its associated regulatory apparatus, including some cases where this dysregulation is directly causative in disease. We will also provide insight into and discussion of possible therapeutic approaches to treat patients with dysregulated RNA polymerase I transcription. This article is part of a Special Issue entitled: Transcription by Odd Pols.