Inhibition of protein phosphatase 2A activity by PI3Kγ regulates β-adrenergic receptor function

Gut microbe-generated phenylacetylglutamine is an endogenous allosteric modulator of β2-adrenergic receptors

Allosteric modulation is a central mechanism for metabolic regulation but has yet to be described for a gut microbiota-host interaction. Phenylacetylglutamine (PAGln), a gut microbiota-derived metabolite, has previously been clinically associated with and mechanistically linked to cardiovascular disease (CVD) and heart failure (HF). Here, using cells expressing β1- versus β2-adrenergic receptors (β1AR and β2AR), PAGln is shown to act as a negative allosteric modulator (NAM) of β2AR, but not β1AR. In functional studies, PAGln is further shown to promote NAM effects in both isolated male mouse cardiomyocytes and failing human heart left ventricle muscle (contracting trabeculae). Finally, using in silico docking studies coupled with site-directed mutagenesis and functional analyses, we identified sites on β2AR (residues E122 and V206) that when mutated still confer responsiveness to canonical β2AR agonists but no longer show PAGln-elicited NAM activity. The present studies reveal the gut microbiota-obligate metabolite PAGln as an endogenous NAM of a host GPCR.

Regulation and role of the PP2A-B56 holoenzyme family in cancer

Protein phosphatase 2A (PP2A) inactivation is common in cancer, leading to sustained activation of pro-survival and growth-promoting pathways. PP2A consists of a scaffolding A-subunit, a catalytic C-subunit, and a regulatory B-subunit. The functional complexity of PP2A holoenzymes arises mainly through the vast repertoire of regulatory B-subunits, which determine both their substrate specificity and their subcellular localization. Therefore, a major challenge for developing more effective therapeutic strategies for cancer is to identify the specific PP2A complexes to be targeted. Of note, the development of small molecules specifically directed at PP2A-B56α has opened new therapeutic avenues in both solid and hematological tumors. Here, we focus on the B56/PR61 family of PP2A regulatory subunits, which have a central role in directing PP2A tumor suppressor activity. We provide an overview of the mechanisms controlling the formation and regulation of these complexes, the pathways they control, and the mechanisms underlying their deregulation in cancer.

Numerous Trigger-like Interactions of Kinases/Protein Phosphatases in Human Skeletal Muscles Can Underlie Transient Processes in Activation of Signaling Pathways during Exercise

Optimizing physical training regimens to increase muscle aerobic capacity requires an understanding of the internal processes that occur during exercise that initiate subsequent adaptation. During exercise, muscle cells undergo a series of metabolic events that trigger downstream signaling pathways and induce the expression of many genes in working muscle fibers. There are a number of studies that show the dependence of changes in the activity of AMP-activated protein kinase (AMPK), one of the mediators of cellular signaling pathways, on the duration and intensity of single exercises. The activity of various AMPK isoforms can change in different directions, increasing for some isoforms and decreasing for others, depending on the intensity and duration of the load. This review summarizes research data on changes in the activity of AMPK, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), and other components of the signaling pathways in skeletal muscles during exercise. Based on these data, we hypothesize that the observed changes in AMPK activity may be largely related to metabolic and signaling transients rather than exercise intensity per se. Probably, the main events associated with these transients occur at the beginning of the exercise in a time window of about 1-10 min. We hypothesize that these transients may be partly due to putative trigger-like kinase/protein phosphatase interactions regulated by feedback loops. In addition, numerous dynamically changing factors, such as [Ca²⁺], metabolite concentration, and reactive oxygen and nitrogen species (RONS), can shift the switching thresholds and change the states of these triggers, thereby affecting the activity of kinases (in particular, AMPK and CaMKII) and phosphatases. The review considers the putative molecular mechanisms underlying trigger-like interactions. The proposed hypothesis allows for a reinterpretation of the experimental data available in the literature as well as the generation of ideas to optimize future training regimens.

Hypoxia Sensing of β-Adrenergic Receptor Is Regulated by Endosomal PI3Kγ

BACKGROUND

Impaired beta-adrenergic receptor (β1 and β2AR) function following hypoxia underlies ischemic heart failure/stroke. Activation of PI3Kγ (phosphoinositide 3-kinase γ) by beta-adrenergic receptor leads to feedback regulation of the receptor by hindering beta-adrenergic receptor dephosphorylation through inhibition of PP2A (protein phosphatase 2A). However, little is known about PI3Kγ feedback mechanism in regulating hypoxia-mediated β1 and β2AR dysfunction and cardiac remodeling.

METHODS

Human embryonic kidney 293 cells or mouse adult cardiomyocytes and C57BL/6 (WT) or PI3Kγ knockout (KO) mice were subjected to hypoxia. Cardiac plasma membranes and endosomes were isolated and evaluated for β1 and β2AR density and function, PI3Kγ activity and β1 and β2AR-associated PP2A activity. Metabolic labeling was performed to assess β1 and β2AR phosphorylation and epinephrine/norepinephrine levels measured post-hypoxia.

RESULTS

Hypoxia increased β1 and β2AR phosphorylation, reduced cAMP, and led to endosomal accumulation of phosphorylated β2ARs in human embryonic kidney 293 cells and WT cardiomyocytes. Acute hypoxia in WT mice resulted in cardiac remodeling and loss of adenylyl cyclase activity associated with increased β1 and β2AR phosphorylation. This was agonist-independent as plasma and cardiac epinephrine and norepinephrine levels were unaltered. Unexpectedly, PI3Kγ activity was selectively increased in the endosomes of human embryonic kidney 293 cells and WT hearts post-hypoxia. Endosomal β1- and β2AR-associated PP2A activity was inhibited upon hypoxia in human embryonic kidney 293 cells and WT hearts showing regulation of beta-adrenergic receptors by PI3Kγ. This was accompanied with phosphorylation of endogenous inhibitor of protein phosphatase 2A whose phosphorylation by PI3Kγ inhibits PP2A. Increased β1 and β2AR-associated PP2A activity, decreased beta-adrenergic receptor phosphorylation, and normalized cardiac function was observed in PI3Kγ KO mice despite hypoxia. Compared to WT, PI3Kγ KO mice had preserved cardiac response to challenge with β1AR-selective agonist dobutamine post-hypoxia.

CONCLUSIONS

Agonist-independent activation of PI3Kγ underlies hypoxia sensing as its ablation leads to reduction in β1- and β2AR phosphorylation and amelioration of cardiac dysfunction.

Stromal Vascular Fraction Restores Vasodilatory Function by Reducing Oxidative Stress in Aging-Induced Coronary Microvascular Disease

Aims: The objective of this study is to identify mechanisms for adipose stromal vascular fraction's (SVF) restorative effects on vasodilation in aging-induced coronary microvascular disease (CMD). We hypothesize that reactive oxygen species (ROS) diminish β1-adrenergic receptor (β1ADR)- and flow-mediated dilation (FMD) in coronary arterioles, reversible by SVF and adipose-derived stem cells (ADSCs). Results: SVF attenuates aging-induced chronic accumulation of ROS and pro-oxidant gene and protein expression with enhancement of antioxidant gene and protein expression and glutathione, but not nitric oxide. ADSCs attenuate hydrogen peroxide while restoring nitric oxide and glutathione. Mass spectrometry of SVF- and ADSC-conditioned media reveals abundant antioxidant proteins suggesting a paracrine mechanism. FMD and β1ADR-mediated dilation diminished with aging, restored with SVF and ADSCs. FMD was restored by a switch in the acute signaling mediator from hydrogen peroxide in aging to peroxynitrite with SVF and ADSCs. Vasorelaxation to β1ADR-agonism was mechanistically linked with hydrogen peroxide, nitric oxide, and glutathione. Exogenous ROS eliminates isoproterenol-mediated dilation in youth that is blocked by inhibition of pro-desensitization and internalization proteins while nitric oxide enhances isoproterenol-mediated dilation in aging. Innovation: We introduce a novel mechanism by which ROS impacts β1ADR trafficking: the ROS/RNS-β1ADR desensitization and internalization axis. Aging-induced ROS shunts β1ADR from the plasma membrane into endosomes. SVF reduces oxidative burden, restoring functional β1ADR. Conclusions: SVF (and ADSCs to a lesser extent) reduce oxidative stress, and restore flow- and β1ADR-mediated vasodilation in aging. SVF represents a promising therapeutic strategy for CMD by addressing root cause of pathology; that is, oxidative stress-mediated hyperconstriction. Antioxid. Redox Signal. 38, 261-281.

Gut Microbiota-Generated Phenylacetylglutamine and Heart Failure

BACKGROUND

The gut microbiota-dependent metabolite phenylacetylgutamine (PAGln) is both associated with atherothrombotic heart disease in humans, and mechanistically linked to cardiovascular disease pathogenesis in animal models via modulation of adrenergic receptor signaling.

METHODS

Here we examined both clinical and mechanistic relationships between PAGln and heart failure (HF). First, we examined associations among plasma levels of PAGln and HF, left ventricular ejection fraction, and N-terminal pro-B-type natriuretic peptide in 2 independent clinical cohorts of subjects undergoing coronary angiography in tertiary referral centers (an initial discovery US Cohort, n=3256; and a validation European Cohort, n=829). Then, the impact of PAGln on cardiovascular phenotypes relevant to HF in cultured cardiomyoblasts, and in vivo were also examined.

RESULTS

Circulating PAGln levels were dose-dependently associated with HF presence and indices of severity (reduced ventricular ejection fraction, elevated N-terminal pro-B-type natriuretic peptide) independent of traditional risk factors and renal function in both cohorts. Beyond these clinical associations, mechanistic studies showed both PAGln and its murine counterpart, phenylacetylglycine, directly fostered HF-relevant phenotypes, including decreased cardiomyocyte sarcomere contraction, and B-type natriuretic peptide gene expression in both cultured cardiomyoblasts and murine atrial tissue.

CONCLUSIONS

The present study reveals the gut microbial metabolite PAGln is clinically and mechanistically linked to HF presence and severity. Modulating the gut microbiome, in general, and PAGln production, in particular, may represent a potential therapeutic target for modulating HF.

REGISTRATION

URL: https://clinicaltrials.gov/; Unique identifier: NCT00590200 and URL: https://drks.de/drks_web/; Unique identifier: DRKS00020915.

Thirty years of SET/TAF1β/I2PP2A: from the identification of the biological functions to its implications in cancer and Alzheimer's disease

The gene encoding for the protein SE translocation (SET) was identified for the first time 30 years ago as part of a chromosomal translocation in a patient affected by leukemia. Since then, accumulating evidence have linked overexpression of SET, aberrant SET splicing, and cellular localization to cancer progression and development of neurodegenerative tauopathies such as Alzheimer's disease. Molecular biology tools, such as targeted genetic deletion, and pharmacological approaches based on SET antagonist peptides, have contributed to unveil the molecular functions of SET and its implications in human pathogenesis. In this review, we provide an overview of the functions of SET as inhibitor of histone and non-histone protein acetylation and as a potent endogenous inhibitor of serine-threonine phosphatase PP2A. We discuss the role of SET in multiple cellular processes, including chromatin remodelling and gene transcription, DNA repair, oxidative stress, cell cycle, apoptosis cell migration and differentiation. We review the molecular mechanisms linking SET dysregulation to tumorigenesis and discuss how SET commits neurons to progressive cell death in Alzheimer's disease, highlighting the rationale of exploiting SET as a therapeutic target for cancer and neurodegenerative tauopathies.

Sex differences in α-adrenergic receptor function contribute to impaired hypothalamic metaplasticity following chronic intermittent ethanol exposure

BACKGROUND

Individuals with alcohol use disorder (AUD) exhibit maladaptive responses of the hypothalamic-pituitary-adrenal (HPA) axis to stress, which has been linked to high rates of relapse to drinking among abstinent individuals. Corticotropin-releasing factor (CRF) parvocellular neuroendocrine cells (PNCs) within the paraventricular nucleus of the hypothalamus (PVN) are critical to stress-induced HPA axis activation. Here, we investigate sex differences in synaptic transmission and plasticity in PNCs following the application of the stress-associated neurotransmitter norepinephrine (NE) in a rat model of AUD.

METHODS

Adult Sprague-Dawley rats were exposed to 40 days of chronic intermittent ethanol (CIE) vapor and 30 to 108 days of protracted withdrawal. We measured changes in holding current, evoked synaptic currents, and short-term glutamatergic plasticity (STP) in putative PNCs following the application of NE (10 μM) with and without the selective α1 adrenergic receptor (AR) antagonist prazosin (10 μM) or the α2AR antagonist atipamezole (10 μM). The experiments were performed using whole-cell patch clamp recordings in slices from CIE rats and air-exposed controls.

RESULTS

NE application caused two distinct effects: a depolarizing, inward, postsynaptic current and a reduction in amplitude of an evoked glutamatergic excitatory postsynaptic current (eEPSC). Both effects were sex- and CIE-specific. Prazosin blocked the postsynaptic inward current, while atipamezole blocked the NE-mediated suppression of eEPSCs. Additionally, STP formation was facilitated following NE application only in stress-naïve males and this response was lost in stressed animals exposed to a 30-min restraint stress following CIE exposure. Furthermore, NE + prazosin restored STP formation in stressed CIE males.

CONCLUSIONS

NE exerts excitatory and inhibitory effects on CRF PVN PNCs, and both effects are influenced by sex and CIE. Behavioral and hormonal responses to stress are influenced by STP formation within the PVN, which is lost following CIE and restored with the preapplication of prazosin. The selective blockade of α1AR may, therefore, ameliorate CIE-induced deficits in HPA responses to stress in a sex-specific manner.

Functional Gly297Ser Variant of the Physiological Dysglycemic Peptide Pancreastatin Is a Novel Risk Factor for Cardiometabolic Disorders

Pancreastatin (PST), a chromogranin A-derived potent physiological dysglycemic peptide, regulates glucose/insulin homeostasis. We have identified a nonsynonymous functional PST variant (p.Gly297Ser; rs9658664) that occurs in a large section of human populations. Association analysis of this single nucleotide polymorphism with cardiovascular/metabolic disease states in Indian populations (n = 4,300 subjects) displays elevated plasma glucose, glycosylated hemoglobin, diastolic blood pressure, and catecholamines in Gly/Ser subjects as compared with wild-type individuals (Gly/Gly). Consistently, the 297Ser allele confers an increased risk (∼1.3-1.6-fold) for type 2 diabetes/hypertension/coronary artery disease/metabolic syndrome. In corroboration, the variant peptide (PST-297S) displays gain-of-potency in several cellular events relevant for cardiometabolic disorders (e.g., increased expression of gluconeogenic genes, increased catecholamine secretion, and greater inhibition of insulin-stimulated glucose uptake) than the wild-type peptide. Computational docking analysis and molecular dynamics simulations show higher affinity binding of PST-297S peptide with glucose-regulated protein 78 (GRP78) and insulin receptor than the wild-type peptide, providing a mechanistic basis for the enhanced activity of the variant peptide. In vitro binding assays validate these in silico predictions of PST peptides binding to GRP78 and insulin receptor. In conclusion, the PST 297Ser allele influences cardiovascular/metabolic phenotypes and emerges as a novel risk factor for type 2 diabetes/hypertension/coronary artery disease in human populations.

Activation of PKC results in improved contractile effects and Ca cycling by inhibition of PP2A-B56α

Protein phosphatase 2A (PP2A) represents a heterotrimer that is responsible for the dephosphorylation of important regulatory myocardial proteins. The present study was aimed to test whether the phosphorylation of PP2A-B56α at Ser⁴¹ by PKC is involved in the regulation of myocyte Ca²⁺ cycling and contraction. For this purpose, heart preparations of wild-type (WT) and transgenic mice overexpressing the non-phosphorylatable S41A mutant form (TG) were stimulated by administration of the direct PKC activator phorbol 12-myristate 13-acetate (PMA), and functional effects were studied. PKC activation was accompanied by the inhibition of PP2A activity in WT cardiomyocytes, whereas this effect was absent in TG. Consistently, the increase in the sarcomere length shortening and the peak amplitude of Ca²⁺ transients after PMA administration in WT cardiomyocytes was attenuated in TG. However, the co-stimulation with 1 µM isoprenaline was able to offset these functional deficits. Moreover, TG hearts did not show an increase in the phosphorylation of the myosin-binding protein C after administration of PMA but was detected in corresponding WT. PMA modulated voltage-dependent activation of the L-type Ca²⁺ channel (LTCC) differently in the two genotypes, shifting V_1/2a by +1.5 mV in TG and by 2.4 mV in WT. In the presence of PMA, I_CaL inactivation remained unchanged in TG, whereas it was slower in corresponding WT. Our data suggest that PKC-activated enhancement of myocyte contraction and intracellular Ca²⁺ signaling is mediated by phosphorylation of B56α at Ser⁴¹, leading to a decrease in PP2A activity.

Cardiac expression of microRNA-7 is associated with adverse cardiac remodeling

Although microRNA-7 (miRNA-7) is known to regulate proliferation of cancer cells by targeting Epidermal growth factor receptor (EGFR/ERBB) family, less is known about its role in cardiac physiology. Transgenic (Tg) mouse with cardiomyocyte-specific overexpression of miRNA-7 was generated to determine its role in cardiac physiology and pathology. Echocardiography on the miRNA-7 Tg mice showed cardiac dilation instead of age-associated physiological cardiac hypertrophy observed in non-Tg control mice. Subjecting miRNA-7 Tg mice to transverse aortic constriction (TAC) resulted in cardiac dilation associated with increased fibrosis bypassing the adaptive cardiac hypertrophic response to TAC. miRNA-7 expression in cardiomyocytes resulted in significant loss of ERBB2 expression with no changes in ERBB1 (EGFR). Cardiac proteomics in the miRNA-7 Tg mice showed significant reduction in mitochondrial membrane structural proteins compared to NTg reflecting role of miRNA-7 beyond the regulation of EGFR/ERRB in mediating cardiac dilation. Consistently, electron microscopy showed that miRNA-7 Tg hearts had disorganized rounded mitochondria that was associated with mitochondrial dysfunction. These findings show that expression of miRNA-7 in the cardiomyocytes results in cardiac dilation instead of adaptive hypertrophic response during aging or to TAC providing insights on yet to be understood role of miRNA-7 in cardiac function.

Aging-Induced Impairment of Vascular Function: Mitochondrial Redox Contributions and Physiological/Clinical Implications

Significance: The vasculature responds to the respiratory needs of tissue by modulating luminal diameter through smooth muscle constriction or relaxation. Coronary perfusion, diastolic function, and coronary flow reserve are drastically reduced with aging. This loss of blood flow contributes to and exacerbates pathological processes such as angina pectoris, atherosclerosis, and coronary artery and microvascular disease. Recent Advances: Increased attention has recently been given to defining mechanisms behind aging-mediated loss of vascular function and development of therapeutic strategies to restore youthful vascular responsiveness. The ultimate goal aims at providing new avenues for symptom management, reversal of tissue damage, and preventing or delaying of aging-induced vascular damage and dysfunction in the first place. Critical Issues: Our major objective is to describe how aging-associated mitochondrial dysfunction contributes to endothelial and smooth muscle dysfunction via dysregulated reactive oxygen species production, the clinical impact of this phenomenon, and to discuss emerging therapeutic strategies. Pathological changes in regulation of mitochondrial oxidative and nitrosative balance (Section 1) and mitochondrial dynamics of fission/fusion (Section 2) have widespread effects on the mechanisms underlying the ability of the vasculature to relax, leading to hyperconstriction with aging. We will focus on flow-mediated dilation, endothelial hyperpolarizing factors (Sections 3 and 4), and adrenergic receptors (Section 5), as outlined in Figure 1. The clinical implications of these changes on major adverse cardiac events and mortality are described (Section 6). Future Directions: We discuss antioxidative therapeutic strategies currently in development to restore mitochondrial redox homeostasis and subsequently vascular function and evaluate their potential clinical impact (Section 7). Antioxid. Redox Signal. 35, 974-1015.

Respiratory syncytial virus induces β2-adrenergic receptor dysfunction in human airway smooth muscle cells

Pharmacologic agonism of the β₂-adrenergic receptor (β₂AR) induces bronchodilation by activating the enzyme adenylyl cyclase to generate cyclic adenosine monophosphate (cAMP). β₂AR agonists are generally the most effective strategy to relieve acute airway obstruction in asthmatic patients, but they are much less effective when airway obstruction in young patients is triggered by infection with respiratory syncytial virus (RSV). Here, we investigated the effects of RSV infection on the abundance and function of β₂AR in primary human airway smooth muscle cells (HASMCs) derived from pediatric lung tissue. We showed that RSV infection of HASMCs resulted in proteolytic cleavage of β₂AR mediated by the proteasome. RSV infection also resulted in β₂AR ligand-independent activation of adenylyl cyclase, leading to reduced cAMP synthesis compared to that in uninfected control cells. Last, RSV infection caused stronger airway smooth muscle cell contraction in vitro due to increased cytosolic Ca²⁺ concentrations. Thus, our results suggest that RSV infection simultaneously induces loss of functional β₂ARs and activation of multiple pathways favoring airway obstruction in young patients, with the net effect of counteracting β₂AR agonist-induced bronchodilation. These findings not only provide a potential mechanism for the reported lack of clinical efficacy of β₂AR agonists for treating virus-induced wheezing but also open the path to developing more precise therapeutic strategies.

Activated Protein Phosphatase 2A Disrupts Nutrient Sensing Balance Between Mechanistic Target of Rapamycin Complex 1 and Adenosine Monophosphate-Activated Protein Kinase, Causing Sarcopenia in Alcohol-Associated Liver Disease

BACKGROUND AND AIMS

Despite the high clinical significance of sarcopenia in alcohol-associated cirrhosis, there are currently no effective therapies because the underlying mechanisms are poorly understood. We determined the mechanisms of ethanol-induced impaired phosphorylation of mechanistic target of rapamycin complex 1 (mTORC1) and adenosine monophosphate-activated protein kinase (AMPK) with consequent dysregulated skeletal muscle protein homeostasis (balance between protein synthesis and breakdown).

APPROACH AND RESULTS

Differentiated murine myotubes, gastrocnemius muscle from mice with loss and gain of function of regulatory genes following ethanol treatment, and skeletal muscle from patients with alcohol-associated cirrhosis were used. Ethanol increases skeletal muscle autophagy by dephosphorylating mTORC1, circumventing the classical kinase regulation by protein kinase B (Akt). Concurrently and paradoxically, ethanol exposure results in dephosphorylation and inhibition of AMPK, an activator of autophagy and inhibitor of mTORC1 signaling. However, AMPK remains inactive with ethanol exposure despite lower cellular and tissue adenosine triphosphate, indicating a "pseudofed" state. We identified protein phosphatase (PP) 2A as a key mediator of ethanol-induced signaling and functional perturbations using loss and gain of function studies. Ethanol impairs binding of endogenous inhibitor of PP2A to PP2A, resulting in methylation and targeting of PP2A to cause dephosphorylation of mTORC1 and AMPK. Activity of phosphoinositide 3-kinase-γ (PI3Kγ), a negative regulator of PP2A, was decreased in response to ethanol. Ethanol-induced molecular and phenotypic perturbations in wild-type mice were observed in PI3Kγ^-/- mice even at baseline. Importantly, overexpressing kinase-active PI3Kγ but not the kinase-dead mutant reversed ethanol-induced molecular perturbations.

CONCLUSIONS

Our study describes the mechanistic underpinnings for ethanol-mediated dysregulation of protein homeostasis by PP2A that leads to sarcopenia with a potential for therapeutic approaches by targeting the PI3Kγ-PP2A axis.

Cofilin: A Promising Protein Implicated in Cancer Metastasis and Apoptosis

Cofilin is an actin-binding protein that regulates filament dynamics and depolymerization. The over-expression of cofilin is observed in various cancers, cofilin promotes cancer metastasis by regulating cytoskeletal reorganization, lamellipodium formation and epithelial-to-mesenchymal transition. Clinical treatment of cancer regarding cofilin has been explored in aspects of tumor cells apoptosis and cofilin related miRNAs. This review addresses the structure and phosphorylation of cofilin and describes recent findings regarding the function of cofilin in regulating cancer metastasis and apoptosis in tumor cells.

The IgG3 subclass of β1-adrenergic receptor autoantibodies is an endogenous biaser of β1AR signaling

Dysregulation of immune responses has been linked to the generation of immunoglobulin G (IgG) autoantibodies that target human β1ARs and contribute to deleterious cardiac outcomes. Given the benefits of β-blockers observed in patients harboring the IgG3 subclass of autoantibodies, we investigated the role of these autoantibodies in human β1AR function. Serum and purified IgG3(+) autoantibodies from patients with onset of cardiomyopathy were tested using human embryonic kidney (HEK) 293 cells expressing human β1ARs. Unexpectedly, pretreatment of cells with IgG3(+) serum or purified IgG3(+) autoantibodies impaired dobutamine-mediated adenylate cyclase (AC) activity and cyclic adenosine monophosphate (cAMP) generation while enhancing biased β-arrestin recruitment and Extracellular Regulated Kinase (ERK) activation. In contrast, the β-blocker metoprolol increased AC activity and cAMP in the presence of IgG3(+) serum or IgG3(+) autoantibodies. Because IgG3(+) autoantibodies are specific to human β1ARs, non-failing human hearts were used as an endogenous system to determine their ability to bias β1AR signaling. Consistently, metoprolol increased AC activity, reflecting the ability of the IgG3(+) autoantibodies to bias β-blocker toward G-protein coupling. Importantly, IgG3(+) autoantibodies are specific toward β1AR as they did not alter β2AR signaling. Thus, IgG3(+) autoantibody biases β-blocker toward G-protein coupling while impairing agonist-mediated G-protein activation but promoting G-protein-independent ERK activation. This phenomenon may underlie the beneficial outcomes observed in patients harboring IgG3(+) β1AR autoantibodies.

PP2A-B55 Holoenzyme Regulation and Cancer

Protein phosphorylation is a post-translational modification essential for the control of the activity of most enzymes in the cell. This protein modification results from a fine-tuned balance between kinases and phosphatases. PP2A is one of the major serine/threonine phosphatases that is involved in the control of a myriad of different signaling cascades. This enzyme, often misregulated in cancer, is considered a tumor suppressor. In this review, we will focus on PP2A-B55, a particular holoenzyme of the family of the PP2A phosphatases whose specific role in cancer development and progression has only recently been highlighted. The discovery of the Greatwall (Gwl)/Arpp19-ENSA cascade, a new pathway specifically controlling PP2A-B55 activity, has been shown to be frequently altered in cancer. Herein, we will review the current knowledge about the mechanisms controlling the formation and the regulation of the activity of this phosphatase and its misregulation in cancer.

Potential roles of PP2A-Rac1 signaling axis in pancreatic β-cell dysfunction under metabolic stress: Progress and promise

Recent estimates by the International Diabetes Federation suggest that the incidence of diabetes soared to an all-time high of 463 million in 2019, and the federation predicts that by 2045 the number of individuals afflicted with this disease will increase to 700 million. Therefore, efforts to understand the pathophysiology of diabetes are critical for moving toward the development of novel therapeutic strategies for this disease. Several contributors (oxidative stress, endoplasmic reticulum stress and others) have been proposed for the onset of metabolic dysfunction and demise of the islet β-cell leading to the pathogenesis of diabetes. Existing experimental evidence revealed sustained activation of PP2A and Rac1 in pancreatic β-cells exposed to metabolic stress (diabetogenic) conditions. Evidence in a variety of cell types implicates modulatory roles for specific signaling proteins (α4, SET, nm23-H1, Pak1) in the functional regulation of PP2A and Rac1. In this Commentary, I overviewed potential cross-talk between PP2A and Rac1 signaling modules in the onset of metabolic dysregulation of the islet β-cell leading to impaired glucose-stimulated insulin secretion (GSIS), loss of β-cell mass and the onset of diabetes. Potential knowledge gaps and future directions in this fertile area of islet biology are also highlighted. It is hoped that this Commentary will provide a basis for future studies toward a better understanding of roles of PP2A-Rac1 signaling module in pancreatic β-cell dysfunction, and identification of therapeutic targets for the treatment of islet β-cell dysfunction in diabetes.

RTK signaling requires C3ar1/C5ar1 and IL-6R joint signaling to repress dominant PTEN, SOCS1/3 and PHLPP restraint

How receptor tyrosine kinase (RTK) growth signaling is controlled physiologically is incompletely understood. We have previously provided evidence that the survival and mitotic activities of vascular endothelial cell growth factor receptor-2 (VEGFR2) signaling are dependent on C3a/C5a receptor (C3ar1/C5ar1) and IL-6 receptor (IL-6R)-gp130 joint signaling in a physically interactive platform. Herein, we document that the platelet derived and epidermal growth factor receptors (PDGFR and EGFR) are regulated by the same interconnection and clarify the mechanism underlying the dependence. We show that the joint signaling is required to overcome dominant restraint on RTK function by the combined repression of tonically activated PHLPP, SOCS1/SOCS3, and CK2/Fyn dependent PTEN. Signaling studies showed that augmented PI-3Kɣ activation is the process that overcomes the multilevel growth restraint. Live-cell flow cytometry and single-particle tracking indicated that blockade of C3ar1/C5ar1 or IL-6R signaling suppresses RTK growth factor binding and RTK complex formation. C3ar1/C5ar1 blockade abrogated growth signaling of four additional RTKs. Active relief of dominant growth repression via joint C3ar1/C5ar1 and IL-6R joint signaling thus enables RTK mitotic/survival signaling.

GPCRs and Insulin Receptor Signaling in Conversation: Novel Avenues for Drug Discovery

Type 2 diabetes is a major health issue worldwide with complex metabolic and endocrine abnormalities. Hyperglycemia, defects in insulin secretion and insulin resistance are classic features of type 2 diabetes. Insulin signaling regulates metabolic homeostasis by regulating glucose and lipid turnover in the liver, skeletal muscle and adipose tissue. Major treatment modalities for diabetes include the drugs from the class of sulfonyl urea, Insulin, GLP-1 agonists, SGLT2 inhibitors, DPP-IV inhibitors and Thiazolidinediones. Emerging antidiabetic therapeutics also include classes of drugs targeting GPCRs in the liver, adipose tissue and skeletal muscle. Interestingly, recent research highlights several shared intermediates between insulin and GPCR signaling cascades opening potential novel avenues for diabetic drug discovery.

Function, Regulation and Biological Roles of PI3Kγ Variants

Phosphatidylinositide 3-kinase (PI3K) γ is the only class IB PI3K member playing significant roles in the G-protein-dependent regulation of cell signaling in health and disease. Originally found in the immune system, increasing evidence suggest a wide array of functions in the whole organism. PI3Kγ occur as two different heterodimeric variants: PI3Kγ (p87) and PI3Kγ (p101), which share the same p110γ catalytic subunit but differ in their associated non-catalytic subunit. Here we concentrate on specific PI3Kγ features including its regulation and biological functions. In particular, the roles of its non-catalytic subunits serving as the main regulators determining specificity of class IB PI3Kγ enzymes are highlighted.

The NMR-based characterization of the FTY720-SET complex reveals an alternative mechanism for the attenuation of the inhibitory SET-PP2A interaction

The su(var)3-9, enhancer of zeste, trithorax (SET)/inhibitor 2 of protein phosphatase 2A (PP2A) oncoprotein binds and inhibits PP2A, composed of various isoforms of scaffolding, regulatory, and catalytic subunits. Targeting SET with a sphingolipid analog drug fingolimod (FTY720) or ceramide leads to the reactivation of tumor suppressor PP2A. However, molecular details of the SET-FTY720 or SET-ceramide, and mechanism of FTY720-dependent PP2A activation, remain unknown. Here, we report the first in solution examination of the SET-FTY720 or SET-ceramide complexes by NMR spectroscopy. FTY720-ceramide binding resulted in chemical shifts of residues residing at the N terminus of SET, preventing its dimerization or oligomerization. This then released SET from PP2ACα, resulting in PP2A activation, while monomeric SET remained associated with the B56γ. Our data also suggest that the PP2A holoenzyme, composed of PP2A-Aβ, PP2A-B56γ, and PP2ACα subunits, is selectively activated in response to the formation of the SET-FTY720 complex in A549 cells. Various PP2A-associated downstream effector proteins in the presence or absence of FTY720 were then identified by stable isotope labeling with amino cells in cell culture, including tumor suppressor nonmuscle myosin IIA. Attenuation of FTY720-SET association by point mutations of residues that are involved in FTY720 binding or dephosphorylation of SET at Serine 171, enhanced SET oligomerization and the formation of the SET-PP2A inhibitory complex, leading to resistance to FTY720-dependent PP2A activation.-De Palma, R. M., Parnham, S. R., Li, Y., Oaks, J. J., Peterson, Y. K., Szulc, Z. M., Roth, B. M., Xing, Y., Ogretmen, B. The NMR-based characterization of the FTY720-SET complex reveals an alternative mechanism for the attenuation of the inhibitory SET-PP2A interaction.

Mechanism of BDNF Modulation in GABAergic Synaptic Transmission in Healthy and Disease Brains

In the mature healthy mammalian neuronal networks, γ-aminobutyric acid (GABA) mediates synaptic inhibition by acting on GABA_A and GABA_B receptors (GABA_AR, GABA_BR). In immature networks and during numerous pathological conditions the strength of GABAergic synaptic inhibition is much less pronounced. In these neurons the activation of GABA_AR produces paradoxical depolarizing action that favors neuronal network excitation. The depolarizing action of GABA_AR is a consequence of deregulated chloride ion homeostasis. In addition to depolarizing action of GABA_AR, the GABA_BR mediated inhibition is also less efficient. One of the key molecules regulating the GABAergic synaptic transmission is the brain derived neurotrophic factor (BDNF). BDNF and its precursor proBDNF, can be released in an activity-dependent manner. Mature BDNF operates via its cognate receptors tropomyosin related kinase B (TrkB) whereas proBDNF binds the p75 neurotrophin receptor (p75^NTR). In this review article, we discuss recent finding illuminating how mBDNF-TrkB and proBDNF-p75^NTR signaling pathways regulate GABA related neurotransmission under physiological conditions and during epilepsy.

GnRH regulates the expression of its receptor accessory protein SET in pituitary gonadotropes

Reproductive function is under the control of the neurohormone GnRH, which activates a G-protein-coupled receptor (GnRHR) expressed in pituitary gonadotrope cells. GnRHR activates a complex signaling network to regulate synthesis and secretion of the two gonadotropin hormones, luteinizing hormone and follicle-stimulating hormone, both regulating gametogenesis and steroidogenesis in gonads. Recently, in an attempt to identify the mechanisms underlying GnRHR signaling plasticity, we identified the first interacting partner of GnRHR, the proto-oncogene SET. We showed that SET binds to intracellular domains of GnRHR to enhance its coupling to cAMP pathway in αT3-1 gonadotrope cells. Here, we demonstrate that SET protein is rapidly regulated by GnRH, which increases SET phosphorylation state and decreases dose-dependently SET protein level. Our results highlight a post-translational regulation of SET protein involving the proteasome pathway. We determined that SET phosphorylation upon GnRH stimulation is mediated by PKC and that PKC mediates GnRH-induced SET down-regulation. Phosphorylation on serine 9 targets SET for degradation into the proteasome. Furthermore, a non-phosphorylatable SET mutant on serine 9 is resistant to GnRH-induced down-regulation. Altogether, these data suggest that GnRH-induced SET phosphorylation on serine 9 mediates SET protein down-regulation through the proteasome pathway. Noteworthy, SET down-regulation was also observed in response to pulsatile GnRH stimulation in LβT2 gonadotrope cells as well as in vivo in prepubertal female mice supporting its physiological relevance. In conclusion, this study highlights a regulation of SET protein by the neurohormone GnRH and identifies some of the mechanisms involved.

G Protein-Coupled Receptor Resensitization Paradigms

Cellular responses to extracellular milieu/environment are driven by cell surface receptors that transmit the signal into the cells resulting in a synchronized and measured response. The ability to provide such exquisite responses to changes in external environment is mediated by the tight and yet, deliberate regulation of cell surface receptor function. In this regard, the seven transmembrane G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors that regulate responses like cardiac contractility, vision, and olfaction including platelet activation. GPCRs regulate these plethora of events through GPCR-activation, -desensitization, and -resensitization. External stimuli (ligands or agonists) activate GPCR initiating downstream signals. The activated GPCR undergoes inactivation or desensitization by phosphorylation and binding of β-arrestin resulting in diminution of downstream signals. The desensitized GPCRs are internalized into endosomes, wherein they undergo dephosphorylation or resensitization by protein phosphatase to be recycled back to the cell membrane as naïve GPCR ready for the next wave of stimuli. Despite the knowledge that activation, desensitization, and resensitization shoulder an equal role in maintaining GPCR function, major advances have been made in understanding activation and desensitization compared to resensitization. However, increasing evidence shows that resensitization is exquisitely regulated process, thereby contributing to the dynamic regulation of GPCR function. In recognition of these observations, in this chapter we discuss the key advances on the mechanistic underpinning that drive and regulate GPCR function with a focus on resensitization.

Proinflammatory Cytokines Mediate GPCR Dysfunction

Proinflammatory reaction by the body occurs acutely in response to injury that is considered primarily beneficial. However, sustained proinflammatory cytokines observed with chronic pathologies such as metabolic syndrome, cancer, and arthritis are detrimental and in many cases is a major cardiovascular risk factor. Proinflammatory cytokines such as interleukin-1, interleukin-6, and tumor necrosis factor α (TNFα) have long been implicated in cardiovascular risk and considered to be a major underlying cause for heart failure (HF). The failure of the anti-TNFα therapy for HF indicates our elusive understanding on the dichotomous role of proinflammatory cytokines on acutely beneficial effects versus long-term deleterious effects. Despite these well-described observations, less is known about the mechanistic underpinnings of proinflammatory cytokines especially TNFα in pathogenesis of HF. Increasing evidence suggests the existence of an active cross-talk between the TNFα receptor signaling and G-protein-coupled receptors such as β-adrenergic receptor (βAR). Given that βARs are the key regulators of cardiac function, the review will discuss the current state of understanding on the role of proinflammatory cytokine TNFα in regulating βAR function.

Dephosphorylation by protein phosphatase 2A regulates visual pigment regeneration and the dark adaptation of mammalian photoreceptors

Resetting of G-protein-coupled receptors (GPCRs) from their active state back to their biologically inert ground state is an integral part of GPCR signaling. This "on-off" GPCR cycle is regulated by reversible phosphorylation. Retinal rod and cone photoreceptors arguably represent the best-understood example of such GPCR signaling. Their visual pigments (opsins) are activated by light, transduce the signal, and are then inactivated by a GPCR kinase and arrestin. Although pigment inactivation by phosphorylation is well understood, the enzyme(s) responsible for pigment dephosphorylation and the functional significance of this reaction remain unknown. Here, we show that protein phosphatase 2A (PP2A) acts as opsin phosphatase in both rods and cones. Elimination of PP2A substantially slows pigment dephosphorylation, visual chromophore recycling, and ultimately photoreceptor dark adaptation. These findings demonstrate that visual pigment dephosphorylation regulates the dark adaptation of photoreceptors and provide insights into the role of this reaction in GPCR signaling.

cAMP assays in GPCR drug discovery

GPCRs are a major family of cell surface receptors and the most druggable protein targets in modern medicine. Recent elucidation of crystal structures of a vast number of GPCRs eludes to the finer details of biased agonism or functional selectivity of many of these receptors and warrants a better understanding of their biological effects as well as therapeutic potential. Receptor function is measured in terms desensitization/resensitization, which provides insights on receptor activation and differential coupling to various G proteins. This review article presents thoughts on the potential of GPCR desensitization assay/cAMP assay, a highly sensitive and versatile platform for high-throughput assays for elucidating novel functions of many orphan GPCRs. Besides, these assays also are very sensitive to screen for agonists, partial agonists, or antagonists and provide a simplified system for novel drug discovery for Gs-coupled GPCRs and understanding their physiological implications.

Aloperine Protects Mice against DSS-Induced Colitis by PP2A-Mediated PI3K/Akt/mTOR Signaling Suppression

Colitis is a major form of inflammatory bowel disease which involved mucosal immune dysfunction. Aloperine is an alkaloid isolated from the shrub Sophora alopecuroides L. and has been recognized as an effective treatment for inflammatory and allergic diseases. The present study aimed to examine the molecular mechanisms underlying aloperine-mediated colitis protection. We found that aloperine treatment improved colitis induced by dextran sodium sulfate (DSS) based on body weight, disease activity index, colonic length, and spleen index. Aloperine also effectively attenuated DSS-induced intestinal inflammation based on the pathological score and myeloperoxidase expression and activity in colon tissues. In addition, aloperine regulated T-cell proportions and promoted Foxp3 expression in the spleens and mesenteric lymph nodes of DSS-induced colitis mice and in the spleens of the Foxp3^GFP mice. Aloperine inhibited Jurkat and mouse naïve T-cell apoptosis. Furthermore, aloperine inhibited PI3K/Akt/mTOR signaling and upregulated PP2A expression in the DSS-induced colitis mice and in Jurkat cells, but LB-100 (PP2A inhibitor) resulted in an elevated Akt activity in Jurkat cells, activated T-cells, and human splenic mononuclear cells. Aloperine inhibited T-cell and lymphocyte proliferation, but LB-100 reverse these effects. In conclusion, aloperine regulates inflammatory responses in colitis by inhibiting the PI3K/Akt/mTOR signaling in a PP2A-dependent manner.

Noncanonical regulation of insulin-mediated ERK activation by phosphoinositide 3-kinase γ

Classically, Class IB phosphoinositide 3-kinase (PI3Kγ) plays a role in ERK activation following G-protein–coupled receptor (GPCR) activation. Here we show that PI3Kγ noncanonically regulates ERK phosphorylation in a kinase-independent mechanism, irrespective of the upstream signals. PI3Kγ sequesters PP2A, allowing sustained ERK function.

Classically Class IB phosphoinositide 3-kinase (PI3Kγ) plays a role in extracellular signal–regulated kinase (ERK) activation following G-protein coupled receptor (GPCR) activation. Knock-down of PI3Kγ unexpectedly resulted in loss of ERK activation to receptor tyrosine kinase agonists such as epidermal growth factor or insulin. Mouse embryonic fibroblasts (MEFs) or primary adult cardiac fibroblasts isolated from PI3Kγ knock-out mice (PI3KγKO) showed decreased insulin-stimulated ERK activation. However, expression of kinase-dead PI3Kγ resulted in rescue of insulin-stimulated ERK activation. Mechanistically, PI3Kγ sequesters protein phosphatase 2A (PP2A), disrupting ERK–PP2A interaction, as evidenced by increased ERK–PP2A interaction and associated PP2A activity in PI3KγKO MEFs, resulting in decreased ERK activation. Furthermore, β-blocker carvedilol-mediated β-arrestin-dependent ERK activation is significantly reduced in PI3KγKO MEF, suggesting accelerated dephosphorylation. Thus, instead of classically mediating the kinase arm, PI3Kγ inhibits PP2A by scaffolding and sequestering, playing a key parallel synergistic step in sustaining the function of ERK, a nodal enzyme in multiple cellular processes.

Phospho-H1 Decorates the Inter-chromatid Axis and Is Evicted along with Shugoshin by SET during Mitosis

Precise control of sister chromatid separation during mitosis is pivotal to maintaining genomic integrity. Yet, the regulatory mechanisms involved are not well understood. Remarkably, we discovered that linker histone H1 phosphorylated at S/T18 decorated the inter-chromatid axial DNA on mitotic chromosomes. Sister chromatid resolution during mitosis required the eviction of such H1S/T18ph by the chaperone SET, with this process being independent of and most likely downstream of arm-cohesin dissociation. SET also directed the disassembly of Shugoshins in a polo-like kinase 1-augmented manner, aiding centromere resolution. SET ablation compromised mitotic fidelity as evidenced by unresolved sister chromatids with marked accumulation of H1S/T18ph and centromeric Shugoshin. Thus, chaperone-assisted eviction of linker histones and Shugoshins is a fundamental step in mammalian mitotic progression. Our findings also elucidate the functional implications of the decades-old observation of mitotic linker histone phosphorylation, serving as a paradigm to explore the role of linker histones in bio-signaling processes.

The heterotrimeric G protein Gβ1 interacts with the catalytic subunit of protein phosphatase 1 and modulates G protein-coupled receptor signaling in platelets

Thrombosis is caused by the activation of platelets at the site of ruptured atherosclerotic plaques. This activation involves engagement of G protein-coupled receptors (GPCR) on platelets that promote their aggregation. Although it is known that protein kinases and phosphatases modulate GPCR signaling, how serine/threonine phosphatases integrate with G protein signaling pathways is less understood. Because the subcellular localization and substrate specificity of the catalytic subunit of protein phosphatase 1 (PP1c) is dictated by PP1c-interacting proteins, here we sought to identify new PP1c interactors. GPCRs signal via the canonical heterotrimeric Gα and Gβγ subunits. Using a yeast two-hybrid screen, we discovered an interaction between PP1cα and the heterotrimeric G protein Gβ₁ subunit. Co-immunoprecipitation studies with epitope-tagged PP1c and Gβ₁ revealed that Gβ₁ interacts with the PP1c α, β, and γ1 isoforms. Purified PP1c bound to recombinant Gβ₁-GST protein, and PP1c co-immunoprecipitated with Gβ₁ in unstimulated platelets. Thrombin stimulation of platelets induced the dissociation of the PP1c-Gβ₁ complex, which correlated with an association of PP1c with phospholipase C β3 (PLCβ3), along with a concomitant dephosphorylation of the inhibitory Ser¹¹⁰⁵ residue in PLCβ3. siRNA-mediated depletion of GNB1 (encoding Gβ₁) in murine megakaryocytes reduced protease-activated receptor 4, activating peptide-induced soluble fibrinogen binding. Thrombin-induced aggregation was decreased in PP1cα^-/- murine platelets and in human platelets treated with a small-molecule inhibitor of Gβγ. Finally, disruption of PP1c-Gβ₁ complexes with myristoylated Gβ₁ peptides containing the PP1c binding site moderately decreased thrombin-induced human platelet aggregation. These findings suggest that Gβ₁ protein enlists PP1c to modulate GPCR signaling in platelets.

A unique microRNA profile in end-stage heart failure indicates alterations in specific cardiovascular signaling networks

It is well established that the gene expression patterns are substantially altered in cardiac hypertrophy and heart failure, however, less is known about the reasons behind such global differences. MicroRNAs (miRNAs) are short non-coding RNAs that can target multiple molecules to regulate wide array of proteins in diverse pathways. The goal of the study was to profile alterations in miRNA expression using end-stage human heart failure samples with an aim to build signaling network pathways using predicted targets for the altered miRNA and to determine nodal molecules regulating individual networks. Profiling of miRNAs using custom designed microarray and validation with an independent set of samples identified eight miRNAs that are altered in human heart failure including one novel miRNA yet to be implicated in cardiac pathology. To gain an unbiased perspective on global regulation by top eight altered miRNAs, functional relationship of predicted targets for these eight miRNAs were examined by network analysis. Ingenuity Pathways Analysis network algorithm was used to build global signaling networks based on the targets of altered miRNAs which allowed us to identify participating networks and nodal molecules that could contribute to cardiac pathophysiology. Majority of the nodal molecules identified in our analysis are targets of altered miRNAs and known regulators of cardiovascular signaling. Cardio-genomics heart failure gene expression public data base was used to analyze trends in expression pattern for target nodal molecules and indeed changes in expression of nodal molecules inversely correlated to miRNA alterations. We have used NF kappa B network as an example to show that targeting other molecules in the network could alter the nodal NF kappa B despite not being a miRNA target suggesting an integrated network response. Thus, using network analysis we show that altering key functional target proteins may regulate expression of the myriad signaling pathways underlying the cardiac pathology.

Scaffolding Function of PI3Kgamma Emerges from Enzyme's Shadow

Traditionally, an enzyme is a protein that mediates biochemical action by binding to the substrate and by catalyzing the reaction that translates external cues into biological responses. Sequential dissemination of information from one enzyme to another facilitates signal transduction in biological systems providing for feed-forward and feed-back mechanisms. Given this viewpoint, an enzyme without its catalytic activity is generally considered to be an inert organizational protein without catalytic function and has classically been termed as pseudo-enzymes. However, pseudo-enzymes still have biological function albeit non-enzymatic like serving as a chaperone protein or an interactive platform between proteins. In this regard, majority of the studies have focused solely on the catalytic role of enzymes in biological function, overlooking the potentially critical non-enzymatic roles. Increasing evidence from recent studies implicate that the scaffolding function of enzymes could be as important in signal transduction as its catalytic activity, which is an antithesis to the definition of enzymes. Recognition of non-enzymatic functions could be critical, as these unappreciated roles may hold clues to the ineffectiveness of kinase inhibitors in pathology, which is characteristically associated with increased enzyme expression. Using an established enzyme phosphoinositide 3-kinase γ, we discuss the insights obtained from the scaffolding function and how this non-canonical role could contribute to/alter the outcomes in pathology like cancer and heart failure. Also, we hope that with this review, we provide a forum and a starting point to discuss the idea that catalytic function alone may not account for all the actions observed with increased expression of the enzyme.

Catestatin Gly364Ser Variant Alters Systemic Blood Pressure and the Risk for Hypertension in Human Populations via Endothelial Nitric Oxide Pathway

Catestatin (CST), an endogenous antihypertensive/antiadrenergic peptide, is a novel regulator of cardiovascular physiology. Here, we report case-control studies in 2 geographically/ethnically distinct Indian populations (n≈4000) that showed association of the naturally-occurring human CST-Gly364Ser variant with increased risk for hypertension (age-adjusted odds ratios: 1.483; P=0.009 and 2.951; P=0.005). Consistently, 364Ser allele carriers displayed elevated systolic (up to ≈8 mm Hg; P=0.004) and diastolic (up to ≈6 mm Hg; P=0.001) blood pressure. The variant allele was also found to be in linkage disequilibrium with other functional single-nucleotide polymorphisms in the CHGA promoter and nearby coding region. Functional characterization of the Gly364Ser variant was performed using cellular/molecular biological experiments (viz peptide-receptor binding assays, nitric oxide [NO], phosphorylated extracellular regulated kinase, and phosphorylated endothelial NO synthase estimations) and computational approaches (molecular dynamics simulations for structural analysis of wild-type [CST-WT] and variant [CST-364Ser] peptides and docking of peptide/ligand with β-adrenergic receptors [ADRB1/2]). CST-WT and CST-364Ser peptides differed profoundly in their secondary structures and showed differential interactions with ADRB2; although CST-WT displaced the ligand bound to ADRB2, CST-364Ser failed to do the same. Furthermore, CST-WT significantly inhibited ADRB2-stimulated extracellular regulated kinase activation, suggesting an antagonistic role towards ADRB2 unlike CST-364Ser. Consequently, CST-WT was more potent in NO production in human umbilical vein endothelial cells as compared with CST-364Ser. This NO-producing ability of CST-WT was abrogated by ADRB2 antagonist ICI 118551. In conclusion, CST-364Ser allele enhanced the risk for hypertension in human populations, possibly via diminished endothelial NO production because of altered interactions of CST-364Ser peptide with ADRB2 as compared with CST-WT.

MYC-dependent recruitment of RUNX1 and GATA2 on the SET oncogene promoter enhances PP2A inactivation in acute myeloid leukemia

The SET (I2PP2A) oncoprotein is a potent inhibitor of protein phosphatase 2A (PP2A) that regulates many cell processes and important signaling pathways. Despite the importance of SET overexpression and its prognostic impact in both hematologic and solid tumors, little is known about the mechanisms involved in its transcriptional regulation. In this report, we define the minimal promoter region of the SET gene, and identify a novel multi-protein transcription complex, composed of MYC, SP1, RUNX1 and GATA2, which activates SET expression in AML. The role of MYC is crucial, since it increases the expression of the other three transcription factors of the complex, and supports their recruitment to the promoter of SET. These data shed light on a new regulatory mechanism in cancer, in addition to the already known PP2A-MYC and SET-PP2A. Besides, we show that there is a significant positive correlation between the expression of SET and MYC, RUNX1, and GATA2 in AML patients, which further endorses our results. Altogether, this study opens new directions for understanding the mechanisms that lead to SET overexpression, and demonstrates that MYC, SP1, RUNX1 and GATA2 are key transcriptional regulators of SET expression in AML.

Phosphorylation of Src by phosphoinositide 3-kinase regulates beta-adrenergic receptor-mediated EGFR transactivation

β2-Adrenergic receptors (β2AR) transactivate epidermal growth factor receptors (EGFR) through formation of a β2AR-EGFR complex that requires activation of Src to mediate signaling. Here, we show that both lipid and protein kinase activities of the bifunctional phosphoinositide 3-kinase (PI3K) enzyme are required for β2AR-stimulated EGFR transactivation. Mechanistically, the generation of phosphatidylinositol (3,4,5)-tris-phosphate (PIP3) by the lipid kinase function stabilizes β2AR-EGFR complexes while the protein kinase activity of PI3K regulates Src activation by direct phosphorylation. The protein kinase activity of PI3K phosphorylates serine residue 70 on Src to enhance its activity and induce EGFR transactivation following βAR stimulation. This newly identified function for PI3K, whereby Src is a substrate for the protein kinase activity of PI3K, is of importance since Src plays a key role in pathological and physiological signaling.

Altered Phosphatidylinositol 3-Kinase and Calcium Signaling in Cardiac Dysfunction After Brain Death in Rats

BACKGROUND

Phosphatidylinositol 3-kinase is involved in myocardial function, including contractility. To date, myocardial regulation by phosphatidylinositol 3-kinase after brain death has not been investigated. The present study using a brain death model was designed to examine the role of phosphatidylinositol 3-kinase in myocardial function after brain death.

METHODS

After anesthesia with sevoflurane, a Fogarty catheter was placed intracranially for induction of brain death. A conductance catheter was inserted into the left ventricle for measurement of myocardial function. Rats were assigned to the following groups: one group undergoing sham operation (with catheter placement but no brain death introduction); one group receiving saline before brain death; and one group receiving wortmannin, an inhibitor of phosphatidylinositol 3-kinase, before brain death. Various measurements, including mean blood pressure, heart rate, maximal rate of rise of left ventricular pressure, and ejection fraction, were obtained every 30 minutes for 6 hours after brain death. The phosphorylation status of Akt and phospholamban was determined 360 minutes after brain death.

RESULTS

After induction of brain death, rats showed significant decreases in blood pressure, maximal rate of rise of left ventricular pressure, and ejection fraction. Inhibition of phosphatidylinositol 3-kinase using wortmannin significantly improved these measurements, resulting in increased survival. Western blot analysis demonstrated that brain death increased Akt phosphorylation and decreased phospholamban phosphorylation; these effects were abolished by wortmannin.

CONCLUSIONS

Inhibition of phosphatidylinositol 3-kinase prevented myocardial dysfunction after brain death in association with inhibition of the decrease in phosphorylation of myocardial phospholamban, characteristic of brain death.

Protein Phosphatase 2A as a Therapeutic Target in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous malignant disorder of hematopoietic progenitor cells in which several genetic and epigenetic aberrations have been described. Despite progressive advances in our understanding of the molecular biology of this disease, the outcome for most patients is poor. It is, therefore, necessary to develop more effective treatment strategies. Genetic aberrations affecting kinases have been widely studied in AML; however, the role of phosphatases remains underexplored. Inactivation of the tumor-suppressor protein phosphatase 2A (PP2A) is frequent in AML patients, making it a promising target for therapy. There are several PP2A inactivating mechanisms reported in this disease. Deregulation or specific post-translational modifications of PP2A subunits have been identified as a cause of PP2A malfunction, which lead to deregulation of proliferation or apoptosis pathways, depending on the subunit affected. Likewise, overexpression of either SET or cancerous inhibitor of protein phosphatase 2A, endogenous inhibitors of PP2A, is a recurrent event in AML that impairs PP2A activity, contributing to leukemogenesis progression. Interestingly, the anticancer activity of several PP2A-activating drugs (PADs) depends on interaction/sequestration of SET. Preclinical studies show that pharmacological restoration of PP2A activity by PADs effectively antagonizes leukemogenesis, and that these drugs have synergistic cytotoxic effects with conventional chemotherapy and kinase inhibitors, opening new possibilities for personalized treatment in AML patients, especially in cases with SET-dependent inactivation of PP2A. Here, we review the role of PP2A as a druggable tumor suppressor in AML.

Bidirectional cross-regulation between ErbB2 and β-adrenergic signalling pathways

AIMS

Despite the observation that ErbB2 regulates sensitivity of the heart to doxorubicin or ErbB2-targeted cancer therapies, mechanisms that regulate ErbB2 expression and activity have not been studied. Since isoproterenol up-regulates ErbB2 in kidney and salivary glands and β2AR and ErbB2 complex in brain and heart, we hypothesized that β-adrenergic receptors (AR) modulate ErbB2 signalling status.

METHODS AND RESULTS

ErbB2 transfection of HEK293 cells up-regulates β2AR, and β2AR transfection of HEK293 up-regulates ErbB2. Interestingly, cardiomyocytes isolated from myocyte-specific ErbB2-overexpressing (ErbB2(tg)) mice have amplified response to selective β2-agonist zinterol, and right ventricular trabeculae baseline force generation is markedly reduced with β2-antagonist ICI-118 551. Consistently, receptor binding assays and western blotting demonstrate that β2ARs levels are markedly increased in ErbB2(tg) myocardium and reduced by EGFR/ErbB2 inhibitor, lapatinib. Intriguingly, acute treatment of mice with β1- and β2-AR agonist isoproterenol resulted in myocardial ErbB2 increase, while inhibition with either β1- or β2-AR antagonist did not completely prevent isoproterenol-induced ErbB2 expression. Furthermore, inhibition of ErbB2 kinase predisposed mice hearts to injury from chronic isoproterenol treatment while significantly reducing isoproterenol-induced pAKT and pERK levels, suggesting ErbB2's role in transactivation in the heart.

CONCLUSION

Our studies show that myocardial ErbB2 and βAR signalling are linked in a feedback loop with βAR activation leading to increased ErbB2 expression and activity, and increased ErbB2 activity regulating β2AR expression. Most importantly, ErbB2 kinase activity is crucial for cardioprotection in the setting of β-adrenergic stress, suggesting that this mechanism is important in the pathophysiology and treatment of cardiomyopathy induced by ErbB2-targeting antineoplastic drugs.

Phosphoinositide 3-kinase: friend and foe in cardiovascular disease

Class I phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases activated by cell membrane receptors, either receptor tyrosine kinases (RTKs) or G protein–coupled receptors (GPCRs), to catalyze the production of the lipid second messenger phosphatidylinositol (3,4,5)-trisphosphate (PIP3). These enzymes engage multiple downstream intracellular signaling pathways controlling cell proliferation, survival and migration. In the cardiovascular system, the four class I PI3K isoforms, PI3Kα, PI3Kβ, PI3Kδ, and PI3Kγ are differentially expressed in distinct cell subsets which include cardiomyocytes, fibroblasts, endothelial, and vascular smooth muscle cells as well as leukocytes, suggesting specific functions for distinct PI3K isoenzymes. During the last decades, genetic disruption studies targeting different PI3K genes have elucidated the contribution of specific isoenzymes to cardiac and vascular function regulation, highlighting both beneficial and maladaptive roles. New layers of complexity in the function of PI3Ks have recently emerged, indicating that distinct PI3K isoforms are interconnected by various crosstalk events and can function not only as kinases, but also as scaffold proteins coordinating key signalosomes in cardiovascular health and disease. In this review, we will summarize major breakthroughs in the comprehension of detrimental and beneficial actions of PI3K signaling in cardiovascular homeostasis, and we will discuss recently unraveled cross-talk and scaffold mechanisms as well as the role of the less characterized class II and III PI3K isoforms.

Defective Resensitization in Human Airway Smooth Muscle Cells Evokes β-Adrenergic Receptor Dysfunction in Severe Asthma

β₂-adrenergic receptor (β₂AR) agonists (β₂-agonist) are the most commonly used therapy for acute relief in asthma, but chronic use of these bronchodilators paradoxically exacerbates airway hyper-responsiveness. Activation of βARs by β-agonist leads to desensitization (inactivation) by phosphorylation through G-protein coupled receptor kinases (GRKs) which mediate β-arrestin binding and βAR internalization. Resensitization occurs by dephosphorylation of the endosomal βARs which recycle back to the plasma membrane as agonist-ready receptors. To determine whether the loss in β-agonist response in asthma is due to altered βAR desensitization and/or resensitization, we used primary human airway smooth muscle cells (HASMCs) isolated from the lungs of non-asthmatic and fatal-asthmatic subjects. Asthmatic HASMCs have diminished adenylyl cyclase activity and cAMP response to β-agonist as compared to non-asthmatic HASMCs. Confocal microscopy showed significant accumulation of phosphorylated β₂ARs in asthmatic HASMCs. Systematic analysis of desensitization components including GRKs and β-arrestin showed no appreciable differences between asthmatic and non-asthmatic HASMCs. However, asthmatic HASMC showed significant increase in PI3Kγ activity and was associated with reduction in PP2A activity. Since reduction in PP2A activity could alter receptor resensitization, endosomal fractions were isolated to assess the agonist ready β₂ARs as a measure of resensitization. Despite significant accumulation of β₂ARs in the endosomes of asthmatic HASMCs, endosomal β₂ARs cannot robustly activate adenylyl cyclase. Furthermore, endosomes from asthmatic HASMCs are associated with significant increase in PI3Kγ and reduced PP2A activity that inhibits β₂AR resensitization. Our study shows that resensitization, a process considered to be a homeostasis maintaining passive process is inhibited in asthmatic HASMCs contributing to β₂AR dysfunction which may underlie asthma pathophysiology and loss in asthma control.

Trafficking of β-Adrenergic Receptors: Implications in Intracellular Receptor Signaling

β-Adrenergic receptors (βARs), prototypical G-protein-coupled receptors, play a pivotal role in regulating neuronal and cardiovascular responses to catecholamines during stress. Agonist-induced receptor endocytosis is traditionally considered as a primary mechanism to turn off the receptor signaling (or receptor desensitization). However, recent progress suggests that intracellular trafficking of βAR presents a mean to translocate receptor signaling machinery to intracellular organelles/compartments while terminating the signaling at the cell surface. Moreover, the apparent multidimensionality of ligand efficacy in space and time in a cell has forecasted exciting pathophysiological implications, which are just beginning to be explored. As we begin to understand how these pathways impact downstream cellular programs, this will have significant implications for a number of pathophysiological conditions in heart and other systems, that in turn open up new therapeutic opportunities.

The N-terminal Set-β Protein Isoform Induces Neuronal Death

Set-β protein plays different roles in neurons, but the diversity of Set-β neuronal isoforms and their functions have not been characterized. The expression and subcellular localization of Set-β are altered in Alzheimer disease, cleavage of Set-β leads to neuronal death after stroke, and the full-length Set-β regulates retinal ganglion cell (RGC) and hippocampal neuron axon growth and regeneration in a subcellular localization-dependent manner. Here we used various biochemical approaches to investigate Set-β isoforms and their role in the CNS, using the same type of neurons, RGCs, across studies. We found multiple alternatively spliced isoforms expressed from the Set locus in purified RGCs. Set transcripts containing the Set-β-specific exon were the most highly expressed isoforms. We also identified a novel, alternatively spliced Set-β transcript lacking the nuclear localization signal and demonstrated that the full-length (∼39-kDa) Set-β is localized predominantly in the nucleus, whereas a shorter (∼25-kDa) Set-β isoform is localized predominantly in the cytoplasm. Finally, we show that an N-terminal Set-β cleavage product can induce neuronal death.

Co-purification of arrestin like proteins with alpha-enolase from bovine myocardial tissues and the possible role in heart diseases as an autoantigen

AIM

Previously, we reported that visual arrestin co-purified with glycolytic enzymes. The aim of this study was to analyze the co-purification of arrestin like proteins (ALP) in bovine cardiac tissues with enolases.

METHODS

The soluble extract of bovine myocardial tissues from different regions such as left and right atriums and ventricles of the bovine heart (n = 3) was analyzed by ACA-34 gel filtration, immuno-affinity column, SDS-PAGE, ELISA, western blot and a sandwich immune assay for quantification of ALP and sequence analysis.

RESULTS

We observed that; 1) The cardiac muscle contained a 50 kDa ALP at a concentration of 751 pg/mg of soluble protein extract, 2) ALP purified, by immunoaffinity, contained alpha-enolase of 48 kDa confirmed by protein sequence analysis; 3) Cardiomyocyte cells exposed to anti arrestin and anti enolase monoclonal antibodies showed decreased proliferation in vitro, 4) High level of autoantibodies were detected by ELISA (3.57% for arrestin and 9.12% for α-enolase) in serum of patients with infarcted heart disease.

CONCLUSION

We suggest a possible interaction between ALP and alpha-enolases yielding a complex that may be involved in the induction of cardiac autoimmune diseases.