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

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Transcriptional regulation by methyltransferases and their role in the heart: highlighting novel emerging functionality

Methyltransferases are a superfamily of enzymes that transfer methyl groups to proteins, nucleic acids, and small molecules. Traditionally, these enzymes have been shown to carry out a specific modification (mono-, di-, or trimethylation) on a single, or limited number of, amino acid(s). The largest subgroup of this family, protein methyltransferases, target arginine and lysine side chains of histone molecules to regulate gene expression. Although there is a large number of functional studies that have been performed on individual methyltransferases describing their methylation targets and effects on biological processes, no analyses exist describing the spatial distribution across tissues or their differential expression in the diseased heart. For this review, we performed tissue profiling in protein databases of 199 confirmed or putative methyltransferases to demonstrate the unique tissue-specific expression of these individual proteins. In addition, we examined transcript data sets from human heart failure patients and murine models of heart disease to identify 40 methyltransferases in humans and 15 in mice, which are differentially regulated in the heart, although many have never been functionally interrogated. Lastly, we focused our analysis on the largest subgroup, that of protein methyltransferases, and present a newly emerging phenomenon in which 16 of these enzymes have been shown to play dual roles in regulating transcription by maintaining the ability to both activate and repress transcription through methyltransferase-dependent or -independent mechanisms. Overall, this review highlights a novel paradigm shift in our understanding of the function of histone methyltransferases and correlates their expression in heart disease.

Activation of the PP2A catalytic subunit by ivabradine attenuates the development of diabetic cardiomyopathy

Hyperglycemia-induced apoptosis plays a critical role in the pathogenesis of diabetic cardiomyopathy (DCM). Our previous study demonstrated that ivabradine, a selective I_f current antagonist, significantly attenuated myocardial apoptosis in diabetic mice, but the underlying mechanisms remained unknown. This study investigated the underlying mechanisms by which ivabradine exerts anti-apoptotic effects in experimental DCM. Pretreatment with ivabradine, but not ZD7288 (an established I_f current blocker), profoundly inhibited high glucose-induced apoptosis via inactivation of nuclear factor (NF)-κB signaling in neonatal rat cardiomyocytes. The effect was abolished by transfection of an siRNA targeting protein phosphatase 2A catalytic subunit (PP2Ac). In streptozotocin-induced diabetic mice, ivabradine treatment significantly inhibited left ventricular hyperpolarization-activated cyclic nucleotide-gated channel 2 (HCN2) and HCN4 (major components of the I_f current), activated PP2Ac, and attenuated NF-κB signaling activation and apoptosis, in line with improved histological abnormalities, fibrosis, and cardiac dysfunction without affecting hyperglycemia. These effects were not observed in diabetic mice with virus-mediated knockdown of HCN2 or HCN4 after myocardial injection, but were alleviated by knockdown of PP2Acα. Molecular docking and phosphatase activity assay confirmed direct binding of ivabradine to, and activation of, PP2Ac. In conclusion, ivabradine may directly activate PP2Ac, leading to inhibition of NF-κB signaling activation, myocardial apoptosis, and fibrosis, and eventually improving cardiac function in experimental DCM. Taken together, the present findings suggest that ivabradine may be a promising drug for treatment of DCM.

Regulation of the phosphoprotein phosphatase 2A system and its modulation during oxidative stress: A potential therapeutic target?

Phosphoprotein phosphatases are of growing interest in the pathophysiology of many diseases and are often the neglected partner of protein kinases. One family member, PP2A, accounts for dephosphorylation of ~55-70% of all serine/threonine phosphosites. Interestingly, dysregulation of kinase signalling is a hallmark of many diseases in which an increase in oxidative stress is also noted. With this in mind, we assess the evidence to support oxidative stress-mediated regulation of the PP2A system In this article, we first present an overview of the PP2A system before providing an analysis of the regulation of PP2A by endogenous inhibitors, post translational modification, and miRNA. Next, a detailed critique of data implicating reactive oxygen species, ischaemia, ischaemia-reperfusion, and hypoxia in regulating the PP2A holoenzyme and associated regulators is presented. Finally, the pharmacological targeting of PP2A, its endogenous inhibitors, and enzymes responsible for its post-translational modification are covered. There is extensive evidence that oxidative stress modulates multiple components of the PP2A system, however, most of the data pertains to the catalytic subunit of PP2A. Irrespective of the underlying aetiology, free radical-mediated attenuation of PP2A activity is an emerging theme. However, in many instances, a dichotomy exists, which requires clarification and mechanistic insight. Nevertheless, this raises the possibility that pharmacological activation of PP2A, either through small molecule activators of PP2A or CIP2A/SET antagonists may be beneficial in modulating the cellular response to oxidative stress. A better understanding of which, will have wide ranging implications for cancer, heart disease and inflammatory conditions.

Global loss of leucine carboxyl methyltransferase-1 causes severe defects in fetal liver hematopoiesis

Leucine carboxyl methyltransferase-1 (LCMT-1) methylates the C-terminal leucine α-carboxyl group of the catalytic subunits of the protein phosphatase 2A (PP2A) subfamily of protein phosphatases, PP2Ac, PP4c, and PP6c. LCMT-1 differentially regulates the formation and function of a subset of the heterotrimeric complexes that PP2A and PP4 form with their regulatory subunits. Global LCMT-1 knockout causes embryonic lethality in mice, but LCMT-1 function in development is unknown. In this study, we analyzed the effects of global LCMT-1 loss on embryonic development. LCMT-1 knockout causes loss of PP2Ac methylation, indicating that LCMT-1 is the sole PP2Ac methyltransferase. PP2A heterotrimers containing the Bα and Bδ B-type subunits are dramatically reduced in whole embryos, and the steady-state levels of PP2Ac and the PP2A structural A subunit are also down ∼30%. Strikingly, global loss of LCMT-1 causes severe defects in fetal hematopoiesis and usually death by embryonic day 16.5. Fetal livers of homozygous lcmt-1 knockout embryos display hypocellularity, elevated apoptosis, and greatly reduced numbers of hematopoietic stem and progenitor cell-enriched Kit+Lin-Sca1+ cells. The percent cycling cells and mitotic indices of WT and lcmt-1 knockout fetal liver cells are similar, suggesting that hypocellularity may be due to a combination of apoptosis and/or defects in specification, self-renewal, or survival of stem cells. Indicative of a possible intrinsic defect in stem cells, noncompetitive and competitive transplantation experiments reveal that lcmt-1 loss causes a severe multilineage hematopoietic repopulating defect. Therefore, this study reveals a novel role for LCMT-1 as a key player in fetal liver hematopoiesis.

Light regulation of nitrate reductase by catalytic subunits of protein phosphatase 2A

PP2A catalytic subunit C2 is of special importance for light/dark regulation of nitrate reductase activity. The level of unmethylated PP2A catalytic subunits decreases in darkness. Protein phosphatase 2A (PP2A) dephosphorylates and activates nitrate reductase (NR) in photosynthetically active tissue when plants are transferred from darkness to light. In the present work, investigation of Arabidopsis thaliana PP2A mutant lines revealed that one of the five PP2A catalytic subunit genes, e.g., C2, was of special importance for NR activation. Impairment of NR activation was, especially pronounced in the c2c4 double mutant. Though weaker, NR activation was also impaired in the c2 single mutant, and c1c2 and c2c5 double mutants. On the other hand, NR activation in the c4c5 double mutant was as efficient as in WT. The c4 single mutant had low PP2A activity, whereas the c2 single mutant possessed WT levels of extractable PP2A activity. PP2A activity was low in both c2c4 and c4c5. Differences in extracted PP2A activity among mutants did not strictly correlate with differences in NR activation, but underpinned that C2 has a special function in NR activation in vivo. The terminal leucine in PP2A catalytic subunits is generally methylated to a high degree, but regulation and impact of methylation/demethylation is barely studied. In WT and PP2A mutants, the level of unmethylated PP2A catalytic subunits decreased during 45 min of darkness, but did not change much when light was switched on. In leucine carboxyl methyl transferase1 (LCMT1) knockout plants, which possess mainly unmethylated PP2A, NR was still activated, although not fully as efficient as in WT.

MFHAS1 suppresses TLR4 signaling pathway via induction of PP2A C subunit cytoplasm translocation and inhibition of c-Jun dephosphorylation at Thr239

TLR4, an important Toll-like receptor in innate immunity, can be activated by LPS and induce proinflammatory cytokines to resist invasion of pathogenic microorganism, but excessive inflammation can trigger tissue injury. Many genes negatively regulate TLR4 signaling pathway. Recent studies found that malignant fibrous histiocytoma amplified sequence 1 (MFHAS1) suppressed the expression of cytokine IL6 in Raw264.7 cells stimulated by LPS, but the mechanisms remained unclear. This study investigated the role of MFHAS1 in TLR4 signaling pathway and the possible mechanisms implicated. The results indicated that the expression of MFHAS1 was significantly increased in cells stimulated with LPS. Up-regulation of MFHAS1 effectively suppressed inflammatory cytokine expression in cells exposed to LPS, whereas down-regulation of MFHAS1 markedly increased inflammatory cytokines expression. Co-immunoprecipitation, pull-down and immunofluorescence tests demonstrated that MFHAS1 combined with the B and C subunits of PP2A and induced cytoplasm translocation of the C subunit, leading to decrease dephosphorylation of c-Jun at Thr239 and increase degradation of c-Jun. Reduction of c-Jun protein results in decreased AP-1 activity, which is independent of inhibition of JNK or p38MAPK phosphorylation. Taken together, these results indicate that MFHAS1 suppresses TLR4 signaling pathway through induction of PP2A C subunit cytoplasm translocation and subsequent c-Jun degradation, leading finally to decrease AP-1 activity and cytokines expression.

Regulation of Ca(2+) transient by PP2A in normal and failing heart

Calcium transient in cardiomyocytes is regulated by multiple protein kinases and phosphatases. PP2A is a major protein phosphatase in the heart modulating Ca²⁺ handling through an array of ion channels, antiporters and pumps, etc. The assembly, localization/translocation, and substrate specificity of PP2A are controlled by different post-translational mechanisms, which in turn are linked to the activities of upstream signaling molecules. Abnormal PP2A expression and activities are associated with defective response to β-adrenergic stimulation and are indication and causal factors in arrhythmia and heart failure.