β 2 -adrenoceptor-induced modulation of transglutaminase 2 transamidase activity in cardiomyoblasts

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

Implications of enigmatic transglutaminase 2 (TG2) in cardiac diseases and therapeutic developments

Cardiac diseases are the leading cause of mortality and morbidity worldwide. Mounting evidence suggests that transglutaminases (TGs), tissue TG (TG2) in particular, are involved in numerous molecular responses underlying the pathogenesis of cardiac diseases. The TG family has several intra- and extracellular functions in the human body, including collagen cross-linking, angiogenesis, cell growth, differentiation, migration, adhesion as well as survival. TGs are thiol- and calcium-dependent acyl transferases that catalyze the formation of a covalent bond between the γ-carboxamide group of a glutamine residue and an amine group, thus increasing the stability, rigidity, and stiffness of the myocardial extracellular matrix (ECM). Excessive accumulation of cross-linked collagen leads to increase myocardial stiffness and fibrosis. Beyond TG2 extracellular protein cross-linking action, mounting evidence suggests that this pleiotropic TG isozyme may also promote fibrotic diseases through cell survival and profibrotic pathway activation at the signaling, transcriptional and translational levels. Due to its multiple functions and localizations, TG2 fulfils critical yet incompletely understood roles in myocardial fibrosis and associated heart diseases, such as cardiac hypertrophy, heart failure, and age-related myocardial stiffness under several conditions. This review summarizes current knowledge and existing gaps regarding the ECM-dependent and ECM-independent roles of TG2 and highlights the therapeutic prospects of targeting TG2 to treat cardiac diseases.

Design, Development, Physicochemical Characterization, and In Vitro Drug Release of Formoterol PEGylated PLGA Polymeric Nanoparticles

Polymeric nanoparticles’ drug delivery systems represent a promising platform for targeted controlled release since they are capable of improving the bioavailability and tissue localization of drugs compared to traditional means of administration. Investigation of key parameters of nanoparticle preparation and their impact on performance, such as size, drug loading, and sustained release, is critical to understanding the synthesis parameters surrounding a given nanoparticle formulation. This comprehensive and systematic study reports for the first time and focuses on the development and characterization of formoterol polymeric nanoparticles that have potential application in a variety of acute and chronic diseases. Nanoparticles were prepared by a variety of solvent emulsion methods with varying modifications to the polymer and emulsion system with the aim of increasing drug loading and tuning particle size for renal localization and drug delivery. Maximal drug loading was achieved by amine modification of polyethylene glycol (PEG) conjugated to the poly(lactic-co-glycolic acid) (PLGA) backbone. The resulting formoterol PEGylated PLGA polymeric nanoparticles were successfully lyophilized without compromising size distribution by using either sucrose or trehalose as cryoprotectants. The physicochemical characteristics of the nanoparticles were examined comprehensively, including surface morphology, solid-state transitions, crystallinity, and residual water content. In vitro formoterol drug release characteristics from the PEGylated PLGA polymeric nanoparticles were also investigated as a function of both polymer and emulsion parameter selection, and release kinetics modeling was successfully applied.

Roles and potential clinical implications of tissue transglutaminase in cardiovascular diseases

Cardiovascular disease (CVD)-related mortality and morbidity are among the most critical disease burdens worldwide. CVDs encompass many diseases and involve complex pathogenesis and pathological changes. While research on these diseases has advanced significantly, treatments and their efficacy remain rather limited. New therapeutic strategies and targets must, therefore, be explored. Tissue transglutaminase (TG2) is pivotal to the pathological development of CVDs, including participating in the cross-linking of extracellular proteins, activation of fibroblasts, hypertrophy and apoptosis of cardiomyocytes, proliferation and migration of smooth muscle cells (SMCs), and inflammatory reactions. Regulating TG2 activity and expression could ensure remarkable improvements in disorders like heart failure (HF), pulmonary hypertension (PH), hypertension, and coronary atherosclerosis. In this review, we summarize recent advances in TG2: we discuss its role and mechanisms in the progression of various CVDs and its potential as a diagnostic and therapeutic target.

Formoterol PLGA-PEG Nanoparticles Induce Mitochondrial Biogenesis in Renal Proximal Tubules

Formoterol is a long-acting β₂ agonist (LABA). Agonism of the β₂-adrenergic receptor by formoterol is known to stimulate mitochondrial biogenesis (MB) in renal proximal tubules and recover kidney function. However, formoterol has a number of cardiovascular side effects that limits its usage. The goal of this study was to design and develop an intravenous biodegradable and biocompatible polymeric nanoparticle delivery system that targets formoterol to the kidney. Poly(ethylene glycol) methyl ether-block-poly(lactide-co-glycolide) nanoparticles containing encapsulated formoterol were synthesized by a modified single-emulsion solvent evaporation technique resulting in nanoparticles with a median hydrodynamic diameter of 442 + 17 nm. Using primary cell cultures of rabbit renal proximal tubular cells (RPTCs), free formoterol, encapsulated formoterol polymeric nanoparticles, and drug-free polymeric nanoparticles were biocompatible and not cytotoxic over a wide concentration range. In healthy male mice, polymeric nanoparticles were shown to localize in tubules of the renal cortex and improved the renal localization of encapsulated formoterol compared to the free formoterol. At a lower total formoterol dose, the nanoparticle localization resulted in increased expression of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), the master regulator of MB, and increased electron transport chain proteins, markers of MB. This was confirmed by direct visual quantification of mitochondria and occurred with both free formoterol and the encapsulated formoterol polymeric nanoparticles. At the same time, localization of nanoparticles to the kidneys resulted in reduced induction of MB markers in the heart. These new nanoparticles effectively target formoterol to the kidney and successfully produce MB in the kidney.

Neurite outgrowth inhibitory levels of organophosphates induce tissue transglutaminase activity in differentiating N2a cells: evidence for covalent adduct formation

Organophosphate compounds (OPs) induce both acute and delayed neurotoxic effects, the latter of which is believed to involve their interaction with proteins other than acetylcholinesterase. However, few OP-binding proteins have been identified that may have a direct role in OP-induced delayed neurotoxicity. Given their ability to disrupt Ca²⁺ homeostasis, a key aim of the current work was to investigate the effects of sub-lethal neurite outgrowth inhibitory levels of OPs on the Ca²⁺-dependent enzyme tissue transglutaminase (TG2). At 1-10 µM, the OPs phenyl saligenin phosphate (PSP) and chlorpyrifos oxon (CPO) had no effect cell viability but induced concentration-dependent decreases in neurite outgrowth in differentiating N2a neuroblastoma cells. The activity of TG2 increased in cell lysates of differentiating cells exposed for 24 h to PSP and chlorpyrifos oxon CPO (10 µM), as determined by biotin-cadaverine incorporation assays. Exposure to both OPs (3 and/or 10 µM) also enhanced in situ incorporation of the membrane permeable substrate biotin-X-cadaverine, as indicated by Western blot analysis of treated cell lysates probed with ExtrAvidin peroxidase and fluorescence microscopy of cell monolayers incubated with FITC-streptavidin. Both OPs (10 µM) stimulated the activity of human and mouse recombinant TG2 and covalent labelling of TG2 with dansylamine-labelled PSP was demonstrated by fluorescence imaging following SDS-PAGE. A number of TG2 substrates were tentatively identified by mass spectrometry, including cytoskeletal proteins, chaperones and proteins involved protein synthesis and gene regulation. We propose that the elevated TG2 activity observed is due to the formation of a novel covalent adduct between TG2 and OPs.

Transglutaminase in Receptor and Neurotransmitter-Regulated Functions

Transglutaminases (TGs) and especially TG2 play important roles in neurotransmitter and receptor signaling pathways. Three different mechanisms by which TG2 interacts with neurotransmitter and receptor signaling systems will be discussed in this review. The first way in which TG2 interacts with receptor signaling is via its function as a guanine nucleotide binding protein (G-protein) coupling to G-protein coupled receptors (GPCRs) to activate down-stream signaling pathways. TG2 can exist in a least two conformations, a closed GTP-bound conformation and an open calcium-bound conformation. In the closed GTP-bound conformation, TG2 is capable of functioning as a G-protein for GPCRs. In the open calcium-bound conformation, TG2 catalyzes a transamidation reaction cross-linking proteins or catalyzing the covalent binding of a mono- or polyamine to a protein. The second mechanism is regulation of the transamidation reaction catalyzed by TG2 via receptor stimulation which can increase local calcium concentrations and thereby increase transamidation reactions. The third way in which TG2 plays a role in neurotransmitter and receptor signaling systems is via its use of monoamine neurotransmitters as a substrate. Monoamine neurotransmitters including serotonin can be substrates for transamidation to a protein often a small G-protein (also known as a small GTPase) resulting in activation of the small G-protein. The transamidation of a monoamine neurotransmitter or serotonin has been designated as monoaminylation or more specifically serotonylation, respectively. Other proteins are also targets for monoaminylation such as fibronectin and cytoskeletal proteins. These receptor and neurotransmitter-regulated reactions by TG2 play roles in physiological and key pathophysiological processes.

Activation of transglutaminase 2 by nerve growth factor in differentiating neuroblastoma cells: A role in cell survival and neurite outgrowth

NGF (nerve growth factor) and tissue transglutaminase (TG2) play important roles in neurite outgrowth and modulation of neuronal cell survival. In this study, we investigated the regulation of TG2 transamidase activity by NGF in retinoic acid-induced differentiating mouse N2a and human SH-SY5Y neuroblastoma cells. TG2 transamidase activity was determined using an amine incorporation and a peptide cross linking assay. In situ TG2 activity was assessed by visualising the incorporation of biotin-X-cadaverine using confocal microscopy. The role of TG2 in NGF-induced cytoprotection and neurite outgrowth was investigated by monitoring hypoxia-induced cell death and appearance of axonal-like processes, respectively. The amine incorporation and protein crosslinking activity of TG2 increased in a time and concentration-dependent manner following stimulation with NGF in N2a and SH-SY5Y cells. NGF mediated increases in TG2 activity were abolished by the TG2 inhibitors Z-DON (Z-ZON-Val-Pro-Leu-OMe; Benzyloxycarbonyl-(6-Diazo-5-oxonorleucinyl)-l-valinyl-l-prolinyl-l-leucinmethylester) and R283 (1,3,dimethyl-2[2-oxo-propyl]thio)imidazole chloride) and by pharmacological inhibition of extracellular signal-regulated kinases 1 and 2 (ERK1/2), protein kinase B (PKB) and protein kinase C (PKC), and removal of extracellular Ca²⁺. Fluorescence microscopy demonstrated NGF induced in situ TG2 activity. TG2 inhibition blocked NGF-induced attenuation of hypoxia-induced cell death and neurite outgrowth in both cell lines. Together, these results demonstrate that NGF stimulates TG2 transamidase activity via a ERK1/2, PKB and PKC-dependent pathway in differentiating mouse N2a and human SH-SY5Y neuroblastoma cells. Furthermore, NGF-induced cytoprotection and neurite outgrowth are dependent upon TG2. These results suggest a novel and important role of TG2 in the cellular functions of NGF.

Role of transglutaminase 2 in A1 adenosine receptor- and β2-adrenoceptor-mediated pharmacological pre- and post-conditioning against hypoxia-reoxygenation-induced cell death in H9c2 cells

Pharmacologically-induced pre- and post-conditioning represent attractive therapeutic strategies to reduce ischaemia/reperfusion injury during cardiac surgery and following myocardial infarction. We have previously reported that transglutaminase 2 (TG2) activity is modulated by the A₁ adenosine receptor and β₂-adrenoceptor in H9c2 cardiomyoblasts. The primary aim of this study was to determine the role of TG2 in A₁ adenosine receptor and β₂-adrenoceptor-induced pharmacological pre- and post-conditioning in the H9c2 cells. H9c2 cells were exposed to 8h hypoxia (1% O₂) followed by 18h reoxygenation, after which cell viability was assessed by monitoring mitochondrial reduction of MTT, lactate dehydrogenase release and caspase-3 activation. N⁶-cyclopentyladenosine (CPA; A₁ adenosine receptor agonist), formoterol (β₂-adrenoceptor agonist) or isoprenaline (non-selective β-adrenoceptor agonist) were added before hypoxia/reoxygenation (pre-conditioning) or at the start of reoxygenation following hypoxia (post-conditioning). Pharmacological pre- and post-conditioning with CPA and isoprenaline significantly reduced hypoxia/reoxygenation-induced cell death. In contrast, formoterol did not elicit protection. Pre-treatment with pertussis toxin (G_i/o-protein inhibitor), DPCPX (A₁ adenosine receptor antagonist) or TG2 inhibitors (Z-DON and R283) attenuated the A₁ adenosine receptor-induced pharmacological pre- and post-conditioning. Similarly, pertussis toxin, ICI 118,551 (β₂-adrenoceptor antagonist) or TG2 inhibition attenuated the isoprenaline-induced cell survival. Knockdown of TG2 using small interfering RNA (siRNA) attenuated CPA and isoprenaline-induced pharmacological pre- and post-conditioning. Finally, proteomic analysis following isoprenaline treatment identified known (e.g. protein S100-A6) and novel (e.g. adenine phosphoribosyltransferase) protein substrates for TG2. These results have shown that A₁ adenosine receptor and β₂-adrenoceptor-induced protection against simulated hypoxia/reoxygenation occurs in a TG2 and G_i/o-protein dependent manner in H9c2 cardiomyoblasts.