Transglutaminase 2-mediated serotonylation in pulmonary hypertension

Overview of Methamphetamine-Associated Pulmonary Arterial Hypertension

TOPIC IMPORTANCE

The global surge in methamphetamine use is a critical public health concern, particularly due to its robust correlation with methamphetamine-associated pulmonary arterial hypertension (MA-PAH). This association raises urgent alarms about the potential escalation of MA-PAH incidence, posing a significant and imminent challenge to global public health.

REVIEW FINDINGS

This comprehensive review meticulously explores MA-PAH, offering insights into its epidemiology, pathophysiology, clinical presentation, diagnostic intricacies, and management strategies. The pathogenesis, yet to be fully described, involves complex molecular interactions, including alterations in serotonin signaling, reduced activity of carboxylesterase 1, oxidative stress, and dysregulation of pulmonary vasoconstrictors and vasodilators. These processes culminate in the structural remodeling of the pulmonary vasculature, resulting in pulmonary arterial hypertension. MA-PAH exhibits a more severe clinical profile in functional class and hemodynamics compared with idiopathic pulmonary arterial hypertension. Management involves a multifaceted approach, integrating pulmonary vasodilators, cessation of methamphetamine use, and implementing social and rehabilitation programs. These measures aim to enhance patient outcomes and detect potential relapses for timely intervention.

SUMMARY

This review consolidates our understanding of MA-PAH, pinpointing knowledge gaps for future studies. Addressing these gaps is crucial for advancing diagnostic accuracy, unraveling mechanisms, and optimizing treatment for MA-PAH, thereby addressing the evolving landscape of this complex health concern.

The janus face of serotonin: Regenerative promoter and chronic liver disease aggravator

The progression of liver diseases, from viral hepatitis and fatty liver disease to cirrhosis and hepatocellular carcinoma (HCC), is the most representative series of pathological events in liver diseases. While serotonin (5-HT) primarily regulates brain functions such as psychology, mood, and appetite in the central nervous system (CNS), peripheral 5-HT plays a crucial role in regulating tumor development, glucose and lipid metabolism, immune function and inflammatory response related to liver diseases. These peripheral physiological processes involving 5-HT are the key mechanisms driving the development of these liver diseases. This study presents an overview of the existing literature, focusing on the role of 5-HT in HCC, cirrhosis, fatty liver disease, viral hepatitis, and liver injury. In summary, while 5-HT promotes liver regeneration, it can also contribute to the progression of chronic liver disease. These findings indicate the potential for the development and use of 5-HT-related drugs for the treatment of liver diseases, including HCC and cirrhosis.

Lung-specific interleukin 6 mediated transglutaminase 2 activation and cardiopulmonary fibrogenesis

Pulmonary hypertension (PH) pathogenesis is driven by inflammatory and metabolic derangements as well as glycolytic reprogramming. Induction of both interleukin 6 (IL6) and transglutaminase 2 (TG2) expression participates in human and experimental cardiovascular diseases. However, little is known about the role of TG2 in these pathologic processes. The current study aimed to investigate the molecular interactions between TG2 and IL6 in mediation of tissue remodeling in PH. A lung-specific IL6 over-expressing transgenic mouse strain showed elevated right ventricular (RV) systolic pressure as well as increased wet and dry tissue weights and tissue fibrosis in both lungs and RVs compared to age-matched wild-type littermates. In addition, IL6 over-expression induced the glycolytic and fibrogenic markers, hypoxia-inducible factor 1α, pyruvate kinase M2 (PKM2), and TG2. Consistent with these findings, IL6 induced the expression of both glycolytic and pro-fibrogenic markers in cultured lung fibroblasts. IL6 also induced TG2 activation and the accumulation of TG2 in the extracellular matrix. Pharmacologic inhibition of the glycolytic enzyme, PKM2 significantly attenuated IL6-induced TG2 activity and fibrogenesis. Thus, we conclude that IL6-induced TG2 activity and cardiopulmonary remodeling associated with tissue fibrosis are under regulatory control of the glycolytic enzyme, PKM2.

Serotonin and the serotonin transporter in the adrenal gland

The adrenal glands are key components of the mammalian endocrine system, helping maintain physiological homeostasis and the coordinated response to stress. Each adrenal gland has two morphologically and functionally distinct regions, the outer cortex and inner medulla. The cortex is organized into three concentric zones which secrete steroid hormones, including aldosterone and cortisol. Neural crest-derived chromaffin cells in the medulla are innervated by preganglionic sympathetic neurons and secrete catecholamines (epinephrine, norepinephrine) and neuropeptides into the bloodstream, thereby functioning as the neuroendocrine arm of the sympathetic nervous system. In this article we review serotonin (5-HT) and the serotonin transporter (SERT; SLC6A4) in the adrenal gland. In the adrenal cortex, 5-HT, primarily sourced from resident mast cells, acts as a paracrine signal to stimulate aldosterone and cortisol secretion through 5-HT4/5-HT7 receptors. Medullary chromaffin cells contain a small amount of 5-HT due to SERT-mediated uptake and express 5-HT1A receptors which inhibit secretion. The atypical mechanism of the 5-HT1A receptors and interaction with SERT fine tune this autocrine pathway to control stress-evoked catecholamine secretion. Receptor-independent signaling by SERT/intracellular 5-HT modulates the amount and kinetics of transmitter release from single vesicle fusion events. SERT might also influence stress-evoked upregulation of tyrosine hydroxylase transcription. Transient signaling via 5-HT3 receptors during embryonic development can limit the number of chromaffin cells found in the mature adrenal gland. Together, this emerging evidence suggests that the adrenal medulla is a peripheral hub for serotonergic control of the sympathoadrenal stress response.

Serotonergic Modulation of Neurovascular Transmission: A Focus on Prejunctional 5-HT Receptors/Mechanisms

5-Hydroxytryptamine (5-HT), or serotonin, plays a crucial role as a neuromodulator and/or neurotransmitter of several nervous system functions. Its actions are complex, and depend on multiple factors, including the type of effector or receptor activated. Briefly, 5-HT can activate: (i) metabotropic (G-protein-coupled) receptors to promote inhibition (5-HT₁, 5-HT₅) or activation (5-HT₄, 5-HT₆, 5-HT₇) of adenylate cyclase, as well as activation (5-HT₂) of phospholipase C; and (ii) ionotropic receptor (5-HT₃), a ligand-gated Na⁺/K⁺ channel. Regarding blood pressure regulation (and beyond the intricacy of central 5-HT effects), this monoamine also exerts direct postjunctional (on vascular smooth muscle and endothelium) or indirect prejunctional (on autonomic and sensory perivascular nerves) effects. At the prejunctional level, 5-HT can facilitate or preclude the release of autonomic (e.g., noradrenaline and acetylcholine) or sensory (e.g., calcitonin gene-related peptide) neurotransmitters facilitating hypertensive or hypotensive effects. Hence, we cannot formulate a specific impact of 5-HT on blood pressure level, since an increase or decrease in neurotransmitter release would be favoured, depending on the type of prejunctional receptor involved. This review summarizes and discusses the current knowledge on the prejunctional mechanisms involved in blood pressure regulation by 5-HT and its impact on some vascular-related diseases.

Implications of Transglutaminase-Mediated Protein Serotonylation in the Epigenetic Landscape, Small Cell Lung Cancer, and Beyond

In the case of small-cell lung carcinoma, the highly metastatic nature of the disease and the propensity for several chromatin modifiers to harbor mutations suggest that epigenetic manipulation may also be a promising route for oncotherapy, but histone deacetylase inhibitors on their own do not appear to be particularly effective, suggesting that there may be other regulatory parameters that dictate the effectiveness of vorinostat's reversal of histone deacetylation. Recent discoveries that serotonylation of histone H3 alters the permissibility of gene expression have led to renewed attention to this rare modification, as facilitated by transglutaminase 2, and at the same time introduce new questions about whether this modification belongs to a part of the concerted cohort of regulator events for modulating the epigenetic landscape. This review explores the mechanistic details behind protein serotonylation and its possible connections to the epigenome via histone modifications and glycan interactions and attempts to elucidate the role of transglutaminase 2, such that optimizations to existing histone deacetylase inhibitor designs or combination therapies may be devised for lung and other types of cancer.

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.

The effect of transglutaminase-2 inhibitor on pulmonary vascular remodeling in rats with pulmonary arterial hypertension

To investigate the relationship between transglutaminase type 2 (TG2) and pulmonary vascular remodeling in the formation of pulmonary arterial hypertension (PAH), and to investigate the effect of the inhibitor cystamine dihydrochloride on pulmonary vascular remodeling in rats with PAH.

Thirty healthy male Sprague Dawley rats were randomly divided into a control group, a PAH model group, and an intervention group. The mean pulmonary artery pressure (mPAP), the right ventricular hypertrophy index (RVHI), the percentage wall thickness of the pulmonary artery (WT%), and the degree of neointimal proliferation were measured, and the pathological changes in the pulmonary tissues were observed.Messenger ribonucleic acid (mRNA) and protein expressions of TG2, 5-hydroxytryptamine transporter (5-HTT), and Rho-associated protein kinase 2 (ROCK2) in the pulmonary tissues of the three groups of rats were detected.

Compared with the control group, the mPAP, RVHI, and WT% were significantly higher in the model group, the degree of neointimal proliferation was significantly increased, and the mRNA and protein expressions of TG2, 5-HTT, and ROCK2 in the pulmonary tissue were significantly increased. Compared with the model group, the mPAP, RVHI, WT%, and the degree of neointimal proliferation were significantly lower in the intervention group, as were the mRNA and protein expressions of TG2, 5-HTT, and ROCK2 in the pulmonary tissue.

The TG2 inhibitor cystamine dihydrochloride can prevent the formation of PAH to some extent. This might be due to the inhibition of the TG2 activity, 5-HTT expression, and possibly the inhibition of RhoA/ROCK signaling pathway activation.

Transglutaminase 2: Development of therapeutic antibodies reveals four inhibitory epitopes and confirms extracellular function in fibrotic remodelling

BACKGROUND AND PURPOSE

Transglutaminase type 2 (TG2) catalyses formation of ε-(γ-glutamyl)-lysine bonds between proteins, including those of the extracellular matrix (ECM). Elevated extracellular TG2 leads to accelerated ECM deposition and reduced clearance that underlies tissue scarring and fibrosis. Many transglutaminase inhibitors exist and allowed for proof-of-concept studies in disease models, but their lack of specificity for the TG2 isoform, and/or poor pharmacokinetic/pharmacodynamic properties have limited their clinical application. We hypothesised that a high affinity TG2-specific antibody could be developed to specifically inhibit extracellular TG2 activity, with characteristics suitable for therapeutic development.

EXPERIMENTAL APPROACH

Individual human TG2 domains were used to immunise mice and generate hybridomas. Supernatants were screened for inhibition of recombinant human TG2 activity, with TG2 specificity determined by ELISA.

KEY RESULTS

Thirteen TG2-specific supernatants inhibited human transamidation activity. Each hybridoma was cloned and antibody mapped to an epitope in the TG2 core domain, using phage display panning of a TG2 fragment library. Four distinct inhibitory epitopes were determined. The most effective antibodies (AB1, DC1 and BB7) bound to amino acids 313-327 (catalytic core), with an IC₅₀ of approximately 10 nM. The antibodies inhibit TG2 in human cells and block ECM accumulation in a primary human proximal tubular epithelial cell model of fibrosis, only 7 antibodies inhibited rat TG2, and all with higher IC₅₀ values.

CONCLUSIONS AND IMPLICATIONS

We identified a preferred inhibitory epitope in human TG2, developed antibodies with required characteristics for clinical development, and established that targeted inhibition of extracellular TG2 transamidation activity is sufficient to modify fibrotic remodelling.

The role of 5-HT metabolism in cancer

Serotonin (5-hydroxytryptamine, 5-HT) metabolism has long been linked to tumorigenesis and tumor progression. Numerous studies have shown the functions of 5-HT and its metabolites in the regulation of tumor biological processes like cell proliferation, invasion, metastasis, tumor angiogenesis and immunomodulatory through multi-step complex mechanisms. Reprogramming of 5-HT metabolism has been revealed in various tumors paving way for development of drugs that target enzymes, metabolites or receptors involved in 5-HT metabolic pathway. However, information on the role of 5-HT metabolism in cancer is scanty. This review briefly describes the main metabolic routes of 5-HT, the role of 5-HT metabolism in cancer and systematically summarizes the most recent advances in 5-HT metabolism-targeted cancer therapy.

An initial assessment of the involvement of transglutaminase2 in eosinophilic bronchitis using a disease model developed in C57BL/6 mice

The detailed pathogenesis of eosinophilic bronchitis (EB) remains unclear. Transglutaminase 2 (TG2) has been implicated in many respiratory diseases including asthma. Herein, we aim to assess preliminarily the relationship of TG2 with EB in the context of the development of an appropriate EB model through ovalbumin (OVA) sensitization and challenge in the C57BL/6 mouse strain. Our data lead us to propose a 50 μg dose of OVA challenge as appropriate to establish an EB model in C57BL/6 mice, whereas a challenge with a 400 μg dose of OVA significantly induced asthma. Compared to controls, TG2 is up-regulated in the airway epithelium of EB mice and EB patients. When TG2 activity was inhibited by cystamine treatment, there were no effects on airway responsiveness; in contrast, the lung pathology score and eosinophil counts in bronchoalveolar lavage fluid were significantly increased whereas the cough frequency was significantly decreased. The expression levels of interleukin (IL)-4, IL-13, IL-6, mast cell protease7 and the transient receptor potential (TRP) ankyrin 1 (TRPA1), TRP vanilloid 1 (TRPV1) were significantly decreased. These data open the possibility of an involvement of TG2 in mediating the increased cough frequency in EB through the regulation of TRPA1 and TRPV1 expression. The establishment of an EB model in C57BL/6 mice opens the way for a genetic investigation of the involvement of TG2 and other molecules in this disease using KO mice, which are often generated in the C57BL/6 genetic background.

The physiology, pathology and potential therapeutic application of serotonylation

The classical neurotransmitter serotonin or 5-hydroxytryptamine (5-HT), synthesized from tryptophan, can be produced both centrally and peripherally. Through binding to functionally distinct receptors, serotonin is profoundly implicated in a number of fundamental physiological processes and pathogenic conditions. Recently, serotonin has been found covalently incorporated into proteins, a newly identified post-translational modification termed serotonylation. Transglutaminases (TGMs), especially TGM2, are responsible for catalyzing the transamidation reaction by transferring serotonin to the glutamine residues of target proteins. Small GTPases, extracellular matrix protein fibronectin, cytoskeletal proteins and histones are the most reported substrates for serotonylation, and their functions are triggered by this post-translational modification. This Review highlights the roles of serotonylation in physiology and diseases and provides perspectives for pharmacological interventions to ameliorate serotonylation for disease treatment.

Novel and atypical pathways for serotonin signaling

Serotonin (5-HT) appeared billions of years before 5-HT receptors and synapses. It is thus not surprising that 5-HT can control biological processes independently of its receptors. One example is serotonylation, which consists of covalent binding of 5-HT to the primary amine of glutamine. Over the past 20 years, serotonylation has been involved in the regulation of many signaling mechanisms. One of the most striking examples is the recent evidence that serotonylation of histone H3 constitutes an epigenetic mark. However, the pathophysiological role of histone H3 serotonylation remains to be discovered. All but one of the 5-HT receptors are G-protein-coupled receptors (GPCRs). The signaling pathways they control are finely tuned, and new, unexpected regulatory mechanisms are being uncovered continuously. Some 5-HT receptors (5-HT_2C, 5-HT₄, 5-HT₆, and 5-HT₇) signal through mechanisms that require neither G-proteins nor β-arrestins, the two classical and almost universal GPCR signal transducers. 5-HT₆ receptors are constitutively activated via their association with intracellular GPCR-interacting proteins (GIPs), including neurofibromin 1, cyclin-dependent kinase 5 (Cdk5), and G-protein-regulated inducer of neurite outgrowth 1 (GPRIN1). Interactions of 5-HT₆ receptor with Cdk5 and GPRIN1 are not concomitant but occur sequentially and play a key role in dendritic tree morphogenesis. Furthermore, 5-HT₆ receptor-mediated G-protein signaling in neurons is different in the cell body and primary cilium, where it is modulated by smoothened receptor activation. Finally, 5-HT_2A receptors form heteromers with mGlu₂ metabotropic glutamate receptors. This heteromerization results in a specific phosphorylation of mGlu₂ receptor on a serine residue (Ser⁸⁴³) upon agonist stimulation of 5-HT_2A or mGlu₂ receptor. mGlu₂ receptor phosphorylation on Ser⁸⁴³ is an essential step in engagement of G_i/o signaling not only upon mGlu₂ receptor activation but also following 5-HT_2A receptor activation, and thus represents a key molecular event underlying functional crosstalk between both receptors.

Targeting SERT promotes tryptophan metabolism: mechanisms and implications in colon cancer treatment

Background

Serotonin signaling has been associated with tumorigenesis and tumor progression. Targeting the serotonin transporter to block serotonin cellular uptake confers antineoplastic effects in various tumors, including colon cancer. However, the antineoplastic mechanism of serotonin transporter inhibition and serotonin metabolism alterations in the absence of serotonin transporter have not been elucidated, especially in colon cancer, which might limit anti-tumor effects associating with targeting serotonin transporter.

Methods

The promotion in the uptake and catabolism of extracellular tryptophan and targeting serotonin transporter was detected by using quantitative reverse-transcription polymerase chain reaction, western blotting and liquid chromatography tandem mass spectrometry. Western blotting Immunoprecipitation and immunofluorescence was utilized to research the serotonylation of mTOR by serotonin and serotonin transporter inhibition. The primary mouse model, homograft model and tissue microarry was used to explore the tryptophan pathway in colon cancer. The cell viability assay, western blotting, xenograft and primary colon cancer mouse model were used to identify whether the combination of sertraline and tryptophan restriction had a synergistic effect.

Results

Targeting serotonin transporter through genetic ablation or pharmacological inhibition in vitro and in vivo induced a compensatory effect by promoting the uptake and catabolism of extracellular tryptophan in colon cancer. Mechanistically, targeting serotonin transporter suppressed mTOR serotonylation, leading to mTOR inactivation and increased tryptophan uptake. In turn, this process promoted serotonin biosynthesis and oncogenic metabolite kynurenine production through enhanced tryptophan catabolism. Tryptophan deprivation, or blocking its uptake by using trametinib, a MEK inhibitor, can sensitize colon cancer to selective serotonin reuptake inhibitors.

Conclusions

The present study elucidated a novel feedback mechanism involved in the regulation of serotonin homeostasis and suggested innovative strategies for selective serotonin reuptake inhibitors-based treatment of colon cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-01971-1.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Transglutaminase 2 as a novel target in chronic kidney disease - Methods, mechanisms and pharmacological inhibition

Chronic kidney disease (CKD) is a global health problem with a prevalence of 10-15%. Progressive fibrosis of the renal tissue is a main feature of CKD, but current treatment strategies are relatively unspecific and delay, but do not prevent, CKD. Exploration of novel pharmacological targets to inhibit fibrosis development are therefore important. Transglutaminase 2 (TG2) is known to be central for extracellular collagenous matrix formation, but TG2 is a multifunctional enzyme and novel research has broadened our view on its extra- and intracellular actions. TG2 exists in two conformational states with different catalytic properties as determined by substrate availability and local calcium concentrations. The open conformation of TG2 depends on calcium and has transamidase activity, central for protein modification and cross-linking of extracellular protein components, while the closed conformation is a GTPase involved in transmembrane signaling processes. We first describe different methodologies to assess TG2 activity in renal tissue and cell cultures such as biotin cadaverine incorporation. Then we systematically review animal CKD models and preliminary studies in humans (with diabetic, IgA- and chronic allograft nephropathy) to reveal the role of TG2 in renal fibrosis. Mechanisms behind TG2 activation, TG2 externalization dependent on Syndecan-4 and interactions between TG and profibrotic molecules including transforming growth factor β and the angiotensin II receptor are discussed. Pharmacological TG2 inhibition shows antifibrotic effects in CKD. However, the translation of TG2 inhibition to treat CKD in patients is a challenge as clinical information is limited, and further studies on pharmacokinetics and efficacy of the individual compounds are required.

Serotonin stimulated parathyroid hormone related protein induction in the mammary epithelia by transglutaminase-dependent serotonylation

Mammary-derived serotonin has been implicated in breast-to-bone communication during lactation by increasing parathyroid hormone related-protein (PTHrP) in the mammary gland. It is well established that PTHrP acts on the bone to liberate calcium for milk synthesis during lactation; however, the mechanism of serotonin’s regulation of PTHrP has not been fully elucidated. Recently, serotonylation has been shown to be involved in a variety of physiological processes mediated by serotonin. Therefore, we investigated whether serotonylation is involved in serotonin’s regulation of PTHrP in the mammary gland using lactogenically differentiated mouse mammary epithelial cells. We investigated the effect of increased intracellular serotonin using the antidepressant fluoxetine or 5-hydroxytryptophan (serotonin precursor), with or without transglutaminase inhibition and the corresponding action on PTHrP induction and activity. Treatment with fluoxetine or 5-hydroxytryptophan significantly increased intracellular serotonin concentrations and subsequently increased PTHrP gene expression, which was reduced with transglutaminase inhibition. Furthermore, we determined that transglutaminase activity is increased with lactogenic differentiation and 5-hydroxytryptophan or fluoxetine treatment. We investigated whether RhoA, Rac1, and Rab4 were potential serotonylation target proteins. We speculate that RhoA is potentially a serotonylation target protein. Our data suggest that serotonin regulates PTHrP induction in part through the process of serotonylation under lactogenic conditions in mouse mammary epithelial cells.

Increased Transglutaminase 2 Expression and Activity in Rodent Models of Obesity/Metabolic Syndrome and Aging

Diastolic dysfunction of the heart and decreased compliance of the vasculature and lungs (i.e., increased organ tissue stiffness) are known features of obesity and the metabolic syndrome. Similarly, cardiac diastolic dysfunction is associated with aging. Elevation of the enzyme transglutaminase 2 (TG2) leads to protein cross-linking and enhanced collagen synthesis and participates as a candidate pathway for development of tissue stiffness. With these observations in mind we hypothesized that TG2 may be elevated in tissues of a rat model of obesity/metabolic syndrome (the ZSF 1 rat) and a mouse model of aging, i.e., the senescent SAMP8 mouse. In the experiments reported here, TG2 expression and activity were found for the first time to be spontaneously elevated in organs from both the ZSF1 rat and the SAMP8 mouse. These observations are consistent with a hypothesis that a TG2-related pathway may participate in the known tissue stiffness associated with cardiac diastolic dysfunction in these two rodent models. The potential TG2 pathway needs better correlation with physiologic dysfunction and may eventually provide novel therapeutic insights to improve tissue compliance.

Role of TG2-Mediated SERCA2 Serotonylation on Hypoxic Pulmonary Vein Remodeling

Sarco-endoplasmic reticulum Ca2+ ATPase (SERCA) pumps take up Ca2+ from the cytoplasm to maintain the balance of intracellular Ca2+. A decline in expression or activity of SERCA results in persistent store-operated calcium entry (SOCE). In cardiomyocytes as well as vascular smooth muscle cells (SMCs), SERCA2 acts as an important regulator of calcium cycling. The purpose of this study is to identify and better understand the role of transglutaminases2 (TG2) as a key factor involved in SERCA2 serotonination (s-SERCA2) and to elucidate the underlying mechanism of action. Human pulmonary venous smooth muscle cell in normal pulmonary lobe were isolated and cultured in vitro. Establishment of hypoxic pulmonary hypertension model in wild type and TG2 knockout mice. SERCA2 serotonylation was analyzed by co-(immunoprecipitation) IP when the TG2 gene silenced or overexpressed under normoxia and hypoxia in vivo and in vitro. Intracellular calcium ion was measured by using Fluo-4AM probe under normoxia and hypoxia. Real-time (RT)-PCR and Western blot analyzed expression of TG2, TRPC1, and TRPC6 under normoxia and hypoxia. Bioactivity of cells were analyzed by using Cell Counting Kit (CCK)-8, flow cytometry, wound healing, RT-PCR, and Western blot under PST-2744 and cyclopiazonic acid. We confirmed that 1) hypoxia enhanced the expression and activity of TG2, and 2) hypoxia increased the basal intracellular Ca2+ concentration ([Ca2+]i) and SOCE through activating TRPC6 on human pulmonary vein smooth muscle cells (hPVSMC). Then, we investigated the effects of overexpression and downregulation of the TG2 gene on the activity of SERCA2, s-SERCA2, basal [Ca2+]i, and SOCE under normoxia and hypoxia in vitro, and investigated the activity of SERCA2 and s-SERCA2 in vivo, respectively. We confirmed that SERCA2 serotonylation inhibited the activity of SERCA2 and increased the Ca2+ influx, and that hypoxia induced TG2-mediated SERCA2 serotonylation both in vivo and in vitro. Furthermore, we investigated the effect of TG2 activity on the biological behavior of hPVSMC by using an inhibitor and agonist of SERCA2, respectively. Finally, we confirmed that chronic hypoxia cannot increase vessel wall thickness, the right ventricular systolic pressure (RVSP), and right ventricular hypertrophy index (RVHI) of vascular smooth muscle-specific Tgm2−/− mice. These results indicated that hypoxia promoted TG2-mediated SERCA2 serotonylation, thereby leading to inhibition of SERCA2 activity, which further increased the calcium influx through the TRPC6 channel. Furthermore, tissue-specific conditional TG2 knockout mice prevents the development of pulmonary hypertension caused by hypoxia. In summary, we uncovered a new target (TG2) for treatment of chronic hypoxic pulmonary hypertension (CHPH).

Glycolysis regulated transglutaminase 2 activation in cardiopulmonary fibrogenic remodeling

The pathophysiology of pulmonary hypertension (PH) and heart failure (HF) includes fibrogenic remodeling associated with the loss of pulmonary arterial (PA) and cardiac compliance. We and others have previously identified transglutaminase 2 (TG2) as a participant in adverse fibrogenic remodeling. However, little is known about the biologic mechanisms that regulate TG2 function. We examined physiological mouse models of experimental PH, HF, and type 1 diabetes that are associated with altered glucose metabolism/glycolysis and report here that TG2 expression and activity are elevated in pulmonary and cardiac tissues under all these conditions. We additionally used PA adventitial fibroblasts to test the hypothesis that TG2 is an intermediary between enhanced tissue glycolysis and fibrogenesis. Our in vitro results show that glycolytic enzymes and TG2 are upregulated in fibroblasts exposed to high glucose, which stimulates cellular glycolysis as measured by Seahorse analysis. We examined the relationship of TG2 to a terminal glycolytic enzyme, pyruvate kinase M2 (PKM2), and found that PKM2 regulates glucose-induced TG2 expression and activity as well as fibrogenesis. Our studies further show that TG2 inhibition blocks glucose-induced fibrogenesis and cell proliferation. Our findings support a novel role for glycolysis-mediated TG2 induction and tissue fibrosis associated with experimental PH, HF, and hyperglycemia.

Serotonylation: Serotonin Signaling and Epigenetics

Serotonylation, the covalent linkage of serotonin to proteins has been discovered more than 60 years ago but only recently the mechanisms and first functions have been elucidated. It has been found that transglutaminases (TG) such as TG2 and the blood coagulation factor XIIIa are the enzymes which catalyze the linkage of serotonin and other monoamines to distinct glutamine (Gln) residues of target proteins. The first target proteins, small G-proteins and extracellular matrix constituents, were found in platelets and are pivotally involved in platelet aggregation and the formation of thrombi. The serotonylation of the same proteins is also involved in insulin secretion and in the proliferation of pulmonary vascular smooth muscle cells and thereby in the pathogenesis of pulmonary arterial hypertension (PAH). Recently histones have been described as targets of serotonylation opening the area of transcriptional control to this posttranslational protein modification. Future studies will certainly reveal further target proteins, signaling pathways, cellular processes, and diseases, in which serotonylation or, more general, monoaminylation is important.

Inhibition of serotonin synthesis: A novel therapeutic paradigm

The rate-limiting enzyme in serotonin synthesis is tryptophan hydroxylase (TPH). There are two independent serotonin systems in the body characterized by two isoforms of TPH, TPH1 and TPH2. While TPH2 synthesizes serotonin in the brain, TPH1 is expressed in the gut and in other peripheral tissues and supplies platelets in the circulation with serotonin. This duality of the serotonin system is enforced by the blood-brain barrier which is impermeable for serotonin. In the brain serotonin acts as neurotransmitter and is a main target for the treatment of psychiatric disorders. In the periphery it is released by platelets at the site of activation and elicits numerous physiological effects. TPH1 deficient mice were shown to be protected from diverse diseases including hemostatic, inflammatory, fibrotic, gastrointestinal, and metabolic disorders and therefore serotonin synthesis inhibition emerged as a reasonable therapeutic paradigm. Recently the first TPH inhibitor, telotristat ethyl, came on the market for the treatment of carcinoid syndrome. This review summarizes the state of development and the therapeutic opportunities of such compounds.

Targeting transglutaminase 2 partially restores extracellular matrix structure but not alveolar architecture in experimental bronchopulmonary dysplasia

The generation, maturation and remodelling of the extracellular matrix (ECM) are essential for the formation of alveoli during lung development. Alveoli formation is disturbed in preterm infants that develop bronchopulmonary dysplasia (BPD), where collagen fibres are malformed, and perturbations to lung ECM structures may underlie BPD pathogenesis. Malformed ECM structures might result from abnormal protein cross-linking, in part attributable to the increased expression and activity of transglutaminase 2 (TGM2) that have been noted in affected patient lungs, as well as in hyperoxia-based BPD animal models. The objective of the present study was to assess whether TGM2 plays a causal role in normal and aberrant lung alveolarization. Targeted deletion of Tgm2 in C57BL/6J mice increased septal thickness and reduced gas-exchange surface area in otherwise normally developing lungs. During aberrant lung alveolarization that occurred under hyperoxic conditions, collagen structures in Tgm2^-/- mice were partially protected from the impact of hyperoxia, where normal dihydroxylysinonorleucine and hydroxylysylpiridinoline collagen cross-link abundance was restored; however, the lung alveolar architecture remained abnormal. Inhibition of transglutaminases (including TGM2) with cysteamine appreciably reduced transglutaminase activity in vivo, as assessed by N^ε -(γ-l-glutamyl)-l-lysine abundance and TGM catalytic activity, and restored normal dihydroxylysinonorleucine and hydroxylysylpiridinoline collagen cross-link abundance under pathological conditions. Furthermore, a moderate improvement in alveoli size and gas-exchange surface density was noted in cysteamine-treated mouse lungs in which BPD was modelled. These data indicate that TGM2 plays a role in normal lung alveolarization, and contributes to the formation of aberrant ECM structures during disordered lung alveolarization.

Adrenal serotonin derives from accumulation by the antidepressant-sensitive serotonin transporter

Adrenal chromaffin cells comprise the neuroendocrine arm of the sympathetic nervous system and secrete catecholamines to coordinate the appropriate stress response. Deletion of the serotonin (5-HT) transporter (SERT) gene in mice (SERT^-/- mice) or pharmacological block of SERT function in rodents and humans augments this sympathoadrenal stress response (epinephrine secretion). The prevailing assumption is that loss of CNS SERT alters central drive to the peripheral sympathetic nervous system. Adrenal chromaffin cells also prominently express SERT where it might coordinate accumulation of 5-HT for reuse in the autocrine control of stress-evoked catecholamine secretion. To help test this hypothesis, we have generated a novel mouse model with selective excision of SERT in the peripheral sympathetic nervous system (SERT^ΔTH), generated by crossing floxed SERT mice with tyrosine hydroxylase Cre driver mice. SERT expression, assessed by western blot, was abolished in the adrenal gland but not perturbed in the CNS of SERT^ΔTH mice. SERT-mediated [³H] 5-HT uptake was unaltered in midbrain, hindbrain, and spinal cord synaptosomes, confirming transporter function was intact in the CNS. Endogenous midbrain and whole blood 5-HT homeostasis was unperturbed in SERT^ΔTH mice, contrasting with the depleted 5-HT content in SERT^-/- mice. Selective SERT excision reduced adrenal gland 5-HT content by ≈ 50% in SERT^ΔTH mice but had no effect on adrenal catecholamine content. This novel model confirms that SERT expressed in adrenal chromaffin cells is essential for maintaining wild-type levels of 5-HT and provides a powerful tool to help dissect the role of SERT in the sympathetic stress response.

The serotonin hypothesis in pulmonary hypertension revisited: targets for novel therapies (2017 Grover Conference Series)

Increased synthesis of serotonin and/or activity of serotonin in pulmonary arteries has been implicated in the pathobiology of pulmonary arterial hypertension (PAH). The incidence of PAH associated with diet pills such as aminorex, fenfluramine, and chlorphentermine initially led to the “serotonin hypothesis of pulmonary hypertension.” Over the last couple of decades there has been an accumulation of convincing evidence that targeting serotonin synthesis or signaling is a novel and promising approach to the development of novel therapies for PAH. Pulmonary endothelial serotonin synthesis via tryptophan hydroxlase 1 (TPH1) is increased in patients with PAH and serotonin can act in a paracrine fashion on underlying pulmonary arterial smooth muscle cells (PASMCs), In humans, serotonin can enter PASMCs via the serotonin transporter (SERT) or activate the 5-HT1B receptor; 5-HT1B activation and SERT activity cooperate to induce PASMC contraction and proliferation via activation of downstream proliferative and contractile signaling pathways. Here we will review the current status of the serotonin hypothesis and discuss potential and novel therapeutic targets.

Mitochondrial dysfunction and pulmonary hypertension: cause, effect, or both

Pulmonary hypertension describes a heterogeneous disease defined by increased pulmonary artery pressures, and progressive increase in pulmonary vascular resistance due to pathologic remodeling of the pulmonary vasculature involving pulmonary endothelial cells, pericytes, and smooth muscle cells. This process occurs under various conditions, and although these populations vary, the clinical manifestations are the same: progressive dyspnea, increases in right ventricular (RV) afterload and dysfunction, RV-pulmonary artery uncoupling, and right-sided heart failure with systemic circulatory collapse. The overall estimated 5-yr survival rate is 72% in highly functioning patients, and as low as 28% for those presenting with advanced symptoms. Metabolic theories have been suggested as underlying the pathogenesis of pulmonary hypertension with growing evidence of the role of mitochondrial dysfunction involving the major proteins of the electron transport chain, redox-related enzymes, regulators of the proton gradient and calcium homeostasis, regulators of apoptosis, and mitophagy. There remain more studies needed to characterize mitochondrial dysfunction leading to impaired vascular relaxation, increase proliferation, and failure of regulatory mechanisms. The effects on endothelial cells and resulting interactions with their microenvironment remain uncharted territory for future discovery. Additionally, on the basis of observations that the "plexigenic lesions" of pulmonary hypertension resemble the unregulated proliferation of tumor cells, similarities between cancer pathobiology and pulmonary hypertension have been drawn, suggesting interactions between mitochondria and angiogenesis. Recently, mitochondria targeting has become feasible, which may yield new therapeutic strategies. We present a state-of-the-art review of the role of mitochondria in both the pathobiology of pulmonary hypertension and potential therapeutic targets in pulmonary vascular processes.

Transglutaminase 2 in pulmonary and cardiac tissue remodeling in experimental pulmonary hypertension

Tissue matrix remodeling and fibrosis leading to loss of pulmonary arterial and right ventricular compliance are important features of both experimental and clinical pulmonary hypertension (PH). We have previously reported that transglutaminase 2 (TG2) is involved in PH development while others have shown it to be a cross-linking enzyme that participates in remodeling of extracellular matrix in fibrotic diseases in general. In the present studies, we used a mouse model of experimental PH (Sugen 5416 and hypoxia; SuHypoxia) and cultured primary human cardiac and pulmonary artery adventitial fibroblasts to evaluate the relationship of TG2 to the processes of fibrosis, protein cross-linking, extracellular matrix collagen accumulation, and fibroblast-to-myofibroblast transformation. We report here that TG2 expression and activity as measured by serotonylated fibronectin and protein cross-linking activity along with fibrogenic markers are significantly elevated in lungs and right ventricles of SuHypoxic mice with PH. Similarly, TG2 expression and activity, protein cross-linking activity, and fibrogenic markers are significantly increased in cultured cardiac and pulmonary artery adventitial fibroblasts in response to hypoxia exposure. Pharmacological inhibition of TG2 activity with ERW1041E significantly reduced hypoxia-induced cross-linking activity and synthesis of collagen 1 and α-smooth muscle actin in both the in vivo and in vitro studies. TG2 short interfering RNA had a similar effect in vitro. Our results suggest that TG2 plays an important role in hypoxia-induced pulmonary and right ventricular tissue matrix remodeling in the development of PH.

Optimization of spirocyclic proline tryptophan hydroxylase-1 inhibitors

As a follow-up to the discovery of our spirocyclic proline-based TPH1 inhibitor lead, we describe the optimization of this scaffold. Through a combination of X-ray co-crystal structure guided design and an in vivo screen, new substitutions in the lipophilic region of the inhibitors were identified. This effort led to new TPH1 inhibitors with in vivo efficacy when dosed as their corresponding ethyl ester prodrugs. In particular, 15b (KAR5585), the prodrug of the potent TPH1 inhibitor 15a (KAR5417), showed robust reduction of intestinal serotonin (5-HT) levels in mice. Furthermore, oral administration of 15b generated high and sustained systemic exposure of the active parent 15a in rats and dogs. KAR5585 was selected for further pharmacological evaluation in disease models associated with a dysfunctional peripheral 5-HT system.

Tryptophan hydroxylase 1 Inhibition Impacts Pulmonary Vascular Remodeling in Two Rat Models of Pulmonary Hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease defined by a chronic elevation in pulmonary arterial pressure with extensive pulmonary vascular remodeling and perivascular inflammation characterized by an accumulation of macrophages, lymphocytes, dendritic cells, and mast cells. Although the exact etiology of the disease is unknown, clinical as well as preclinical data strongly implicate a role for serotonin (5-HT) in the process. Here, we investigated the chronic effects of pharmacological inhibition of tryptophan hydroxylase 1 (TPH1), the rate-limiting enzyme in peripheral 5-HT biosynthesis, in two preclinical models of pulmonary hypertension (PH), the monocrotaline (MCT) rat and the semaxanib (SUGEN, Medinoah, Suzhou, China)-hypoxia rat. In both PH models, ethyl (S)-8-(2-amino-6-((R)-1-(5-chloro-[1,1'-biphenyl]-2-yl)-2,2,2-trifluoroethoxy)pyrimidin-4-yl)-2,8-diazaspiro[4.5]decane-3-carboxylate and ethyl (S)-8-(2-amino-6-((R)-1-(3',4'-dimethyl-3-(3-methyl-1 H-pyrazol-1-yl)-[1,1'-biphenyl]-4-yl)-2,2,2-trifluoroethoxy)pyrimidin-4-yl)-2,8-diazaspiro[4.5]decane-3-carboxylate, novel orally active TPH1 inhibitors with nanomolar in vitro potency, decreased serum, gut, and lung 5-HT levels in a dose-dependent manner and significantly reduced pulmonary arterial pressure, and pulmonary vessel wall thickness and occlusion in male rats. In the MCT rat model, decreases in lung 5-HT significantly correlated with reductions in histamine levels and mast cell number (P < 0.001, r² = 0.88). In contrast, neither ambrisentan nor tadalafil, which are vasodilators approved for the treatment of PAH, reduced mast cell number or 5-HT levels, nor were they as effective in treating the vascular remodeling as were the TPH1 inhibitors. When administered in combination with ambrisentan, the TPH1 inhibitors showed an additive effect on pulmonary vascular remodeling and pressures. These data demonstrate that in addition to reducing vascular remodeling, TPH1 inhibition has the added benefit of reducing the perivascular mast cell accumulation associated with PH.

Emerging concepts in smooth muscle contributions to airway structure and function: implications for health and disease

Airway structure and function are key aspects of normal lung development, growth, and aging, as well as of lung responses to the environment and the pathophysiology of important diseases such as asthma, chronic obstructive pulmonary disease, and fibrosis. In this regard, the contributions of airway smooth muscle (ASM) are both functional, in the context of airway contractility and relaxation, as well as synthetic, involving production and modulation of extracellular components, modulation of the local immune environment, cellular contribution to airway structure, and, finally, interactions with other airway cell types such as epithelium, fibroblasts, and nerves. These ASM contributions are now found to be critical in airway hyperresponsiveness and remodeling that occur in lung diseases. This review emphasizes established and recent discoveries that underline the central role of ASM and sets the stage for future research toward understanding how ASM plays a central role by being both upstream and downstream in the many interactive processes that determine airway structure and function in health and disease.

Transglutaminase is a Critical Link Between Inflammation and Hypertension

Background

The pathogenesis of essential hypertension is multifactorial with different underlying mechanisms contributing to disease. We have recently shown that TNF superfamily member 14 LIGHT (an acronym for homologous to lymphotoxins, exhibits inducible expression, and competes with herpes simplex virus glycoprotein D for herpes virus entry mediator, a receptor expressed by T lymphocytes, also known as TNFSF14) induces hypertension when injected into mice. Research reported here was undertaken to examine the role of transglutaminase (TGase) in LIGHT‐induced hypertension.

Methods and Results

Initial experiments showed that plasma and kidney TGase activity was induced by LIGHT infusion (13.91±2.92 versus 6.75±1.92 mU/mL and 19.86±3.55 versus 12.00±0.97 mU/10 μg) and was accompanied with hypertension (169±7.16 versus 117.17±11.57 mm Hg at day 14) and renal impairment (proteinuria, 61.33±23.21 versus 20.38±9.01 μg/mg; osmolality, 879.57±93.02 versus 1407.2±308.04 mmol/kg). The increase in renal TGase activity corresponded to an increase in RNA for the tissue TGase isoform, termed TG2. Pharmacologically, we showed that LIGHT‐induced hypertension and renal impairment did not occur in the presence of cystamine, a well‐known competitive inhibitor of TGase activity. Genetically, we showed that LIGHT‐mediated induction of TGase, along with hypertension and renal impairment, was dependent on interleukin‐6 and endothelial hypoxia inducible factor‐1α. We also demonstrated that interleukin‐6, endothelial hypoxia inducible factor‐1α, and TGase are required for LIGHT‐induced production of angiotensin receptor agonistic autoantibodies.

Conclusions

Thus, LIGHT‐induced hypertension, renal impairment, and production of angiotensin receptor agonistic autoantibodies require TGase, most likely the TG2 isoform. Our findings establish TGase as a critical link between inflammation, hypertension, and autoimmunity.

Role of tissue transglutaminase-2 (TG2)-mediated aminylation in biological processes

Post-translational modification (PTM) is an important mechanism in modulating a protein's structure and can lead to substantial diversity in biological function. Compared to other forms of PTMs such as phosphorylation, acetylation and glycosylation, the physiological significance of aminylation is limited. Aminylation refers to the covalent incorporation of biogenic/polyamines into target protein by calcium-dependent transglutaminases (TGs). The development of novel and more sensitive techniques has led to more proteins identified as tissue transglutaminase (TG2) substrates and potential targets for aminylation. Many of these substrate proteins play a role in cell signaling, cytoskeleton organization, muscle contraction, and inflammation. TG2 is well studied and widely expressed in a variety of tissues and will be the primary focus of this review on recent advance in transglutaminase-mediated aminylation.

The Extracellular Matrix in Bronchopulmonary Dysplasia: Target and Source

Bronchopulmonary dysplasia (BPD) is a common complication of preterm birth that contributes significantly to morbidity and mortality in neonatal intensive care units. BPD results from life-saving interventions, such as mechanical ventilation and oxygen supplementation used to manage preterm infants with acute respiratory failure, which may be complicated by pulmonary infection. The pathogenic pathways driving BPD are not well-delineated but include disturbances to the coordinated action of gene expression, cell-cell communication, physical forces, and cell interactions with the extracellular matrix (ECM), which together guide normal lung development. Efforts to further delineate these pathways have been assisted by the use of animal models of BPD, which rely on infection, injurious mechanical ventilation, or oxygen supplementation, where histopathological features of BPD can be mimicked. Notable among these are perturbations to ECM structures, namely, the organization of the elastin and collagen networks in the developing lung. Dysregulated collagen deposition and disturbed elastin fiber organization are pathological hallmarks of clinical and experimental BPD. Strides have been made in understanding the disturbances to ECM production in the developing lung, but much still remains to be discovered about how ECM maturation and turnover are dysregulated in aberrantly developing lungs. This review aims to inform the reader about the state-of-the-art concerning the ECM in BPD, to highlight the gaps in our knowledge and current controversies, and to suggest directions for future work in this exciting and complex area of lung development (patho)biology.

The pathophysiology of pulmonary hypertension in left heart disease

Pulmonary hypertension (PH) is characterized by elevated pulmonary arterial pressure leading to right-sided heart failure and can arise from a wide range of etiologies. The most common cause of PH, termed Group 2 PH, is left-sided heart failure and is commonly known as pulmonary hypertension with left heart disease (PH-LHD). Importantly, while sharing many clinical features with pulmonary arterial hypertension (PAH), PH-LHD differs significantly at the cellular and physiological levels. These fundamental pathophysiological differences largely account for the poor response to PAH therapies experienced by PH-LHD patients. The relatively high prevalence of this disease, coupled with its unique features compared with PAH, signal the importance of an in-depth understanding of the mechanistic details of PH-LHD. The present review will focus on the current state of knowledge regarding the pathomechanisms of PH-LHD, highlighting work carried out both in human trials and in preclinical animal models. Adaptive processes at the alveolocapillary barrier and in the pulmonary circulation, including alterations in alveolar fluid transport, endothelial junctional integrity, and vasoactive mediator secretion will be discussed in detail, highlighting the aspects that impact the response to, and development of, novel therapeutics.

Tissue transglutaminase promotes serotonin-induced AKT signaling and mitogenesis in pulmonary vascular smooth muscle cells

Tissue transglutaminase 2 (TG2) is a multifunctional enzyme that cross-links proteins with monoamines such as serotonin (5-hydroxytryptamine, 5-HT) via a transglutamidation reaction, and is associated with pathophysiologic vascular responses. 5-HT is a mitogen for pulmonary artery smooth muscle cells (PASMCs) that has been linked to pulmonary vascular remodeling underlying pulmonary hypertension development. We previously reported that 5-HT-induced PASMC proliferation is inhibited by the TG2 inhibitor monodansylcadaverine (MDC); however, the mechanisms are poorly understood. In the present study we hypothesized that TG2 contributes to 5-HT-induced signaling pathways of PASMCs. Pre-treatment of bovine distal PASMCs with varying concentrations of the inhibitor MDC led to differential inhibition of 5-HT-stimulated AKT and ROCK activation, while p-P38 was unaffected. Concentration response studies showed significant inhibition of AKT activation at 50 μM MDC, along with inhibition of the AKT downstream targets mTOR, p-S6 kinase and p-S6. Furthermore, TG2 depletion by siRNA led to reduced 5-HT-induced AKT activation. Immunoprecipitation studies showed that 5-HT treatment led to increased levels of serotonylated AKT and increased TG2-AKT complex formations which were inhibited by MDC. Overexpression of TG2 point mutant cDNAs in PASMCs showed that the TG2 C277V transamidation mutant blunted 5-HT-induced AKT activation and 5-HT-induced PASMC mitogenesis. Finally, 5-HT-induced AKT activation was blunted in SERT genetic knock-out rat cells, but not in their wild-type counterpart. The SERT inhibitor imipramine similarly blocked AKT activation. These results indicate that TG2 contributes to 5-HT-induced distal PASMC proliferation via promotion of AKT signaling, likely via its serotonylation. Taken together, these results provide new insight into how TG2 may participate in vascular smooth muscle remodeling.

Role of hypoxia-induced transglutaminase 2 in pulmonary artery smooth muscle cell proliferation

We previously reported that transglutaminase 2 (TG2) activity is markedly elevated in lungs of hypoxia-exposed rodent models of pulmonary hypertension (PH). Since vascular remodeling of pulmonary artery smooth muscle cells (PASMCs) is important in PH, we undertook the present study to determine whether TG2 activity is altered in PASMCs with exposure to hypoxia and whether that alteration participates in their proliferative response to hypoxia. Cultured distal bovine (b) and proximal human (h) PASMCs were exposed to hypoxia (3% O2) or normoxia (21% O2). mRNA and protein expression were determined by PCR and Western blot analyses. TG2 activity and function were visualized and determined by fluorescent labeled 5-pentylamine biotin incorporation and immunoblotting of serotonylated fibronectin. Cell proliferation was assessed by [(3)H]thymidine incorporation assay. At 24 h, both TG2 expression and activity were stimulated by hypoxia in bPASMCs. Activation of TG2 by hypoxia was blocked by inhibition of the extracellular calcium-sensing receptor or the transient receptor potential channel V4. In contrast, TG2 expression was blocked by inhibition of the transcription factor hypoxia-inducible factor-1α, supporting the presence of separate mechanisms for stimulation of activity and expression of TG2. Pulmonary arterial hypertension patient-derived hPASMCs were found to proliferate significantly more rapidly and respond to hypoxia more strongly than control-derived hPASMCs. Similar to bovine cells, hypoxia-induced proliferation of patient-derived cells was blocked by inhibition of TG2 activity. Our results suggest an important role for TG2, mediated by intracellular calcium fluxes and HIF-1α, in hypoxia-induced PASMC proliferation and possibly in vascular remodeling in PH.