Transglutaminase 2-mediated serotonylation in pulmonary hypertension.

The monoamine serotonin (5-HT) has been previously implicated in pulmonary arterial remodeling and is considered a potential therapeutic target for the disease pulmonary arterial hypertension (PAH). More recently, it has been recognized that the enzyme tissue transglutaminase (TG2) mediates cross-linking of proteins with 5-HT, a posttranslational process of monoaminylation known as "serotonylation." TG2 activity and serotonylation of protein participate in both smooth muscle proliferation and contraction produced by 5-HT. Indeed, markedly increased TG2 activity has now been identified in lung tissue of an experimental rodent model of pulmonary hypertension, and elevated serotonylation of fibronectin and the signaling molecule Rho, downstream products of transglutamidation, have been found in blood of patients with PAH. The basic mechanism by which TG2 is activated and the potential role(s) of serotonylated proteins in pulmonary hypertension remain a mystery. In the present review we have tried to address the current understanding of 5-HT metabolism in pulmonary hypertension and relate it to what is currently known about the evolving cellular process of serotonylation.

Phospholipase D signaling in serotonin-induced mitogenesis of pulmonary artery smooth muscle cells.

We have previously reported the participation of mitogen-activated protein, Rho, and phosphoinositide-3 (PI3) kinases in separate pathways in serotonin (5-HT)-induced proliferation of pulmonary artery smooth muscle cells (SMCs). In this study, we investigated the possible participation of phospholipase D (PLD) and phosphatidic acid (PA) in this growth process. 5-HT stimulated a time-dependent increase in [(3)H]phosphatidylbutanol and PA generation. Exposure of SMCs to 1-butanol or overexpression of an inactive mutant of human PLD1R898R blocked 5-HT-induced proliferation. Furthermore, 1-butanol inhibited 5-HT activation of S6K1 and S6 protein, downstream effectors of mammalian target of rapamycin (mTOR), by 80 and 72%, respectively, and partially blocked activation of extracellular signal-regulated kinase (ERK) by 30% but had no effect on other associated signaling pathways. Exogenous PA caused cellular proliferation and revitalized cyclin D1 expression by 5-HT of the 1-butanol-treated cells. PA also reproduced activations by 5-HT of mTOR, S6K1, and ERK. Transfection with inactive human PLD1 reduced 5-HT-induced activation of S6K1 by approximately 50%. Inhibition of 5-HT receptor 2A (R 2A) with ketaserin blocked PLD activation by 5-HT. Inhibition with PI3-kinase inhibitor failed to block either activation of PLD by 5-HT or PA-dependent S6K1 phosphorylation. Taken together, these results indicate that ligation of the 5-HTR 2A by 5-HT initiates PLD activation in SMCs, and that its product, PA, is an early signaling molecule in 5-HT-induced pulmonary artery SMC proliferation. Signaling by PA produces its downstream effects primarily through the mTOR/S6K1 pathway and to a lesser extent through the ERK pathway. Hydrolysis of cell membrane lipid may be important in vascular effects of 5-HT.

Plasma antioxidants in subjects before hematopoietic stem cell transplantation.

UNLABELLED: Chemo-irradiation induced oxidative damage to vascular endothelium may contribute to pulmonary complications of hematopoietic stem cell transplantation (HSCT). We measured antioxidants, markers of oxidative stress and plasma antioxidant capacity in plasma or serum from 24 subjects at day 7 before HSCT and 20 control subjects. The plasma concentration of extracellular glutathione peroxidase (GPX-3) was significantly reduced in the HSCT subjects compared with controls (HSCT: 98+/-42 microg/ml, control: 169+/-56 microg/ml, P<0.0001). The concentration of gamma-tocopherol was significantly higher in the HSCT subjects compared with controls (HSCT: 207+/-103 microg/dl; CONTROL: 98+/-52 microg/dl; P=0.0002). The plasma concentrations of protein carbonyl, nitrotyrosine, malondialdehyde, alpha-tocopherol, vitamin A, homocysteine, cysteine and cysteinylglycine did not differ between HSCT and control subjects. Plasma from HSCT subjects was as effective as control plasma in quenching menadione-induced intracellular reactive oxygen species production in human microvascular endothelial cells. In summary, subjects before HSCT have significantly reduced plasma concentrations of GPX-3, elevated plasma gamma-tocopherol yet retains the ability to quench an acute oxidative stress. These changes may play a role in chronic oxidative stress in the HSCT population.

Regulation of PDGF production and ERK activation by estrogen is associated with TSC2 gene expression.

Mechanisms that regulate the growth response to estrogen (17beta-estradiol, E2) are poorly understood. Recently, loss of function of the tuberous sclerosis complex 2 (TSC2) gene has been associated with E2-related conditions that are characterized by benign cellular proliferation. We examined the growth response to E2 in vascular smooth muscle cells (VSMCs) that possess wild-type TSC2 and compared them with ELT-3 smooth muscle cells that do not express TSC2. In TSC2-expressing VSMCs, growth inhibition in response to E2 was associated with downregulation of platelet-derived growth factor (PDGF), PDGF receptor (PDGFR), and limited activation of extracellular signal-regulated kinase (ERK). In contrast, the growth-promoting effect of E2 in TSC2-null ELT-3 cells was associated with induction of PDGF, robust phosphorylation of PDGFR, and sustained activation of ERK. Furthermore, in ELT-3 cells, cellular growth and ERK activation by E2 were inhibited by the PDGFR inhibitor tyrphostin AG 17 and by PDGF-neutralizing antibody. These results demonstrate that autocrine production of PDGF and augmentation of the ERK pathway leads to estrogen-induced cellular proliferation in TSC2-null cells, a pathway that was downregulated in cells that express TSC2. Understanding the mechanisms that regulate the diverse responses to the steroid hormone estrogen could lead to novel approaches to the treatment of estrogen-related diseases that are characterized by aberrant cell proliferation.

Bleomycin upregulates gene expression of angiotensin-converting enzyme via mitogen-activated protein kinase and early growth response 1 transcription factor.

Pulmonary fibrosis is a progressive disorder characterized by the loss of alveolar architecture through epithelial and endothelial cell apoptosis and fibroblast proliferation. Recent studies showed that angiotensin-converting enzyme (ACE) activity is increased in fibrotic tissues, and ACE inhibitors administered in vivo ameliorate fibrosis, suggesting that ACE may play a critical role. However, the regulation of ACE expression is not well understood. In the present study, we demonstrate that bleomycin, a chemotherapeutic agent which induces pulmonary fibrosis in animals and humans, increases gene expression of ACE. Treatment of primary bovine pulmonary artery endothelial cells with 0.1 to 1.0 microg/ml bleomycin increased ACE enzymatic activity and ACE mRNA, as monitored by hippuryl-L-histidyl-L-leucine assay and competitive quantitative reverse transcriptase polymerase chain reaction (RT-PCR), respectively. Luciferase reporter constructs showed that upregulation of ACE transcription by bleomycin is mediated through element(s) in the 97-bp ACE promoter. Bleomycin activated p42/p44 mitogen-activated protein kinase (MAPK) and induced nuclear translocation and activation of the early growth response (Egr)-1 transcription factor, a factor previously shown to positively regulate ACE expression. The MAPK kinase1/2 (MEK1/2) inhibitor U0126 blocked MAPK and Egr-1 activation by bleomycin, suggesting that Egr-1 activation is MAPK dependent. These data provide the first evidence that bleomycin activates ACE gene expression through the MAPK pathway and Egr-1.

H(2)O(2) signals 5-HT-induced ERK MAP kinase activation and mitogenesis of smooth muscle cells.

Our previous studies have shown that 5-hydroxytryptamine (5-HT) induces cellular hyperplasia/hypertrophy through protein tyrosine phosphorylation, rapid formation of superoxide (O(2)(-)), and extracellular signal-regulated kinase (ERK)1/ERK2 mitogen-activated protein (MAP) kinase activation. Intracellularly released O(2)(-) is rapidly dismuted by superoxide dismutase (SOD) to H(2)O(2), another possible cellular growth mediator. In the present study, we assessed whether H(2)O(2) participates in 5-HT-induced mitogenic signaling. Inactivation of cellular Cu/Zn SOD by copper-chelating agents inhibited 5-HT-induced DNA synthesis of bovine pulmonary artery smooth muscle cells (BPASMCs). Infection of BPASMCs with an adenovirus containing catalase inhibited both ERK1/ERK2 MAP kinase activation and DNA synthesis induced by 5-HT. Although we could not find evidence of p38 MAP kinase activation by 5-HT, SB-203580 and SB-202190, reported inhibitors of p38 MAP kinase, inhibited the 5-HT-induced growth of BPASMCs. However, these inhibitors also inhibited 5-HT-induced O(2)(-) release. Thus quenching of O(2)(-) may be their mechanism for inhibition of cellular growth unrelated to p38 MAP kinase inhibition. These data indicate that generation of O(2)(-) in BPASMCs in response to 5-HT is followed by an increase in intracellular H(2)O(2) that mediates 5-HT-induced mitogenesis through activation of ERK1/ERK2 but not of p38 MAP kinase.

Oxidative protein cross-linking reactions involving L-tyrosine in transforming growth factor-beta1-stimulated fibroblasts.

The mechanisms by which ligand-stimulated generation of reactive oxygen species in nonphagocytic cells mediate biologic effects are largely unknown. The profibrotic cytokine, transforming growth factor-beta1 (TGF-beta1), generates extracellular hydrogen peroxide (H2O2) in contrast to intracellular reactive oxygen species production by certain mitogenic growth factors in human lung fibroblasts. To determine whether tyrosine residues in fibroblast-derived extracellular matrix (ECM) proteins may be targets of H2O2-mediated dityrosine-dependent cross-linking reactions in response to TGF-beta1, we utilized fluorophore-labeled tyramide, a structurally related phenolic compound that forms dimers with tyrosine, as a probe to detect such reactions under dynamic cell culture conditions. With this approach, a distinct pattern of fluorescent labeling that seems to target ECM proteins preferentially was observed in TGF-beta1-treated cells but not in control cells. This reaction required the presence of a heme peroxidase and was inhibited by catalase or diphenyliodonium (a flavoenzyme inhibitor), similar to the effect on TGF-beta1-induced dityrosine formation. Exogenous addition of H2O2 to control cells that do not release extracellular H2O2 produced a similar fluorescent labeling reaction. These results support the concept that, in the presence of heme peroxidases in vivo, TGF-beta1-induced H2O2 production by fibroblasts may mediate oxidative dityrosine-dependent cross-linking of ECM protein(s). This effect may be important in the pathogenesis of human fibrotic diseases characterized by overexpression/activation of TGF-beta1.

Reactive oxygen species in cell signaling.

Reactive oxygen species (ROS) are generated as by-products of cellular metabolism, primarily in the mitochondria. When cellular production of ROS overwhelms its antioxidant capacity, damage to cellular macromolecules such as lipids, protein, and DNA may ensue. Such a state of "oxidative stress" is thought to contribute to the pathogenesis of a number of human diseases including those of the lung. Recent studies have also implicated ROS that are generated by specialized plasma membrane oxidases in normal physiological signaling by growth factors and cytokines. In this review, we examine the evidence for ligand-induced generation of ROS, its cellular sources, and the signaling pathways that are activated. Emerging concepts on the mechanisms of signal transduction by ROS that involve alterations in cellular redox state and oxidative modifications of proteins are also discussed.

Regulation of STAT3 by direct binding to the Rac1 GTPase.

The signal transducers and activators of transcription (STAT) transcription factors become phosphorylated on tyrosine and translocate to the nucleus after stimulation of cells with growth factors or cytokines. We show that the Rac1 guanosine triphosphatase can bind to and regulate STAT3 activity. Dominant negative Rac1 inhibited STAT3 activation by growth factors, whereas activated Rac1 stimulated STAT3 phosphorylation on both tyrosine and serine residues. Moreover, activated Rac1 formed a complex with STAT3 in mammalian cells. Yeast two-hybrid analysis indicated that STAT3 binds directly to active but not inactive Rac1 and that the interaction occurs via the effector domain. Rac1 may serve as an alternate mechanism for targeting STAT3 to tyrosine kinase signaling complexes.

Ras-dependent and -independent regulation of reactive oxygen species by mitogenic growth factors and TGF-beta1.

Mitogenic growth factors and transforming growth factor beta1 (TGF-beta1) induce the generation of reactive oxygen species (ROS) in nonphagocytic cells, but their enzymatic source(s) and regulatory mechanisms are largely unknown. We previously reported on the ability of TGF-beta1 to activate a cell surface-associated NADH:flavin:O(2) oxidoreductase (NADH oxidase) that generates extracellular H(2)O(2). In this study, we compared the ROS-generating enzymatic systems activated by mitogenic growth factors and TGF-beta1 with respect to the primary reactive species produced (O(2)(.-) vs. H(2)O(2)), the site of generation (intracellular vs. extracellular) and regulation by Ras. We find that the mitogenic growth factors PDGF-BB, FGF-2, and TGF-alpha (an EGF receptor ligand) are able to rapidly (within 5 min) induce the generation of intracellular O(2)(.-) without detectable NADH oxidase activity or extracellular H(2)O(2) release. In contrast, TGF-beta1 does not stimulate intracellular O(2)(.-) production and the delayed induction of extracellular H(2)O(2) release is not associated with O(2)(.-) production. Expression of dominant-negative Ras (N17Ras) protein by herpes simplex virus-mediated gene transfer blocks mitogen-stimulated intracellular O(2)(.-) generation but has no effect on TGF-beta1-induced NADH oxidase activation/H(2)O(2) production. These results demonstrate that there are at least two distinctly different ROS-generating enzymatic systems in lung fibroblasts regulated by mitogenic growth factors and TGF-beta1 via Ras-dependent and -independent mechanisms, respectively. In addition, these findings suggest that endogenous production of ROS by growth factors/cytokines may have different biological effects depending on the primary reactive species generated and site of production.

Transforming growth factor-beta 1-induced activation of the ERK pathway/activator protein-1 in human lung fibroblasts requires the autocrine induction of basic fibroblast growth factor.

Transforming growth factor-beta (TGF-beta) is involved in multiple processes including cell growth and differentiation. In particular, TGF-beta has been implicated in the pathogenesis of fibrotic lung diseases. In this study, we examined regulation of the mitogen-activated protein kinase pathway by TGF-beta1 in primary human lung fibroblasts. TGF-beta1 treatment resulted in extracellular signal-regulated kinase (ERK) pathway activation in a delayed manner, with maximal activity at 16 h. ERK activation occurred concomitantly with the induction of activator protein-1 (AP-1) binding, a nuclear factor required for activation of multiple genes involved in fibrosis. AP-1 binding was dependent on ERK activation, since the MEK-1 (mitogen-activated protein kinase kinase) inhibitor PD98059 inhibited TGF-beta1-induced binding. Induction of the receptor tyrosine kinase-linked growth factor, basic fibroblast growth factor (bFGF) protein expression temporally paralleled the activation of ERK/AP-1. Induction of AP-1 by TGF-beta1-conditioned medium was observed at 2 h, similar to AP-1 induction in response to exogenous bFGF. Dependence of ERK/AP-1 activation on bFGF induction was demonstrated by inhibition of TGF-beta1-induced ERK/AP-1 activation when conditioned medium from TGF-beta1-treated cells was incubated with bFGF-neutralizing antibody. Together, these results demonstrate that TGF-beta1 regulates the autocrine induction of bFGF, resulting in activation of the ERK mitogen-activated protein kinase pathway and induction of AP-1 binding.

Serotonin stimulates mitogen-activated protein kinase activity through the formation of superoxide anion.

Our previous studies have shown that, through an active transport process, serotonin (5-HT) rapidly elevates O(-)(2). formation, stimulates protein phosphorylation, and enhances proliferation of bovine pulmonary artery smooth muscle cells (SMCs). We presently show that 1 microM 5-HT also rapidly elevates phosphorylation and activation of the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase (ERK) 1 and ERK2 of SMCs, and the enhanced phosphorylation is blocked by the antioxidants Tiron, N-acetyl-L-cysteine (NAC), and Ginkgo biloba extract. Inhibition of MAP kinase with PD-98059 failed to block enhanced O(-)(2). formation by 5-HT. Chinese hamster lung fibroblasts (CCL-39 cells), which demonstrate both 5-HT transporter and receptor activity, showed a similar response to 5-HT (i.e., enhanced mitogenesis, O(-)(2). formation, and ERK1 and ERK2 phosphorylation and activation). Unlike SMCs, they also responded to 5-HT receptor agonists. We conclude that downstream signaling of MAP kinase is a generalized cellular response to 5-HT that occurs secondary to O(-)(2). formation and may be initiated by either the 5-HT transporter or receptor depending on the cell type.

Reduction of endothelial cell related TGFbeta activity by thiols.

Transforming growth factor beta (TGFbeta) may play an important role in diseases characterized by pulmonary fibrosis. We have previously demonstrated that thiols inhibit the pro-oxidant effects of TGFbeta1 in bovine pulmonary artery endothelial cells (BPAEC). To help define the mechanism of this observation we have examined the effect of reduced (GSH) and oxidized (GSSG) glutathione, N-acetyl cysteine (NAC) and cysteine (CYS) on the biological activity of a) TGFbeta released by bovine pulmonary artery endothelial cells (BPAEC) into culture medium, and b) commercially available porcine platelet TGFbeta1. The biological activity of TGFbeta (following activation) released into the medium from cultured BPAEC was significantly reduced when the cells were cultured in the presence of 10 mM GSH or 10 mM NAC for 24 h (10 mM GSH: 85.7 +/- 50 pg/ml/10(6) cells and 10 mM NAC: 127.3 +/- 35 pg/ml/10(6) cells, compared with control: 541 +/- 8.9 pg/ml/10(6) cells; p < 0.05). Thiols (10 mM GSH, 10 mM NAC and 5 mM cysteine), added directly to cell-free conditioned medium or to a commercially available preparation of porcine platelet TGFbeta1 for 6-24 h had a similar inhibitory effect on the biological activity of TGFbeta and altered the structure of porcine platelet TGFbeta1 as determined by mass spectroscopy. These thiols failed to reduce the expression of TGFbeta mRNA in BPAEC as measured by a competitive polymerase chain reaction assay. Incubating endothelial cells or cell-free conditioned medium with GSSG did not alter the biological activity of TGFbeta. Lower doses of thiols (0.1-1 mM), that we have shown inhibit the antiproliferative and pro-oxidant effects of exogenous TGFbeta1 on BPAEC, had no direct effect on TGFbeta bioactivity. In summary, thiols are capable of reducing the effects of TGFbeta in biological systems through a direct effect on the TGFbeta molecule. However, this action appears to be dose-dependent, and at low doses (0.1-1 mM) thiols may also inhibit the actions of exogenous TGFbeta1 in cell culture through a mechanism involving the cellular redox status.

Activation of the JAK-STAT pathway by reactive oxygen species.

Reactive oxygen species (ROS) play an important role in the pathogenesis of many human diseases, including the acute respiratory distress syndrome, Parkinson's disease, pulmonary fibrosis, and Alzheimer's disease. In mammalian cells, several genes known to be induced during the immediate early response to growth factors, including the protooncogenes c-fos and c-myc, have also been shown to be induced by ROS. We show that members of the STAT family of transcription factors, including STAT1 and STAT3, are activated in fibroblasts and A-431 carcinoma cells in response to H2O2. This activation occurs within 5 min, can be inhibited by antioxidants, and does not require protein synthesis. STAT activation in these cell lines is oxidant specific and does not occur in response to superoxide- or nitric oxide-generating stimuli. Buthionine sulfoximine, which depletes intracellular glutathione, also activates the STAT pathway. Moreover, H2O2 stimulates the activity of the known STAT kinases JAK2 and TYK2. Activation of STATs by platelet-derived growth factor (PDGF) is significantly inhibited by N-acetyl-L-cysteine and diphenylene iodonium, indicating that ROS production contributes to STAT activation in response to PDGF. These findings indicate that the JAK-STAT pathway responds to intracellular ROS and that PDGF uses ROS as a second messenger to regulate STAT activation.

Upregulated expression of fibroblast growth factor (FGF) receptors by transforming growth factor-beta1 (TGF-beta1) mediates enhanced mitogenic responses to FGFs in cultured human lung fibroblasts.

Transforming growth factor-beta1 (TGF-beta1) is a multifunctional cytokine that induces mesenchymal cell proliferation in vivo while inhibiting growth of most cells directly via serine-threonine receptor(s) binding/activation in vitro. In this study, the ability of TGF-beta1 to regulate the receptor expression of classical mitogenic growth factors that bind receptor tyrosine kinases was examined. TGF-beta1 markedly increased the protein expression of the fibroblast growth factor (FGF) receptors FGFR-1 (Flg) and FGFR-2 (Bek) in a time- and dose-dependent manner in human lung fibroblasts. This resulted in a potentiation of the mitogenic response of multiple FGF ligands that bind to these receptors. TGF-beta1 had no effect on epidermal growth factor (EGF) or platelet-derived growth factor (PDGF)-beta receptor expression and the mitogenic responses mediated by specific ligands for these receptors were not increased. These results demonstrate a novel action of TGF-beta1 to selectively upregulate the expression of FGF receptor family members leading to enhanced mitogenesis by FGFs.

Superoxide scavenging effect of Ginkgo biloba extract on serotonin-induced mitogenesis.

We have reported previously that serotonin (5-HT) stimulates the mitogenesis of bovine pulmonary artery smooth muscle cells (SMCs) through active transport of 5-HT and cellular signaling that includes elevation of superoxide (O2.-) and enhancement of protein tyrosine phosphorylation. Ginkgo biloba extract 501 (EGb 501), which has been demonstrated to act as an antioxidant, was found to block both the elevated O2.- and the proliferative and hypertrophic influences of 5-HT on SMCs, but not to directly inhibit the associated activation of NAD(P)H oxidase or the stimulation of phosphorylation of GTPase-activating protein (GAP). A similar effect of Ginkgo biloba extract 501 occurred on Chinese hamster lung fibroblasts (CCL-39), where 5-HT receptor, as opposed to transporter, action has been associated with mitogenesis. We conclude from these studies that Ginkgo biloba extract 501 quenches O2.- formation by 5-HT, thereby blocking its mitogenic effect. Stimulation of protein tyrosine phosphorylation of GAP by 5-HT appears to precede the elevation of O2.-.