Cardiac STAT3 Deficiency Impairs Contractility and Metabolic Homeostasis in Hypertension

Signal transducer and activator of transcription 3 (STAT3) protects the heart from acute ischemic stress. However, the importance of STAT3 to the heart in chronic stress, such as hypertension, is not known. To study this, we used cardiomyocyte-targeted STAT3 knockout (KO) mice and Angiotensin II (ANG II) infusion by osmotic minipumps. After 4 weeks, ANG II induced similar cardiac hypertrophy in wild type (WT) and cardiac Cre-expressing control (CTRL) mice with no impairment of cardiac function. In contrast, STAT3 KO mice exhibited reduced contractile function but similar hypertrophy to CTRL mice. Ejection fraction and fractional shortening decreased by 22.5 and 27.3%, respectively. Since STAT3 has direct protective effects on mitochondrial function, we examined rates of glucose and oleate oxidation by isolated perfused hearts using a Langendorff system. Hearts of ANG II-treated STAT3 KO and CTRL mice had similar rates of oleate oxidation as saline-infused WT mice. Rates of glucose oxidation were similar between hearts of WT plus saline and CTRL plus ANG II mice; however, glucose oxidation was increased by 66% in hearts of ANG II-treated STAT3 KO mice. The ratio of maximal ATP yield from glucose to fatty acid oxidation was 21.1 ± 3.1 in hearts of ANG II-treated STAT3 KO mice vs. 12.6 ± 2.2 in hearts of ANG II-treated CTRL mice. Lactate production was also elevated in hearts of ANG II-treated STAT3 KO mice by 162% compared to ANG II-treated CTRL mice. Our findings indicate that STAT3 is important for maintaining contractile function and metabolic homeostasis in the hypertensive heart, and STAT3 deficiency promotes a switch toward glucose utilization.

Molecular mechanisms and cell signaling of 20-hydroxyeicosatetraenoic acid in vascular pathophysiology

Cytochrome P450s enzymes catalyze the metabolism of arachidonic acid to epoxyeicosatrienoic acids (EETs), dihydroxyeicosatetraenoic acid and hydroxyeicosatetraeonic acid (HETEs). 20-HETE is a vasoconstrictor that depolarizes vascular smooth muscle cells by blocking K+ channels. EETs serve as endothelial derived hyperpolarizing factors. Inhibition of the formation of 20-HETE impairs the myogenic response and autoregulation of renal and cerebral blood flow. Changes in the formation of EETs and 20-HETE have been reported in hypertension and drugs that target these pathways alter blood pressure in animal models. Sequence variants in CYP4A11 and CYP4F2 that produce 20-HETE, UDP-glucuronosyl transferase involved in the biotransformation of 20-HETE and soluble epoxide hydrolase that inactivates EETs are associated with hypertension in human studies. 20-HETE contributes to the regulation of vascular hypertrophy, restenosis, angiogenesis and inflammation. It also promotes endothelial dysfunction and contributes to cerebral vasospasm and ischemia-reperfusion injury in the brain, kidney and heart. This review will focus on the role of 20-HETE in vascular dysfunction, inflammation, ischemic and hemorrhagic stroke and cardiac and renal ischemia reperfusion injury.

Pivotal Importance of STAT3 in Protecting the Heart from Acute and Chronic Stress: New Advancement and Unresolved Issues

The transcription factor, signal transducer and activator of transcription 3 (STAT3), has been implicated in protecting the heart from acute ischemic injury under both basal conditions and as a crucial component of pre- and post-conditioning protocols. A number of anti-oxidant and antiapoptotic genes are upregulated by STAT3 via canonical means involving phosphorylation on Y705 and S727, although other incompletely defined posttranslational modifications are involved. In addition, STAT3 is now known to be present in cardiac mitochondria and to exert actions that regulate the electron transport chain, reactive oxygen species production, and mitochondrial permeability transition pore opening. These non-canonical actions of STAT3 are enhanced by S727 phosphorylation. The molecular basis for the mitochondrial actions of STAT3 is poorly understood, but STAT3 is known to interact with a critical subunit of complex I and to regulate complex I function. Dysfunctional complex I has been implicated in ischemic injury, heart failure, and the aging process. Evidence also indicates that STAT3 is protective to the heart under chronic stress conditions, including hypertension, pregnancy, and advanced age. Paradoxically, the accumulation of unphosphorylated STAT3 (U-STAT3) in the nucleus has been suggested to drive pathological cardiac hypertrophy and inflammation via non-canonical gene expression, perhaps involving a distinct acetylation profile. U-STAT3 may also regulate chromatin stability. Our understanding of how the non-canonical genomic and mitochondrial actions of STAT3 in the heart are regulated and coordinated with the canonical actions of STAT3 is rudimentary. Here, we present an overview of what is currently known about the pleotropic actions of STAT3 in the heart in order to highlight controversies and unresolved issues.

Temporal cardiac remodeling post-myocardial infarction: dynamics and prognostic implications in personalized medicine

Despite dramatic improvements in short-term mortality rates following myocardial infarction (MI), long-term survival for MI patients who progress to heart failure remains poor. MI occurs when the left ventricle (LV) is deprived of oxygen for a sufficient period of time to induce irreversible necrosis of the myocardium. The LV response to MI involves significant tissue, cellular, and molecular level modifications, as well as substantial hemodynamic changes that feedback negatively to amplify the response. Inflammation to remove necrotic myocytes and fibroblast activation to form a scar are key wound healing responses that are highly variable across individuals. Few biomarkers of early remodeling stages are currently clinically adopted. The discovery of underlying pathophysiological mechanisms and associated novel biomarkers has the potential of improving prognostic capability and therapeutic monitoring. Combining these biomarkers with other prominent ones could constitute a powerful diagnostic and prognostic tool that directly reflects the pathophysiological remodeling of the LV. Understanding temporal remodeling at the tissue, cellular, and molecular level and its link to a well-defined set of biomarkers at early stages post-MI is a prerequisite for improving personalized care and devising more successful therapeutic interventions. Here we summarize the integral mechanisms that occur during early cardiac remodeling in the post-MI setting and highlight the most prominent biomarkers for assessing disease progression.

High-fat diet induces cardiac remodelling and dysfunction: assessment of the role played by SIRT3 loss

Mitochondrial dysfunction plays an important role in obesity-induced cardiac impairment. SIRT3 is a mitochondrial protein associated with increased human life span and metabolism. This study investigated the functional role of SIRT3 in obesity-induced cardiac dysfunction. Wild-type (WT) and SIRT3 knockout (KO) mice were fed a normal diet (ND) or high-fat diet (HFD) for 16 weeks. Body weight, fasting glucose levels, reactive oxygen species (ROS) levels, myocardial capillary density, cardiac function and expression of hypoxia-inducible factor (HIF)-1α/-2α were assessed. HFD resulted in a significant reduction in SIRT3 expression in the heart. Both HFD and SIRT3 KO mice showed increased ROS formation, impaired HIF signalling and reduced capillary density in the heart. HFD induced cardiac hypertrophy and impaired cardiac function. SIRT3 KO mice fed HFD showed greater ROS production and a further reduction in cardiac function compared to SIRT3 KO mice on ND. Thus, the adverse effects of HFD on cardiac function were not attributable to SIRT3 loss alone. However, HFD did not further reduce capillary density in SIRT3 KO hearts, implicating SIRT3 loss in HFD-induced capillary rarefaction. Our study demonstrates the importance of SIRT3 in preserving heart function and capillary density in the setting of obesity. Thus, SIRT3 may be a potential therapeutic target for obesity-induced heart failure.

Applying fractal dimension and image analysis to quantify fibrotic collagen deposition and organization in the normal and hypertensive heart

Hearts of mice with reduction of function mutation in STAT3 (SA/SA) develop fibrotic collagen foci and reduced systolic function with hypertension. This model was used to determine if fractal dimension and image analysis can provide a quantitative description of myocardial fibrosis using routinely prepared trichome-stained material. Collagen was characterized by relative density [integrated optical density/area (IOD/A)] and fractal dimension (D), an index of complexity. IOD/A of collagen in wild type mice increased with hypertension while D decreased, suggesting tighter collagen packing that could eventually stiffen the myocardium as in diastolic heart failure. Reduced STAT3 function caused modest collagen fibrosis with increased IOD/A and D, indicating more tightly packed, but more disorganized collagen than normotensive and hypertensive controls. Hypertension in SA/SA mice resulted in large regions where myocytes were lost and replaced by fibrotic collagen characterized by decreased density and increased disorder. This indicates that collagen associated with reparative fibrosis in SA/SA hearts experiencing hypertension was highly disorganized and more space filling. Loss of myocytes and their replacement by disordered collagen fibers may further weaken the myocardium leading to systolic heart failure. Our findings highlight the utility of image analysis in revealing importance of a cellular protein for normal and reparative extracellular matrix deposition.

Loss of STAT3 in mouse embryonic fibroblasts reveals its Janus-like actions on mitochondrial function and cell viability

STAT3 has been implicated in mitochondrial function; however, the physiological relevance of this action is not established. Here we studied the importance of STAT3 to the cellular response to stimuli, TNFα and serum deprivation, which increase mitochondrial reactive oxygen species (ROS) formation. Experiments were performed using wild type (WT) and STAT3 knockout (KO) mouse embryonic fibroblasts (MEF). Both WT and STAT3 KO MEF expressed similar levels of tumor necrosis factor receptor 1 (TNFR1) and exhibited comparable IκBα degradation with TNFα. However, in the absence of STAT3 nuclear accumulation of NFκB p65 with TNFα was attenuated and induction of the survival protein c-FLIPL was eliminated. Nonetheless, WT MEF were more sensitive to TNFα-induced death which was attributed to necrosis. Deletion of STAT3 decreased ROS formation induced by TNFα and serum deprivation. STAT3 deletion was associated with lower levels of complex I and rates of respiration. Relative to WT cells, mitochondria of STAT3 KO cells released significantly more cytochrome c in response to oxidative stress and had greater caspase 3 cleavage due to serum deprivation. Our findings are consistent with STAT3 being important for mitochondrial function and cell viability by ensuring mitochondrial integrity and the expression of pro-survival genes.

Abstract 387: Role of STAT3 in Collagen Deposition and Organization in the Normal and Hypertensive Heart

Hypertension causes concentric hypertrophy of the left ventricle that may progress to systolic heart failure by means not defined. We reported that mice with a S727A reduced function mutation in STAT3 have reduced systolic function with angiotensin II (Ang II)-induced hypertension and foci of fibrotic collagen in the myocardium as occurs in hypertensive heart disease. We applied image analysis of relative density (IOD/A) and fractal dimension (D) to quantify foci. Hypertension in WT mice did not affect % area and relative density, but D values decreased (P<0.001) suggesting tighter packing of fibrotic collagen stiffening the myocardium. SA/SA+saline hearts showed significant increases in fibrotic collagen (P=0.025) and density (P<0.001) vs. WT hearts. D values indicate an increase (P=0.002) in chaos of collagen matrix. In SA/SA mice Ang II caused remodeling of myocardial collagen with loss of myocytes within fibrotic foci. Hypertension in SA/SA hearts induced further increase (P<0.001) in collagen, while the relative density was significantly less (P=0.015) than the SA/SA+saline group but greater than WT+saline and WT+Ang II groups. The decrease in collagen density was associated with a large (P<0.001) increase in D values, indicating collagen in fibrotic foci was highly disorganized and more space filling. Loss of myocytes and their replacement by chaotic and randomly organized collagen may further weaken the myocardium leading to systolic heart failure. Our results show that STAT3 is essential in normal regulation and organization of the collagen matrix in the heart. Suppression of STAT3 results in fibrotic lesions in the myocardium that are exacerbated by hypertension.

Differential STAT3 signaling in the heart: Impact of concurrent signals and oxidative stress

Multiple lines of evidence suggest that the transcription factor STAT3 is linked to a protective and reparative response in the heart. Thus, increasing duration or intensity of STAT3 activation ought to minimize damage and improve heart function under conditions of stress. Two recent studies using genetic mouse models, however, report findings that appear to refute this proposition. Unfortunately, studies often approach the question of the role of STAT3 in the heart from the perspective that all STAT3 signaling is equivalent, particularly when it comes to signaling by IL-6 type cytokines, which share the gp130 signaling protein. Moreover, STAT3 activation is typically equated with phosphorylation of a critical tyrosine residue. Yet, STAT3 transcriptional behavior is subject to modulation by serine phosphorylation, acetylation, and redox status of the cell. Unphosphorylated STAT3 is implicated in gene induction as well. Thus, how STAT3 is activated and also what other signaling events are occurring at the same time is likely to impact on the outcome ultimately linked to STAT3. Notably STAT3 may serve as a scaffold protein allowing it to interact with other singling pathways. In this context, canonical gp130 cytokine signaling may function to integrate STAT3 signaling with a protective PI3K/AKT signaling network via mutual involvement of JAK tyrosine kinases. Differences in the extent of integration may occur between those cytokines that signal through gp130 homodimers and those through heterodimers of gp130 with a receptor α chain. Signal integration may have importance not only for deciding the particular gene profile linked to STAT3, but for the newly described mitochondrial stabilization role of STAT3 as well. In addition, disruption of integrated gp130-related STAT3 signaling may occur under conditions of oxidative stress, which negatively impacts on JAK catalytic activity. For these reasons, understanding the importance of STAT3 signaling to heart function requires a greater appreciation of the plasticity of this transcription factor in the context in which it is investigated.

AAV-mediated gene therapy for heart failure: enhancing contractility and calcium handling

Heart failure is a progressive, debilitating disease that is characterized by inadequate contractility of the heart. With an aging population, the incidence and economic burden of managing heart failure are anticipated to increase substantially. Drugs for heart failure only slow its progression and offer no cure. However, results of recent clinical trials using recombinant adeno-associated virus (AAV) gene delivery offer the promise, for the first time, that heart failure can be reversed. The strategy is to improve contractility of cardiac muscle cells by enhancing their ability to store calcium through increased expression of the sarco(endo)plasmic reticulum Ca(2+)-ATPase pump (SERCA2a). Preclinical trials have also identified other proteins involved in calcium cycling in cardiac muscle that are promising targets for gene therapy in heart failure, including the following: protein phosphatase 1, adenylyl cyclase 6, G-protein-coupled receptor kinase 2, phospholamban, SUMO1, and S100A1. These preclinical and clinical trials represent a "quiet revolution" that may end up being one of the most significant and remarkable breakthroughs in modern medical practice. Of course, a number of uncertainties remain, including the long-term utility and wisdom of improving the contractile performance of "sick" muscle cells. In this regard, gene therapy may turn out to be a way of buying additional time for actual cardiac regeneration to occur using cardiac stem cells or induced pluripotent stem cells.

Dancing rhinos in stilettos: The amazing saga of the genomic and nongenomic actions of STAT3 in the heart

A substantial body of evidence has shown that signal transducer and activator of transcription 3 (STAT3) has an important role in the heart in protecting the myocardium from ischemia and oxidative stress. These actions are attributed to STAT3 functioning as a transcription factor in upregulating cardioprotective genes. Loss of STAT3 has been implicated as well in the pathogenesis of heart failure and, in that context and in addition to the loss of a cardioprotective gene program, nuclear STAT3 has been identified as a transcriptional repressor important for the normal functioning of the ubiquitin-proteasome system for protein degradation. The later finding establishes a genomic role for STAT3 in controlling cellular homeostasis in cardiac myocytes independent of stress. Surprisingly, although a well-studied area, very few downstream gene targets of STAT3 in the heart have been definitively identified. In addition, STAT3 is now known to induce gene expression by noncanonical means that are not well characterized in the heart. On the other hand, recent evidence has shown that STAT3 has important nongenomic actions in cardiac myocytes that affect microtubule stability, mitochondrial respiration, and autophagy. These extranuclear actions of STAT3 involve protein–protein interactions that are incompletely understood, as is their regulation in both the healthy and injured heart. Moreover, how the diverse genomic and nongenomic actions of STAT3 crosstalk with each other is unchartered territory. Here we present an overview of what is and is not known about both the genomic and nongenomic actions of STAT3 in the heart from a structure-function perspective that focuses on the impact of posttranslational modifications and oxidative stress in regulating the actions and interactions of STAT3. Even though we have learnt a great deal about the role played by STAT3 in the heart, much more awaits to be discovered.

Role of STAT3 in angiotensin II-induced hypertension and cardiac remodeling revealed by mice lacking STAT3 serine 727 phosphorylation

STAT3 is involved in protection of the heart provided by ischemic preconditioning. However, the role of this transcription factor in the heart in chronic stresses such as hypertension has not been defined. We assessed whether STAT3 is important in hypertension-induced cardiac remodeling using mice with reduced STAT3 activity due to a S727A mutation (SA/SA). Wild type (WT) and SA/SA mice received angiotensin (ANG) II or saline for 17 days. ANG II increased mean arterial and systolic pressure in SA/SA and WT mice, but cardiac levels of cytokines associated with heart failure were increased less in SA/SA mice. Unlike WT mice, hearts of SA/SA mice showed signs of developing systolic dysfunction as evidenced by reduction in ejection fraction and fractional shortening. In the left ventricle of both WT and SA/SA mice, ANG II induced fibrosis. However, fibrosis in SA/SA mice appeared more extensive and was associated with loss of myocytes. Cardiac hypertrophy as indexed by heart to body weight ratio and left ventricular anterior wall dimension during diastole was greater in WT mice. In WT+ANG II mice there was an increase in the mass of individual myofibrils. In contrast, cardiac myocytes of SA/SA+ANG II mice showed a loss in myofibrils and myofibrillar mass density was decreased during ANG II infusion. Our findings reveal that STAT3 transcriptional activity is important for normal cardiac myocyte myofibril morphology. Loss of STAT3 may impair cardiac function in the hypertensive heart due to defective myofibrillar structure and remodeling that may lead to heart failure.

Abstract 71: STAT3 Affects Myofibrillar Structure and Its Loss May Contribute to Heart Failure in Hypertension

How hypertension causes heart failure is not known. Since patients with heart failure have reduced cardiac STAT3 and STAT3 KO mice develop heart failure with age, we tested the hypothesis that reduced STAT3 transcriptional activity contributes at an early stage to remodeling that precedes heart failure in hypertension using SA mice with a STAT3 S727A mutation. SA and wild type (WT) mice received angiotensin (A) II (1000 ng/kg/min) or saline (S) for 17 days. Hearts of WT and SA mice had similar levels of STAT3-induced protective proteins Bcl-xL and SOD2, and unlike STAT3 KO mice, cardiac miR-199a levels were not increased in SA mice. AII increased systolic blood pressure measured by telemetry in SA (124 ± 1 to 167 ± 3) and WT (122 ± 3 to 162 ± 3) mice to the same extent. AII increased cardiac levels of cytokines (pg/μg protein) associated with heart failure in both WT and SA mice, but significantly less so (P<0.05) in SA mice; IL-6, 13.6 ± 1.4 vs. 9.1 ± 0.6; TGFβ, 56 ± 4 vs. 38 ± 3 and MCP1 35 ± 2 vs. 22 ± 2. Compared to WT mice, hearts of SA mice showed signs of developing systolic dysfunction with AII as seen by a significant (P<0.05) reduction in ejection fraction (63.7 ± 7.1 to 51.7 ± 6.9) and fractional shortening (34.3 ± 4.9 to 26.4 ± 4.3). AII caused fibrosis in the left ventricle of both WT and SA mice characterized by cardiac myocyte loss and increased % collagen: WT+S, 5.59 ± 0.34; WT+AII, 15.70 ± 1.87; SA+S, 6.70 ± 0.40; SA+AII, 16.50 ± 1.91. In WT+AII mice there was a nonsignificant trend towards a loss of myofibrillar content of cardiac myocytes, but an increase in the mass of the myofibrils (IOD/myofibrillar area). In contrast, cardiac myocytes of SA+AII mice had a significant (P<0.001) % loss in myofibrils (5.71 ± 0.28) compared to SA+S (0.75 ± 0.07), WT+S (0.80 ± 0.06) and WT+AII (1.54 ± 0.10) mice. In addition, the mass of the myofibrils in SA+AII mice (6.01 ± 0.07) was significantly less (P<0.001) than those of SA+S mice (6.46 ± 0.04), although greater than WT+S (4.85 ± 0.06) or WT+AII (5.27 ± 0.08) mice. Our findings reveal that STAT3 transcriptional activity is important for proper morphology of the myofibrils of cardiac myocytes. Loss of STAT3 activity may impair cardiac function in the hypertensive heart due to defective myofibrillar structure and remodeling that may lead to heart failure.

Abstract 219: Acyloxy Nitroso Compounds Inhibit LIF Signaling in Endothelial Cells and Cardiac Myocytes: Evidence That STAT3 Signaling is Redox-Sensitive

Nitroxyl donors have positive inotropic and lusitropic effects that may have therapeutic potential for treating heart failure, but their effects on other signaling pathways are not well studied. Previously we showed that leukemia inhibitory factor (LIF)-induced signaling is inhibited by oxidative stress, which targets JAK1 activation. In addition, we recently reported evidence for a cysteine-based redox switch in the JH1 catalytic domain of JAK 1 and 2. Since nitroxyl is thiophylic, we postulated that nitroxyl donors would inhibit LIF-induced JAK-STAT3 activation. Pretreatment of human microvascular endothelial cells (HMEC-1) or neonatal rat ventricular myocytes with the nitroxyl donors Angeli’s salt and the recently-described nitrosocyclohexyl acetate (NCA) inhibited LIF-induced STAT3 activation as indexed by Y705 phosphorylation. Pretreatment of HMEC-1 with NCA also blocked LIF-induced expression of the inflammation-related genes, ICAM-1 and CEBPD. The related acyloxy nitroso compound 1-nitrosocyclohexyl pivalate (NCP), which is not a nitroxyl donor, was equally effective in inhibiting STAT3 activation, suggesting that these compounds were acting as electrophiles on a thiolate residue. NCA had no effect on the catalytic activity of JAK1 and modestly affected JAK1-induced tyrosine phosphorylation of the LIF receptor. Thus, the JH1 redox switch is not a direct target of the acyloxy nitroso compound. However, pretreatment of recombinant human STAT3 with NCA or NCP reduced the labeling of free sulfhydryl residues with fluorescein-5-maleimide. In addition, we document by dimedone labeling that oxidation of STAT3 is associated with formation of sulfenic acid residues, a hallmark of redox-sensitive proteins. Altogether our evidence indicates that STAT3 has redox-sensitive cysteines that regulate its activation and are targeted by nitroxyl donors and related acyloxy nitroso compounds. These findings raise the possibility of new therapeutic strategies to target STAT3 signaling.

Depletion of cellular glutathione modulates LIF-induced JAK1-STAT3 signaling in cardiac myocytes

Previously we reported that the sesquiterpene lactone parthenolide induces oxidative stress in cardiac myocytes, which blocks Janus kinase (JAK) activation by the interleukin 6 (IL-6)-type cytokines. One implication suggested by this finding is that IL-6 signaling is dependent upon cellular anti-oxidant defenses or redox status. Therefore, the present study was undertaken to directly test the hypothesis that JAK1 signaling by the IL-6-type cytokines in cardiac myocytes is impaired by glutathione (GSH) depletion, since this tripeptide is one of the major anti-oxidant molecules and redox-buffers in cells. Cardiac myocytes were pretreated for 6h with l-buthionine-sulfoximine (BSO) to inhibit GSH synthesis. After 24h, cells were dosed with the IL-6-like cytokine, leukemia inhibitory factor (LIF). BSO treatment decreased GSH levels and dose-dependently attenuated activation of JAK1, Signal Transducer and Activator of Transcription 3 (STAT3), and extracellular signal regulated kinases 1 and 2 (ERK1/2). Addition of glutathione monoethyl ester, which is cleaved intracellularly to GSH, prevented attenuation of LIF-induced JAK1 and STAT3 activation, as did the reductant N-acetyl-cysteine. Unexpectedly, LIF-induced STAT1 activation was unaffected by GSH depletion. Evidence was found that STAT3 is more resistant than STAT1 to intermolecular disulfide bond formation under oxidizing conditions and more likely to retain the monomeric form, suggesting that conformational differences explain the differential effect of GSH depletion on STAT1 and STAT3. Overall, our findings indicate that activation of both JAK1 and STAT3 is redox-sensitive and the character of IL-6 type cytokine signaling in cardiac myocytes is sensitive to changes in the cellular redox status. In cardiac myocytes, activation of STAT1 may be favored over STAT3 under oxidizing conditions due to GSH depletion and/or augmented reactive oxygen species production, such as in ischemia-reperfusion and heart failure.

Acyloxy nitroso compounds inhibit LIF signaling in endothelial cells and cardiac myocytes: evidence that STAT3 signaling is redox-sensitive

We previously showed that oxidative stress inhibits leukemia inhibitory factor (LIF) signaling by targeting JAK1, and the catalytic domains of JAK 1 and 2 have a cysteine-based redox switch. Thus, we postulated that the NO sibling and thiophylic compound, nitroxyl (HNO), would inhibit LIF-induced JAK-STAT3 activation. Pretreatment of human microvascular endothelial cells (HMEC-1) or neonatal rat cardiomyocytes with the HNO donors Angeli’s salt or nitrosocyclohexyl acetate (NCA) inhibited LIF-induced STAT3 activation. NCA pretreatment also blocked the induction of downstream inflammatory genes (e.g. intercellular adhesion molecule 1, CCAAT/enhancer binding protein delta). The related 1-nitrosocyclohexyl pivalate (NCP; not a nitroxyl donor) was equally effective in inhibiting STAT3 activation, suggesting that these compounds act as thiolate targeting electrophiles. The JAK1 redox switch is likely not a target of acyloxy nitroso compounds, as NCA had no effect on JAK1 catalytic activity and only modestly affected JAK1-induced phosphorylation of the LIF receptor. However, pretreatment of recombinant human STAT3 with NCA or NCP reduced labeling of free sulfhydryl residues. We show that NCP in the presence of diamide enhanced STAT3 glutathionylation and dimerization in adult mouse cardiac myocytes and altered STAT3 under non-reducing conditions. Finally, we show that monomeric STAT3 levels are decreased in the Gαq model of heart failure in a redox-sensitive manner. Altogether, our evidence indicates that STAT3 has redox-sensitive cysteines that regulate its activation and are targeted by HNO donors and acyloxy nitroso compounds. These findings raise the possibility of new therapeutic strategies to target STAT3 signaling via a redox-dependent manner, particularly in the context of cardiac and non-cardiac diseases with prominent pro-inflammatory signaling.

Identification of a redox-sensitive switch within the JAK2 catalytic domain

Four cysteine residues (Cys866, Cys917, Cys1094, and Cys1105) have direct roles in cooperatively regulating Janus kinase 2 (JAK2) catalytic activity. Additional site-directed mutagenesis experiments now provide evidence that two of these residues (Cys866 and Cys917) act together as a redox-sensitive switch, allowing JAK2's catalytic activity to be directly regulated by the redox state of the cell. We created several variants of the truncated JAK2 (GST/(NΔ661)rJAK2), which incorporated cysteine-to-serine or cysteine-to-alanine mutations. The catalytic activities of these mutant enzymes were evaluated by in vitro autokinase assays and by in situ autophosphorylation and transphosphorylation assays. Cysteine-to-alanine mutagenesis revealed that the mechanistic role of Cys866 and Cys917 is functionally distinct from that of Cys1094 and Cys1105. Most notable is the observation that the robust activity of the CC866,917AA mutant is unaltered by pretreatment with dithiothreitol or o-iodosobenzoate, unlike all other JAK2 variants previously examined. This work provides the first direct evidence for a cysteine-based redox-sensitive switch that regulates JAK2 catalytic activity. The presence of this redox-sensitive switch predicts that reactive oxygen species can impair the cell's response to JAK-coupled cytokines under conditions of oxidative stress, which we confirm in a murine pancreatic β-islet cell line.

Selenate enhances STAT3 transcriptional activity in endothelial cells: differential actions of selenate and selenite on LIF cytokine signaling and cell viability

Sodium selenate may have utility in treating Alzheimer's disease and diabetes; however, its impact on the associated proinflammatory cytokine signaling of endothelial cells has not been investigated. We report that treatment of human microvascular endothelial cells with sodium selenate at a pharmacological dose (100 μM) enhanced tyrosine phosphorylation of nuclear STAT3 on Y705 in response to IL-6-type cytokine, leukemia inhibitory factor (LIF), indicative of enhanced STAT3 activity. Accordingly, STAT3 nuclear binding to DNA was increased, as well as LIF-induced gene expression of chemokine (C-C motif) ligand 2 (CCL2). CCL2 plays a key role in inflammatory processes associated with neuronal degenerative and vascular diseases. The enhancing action of selenate on LIF-induced STAT3 Y705 phosphorylation was replicated by vanadate and a specific inhibitor of protein tyrosine phosphatase, non-receptor type 1 (PTP1B). Moreover, we observed that selenite, the cellular reduction bioproduct of selenate but not selenate itself, inhibited enzymatic activity of human recombinant PTP1B. Our findings support the conclusion that in human microvascular endothelial cells selenate has a vanadate-like effect in inhibiting PTP1B and enhancing proinflammatory STAT3 activation. These findings raise the possibility that beneficial actions of supranutritional levels of selenate for treating Alzheimer's and diabetes may be offset by a proinflammatory action on endothelial cells.

A novel U-STAT3-dependent mechanism mediates the deleterious effects of chronic nicotine exposure on renal injury

Previous data from our group have demonstrated (Arany I, Grifoni S, Clark JS, Csongradi, Maric C, Juncos LA. Am J Physiol Renal Physiol 301: F125-F133, 2011) that chronic nicotine (NIC) exposure exacerbates acute renal ischemic injury (AKI) in mice that could increase the risk for development and progression of chronic kidney disease (CKD). It has been shown that proximal tubules of the kidney can acquire characteristics that may compromise structural recovery and favor development of inflammation and fibrosis following injury. Chronic NIC exposure can amplify this epithelial process although the mechanism is not identified. Recently, the unphosphorylated form of signal transducer and activator of transcription-3 (U-STAT3) has emerged as a noncanonical mediator of inflammation and fibrosis that may be responsible for the effects of chronic NIC. We found that levels of transforming growth factor β-1 (TGF-β1), α-smooth muscle actin (α-SMA), fibronectin, monocyte chemotactic protein-1 (MCP-1), and expression of U-STAT3 were increased in the ischemic kidneys of NIC-exposed mice. Chronic NIC exposure also increased TGF-β1-dependent F-actin reorganization, vimentin, fibronectin, and α-SMA expression as well as promoter activity of α-SMA and MCP-1 without significant loss of epithelial characteristics (E-cadherin) in cultured renal proximal tubule cells. Importantly, transduction of cells with a U-STAT3 mimetic (Y705F-STAT3) augmented stress fiber formation and also amplified NIC+TGF-β1-induced expression of α-SMA, vimentin, fibronectin, as well as promoter activity of α-SMA and MCP-1. Our results reveal a novel, chronic NIC-exposure-related and U-STAT3-dependent mechanism as mediator of a sustained transcription of genes that are linked to remodeling and inflammation in the kidney during injury. This process may facilitate progression of AKI to CKD. The obtained data may lead to devising therapeutic methods to specifically enhance the protective and/or inhibit adverse effects of STAT3 in the kidney.

Calyculin A reveals serine/threonine phosphatase protein phosphatase 1 as a regulatory nodal point in canonical signal transducer and activator of transcription 3 signaling of human microvascular endothelial cells

Vascular inflammation is initiated by stimuli acting on endothelial cells. A clinical feature of vascular inflammation is increased circulating interleukin 6 (IL-6) type cytokines such as leukemia inhibitory factor (LIF), but their role in vascular inflammation is not fully defined. IL-6 type cytokines activate transcription factor signal transducer and activator of transcription 3 (STAT3), which has a key role in inflammation and the innate immune response. Canonical STAT3 gene induction is due to phosphorylation of (1) Y705, leading to STAT3 dimerization and DNA binding and (2) S727, enhancing homodimerization and DNA binding by recruiting p300/CBP. We asked whether enhancing S727 STAT3 phosphorylation using the protein phosphatase 1 (PP1) inhibitor, calyculin A, would enhance LIF-induced gene expression in human microvascular endothelial cells (HMEC-1). Cotreatment with calyculin A and LIF markedly increased STAT3 S727 phosphorylation, without affecting the increase in the nuclear fraction of STAT3 phosphorylated on Y705. PP2A inhibitors, okadaic acid and fostriecin, did not enhance STAT3 S727 phosphorylation. Surprisingly, calyculin A eliminated LIF-induced gene expression: (1) calyculin A reduced binding of nuclear extracts to a STAT3 consensus site, thereby reducing the overall level of binding observed with LIF; and (2) calyculin A caused p300/CBP phosphorylation, thus resulting in reduced acetylation activity and degradation. Together, these findings reveal a pivotal role of a protein serine/threonine phosphatases that is likely PP1 in HMEC in controlling STAT3 transcriptional activity.