Unrestrained p38 MAPK activation in Dusp1/4 double-null mice induces cardiomyopathy

MicroRNA-200c modulates DUSP-1 expression in diabetes-induced cardiac hypertrophy

Mitogen-activated protein kinases (MAPKs) (ERK1/2, JNK, and p38) are upregulated in diabetic cardiomyopathy (DCM). Dual-specific phosphatase-1 (DUSP-1) has been reported to regulate the activity of MAPKs in cardiac hypertrophy; however, the role of DUSP-1 in regulating MAPKs activity in DCM is not known. MicroRNAs have been reported to regulate the expression of several genes in hypertrophied failing hearts. However, little is known about the microRNAs regulating DUSP-1 expression in diabetes-related cardiac hypertrophy. In the present study, we investigated the role of DUSP-1 and miR-200c in diabetes-induced cardiac hypertrophy. DCM was induced in Wistar rats by low-dose Streptozotocin high-fat diet for 12 weeks. Cardiac expression of ERK, p-38, JNK, DUSP-1, miR-200c, and hypertrophy markers (ANP and β-MHC) was studied in DCM in control rats and in high-glucose (HG)-treated rat neonatal cardiomyocytes. miR-200c inhibition was performed to validate DUSP-1 as target. A significant increase in phosphorylated ERK, p38, and JNK was observed in DCM model and in HG-treated cardiomyocytes (p < 0.05). Expression of DUSP-1 was significantly decreased in diabetes group and in HG-treated cardiomyocytes (p < 0.05). Increased expression of miR-200c was observed in DCM model and in HG-treated cardiomyocytes (p < 0.05). Inhibition of miR-200c induces the expression of the DUSP-1 causing decreased expression of phosphorylated ERK, p38, and JNK and attenuated cardiomyocyte hypertrophy in HG-treated cardiomyocytes. miR-200c plays a role in diabetes-associated cardiac hypertrophy by modulating expression of DUSP-1.

Cardiac-restricted Overexpression of TRAF3 Interacting Protein 2 (TRAF3IP2) Results in Spontaneous Development of Myocardial Hypertrophy, Fibrosis, and Dysfunction

TRAF3IP2 (TRAF3 interacting protein 2; previously known as CIKS or Act1) is a key intermediate in the normal inflammatory response and the pathogenesis of various autoimmune and inflammatory diseases. Induction of TRAF3IP2 activates IκB kinase (IKK)/NF-κB, JNK/AP-1, and c/EBPβ and stimulates the expression of various inflammatory mediators with negative myocardial inotropic effects. To investigate the role of TRAF3IP2 in heart disease, we generated a transgenic mouse model with cardiomyocyte-specific TRAF3IP2 overexpression (TRAF3IP2-Tg). Echocardiography, magnetic resonance imaging, and pressure-volume conductance catheterization revealed impaired cardiac function in 2-month-old male transgenic (Tg) mice as evidenced by decreased ejection fraction, stroke volume, cardiac output, and peak ejection rate. Moreover, the male Tg mice spontaneously developed myocardial hypertrophy (increased heart/body weight ratio, cardiomyocyte cross-sectional area, GATA4 induction, and fetal gene re-expression). Furthermore, TRAF3IP2 overexpression resulted in the activation of IKK/NF-κB, JNK/AP-1, c/EBPβ, and p38 MAPK and induction of proinflammatory cytokines, chemokines, and extracellular matrix proteins in the heart. Although myocardial hypertrophy decreased with age, cardiac fibrosis (increased number of myofibroblasts and enhanced expression and deposition of fibrillar collagens) increased progressively. Despite these adverse changes, TRAF3IP2 overexpression did not result in cell death at any time period. Interestingly, despite increased mRNA expression, TRAF3IP2 protein levels and activation of its downstream signaling intermediates remained unchanged in the hearts of female Tg mice. The female Tg mice also failed to develop myocardial hypertrophy. In summary, these results demonstrate that overexpression of TRAF3IP2 in male mice is sufficient to induce myocardial hypertrophy, cardiac fibrosis, and contractile dysfunction.

Cardiomyocyte GTP Cyclohydrolase 1 Protects the Heart Against Diabetic Cardiomyopathy

Diabetic cardiomyopathy increases the risk of heart failure and death. At present, there are no effective approaches to preventing its development in the clinic. Here we report that reduction of cardiac GTP cyclohydrolase 1 (GCH1) degradation by genetic and pharmacological approaches protects the heart against diabetic cardiomyopathy. Diabetic cardiomyopathy was induced in C57BL/6 wild-type mice and transgenic mice with cardiomyocyte-specific overexpression of GCH1 with streptozotocin, and control animals were given citrate buffer. We found that diabetes-induced degradation of cardiac GCH1 proteins contributed to adverse cardiac remodeling and dysfunction in C57BL/6 mice, concomitant with decreases in tetrahydrobiopterin, dimeric and phosphorylated neuronal nitric oxide synthase, sarcoplasmic reticulum Ca(2+) handling proteins, intracellular [Ca(2+)]i, and sarcoplasmic reticulum Ca(2+) content and increases in phosphorylated p-38 mitogen-activated protein kinase and superoxide production. Interestingly, GCH-1 overexpression abrogated these detrimental effects of diabetes. Furthermore, we found that MG 132, an inhibitor for 26S proteasome, preserved cardiac GCH1 proteins and ameliorated cardiac remodeling and dysfunction during diabetes. This study deepens our understanding of impaired cardiac function in diabetes, identifies GCH1 as a modulator of cardiac remodeling and function, and reveals a new therapeutic target for diabetic cardiomyopathy.

Physiological and pathological cardiac hypertrophy

The heart must continuously pump blood to supply the body with oxygen and nutrients. To maintain the high energy consumption required by this role, the heart is equipped with multiple complex biological systems that allow adaptation to changes of systemic demand. The processes of growth (hypertrophy), angiogenesis, and metabolic plasticity are critically involved in maintenance of cardiac homeostasis. Cardiac hypertrophy is classified as physiological when it is associated with normal cardiac function or as pathological when associated with cardiac dysfunction. Physiological hypertrophy of the heart occurs in response to normal growth of children or during pregnancy, as well as in athletes. In contrast, pathological hypertrophy is induced by factors such as prolonged and abnormal hemodynamic stress, due to hypertension, myocardial infarction etc. Pathological hypertrophy is associated with fibrosis, capillary rarefaction, increased production of pro-inflammatory cytokines, and cellular dysfunction (impairment of signaling, suppression of autophagy, and abnormal cardiomyocyte/non-cardiomyocyte interactions), as well as undesirable epigenetic changes, with these complex responses leading to maladaptive cardiac remodeling and heart failure. This review describes the key molecules and cellular responses involved in physiological/pathological cardiac hypertrophy.

DUSP8 Regulates Cardiac Ventricular Remodeling by Altering ERK1/2 Signaling

RATIONALE

Mitogen-activated protein kinase (MAPK) signaling regulates the growth response of the adult myocardium in response to increased cardiac workload or pathological insults. The dual-specificity phosphatases (DUSPs) are critical effectors, which dephosphorylate the MAPKs to control the basal tone, amplitude, and duration of MAPK signaling.

OBJECTIVE

To examine DUSP8 as a regulator of MAPK signaling in the heart and its impact on ventricular and cardiac myocyte growth dynamics.

METHODS AND RESULTS

Dusp8 gene-deleted mice and transgenic mice with inducible expression of DUSP8 in the heart were used here to investigate how this MAPK-phosphatase might regulate intracellular signaling and cardiac growth dynamics in vivo. Dusp8 gene-deleted mice were mildly hypercontractile at baseline with a cardiac phenotype of concentric ventricular remodeling, which protected them from progressing towards heart failure in 2 surgery-induced disease models. Cardiac-specific overexpression of DUSP8 produced spontaneous eccentric remodeling and ventricular dilation with heart failure. At the cellular level, adult cardiac myocytes from Dusp8 gene-deleted mice were thicker and shorter, whereas DUSP8 overexpression promoted cardiac myocyte lengthening with a loss of thickness. Mechanistically, activation of extracellular signal-regulated kinases 1/2 were selectively increased in Dusp8 gene-deleted hearts at baseline and following acute pathological stress stimulation, whereas p38 MAPK and c-Jun N-terminal kinases were mostly unaffected.

CONCLUSIONS

These results indicate that DUSP8 controls basal and acute stress-induced extracellular signal-regulated kinases 1/2 signaling in adult cardiac myocytes that then alters the length-width growth dynamics of individual cardiac myocytes, which further alters contractility, ventricular remodeling, and disease susceptibility.

p38γ and δ promote heart hypertrophy by targeting the mTOR-inhibitory protein DEPTOR for degradation

Disrupted organ growth leads to disease development. Hypertrophy underlies postnatal heart growth and is triggered after stress, but the molecular mechanisms involved in these processes are largely unknown. Here we show that cardiac activation of p38γ and p38δ increases during postnatal development and by hypertrophy-inducing stimuli. p38γ/δ promote cardiac hypertrophy by phosphorylating the mTORC1 and mTORC2 inhibitor DEPTOR, which leads to its degradation and mTOR activation. Hearts from mice lacking one or both kinases are below normal size, have high levels of DEPTOR, low activity of the mTOR pathway and reduced protein synthesis. The phenotype of p38γ/δ^−/− mice is reverted by overactivation of mTOR with amino acids, shRNA-mediated knockdown of Deptor, or cardiomyocyte overexpression of active p38γ and p38δ. Moreover, in WT mice, heart weight is reduced by cardiac overexpression of DEPTOR. Our results demonstrate that p38γ/δ control heart growth by modulating mTOR pathway through DEPTOR phosphorylation and subsequent degradation.

mTOR signalling pathway is a critical regulator of cardiac hypertrophy. Here the authors show that two kinases, p38γ and p38δ, control heart growth by promoting mTOR activity via phosphorylation and consequent proteasome degradation of mTOR inhibitor DEPTOR, extending our knowledge of cardiac hypertrophy regulation.

Dual-Specificity Phosphatase 4 Regulates STAT5 Protein Stability and Helper T Cell Polarization

Immune responses are critically regulated by the functions of CD4 helper T cells. Based on their secreted cytokines, helper T cells are further categorized into different subsets like Treg or Th17 cells, which suppress or promote inflammatory responses, respectively. Signals from IL-2 activate the transcription factor STAT5 to promote Treg but suppress Th17 cell differentiation. Our previous results found that the deficiency of a dual-specificity phosphatase, DUSP4, induced STAT5 hyper-activation, enhanced IL-2 signaling, and increased T cell proliferation. In this report, we examined the effects of DUSP4 deficiency on helper T cell differentiation and STAT5 regulation. Our in vivo data showed that DUSP4 mice were more resistant to the induction of autoimmune encephalitis, while in vitro differentiations revealed enhanced iTreg and reduced Th17 polarization in DUSP4-deficient T cells. To study the cause of this altered helper T cell polarization, we performed luciferase reporter assays and confirmed that, as predicted by our previous report, DUSP4 over-expression suppressed the transcription factor activity of STAT5. Surprisingly, we also found that DUSP4-deficient T but not B cells exhibited elevated STAT5 protein levels, and over-expressed DUSP4 destabilized STAT5 in vitro; moreover, this destabilization required the phosphatase activity of DUSP4, and was insensitive to MG132 treatment. Finally, domain-mapping results showed that both the substrate-interacting and the phosphatase domains of DUSP4 were required for its optimal interaction with STAT5, while the coiled-coil domain of STAT5 appeared to hinder this interaction. Our data thus provide the first genetic evidence that DUSP4 is important for helper T cell development. In addition, they also help uncover the novel, DUSP4-mediated regulation of STAT5 protein stability.

Modulation of p38 kinase by DUSP4 is important in regulating cardiovascular function under oxidative stress

Over-activation of p38 is implicated in many cardiovascular diseases (CVDs), including myocardial infarction, hypertrophy, heart failure, and ischemic heart disease. Numerous therapeutic interventions for CVDs have been directed toward the inhibition of the p38 mitogen-activated protein kinase activation that contributes to the detrimental effect after ischemia/reperfusion (I/R) injuries. However, the efficacy of these treatments is far from ideal, as they lack specificity and are associated with high toxicity. Previously, we demonstrated that N-acetyl cysteine (NAC) pretreatment up-regulates DUSP4 expression in endothelial cells, regulating p38 and ERK1/2 activities, and thus providing a protective effect against oxidative stress. Here, endothelial cells under hypoxia/reoxygenation (H/R) insult and isolated heart I/R injury were used to investigate the role of DUSP4 in the modulation of the p38 pathway. In rat endothelial cells, DUSP4 is time-dependently degraded by H/R (0.25 ± 0.07-fold change of control after 2h H/R). Its degradation is closely associated with hyperphosphorylation of p38 (2.1 ± 0.36-fold change) and cell apoptosis, as indicated by the increase in cells immunopositive for cleaved caspase-3 (12.59 ± 3.38%) or TUNEL labeling (29.46 ± 3.75%). The inhibition of p38 kinase activity with 20 µM SB203580 during H/R prevents H/R-induced apoptosis, assessed via TUNEL (12.99 ± 1.89%). Conversely, DUSP4 gene silencing in endothelial cells augments their sensitivity to H/R-induced apoptosis (45.81 ± 5.23%). This sensitivity is diminished via the inhibition of p38 activity (total apoptotic cells drop to 17.47 ± 1.45%). Interestingly, DUSP4 gene silencing contributes to the increase in superoxide generation from cells. Isolated Langendorff-perfused mouse hearts were subjected to global I/R injury. DUSP4(-/-) hearts had significantly larger infarct size than WT. The increase in I/R-induced infarct in DUSP4(-/-) mice significantly correlates with reduced functional recovery (assessed by RPP%, LVDP%, HR%, and dP/dtmax) as well as lower CF% and a higher initial LVEDP. From immunoblotting analysis, it is evident that p38 is significantly overactivated in DUSP4(-/-) mice after I/R injury. The activation of cleaved caspase-3 is seen in both WT and DUSP4(-/-) I/R hearts. Infusion of a p38 inhibitor prior to ischemia and during the reperfusion improves both WT and DUSP4(-/-) cardiac function. Therefore, the identification of p38 kinase modulation by DUSP4 provides a novel therapeutic target for oxidant-induced diseases, especially myocardial infarction.

Imbalanced signal transduction in regulatory T cells expressing the transcription factor FoxP3

FoxP3(+) T regulatory (Treg) cells have a fundamental role in immunological tolerance, with transcriptional and functional phenotypes that demarcate them from conventional CD4(+) T cells (Tconv). Differences between these two lineages in the signaling downstream of T-cell receptor-triggered activation have been reported, and there are different requirements for some signaling factors. Seeking a comprehensive view, we found that Treg cells have a broadly dampened activation of several pathways and signaling nodes upon TCR-mediated activation, with low phosphorylation of CD3ζ, SLP76, Erk1/2, AKT, or S6 and lower calcium flux. In contrast, STAT phosphorylation triggered by interferons, IL2 or IL6, showed variations between Treg and Tconv in magnitude or choice of preferential STAT activation but no general Treg signaling defect. Much, but not all, of the Treg/Tconv difference in TCR-triggered responses could be attributed to lower responsiveness of antigen-experienced cells with CD44(hi) or CD62L(lo) phenotypes, which form a greater proportion of the Treg pool. Candidate regulators were tested, but the Treg/Tconv differential could not be explained by overexpression in Treg cells of the signaling modulator CD5, the coinhibitors PD-1 and CTLA4, or the regulatory phosphatase DUSP4. However, transcriptome profiling in Dusp4-deficient mice showed that DUSP4 enhances the expression of a segment of the canonical Treg transcriptional signature, which partially overlaps with the TCR-dependent Treg gene set. Thus, Treg cells, likely because of their intrinsically higher reactivity to self, tune down TCR signals but seem comparatively more attuned to cytokines or other intercellular signals.

Counteracting Protein Kinase Activity in the Heart: The Multiple Roles of Protein Phosphatases

Decades of cardiovascular research have shown that variable and flexible levels of protein phosphorylation are necessary to maintain cardiac function. A delicate balance between phosphorylated and dephosphorylated states of proteins is guaranteed by a complex interplay of protein kinases (PKs) and phosphatases. Serine/threonine phosphatases, in particular members of the protein phosphatase (PP) family govern dephosphorylation of the majority of these cardiac proteins. Recent findings have however shown that PPs do not only dephosphorylate previously phosphorylated proteins as a passive control mechanism but are capable to actively control PK activity via different direct and indirect signaling pathways. These control mechanisms can take place on (epi-)genetic, (post-)transcriptional, and (post-)translational levels. In addition PPs themselves are targets of a plethora of proteinaceous interaction partner regulating their endogenous activity, thus adding another level of complexity and feedback control toward this system. Finally, novel approaches are underway to achieve spatiotemporal pharmacologic control of PPs which in turn can be used to fine-tune misleaded PK activity in heart disease. Taken together, this review comprehensively summarizes the major aspects of PP-mediated PK regulation and discusses the subsequent consequences of deregulated PP activity for cardiovascular diseases in depth.

Diet-induced obesity promotes altered remodeling and exacerbated cardiac hypertrophy following pressure overload

Heart failure (HF) is the end stage of cardiovascular disease, in which hypertrophic remodeling no longer meets cardiac output demand. Established animal models of HF have provided insights into disease pathogenesis. However, these models are developed on dissimilar metabolic backgrounds from humans – patients with HF are frequently overweight or obese, whereas animal models of HF are typically lean. Thus, we aimed to develop and investigate model for cardiac hypertrophy and failure that also recapitulates the cardiometabolic state of HF in humans. We subjected mice with established diet-induced obesity (DIO) to cardiac pressure overload provoked by transverse aortic constriction (TAC). Briefly, we fed WT male mice a normal chow or high-fat diet for 10 weeks prior to sham/TAC procedures and until surgical follow-up. We then analyzed cardiac hypertrophy, mechanical function, and electrophysiology at 5–6 weeks after surgery. In DIO mice with TAC, hypertrophy and systolic dysfunction were exacerbated relative to chow TAC animals, which showed minimal remodeling with our moderate constriction intensity. Normalized heart weight was 55.8% greater and fractional shortening was 30.9% less in DIO TAC compared with chow TAC hearts. However, electrophysiologic properties were surprisingly similar between DIO sham and TAC animals. To examine molecular pathways activated by DIO and TAC, we screened prohypertrophic signaling cascades, and the exacerbated remodeling was associated with early activation of the c-Jun-N-terminal kinase (JNK1/2) signaling pathway. Thus, DIO aggravates the progression of hypertrophy and HF caused by pressure overload, which is associated with JNK1/2 signaling, and cardiometabolic state can significantly modify HF pathogenesis.

Toll-like receptor 5 deficiency exacerbates cardiac injury and inflammation induced by myocardial ischaemia-reperfusion in the mouse

In the present study, we report that genetic deletion of toll-like receptor 5 (TLR5) is associated with increased myocardial injury and dysfunction following ischaemia-reperfusion, by enhancing cardiac oxidative stress and p38 activation. TLR5 may thus convey cardioprotective signals during myocardial infarction (MI).

The dual-specificity phosphatase 2 (DUSP2) does not regulate obesity-associated inflammation or insulin resistance in mice

Alterations in the immune cell profile and the induction of inflammation within adipose tissue are a hallmark of obesity in mice and humans. Dual-specificity phosphatase 2 (DUSP2) is widely expressed within the immune system and plays a key role promoting immune and inflammatory responses dependent on mitogen-activated protein kinase (MAPK) activity. We hypothesised that the absence of DUSP2 would protect mice against obesity-associated inflammation and insulin resistance. Accordingly, male and female littermate mice that are either wild-type (wt) or homozygous for a germ-line null mutation of the dusp2 gene (dusp2^−/−) were fed either a standard chow diet (SCD) or high fat diet (HFD) for 12 weeks prior to metabolic phenotyping. Compared with mice fed the SCD, all mice consuming the HFD became obese, developed glucose intolerance and insulin resistance, and displayed increased macrophage recruitment and markers of inflammation in epididymal white adipose tissue. The absence of DUSP2, however, had no effect on the development of obesity or adipose tissue inflammation. Whole body insulin sensitivity in male mice was unaffected by an absence of DUSP2 in response to either the SCD or HFD; however, HFD-induced insulin resistance was slightly, but significantly, reduced in female dusp2^−/− mice. In conclusion, DUSP2 plays no role in regulating obesity-associated inflammation and only a minor role in controlling insulin sensitivity following HFD in female, but not male, mice. These data indicate that rather than DUSP2 being a pan regulator of MAPK dependent immune cell mediated inflammation, it appears to differentially regulate inflammatory responses that have a MAPK component.

Activation of the retinoid X receptor modulates angiotensin II-induced smooth muscle gene expression and inflammation in vascular smooth muscle cells

The retinoid X receptor (RXR) partners with numerous nuclear receptors, such as the peroxisome proliferator activated receptor (PPAR) family, liver X receptors (LXRs), and farnesoid X receptor (FXR). Although each heterodimer can be activated by specific ligands, a subset of these receptors, defined as permissive nuclear receptors, can also be activated by RXR agonists known as rexinoids. Many individual RXR heterodimers have beneficial effects in vascular smooth muscle cells (SMCs). Because rexinoids can potently activate multiple RXR pathways, we hypothesized that treating SMCs with rexinoids would more effectively reverse the pathophysiologic effects of angiotensin II than an individual heterodimer agonist. Cultured rat aortic SMCs were pretreated with either an RXR agonist (bexarotene or 9-cis retinoic acid) or vehicle (dimethylsulfoxide) for 24 hours before stimulation with angiotensin II. Compared with dimethylsulfoxide, bexarotene blocked angiotensin II-induced SM contractile gene induction (calponin and smooth muscle-α-actin) and protein synthesis ([(3)H]leucine incorporation). Bexarotene also decreased angiotensin II-mediated inflammation, as measured by decreased expression of monocyte chemoattractant protein-1 (MCP-1). Activation of p38 mitogen-activated protein (MAP) kinase but not extracellular signal-related kinase (ERK) or protein kinase B (Akt) was also blunted by bexarotene. We compared bexarotene to five agonists of nuclear receptors (PPARα, PPARγ, PPARδ, LXR, and FXR). Bexarotene had a greater effect on calponin reduction, MCP-1 inhibition, and p38 MAP kinase inhibition than any individual agonist. PPARγ knockout cells demonstrated blunted responses to bexarotene, indicating that PPARγ is necessary for the effects of bexarotene. These data demonstrate that RXR is a potent modulator of angiotensin II-mediated responses in the vasculature, partially through inhibition of p38.

Genome-wide DNA methylation profiles and their relationships with mRNA and the microRNA transcriptome in bovine muscle tissue (Bos taurine)

DNA methylation is a key epigenetic modification in mammals and plays important roles in muscle development. We sampled longissimus dorsi muscle (LDM) from a well-known elite native breed of Chinese Qinchuan cattle living within the same environment but displaying distinct skeletal muscle at the fetal and adult stages. We generated and provided a genome-wide landscape of DNA methylomes and their relationship with mRNA and miRNA for fetal and adult muscle studies. Integration analysis revealed a total of 77 and 1,054 negatively correlated genes with methylation in the promoter and gene body regions, respectively, in both the fetal and adult bovine libraries. Furthermore, we identified expression patterns of high-read genes that exhibit a negative correlation between methylation and expression from nine different tissues at multiple developmental stages of bovine muscle-related tissue or organs. In addition, we validated the MeDIP-Seq results by bisulfite sequencing PCR (BSP) in some of the differentially methylated promoters. Together, these results provide valuable data for future biomedical research and genomic and epigenomic studies of bovine skeletal muscle that may help uncover the molecular basis underlying economically valuable traits in cattle. This comprehensive map also provides a solid basis for exploring the epigenetic mechanisms of muscle growth and development.

Global MEF2 target gene analysis in cardiac and skeletal muscle reveals novel regulation of DUSP6 by p38MAPK-MEF2 signaling

MEF2 plays a profound role in the regulation of transcription in cardiac and skeletal muscle lineages. To define the overlapping and unique MEF2A genomic targets, we utilized ChIP-exo analysis of cardiomyocytes and skeletal myoblasts. Of the 2783 and 1648 MEF2A binding peaks in skeletal myoblasts and cardiomyocytes, respectively, 294 common binding sites were identified. Genomic targets were compared to differentially expressed genes in RNA-seq analysis of MEF2A depleted myogenic cells, revealing two prominent genetic networks. Genes largely associated with muscle development were down-regulated by loss of MEF2A while up-regulated genes reveal a previously unrecognized function of MEF2A in suppressing growth/proliferative genes. Several up-regulated (Tprg, Mctp2, Kitl, Prrx1, Dusp6) and down-regulated (Atp1a2, Hspb7, Tmem182, Sorbs2, Lmod3) MEF2A target genes were chosen for further investigation. Interestingly, siRNA targeting of the MEF2A/D heterodimer revealed a somewhat divergent role in the regulation of Dusp6, a MAPK phosphatase, in cardiac and skeletal myogenic lineages. Furthermore, MEF2D functions as a p38MAPK-dependent repressor of Dusp6 in myoblasts. These data illustrate that MEF2 orchestrates both common and non-overlapping programs of signal-dependent gene expression in skeletal and cardiac muscle lineages.

Redox activation of DUSP4 by N-acetylcysteine protects endothelial cells from Cd²⁺-induced apoptosis

Redox imbalance is a primary cause of endothelial dysfunction (ED). Under oxidant stress, many critical proteins regulating endothelial function undergo oxidative modifications that lead to ED. Cellular levels of glutathione (GSH), the primary reducing source in cells, can significantly regulate cell function via reversible protein thiol modification. N-acetylcysteine (NAC), a precursor for GSH biosynthesis, is beneficial for many vascular diseases; however, the detailed mechanism of these benefits is still not clear. From HPLC analysis, NAC significantly increases both cellular GSH and tetrahydrobiopterin levels. Immunoblotting of endothelial NO synthase (eNOS) and DUSP4, a dual-specificity phosphatase with a cysteine as its active residue, revealed that both enzymes are upregulated by NAC. EPR spin trapping further demonstrated that NAC enhances NO generation from cells. Long-term exposure to Cd(2+) contributes to DUSP4 degradation and the uncontrolled activation of p38 and ERK1/2, leading to apoptosis. Treatment with NAC prevents DUSP4 degradation and protects cells against Cd(2+)-induced apoptosis. Moreover, the increased DUSP4 expression can redox-regulate the p38 and ERK1/2 pathways from hyperactivation, providing a survival mechanism against the toxicity of Cd(2+). DUSP4 gene knockdown further supports the hypothesis that DUSP4 is an antioxidant gene, critical in the modulation of eNOS expression, and thus protects against Cd(2+)-induced stress. Depletion of intracellular GSH by buthionine sulfoximine makes cells more susceptible to Cd(2+)-induced apoptosis. Pretreatment with NAC prevents p38 overactivation and thus protects the endothelium from this oxidative stress. Therefore, the identification of DUSP4 activation by NAC provides a novel target for future drug design.

Dual-specificity phosphatases as molecular targets for inhibition in human disease

SIGNIFICANCE

The dual-specificity phosphatases (DUSPs) constitute a heterogeneous group of cysteine-based protein tyrosine phosphatases, whose members exert a pivotal role in cell physiology by dephosphorylation of phosphoserine, phosphothreonine, and phosphotyrosine residues from proteins, as well as other non-proteinaceous substrates.

RECENT ADVANCES

A picture is emerging in which a selected group of DUSP enzymes display overexpression or hyperactivity that is associated with human disease, especially human cancer, making feasible targeted therapy approaches based on their inhibition. A panoply of molecular and functional studies on DUSPs have been performed in the previous years, and drug-discovery efforts are ongoing to develop specific and efficient DUSP enzyme inhibitors. This review summarizes the current status on inhibitory compounds targeting DUSPs that belong to the MAP kinase phosphatases-, small-sized atypical-, and phosphatases of regenerating liver subfamilies, whose inhibition could be beneficial for the prevention or mitigation of human disease.

CRITICAL ISSUES

Achieving specificity, potency, and bioavailability are the major challenges in the discovery of DUSP inhibitors for the clinics. Clinical validation of compounds or alternative inhibitory strategies of DUSP inhibition has yet to come.

FUTURE DIRECTIONS

Further work is required to understand the dual role of many DUSPs in human cancer, their function-structure properties, and to identify their physiologic substrates. This will help in the implementation of therapies based on DUSPs inhibition.

p38α regulates SERCA2a function

cAMP-dependent protein kinase (PKA) regulates the L-type calcium channel, the ryanodine receptor, and phospholamban (PLB) thereby increasing inotropy. Cardiac contractility is also regulated by p38 MAPK, which is a negative regulator of cardiac contractile function. The aim of this study was to identify the mechanism mediating the positive inotropic effect of p38 inhibition. Isolated adult and neonatal cardiomyocytes and perfused rat hearts were utilized to investigate the molecular mechanisms regulated by p38. PLB phosphorylation was enhanced in cardiomyocytes by chemical p38 inhibition, by overexpression of dominant negative p38α and by p38α RNAi, but not with dominant negative p38β. Treatment of cardiomyocytes with dominant negative p38α significantly decreased Ca(2+)-transient decay time indicating enhanced sarco/endoplasmic reticulum Ca(2+)-ATPase function and increased cardiomyocyte contractility. Analysis of signaling mechanisms involved showed that inhibition of p38 decreased the activity of protein phosphatase 2A, which renders protein phosphatase inhibitor-1 phosphorylated and thereby inhibits PP1. In conclusion, inhibition of p38α enhances PLB phosphorylation and diastolic Ca(2+) uptake. Our findings provide evidence for novel mechanism regulating cardiac contractility upon p38 inhibition.

Myocardial loss of IRS1 and IRS2 causes heart failure and is controlled by p38α MAPK during insulin resistance

Cardiac failure is a major cause of death in patients with type 2 diabetes, but the molecular mechanism that links diabetes to heart failure remains unclear. Insulin resistance is a hallmark of type 2 diabetes, and insulin receptor substrates 1 and 2 (IRS1 and IRS2) are the major insulin-signaling components regulating cellular metabolism and survival. To determine the role of IRS1 and IRS2 in the heart and examine whether hyperinsulinemia causes myocardial insulin resistance and cellular dysfunction via IRS1 and IRS2, we generated heart-specific IRS1 and IRS2 gene double-knockout (H-DKO) mice and liver-specific IRS1 and IRS2 double-knockout (L-DKO) mice. H-DKO mice had reduced ventricular mass; developed cardiac apoptosis, fibrosis, and failure; and showed diminished Akt→forkhead box class O-1 signaling that was accompanied by impaired cardiac metabolic gene expression and reduced ATP content. L-DKO mice had decreased cardiac IRS1 and IRS2 proteins and exhibited features of heart failure, with impaired cardiac energy metabolism gene expression and activation of p38α mitogen-activated protein kinase (p38). Using neonatal rat ventricular cardiomyocytes, we further found that chronic insulin exposure reduced IRS1 and IRS2 proteins and prevented insulin action through activation of p38, revealing a fundamental mechanism of cardiac dysfunction during insulin resistance and type 2 diabetes.

Altered gene expression patterns in muscle ring finger 1 null mice during denervation- and dexamethasone-induced muscle atrophy

Muscle atrophy can result from inactivity or unloading on one hand or the induction of a catabolic state on the other. Muscle-specific ring finger 1 (MuRF1), a member of the tripartite motif family of E3 ubiquitin ligases, is an essential mediator of multiple conditions inducing muscle atrophy. While most studies have focused on the role of MuRF1 in protein degradation, the protein may have other roles in regulating skeletal muscle mass and metabolism. We therefore systematically evaluated the effect of MuRF1 on gene expression during denervation and dexamethasone-induced atrophy. We find that the lack of MuRF1 leads to few differences in control animals, but there were several significant differences in specific sets of genes upon denervation- and dexamethasone-induced atrophy. For example, during denervation, MuRF1 knockout mice showed delayed repression of metabolic and structural genes and blunted induction of genes associated with the neuromuscular junction. In the latter case, this pattern correlates with blunted HDAC4 and myogenin upregulation. Lack of MuRF1 caused fewer changes in the dexamethasone-induced atrophy program, but certain genes involved in fat metabolism and intracellular signaling were affected. Our results demonstrate a new role for MuRF1 in influencing gene expression in two important models of muscle atrophy.

Phenotypic screening with human iPS cell-derived cardiomyocytes: HTS-compatible assays for interrogating cardiac hypertrophy

A major hurdle for cardiovascular disease researchers has been the lack of robust and physiologically relevant cell-based assays for drug discovery. Derivation of cardiomyocytes from human-induced pluripotent stem (iPS) cells at high purity, quality, and quantity enables the development of relevant models of human cardiac disease with source material that meets the demands of high-throughput screening (HTS). Here we demonstrate the utility of iPS cell-derived cardiomyocytes as an in vitro model of cardiac hypertrophy. Exposure of cardiomyocytes to endothelin 1 (ET-1) leads to reactivation of fetal genes, increased cell size, and robust expression of B-type natriuretic peptide (BNP). Using this system, we developed a suite of assays focused on BNP detection, most notably a high-content imaging-based assay designed for phenotypic screening. Miniaturization of this assay to a 384-well format enabled the profiling of a small set of tool compounds known to modulate the hypertrophic response. The assays described here provide consistent and reliable results and have the potential to increase our understanding of the many mechanisms underlying this complex cardiac condition. Moreover, the HTS-compatible workflow allows for the incorporation of human biology into early phases of drug discovery and development.

Signal-dependent repression of DUSP5 by class I HDACs controls nuclear ERK activity and cardiomyocyte hypertrophy

Cardiac hypertrophy is a strong predictor of morbidity and mortality in patients with heart failure. Small molecule histone deacetylase (HDAC) inhibitors have been shown to suppress cardiac hypertrophy through mechanisms that remain poorly understood. We report that class I HDACs function as signal-dependent repressors of cardiac hypertrophy via inhibition of the gene encoding dual-specificity phosphatase 5 (DUSP5) DUSP5, a nuclear phosphatase that negatively regulates prohypertrophic signaling by ERK1/2. Inhibition of DUSP5 by class I HDACs requires activity of the ERK kinase, mitogen-activated protein kinase kinase (MEK), revealing a self-reinforcing mechanism for promotion of cardiac ERK signaling. In cardiac myocytes treated with highly selective class I HDAC inhibitors, nuclear ERK1/2 signaling is suppressed in a manner that is absolutely dependent on DUSP5. In contrast, cytosolic ERK1/2 activation is maintained under these same conditions. Ectopic expression of DUSP5 in cardiomyocytes results in potent inhibition of agonist-dependent hypertrophy through a mechanism involving suppression of the gene program for hypertrophic growth. These findings define unique roles for class I HDACs and DUSP5 as integral components of a regulatory signaling circuit that controls cardiac hypertrophy.