Recent Insights into the Role of PPARs in Disease

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that play important roles in cell proliferation, differentiation, metabolism, and cancer [...].

Molecular Mechanisms of Cardiac Development and Disease

During development, the heart is the first organ to form and function [...].

Cardiomyocyte-Specific Wt1 Is Involved in Cardiac Metabolism and Response to Damage

The Wilms tumor suppressor gene (Wt1) encodes a C2H2-type zinc-finger transcription factor that participates in transcriptional regulation, RNA metabolism, and protein-protein interactions. WT1 is involved in the development of several organs, including the kidneys and gonads, heart, spleen, adrenal glands, liver, diaphragm, and neuronal system. We previously provided evidence of transient WT1 expression in about 25% of cardiomyocytes of mouse embryos. Conditional deletion of Wt1 in the cardiac troponin T lineage caused abnormal cardiac development. A low expression of WT1 has also been reported in adult cardiomyocytes. Therefore, we aimed to explore its function in cardiac homeostasis and in the response to pharmacologically induced damage. Silencing of Wt1 in cultured neonatal murine cardiomyocytes provoked alterations in mitochondrial membrane potential and changes in the expression of genes related to calcium homeostasis. Ablation of WT1 in adult cardiomyocytes by crossing αMHC^MerCreMer mice with homozygous WT1-floxed mice induced hypertrophy, interstitial fibrosis, altered metabolism, and mitochondrial dysfunction. In addition, conditional deletion of WT1 in adult cardiomyocytes increased doxorubicin-induced damage. These findings suggest a novel role of WT1 in myocardial physiology and protection against damage.

Pharmacological Utility of PPAR Modulation for Angiogenesis in Cardiovascular Disease

Peroxisome proliferator activated receptors, including PPARα, PPARβ/δ, and PPARγ, are ligand-activated transcription factors belonging to the nuclear receptor superfamily. They play important roles in glucose and lipid metabolism and are also supposed to reduce inflammation and atherosclerosis. All PPARs are involved in angiogenesis, a process critically involved in cardiovascular pathology. Synthetic specific agonists exist for all PPARs. PPARα agonists (fibrates) are used to treat dyslipidemia by decreasing triglyceride and increasing high-density lipoprotein (HDL) levels. PPARγ agonists (thiazolidinediones) are used to treat Type 2 diabetes mellitus by improving insulin sensitivity. PPARα/γ (dual) agonists are supposed to treat both pathological conditions at once. In contrast, PPARβ/δ agonists are not in clinical use. Although activators of PPARs were initially considered to have favorable effects on the risk factors for cardiovascular disease, their cardiovascular safety is controversial. Here, we discuss the implications of PPARs in vascular biology regarding cardiac pathology and focus on the outcomes of clinical studies evaluating their benefits in cardiovascular diseases.

Editorial (Preface) "Cells/Cells of the Cardiovascular System-Editorial Highlights 2020-2021: The Book Selection"

This introduction provides a preface to the section on "Cells of the Cardiovascular System" in the book entitled "Editor's Choice Articles in 2020-2021" [...].

Peroxisome Proliferator-Activated Receptors and the Hallmarks of Cancer

Peroxisome proliferator-activated receptors (PPARs) function as nuclear transcription factors upon the binding of physiological or pharmacological ligands and heterodimerization with retinoic X receptors. Physiological ligands include fatty acids and fatty-acid-derived compounds with low specificity for the different PPAR subtypes (alpha, beta/delta, and gamma). For each of the PPAR subtypes, specific pharmacological agonists and antagonists, as well as pan-agonists, are available. In agreement with their natural ligands, PPARs are mainly focused on as targets for the treatment of metabolic syndrome and its associated complications. Nevertheless, many publications are available that implicate PPARs in malignancies. In several instances, they are controversial for very similar models. Thus, to better predict the potential use of PPAR modulators for personalized medicine in therapies against malignancies, it seems necessary and timely to review the three PPARs in relation to the didactic concept of cancer hallmark capabilities. We previously described the functions of PPAR beta/delta with respect to the cancer hallmarks and reviewed the implications of all PPARs in angiogenesis. Thus, the current review updates our knowledge on PPAR beta and the hallmarks of cancer and extends the concept to PPAR alpha and PPAR gamma.

The Senescence Markers p16INK4A, p14ARF/p19ARF, and p21 in Organ Development and Homeostasis

It is widely accepted that senescent cells accumulate with aging. They are characterized by replicative arrest and the release of a myriad of factors commonly called the senescence-associated secretory phenotype. Despite the replicative cell cycle arrest, these cells are metabolically active and functional. The release of SASP factors is mostly thought to cause tissue dysfunction and to induce senescence in surrounding cells. As major markers for aging and senescence, p16INK4, p14ARF/p19ARF, and p21 are established. Importantly, senescence is also implicated in development, cancer, and tissue homeostasis. While many markers of senescence have been identified, none are able to unambiguously identify all senescent cells. However, increased levels of the cyclin-dependent kinase inhibitors p16INK4A and p21 are often used to identify cells with senescence-associated phenotypes. We review here the knowledge of senescence, p16INK4A, p14ARF/p19ARF, and p21 in embryonic and postnatal development and potential functions in pathophysiology and homeostasis. The establishment of senolytic therapies with the ultimate goal to improve healthy aging requires care and detailed knowledge about the involvement of senescence and senescence-associated proteins in developmental processes and homeostatic mechanism. The review contributes to these topics, summarizes open questions, and provides some directions for future research.

Small Activating RNAs: Towards the Development of New Therapeutic Agents and Clinical Treatments

Small double-strand RNA (dsRNA) molecules can activate endogenous genes via an RNA-based promoter targeting mechanism. RNA activation (RNAa) is an evolutionarily conserved mechanism present in diverse eukaryotic organisms ranging from nematodes to humans. Small activating RNAs (saRNAs) involved in RNAa have been successfully used to activate gene expression in cultured cells, and thereby this emergent technique might allow us to develop various biotechnological applications, without the need to synthesize hazardous construct systems harboring exogenous DNA sequences. Accordingly, this thematic issue aims to provide insights into how RNAa cellular machinery can be harnessed to activate gene expression leading to a more effective clinical treatment of various diseases.

PPARs and Angiogenesis-Implications in Pathology

Peroxisome proliferator-activated receptors (PPARs) belong to the family of ligand-activated nuclear receptors. The PPAR family consists of three subtypes encoded by three separate genes: PPARα (NR1C1), PPARβ/δ (NR1C2), and PPARγ (NR1C3). PPARs are critical regulators of metabolism and exhibit tissue and cell type-specific expression patterns and functions. Specific PPAR ligands have been proposed as potential therapies for a variety of diseases such as metabolic syndrome, cancer, neurogenerative disorders, diabetes, cardiovascular diseases, endometriosis, and retinopathies. In this review, we focus on the knowledge of PPAR function in angiogenesis, a complex process that plays important roles in numerous pathological conditions for which therapeutic use of PPAR modulation has been suggested.

Transitioning from pharmaceutical opioids: A discrete-time survival analysis of heroin initiation in suburban/exurban communities

INTRODUCTION

Research identifying pathways to heroin use has typically been conducted among urban populations. This study examined heroin initiation following pharmaceutical opioid use in three suburban/exurban Southern California counties.

METHODS

Interviewer-administered surveys collected data among 330 participants (65.9 % male; 63.9 % non-Hispanic white) whose initial use of any opioid was a pharmaceutical opioid. Retrospective discrete-time survival analysis identified predictors of heroin initiation, measured as self-reported age of first heroin use.

RESULTS

Median age of first pharmaceutical opioid use was 17 years; 50.6 % initially acquired pharmaceutical opioids from an illicit source, 56.7 % first used pharmaceutical opioids for recreational purposes, and 86 % initiated heroin use. Average time from first pharmaceutical opioid use to first heroin use was 8.2 years. Drug/alcohol treatment (adjusted Hazard Ratio [aHR]: 0.67, 95 % CI: 0.50, 0.88) was associated with delayed time to heroin initiation. Obtaining opioids from non-medical sources (aHR: 2.21, 95 % CI: 1.55, 3.14) was associated with accelerated time to heroin initiation. Reporting supply problems with obtaining pharmaceutical opioids (e.g., unable to acquire pharmaceutical opioids) was associated with accelerated time to heroin initiation, but the magnitude of this effect was dependent on one's history of methamphetamine use (p < 0.05).

CONCLUSIONS

Time to heroin initiation following pharmaceutical opioid use was accelerated among those reporting supply problems and delayed among those with exposure to substance use treatment. Interventions interrupting supply of opioids might benefit from coordination with evidence-based medication-assisted treatment to minimize the risk of transitioning to heroin use, particularly among those with a long history of non-prescribed pharmaceutical opioid use.

"Generally, you get 86'ed because you're a liability": An application of Integrated Threat Theory to frequently witnessed overdoses and social distancing responses

While rates of opioid overdose deaths in North American have increased exponentially in recent years, most overdoses are not fatal, especially when witnesses are present and can intervene. Previous research has found that some people who use drugs [PWUDs] trained in overdose response might cut social ties with frequent overdosers, leading to more solitary opioid use and risk of death if someone overdoses alone. To examine the phenomenon of social distancing of people who overdose frequently, we used data from fifty-two in-depth qualitative interviews collected in Southern California with PWUDs who had recently witnessed an opioid overdose. Transcripts were reviewed and coded thematically, using the Integrated Threat Theory (ITT) to conceptualize the observed phenomenon. ITT outlines how realistic and symbolic threats are experienced by a group. We found that while some participants acknowledged the role of adulterated street drugs in overdoses, individualized blame was nonetheless imposed. Accusations of careless drug use practices fostered negative stereotyping towards frequent overdosers. This was attributed to the need to summon 911 for rescue, which often resulted in police dispatch. The intergroup relationship between police and PWUDs is precarious as police pose realistic threats onto PWUDs - such as incarceration, eviction, and manslaughter charges - leading to intragroup anxiety among PWUDs about future overdose events, and labelled frequent overdosers as liabilities. These threats, and inter/intra-group conflict, explained one reason how and why non-fatal overdoses led to social distancing events. People who overdose frequently were also accused of breaking the norm of drug user surreptitiousness; a symbolic threat that endangered the group due to police exposure. Social distancing might dampen exposure to the protective effect of peer-led interventions such as take-home naloxone programs, increasing risk of overdose death. This phenomenon highlights how intergroup dynamics are driving intragroup processes. Suggestions for tailoring public health interventions are discussed.

Defined p16High Senescent Cell Types Are Indispensable for Mouse Healthspan

The accumulation of senescent cells can drive many age-associated phenotypes and pathologies. Consequently, it has been proposed that removing senescent cells might extend lifespan. Here, we generated two knockin mouse models targeting the best-characterized marker of senescence, p16^Ink4a. Using a genetic lineage tracing approach, we found that age-induced p16^High senescence is a slow process that manifests around 10-12 months of age. The majority of p16^High cells were vascular endothelial cells mostly in liver sinusoids (LSECs), and to lesser extent macrophages and adipocytes. In turn, continuous or acute elimination of p16^High senescent cells disrupted blood-tissue barriers with subsequent liver and perivascular tissue fibrosis and health deterioration. Our data show that senescent LSECs are not replaced after removal and have important structural and functional roles in the aging organism. In turn, delaying senescence or replacement of senescent LSECs could represent a powerful tool in slowing down aging.

Context-dependent regulation of endothelial cell metabolism: differential effects of the PPARβ/δ agonist GW0742 and VEGF-A

Peroxisome proliferator activated receptor β/δ (PPARβ/δ) has pro-angiogenic functions, but whether PPARβ/δ modulates endothelial cell metabolism to support the dynamic phenotype remains to be established. This study characterised the metabolic response of HUVEC to the PPARβ/δ agonist, GW0742, and compared these effects with those induced by VEGF-A. In HUVEC monolayers, flux analysis revealed that VEGF-A promoted glycolysis at the expense of fatty acid oxidation (FAO), whereas GW0742 reduced both glycolysis and FAO. Only VEGF-A stimulated HUVEC migration and proliferation whereas both GW0742 and VEGF-A promoted tubulogenesis. Studies using inhibitors of PPARβ/δ or sirtuin-1 showed that the tubulogenic effect of GW0742, but not VEGF-A, was PPARβ/δ- and sirtuin-1-dependent. HUVEC were reliant on glycolysis and FAO, and inhibition of either pathway disrupted cell growth and proliferation. VEGF-A was a potent inducer of glycolysis in tubulogenic HUVEC, while FAO was maintained. In contrast, GW0742-induced tubulogenesis was associated with enhanced FAO and a modest increase in glycolysis. These novel data reveal a context-dependent regulation of endothelial metabolism by GW0742, where metabolic activity is reduced in monolayers but enhanced during tubulogenesis. These findings expand our understanding of PPARβ/δ in the endothelium and support the targeting of PPARβ/δ in regulating EC behaviour and boosting tissue maintenance and repair.

PPAR Beta/Delta and the Hallmarks of Cancer

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone receptor family. Three different isoforms, PPAR alpha, PPAR beta/delta and PPAR gamma have been identified. They all form heterodimers with retinoic X receptors to activate or repress downstream target genes dependent on the presence/absence of ligands and coactivators or corepressors. PPARs differ in their tissue expression profile, ligands and specific agonists and antagonists. PPARs attract attention as potential therapeutic targets for a variety of diseases. PPAR alpha and gamma agonists are in clinical use for the treatment of dyslipidemias and diabetes. For both receptors, several clinical trials as potential therapeutic targets for cancer are ongoing. In contrast, PPAR beta/delta has been suggested as a therapeutic target for metabolic syndrome. However, potential risks in the settings of cancer are less clear. A variety of studies have investigated PPAR beta/delta expression or activation/inhibition in different cancer cell models in vitro, but the relevance for cancer growth in vivo is less well documented and controversial. In this review, we summarize critically the knowledge of PPAR beta/delta functions for the different hallmarks of cancer biological capabilities, which interplay to determine cancer growth.

Vascular PPARβ/δ Promotes Tumor Angiogenesis and Progression

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors, which function as transcription factors. Among them, PPARβ/δ is highly expressed in endothelial cells. Pharmacological activation with PPARβ/δ agonists had been shown to increase their angiogenic properties. PPARβ/δ has been suggested to be involved in the regulation of the angiogenic switch in tumor progression. However, until now, it is not clear to what extent the expression of PPARβ/δ in tumor endothelium influences tumor progression and metastasis formation. We addressed this question using transgenic mice with an inducible conditional vascular-specific overexpression of PPARβ/δ. Following specific over-expression of PPARβ/δ in endothelial cells, we induced syngenic tumors. We observed an enhanced tumor growth, a higher vessel density, and enhanced metastasis formation in the tumors of animals with vessel-specific overexpression of PPARβ/δ. In order to identify molecular downstream targets of PPARβ/δ in the tumor endothelium, we sorted endothelial cells from the tumors and performed RNA sequencing. We identified platelet-derived growth factor receptor beta (Pdgfrb), platelet-derived growth factor subunit B (Pdgfb), and the tyrosinkinase KIT (c-Kit) as new PPARβ/δ -dependent molecules. We show here that PPARβ/δ activation, regardless of its action on different cancer cell types, leads to a higher tumor vascularization which favors tumor growth and metastasis formation.

Echocardiographic and Histological Examination of Cardiac Morphology in the Mouse

An increasing number of genetically modified mouse models has become available in recent years. Moreover, the number of pharmacological studies performed in mice is high. Phenotypic characterization of these mouse models also requires the examination of cardiac function and morphology. Echocardiography and magnetic resonance imaging (MRI) are commonly used approaches to characterize cardiac function and morphology in mice. Echocardiographic and MRI equipment specialized for use in small rodents is expensive and requires a dedicated space. This protocol describes cardiac measurements in mice using a clinical echocardiographic system with a 15 MHz human vascular probe. Measurements are performed on anesthetized adult mice. At least three image sequences are recorded and analyzed for each animal in M-mode in the parasternal short-axis view. Afterwards, cardiac histological examination is performed, and cardiomyocyte diameters are determined on hematoxylin-eosin- or wheat germ agglutinin (WGA)-stained paraffin sections. Vessel density is determined morphometrically after Pecam-1 immunostaining. The protocol has been applied successfully to pharmacological studies and different genetic animal models under baseline conditions, as well as after experimental myocardial infarction by the permanent ligation of the left anterior descending coronary artery (LAD). In our experience, echocardiographic investigation is limited to anesthetized animals and is feasible in adult mice weighing at least 25 g.

Andersen's syndrome mutants produce a knockdown of inwardly rectifying K+ channel in mouse skeletal muscle in vivo

Andersen's syndrome (AS) is a rare autosomal disorder that has been defined by the triad of periodic paralysis, cardiac arrhythmia, and developmental anomalies. AS has been directly linked to over 40 different autosomal dominant negative loss-of-function mutations in the KCNJ2 gene, encoding for the tetrameric strong inward rectifying K⁺ channel K_IR2.1. While K_IR2.1 channels have been suggested to contribute to setting the resting membrane potential (RMP) and to control the duration of the action potential (AP) in skeletal and cardiac muscle, the mechanism by which AS mutations produce such complex pathophysiological symptoms is poorly understood. Thus, we use an adenoviral transduction strategy to study in vivo subcellular distribution of wild-type (WT) and AS-associated mutant K_IR2.1 channels in mouse skeletal muscle. We determined that WT and D71V AS mutant K_IR2.1 channels are localized to the sarcolemma and the transverse tubules (T-tubules) of skeletal muscle fibers, while the ∆314-315 AS K_IR2.1 mutation prevents proper trafficking of the homo- or hetero-meric channel complexes. Whole-cell voltage-clamp recordings in individual skeletal muscle fibers confirmed the reduction of inwardly rectifying K⁺ current (I_K1) after transduction with ∆314-315 K_IR2.1 as compared to WT channels. Analysis of skeletal muscle function revealed reduced force generation during isometric contraction as well as reduced resistance to muscle fatigue in extensor digitorum longus muscles transduced with AS mutant K_IR2.1. Together, these results suggest that K_IR2.1 channels may be involved in the excitation-contraction coupling process required for proper skeletal muscle function. Our findings provide clues to mechanisms associated with periodic paralysis in AS.

CDK9 Regulates Apoptosis of Myoblast Cells by Modulation of microRNA-1 Expression

Cdk9 is the catalytic core of the positive transcription elongation factor b (P-TEFb) and regulates transcriptional elongation factors by phosphorylation of RNA pol II. Apart from its role on myogenic gene expression, Cdk9 regulation of muscle-specific microRNAs in the early stage of cardiomyogenesis is poorly understood. Here we demonstrate that Cdk9 not only regulates myogenic transcription factors, but also controls muscle-specific microRNAs. During cardiac differentiation of mouse embryonic stem cells, high Cdk9 expression preceded up-regulation of miR-1. To investigate potential regulatory roles of Cdk9 on cardiac microRNAs and myogenesis genes, we overexpressed Cdk9 in myoblast C2C12 cells, which resulted in significant induction of miR-1 and miR-206, while miR-133 was downregulated. Moreover, expression levels of MyoD and Srf, key regulators of myogenesis, also increased in cells with overexpression of Cdk9. We further observed Cdk9-mediated apoptosis in C2C12 cells corresponding to induction of miR-1 expression levels. Thus, Cdk9 plays a complex role in myocyte progenitor differentiation and apoptosis by regulating myogenic protein and muscle-specific microRNA expression. J. Cell. Biochem. 119: 547-554, 2018. © 2017 Wiley Periodicals, Inc.

Dnmt2/Trdmt1 as Mediator of RNA Polymerase II Transcriptional Activity in Cardiac Growth

Dnmt2/Trdmt1 is a methyltransferase, which has been shown to methylate tRNAs. Deficient mutants were reported to exhibit various, seemingly unrelated, defects in development and RNA-mediated epigenetic heredity. Here we report a role in a distinct developmental regulation effected by a noncoding RNA. We show that Dnmt2-deficiency in mice results in cardiac hypertrophy. Echocardiographic measurements revealed that cardiac function is preserved notwithstanding the increased dimensions of the organ due to cardiomyocyte enlargement. Mechanistically, activation of the P-TEFb complex, a critical step for cardiac growth, results from increased dissociation of the negatively regulating Rn7sk non-coding RNA component in Dnmt2-deficient cells. Our data suggest that Dnmt2 plays an unexpected role for regulation of cardiac growth by modulating activity of the P-TEFb complex.

Police bribery and access to methadone maintenance therapy within the context of drug policy reform in Tijuana, Mexico

AIMS

In 2009, Mexico passed legislation to decriminalize drug possession and improve access to addiction treatment. We undertook research to assess the implementation of the reform among a cohort of people who inject drugs (PWID) in Tijuana. This study specifically sought to determine whether discretionary policing practices like extortion impact access to methadone maintenance therapy (MMT) in Tijuana, a city characterized by high levels of drug-related harms.

METHODS

Generalized estimating equation analyses were used to construct longitudinal confounding models to determine the association between paying a police bribe and MMT enrolment among PWID in Tijuana enrolled in a prospective cohort study. Outcome of interest was MMT enrolment in the past six months. Data on police interactions and MMT enrolment were also obtained.

RESULTS

Between October, 2011 and September, 2013, 637 participants provided 1825 observations, with 143 (7.8%) reports of MMT enrolment during the study period. In a final confounding model, recently reporting being forced to pay a bribe to police was significantly associated with an increased likelihood of accessing MMT (adjusted odds ratio=1.69, 95% confidence interval: 1.02-2.81, p=0.043). However, in 56 (39.2%) cases, MMT enrolment ceased within six months. The majority of participant responses cited the fact that MMT was too expensive (69.1%).

DISCUSSION

Levels of MMT access were low. PWID who experienced police extortion were more likely to access MMT at baseline, though this association decreased during the study period. Coupled with the costs of MMT, this may compromise MMT retention among PWID.

TRF2 acts as a transcriptional regulator in tumor angiogenesis

We recently showed that telomeric repeat-binding factor 2 (TRF2) regulates gene expression to promote angiogenesis. We found that TRF2 is highly expressed in tumor vessels and transcriptionally activates platelet-derived growth factor receptor β to promote endothelial cell angiogenic properties independently of its function in telomere protection. This work identifies TRF2 as a promising dual target for cancer therapy.

The Telomeric Protein TRF2 Regulates Angiogenesis by Binding and Activating the PDGFRβ Promoter

Telomeric repeat binding factor 2 (TRF2), which plays a central role in telomere capping, is frequently increased in human tumors. We reveal here that TRF2 is expressed in the vasculature of most human cancer types, where it colocalizes with the Wilms' tumor suppressor WT1. We further show that TRF2 is a transcriptional target of WT1 and is required for proliferation, migration, and tube formation of endothelial cells. These angiogenic effects of TRF2 are uncoupled from its function in telomere capping. Instead, TRF2 binds and transactivates the promoter of the angiogenic tyrosine kinase platelet-derived growth factor receptor β (PDGFRβ). These findings reveal an unexpected role of TRF2 in neoangiogenesis and delineate a distinct function of TRF2 as a transcriptional regulator.

The podocyte protein nephrin is required for cardiac vessel formation

Nephrin (NPHS1) has been described as an important structural protein of kidney podocytes. Mutations in this gene lead to the Finnish-type congenital nephrotic syndrome. More recently, a role of nephrin as a signalling molecule in kidney podocytes has been identified. Here, we show that nephrin not only has a function in kidney podocytes, but is also required for cardiovascular development. Nephrin is expressed in the epicardium and coronary vessels during human and mouse embryonic development. Nephrin knockout embryos showed abnormal epicardial cell morphology and, at later stages of development, a reduced number of coronary vessels due to increased apoptosis, and in addition, cardiac fibrosis. Connexin 43, which is required for coronary vessel formation, was downregulated in nephrin knockout embryos. Expression of the p75NTR neurotrophin receptor, a known mediator of apoptosis, was increased in mutants. Furthermore, co-immunoprecipitation studies demonstrated a direct interaction of nephrin with p75NTR. Primary nephrin-deficient cardiac cells showed a 5-fold higher rate of apoptosis in response to progenitor of nerve growth factor compared with wild-type cells, which could be rescued by RNAi against p75NTR. Taken together, our data demonstrate that nephrin directly interacts with p75NTR and reveal an important role for nephrin in murine cardiac development by permitting survival of cardiovascular progenitor cells.

Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is highly expressed in liposarcoma and promotes migration and proliferation

Aberrations of specialized metabolic pathways might be implicated in the development of neoplasias. Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors with important functions in metabolism. PPARβ/δ and PPARγ act in the proliferation and differentiation of adipose tissue progenitor cells. Thus, a potential use of PPARγ agonists for the treatment of liposarcoma had been suggested, but clinical trials failed to detect beneficial effects. We show here that PPARδ is highly expressed in liposarcoma compared to lipoma and correlates with proliferation. Stimulation of liposarcoma cell lines with a specific PPARδ agonist increases proliferation, which is abolished by a PPARδ-siRNA or a specific PPARδ antagonist. Expression of the adipose tissue secretory factor leptin is lower in liposarcoma compared to lipoma and leptin reduces proliferation of liposarcoma cell lines. PPARδ activation stimulates cell migration whereas leptin diminishes it. We demonstrate that PPARδ directly represses leptin as: (a) leptin becomes down-regulated upon PPARδ activation; (b) PPARδ represses leptin promoter activity in different sarcoma cell lines; (c) deletion of a PPAR/RxR binding element in the leptin promoter abolishes repression by PPARδ; and (d) in chromatin immunoprecipitation we confirm in vivo binding of PPARδ to the leptin promoter. Our data suggest inhibition of PPARδ as a potential novel strategy to reduce liposarcoma cell proliferation.

Enhanced L-type calcium currents in cardiomyocytes from transgenic rats overexpressing SERCA2a

BACKGROUND

Previous research reported that transgenic rats overexpressing the sarco(endo)plasmic reticulum Ca(2+)-ATPase SERCA2a exhibit improved contractile function of the myocardium. Furthermore, impaired Ca(2+) uptake and reduced relaxation rates in rats with diabetic cardiomyopathy were partially rescued by transgenic expression of SERCA2a in the heart.

OBJECTIVE

To explore whether enhanced Ca(2+) cycling in the cardiomyocytes of SERCA2a transgenic rats is associated with changes in L-type Ca(2+) (I(Ca-L)) currents.

METHODS

The patch-clamp technique was used to measure whole-cell currents in cardiomyocytes from transgenic rats overexpressing SERCA2a and from wild-type (nontransgenic) animals.

RESULTS

The amplitudes of I(Ca-L) currents at depolarizing pulses ranging from -45 mV to 0 mV (350 ms duration, 1 Hz) were significantly higher in cardiomyocytes of SERCA2a transgenic rats than in nontransgenic rats (1985±48 pA [n=32] versus 1612±55 pA [n=28], respectively). The inactivation kinetics of I(Ca-L) showed subtle differences with increased tau fast and tau slow decay constants in cardiomyocytes of SERCA2a transgenic animals. Beta-adrenergic stimulation with 50 nM isoproterenol reduced tau fast and tau slow decay constants in cardiomyocytes of transgenic rats to values that were not significantly different from those in normal cardiomyocytes. Furthermore, isoproterenol enhanced I(Ca-L) currents 3.2-fold and 2.3-fold in cardiomyocytes with and without the SERCA2a transgene, respectively, and this effect was abolished by buffering intracellular Ca(2+) with BAPTA.

CONCLUSIONS

These findings indicate that enhanced Ca(2+) cycling in the hearts of SERCA2a transgenic rats, both under normal conditions and during beta-adrenergic stimulation, involves changes in I(Ca-L) currents. Modified I(Ca-L) kinetics may contribute, to some extent, to the improved contractile function of the myocardium of transgenic rats.

Peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) acts as regulator of metabolism linked to multiple cellular functions

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors. They function as ligand activated transcription factors. They exist in three isoforms, PPARalpha, PPARbeta (formerly PPARdelta), and PPARgamma. For all PPARs lipids are endogenous ligands, linking them directly to metabolism. PPARs form heterodimers with retinoic X receptors, and, upon ligand binding, modulate gene expression of downstream target genes dependent on the presence of co-repressors or co-activators. This results in cell-type specific complex regulations of proliferation, differentiation and cell survival. Specific synthetic agonists for all PPARs are available. PPARalpha and PPARgamma agonists are already in clinical use for the treatment of hyperlipidemia and type 2 diabetes, respectively. More recently, PPARbeta activation came into focus as an interesting novel approach for the treatment of metabolic syndrome and associated cardiovascular diseases. Although the initial notion was that PPARbeta is expressed ubiquitously, more recently extensive investigations have been performed demonstrating high PPARbeta expression in a variety of tissues, e.g. skin, skeletal muscle, adipose tissue, inflammatory cells, heart, and various types of cancer. In addition, in vitro and in vivo studies using specific PPARbeta agonists, tissue-specific over-expression or knockout mouse models have demonstrated a variety of functions of PPARbeta in adipose tissue, muscle, skin, inflammation, and cancer. We will focus here on functions of PPARbeta in adipose tissue, skeletal muscle, heart, angiogenesis and cancer related to modifications in metabolism and the identified underlying molecular mechanisms.

Peroxisome proliferator-activated receptor beta stimulation induces rapid cardiac growth and angiogenesis via direct activation of calcineurin

AIMS

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors. PPARbeta agonists were suggested as potential drugs for the treatment of metabolic syndrome, but effects of PPARbeta activation on cardiac growth and vascularization are unknown. Thus, we investigated the consequences of pharmacological PPARbeta activation on the heart and the underlying molecular mechanisms.

METHODS AND RESULTS

Male C57/Bl6 mice were injected with the specific PPARbeta agonists GW0742 or GW501516, or vehicle. Cardiomyocyte size and vascularisation were determined at different time points. Expression differences were investigated by quantitative reverse transcriptase-polymerase chain reaction and western blotting. In addition, the effects of PPARbeta stimulation were compared with hearts of mice undergoing long-term voluntary exercise or pharmacological PPARalpha activation. Five hours after GW0742 injection, we detected an enhanced angiogenesis compared with vehicle-injected controls. After 24 h, the heart-to-body weight ratios were higher in mice injected with either GW0742 or GW501516 vs. controls. The increased heart size was due to cardiomyocyte enlargement. No signs of pathological cardiac hypertrophy (i.e. apoptosis, fibrosis, or deteriorated cardiac function) could be detected. The effects are mediated via calcineurin A (CnA) activation as: (i) CnA was upregulated, (ii) GW0742 administration or co-transfection of PPARbeta significantly stimulated the activity of the CnA promoter, (iii) PPARbeta protein bound directly to the CnA promoter, (iv) the CnA target genes NFATc3, Hif-1alpha, and Cdk 9 were upregulated in response to PPARbeta stimulation, and (v) the inhibition of CnA activity by cyclosporine A abolished the hypertrophic and angiogenic responses to PPARbeta stimulation.

CONCLUSION

Our data suggest PPARbeta pharmacological activation as a novel approach to increase cardiac vascularization and cardiac muscle mass.

Role of the Wilms' tumour transcription factor, Wt1, in blood vessel formation

Blood vessel formation is important for normal organ development and tumour growth. A highly specialised developmental program of vessel formation exists in the heart and is essential for normal cardiogenesis. From mouse models, it became clear that the Wilms' tumour protein Wt1 is required for normal heart development. Originally identified as a tumour suppressor gene based on its mutational inactivation in Wilms' tumour or nephroblastoma, Wt1 is nowadays recognised to have much broader functions in organogenesis and pathophysiology. The multiple tasks of Wt1 are not only limited to the kidney but involve the heart and vascular system as well. In this review, we focus on recent findings about the importance of Wt1 in heart and coronary vessel development and the identified molecular mechanisms. In addition, we discuss the implication of Wt1 in the vascular response to myocardial ischaemia and its oncogenic potential as a promoter of tumour angiogenesis.

A novel Wilms' tumor 1 gene mutation in a child with severe renal dysfunction and persistent renal blastema

The Wilms' tumor suppressor gene WT1 is an important regulator of development. Mutations in this gene have been associated with Wilms' tumor, Frasier syndrome, and Denys-Drash syndrome, as well as isolated glomerular disease. Here we report the case of a 4-month-old girl, who presented with end-stage renal disease, thrombopenia, anemia, and cardiac hypertrophy accompanied by severe hypertension. Histological analysis of kidney biopsies revealed a massive and diffuse nephroblastomatosis with a dramatic reduction in the number of glomeruli. Although no normal cortical nephrons could be detected, medullary organization was nearly normal. Sequence analysis demonstrated a heterozygous nonsense mutation in exon 9 of WT1, which leads to a truncation of the WT1 protein at the beginning of zinc finger 3. Given the requirement of WT1 for normal development of the kidney and heart, these data raise the hypothesis that the mutation identified was responsible for the severe phenotype observed in our patient.

The Wilms' tumor suppressor WT1 is associated with melanoma proliferation

Expression of the Wilms' tumor suppressor WT1 has been demonstrated in a variety of tumors and tumor cell lines, e.g., in breast cancer and melanoma cell lines. Its role is controversial, with evidence for both tumor-promoting and tumor-suppressing activities. In this paper, we show that WT1 is expressed in malignant melanoma in >80% of the tumor cells, but not in normal skin or benign melanocytic nevi in vivo. We detected an unusual shift of WT1 isoforms towards WT1(+17AA/+KTS) in melanoma. WT1 shared an overlapping expression with proliferating nuclear cell antigen and with Nestin and Zyxin, which are involved in melanoma cell proliferation. To investigate whether WT1 is directly involved in melanoma cell proliferation, we made use of an RNAi approach in vitro. WT1 silencing significantly reduced the expression of Nestin and Zyxin and resulted in inhibition of melanoma cell proliferation as determined by a reduced BrdU incorporation. These findings suggest a direct role of WT1 in melanoma proliferation, which might be mediated via Nestin and Zyxin. Furthermore, expression of WT1 in vivo clearly discriminates between benign acquired nevi and malignant melanomas and appears to be correlated with melanocytic atypia and malignancy.

The Wilms tumor suppressor Wt1 promotes cell adhesion through transcriptional activation of the alpha4integrin gene

Cell-matrix interaction through specific adhesion molecules is a critical step during organ development. In addition, down-regulation of cell adhesion receptors may promote tumor invasion and metastasis. We show here that the Wilms tumor suppressor Wt1, which is necessary for normal development of the epicardium, coronary vessels, genitourinary system, and other tissues, activates transcription of the alpha4integrin gene. Binding of the Wt1(-KTS) form, which is transcriptionally active, to the proximal alpha4integrin promoter was demonstrated by electrophoretic mobility shift assay and chromatin immunoprecipitation. A reporter construct harboring approximately 1.9 kb of the human alpha4integrin gene promoter was activated significantly by transient co-transfection of a Wt1(-KTS) expression plasmid. Introducing mutations in two identified Wt1(-KTS) binding motifs in the proximal promoter of the alpha4integrin gene abrogated this stimulatory effect. Endogenous alpha4integrin transcripts were increased more than 3-fold in human embryonic kidney 293 cells with stable expression of the Wt1(-KTS) protein. Wt1-overexpressing cells showed augmented adhesion to the alpha4integrin ligand vascular cell adhesion molecule-1 that was abolished upon incubation with an inhibitory alpha4integrin antibody. Double immunofluorescent staining revealed co-localization of Wt1 and alpha4integrin in the developing epicardium of mouse embryos. Cardiac expression of alpha4integrin was reduced significantly in embryos with a homozygous Wt1 defect (Wt1-/-). These findings demonstrate that Wt1 can support cell adhesion through enhanced expression of alpha4integrin. This transcriptional activation of the alpha4integrin gene by Wt1(-KTS) might contribute to normal formation of the epicardium and other tissues in the developing embryo.

Intermediate filament protein nestin is expressed in developing kidney and heart and might be regulated by the Wilms' tumor suppressor Wt1

Nestin is an intermediate filament protein originally described in neural stem cells and a variety of progenitor cells. More recently, nestin was detected in rat kidney podocytes. We show here that nestin is expressed in a developmentally regulated pattern in the kidney. Nestin was detected by immunohistochemistry in the condensing mesenchyme surrounding the ureter, in developing glomeruli, in podocytes of the adult kidney, and in a podocyte cell line. Nestin shared a striking overlap in expression with the Wilms' tumor suppressor Wt1. Nestin was significantly upregulated in a cell line with inducible Wt1 expression upon induction of Wt1. Cotransfection experiments in human embryonic kidney cells (HEK293) revealed stimulation of a nestin intron 2 enhancer element up to six-fold by the Wt1(-KTS) splice variant. Nestin expression was significantly reduced in an inducible mouse model of glomerular disease. This model is based on podocyte-specific overexpression of Pax2 and associated with a loss of Wt1 expression. Furthermore, also in the developing heart, nestin was found in an overlapping pattern with Wt1 in the epicardium and the forming coronary vessels. Strikingly, in the hearts of Wt1 knockout mice, nestin was barely detectable compared with the hearts of wild-type embryos. Our results show that nestin is expressed at different stages of kidney and cardiac development and suggest that its expression in these organs might be regulated by the Wilms' tumor suppressor Wt1.

An Inducible Mouse Model for PAX2-Dependent Glomerular Disease: Insights into a Complex Pathogenesis

Pax2 is a transcription factor with important functions during kidney development . Ectopic expression of Pax2 in podocytes has been reported in various glomerular diseases , but the functional relevance remains unknown. We developed an inducible mouse model that allows activation of Pax2 specifically in podocytes. Persistent expression of Pax2 did not interfere with the initial differentiation of podocytes, but mice ectopically expressing PAX2 developed end-stage renal failure soon after birth. Similarly, activation of PAX2 in healthy adult animals resulted in renal disease within 3 weeks after podocyte-specific induction of a deleter Cre. PAX2 activation caused repression of the podocyte key regulator molecule Wt1 and consequently a dramatic reduction of nephrin expression. Recruitment of the groucho-related protein TLE4 may be involved in converting Pax2 into a transcriptional repressor of Wt1. Finally, treatment of mice with an angiotensin-converting enzyme (ACE) inhibitor normalized renal function and induced upregulation of the important structural molecule nephrin via a Wt1-independent pathway. Our data demonstrate the functional significance of PAX2 reexpression in mature podocytes for the development of glomerular diseases and suggest that reactivation of PAX genes in terminally differentiated cells leads to a more dedifferentiated phenotype.

Coronary vessel development requires activation of the TrkB neurotrophin receptor by the Wilms' tumor transcription factor Wt1

The formation of intramyocardial blood vessels is critical for normal heart development and tissue repair after infarction. We report here expression of the Wilms' tumor gene-1, Wt1, in coronary vessels, which could contribute to the defective cardiac vascularization in Wt1-/- mice. Furthermore, the high-affinity neurotrophin receptor TrkB, which is expressed in the epicardium and subepicardial blood vessels, was nearly absent from Wt1-deficient hearts. Activation of Wt1 in an inducible cell line significantly enhanced TrkB expression. The promoter of NTRK2, the gene encoding TrkB, was stimulated approximately 10-fold by transient cotransfection of a Wt1 expression construct. The critical DNA-binding site for activation of the NTRK2 promoter by Wt1 was delineated by DNase I footprint analysis and electrophoretic mobility shift assay. Transgenic experiments revealed that the identified Wt1 consensus motif in the NTRK2 promoter was necessary to direct expression of a reporter gene to the epicardium and the developing vasculature of embryonic mouse hearts. Finally, mice with a disrupted Ntrk2 gene lacked a significant proportion of their intramyocardial blood vessels. These findings demonstrate that transcriptional activation of the TrkB neurotrophin receptor gene by the Wilms' tumor suppressor Wt1 is a crucial mechanism for normal vascularization of the developing heart.

A splice variant of the Wilms' tumour suppressor Wt1 is required for normal development of the olfactory system

Neuronal lineage formation in the developing olfactory epithelium has been extensively studied at the cellular level, but little is known about the genes that control proliferation and differentiation of neuronal progenitor cells. Here, we report that the Wilms' tumour zinc-finger protein, Wt1, is required for normal formation of the olfactory epithelium. Wt1 was detected by immunohistochemistry in the developing olfactory epithelium of wild-type embryos between gestational days E9.5 and E18.5. Embryos with complete lack of Wt1 and embryos with selective ablation of the alternatively spliced Wt1(+KTS) isoform both had thinner olfactory epithelia and fewer neuronal progenitor cells than do normal animals. Mash1 and neurogenin 1, two basic helix-loop-helix transcription factors with critical functions during olfactory neuron development, were reduced in the Wt1(+KTS)-/- mutants compared with the wild-type mice. Stable expression of the Wt1(+KTS) isoform, but not of the Wt1(-KTS) variant, upregulated Mash1 mRNA and protein in vitro. The olfactory epithelia of mouse embryos, which lacked the Wt1(-KTS) protein, appeared normal. However, formation of the neural retina was severely impaired in the Wt1(-KTS)-/- mutants. These findings demonstrate that the Wt1(+KTS) protein, which has been proposed to play a role in mRNA processing, acts upstream of Mash1 to promote the development of the olfactory epithelium. Furthermore, neuron formation depends on distinct functions of alternatively spliced Wt1 products in the embryonic retina and the olfactory epithelium.

The major podocyte protein nephrin is transcriptionally activated by the Wilms' tumor suppressor WT1

NPHS1 encodes the structural protein nephrin, which has a crucial role in the filtration barrier of the glomerular podocyte. Mutations or deregulation of NPHS1 are associated with a variety of renal diseases, including the Finnish type congenital nephrotic syndrome. This study analyzed a potential regulation of nephrin by the Wilms' tumor protein, Wt1. Using an inducible U2OS osteosarcoma cell line, it is shown that upon Wt1 induction, endogenous nephrin mRNA becomes highly upregulated. Co-transfection studies demonstrate that Wt1 can activate the nephrin promoter >10-fold. DNase footprinting and mutation analysis identify a Wt1 responsive element in the nephrin promoter, which is required for the binding of Wt1 protein. Mutations or deletion of this Wt1 responsive element completely abolished transactivation of the nephrin promoter by Wt1. Moreover, transgenic analysis demonstrates the requirement of the identified binding site to direct podocyte-specific expression of a reporter gene in transgenic mice, thus confirming the importance of this site for the regulation of nephrin in vivo. Finally, it is shown that nephrin expression is lowest in kidneys of mice that lack specifically the Wt1(-KTS) splice variant, but in comparison with wild-type littermates, it is also reduced in animals with disruption of the Wt1(+KTS) splice variant. Taken together, these data identify nephrin as a direct transcriptional target for Wt1 and underline the importance of Wt1 as a key regulator in podocyte function.

Oxygen-regulated expression of the Wilms' tumor suppressor Wt1 involves hypoxia-inducible factor-1 (HIF-1)

The Wilms' tumor gene Wt1 is unique among tumor suppressors because of its requirement for the development of certain organs. We recently described de novo expression of Wt1 in myocardial blood vessels of ischemic rat hearts. The purpose of this study was to analyze the mechanism(s) of hypoxic/ischemic induction of Wt1. We show here that Wt1 mRNA and protein is up-regulated in the heart and kidneys of rats exposed to normobaric hypoxia (8% O2). Ectopic Wt1 immunoreactivity was detected in renal tubules of hypoxic rats, which also expressed the antiapoptotic protein Bcl-2 and contained significantly fewer TUNEL-positive cells than in normoxic kidneys. Wt1 expression was enhanced in the osteosarcoma line U-2OS and in Reh lymphoblast cells that were grown either at 1% O2 or in the presence of CoCl2 and desferrioxamine, respectively. The promoter of the Wt1 gene was capable of mediating expression of a luciferase reporter in response to hypoxia. We identified a hypoxia-responsive element in the Wt1 sequence that bound to hypoxia-inducible factor-1 (HIF-1) and was required for activation of the Wt1 promoter by CoCl2 and HIF-1. These findings demonstrate that Wt1 expression can be stimulated by hypoxia, which involves activation of the Wt1 promoter by HIF-1.

1,25-dihydroxyvitamin D3-induced apoptosis of retinoblastoma cells is associated with reciprocal changes of Bcl-2 and bax

The active vitamin D metabolite 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) and related substances have previously been tested in tissue culture and animal models of retinoblastoma for their use as anti-tumor drugs. However, despite of the potential therapeutic value, the molecular mechanisms through which 1,25-(OH)(2)D(3) inhibits the growth of retinoblastoma cells are incompletely understood. To elucidate possible signalling pathways for the anti-proliferative action of vitamin D compounds in retinal tumor cells, we analyzed the effect of 1,25-(OH)(2)D(3) and its synthetic analogue KH1060 on the growth of human retinoblastoma-derived Y79 cells. Vitamin D receptor (VDR) mRNA was detected by reverse transcription PCR in Y79 cells and in tissue specimens of human retinoblastoma. VDR transcripts were confirmed at the protein level by strong immunostaining of solid retinal tumors for VDR. Incubation with 1,25-(OH)(2)D(3) and KH1060 (10(-10)-10(-6)moll(-1)) decreased the number of Y79 cells in a timely and dose-dependent manner. Treatment with 1,25-(OH)(2)D(3) (10(-10)moll(-1)) for 24 hr caused cell cycle arrest in the G0/1 phase. Apoptosis of Y79 cells in response to 1,25-(OH)(2)D(3) was demonstrated by the means of TdT-dUTP terminal nick-end labelling (TUNEL), annexin V staining, and detection of DNA fragmentation on agarose gels. 1,25-(OH)(2)D(3)-induced programmed death of Y79 cells was accompanied by a concentration-dependent increase in Bax protein and a reduction in Bcl-2 content. These findings suggest that 1,25-(OH)(2)D(3) inhibits the growth of retinoblastoma cells by causing cell cycle arrest and apoptosis. 1,25-(OH)(2)D(3)-induced programmed death of retinoblastoma cells appears to involve reciprocal changes in Bcl-2 and Bax proteins.

Characterization of stretch-activated ion currents in isolated atrial myocytes from human hearts

To explore further the mechanisms that may underlie cardiac arrhythmia, we analysed stretch-activated ion currents in human atrial myocytes. Longitudinal stretch of freshly isolated atrial myocytes prolonged the duration of action potentials, depolarized the resting membrane potential and caused extra action potentials. Under voltage-clamp conditions, the amplitude of stretch-induced transmembrane currents increased reversibly with the intensity of stretch. Stretch-activated currents ( I(SAC)) had a reversal potential of 0 mV and were insensitive to substitution of Cl(-) with aspartate ions in the extracellular fluid. I(SAC) was suppressed by 5 micro M gadolinium (Gd(3+)). Furthermore, mechanical stretch decreased transmembrane ion fluxes through L-type calcium channels (I(Ca,L)). This reduction of I(Ca,L) was inhibited by dialysing the cells for 5 min with 5 mM BAPTA prior to application of stretch. In contrast, both BAPTA and removal of Ca(2+) from the extracellular bathing solution had no significant effect on stretch activation of I(SAC). These findings suggest that non-selective cation channels in human atrial myocytes are sensitive to mechanical stimulation. We propose that activation of transmembrane influx of cations, preferentially Na(+), by local stretch may play a role in cardiac arrhythmia.

The complex life of WT1

The Wilms' tumour gene, WT1, encodes a zinc-finger transcription factor that is inactivated in a subset of Wilms' tumours. Mutation analysis in human patients and genetic experiments in mice have revealed that WT1 has a role much wider than just tumour suppression. Alternative splicing, RNA editing, and the use of alternative translation initiation sites generate a multitude of isoforms, which seem to have overlapping but also distinct functions during embryonic development and the maintenance of organ function. Recently, mouse strains lacking the WT1(-KTS) or WT1(+KTS) splice variants of exon 9 were generated. More severe defects of kidneys and gonads are found in mice lacking the WT1(-KTS) variant. Animals lacking the WT1(+KTS) variant show disturbed podocyte function and male-to-female sex reversal. Alternative splicing of exon 5, however, might not modify WT1 function dramatically. Recently, it was also described that reduction of WT1 levels in the kidney results in glomerulosclerosis and upregulation of WT1 in the heart might contribute to neovascularization after infarction.

Mechanically induced potentials in atrial fibroblasts from rat hearts are sensitive to hypoxia/reoxygenation

Membrane potential changes of atrial fibroblasts in response to mechanical stress have been considered to modulate the rhythmic electrical activity of healthy hearts. Our recent findings suggest that cardiac arrhythmia after infarction is related to enhanced susceptibility of the fibroblasts to physical stretch. In this study, we analysed the effect of hypoxia/reoxygenation, which are major components of tissue ischemia/reperfusion, on the membrane potential of atrial fibroblasts. Intracellular microelectrode recordings were performed together with isometric force measurements on isometrically contracting right atrial tissue preparations from adult rats. Lowering the oxygen tension in the perfusate from 80 kPa to 3.5 kPa reduced active force development and decreased the resting membrane potential of the cardiac fibroblasts from -23+/-5 mV to -5+/-2 mV ( n=35). Application of gadolinium (40 microM) to inhibit non-selective cation channels prevented hypoxia-induced membrane depolarization of the fibroblasts. Reoxygenation of the myocardial tissue resulted in a transient increase of the resting membrane potential to maximally -60+/-8 mV. These findings indicate that transmembrane currents in atrial fibroblasts are sensitive to changes in tissue oxygenation. In conclusion, altered electro-mechanical function of the ischemic heart may possibly involve changes of the membrane potential of the cardiac fibroblasts.

The Wilms' tumor suppressor Wt1 encodes a transcriptional activator of the class IV POU-domain factor Pou4f2 (Brn-3b)

The Wilms' tumor gene Wt1 encodes a zinc finger protein, which is required for normal formation of the genitourinary system and mesothelial tissues. Our recent findings indicate that Wt1 also plays a critical role in the development of ganglion cells in the vertebrate retina. Here we show that the POU-domain factor Pou4f2 (formerly Brn-3b), which is necessary for retinal ganglion cell survival, is up-regulated in human embryonic kidney (HEK)293 cells with stable Wt1 expression. Consistent with our previous observations of increased Pou4f2 mRNA in stably Wt1-transfeced HEK293 cells [EMBO J. 21 (2002) 1398], endogenous Pou4f2 was also elevated at the protein level in the HEK293 transfectants as well as in U2OS osteosarcoma cells that expressed an inducible Wt1 isoform. Transient co-transfection of a Wt1 expression construct activated a Pou4f2 promoter-reporter construct approximately 4-fold. Stimulation of the Pou4f2 promoter required a Wt1 binding element that was similar to a degenerative consensus site previously identified in other Wt1 responsive genes. Double-immunofluorescent labeling revealed co-expression of Pou4f2 and Wt1 in glomerular podocytes of adult kidney and in developing retinal ganglion cells of mouse embryos. Pou4f2 immunoreactivity was absent from the retinas of Wt1(-/-) embryos. In conclusion, we identified Pou4f2 as a novel downstream target gene of Wt1. Co-localization of both proteins in glomerular podocytes of the kidney and in developing retinal ganglion cells suggests a role for Wt1-Pou4f2 interaction in these tissues.

Time-dependent changes of the susceptibility of cardiac contractile function to hypoxia-reoxygenation after myocardial infarction in rats

In this study we analyzed the susceptibility of contractile function of the myocardium to hypoxia-reoxygenation after infarction. For this purpose, the contractility of isolated papillary muscles from rats was studied at high oxygen tension (pO2 80 kPa) and during hypoxia (pO2 3 kPa) with subsequent reoxygenation at variable intervals between 15 h and 9 weeks after permanent ligation of the left coronary artery. Hypoxic exposure reduced the contractile performance of the preparations to a similar extent in both groups. Notably, the contractility and, in particular, the relaxation rates recovered more completely from hypoxia in the hypertrophied myocardium of rats with coronary artery ligation than in sham-operated (SO) animals. The recovery of contractile function was improved maximally between 6 and 9 weeks after myocardial infarction (MI). The lower sensitivity of the (post)ischemic myocardium to hypoxia-reoxygenation correlated with enhanced left ventricular glutathione peroxidase (GSH-Px) activity (15 h to 9 weeks post-MI) and 2-3-fold increased expression levels (15 h to 6 weeks post-MI) of the 72 kDa heat shock protein (HSP72) in the papillary muscles. These findings suggest that the greater antioxidant potential and, possibly, stimulation of HSPs contribute to the sustained tolerance of the myocardium to hypoxia-reoxygenation injury after infarction.

Transgenic overexpression of the sarcoplasmic reticulum Ca2+ATPase improves reticular Ca2+ handling in normal and diabetic rat hearts

Slowed relaxation in diabetic cardiomyopathy (CM) is partially related to diminished expression of the sarcoplasmic reticulum (SR) Ca2+-ATPase SERCA2a. To evaluate the impact of SERCA2a overexpression on SR Ca2+ handling in diabetic CM, we 1) generated transgenic rats harboring a human cytomegalovirus enhancer/chicken beta-actin promotor-controlled rat SERCA2 transgene (SERCA2-TGR), 2) characterized their SR phenotype, and 3) examined whether transgene expression may rescue SR Ca2+ transport in streptozotocin-induced diabetes. The transgene was expressed in all heart chambers. Compared to wild-type (WT) rats, a heterozygous line exhibited increased SERCA2 mRNA (1.5-fold), SERCA2 protein (+26%) and SR Ca2+ uptake (+37%). Phospholamban expression was not altered. In SERCA2-TGR, contraction amplitude (+48%) and rates of contraction (+34%) and relaxation (+35%) of isolated papillary muscles (PM) were increased (P2+ uptake and SERCA2 protein of SERCA2-TGR were 1.3-fold higher (P2+ uptake, accelerates relaxation and compensates, in part, for depressed Ca2+ uptake in diabetic CM. Therefore, SERCA2 expression might constitute an important therapeutic target to rescue cardiac SR Ca2+ handling in diabetes.

The Wilms' tumor suppressor Wt1 is associated with the differentiation of retinoblastoma cells

We have demonstrated recently that Wilms' tumor suppressor 1 (Wt1),in addition to its role in genitourinary formation,is required for the differentiation of ganglion cells in the developing retina. Here we provide further evidence that Wt1 is associated with neuronal differentiation. Thus, the retinoblastoma-derived human cell line, Y-79, contained robust amounts of Wt1 mRNA and protein. Wt1 expression was down-regulated upon laminin-induced differentiation of Y-79 into neuron-like cells. Inhibition of Wt1 with antisense oligonucleotides dramatically reduced the capacity of undifferentiated Y-79 cells to undergo neuronal differentiation, whereas sense and missense oligonucleotides had no effect. Wt1 immunoreactivity was also detected in solid retinoblastomas, in which it resided mainly in areas with moderate proliferative activity. These findings suggest a role for Wt1 in the differentiation of retinoblastoma cells. Furthermore, Wt1 expression in retinoblastoma may reflect the potential of these tumors to initiate the early steps of neuronal differentiation.