Michael Tranter

Probenecid: novel use as a non-injurious positive inotrope acting via cardiac TRPV2 stimulation.

Probenecid is a highly lipid soluble benzoic acid derivative originally used to increase serum antibiotic concentrations. It was later discovered to have uricosuric effects and was FDA approved for gout therapy. It has recently been found to be a potent agonist of transient receptor potential vanilloid 2 (TRPV2). We have shown that this receptor is in the cardiomyocyte and report a positive inotropic effect of the drug. Using echocardiography, Langendorff and isolated myocytes, we measured the change in contractility and, using TRPV2(-/-) mice, proved that the effect was mediated by TRPV2 channels in the cardiomyocytes. Analysis of the expression of Ca(2+) handling and β-adrenergic signaling pathway proteins showed that the contractility was not increased through activation of the β-ADR. We propose that the response to probenecid is due to activation of TRPV2 channels secondary to SR release of Ca(2+).

Coordinated post-transcriptional regulation of Hsp70.3 gene expression by microRNA and alternative polyadenylation.

Heat shock protein 70 (Hsp70) is well documented to possess general cytoprotective properties in protecting the cell against stressful and noxious stimuli. We have recently shown that expression of the stress-inducible Hsp70.3 gene in the myocardium in response to ischemic preconditioning is NF-κB-dependent and necessary for the resulting late phase cardioprotection against a subsequent ischemia/reperfusion injury. Here we show that the Hsp70.3 gene product is subject to post-transcriptional regulation through parallel regulatory processes involving microRNAs and alternative polyadenylation of the mRNA transcript. First, we show that cardiac ischemic preconditioning of the in vivo mouse heart results in decreased levels of two Hsp70.3-targeting microRNAs: miR-378* and miR-711. Furthermore, an ischemic or heat shock stimulus induces alternative polyadenylation of the expressed Hsp70.3 transcript that results in the accumulation of transcripts with a shortened 3′-UTR. This shortening of the 3′-UTR results in the loss of the binding site for the suppressive miR-378* and thus renders the alternatively polyadenylated transcript insusceptible to miR-378*-mediated suppression. Results also suggest that the alternative polyadenylation-mediated shortening of the Hsp70.3 3′-UTR relieves translational suppression observed in the long 3′-UTR variant, allowing for a more robust increase in protein expression. These results demonstrate alternative polyadenylation of Hsp70.3 in parallel with ischemic or heat shock-induced up-regulation of mRNA levels and implicate the importance of this process in post-transcriptional control of Hsp70.3 expression.

NF-κB driven cardioprotective gene programs; Hsp70.3 and cardioprotection after late ischemic preconditioning

It has been shown that the transcription factor NF-kappaB is necessary for late phase cardioprotection after ischemic preconditioning (IPC) in the heart, and yet is injurious after ischemia/reperfusion (I/R). However the downstream gene expression programs that underlie the contribution of NF-kappaB to cardioprotection after late IPC are incompletely understood. The objective of this study was to delineate the specific genes that are regulated by NF-kappaB immediately after a late IPC stimulus and validate the methodology for the identification of NF-kappaB-dependent genes that contribute to cardioprotection. A directed microarray analysis identified 238 genes as up or downregulated in an NF-kappaB-dependent manner 3.5h after late IPC. Among these are several genes previously implicated in late IPC. Gene ontological analysis showed that the most significant group of NF-kappaB-dependent genes are heat shock response genes, including the genes encoding Hsp70.1 and Hsp70.3. Though an Hsp70.1/70.3 double knockout failed to exhibit cardioprotection, late IPC was intact in the Hsp70.1 single knockout. After I/R, the Hsp70.1/70.3 double knockout and the Hsp70.1 single knockout had significantly increased and reduced infarct size, respectively. These results delineate the immediate NF-kappaB-dependent transcriptome after late IPC. One of the major categories of NF-kappaB-dependent genes induced by late IPC is the heat shock response. The results of infarct studies confirm that Hsp70.3 is protective after IPC. However, though Hsp70.1 and Hsp70.3 are coordinately regulated, their functions are opposing after I/R injury.

Identification of a NF-κB cardioprotective gene program: NF-κB regulation of Hsp70.1 contributes to cardioprotection after permanent coronary occlusion

The transcription factor Nuclear Factor Kappa B (NF-κB) has been shown to be cardioprotective after permanent coronary occlusion (PO) and late ischemic preconditioning (IPC), and yet it is cell injurious after ischemia/reperfusion (I/R) in the heart. There is limited information regarding NF-κB-dependent cardioprotection, and the NF-κB-dependent genes that contribute to the cardioprotection after PO are completely unknown. The objective of the study was to identify NF-κB-dependent genes that contribute to cardioprotection after PO. Microarray analysis was used to delineate genes that potentially contribute to the NF-κB-dependent cardioprotection by determining the overlap between the set of PO regulated genes and genes regulated by NF-κB, using mice with genetic abrogation of NF-κB activation in the heart. This analysis identified 16 genes as candidates for NF-κB-dependent effects after PO. This set of genes overlaps with, but is significantly different from the set of genes we previously identified as regulated by NF-κB after IPC. The genes encoding heat shock protein 70.3 (hspa1a) and heat shock protein 70.1 (hspa1b) were the most significantly regulated genes after PO and were up-regulated by NF-κB. Results using knockout mice show that Hsp70.1 contributes to NF-κB-dependent cardioprotection after PO and likely underlies, at least in part, the NF-κΒ-dependent cardioprotective effect. Our previous results show that Hsp70.1 is injurious after I/R injury. This demonstrates that, like NF-κB itself, Hsp70.1 has antithetical effects on myocardial survival and suggests that this may underlie the similar antithetical effects of NF-κB after different ischemic stimuli. The significance of the research is that understanding the gene network regulated by NF-κB after ischemic insult may lead to identification of therapeutic targets more appropriate for clinical development.

In Vivo Delivery of Nucleic Acids via Glycopolymer Vehicles Affords Therapeutic Infarct Size Reduction In Vivo

Using a new class of nontoxic and degradable glycopolymer-based vehicles termed poly(glycoamidoamine)s, we demonstrate virus-like delivery efficacy of oligodeoxynucleotide (ODN) decoys to cardiomyoblasts (H9c2), primary cardiomyocytes, and the mouse heart. These glycopolymers bind and compact ODN decoys into nanoparticle complexes that are internalized by the cell membrane and mediate nuclear uptake of DNA in 90+% of cultured primary cardiomyocytes and 87% of the mouse myocardium. Experimental results reveal that decoys delivered via these glycopolymers block the activation of the transcription factor NF-κB, a major contributor to ischemia/reperfusion injury. Decoy complexes formed with glycopolymer T4 significantly blocked downstream gene expression of Cox-2 and limited myocardial infarction in vivo, phenocopying a transgenic mouse model. These promising delivery vehicles may facilitate high-throughput genetic approaches in animal models. Additionally, the low toxicity, biodegradation, and outstanding delivery efficacy suggest that these nanomedicines may be clinically applicable for gene regulatory therapy.

Acute consumption of a high-fat diet prior to ischemia-reperfusion results in cardioprotection through NF-κB-dependent regulation of autophagic pathways

Previous studies have demonstrated improvement of cardiac function occurs with acute consumption of a high-fat diet (HFD) after myocardial infarction (MI). However, no data exist addressing the effects of acute HFD upon the extent of injury after MI. This study investigates the hypothesis that short-term HFD, prior to infarction, protects the heart against ischemia-reperfusion (I/R) injury through NF-κB-dependent regulation of cell death pathways in the heart. Data show that an acute HFD initiates cardioprotection against MI (>50% reduction in infarct size normalized to risk region) after 24 h to 2 wk of HFD, but protection is completely absent after 6 wk of HFD, when mice are reported to develop pathophysiology related to the diet. Furthermore, cardioprotection after 24 h of HFD persists after an additional 24 h of normal chow feeding and was found to be dependent upon NF-κB activation in cardiomyocytes. This study also indicates that short-term HFD activates autophagic processes (beclin-1, LC-3) preischemia, as seen in other protective stimuli. Increases in beclin-1 and LC-3 were found to be NF-κB-dependent, and administration of chloroquine, an inhibitor of autophagy, abrogated cardioprotection. Our results support that acute high-fat feeding mediates cardioprotection against I/R injury associated with a NF-κB-dependent increase in autophagy and reduced apoptosis, as has been found for ischemic preconditioning.

Increased fibrosis and progression to heart failure in MRL mice following ischemia/reperfusion injury

The cardiac regenerative capacity of MRL/MpJ mouse remains a controversy. Although the MRL mouse has been reported to exhibit minimal scarring and subsequent cardiac regeneration after cryoinjury of the right ventricle, multiple studies have been unable to replicate this cardiac regenerative capacity after both cryogenic and coronary ligation cardiac injury. Therefore, we evaluated the cardiac regenerative wound-healing response and functional recovery of MRL mice compared to C57 mice, in response to a clinically relevant left ventricular (LV) coronary ligation. Male MRL/MpJ+/+ and C57BL/6 mice underwent left coronary artery ligation followed by reperfusion. Cardiac function was evaluated by echocardiography [LV ejection fraction (LVEF), LV end-diastolic volume (LVEDV), LV mass, wall thickness] at 24 hours post-ischemia and weekly for 13 weeks thereafter. Hearts were also analyzed histologically for individual cardiomyocyte hypertrophy and cardiac fibrosis. Our results show that contrary to prior reports of cardiac regenerations, MRL mice progress to heart failure more rapidly following I/R injury as marked by a significant decrease in LVEF, increase in LVEDV, LV mass, individual myocyte size, and fibrosis in the post-ischemic myocardium. Therefore, we conclude that MRL mice do not exhibit regeneration of the LV or enhanced functional improvement in response to coronary ligation. However, unlike prior studies, we matched initial infarct size in MRL and C57 mice, used high frequency echocardiography, and histological analysis to reach this conclusion. The prospect of cardiac regeneration after ischemia in MRL mice seems to have attenuated interest, given the multiple negative studies and the promise of stem cell cardiac regeneration. However, our novel observation that MRL may possess an impaired compensated hypertrophy response makes the MRL mouse strain an interesting model in the study of cardiac hypertrophy.

Differential Translocation of Nuclear Factor-KappaB in a Cardiac Muscle Cell Line Under Gravitational Changes

Microgravity (micro-g) environments have been shown to elicit dysregulation of specific genes in a wide assay of cell types. It is known that the activation of transcription factors and molecular signaling pathways influence various physiological outcomes associated with stress and adaptive responses. Nuclear factor-kappa B (NF-kappaB) is one of the most prevailing oxidation-sensitive transcription factors. It is hypothesized that simulated microgravity would activate NF-kappaB and its downstream transcriptional networks, thus suggesting a role for NF-kappaB in microgravity induced muscle atrophy. To investigate the activation of NF-kappaB in a rat cardiac cell line (H9c2) under micro-g, rotating wall vessel bioreactors were used to simulate micro-g conditions. Western blotting revealed that mean nuclear translocation of NF-kappaB p65 subunit was 69% for micro-g and 46% for unit-g dynamic control as compared with a 30 min TNF-alpha positive control (p<0.05, n=3). The results from western blots were confirmed by enzyme-linked immunosorbent assay, which showed 66% for micro-g and 45% for dynamic control as compared with positive control (p<0.05, n=3). These results show significant differential translocation of NF-kappaB p65 under simulated micro-g. These results may be expanded upon to explain physiological changes such as muscle atrophy and further identify the regulatory pathways and effector molecules activated under exposure to micro-g.

Probenecid as a Noninjurious Positive Inotrope in an Ischemic Heart Disease Murine Model

The current therapeutic options for acute decompensated heart failure are limited to afterload reducers and positive inotropes. The latter increases myocardial contractility through changes in myocyte calcium (Ca2+) handling (mostly through stimulation of the β-adrenergic pathways [β-ADR]) and is associated with paradoxical effects of arrhythmias, cell death, and subsequently increased mortality. We have previously demonstrated that probenecid can increase cytosolic Ca2+ levels in the cardiomyocyte resulting in an improved inotropic response in vitro and in vivo without activating the β-ADR system. We hypothesize that, in contrast to other commonly used inotropes, probenecid functions through a system separate from that of β-ADR and hence will increase contractility and improve function without damaging the heart. Furthermore, our goal was to evaluate the effect of probenecid on cell death in vitro and its use in vivo as a positive inotrope in a mouse model of ischemic cardiomyopathy. Herein, we demonstrate that probenecid induced an influx of Ca2+ similar to isoproterenol, but does not induce cell death in vitro. Through a series of in vivo experiments we also demonstrate that probenecid can be used at various time points and with various methods of administration in vivo in mice with myocardial ischemia, resulting in improved contractility and no significant difference in infarct size. In conclusion, we provide novel data that probenecid, through its activity on cellular Ca2+ levels, induces an inotropic effect without causing or exacerbating injury. This discovery may be translatable if this mechanism is preserved in man.

Activation of HuR downstream of p38 MAPK promotes cardiomyocyte hypertrophy

The RNA binding protein Human antigen R (HuR) interacts with specific AU-rich domains in target mRNAs and is highly expressed in many cell types, including cardiomyocytes. However, the role of HuR in cardiac physiology is largely unknown. Our results show that HuR undergoes cytoplasmic translocation, indicative of its activation, in hypertrophic cardiac myocytes. Specifically, HuR cytoplasmic translocation is significantly increased in NRVMs (neonatal rat ventricular myocytes) following treatment with phenylephrine or angiotensin II, agonists of two independent Gαq-coupled GPCRs known to induce hypertrophy. This Gq-mediated HuR activation is dependent on p38 MAP kinase, but not canonical Gq-PKC signaling. Furthermore, we show that HuR activation is necessary for Gq-mediated hypertrophic growth of NRVMs as siRNA-mediated knockdown of HuR inhibits hypertrophy as measured by cell size and expression of ANF (atrial natriuretic factor). Additionally, HuR overexpression is sufficient to induce hypertrophic cell growth. To decipher the downstream mechanisms by which HuR translocation promotes cardiomyocyte hypertrophy, we assessed the role of HuR in the transcriptional activity of NFAT (nuclear factor of activated T cells), the activation of which is a hallmark of cardiac hypertrophy. Using an NFAT-luciferase reporter assay, we show an acute inhibition of NFAT transcriptional activity following pharmacological inhibition of HuR. In conclusion, our results identify HuR as a novel mediator of cardiac hypertrophy downstream of the Gq-p38 MAPK pathway, and suggest modulation of NFAT activity as a potential mechanism.