Characterization of adenosine transport in H9c2 cardiomyoblasts

Targeting myocardial equilibrative nucleoside transporter ENT1 provides cardioprotection by enhancing myeloid Adora2b signaling

Previous studies implicate extracellular adenosine signaling in attenuating myocardial ischemia and reperfusion injury (IRI). This extracellular adenosine signaling is terminated by its uptake into cells by equilibrative nucleoside transporters (ENTs). Thus, we hypothesized that targeting ENTs would function to increase cardiac adenosine signaling and concomitant cardioprotection against IRI. Mice were exposed to myocardial ischemia and reperfusion injury. Myocardial injury was attenuated in mice treated with the nonspecific ENT inhibitor dipyridamole. A comparison of mice with global Ent1 or Ent2 deletion showed cardioprotection only in Ent1-/- mice. Moreover, studies with tissue-specific Ent deletion revealed that mice with myocyte-specific Ent1 deletion (Ent1loxP/loxP Myosin Cre+ mice) experienced smaller infarct sizes. Measurements of cardiac adenosine levels demonstrated that postischemic elevations of adenosine persisted during reperfusion after targeting ENTs. Finally, studies in mice with global or myeloid-specific deletion of the Adora2b adenosine receptor (Adora2bloxP/loxP LysM Cre+ mice) implied that Adora2b signaling on myeloid-inflammatory cells in cardioprotection provided by ENT inhibition. These studies reveal a previously unrecognized role for myocyte-specific ENT1 in cardioprotection by enhancing myeloid-dependent Adora2b signaling during reperfusion. Extension of these findings implicates adenosine transporter inhibitors in cardioprotection against ischemia and reperfusion injury.

Systematic transcriptomic and phenotypic characterization of human and murine cardiac myocyte cell lines and primary cardiomyocytes reveals serious limitations and low resemblances to adult cardiac phenotype

BACKGROUND

Cardiac cell lines and primary cells are widely used in cardiovascular research. Despite increasing number of publications using these models, comparative characterization of these cell lines has not been performed, therefore, their limitations are undetermined. We aimed to compare cardiac cell lines to primary cardiomyocytes and to mature cardiac tissues in a systematic manner.

METHODS AND RESULTS

Cardiac cell lines (H9C2, AC16, HL-1) were differentiated with widely used protocols. Left ventricular tissue, neonatal primary cardiomyocytes, and human induced pluripotent stem cell-derived cardiomyocytes served as reference tissue or cells. RNA expression of cardiac markers (e.g. Tnnt2, Ryr2) was markedly lower in cell lines compared to references. Differentiation induced increase in cardiac- and decrease in embryonic markers however, the overall transcriptomic profile and annotation to relevant biological processes showed consistently less pronounced cardiac phenotype in all cell lines in comparison to the corresponding references. Immunocytochemistry confirmed low expressions of structural protein sarcomeric alpha-actinin, troponin I and caveolin-3 in cell lines. Susceptibility of cell lines to sI/R injury in terms of viability as well as mitochondrial polarization differed from the primary cells irrespective of their degree of differentiation.

CONCLUSION

Expression patterns of cardiomyocyte markers and whole transcriptomic profile, as well as response to sI/R, and to hypertrophic stimuli indicate low-to-moderate similarity of cell lines to primary cells/cardiac tissues regardless their differentiation. Low resemblance of cell lines to mature adult cardiac tissue limits their potential use. Low translational value should be taken into account while choosing a particular cell line to model cardiomyocytes.

Construction of cardiomyoblast sheets for cardiac tissue repair: comparison of three different approaches

Recently, cell sheet engineering has emerged as one of the most accentuated approaches of tissue engineering and cardiac tissue is the pioneering application area of cell sheets with clinical use. In this study, we cultured rat cardiomyoblasts (H9C2 cell line) to obtain cell sheets by using three different approaches; using (1) thermo-responsive tissue culture plates, (2) high cell seeding density/high serum content and (3) ascorbic acid treatment. To compare the outcomes of three methods, morphologic examination, immunofluorescent stainings and live/dead cell assay were performed and the effects of serum concentration and ascorbic acid treatment on cardiac gene expressions were examined. The results showed that cardiomyoblast sheets were successfully obtained in all approaches without losing their integrity and viability. Also, the results of RT-PCR analysis showed that the types of tissue culture surface, cell seeding density, serum concentration and ascorbic acid treatment affect cardiac gene expressions of cells in cell sheets. Although three methods were succeeded, ascorbic acid treatment was found as the most rapid and effective method to obtain cell sheets with cardiac characteristics.

Effects of ticagrelor on the sodium/calcium exchanger 1 (NCX1) in cardiac derived H9c2 cells

Ticagrelor is a direct acting and reversibly binding P2Y₁₂ antagonist approved for the prevention of thromboembolic events. Clinical effects of ticagrelor cannot be simply accounted for by pure platelet inhibition, and off-target mechanisms can potentially play a role. In particular, recent evidence suggests that ticagrelor may also influence heart function and improve the evolution of myocardial ischemic injury by more direct effects on myocytes. The cardiac sodium/calcium exchanger 1 (NCX1) is a critical player in the generation and control of calcium (Ca²⁺) signals, which orchestrate multiple myocyte activities in health and disease. Altered expression and/or activity of NCX1 can have profound consequences for the function and fate of myocytes. Whether ticagrelor affects cardiac NCX1 has not been investigated yet. To explore this hypothesis, we analyzed the expression, localization and activity of NCX1 in the heart derived H9c2-NCX1 cells following ticagrelor exposure. We found that ticagrelor concentration- and time-dependently reduced the activity of the cardiac NCX1 in H9c2 cells. In particular, the inhibitory effect of ticagrelor on the Ca²⁺-influx mode of NCX1 was evident within 1 h and further developed after 24 h, when NCX1 activity was suppressed by about 55% in cells treated with 1 μM ticagrelor. Ticagrelor-induced inhibition of exchanger activity was reached at clinically relevant concentrations, without affecting the expression levels and subcellular distribution of NCX1. Collectively, these findings suggest that cardiac NCX1 is a new downstream target of ticagrelor, which may contribute to the therapeutic profile of ticagrelor in clinical practice.

Equilibrative Nucleoside Transporters 1 and 4: Which One Is a Better Target for Cardioprotection Against Ischemia-Reperfusion Injury?

The cardioprotective effects of adenosine and adenosine receptor agonists have been studied extensively. However, their therapeutic outcomes in ischemic heart disease are limited by systemic side effects such as hypotension, bradycardia, and sedation. Equilibrative nucleoside transporter (ENT) inhibitors may be an alternative. By reducing the uptake of extracellular adenosine, ENT1 inhibitors potentiate the cardioprotective effect of endogenous adenosine. They have fewer systemic side effects because they selectively increase the extracellular adenosine levels in ischemic tissues undergoing accelerated adenosine formation. Nonetheless, long-term inhibition of ENT1 may adversely affect tissues that have low capacity for de novo nucleotide biosynthesis. ENT1 inhibitors may also affect the cellular transport, and hence the efficacy, of anticancer and antiviral nucleoside analogs used in chemotherapy. It has been proposed that ENT4 may also contribute to the regulation of extracellular adenosine in the heart, especially under the acidotic conditions associated with ischemia. Like ENT1 inhibitors, ENT4 inhibitors should work specifically on ischemic tissues. Theoretically, ENT4 inhibitors do not affect tissues that rely on ENT1 for de novo nucleotide synthesis. They also have no interaction with anticancer and antiviral nucleosides. Development of specific ENT4 inhibitors may open a new avenue in research on ischemic heart disease therapy.

Nucleoside/nucleobase transporters: drug targets of the future?

BACKGROUND

Nucleoside/nucleobase transporters have been investigated since the 1960s. In particular, equilibrative nucleoside transporters were thought to be valuable drug targets, since they are involved in various kinds of viral and parasitic diseases as well as cancers.

DISCUSSION

In the postgenomic era multiple transporters, including different subtypes, have been cloned and characterized on the molecular level. In this article we summarize recent advances regarding structure, function and localization of nucleoside/nucleobase transporters as well as the pharmacological profile of selected drugs.

CONCLUSION

Knowledge of the different kinetic properties and structural features of nucleoside transporters can either be used for the rational design of therapeutics directly targeting the transporter itself or for the delivery of drugs using the transporter as a port of entry into the target cell. Equilibrative nucleoside transporters are of considerable pharmacological interest as drug targets for the development of drugs tailored to each patient's need for the treatment of cardiac disease, cancer and viral infections.

Equilibrative nucleoside transporter 1 plays an essential role in cardioprotection

To better understand the role of equilibrative nucleoside transporters (ENT) in purine nucleoside-dependent physiology of the cardiovascular system, we investigated whether the ENT1-null mouse heart was cardioprotected in response to ischemia (coronary occlusion for 30 min followed by reperfusion for 2 h). We observed that ENT1-null mouse hearts showed significantly less myocardial infarction compared with wild-type littermates. We confirmed that isolated wild-type adult mouse cardiomyocytes express predominantly ENT1, which is primarily responsible for purine nucleoside uptake in these cells. However, ENT1-null cardiomyocytes exhibit severely impaired nucleoside transport and lack ENT1 transcript and protein expression. Adenosine receptor expression profiles and expression levels of ENT2, ENT3, and ENT4 were similar in cardiomyocytes isolated from ENT1-null adult mice compared with cardiomyocytes isolated from wild-type littermates. Moreover, small interfering RNA knockdown of ENT1 in the cardiomyocyte cell line, HL-1, mimics findings in ENT1-null cardiomyocytes. Taken together, our data demonstrate that ENT1 plays an essential role in cardioprotection, most likely due to its effects in modulating purine nucleoside-dependent signaling and that the ENT1-null mouse is a powerful model system for the study of the role of ENTs in the physiology of the cardiomyocyte.

Myostatin represses physiological hypertrophy of the heart and excitation-contraction coupling

Although myostatin negatively regulates skeletal muscle growth, its function in heart is virtually unknown. Herein we demonstrate that it inhibits basal and IGF-stimulated proliferation and differentiation and also modulates cardiac excitation-contraction (EC) coupling. Loss of myostatin induced eccentric hypertrophy and enhanced cardiac responsiveness to beta-adrenergic stimulation in vivo. This was due to myostatin null ventricular myocytes having larger [Ca(2+)](i) transients and contractions and responding more strongly to beta-adrenergic stimulation than wild-type cells. Enhanced cardiac output and beta-adrenergic responsiveness of myostatin null mice was therefore due to increased SR Ca(2+) release during EC coupling and to physiological hypertrophy, but not to enhanced myofilament function as determined by simultaneous measurement of force and ATPase activity. Our studies support the novel concept that myostatin is a repressor of physiological cardiac muscle growth and function. Thus, the controlled inhibition of myostatin action could potentially help repair damaged cardiac muscle by inducing physiological hypertrophy.

High D-glucose reduces SLC29A1 promoter activity and adenosine transport involving specific protein 1 in human umbilical vein endothelium

High D-glucose reduces human equilibrative nucleoside transporter 1 (hENT1)-mediated adenosine uptake involving endothelial nitric oxide synthase (eNOS), mitogen-activated protein (MAP) kinase kinases 1 and 2/MAP kinases p42/44 (MEK/ERKs), and protein kinase C (PKC) activation in human umbilical vein endothelium (HUVEC). Since NO represses SLC29A1 gene (hENT1) promoter activity we studied whether D-glucose-reduced hENT1-adenosine transport results from lower SLC29A1 expression in HUVEC primary cultures. HUVEC incubation (24 h) with high D-glucose (25 mM) reduced hENT1-adenosine transport and pGL3-hENT1(-1114) construct SLC29A1 reporter activity compared with normal D-glucose (5 mM). High D-glucose also reduced pGL3-hENT1(-1114) reporter activity compared with cells transfected with pGL3-hENT1(-795) construct. N(G)-nitro-L-arginine methyl ester (L-NAME, NOS inhibitor), PD-98059 (MEK1/2 inhibitor), and/or calphostin C (PKC inhibitor) blocked D-glucose effects. Insulin (1 nM) and phorbol 12-myristate 13-acetate (PMA, 100 nM, PKC activator), but not 4alpha-phorbol 12,13-didecanoate (4alphaPDD, 100 nM, PMA less active analogue) reduced hENT1-adenosine transport. L-NAME and PD-98059 blocked insulin effects. L-NAME, PD-98059, and calphostin C increased hENT1 expression without altering protein or mRNA stability. High D-glucose increased Sp1 transcription factor protein abundance and binding to SLC29A1 promoter, phenomena blocked by L-NAME, PD-98059, and calphostin C. Sp1 overexpression reduced SLC29A1 promoter activity in normal D-glucose, an effect reversed by L-NAME and further reduced by S-nitroso-N-acetyl-L,D-penicillamine (SNAP, NO donor) in high D-glucose. Thus, reduced hENT1-mediated adenosine transport in high D-glucose may result from increased Sp1 binding to SLC29A1 promoter down-regulating hENT1 expression. This phenomenon depends on eNOS, MEK/ERKs, and PKC activity, suggesting potential roles for these molecules in hyperglycemia-associated endothelial dysfunction.

Physiology of Nucleoside Transporters: Back to the Future

Nucleoside transporters (NTs) are integral membrane proteins responsible for mediating and facilitating the flux of nucleosides and nucleobases across cellular membranes. NTs are also responsible for the uptake of nucleoside analog drugs used in the treatment of cancer and viral infections, and they are the target of certain compounds used in the treatment of some types of cardiovascular disease. The important role of NTs as drug transporters and therapeutic targets has necessarily led to intense interest into their structure and function and the relationship between these proteins and drug efficacy. In contrast, we still know relatively little about the fundamental physiology of NTs. In this review, we discuss various aspects of the physiology of NTs in mammalian systems, particularly noting tissues and cells where there has been little recent research. Our central thesis is reference back to some of the older literature, combined with current findings, will provide direction for future research into NT physiology that will lead to a fuller understanding of the role of these intriguing proteins in the everyday lives of cells, tissues, organs, and whole animals.