BackgroundCardiomyocyte hypertrophy is associated with changes in gene expression. Extracellular signal-regulated kinases 1/2 (ERK1/2) and RhoA [activated by hypertrophic agonists (e.g. endothelin-1)] regulate gene expression and are implicated in the response, but their relative significance in regulating the cardiomyocyte transcriptome is unknown. Our aim was to establish the significance of ERK1/2 and/or RhoA in the early cardiomyocyte transcriptomic response to endothelin-1.Methods/Principal FindingsCardiomyocytes were exposed to endothelin-1 (1 h) with/without PD184352 (to inhibit ERK1/2) or C3 transferase (C3T, to inhibit RhoA). RNA expression was analyzed using microarrays and qPCR. ERK1/2 signaling positively regulated ∼65% of the early gene expression response to ET-1 with a small (∼2%) negative effect, whereas RhoA signaling positively regulated ∼10% of the early gene expression response to ET-1 with a greater (∼14%) negative contribution. Of RNAs non-responsive to endothelin-1, 66 or 448 were regulated by PD184352 or C3T, respectively, indicating that RhoA had a more significant effect on baseline RNA expression. mRNAs upregulated by endothelin-1 encoded a number of receptor ligands (e.g. Ereg, Areg, Hbegf) and transcription factors (e.g. Abra/Srf) that potentially propagate the response.Conclusions/SignificanceERK1/2 dominates over RhoA in the early transcriptomic response to endothelin-1. RhoA plays a major role in maintaining baseline RNA expression but, with upregulation of Abra/Srf by endothelin-1, RhoA may regulate changes in RNA expression over longer times. Our data identify ERK1/2 as a more significant node than RhoA in regulating the early stages of cardiomyocyte hypertrophy.

Krüppel-like transcription factors (Klfs) modulate fundamental cell processes. Cardiac myocytes are terminally-differentiated, but hypertrophy in response to stimuli such as endothelin-1. H2O2 or cytokines promote myocyte apoptosis. Microarray studies of neonatal rat myocytes identified several Klfs as endothelin-1-responsive genes. We used quantitative PCR for further analysis of Klf expression in neonatal rat myocytes. In response to endothelin-1, Klf2 mRNA expression was rapidly increased (∼ 9-fold; 15–30 min) with later increases in expression of Klf4 and Klf6 (∼ 5-fold; 30–60 min). All were regulated as immediate early genes (cycloheximide did not inhibit the increases in expression). Klf5 expression was increased at 1–2 h (∼ 13-fold) as a second phase response (cycloheximide inhibited the increase). These increases were transient and attenuated by U0126. H2O2 increased expression of Klf2, Klf4 and Klf6, but interleukin-1β or tumor necrosis factor α downregulated Klf2 expression with no effect on Klf4 or Klf6. Of the Klfs which repress transcription, endothelin-1 rapidly downregulated expression of Klf3, Klf11 and Klf15. The dynamic regulation of expression of multiple Klf family members in cardiac myocytes suggests that, as a family, they are actively involved in regulating phenotypic responses (hypertrophy and apoptosis) to extracellular stimuli.

The canonical pathway of regulation of the GCK (germinal centre kinase) III subgroup member, MST3 (mammalian Sterile20-related kinase 3), involves a caspase-mediated cleavage between N-terminal catalytic and C-terminal regulatory domains with possible concurrent autophosphorylation of the activation loop MST3(Thr178), induction of serine/threonine protein kinase activity and nuclear localization. We identified an alternative ‘non-canonical’ pathway of MST3 activation (regulated primarily through dephosphorylation) which may also be applicable to other GCKIII (and GCKVI) subgroup members. In the basal state, inactive MST3 co-immunoprecipitated with the Golgi protein GOLGA2/gm130 (golgin A2/Golgi matrix protein 130). Activation of MST3 by calyculin A (a protein serine/threonine phosphatase 1/2A inhibitor) stimulated (auto)phosphorylation of MST3(Thr178) in the catalytic domain with essentially simultaneous cis-autophosphorylation of MST3(Thr328) in the regulatory domain, an event also requiring the MST3(341–376) sequence which acts as a putative docking domain. MST3(Thr178) phosphorylation increased MST3 kinase activity, but this activity was independent of MST3(Thr328) phosphorylation. Interestingly, MST3(Thr328) lies immediately C-terminal to a STRAD (Sterile20-related adaptor) pseudokinase-like site identified recently as being involved in binding of GCKIII/GCKVI members to MO25 scaffolding proteins. MST3(Thr178/Thr328) phosphorylation was concurrent with dissociation of MST3 from GOLGA2/gm130 and association of MST3 with MO25, and MST3(Thr328) phosphorylation was necessary for formation of the activated MST3–MO25 holocomplex.

The Mdm2 ubiquitin ligase is an important regulator of p53 abundance and p53-dependent apoptosis. Mdm2 expression is frequently regulated by a p53 Mdm2 autoregulatory loop whereby p53 stimulates Mdm2 expression and hence its own degradation. Although extensively studied in cell lines, relatively little is known about Mdm2 expression in heart where oxidative stress (exacerbated during ischemia-reperfusion) is an important pro-apoptotic stimulus. We demonstrate that Mdm2 transcript and protein expression are induced by oxidative stress (0.2 mM H 2 O 2 ) in neonatal rat cardiac myocytes. In other cells, constitutive Mdm2 expression is regulated by the P1 promoter (5 to exon 1), with inducible expression regulated by the P2 promoter (in intron 1). In myocytes, H 2 O 2 increased Mdm2 expression from the P2 promoter, which contains two p53-response elements (REs), one AP-1 RE, and two Ets REs. Exposure of cardiac myocytes to sufficiently high levels of reactive oxygen species (ROS) 6 such as H 2 O 2 leads to their death (1-3), and this probably involves a continuum from apoptosis to necrosis, depending on the severity of the oxidative stress (4). In aerobic tissues such as the heart, the mitochondria probably represent a significant source of ROS, and increased ROS production by these organelles during hypoxia and ischemia-reperfusion injury may be particularly important in myocardial injury (5, 6). However, at lower concentrations, ROS have been reported to promote either growth of the cardiac myocyte (7) or to induce "preconditioning" (8), either of which potentially increases the ability of the cardiac myocyte to survive cytotoxic stresses. H 2 O 2 -induced oxidative stress simultaneously stimulates a number of potentially pro-apoptotic and cytoprotective signaling pathways in the whole heart or cardiac myocytes (9), and the final outcome (cell death or survival) could depend on which signaling pathway(s) predominates and endures. As shown by our microarray studies, H 2 O 2 can positively and negatively regulate global gene expression in cardiac myocytes (3, 10). One gene consistently up-regulated by H 2 O 2 in rat cardiac myocytes at toxic and nontoxic concentrations is the orthologue of transformed mouse 3T3 cell double minute 2 (Mdm2) (3, 10), a proto-oncogene (11, 12), to which we will refer as Rdm2. 7 The human orthologue will be abbreviated as HDM2. The Mdm2 protein binds to the pro-apoptotic p53 tumor suppressor transcription factor to inhibit its transactivating activity (13). Perhaps more importantly, Mdm2 is a ubiquitin-protein isopeptide ligase that ubiquitinates p53 and other proteins (13), thus promoting their proteasomal degradation. Indeed, the stability of p53 protein appears to be of major importance in controlling its abundance (13). In addition, Mdm2 may autoubiquitinate to promote its own degradation (14, 15).Regulation of Mdm2 expression is complex and involves two alternative promoters. The P1 promoter lies 5Ј to exon 1 and the P2 promoter lies within intron 1 (16). P1 primarily regulates Th...

BackgroundChanges in cellular phenotype result from underlying changes in mRNA transcription and translation. Endothelin-1 stimulates cardiomyocyte hypertrophy with associated changes in mRNA/protein expression and an increase in the rate of protein synthesis. Insulin also increases the rate of translation but does not promote overt cardiomyocyte hypertrophy. One mechanism of translational regulation is through 5' terminal oligopyrimidine tracts (TOPs) that, in response to growth stimuli, promote mRNA recruitment to polysomes for increased translation. TOP mRNAs include those encoding ribosomal proteins, but the full panoply remains to be established. Here, we used microarrays to compare the effects of endothelin-1 and insulin on the global transcriptome of neonatal rat cardiomyocytes, and on mRNA recruitment to polysomes (i.e. the translatome).ResultsGlobally, endothelin-1 and insulin (1 h) promoted >1.5-fold significant (false discovery rate < 0.05) changes in expression of 341 and 38 RNAs, respectively. For these transcripts with this level of change there was little evidence of translational regulation. However, 1336 and 712 RNAs had >1.25-fold significant changes in expression in total and/or polysomal RNA induced by endothelin-1 or insulin, respectively, of which ~35% of endothelin-1-responsive and ~56% of insulin-responsive transcripts were translationally regulated. Of mRNAs for established proteins recruited to polysomes in response to insulin, 49 were known TOP mRNAs with a further 15 probable/possible TOP mRNAs, but 49 had no identifiable TOP sequences or other consistent features in the 5' untranslated region.ConclusionsEndothelin-1, rather than insulin, substantially affects global transcript expression to promote cardiomyocyte hypertrophy. Effects on RNA recruitment to polysomes are subtle, with differential effects of endothelin-1 and insulin on specific transcripts. Furthermore, although insulin promotes recruitment of TOP mRNAs to cardiomyocyte polysomes, not all recruited mRNAs are TOP mRNAs.