The contractile cells in the heart (the cardiac myocytes) are terminally differentiated. In response to pathophysiological stresses, cardiac myocytes undergo hypertrophic growth or apoptosis, responses associated with the development of cardiac pathologies. There has been much effort expended in gaining an understanding of the stimuli which promote these responses, and in identifying the intracellular signaling pathways which are activated and potentially involved. These signaling pathways presumably modulate gene and protein expression to elicit the end-stage response. For the regulation of gene expression, the signal may traverse the cytoplasm to modulate nuclear-localized transcription factors as occurs with the mitogen-activated protein kinase or protein kinase B/Akt cascades. Alternatively, the signal may promote translocation of transcription factors from the cytoplasm to the nucleus as is seen with the calcineurin/NFAT and JAK/STAT systems. We present an overview of the principal signaling pathways implicated in the regulation of gene expression in cardiac myocyte pathophysiology, and summarize the current understanding of these pathways, the transcription factors they regulate and the changes in gene expression associated with the development of cardiac pathologies. Finally, we discuss how intracellular signaling and gene expression may be integrated to elicit the overall change in cellular phenotype.
Background: Endothelin-1 stimulates Gq protein-coupled receptors to promote proliferation in dividing cells or hypertrophy in terminally differentiated cardiomyocytes. In cardiomyocytes, endothelin-1 rapidly (within minutes) stimulates protein kinase signaling, including extracellularsignal regulated kinases 1/2 (ERK1/2; though not ERK5), with phenotypic/physiological changes developing from approximately 12 h. Hypertrophy is associated with changes in mRNA/protein expression, presumably consequent to protein kinase signaling, but the connections between early, transient signaling events and developed hypertrophy are unknown.
Systemic hypertension increases cardiac workload causing cardiomyocyte hypertrophy and increased cardiac fibrosis. An underlying feature is increased production of reactive oxygen species. Redox-sensitive ASK1 (apoptosis signal-regulating kinase 1) activates stress-regulated protein kinases (p38-MAPK [mitogen-activated protein kinases] and JNKs [c-Jun N-terminal kinases]) and promotes fibrosis in various tissues. Here, we determined the specificity of ASK1 signaling in the heart, with the hypothesis that ASK1 inhibitors may be used to manage fibrosis in hypertensive heart disease. Using immunoblotting, we established that moderate levels of H
2
O
2
activate ASK1 in neonatal rat cardiomyocytes and perfused rat hearts. ASK1 was activated during ischemia in adult rat hearts, but not on reperfusion, consistent with activation by moderate (not high) reactive oxygen species levels. In contrast, IL (interleukin)-1β activated an alternative kinase, TAK1 (transforming growth factor–activated kinase 1). ASK1 was not activated by IL1β in cardiomyocytes and activation in perfused hearts was due to increased reactive oxygen species. Selonsertib (ASK1 inhibitor) prevented activation of p38-MAPKs (but not JNKs) by oxidative stresses in cultured cardiomyocytes and perfused hearts. In vivo (C57Bl/6J mice with osmotic minipumps for drug delivery), selonsertib (4 mg/[kg·d]) alone did not affect cardiac function/dimensions (assessed by echocardiography). However, it suppressed hypertension-induced cardiac hypertrophy resulting from angiotensin II (0.8 mg/[kg·d], 7d), with inhibition of
Nppa/Nppb
mRNA upregulation, reduced cardiomyocyte hypertrophy and, notably, significant reductions in interstitial and perivascular fibrosis. Our data identify a specific reactive oxygen species→ASK1→p38-MAPK pathway in the heart and establish that ASK1 inhibitors protect the heart from hypertension-induced cardiac remodeling. Thus, targeting the ASK1→p38-MAPK nexus has potential therapeutic viability as a treatment for hypertensive heart disease.
Inhibition of glycogen synthase kinase 3beta (GSK3beta) as a consequence of its phosphorylation by protein kinase B/Akt (PKB/Akt) has been implicated in cardiac myocyte hypertrophy in response to endothelin-1 or phenylephrine. We examined the regulation of GSK3alpha (which we show to constitute a significant proportion of the myocyte GSK3 pool) and GSK3beta in cardiac myocytes. Although endothelin increases phosphorylation of GSK3 and decreases its activity, the response is less than that induced by insulin (which does not promote cardiac myocyte hypertrophy). GSK3 phosphorylation induced by endothelin requires signalling through the extracellular signal-regulated kinase 1/2 (ERK1/2) cascade and not the PKB/Akt pathway, whereas the reverse is true for insulin. Cardiac myocyte hypertrophy involves changes in morphology, and in gene and protein expression. The potent GSK3 inhibitor 1-azakenpaullone increases myocyte area as a consequence of increased cell length whereas phenylephrine increases both length and width. Azakenpaullone or insulin promotes AP1 transcription factor binding to an AP1 consensus oligonucleotide, but this was significantly less than that induced by endothelin and derived principally from increased binding of JunB protein, the expression of which was increased. Azakenpaullone promotes significant changes in gene expression (assessed by Affymetrix microarrays), but the overall response is less than with endothelin and there is little overlap between the genes identified. Thus, although GSK3 may contribute to cardiac myocyte hypertrophy in some respects (and presumably plays an important role in myocyte metabolism), it does not appear to contribute as significantly to the response induced by endothelin as has been maintained.
G-protein-coupled receptor agonists are powerful stimulators of mitogen-activated protein kinase (MAPK) cascades in cardiac myocytes. However, little is known regarding the physiological activation of enzymes downstream of MAPKs. We examined the activation of mitogen- and stress-activated protein kinase-1 (MSK1), a downstream target of MAPKs, in adult rat cardiac myocytes by phenylephrine and endothelin-1. Both agonists induced the phosphorylation of MSK1 at Thr-581 and Ser-376 but not at Ser-360. Maximal phosphorylation was observed at 10-15 min after stimulation and it correlated with increased activity. Maximal activation of MSK1 in adult cardiomyocytes temporally coincided with maximal p38 MAPK activation while activation of the extracellular-signal-regulated kinase (ERK) cascade was more rapid. Phosphorylation and activation of MSK1 was completely inhibited by either PD98059 (ERK1/2 pathway inhibitor) or SB203580 (p38 MAPK inhibitor) alone. These data demonstrate that MSK1 activation in adult rat cardiac myocytes by G-protein-coupled receptor agonists requires the simultaneous activation of both the ERK and p38 MAPK pathways. However, the lack of phosphorylation at Ser-360, an identified phosphorylation site targeted by MAPKs, may indicate that MSK1 is not a direct substrate of ERK1/2 and p38 MAPK in adult rat cardiomyocytes.
Oxidative stress promotes cardiac myocyte death and has been implicated in the pathogenesis of many cardiovascular diseases. Bcl-2 family proteins are key regulators of the apoptotic response, while their functions can be regulated by post-transcriptional modifications including phosphorylation, dimerization or proteolytic cleavage. This study used adult cardiac myocytes to test the hypothesis that activation of specific kinase signalling pathways by oxidative stress may modulate either the expression or the phosphorylation of Bcl-2, with the resulting effect of a decrease or increase in its anti-apoptotic function. Stimulation of cardiac myocytes with 0.2 mM H(2)O(2), which induces apoptosis, resulted in a marked down-regulation of Bcl-2 protein simultaneously with an increase in its phosphorylation. Inhibition of p38-MAPK resulted in attenuation of Bcl-2 phosphorylation, whereas inhibition of ERK1/2, JNKs or PI-3-K had no effect. These data suggest that activation of p38 MAPK by oxidative stress results in the phosphorylation and degradation of Bcl-2 and the inactivation of its anti-apoptotic activity.
The present study aimed to examine the expression and activation of MAPKs (p38 MAPK, ERK1/2, and JNKs) in red blood cells (RBCs) of the gilthead sea bream, Sparus aurata, during thermal stress and investigate their involvement in the expression of heat shock proteins. The data showed that only p38 MAPK is detected in RBCs of Sparus aurata and it is phosphorylated and activated during exposure to increased temperature. Induction of Hsp70 in thermally stressed RBCs was abolished in the presence of the p38 MAPK inhibitor, SB203580, suggesting the involvement of the kinase in this response. This mechanism might play a cytoprotective role in the RBCs of the gilthead sea bream.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.