Dissociation of p44 and p42 Mitogen-activated Protein Kinase Activation from Receptor-induced Hypertrophy in Neonatal Rat Ventricular Myocytes

Abstract

In response to hormones and mechanical stretch, neonatal rat ventricular myocytes exhibit a hypertrophic response that is characterized by induction of cardiac-specific genes and increased myocardial cell size. Hypertrophic stimuli also activate mitogen-activated protein kinase (MAPK), an enzyme thought to play a central role in the regulation of cell growth and differentiation. To determine if MAPK activation is sufficient for acquisition of the molecular and morphological features of cardiac hypertrophy we compared four agonists that stimulate G protein-coupled receptors. Whereas phenylephrine and endothelin transactivate cardiac-specific promoter/luciferase reporter genes, increase atrial natriuretic factor (ANF) expression, and promote myofilament organization, neither carbachol nor ATP induces these responses. Interestingly, all four agonists activate both the p42 and the p44 isoforms of MAPK. Furthermore, the kinetics of MAPK activation are not different for the hypertrophic agonist phenylephrine and the nonhypertrophic agonist carbachol. Transient transfection of myocytes with dominant-interfering mutants of p42 and p44 MAPK failed to block phenylephrine-induced ANF expression, although Ras-induced gene expression was inhibited by expression of the mutant MAPK constructs. Moreover, PD 098059, an inhibitor of MAPK kinase, blocked phenylephrine-stimulated MAPK activity but not ANF reporter gene expression. Thus, MAPK activation is not sufficient for G protein receptor-mediated induction of cardiac cell growth and gene expression and is apparently not required for transcriptional activation of the ANF gene.

Specific role of the extracellular signal-regulated kinase pathway in angiotensin II-induced cardiac hypertrophy in vitro

“…Constitutively active MEK1-induced ANF-Luc activation was not inhibited by 30 µM PD98059 (results not shown), confirming that PD98059 acts specifically on MEK1 and does not affect downstream signalling. In contrast, phenylephrine-induced ANF-Luc activation was not affected by PD98059 ( Figure 3C) or dnMEK1 ( Figure 3D), confirming a previous study [21]. These results indicate that activation of the MEK1-ERK pathway is necessary and sufficient for Ang II-induced activation of the ANF promoter but is dispensable for phenylephrine-induced activation of the ANF promoter.…”

“…In cultured cardiac myocytes, ERKs are activated by hypertrophic stimuli such as angiotensin II (Ang II) [10], phenylephrine [11,12], endothelin 1 [12][13][14] and PMA [13], as well as cellular stresses such as mechanical stretching [15,16] or osmotic shock [17,18]. Although the role of ERKs in phenylephrineinduced cardiac hypertrophy has been studied by several investigators with different methods, reported roles of ERKs in cardiac hypertrophy vary substantially [19][20][21][22][23]. Furthermore, recent evidence indicates that agonists for the G q -coupled receptor activate not only ERKs but also other members of the MAP kinase family, namely JNK and p38 [17,[24][25][26][27][28], and that these MAP kinases are also important in cardiac hypertrophy [25,26,[29][30][31][32] (reviewed in [5]).…”

Mechanical Strain Increases Expression of the Brain Natriuretic Peptide Gene in Rat Cardiac Myocytes

“…In this study, the functional activity of the various pharmacological antagonists closely paralleled their ability to inhibit MAPK activity in these cultures. By inference, this suggests that MAPK participates in the signaling cascade linking mechanical strain to acquisition of the hypertrophic phenotype, a hypothesis that draws support from the studies of others (31)(32)(33). The activity of the inhibitors did not correlate well with their ability to inhibit JNK/SAPK activity.…”

“…Moreover, antisense oligonucleotides directed against MAPK have been shown to reduce phenylephrine-dependent increments in ANP mRNA levels and ANP promoter activity while, at the same time, suppressing sarcomerogenesis and increments in cell size that typically accompany hypertrophy induced by this agent (33). However, Post et al (32) recently reported that activation of MAPK does not routinely parallel the development of hypertrophy in the cultured myocyte model, and in their hands, dominant-negative mutants of the MAPK pathway failed to suppress phenylephrine-mediated activation of a transfected ANP promoter.…”

Cardiotrophin 1 (CT-1) Inhibition of Cardiac Myocyte Apoptosis via a Mitogen-activated Protein Kinase-dependent Pathway

“…Transfection and Immunostaining-For calcium phosphate transfections, purified cardiac myocytes were exposed to a cDNA-calcium phosphate precipitate 24 h after plating (11). A firefly luciferase cDNA controlled by a Rous sarcoma virus promoter and pcDNA3 backbone vector (Invitrogen) or pcDNA3 containing the MEK1 K97M mutant (12) (a kind gift from Natalie Ahn, University of Colorado, Boulder, CO) were co-transfected.…”