Adrenergic receptor stimulation of the mitogen-activated protein kinase cascade and cardiac hypertrophy

Abstract

Phenylephrine and noradrenaline (alpha-adrenergic agonism) or isoprenaline (beta-adrenergic agonism) stimulated protein synthesis rates, increased the activity of the atrial natriuretic factor gene promoter and activated mitogen-activated protein kinase (MAPK). The EC50 for MAPK activation by noradrenaline was 2-4 microM and that for isoprenaline was 0.2-0.3 microM. Maximal activation of MAPK by isoprenaline was inhibited by the beta-adrenergic antagonist, propranolol, whereas the activation by noradrenaline was inhibited by the alpha1-adrenergic antagonist, prazosin. FPLC on a Mono-Q column separated two peaks of MAPK (p42MAPK and p44MAPK) and two peaks of MAPK-activating activity (MEK) activated by isoprenaline or noradrenaline. Prolonged phorbol ester exposure partially down-regulated the activation of MAPK by noradrenaline but not by isoprenaline. This implies a role for protein kinase C in MAPK activation by noradrenaline but not isoprenaline. A role for cyclic AMP in activation of the MAPK pathway was eliminated when other agonists that elevate cyclic AMP in the cardiac myocyte did not activate MAPK. In contrast, MAPK was activated by exposure to ionomycin, Bay K8644 or thapsigargin that elevate intracellular Ca2+. Furthermore, depletion of extracellular Ca2+ concentrations with bis-(o-aminophenoxy)ethane-NNN'N'-tetra-acetic acid (BAPTA) or blocking of the L-type Ca2+ channel with nifepidine or verapamil inhibited the response to isoprenaline without inhibiting the responses to noradrenaline. We conclude that alpha- and beta-adrenergic agonists can activate the MEK/MAPK pathway in the heart by different signalling pathways. Elevation of intracellular Ca2+ rather than cyclic AMP appears important in the activation of MAPK by isoprenaline in the cardiac myocyte.

Both Gs and Gi Proteins Are Critically Involved in Isoproterenol-induced Cardiomyocyte Hypertrophy

“…As reported previously, ERK activation is required for cell growth including cardiac hypertrophy (6,7,34,61,62). We also showed in the present study that the signal transduction pathway leading to activation of ERKs is important for ISO-induced protein synthesis in cardiac myocytes.…”

“…Although there are great differences between signaling steps and protein synthesis in many aspects such as the basal levels, the responsive ability to stimulation, and the methods of examination among them, it is also possible that the initiation of protein synthesis needs a much greater fold-increase in the activity of signaling molecules. Many previous studies also showed that signaling steps such as ERKs are activated more than severalfold by stimulation, whereas an increase in the protein synthesis rate is less than 50% (6,7,62,63). Although it is unknown at present why there are big differences in the time course for protein synthesis and activation of signaling molecules, there are several possibilities.…”

“…In human endothelial cells, down-regulation of the ␣ subunit of G s (G s␣ ) abolishes ␤-AR-mediated ERK activation by ISO (25), suggesting that G s␣ -dependent cAMP elevation and PKA activation are responsible for the activation of ERKs. We and others have also demonstrated that ␤-AR agonists including ISO significantly activate ERKs and increase protein synthesis through cAMP/PKA in cardiac myocytes (6,7). However, the molecular mechanisms of ␤-AR agonist-induced ERK activation remain largely unknown.…”

“…It has been reported that PHE evokes hypertrophic responses in the cardiac myocytes of neonatal rats (5) and that expression of constitutively active ␣-AR induces cardiac hypertrophy in adult mice (8). Both in vivo and in vitro studies demonstrate that ISO also stimulates expression of proto-oncogenes in cardiomyocytes and induces cardiac hypertrophy (6,9,10). Activation of each AR evokes specific intracellular signals (4,11).…”

“…1 AR agonists such as norepinephrine (NE) (␣ and ␤), phenylephrine (PHE) (␣), and isoproterenol (ISO) (␤) have been reported to induce cardiomyocyte hypertrophy (2)(3)(4)(5)(6)(7). Prolonged infusion of subpressor doses of NE increases the mass of the myocardium and the thickness of the left ventricular wall, suggesting that NE has direct hypertrophic effects on cardiac myocytes without affecting afterload (2).…”