Akt-mediated Cardiomyocyte Survival Pathways Are Compromised by Gαq-induced Phosphoinositide 4,5-Bisphosphate Depletion

Abstract

Expression of the wild type ␣ subunit of G q (G q WT) in cardiomyocytes induces hypertrophy, whereas a constitutively active G␣ q subunit (G q Q209L) induces apoptosis. Akt phosphorylation increases with G q WT expression but is markedly attenuated in cardiomyocytes expressing G q Q209L or in those expressing G q WT and treated with agonist. A membrane-targeted Akt rescues G q Q209L-expressing cardiomyocytes from apoptotic cell death. In contrast, leukemia inhibitory factor fails to activate Akt or promote cell survival in these cells. Association of Akt and PDK-1 with the membrane is also diminished in G q Q209L-expressing cardiomyocytes. Phosphatidylinositol 3,4,5-trisphosphate (PIP 3 ), the primary regulator of Akt, increases significantly in G q WTexpressing cells but not in cardiomyocytes expressing G q Q209L. Levels of phosphatidylinositol 4,5-bisphosphate (PIP 2 ), the immediate precursor of PIP 3 , are also markedly lower in G q Q209L-expressing compared to control cells. Expression of a G q Q209L mutant that has diminished capacity to activate phospholipase C does not decrease PIP 2 or Akt or induce apoptosis. In transgenic mice with cardiac G␣ q overexpression, heart failure and increased cardiomyocyte apoptosis develop during the peripartal period. Akt phosphorylation and PIP 2 levels decrease concomitantly. Our findings suggest that an Akt-mediated cell survival pathway is compromised by the diminished availability of PIP 2 elicited by pathological levels of G q activity.G␣ q signaling plays a key role in the hypertrophic growth of cardiomyocytes. Hormones that act through receptors coupled to the heterotrimeric protein G q (e.g. norepinephrine, PGF 2␣ , 1 endothelin, and angiotensin II) induce cardiomyocyte hypertrophy in vitro (1). Further evidence for a physiological role of G␣ q activation is that blocking G q signaling in vivo prevents pressure-overload hypertrophy (2, 3). Hypertrophy can transition to heart failure under conditions of enhanced G␣ q signaling induced by overexpression of G q -coupled receptors, of phospholipase C-␤ (PLC), of constitutively active G␣ q , or of wild type G␣ q in combination with stress (4 -11). Thus studies in isolated cardiomyocytes and in transgenic animals have led to the conclusion that although G␣ q signaling is critical for cardiac hypertrophy, prolonged or marked enhancement of G q activity is deleterious to the heart.Our laboratory has characterized a model of in vitro cardiomyocyte hypertrophy and apoptosis by expressing the ␣ subunit of G q (8). When the wild type ␣ subunit (G q WT) is expressed by adenoviral infection, neonatal rat ventricular myocytes (NRVMs) undergo hypertrophy (8). In contrast, when the constitutively active ␣ subunit (G q Q209L) is expressed, or G q WTexpressing cardiomyocytes are treated with G q -coupled receptor agonists, the NRVMs undergo apoptosis (8,12). Varying the amount of G␣ q signaling in myocytes therefore results in distinct cellular phenotypes. Although modest levels of signaling induce hypertrophy, at higher levels su...

Ca2+ Dysregulation Induces Mitochondrial Depolarization and Apoptosis

“…As we have previously reported (15), constitutively activated Akt (myr-Akt) rescues cells from apoptosis induced by Q209L expression (Fig. 8A).…”

“…We also reported that constitutively activated Akt inhibits development of apoptosis in this system (15). Neither the mechanisms underlying induction of the PT-pore nor the mechanisms of Akt-mediated protection have been fully elucidated.…”

“…The proapoptotic BH-3 only protein, Nix, was subsequently implicated in G␣ q -induced apoptosis and heart failure development in vivo (4). In addition we reported that Akt activity was compromised and that constitutively activated Akt rescued cells from Q209L-induced apoptosis (15). The predictive potential of the Q209L cell model encouraged us to utilize this paradigm to further evaluate mechanisms contributing to PT-pore opening and to identify mechanisms of Akt-mediated protection.…”

Kisspeptin-10-Induced Signaling of GPR54 Negatively Regulates Chemotactic Responses Mediated by CXCR4: a Potential Mechanism for the Metastasis Suppressor Activity of Kisspeptins

“…For instance, it was shown that the activation of Gq-coupled M1, M3, and M5 muscarinic receptors but not Gi-coupled M2 and M4 receptors could protect cells from apoptosis induced by etoposide or UV (20,23,24). Other reports suggest instead that signaling of Gq, either activated by a GPCR or in the form of a constitutively active mutant (CAM), can inhibit the activation of Akt induced by growth factors and trigger apoptosis (25)(26)(27)(28)(29). Additional studies showed an opposite effect for Gqcoupled receptors (phosphorylation of Akt and inhibition of apoptosis) and CAMs of Gq (reduction of phosphorylation of Akt and apoptosis; ref.…”

Depletion of PtdIns(4,5)P2 underlies retinal degeneration in Drosophila trp mutants

“…Similar pathways could also play important roles in retinal degeneration associated with mutations in PI recycling genes, such as rdgB, cds, rdgA and dpis. Interestingly, PtdIns(4,5)P 2 depletion mediated via effects of RhoA on PtdIns(4)P5-kinase has been implicated in cardiac hypertrophy (Howes et al, 2003;Miyamoto et al, 2010). Cardiomyocytes are also known to express high levels of TRP channels which play important roles in their functioning and maintenance (Vennekens, 2011;Watanabe et al, 2008).…”