Inducible cAMP Early Repressor (ICER) Is a Negative-Feedback Regulator of Cardiac Hypertrophy and an Important Mediator of Cardiac Myocyte Apoptosis in Response to β-Adrenergic Receptor Stimulation

Abstract

Abstract-Although stimulation of the ␤-adrenergic receptor increases levels of cAMP and activation of the cAMP response element (CRE) in cardiac myocytes, the role of the signaling mechanism regulated by cAMP in hypertrophy and apoptosis is not well understood. In this study we show that protein expression of inducible cAMP early repressor (ICER), an endogenous inhibitor of CRE-mediated transcription, is induced by stimulation of isoproterenol (ISO), a ␤-adrenergic agonist with a peak at Ϸ12 hours and persisting for more than 24 hours in neonatal rat cardiac myocytes. ICER is also upregulated by phenylephrine but not by endothelin-1. Continuous infusion of ISO also increased ICER in the rat heart in vivo. Overexpression of ICER significantly attenuated ISO-and phenylephrine-induced cardiac hypertrophy but did not inhibit endothelin-1-induced cardiac hypertrophy. Overexpression of ICER also stimulated cardiac myocyte apoptosis. Antisense inhibition of ICER significantly enhanced ␤-adrenergic hypertrophy, whereas it significantly inhibited ␤-adrenergic cardiac myocyte apoptosis, suggesting that endogenous ICER works as an important regulator of cardiac hypertrophy and apoptosis. Inhibition of CRE-mediated transcription by dominant-negative CRE binding protein inhibited cardiac hypertrophy, whereas it stimulated cardiac myocyte apoptosis, thereby mimicking the effect of ICER. Both ISO and ICER reduced expression of Bcl-2, an antiapoptotic molecule, whereas antisense ICER prevented ISO-induced downregulation of Bcl-2. These results suggest that ICER is upregulated by cardiac hypertrophic stimuli increasing CRE-mediated transcription in cardiac myocytes and acts as a negative regulator of hypertrophy and a positive mediator of apoptosis, in part through both inhibition of CRE-mediated transcription and downregulation of Bcl-2. Key Words: cardiac hypertrophy Ⅲ apoptosis Ⅲ gene regulation Ⅲ cell signaling/signal transduction A ccumulating evidence suggests that chronic stimulation of ␤-adrenergic receptor (␤-AR) in patients causes progressive cardiac dysfunction, cell loss, and cardiac chamber remodeling. Consistent with this notion, it has been demonstrated that stimulation of the ␤-AR causes hypertrophy and apoptosis in cardiac myocytes. [1][2][3][4] Thus, understanding the signaling mechanism that mediates growth and death of cardiac myocytes by ␤-AR stimulation may lead to a better treatment for patients with heart failure.Although cAMP is one of the most well-known signaling molecules activated by ␤-AR stimulation, 5 its mechanism of action in cardiac hypertrophy and apoptosis is not fully understood. Increased production of cAMP by enhanced activities of adenylyl cyclase leads to activation of protein kinase A (PKA), which in turn causes phosphorylation/activation of cAMP response element (CRE) binding protein (CREB) and subsequent gene expression through CRE-mediated transcription. 6 It has recently been shown that activation of PKA in the mouse heart causes cardiac myocyte hypertrophy and dilated cardiomyopathy...

Functional Role of Phosphodiesterase 3 in Cardiomyocyte Apoptosis

“…Similar to previous observations, 23 this Ad-ICER-AS can efficiently block ICER induction by isoproterenol and Ang II (data not shown). We found that expression of ICER-AS via adenovirus (Ad-ICER-AS) completely blocked Ang II-and isoproterenol-induced cardiomyocyte apoptosis, analyzed by both TUNEL ( Figure 7A) and ELISA methods ( Figure 7C).…”

“…For example, transgenic mice with cardiac-specific overexpression of DN-CREB through the predevelopment and postdevelopment stages of hearts developed dilated cardiomyopathy, 30 likely because of impairment of the effects of CREB on normal cell growth and survival. However, our findings and those of others 23 have shown that under elevated neurohormonal stimulation such as Ang II or isoproterenol in vitro or in vivo, CREB becomes proapoptotic via induction of ICER. The prosurvival effect of CREB is mediated by upregulation of antiapoptotic molecules such as Bcl-2 and inhibitor of apoptosis protein-2, 23,31 and the proapoptotic effects of CREB can be mediated by induction of ICER, which antagonizes antiapoptotic molecule expression.…”

“…Bcl-2 is a well-known antiapoptotic molecule, and its expression can be repressed by ICER. 23 We observed decreased Bcl-2 protein levels by PDE3 inhibition but not PDE4 inhibition ( Figure 6C). These data suggest that PDE3 and PDE4 may regulate different pools of cAMP, which have distinct effects on CREB activation, ICER induction, and Bcl-2 expression.…”

“…23 CREB activation can be induced via phosphorylation of serine 133 by several serine/threonine kinases, including cAMP-dependent PKA. Thus, it is likely that reduction of PDE3 induces ICER and promotes apoptosis by activation of CREB in a cAMP/PKA-dependent manner.…”

A positive feedback loop contributes to the deleterious effects of angiotensin

“…G q activity has been inhibited in transgenic mice by overexpression of the carboxyl-terminal peptide of G ␣q ; inhibitor-expressing transgenic mice have a reduced hypertrophic response to aortic constriction (12), demonstrating that G q activation is necessary for this hypertrophic response. Overexpression and knockout of the AT 1 receptor produce phenotypes that are consistent with the G q data (13,14 (18). These data raise some interesting questions.…”