Published online before print June 5, 2003, doi: 10.1161/01.RES.0000079794.57578.F1
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© 2003 American Heart Association, Inc.
Molecular Medicine |
Inducible cAMP Early Repressor (ICER) Is a Negative-Feedback Regulator of Cardiac Hypertrophy and an Important Mediator of Cardiac Myocyte Apoptosis in Response to ß-Adrenergic Receptor Stimulation
From the Cardiovascular Research Institute, Department of Cell Biology and Molecular Medicine and Department of Medicine (H.T., M.N., K.K., J.S.); Department of Gynecology, Obstetrics and Women’s Health (G.Y., C.A.M.), University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, NJ.
Correspondence to Junichi Sadoshima, MD, PhD, Cardiovascular Research Institute, UMDNJ, New Jersey Medical School, 185 S Orange Ave, MSB G-609, Newark, NJ 07103. E-mail Sadoshju{at}umdnj.edu
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Abstract |
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Although stimulation of the ß-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 ß-adrenergic agonist with a peak at
12 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 ß-adrenergic hypertrophy, whereas it significantly inhibited ß-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 • apoptosis • gene regulation • cell signaling/signal transduction
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Introduction |
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Accumulating evidence suggests that chronic stimulation of ß-adrenergic receptor (ß-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 ß-AR causes hypertrophy and apoptosis in cardiac myocytes.1–4 Thus, understanding the signaling mechanism that mediates growth and death of cardiac myocytes by ß-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 ß-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.7 By contrast, overexpression of adenylyl cyclase type VI does not induce hypertrophy and is rather cardioprotective, despite enhancement of responsiveness to catecholamine.8 Similarly, conflicting results have been reported regarding the role of cAMP in cardiac myocyte apoptosis.3,9 One mechanism, which potentially explains complex cardiac myocyte responses to cAMP, may be that there exists a cAMP-inducible modulator of cardiac hypertrophy and apoptosis, whose expression level varies under different experimental conditions.
Inducible cAMP early repressor (ICER) is a member of the CREB and CRE modulator (CREM) family of transcription factors, which bind to CREs.10 ICER is generated in an inducible manner when an internal promoter of CREM gene, containing CRE sites, is stimulated by increased levels of cAMP.10 Because ICER consists of only the DNA-binding domain identical to the one in CREM and lacks the transactivation domain, ICER serves as a dominant-negative of CREM/CREB-mediated transcription.10 As expected from its function as a nuclear antagonist of CRE, ICER suppresses cell proliferation in cancer cell lines.11–13 Importantly, the responses of cancer cells to cAMP vary substantially depending on the extent of ICER expression.13 Although it has been shown that expression of ICER is induced by cAMP in cardiac myocytes,14 little is known about the function of ICER in these cells.
Considering the possible importance of CREB/CRE-mediated transcription in cardiac gene expression15 and the significant effect of ICER on cAMP-regulated cell growth in other cell types, we hypothesized that ICER is an important modulator of hypertrophy and apoptosis in cardiac myocytes during ß-AR stimulation. We examined whether expression of ICER is induced in cardiac myocytes in response to ß-AR stimulation and whether expression of ICER affects ß-adrenergic cardiac hypertrophy and apoptosis.
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Materials and Methods |
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Plasmids
cDNA encoding ICER-II
and antisense ICER-II were subcloned into pSG5 (Stratagene). Although four isoforms exist in the ICER family (ICER-I, ICER-I, ICER-II, and ICER-II), the function of each isoform thus far identified is indistinguishable.10 Antisense ICER-II inhibits expression of all isoforms of ICER.12
Adenovirus Vectors
Adenovirus harboring ICER-II (Ad-RSV-ICER) was a gift from Dr L. Kaczmarek (Nencki Institute, Warsaw, Poland). Adenovirus harboring antisense ICER (Ad-CMV-ASICER) was generated using the Adeno-X expression system (Clontech), with the entire coding sequence of ICER subcloned in reverse orientation. Adenovirus vector expressing a mutant of CREB (Ad-CMV-CREBM1), in which Ser133 was changed to alanine,16 was a gift from Dr A.J. Zeleznik (University of Pittsburgh, Pa). Adenovirus harboring ß galactosidase (Ad-RSV or CMV-LacZ) was used as a control virus.
Cell Cultures
Primary cultures of cardiac ventricular myocytes from 1-day-old Crl:(WI)BR-Wistar rats (Charles River Laboratories, Wilmington, Mass) were prepared as described.17 Myocytes were cultured with serum for the initial 24 hours and then in serum-free conditions for 48 hours before experiments.
Reverse Transcription–Polymerase Chain Reaction
Total RNA was subjected to reverse transcription–polymerase chain reaction (RT-PCR). The PCR primers were as follows: ICER: sense 5'-GTAACTGGAGATGAAACTGA-3', antisense 5'-GACACTTGACATACTCTTTC-3'; c-fos: sense 5'-CTTTCCTACT-ACCATTCCCCA G-3', antisense 5'-GCAGCCATCTTATTCCTTT-CCC3'; glyceraldehyde-3-phosphate dehydrogenase (GAPDH): sense 5'-TTCTTGTGCAGTGCCAGCCTCGTC-3'; antisense 5'-TAGGAACAGGGAAGGCCATGCCAG-3'.
Immunostaining
Myocytes were fixed in PBS containing 3.7% formaldehyde. Immunostaining was performed using anti-ICER antibody (1:1000)10,18 or anti-sarcomeric myosin antibody (MF20, 1:100). TUNEL staining was performed using the In Situ Cell Death Detection kit (Roche).
Transient Transfection and Reporter Gene Assays
A plasmid containing a 638-bp fragment of the rat atrial natriuretic factor (ANF) promoter linked to firefly luciferase has been described.17 To determine whether CRE and CREB are activated, the PathDetect cis- and trans-reporting systems (Stratagene) were used, respectively. Bcl-2-luciferase was provided by Dr D.J. Tindall (Mayo Medical School, Rochester, Minn).19 An SV40-ß galactosidase construct was cotransfected to determine the transfection efficiency.
Immunoblot Analyses
Cells were scraped in a whole-cell lysis buffer (2% SDS, 10% glycerol, 30 mmol/L Tris, pH 6.8, 100 mmol/L DTT). The nuclear fraction was isolated as described.17 Bcl-2 antibody was obtained from Transduction laboratory. 
-sarcomeric actin antibody was purchased from Sigma.
Measurement of the Myocyte Cell Surface Area and Total Protein/DNA Content
Cardiac myocyte size and total protein/DNA content were determined as described previously.2
Analysis of DNA Fragmentation by ELISA
Cytoplasmic accumulation of histone-associated DNA fragments were quantitated by the Cell Death Detection ELISA (Roche).
In Vivo Administration of Isoproterenol
Either ISO (2.4 mg/kg per day) or PBS was continuously infused into adult Crl:(WI) BR-Wistar male rats (350 to 400 g) through osmotic pumps.2 Experiments involving animals were approved by the Institutional Animal Care and Use Committee.
Statistics
All values are expressed as mean±SEM. Statistical analyses were performed using ANOVA and the Tukey post-test procedure with a P<0.05 considered significant.
An expanded Materials and Methods section is available online at http://www.circresaha.org.
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Results |
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ISO Rapidly Induces ICER mRNA and Protein Expression in Cardiac Myocytes
Stimulation of ß-ARs in cardiac myocytes with ISO (10 µmol/L) but not with vehicle (PBS) induced mRNA expression of ICER-I and ICER-II with a peak at 60 minutes (Figure 1A). Significant upregulation was also observed at 12 hours. ISO dose-dependently induced upregulation of ICER-I and ICER-II with an EC50 at
1 µmol/L (Figure 1B). Forskolin (10 µmol/L), an activator of adenylyl cyclase, and phenylephrine (PE, 10 µmol/L), an agonist for 1-adrenergic receptors, also induced ICER-I and ICER-II mRNA. By contrast, endothelin-1 (ET, 100 nmol/L), another hypertrophic stimulus, failed to induce them (Figure 1C). To test whether stimulation of ß-AR causes upregulation of ICER in vivo, continuous infusion of ISO was conducted in adult rats.2 Significant increases in left ventricular weight/body weight were observed at 24 and 48 hours. mRNA expression of ICER-I and ICER-II was upregulated within 3 to 6 hours, reached a peak at 6 to 12 hours, and persisted for 24 to 48 hours (Figure 1D).
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Protein expression of ICER was confirmed by immunostaining. After 6 hours of stimulation with ISO, the immunoreactivity of ICER (ICER-ir) was increased in the center of cardiac myocytes (Figure 2A). Increased staining of ICER-ir was observed 24 to 48 hours after ISO stimulation. Bright nuclear signal was observed in myocytes transduced with Ad-ICER, whereas there was no signal in ISO-stimulated myocytes stained with secondary antibody alone. Confocal microscopic analyses confirmed that ISO upregulates nuclear expression of ICER-ir (Figure 2B). To confirm that ISO upregulates ICER, the nuclear fraction was subjected to immunoblot analyses. ISO increased nuclear expression of ICER within 4 hours, and the upregulation persisted for 24 to 48 hours (Figure 2C). At 12 hours, ISO increased ICER to 31% of the level induced by Ad-ICER. ISO did not affect expression of CREM.
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ICER Attenuates ISO-Induced Cardiac Hypertrophy
We examined the effect of transduction with Ad-ICER on ISO-induced cardiac hypertrophy. ISO dose-dependently increased cardiac myocyte size and total protein/DNA content, confirming our previous results.2 Transduction of Ad-ICER significantly inhibited ISO-induced increases in cardiac myocyte cell size (83% inhibition versus 1 µmol/L ISO and 39% inhibition versus 10 µmol/L ISO), whereas that of ß galactosidase did not affect them (Figures 3A and 3B). Similarly, transduction of Ad-ICER did not affect the protein/DNA content in unstimulated cardiac myocytes, whereas it significantly inhibited ISO-induced increases in the protein/DNA content (100% inhibition versus 1 µmol/L ISO and 30% inhibition versus 10 µmol/L ISO) (Figure 3B). By contrast, transduction of ß galactosidase did not significantly affect either basal or ISO-induced increases in the cell size or protein/DNA content (Figure 3B). Overexpression of ICER did not significantly affect basal ANF transcription, whereas it completely inhibited ISO-induced increases in ANF transcription (Figure 3C). Adenovirus-mediated overexpression of ICER also completely inhibited ISO-induced c-fos mRNA expression, whereas that of ß galactosidase did not affect it (Figure 3D). These results suggest that ICER significantly inhibits ISO-induced hypertrophy and upregulation of some hypertrophy marker genes.
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ICER Stimulates Cardiac Myocyte Apoptosis
We examined the effect of ICER on cardiac myocyte apoptosis. Forty-eight hours of stimulation with ISO increased nuclear fragmentation of cardiac myocytes determined by triple staining with anti-MF20 antibody, TUNEL, and DAPI (Figures 4A and 4B). Quantitative analyses by ELISA showed that ISO dose-dependently increased cytoplasmic accumulation of mono- and oligo-nucleosomes (Figure 4C). Transduction of Ad-ICER, but not of control virus, increased the number of TUNEL-positive myocytes (Figures 4A and 4B) and DNA fragmentation (Figure 4C). These results suggest that ICER stimulates cardiac myocyte apoptosis. Combined treatment with ISO and ICER slightly increased apoptosis compared with individual treatment. However, their effect was less than additive (Figure 4C), consistent with the notion that ISO and ICER in part share the mechanism inducing cardiac myocyte apoptosis.
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Endogenous ICER Negatively Regulates ISO-Induced Cardiac Hypertrophy While It Mediates ISO-Induced Cardiac Myocyte Apoptosis
Although the aforementioned results suggest that exogenously applied ICER inhibits ß-adrenergic cardiac hypertrophy while it promotes apoptosis, they may not prove the importance of endogenous ICER. To selectively abolish expression of endogenous ICER, we generated adenovirus vector harboring antisense ICER (Ad-ASICER). Transduction of cardiac myocytes with Ad-ASICER abolished ISO-induced upregulation of ICER in the nucleus without affecting expression of CREM (Figure 5A). Transduction of Ad-ASICER did not significantly affect the protein/DNA content in unstimulated cardiac myocytes. By contrast, ASICER, but not control ß galactosidase, significantly enhanced ISO-induced increases in protein/DNA content (Figure 5B). Ad-ASICER treatment also significantly enhanced ISO-induced increases in ANF transcription without affecting basal ANF transcription (Figure 5C), whereas sense plasmid of ICER abolished ISO-induced increases in ANF transcription (Figure 5C). These results suggest that endogenous ICER acts as a negative-feedback mechanism of ISO-induced cardiac hypertrophy.
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Interestingly, transduction of ASICER, but not that of ß galactosidase, significantly reduced increases in DNA fragmentation caused by 1 and 10 µmol/L ISO (85% reduction versus 1 µmol/L ISO and 48% reduction versus 10 µmol/L ISO) (Figure 5D). These results suggest that ICER plays an important role in mediating cardiac myocyte apoptosis by ß-AR stimulation.
ICER Is a Negative-Feedback Regulator for Cardiac Hypertrophy by Phenylephrine but not by Endothelin-1
To test the specificity of actions of ICER, we examined the effect of ICER and ASICER on PE- and ET-induced cardiac hypertrophy. Transduction of Ad-ICER inhibited increases in cell size and protein/DNA content by PE (10 µmol/L) by 53% and 52%, respectively, but not by ET (100 nmol/L) (online Figure 2, available at http://www.circresaha.org). Transduction of Ad-ASICER enhanced increases in cell size and protein/DNA content by PE but not by ET (online Figure 2). These results suggest that ICER works a negative-feedback regulator in ISO- and PE-induced cardiac hypertrophy but not in an ET-induced one.
Overexpression of ICER Inhibits CRE-Mediated Gene Transcription in Cardiac Myocytes
It has been suggested that ICER acts as a dominant-negative molecule of CREB.10 We examined whether ICER inhibits CRE-mediated gene transcription in cardiac myocytes. Reporter gene assays showed that ICER, but not control vector, completely inhibited ISO-induced activation of CRE-mediated gene transcription in cardiac myocytes (Figure 6A). By contrast, ICER did not affect ISO-induced increases in CREB phosphorylation, as determined by Path Detect reporter gene assays as well as by immunoblotting with anti-phospho-CREB antibody (Figures 6B and 6C). These results suggest that ICER competes with CREB at the level of CRE-mediated transcription without affecting CREB phosphorylation. This raises the possibility that the effect of ICER on cardiac hypertrophy and apoptosis may be mediated through its effects on CRE.
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Dominant-Negative CREB Mimics the Effect of ICER on Cardiac Hypertrophy and Apoptosis
To test the role of CRE in mediating ß-adrenergic cardiac hypertrophy and apoptosis, we used dominant-negative CREB, which is known to block transcriptional activities of CREB/CREM, a major CRE-activating transcription factor. We transduced adenovirus vector harboring dominant-negative CREB (DNCREB) or Ad-LacZ into myocytes and performed immunoblot analyses using anti-phospho-CREB antibody. Overexpression of DNCREB, but not ß galactosidase, abolished ISO-stimulated CREB phosphorylation (Figure 7A) and ISO-induced activation of CRE-mediated transcription (Figure 7B). DNCREB, but not ß galactosidase, also attenuated ISO-induced increases in cardiac myocyte cell size by 68% (Figure 7C) as well as protein/DNA content by 44% (data not shown). Furthermore, expression of DNCREB, but not ß galactosidase, induced DNA fragmentation in cardiac myocytes (Figure 7D), suggesting that inhibition of transcription through CRE stimulates apoptosis. These results are consistent with the notion that inhibition of transcription through CRE may in part mediate inhibition of hypertrophy and stimulation of apoptosis by ICER in cardiac myocytes.
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ICER Mediates ISO-Induced Downregulation of Bcl-2
The results presented thus far suggest that ICER induces apoptosis in part through inhibition of CRE-mediated transcription. However, it alone may not be sufficient for ISO-induced apoptosis, because the net effect of ISO on CRE-mediated transcription is positive at least in the initial phase (Figure 6A). Thus, we examined additional mechanisms by which ICER mediates ISO-induced apoptosis. Interestingly, ISO significantly decreased expression of Bcl-2, an important cell survival factor, by 37% (Figure 8A). Transduction of Ad-ICER also reduced expression of Bcl-2 by 43%. Transduction of Ad-ASICER inhibited ISO-induced decreases in Bcl-2 expression. Reblotting of the same filter with anti--sarcomeric actin antibody showed that similar amounts of protein were loaded in each lane. The effect of ISO and ICER on promoter activities of Bcl-2 was determined by reporter gene assays. Both ISO and ICER inhibited the promoter activity of Bcl-2 by 45% and 59%, respectively, whereas ASICER reverted the ISO-induced inhibition (Figure 8B). These results suggest that ICER negatively regulates transcription of Bcl-2 and that ICER mediates ISO-induced downregulation of Bcl-2. The effect of ICER on Bcl-2 is specific, because transduction of Ad-ICER or Ad-ASICER did not affect Ser483 phosphorylation or total amount of Akt, a cell survival kinase, at basal conditions or in response to ISO (10 µmol/L, 30 minutes or 24 hours) (Figure 8C and data not shown).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Accumulating evidence suggests that stimulation of ß-AR induces both hypertrophy and apoptosis in cardiac myocytes.1–4,20 The role of cAMP in hypertrophy and apoptosis of cardiac myocytes is complex, which may be in part attributable to the presence of various effector molecules of cAMP, either positively or negatively modulating growth and death of cardiac myocytes. In this study we show that ICER is rapidly upregulated by stimulation of ß-AR in cardiac myocytes and works as a negative regulator of hypertrophy as well as a positive mediator of apoptosis.
Our results suggest that both mRNA and protein expression of ICER are rapidly induced by stimulation of ß-AR in cardiac myocytes. It has been shown that ICER is degraded by the ubiquitin-proteasome pathway when it is phosphorylated by ERKs.18 Interestingly, protein expression of ICER persisted more than 24 hours after ß-AR stimulation in cardiac myocytes. It has been suggested that increases in CRE-mediated transcription by ß-AR stimulation go back to baseline levels despite the continued presence of CREB phosphorylation in cardiac myocytes.21 Robust and sustained activation of ICER by ISO may explain such inhibition of CRE.
The EC50 of ISO for hypertrophy in cultured cardiac myocytes is 1 to 2 µmol/L and coincides with that for ICER induction.2,22 Although 1 to 10 µmol/L of ISO has been commonly used to induce either hypertrophy or apoptosis in cultured cardiac myocytes,2,3,23 the dosage ISO required for inducing these cellular responses is higher than that for increased contractility and cAMP production.24,25 Cellular responses requiring sustained activation of ßARs may need higher dosages of ISO to overcome desensitization or downregulation of ßARs. However, it is possible that induction of ICER by ISO may require additional cAMP-independent mechanisms, which converge with cAMP-dependent mechanisms at the level of CRE.
Endogenous ICER acts as an important negative regulator of hypertrophy, because antisense inhibition of ICER expression enhanced ß-adrenergic increases in both protein/DNA content and transcription of ANF, a cAMP-responsive gene. It has recently been shown that suppressor of cytokines signaling 3 (SOCS3) works as an inducible negative regulator of cardiac hypertrophy in response to leukemia inhibitory factor stimulation.26 To our knowledge, ICER is the first inducible negative regulator of cardiac hypertrophy identified for ß-ARs. Importantly, although SOCS3 blocks a proximal event of the leukemia inhibitory factor signaling by blocking janus kinase (JAK),26 ICER blocks a more distal event of the ß-AR signaling by inhibiting transcription through CRE.
The results of reporter gene analyses indicated that ICER potently blocks CRE-mediated gene expression in cardiac myocytes. Furthermore, expression of dominant-negative CREB, another method known to block CRE-mediated gene expression,15,16 mimicked the effect of ICER on ß-adrenergic cardiac hypertrophy. Thus, the effect of ICER may be in part mediated through its inhibitory effects on CRE. To our knowledge, the role of CRE in ß-adrenergic cardiac hypertrophy has not been clearly demonstrated. It has recently been shown that cardiac-specific overexpression of dominant-negative CREB in transgenic mice causes dilated cardiomyopathy.15 Although whether hypertrophy of individual cardiac myocytes is impaired has not been described,15 failure to mediate postnatal hypertrophy may lead to cardiac dilation in those mice. It has been suggested that a CREB-mediated mechanism plays an important role in determining the cell size of embryonic fibroblasts in mice.27 Taken together, our results suggest that the CREB/CRE plays an important role in mediating ß-adrenergic cardiac hypertrophy, and ICER negatively regulates cardiac hypertrophy at least in part by inhibiting transcription through CREB/CRE. It has been shown that the antiproliferative effects of ICER are mediated through inhibition of cyclin A in pituitary cells.11 ICER negatively regulates expression of tyrosine hydroxylase,28 the rate-limiting enzyme for catecholamine biosynthesis, as well as ß-ARs29 in neuronal cells. It is unlikely, however, that downregulation of the whole ß-AR system is responsible for inhibition of hypertrophy by ICER, because ISO-induced phosphorylation of CREB is maintained after ICER overexpression (Figures 6B and 6C). At present, besides ANF and c-fos, we do not know if expression of other genes is affected by ICER. Identifying the genes affected by ICER may lead to a better understanding of the molecular mechanism of actions of ICER against ß-adrenergic cardiac hypertrophy.
Another important finding in this report is that ICER induces cardiac myocyte death, consistent with apoptosis. Our results also suggest that ICER is an important mediator of ß-adrenergic cardiac myocyte apoptosis. Two lines of evidence support this notion. First, both ISO stimulation and overexpression of ICER cause a similar extent of cell death consistent with apoptosis in cardiac myocytes, and ISO stimulation in the presence of ICER overexpression fails to show additive effects on apoptosis, suggesting that both stimuli share mechanisms inducing apoptosis. Second, ISO-induced increases in cardiac myocyte apoptosis are significantly inhibited when expression of ICER is inhibited by Ad-ASICER. Although antiapoptotic actions of ICER have been shown in leukemia cells,30 to our knowledge, proapoptotic effects of ICER have not been clearly demonstrated in any other cell types.
It has been suggested that CREB is phosphorylated by upstream kinases, including PKA, ERK-p90RSK2, or Akt, and mediates antiapoptotic or cell survival mechanisms.31 For example, CREB mediates expression of Bcl-2, an important antiapoptotic molecule, in cardiac myocytes.32 Consistent with this notion, overexpression of dominant-negative CREB alone was sufficient to induce apoptosis in cardiac myocytes, thereby mimicking the effect of ICER. Thus, it is likely that inhibition of CREB/CRE by ICER in part mediates increases in apoptosis. These notions are also consistent with development of heart failure in the dominant-negative CREB mice.
Previous studies have suggested that cAMP-dependent signaling mechanisms mediate cardiac myocyte apoptosis by ß-AR stimulation despite activation of CREB/CRE.3 Although CRE-mediated transcription may be shut down after transient activation,21 the effect on inhibition of CRE-mediated transcription alone may not explain the proapoptotic effects of ISO. We found that both ISO and ICER downregulated Bcl-2 expression, and Ad-ASICER reversed ISO-induced downregulation of Bcl-2. Thus, we propose that induction of ICER would explain how ß-AR is able to induce apoptosis despite initial activation of CRE-mediated transcription and that ICER mediates ß-adrenergic apoptosis at least in part through downregulation of Bcl-2. It should be noted that ICER may negatively affect cell survival not only through inhibition of CREB-mediated transcription but also through its modulatory effects on other transcription factors.33
In summary, our results suggest that stimulation of the ß-AR rapidly induces an immediate early transcriptional regulator, which acts as both a negative regulator of cardiac hypertrophy as well as a positive mediator of apoptosis. Although inhibition of ß-adrenergic cardiac hypertrophy by ICER may be protective through prevention of cardiac remodeling, it can be detrimental if it interferes with an adaptive phase of hypertrophy. By contrast, promotion of apoptosis by ICER is likely to consistently stimulate functional deterioration. In this regard, ICER may be an important target for the treatment of ß-adrenergic cardiac hypertrophy and congestive heart failure.
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Acknowledgments |
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This work was supported by grants from the NIH (HL59139, HL67724, HL67727, and HL69020) and the AHA (9950673N and 0340123N).
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Footnotes |
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Original received December 23, 2002; revision received May 21, 2003; accepted May 21, 2003.
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