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Vol. 19, Issue 10, 4141-4153, October 2008
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*Division of Cardiology, School of Medicine, University of Colorado Health Sciences Center, Denver, CO 80262; and
Gilead Colorado, Inc., Westminster, CO 80021
Submitted December 6, 2007;
Revised May 27, 2008;
Accepted June 26, 2008
Monitoring Editor: Sandra L. Schmid
| ABSTRACT |
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| INTRODUCTION |
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At the cellular level, myocardial failure and myocyte hypertrophy are characterized by an increase in cell size and by changes in the gene expression of many components of the heart, including the contractile apparatus. These molecular changes have been described as a recapitulation of a "fetal" gene program (FGP) because many embryonically expressed genes that are down-regulated postnatally are reactivated, whereas several "adult" genes are repressed (Lompre et al., 1979
; Abraham et al., 2002
). Of the changes that are observed in failing hearts, increases in β myosin heavy chain (β-MyHC), skeletal
-actin, and atrial natriuretic peptide (ANP), with coordinate decreases in
myosin heavy chain (
MyHC) and sarcoplasmatic reticulum ATPase 2a (SERCA), are perhaps the most widely recognized.
In the work presented here, we show the class II HDAC HDAC5 interacts with YY1 in muscle cells and that this interaction is necessary for YY1 to function as a transcription repressor of cardiac-specific promoters. Moreover, we show that overexpression of YY1 in cardiac myocytes prevents HDAC5 nuclear export in response to hypertrophic stimuli and prevents induction of the fetal gene program. In addition, YY1 overexpression blocks increases in cell size that result from
-adrenergic receptor (
-AR)-mediated hypertrophy through a mechanism that involves interaction with and retention of HDAC5 in the nucleus. Together, these data suggest that YY1 functions in concert with HDAC5 to maintain gene-specific transcriptional repression in cardiac cells.
| MATERIALS AND METHODS |
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-actinin (A 7811) antibody were purchased from Sigma-Aldrich (St. Louis, MO). The horseradish peroxidase (115-035-146) anti-mouse and anti-rabbit were purchased from Jackson ImmunoResearch Laboratories (West Grove, PA). Alexa Fluor 594 (A11032) anti-mouse was purchased from Invitrogen (Carlsbad, CA).
DNA and Adenovirus Constructs
The YY1-green fluorescent protein (GFP) adenovirus construct was a gift from Dr. Aristidis Moustakas (Ludwig Institute of Cancer Research, Uppsala, Sweden) and the YY1-FLAG 174–200 deletion construct was a gift of Dr. Edward Seto (University of South Florida, Tampa, FL). YY1-GAL4 wild-type control and deletion constructs were a gift of Dr. Michael Atchison (University of Pennsylvania, Philadelphia, PA) and are described in detail in Bushmeyer et al. (1995)
and Bushmeyer and Atchison (1998)
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Cell Culture and Adenoviral Infection
Neonatal rat cardiac myocytes (NRVMs) were prepared according to the method described in Waspe et al. (1990)
. Cells were infected with an adenovirus expressing YY1-GFP and/or HDAC5-FLAG or with a control adenovirus at a multiplicity of infection of 7 plaque-forming units/cell.
Real-Time Polymerase Chain Reaction (PCR)
Total RNA was extracted by TRIzol (Invitrogen). 0.5 µg of RNA was reverse transcribed into cDNA using the SuperScript III first-strand cDNA synthesis kit (Invitrogen). Typically, 0.1 ng of cDNA, 12.5 nM of each primer, and Power SYBER Green PCR Master Mix (Applied Biosystems, Foster City, CA) were used in the reverse transcription (RT)-PCR reactions. Reactions were performed using the ABI7300 system. The primers used are presented on Table 1.
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Nuclear and Cytoplasmic Fractionation
Nuclear and cytoplasmic fractionation were performed using the NE-PER kit (Pierce Chemical, Rockford, IL) according to manufacturer's recommendations.
Western Blots
Western blots were performed essentially as described previously (Sucharov et al., 2003
). Antibodies were diluted 1:1000 in 1x Tris-buffered saline (20 mM Tris and 500 mM NaCl, pH 7.5) containing 3% bovine serum albumin (BSA) and 0.1% Tween and incubated with the blot overnight at 4°C.
Immunoprecipitation/Immunoblotting
Immunoprecipitation experiments were done using antibodies described in the text. Experiments were done according to Santa Cruz Biotechnology recommendations, with minor modifications. After four washes with 1x radioimmunoprecipitation assay buffer (Calalb et al., 1995
), the sample was incubated with 2–3x packed volume with 2x sample buffer (Bio-Rad, Hercules, CA) at room temperature for 30 min.
Immunofluorescence
Immunofluorescence was performed according to Harrison et al. (2004)
. Cells were washed with Tris-buffered saline/Tween 20 (TBST) and fixed with 10% formaldehyde for 20 min. Cells were again washed with TBST and incubated with 0.1% Triton X for 30 min. Cells were then blocked with 1% BSA in TBST for 1 h followed by 1-h incubation with 1:500 dilution of the FLAG antibody. Cells incubated with 1:1000 dilution of Alexa 594 anti-mouse antibody and 2 µg/ml Hoechst staining for 1 h. Images were captured at a 40x magnification with a fluorescence microscope (Nikon E800) equipped with a digital camera (AxioCam) and Axiovision, version 3.0.6.36 imaging software (Carl Zeiss, Thornwood, NY).
COS Cell Transfection
COS cells were transfected with Lipofectamine 2000 (Invitrogen). Briefly, 8.4 µg of total DNA was combined with 25 µl of Lipofectamine according to manufacturer's recommendations.
Cardiac Myocyte Transfection
Cardiac myocyte transfections were done using the nucleofaction protocol (Amaxa Biosystems, Gaithersburg, MD). This methodology results in
50% transfection efficiency. Briefly, 2 x 106 cells were transfected with 4 µg of plasmid DNA according to the manufacturer's recommendations.
YY1 and HDAC5 Small Interfering RNA (siRNA) Transfection
YY1 siRNA was purchased from Ambion (Austin, TX; catalog no. 16704), and HDAC5 siRNA was purchased from Thermo Fisher Scientific (Waltham, MA). siRNAs were transfected using the nucleofaction protocol (HDAC5 and YY1) or oligofectamine methodology (YY1) (Invitrogen). In both cases, 20 µM siRNA oligonucleotide was used. All results were compared with transfections containing a negative control siRNA (Ambion; catalog no. 4611).
Chromatin Immunoprecipitation (ChIP)
ChiPs were performed using the ChIP assay kit (Millipore, Billerica, MA). Cells (1 x 106) were used for each condition. Cells were sonicated four times with 10-min pulses at 40% of the power. The resulting DNA–protein complex was immunoprecipitated with YY1 agarose-conjugated antibody (Santa Cruz Biotechnology; sc-281), immunoglobulin G (IgG), or RNA polymerase antibody kit (Millipore). Cross-link was reversed and protein was digested with proteinase K. DNA was analyzed by PCR. Primers used in the PCR reaction are described in Table 2.
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| RESULTS |
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-actin), but it up-regulated the expression of the adult isoforms
MyHC and SERCA2A (Figure 1).
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-adrenergic receptor agonist PE 24 h after infection. Activation of the
-adrenergic receptor has been shown to increase myocyte cell size and to activate the fetal gene program, with repression of
MyHC and SERCA2A, and up-regulation of ANP, BNP, βMyHC, and skeletal
-actin (Patten et al., 1996
-actinin antibody. As shown in Figure 2 sarcomeric organization was present in PE-treated cells and in cells infected with YY1 adenovirus, suggesting that YY1 induces sarcomeric organization in the absence of hypertrophic stimuli.
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MyHC mRNA. In addition, cells transfected with the YY1 siRNA showed an increase in protein content (Figure 3C) and in cell size (Figure 3, D and E), consistent with a hypertrophic response. Together, these data suggest that YY1 is required to maintain a normal myocyte phenotype by repressing the fetal gene program and preventing myocyte hypertrophy.
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MyHC and BNP Promoters
MyHC and BNP promoter constructs in electrophoretic mobility shift assays (Bhalla et al., 2001
MyHC and BNP genes. As shown in Figure 4, these promoters are bound by YY1 in these cells. ChIP experiments were performed six different times. No consistent difference was observed in the level of amplified product. This suggests that YY1 function in regulating gene expression likely results from a mechanism that is independent of YY1 occupancy of the promoter.
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MyHC and BNP primers, showing that PCR amplification is in fact due to YY1 interaction with these promoters. Protein A (data not shown) or IgG was used as control for the immunoprecipitation experiments. RNA polymerase antibody was used as a positive control for the experiments, whereas
-amylase primer was used as a negative control due to lack of interaction of YY1 with the amylase promoter (Caretti et al., 2004
YY1 Interacts with HDAC5 in NRVMs and in Rat Hearts
Because the activity of YY1 is known to depend on its interaction with other transcriptional regulators, we were interested to determine what cofactors are involved in the YY1-mediated regulation of the fetal gene program in cardiac cells. Previous work with HeLa cells demonstrated that YY1 can interact directly with class I HDACs (Yao et al., 2001
). Furthermore, the class II HDAC HDAC5 has been shown to be regulated during hypertrophy (Zhang et al., 2002
), and phosphorylation of HDAC5 in response to PE stimulation results in its nuclear export and transcription derepression (Zhang et al., 2002
). To test whether YY1 interacts with HDAC5, NRVMs were infected with YY1-GFP adenovirus construct. Cells were harvested and immunoprecipitation experiments were performed with an anti-YY1 antibody. As shown in Figure 5A, YY1 and HDAC5 coimmunoprecipitated in untreated but not in PE-treated cells, suggesting that PE treatment prevents their interaction. To demonstrate that this interaction is also observed in vivo, protein extracts were prepared from rat heart tissue, and immunoprecipitation experiments were performed with anti-YY1 and anti-HDAC5 antibodies. As seen in NRVMs, endogenous YY1 and HDAC5 also coimmunoprecipitated, suggesting that they interact in vivo (Figure 5B). This interaction of YY1 with HDACs seems to be specific to HDAC5 in these cells because no interaction of YY1 with HDAC 4 or HDAC9 was observed in these experiments (Figure 5B).
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YY1 Reduces PE-mediated HDAC5 Phosphorylation
Because YY1 overexpression prevents the activated nuclear export of HDAC5 in cardiac cells, we hypothesized that the binding interaction of YY1 with HDAC5 prevents HDAC5 phosphorylation at the amino acids that are specifically phosphorylated in response to hypertrophic stimuli. To test this hypothesis, we first examined the level of HDAC5 phosphorylation in cells overexpressing YY1. As shown in Figure 7, A and B, overexpression of YY1 results in a dramatic reduction in the phosphorylation levels of HDAC5-FLAG or endogenous HDAC5. We then tested whether YY1 interacts with a region of HDAC5 that includes the serine residues (Ser259 and Ser496) that are phosphorylated in response to PE. By transfecting an HDAC5 260–615 deletion construct and the full-length YY1 into COS cells, we show that YY1 does not bind the HDAC5 deletion construct in coimmunoprecipitation assays (Figure 7, C and D), suggesting that this region of HDAC5 is necessary for YY1 interaction. Together, these data suggest that YY1 prevents HDAC5 nuclear export by directly binding its activation domain and preventing its phosphorylation.
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-actin was still repressed by YY1, suggesting that the mechanism of YY1 repression of skeletal
-actin is not mediated by HDAC5. | DISCUSSION |
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-AR–stimulated cardiac cells through interaction with HDAC5. HDACs promote deacetylation of histones, which tightens the chromatin structure around genes and results in transcription repression at those sites (Courey and Jia, 2001An interaction between YY1 and HDAC5 may lead to the transcriptional repression of fetal genes by causing the recruitment of HDAC5 to the promoter regions of these genes. Consistent with this model, the down-regulation of YY1 by siRNA is sufficient to induce the expression of the fetal gene isoforms. When levels of YY1 are synthetically reduced, HDAC5 would have no mechanism to specifically interact to these promoters and transcription of fetal genes would increase. This is evidenced by the lack of further increase in gene expression in response to PE treatment. Furthermore, the YY1 deletion construct that lacks interaction with HDAC5 functions as transcription activator, suggesting that the ability of YY1 to act as a repressor is dependent on HDAC5 interaction.
These data suggest that YY1 is critically important to the maintenance of normal, adult gene expression patterns. By acting to repress fetal genes, YY1 effectively prevents a regression to fetal programs and promotes the continued terminal differentiation of cardiac cells. Consistent with this idea that YY1 acts as a global regulator of differentiation, YY1 has been identified as a homologue of polycomb group (PcG) proteins and has been shown to be involved in development (Atchison et al., 2003
). During the development/differentiation process, there is a fundamental mechanistic need to maintain key transcription patterns throughout the development and lifetime of an organism. PcG proteins have been shown to be an essential component of the maintenance of transcription repression in development and differentiation. PcG proteins can repress transcription by generating chromatin structures that are refractory to gene expression (reviewed in Levine et al., 2004
).
ChIP assays done with an anti-YY1 antibody indicate that the interaction of YY1 with the endogenous fetal promoters is unchanged in response to PE treatment, suggesting that the activity of YY1 is not simply a result of YY1 binding to these promoters. Instead, the activity of YY1 at these promoters likely results from an interaction with other factors that bind to and/or modify YY1. Alternatively, retention of HDAC5 in the nucleus of cells overexpressing YY1 is likely to promote transcription repression not only through interaction with YY1 on the promoter region of the various genes, but also through interaction with other transcription factors known to be important in the regulation of these genes, i.e., MEF2. In heart cells, the interaction of YY1 with HDAC5 seems to be critical for the controlled repression of fetal genes. However, the factors that mediate the affinity of YY1 for HDAC5 would also be critical for the proper regulation of development in the mammalian system. In experiments with undifferentiated and differentiated H9C2 cells, YY1 acts as a transcription repressor only in differentiated cells (Sucharov et al., 2006
).
Last, we show that HDAC5 is necessary for YY1 function as a transcription repressor of the fetal isoforms in cardiac cells. YY1 has been shown to interact with class I HDACs in HeLa cells and this interaction occurs in two regions of YY1, the 170–200 and 261–333 (Yao et al., 2001
). YY1 interaction with HDAC5 occurs through the 170–200 but not the 261–333 region (Figure 8A). It is possible that other domains of YY1 are important for interaction with class II HDACs. However, our results suggest that deletion of the 170–200 region is the only one required for the interaction to occur. Although we showed that HDAC5 is necessary for YY1 to function as a transcription repressor, it is possible that in cardiac cells interaction of YY1 and class I HDACs is important for regulation of a subset of genes. Interestingly, repression of skeletal
-actin seems to be independent of HDAC5, suggesting that other mechanisms are involved in YY1 function as a repressor. Changes in gene expression in response to HDAC5 down-regulation are part of different study (Dockstader and Sucharov, unpublished data). The ability of YY1 to control fetal gene expression does not seem to be limited to an interaction with HDAC5. YY1 regulation of the adult isoforms of gene expression,
MyHC and SERCA, is the opposite of what is seen for the fetal isoforms. YY1 up-regulates expression of these genes independently of its interaction with HDAC5, and repression of these genes in response to PE is also independent of the YY1–HDAC5 interaction (Figure 8C). However, in response to down-regulation of HDAC5,
MyHC gene expression does not change in cells overexpressing YY1, whereas SERCA levels are up-regulated (Figure 8E). This suggests that regulation of the adult isoforms by YY1 is mediated by factors other than HDAC5. Down-regulation of HDAC5 did not have an effect on YY1 repression of skeletal
-actin, whereas deletion of the region that interacts with HDAC5 resulted in up-regulation of skeletal
-actin gene expression by YY1. This suggests that YY1 down-regulation of skeletal
-actin is likely due to interaction with a different HDAC. We had shown previously that YY1 represses human
MyHC promoter activity and gene expression (Sucharov et al., 2003
; Mariner et al., 2005
). In the previous experiments, NRVMs were infected or transfected for only 48 h, whereas the cells were infected for 72 h (24 h of infection previous to treatment and 48 h of PE treatment) in the experiments shown here. Future experiments in transgenic animal will allow us to further investigate these phenomena.
As a result of the work described here, we postulate that the increase in YY1 expression is a protective mechanism of the cell to prevent further increases in gene expression. We have recently shown that YY1 expression is up-regulated in human heart cells, and in a transgenic model of hypertrophic cardiomyopathy (Sucharov et al., 2003
). Future in vivo studies will allow us to gain a better understanding of YY1 function during hypertrophy and heart failure. Our results show that YY1 is an important factor in preventing cardiac hypertrophy through a mechanism that involves inhibition of HDAC5 nuclear export.
| ACKNOWLEDGMENTS |
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| Footnotes |
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Address correspondence to: Carmen C. Sucharov (kika.sucharov{at}uchsc.edu)
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