From the Section of Molecular Cardiology, Division of
Cardiology and Cardiovascular Research Center, University of
Cincinnati, Cincinnati, Ohio 45267 and the § Department of
Molecular Biology and Oncology, University of Texas Southwestern
Medical Center, Dallas, Texas 75235
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ABSTRACT |
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To understand smooth muscle-specific gene
expression, we have focused our studies on the smooth muscle myosin
heavy chain (SMHC) gene, a smooth muscle-specific marker. In this
study, we demonstrate that the SMHC promoter region (1594 to
1462
base pairs) containing the A/T-rich element can activate the
heterologous thymidine kinase promoter in smooth muscle cells, but not
in fibroblasts. Mutations of this A/T-rich element decreased SMHC
promoter activity significantly. Both gel mobility shift assays and
DNase I footprinting revealed that this region binds to specific
protein complexes from smooth muscle nuclear extracts, whereas nuclear
extracts from skeletal muscle and fibroblasts produced a different
binding pattern. We also demonstrate that the protein complex obtained from smooth muscle nuclear extract reacts with MEF2B-specific antibody,
but not with antibodies specific to MEF2A, MEF2C, or MEF2D, suggesting
that only MEF2B protein binds to the A/T-rich element. Furthermore,
MEF2B overexpression in smooth muscle cells up-regulated the SMHC
promoter, suggesting that MEF2B is important for SMHC gene regulation.
This is the first report demonstrating a role for MEF2 factors in
smooth muscle-specific gene expression.
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INTRODUCTION |
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Smooth muscle cells have been the subject of intense study because abnormal growth and proliferation of smooth muscle cells are involved in the pathogenesis of both atherosclerosis (1) and restenosis following percutaneous transluminal coronary angioplasty or atherectomy. The hallmark of restenosis following percutaneous transluminal coronary angioplasty or coronary atherectomy is vascular smooth muscle cell proliferation and migration causing obstructive lesions (2). Numerous observations suggest that smooth muscle cells in vascular lesions undergo phenotypic modulations from a contractile to a synthetic phenotype. These proliferating smooth muscle cells secrete increased extracellular matrix and express several embryonic markers, but lose SM-2 myosin present in mature smooth muscle cells, more closely resembling immature fetal smooth muscle cells. These studies suggest that upon injury, adult smooth muscle cells recapitulate many aspects of embryonic development during their remodeling. Therefore, a better understanding of smooth muscle development and differentiation is important to define the vascular disease process. From this standpoint, a central question in vascular biology relates to understanding the transcriptional control mechanisms underlying smooth muscle growth and differentiation.
To date, transcription factors that are responsible for smooth muscle commitment and differentiation or modulation of the smooth muscle cell phenotype have not been identified (3). The transcription factors that regulate skeletal muscle development and differentiation are better understood and may provide useful paradigms for elucidating control of gene expression in smooth muscle. In recent years, skeletal muscle has become the paradigm for understanding tissue-specific gene activation and cell differentiation due to the discovery of a set of master regulatory genes, namely the MyoD family, which includes MyoD, myogenin, myf-5, and MRF-4/herculin/myf-6 (4). The myogenic factors are unique in their abilities to orchestrate an entire program of skeletal muscle-specific gene activation when introduced into diverse cell types. However, the members of the MyoD gene family are not expressed in smooth muscle, and related helix-loop-helix transcription factors controlling smooth muscle cell differentiation have not yet been identified. This raises the possibility that there are other types of transcription factors involved in smooth muscle myogenesis.
A second class of transcription factors, namely MEF2 (myocyte-specific enhancer factor 2)/RSRF (related to serum response factor), has been implicated in striated muscle differentiation and transcriptional control (5). The MEF2 family includes four genes, MEF2A (6), MEF2B (7), MEF2C (8), and MEF2D (9-11). Each of the MEF2 proteins cloned thus far appears to be subject to complex forms of regulation at different levels. Members of this gene family are transcribed in a wide range of cell types including skeletal, cardiac, and smooth muscles as well as brain and spleen. MEF2 was originally described as a DNA-binding activity present in differentiating myotubes (12). The MEF2-binding site, CTA(A/T)4TA(G/A), has been shown to be important for transcriptional regulation of many cardiac and skeletal muscle-specific genes (12-16). It has also been demonstrated that MEF2 transcription factors can interact with myogenic bHLH1 proteins (17) to synergistically activate muscle-specific genes. Recent studies in the fruit fly Drosophila melanogaster showed that MEF2 is indispensable for muscle development since disruption of the single MEF2 gene, D-MEF2, causes lethality with abnormalities in cardiac, somatic, and visceral muscle development. This demonstrates that MEF2 plays an important role in all three muscle lineages (18, 19). MEF2A, MEF2B, and MEF2D are also expressed (mRNA and protein) in cultured smooth muscle cells and in the adult rat aorta, and their expression is increased in neointimal smooth muscle cells during vascular remodeling (20). However, the exact role of MEF2 transcription factors in regulating smooth muscle gene expression remains to be determined.
Toward understanding smooth muscle-specific gene expression, we have
recently isolated and characterized the rabbit smooth muscle myosin
heavy chain (SMHC) gene promoter (21), whose expression is highly
tissue-specific. We have shown that the promoter region extending to
2266 bp is highly active in cultured rat aortic smooth muscle cells,
but not in other cell types. Promoter deletion analyses identified a
region between
1548 and
1392 bp as important for high level
promoter activity. This region includes a MEF2-like A/T-rich element
located at
1540 bp that binds to a specific protein complex in
nuclear extracts from vascular smooth muscle cells (21). The goal of
this study was to examine the nature of protein binding to this element
and its functional relevance to SMHC promoter activity. In this study,
we demonstrate that an antibody specific to MEF2B protein supershifted
the protein complexes formed on the A/T-rich region from smooth muscle
nuclear extracts. However, MEF2B does not appear to bind this A/T-rich element directly. We further show that overexpression of MEF2B protein
in smooth muscle cells up-regulates SMHC gene promoter activity. This
is the first report demonstrating a role for MEF2 in smooth
muscle-specific gene expression.
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MATERIALS AND METHODS |
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Cell Culture-- Smooth muscle cells from rat thoracic aorta were isolated and cultured by a modification of the procedures described by Owens et al. (22). Male Sprague-Dawley rats (4-6 weeks old) were anesthetized with an intraperitoneal injection of sodium pentobarbital, and the descending thoracic aorta was excised aseptically. Two vessels were placed in Hanks' balanced salt solution (Life Technologies, Inc.) with 1% antibiotic/antimycotic (Life Technologies, Inc.), cleaned free of adhering fat and connective tissue by blunt dissection, and opened longitudinally. After preincubation of the vessels for 15-25 min at 37 °C in a 5% CO2 and 95% air atmosphere in Hank's balanced salt solution in the presence of 1 mg/ml collagenase (2219 units/mg; Worthington) and 0.16-0.2 mg/ml elastase (4.2 units/mg; Worthington) with 1% antibiotic/antimycotic, the adventitia was carefully stripped off under a dissecting microscope, and the luminal surface was scraped with the convex side of curved forceps to remove endothelial cells. The resulting aorta pieces were placed into fresh collagenase/elastase solution, minced into 1-2-mm2 pieces, and incubated (37 °C, 5% CO2 and 95% air) for an additional 80-110 min. The dissociated cells were separated from the undigested tissue by filtration through an 85-µm stainless steel screen, and fetal calf serum (Life Technologies, Inc.) was added to a final concentration of 30%. The isolated cells were collected by sedimentation at 1500 rpm for 6 min and resuspended in Medium 199 (Life Technologies, Inc.) containing 10% fetal calf serum and the above antibiotics. Cells were seeded into plastic tissue culture dishes (Falcon) and cultured at 37 °C in a humidified atmosphere of 5% CO2 and 95% air with medium changes three times weekly. The confluent cultures were usually obtained after 4-8 days. The confluent cultures were washed with 1 × phosphate-buffered saline, harvested with 0.05% trypsin and 0.53 mM EDTA, and replated at a 1:5 or 1:6 split area ratio into 90-mm tissue culture plates. Rat aortic smooth muscle cells after the first passage were utilized for in vitro transfection studies and for isolation of nuclear extracts. For C2C12 and Sol-8 myotubes, cells were allowed to propagate in growth medium for 8-10 h, and then growth medium was removed and replaced with differentiation medium. C2C12 and Sol-8 myotubes were harvested 36-48 h after the fusion medium change.
Plasmid Construction--
Unidirectional deletions of the SMHC
promoter were produced using the Erase-a-base system (Promega) in the
pJRCATX vector (21). Heterologous SMHC-TK promoter constructs were made
by ligating a 133-bp fragment generated by PCR corresponding to the 1594 to
1462 bp region to pBLCAT5. This produced constructs containing both sense and antisense orientations. Orientation of the
SMHC fragment was confirmed by SnaBI digestion, which is unique to this fragment. The MEF2 expression vectors for MEF2C (pCDNA1-MEF2C) (11) and MEF2B (pCGNR2-MEF2B) were described earlier
(23). Constructs pJRCATX, pBLCAT5, pBLCAT6, and pSV2CAT were used as positive and negative controls in transfection analyses (24).
Site-directed Mutagenesis of the A/T-rich (MEF2-like) Element in
the SMHC Promoter--
A single base substitution of the A/T-rich
element was performed by a two-step symmetrical PCR procedure using a
pair of complementary template-mismatched central primers containing
the desired sequence (25). Briefly, the first PCR was done with a
primer (5-CCCGGGATCCCCTGTAGGGA-3
) that hybridizes to a
region encompassing the BamHI site at
1584 bp and a primer
(5
-GTATTCATATAAGACACC-3
) that includes the mutation of
the A/T-rich sequence. A second reaction was performed with a primer
(5
-TTCCTGCAGGAATTCCCAGC-3
) that hybridizes to a region encompassing the EcoRI site at
1224 bp and a primer
(5
-GGTGTCTTATATGAATAC-3
) that is the complementary
sequence of the above-mentioned MEF2-like (A/T-rich) mutated sequence.
The products of both reactions were mixed together, and a third
reaction was carried out with the underlined primers. PCR products
corresponding to the full-length product was cloned once into plasmid
pCRII (Invitrogen) and digested with BamHI and
EcoRI, and then the BamHI-EcoRI and
EcoRI-HindIII (which contains the proximal
promoter region including the TATA box) fragments were subcloned into
the pJRCATX vector. The presence of the correct mutations was verified
by sequence analysis before cloning into pJRCATX.
Western Blot Analyses-- For Western blots, equivalent quantities of nuclear extracts from (second passage) primary rat aortic smooth muscle cells were separated through a 10% SDS-polyacrylamide gel as described previously (20). Proteins were then transferred to nitrocellulose, incubated with polyclonal antibodies at 1:1000 dilutions, and detected with an enhanced chemiluminescence kit (ECL, Amersham Corp.). Antibodies to MEF2A (GST-MEF2A-(129-263)) and MEF2B (GST-MEF2B-(88-365)) were obtained from Dr. Yie-Teh Yu. MEF2A and MEF2C were from the laboratory of Olson and co-workers (20). The rabbit antisera to MEF2B and MEF2D were provided by Dr. Ron Prywes and have been described previously (23).
DNA Transfections--
Transient transfections of the SMHC
promoter constructs were performed using the calcium phosphate
coprecipitation method (21). Briefly, duplicate dishes of cells (5 × 105 cells/dish) were transfected with 15 µg of the
SMHC constructs plus 5 µg of pMSV-
gal into cultures of rat aortic
smooth muscle cells and NIH3T3 fibroblasts. To determine the role of
MEF2 proteins, cotransfections were performed using 5 µg of the MEF2
expression plasmids pCGNR2-MEF2B (23) and pCDNA1-MEF2C (11). DNA
used for transfections was purified by two successive CsCl2
density gradient centrifugations. After 4-14 h, cells were washed
twice with phosphate-buffered saline (137 mM NaCl, 27 mM KCl, 8 mM Na2HPO4, and 1.5 mM KH2PO4), and fresh
growth medium (10% fetal calf serum) was added. Rat aortic smooth
muscle cells and NIH3T3 fibroblasts were harvested 48 h
post-transfection.
Chloramphenicol Acetyltransferase (CAT) Assays--
The
transfected cells were rinsed twice with cold Hanks' balanced salt
solution and harvested in release buffer (40 mM Tris-HCl, pH 7.4, 1 mM EDTA, and 150 mM NaCl). The cells
were lysed in 250 mM Tris-HCl, pH 7.5, by three cycles of
freeze-thawing, and the cell lysate was used for the enzyme assays.
-Galactosidase assays were performed on 30% of the cell extract to
provide values of relative transfection efficiency. The resulting
values were used to normalize the amount of extract added to the
subsequent CAT assays. The reaction mixture contained 20 µl of the
extract in a 150-µl assay containing 4 mM acetyl coenzyme
A (Sigma) and 0.05 mCi of [14C]chloramphenicol (54 mCi/mmol; Amersham Corp.) in 250 mM Tris-HCl, pH 7.8. The
reaction was stopped and extracted with 1 ml of ethyl acetate. The
organic phase was dried, suspended in 20 µl of ethyl acetate, and
spotted onto a silica gel thin-layer plate. The chromatogram was
developed in a chloroform/methanol (95:5) system, dried, and autoradiographed. For quantitating the conversion of chloramphenicol to
its acetylated forms, the spots were excised, and the radioactivity was
measured in a liquid scintillation counter. CAT activity for each
promoter construct was determined by at least three separate transfection experiments using two different plasmid preparations. The
construct pSV2CAT was used as a positive control (26).
Preparation of In Vitro Translated MEF2 Proteins, Vascular Smooth
Muscle Nuclear Extracts, and Gel Mobility Shift
Assays--
In vitro transcriptions and translations
were performed together in a coupled rabbit reticulocyte lysate system
(Promega) according to the manufacturer's recommended conditions. The
T7 promoter was used in plasmids pCDNA1-MEF2A, pCDNA1-MEF2C,
pCDNA1-MEF2D1a or pCDNA1-MEF2D1b, and pCI-MEF2B (described in Refs.
8, 11, and 27) to give transcription products followed by
translation products of MEF2A, MEF2C, MEF2D, and MEF2B, respectively.
Nuclear extracts from cultured rat aortic smooth muscle cells were
prepared essentially as described by Dignam et al. (28). The
protein concentration was determined by the Bradford assay (29). Gel mobility shift assays were performed as described previously (21). A
133-bp fragment that contains the A/T-rich element (see Fig. 1B) was 3-end-labeled with [
-32P]dATP
using DNA polymerase (Klenow fragment, New England Biolabs Inc).
Briefly, 10-20 µg of nuclear extract was incubated in a total volume
of 30 µl for 30 min at room temperature in the presence of 1 ng of
radiolabeled fragment or double-stranded oligonucleotides and 2 µg of
poly(dI-dC) and analyzed by electrophoresis on 4 and 5% polyacrylamide
gels. Competition experiments were performed with a 100-500-fold molar
excess of specific or nonspecific unlabeled DNA oligomers. The
oligonucleotides used as probes or competitors in the gel mobility
shift assays were as follows: SMHC-A/T-rich, CTGGGGGTATTAATATAAGACACCC; µA/T-rich (which contains three
nucleotide substitutions),
CTGGGGGCATTCCTATAAGACACCC;
A/T-rich
(which contains a point mutation),
CTGGGGGTATTCATATAAGACACCC; E-box (E3), CCACGCACACGTGGTTGCAG; MCK-MEF2, GCTCTAAAAATAACCCTG; and
MHC-MEF2, CTTTCAGATTAAAAATAACTAAGG. To determine whether MEF2
protein families bind to the A/T-rich element in the 133-bp fragment,
antibodies that react specifically to the MEF2A, MEF2B, MEF2C, and
MEF2D isoforms were incubated together with the DNA and nuclear
extracts in the gel shift reaction. MEF2A, MEF2B, MEF2C, or MEF2D
antibodies (1-2 µl) were added to a 30-µl gel shift reaction and
incubated at 4 °C for 2 h. Following electrophoresis, the gels
were stained in 10% acetic acid and 30% methanol, dried, and
autoradiographed.
DNase I Footprinting--
DNase I footprinting was performed on
the 1594 to
1462 bp promoter region using nuclear extracts from
cultured rat aortic smooth muscle cells. Briefly, a 133-bp probe
labeled on only one strand was prepared by PCR using an end-labeled
sequence primer. 120 µg of smooth muscle nuclear extract and 4 ng of
the labeled probe were incubated in the same buffer as that used in the
gel mobility shift assays for 10 min at room temperature and for an additional 10 min on ice. 50 µl of DNase I (Worthington) at a concentration of 10 µg/ml in 10 mM Tris-HCl, pH 8.0, 10 mM MgCl2, and 1 mM
CaCl2 was added to the binding mixture and incubated for
90 s at room temperature. This reaction was terminated by the
addition of 100 µl of stop solution (200 mM NaCl, 30 mM EDTA, and 1% SDS), phenolized, and
ethanol-precipitated. The samples were heat-denatured and loaded onto a
6% sequencing gel. The A + G ladders were generated by the
Maxam-Gilbert chemical sequencing method (30).
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RESULTS |
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The Rabbit SMHC Promoter Region (1594 to
1462 bp) Can Activate
the Heterologous TK Promoter in Smooth Muscle Cells--
Using
promoter deletion mapping analyses, we have previously demonstrated
that the region between
1594 and
1392 bp is essential for high
level SMHC promoter activity in cultured smooth muscle cells (21).
Deletion of this region resulted in a 52% decrease in CAT activity in
smooth muscle cells. This region includes an A/T-rich element
(5
-TATTAATATA-3
) located at
1540 bp (Fig. 1A). To determine whether this
region contained positive regulatory (enhancer-like) elements, the DNA
from the
1594 to
1462 bp region was cloned upstream of the
heterologous TK promoter-CAT plasmid (pBLCAT5) both in the sense and
antisense orientations. These heterologous promoter constructs together
with control vectors were transiently transfected into primary cultures
of rat aortic smooth muscle cells and NIH3T3 fibroblast cells. The
basal TK promoter pBLCAT5 produced low levels of CAT activity in smooth muscle and NIH3T3 fibroblasts (Fig.
2B). On the other hand,
inclusion of the SMHC promoter region (
1594 to
1462 bp) increased
TK promoter activity significantly in smooth muscle cells. The sense
orientation produced an 8-10-fold increase in TK-CAT expression,
whereas the antisense orientation produced an increase of 3-4-fold
compared with the basal TK promoter pBLCAT5 (Fig. 2). However, the same heterologous promoter constructs did not produce elevated CAT expression in NIH3T3 fibroblasts (Fig. 2). Thus, the SMHC promoter region (
1594 to
1462 bp) appears to function as a positive
regulatory element in smooth muscle cells.
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DNase I Footprinting Reveals That the A/T-rich Element Located at
1540 bp Binds Nuclear Proteins from Smooth Muscle Cells--
The
SMHC promoter region (
1594 to
1462 bp) described above includes an
A/T-rich element (5
-TATTAATATA-3
) located at
1540 bp (Fig.
1A). To determine the nature of protein-binding sites within
the
1594 to
1462 bp DNA region, in vitro DNase I
footprint analysis was carried out using nuclear extracts from cultured rat aortic smooth muscle cells. A 133-bp probe (
1594 to
1462 bp)
was generated in which one of the amplification primers was 5
-end-labeled with [
-32P]ATP and T4 polynucleotide
kinase. The same DNA (end-labeled in the sense strand) was used to
generate a Maxam-Gilbert chemical-degradation sequencing ladder (Fig.
3, first and fourth
lanes and fifth and eighth lanes). DNase I
footprinting on both sense and antisense strands showed strong
protection of the A/T-rich region (Fig. 3, third and
seventh lanes). The footprint spans a large region (GGGGGTATTAATATAA) that includes the A/T-rich element and several flanking nucleotides, suggesting that more than one protein is binding
to this region. Footprinting analyses also showed a weak protection of
the E-box located at
1516 bp in the antisense strand, but absent in
the sense strand (Fig. 3).
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Mutations of the A/T-rich Element Produced a Significant Decrease
in SMHC Promoter Activity in Vascular Smooth Muscle Cells--
To
determine whether the A/T-rich element (located at 1540 bp) is
critical for SMHC promoter activity, the A/T-rich element was modified
in the p1548-CAT plasmid using site-directed mutagenesis. A point
mutation was introduced into the A/T-rich element
(GTATTAAT ATA
GTATTCATATA) where a single
nucleotide was modified from adenine to cytidine. The
introduction of C in the place of A abolished MEF2-binding activity in
the MCK promoter (31). The CAT reporter constructs containing the
wild-type SMHC promoter (p1548-CAT) and the mutated A/T-rich element
(p1548-CATµA/T) were transfected into primary cultures of rat aortic
smooth muscle cells, and CAT reporter activity was determined. The
wild-type SMHC promoter produced a 20-fold increase in CAT activity
over the promoterless CAT vector (pJRCATX) (Fig.
4). Conversely, mutation of the A/T-rich element produced a 6.5-fold reduction (68% decrease) in reporter activity (Fig. 4). These results indicate that the A/T-rich element is
critical for maximal SMHC promoter activity.
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MEF2B-specific Antibody Supershifts the Protein Complexes That Bind
to the A/T-rich Element--
To determine precisely what protein
factors bind to the A/T-rich element, we performed gel shift analyses
using nuclear extracts from smooth, skeletal, and nonmuscle cells. The
133-bp DNA fragment (1594 to
1462 bp) that contained the A/T-rich
element (5
-TATTAATATA-3
) was used as a probe. As shown in Fig.
5A, the gel shift analyses revealed three closely migrating protein complexes with the nuclear extracts from vascular smooth muscle cells (lanes 1 and
2). However, the same probe produced a different protein
binding pattern with nuclear extracts from Sol-8 myotubes,
C2C12 myotubes, and NIH3T3 fibroblasts, where
only a single protein complex was observed in
C2C12 and NIH3T3 fibroblast extracts (Fig.
5B). Interestingly, a 100-fold molar excess of an
oligonucleotide containing the A/T-rich element
(5
-CTGGGGGTATTAATATAAGACACCC-3
) abolished all three protein complexes
(Fig. 5A, lane 3). However, an A/T-rich oligomer containing three nucleotide substitutions, µA/T rich
(5
-CTGGGGGCATTCCTATAAGACACCC-3
), did not
compete protein binding at a 100-fold excess (lane 4). In
addition, we performed competition assays using an oligonucleotide containing a point mutation in the A/T-rich core sequence (which was
shown to decrease SMHC promoter activity in the transient transfection
analyses) (Fig. 4). This oligonucleotide also failed to compete protein
binding at 10-, 20-, and 100-fold molar excesses (data not shown),
suggesting that any modification to the A/T-rich core region abolishes
protein binding. An E-box oligonucleotide corresponding to the E-box
sequence located at
1516 bp did not act as a competitor (Fig.
5B, lane 2). To our surprise, a MEF2 consensus
oligonucleotide derived from the MCK promoter
(5
-GATCCTCGCTCTAAAAATAACCCTGTC-3
) did not compete protein binding
(lanes 5 and 6).
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The MEF2 Family of Transcription Factors (MEF2A, MEF2B, MEF2C, and MEF2D) Is Present in Nuclear Extracts from Cultured Smooth Muscle Cells-- To determine whether MEF2 transcription factors are present in smooth muscle nuclear extracts, we performed Western blot analyses using MEF2 isoform-specific antibodies. As shown in Fig. 6, all four MEF2 isoforms (A, B, C, and D) were present in the nuclear extract from early passage rat aortic smooth muscle cells. Therefore, it appears that primary rat aortic smooth muscle cells express all of the MEF2 isoforms.
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The MEF2 Family Members (MEF2A, MEF2B, MEF2C, and MEF2D) Can Bind
to the A/T-rich Element in the SMHC Gene Promoter--
To determine
whether the A/T-rich element in the SMHC promoter can bind directly to
each member of the MEF2 family, we performed gel mobility shift assays
using in vitro translated proteins for MEF2A, MEF2B, MEF2C,
and MEF2D with a 25-bp oligomer containing the A/T-rich element. In
parallel, gel mobility shift assays were performed with the MCK-MEF2
and MHC-MEF2 site oligomers as high and low affinity controls,
respectively. As shown in Fig. 7, each of
the MEF2 family members can bind to the A/T-rich oligomers from the
SMHC promoter. The binding of the MEF2B in vitro translated product to each of the probes was somewhat more diffuse when compared with the other MEF2 translation products, consistent with its previous
description (27). However, it is important to note that the ratio of
each of the different MEF2 translation products is similar between the
three different probes. This suggests that the minimal A/T-rich site
itself does not direct the preferential binding of MEF2B over the other
MEF2 proteins.
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Cotransfection of the MEF2-B Expression Vector Together with the SMHC Promoter Increases Reporter Activity in Vascular Smooth Muscle Cells, but Not in NIH3T3 Fibroblasts-- To determine whether MEF2B protein can positively regulate the SMHC promoter, we cotransfected the SMHC promoter constructs (p1548-CAT, p1392-CAT, and pJRCATX) together with the MEF2B expression vector pCGNR2 (23) into cultured rat aortic smooth muscle cells and NIH3T3 fibroblasts. In response to MEF2B overexpression, the construct p1548-CAT, which includes the MEF2B-binding site, showed a 61% increase in promoter activity, whereas the promoter construct p1392-CAT, which lacks the A/T-rich element, failed to show an increase in promoter activity in cultured rat aortic smooth muscle cells (Fig. 8). On the other hand, MEF2B cotransfected with p1548-CAT and p1392-CAT into NIH3T3 fibroblasts did not significantly increase SMHC promoter activity, which is very low in fibroblasts. Cotransfection of the MEF2C expression vector (pCDNA1-MEF2C) failed to increase SMHC promoter activity significantly both in smooth muscle cells and in NIH3T3 cells (data not shown). These results demonstrate that MEF2B can up-regulate SMHC promoter activity in smooth muscle cells, but not in NIH3T3 fibroblast cells. These data further suggest that MEF2B function is dependent upon other tissue-specific cofactors.
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DISCUSSION |
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The goal of this study was to characterize
cis-regulatory elements important for SMHC gene expression.
In this study, we demonstrated that the SMHC promoter region (1594 to
1462 bp), which contains the A/T-rich element, is capable of
activating a heterologous (TK) promoter in smooth muscle cells, but not
in fibroblasts. Point mutation of this A/T-rich element reduces SMHC
promoter activity significantly in vascular smooth muscle cells. Gel
mobility shift analyses demonstrated that the probe (133 bp) containing the A/T-rich element binds to three closely migrating protein complexes
from smooth muscle cell nuclear extracts, whereas nuclear extracts from
skeletal muscle showed a different protein binding pattern. We further
demonstrated that the protein complexes were supershifted by an
antibody specific to MEF2B protein, indicating that MEF2B is a
component of a smooth muscle-specific complex that binds the A/T-rich
element in the SMHC promoter. By Western blot analyses, we demonstrated
that all four MEF2 isoforms (A, B, C, and D) were present in nuclear
extracts from rat aortic smooth muscle cells. Therefore, the
preferential binding of MEF2B to the A/T-rich element is not simply a
reflection of its abundance relative to the other MEF2 factors.
Furthermore, our data suggest that MEF2B overexpression can activate
the SMHC promoter in smooth muscle cells, but not in NIH3T3 fibroblast
cells. These findings taken together suggest that MEF2B protein may
play an important role in regulating SMHC gene expression in smooth
muscle cells.
In this study, we have shown that the antibody specific to MEF2B protein can recognize the protein complexes formed on the A/T-rich element. This is a novel finding and suggests that MEF2 transcription factors might be important in the regulation of smooth muscle-specific gene expression. MEF2B mRNA is expressed in brain, heart, and skeletal muscle (7) and in smooth muscle (20). However, the precise role of MEF2B protein in transcriptional regulation is not well understood. Initial studies showed that MEF2B protein did not bind DNA efficiently (6, 7). Nevertheless, forced expression of MEF2B resulted in expression of a reporter gene containing multimers of the MEF2 recognition sequence. These studies suggested that MEF2B may be heterodimerizing with other MEF2 isoforms to become a transactivator. Recently, Molkentin et al. (27) demonstrated that MEF2B protein can bind directly to the MCK-MEF2 consensus sequence and that MEF2 transcription factors can interact with the MyoD-E12 protein complex and cooperate in activating muscle-specific gene expression.
In this study, we observed that the probe containing the A/T-rich region revealed three closely migrating protein complexes only in smooth muscle nuclear extracts. Interestingly, all three protein complexes were supershifted by MEF2B antibody. This raises the possibility that MEF2B protein may be complexing with other proteins to produce these complexes. Alternatively, we should also consider the possibility that these multiple protein complexes are due to phosphorylated forms of MEF2B protein (27). Although MEF2B protein is known to be expressed in Sol-8 and C2C12 skeletal muscle cells, the nuclear extracts from these cells did not produce the same protein binding pattern. This finding also supports the idea that MEF2B binding may depend upon cell type-specific interacting proteins. DNase I footprinting with smooth muscle nuclear extracts revealed extended protection including the A/T-rich element and its flanking nucleotides, further supporting multiple protein binding in the A/T-rich region.
A double-stranded oligonucleotide containing the A/T-rich element
effectively competed all three protein complexes, whereas the
oligonucleotides containing mutations of the A/T-rich core sequence
(TATTAAT) failed to act as competitors. These results suggest that
direct protein interaction with the A/T-rich element is critical for
the formation of the three protein complexes. In this study, we have
shown that in vitro translated MEF2 family members (MEF2A,
MEF2B, MEF2C, and MEF2D) can directly bind to the A/T-rich element.
However, the binding affinity for the SMHC-A/T-rich element is much
lower compared with the MCK-MEF2 consensus element, but comparable to
the MHC-MEF2 site. Although other MEF2 family members (MEF2A, MEF2C,
and MEF2D) are also expressed in smooth muscle cells, they do not bind
to this A/T-rich element in smooth muscle nuclear extracts, which
suggests that MEF2B binding specificity may depend upon the interacting
proteins present in smooth muscle cells.
MADS box proteins have been shown to be highly interactive molecules in both yeast and higher organisms (reviewed in Ref. 32). Recently, it has been shown that the MADS box protein MEF2C physically interacts with the myogenic bHLH proteins as a heterodimer with E-box proteins (17). The MADS box protein SRF has also been shown to physically interact with this same heterodimer formed between the myogenic bHLH proteins and E-box proteins (33). Furthermore, MEF2 proteins have also been shown to interact with the bHLH protein Twist (34) as well as with the bHLH protein MASH1 (35). Together, these results suggest that MADS box proteins are interactive molecules that complex with other cell type-specific transcription factors to direct tissue-restricted gene expression.
To date, transcription factors that are unique to smooth muscle cells
have not been identified, and smooth muscle-specific gene expression
remains underexplored. However, the recent isolation and
characterization of the smooth muscle-specific gene promoters for
smooth muscle -actin (36-38), smooth muscle
-actin (39), SM22
(40, 41), calponin (42, 43), and SMHC provide unique opportunities for
dissecting the various cis- and trans-regulatory factors involved in smooth muscle cell-specific gene expression. Studies using the above-mentioned promoters have identified a number of
known cis-elements including the CArG box, E-box,
GATA-binding site, AP-2, SP1, and A/T-rich element. In particular, CArG
elements that bind SRF have been shown to be important for smooth
muscle
- and
-actin gene expression (36, 37, 44). Similarly, CArG
box elements are found in the SM22
proximal promoter and may be
important for its regulation in myogenic lineages (41). On the other
hand, there are no CArG elements in the human calponin gene, which is
expressed exclusively in smooth muscle-containing tissues in adult
stages.
The SMHC promoter region (1281 to
1038 bp) contains three CArG
box-like elements (Fig. 1) and is conserved between mouse, rat, and
rabbit promoters. This region is found to be necessary for maximal SMHC
promoter activity (45, 46).2
The CArG box element is known to bind to SRF or related factors; however, the precise role of SRF transcription factors in smooth muscle-specific gene expression remains to be explored. Future experiments will be focused toward identifying
cis/trans-factors responsible for smooth muscle
cell-specific gene expression.
In summary, the data presented here suggest that MEF2B protein may play an important role in regulating SMHC gene expression in smooth muscle cells. We propose that MEF2B protein may interact with other transcription factors in controlling the smooth muscle-specific expression of this gene. Future experiments will attempt to identify and characterize the protein factors that interact with MEF2B protein.
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ACKNOWLEDGEMENTS |
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We thank Dr. Ron Prywes for MEF2 expression vectors (pCGNR2) and antibodies to MEF2B and MEF2D and Dr. Yie-Teh Yu for antibodies to MEF2A and MEF2B. We thank Alla Zilberman for expert technical assistance.
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FOOTNOTES |
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* This work was supported by National Institutes of Health Grant R01-HL-38355.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ To whom correspondence should be addressed: Div. of Cardiology and Cardiovascular Research Center, University of Cincinnati, 231 Bethesda Ave., Cincinnati, OH 45267. Tel.: 513-558-3080; Fax: 513-558-2002.
1 The abbreviations used are: bHLH, basic helix-loop-helix; SMHC, smooth muscle myosin heavy chain; bp, base pair(s); TK, thymidine kinase; PCR, polymerase chain reaction; GST, glutathione S-transferase; CAT, chloramphenicol acetyltransferase; MCK, muscle creatine kinase; SRF, serum response factor.
2 Zilberman, A., Dave, V., Miano, J., Olson, E. N., and Periasamy, M. (1998) Circ. Res., in press.
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