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Volume 270,
Number 42,
Issue of October 20, 1995 pp. 25266-25272
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
Promoter Analysis
of Zfp-36, the Mitogen-inducible Gene Encoding the Zinc Finger
Protein Tristetraprolin (*)
(Received for publication, June 5,
1995; and in revised form, August 9, 1995)
Wi S.
Lai (§),
,
Michael
J.
Thompson (¶),
,
Gregory A.
Taylor (**),
,
Yi
Liu
(1),
Perry
J.
Blackshear (§§)
From the Howard Hughes Medical Institute Laboratories, Section
of Diabetes and Metabolism, Division of Endocrinology, Metabolism and
Nutrition, Departments of Medicine and Biochemistry, and the Sarah W.
Stedman Center for Nutritional Studies, Duke University Medical Center,
Durham, North Carolina 27710 and theDepartment of
Microbiology, Duke University, Durham, North Carolina 27710
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
FOOTNOTES
ACKNOWLEDGEMENTS
REFERENCES
ABSTRACT
The gene encoding the putative zinc finger protein
tristetraprolin (TTP), Zfp-36, is rapidly induced by a variety
of mitogens and growth factors. We show here that 77 base pairs 5` of
the transcription start site are sufficient for full serum inducibility
of the mouse Zfp-36 promoter. This region of the promoter
includes consensus sequences for the binding of the transcription
factors EGR-1, AP2, and Sp1. In addition, we have identified a
previously undescribed element, TTP promoter element 1 (TPE1); this
10-base pair sequence includes a palindrome and is identical in the
human, bovine, and mouse genes. Each of the three binding elements,
EGR-1, AP2, and TPE1, contribute to the serum induction of Zfp-36 and can confer serum-inducible expression on a heterologous
minimal promoter. Gel mobility shift assays demonstrated the formation
of complexes consisting of this region of the promoter and cellular
nuclear proteins and demonstrated that the extent of complex formation
could be altered by treatment of the cells with serum or insulin. These
results suggest that the response of Zfp-36 to serum and other
mitogens is mediated by a series of cis-acting elements acting
in concert to confer full inducible transcription of this gene.
INTRODUCTION
An early response of many cells to serum and polypeptide growth
factors is the activated transcription of specific genes in the absence
of de novo protein synthesis. Many of these immediate-early
response genes encode regulatory proteins that mediate growth
responses, including transcription factors that modulate the expression
of other genes. These genes encode a number of well studied
transcription regulators such as the fos and jun families. One interesting member of the immediate-early response
class of genes encodes tristetraprolin (TTP), ( )(1, 2) a zinc finger protein also known
as Nup475 (3) and TIS11(4) . This protein, which is
encoded by the gene Zfp-36(2) , is expressed at very
low levels in quiescent fibroblasts but is rapidly induced by serum,
polypeptide growth factors, and phorbol 12-myristate 13-acetate (PMA) (1, 3, 4) . This expression is transient,
with most mRNA disappearing after 2 h. In these cells, induction of Zfp-36 expression is independent of protein synthesis, and
treatment with cycloheximide results in superinduction of the gene (1, 3, 4, 5, 6) The
mechanisms that control the activation of Zfp-36 transcription
have not been defined. To identify sequence elements involved in the
transcriptional control of TTP synthesis, we have characterized the
5`-flanking region of Zfp-36 using site-directed mutagenesis
and deletional analysis. Several potential binding sites for known
transcription factors have been identified in this study as
contributing to the serum-stimulated activity of this promoter. In
addition, we have identified and characterized a previously unknown
promoter element that appears to function as a transcriptional enhancer
and that participates in the regulation of serum-induced Zfp-36 transcription.
EXPERIMENTAL PROCEDURES
Library Screening and SubcloningA Balb/c mouse genomic library (Clontech, Palo Alto, CA) was
screened using the mouse TTP cDNA (1) as a probe. Pure plaques
positively hybridizing to the probe were subjected to Southern mapping
analysis. Restriction fragments from two clones overlapping within the
intron and 1 kb of TTP protein coding sequence (clone 13 extended 5`,
clone 7 extended 3` from the overlapping sequence) were subcloned into
pBS (Stratagene, La Jolla, CA). An 8-kb SstI
fragment from clone 13 was ligated into the SstI cloning site
in pBS , and another plasmid was made by ligating a
3-kb XhoI fragment from clone 7 into the SalI site of
pBS . The inclusion of the entire mRNA coding region
was confirmed by dideoxy sequencing. Five kb of 5`-flanking region was
also sequenced. The entire mouse genomic sequence has been deposited in
GenBank (accession number L42317). A second mouse genomic
library (129 sv, Stratagene) was also screened to obtain a 3.8-kb Sau3aI-SalI fragment, which contained 0.9 kb of the
5`-flanking region, the entire mRNA coding sequence, and a single
intron. DNA sequence analysis revealed only a 2-bp difference in the
5`-flanking region when compared with the clone from the Balb/c
library. This clone was used for promoter activity analysis as well as
for the construction of a gene-targeting vector to create knockout
mice. ( )A human genomic clone was obtained by screening a
human placental genomic library (Clontech) with the human TTP cDNA (2) (GenBank accession number for the human
genomic sequence: M19844). The bovine TTP cDNA was obtained from a
bovine aorta epithelial cell cDNA library (Stratagene) using the human
TTP cDNA as a probe(2) . The bovine cDNA was then used to
screen a bovine liver genomic library (Stratagene). The bovine genomic
sequence has been deposited in GenBank (accession number
L42319). Base pair numbers in this report refer to the transcription
initiation site in the mouse genomic clone as determined by primer
extension assays (data not shown). The position of the single intron
was determined by sequence comparison of the genomic clones with the
cDNAs and had the conventional sites for splice donors and
acceptors(7) . Southern mapping, subcloning, and DNA sequencing
were performed by standard techniques(8) .
Plasmid Constructions
Reporter ConstructsZfp-36 promoter-driven human growth hormone reporter plasmids were
created by ligating restriction fragments from genomic clone 13 to the
5` end of a promoterless human growth hormone gene. Three unique
restriction fragments extending upstream from the TTP mRNA translation
start site (NcoI, 5 kb; XhoI-NcoI, 2.1 kb;
and SstII-NcoI, 137 bp) were excised, filled in with
dNTPs, and inserted into the blunted BamHI cloning site of
pØGH (Nichols Institute Diagnostics, San Juan Capistrano, CA) to
produce 5kb , 2.1kb , and 137bp ,
respectively. These plasmids contained genomic DNA beginning at
-5026, -2057, and -77 relative to the transcription
initiation site and extending to +60.
TTP mRNA Expression ConstructsGenomic Zfp-36 constructs in vector pBS were made by ligating 5`
TTP fragments from clone 13 and a 3` fragment from clone 7 at the SstI site (located at base pair 330 of the mouse TTP
cDNA(1) . Correct annealing was confirmed by comparing the
sequence with that of a Sau3AI-SalI fragment of the
129sv genomic clone, which contains a 0.9-kb 5`-flanking region, the
intron, and the entire mRNA coding sequence that includes the
endogenous polyadenylation signal and three consensus sequences for
mRNA instability(1) . Plasmids TTP and
TTP constructed in such a manner contained 1.7 kb and
77 bp, respectively, of 5`-flanking sequences. Intronless constructs
TTP -Int and TTP -Int were made using a
1.7-kb EcoRV-NcoI or a 137-bp SstII-NcoI promoter fragment ligated to the NcoI site of the cDNA.In order to test potential promoter
elements, Glo48TTP was constructed as follows. The mouse TTP cDNA was
inserted into the EcoRI cloning site of pBS ,
and a synthetic 48-mer double-stranded human -globin promoter (9) with XbaI and BamHI coherent termini was
ligated 5` to the TTP cDNA at the XbaI-BamHI site of
the vector. Synthetic double-stranded serum response element (SRE,
5`-tcgacAGGATGTCCATATTAGG-3`; (10) ), EGR-1
(5`-tcgacGCGGGGGCG-3`; (11) ), AP2
(5`-tcgacTCTAGTGGCCACGCCCCCAGGC-3`; (12) ), and TTP promoter
element 1 (TPE1, 5`-tcgacCGTCCCGGAAGC-3`; this paper) DNA binding
sequences with SalI coherent termini were ligated individually
upstream of the minimal -globin promoter at the SalI
cloning site to create reporter constructs containing these elements.
All insertions were confirmed by dideoxy sequencing (U.S. Biochemical
Corp.).
Mutant ConstructsSite-specific deletions or
substitutions in the TTP upstream sequence of the TTP
construct were generated by using the Site-directed in vitro Mutagenesis System (Amersham Corp.). Deletion primers for each
specific site are: dEGR-1, vector
AAGCT( CCGCGGGGGCGCG)TCCCGGAAGCTCTAGTGG ;
dTPE1, CCGCGGGGGCG(CGTCCCGGAAGCTC)TACGTACCACGCCCCCAGG ;
dAP2, GAAGCTCTAGTGGCCTCG(CCCCC)AGGCCGCCCCATAAAAGG ;
dSP1, GTGGCCACGCCCCCAGG(CCGCCC)CATAAAAGGAGAAAGCTCCT ;
dTATA, CACGCCCCAGGCCGCCC(CATAAAAGG)AGAAAGCTCCTGCTG .
Sequences in parenthesis were to be deleted. Additional site-directed
mutations (underlined) were introduced for cloning purposes. All
constructs were sequenced to confirm that the deletions and mutations
had taken place.
Cell Culture and TransfectionsPrimary embryonic chicken fibroblasts (CEF) were isolated by
trypsinizing tissues (except for the internal organs and the head) of
11-14-day chicken embryos as described by Bergsma et al.(13) and Carroll et al.(14) and
maintained in minimal essential medium (Life Technologies, Inc.)
supplemented with 10% tryptose phosphate broth, 5% fetal bovine serum
(FBS, Life Technologies, Inc.), 2 mML-glutamine, 100
units/ml penicillin, and 100 µg/ml streptomycin.HIR3.5
cells(15) , a generous gift from Dr. J. Whittaker (State
University of New York, Stoney Brook, NY), were grown as described (16) . Transient transfections were performed with plasmid
DNA in calcium-phosphate precipitates prepared in a transfection
solution containing 140 mM NaCl, 25 mM HEPES, 0.12
mM CaCl , and 0.75 mM sodium phosphate.
CEF cells were plated 1 day before the transfection in 10-cm tissue
culture dishes at a density of 3 10 cells/dish.
After removing the culture medium, 1 ml of transfection mixture
containing 15 µg of test plasmid and 5 µg of pXGH5 was added to
each dish for 30 min at room temperature. The cells were then incubated
at 37 °C following the addition of 9 ml of culture medium. Four h
after incubation with the plasmids, the cells were treated for 4 min
with 4 ml of 10% glycerol in HEPES-buffered saline and washed two times
with phosphate-buffered saline to remove the remaining precipitate. A
24-h incubation in complete culture medium followed to allow the
transfected DNA to be expressed. The cells were then incubated for 24 h
in medium containing 0.5% FBS to synchronize cells into quiescence and
were harvested to prepare total cellular RNA. Plasmid pXGH5 encoding
human growth hormone driven by the mouse metallothionein-1 promoter (17) was co-transfected as an internal control for transfection
efficiency. Growth hormone released was measured by the HGH transient
gene expression assay (Nichols Institute Diagnostics). HIR3.5 cells
were plated at the density of 1 10 cells/dish and
were transfected with 10 µg of test plasmid as described above.
RNA Preparation and Northern Blot AnalysisTotal cellular RNA was prepared following the method of
Chomczynski and Sacchi (18) with modifications by Xie and
Rothblum (19) . Northern blots were prepared as described
before(1) . Blots were hybridized to random primed
- P-labeled (Stratagene) mouse TTP cDNA, and the
presence of TTP mRNA was quantified using a PhosphorImager (Molecular
Dynamics, Sunnyvale, CA).
Nuclear Extracts and Gel Mobility Shift AssaysNuclear extracts from HIR 3.5 cells and CEF cells were
prepared for gel mobility shift assays as described
previously(20) . Briefly, 10 µl of binding buffer (10
mM Tris (pH 7.5), 1 mM EDTA, 5% glycerol) containing
1 mM dithiothreitol, 1 µg of poly(dI-dC) (Pharmacia
Biotech Inc.), and 20 10 cpm of
- P-labeled probe, was added to 5-12 µg of
nuclear protein (as indicated in the figure legends), diluted to 10
µl in nuclear extract buffer (20 mM Tris (pH 7.9), 20%
(v/v) glycerol, 100 mM KCl, 0.2 mM EDTA, 0.5
mM dithiothreitol, 0.5 mM phenylmethylsulfonyl
fluoride, 5 mM NaF, 0.5 mM
Na Vo ), and then subjected to binding reactions
and electrophoresis(20) .The following synthetic
oligonucleotides containing potential sequences for DNA binding factors
were used in this study. For EGR1-AP2, two complementary synthetic
oligonucleotides were annealed to form a double-stranded
oligonucleotide corresponding to mouse Zfp-36 nucleotides
-77 to -37. A 5-base single-stranded tail (SalI
site) was included at the 5` end, and a 6-bp double-stranded EcoRV site was included at the other. For TPE1, two
complementary synthetic oligonucleotides were annealed to form a 22-bp
double-stranded oligonucleotide corresponding to bases -68 to
-55 of mouse Zfp-36, with cloning sequences for HindIII and SalI site at either end. To ensure
sequence fidelity, each oligonucleotide pair was cloned into Bluescribe
and sequenced. Double-stranded oligonucleotides were released by
digesting the plasmid with appropriate restriction enzymes, and the
ends were filled in with [ - P]dCTP (DuPont
NEN) and unlabeled dATP, dGTP, and dTTP (Life Technologies, Inc.).
Unlabeled dCTP was subsequently added to the reaction. The labeled DNA
was separated from unincorporated radioactivity and the cloning vector
by acrylamide gel purification. Competitor fragments containing
putative binding sequences for EGR-1 (tcgacGCGGGGGCG), AP2
(tcgaCCCCCAGGC), TPE1 (AGCTTCGTCCCGGAAGCTCG), and mutant
TPE1(AGCTTCGTAACTTAAGCTCG) were made by annealing the complementary
strands and filling in the ends with dNTPs using the Klenow fragment of
DNA polymerase I.
RESULTS
TTP Promoter AnalysisFor convenience, we will
refer to the 5`-flanking region of Zfp-36 as the ``TTP
promoter.'' TTP promoter activity upstream of the translation
start site was first tested by fusing 5-kb NcoI and 2.1-kb XhoI-NcoI fragments of the mouse genomic sequence to
the human growth hormone gene, followed by assay of growth hormone (GH)
released into the medium following transient transfection of the
plasmids into HIR3.5 cells. Both the 5-kb and the 2.1-kb promoter
sequences efficiently drove the expression of the GH gene (Fig. 1A). GH secretion from these constructs was
60-100-fold greater than that of the promoterless plasmid
pØGH. Sequential deletions of 5` sequences from the 2.1-kb
promoter were then made to determine the minimal promoter length
conferring full expression activity on the reporter construct. A 137-bp
fragment (SstII-NcoI) containing only 77 bp upstream
of the transcription initiation site was found to promote GH expression
with similar effectiveness to the longer promoter constructs (Fig. 1B). These results suggested that this region of
the TTP promoter contained the essential elements for high level
expression under these experimental conditions.
Figure 1:
Mouse TTP promoter-driven expression of
human growth hormone. A, HIR3.5 cells were transfected with
plasmids containing 5 kb (NcoI-NcoI), or 2.1 kb (XhoI-NcoI), of genomic sequence 5` of the TTP
translation initiation site linked to the promoterless growth hormone
gene (pØGH). Medium was sampled 48 h after transfection, and
immunoreactive growth hormone was measured. The results shown are the
means ± S.D. from three plates of cells in each assay; data from
two independent experiments are shown. B, identical
experiments were performed in which CEF cells were transfected with
plasmids containing either 2.1 kb or 137 bp of genomic mouse TTP
sequence 5` of the translation initiation site linked to the
promoterless growth hormone gene (pØGH). Data shown are from
three independent experiments. Individual experiments are indicated by
the Roman numerals at the bottom of the
graphs.
Promoter Sequences Required for TTP
ExpressionBoth the GH mRNA and protein are stable when
expressed in the cell lines we have tested. This fundamental difference
with the very labile TTP mRNA (1) made it difficult to measure
the responsiveness of the TTP promoter to extracellular stimulation
using the GH reporter system, since GH mRNA levels remained high even
when the transfected cells were made quiescent by serum deprivation. A
series of constructs containing the complete mouse Zfp-36 genomic sequence 3` from the transcription start site and varying
lengths of promoter was created (Fig. 2). We chose CEF cells to
study the expression of transiently transfected mammalian Zfp-36 because the endogenous chicken TTP mRNA was not detectable by the
mammalian TTP cDNA probes under the hybridization conditions used (see
below). Mouse Zfp-36 expression in transfected CEF cells
showed a time course of mRNA accumulation and degradation (data not
shown) similar to that of the endogenous mouse mRNA in HIR3.5
cells(1) . Truncation of the TTP promoter to 1.7 kb, 0.9 kb, or
137 bp resulted in no decrease in serum-induced mRNA expression (Fig. 2B). In addition, transfection of human ZFP-36 constructs containing 1.4-kb and 151-bp 5` sequences
resulted in similar patterns of serum-inducible expression (data not
shown). From these findings, we concluded that the mouse TTP promoter
sequences that are essential for serum responsiveness are located
within a 137-bp region 5` of the translation start site or 77 bp 5` of
the transcription initiation site.
Figure 2:
5`
sequences of the TTP promoter required for serum induction. A,
diagrammatic representation of the mouse TTP gene. B, CEF
cells were transfected with plasmids containing the full mouse mRNA
coding region, the single TTP intron, and various lengths of genomic
sequence 5` of the initiator codon: 1.7 kb (EcoRV), 0.9 kb (Sau3AI), and 137 bp (SstII) as indicated. 24 h after
transfection, the cells were serum-deprived for 24 h and then treated
with control conditions (C) or stimulated with 10% FBS (S) for 60 min. Each lane was loaded with 20 µg of total
cellular RNA, and the Northern blot was hybridized with the P-labeled TTP cDNA probe. Equivalent RNA loading in this
and subsequent figures was confirmed by acridine orange staining (data
reviewed but not shown). The positions of the 18 S and 28 S ribosomal
RNAs are indicated.
Computer analysis of the 137 bp
5` of the translational start site revealed a consensus H2A (+46
to +51) binding sequence for the eukaryotic core histone dimer
H2A-H2B(21, 22) , and a consensus c-fos.5 binding site
(+52 to +59)(23) . Neither the H2A site nor the
c-fos.5 site were present in the human and bovine TTP sequences, and
neither mutation of the putative H2A binding site (from CCATTC to
GTCGAC) nor the putative c-fos.5 sequence (from GCGCCACC to GGTCGACC)
affected either the basal or serum-stimulated expression of mouse TTP
(data not shown). The human, bovine, and mouse sequences were highly
conserved in the proximal 5`-flanking region (Fig. 3A).
Sequence analysis revealed consensus motifs for several transcription
factors in the 5`-proximal region of all three TTP promoters, including
sites for the TATA binding protein and Sp1 binding protein (Fig. 3A). Deletion of these sites from the mouse construct
containing the 137-bp 5`-proximal sequence produced a predictable
inhibition of TTP mRNA expression (Fig. 3B). Consensus
sequences for the binding of transcription factors EGR-1 (11) and AP2 (12, 24) were also found in the
proximal promoter region of the three animal species (Fig. 3A). To assess the contribution of the EGR-1 and
AP2 elements to the induction of TTP by serum, deletion of each of
these elements was made in the mouse TTP construct containing the
137-bp 5`-proximal sequence. The effects of these deletion mutations on
serum induction of TTP mRNA expression by these plasmids were measured
in starved CEF cells after transient transfection. Deletion of either
of the two elements resulted in a reduced level of TTP mRNA
accumulation induced by serum. Serum-stimulated TTP mRNA expression in
cells expressing the deleted EGR-1 or AP2 constructs was decreased to
35 and 18%, respectively, of the wild-type level (Fig. 3C).
Figure 3:
Deletion analysis of the mouse TTP
promoter. Panel A, sequences of the first 100 bp of TTP
promoter from human, bovine, and mouse are compared. The consensus
sequences for the EGR-1, TPE1, AP2, and Sp1 elements and the TATA box
are indicated. The mouse TPE1 sequence shown was from the Balb/c clone.
The analogous sequence from the 129sv mouse genomic clone was identical
to the human and bovine sequences. Panel B, TTP plasmids
containing 77 bp of genomic sequence 5` of the TTP mRNA transcription
start site (TTP ) deleted of the TTP intron (-Int), SP1, or TATA sequences, or deleted of the TTP intron (-Int), EGR-1, or AP2 sequences (panel C) were
transfected into CEF cells. 24 h after transfection, the cells were
deprived of serum for 24 h and then treated with control conditions (C) or 10% FBS (S) for 60 min. Northern blot analyses
were performed as described in the legend to Fig. 2.
TTP mRNA expression from constructs
lacking the TTP intron was also greatly attenuated when compared with
the corresponding constructs containing the same length of 5` sequence (Fig. 3, B and C). Full expression of human
and bovine TTP constructs also required the presence of the intron
(data not shown). The relationship between the promoter and the intron
in the expression of Zfp-36 is not understood at present;
these potential interactions are the subject of ongoing experimentation
and will not be discussed further here. Analysis of promoter
sequences in this region also revealed a previously undescribed 10-bp
sequence, TPE1, that was identical in the promoters of all three animal
species (Fig. 3A). When this region was deleted from
the mouse construct containing 137 bp of 5` DNA sequence, serum-induced
TTP expression was decreased by 75% (Fig. 4A). In order to
rule out the possibility that the diminished expression was due to
nonspecific effects resulting from a shortened construct, we created a
site-specific mutation at the palindromic sequence of TPE1 from TCCCGGA
to TAACTTA. Serum-induced expression of the substitution mutant
construct was decreased to a similar extent as that of the deletion
mutant (Fig. 4A). Mutation of the same 4 bp also had
profound effects on the expression of TTP from longer constructs
containing 1.7 kb or 0.9 kb of promoter sequence (Fig. 4B, Table 1, Group 1), indicating that this
palindrome is one of the key sites for the activation of the TTP gene
during serum stimulation.
Figure 4:
Effect of deletion or mutation of the TPE1
sequence on the expression of mouse TTP. A, CEF cells were
transfected with the mouse TTP plasmid containing the wild-type 137 bp
5`-proximal sequence or the same sequence with the TPE1 site deleted (delTPE1) or mutated from TCCCGGA to TAACTTA ( TPE1). B, CEF cells were transfected with mouse
TTP plasmids containing varying lengths of promoter with ( TPE1) or without (Wt) the analogous mutation of
the TPE1 site described above. Cell treatments, abbreviations, and
other details are as described in the legend to Fig. 2.
The above results suggest that the EGR-1,
TPE1, and AP2 elements within the first 77 bp upstream of the mouse TTP
transcription start site all contribute to transcriptional activation
of Zfp-36 by serum in CEF cells (Table 1, Group 2).
Since deletion or mutation of each single element reduced, but did not
abolish, serum-induced expression, we examined whether any of these
elements was able to impart serum-responsiveness to a minimal promoter.
We therefore constructed plasmids containing a single copy of EGR-1,
AP2, or TPE1 5` of a hybrid insert, Glo48TTP. This hybrid insert was
constructed of a 48-mer 5`-flanking sequence from the human
-globin gene and the mouse TTP cDNA. Function of this minimal
human -globin promoter has only been observed in vectors that
include an enhancer element(9, 25) . The 5` 48-mer
only contained a TATA box and no other promoter elements. We did not
include the TTP intron in these plasmids because this intron may also
contain enhancer activities (Fig. 3, B and C).
A plasmid with the c-fos SRE sequence inserted 5` to Glo48TTP served as
a positive control, since the SRE has been demonstrated to be
sufficient for the induction by serum in heterologous
constructs(10) . Using transient transfection in CEF cells, we
tested the ability of these putative elements, EGR-1, TPE1 and AP2, to
confer serum responsiveness on the silent human -globin promoter.
The expression of Glo48TTP plasmids containing each of the test
sequences was induced by serum as compared with the plasmid lacking
these sequences (Fig. 5), both in the presence and absence of
cycloheximide. In addition, the presence of all three putative enhancer
elements together resulted in a 1.5-fold increase in expression
compared with the sum of expression from each element individually (Fig. 5). Finally, the reverse orientation of the TPE1 element
resulted in a similar increase in serum-induced expression to that of
the forward construct (data not shown).
Figure 5:
Effect of TTP promoter elements on the
activity of a silent promoter. CEF cells were transfected with Glo48TTP
alone(-), or with Glo48TTP constructs containing a single copy of
the indicated TTP promoter elements or the c-fos SRE. Cells
were deprived of serum for 24 h after 24 h of recovery from
transfection; FBS (10%) was then added to the treatment group (S) for 60 min at 37 °C, compared with control conditions (C). The position of the TTP mRNA is indicated. Other details
describing the Northern blotting are contained in the legend to Fig. 2.
Binding of TPE1 by Nuclear ExtractsOur transient
transfection analysis of the 77-bp minimal TTP promoter identified
three DNA elements that contribute to its serum responsiveness other
than the intron and general factor binding sites such as the TATA box.
These include two known DNA-protein binding motifs (EGR-1 and AP2) as
well as a previously undescribed element (TPE1), all of which are
highly conserved in three mammalian species (Fig. 3A).
To determine if nuclear proteins could associate with these elements,
we performed DNA mobility shift analyses using nuclear extracts from
both serum- or insulin-treated and quiescent cells. When a radiolabeled
probe (EGR1-AP2) containing the EGR-1, TPE1, and AP2 elements was used
in gel shift assays, four major protein-probe complexes were identified (Fig. 6A). One of these complexes (C4) showed a reproducible
decrease in intensity following 10 min of treatment of the cells with
insulin or serum, whereas complexes C1 and C2 increased following
insulin or serum stimulation. Double-stranded oligonucleotides encoding
the individual EGR-1, TPE1, and AP2 sites were then used as competitors
in gel shift assays with the EGR1-AP2 probe. No specific DNA-protein
complex was displaced by the EGR-1 oligonucleotide (not shown). The top
three complexes (C1, C2, and C3) were decreased in the presence of the
AP2 competitor, although higher concentrations of the AP2 competitor
were required to completely abolished C3. The TPE1 competitor
oligonucleotide specifically abolished C4. We also observed that the
intensity of C1 and C2 increased when C4 was competed by the TPE1
oligonucleotide competitor.
Figure 6:
Gel mobility shift assay for nuclear
proteins binding to the TTP promoter. Panel A, nuclear
extracts (12 µg of protein) from HIR3.5 cells treated for 10 min
with 70 nM insulin (I) or control conditions (C) were then allowed to bind to the EGR1-AP2 probe containing
-35 to -77 of the mouse TTP promoter. These assays were
performed with or without specific oligonucleotide competitors
comprising either the AP2 or TPE1 sites, as indicated at the top of the gel. C1-C4 represent DNA-protein complexes 1-4
as described in the text. Panel B, nuclear extracts (5 µg
of protein) from CEF cells treated for 10 min with 10% FBS (S)
or control conditions (C) were allowed to bind to the TPE1
probe. Panel C, nuclear extracts (5 µg of protein) from
HIR3.5 cells treated with control conditions (C) were allowed
to bind to the wild-type or the mutant ( TPE1) TPE1 probes. All
lanes contained 20 10 cpm of P-labeled
double-stranded oligonucleotide, in the presence of poly(dI-dC) at a
final concentration of 50 µg/ml. The unlabeled competitors were
present at 1 µg/reaction when indicated. Assay conditions are
described under ``Experimental Procedures.'' The sequences of
the probes used were: EGR1-AP2,
ctagaGCGGGGGCGCGTCCCGGAAGCTCTAGTGGCCACGCCCCCAGGCgatatc; TPE1,
tcgacGTCCGGGAAGCGtcga; TPE1, tcgacGTAAGTTAAGCGtcga, where the underlined bases indicate the core sequences of the consensus
protein binding sites, and the lowercase bases indicate
portions of the restriction sites used for
subcloning.
Wild-type and mutant TPE1
double-stranded oligonucleotides were next radiolabeled for use as
probes in gel shift assays. Nuclear extracts from both serum-treated
and control cells produced a single DNA-protein complex with the
wild-type TPE1 probe, which corresponded in eletrophoretic mobility to
complex C4 seen with the EGR1-AP2 probe. As with the EGR1-AP2 probe,
formation of this complex was decreased in extracts from serum-treated
cells. A mutant TPE1 oligonucleotide competitor containing base
changes from TCCCGGA to TAACTTA had no effect on formation of the TPE1
complex (Fig. 6B). The mutant TPE1 oligonucleotide
probe was also directly radiolabeled and shown to be unable to form
this complex (Fig. 6C). These results suggested that
the TPE1 element was recognized by one or more specific nuclear
proteins. The small but consistent decrease in the intensity of the
TPE1 complex following serum treatment, seen with both the EGR1-AP2
(C4) and the TPE1-specific probe, suggests that serum treatment might
modify these binding proteins in such a way as to decrease their
binding to the TPE1 element. Similar decreases in intensity of complex
C4 were seen when HIR3.5 cells were treated with either insulin (70
nM) (Fig. 6A) or PMA (1.6 µM)
(not shown) for 10 min.
DISCUSSION
These studies establish that the first 77 bp 5` upstream of
the transcriptional start site are sufficient for maximal serum
induction of the mouse TTP gene (Zfp-36) when expressed in CEF
cells; deletions 3` of this point dramatically decrease serum
inducibility of this gene. Within this minimal effective promoter, we
have also identified several putative transcription factor binding
sites in the mouse TTP promoter, all of which are present in the human
and bovine genes. The presence of each is necessary for the full,
serum-inducible expression of the gene. Finally, we have identified
binding proteins in cell nuclear extracts that bind specifically to
some of these DNA motifs and whose binding is altered by prior
treatment of the cells with mitogens. These studies have begun to
evaluate the mechanisms by which serum and other mitogens rapidly and
dramatically stimulate the transcription of this immediate-early
response gene. One conserved transcription factor binding site
identified in the present study is the Sp1 site, located at -35
to -30 5` of the transcription start site in the mouse promoter.
Sp1 is a well characterized zinc finger-containing transcription
factor, which enhances transcription by RNA polymerase II from
promoters that contain at least one properly positioned GGGCGG
hexanucleotide (for review, see (26) ). Sp1 has been shown to
regulate transcription of certain proto-oncogenes (27, 28) and growth factor genes(29) . When
the consensus binding sequence for Sp1 was deleted from the
TTP plasmid, an 80% decrease in TTP expression
resulted, indicating that the TTP promoter is Sp1-responsive (Fig. 4B); however, we have not demonstrated directly
that Sp1 binds to its hexanucleotide binding site in the TTP promoter.
The TTP promoter construct with the Sp1 site deleted remained
serum-responsive but to a lesser extent than the wild-type construct,
implicating other promoter elements in the serum-induced expression of
TTP mRNA. EGR-1 is another zinc finger-containing transcription
factor, also known as NGF1-A(30) , KROX24(31) ,
TIS-8(4) , and Zif268(32) . It binds to a GC-rich
consensus sequence, GCGGGGGCG, that is found in the 5`-flanking regions
of many genes involved in cell growth such as proto-oncogenes and genes
encoding mitogens and mitogen receptors. Several immediate-early
response genes also have EGR-1 binding sites in their
promoters(33) . Deletion of the EGR-1 binding sequence from the
TTP promoter decreased its serum-stimulated expression by 65%. Although
we found no direct evidence of EGR-1 binding to the TTP promoter in our
gel shift assays, our deletional analysis indicates that EGR-1 may
contribute to the regulation of TTP expression. The AP2 consensus
binding sequence GCCNNNGGC (34) is present in the minimal
effective promoter of TTP from all three animal species tested. This
sequence, when bound by AP2 homodimers, has been identified as a
control element for several viral and cellular
genes(24, 34) . AP2 mediates regulation of gene
expression in response to a number of different signal transduction
pathways(35) . The activity of AP2 is increased in response to
treatment of cells with phorbol esters and agents that elevate cAMP
levels(34, 35, 36) . When the AP2 consensus
sequence in the mouse TTP promoter was deleted, induction of TTP
expression by serum was decreased by 72%. We have previously shown that
PMA could induce TTP expression(1) , making it possible that
the AP2 binding site was involved in PMA-induced TTP expression.
However, when the AP2 binding sequence was deleted from the
TTP construct, PMA still induced TTP expression to a
similar extent as the serum-induced response (data not shown). These
results indicate that the PMA-stimulated TTP expression does not depend
solely upon the consensus AP2 binding sequence in the TTP promoter. We also identified a previously undescribed promoter element at
-66 to -60 5` of the cap site in the mouse gene that we
have called TPE1. It contains a palindromic element with dyad symmetry,
TCC(C/G)GGA. It is perfectly conserved in the TTP promoter from all
three animal species we have studied. Deletion or mutation of this
element led to a 75% decrease in serum-induced TTP expression. Mutation
of the TPE1 palindrome also severely impaired serum responsiveness of
the TTP promoter when introduced into longer promoter constructs. Both
orientations of the TPE1 element could also confer serum responsiveness
to the silent promoter Glo48, indicating that this element behaves as a
transcriptional enhancer. The TPE1 palindrome also appears to
represent a binding site for a nuclear protein, as demonstrated by gel
shift analysis. Mutations within the palindrome that impaired serum
responsiveness of the promoter also abolished binding of this nuclear
protein. In addition, nuclear extracts from cells stimulated with serum
or other mitogens for 10 min showed a small but consistent decrease in
band intensity in the gel shift assays. This suggests, but does not
prove, that the TPE1 element binding protein is a target of the
signaling cascade responsible for serum induction of the TTP promoter. We have located consensus TPE1 binding sites in the promoters of a
number of genes, including c-ha-ras1(27) , mdm2(37) , and fosb(38) . fosb is
induced by serum with early response gene characteristics. It will be
interesting to investigate the possibility that this motif is involved
in the regulated expression of these and other genes. Because the
serum-responsive region in the TTP promoter contains multiple potential
promoter elements, it is likely that a number of nuclear proteins
interact in its regulation. The DNA mobility shift assay using the
EGR1-AP2 probe provides evidence to suggest that several proteins
interact with the -77 to -35 sequence to mediate the serum
response. We observed small but consistent changes in the intensity of
a number of protein-DNA complexes using nuclear extracts from
serum-treated and control cells. Serum-induced transcriptional
activation of TTP is likely to result from post-transcriptional
modification of pre-existing nuclear protein complexes, since TTP gene
transcription is very rapidly stimulated by insulin or serum, even when
cells have been pretreated with the protein synthesis inhibitor
cycloheximide(1) . Post-transcriptional modification of such
factors following serum treatment could result in changes in the
affinity of these factors for their DNA binding sites. Isolation and
characterization of the TPE1 protein should lead to a better
understanding of the transcription factor interactions that are
involved in the regulation of TTP expression. Our results suggest
cooperative interactions among nuclear proteins binding to closely
positioned cis-acting elements in the TTP promoter. For
example, the EGR-1, TPE1, and AP2 elements from the TTP promoter each
conferred only low levels of serum-inducible expression of
-globin-TTP hybrid constructs, but together they produced greater
than additive expression. The gel mobility shift data also suggest the
possibility of cooperative interactions among several transcription
factors; for example, there was a reciprocal increase in the binding of
nuclear proteins to the AP2 binding site and decreased protein binding
to the TPE1 site. Finally, the presence of the single intron in the TTP
gene markedly enhanced serum induction of the gene; we suspect this is
due to enhanced transcription, although we cannot exclude effects of
the intron on processing or stability of the mRNA(39) . Further
experimentation will be necessary to determine whether the intron
and/or proteins that bind to it interact in some way with the other
serum-responsive elements of the TTP promoter.
FOOTNOTES
- *
- The costs of publication of this article were
defrayed in part by the payment of page charges. This article must
therefore by hereby marked ``advertisement'' in
accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The nucleotide sequence(s) reported in this paper has been submitted
to the GenBank(TM)/EMBL Data Bank with accession number(s)
M19844[GenBank],
L42319[GenBank], and
L42317[GenBank]. - §
- An
associate of the Howard Hughes Medical Institute.
- ¶
- Supported by National Institutes of Health
Grant K11-DK02227-02.
- **
- Supported in part by
the Howard Hughes Medical Institute Laboratory Graduate Student's
Support Fund. Current address: ABL-Basic Science Research Program,
NCI-Frederick Cancer Research and Development Center, Frederick, MD
21702.
- §§
- An investigator of the Howard Hughes
Medical Institute. To whom correspondence should be addressed: P.O. Box
3897, Duke University Medical Center, Durham, NC 27710. Tel.:
919-684-8760; Fax: 919-684-5458.
- (
) - The
abbreviations used are: TTP, tristetraprolin; PMA, phorbol 12-myristate
13-acetate; GH, growth hormone; CEF, chicken embryonic fibroblasts;
FBS, fetal bovine serum; TPE1, TTP promoter element 1; SRE, c-fos serum-responsive element; bp, base pair(s); kb, kilobase pair(s);
EGR, early growth response.
- (
) - G. A. Taylor, D. M.
Lee, W. S. Lai, M. J. Thompson, D. D. Patel, D. I. Schenkman, B. F.
Haynes, and P. J. Blackshear, submitted for publication.
ACKNOWLEDGEMENTS
We are very grateful to Dr. Jonathan Whittaker for the
HIR3.5 cells. We also thank Jane S. Tuttle for preparing and
maintaining the CEF cells.
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