(Received for publication, October 20, 1995; and in revised form, December 29, 1995)
From the
In an attempt to understand the molecular mechanism regulating the expression of the gene coding for human hepatocyte growth factor-like protein/macrophage stimulating protein (HGFL), our laboratory has isolated and characterized approximately 4200 bp of the 5`-flanking region of the HGFL gene. To determine the location of sites which may be critical for the function of the HGFL gene promoter, we constructed a series of hybrid genes containing serial deletions of this region attached to the coding sequences for chloramphenicol acetyltransferase. Expression of these chimeric plasmids was examined by transient transfection of HepG2 and 293 cells. Our results suggest that the transcriptional activity of the HGFL promoter is modulated in HepG2 cells by one positive element at position -135 to -105 (-135/-105). In contrast, only background levels of chloramphenicol acetyltransferase expression have been detected in 293 cells. The -135/-105 region appears to bind a liver-specific transcription factor essential for expression of this gene. Gel mobility shift experiments with antibodies against hepatocyte nuclear factor-4 (HNF-4) and transactivation of the HGFL promoter by a HNF-4 cDNA expression vector suggest that HNF-4 binds to the -135/-105 region and is responsible for the liver-specific expression of HGFL.
We previously isolated a human gene which is located at the
D3F15S2 locus on human chromosome 3 (3p21), a region believed to code
for one or more tumor suppressor genes because this area is deleted in
DNA from small cell lung carcinomas, renal carcinomas, and other forms
of cancer(1, 2) . The corresponding cDNA for this gene
codes for a protein with a domain structure similar to that of a known
growth factor, hepatocyte growth factor (HGF). ()Both
proteins contain four kringle domains followed by a serine
protease-like domain and are approximately 50% identical. Based on
these similarities, the protein encoded by this cDNA was designated as
``hepatocyte growth factor-like protein'' (HGFL).
HGF is a multifunctional protein that elicits different biological responses in a cell-type and tissue-specific manner(3) . HGF can function as either a growth factor or tumor suppressor for a broad spectrum of tissues and cell types including various epithelial cells. HGF has been shown to be identical to scatter factor (SF), a mesenchymal cell-derived cytokine that dissociates cohesive colonies of epithelial cells into individual units(4) . The cellular responses of HGF/SF are elicited by c-met, a tyrosine kinase receptor involved in signal transduction(5) . Recently, HGF/SF was shown to be required in mice for liver and placental development, since mice lacking this gene die in utero with defects primarily affecting these organs(6, 7) .
The human HGFL gene is 4960 base pairs (bp) in length (from the codon for the putative initiator methionine to the polyadenylation site), containing 18 exons and 17 intervening sequences(2) . The expression pattern of HGFL has been shown by in situ hybridization analysis of mouse tissues to be restricted to hepatocytes in the liver(8) . The translated amino acid sequence of the gene and cDNA coding for human HGFL predict a protein of 80,325 dalton molecular mass containing 711 amino acids(2) . Western analysis using polyclonal antibodies to HGFL has shown that this protein is secreted and is present in human, mouse, and rat plasma with a molecular mass of approximately 90,000 daltons(9) .
Although the precise biological function(s) of HGFL remains to be elucidated, two functions for HGFL have been reported. The HGFL protein has been shown to function as an inflammatory mediator based on the identity of its translated amino acid sequence with the amino acid sequence of macrophage-stimulating protein (MSP)(9, 10) . This function is further enforced by the observation that conditioned medium from COS-7 cells expressing human HGFL mRNA specifically contained a factor which activates macrophages(10) . Based on the structural homology to a known growth factor (HGF), HGFL may also exert it effects as a regulator of cell growth. Gaudino et al.(11) have shown that recombinant HGFL induces phosphorylation of RON, a tyrosine kinase receptor homologue to the HGF receptor gene (c-met), in the epithelial cell line T47D. This phosphorylation is followed by a stimulation in DNA synthesis.
Apart from the progress on identifying a putative function for the HGFL protein, no information regarding the regulation of the expression of the HGFL gene is yet available. A wide range of liver-specific gene products have been found to be controlled at the level of transcription(12) . This control may be governed by the interaction of cis-acting DNA sequences in the 5`-flanking region of many liver-specific genes and their cognate trans-acting factors. In this report, we have used this strategy to investigate the regulation of the expression of the HGFL gene. We have cloned and characterized the 5`-flanking region of the HGFL gene. A positive regulatory element has been identified using transient transfection analyses into various cell lines with HGFL promoter-CAT chimeric plasmids. Sequence and transfection analyses suggested that HNF-4 may play a critical role in HGFL regulation. HNF-4 is a liver-enriched transcription factor in the steroid/thyroid/retinoic acid superfamily which has been shown to regulate liver-specific expression of a variety of genes. Antibody reactivity and competition gel mobility shift assays demonstrate that HNF-4 binds to the HGFL promoter and transactivation experiments show that HNF-4 is sufficient for HGFL reporter gene expression.
Figure 1:
The DNA
sequence of the 5`-flanking region including the first exon and the 5`
end of the first intervening sequence of the gene coding for human HGFL
protein. The adenine nucleotide of the initiator methionine is
designated as nucleotide 1; nucleotides upstream from this site are
numbered in a negative fashion, and those downstream are positive.
Numbering of nucleotides is shown on the right and indicates the
nucleotide immediately to the left. The number in parentheses
represents the numbering of these nucleotides in Han et al.(2) with the first nucleotide of this sequence represented
by an arrow. Asterisks represent putative transcription start
sites. The first exon is identified with the translated amino acid
sequence above the DNA sequence. The splice site for the first
intervening sequence is between nucleotides 52 and 53 and interrupts
the codon between the first and second base. Underlined sequences represent repetitive DNA; nucleotides -3613 to
-3321 code for an Alu repetitive element. Sequence represented by boldface type is similar to regions in the 5`-flanking region
of the mouse gene coding for HGFL. Nucleotides -1860 to
-1734 and -1702 to -1631 are both 70.8% identical to
nucleotides 847-972 and 757-827, respectively, on the
complementary strand in the mouse gene(1) . Nucleotides
-1018 to -836, -560 to -342 and -164 to
-1 are 78.3, 67.3, and 77.2% (respectively) to nucleotides
12-189, 465-683, and 1118-1277 in the mouse gene,
respectively. Putative transcription factor binding sites are boxed and
represented within the first kilobase of sequence upstream of iMet: ERE-half, estrogen response element half-site (identical to
type 2 nuclear receptor half-sites); IRE, -interferon
response element; RXR, 9-cis-retinoic acid response
element; T3, thyroid hormone response element; GMCSF,
granulocyte/macrophage colony-stimulating factor; LF-A1,
HNF-4/LF-A1 binding site; C/EBP, CCAAT-enhancer binding
protein; CRE, cAMP response element; alpha INF,
-interferon binding site; NF-Il
, a
response element for a nuclear factor that stimulates interleukin-6;
and GRE, glucocorticoid receptor
element.
The pL5CAT(-3049/+1) plasmid which contains nucleotides from -3049 to +1 of the HGFL promoter, was constructed by PCR amplification using primers a and t (Table 1), followed by digestion with BamHI and XhoI and ligation into the corresponding sites of pBLCAT6. Plasmids pL5CAT(-1554/+1) and pL5CAT(-1348/+1) were cloned into pBLCAT6 after PCR amplification using primers b and t followed by restriction endonuclease digestion with XbaI and XhoI or BglII and XhoI, respectively. Clones pL5CAT(-1233/+1), (-1075/+1), (-913/+1), (-793/+1) and (-554/+1) were all PCR-amplified with the reverse primer, t, and primers c, d, e, f, and g respectively, followed by digestion with XbaI and XhoI and ligation into pBLCAT6. pL5CAT(-272/+1) was created by digestion of the -793/+1 PCR product with PstI and XhoI and contains nucleotides -272 to +1 of the HGFL promoter. Constructs pL5CAT(-135/+1), (-104/+1), (-69/+1) and (-56/+1) were made by PCR amplification using the primers h, i, j, and k, respectively with primer t and digestion with XbaI and XhoI. For pL5CAT(-25/+1), primers l and m were annealed and digested with XbaI and XhoI followed by insertion into pBLCAT6. All of the above mentioned constructs have the same end terminus at +1 in the HGFL promoter (see Fig. 2). Plasmids containing PCR products were confirmed by restriction enzyme and DNA sequence analyses.
Figure 2:
Structure and activity of the human HGFL promoter-CAT chimeric plasmids. A, a partial
restriction map of the HGFL gene is shown above the bar.
Solid boxes represent exons, open boxes contain
5`-flanking DNA and intervening sequences. Blackened bars indicate the 5`-flanking regions of the HGFL promoter
which were cloned into pBLCAT6. Numbers beside the bar in parentheses
refer to the exact nucleotide positions of each end of the DNA
fragments cloned with respect to the A of iMet at +1 as in Fig. 1. B, the pBLCAT6 constructs containing variable
amounts of the 5`-flanking region of the human HGFL gene (as
shown in A) were transfected into HepG2 (striped
bars) and 293 (solid bars) cells. After transfection, the
amount of CAT protein produced was quantified by an ELISA and
normalized for -galactosidase activity. In each assay, the
relative activity of pL5CAT(-1554/-43) was set at
100.
The HGFL promoter regions for plasmids pL5CAT(-1554/-43) and (-43/-1554) were amplified using the n and o primers followed by digestion with SstI and XbaI and ligation into pBluescript. The insert was excised by digesting with HindIII and cloned into pBLCAT6 in both orientations. This same fragment, containing nucleotides -1554 to -43 was cloned into pBluescript downstream of a BamHI/XbaI 2.5-kb fragment from pL5Bam6 containing nucleotides -4154 to -1555. The entire 4.1-kb insert was excised by digestion with HindIII and cloned into pBLCAT6 to create pL5CAT(-4154/-43).
pL5CAT5(-135/-105) was created by annealing oligonucleotides p and q, followed by digestion with HindIII and XbaI and ligation into pBLCAT5(16) . pBLCAT5 contains the herpes simplex virus tk promoter upstream of the CAT reporter gene. pL5CAT5(-135/-105mut) was synthesized in an analogous manner with primers r and s. The plasmid pMT2.HNF4 (17) was described elsewhere and was a generous gift from Dr. Francis M. Sladek (University of California, Riverside).
With the exception of pL5CAT(-4154/-43) and pL5CAT(-1554/-43) which have their 3` end point at -43, all of the other HGFL promoter-CAT plasmids have the same end point at nucleotide +1 (Fig. 1). pL5CAT(-43/-1554) has the same HGFL promoter sequence as pL5CAT(-1554/-43) but cloned in the reverse orientation. The HGFL promoter region of pL5CAT(-43/-1554) was inactive in either cell type. pL5CAT(-1554/-43) gave results similar to pL5CAT(-1554/+1) which contains an additional 43 bp suggesting that these extra bases were not important for CAT protein production from these constructs.
As shown in Fig. 2B, the region containing nucleotides from -135 to +1 of the HGFL promoter appeared to contain the minimal amount of flanking DNA required to give strong liver-specific promoter activity. When the sequence between -135/-105 was deleted to create pL5CAT(-104/+1), CAT protein production decreased to approximately background levels. Another less prominent positive regulatory element may also be present in the region between nucleotides -1075 and -914. CAT protein production was reduced by approximately 25% when this region is deleted (Fig. 2B). Further upstream, the nucleotides between -4154 to -3050 appear to contain a negative regulatory element since removal of this region relieves transcriptional suppression.
Figure 3: Primer extension analysis. Primer extension was performed using RNA isolated from HepG2 cells transfected with pL5CAT(-1348/+1) and a radiolabeled oligonucleotide complementary to nucleotides 59-88 of the CAT gene of pBLCAT6 (lane 2). Lane 1 is a primer extension reaction with the same primer as in lane 2 with tRNA as a template. The cDNA products were resolved on a 6% denaturing polyacrylamide gel along with a sequencing ladder (GATC) for exact size determination of resulting cDNA products. The sizes of the primer extension products are indicated in base pairs.
Figure 4:
Gel mobility shift assays with the
-135/-105 region. Double-stranded P-labeled
oligonucleotides encompassing the -135/-105 region of the HGFL promoter were incubated in the presence of crude nuclear
extracts from the cell lines indicated. Lane 1, radiolabeled
probe only. Lanes 2 and 8, probe with HepG2 nuclear
extracts. Lanes 3-5, probe incubated with HepG2 nuclear
extracts containing 10-, 50-, and 100-fold excess of unlabeled
-135/-105 fragment. Lanes 6 and 7, probe
with HepG2 cell extracts and 50- and 100-fold molar excess of an
unlabeled oligonucleotide containing a binding site for hepatocyte
nuclear factor-1 (HNF1), used as a nonspecific competitor. Lane 9, probe with 293 nuclear extracts. C indicates
specific protein-DNA complexes, and F represents unbound
probe.
Since the region between nucleotides -135/-105 appeared to be crucial for the expression of the HGFL promoter-CAT constructs in HepG2 cells, the nucleotide sequence in this region was examined for known protein binding motifs. The most striking putative transcription factor binding site in this area was for LF-A1 (Fig. 1)(22) . LF-A1 is thought to be identical to HNF-4 based on DNA binding data and on antiserum reactivity(26) .
To test indirectly if the protein binding
to the -135/-105 was HNF-4, oligonucleotides were
synthesized complementary to a reported HNF-4 consensus sequence (21) and to a region of the -AT promoter
containing nucleotides -130 to -101 which has been shown to
bind HNF-4(22) . These oligonucleotides were used in
competition gel mobility shift experiments shown in Fig. 5A. The
-AT competitor almost
completely competed for the wild type -135/-105 region at
50- and 100-fold molar excess (lanes 4 and 5). The
HNF-4 consensus competitor competed to a lesser extent (lanes
6-8).
Figure 5:
Gel
mobility competition experiments with the -135/-105 region. A, the -135/-105 region of the HGFL promoter was used in competition experiments with sequences from
the -AT promoter and a consensus sequence for HNF-4. Lane1, no competitor. Lane 2, with 50-fold excess
self-competitor. Lanes 3-5, with 10-, 50-, and 100-fold
excess
-AT competitor sequence. Lanes
6-8, with 10-, 50-, and 100-fold excess HNF-4 competitor
sequence. All gel retardation experiments were performed with nuclear
extracts prepared from HepG2 cells. C, protein-DNA complex; F, free probe. B, the -135/-105 region of HGFL 5`-flanking DNA was used in gel mobility shift assays
with 100-fold excess (100X) competitors. Lanes 1 and 11, no competitor present. Lanes 2 and 12,
100-fold excess of the wild type (WT) -135/-105
sequence. Lanes 3-10, 100-fold excess of mutants
1-8, respectively (refer to Table 2for mutant sequences). Lanes 13 and 14, 100-fold excess of mutants 9 and 10.
All other notations are as in A.
To examine the DNA sequence requirements for protein binding to the -135/-105 region, mutant oligonucleotides were synthesized and tested for their ability to compete for trans-acting factor binding in gel mobility shift assays. Table 2shows the sequence of the mutant oligonucleotides used in the assay, and Fig. 5B shows the results of the competition experiments. At 100-fold molar excess, mutants 4, 5, and 6 (lanes 6-8) were unable to compete for protein binding to the -135/-105 region of the HGFL promoter. Mutant 2 almost completely competed for specific protein binding (lane 4) and mutants 1, 3, 7, 8, 9, and 10 competed to a lesser extent (lanes 3, 5, 9, 10, 13, and 14, respectively). Therefore it appears that the most important sequence involved in protein binding to the -135/-105 region is TCAGGTCAG at nucleotides -126 to -118 (Table 2; Fig. 1). However, the remaining nucleotides in this region also appear to be involved in protein binding but to a lesser extent.
Figure 6: HNF-4 binding to the -135/-105 region of the HGFL promoter. A, gel retardation assays with the -135/-105 HGFL promoter sequence and nuclear extracts from HepG2 cells. Lane1, probe alone. Lanes 2-4, probe with HepG2 extracts. Lane 3, with the addition of an antibody against HGFL. Lane 4, with an antisera against HNF-4. A - indicates no antibody present. The letters F, C, and S represent free probe, protein-DNA complex, and antibody supershift, respectively. B, the -135/-105 region was tested for protein binding with 293 extracts made from cells transfected with the expression vector, pMT2.HNF-4 which contains a cDNA for HNF-4. Gel mobility shift experiments were performed with extracts from HepG2 cells (lane 1), from 293 cells (lane 2), and from 293 cells transfected with the expression vector (lane 3). All other notations are as in A.
As further evidence for HNF-4 binding to the -135/-105 region of the HGFL promoter, 293 cells were transfected with an expression vector for HNF-4, pMT2.HNF-4(17) . Nuclear extracts were prepared from the transfected 293 cells and tested for binding to the -135/-105 region (Fig. 6B). Extracts prepared from untransfected 293 cells were unable to retard the mobility of the -135/-105 region (lane 2), whereas extracts from 293 cells transfected with pMT2.HNF-4 (lane 3) produced a mobility shift identical to that of extracts from HepG2 cells (lane 1).
Figure 7:
Heterologous promoter activity of the
-135/-105 region. The -135/-105 region of the HGFL promoter was inserted upstream of the herpes simplex
virus tk promoter in pBLCAT5 to create pL5CAT5(-135/-105).
A mutant -135/-105 region which changes 3 bp (GGT to TTG at
nucleotides -123 to -121; Table 2, mutant 7) was also
cloned 5` to the tk promoter in pBLCAT5 to create
pL5CAT5(-135/-105mut). The amount of CAT protein produced
was determined after transfection into HepG2 (stripped bars)
and 293 (solid bars) cells and normalized for
-galactosidase activity. In each cell type, the activity of
pL5CAT5 was set at 1. The fold stimulation of CAT protein production of
the -135/-105 and -135/-105mut clones over
pBLCAT5 after transfection into is shown. The experiments were
performed with and without co-transfections of an HNF-4 cDNA expression
vector, pMT2.HNF-4.
An expression vector for HNF-4, pMT2.HNF-4(17) , was also cotransfected with either the wild type or mutant heterologous promoter constructs (Fig. 7). In HepG2 cells, increased HNF-4 expression was only moderately able to increase transcription from the wild type -135/-105 promoter. In 293 cells, the expression of HNF-4 was able to stimulate transcription from pL5CAT5(-135/-105) over 10-fold compared to pL5CAT(-135/-105) alone, whereas the mutant was only slightly stimulated. The mutant -135/-105 sequence in either cell type was not stimulated to an appreciable extent with the HNF-4 expression vector compared to the wild type sequence.
The HGFL gene was identified based on its similarity to a probe coding for the kringle domains of bovine prothrombin (2) and named for its structural similarity to HGF. Based on the translated cDNA sequence for this gene, it has become apparent that HGFL protein is identical to MSP(9, 27) . HGFL/MSP has been shown to be a chemoattractant for mouse resident peritoneal macrophages(28) , to induce morphological changes in macrophages(10) , and to inhibit induction of nitric oxide production by lipopolysaccharide- or cytokine-induced macrophages(29) . Furthermore, the expression of HGFL has been shown to be up-regulated during liver regeneration and inflammation(30) . This investigation has focused on understanding the factors involved in the regulation of the HGFL gene.
Transient transfection analyses with sequential deletions
of the HGFL flanking DNA fused 5` to the CAT gene were
performed to identify potential regulatory sequences governing the
expression of this gene. Since the endogenous transcriptional start
point (tsp) of this gene could not be precisely determined due to the
duplicated copies of this locus()(32) , the 3` end
point of the majority of deletion constructs was set at the A
nucleotide of the potential iMet (+1 in Fig. 1) with a few
exceptions. The plasmids pL5CAT(-4154/-43) and
pL5CAT(-1554/-43) have their 3` end point at the A
nucleotide (nucleotide -43 in Fig. 1) of an inframe ATG
codon upstream of the ATG at +1. An upstream ATG codon is also
present in the mouse; however, this codon is not in frame. Transient
transfection analyses of pL5CAT(-1554/+1) and
pL5CAT(-1554/-43) gave indistinguishable results (Fig. 2B), suggesting that these first 43 bp were not
necessary for the transcription of CAT. From these results, it may be
inferred that the tsp is not within the first 43 bp, however, an
alternative start site can not be excluded.
In order to determine
the tsp of the HGFL gene, primer extension analyses were
performed using RNA isolated from HepG2 cells transfected with the
chimeric HGFL promoter-CAT construct
pL5CAT(-1348/+1). This allowed us to assay for RNA initiated
specifically from the HGFL promoter and not from RNA initiated
from highly homologous regions of the amplified chromosome 1
loci(32) . The sequence of one copy of a homologous gene has
been determined and was found to be 97% identical to the
gene coding for HGFL, including 2200 bp of the 5`-flanking
sequence. All but the 3` end of the HGFL gene is homologous to
sequences on chromosome 1(2) . Primer extension experiments
performed on total RNA from HepG2 cells using endogenous HGFL sequence primers resulted in a multitude of putative start
sites
most likely originating from the combination of HGFL and related chromosome 1 sequence start points. The two
bands obtained in Fig. 3correspond to RNA transcripts
potentially initiated from -75 and -76 bp upstream of the
iMet of the HGFL promoter (Fig. 1). This same
combination of bands was observed using several HGFL promoter-CAT constructs. The tsp identified in the human HGFL flanking DNA is within 20 bp of the unique tsp identified in the
(nonamplified) mouse gene (1) when the two sequences are
aligned at the iMet residue.
Transient transfection analyses implicated several regions which may be critical for expression of HGFL. One region, between nucleotides -135 to +1, contained the minimal sequence of the human HGFL flanking DNA that could drive cell type-specific expression (Fig. 2B). Given the results of the primer extension experiments, this suggests that the minimal promoter required may only span 60 bp from nucleotides -135 to -75. Furthermore, there is no apparent TATA box or Sp1 site near the tsp (Fig. 1). TATA-less promoters are found in many housekeeping genes (e.g. genes encoding the enzymes of intermediary metabolism). Genes with TATA boxes in the promoter region generally initiate transcription at well defined sites, whereas transcription of TATA-less promoters generally occurs over an extended region. The primer extension results presented in Fig. 3suggest a well defined tsp contrary to the ambiguous tsp of many promoters devoid of TATA boxes.
When comparing the mouse and human HGFL sequences, several areas of homology were observed; one of which overlaps with the -135 to +1 region. Nucleotides -164 to +1 have a high degree of similarity, approximately 76%, between the two species. Furthermore, the -135/-105 region is 71% identical between the mouse and human. Similarities in DNA sequence among different species are thought to represent important regulatory regions.
The -135/-105
region appeared to be the most critical region implicated in our
transient transfection studies since it is both required for promoter
activity and is able to confer this activity in a cell type-specific
manner. Among the multiple putative binding motifs in the
-135/-105 region, sequence analyses identified a putative
recognition site for LF-A1/HNF-4 in the -135/-105 region.
HNF-4 is a potent transcriptional activator that controls the
expression of a variety of genes including the liver-specific
expression of the transthyretin(33) ,
-AT(22) , and apolipoprotein CIII (26) genes. Due to the presence of this binding sequence,
oligonucleotides were made complementary to a reported HNF-4 consensus
sequence (21) and to the 5` region of the
-AT
promoter where HNF-4 has been reported to bind(22) .
Competition experiments were performed with both of these regions
against the -135/105 promoter region of HGFL. The
results depicted in Fig. 5A show that both competitors
were capable of binding the same protein as the -135/-105
region indicating that HNF-4 was binding the HGFL promoter
region. Recently, however, a more refined consensus sequence for HNF-4
binding sequence has been reported by Jiang et
al.(34) . This sequence is a direct repeat of AGGTCA
separated by one nucleotide, referred to as a DR+1 element. Close
inspection of the -135/-105 region shows that a very near
match to this consensus sequence is found at nucleotides -131 to
-121 containing the sequence 5`-AGGTCTCAGGTCA-3`. This may
explain why the consensus sequence (5`-GGCAAGGTTCATATTTGTGTAG-3`)
chosen in our competition experiments for HNF-4 may not compete to a
larger extent.
In an attempt to define the nucleotides involved in the binding of protein to the -135/-105 region, several mutant oligonucleotides were obtained and used in the competition gel mobility shift experiments shown in Fig. 5B. Although most the nucleotides appeared to be important for binding, the sequence encompassing nucleotides -126 to -118 containing the sequence TCAGGTCAG was found to be most crucial. The most important nucleotide sequence involved in potential HNF-4 binding contains one copy of the AGGTCA motif.
In order to confirm that HNF-4 was
responsible for binding and conferring liver-specific expression to the HGFL promoter, a HNF-4 cDNA expression vector was transfected
into 293 cells. Extracts from these cells were now capable of binding
to the -135/-105 region as shown in Fig. 6B. Furthermore, antibodies directed specifically
against HNF-4 were able to completely supershift the protein-DNA
complex formed in endogenous HepG2 cell extracts (Fig. 6A), suggesting that all the protein complexes
contain at least one HNF-4 monomer. These results indicate that HNF-4
does in fact bind to this region. HNF-4 thus far has been reported to
bind exclusively as a homodimer(26, 34) . The results
of our supershift experiments, in which all of the protein-DNA complex
is recognized, are consistent with this observation, although
heterodimer formation with another protein(s) cannot be excluded.
Interestingly, HNF-4 is not exclusively found in the liver, but its
expression has also been reported in intestine and kidney
tissue(21, 35) . Preliminary northern analyses of
HepG2 and 293 RNA indicate that HNF-4 is not present in the kidney
derived 293 cells used in our studies.
Further experiments were undertaken to demonstrate that not only could HNF-4 bind to the -135/-105 region, but that binding of this protein resulted in transcriptional activation. Cotransfection experiments with the vector pBLCAT5 and pMT2.HNF-4 gave results identical to those of pBLCAT5 alone in both cell types. Transactivation experiments shown in Fig. 7demonstrated that the -135/-105 region was sufficient to promote activation of a heterologous promoter construct. More importantly, HNF-4 expression was solely able to activate transcription of CAT through the -135/-105 promoter region in 293 cells. These results demonstrate that HNF-4 is necessary and sufficient to both bind to (Fig. 6) and activate transcription from the -135/-105 region of the HGFL promoter.
A mutant -135/-105 sequence which changes the important HNF-4 AGGTCA motif to ATTGCA, decreased transcription of CAT in HepG2 cells compared to the wild type sequence. This mutant sequence was shown not to compete, even at 100-fold molar excess, with the wild type sequence in gel mobility shift assays (mutant 7, Table 2and Fig. 5B). In contrast to the wild type -135/-105 heterologous promoter construct which did not stimulate transcription in 293 cells, the mutated sequence was able to confer some transcriptional activity in 293 cells. The activity in both cell types, however, appeared not to be stimulated to a significant extent by HNF-4. Gel mobility shift experiments performed with this mutant sequence showed that protein was capable of binding to the mutated sequence in both cell extracts (data not shown) and may account for the activation seen. The protein-DNA complex formed with the wild type sequence had a different electrophoretic mobility (larger) then the mutant complex. Furthermore, the activity was approximately equivalent (5.7- versus 4.2-fold) in both HepG2 and 293 cells. This was not entirely unexpected since weak binding to direct repeats (DR+1 elements) or near matches to these repeats (as is apparent in this case) has been reported for family members of the type 2 nuclear receptor family, specifically 9-cis-retinoic acid response element homo- and heterodimers (31, 34, 36) . Initial attempts investigating the protein/DNA complexes formed on the -135/+1 promoter region using DNaseI footprinting analyses with extracts from 293 cells with and without overexpression of HNF-4 have been unsuccessful thus far. However, it appears as though the majority of the -135/+1 region of the promoter may be occupied in various cell types. Efforts are underway to purify HNF-4 and to more precisely map the important contact residues for HNF-4 binding.
After
examination of the DNA sequence of the 5`-flanking region of the human HGFL gene, various putative regulatory elements found in
inducible genes were identified (Fig. 1). Of these elements,
there are several potential liver-specific C/EBP transcription factors
and multiple potential HNF-4 binding sites. Both of these proteins have
been found to regulate a number of liver-specific promoters. There are
a number of potential regulators in the -135 to -105
region. There are various hormone responsive elements including: a
putative estrogen response element half-site, a site for regulation by
retinoic acid and/or derivatives of retinoic acid through the
9-cis-retinoic acid response element, a site for the thyroid hormone
binding and a -interferon response element. Since all of these
putative binding sites occur within or overlap the
-135/-105 region, it is tempting to speculate that the
levels of HGFL may be regulated by the competition or availability of
these factors with their recognition sequences.
Based on our results, it can be concluded that HNF-4 is necessary and sufficient for the liver-specific expression of HGFL in HepG2 cells. Antibody reactivity and transactivation experiments conclude that HNF-4 binds to the -135/-105 region and is the sole factor required for stimulating transcription from the wild type -135/-105 region in 293 cells. Furthermore, mutations of this sequence result in the lose of HNF-4 binding and tissue specificity. These studies represent an initial effort to unravel the mechanisms governing expression of HGFL and provides a basis for further study of transcriptional regulation of this gene. Current studies are under way to determine the inducibility of this gene, to more precisely map protein/DNA contact sites and to further map upstream regulators.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBank(TM)/EMBL Data Bank with accession number(s) U37055[GenBank].