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Molecular and Cellular Biology, February 2001, p. 1393-1403, Vol. 21, No. 4
Laboratory of Metabolism, Division of Basic
Sciences, National Cancer Institute,1 and
Metabolic Disease Branch, National Heart, Lung and Blood
Institute,2 National Institutes of Health,
Bethesda, Maryland 20892, and Office of Laboratory Animal
Resources, National Cancer Institute, National Institutes of
Health, Frederick, Maryland 217023
Received 5 July 2000/Returned for modification 29 August
2000/Accepted 31 October 2000
The numerous functions of the liver are controlled primarily at the
transcriptional level by the concerted actions of a limited number of
hepatocyte-enriched transcription factors (hepatocyte nuclear factor
1 The adult liver executes numerous
functions that are essential for metabolic homeostasis including plasma
protein synthesis; carbohydrate, lipid, and amino acid metabolism; and
xenobiotic metabolism. The majority of these functions are performed by
hepatocytes. Hepatocyte-enriched transcription factors control
transcription of genes that are preferentially expressed in liver
(5). Our understanding of the role of liver-enriched
transcription factors in gene expression has largely been developed
using transfection assays in cultured cells. Cis-acting
elements for transcription factor binding have been characterized using
this approach in conjunction with in vitro DNA binding assays. However,
many genes have regulatory elements for several transcription factors,
and it becomes difficult to assess which factor predominates in vivo. Moreover, it is becoming increasingly clear that the in vitro regulation of a gene is not always reflected by the in vivo situation. For example, numerous hepatic genes are transactivated by hepatocyte nuclear factor 3 HNF4 To determine the role of HNF4 Targeting of the HNF4
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.4.1393-1403.2001
Hepatocyte Nuclear Factor 4
(Nuclear Receptor
2A1) Is Essential for Maintenance of Hepatic Gene Expression
and Lipid Homeostasis

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ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
[HNF1
], -1
, -3
, -3
, -3
, -4
, and -6 and members
of the c/ebp family). Of these, only HNF4
(nuclear receptor 2A1) and
HNF1
appear to be correlated with the differentiated phenotype of
cultured hepatoma cells. HNF1
-null mice are viable, indicating that
this factor is not an absolute requirement for the formation of an
active hepatic parenchyma. In contrast, HNF4
-null mice die during
embryogenesis. Moreover, recent in vitro experiments using tetraploid
aggregation suggest that HNF4
is indispensable for hepatocyte
differentiation. However, the function of HNF4
in the maintenance of
hepatocyte differentiation and function is less well understood. To
address the function of HNF4
in the mature hepatocyte, a conditional
gene knockout was produced using the Cre-loxP system. Mice lacking
hepatic HNF4
expression accumulated lipid in the liver and exhibited
greatly reduced serum cholesterol and triglyceride levels and increased
serum bile acid concentrations. The observed phenotypes may be
explained by (i) a selective disruption of very-low-density lipoprotein
secretion due to decreased expression of genes encoding apolipoprotein
B and microsomal triglyceride transfer protein, (ii) an increase in
hepatic cholesterol uptake due to increased expression of the major
high-density lipoprotein receptor, scavenger receptor BI, and (iii) a
decrease in bile acid uptake to the liver due to down-regulation of the
major basolateral bile acid transporters sodium taurocholate
cotransporter protein and organic anion transporter protein 1. These
data indicate that HNF4
is central to the maintenance of hepatocyte
differentiation and is a major in vivo regulator of genes involved in
the control of lipid homeostasis.
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INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
(HNF3
) in vitro but deletion of HNF3
in the mature hepatocyte using conditional gene targeting in mice has minimal
consequences (54).
is a highly conserved member of the nuclear receptor
superfamily (NR2A1). It is expressed at the highest levels in the liver, kidney, intestine, and pancreas in mammals and in the homologous structures in invertebrates (39, 47, 69). This conserved expression pattern suggests that HNF4
fulfills an essential role in
development, organogenesis, and maintenance of organ function. HNF4
can activate gene transcription in the absence of exogenous ligands
(25, 48, 49). However, HNF4
binding activity may be
modulated by fatty acyl-coenzyme A (CoA) thioesters, which may act as
agonistic or antagonistic ligands depending on chain length and
degree of saturation (17), and also by protein kinase A-mediated phosphorylation (57). This suggests that
HNF4
may be responsive to and, by implication, important in the
control of metabolic status. To add further weight to the proposal that HNF4
is involved in metabolic homeostasis, mutations in the HNF4
gene cause the disorder maturity onset diabetes of the young (MODY1) (67). Finally, a large number of putative HNF4
target
genes have been identified, including those encoding several
apolipoproteins, blood coagulation factors, and enzymes involved in
lipid, amino acid, and glucose metabolism (47).
in gene expression in an intact animal
model and to circumvent the embryonic lethality of a standard gene
knockout, conditional liver-specific disruption of the HNF4
gene was
carried out using the Cre-loxP method with an albumin-Cre transgene.
Here, data that strongly suggest a central role for hepatic HNF4
in
the maintenance of lipid homeostasis are presented.
![]()
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
gene.
A mouse ES-129/SvJ genomic
DNA bacterial artificial chromosome library (Genome Systems, St. Louis,
Mo.) was screened with the rat HNF4
cDNA (a generous gift from
Francis Sladek). A 5.5-kb EcoRI fragment containing exons 4, 5, and 6 of the mouse HNF4
gene was subcloned into a modified
pGEM-3Z vector (Promega, Madison, Wis.) in which the polylinker from
SacI to XbaI had been destroyed. A
double-stranded oligonucleotide,
5'-TCGAATATAACTTCGTATAATGTATGCTATACGAAGTTATTAAGCTTCCCGGGG-3' and
5'-TCGACCCCGGGAAGCTTAATAACTTCGTATAGCATACATTATACGAAGTTATAT-3' (TCGA-loxP-HindIII-SmaI-SalI),
was cloned into the SalI site of the polylinker (5' of exon
4). Insertion and orientation of the linker were confirmed by
sequencing. A 2-kb SacI fragment containing a neomycin
resistance cassette flanked by two loxP motifs was cloned between the
two SacI sites in intron 5 (deleting 178 bp of the intron
and introducing an additional EcoRI site). This fragment was
cloned such that all three loxP sites were in the same orientation. A
blunt-ended, 0.75-kb EcoRI fragment from intron 3 (derived
from a different subclone) was cloned into the SmaI site
introduced with the first loxP oligonucleotide in order to ensure the
inclusion of this loxP site during homologous recombination. Finally, a
thymidine kinase gene cassette was cloned into the SalI site
of the polylinker. Embryonic stem (ES) cells (RW4; Genome Systems) were
electroporated with the linearized targeting construct, and 23 (out of
320 ES cells screened) homologous recombinants were identified using a
flanking probe (probe 1; Fig 1). Of
these, eight contained all three loxP sites. Clone 300 was injected
into C57BL/6 blastocysts. The blastocysts were then inserted into
foster NIH Swiss mice. Chimeric male mice were crossed with C57BL/6
females to generate heterozygous targeted/wild-type (t/+)
mice. Deletion of the neomycin cassette in vivo was achieved by
crossing the mice with an EIIaCre transgenic line (27) as
described in Results.

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FIG. 1.
Gene targeting and conditional deletion of exons 4 and 5 of the HNF4
gene. (A) Restriction maps of the wild-type allele,
targeting vector, and targeted allele. P1, P2, and P3, probes used to
assess recombination events; open boxes, neomycin (Neo) and thymidine
kinase (TK) positive- and negative-selection cassettes, respectively;
shaded boxes, exons, numbered as described by Taraviras et al.
(55); arrowheads, position and orientation of loxP sites.
The upside-down Neo shows that the Neo cassette is transcribed in an
orientation opposite to that of the HNF4
gene. Restriction sites: E,
EcoRI; H, HindIII; B, BamHI; K,
KpnI; S, SacI. Sites introduced in the targeting
vector are italicized. (B) Southern blot analysis of homologous
recombination in ES cells electroporated with the targeting vector.
Hybridizing fragments of wild-type (±) and targeted (t)
alleles and their respective sizes are indicated. (C) Crosses between
mice heterozygous for the targeted allele failed to yield targeted
homozygotes (see text). In order to generate a viable, conditional
knockout mouse line, heterozygous (t/+) males were crossed
with EIIaCre females in order to generate both floxed (fl)
and null (
) alleles. Shown is recombination between loxP sites 2 and
3 (although all recombination events, i.e., recombination between loxP
sites 1 and 2, 2 and 3, and 1 and 3, were observed). (D) Southern blot
analysis of tail DNA from pups derived from the cross described for
panel C. Hybridizing fragments of wild-type (±), targeted
(t), floxed (fl), null (-), and neomycin
(neo) alleles and their respective sizes are indicated. The
floxed, null, and neomycin alleles are the result of recombination
between loxP sites 2 and 3, 1 and 3, and 1 and 2, respectively. (E) PCR
genotyping of mice. (Top) HNF4
genotyping. Tail DNA was amplified
using primers flanking loxP site 1 (A) in the floxed allele, yielding
products of 241 and 180 bp for the floxed and wild-type alleles,
respectively. (Bottom) Cre genotyping. mEH primers served as a positive
control for amplification, yielding a fragment of 341 bp in all
samples. The presence of the Cre transgene was indicated by the
amplification of an additional band of 411 bp with Cre-specific
primers.
Breeding scheme to produce animals with a liver-specific deletion
of HNF4
.
Heterozygous animals carrying one floxed
(fl; flanked by loxP) and one wild-type allele were crossed
with animals hemizygous for the previously described albumin-Cre
transgene (AlbCre) (66), kindly provided by Derek LeRoith.
Heterozygous (fl/+) animals carrying one copy of the AlbCre
transgene were then interbred with fl/+ littermates lacking
Cre to generate HNF4
liver knockout mice (H4LivKO) and littermate
control mice (H4Flox, AlbCre, and wild type, as described below in Results).
PCR genotyping of mice for HNF4
and AlbCre.
Genomic DNA
was isolated from mouse tails as described previously
(26). For HNF4
gene diagnostic PCR, approximately 50 ng
of tail DNA was amplified in a 50-µl final reaction mixture containing 1.5 mM MgCl2, 0.2 mM deoxynucleoside
triphosphates (dNTPs), 2.5 U of AmpliTaq, and 0.3 µM (each) HNF4
gene-specific primers prH4GT-LP1-F1 (5'-AGAATGACCCTGAAGCACCAGG-3')
and prH4GT-LP1-R1 (5'-GCCAGAGGTCTGTGAAACAAGG-3').
Cycling conditions were 94°C for 4 min and then 30 cycles of
94°C for 30 s, 55°C for 30 s, and 72°C for 30 s,
followed by a 10-min extension at 72°C. These primers amplify the
region surrounding loxP site 1 (Fig. 1) in the flox allele, yielding
products of 241 and 180 bp for the floxed and wild-type alleles, respectively.
Serum and organ collection.
Mice were anesthetized with
2.5% Avertin and decapitated, and trunk blood was collected in serum
tubes. Serum was separated by centrifugation at 7,000 × g for 5 min and stored at
20°C prior to analysis. Serum was
assayed for total cholesterol, high-density lipoprotein (HDL)
cholesterol, triglycerides, and alanine aminotransferase using standard
methodology (Anilytics Inc., Gaithersburg, Md.). Selected tissues were
collected, weighed, and either fixed immediately for histological
analysis or snap-frozen in liquid nitrogen for preparation of RNA or nuclei.
FPLC analysis of plasma lipids. Total cholesterol and triglyceride (Sigma, St. Louis, Mo.), as well as free cholesterol and phospholipid (Wako, Osaka, Japan), concentrations were measured in 12-µl aliquots of plasma using commercial kits and the Hitachi 911 automated chemistry analyzer (Boehringer Mannheim, Indianapolis, Ind.). Plasma lipoproteins were analyzed by gel filtration on two Superose 6 columns in series (fast-protein liquid chromatography [FPLC]; Pharmacia, Piscataway, N.J.) at 0.3 ml/min in phosphate-buffered saline containing 0.1 mM EDTA and 0.02% sodium azide (28). Mouse apolipoproteins A-I, A-II, Cs, E, and B were identified by Western blotting in plasma and FPLC fractions using a mixture of polyclonal rabbit anti-mouse immunoglobulin G antisera raised against purified apolipoproteins (Biodesign, Saco, Maine).
Pathology. Livers from 45-day-old representative H4LivKO and H4Flox control mice were fixed in 10% neutral buffered formalin and embedded in paraffin, and sections cut at a thickness of 4 to 6 µm were stained with hematoxylin and eosin (H & E). Selected livers were fixed in alcoholic formalin or frozen in Tissue-Tek O. C. T. compound (Miles, Inc., Elkhart, Ind.) for histochemistry of glycogen and fat by periodic acid-Schiff (PAS) and oil red O staining, respectively. Pieces of liver were fixed in 2.5% glutaraldehyde and postfixed in osmium tetroxide, and thin sections were stained with uranyl nitrate and lead citrate for ultrastructural studies.
Northern blot analysis. Tissues were crushed on dry ice in a mortar and pestle, and total RNA was extracted with Ultraspec reagent (Biotecx, Houston, Tex.). RNA was separated on 1% agarose-0.22 M formaldehyde gels and transferred to GeneScreen Plus membranes (Dupont, Wilmington, Del.) by capillary blotting in 10× SSC (1× SSC is 0.15 M NaCl plus 0.015 M sodium citrate) overnight. Blots were hybridized at 42°C in UltraHyb (Ambion Inc., Austin, Tex.) with random-primer-labeled cDNA probes. Probes for rat cDNAs were generously provided by Kiyoto Motojima (ApoA-I, -A-IV, -C-II, and -C-III) (41), Johann Auwerx (ApoB) (52), and Daniel Kelly (midchain acyl-CoA dehydrogenase [MCAD]) (11). All other probes were amplified from a mouse liver cDNA library using gene-specific primers and cloned into pCR TOPO II (Stratagene, La Jolla, Calif.). The probes were quality controlled by sequencing.
Western blot analysis.
Nuclei were prepared according to the
method of Wu (65). Protein concentration was measured
using the BCA kit (Pierce Chemical Co., Rockford, Ill.). Total nuclear
protein was separated on a sodium dodecyl sulfate-10% polyacrylamide
gel electrophoresis gel, transferred to nitrocellulose, and probed with
an antibody raised to a C-terminal peptide of HNF4
(Santa Cruz
Biotechnology, Inc., Santa Cruz, Calif.). Equal loading of nuclear
protein was demonstrated by reprobing the membrane with an antibody to
histone H1 (a generous gift from Michael Bustin) (2).
Bound antibody was detected with a horseradish peroxidase-conjugated
secondary antibody (KPL, Gaithersburg, Md.), and immune complexes were
developed with the ECL reagent (Pharmacia).
| |
RESULTS |
|---|
|
|
|---|
Generation of a conditional null allele of the HNF4
gene.
A
triple-lox targeting vector was constructed; in this vector a
phosphoribosyltransferase (neo) selection cassette flanked by loxP sites was introduced into intron 5 of the mouse HNF4
gene
along with a third loxP site in intron 3 (Fig. 1A). Following standard
electroporation and culture of ES cells, homologous recombinants were
identified by Southern blotting of genomic DNA. All three loxP sites
were incorporated into the HNF4
locus in 8 of 320 ES cells screened
(Fig. 1B). A single clone (clone 300) was injected into C57BL/6
blastocysts to generate chimeric mice. The triple-lox, targeted allele
(t) was transmitted through the germ line. However, extensive intercrosses between heterozygous (t/+) mice
failed to yield homozygous targeted mice. Thus, the targeted allele was nonfunctional, most likely due to interference with HNF4
gene expression by the neo cassette.
t/+ × EIIaCre+/
).
This indicated the presence of all possible deletion patterns (Fig.
1D). Chimeric mice were selected according to the observed pattern of
deletion and crossed to wild-type mice in order to generate lines
carrying floxed (male 44) and null (male 45) alleles of the HNF4
gene. As predicted, the floxed allele, containing just small loxP sites
flanking exons 4 and 5 of the HNF4
gene (Fig. 1C), was active and
viable at homozygosity.
Deletion of exons 4 and 5 is predicted to result in the splicing of
exon 3 to exon 6. This results in a frameshift, leading to a premature
stop codon. Translation of the truncated mRNA results in an N-terminal
peptide of just 128 amino acid residues (118 residues of HNF4
followed by 10 missense residues). While this protein retains both zinc
finger motifs, it lacks both the A and T boxes necessary for
high-affinity binding to DNA (14, 18), along with the
ligand-binding and activation function 2 domains. Significantly, a
mutant HNF4
protein containing 125 N-terminal residues fails to bind
DNA at room temperature, although some low-affinity binding is observed
at 4°C (18). The putative truncated protein also lacks
sequences required for the recruitment of coactivator proteins CREB
binding protein (CBP), SRC-1, and GRIP1 (10, 61, 68).
Given that CBP-mediated acetylation of HNF4
is required for its
retention in the nucleus (50), it is likely that any truncated protein produced would be quickly exported to the cytoplasm and therefore unable to interfere with transcription. While we cannot
rule out subtle dominant-negative effects due to retention of some DNA
binding, heterozygous animals were normal for all parameters measured
(not shown), suggesting that this is not the case. Moreover, consistent
with the embryonic lethality of germ line inactivation of HNF4
,
crosses between the heterozygous null mice produced here
(HNF4
+/
) failed to yield homozygous knockouts. Thus,
the available evidence strongly suggests that deletion of exons 4 and 5 results in a null allele.
High-efficiency, liver-specific disruption of HNF4
.
To
examine the role of HNF4
in the maintenance of hepatic gene
expression and liver function, the active floxed allele was crossed
into mice hemizygous for an albumin-Cre transgene (AlbCre). that
express Cre exclusively in the postpartum liver (66). The (HNF4
fl/+;
AlbCre+/
)F1 mice so generated were
interbred with HNF4
fl/+ littermates
lacking the AlbCre transgene. This breeding scheme yielded
(HNF4
fl/fl;
AlbCre+/
)F2 mice (designated
H4LivKO) and three groups of littermate control mice,
HNF4
fl/fl; AlbCre
/
(H4Flox),
HNF4
+/+; AlbCre+/
(AlbCre), and
HNF4
+/+; AlbCre
/
(wild
type). Genotypes of all mice were assessed by PCR analysis of tail DNA
(Fig. 1E). All alleles were inherited in a Mendelian fashion (Fig. 1E),
suggesting that neither the floxed allele, the AlbCre transgene, nor
the combination thereof resulted in any prenatal lethality.
locus, Northern blots of total liver RNA
from 2-, 4-, and 6-week-old H4LivKO and control animals were probed
with a fragment of the HNF4
cDNA homologous to exons 4 and 5 (Fig.
2A). Deletion of the floxed exons of
HNF4
was about 50% by 4 weeks of age, and, by 6 weeks of age, mRNA
signal intensity in H4LivKO livers was below the limit of detection
(Fig. 2A and B). Thus, intact HNF4
mRNA containing exons 4 and 5 is
reduced to <10% that seen in control livers at 6 weeks of age. This
time course is similar to that reported for a retinoid-X receptor-
(RXR
) floxed allele using the albumin promoter driving Cre
expression (60). All further experiments were conducted
using 45-day-old mice.
|
cDNA revealed that H4LivKO mice expressed an mRNA that migrated with
greater mobility than that from wild-type, AlbCre, or H4Flox mice (Fig.
2B). Reverse transcription-PCR of HNF4
cDNA from H4LivKO liver RNA
failed to detect the full-length transcript. Sequencing the cloned,
truncated cDNA revealed that the expected exon 3-to-exon 6 splice event
had occurred (not shown). The steady-state level of the truncated
HNF4
mRNA in H4LivKO hepatocytes is about 30% that seen for the
undisrupted message in controls. This is consistent with the suggestion
that HNF4
regulates its own expression (51) and/or that
the abnormal transcript is less stable than the wild-type message.
Disruption of HNF4
RNA resulted in a corresponding loss of
full-length protein as evidenced by Western blots of nuclear protein probed with an antibody raised to a C-terminal peptide of HNF4
(Fig.
2C). Equal loading of nuclear protein was demonstrated by reprobing the
membrane with an antibody to histone H1. Thus, H4LivKO mouse liver
nuclei contain less than 10% of the immunoreactive HNF4
protein
present in controls.
To confirm that deletion of exons 4 and 5 was restricted to the liver,
RNA from kidney and intestine, which also express HNF4
, was
subjected to Northern blotting and probed with a probe consisting of
exons 4 and 5. As expected, H4LivKO mice expressed full-length HNF4
mRNA in both tissues at levels similar to those found in wild-type,
AlbCre, and H4Flox mice (Fig. 2D). Thus, HNF4
mRNA and protein were
efficiently and specifically disrupted in the livers of adult H4LivKO
mice by 45 days of age.
Hepatomegaly, hepatocyte hypertrophy, and abnormal glycogen and
lipid deposition in livers of H4LivKO mice.
To begin to assess the
influence of disruption of HNF4
on hepatic function, liver weights
and pathology were examined. Livers from 45-day-old H4LivKO mice were
significantly enlarged relative to those of controls (Table
1) and had a visibly gray, mottled appearance. Marked pathological lesions were evident in H4LivKO mouse
livers but were absent in controls (Fig.
3). These lesions were more severe in
male mice than in females. Centrilobular hepatocytes in H4LivKO mice were invariably hypertrophic, with pale eosinophilic intracytoplasmic inclusions (Fig. 3B and C). Hepatocytes throughout the
liver lobule were markedly vacuolated (Fig. 3B). Although the material
accumulating in the vacuoles had the typical morphology of glycogen, it
did not stain PAS-positive in alcoholic formalin-fixed sections (Fig.
3D and E). Rather, the majority of this material stained positive for
fat with oil red O stain (Fig. 3F and G).
|
|
Liver-specific deletion of HNF4
alters serum lipid levels.
Consistent with the unusual fatty-liver phenotype observed in
histological sections of H4LivKO livers, total cholesterol, HDL
cholesterol, and triglyceride levels in sera from H4LivKO mice were
dramatically reduced relative to those in sera from controls (Table 1).
Conversely, serum bile acid concentrations were markedly elevated
(Table 1). These alterations in serum lipid profiles could be due to
either a generalized liver failure or to a more specific defect in
lipid transport and metabolism. Mild liver dysfunction was indicated by
a small elevation in the level of alanine aminotransferase in serum
from H4LivKO mice (Table 1). However, the levels of albumin,
nonesterified fatty acids, and glucose were indistinguishable from
those in controls, suggesting the retention of many liver functions
(Table 1). Thus, at this time point, disruption of HNF4
in the liver
appears to induce a specific failure of normal hepatic lipid metabolism
and/or transport rather than a generalized liver failure.
FPLC analysis of serum lipids.
To further characterize the
observed alterations in blood lipid composition, plasma from H4LivKO
and H4Flox mice was subjected to FPLC analysis. Compared to controls,
H4LivKO mice had significantly reduced plasma cholesterol (
61%) and
phospholipids (
53%) (Fig. 4).
Moreover, the elution profile of the lipids was altered (Fig. 4). Thus,
both plasma low-density lipoprotein (LDL) and HDL cholesterol were
dramatically decreased in H4LivKO mice compared to controls. Moreover,
HDL cholesterol from H4LivKO plasma eluted later than that from
controls and contained a significant amount of very-late-eluting, smaller HDL. This elution profile is indicative of production of
unusually small, lipid-poor HDL particles. Western blot analysis of
lipoprotein content in whole plasma from H4LivKO mice indicated that
these mice exhibit reduced ApoB100, ApoA-II and ApoC content, while
apolipoproteins A-I, E, and B48 are unaffected (Fig. 4, inset).
Strikingly, HDL cholesterol from H4LivKO mice is virtually devoid of
ApoA-II, with ApoA-I as the sole apolipoprotein component (Fig. 4,
inset).
|
HNF4
regulates hepatic genes involved in lipid metabolism and
transport.
The HNF4
-mediated transactivation of numerous genes
involved in lipid metabolism and transport has been demonstrated using cell culture systems. These include genes for ApoA-I (16,
34), ApoA-II (44), ApoA-IV (21), ApoB
(37), ApoC-II (19), ApoC-III (38), and ApoE (9); MCAD (3, 4);
microsomal triglyceride transfer protein (MTP) (15); and
cholesterol 7
-hydroxylase (CYP7A) (8, 53). To examine
whether the expression of these putative target genes was affected by
disruption of the hepatic HNF4
gene, liver RNA from 45-day-old
H4LivKO mice and controls was analyzed (Fig.
5). Strikingly, steady-state mRNA levels
for apolipoproteins A-II, A-IV, C-II, and C-III and MTP (Fig. 5A) and
CYP7A1 (Fig. 5E) were drastically reduced in H4LivKO livers compared to
those in controls. In contrast, steady-state mRNA levels for ApoA-I and
ApoE were relatively unaffected by disruption of the HNF4
gene (Fig.
5A), even though both of the genes are HNF4
responsive in transient
transfection assays (9, 16, 42). Paradoxically, while MCAD
is positively regulated by HNF4
in cultured cells (4),
it is induced by disruption of the HNF4
gene (Fig. 5D). To further
examine the influence of the deletion of hepatic HNF4
on lipid
metabolism, the expression of numerous other key genes involved in
lipid transport and metabolism was examined. Expression of the LDL
receptor and ATP-binding cassette 1, both of which are essential lipid
and cholesterol transporters, was unaffected, while expression of the
major HDL receptor, SR-BI, was induced (Fig. 5B). The expression levels
of several nuclear receptors implicated in the control of lipid
homeostasis, RXR
, pregnane-X receptor, farsenoid-X receptor (FXR),
oxysterol receptor-
, and liver receptor homologue 1, were unchanged
by deletion of HNF4
(Fig. 5C). Expression of the nuclear receptor
small heterodimer partner was variable (Fig. 5C). In contrast,
expression of peroxisome proliferator-activated receptor
(PPAR
)
was lower in H4LivKO livers than in controls (Fig. 5D). Given this
decrease in PPAR
, it is interesting to note that expression of some
PPAR
target genes (carnitoyl-palmitoyl transferase-II, MCAD, and
3-hydroxy-3-methylglutaryl CoA synthase genes) is enhanced in H4LivKO
livers relative to controls (Fig. 5D). The increased levels of serum
bile acids suggested that these mice might be deficient in one or more
pathways of bile acid trafficking. Consistent with this hypothesis,
levels of sodium taurocholate cotransporter protein (Ntcp), organic
anion transporter protein 1, liver fatty acid binding protein (L-FABP), and multidrug resistance protein 2 mRNA were markedly decreased in
H4LivKO mice relative to controls (Fig. 5E). In contrast, the steady-state level of bile salt export pump (BSEP) mRNA was mildly elevated (Fig. 5E). Finally, expression of the key lipogenic genes encoding fatty acid synthase, sterol receptor element binding protein
1c, and spot-14 genes was unaffected in H4LivKO livers (Fig. 5F). Thus,
consistent with the histological and serological alterations observed
in these mice, disruption of hepatic HNF4
results in altered
expression of genes involved in several pathways of lipid metabolism
and transport. Conversely, the unchanged expression of other hepatic
genes again argues that the observed dyslipidemia is due to disruption
in a specific pathway(s) and not due to a generalized liver failure.
|
Lethality associated with disruption of hepatic HNF4
.
To
assess the influence of HNF4
deletion in liver on mouse development,
mice were weighed at 10 days and 2 weeks of age and each week
thereafter. The H4LivKO mice had no observable phenotype until they
reached 5 weeks of age when they lost weight compared with the
wild-type, AlbCre transgenic, and H4Flox mice (not shown). This weight
loss coincided with the complete loss of HNF4
in the livers (between
4 and 6 weeks). To assess whether this weight loss was terminal, a
cohort of mice was allowed to develop without further interference. In
this group mortality reached >70% by 8 weeks of age. Thus, while we
are as yet unable to explain the observed mortality at the functional
level, this indicates that HNF4
activity is essential for the proper
functioning of the liver.
| |
DISCUSSION |
|---|
|
|
|---|
The presence of functional HNF4
response elements in the
promoters of numerous genes encoding metabolic enzymes suggests that
HNF4
fulfills an essential role in cellular metabolism. Moreover,
this nuclear receptor can respond to metabolic status through fatty
acyl-CoA thioesters and cyclic AMP-dependent phosphorylation, suggesting that it may integrate signals from different pathways to
fine tune the storage, import, and export of metabolic products (17, 57). Consistent with this hypothesis, the data
presented here demonstrate that HNF4
is indispensable for the
constitutive expression of several key hepatic genes involved in lipid
transport and metabolism. This leads to profound decreases in
cholesterol, HDL cholesterol, and triglycerides and a large increase in
bile acids in the serum of mice lacking hepatic HNF4
. These data
provide the first direct in vivo evidence for the role that HNF4
plays in controlling hepatic lipid metabolism and transport in the adult.
H4LivKO mice are deficient in expression of genes required for VLDL secretion. Serum lipid levels are controlled in large part by the balance between hepatic cholesterol synthesis, degradation, export, and uptake. In this regard, it is salient to note that H4LivKO mice display decreased expression of two genes essential for very-low-density lipoprotein (VLDL) secretion (ApoB and MTP). While expression of ApoB in the H4LivKO mice is variable, expression of MTP is consistently and strongly decreased. Studies with hepatocyte-specific MTP knockout mice (6, 43) and MTP/ApoB double-heterozygous null animals (31) indicate that the concentration of MTP within the endoplasmic reticulum is the major determinant of hepatic VLDL secretion. Thus, it seems likely that the accumulation of lipid in hepatocytes of H4LivKO mice is due, at least partially, to a defect in VLDL secretion.
H4LivKO mice display increased expression of SR-BI, required for
HDL uptake.
SR-BI appears to be the major mediator of selective
cholesterol uptake from HDL by hepatocytes and, in combination with the LDL receptor, may also participate in cholesterol uptake from non-HDL
cholesterol (20). SR-BI is subject to feedback regulation in response to changes in cellular cholesterol stores
(20). However, most work on the regulation of SR-BI
expression has been conducted with macrophages and steroidigenic
tissues, and little is known about control of hepatic expression.
Interestingly, expression of the macrophage homologue of SR-BI, CD36
(FAT/CLA-1), is induced by PPAR
and -
ligands (7).
Given the increases in PPAR
target gene expression observed here and
the fact that HNF4
and PPAR
compete for binding to a number of
promoters (42, 45, 64), one may speculate that PPAR
also regulates hepatic SR-BI expression. Experiments are under way to
determine whether SR-BI is a direct target of HNF4
or whether the
observed increase in expression is a secondary consequence of altered
lipid homeostasis (e.g., due to changes in intracellular lipid levels).
What is clear, however, is that expression of SR-BI is markedly
increased in H4LivKO mice. Thus, it is likely that selective
cholesterol uptake is increased in H4LivKO livers.
appears to have opposite effects on
the expression of genes involved in VLDL secretion (down-regulation) and HDL cholesterol uptake (up-regulation), it is tempting to speculate
that the overall effect of HNF4
may be to promote the net efflux of
cholesterol from the liver. Direct measurements of VLDL secretion and
HDL uptake in H4LivKO mice to definitively address these questions are
under way.
H4LivKO mice display gene expression patterns indicative of
defective bile acid homeostasis.
Expression of CYP7A1, which
catalyzes the first, rate-limiting step in the neutral pathway of bile
acid synthesis, is reduced in H4LivKO livers relative to controls.
Thus, the in vivo data presented here are in close agreement with in
vitro studies implicating HNF4
in the transcriptional regulation of
CYP7A1 (8, 53). Similarly, in the absence of hepatic
HNF4
, expression levels of both Ntcp and organic anion transporter
protein 1 are markedly down-regulated. These proteins are the major
mediators of hepatic basolateral bile acid uptake via sodium-dependent
and -independent pathways, respectively (35). Consistent
with the increased serum bile acid levels observed, it is likely that
H4LivKO mice are unable to efficiently take up bile acids from the
blood. Conversely, BSEP, the major canilicular bile acid export protein
(12), is expressed at moderately elevated levels in
H4LivKO mice. This suggests that clearance of bile acids from the liver
into the gall bladder may be increased in H4LivKO mice. BSEP expression is induced by bile acids via the nuclear receptor FXR, while Ntcp expression is repressed by the same pathway (46). It is
possible, therefore, that the altered expression of BSEP and Ntcp seen
here may be due to elevated liver bile acid concentrations. However, several lines of evidence suggest that this is not the case. Thus, expression of oxysterol receptor-
, which is also induced by
increased intracellular bile acid concentrations via FXR
(46), is unchanged in H4LivKO livers relative to that in
controls. Moreover, H4LivKO mice show none of the severe hepatotoxicity
characteristic of elevated bile acid concentrations. Taken together,
these observations suggest that the alterations in expression of bile
acid transporters cannot be fully explained by indirect mechanisms and
suggest that some of these genes may be novel targets for HNF4
.
Decreased expression of PPAR
but increased expression of PPAR
target genes in H4LivKO mice.
PPAR
is consistently
down-regulated in H4LivKO mouse liver. Although no measurements of
glucocorticoids were performed, PPAR
is known to be positively
regulated by corticosterone (29, 30). Given that
cholesterol is an essential precursor for the formation of
glucocorticoids, it seems likely that the observed down-regulation is
due to alterations in circulating hormone levels. Despite the decreased
steady-state levels of PPAR
mRNA, many prototypical PPAR
target
genes are induced in the H4LivKO mouse. Therefore, PPAR
may not be
limiting for target gene induction, and increases in the intracellular
concentration of a PPAR
agonist(s) such as free fatty acids may
account for this effect.
H4LivKO mice show an atheroprotective serum lipid profile.
It
is interesting to note that the overall effect of disruption of hepatic
HNF4
activity is to reduce circulating lipid levels. This is
consistent with the proposal that polyunsaturated fatty acids and
saturated (C18:O) fatty acids act as antagonists of HNF4
, as diets rich in these fats also decrease blood lipid levels (13, 17). Similarly, dietary fats that increase lipid
levels in blood were shown to act, via their acyl-CoA thiosters, as
agonists for HNF4
(17). Thus, the data presented here
support the proposal that the influence of dietary composition on
circulating lipid levels may be mediated, at least in part, by HNF4
.
By inference, this suggests that HNF4
may be important in the
etiology of dyslipidemias.
-null mice. The widely prescribed hypolipidemic fibrates actually decrease the ratio of LpA-I to LpA-I/A-II (36, 59). Thus, HNF4
antagonists may offer
significant advantages over fibrates in the treatment of
hyperlipidemias. Moreover, the observed decreases in ApoA-II, ApoA-IV,
ApoC-III, and MTP found in the H4LivKO mice are consistent with the
lower lipid levels in serum from the Apo-null (33, 62, 63)
and MTP liver conditional-null (43) mice, all of which all
display, to different degrees, decreased serum cholesterol levels.
HNF4
is essential for constitutive expression of genes with
complex promoters.
The data presented here demonstrate that
HNF4
is indispensable for the expression of Apo-AII, Apo-CII,
Apo-CIII, L-FABP, and MTP. Interestingly, transcription factors in
addition to HNF4
have been shown by transient transfection studies
to play a role in regulating the corresponding promoters (19, 21,
24, 25, 38, 42, 44, 56, 58). For example, the ApoC-II promoter is activated by RXR
/T3R
and this increase is accentuated by the
synergistic action of HNF4
and ARP-1 (19). Thus, while it remains possible that other transcription factors may contribute to
the regulation of the genes under normal physiological conditions, HNF4
is absolutely required for their constitutive expression in the
fully differentiated mouse liver. The dominant influence of HNF4
on
the expression of some genes with complex promoters is consistent with
the proposal that HNF4
-recruited chromatin-remodeling activity may
be a prerequisite for the binding of other transcription factors
(32). Alternatively, the dominant effect of HNF4
may be
explained by the presence of a transcription factor cascade, whereby
HNF4
controls the expression of other factors required for
expression of a given gene. Indeed, HNF1
, a direct transcriptional target of HNF4
(23), has recently been shown to control
the expression of L-FABP (1).
Dichotomy of HNF4
function in adult versus fetal
hepatocytes?
The gene expression patterns reported here are
broadly similar to those described recently for HNF4
-null livers
produced by tetraploid rescue (32). However, significant
differences are apparent. Disruption of HNF4
in the adult hepatocyte
virtually abolishes ApoA-IV expression but does not significantly
influence ApoA-I, while this pattern is reversed in the developing
hepatocyte. Similarly, expression of pregnane-X receptor, which is
abolished in fetal HNF4
-null livers (32), is unaffected
by disruption of HNF4
in the fully differentiated liver. Thus,
HNF4
appears to function differently in the maintenance of hepatic
differentiation than in the establishment of the differentiated state.
A similar disparity in the function of HNF3
in the developing liver
and the fully differentiated hepatocyte has recently been published (54). It seems likely that more examples of this
functional dichotomy may become apparent as conditional gene targeting
is applied to examine the role of transcription factors in adult tissues.
Decreased hepatic HNF4
activity in MODY1 patients may explain
decreases in serum triglyceride levels.
While the conditional
HNF4
-null mouse, not surprisingly, does not resemble MODY1 subjects
in terms of hyperglycemia, the evidence presented here suggests that
the altered triglyceride levels observed in MODY1 subjects are a direct
consequence of impaired hepatic HNF4
function. The demonstration
that hepatic HNF4
deficiency in the mouse results in lower
triglyceride levels in the serum suggests the possibility that
triglyceride levels may be an early diagnostic test for the onset of
diabetes in humans. It is still not clear whether diabetes associated
with HNF4
haploinsufficiency in humans is due to decreased HNF4
activity in the
cell itself, particularly as this factor is
expressed at very low levels in adult pancreas (39).
Alternatively, perturbation of HNF4
activity in another organ such
as the liver may lead to the exposure of the pancreas to a metabolite
that gradually disrupts the function of the
cell. The phenotype of
the conditional hepatic HNF4
-null mouse provides evidence that, at
least in mice, lack of HNF4
activity in the liver does not produce
the characteristic hyperglycemia of diabetes mellitus. To directly test
the function of HNF4
in pancreatic
cells, experiments using the
H4Flox line crossed with a
-cell Cre-expressing transgenic line are
under way (22).
gene was developed. Disruption of the HNF4
gene specifically
in hepatocytes indicates that HNF4
is central to the maintenance of
hepatic function and is a major in vivo regulator of genes involved in
the control of lipid homeostasis. Further analysis of this model should
aid in the elucidation of the role that HNF4
plays in the
transcriptional control of liver function. The conditionally null
allele described here should be useful in dissecting the role that
HNF4
plays in gene regulation in other tissues and in the etiology
of MODY1.
| |
ACKNOWLEDGMENTS |
|---|
We thank Roberta Smith, Keith Rogers, and Kunio Nagashina for assistance with the liver pathology and Derek LeRoith for providing the AlbCre mice. We also thank Johan Auwerx and Shioko Kimura for review of the manuscript and Chris Sinal and Taro Akiyama for useful discussions.
| |
FOOTNOTES |
|---|
* Corresponding author. Mailing address: Building 37, Room 3E-24, National Institutes of Health, Bethesda, MD 20892. Phone: (301) 496-9067. Fax: (301) 496-8419. E-mail: fjgonz{at}helix.nih.gov.
Present address: Institute of Molecular Biology, Academia Sinica,
Taipei 115, Taiwan.
| |
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