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Oncogenic Virus Unit, CNRS URA 1644, Biotechnology Department, Pasteur Institute, Paris, France.
Correspondence to Dr. Marco Pontoglio, Unité des Virus Oncogènes, CNRS URA 1644, Département des Biotechnologies, Institut Pasteur, 25, rue du Dr Roux, 75724 Paris Cedex 15, France. Phone: 33 1 456 88514; Fax: 33 1 406 13033; E-mail: marcop{at}pasteur.fr
| Abstract |
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-fibrinogen, and
1-antitrypsin. HNF1 is expressed in the
liver, digestive tract, pancreas, and kidney. Mice lacking HNF1 exhibit
hepatic, pancreatic, and renal dysfunctions. HNF1-deficient mice fail to
express the hepatic phenylalanine hydroxylase gene, giving rise to
hyperphenylalaninemia. Renal proximal tubular reabsorption of glucose,
phosphate, arginine, and other metabolites is affected, producing severe renal
glucosuria, phosphaturia, and amino aciduria. Homozygous mutant mice also
exhibit a dramatic insulin secretion defect. This dysfunction resembles that
exhibited by patients with maturity-onset diabetes mellitus of the young type
3, who carry mutations in the human HNF1 gene in the heterozygous state. These
data show that HNF1 is a major regulator of glucose homeostasis, regulating
the expression of genes that are expressed in the liver, kidney, and
pancreas. | Introduction |
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and
HNF1
or LF-B1 and LB-B3, respectively)
(1,2,3,4,5),
HNF3
, -
, and -
(6), HNF4
(7), and HNF6
(8). HNF1 was first identified
as a DNA-binding activity capable of interacting with the promoter of several
hepatic genes, including albumin,
1-antitrypsin, and
-fibrinogen.
HNF1 is a dimeric protein functionally composed of three domains,
i.e., an amino-terminal dimerization domain composed of 33 amino
acids, a DNA-binding domain belonging to the homeobox family, and a
carboxyl-terminal domain that is essential for transactivation of target
promoters.
Among vertebrates, the closely related protein vHNF1 has also been
characterized
(4,5).
HNF1 and vHNF1 share strong homologies in both the dimerization domain and the
DNA-binding domain. These homologies enable the two proteins to form
heterodimers and bind to the same DNA sequences. However, HNF1 seems to be a
more potent transactivator than is vHNF1 in transient transfection assays.
Another partner for HNF1, termed the dimerization cofactor for HNF1, has also
been cloned (9). This protein
binds to the amino-terminal domain of both HNF1 and vHNF1, stabilizing the
homo- and heterodimers and somewhat increasing their transactivation
potential. Surprisingly, the dimerization cofactor for HNF1 has also been
shown to be involved in the enzymatic dehydration of
pterin-4
-carbinolamine to dihydrobiopterin, a cofactor involved in the
enzymatic hydroxylation of phenylalanine and tryptophan
(10,11).
Studies have shown that HNF1 and vHNF1 are expressed in polarized epithelia of different organs, including the liver, digestive tract, pancreas, and kidney (12,13,14,15). The expression of vHNF1 overlaps with that of HNF1 with the exception of lung, where only vHNF1 is expressed. Conversely, vHNF1 is very weakly expressed in liver, where HNF1 constitutes >95% of the total HNF1-like protein. In other organs, HNF1 and vHNF1 are more or less equally expressed. In the kidney, HNF1 expression is confined to proximal tubules. In fact, immunofluorescence studies show that HNF1-positive nuclei are observed only in tubules that are also positive for villin, a marker of proximal tubules (Figure 1). In contrast, vHNF1 is also expressed in distal tubules (12,13,14,15).
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A significant difference between HNF1 and vHNF1 is the onset of their expression during embryonic development. vHNF1 is expressed in the extraembryonic visceral endoderm, in the neural tube, and in the primitive gut at a developmental stage when HNF1 expression has not yet been turned on (16,17,18). In fact, HNF1 expression is activated only during organogenesis. Again, during liver and renal development, vHNF1 is expressed from the first stages of organogenesis, whereas HNF1 is turned on later, when differentiation is more advanced. For example, during kidney development, vHNF1 is already activated in the swelling ureteric bud and in the surrounding metanephric mesenchyme, whereas HNF1 becomes detectable during formation of the S-shaped structures (19). The differences between HNF1 and vHNF1 in their expression patterns and timing during development are also reflected in the outcomes of gene knockout experiments in mice. Although vHNF1-/- mice embryos die at day 6.5 to 7.0 after conception, because of a defect in extraembryonic visceral endoderm differentiation (17,18), disruption of the mouse HNF1 gene leads to a complex set of postnatal dysfunctions that affect most of the organs in which HNF1 is expressed (13). HNF1-/- mice do not die as embryos but exhibit hepatic, pancreatic, and renal dysfunctions that significantly affect the growth and life spans of the mutant animals.
| Liver Dysfunction |
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1-antitrypsin, and
-fibrinogen, exhibited only moderate decreases
(two- to fourfold) in their transcription rates. The corollary of these
observations is that the lack of HNF1 results in selective and drastic
inactivation of only a small subset of hepatic genes. The PAH gene, which is
normally responsive to glucocorticoids and cAMP, cannot be induced by any
hormonal treatment in the liver of HNF1-deficient animals
(21). This lack of
inducibility is probably attributable to the fact that the PAH gene in the
hepatocytes of HNF1-deficient mice is characterized by an inactive chromatin
structure in which several nuclease-hypersensitive sites, located in the
transcription control regions of the PAH gene, have disappeared. These
nuclease-sensitive sites contain several HNF1-binding sites and should play an
important role in the transcriptional activation of the PAH gene. In addition
to the poorly accessible chromatin status, PAH transcription control regions
are characterized by hypermethylation of CpG residues in the DNA sequence.
Therefore, the lack of HNF1 prevents remodeling and demethylation of the PAH
locus. In this context, HNF1 seems to play an essential role by inducing
chromatin remodeling events that render the transcription control region of
the PAH gene accessible to the transcription machinery. It will be important
to elucidate the mechanisms involved in the HNF1-dependent demethylation and
chromatin remodeling processes. In the kidney, where relatively high levels of vHNF1 persist in HNF1-/- mice, the normal low expression level of PAH is not affected. This suggests that vHNF1 might have taken over the role of HNF1 for this function in the kidney.
| Kidney Dysfunction |
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Phlorizine, which is a glucoside capable of inhibiting glucose reabsorption, is able to bind irreversibly to glucose cotransporters. The use of this inhibitor has demonstrated that brush border membranes of HNF1-/- animals exhibit large decreases in the number of phlorizine-binding sites (13). Glucose reabsorption is accomplished by two distinct cotransporters, termed sodium glucose transporter type 1 (SGLT1) and SGLT2 (22,23,24,25). SGLT2 is preferentially expressed in the initial portion (S1 and S2) of proximal tubules. There the concentration of glucose is still relatively high and SGLT2 is responsible for so-called high-capacity/low-affinity glucose transport. In the more distal part of the proximal tubule (S3), the glucose concentration is decreased and SGLT1, which is preferentially expressed in this segment, performs low-capacity/high-affinity transport. SGLT1 and SGLT2 have different glucose/Na+ stoichiometries (1:2 and 1:1 for SGLT1 and SGLT2, respectively) (20). This enables SGLT1 to reabsorb glucose against the much stronger glucose gradient that is characteristic of the more distal part of proximal tubules.
Northern blot analysis of renal mRNA has demonstrated that SGLT1 is normally expressed, whereas transcription of the SGLT2 gene is severely affected, in the kidneys of HNF1-deficient animals (26). In addition, isolated tubules are characterized by a reduced capacity to reabsorb glucose, phosphate, and arginine but not alanine, an amino acid that is reabsorbed normally in vivo in HNF1-/- animals (unpublished results).
| Pancreas Dysfunction |
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-cells have a major defect in glucose-sensing. In addition, metabolic
studies have demonstrated that the flux of glucose through glycolysis in
islets from mutant mice is reduced, producing blunted increases in ATP levels
in response to glucose exposure
(31). HNF1-deficient mice exhibit defective insulin secretion as well as a major defect in renal glucose reabsorption. The combination of these two dysfunctions establishes a novel dynamic equilibrium for glucose homeostasis. Renal glucosuria at least partially compensates for the defective insulin secretion.
| MODY3 Mutations |
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| Conclusion |
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-cell glucose-sensing, and renal proximal tubular
reabsorption of glucose and several other metabolites. For these reasons, HNF1
can be considered a transcription factor at the crossroads of the regulation
of glucose homeostasis. | References |
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-carbinolamine dehydratase, a component of
the phenylalanine hydroxylation system, and DCoH, a transregulator of
homeodomain proteins. Proc Natl Acad Sci USA89
: 11891-11894,1992
-carbinolamine dehydratase from rat and human liver:
Purification, characterization, and complete amino acid sequence. J
Biol Chem 268:4828
-4831, 1993
homeoprotein gene during mouse organogenesis. Mech
Dev 89: 211-213,1999[Medline]
controls renal glucose reabsorption in mouse and man. Embo
Rep 2000, in press
gene in maturity-onset diabetes of the young (MODY3).
Nature (Lond) 384:455
-458, 1996[Medline]
-cell glycolytic signaling in hepatocyte nuclear factor-1
-deficient mice. J Biol Chem273
: 24457-24464,1998This article has been cited by other articles:
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