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J Biol Chem, Vol. 273, Issue 47, 30843-30846, November 20, 1998
From The Johns Hopkins University, Baltimore, Maryland 21205
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ABSTRACT |
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The expressions and activities of several
CCAAT/enhancer-binding proteins (C/EBP) isoforms fluctuate in the
regenerating liver. The physiological implications of these variations
in C/EBP function remain poorly characterized in the setting of
regeneration. However, lessons learned in various hepatocyte cell lines
and by studying primary hepatocytes from transgenic C/EBP Liver regeneration is a complex physiological response to liver
injury during which the surviving, mature hepatocytes and liver
nonparenchymal cells proliferate to reconstitute the pre-morbid mass of
the damaged organ. The liver is one of the few tissues in adult
vertebrates that retains the capacity for regeneration. However, such
compensatory hyperplasia requires careful orchestration to assure that
vital liver-specific functions are preserved while hepatocytes, which
generally do not proliferate in adult animals, replicate. The study of
organisms that are as divergent as urodeles and humans has identified
common, fundamental processes that are required for the regeneration of
any tissue. This work indicates that regeneration occurs only if
injury-related changes in the local extracellular environment motivate
differentiated, but proliferation-competent, cells to reenter the cell
cycle while the normal homeostatic mechanisms that couple cell cycle
reentry to cell death are suspended (1-3). Although much has been
learned about the mechanisms that regulate hepatic regeneration, many
aspects of the process remain poorly understood.
CCAAT/enhancer-binding proteins
(C/EBPs)1 are known to play
important roles in regulating the expression of multiple
hepatocyte-specific genes (4). These transcription factors also help to
control hepatocyte progression through the cell cycle (5, 6). Thus, the
C/EBPs are likely to be important targets for regulation during liver
regeneration. Consistent with this concept, variations in the
expression of C/EBP mRNAs, proteins, and DNA binding activities have been documented during liver regeneration. C/EBP The importance of these reciprocal variations in C/EBP There are several mechanisms by which the C/EBPs may regulate
hepatocyte proliferation. C/EBP In addition to promoting hepatocyte proliferation, changes in the
relative expression of different C/EBP isoforms may affect global
changes in hepatocyte gene expression that permit these cells to
survive the "stress" of regeneration. For example, C/EBP Although liver regeneration imposes a tremendous drain on hepatocyte
ATP stores, hepatocyte necrosis is not normally increased by PH.
Changes in the expression of phosphoenolpyruvate carboxykinase, glycogen synthase, and many other genes that regulate hepatic metabolism occur quickly after PH (45-47). C/EBP Successful regeneration of the liver requires much more than the
proliferation of hepatocytes. Cells that form supporting structures,
such as bile ducts and blood vessels, must also replicate. In addition,
new matrix must be deposited to provide a scaffolding for the
"rebuilding" effort. Furthermore, evidence that hepatocyte proliferation is driven largely by factors that are produced within the
regenerating liver implies that de novo synthesis of
autocrine and paracrine growth regulatory factors must follow liver
injury (2). The importance of the C/EBPs in regulating these aspects of
the hepatic regenerative response has barely been evaluated.
Matrix Production
Workers in the field of hepatic fibrogenesis have long recognized
that inflammation initiates fibrosis. The molecular basis for this
observation is being discovered. Injury-related cytokines, such as
TNF Production of Paracrine and Autocrine Growth Regulators
Hepatocyte Growth Factor (HGF)--
Stellate cells are also the
principal source of the hepatocyte mitogen, HGF, in the regenerating
liver (2). The regulatory regions of the HGF gene contain C/EBP
consensus motifs (52), and recently, C/EBP TNF C/EBP function is regulated at many levels, consistent with the
growing body of evidence which suggests that these factors regulate
critical events that are involved in cellular proliferation, differentiation, and survival. Several hormones (e.g.
insulin, glucagon, epinephrine, norepinephrine, and corticosteroids)
that increase in the blood after PH (2) are known to regulate either the expression or activity of various C/EBP isoforms. For example, insulin has been shown to alter the turnover of the 40-42 kDa C/EBP Several hepatocyte mitogens that regulate the hepatocyte proliferative
response during regeneration (including epidermal growth factor,
transforming growth factor Thus, many factors that regulate the proliferation and differentiated
functions of hepatocytes during the regenerative response are also
known to effect the biological activity of the C/EBPs. Given this and
the fact that the C/EBPs regulate the proliferation and differentiation
of many types of cells, these transcription factors are likely to be
important downstream targets for growth factor-initiated signals that
drive the regeneration of the liver after it has been injured (Fig.
1). Definitive proof of this concept as
well as progress in delineating the relative importance of various
C/EBP family members during liver regeneration is now possible by
studying the regenerative response to PH in transgenic mice strains
with targeted deletion(s) of specific C/EBP genes.
-deficient
mice suggest that the C/EBP isoforms are likely to influence
proliferation, differentiated gene expression, and survival in mature,
adult hepatocytes. In addition, these factors are potentially important modulators of liver nonparenchymal cell genes, including those that
encode matrix molecules and growth factors that are required for
successful liver regeneration. The possibility that members of the
C/EBP family of transcription factors actively participate in many
aspects of the regenerative response to liver injury is strengthened by
growing evidence that many hepatocyte mitogens and co-mitogens regulate
C/EBP activity. Furthermore, the C/EBPs themselves appear to regulate
the expression of some of these growth regulators.
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INTRODUCTION
Top
Abstract
Introduction
References
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CCAAT/Enhancer-binding Proteins and Hepatocyte
Proliferation
is the predominant C/EBP isoform that is expressed by adult hepatocytes in
healthy livers. During the initial 24 h after 70% (partial) hepatectomy (PH), levels of C/EBP
mRNA and protein decline
transiently. This is associated with decreased binding activity of
C/EBP
homodimers in gel mobility shift assays. Decreases in C/EBP
are preceded by the induction of other C/EBP isoforms. The level of
C/EBP
and C/EBP
mRNAs, proteins, and DNA binding activities
increases in the early pre-replicative period following PH and returns
to base line as hepatocytes enter S phase (7-10).
and other
C/EBP isoforms in regulating hepatocyte proliferation during liver
regeneration has been debated. Present controversy stems from an
apparently poor correlation between the induction of C/EBP
DNA
binding activity and hepatocyte proliferation in transgenic mice
strains with disruption of certain cytokines or cytokine receptors.
Mice homozygous for a deletion in the gene for the type 1 TNF receptor
have extremely limited hepatocyte proliferation after PH and yet
exhibit apparently normal induction of total C/EBP DNA binding activity
during the early pre-replicative period (11). In contrast, transgenic
mice lacking the gene for type 2 TNF receptors demonstrate normal
hepatocyte proliferation after PH but decreased induction of C/EBP
DNA binding activity (12). Furthermore, liver regeneration is severely
inhibited in IL-6-null mice, although these animals exhibit normal
induction of C/EBP
mRNAs after PH (13). However, because C/EBP
function is regulated, to a large extent, by post-transcriptional
modifications of the C/EBPs themselves, as well as by their
interactions with specific dimerization partners (14-21), the former
observations do not preclude an important role for the C/EBPs as
regulators of hepatocyte proliferation during liver regeneration.
Indeed, other evidence supports the biological importance of the C/EBPs
as physiologically relevant regulators of hepatocyte proliferation.
Transgenic mice with a deleted C/EBP
gene have increased hepatocyte
proliferation at birth (22, 23). Because these animals do not survive
long enough to permit evaluation of their response to PH, whether or not C/EBP
deletion enhances hepatocyte proliferation during liver regeneration has not been tested. However, hepatocytes isolated from
these mice do have increased proliferative activity in culture (24).
Conversely, enforced overexpression of C/EBP
has been shown to
inhibit proliferation in several different hepatocyte cell lines (5,
25, 26). Thus, it seems reasonable to conclude that changes in the
relative amounts (or activities) of C/EBP
and other C/EBP isoforms
influence the proliferative activity of hepatocytes during liver
regeneration. However, because of the complexity of the C/EBP family of
transcription factors, the multiple extracellular signals that regulate
C/EBP activities, and the fact that hepatocytes co-express several
different C/EBP isoforms, the role of each C/EBP family member in the
regulation of hepatocyte proliferation during liver regeneration
remains uncertain. Now that transgenic mice have been developed with
targeted disruption of different C/EBP genes, it will be possible to
use these mice to delineate the relative importance of different C/EBP isoforms as regulators of the hepatocyte proliferative response in vivo. Indeed, Greenbaum and colleagues have reported
preliminary evidence that C/EBP
knock-out mice exhibit impaired
hepatocyte proliferation and decreased liver regeneration after
PH.2
is known to stabilize p21/WAF, a
protein that inhibits transition from the pre-replicative
(G1) period into S phase (5). Because induction of C/EBP
and C/EBP
during the early pre-replicative period may inhibit
subsequent C/EBP
activity by repressing C/EBP
gene expression,
reciprocal variations in C/EBP isoforms could release hepatocytes from
C/EBP
-mediated cell cycle arrest. There is also some evidence that
C/EBP
may positively regulate the expression of genes that promote
cell cycle progression. For example, C/EBP consensus sequences have been identified in the regulatory regions of cyclin D2 (27) and cyclin
A (28) and may also interact with the retinoblastoma protein (RB)
(29).
and
-
are important in modulating the expression of various acute phase
response genes, including those such as inducible nitric oxide synthase
(iNOS) (30-32), that are thought to protect hepatocytes from
cytokine-mediated toxicity (33-37). The activity of iNOS and production of nitric oxide (NO) increase in hepatocytes in the mid-late
pre-replicative period following PH (38-40). NO has diverse biological
actions and has recently been shown to inhibit caspase 3 activation by
TNF
(33, 41). Given the pivotal role that caspase 3 plays in
apoptosis (42-44), C/EBP
-mediated induction of NO production may be
one of the mechanisms that prevents injury-related cytokines from
causing hepatocyte apoptosis in the regenerating liver. Preliminary
experiments in iNOS-deficient transgenic mice support this
view.3
and -
are
important transcriptional regulators of many of the genes that are
involved in metabolism (48-50). As such, these transcription factors
modify hepatocyte substrate utilization to assure energy homeostasis during the period of intense proliferative activity. Because failure to
meet the increased ATP requirements during regenerative growth could
lead to hepatocyte ATP depletion and necrosis, successful regulation of
C/EBP activity may be important for thwarting hepatocyte necrosis
during the regenerative response to injury.
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C/EBPs and Liver Nonparenchymal Cells
and IL-6, and oxidant stress are known to induce several
transcription factors, including C/EBP
, that activate transcription
of the type I collagen gene in stellate cells (2). C/EBP sites have
also been identified in the promoters of metalloproteinases (51). Thus,
the C/EBPs are likely to regulate matrix production and remodeling by
hepatic nonparenchymal cells during liver regeneration.
was shown to
trans-activate HGF gene expression. Furthermore, recombinant
TNF
induces a dose-dependent increase in IL-6 expression
in primary stellate cell cultures. This is followed by increased
C/EBP
DNA binding activity and induction of HGF mRNA
expression.4 These
observations suggest that the C/EBPs play a role in regulating the
production of paracrine growth factors in the regenerating liver.
Definitive proof of this concept awaits analysis of HGF gene induction
in the liver remnants of C/EBP
knock-out mice that have been
subjected to PH.
and TNF-regulated Cytokines--
Injury-related cytokines,
particularly TNF
and IL-6, are now widely acknowledged to play
pivotal parts in the initiation of hepatocyte proliferation after PH
(11, 13, 54, 55), although the relative importance and precise roles of
these factors during liver regeneration remain a topic of considerable
debate. Binding sites for C/EBP
have been identified in the
promoters of a large number of cytokine genes, including TNF
, IL-1,
-6, and -8, and granulocyte colony-stimulating factor (56-59).
Transfection studies with TNF
reporter gene constructs have
demonstrated that C/EBP
is much more active than C/EBP
as an
inducer of TNF gene transcription. Furthermore, C/EBP
and -
are
the predominate C/EBP isoforms in activated macrophages, which produce
large amounts of TNF and TNF-inducible cytokines. Consistent with the
concept that C/EBP
is important for TNF induction,
inflammation-related increases in serum TNF
concentrations are
blunted in C/EBP
-deficient transgenic mice. This observation
suggests that decreased production of TNF
may contribute to the
impaired liver regeneration that Greenbaum's group has noted in
C/EBP
knock-out mice. Decreases in TNF
, in turn, would be
expected to influence the relative abundance of C/EBP family members in
the nuclei of hepatocytes and other TNF target cells because TNF or
peptide ligands that specifically activate type 1 or type 2 TNF
receptors induce an almost instantaneous redistribution of preformed
C/EBP
and C/EBP
from the cytosol into the nucleus (18). Although
C/EBP
was initially proposed as a potential mediator of the
TNF-dependent induction of IL-6, which occurs after PH (11,
54), studies using mice homozygous for a deletion in the gene for
C/EBP
demonstrate that C/EBP
is not essential for IL-6 induction
because these mice actually develop increased circulating levels of
IL-6 as they age (60). Given this, it is unlikely that decreased
induction of IL-6 contributes to the impaired liver regeneration that
Greenbaum's group has noted in the C/EBP
knock-out mice. Rather,
these observations demonstrate that organisms have developed many
mechanisms to preserve cytokine induction when any given
cytokine-induced transcription factor becomes deficient. The latter may
help to explain why IL-6 induction is not entirely abolished in
transgenic TNF receptor type 1-null mice (11) or in rats that were
pretreated with anti-TNF antibodies before PH (54). Similarly, because
IL-6 induces C/EBP
and C/EBP
regulates the transcription of
multiple cytokines, including TNF
, it is difficult to assure that
IL-6 is the only cytokine that is deficient in transgenic mice in which
the IL-6 gene has been disrupted experimentally.
![]()
Regulation of C/EBP Activity during Liver Regeneration
proteins, accelerating their degradation (61). Cyclic AMP is known to
increase transcription of C/EBP
and -
(61-63). Hence, glucagon
and epinephrine, which increase adenylyl cyclase activity and promote
the accumulation of hepatic cAMP in the early pre-replicative period,
are likely to play a role in the induction of these C/EBP isoforms
after PH. The cAMP-dependent kinase, protein kinase A, has
also been shown to influence the function of C/EBP
protein by
phosphorylating Ser299 (21). Norepinephrine and other
-adrenergic agents that promote increases in intracellular calcium
and activate calmodulin-sensitive kinases in the regenerating liver may
also regulate C/EBP
because there is evidence that Cam kinase II
phosphorylation of C/EBP
alters its DNA binding activity (20).
Regenerative increases in circulating corticosteroids may contribute to
increase the expression of C/EBP
mRNAs that have been observed
in the regenerating liver (63).
, and HGF) may also influence C/EBP
function. These growth factors activate receptor tyrosine kinases that
couple to mitogen-activated kinases (64), which are known to
phosphorylate C/EBP
(65). Injury-related cytokines are also likely
to play important roles in regulating C/EBP function in the
regenerating liver. For example, IL-6 and IL-1
are well established
inducers of C/EBP
and C/EBP
mRNA expression, respectively (53), whereas TNF
has been shown to increase the DNA binding activity of C/EBP
and C/EBP
by affecting the nuclear localization of these proteins (18). The latter appears to have physiological relevance for liver regeneration because pretreatment with anti-TNF antibodies inhibits the nuclear accumulation of these C/EBP isoforms and decreases their participation in DNA complex formation after PH
(46).

View larger version (30K):
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Fig. 1.
After PH, there is transient inhibition of
C/EBP
activity and induction of C/EBP
and -
activities.
These reciprocal variations in C/EBP family members modulate gene
expression in different liver cell populations. Some of the phenotypic
consequences in hepatocytes and liver nonparenchymal cells
(e.g. macrophages, endothelial cells, stellate cells, and
cholangiocytes) are indicated.
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FOOTNOTES |
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* This minireview will be reprinted in the 1998 Minireview Compendium, which will be available in December, 1998. This is the fourth article of five in the "Biological Role of the Isoforms of C/EBP Minireview Series."
To whom correspondence should be addressed: 912 Ross Bldg., The
Johns Hopkins University, 720 Rutland St., Baltimore, MD 21205. Tel.:
410-955-7316; Fax: 410-955-9677; E-mail: amdiehl{at}welchlink.welch.jhu.edu.
The abbreviations used are: C/EBP, CCAAT/enhancer-binding protein; PH, 70% (partial) hepatectomy; TNF, tumor necrosis factor; IL, interleukin; iNOS, inducible nitric oxide synthase; NO, nitric oxide; HGF, hepatocyte growth factor.
2 L. Greenbaum, FASEB Conference, Snomass, CO, July, 1998.
3 A. M. Diehl and R. Rai, unpublished data.
4 R. Rai, F. Anania, S. Q. Yang, H. Z. Lin, J. J. Potter, and A. M. Diehl, submitted for publication.
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