Liver Regeneration
Nelson Fausto,1 Jean S. Campbell,1 and Kimberly J. Riehle1,2
During liver regeneration after partial hepatectomy, normally quiescent hepatocytes undergoone or two rounds of replication to restore the liver mass by a process of compensatoryhyperplasia. A large number of genes are involved in liver regeneration, but the essentialcircuitry required for the process may be categorized into three networks: cytokine, growthfactor and metabolic. There is much redundancy within each network, and intricate interactionsexist between them. Thus, loss of function from a single gene rarely leads to completeblockage of liver regeneration. The innate immune system plays an important role in theinitiation of liver regeneration after partial hepatectomy, and new cytokines and receptorsthat participate in initiation mechanisms have been identified. Hepatocytes primed by theseagents readily respond to growth factors and enter the cell cycle. Presumably, the increasedmetabolic demands placed on hepatocytes of the regenerating liver are linked to the machineryneeded for hepatocyte replication, and may function as a sensor that calibrates theregenerative response according to body demands. In contrast to the regenerative processafter partial hepatectomy, which is driven by the replication of existing hepatocytes, liver
repopulation after acute liver failure depends on the differentiation of progenitor cells. Suchcells are also present in chronic liver diseases, but their contribution to the production ofhepatocytes in those conditions is unknown. Most of the new knowledge about the molecularand cellular mechanisms of liver regeneration is both conceptually important anddirectly relevant to clinical problems. (HEPATOLOGY 2006;43:S45-S53.)
The evolution of ideas pertaining to the mechanismsof liver regeneration may be categorizedinto three phases: (1) the original view that a singlehumoral agent could function as a key, capable of
unlocking all of the events required for liver regeneration;
(2) the idea that the activation of one pathway involvingmultiple components could be responsible for regeneration;and (3) the more recent idea that the activity ofmultiple pathways is required for liver regeneration.1-3
This last formulation is still an oversimplification of amore complex reality, as liver regeneration does require
the activation of multiple pathways, but these pathways
do not act independently of each other. The patterns of
interaction between pathways are particularly complex
because they may involve simultaneous and/or sequential
modes of operation, may occur in different liver cell types,
and may be present only at certain stages of liver regeneration.
The recent literature is replete with data showing that
the ablation of genes involved in different pathways can
inhibit liver regeneration, leading to the notion that this
process requires the activation of dozens of different pathways.
An alternative view, which we propose, is that the#p#分頁標(biāo)題#e#
essential circuitry required for liver regeneration is encompassed
by three types of pathways: cytokine, growth
factor, and metabolic networks that link liver function
with cell growth and proliferation (Fig. 1). A characteristic
feature of these networks is that redundancy exists
among the intracellular components of each network,
such that loss of an individual gene rarely leads to complete
inhibition of liver regeneration. Instead, a change in
the timing of hepatocyte DNA replication or mortality in
Abbreviations: PH, partial hepatectomy; NPC, non-parenchymal cells; FoxM1b,
Forkhead boxM 1b; KO, knockout; TNF, tumor necrosis factor; IL-6, interleukin-
6; NF-
B, nuclear factor kappa B; STAT3, signal transducer and activator of
transcription 3; TNFR1, type I TNF-receptor; TGF, transforming growth factor
alpha; EGF, epidermal growth factor; HGF, hepatocyte growth factor; OSM, oncostatin
M; LPS, lipopolysaccharide; TLR, Toll-like receptor; MyD88, myeloid
differentiation factor 88; cdk, cyclin-dependent kinase; EGFR, EGF receptor; ERK,
extracellular signal-regulated kinase; HB-EGF, heparin-binding EGF-like growth
factor; AR, amphiregulin; TCPOBOP, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene;
CAR, constitutive androstane receptor; mTOR, mammalian target of rapamycin;
MMP, matrix metalloproteinases; TIMP, tissue inhibitor of
metalloproteinase; TACE, TGF-converting enzyme.
From the Departments of 1Pathology and 2Surgery, University of Washington
School of Medicine, Seattle, WA.
Supported by funds from NIH grants CA-23226 and CA-074131 and the
American College of Surgeons Resident Research Scholarship (K.J.R.).
Address reprint requests to: Nelson Fausto, M.D., Department of Pathology,
University of Washington School of Medicine, Box 357470, C-516 Health Sciences
Building, Seattle, WA, 98195-7470. E-mail: [email protected]; fax:
206-543-3644.
Copyright © 2006 by the American Association for the Study of Liver Diseases.
Published online in Wiley InterScience (www.interscience.wiley.com).
DOI 10.1002/hep.20969
Potential conflict of interest: Nothing to report.
S45
only a fraction of the animals carrying the defect is typically
seen. No single genetically modified mouse model
demonstrates 100% mortality and a complete blockage of
both DNA replication and cell proliferation after twothirds
partial hepatectomy (PH). Thus, using criteria established
by genetic studies in other organisms, no single
gene can be considered “essential” for liver regeneration.
Most of the data that we discuss in this review are based
on studies of PH in genetically modified mice. It should
be noted that the original technique of Higgins and
Anderson for PH in rats4 must be modified to be safely
and reproducibly performed in mice.5 Ligating both the#p#分頁標(biāo)題#e#
left and median lobes together (as done in rats) causes
necrosis in the remaining right lobe in a mouse, presumably
from vascular obstruction. Thus, data reporting the
effect of a gene on mouse mortality after PH need to be
carefully interpreted.
Before discussing cytokine, growth factor, and metabolic
pathways active during liver regeneration, we comment
briefly on the definition of the term regeneration as it
applies to liver growth processes, about the cell types responsible
for this growth, and how synchronous hepatocyte
division is coordinated after PH.
Regeneration Is Compensatory Growth of
the Liver
In biological terms, regeneration means the reconstitution
of a structure that has been excised, such as the complete
re-growth of the limb of a newt, including skin,
muscle, and digits. Regeneration of a lost limb starts with
the formation of a blastema at the cut surface, which
contains dedifferentiated cells with broad differentiation
potential.6 Liver “regeneration” after PH is a very different
process, in which the excised parts do not grow back.
Rather, the remaining liver expands in mass to compensate
for lost tissue. Thus, liver regeneration is technically a
process of compensatory growth rather than regeneration.
As such, it does not follow the same general steps involved
in true regenerative processes, and formation of a blastema
containing dedifferentiated cells does not occur.
An important distinction must be made regarding the
origin of the cells that replace missing hepatocytes after
PH and in the growth processes that follow parenchymal
cell necrosis.7,8 After PH or CCl4-induced injury, liver
mass is replenished by replication of existing hepatocytes,
without activation of a progenitor cell compartment.9 In
the regeneration of the liver that follows loss of parenchymal
cells induced by other toxins, such as galactosamine,
replication and differentiation of intrahepatic progenitor
cells occurs.10-13 The extent to which these cells contribute
to regeneration varies according to the nature of the
injury, doses of inducing agents, or other experimental
variables. It is important to mention that we do not find
any compelling evidence that bone marrow cells generate
significant numbers of hepatocytes in in vivo physiological
or pathological hepatic growth processes.8 Conversely,
cells originating in the bone marrow may generate 20% or
more of the endothelial cells and other nonparenchymal
cells (NPCs) during liver regeneration.14
Autonomy and Timing of Regeneration
The extent and timing of liver regeneration are known
to vary according to circadian rhythms15; a recent study
has identified a mechanism by which these rhythms control
hepatocyte proliferation after PH.16,17 In these experiments,#p#分頁標(biāo)題#e#
the peak of DNA replication after PH in mice
always occurred 36 hours after the operation, regardless of
the time of the day at which the procedure was performed.
The entry of cells that had replicated theirDNA(G2 cells)
into mitosis, however, always occurred at the same time of
day. This finding suggests that a circadian clock controls
the G2/M transition; the authors implicate WEE1 kinase
as a candidate circadian regulator of cell division. WEE1
phosphorylates Cdc2 kinase, disrupting the activity of the
Cdc2/cyclin B1 complex, which participates in hepatocyte
mitosis. Expression of Wee1 follows a circadian cycle,
and entry of G2 cells into mitosis varies inversely with
levels of WEE1 after PH. This study is consistent with
studies in Forkhead boxM 1b (Foxm1b) knockout (KO)
mice, which display a deficit inDNAreplication after PH.
FoxM1b regulates entry into M-phase by coordinating
induction of cyclin B1 and activation of cdc25b to dephosphorylate
Cdc2.18
Fig. 1. Cytokine, growth factor, and metabolic networks during liver
regeneration. Efficient liver regeneration involves three networks; cytokine
(yellow), growth factor (red), and metabolic (white). Representative
molecules that participate in each network are shown with activation
profiles drawn as a waves, indicating that networks are only transiently
activated after PH (see text for details).
S46 FAUSTO, CAMPBELL, AND RIEHLE HEPATOLOGY, February 2006
A fascinating counterpart to these observations is that
the timing of DNA replication, which is not under the
control of circadian rhythms, appears to be an intrinsic
property of hepatocytes. Rats and mice differ in the timing
of DNA replication after PH, which is 12 to 16 hours
earlier in rats. Weglarz and Sandgren transplanted rat
hepatocytes into the livers of mice after PH and found
that rat hepatocytes replicated earlier than mouse hepatocytes
in the resultant chimeric liver.19 These results indicate
that the timing of hepatocyte DNA replication after
PH is an autonomous process, primarily guided by intrinsic
signals.
The Cytokine Network and the Initiation of
Liver Regeneration
A wide variety of genes are differentially expressed during
the first few hours after PH (the “priming phase”);
many of these genes are involved in a cytokine network.
Evidence for the importance of cytokines during this
phase of regeneration includes (1) increases in liver
mRNA and serum levels of tumor necrosis factor (TNF)
and interleukin-6 (IL-6) after PH20-22; (2) activation of
the transcription factors nuclear factor-kappa B (NF-
B)
and signal transducer and activator of transcription 3
(STAT3)23,24; (3) inhibition of DNA replication by anti-
TNF antibodies20; (4) blockage of liver regeneration in#p#分頁標(biāo)題#e#
Il-6 and TNF receptor type I (Tnfr1) KO mice25,26; and
(5) correction of the defect in Tnfr1 KO mice by IL-6
injection.26 The cytokine network is initiated through the
binding of TNF to TNFR1, leading to activation of
NF-
B in NPCs, production of IL-6, and activation of
STAT3 in hepatocytes (Fig. 2). One important STAT3
target gene is Socs3, which acts in a feedback loop to
prevent ongoing activation of IL-6 signaling by inhibiting
STAT3 phosphorylation.27
Further evidence that cytokines are important for regeneration
arises from the fact that certain cytokines have
the ability to prime resting hepatocytes for cell division
without PH. Hepatocytes in the normal liver are quiescent
(G0 phase) and exhibit only a minimal response to
potent in vitro mitogens, such as transforming growth
factor alpha (TGF), epidermal growth factor (EGF),
and hepatocyte growth factor (HGF). However, growth
factor infusion into rats preceded by a single TNF injection
induces replication in up to 40% of hepatocytes in
the normal liver.28
Although there is no dispute that a cytokine network
including the components mentioned above is activated
within 30 minutes after PH; there is much debate regarding
the precise roles played by individual cytokines. The
increase of serum TNF after PH has not been universally
observed, although it appears to be higher in rats than in
Fig. 2. Cytokine pathways activated during liver regeneration. The
figure illustrates interactions in cytokine pathways between Kupffer cells
and hepatocytes in the regenerating liver (other non-parenchymal cells
also may be involved). TNF binds its type I receptor on Kupffer cells,
leading to the activation of NF-
B. C3a, C5a, and MyD88 also can
activate NF-
B after PH. Il-6 and Tnf are both NF-
B target genes; IL-6
is subsequently released into the serum, and binds to its receptor, a
complex of gp80 and gp130 subunits, on hepatocytes. Activation of
gp130 leads to phosphorylation of STAT3 monomers by Janus-associated
kinases (JAKs). STAT3 then homodimerizes and translocates to the
nucleus, where it induces transcription of a number of target genes,
including Socs3, which then inhibits further STAT3 phosphorylation. SCF
also may activate STAT3 after PH. In parallel with STAT3 phosphorylation,
gp130 activation also leads to a signaling cascade involving the phosphorylation
of ERK1/2 and the upregulation of multiple genes important
for regeneration. TNF, tumor necrosis factor; NF-
B, nuclear factorkappaB;
MyD88, myeloid differentiation factor 88; PH, partial hepatectomy;
IL-6, interleukin-6; STAT3, signal transducer and activator of
transcription 3; SCF, stem cell factor.
HEPATOLOGY, Vol. 43, No. 2, Suppl. 1, 2006 FAUSTO, CAMPBELL, AND RIEHLE S47
mice, and although TnfrI KO mice have multiple deficits#p#分頁標(biāo)題#e#
after PH, Tnf KO mice appear to regenerate normally.
29,30 These data suggest that TNF itself may not be
required, because other ligands can signal through
TNFR1, such as lymphotoxin alpha. Indeed, Knight and
Yeoh31 showed that hepatocyteDNAreplication after PH
is inhibited in Lt/Tnf double KO mice.
The precise role of IL-6 in liver regeneration has been
particularly difficult to define. It has been calculated that
almost 40% of the immediate early genes expressed in the
regenerating liver32,33 may be IL-6 dependent,34 suggesting
that the role of IL-6 in this process is complex. The
primary function of IL-6 in regeneration was originally
shown to be proliferative, as Il-6 KO mice had a striking
deficit in DNA replication after PH, but subsequent data
have suggested that IL-6 also has anti-apoptotic and hepatocyte
survival activity.35-39 The proliferative effects of
IL-6 have undergone further scrutiny, as some groups
have reported that IL-6 KO mice and mice deficient in
gp130 (a necessary component of the receptor complex
for the IL-6 ligand family) have no defects in DNA replication
after PH,39 in direct contrast to the original report.
25 These discrepancies may reflect a lack of
standardization of experimental conditions, including
surgical techniques, anesthetic agents, strain of mice used,
or animal maintenance. In addition, it appears that the
precise level of IL-6 present after PH may be critical in
determining its effects.35,40
Stem cell factor (SCF) and oncostatinM(OSM) are 2
molecules that may modulate or enhance the effects of
IL-6 during liver regeneration. SCF restores DNA replication
in Il-6 KO mice after PH,41 and administration of
OSM can correct the deficient regeneration seen in Il-6
knockout mice after CCl4-induced injury.42 Conversely,
IL-6 cannot restore the defective regeneration after CCl4
that is seen in mice deficient for the OSM receptor. The
effects of these cytokines are at least in part redundant, as
IL-6, SCF, and OSM can all activate STAT3 in hepatocytes,
but their intracellular signaling pathways must diverge
at some point to explain their apparent differences
in biological activity.
Triggering the Cytokine Cascade: Role of
Components of the Innate Immune System
Because cytokine activation participates in the initiation
of liver regeneration, identifying the mechanisms
that trigger the activation of this network is important. A
logical candidate for a master upstream molecule is lipopolysaccharide
(LPS), which is released from enteric bacteria
into the portal circulation.43 Indeed, Cornell et al.44
found that rats with restricted production of LPS and
mice that are naturally hypo-responsive to LPS (C3H/
HeJmice) have a delay in regeneration after PH. The LPS#p#分頁標(biāo)題#e#
resistance of C3H/HeJ mice45 was later found to be the
consequence of a point mutation in the gene for Toll-like
receptor 4 (TLR4), a member of a class of receptors that
bind various microbial products. LPS binding to TLR4
activates multiple intracellular signaling pathways, some
of which are dependent on myeloid differentiation factor
88 (MyD88), an adapter protein that mediates intracellular
signals from several TLRs.46 Contrary to the hypothesis
that LPS is the key initiator of regeneration and the
findings in C3H/HeJ mice, Tlr4 KO mice showed no
abnormalities after PH. Mice deficient for TLR2, TLR9,
or CD14 also had normal cytokine activation and regeneration
after PH. By contrast, Myd88 KO mice failed to
activate TNF and IL-6.47,48 STAT3 activation and expression
of important STAT3 target genes, such as Socs3
and acute phase response genes, were also blocked in
Myd88 KO mice after PH. Identifying the ligand and
receptor that signal through MyD88 early after PH is an
exciting challenge, and perhaps in doing so the mechanisms
that initiate the liver regeneration cytokine cascade
will be identified.
Other components of the innate immune system appear
to be critical for normal regeneration as well; mice
deficient in the C3 and C5 components of complement
display significant deficits after PH.49 In these animals,
diminished activation of the cytokine pathway is manifested
by lack of increases in TNF and IL-6 levels, and in
impaired NF-
B and STAT3 activity. Whether and how
these two aspects of innate immunity, TLR-MyD88 signaling
and the complement cascade, converge to initiate
cytokine signaling in liver regeneration is not clear.
Growth Factors and Cell Cycle Progression
The cytokine network acts at the priming phase of liver
regeneration, which corresponds to the passage of quiescent
hepatocytes into the cell cycle (G0 to G1). Cell cycle
progression is then driven by growth factors, which override
a restriction point in late G1. Passage from G1 to S
phase is associated with Rb phosphorylation, increased
expression of the Rb family member p107 and of cyclins
D, E, and A, and formation of cdk4/cyclin D and cdk2/
cyclin E complexes.50-52
HGF and the EGF receptor (EGFR) ligand family are
important growth factors that drive cell cycle progression
during liver regeneration.53,54 HGF is produced by mesenchymal
cells and acts on hepatocytes in a paracrine or
endocrine fashion. Its effects are multiple and have been
grouped into morphogenic, motogenic, and mitogenic
categories. Studies of liver regeneration in mice with hepatocyte-
specific deletion of c-met, the gene for the HGF
receptor, were recently conducted.55,56 Borowiak et al.56
S48 FAUSTO, CAMPBELL, AND RIEHLE HEPATOLOGY, February 2006#p#分頁標(biāo)題#e#
demonstrated that HGF/c-met signaling is essential for
cell cycle entry after PH, and that it is responsible for the
activation of extracellular signal–regulated kinase 1/2
(ERK1/2). In contrast, Huh et al.55 reported that hepatocyte
c-met–deficient mice had massive mortality after PH,
and thus examined the role of this pathway in other liver
injury models. They conclude that HGF/c-met signaling
is important in hepatoprotection from apoptosis, and in
facilitating healing after CCl4 administration. The discrepancy
in post-operative survival between the two reports
is most likely related to the different surgical
techniques used by the two groups, as noted by Borowiak
et al.56 Until additional data become available, deciding
whether HGF/c-met signaling functions primarily in mitogenesis,
or whether it maintains hepatocyte homeostasis
and thus facilitates cell replication, is not possible.
The family of ligands that bind the EGFR, in addition to
EGF, includes TGF, heparin-binding EGF-like growth
factor (HB-EGF), and amphiregulin (AR). TGF is an autocrine
growth factor, both produced by and active on hepatocytes.
57 Although TGF has effects on cell motility and
vascularization, its main effect is the stimulation of cell proliferation.
Transgenic mice that overexpress TGF display
constitutive hepatocyte proliferation and eventually develop
cancer.58 Tgf expression increases after PH in wild-type
mice, but Tgf KO mice have no defects in liver regeneration.
59 The normal regeneration seen in these animals is
likely a consequence of compensation by other EGFR ligands,
although the roles of these growth factors afterPHare
not entirely redundant, as discussed below.
HB-EGF is expressed earlier than HGF and TGF
after PH and appears to have a unique role in liver regeneration.
60,61 A 30% PH does not result in coordinated
DNA replication, despite activation of the cytokine cascade.
62,63 A single injection of HB-EGF 24 hours after
30% PH can override this blockage between priming and
cell cycle progression, eliciting a wave of DNA replication.
Interestingly, this effect cannot be accomplished by
similarly injecting HGF or TGF.63 In addition, Hb-egf
KO mice have a delay in DNA replication after 70% PH,
although this deficiency is partially compensated by an
earlier increase in Tgf expression in these animals.
AR also appears to contribute to regeneration, as mice
deficient for this growth factor have a significant deficit in
DNA replication after PH.54,64 Direct comparisons between
AR and other EGFR ligands have not been made,
but it is likely that the different growth factors have independent
but partially overlapping functions in liver regeneration.
The complexity of EGFR signaling after PH is#p#分頁標(biāo)題#e#
most likely attributable to the number of EGF ligands
involved, their specificity for different receptor heterodimers,
and the nuances of subsequent activation of
intracellular signaling pathways.54,65
Both c-met and the EGFR are receptor tyrosine kinases,
which recruit enzymes and scaffolding proteins to
phosphorylated intracellular domains of each receptor.
Multiple intracellular signaling pathways are thus activated,
which regulate amultitude of transcription factors,
initiate translation, and regulate metabolic pathways (Fig.
3). One mitogenic signal transduction pathway that is of
particular interest, because it may integrate cytokine signals
as well as growth factor signals, is the Ras-Raf-MEK
cascade, which results in the activation of ERK1/2.
ERK1/2 activation is correlated with hepatocyte DNA
replication in vivo and hepatocyte proliferation in
vitro.66-69 Moreover, growth factors such as HGF and
TGF and cytokines such as TNF and IL-6 stimulate
ERK1/2 activity in primary hepatocytes and hepatocyte
cell lines.70-72
A family of nuclear receptor ligands, including thyroxine,
1,2-bis [2-(3.5-dichloropyridyloxy)] benzene (TCPOBOP),
and retinoic acid, appears to be primary hepatocyte mitogens
in vivo, capable of inducing replication without tissue
loss. TCPOBOP, which binds the constitutive
androstane receptor (CAR), in particular has been shown
to stimulate hepatocyte DNA synthesis independent of
many molecules that are critical to regeneration after PH,
including TNF, IL-6, NF-
B, STAT3, and cyclin
D1.73,74 However, the pathways through which CAR activation
directs hepatocyte replication are yet undefined.
Fig. 3. Growth factor signaling pathways during liver regeneration.
Stimulation of the tyrosine kinase receptors for HGF (c-met), and the EGF
ligands, TGF, HB-EGF, and AR, (EGFR) activates numerous intracellular
signaling pathways that regulate transcription factors involved in liver
regeneration. mTOR and its inhibitor rapamycin modulate translational
control of these pathways. HGF, hepatocyte growth factor; EGF, epidermal
growth factor; TGF, transforming growth factor alpha; HB-EGF,
heparin-binding EGF-like growth factor; AR, amphiregulin; mTOR, mammalian
target of rapamycin.
HEPATOLOGY, Vol. 43, No. 2, Suppl. 1, 2006 FAUSTO, CAMPBELL, AND RIEHLE S49
Metabolic Pathways and Liver Regeneration
Liver regeneration after PH is a perfectly calibrated
response whose apparent sensor is the body’s requirement
for liver function. Identifying molecular mechanisms that
account for the capacity of the liver to modulate its
growth in accordance to the needs of the whole organism
has been difficult. Nevertheless, the increased metabolic
demands imposed on the liver remnant after PH are likely#p#分頁標(biāo)題#e#
connected with activation of the machinery directly involved
in DNA replication.
The administration of an amino acid mixture to intact
rats induces a wave of hepatocyte replication, and protein
deprivation blocks liver regeneration after PH.75,76 Recent
studies have shown that amino acids regulate hepatocyte
proliferation through cyclin D1 expression.77 The
initiation of protein translation is a critical control point
that may integrate nutrient and energy levels with mitogenic
signals.78 After PH, the activity of p70 S6 kinase
increases, and the activation 4E-BP1 (a translational repressor)
decreases, leading to an increase in translation.
Both of these proteins are thought to be downstream effectors
of mTOR (mammalian target of rapamycin),
which is part of a complex that senses nutrient or energy
status, and also integrates growth factor signals, resulting
in the regulation of protein translation and cell
growth.79,80 The importance of translation in liver regeneration
has been illustrated by a study of PH in S6 KO
mice, in which a near complete loss of hepatocyte DNA
replication was demonstrated.81 The mTOR complex
may regulate liver regeneration by modulating cell growth
and proliferation in response to the energy demands of the
remaining liver, given that rapamycin, an inhibitor of
mTOR, inhibits DNA replication after PH.81-83 If so,
liver regeneration poses a challenge for this complex, because
nutrient-sensing mechanisms in mammalian cells
appear to modulate cell growth, depending on the availability
of nutrients. After PH, the liver needs to regulate
systemic nutrient homeostasis while its own cells are undergoing
cell growth and proliferation.
Interactions Between Cytokines and Growth
Factors During Liver Regeneration
We have thus far discussed separately the cytokine,
growth factor, and metabolic pathways active after PH,
but these pathways interact during different phases of
liver regeneration. One important linkage between cytokines
and growth factors may be the activation of matrix
metalloproteinases (MMPs) by cytokines, such as TNF.
The activity of several MMPs increases after PH,84,85 and
a recent study in tissue inhibitor of metalloproteinases 3
(Timp3) KO mice implicates an MMP called TGF converting
enzyme (TACE, also known as ADAM17).86
TIMP3 is thought to be a specific inhibitor of TACE, and
Timp3 KO mice displayed elevated hepatic TNF protein
levels and an earlier entry into S phase after PH than
wild-type mice. However, 144 hours after PH, a proportion
of these animals die, possibly because of loss of regulation
of TNF signaling pathways.
We recently demonstrated that TNF itself can activate
TACE, resulting in release of TGF, activation of EGFR,#p#分頁標(biāo)題#e#
and cell proliferation in cultured hepatocytes.70 The sequential
activation of cytokine and growth factor receptors
may stimulate numerous intracellular signaling
pathways needed for cell survival and proliferation (Fig.
4). Because TACE has been shown to cleave the precursor
forms of cytokines and many EGFR ligands,87 including
HB-EGF, AR, and TGF, further studies are needed to
elucidate its role(s) in liver regeneration.
In elegant studies of the proliferation of hepatocytes
co-cultured with liver epithelial cells, Serandour et al.87
showed that EGF alone does not initiate hepatocyte replication,
but exposure of the system to EGF and TNF
induces replication in up to 30% of the hepatocytes. The
authors demonstrate that the primary effect of TNF is to
activate MMPs, which subsequently degrade components
of the extracellular matrix, allowing hepatocyte proliferation.
This regimen of EGF and TNF was capable of inducing
repeated waves of hepatocyte replication if given
over multiple 10-day cycles. These remarkable results are
an important extension of the above-mentioned data on
Fig. 4. Mechanism for EGFR activation through TACE activity. TNF
binding to its receptor activates NF-
B and Akt, stimulating cytokine
production and survival pathways (left panel). TNF also activates TACE,
which cleaves membrane-bound TGF. The cleaved, active TGF molecule
binds to and activates the EGFR, a receptor tyrosine kinase, leading
to downstream activation of ERK1/2 (right panel). Cooperation between
cytokine (TNF) and growth factor (EGF ligands) signaling activates pathways
that are needed for hepatocyte survival, growth, and proliferation.
EGFR, epidermal growth factor receptor; TACE, transforming growth factor
alpha converting enzyme; NF-
B, nuclear factor-kappaB; TNF, tumor
necrosis factor; TGF, transforming growth factor alpha; ERK1/2, extracellular
signal-regulated kinase 1/2.
S50 FAUSTO, CAMPBELL, AND RIEHLE HEPATOLOGY, February 2006
the interactions between TNF and EGFR signal transduction,
and provide an interesting in vitro system for
studying liver regeneration.
Concluding Remarks
During the last decade, a surge of interest in the mechanisms
of liver regeneration has been seen, generated from
both a biological science and a clinical perspective. Laboratory
scientists, at long last, realized that liver regeneration
constitutes a unique model to study signal
transduction and cell cycle events in a synchronized manner
in vivo. From a clinical perspective, understanding the
mechanisms of liver regeneration is crucial for the appropriate
management of and the development of new therapies
for a number of important conditions, such as acute
liver failure and cirrhosis. The interaction between basic
biological knowledge and clinical issues is particularly#p#分頁標(biāo)題#e#
close in studies dealing with liver progenitor cells and liver
repopulation during liver development and hepatic disease.
The regulation of growth of liver transplants, and particularly
that of the donor liver in living donor transplantation,
appears to follow the same principles as those that
regulate liver regeneration after PH in laboratory animals.
89,90 What is needed, however, is a more vigorous
effort to apply the knowledge gained in experimental
work to solve clinical problems, such as the failure of
small-for-size transplants, and to obtain more rapid liver
growth after transplantation. Another important issue is
the replicative activity of hepatocytes during the progression
of cirrhosis.91 Do hepatocytes at the late stages of
cirrhosis truly exhaust their replicative capacity? If this is
the case, does the “replicative senescence” of hepatocytes
contradict the experimental data obtained by serial cell
transplantation in mice, which demonstrated that hepatocytes
are capable of at least 80 doublings92? And finally,
would the restoration of proliferative activity in these
hepatocytes correct functional defects, or might it lead to
enhanced tumorigenesis?
An important gap in our knowledge is the lack of understanding
of the factors that determine whether hepatocyte
production during diverse regenerative processes
originates from the replication of mature hepatocytes, as
in liver regeneration after PH, or the differentiation of
liver progenitor cells, as occurs after massive liver necrosis.
Moreover, more knowledge needs to be gained on the role
of the innate immune system and lymphoid cells in the
initiation of these processes. Major advances in the genetic
manipulation of mice now make possible the construction
of appropriate animal models to study
unresolved issues of major biological and clinical importance
involving the molecular and cellular aspects of liver
regeneration.
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