Download Viral hepatitis and fatty liver disease: how an

Biochem. J. (2008) 416, e15–e17 (Printed in Great Britain)
e15
doi:10.1042/BJ20081916
COMMENTARY
Viral hepatitis and fatty liver disease: how an unwelcome guest
makes pâté of the host
Andrew J. BROWN1
BABS, School of Biotechnology and Biomolecular Sciences, Biosciences Building D26, University of New South Wales, Sydney, NSW 2052, Australia
HBV and HCV (hepatitis B and C viruses respectively) affect
hundreds of millions of people globally, and are a major cause of
chronic liver disease, including NAFLD (non-alcoholic fatty liver
disease). Previous work on HCV-associated fatty liver disease
has implicated two transcription factors that are important in
lipid metabolism, SREBP1c (sterol-regulatory-element-binding
protein 1c) and the LXRα (liver X receptor α). HBV-associated
fatty liver disease has been less well-studied. New work from
Kim and colleagues in this issue of the Biochemical Journal has
provided new insight into how HBV causes fatty liver disease.
Investigating HBV’s so-called X gene product (HBx), they report
that this viral protein directly binds to LXRα in the host liver
cells to up-regulate the lipogenic transcription factor, SREBP1c.
Also discussed in this commentary is another way that viruses
such as HBV and HCV could induce SREBP1c-mediated lipogenesis, via the PI3K (phosphoinositide 3-kinase)–Akt signalling
pathway.
It is a curious thought that a virus can commandeer the lipid
production machinery in the host’s liver to make their own version
of pâté de foie gras. Fatty liver disease (hepatic steatosis) is a
common consequence of infection by HBV or HCV (hepatitis B
or C viruses respectively), occurring in roughly a quarter to a half
of cases. These viruses are a major global cause of morbidity and
mortality, chronically infecting more than half a billion people
around the world. From fatty liver disease, a subset of infected
individuals will go on to develop one of the most prevalent
forms of human cancer, hepatocellular carcinoma [1,2]. Although
causing similar disease outcomes, HBV and HCV are very
different viruses, belonging to distinct families (Hepadnaviridae
and Flaviviridae respectively). Whereas HCV is a positive-sense
single-stranded RNA virus, HBV is made of circular partially
double-stranded DNA. The genome of HBV encodes four overlapping open reading frames that are translated to make the viral
core protein, the surface proteins, a reverse transcriptase, and the
so-called X gene product of the human hepatitis B virus (HBx).
Although the precise function of HBx remains elusive, it appears
to be a multifunctional regulator that modulates many cellular
processes by directly or indirectly interacting with host factors
[1]. In this issue of the Biochemical Journal, Kim and colleagues
[2] reveal novel molecular insights into how HBx may contribute
to fatty liver disease induced by HBV infection.
A fatty liver could be created in a number of ways, including
decreased secretion of lipoproteins, reduced fat catabolism and
increased fat synthesis (steatosis). There is some evidence that
HBV and HCV may affect all three processes. However, the focus
of the paper by Kim and colleagues [2] is on the steatosis side of
the equation. The chief controllers of fat manufacture in animal
cells are the SREBP (sterol-regulatory-element-binding protein)
family of transcription factors. SREBP2 is principally in charge of
regulating cholesterol synthesis and uptake, whereas SREBP1c’s
primary responsibility is controlling the synthesis of fatty acids
and their constituent lipids. As the storage form of fatty
acids, triacylglycerol is the major lipid found in fatty liver
disease, accumulating in vacuoles in the hepatocyte. Targets of
SREBP1c include genes encoding key enzymes involved in fatty
acid synthesis, ACC1 (acetyl-CoA carboxylase 1) and FAS
(fatty acid synthase). SREBP1c also binds to its own promoter
in a feed-forward loop. In addition, both SREBP1c and FAS gene
expression are also positively regulated via LXR (liver X receptor)
[3].
LXR is a nuclear receptor which complexes with the RXR
(retinoid X receptor) to form a heterodimer. Two LXR isoforms
have been characterized. LXRα is abundantly expressed in the
liver and is also detected in other tissues (intestine, adipose
tissue, spleen and macrophages), whereas LXRβ is ubiquitously
expressed. Oxygenated forms of cholesterol, so-called oxysterols
such as (24S),25-epoxycholesterol, are the natural ligands that
activate LXR [4]. It is known that certain synthetic LXR agonists,
such as T0901317, induce dramatic liver steatosis akin to that
seen in viral hepatitis. Another impetus for the research by Kim
et al. [2] was their previous findings that HBx induces hepatic
steatosis via transcriptional activation of SREBP1c and PPARγ
(peroxisome-proliferator-activated receptor γ ), a nuclear receptor
best known for its role in adipogenesis [5].
In their study [2], Kim and colleagues employed a combination
of molecular, cellular and animal models to investigate the
influence of HBx on LXRα-mediated SREBP1c induction. Using
a luciferase reporter assay in a variety of human liver cell lines,
they showed that HBx increased LXRα transcriptional activity.
This effect was most pronounced when cells were also treated
with an LXR agonist (T0901317 or an oxysterol). Another clear
indication for the involvement of LXRα was that the induction by
HBx of SREBP1c and FAS gene transcription was abolished
by small-interfering RNA directed against LXRα. Moreover, Kim
et al. [2] provide evidence that HBx and LXRα co-localize within
the nucleus and form a transcriptional protein complex on the
LXREs (LXR-response elements) in the promoter of SREBP1c.
1
Key words: complex 1 of the mammalian target of rapamycin
(mTORC1), hepatic steatosis, hepatitis B virus (HBV), oxysterol,
statin, steatohepatitis.
email [email protected]
c The Authors Journal compilation c 2008 Biochemical Society
e16
A. J. Brown
Figure 1 A schematic representation of how HBV induces lipogenic gene
expression via its protein HBx
The promoters of both lipogenic genes, SREBP1c and FAS , contain at least one LXRE and SRE.
Mechanism 1: as featured in the article by Kim et al. [2], HBx up-regulates lipogenic genes by
binding directly to the ligand-binding domain of LXRα. The star symbolizes the LXR ligand which
can either be an endogenously produced oxysterol or an administered synthetic agonist. The
transcriptional co-activator, ASC2, enhances the effect of HBx on LXRα-mediated transactivation
of lipogenic genes. Mechanism 2: HBx may also induce lipogenesis by increasing activation of
SREBP1c through stimulation of the PI3K–Akt signalling pathway. This may occur by one or
more different ways: (2a) stimulated activation of SREBP1c by Akt; or (2b) a downstream target
via mTORC1; and/or (2c) increased stability of the nuclear SREBP1c via a GSK3β-mediated
pathway. HBx is a small protein of 154 amino acids and is depicted as a spiky ball to signify its
viral origin. See the text for more details.
Specifically, HBx appears to interact with the ligand-binding
domain of LXRα. The ability of HBx to up-regulate lipogenic
genes (SREBP1c or FAS) via LXRα was potentiated by ASC2
(activating signal co-integrator-2), a transcriptional co-activator
that mediates the transactivation of nuclear receptors including
LXRα [6] (see Figure 1). Kim et al. [2] also presented data from
an HBx-transgenic mouse model, where they demonstrated increased levels of SREBP1 and FAS protein in response to HBx.
However, definitive proof for the involvement of LXRα in
HBx-mediated lipogenesis in vivo would require additional control experiments. For example, is the lipogenic effect abolished
if HBx-transgenic mice are crossed with LXRα-null animals?
Overall, the studies of Kim et al. [2] firmly place HBx, LXRα and
SREBP1c in the frame for the hepatic lipid accumulation often
observed with HBV infection. This work also adds LXRα to a
growing list of transcription factors with which HBx has been
shown to interact [1].
The link between LXR, SREBP1c and steatosis has been
noted before with HCV infection. HCV ‘non-structural protein2’ activated SREBP1c and FAS transcription which involved the
LXREs and SREs (sterol-response elements) in the SREBP1c
promoter [7]. In addition, HCV core protein and non-structural
protein-4b also activated SREBPs [8]. In the light of Kim et al.’s
study on HBx [2], it would be interesting to determine whether
any of these HCV proteins also act through binding to LXRα.
Therefore different viruses may cause lipid accumulation by
common or at least similar pathways, although a particular virus
may harness more than one strategy to do so.
Besides inducing lipogenic genes, LXRα also represses various
inflammatory genes in macrophages. Kim et al. [2] make the point
that it is difficult to predict whether the inflammatory nature of
chronic HBV infection might be offset to some extent by the antiinflammatory effect of LXRα activation by HBx. This may well
be another cunning trick by the virus to weaken the host’s ability
to fight it. LXRα also induces an array of genes involved in other
aspects of lipid and lipoprotein metabolism, such as cholesterol
export from the cell (ATP-binding cassette proteins ABCA1 and
c The Authors Journal compilation c 2008 Biochemical Society
ABCG1), apolipoproteins (apoE, apoD and apoAIV), transporters
and enzymes involved in lipoprotein metabolism (cholesteryl
ester and phospholipid transfer proteins, and lipoprotein lipase),
as well as genes involved in enhancing bile acid transport and
metabolism. Since HBV infection is associated with general liver
pathology, and given the complexity of LXRα responses, it is
difficult to extrapolate how HBx-mediated activation of LXRα in
the liver might have an impact on overall lipoprotein metabolism.
Another way that viruses such as HBV and HCV could induce lipogenesis is via the PI3K (phosphoinositide 3-kinase)–Akt
signalling pathway. The PI3K–Akt pathway is commonly activated in response to growth factors and hormones such as insulin
that act through cell-surface receptors. Virtually all mammalian
viruses, both DNA and RNA, activate this pathway at some point
in their life cycle to benefit from the growth, metabolic, survival
and translational functions that the pathway controls [9]. Indeed,
HBx protects against apoptotic cell death by exploiting PI3K–Akt
signalling in the host’s liver cells [10]. Importantly, PI3K–
Akt signalling also activates SREBPs ([11,12] and references
therein), suggesting another mechanism through which viruses
can up-regulate lipogenesis. Activation of SREBPs occurs via
some elegant cell biology, involving regulated transport of the
inactive membrane-bound precursor from the ER (endoplasmic
reticulum) to the Golgi apparatus, where proteolytic processing
occurs to release the truncated active transcription factor. The
best documented regulation is via feedback inhibition of ERto-Golgi transport of precursor SREBP by sterols (cholesterol
and oxysterols) [3]. Beyond the familiar lipid end-products, it
is now becoming increasingly clear that PI3K–Akt signalling
represents another important input into the regulation of SREBP
activation. Thus we have shown that the PI3K–Akt pathway is
involved in the ER-to-Golgi trafficking of SREBP2 [11]. Others
have shown that PI3K–Akt signalling stabilizes nuclear SREBP1c
[via phosphorylation by the Akt substrate GSK3β (glycogen
synthase kinase-3β)], and stimulates lipogenesis via activation of
an Akt downstream target, mTORC1 (complex 1 of the mammalian target of rapamycin) ([12] and references therein) (Figure 1).
However, it should be noted that SREBP2 transport and processing
are unaffected by rapamycin treatment [13], indicating that PI3K–
Akt signalling may activate SREBP1c and SREBP2 via different
means. Irrespective of the precise mechanism(s) involved, it is
likely that many viruses, including HBV and HCV, will also
stimulate lipid synthesis via PI3K–Akt signalling.
In terms of clinical implications for the paper by Kim et al. [2],
there is ongoing interest in LXR agonists as a potential drug treatment for various human diseases, notably cardiovascular disease.
First-generation synthetic LXR agonists have shown beneficial
effects in various animal models of human disease, and yet have
not progressed to the clinic owing to problems with hepatic
steatosis. Second-generation agonists are under investigation and
hold promise in avoiding these deleterious lipogenic side-effects.
Kim et al. [2] found that HBx enhanced the transcriptional activity
of LXRα in the absence of added LXR agonist, suggesting that
there may be sufficient endogenous ligand available to activate
LXRα. It should be noted that endogenous oxysterol ligands have
an advantage over their synthetic non-steroidal counterparts: they
suppress the proteolytic activation of SREBP1c which helps to
counteract the stimulation of SREBP1c gene expression via LXR.
(24S),25-Epoxycholesterol is a potent LXR ligand produced in
human liver by the same pathway that synthesizes cholesterol,
and can thus be inhibited by the statin class of drugs [4]. It would
be interesting to determine whether a statin would abolish the
effect of HBx on LXRα-mediated SREBP1c induction in liver
cells. Conversely, statin treatment may run the risk of increasing
local production of other oxysterol ligands for LXRα, derived
Commentary
from cholesterol, by increasing hepatic clearance of circulating
lipoproteins.
In conclusion, the study by Kim et al. [2] has shed new light
on the mechanism by which a prevalent human pathogen, HBV,
causes lipid accumulation in the liver. LXRα represents a vital
link between the viral protein, HBx, and the host’s lipogenic transcription factor, SREBP1c, and may be the missing ingredient in
HBV’s recipe for fatty liver disease.
REFERENCES
1 Nguyen, D. H., Ludgate, L. and Hu, J. (2008) Hepatitis B virus–cell interactions and
pathogenesis. J. Cell. Physiol. 216, 289–294
2 Kim, K., Kim, K. H., Kim, H. H. and Cheong, J. (2008) Hepatitis B virus X protein induces
lipogenic transcription factor SREBP1 and fatty acid synthase through the activation of
nuclear receptor LXRα. Biochem. J. 416, xxxx–xxxx
3 Horton, J. D., Goldstein, J. L. and Brown, M. S. (2002) SREBPs: activators of the
complete program of cholesterol and fatty acid synthesis in the liver. J. Clin. Invest. 109,
1125–1131
4 Brown, A. J. (2008) 24(S ),25-Epoxycholesterol: a messenger for cholesterol
homeostasis. Int. J. Biochem, Cell Biol., doi:10.1016/j.biocel.2008.05.029
5 Kim, K. H., Shin, H. J., Kim, K., Choi, H. M., Rhee, S. H., Moon, H. B., Kim, H. H., Yang,
U. S., Yu, D. Y. and Cheong, J. (2007) Hepatitis B virus X protein induces hepatic steatosis
via transcriptional activation of SREBP1 and PPARγ . Gastroenterology 132, 1955–1967
e17
6 Son, Y. L., Park, O. G., Kim, G. S., Lee, J. W. and Lee, Y. C. (2008) RXR heterodimerization
allosterically activates LXR binding to the second NR box of activating signal
co-integrator-2. Biochem. J. 410, 319–330
7 Oem, J. K., Jackel-Cram, C., Li, Y. P., Zhou, Y., Zhong, J., Shimano, H., Babiuk, L. A. and
Liu, Q. (2008) Activation of sterol regulatory element-binding protein 1c and fatty acid
synthase transcription by hepatitis C virus non-structural protein 2. J. Gen. Virol. 89,
1225–1230
8 Waris, G., Felmlee, D. J., Negro, F. and Siddiqui, A. (2007) Hepatitis C virus induces
proteolytic cleavage of sterol regulatory element binding proteins and stimulates their
phosphorylation via oxidative stress. J. Virol. 81, 8122–8130
9 Buchkovich, N. J., Yu, Y., Zampieri, C. A. and Alwine, J. C. (2008) The TORrid affairs of
viruses: effects of mammalian DNA viruses on the PI3K–Akt–mTOR signalling pathway.
Nat. Rev. Microbiol. 6, 266–275
10 Lee, Y. I., Kang-Park, S. and Do, S. I. (2001) The hepatitis B virus-X protein activates a
phosphatidylinositol 3-kinase-dependent survival signaling cascade. J. Biol. Chem. 276,
16969–16977
11 Du, X., Kristiana, I., Wong, J. and Brown, A. J. (2006) Involvement of Akt in ER-to-Golgi
transport of SCAP/SREBP: a link between a key cell proliferative pathway and membrane
synthesis. Mol. Biol. Cell 17, 2735–2745
12 Porstmann, T., Santos, C. R., Griffiths, B., Cully, M., Wu, M., Leevers, S., Griffiths, J. R.,
Chung, Y. L. and Schulze, A. (2008) SREBP activity is regulated by mTORC1 and
contributes to Akt-dependent cell growth. Cell Metab. 8, 224–236
13 Sharpe, L. J. and Brown, A. J. (2008) Rapamycin down-regulates LDL-receptor
expression independently of SREBP-2. Biochem. Biophys. Res. Commun. 373,
670–674
Received 22 September 2008; accepted 30 September 2008
Published on the Internet 12 November 2008, doi:10.1042/BJ20081916
c The Authors Journal compilation c 2008 Biochemical Society