Dr. Jekyll and Mr. Hyde: A Paradoxical Oncogenic and
Tumor Suppressive Role of Signal Transducer and
Activator of Transcription 3 in Liver Cancer
See Article on Page 164
U
nrestrained activation of the signal transducer
and activator of transcription (Stat) signaling
cascade frequently occurs in a wide variety of
tumor types.1-3 In this pathway, binding of extracellular
ligands such as cytokines, hormones, and growth factors
to their specific receptors leads to the activation of Janus
tyrosine kinases (Jak1, Jak2, Jak3, and Tyk2).2,3 These
tyrosine kinases are able to phosphorylate a single
tyrosine residue of each Stat protein. Similarly, nonreceptor tyrosine kinase such as Abelson murine
leukemia viral oncogene homolog (c-ABL) and
Schmidt-Ruppin A-2 viral oncogene homolog (c-SRC)
can directly phosphorylate Stat proteins in the absence
of ligand-induced receptor signaling.2,3 Once phosphorylated, Stat proteins homodimerize or heterodimerize
and translocate into the nucleus, where they activate a
number of target genes involved in cell proliferation,
survival, angiogenesis, invasion, and metastasis.2,3 Nuclear
localization of Stat proteins also results in the transcriptional activation of three families of inhibitory proteins,
namely, the protein inhibitors of activated Stats (PIAS), the
SH2-containing phosphatases (SHP), and the suppressors
of cytokine signaling (SOCS).2-4 Induction of PIAS, SHP,
and SOCS proteins following Stat activation represents an
efficient negative feedback loop mechanism limiting the
magnitude of Stat effects on target cells.2-4
In the liver, the best-characterized member of the
Stat pathway is Stat3. Under physiological conditions,
Stat3 is required for liver regeneration due to its ability
to stimulate hepatic cell proliferation and survival.5 In
hepatocellular carcinoma (HCC), presumably due to
the loss of PIAS, SHP, and SOCS genes via promoter
Abbreviations: HCC, hepatocellular carcinoma; Jak, Janus tyrosine kinase;
NF-jB, nuclear factor kappa-B; PIAS, protein inhibitors of activated Stats;
SHP, SH2-containing phosphatases; SOCS, suppressors of cytokine signaling;
Stat, signal transducer and activator of transcription.
Address reprint requests to: Diego F. Calvisi, M.D., Institut für Pathologie,
Ernst-Moritz-Arndt-Universita¨t, Friedrich-Lo¨ffler-Straße 23e, 17489 Greifswald,
Germany. E-mail: [email protected]; fax: 0049-3834-865701.
C 2011 by the American Association for the Study of Liver Diseases.
Copyright V
View this article online at wileyonlinelibrary.com.
DOI 10.1002/hep.24435
Potential conflict of interest: Nothing to report.
hypermethylation and/or the elevated synthesis of cytokines and growth factors that are able to activate the
Stat cascade, Stat3 activity is almost ubiquitously elevated and unconstrained.6-8 Of note, high levels of
Stat3 were detected in a vast HCC collection regardless of the liver tumor etiology, indicating that uncontrolled activation of Stat3 is a universal event in hepatocarcinogenesis.6 Also, it has been found that Stat3
levels are directly correlated with HCC biological
aggressiveness and inversely with survival of patients
with HCC, pointing to an important role of Stat3 in
liver cancer prognosis.6,7 Subsequent in vitro and
in vivo studies have shown that suppression of Stat3
enhances the chemosensitivity of HCC cells and suppresses the growth and metastatic properties of human
HCC in xenograft models.9,10 Also, hepatocyte-specific
Stat3-deficient mice were found to exhibit more than a
six-fold reduction in HCC load compared to Stat3
wild-type mice when subjected to diethylnitrosamine
treatment.8 In the latter model, disruption of the inhibitor of nuclear factor kappa-B kinase subunit beta–
nuclear factor kappa-B (NF-jB) axis was the causative
event leading to aberrant Stat3 activation, suggesting
that the NF-jB pathway is a major negative regulator
of Stat3 in HCC.8 Furthermore, carcinogen-induced
HCC development in mice was significantly enhanced
by heterozygous deletion of the Stat3 inhibitor
SOCS1.11 Similarly, genetic knockout of the SOCS3
gene in the mouse liver resulted in prolonged Stat3
phosphorylation and increased expression of Stat3 target
genes, ultimately leading to rapid HCC development
when SOCS3 knockout mice were subjected to chemically induced hepatocarcinogenesis.12,13 Altogether, these
data establish Stat3 as a bona fide oncogene in the liver
and suggest that Stat3 might be a critical target for
innovative therapeutical approaches in HCC.
In this issue of HEPATOLOGY, the report by
Schneller et al. provides new, significant experimental
data on the role of Stat3 in liver carcinogenesis.14 In
particular, the authors investigate the function of Stat3
in HCC progression in the presence of other molecular alterations, namely the activation of the rat sarcoma
viral oncogene homolog (Ras) oncogenic cascade and
the loss of expression of the p14ARF/p19ARF tumor
9
10
CALVISI
HEPATOLOGY, July 2011
Fig. 1. Schematic representation of the hypothetical interplays between Stat3 and p19ARF (and its human homolog, p14ARF) in hepatocarcinogenesis induced by activated Ras. (A) In the presence of p19ARF (p19ARFþ/þ), overexpression of constitutive active Stat3 (Ca-Stat3) promotes
liver tumor development, whereas both genetic disruption of Stat3 (Stat3/) or overexpression of unphosphorylated Stat3 (U-Stat3) drive liver
tumor suppression. In this genetic background, p19ARF sequesters an unknown factor termed ARF-X. (B) In the absence of p19ARF (p19ARF/),
U-Stat3 functionally interacts with ARF-X to trigger an oncogenic program. Similarly, genetic inactivation of Stat3 induces hepatocarcinogenesis in
the p19ARF/ background. In striking contrast, overexpression of Ca-Stat3 induces tumor suppression, presumably through the activation of an
alternative group of Stat3-specific target genes with antioncogenic activity.
suppressor gene. Unrestricted activation of the Ras
pathway via suppression of Ras cellular inhibitors and
epigenetic silencing of p14ARF (the human homolog of
mouse p19ARF) are two recurrent molecular events in
human hepatocarcinogenesis.6,15 In their present investigation, Schneller et al. used an established mouse tumor transplantation model lacking p19ARF and transformed by oncogenic v-Ha-Ras that was retrovirally
infected with various Stat3 gene variants. Interestingly,
the results show that constitutive active Stat3 (referred
to as Ca-Stat3) exerts a tumor-suppressive role in Rastransformed p19ARF/ hepatocytes, whereas the
expression of Stat3 lacking phosphorylation at the Tyr705 and Ser-727 residues (referred to as U-Stat3)
enhances liver tumor formation.14 Similarly, Ras-transformed hepatocytes lacking both Stat3 and p19ARF displayed an increase in tumor growth compared to those
expressing Stat3, implying an unexpected tumor suppressor activity of Stat3 in cells lacking p19ARF.14 Of
note, endogenous expression of p19ARF led to either
augmented or reduced HCC progression after expression of Ca-Stat3 or U-Stat3, respectively, in Ras-trans-
formed hepatocytes. Furthermore, the analysis of diethylnitrosamine-induced liver tumors showed a
remarkable up-regulation of p19ARF in Stat3 wild-type
mice, whereas a significant reduction of p19ARF levels
characterized HCC in Stat3-deleted mice following the
same carcinogenesis protocol.14
The results obtained in the mouse models were successfully recapitulated by Schneller et al. in human
HCC cells. Indeed, silencing of p14ARF in the Hep3B
cell line via short hairpin RNAs was associated with
reduced tyrosine phosphorylated levels of Stat3 during
tumor growth in order to circumvent the tumor-suppressive function of Stat3.14 Subsequent experiments
aimed at inhibiting the Janus tyrosine kinases revealed
that Jak caused tyrosine phosphorylation and activation
of Stat3 independently of p14ARF levels, implying that
p14ARF modulates the oncogenic function of tyrosinephosphorylated Stat3 downstream of Jak.14
In summary, this interesting study implies the existence of pro- and antioncogenic roles played by Stat3
in Ras-induced liver cancer that directly depend on
p19ARF/p14ARF expression (Fig. 1). In accordance with
HEPATOLOGY, Vol. 54, No. 1, 2011
the results obtained by Schneller et al., it has been previously shown that the HepG2 and PLC/PRF/5 HCC
cell lines, which are p14ARF-negative, are partly resistant to treatment with the Stat3 inhibitor NSC 74859.
On the other hand, Huh-7 and SNU-398 cells, which
express p14ARF, showed a remarkable decline in cell
proliferation after the same therapeutic approach.10
Therefore, the present data also envisage a predictive
value of Stat3 and p14ARF status in the treatment of
human HCC with Stat3 inhibitors.
The apparently paradoxical tumor-suppressive role
of Stat3 in liver cancer is in accordance with cumulative findings in other tumor types. In glioblastoma,
deficiency of the phosphatase and tensin homolog
tumor suppressor (PTEN) led to astrocyte malignant
transformation upon Stat3 inhibition, arguing for an
antioncogenic function of Stat3.16 Similarly, overexpression of Ca-Stat3 was able to consistently block cmyc–induced transformation of p53/ mouse fibroblasts.17 A dual role of Stat3 was also described in a
well-characterized mouse model of intestinal cancer.
In ApcMin/þ (adenatomous polyposis coli) mice, Stat3
promoted early adenoma formation, whereas Stat3
deficiency triggered rapid tumor progression and
invasion.18 The latter observation suggests that, similar to other genes, Stat3 might play diverse and seemingly paradoxical roles in cancer, reflecting specific
requirements during different stages of tumorigenesis.
According to this hypothesis, inactivation of the
E-cadherin (CDH1) tumor suppressor gene was predominantly detected in HCC with no vascular invasion, whereas increased CDH1 expression was a predominant feature of liver tumors with marked
vascular invasion and adverse prognosis.19 Similarly,
although lysyl oxidase (LOX) methylation and downregulation has been found in various tumor types,
LOX up-regulation is a hallmark of highly aggressive
tumors, due to its ability to confer migration and
invasive advantages to cancer cells during hypoxiainduced metastasis.20
Although the study by Schneller et al. has significantly improved our knowledge on the role, or roles,
of Stat3 in liver cancer, many questions on this topic
remain unanswered. Indeed, although it is clear that
Stat3 activation possesses oncogenic and antioncogenic
properties in the liver, the genes that cooperate with
p14ARF/p19ARF to modulate Stat3 functions remain
poorly delineated. In their work, Schneller et al.
hypothesize the presence of a putative transcription
factor, designated as ARF-X, which might mediate the
p14ARF/p19ARF control over Stat3 transcriptional activity.14 Additional experiments are required to address
CALVISI
11
this issue. Because a negative modulation by the NF-jB
over Stat3 in experimental hepatocarcinogenesis has been
demonstrated,8 it would be significant to determine
whether NF-jB contributes to Stat3 regulation by
p14ARF/p19ARF. Furthermore, the genes that modulate
the function of Stat3 in the liver independent of p14ARF/
p19ARF deserve additional investigation. In this regard,
preliminary data by Schneller et al. suggest that the tumor-suppressive role of Stat3 in the liver is independent
of the activity of both p16INK4A and p53 tumor suppressors.14 Also, it is worth mentioning that HCC cells with
a disrupted transforming growth factor beta pathway
have been found to be peculiarly sensitive to treatments
with Stat3 inhibitors, implying the existence of a crosstalk between these two signaling cascades.10 Finally, it
will be pivotal to determine the cellular (proliferation,
apoptosis, senescence) events as well as the molecular
mechanisms whereby the Stat3 function is modulated by
its interactors in liver cancer.
Note added in proof:
Since this Editorial was originally submitted, a
recent study from Wang et al.21 has shown that hepatocyte-specific Stat3 knockout mice are resistant to
DEN-induced hepatic carcinogenesis but more susceptible to CCl4-induced liver fibrosis and liver tumor development than wild-type mice, suggesting that Stat3
could either promote or inhibit liver tumorigenesis in
different models. These observations further substantiate the notion of the paradoxical oncogenic and tumor
suppressive role of Stat3 in liver cancer.
DIEGO F. CALVISI, M.D.
Pathology Institute
Ernst-Moritz-Arndt-Universita¨t
Greifswald, Germany
References
1. Bromberg JF. Activation of STAT proteins and growth control. Bioessays 2001;23:161-169.
2. Klampfer L. Signal transducers and activators of transcription (STATs):
Novel targets of chemopreventive and chemotherapeutic drugs. Curr
Cancer Drug Targets 2006;6:107-121.
3. Yang J, Stark GR. Roles of unphosphorylated STATs in signaling. Cell
Res 2008;18:443-451.
4. Wormald S, Hilton DJ. Inhibitors of cytokine signal transduction.
J Biol Chem 2004;279:821-824.
5. Taub R. Liver regeneration: from myth to mechanism. Nat Rev Mol
Cell Biol 2004;5:836-847.
6. Calvisi DF, Ladu S, Gorden A, Farina M, Conner EA, Lee JS, et al.
Ubiquitous activation of Ras and Jak/Stat pathways in human HCC.
Gastroenterology 2006;130:1117-1128.
7. Wurmbach E, Chen YB, Khitrov G, Zhang W, Roayaie S, Schwartz M,
et al. Genome-wide molecular profiles of HCV-induced dysplasia and
hepatocellular carcinoma. Hepatology 2007;45:938-947.
12
CALVISI
8. He G, Yu GY, Temkin V, Ogata H, Kuntzen C, Sakurai T, et al. Hepatocyte IKKbeta/NF-kappaB inhibits tumor promotion and progression
by preventing oxidative stress-driven STAT3 activation. Cancer Cell
2010;17:286-297.
9. Lau CK, Yang ZF, Lam SP, Lam CT, Ngai P, Tam KH, et al. Inhibition of Stat3 activity by YC-1 enhances chemo-sensitivity in hepatocellular carcinoma. Cancer Biol Ther 2007;6:1900-1907.
10. Lin L, Amin R, Gallicano GI, Glasgow E, Jogunoori W, Jessup JM,
et al. The STAT3 inhibitor NSC 74859 is effective in hepatocellular
cancers with disrupted TGFbeta signaling. Oncogene 2009;28:961-972.
11. Yoshida T, Ogata H, Kamio M, Joo A, Shiraishi H, Tokunaga Y, et al.
SOCS1 is a suppressor of liver fibrosis and hepatitis-induced carcinogenesis. J Exp Med 2004;199:1701-1707.
12. Riehle KJ, Campbell JS, McMahan RS, Johnson MM, Beyer RP, Bammler TK, et al. Regulation of liver regeneration and hepatocarcinogenesis
by suppressor of cytokine signaling 3. J Exp Med 2008;205:91-103.
13. Ogata H, Chinen T, Yoshida T, Kinjyo I, Takaesu G, Shiraishi H, et al.
Loss of SOCS3 in the liver promotes fibrosis by enhancing STAT3mediated TGF-beta1 production. Oncogene 2006;25:2520-2530.
14. Schneller D, Machat G, Sousek A, Proell V, van Zijl F, Zulehner G,
et al. p19ARF/p14ARF controls oncogenic functions of Stat3 in hepatocellular carcinoma. HEPATOLOGY 2011;54:164-172.
HEPATOLOGY, July 2011
15. Tannapfel A, Busse C, Weinans L, Benicke M, Katalinic A, Geissler F,
et al. INK4a-ARF alterations and p53 mutations in hepatocellular carcinomas. Oncogene 2001;20:7104-7109.
16. de la Iglesia N, Konopka G, Puram SV, Chan JA, Bachoo RM, You
MJ, et al. Identification of a PTEN-regulated STAT3 brain tumor suppressor pathway. Genes Dev 2008;22:449-462.
17. Ecker A, Simma O, Hoelbl A, Kenner L, Beug H, Moriggl R, et al.
The dark and the bright side of Stat3: proto-oncogene and tumor-suppressor. Front Biosci 2009;14:2944-2958.
18. Musteanu M, Blaas L, Mair M, Schlederer M, Bilban M, Tauber S,
et al. Stat3 is a negative regulator of intestinal tumor progression in
Apc(Min) mice. Gastroenterology 2010;138:1003-1011.
19. Wei Y, Van Nhieu JT, Prigent S, Srivatanakul P, Tiollais P, Buendia
MA. Altered expression of E-cadherin in hepatocellular carcinoma: correlations with genetic alterations, beta-catenin expression, and clinical
features. HEPATOLOGY 2002;36:692-701.
20. Payne SL, Hendrix MJ, Kirschmann DA. Paradoxical roles for lysyl
oxidases in cancer--a prospect. J Cell Biochem 2007;101:13381354.
21. Wang H, Lafdil F, Wang L, Park O, Yin S, Niu J, et al. Hepatoprotective versus oncogenic functions of STAT3 in liver tumorigenesis in
mice. Am J Pathol 2011 (in press).