Introduction
Primary liver cancer is the 6th most commonly
diagnosed cancer and the 3rd leading cause of cancer
death worldwide. Incidence and mortality are higher among men and in low
to moderate income countries (1). Hepatocellular carcinoma (HCC),
accounting for 75–85% of primary liver cancers, develops in the
context of chronic liver diseases, such as hepatitis and/or metabolic
dysfunction. HCC is increasingly associated with obesity, insulin
resistance and the metabolic syndrome and has limited therapeutic
options (2, 3). The signalling pathways most frequently involved in
hepatocarcinogenesis include Wnt/β-catenin, mTOR, IL-6, TGF-β, Ras, Rb,
HGF/c-Met, and IGF1, which converge and modulate the activity of the
NF-κB, p53, Stat3, c-Myc and AP-1 transcription factors (4-6).
The Jun (c-Jun, JunB, JunD) and Fos (c-Fos, FosB, Fra-1, Fra-2) proteins
are components of the dimeric Activator Protein-1 (AP-1) transcription
factor complex (7). While Jun proteins can form homo- or heterodimers,
Fos proteins can only form heterodimers with a Jun protein. The AP-1
dimer combinations that co-exist in a given cell/biological context,
together with dimer-specific variation in DNA sequence affinity and/or
co-activator/repressor recruitment, determine the target genes that are
positively or negatively regulated by AP-1. The AP-1 dimer pool is
modulated by various signals, such as growth factors, inflammatory
cytokines, mechanical and oxidative stress, and plays important roles in
many diseases including cancer (7, 8). In genetically engineered mouse
models (GEMMs), liver-specific inactivation of c-Jun revealed its
essential role in liver regeneration (9), steatohepatitis (10),
hepatocyte survival during acute hepatitis (11), endoplasmic reticulum
(ER) stress (12), and liver cancer (13-18). In HCC, c-Jun promotes the
survival of diethylnitrosamine (DEN)-induced pre-neoplastic hepatocytes
by repressing c-Fos expression (16), while c-Fos is needed for
DEN-induced hepatocarcinogenesis when the c-jun gene is intact
(19). Furthermore, doxycycline (Dox)-switchable c-Fos expression in
adult hepatocytes (c-Foshep) leads to reversible liver
inflammation, accumulation of toxic oxysterols and bile acids,
activation of the DNA damage response (DDR), premalignant transformation
and enhanced DEN-induced HCC (19). The hepatic functions of the other
Jun and Fos proteins are less studied, especially in cancer.
Hepatocyte-specific JunB inactivation increases liver damage during
acute hepatitis, an effect that is largely counteracted by the
pro-inflammatory role of JunB in hepatic NK/NKT cells (20). JunD
knock-out mice are protected from chemically-induced liver fibrosis (21)
and high fat diet (HFD)-induced hepatosteatosis (22). On the other hand,
loss of Fra-1 sensitizes, while hepatic Fra-1, but not Fra-2, expression
protects from acetaminophen-induced liver damage, an acute liver failure
paradigm (23). Interestingly, Fra-1 and Fra-2 play redundant functions
in hepatic lipid metabolism: Fra-1 or Fra-2 expression in hepatocytes
prevented and could even revert HFD-induced hepatosteatosis by
suppressing the transcription of the nuclear receptor PPARγ, a central
regulator of lipid metabolism, while single inactivation of either of
the two genes had no effect (22, 24). In contrast, c-Fos activated
hepatic Pparg transcription, while it suppressed another nuclear
receptor LXRα, responsible for increased hepatic cholesterol and
oxysterols (19, 22). Thus, Fra-1/2- and c-Fos-containing AP-1 dimers
exert antagonistic effects on the pparg2 promoter and lipid
handling in the liver. When selected Jun and Fos monomers were tethered
by a flexible polypeptide to force specific AP-1 pairing in a
“single-chain” approach (25), and expressed in Dox-switchable
AP-1hep mice, c-Jun~Fra-2 dimers
inhibited, whereas c-Jun~c-Fos,
JunB~c-Fos, and JunD~c-Fos dimers
activated PPARγ expression and signalling (22, 24).
In this study, we show that hepatic expression of
c-Jun~Fra-2 dimers results in spontaneous and reversible
HCC formation, while mice expressing Fra-1/2 monomers or
c-Jun~Fra-1 dimers remained tumor-free.
c-Jun~Fra-2 dimers promote tumorigenesis in murine and
human liver cells, in significant part through direct transcriptional
activation of c-myc expression. Furthermore, we show that
established tumors are largely addicted to c-Jun~Fra-2
and sensitive to JQ-1, a BET bromodomain inhibitor that inhibits c-Myc
activity.