Mitochondrial apoptotic pathways
Mitochondrial dysfunction is one of the
crucial events involved in apoptotic cell death. Accumulating evidence suggests
that apoptotic pathways converge at the mitochondria; signaling is initiated
via a series of molecular events that culminate in the release of death factors
(21). Gene polymorphisms in mitochondrial apoptotic pathways usually play a
role in the apoptosis of hepatoma cells.
Heat shock protein 70 (HSP70), abundantly
expressed in human tumors, protects cells from a wide range of apoptotic
stimuli. HSP70 potentially modulates mitochondrial protein apoptosis-inducing
factor (AIF)-induced cell death. Furthermore, HSP70 silencing has been shown to
dramatically increase AIFDelta1-120 mutant (an active form of AIF after
deletion of the mitochondria localization domain)-induced apoptosis (22). On
the other hand, the potential pro-apoptotic molecule in HCC cells exhibits a
gene polymorphism associated with mitochondrion. Fragile histidine triad (FHIT)
is a confirmed tumor suppressor gene in HCC. Yu and colleagues (23) found that
transactivator of transcription (TAT)-FHIT robustly inhibits growth and induces
apoptosis of HCC cells in vitro;
furthermore, they demonstrated that both exogenous and intrinsic apoptotic
pathways are involved in TAT-FHIT-mediated apoptosis, and that this effect may
be partially attenuated by mitochondrial protector TAT-BH4. These findings
indicate that mitochondria play a critical role in TAT-FHIT-mediated
pro-apoptotic effects in cancer cells. TAT-FHIT is a potential pro-apoptotic
molecule in HCC cells. The tumor suppressive mechanism of the tyrosine
aminotransferase gene has also been shown to play a
pro-apoptotic role in a mitochondrial-dependent manner by promoting
cytochrome-c release and activating caspase-9 and PARP (24). Moreover, a novel
gene, homo sapiens longevity assurance homologue 2 (LASS2), has been isolated
from a human liver cDNA library, a human homologue of the yeast longevity
assurance gene LAG1 (Saccharomyces cerevisiae longevity assurance gene 1) (25).
The LASS2 gene may increase intracellular H(+) of HCC cells via interaction
with V-ATPase, thereby inducing cell apoptosis through the mitochondrial
pathway.
Transforming growth factor-beta (TGF-beta)
is a potent inhibitor of hepatocyte growth (26), and is known to induce apoptosis
in both hepatocytes and HCC cell lines (27,28). One of the mediators of the
TGF-beta pathway is the Smad family of genes, and candidate tumor suppressor
genes in Smad2 and Smad4 gene mutations have been identified in HCC; these
mutations were suspected to result from oxidative stress, as observed in mtDNA
(29). More recently, Zhang and colleagues (30) found that TGF-beta1 treatment
of Huh7 cells resulted in increased PDCD4 expression and apoptosis, concomitant
with mitochondrial events. The human HCC cell line Huh7 transfected with PDCD4
resulted in increases of cytosolic cytochrome c and mitochondrial Bax, which was accompanied with decreases
Figure
1. Genes in the TGF-beta pathway or
downstream of TGF-beta promote apoptosis due to mitochondrial changes. One of
the mediators of the TGF-beta pathway is the Smad family of genes, and
candidate tumor suppressor genes in Smad2 and Smad4 gene mutations have been
identified in HCC; these mutations were suspected to result from oxidative
stress, as observed in mtDNA. The human HCC cell line Huh7 transfected with
PDCD4 resulted in increases of cytosolic cytochrome c and mitochondrial Bax, which
accompanied with decreases of mitochondrial cytochrome c and cytosolic Bax,
then inducing apoptosis.
of
mitochondrial cytochrome c and cytosolic Bax, then inducing apoptosis. Thus,
genes in the TGF-beta pathway or downstream of TGF-beta could promote apoptosis
due to mitochondrial changes (Figure 1).
The tumor suppressor p53 gene plays a major
role in preventing tumorigenesis by responding to both cellular stress and DNA
damage; p53 mutation is frequently associated with oncogenesis (31,32). In some
HCC cell lines, its activation leads to the induction of pro-apoptotic Bax
expression (33,34), whereas pro-apoptotic Bax also antagonizes anti-apoptotic
Bcl-2 and other anti-apoptotic molecules. The mitochondria play a central role
in apoptosis, which is regulated by members of the Bcl-2 family through an
intrinsic apoptosis pathway (35-37). In p53-deficient Hep3B cells, apoptosis
induced by pectenotoxin-2 (PTX-2) is associated with the down-regulation of
anti-apoptotic Bcl-2 members (Bcl-2 and Bcl-xL) and IAP family proteins, and
the up-regulation of pro-apoptotic Bax protein, tumor necrosis factor-related
apoptosis-inducing ligand (TRAIL)-receptor 1/receptor 2 (DR4/DR5), and
mitochondrial dysfunction (38).
In contrast, interference with the p53
family network contributes to a gain of oncogenic function of mutant p53 in HCC
via mitochondrial apoptotic pathway. Mutant forms of p53, including p53R143A,
p53R175D, p53R175H, p53R248W, and p53R273H, can acquire anti-apoptotic
gain-of-function in HCC by repressing the activity of genes that regulate both
the extrinsic apoptosis pathway initiated by ligation of death receptors and
the intrinsic/mitochondrial apoptosis pathway. Furthermore, p53
gain-of-function mutants significantly decrease activation of pro-apoptotic
target genes by wild-type p53, TAp63, and TAp73. This contributes to the
ability of cancer cells to withstand DNA damage-induced apoptosis (39). TAp63
and TAp73 are the promoters of the Trp63 gene and Trp73 gene, respectively. In
general, Trp63 gene and Trp73 gene have two respective promoters that drive the
expression of two major p63 and p73 isoform subfamilies: TA and N-terminally
truncated (DeltaN). TAp63 and TAp73 isoforms exhibit pro-apoptotic activities,
whereas members of DeltaNp63 and DeltaNp73 subfamily exhibit antiapoptotic
functions. Müller and colleagues (40) found that TAp73 beta could transactivate
the CD95 gene via the p53-binding site in the first intron. Inhibition of CD95
gene transactivation was one mechanism by which DeltaNp73 functionally
inactivated the tumor suppressor action of p53 and TAp73 beta. Concomitantly,
DeltaNp73 inhibited the apoptosis emanating from mitochondria. Thus, DeltaNp73
expression in tumors selects against both the death receptor and the
mitochondrial apoptosis activity of TAp73 beta. In HCC DeltaNp63alpha and
DeltaNp73 beta expression interferes
Figure
2. Mfn2 exerts apoptotic effects and
inhibits proliferation in HCC cells by the mitochondrial apoptotic pathway.
Overexpression of mfn2 also induced cytochrome c release to the cytoplasm by
enhancing Bax translocation from the cytoplasm to the mitochondrial membrane.
Furthermore, HBx was shown to inhibit p53-mediated up-regulation of mfn2 in HCC
cells.
with
both the death receptor and the mitochondrial apoptosis activity of the TA
isoforms (41,42). DeltaNp63alpha could inhibit the activation of p53 family
target genes (41), whereas DeltaNp73beta is oncogenic in HCC by blocking
apoptosis signaling via death receptors and mitochondria (42). These findings
suggest that DeltaNp63 and DeltaNp73 isoforms repress apoptosis-related genes
of mitochondrial apoptosis signaling pathways.
The mitochondrial GTPase mitofusin-2 gene
(mfn2), also named hyperplasia suppressor gene (HSG), located on chromosome
1p22.3, encodes a mitochondrial membrane protein that participates in
mitochondrial fusion and contributes to the maintenance of the mitochondrial
network (43,44). It exhibits a potential apoptotic effect mediated by the
mitochondrial apoptotic pathway (45-47). Qu and colleagues (48) reported that
heterozygosity (LOH) at mfn2 gene in HCC is associated with clinicopathological
features of patients, including tumor size, age, capsule, differentiation, and
t HBV infection. Mfn2 was identified as a novel direct target of p53 that could
also exert apoptotic effects via Bax signaling and potentially inhibits
proliferation in HCC cells (49,50). Overexpression of mfn2 also induced
cytochrome c release to the cytoplasm by enhancing Bax translocation from the
cytoplasm to the mitochondrial membrane (50). Furthermore, the relationship
between mfn2 and HBx was examined. HBx was
shown to inhibit p53-mediated up-regulation of mfn2 in HCC cells, though HBx exhibited little effect on the expression of mfn2
or P53. (51) (Figure 2).
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