2.1 The intrinsic pathway
The occurrence of the intrinsic apoptosis signal pathway of apoptosis
mainly involves various non-receptor-mediated stimuli, generating
intracellular signals, and directly acting on intracellular target
(mitochondria), leading to apoptosis. Stimulants that cause endogenous
apoptosis act in an active or passive manner. Passive signals include
the loss of specific growth factors, hormones and cytokines, leading to
an unlimited death program, thereby triggering cell
apoptosis(Papaianni et al., 2015). Other
stimuli that act in an active manner include but are not limited to
radiation, toxins, hypoxia, high fever, viral infections and free
radicals.
The mitochondrial pathway in apoptosis is mainly regulated by members of
the Bcl-2 family proteins(Ashkenazi &
Salvesen, 2014; Papaianni et al., 2015).
Bcl-2 family proteins have pro-apoptotic or anti-apoptotic effects. So
far, 25 genes have been identified in the Bcl-2 family. Anti-apoptotic
proteins include Bcl-2, Bcl-XL, Bcl-x, Bcl-XS, Bcl-w and
BAG, pro-apoptotic proteins include Bax, Bak, Bid, Bad, Bim, Bcl-10, Bik
and Blk(Borner & Andrews, 2014;
Kilbride & Prehn, 2013;
Wong & Puthalakath, 2008). These
proteins all play a role in determining whether a cell undergoes
apoptosis. Studies have found that the main mechanism of Bcl-2 family
protein function is to modulate the mitochondrial membrane permeability
to regulate the release of cytochrome c (Cyt-c).
The stimulus causes changes in the inner mitochondrial membrane, leading
to the opening of the permeability transition pore (PTP), the loss of
mitochondrial membrane potential (MMP), and the release of two groups of
pro-apoptotic proteins from the membrane space into the
cytoplasm(Tait & Green, 2010) (Figure
3). The first group contains Cyt-c, DIABLO/Smac and serine protease
HtraA2/Omi(Garrido et al., 2006;
van Loo et al., 2002). These proteins
activate the caspases-dependent mitochondrial pathway, cyt-c binds and
activates apaf-1 and procaspase-9 to form
“apoptosome”(Yuan & Akey, 2013). The
accumulation of procaspase-9 leads to the activation of caspase-9, and
caspase-9 triggers the caspase cascade, then the activated caspase-3/6/7
executes apoptosis. DIABLO/Smac and serine protease HtrA2/Omi have been
reported to inhibit IAPs (inhibitors of apoptotic protein) and promote
apoptosis(Schimmer, 2004). The second
group of pro-apoptotic proteins includes AIF (apoptosis-inducing factor)
and EndoG (endonuclease G), and the functions of AIF and EndoG are
independent of caspases. These two proteins are released from the
mitochondria when apoptosis occurs, and as a late event, it occurs in
the dying cells. AIF translocates into the nucleus, causing DNA
fragmentation and condensation of peripheral nuclear
chromatin(Joza et al., 2001). EndoG can
also be translocated into the nucleus to cut off nuclear chromatin and
produce oligomeric nucleosome DNA
fragments(Jang et al., 2015). As a site
for protein synthesis and processing, the ER is imbalanced due to
physical and chemical stimulation or infection of pathogenic
microorganisms, resulting in the inability of protein folding or
misfolding. When these conformationally dissimilated proteins accumulate
in the ER and cannot be effectively eliminated, the cells will initiate
an unfolded protein response (UPR). Long-term activation of UPR can
trigger cell apoptosis through IRE1 (inositol requiring protein-1), PERK
(protein kinase RNA-like ER kinase) and ATF6 (activating transcription
factor-6) pathways, which are connected with mitochondria-mediated
apoptosis pathway(Verma & Datta, 2012).
First, phosphorylated IRE1 recruits TRAF2 (TNF receptor-associated
factor 2) and triggers a cascade of phosphorylation events, such as the
activation of ASK1 (apoptosis signaling kinase 1), and ultimately
phosphorylates and activates JNK. Then, JNK activates pro-apoptotic
genes Bax, Bak, Bim, PUMA and NOXA, these genes are transferred to the
mitochondrial to initiate apoptosis(Upton
et al., 2012). The homomultimerization and autophosphorylation of PERK
lead to phosphorylation of eIF2α (eukaryotic translation initiation
factor 2α), which increases the translation of ATF4 (activating
transcription factor 4). Then, ATF4 upregulates the expression of CHOP
(C/EBP-homologous protein) and activates
apoptosis(Tabas & Ron, 2011). CHOP can
reduce the expression of anti-apoptotic genes Bcl-2 and
Bcl-XL(Puthalakath et
al., 2007). ATF6 binds to the ER membrane, but when protein homeostasis
is disrupted, ATF6 then migrates to the Golgi apparatus to undergo
cleavage, first by Site 1 Protease (S1P) and then by Site 2 Protease
(S2P)(Ye et al., 2000), cleaved ATF6 can
also promote cell apoptosis via upregulating of
CHOP(Hirsch, Weiwad, Prell, & Ferrari,
2014; Morishima, Nakanishi, & Nakano,
2011).