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Schematic representation of interaction of flavonoids with cell signalling 
pathways. As shown in figure, flavonoids activate various signalling 
pathways such as ERK, Akt/PKB, PI3K, and PKC to improve the cell survival. 
The symbol arrows show the activation, and the symbol box drawings light up 
and horizontal show deactivation of various signalling pathways. ERK 
extracellular signal-regulated protein kinase; JNK c-Jun N-terminal kinase; 
PI3K phosphatidylinositol-3 kinase; PKC protein kinase C; Akt/PKB protein 
kinase B; ARE antioxidant response element; CREB cAMP response 
element-binding protein; Nrf2 nuclear factor (erythroid-derived 2)-like 2; 
PM plasma membrane

Quercetin, a most abundant flavonoid found in many fruits and vegetables 
and EGCG, abundantly found in green tea, both inhibited H2O2-induced 
phosphorylation of JNK and p38 MAPK pathway after 60 min of exposure. Both 
quercetin and EGCG also inhibit H2O2-induced caspase-3 activation at the 
concentrations between 1 and 50 μM/L (Choi et al. 2005). Thus, MAPK-related 
signalling may regulate expression of apoptotic genes, preventing 
apoptosis, and promoting cell survival. Another observation demonstrates 
that EGCG at the concentrations between 5 and 25 μM/L inhibits angiotensin 
II-induced endothelial stress fibre formation and hyperpermeability via 
inactivation of p38/heat shock protein 27 (HSP27) pathway and suggests that 
EGCG may protect against endothelial barrier dysfunction and injury (Yang 
et al. 2010).

The flavonoids hesperetin and its structural counterparts, isorhamnetin, 
and isosakuranetin differentially activated pro-survival signalling 
molecules, including PI3K/Akt and other protein kinases. In nervous 
tissues, the hesperetin (100 nmol/L) and its metabolites 5-nitro-hesperetin 
were effective at preventing neuronal apoptosis via a mechanism involving 
both Akt/PKB activation/phosphorylation and also via an activation of 
ERK1/2 (Vauzour et al. 2007).

Myricetin induces cell survival via signal transduction pathway involving 
Akt activation. Cells induced with H2O2-induced apoptosis were rescued by 
myricetin (30 μM) treatment, and this survival mechanism was inhibited by 
the specific PI3K inhibitor (Kyoung et al. 2010). These observations 
suggest that PI3K/Akt and MAPK are the main signalling pathways by which 
myricetin prevents oxidative stress-induced apoptosis (Kyoung et al. 2010).

Brain injury induces acute inflammation, thereby exacerbating poststroke 
neuronal damage.1, 2, 3, 4 Although central nervous system (CNS) is known 
for its limited reparative capacity, inflammation is a strong stimulus for 
reparative mechanisms including activation of neurogenesis. However, the 
latter results in low survival of newly generated neural stem cells.5 These 
findings indicate the relevance of endogenous regulatory and/or 
environmental factors for survival and differentiation of neural stem cells.

HIF-1α stabilization and NF-κB activation may also have a role in promoting 
the survival of cancer cells, angiogenesis, neovascularization, glycolytic 
ATP generation, and tumor invasion. Therefore, hypoxia-induced mROS may 
promote cancer development and progression. However, overgeneration of 
mROS, occurring after mGSH depletion or by blocking mitochondrial 
respiration (84, 114), may sensitize tumor cells by inhibiting the NF-κB 
survival pathway (Fig. 8). Because hypoxia is expected to affect 
predominantly cells from solid tumors, more than cells from healthy 
tissues, the combination of mGSH depletion, or strategies that increase 
mROS, and hypoxia may be an interesting approach in cancer therapy that 
deserves further study.

*mGSH depletion sensitizes tumor cells to hypoxia.* HIF-1α stabilization 
and NF-κB activation participate in promoting survival of cancer cells 
under hypoxic conditions. However, overgeneration of mitochondrial ROS, as 
obtained after mGSH depletion, may sensitize tumor cells by inhibiting the 
NF-κB survival pathway, despite HIF stabilization.

In addition to marshalling immune and inflammatory responses, transcription 
factors of the NF-κB family control cell survival. This control is crucial 
to a wide range of biological processes, including B and T lymphopoiesis, 
adaptive immunity, oncogenesis and cancer chemoresistance. During an 
inflammatory response, NF-κB activation antagonizes apoptosis induced by 
tumor necrosis factor (TNF)-α, a protective activity that involves 
suppression of the Jun N-terminal kinase (JNK) cascade. This suppression 
can involve upregulation of the Gadd45-family member Gadd45β/Myd118, which 
associates with the JNK kinase MKK7/JNKK2 and blocks its catalytic 
activity. Upregulation of XIAP, A20 and blockers of reactive oxygen species 
(ROS) appear to be important additional means by which NF-κB blunts JNK 
signaling. These recent findings might open up entirely new avenues for 
therapeutic intervention in chronic inflammatory diseases and certain 
cancers; indeed, the Gadd45β-MKK7 interaction might be a key target for 
such intervention.

In multicellular organisms, cells are constantly faced with the choice of 
whether to live or die. The decision requires integration of a complex 
network of intracellular and extracellular signals, and making the right 
decision is essential for survival of these organisms. Programmed cell 
death (PCD) is crucial to tissue homeostasis, organ development and the 
elimination of defective or `dangerous' cells, such as cancerous and 
virus-infected cells (Danial and Korsmeyer, 2004; Rathmell and Thompson, 
2002). Underscoring the importance of this process, numerous diseases arise 
from defects in the pathways controlling PCD. For instance, defective and 
excessive cell death respectively contribute to cancer and 
neurodegenerative disorders such as Alzheimer's disease (Danial and 
Korsmeyer, 2004; Rathmell and Thompson, 2002). Ultimately, the balance 
between life and death might depend on the ability of the cell to sustain 
activation of transcription factors of the NF-κB family.

TNFR1-induced pathways modulating apoptosis. Formation of complex I leads 
to NF-κB activation, Gadd45β induction, JNK inhibition and cell survival. 
Formation of complex II leads to caspase-8/10-mediated cleavage of Bid into 
tBid, which then targets mitochondria to induce cytochrome c release and, 
ultimately, cell death. The figure also depicts JNK activation, which 
results in formation of jBid; this promotes PCD by triggering release of 
Smac/Diablo into the cytosol, inhibiting the TRAF2-IAP1 complex and 
consequently activating caspase-8. The Gadd45β-MKK7 interaction linking the 
JNK and NF-κB pathways is also shown.

Numerous studies have shown that NF-κB has anti-apoptotic effects that have 
been implicated in a variety of biological processes. In the B-cell 
lineage, this activity is required for completion of various developmental 
steps, including differentiation into mature IgMlow/IgDhigh cells, as well 
as the response of these cells to antigen and CD40 costimulation (Gilmore 
et al., 2004; Gerondakis and Strasser, 2003). Likewise, during an immune 
reaction, survival of naive T cells depends on NF-κB activation by the 
T-cell receptor (TCR) and CD28 stimulation (Green, 2003; Zheng et al., 
2003; Kane et al., 2002). NF-κB also plays an important pro-survival role 
during thymocyte development (Voll et al., 2000; Boothby et al., 1997; 
Esslinger et al., 1997).

The pro-survival activity of NF-κB also plays a crucial role in viral 
pathogenesis (reviewed by Kucharzack et al., 2003). Indeed, the need for an 
inducible gene expression program to maintain cell survival might have 
originally evolved as a mechanism for disposing of infected cells that, 
because of viral takeover, exhibit grossly defective transcription. Not 
surprisingly, many viruses have adapted to this host defense mechanism by 
developing their own anti-apoptotic strategies or acquiring genes that 
either induce or mimic NF-κB. Examples of such genes, many of which are 
implicated in viral oncogenesis, include: v-FLIP of human herpesvirus 8 
(HHV-8), which is linked to Kaposi's sarcoma and lymphoma; Tax of HTLV-1, 
which causes adult T-cell leukemia (ATL); and of course v-Rel, which is 
encoded by the avian retrovirus REV-T (Kucharczak et al., 2003; Benedict et 
al., 2002).

The suppression of TNF-α-induced apoptosis by NF-κB is crucial for the 
survival of the organism and its response to injury. In mice lacking RelA, 
liver apoptosis and embryonic lethality are rescued by deletion of TNFR1 
(Alcamo et al., 2001; Doi et al., 1999). The resistance to TNF-α-induced 
apoptosis that NF-κB confers on the liver has also been observed in adults 
(Chaisson et al., 2002; Maeda et al., 2003). Overactivation of NF-κB by 
TNF-α can be detrimental too. For instance, when caused by loss of the 
de-ubiquitinase CYLD, it inappropriately blocks apoptosis, thereby 
promoting oncogenesis (Brummelkamp et al., 2003; Kovalenko et al., 2003; 
Trompouki et al., 2003). NF-κB-mediated inhibition of TNFR-induced PCD is 
also involved in chronic inflammatory diseases (Liu and Pope, 2003) (see 
below).

The relevance of the JNK cascade to apoptosis signaling is highlighted by 
the finding that activation of this cascade is controlled by NF-κB. Indeed, 
suppression of NF-κB by ablation of RelA or IKKβ, or expression of IκBαM, 
leads to persistent (rather then transient) JNK induction by TNF-α, and it 
seems to be the persistence of this induction that ultimately causes the 
cell to succumb to PCD (De Smaele et al., 2001; Javelaud and Besancon, 
2001; Tang et al., 2001) (see also Franzoso et al., 2003). Caspases can 
activate various MAPKKKs (Davis, 2000; Roulston et al., 1998), but the 
effects of NF-κB on JNK signaling are not affected by protective cell 
treatment with the caspase blocker z-VADfmk and so do not appear to be a 
secondary consequence of caspase inhibition (Javelaud and Besancon, 2001; 
Franzoso et al., 2003). In short, the containment of the JNK cascade is 
crucial for the control of TNF-α-induced apoptosis, and this critically 
depends on NF-κB. Curiously, although confirming the inhibitory effects of 
NF-κB on JNK signaling, another study has suggested that, in TNF-α-treated 
NF-κB-deficient cells, persistent JNK activation promotes cell survival 
(Reuther-Madrid et al., 2002). The bases for the discrepancy with other 
studies are not clear.
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