Download Document

Ms. Bushra ALLAH RAKHA
Ph.D. SCHOLAR
DEPARTMENT OF WILDLIFE MANAGEMENT
PMAS-AAUR
 ROS
are generally perceived as toxicants that
induce various deleterious effects, like cell
dysfunction,
death
or
malignant
transformation.
 The toxic potential of ROS is used by the
innate immune defense as a powerful
weapon against pathogens.
 If pathogens may elude their recognition as
“non-self” by the sophisticated adaptive
immune system due to their plasticity and
adaptive mechanisms, they cannot escape
the rough chemical attack of ROS.




Cellular redox balance is maintained by a powerful
antioxidant system that “neutralizes” ROS.
It consists of SOD, catalase, the glutathione system
(glutathione, glutathione reductase, peroxidase and
transferase), the thioredoxin system (thioredoxins,
thioredoxin peroxidase and peroxiredoxins), vitamin E
and C.
Subtle intra- and extracellular mechanisms related to
metal-binding proteins (transferring, albumin, ferritin
etc.) and various metabolites (uric acid, bilirubin,
pyruvate etc) are also active.
It is noteworthy that cellular targets attacked and
damaged by ROS (lipids, proteins, sugars, nucleic
acids) contribute themselves to ROS detoxification
and
represent
therefore
sacrifice
cellular
components.
 Oxidative
stress generally describes a
condition in which cellular antioxidant
defense mechanisms are insufficient to
inactivate ROS, or excessive ROS are
produced, or both.
 It
is well-documented that significant
oxidative stress carries out severe damage to
lipids, proteins, sugars and nucleic acid bases
which compromises cell viability and
functions.
Oxidative damage of any cellular constituent, if
unchecked, can theoretically contribute to
disease development.
 Indeed, an increasing amount of evidence
suggests that oxidative stress is linked, more or
less directly, to either primary or secondary
pathophysiologic mechanisms of several acute
and chronic human diseases.
 cancer cells develop an enhanced constitutive
oxidative stress that sustains tumor growth and
shields these cells against pro-apoptotic signals,
thus promoting tumor progression.


Rac1 is a small G-protein in the Rho family that drives Actin
polymerization and formation of lamellipodia, promotes cell-cell
adhesion, breakdown and migration of different carcinoma cells.

In other epithelial cell lines, Rac activation also plays a role in
scattering after distinct stimuli such as Growth factor stimulation
or Integrin engagement.

It also plays a critical role in processes, such as control of cell
morphology, transcriptional activation, and apoptosis signaling.

The broad range of events controlled by this GTPase requires
regulation of its interactions with multiple downstream targets.

Rac1 is activated by GEF (Guanine Nucleotide Exchange Factors),
in particular ARHGEF6 and repressed by GAPs (GTPase-Activating
Proteins), specifically RacGAP.

Rac1 GTPase mediates key cellular processes in response to
upstream regulators such as Growth Factors, Integrins and HA
(Hyaluronic Acid)-binding receptor CD44.




Signaling pathways that are regulated by Rho family
members play an important role in several
pathological
conditions,
including
cancer,
inflammation, and bacterial infections.
Rac controls the generation of ROS , both in
leukocytes and nonhematopoietic cells and is
necessary for Cadherin-dependent adhesion.
Rac activation is required for the full transformed
phenotype induced by oncogenes such as TIAM1 and
Ras. In addition, Rac activation perturbs cadherin
contacts with a concomitant change in cell shape,
including formation of lamellae/protusions and
conversion to a fibroblastic morphology.
Translocation of Rac to the plasma membrane is also
required for assembly and activation of the NADPH
Oxidase complex.
The importance of Rac1 in neoplastic
transformation is indicated by the finding that
Rac1
is
necessary
for
K-Ras-induced
transformation of fibroblasts.
 Nuclear factor kappaB (NFκB) is also implicated
in cancer development and progression.
 NFκB is a ubiquitously expressed transcription
factor which regulates over 200 different genes
involved in numerous pathways including
inflammation, apoptosis/survival, cell cycle
progression and migration.
 Interestingly, many cancers have been shown to
rely on constitutive NFκB signaling, which is now
referred to as NFκB addiction.

Protein complex that controls the transcription
of DNA.
 NF-κB is found in almost all animal cell types and
is involved in cellular responses to stimuli such
as stress, cytokines, free radicals, UV radiation,
oxidized LDL, and bacterial or viral antigens.
 NF-κB plays a key role in regulating the immune
response to infection (κ light chains are critical
components of immunoglobulins).
 Incorrect regulation of NF-κB has been linked to
cancer, inflammatory and autoimmune diseases,
septic shock, viral infection, and improper
immune development.
 NF-κB has also been implicated in processes of
synaptic plasticity and memory.




NF-κB (green) heterodimerizes with RelB (cyan) to form a
ternary complex with DNA (orange) that promotes gene
transcription.
NF-κB is important in regulating cellular responses because
it belongs to the category of "rapid-acting" primary
transcription factors, i.e., transcription factors that are
present in cells in an inactive state and do not require new
protein synthesis in order to become activated (other
members of this family include transcription factors such
as c-Jun, STATs, and nuclear hormone receptors).
This allows NF-κB to be a first responder to harmful
cellular stimuli. Known inducers of NF-κB activity are
highly variable and include reactive oxygen species (ROS),
tumor necrosis factor alpha (TNFα), interleukin 1-beta (IL1β), bacterial lipopolysaccharides (LPS), isoproterenol,
cocaine, and ionizing radiation.
NF-κB is widely used by eukaryotic cells as a
regulator of genes that control cell proliferation
and cell survival.
 Active NF-κB turns on the expression of genes
that keep the cell proliferating and protect the
cell from conditions that would otherwise cause
it to die via apoptosis.
 Defects
in NF-κB results in increased
susceptibility to apoptosis leading to increased
cell death.
 This is because NF-κB regulates anti-apoptotic
genes especially the TRAF1 and TRAF2 and,
therefore, checks the activities of the caspase
family of enzymes, which are central to most
apoptotic processes.

 In
tumor cells, NF-κB is active either due to
mutations in genes encoding the NF-κB
transcription factors themselves or in genes
that control NF-κB activity (such as IκB
genes).
 Some tumor cells secrete factors that cause
NF-κB to become active. Blocking NF-κB can
cause tumor cells to stop proliferating, to
die, or to become more sensitive to the
action of anti-tumor agents.

The Wnt signaling pathways are a group of signal transduction
pathways made of proteins that pass signals from outside of a
cell through cell surface receptors to the inside of the cell.

Three signaling pathways:

The canonical Wnt pathway

The noncanonical planar cell polarity pathway

The noncanonical Wnt/calcium pathway.

All three Wnt signaling pathways are activated by the binding of
a Wnt-protein ligand to a Frizzled family receptor, which passes
the biological signal to the protein Dishevelled inside the cell.

The canonical Wnt pathway leads to regulation of gene
transcription.

The noncanonical planar cell polarity pathway regulates the
cytoskeleton that is responsible for the shape of the cell

The noncanonical Wnt/calcium pathway regulates calcium inside
the cell.
 Wnt
signaling was first identified for its role
in carcinogenesis, but has since been
recognized for its function in embryonic
development.
 The clinical importance of this pathway has
been demonstrated by mutations that lead to
a variety of diseases, including breast and
prostate cancer, colorectal cancer,
glioblastoma, type II diabetes.
Colorectal cancer, also known as colon cancer,
rectal cancer, or bowel cancer, is a cancer from
uncontrolled cell growth in the colon or rectum
(parts of the large intestine), or in the appendix.
 The symptoms and signs of colorectal cancer
depend on the location of tumor in the bowel,
and whether it has spread elsewhere in the body
(metastasis).
 The classic warning signs include: worsening
constipation, blood in the stool, decrease in
stool calibre, loss of appetite, loss of weight,
and nausea or vomiting in someone over 50 years
old.

Greater than 75-95% of colon cancer occurs in
people with little or no genetic risk.
 Other risk factors include older age, male
gender, high intake of fat, alcohol or red meat,
obesity, smoking and a lack of physical exercise.
 Approximately 10% of cases are linked to
insufficient activity.
 Those with a family history in two or more firstdegree relatives have a two to threefold greater
risk of disease and this group accounts for about
20% of all cases.
 A number of genetic syndromes are also
associated with higher rates of colorectal cancer.

 Colorectal
cancer (CRC) derives from normal
colonic mucosa by stepwise accumulation of
somatic genetic alterations, starting with
inactivation of the tumor suppressor gene
Adenomatous Polyposis Coli (APC) or
activation of the oncogene ß-catenin
(CTNNB1).
 APC is a negative regulator of WNT signaling
and its mutational inactivation leads to
accumulation of β-catenin, which associates
with transcription factors of the T-cell Factor
(TCF) family to activate WNT target genes.
 RAC1
acts downstream of constitutive WNT
signaling to promote progenitor cell
proliferation and expansion of the LGR5 ISC
population.
 Mechanistically, these phenotypes depend on
RAC1-driven production of ROS and NF-kB
signaling.
 The role of ROS in stem cell maintenance,
cellular transformation, and CSC survival
appears to be context and tissue specific.
 In
proliferative neural stem cells, high levels
of ROS regulate self-renewal by driving
PI3K/AKT signaling.
 Conversely, hematopoietic stem cells are
sensitive to ROS levels and higher levels limit
their lifespan.
 RAC1 is also required for efficient NF-kB
signaling after Apc loss and, importantly,
constitutive activation of NF-kB in the
absence of RAC1 partially rescued the
attenuated proliferation and ISC expansion
phenotypes.
 RAC1
deficiency prevents this by impairing
NF-kB signaling, despite b-catenin nuclear
accumulation.
 The finding that ROS and NF-kB both play
important roles in intestinal tumor initiation
indicates a role for inflammation in this
process.
 Rac1
deletion
did
not
completely
recapitulate
this,
the
comprehensive
suppression of tumor formation suggests that
RAC1 is required for transformation after Apc
loss.
 ROS and NF-kB are two parallel pathways
involved in promoting ISC/progenitor
proliferation downstream of RAC1.
 It should be noted that activation of ROS or
NF-kB was not sufficient to completely
rescue the phenotypes associated with Rac1
loss.
 This
is important in two ways;
 1st: how robust the requirement for RAC1 is
to intestinal tumor formation. As a key
signaling node that integrates numerous
downstream signaling pathways, the
potential therapeutic benefits of targeting it
should be high.
 Also, it would likely increase the range of
tumors that would be sensitive to its
inhibition and reduce the scope for drug
resistance to develop.
 2nd:
it indicates that the RAC1 phenotype
may also involve additional downstream
pathways.
 In summary, downstream of WNT/MYC
signaling after Apc loss that is crucial for
hyperproliferation and hence tumorigenesis.




Buczacki, S.J., Zecchini, H.I., Nicholson, A.M., Russell, R.,
Vermeulen, L., Kemp, R., and Winton, D.J. (2013).
Intestinal label-retaining cells are secretory precursors
expressing Lgr5. Nature 495, 65–69.
Cancer Genome Atlas Network. (2012). Comprehensive
molecular characterization of human colon and rectal
cancer. Nature 487, 330–337.
Zhou, C., Licciulli, S., Avila, J.L., Cho, M., Troutman, S.,
Jiang, P., Kossenkov, A.V., Showe, L.C., Liu, Q., Vachani,
A., et al. (2013). The Rac1 splice form Rac1b promotes Kras-induced lung tumorigenesis. Oncogene 32, 903–909.
ROS Production and NF-kB Activation Triggered by RAC1
Facilitate WNT-Driven Intestinal Stem Cell Proliferation
and Colorectal Cancer Initiation. Myant et al., 2013. Cell
Stem Cell. 2013.