Download Neural Stem and Progenitor Cells as Possible Seedbeds for Cancer

JSCG
Journal of Stem Cells and Genetics
Short Communication
Open Access
“Destined to Die”, Neural Stem and Progenitor Cells as Possible
Seedbeds for Cancer: A Hypothesis and Theoretical Model1
Kumar Mallik M*
Department of Laboratory Medicine
*Corresponding Author:
Kumar Mallik M
Consultant Cytopathologist, Department of Laboratory Medicine, Mubarak Al Kabeer Hospital, Hawally, Kuwait
Email: [email protected]
Received on: January 23, 2017 | Accepted on: January 24, 2017 | Published on: February 14, 2017
Citation: Kumar Mallik M. “Destined to Die”, Neural Stem and Progenitor Cells as Possible Seedbeds for Cancer: A Hypothesis and Theoretical Model1. J Stem Cells and
Genetics 2017; 1(1): 10-16. doi: 10.00000/jscg.2017.103
Copyright: © 2017 Kumar Mallik M. This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 International License (CCBY) (http://creativecommons.org/licenses/by/4.0/) which permits commercial use, including reproduction, adaptation, and distribution of the article provided the
original author and source are credited.
Published by Scientific Synergy Publishers
Introduction:
The ability to resist apoptotic and non-apoptotic cell death is considered to be a hallmark of cancer because all cancers share
this characteristic [Hanahan and Weinberg, 2011]. How this particular hallmark facilitates the accumulation of oncogenic
mutations without triggering apoptosis during the process of initiation and progression of the neoplasm and how it protects
cancer cells from succumbing to anti-neoplastic therapies are well known [Wong 2011]. Many of the molecular mechanisms
which contribute towards the empowerment of cancer cells with enhanced survival capabilities are well characterized [Allison
et al 2012, Wong 2011]. Cancers develop as cancerous mutations accumulate. Some of these mutations contribute towards
apoptotic resistance exhibited by different cancers. However acquirement of mutations indicates damages to the DNA. Such
damages provoke apoptotic responses [Nowasheen et al 2012]. Therefore cells which serve as origins of neoplasms should
already have heightened survival capabilities in order to survive the accumulation of mutations. The hypothesis and model
presented in this article is an attempt to envisage a common mechanism through which certain cells may acquire increased
apoptotic resistance and thus become ideally suited to serve as the seedbeds of cancers. The cell of origin of cancers has been a
subject of intense speculation and debate with most of the attention being directed towards the multipotent stem cells and the
oligopotent progenitor cells [Visvader and Lindeman 2012]. An oncogenic mutation might initiate a neoplastic process whose
further development and subsequent progress depend upon the accumulation of further genetic and epigenetic aberrations
[Hanahan and Wienberg 2011]. Cell proliferations and apoptosis are often closely related. Both during physiological and
pathological conditions some of the cells in proliferating compartments undergo apoptosis [Al Enezi 2004].
In light of the above mentioned scenario, the hypothesis
is essential to identify a situation where a proliferation
presented here is centered on cells which are present in a
associated apoptotic process might get aborted due to an
proliferating cellular compartment but are destined to undergo
apoptotic roadblock, leading to the generation of the
apoptosis. The article hypothesizes that if one of the above
hypothetical population of cells which “were destined to die but
mentioned cells was to face an apoptotic roadblock it would be
survived”. Dissecting the available knowledge related to the
unable to complete the apoptotic process. As consequence it
molecular machinery which underlies the apoptosis that occurs
would survive amongst the population of proliferating cells as
concurrently with neural progenitor cell proliferation
one which was destined to die but survived [referred to WDDS
[Bieberich et al 2003] provides with a prospect of analyzing a
cell in this article]. The molecular changes which the cell
situation during which the apoptotic process might face a
encounters before encountering the road block could be
roadblock which might lead to the generation of the above
equated with a “cellular near death experience” which might
mentioned WDDS cells. The apoptosis in the proliferating
significantly alter the molecular circuitry of the cell. These cells,
neural progenitor cells during murine brain development
as a result of the above, might become more resistant to
depends on an asymmetric distribution of Prostate apoptosis
apoptosis which could serve them well for the purpose of
response proteins [Par-4] and Nestin. Although there is a rise in
becoming the seedbeds of cancer development. In order to
ceramide concentration in all the cells during the proliferative
substantiate the feasibility of existence of such a phenomenon it
process and elevated ceramide levels are necessary for cell
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death to occur, following the asymmetric cell division, only
those cells which receive Par-4 proteins die while those which
receive Nestin survive [Bieberich et al 2003]. Par-4 protein was
first described as an apoptosis inducing protein in prostatic
carcinomas [Sells et al 1997] but since then their roles as an
apoptotic modulating protein have been demonstrated in a
wide range of biological and pathological conditions and a
variety of molecular mechanisms underlie their role in
apoptosis [Hebbar et al 2012]. Importantly, Par-4 protein needs
to gain access inside the nuclei in order to perform its proapoptotic functions. However in situations with hyperactivity
of Akt protein, a particular post-translational modification of
the Par-4 protein occurs, which prevents it from entering the
nucleus and this abrogates its ability to induce apoptosis
[Goswami et al 2005].
Hyperactivity of Akt, following PTEN [Phosphatase and
Tenascin homologue] mutations is a common occurrence in
many cancers. These mutations commonly occur in a wide
range of tumors including brain tumors like glioblastomas [The
cancer genome atlas research network 2008]. Neural stem cells
[NSCs] and neural progenitor cells [NPCs] are both considered
as entities which might serve as origins of various brain tumors
[Walters et al 2010]. From the behavioral and conceptual angle
NPCs have more limited self-renewal capabilities as compared
to NSCs and are oligopotent as compared to the multipotent
NSCs [Seaberg and Van Der Kooy 2003]. In light of the cancer
stem cell [CSC] theory the above paradigm acquires profound
importance since CSCs have been shown to be critical
components of brain tumors [Stiles and Rowitch 2008]. Cancer
stem cells have self-renewal and multi-differentiation potential
along with greater abilities to resist apoptosis which make
them ideally suited to serve as the seedbeds for tumor
recurrence [Allison et al 2012].
Putting all the above mentioned facts, a theoretical model is
being proposed which constitutes a scenario wherein a PTEN
mutation or any other genetic alteration creates a hyperactive
Akt status amongst a population of neural stem/progenitor
cells. The resulting Akt hyperactivity would prevent Par-4
proteins from inducing the apoptotic process amongst the
proliferating cell population and this might lead to the
generation of a population of cells with stem cell properties
which were destined to die but which managed to survive;
[WDDS] cells. The events which might be triggered as a result of
the above have been envisaged on the basis of the knowledge
pertaining to the interaction between Par4 protein and p62
protein. The p62 protein is a vital protein interaction partner of
Par4 [Moscat et al 2009]. It serves as a crucial signaling hub in a
wide range of cellular functions. Its role in modulating the
process of oncogenesis has been demonstrated in various
cancers [Moscat and Diaz Meco 2012]. The article speculates
that the binding of Par-4 to p62 in a cell which was destined to
die might be able to alter the cells’ molecular machinery, since
p62 is a vital signaling hub. The model presented here is not an
attempt to speculate upon the possible existence of a mechanism
involving the initiation of oncogenic process within the neural
progenitor cell population during embryogenesis. However,
since examples of close analogy are known to exist between
embryogenesis and oncogenesis [Monk and Holding 2001], the
model presented here may be studied as a paradigm of a
J Stem Cells and Genetics 2017
situation in which an apoptotic roadblock could initiate a
sequence of events driven by molecular interactions that might
facilitate the process of oncogenesis. In the following sections,
firstly the available knowledge based on which the basic tenets
of the hypothesis and theoretical model have been constructed
will be presented before describing them. In a subsequent
section, some experimental strategies which might query the
various aspects of the model will be put forth followed by a
section on discussion.
Available knowledge based on which the
basic tenets of the hypothesis and the
theoretical model has been constructed.
Neural stem cells and neural progenitor cells: Neural stem
cells have unlimited self renewal potential in vivo and in vitro
as well as the potentiality to differentiate into all three neural
lineages whereas the progenitor cells have limited capacities
for self renewal and ability to differentiate into one restricted
lineage [Seaberg and Van Der Kooy 2003]. Both have been
shown to posses the potential to initiate brain tumors [Walters
et al 2010].
The role of asymmetric distribution of PAR-4 protein in
apoptosis during neuronal progenitor cell proliferation:
Apoptosis occurs during neural progenitor cell proliferation,
wherein, following an asymmetric cell division, one daughter
cell dies while the other survives to further proliferate or
differentiate [Kuan et al 2000] [Sommer and Rao 2002].
Bieberich et al have shown that during the E12-E16 stage of
brain development in mice, the levels of brain ceramide and
prostate apoptosis response protein-4 [PAR-4] are elevated
with the concurrent presence of numerous apoptotic cells in
the sub ventricular and ventricular zones [Bieberich et al 2001].
PAR-4/ceramide induced apoptosis is thus the predominant
mechanism through which active cell death occurs in the
developing mouse brain. In a later study the same group of
authors has established an elegant mechanism through which
the fate of proliferating neural precursor cells is decided by an
asymmetric cell division during which the asymmetric
distribution of PAR-4 and Nestin occur [Bieberich et al 2003].
Although there is an increase in ceramide biosynthesis in all the
cells, some cells express PAR-4 without expressing Nestin,
while the others express Nestin and not PAR-4. Thus there are
two groups of cells; PAR-4 positive, Nestin negative and PAR-4
negative, Nestin positive. Using TUNEL staining for identifying
apoptotic cells, the authors demonstrated that the PAR-4 [+],
Nestin [-] cells undergo apoptosis while the PAR-4 [-], Nestin
[+] cells survive [Figure1].
The pro-apoptotic effect of PAR-4 protein and the role of
AKT in regulating its pro-apoptotic effect: The PAR-4 gene
was first identified as one which is up regulated during
apoptosis of prostate cancer cells [Sells et al 1994]. The gene
encodes for a multi domain protein comprising of 343 amino
acids which can induce apoptosis in different ways depending
upon the cell, the situation and the apoptotic stimuli. Inhibition
of NFKB signaling pathway is one of the frequently utilized
mechanisms by Par-4 protein in this regard [Hebbar et al 2012].
PAR-4 is a cytosolic protein which needs to get translocated to
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the nucleus in order to induce the apoptotic process [Hebbar et
al 2012]. Endogenous Akt was shown to be a binding partner of
PAR-4 through co-imunoprecipitation experiments performed
on human prostate cancer cells PC3 [Goswami et al 2005]. Akt
binds PAR-4 at its Lucien zipper domain and phosphorylates it
on its S-249 [Seine 249 residue].This prevents the nuclear
translocation of PAR-4 and its apoptosis inducing effect. The
binding of Akt to PAR-4, and the phosphorylation of its S-249
residue is essential for the inhibitory effect of AKT-1 in PAR-4
induced apoptosis. [Figure2] [Goswami et al 2005] [Hebbar et
al 2012]. It was also shown that 14-3-3 proteins, also bind to
PAR-4 in an AKT dependent manner and contributes towards
the effect. [Goswami et al 2005].
The interactions between PAR4 and p62 protein: The p62
protein [also called Sequestosome 1], is a critical signaling
protein. It utilizes PBI as an important protein protein
interaction module to form a complex with PAR4 and atypical
protein kinase c ζ [Moscat et al 2009]. A ternary complex is
formed between PAR-4, PKC ζ and p62 in which there is a direct
interaction between PAR4 and p62 [Chang et al 2002]. In fact
p62 inhibits the PAR-4 mediated inhibition of PKC ζ. Since PKC
ζ is supportive of NFKB signaling, the PAR-4- PKC ζ interaction
inhibits it and contributes towards its pro apoptotic effects. p62
inhibits this interaction and promotes NFKB signaling. The
above mentioned ternary complex appears to play a crucial role
in the fine tuning of apoptosis-survival balance. The significance
of this becomes multifold when one considers the fact that p62
is a multi-domain protein and these domains allow it to interact
with a wide array of other proteins and thus act as a signaling
hub involved in a wide range of biological functions like receptor
endocytosis, cell growth, mitosis, autophagy, proteasomal
degradation and control of the levels of reactive oxygen species.
A number of signaling pathways like mTOR and NFKB also
utilize this hub [Moscat and Diaz-Meco 2009]. Needless to say,
p62, play a critical role in cell survival, proliferation and
oncogenesis. The regulation of ROS levels is crucial for the selfrenewal and proliferation of neural stem cells [LaBelle et al
2011]. Since p62 is important for ROS regulation through its
interaction with Keap and NRF2 proteins [Taguchi et al 2011],
it is possible that p62 might have a role to play in stem cell/
progenitor cell self-renewal and cancer stem cell bio dynamics.
Since cancer is a disease which involves fine adjustments in
wide range of biological processes to prevent the cell from
undergoing apoptosis, the alterations at the p62 signaling hub
occurring as a result of dysfunctional interactions with protein
partners like PAR 4 might have very significant effects in the
process of oncogenesis.
PAR-4 and resistance to therapy in gliomas: In a recent
article, Zhuang et al have shown that PAR-4 mediated apoptosis
is important for the Temozolomide induced killing of glioma
cells and association with prion proteins.PAR-4 inhibits this
process and promotes chemoresistance to Temozolomide.
[Zhuang et al 2012]. This indicates a potential connection
between the pro-apoptotic role of PAR-4 proteins during neural
progenitor cell proliferation and its possible effect on drug
tolerance mechanisms to Temozolomide. It also brings about an
intriguing element of functional paradox with Par-4 having the
capability of acting both in a pro-apoptotic and anti-apoptotic
manner.
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Akt hyperactivity in brain tumors: A number of brain tumors
show hyperactivity of Akt. In glioblastomas, for example,
mutations in the PTEN gene can occur in 36 % of cases leading
to Akt hyperactivity. The others changes which could also lead
to hyperactivity in Akt are mutations in PI3K [15%] and
amplifications in AKT gene in 2%. [The Cancer Genome Atlas
Research Network 2008]. In the light of the above findings it is
tempting to envisage a possible sequence of events which might
occur if a neural stem cells/progenitor cell undergoes a PTEN
mutation leading to AKT hyperactivity leading to abrogation of
the ability of Par-4 protein to promote apoptosis.
The hypothesis and the model
The hypothesis which is being proposed is as follows;
[Figure 3]. The apoptotic process, which affects some of the
cells among a population of proliferating cells might face a
roadblock, potentially leading to the generation of a population
of cells which were derived from a cell or cells which “were
destined to die but survived”[WDSS] cells. Assuming that these
cells go down the apoptotic pathway for certain duration of
time and subsequently survive, it is possible that their molecular
machinery gets significantly altered in a way which provides
them greater ability to resist apoptosis i.e. Greater survival
ability. Such ability would provide the cell with an advantage
because it would enhance its chances of resisting potentially
apoptosis triggering oncogenic events during the process of
initiation of cancer. This would make these cell ideally suited
for serving as seedbeds of cancer. In order to study the feasibility
of such a phenomenon a biological situation has been selected
and analyzed from a theoretical perspective to generate a
theoretical model to speculate on the possibilities which might
arise as a result of an apoptotic process which might get aborted
due to some pre-existing molecular anomaly which could have
resulted from a prior mutation.
The platform on which this model has been built is a
population of proliferating neural progenitor cells during the
process of mouse brain development. Some of these cells
undergo apoptosis. Since the PAR-4 protein is essential for this
apoptosis, preventing the PAR-4 protein from performing its
apoptotic function might act as an apoptotic roadblock which
could lead to the generation of a population of “were destined
to die but survived” [WDSS] cells. The sequence of possible
changes has been described below.
If a neural progenitor cell undergoes a mutation, like a PTEN
mutation that leads to hyperactivity of the AKT protein a
sequence of events might occur which has been outlined in figure
4. Asymmetric cell divisions occur during neural progenitor cell
proliferation and it is the asymmetric distribution of PAR4
proteins and Nestin between the daughter cells which determines
their fate. The cell receiving PAR4 undergoes a ceramide
mediated apoptosis, whereas the one which receives Nestin
survives. The exact mechanism of portioning out these proteins
into the two cells is unknown. In case a neural stem/progenitor
cell suffers a mutation like a mutation in PTEN gene, it should
experience the effects of AKT hyperactivity. One of the effects of
AKT hyperactivity is the phosphorylation of the PAR-4 protein at
the seine 249 residue which prevents it from gaining access into
the nucleus, which is necessary for it to induce apoptosis through
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its interference with NFKB signaling. 14-3-3 proteins bind to
PAR-4 and sequester it within the cytoplasm Since PAR4
mediated signaling is necessary for the death of neural cells, the
above sequence of events will produce a population of PAR4
positive cells which were destined to die but did not because of
reasons stated above. These has been referred to as “were
destined to die but survived” [WDDS] cells. PAR4 has a number
of protein interaction partners. p62 is one of them . The reason
for highlighting its relation to the p62 protein is because p62 is
an important signaling hub with critical influence on an array of
biological functions and signaling pathways. It is unknown
whether the WDDS cells have any mechanism through which the
excess PAR-4 protein could be inactivated. If not, PAR-4 is likely
to bind to p62 and form a ternary complex comprising of p62,
PAR-4 and PKCζ. It is known that p62 antagonizes the proapoptotic effect of PAR-4 which the latter achieves by inhibiting
PKCζ. However in this scenario PAR-4 is unable to act in a proapoptotic way because of reasons mentioned above. It is possible
that as a result of this abnormal binding of PAR-4, the biological
functions in which p62 serves as an important signaling might
get altered. These functions involve the participation of the p62
hub in NFKB signaling, MTOR signaling, cell growth, mitosis,
autophagy, proteasomal degradation, cell survival and regulation
of the levels of reactive oxygen species. This could alter the fine
tuning of various biological mechanisms, possibly allowing
accumulation of mutations without triggering apoptosis and
these could be crucial in the oncogenic pathway. The different
tenets involved in the construction of this hypothetical model
could be investigated through experimental approaches and
strategies. Some of these have been outlined below.
Experimental strategies to study the
feasibility of the underlying hypothesis
and the model
The experimental strategies which are being proposed
below attempt to find the answer to the following set of
questions.
i] Is the PAR-4 protein able to trigger apoptosis amongst
neural stem/progenitor cells which have hyperactive Akt?
ii] In case a hyperactive Akt status prevents PAR-4 from
triggering apoptosis is it possible to isolate these PAR-4 positive
neural stem/progenitor cells, and set up separate cultures of
Par4 and negative cells in PTEN wild type and PTEN mutant
mice? [See below]
iii] Are there any differences between the above groups of
cells with respect to their proliferation, clonogenicity,
differentiation and ability to transform to transformed and
neoplastic cells?
iv] Is there any association between PAR-4 proteins and
p62 in these cells?
v] Are the p62 associated cellular processes different in the
above cells?.
i] Is the PAR-4 protein able to trigger apoptosis amongst neural
stem/progenitor cells which have hyperactive Akt?
During E12-E18 of mouse brain development the levels of
ceramide and PAR-4 proteins are elevated.
J Stem Cells and Genetics 2017
[Bieberich et al 2001]. On the other hand PTEN can be
conditionally deleted in different parts of the embryonic mice
brain using the Cre-LoxP technology. [Groszer et al 2001] PTEN
deletion is just complete around E14.5. [Groszer et al 2006]. By
crossing Pten conditional knockout mice with the Nestin-Cre
line, it was possible to generate mutant mice with increased
brain size and similar to macrocephalic phenotypes found in
humans with inherited PTEN mutations. The increase in size is
due to increase in cell proliferation, increase in cell size and
decrease in cell death. In the proposed experiments it should be
possible to generate neurophere cultures from PTEN mutant
and PTEN wild type mice as indicated in the studies above.
TUNEL staining may be used to determine the proportion of
apoptotic cells amongst the cells constituting the neurosphere
The AKT phosphorylation status in these cells may also be
compared by lysing the cells and subsequently immunoblotting
with antibodies against Akt phosphorylated at specific residues
which is indicative of their activity [Oteagi et al 2006] . With
help of TUNEL staining the proportion of apoptotic cells within
the two populations can be quantified and compared. The level
of endogenous ceramide within the neurosphere cultures is
determined and the dependence of the apoptosis on ceramide
is queried using a ceramide synthase inhibitor like fumosin
[Bieberich et al 2003].
Following this immunofluorescence microscopy may be
performed for TUNEL, PAR4, Nestin, p62 and with the help of
immunofluorescence overlays the co-expression of these
proteins in the cells may be determined. Then it may be
determined whether the asymmetric distribution of Par-4 and
Nestin occurs in the above proposed set up and whether the
Par-4 positive cells survive amongst the PTEN mutated Akt
hyperactive population.
ii] In case a hyperactive Akt status prevents Par-4 from triggering
apoptosis, is it possible to isolate these PAR-4 positive neural
stem/progenitor cells and culture them?
Laser capture microdissection [LMD] technique may be
used in culture cells to isolate the populations of cells chosen
according to their immune profile [Mustafa et al 2012]. Similarly
in the proposed experimental set up LMD could be used to
isolate the following groups of
a] Par4 positive, PTEN positive [wildtype mice]
b] Par4 positive, PTEN negative [mutant mice]
c] Par4 negative, PTEN negative [mutant mice]
d] Par negative, PTEN positive [wildtype mice].
The hypothesized WDSS cells should be in the PAR4
positive, PTEN negative group. The Par 4
Positivism, PTEN negativism and the hyperactivity of Akt
protein should be confirmed from cellular lysates of these cells
using immunoprecipitation techniques with appropriate
antibodies.
iii] Are there any differences between the above groups of cells
with respect to their proliferation, clonogenicity, differentiation
and ability to transform to transformed and neoplastic cells?
In the proposed experimental set up the dynamic properties
of proliferation, clonogenicity and differentiation capabilities
are to be studied in the different cell populations. Queries may
also be made with regard to the transformation potential of the
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different groups of cells. For example, attempts may be made to
transform the neural stem/progenitor cells with the help of
retroviruses containing the constitutively active mutation of
epidermal growth factor receptor EGFRvIII as described by
Ligon et al on INK4/ARF double negative neural stem cells
[Ligon et al 2007]. Using such a method it might be possible to
detect differences between the groups of cells with respect to
the ease or difficulty or to that matter feasibility of them being
transformed. In case there is transformation in the population
of cells, the tumorogenous capabilities may be studied by
injecting them into the brains or subcutaneous tissues of severe
combined immune deficient mice.
iv] Is there any association between PAR-4 proteins and p62 in
these cells?
In the course of the hypothesis a great deal of importance
has been directed towards the possible interactions between
PAR4 and P62 and how this interaction could influence the
biological activities in which p62 plays crucial roles. These are
regulation of cell size, mTOR pathway, autophagy, regulation of
levels of reactive oxygen species [ROS] and NFKB activity
among others. The following investigations could throw a
significant amount of light on the concerned activities in the
proposed set of experiments. The references in paranthesis
indicate that different groups who have employed this methods
to gain insights into various biological and pathological
scenarios
a] Compare the localization of p62 as cytosolic speckles and
aggregates which is indicative of the abilities of p62 in cellular
functions of protein aggregations and proteasomal degradations
as well as response to hypoxia [Moscat and Diaz Meco 2009]
[Ratanen et al 2013]
b] Compare the differences in leucine induced increase in
cell size [Duran et al 2011]
c] Correlation between p62 and S6Kinase1 [S6K1] levels
and comparing the phosphorylation of S6K and 4EBP1 to
estimate the activity of the mTOR pathway. [Duran et al 2011]
d] Compare the autophagic response to leucine depletion
by estimating the accumulation of conjugated forms of LC3.
[Duran et al 2011]
e] Comparison of the levels of endogenous ROS between
the cell population using ROS sensitive dyes like DCFDA,
hydroethidine and HPF-ARF [Taguchi et al 2011]
f] Quantify and compare the NFKB activity. The estimation
of reporter gene activity may be utilized to evaluate the NFKB
signaling activity status in these cells. [Widera et al 2006]
Discussion
Cancer cells have heightened survival capabilities [Hanahan
and Weinberg 2011] and the cancer stem cells are more resistant
to apoptotic stimuli as compared to other non-stem neoplastic
cells [Allison et al 2012]. It is known that the above cancer
hallmark facilitates the process of neoplastic initiation and
progression and is crucial for cancer stem cells to survive antineoplastic therapies [Hanahan and Wienberg 2011]. However
the underlying mechanism/s which uniformly empowers
neoplastic cells from all cancers with survival capabilities, from
the time of its initiation through its progression are unknown.
J Stem Cells and Genetics 2017
The hypothesis and theoretical model proposed here is an
attempt to present a possibility which might be able to explain
the above phenomenon. In the current article it has been
hypothesized that cells which survive an apoptotic process due
to a pre-existing apoptotic roadblock among a population of
proliferating cells might be more resistant to subsequent
apoptotic stimuli as compared to other cells. In an attempt to
construct a model to show how the hypothesized phenomenon
might work, the population of proliferating cells which has been
utilized to build a theoretical model is the one consisting of
proliferating neural progenitor cells where the apoptosis
determinant is the Par-4 protein [Bieberich et al 2001 and
Bieberich et al 2003] and the roadblock is a hyperactive Akt
status [2005]. Non-neoplastic proliferations occur throughout
the life of an organism and these proliferations could be
physiological or pathological by nature. Also apoptosis of some of
these proliferating cells do occur [Alenzi 2004]. If one of these
apoptotic cells was to face an apoptotic roadblock which prevents
it from completing this process, it might survive and according to
the proposed hypothesis even have greater survival capabilities
than the others in resisting subsequent apoptotic stimuli which
could facilitate the process of neoplastic initiation in these cells.
The neural progenitor cells have been chosen as an example
because the molecular determinants of the apoptotic process are
known. It is also possible to theoretically analyze the effects of a
possible roadblock which might obstruct this apoptotic process.
In no way is it an attempt to try and pin-point the cell of origin or
oncogenic mechanisms which are associated with any tumor in
particular although progenitor cells have been implicated in the
pathogenesis of oligodendrogliomas [Persson et al 2010]. It is
rather a theoretical analysis of a possible sequence of events in a
situation in which at least some of the initial events have been
experimentally established. Neurogenesis continues to occur
during adulthood in mammals in the subventricular zone of the
lateral ventricle and the subgranular layers of the dentate gyrus
of the hippocampus [Reynolds and Weiss 1992, Alvarez-Buyalla
2004]. Although an apoptotic regulatory process involving
asymmetric distribution of Par-4 has not been described in adult
neurogenesis other yet undiscovered mechanisms might be in
place since apoptosis occurs during adult neurogenesis [Sierra et
al 2010]. Moreover many of the cellular mechanisms evident
during embryogenesis may get re-initiated during the process of
cancer development. [Monk and Holding 2001].
In this model, the molecular events which might occur
before the apoptotic roadblock comes into effect are unknown.
Apart from the asymmetric distribution of Par-4 and Nestin
between the two daughter cells which determines which of the
two is destined to die, no other molecular changes are yet
known. Nestin in itself is known to show pro-survival effects
under certain circumstances through its interactions with the
pro-survival protein bcl-2 [Piras et al 2011] but the effect of its
absence in “WDSS cells” is currently beyond the scope of
available knowledge. On the other hand the model is unable to
demonstrate, any specific end to end sequence of molecular
events which shows how the so called “WDSS cell” might carry
an advantage over the other normally proliferating cells in
being the preferred sites for the neoplastic process to begin.
The model has been constructed by translating and
integrating knowledge derived from completely different
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cellular systems. The effect of hyperactive Akt on preventing
Par-4 mediated apoptosis has been studied and demonstrated
on prostatic carcinoma cells [Goswami et al 2005]. Whether a
similar mechanism blocks the pro-apoptotic effect of Par-4
protein in neural progenitors requires to be investigated. It is
known that a hyperactive Akt decreases the rate of apoptosis in
proliferating neural stem cells [Groszger et al 2006] but there is
no evidence as yet to show that Par-4 protein and Akt
interactions could modulate this process.
It should also be noted that Par-4 knockout mice do not
develop with any brain abnormality [Garcia-Cao et al 2003].
This indicates that some other protein could replace Par-4,
functionally with regard to its pro-apoptotic role during the
embryologic neurogenic process. However the model is based
on a situation where the downstream effects have been
envisaged based on the presence of Par-4 and possible
downstream effects based on its protein interaction
partnerships with p62. The hypothesis is based on the premise
that the “near death” experience encountered by the destined
to die cells makes them more resistant to apoptosis. Although
the possible interaction of Par-4 with p62 protein opens up a
myriad of theoretical possibilities related to the perturbation of
a crucial cell signaling hub [with cell survival being one of the
functions which is regulated through this hub] the model is
unable to show how this interaction could actually translate
into a situation which provides the cell with enhanced
capabilities to withstanding genetic and epigenetic changes
without undergoing apoptosis and accumulating them in the
process of neoplastic development. Moreover the hypothesis is
an attempt to envisage a condition which not only explains why
apoptotic resistance is a hallmark prevalent in all cancers but
also to show how this facilitates the initiation of cancers through
the accumulation of mutation. However in order to construct
the model an apoptotic roadblock which commonly occurs as a
result of a mutation [PTEN in this case] has been utilized. Indeed
it is possible that this lone mutation might not have initiated an
apoptotic response in the first place but by creating an apoptotic
roadblock in a cell which was destined to die, alter the molecular
circuitry which allows the accumulation of oncogenic mutations
without triggering apoptosis.
It also needs to be seen what an excess of Par4 bound to
p62 and PKCζ can do to the entire signaling hub in which p62 is
a nodal point. The biological functions in which this hub
participates in are varied, but of particular interest is its role in
survival responses, mTOR signaling and autophagy [Lowe et al
2004, Moscat et al 2009, Moscat et al 2012, and Duran et al
2011]. In addition this hub is also important for regulation of
ROS [Rantanen et al 2013]. Recently ROS has been shown to
regulate a number of stem cell properties in the neural stem cell
population [la Belle et al 2011]. Par-4 protein is indeed a proapoptotic protein but at the same time we need to know what
its effect on neural stem/progenitor cell population might be
when its pro-apoptotic potential is blunted. Its role in a cell’s
molecular machinery is often context dependent [ Hebbar et al
2012]. A noteworthy role of Par-4 protein which has been
recently described is its role in mediating resistance to
Temozolomide in glioblastomas, [Zhuang et al 2012] an
intriguing new finding in view of the general pro-apoptotic role
of this protein. A number of experimental strategies have been
J Stem Cells and Genetics 2017
put forward to investigate the various tenets of the hypothesis.
The hypothesis has been constructed on the basis of assumptions
from varied cellular contexts and thus it is essential to test out
the different tenets of the hypothesis and the theoretical model
which has been proposed. Recent advances in laser capture
micro-dissection technology could be of great help to isolate the
so called “WDDS” cells and compare them with the other cells.
Tumor recurrences continue to frustrate the efforts directed
towards the achievement of effective management strategies in
most cancers. The ability to resist therapy mediated apoptosis
is the reason behind most tumor recurrences. In spite of the
criticisms which might be put forward against some of the
tenets of the models, the hypothesis and theoretical model is an
attempt towards the development of a unified concept to
explain this cancer hallmark in a novel manner with the hope of
generating interest within the research community to pursue
this paradigm through well planned experimental strategies.
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