cancer stem cell ( csc ) theory states that tumors are
organized in the same hierarchical manner as normal
tissues, utilizing a sub-population of tumorigenic stem-like
cells that generate the a lot of differentiated nontumorigenic
tumor cells. cscs are chemoresistant and seem to firmly be
chargeable for tumor recurrence and formation of
metastases. thus, the study of those cells could lead
to firmly crucial advances within the whole understanding of tumor biology
and even as to firmly innovative and a lot of effective therapies. lung
cancer represents the leading cause of cancer-related
mortality worldwide. despite improvements in medical
and surgical management, patient survival rates remain
stable at b15%, calling for innovative strategies that
could contribute to enhance patient outcome. the discovery
of lung cscs and therefore the possibility to firmly characterize their
biological properties could give powerful translational
tools to enhance the clinical outcome of patients with lung
cancer. during this report, we review what exactly is known about
lung cscs and discuss the diagnostic, prognostic and
therapeutic prospective of those findings.
oncogene ( 2010 ) 29, 4625–4635 ; doi :10. 1038/onc. 2010. 207 ;
printed on-line 7 june 2010
keywords : lung cancer ; cancer stem cells ; therapy ; stem
cells
cancer stem cells
tumor cells show a wide spectrum of useful and
morphological heterogeneity. even cells associated with a single
tumor lesion vary in his or her differentiation level, proliferation
capability and tumorigenicity. this phenomenon
could well be explained from the presence of ‘cancer stem
cells’ ( cscs ) ( reya et al. , 2001 ; wang and dick, 2005 ;
clarke et al. , 2006 ) just like the driving force of tumorigenesis.
the csc model implies a hierarchical organization
inside the tumor during which a restricted variety of cscs
represents the apex of one's hierarchy. similar to firmly normal
stem cells, cscs have the ability to firmly self-renew and
endure asymmetric divisions, thereby giving rise to firmly a
differentiated progeny that represents the majority of the tumor
population. these key features enable cscs to firmly initiate
tumors and promote cancer progression.
the very first few direct experimental proof supporting the
plan of csc-driven tumorigenesis came from acute
myeloid leukemia ( lapidot et al. , 1994 ), the initiating
cells of that utilize a phenotype ( that would be, cd34þ ;
cd38_ ) similar to firmly normal hematopoietic stem cells
( bonnet and dick, 1997 ). despite a few technical
limitations, a growing variety of reports support the
existence of cscs in solid tumors. specific biomarkers
for detection and isolation of cscs are suggested
for all those major tumor types, as well as lung cancer ( eramo
et al. , 2008 ; visvader and lindeman, 2008 ).
besides the expression of surface proteins, cscs may
too share useful features with tissue stem cells, such
as their ability to firmly actively exclude the dye hoechst 33342,
that defines them as side-population ( sp ) cells in flow
cytometric assays ( storms et al. , 1999 ) and high aldehyde
dehydrogenase ( aldh ) activity ( ginestier et al. , 2007 ).
in parallel in the growing variety of potential surface
markers, it's obtaining evident that the known markers are
not continuously ideal to firmly sort for our csc population. a good
example happens to be the expression of cd133, that was initially
reported becoming a reliable marker for glioblastoma
and medulloblastoma cscs ( singh et al. , 2003, 2004 ).
though, freelance studies have shown that cd133-
negative glioblastoma cells will too establish a tumor
in recipient mice with similar efficiencies compared
with cd133þ cells ( beier et al. , 2007 ) which cd133 is
highly expressed on nontransformed neural progenitor
cells ( lee et al. , 2005 ). in line with tumor heterogeneity,
the phenotype of cscs isn't uniform, even when they
originate due to same tumor subtype. this can be underlined
by studies using colorectal carcinoma as the
model system. we along with other people found that colorectal
cscs are enriched within the whole cd133þ sub-population
( o’brien et al. , 2007 ; ricci-vitiani et al. , 2007 ; todaro
et al. , 2007 ), whereas dalerba et al. ( 2007 ) reported an
enrichment of cscs within the whole epcamhi, cd44hi subpopulation
of colon cancer cells. the image is further
difficult by a recent study showing that colorectal
cscs are highly enriched within the whole aldh-positive cell
population. during this report, the tumorigenicity of aldhpositive
cells is just modestly increased using cd133
and cd44 expression being a second marker and get a more
stringent choice ( huang et al. , 2009 ). upon the other
hand, a mix of markers could be needed to
isolate a highly enriched breast csc population. breast
cscs are reported that should be enriched in each cd44þ cd24_/low
( al-hajj et al. , 2003 ) and aldh-positive subfractions.
though the overlap of one's 2 populations was surprisingly
low ( zero, 1–1. 2% ), cells displaying the combined
phenotype turned purpose is to be highly tumorigenic when
transplanted into your recipient mouse ( ginestier et al. , 2007 ).
the marked heterogeneity inside csc sub-populations
underlines the necessity to look for a lot of specific only markers,
or to firmly define new marker combinations for our prospective
isolation of cscs in solid tumors.
recently developed transgenic mouse models
provided a lot of hints relating to the potential identity of the
csc population in colon and lung cancer. targeted
oncogenic transformation of adult stem or progenitor
cells led in the formation of colon adenomas ( barker
et al. , 2009 ; zhu et al. , 2009 ) or lung adenocarcinomas
( acs ) ( kim et al. , 2005 ). though these results underline
the pivotal role of stem/progenitor cells in
carcinogenesis, the notion of ‘cancer stem cell’ will not imply that tumorigenic cells essentially need to derive
from tissue stem cells. it is likewise probable that cscs develop
from a lot of restricted progenitor cells that receive
reprogramming ‘hits’, and thereby regain the stem cell
trait of unlimited self-renewal capability ( clarke et al. ,
2006 ). though there's substantial proof for csc
relevance in human carcinomas, several problems have to be compelled to be
thought of, as deeply discussed utilizing a recent review
( visvader and lindeman, 2008 ). initial of all, the best
practical in vivo assay for cscs is their ability to
recapitulate the patient tumor in animal models.
but, animal models don't entirely mimic the
human tumor microenvironment and poorly tumorigenic
cscs can be inadvertently selected during
xenotransplantation. besides the problem of species specificity,
tissue specificity conjointly has as being thought of for
in vivo transplantation of cscs. so, orthotopic
tumors might higher reproduce the csc niche, especially
in the event the human microenvironment is mimicked by the
use of humanized supports or patient-derived accessory
cells, like fibroblasts or mesenchymal stem cells
( kuperwasser et al. , 2004 ). the impact as to the microenvironment
on tumor growth is underlined via the fact
that subcutaneous implantation of cscs results in less
efficient engraftment than brain or renal capsule injection
for gliomas or colorectal cscs, respectively. finally, the
presence of residual immune effector cells in recipient
mice may additionally influence the efficiency of human cell
engraftment in nod/scid mice, whereas nod/scidil2rgnull
mice, recently introduced for csc transplantation,
represent a a lot of permissive environment ( shultz
et al. , 2005 ). by the alternative hand, it is likewise true that
immune cells utilize a role within the progression of human
tumors. these limitations of mouse models really should be kept
on your mind when csc behavior, frequency in tumors, growth
requirements or any other properties linked out to niche interaction
are investigated. moreover, till currently, no complete
agreement relating to csc frequency or phenotype has
been reached. but, though a few controversies
concerning cscs in solid tumors are raised, their
biological relevance and also the immense therapeutic potential
of the targeting generally are not seriously questioned ( hill,
2006 ; visvader and lindeman, 2008 ; zhou et al. , 2009 ).
lung cancer
lung cancer is that the most common cause of cancer-related
mortality worldwide, with 160 000 deaths in the
u. s. in 2008 but a poor 5-year survival rate,
that remains stable at 15% ( jemal et al. , 2008 ). it has
been estimated that 9 from 10 lung cancer cases are
directly caused by smoking. alternative risk factors for lung
cancer are exposure out to asbestos and, to the lesser extent,
out to radon, arsenic, chromium, nickel, vinyl chloride and
ionizing radiation or a few preexisting nonmalignant
lung diseases ( dubey and powell, 2009 ). different
known or unknown etiologies may account for the
occurrence of totally different kinds of lung cancer, the
heterogeneity of that has vital diagnostic, prognostic
and therapeutic implications ( borczuk et al. , 2009 ).
out to facilitate treatment and prognostic choices, lung
cancer is categorized into little cell lung
carcinoma ( sclc ) and non-sclc ( nsclc ). nsclc
might well be any distinguished into 3 major histological
categories : ac, squamous cell carcinoma and enormous cell
carcinoma ( collins et al. , 2007 ). sclc accounts for
b20% as to the pulmonary tumors. despite a generally
sensible initial response out to chemotherapy, it really has a
notably poor prognosis, owing to early extra
thoracic dissemination and frequent disease relapse.
sclc predominately localizes out to midlevel bronchioles
and displays neuroendocrine differentiation, suggesting
that transformed pulmonary neuroendocrine cells might
offer rise out to this way of lung cancer ( giangreco et al. ,
2007 ). with b40% prevalence, adenocarcinoma ( ac ) is
possibly the most common kind of nsclc in each smokers and
nonsmokers. it develops mostly direct from junction
amongst the terminal bronchiole and also the alveolus,
termed ‘bronchoalveolar duct junction’, and displays
either airway or alveolar differentiation, or both
( giangreco et al. , 2007 ). squamous cell carcinoma has
a 25% prevalence rate and is highly associated with
tobacco smoking. it arises within the proximal airways down
out to the second or third bifurcation and is never observed
distally. compared with alternative kinds of nsclcs, large
cell carcinoma ( 10% prevalence ) comprises a category of
rather poorly differentiated and fewer aggressive tumors,
possibly the most frequent subtype of which is certainly giant cell
neuroendocrine carcinoma ( giangreco et al. , 2007 ).
analysis as to the genetic alterations occurring in lung
cancer has shown that histopathological differences are
in line with genetic heterogeneity as to the disease ( minna
et al. , 2002 ; sekido et al. , 2003 ). further genetic
differences are found among the 3 main
kinds of nsclcs ( minna et al. , 2002 ; sekido et al. ,
2003 ). consistent along with the genetic heterogeneity,
expression studies haven't no more than unveiled profound
differences between sclc and nsclc but as well as within
totally different nsclc subtypes ( bianchi et al. , 2008 ).
but, till recently, all nsclc lung cancer forms
are treated with similar approaches regardless of
their biological differences ( minna et al. , 2002 ; collins
et al. , 2007 ; dubey and powell, 2008 ; tiseo et al. , 2009 ).
therefore, it's probable that the poor lung cancer response rates
can be due, a minimum of partly, to the too homogeneous
treatment approach applied to yesteryear obtain a highly heterogeneous
disease ( borczuk et al. , 2009 ). no more than recently,
lung cancer heterogeneity has started out to gain therapeutic
relevance, as documented via the try out to propose
preferential chemotherapeutic choices for every tumor
histotype ( tiseo et al. , 2009 ) ( scagliotti et al. , 2009 ).
besides histology-based approaches, ongoing efforts are
underway out to establish clinically relevant biological properties of lung tumors which can facilitate individualized
therapy. during this direction, completely different expression
profiles have conjointly been found inside a similar tumor
subtype, and could well be correlated with patient survival
( bhattacharjee et al., 2001 ). to firmly define a lot of personalized
therapies, each genetic and proteomic signatures of
completely different lung tumor subtypes be required to be determined
and correlated with tumor histotype, clinical outcome
and treatment response. by way of example, it must been found
that, inside acs, kras mutations are a lot of common
in tumors from patients with tobacco exposure and are
related to worse prognosis and primary resistance
to firmly treatment. in distinction, tumors from nonsmokers are
a lot of frequently related to molecular alterations
involving the epidermal growth issue receptor and with
longer overall survival ( pao et al., 2004 ; janne et al.,
2005 ; riely et al., 2009 ). coming from the therapeutic point
of read, patients with epidermal growth issue receptor
mutations show increased sensitivity to firmly epidermal
growth issue receptor inhibitors, whereas krasbearing
tumors are resistant ( pao et al., 2005 ). so,
epidermal growth issue receptor and kras
mutations define 2 distinct populations of nsclc
patients with completely different natural histories and responses to
targeted therapy ( bianchi et al., 2008 ; lantuejoul et al.,
2009 ; riely et al., 2009 ). these evidences strongly suggest
that a deep knowledge just about every one of them patient tumor will predict
treatment response to enhance patient outcome, calling
to produce a robust effort toward customized therapy. in the
try to enhance lung cancer patient outcome, csc
analysis would possibly utilize a central role not solely within the whole search
for new anticancer medicine but additionally within the whole diagnosis and
monitoring of one's therapeutic success as discussed later.
but, the relative low frequency of cscs in tumors
might complicate their investigation. by way of example, the
current methodology for molecular profiling of tumors
won't analyze cell sub-populations and may even provide
misleading results as a result of the signal connected to firmly csc gene
expression is extremely diluted.
adult progenitor cells of one's lung
cscs share variety of features with normal tissue
stem or progenitor cells, regardless of whether or not they
originate directly from stem cells gaining tumorigenic
hits or by reprogramming of a lot of differentiated cells. in
adult organisms, tissue homeostasis is maintained by
stem and progenitor cells which may have the ability to firmly divide
throughout life and replace dying or damaged cells.
stem cell populations are well characterized
in many organs by having high rate of homeostatic
proliferation, inclusive of the hematopoietic system, skin,
gut and hair follicle ( blau et al., 2001 ; barker et al.,
2008 ; fuchs, 2009 ). a classical model associated with a ‘stem cell
hierarchy’ is well established regarding the maintenance of the
hematopoietic system ( orkin and zon, 2002 ; iwasaki
and akashi, 2007 ) and of course the little intestine ( barker et al.,
2008 ; fuchs, 2009 ).
in organs with slow turnover rates, inclusive of the lung,
liver and pancreas, the terminology developed to firmly define
the ‘hierarchical stem cell’ model of rapid renewing
tissues has that should be refined. cellular turnover in the
adult lung tissue is incredibly slow when compared with the
little intestine ( blenkinsopp, 1967 ; barker et al., 2008 ;
rawlins, 2008 ), and just extreme conditions such as
pollutant- or pathogen-induced injuries spur a massive
lung cell proliferation. throughout organ regeneration, lung
cells that sometimes fulfil differentiated functions in the
normal tissue actively begin to firmly proliferate and act as
progenitor cells that may provide rise to firmly many cell types
( stripp and reynolds, 2008 ). utilizing a well known lung injury
model, mice are treated with naphthalene, that leads
in the ablation of clara cells ( mahvi et al., 1977 ).
cell proliferation starts 48–72 h once injury and the
epithelium is fully regenerated once 2–4 weeks
( stripp et al., 1995 ; van winkle et al., 1995 ). the
mechanism of how the clara cell population is regenerated
depends inside the region of one's lung ( see figure 1 ).
within the whole trachea and bronchi, cytokeratin 14-positive cells
are most most likely basal cells that may behave as self-renewing
progenitors and provides rise to firmly clara-like and ciliated cells
( hong et al., 2004a, b ). within the whole a lot of distal regions of the
lung, naphthalene-resistant clara cells begin to firmly proliferate
and regenerate the damaged tissue. these rare ‘variant’
clara cells ( clarav ), accumulate in specialized stem cell
niches shut in the neuroendocrine bodies and at the
bronchoalveolar duct junctions ( reynolds et al., 2000 ;
giangreco et al., 2002 ). upon injury, a subset of clarav
cells found within the whole bronchoalveolar duct junctions show a
characteristic expression pattern comprising each clara and
alveolar cell proteins ( ccsp and sp-c ) in combination
with cd34 and sca-1, 2 common stem cell markers.
these cells were named bronchoalveolar stem cells
( bascs ), as they actually utilize a substantial clonogenic survival
capability, are ready to self-renew and could provide rise to firmly clara
and alveolar cells in vitro ( kim et al., 2005 ). during this report,
we can refer to firmly ccsp/sp-c positive cells as bascs,
though the ability of such cells to firmly self-renew and give
rise to firmly completely different lineages in vivo has not been formally
proven, and recent cell-tracing experiments have suggested
that bascs contribute in the maintenance and
repair of one's proximal airways, however not of one's alveolar
epithelium ( giangreco et al., 2009 ; rawlins et al., 2009 ).
but, albeit their exact purpose in maintenance
and repair of one's lung isn't properly understood
however, there's growing proof that adenomas and acs
arise from transformed bascs, at the very least in mouse models.
potential lung cancer-initiating cells within the whole mouse
though the identity of tissue stem cells within the whole lung is
controversial, bascs drive the tumorigenic method in
many mouse models of lung acs. as described
before, kras is mutated utilizing a substantial number
of nsclcs. so, transgenic mice expressing
oncogenic variants of kras ( inclusive of kras( g12d ) ) represent necessary animal models for lung
cancer, particularly acs. many lines of proof point
toward the importance of bascs regarding the induction of
kras( g12d )-induced acs : ( 1 ) it was actually shown that the
variety of bascs was expanded in ac precursors of
mice expressing an inducible style of kras( g12d )
( jackson et al., 2001 ; kim et al., 2005 ). ( 2 ) isolated
transgenic bascs displayed enhanced in vitro proliferation
capability when compared with a lot of differentiated
cells of the very same tissue. ( 3 ) whenever the variety of bascs
were elevated by naphthalene injury until that induction
of one's transgene, tumor growth was strongly
enhanced compared with management animals ( jackson
et al., 2001 ; kim et al., 2005 ). such results counsel that
these progenitor cells utilize a key role within the onset of
kras-induced lung cancer. in line basic experiments,
it was actually shown that inducible p38 knockout mice
were a lot of susceptible to firmly kras-induced formation of
acs. p38 supports the differentiation and suppresses the
proliferation of bascs, that accumulated within the lungs
of p38-deficient mice even while not the ectopic expression
of mutated kras ( ventura et al., 2007 ). this
accumulation may justify the higher susceptibility of
kras-transgenic/p38-deficient mice for lung acs when
compared with p38-proficient mice. bascs conjointly have
a very important role in alternative tumor model systems.
for instance, mice knocked in for the oncogenic form
of p27kip ( p27ck_ ) displayed a high incidence of
spontaneous hyperplastic lesions in many tissues,
as well as the retina, pituitary gland, ovary, adrenal
gland, spleen and lung. when compared with wild-type
mice, these mice had an abnormal high variety of
bascs in terminal bronchioles and eventually developed
ac ( besson et al., 2007 ). moreover, utilizing a mouse
model within which pten ( phosphatase and tensin homolog )
might well be deleted in sp-c-expressing cells, the number
of bascs and sp cells was considerably increased.
nearly all mice in whom pten was deleted spontaneously
developed lung cancer and, strikingly, like in
the human system, kras was mutated in 30% of acs
( yanagi et al., 2007 ). in summary, these reports suggest
a very important role for bascs in the first steps of lung
tumorigenesis. there could be a similar behavior between
normal lung stem cells in injured tissues and transformed
bascs in tumors. once lung injury, normal
lung stem cells survive and proliferate whereas giving
rise to firmly specialized cells that repair the damaged tissues.
this method is induced by external stimuli other then strictly
regulated inside the stem cell niche, that guarantees
that stem cell expansion occurs provided that needed.
in distinction, the expansion of transformed cscs during
lung tumorigenesis seems rather unrestrained, and
poorly regulated by signals coming due to niche.
in spite of this, it still has to firmly be proven whether or not transformed
basc-like cells conjointly fulfill all criteria to becoming cscs, as
there's hardly any proof of the sub-population in the basc
phenotype able to firmly establish a histophenocopy of the
initial tumor in secondary and tertiary hosts.
human lung cscs
following the growing enthusiasm regarding the csc model in
solid tumors and therefore the new insights straight into the biology of
normal cells and cscs within the mouse airway, research
interest has recently started to firmly rise for stem cells in
human lung cancer. tumorigenic human lung cancer
cells are isolated using completely different approaches
from each cell lines and primary tumors ( see table 1 ).
the very first few approach was primarily based upon the sp phenotype
( low hoechst 33342 staining pattern ) of stem cells. sp
lung cancer cells isolated from h460, h23, htb-58,
a549, h441 and h2170 cell lines, displayed increased
invasiveness, higher resistance to firmly chemotherapeutic
medicine and were a lot of tumorigenic in vivo when
compared with non-sp cells ( ho et al., 2007 ). on the
basis of one's widely accepted hypothesis that cscs of
completely different origins are endowed with increased drug
resistance, the second approach used to firmly enrich human
lung cscs from cell lines was primarily based onto their inherent
resistance to firmly cisplatin, doxorubicin or etoposide
treatment. drug-surviving cells exhibited many csc
features, an example would be high clonogenic capability, enrichment in
sp cells, expression of embryonic stem cell markers,
capability for self-renewal and generation of differentiated
progeny and high tumorigenicity ( levina et al.,
2008 ). the third approach leading to firmly the isolation of
lung cscs was primarily based on increased aldh activity
previously shown in stem cell populations of several
human cancers. aldh-positive cells isolated from lung
cancer cell lines displayed features of cscs each in vitro
( invasive properties, expression of stem cell markers )
and in vivo ( ability to firmly generate tumors ) ( jiang et al.,
2009 ). the very first few isolation and expansion of lung cscs
from primary patient tumors was reported by our group
( eramo et al., 2008 ). in your study, human lung cscs
were isolated upon the basis with the ability to firmly survive
underneath serum-free conditions and proliferate as cellular
clusters referred to as ‘tumor spheres’. this experimental
strategy represents one of the best approach to this point to firmly obtain the
unlimited expansion of one's tumorigenic lung cancer
cell population from primary patients, providing a
powerful tool to firmly permit in depth studies on these
cells. owing to firmly the low frequency of lung cscs within
primary tumor tissues, in depth investigation on these
cells wouldn't be potential within the absence with the in vitro
expansion. upon the basis in our approach, high numbers
of lung cscs can be generated and extensively
characterised. spheres due to major subtypes of lung
cancer ( an example would be sclc, ac, squamous cell carcinoma
and huge cell carcinoma ) were found to firmly possess csc
properties, each in vitro ( expression of one's csc marker
cd133, unlimited proliferative potential, extended selfrenewal and differentiation ability ) and in vivo ( high
tumorigenic potential, capability to firmly recapitulate tumor
heterogeneity and mimic the histology as to the specific
tumor subtype from that cscs were derived ). in
addition, lung cancer ‘spheres’ were extremely resistant
to firmly most typical medication presently utilized treat lung
cancer patients, calling for your intense effort in testing
each typical and innovative medication by the tumorigenic
lung cancer cell population. this tumor cell
population was too characterised by your expression of
embryonic genes, an example would be oct-4 and nanog, thus
confirming the undifferentiated phenotype of isolated
cancer cells. oct-4 expression was too recently reported
to firmly characterize cd133-positive tumor cells in freshly
isolated tumors, and also to be essential in maintaining the
stem-like properties of isolated cells, an example would be invasion
and self-renewal ( chen et al. , 2008 ). recently, the
relevance of cd133-positive cells in tumorigenicity
and chemoresistance of lung cancer has also been further
confirmed ( bertolini et al. , 2009 ). cd133þ cells isolated
from primary tumors displayed increased tumorigenicity
and expression of stemness, adhesion, motility and drug
efflux genes in comparison in the corresponding
cd133_ tumor cells. additionally, cd133þ cells survived
cisplatin treatment when in vitro drug exposure of the
a549 cell line as well as primary tumor-derived mouse
xenograft when cisplatin administration. importantly,
the expression of cd133 in tumors was linked to firmly shorter
progression-free survival of nsclc patients treated
with platinum-based regimens, so providing the first
proof as to the relevance of cd133þ tumor-initiating
cells for prediction of chemoresponse and prognosis of
lung cancer patients. though these and alternative encouraging
results are commencing to clarify totally different aspects of
lung cscs, additional investigation will still be needed to
obtain a deep elucidation by the biology of these
vital cells.
cscs and the potential implications regarding the development
of innovative cancer treatments
advances in csc analysis may facilitate to firmly establish
successful therapies against leukemia and solid tumors.
typical antineoplastic agents utilized fight cancer
mainly hit the malignant cells by 2 totally different mechanisms.
one category of typical chemotherapeutic drugs
induces dna injury, whereas the second drug impairs
mitosis or dna replication. unfortunately, this therapeutic
combination are only able to prolong the patient’s
survival for a couple of months. one reason to get this poor
medical outcome can be the presence of drug- and
radiation-resistant cscs. there may be many indications
regarding the higher resistance of cscs to firmly conventional
chemotherapy or radiotherapy in vivo. in tumor biopsies
from breast cancer patients treated with conventional
chemotherapy, a better frequency of potential cscs
( cd24low/cd44þ ) was detected when compared with
pretreatment biopsies ( yu et al. , 2007 ; li et al. , 2008 ;
creighton et al. , 2009 ). similar results were obtained in
mouse xenograft models of glioblastoma and colorectal
cancer ( bao et al. , 2006 ; dylla et al. , 2008 ). we
previously reported that lung cscs isolated from
primary tumors too show high resistance to firmly standard
chemotherapy in vitro ( eramo et al. , 2008 ), suggesting
that these cells exhibit highly efficient intrinsic resistance
mechanisms. moreover, lung cancer mouse xenografts
treated with cisplatin regimens in vivo showed enrichment
of specific sub-populations with stem cell phenotype
( cd133þ/abcg2þ/cxcr4þ ) ( bertolini et al. ,
2009 ).
the combinations of slow cell-cycle progression and
efficient dna repair machinery are potential resistance
mechanisms of cscs. in distinction towards the majority of
tumor progenitors and precursors, cscs showed increased
quiescence in vivo and in vitro, suggesting poor
responses to firmly typical treatments, that primarily
kill proliferating cells ( holyoake et al. , 1999 ; guan
et al. , 2003 ; ishikawa et al. , 2007 ; cicalese et al. , 2009 ).
additionally, it really has been reported that glioblastoma stem
cells exhibit active dna repair mechanisms, which
permit them to be able to recognize and repair radiation-induced
dna injury efficiently ( bao et al. , 2006 ). the accumulating
proof that the csc sub-population of tumor
cells is resistant to firmly typical anticancer therapy can
make a case for the frequent relapse when a very good initial response
to firmly chemotherapy was observed in patients ( see figure 2 ).
to get this reason, innovative therapeutic approaches
to firmly kill or disarm cscs can be the secret for cure solid
cancers. targeted treatment of cscs is an ambitious
approach, as these most most likely represent a heterogeneous
population of cells with totally different sensitivities
to firmly chemotherapeutic medication that need to be combined
to obtain full antitumor effects. in distinction to firmly the
induction of apoptosis within the majority of tumor cells
observed with typical chemotherapeutic agents,
targeted elimination of cscs may spur slow but
sustainable tumor eradication. this implicates that most
preclinical xenograft models utilized check the efficacy
of chemical compounds or biomolecules need to be
modified. these adjustments embrace not solely longer
observation periods but as well as a faithful analysis of
treated tumors for cells expressing potential csc
markers at intermediate finish points. monitoring druginduced
alterations within the csc compartment may be
a useful indicator for general anticancer efficacy of the
compound. as cscs share many resistance mechanisms
with normal tissue stem cells, there may be a high risk to firmly hit
normal stem cells by a targeted anti-csc therapy with
disastrous consequences regarding the patient. thus, it is
vital to firmly style new therapeutic approaches to
selectively hit csc-specific pathways, whereas sparing
normal stem cells. up to now, variety of preclinical studies
are performed using totally different strategies to firmly target cscs. these embody the specific elimination of cscs
with selective targeting ( schatton et al. , 2009 ) or
sensitization of cscs out to typical chemotherapy
and differentiation therapies. in a few of those innovative
therapeutic approaches, antibodies were designed to neutralize
autocrine signaling mediators vital for csc
growth and chemoresistance, an example would be cd123 ( interleukin-
3 receptor ) in acute myeloid leukemia ( jin et al. , 2009 )
and interleukin-4 in colorectal cancer ( todaro et al. ,
2007, 2008 ). alternative antibodies interfere in the communication
of cscs with noncancerous tissue, and thereby
forestall csc localization with their niche ( jin et al. , 2006 ;
yang et al. , 2008 ). similarly, inhibitors blocking cxcr4
impair the interaction of acute myeloid leukemia stem
cells in the microenvironment within the whole bone marrow,
by having consequent sensitization out to targeted therapy ( zeng
et al. , 2009 ). because we are part of a glioblastoma model, it was eventually shown that
endothelial cells promote the propagation of glioblastoma
stem cells. subsequently, antiangiogenic therapy using
vascular endothelial growth issue inhibitors depleted
not no more than tumor vascularization but as well as ablated cscs in
the xenograft ( calabrese et al. , 2007 ).
another approach for innovative therapy is the
targeted inhibition of vital signal transduction
pathways and transcription factors highly active in
cscs, together with embryonic pathways. pharmacological
inhibition on your hedgehog pathway reduced the growth
of lung tumor cells in xenograft models, suggesting an
vital role with this pathway in lung cscs also
( watkins et al. , 2003 ). a recent study showed that knock
down on your transcription issue oct-4 led out to apoptosis
associated with a csc-like population of lung cancer cells ( hu et al. ,
2008 ). this finding is of explicit interest as oct-4 is
expressed in potential human lung progenitor cells
that show a few characteristics of murine bascs
( ling et al. , 2006 ; chen et al. , 2007 ).
cscs will divide asymmetrically and provides rise out to more
differentiated progeny that lose the ability out to divide
indefinitely. differentiation therapy aims at converting
tumorigenic cscs with their nontumorigenic progeny. it
was shown that treatment with bone morphogenic
protein 4 reduced the tumor-initiating cell pool in a
glioma model and markedly slowed down tumor growth
in vivo while not toxic aspect effects ( piccirillo et al. , 2006 ).
finally, cscs represent a very good model system out to test
new medication for antitumor treatments. in spite of this, until
recently, many technical difficulties restricted the use of
cscs for high throughput screenings of chemical
compounds. the largest problem is perhaps the rarity
of cscs. thus, it's nearly not possible out to use sorted
cscs from primary human tumors for large-scale
experiments. out to overcome these technical limitations,
one potential resolution is that the generation of ‘induced’ cscs.
gupta et al. ( 2009 ) demonstrated that immortalized
human mammary epithelial cells underwent epithelial–
mesenchymal transition, acquired tumorigenicity, gained
the ability out to grow as mammospheres and acquired the
expression of csc marker proteins in the event the expression of
e-cadherin was knocked down with short hairpin rna.
these cells were used for high-content screenings and
identified compounds selectively abolishing the cd44þ/
cd24low cell population in vitro, leading to some marked
reduction in tumorigenicity of pretreated cell lines in vivo.
alternative experimental strategies will overcome the need
out to genetically modify cells to get sufficient numbers
of csc-like cells for in vitro assays. for a few tumors,
it's attainable out to propagate primary cancer cells in
spheroid cultures ( reynolds and weiss, 1992 ), which
might enable intensive csc characterization in vitro. these
cultures contain defined growth factors supporting the
proliferation of undifferentiated stem-like cells endowed
with high clonogenic capability and therefore the ability out to generate
tumor xenografts recapitulating the initial tumor ( singh
et al. , 2004 ; ricci-vitiani et al. , 2007, todaro et al. ,
2007 ). recently, pollard et al. described a cultivation
technique for primary glioblastoma cells using laminincoated
flasks and an analogous medium used for spheroid
cultures. below these conditions, the cells adhere but
maintain a csc-like character ( pollard et al. , 2009 ).
in spite of this, these approaches conjointly harbor variety of
technical difficulties. at present, csc spheres from
epithelial tumors are problematic to establish, significantly in
the case of lung cancer. therefore, the use of csc spheroids
from primary tumors can be restricted to some few specialized
laboratories. another potential limitation is the idea that an
average of 15–20% of cells in tumor spheres are cscs
( eramo et al. , 2008 ). as elegantly shown in mouse breast
cancer, the speed of symmetric divisions depends on the
genomic alterations present in tumors. therefore, though a
normal sphere might contain as low collectively stem cell, the
proportion of cscs in tumor spheres can be very
variables ( cicalese et al. , 2009 ; pece et al. , 2010 ). despite
all of those limitations, csc cultures represent an
attention-grabbing in vitro model system for systematic highcontent
drug-screening approaches aiming to look for new
compounds targeting cscs as well as their direct progeny.
lung cscs : implications for diagnosis, prognosis and
monitoring of therapeutic response
as discussed within the whole previous paragraph, lung csc
analysis might supply a relevant contribution out to the
establishment of innovative tumor eradicating therapies.
besides therapy, knowledge derived direct from csc field
of investigation goes out to cause crucial advances
in lung cancer diagnosis, prognosis and monitoring of
patient response out to treatment. inside the basis of the
assumption that cscs are additional aggressive in comparison to the bulk
tumor cell population, tumor malignancy can be related
in the grade of cell differentiation and the abundance
on your csc fraction among the bulk tumor cells. the
possibility out to isolate, expand and characterize cscs
from totally different tissues has enabled the identification of csc-associated antigens and consequently the immunodetection
of tumorigenic cells at intervals patient tumors.
the estimation of csc frequency within the whole tumor mass
could be accustomed being a diagnostic tool out to define tumor grade
and staging a lot of precisely. recent studies have highlighted
the growing interest within the whole exploitation of lung
csc knowledge regarding the improvement of lung cancer
diagnosis. it's been reported that the expression of
csc-associated markers aldh1 or sox2 positively
correlated with higher stage and grade in numerous lung
tumors ( jiang et al. , 2009 ; sholl et al. , 2009 ). additional
csc-associated markers, embryonic genes or combinations
of multiple markers may well be relevant out to generate
a lot of informative lung cancer diagnoses. highly
descriptive, csc-considering diagnoses may well be the
beginning purpose for improved and a lot of tailored lung
cancer treatments just like the therapeutic alternative has out to be
strictly linked out to the kind and stage on your tumor. it is
noteworthy that a correct and detailed diagnosis is
relevant out to define tumor stage conjointly out to predict the chance
of metastatic dissemination of tumors. during this context,
the so-called circulating tumor cells can possibly be of high
importance. circulating tumor cells are cancer cells that
are detectable in patient blood and indicative of tumor
dissemination potential ( pantel et al. , 2009 ). an early
spread of tumor cells is sometimes undetected by current
imaging technologies in patients with cancer. so,
completely different detection approaches are presently being set up
as early metastasis-sensitive ways. these methods
rely by the immunological, cytometrical or molecular
detection of circulating tumor cells ( pantel et al. , 2009 ).
every style of tumor cell could be ‘circulating’. but, it
is probable that simply a restricted fraction of those may
migrate across the blood stream, colonize alternative tissues
and initiate a whole new tumor mass in another location,
thereby giving rise out to metastatic lesions. as shown for
breast cancer, these cells are currently largely believed out to be
compatible with cscs ( ross and slodkowska, 2009 ;
theodoropoulos et al. , 2009 ). these findings confer key
relevance out to cscs conjointly within the whole topic of circulating tumor
cells and within the whole differential diagnosis of localized or
premetastatic disease. so, circulating cscs
may well be used as prognostic tools, representing predictors
of tumor progression. ( aktas et al. , 2009 ). the
investigation on csc-related factors linked out to patient
prognosis and likewise predictive of patient response to
treatments will just be crucial for second-hand of alternative
therapeutic choices. many publications recommend that
csc-related antigen expressions within the whole tumor mass
represent relevant prognostic indicators in various
cancers ( maeda et al. , 2008 ; pallini et al. , 2008 ; song
et al. , 2008 ; zeppernick et al. , 2008 ).
in pulmonary tumors, higher aldh1 expression was
linked out to poor prognosis in patients with early-stage
lung cancer ( jiang et al. , 2009 ). when treatment has
started, many biomarkers are designed to confirm the
impact on your therapeutic regimen on each, tumor and
patient at early time points, to make sure that ineffective or highly
toxic approaches could be rapidly substituted with a
second-line therapy. along at the finish of treatment, specific
biomarkers are presently designed to monitor over time
disease regression, progression or relapse.
current biomarkers out to monitor treatment response
would be the abundance of tumor-derived cells in patient
blood or plasma levels of tumor-derived proteins, such
as prostate-specific antigen for prostate cancer, ca19. 9
for gastric cancer, ca125 for ovarian cancer or cea
and cytokeratins for alternative tumors, together with lung cancer
( pantel et al. , 2009 ). as tumor relapse depends on
the presence of tumorigenic cells, a careful monitoring
of tumor response out to treatments ought to embody the
detection of csc-related biomarkers. the fact is, treatment
associated with a tumor with standard anticancer medicine might
result in partial reduction on your tumor mass, representing
an indicator of excellent response making use of the current
standards of tumor monitoring. but, just like the drug
would possibly preferentially kill the differentiated cells, it is
probable that the csc frequency wouldn't be reduced
at intervals the tumor, other then rather increased ( bao et al. , 2006 ;
ishikawa et al. , 2007 ). so, the analysis of cscbased
biomarkers looks crucial out to dissect among
transient tumor-debulking activity and long-lasting
tumor-eradicating treatments.
conclusions
despite the event of targeted therapies, clinical
oncologists have experienced a restricted improvement in
the prognosis of lung cancer patients. it's seemingly that
an improvement in lung cancer patient outcome may
need the introduction of a lot of effective innovative
agents with increased activity against drug-resistant
tumorigenic lung cscs. we envision that future
investigations on cscs routinely isolated from
cancer patients would pave the means out to innovative
knowledge, leading in flip out to novel approaches out to fight
lung cancer. this could result in considerable changes
into your clinical follow, who can need a more
intense role of molecular pathologists out to provide
info for a lot of effective and fewer destructive
personalised therapies.
References
Aktas B, Tewes M, Fehm T, Hauch S, Kimmig R, Kasimir-Bauer S.
(2009). Stem cell and epithelial-mesenchymal transition markers are
frequently overexpressed in circulating tumor cells of metastatic
breast cancer patients. Breast Cancer Res 11: R46.
Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ,
Clarke MF. (2003). Prospective identification of tumorigenic breast
cancer cells. Proc Natl Acad Sci USA 100: 3983–3988.
Bao S, Wu Q, McLendon RE, Hao Y, Shi Q, Hjelmeland AB et al.
(2006). Glioma stem cells promote radioresistance by preferential
activation of the DNA damage response. Nature 444: 756–760.
Barker N, Ridgway RA, van Es JH, van de Wetering M, Begthel H,
van den Born M et al. (2009). Crypt stem cells as the cells-of-origin
of intestinal cancer. Nature 457: 608–611.
Barker N, van de Wetering M, Clevers H. (2008). The intestinal stem
cell. Genes Dev 22: 1856–1864.
Beier D, Hau P, Proescholdt M, Lohmeier A, Wischhusen J, Oefner PJ
et al. (2007). CD133(+) and CD133(_) glioblastoma-derived cancer
stem cells show differential growth characteristics and molecular
profiles. Cancer Res 67: 4010–4015.
Bertolini G, Roz L, Perego P, Tortoreto M, Fontanella E, Gatti L
et al. (2009). Highly tumorigenic lung cancer CD133+ cells display
stem-like features and are spared by cisplatin treatment. Proc Natl
Acad Sci USA 106: 16281–16286.
Besson A, Hwang HC, Cicero S, Donovan SL, Gurian-West M,
Johnson D et al. (2007). Discovery of an oncogenic activity in
p27Kip1 that causes stem cell expansion and a multiple tumor
phenotype. Genes Dev 21: 1731–1746.
Bhattacharjee A, Richards WG, Staunton J, Li C, Monti S, Vasa P
et al. (2001). Classification of human lung carcinomas by mRNA
expression profiling reveals distinct adenocarcinoma subclasses.
Proc Natl Acad Sci USA 98: 13790–13795.
Bianchi F, Nicassio F, Di Fiore PP. (2008). Unbiased vs biased
approaches to the identification of cancer signatures: the case of
lung cancer. Cell Cycle 7: 729–734.
Blau HM, Brazelton TR, Weimann JM. (2001). The evolving concept
of a stem cell: entity or function? Cell 105: 829–841.
Blenkinsopp WK. (1967). Proliferation of respiratory tract epithelium
in the rat. Exp Cell Res 46: 144–154.
Bonnet D, Dick JE. (1997). Human acute myeloid leukemia is
organized as a hierarchy that originates from a primitive hematopoietic
cell. Nat Med 3: 730–737.
Borczuk AC, Toonkel RL, Powell CA. (2009). Genomics of lung
cancer. Proc Am Thorac Soc 6: 152–158.
Calabrese C, Poppleton H, Kocak M, Hogg TL, Fuller C, Hamner B
et al. (2007). A perivascular niche for brain tumor stem cells. Cancer
Cell 11: 69–82.
Chen Y, Chan VS, Zheng B, Chan KY, Xu X, To LY et al. (2007).
A novel subset of putative stem/progenitor CD34+Oct-4+ cells is
the major target for SARS coronavirus in human lung. J Exp Med
204: 2529–2536.
Chen YC, Hsu HS, Chen YW, Tsai TH, How CK, Wang CY et al.
(2008). Oct-4 expression maintained cancer stem-like properties in
lung cancer-derived CD133-positive cells. PLoS One 3: e2637.
Cicalese A, Bonizzi G, Pasi CE, Faretta M, Ronzoni S, Giulini B
et al. (2009). The tumor suppressor p53 regulates polarity of selfrenewing
divisions in mammary stem cells. Cell 138: 1083–1095.
Clarke MF, Dick JE, Dirks PB, Eaves CJ, Jamieson CH, Jones DL
et al. (2006). Cancer stem cells—perspectives on current status and
future directions: AACR Workshop on cancer stem cells. Cancer
Res 66: 9339–9344.
Collins LG, Haines C, Perkel R, Enck RE. (2007). Lung cancer:
diagnosis and management. Am Fam Physician 75: 56–63.
Creighton CJ, Li X, Landis M, Dixon JM, Neumeister VM, Sjolund A
et al. (2009). Residual breast cancers after conventional therapy
display mesenchymal as well as tumor-initiating features. Proc Natl
Acad Sci USA 106: 13820–13825.
Dalerba P, Dylla SJ, Park IK, Liu R, Wang X, Cho RW et al. (2007).
Phenotypic characterization of human colorectal cancer stem cells.
Proc Natl Acad Sci USA 104: 10158–10163.
Dubey S, Powell CA. (2008). Update in lung cancer 2007. Am J Respir
Crit Care Med 177: 941–946.
Dubey S, Powell CA. (2009). Update in lung cancer 2008. Am J Respir
Crit Care Med 179: 860–868.
Dylla SJ, Beviglia L, Park IK, Chartier C, Raval J, Ngan L et al.
(2008). Colorectal cancer stem cells are enriched in xenogeneic
tumors following chemotherapy. PLoS One 3: e2428.
Eramo A, Lotti F, Sette G, Pilozzi E, Biffoni M, Di Virgilio A et al.
(2008). Identification and expansion of the tumorigenic lung cancer
stem cell population. Cell Death Differ 15: 504–514.
Fuchs E. (2009). The tortoise and the hair: slow-cycling cells in the
stem cell race. Cell 137: 811–819.
Giangreco A, Arwert EN, Rosewell IR, Snyder J, Watt FM,
Stripp BR. (2009). Stem cells are dispensable for lung homeostasis
but restore airways after injury. Proc Natl Acad Sci USA 106:
9286–9291.
Giangreco A, Groot KR, Janes SM. (2007). Lung cancer and
lung stem cells: strange bedfellows? Am J Respir Crit Care Med
175: 547–553.
Giangreco A, Reynolds SD, Stripp BR. (2002). Terminal bronchioles
harbor a unique airway stem cell population that localizes to the
bronchoalveolar duct junction. Am J Pathol 161: 173–182.
Ginestier C, Hur MH, Charafe-Jauffret E, Monville F, Dutcher J,
Brown M et al. (2007). ALDH1 is a marker of normal and
malignant human mammary stem cells and a predictor of poor
clinical outcome. Cell Stem Cell 1: 555–567.
Guan Y, Gerhard B, Hogge DE. (2003). Detection, isolation,
and stimulation of quiescent primitive leukemic progenitor cells
from patients with acute myeloid leukemia (AML). Blood 101:
3142–3149.
Gupta PB, Onder TT, Jiang G, Tao K, Kuperwasser C, Weinberg RA
et al. (2009). Identification of selective inhibitors of cancer stem cells
by high-throughput screening. Cell 138: 645–659.
Hill RP. (2006). Identifying cancer stem cells in solid tumors: case not
proven. Cancer Res 66: 1891–1895.
Ho MM, Ng AV, Lam S, Hung JY. (2007). Side population in human
lung cancer cell lines and tumors is enriched with stem-like cancer
cells. Cancer Res 67: 4827–4833.
Holyoake T, Jiang X, Eaves C, Eaves A. (1999). Isolation of a highly
quiescent subpopulation of primitive leukemic cells in chronic
myeloid leukemia. Blood 94: 2056–2064.
Hong KU, Reynolds SD, Watkins S, Fuchs E, Stripp BR. (2004a).
Basal cells are a multipotent progenitor capable of renewing the
bronchial epithelium. Am J Pathol 164: 577–588.
Hong KU, Reynolds SD, Watkins S, Fuchs E, Stripp BR. (2004b).
in vivo differentiation potential of tracheal basal cells: evidence for
multipotent and unipotent subpopulations. Am J Physiol Lung Cell
Mol Physiol 286: L643–L649.
Hu T, Liu S, Breiter DR, Wang F, Tang Y, Sun S. (2008). Octamer 4
small interfering RNA results in cancer stem cell-like cell apoptosis.
Cancer Res 68: 6533–6540.
Huang EH, Hynes MJ, Zhang T, Ginestier C, Dontu G, Appelman H
et al. (2009). Aldehyde dehydrogenase 1 is a marker for normal and
malignant human colonic stem cells (SC) and tracks SC overpopulation
during colon tumorigenesis. Cancer Res 69: 3382–3389.
Ishikawa F, Yoshida S, Saito Y, Hijikata A, Kitamura H, Tanaka S
et al. (2007). Chemotherapy-resistant human AML stem cells
home to and engraft within the bone-marrow endosteal region.
Nat Biotechnol 25: 1315–1321.
Iwasaki H, Akashi K. (2007). Myeloid lineage commitment from the
hematopoietic stem cell. Immunity 26: 726–740.
Jackson EL, Willis N, Mercer K, Bronson RT, Crowley D, Montoya
R et al. (2001). Analysis of lung tumor initiation and progression
using conditional expression of oncogenic K-ras. Genes Dev 15:
3243–3248.
Janne PA, Engelman JA, Johnson BE. (2005). Epidermal growth
factor receptor mutations in non-small-cell lung cancer: implications
for treatment and tumor biology. J Clin Oncol 23: 3227–3234.
Jemal A, Thun MJ, Ries LA, Howe HL, Weir HK, Center MM
et al. (2008). Annual report to the nation on the status of cancer,
1975–2005, featuring trends in lung cancer, tobacco use, and
tobacco control. J Natl Cancer Inst 100: 1672–1694.
Jiang F, Qiu Q, Khanna A, Todd NW, Deepak J, Xing L et al. (2009).
Aldehyde dehydrogenase 1 is a tumor stem cell-associated marker in
lung cancer. Mol Cancer Res 7: 330–338.
Jin L, Hope KJ, Zhai Q, Smadja-Joffe F, Dick JE. (2006). Targeting of
CD44 eradicates human acute myeloid leukemic stem cells. Nat Med
12: 1167–1174.
Jin L, Lee EM, Ramshaw HS, Busfield SJ, Peoppl AG, Wilkinson L
et al. (2009). Monoclonal antibody-mediated targeting of CD123,
IL-3 receptor alpha chain, eliminates human acute myeloid leukemic
stem cells. Cell Stem Cell 5: 31–42.
Kim CF, Jackson EL, Woolfenden AE, Lawrence S, Babar I, Vogel S
et al. (2005). Identification of bronchioalveolar stem cells in normal
lung and lung cancer. Cell 121: 823–835.
Kuperwasser C, Chavarria T, Wu M, Magrane G, Gray JW,
Carey L et al. (2004). Reconstruction of functionally normal and
malignant human breast tissues in mice. Proc Natl Acad Sci USA
101: 4966–4971.
Lantuejoul S, Salameire D, Salon C, Brambilla E. (2009). Pulmonary
preneoplasia—sequential molecular carcinogenetic events. Histopathology
54: 43–54.
Lapidot T, Sirard C, Vormoor J, Murdoch B, Hoang T, Caceres-
Cortes J et al. (1994). A cell initiating human acute myeloid
leukaemia after transplantation into SCID mice. Nature 367:
645–648.
Lee A, Kessler JD, Read TA, Kaiser C, Corbeil D, Huttner WB et al.
(2005). Isolation of neural stem cells from the postnatal cerebellum.
Nat Neurosci 8: 723–729.
Levina V, Marrangoni AM, DeMarco R, Gorelik E, Lokshin AE.
(2008). Drug-selected human lung cancer stem cells: cytokine
network, tumorigenic and metastatic properties. PLoS One
3: e3077.
Li X, Lewis MT, Huang J, Gutierrez C, Osborne CK, Wu MF et al.
(2008). Intrinsic resistance of tumorigenic breast cancer cells to
chemotherapy. J Natl Cancer Inst 100: 672–679.
Ling TY, Kuo MD, Li CL, Yu AL, Huang YH, Wu TJ et al. (2006).
Identification of pulmonary Oct-4+ stem/progenitor cells and
demonstration of their susceptibility to SARS coronavirus (SARSCoV)
infection in vitro. Proc Natl Acad Sci USA 103: 9530–9535.
Maeda S, Shinchi H, Kurahara H, Mataki Y, Maemura K, Sato M
et al. (2008). CD133 expression is correlated with lymph node
metastasis and vascular endothelial growth factor-C expression in
pancreatic cancer. Br J Cancer 98: 1389–1397.
Mahvi D, Bank H, Harley R. (1977). Morphology of a naphthaleneinduced
bronchiolar lesion. Am J Pathol 86: 558–572.
Minna JD, Roth JA, Gazdar AF. (2002). Focus on lung cancer. Cancer
Cell 1: 49–52.
O’Brien CA, Pollett A, Gallinger S, Dick JE. (2007). A human colon
cancer cell capable of initiating tumour growth in immunodeficient
mice. Nature 445: 106–110.
Orkin SH, Zon LI. (2002). Hematopoiesis and stem cells: plasticity
versus developmental heterogeneity. Nat Immunol 3: 323–328.
Pallini R, Ricci-Vitiani L, Banna GL, Signore M, Lombardi D,
Todaro M et al. (2008). Cancer stem cell analysis and clinical
outcome in patients with glioblastoma multiforme. Clin Cancer Res
14: 8205–8212.
Pantel K, Alix-Panabieres C, Riethdorf S. (2009). Cancer micrometastases.
Nat Rev Clin Oncol 6: 339–351.
Pao W, Miller V, Zakowski M, Doherty J, Politi K, Sarkaria I et al.
(2004). EGF receptor gene mutations are common in lung cancers
from ‘never smokers’ and are associated with sensitivity of tumors to
gefitinib and erlotinib. Proc Natl Acad Sci USA 101: 13306–13311.
Pao W, Wang TY, Riely GJ, Miller VA, Pan Q, Ladanyi M et al.
(2005). KRAS mutations and primary resistance of lung adenocarcinomas
to gefitinib or erlotinib. PLoS Med 2: e17.
Pece S, Tosoni D, Confalonieri S, Mazzarol G, Vecchi M, Ronzoni S
et al. (2010). Biological and molecular heterogeneity of breast
cancers correlates with their cancer stem cell content. Cell 140:
62–73.
Piccirillo SG, Reynolds BA, Zanetti N, Lamorte G, Binda E, Broggi G
et al. (2006). Bone morphogenetic proteins inhibit the tumorigenic
potential of human brain tumour-initiating cells. Nature 444:
761–765.
Pollard SM, Yoshikawa K, Clarke ID, Danovi D, Stricker S, Russell R
et al. (2009). Glioma stem cell lines expanded in adherent culture
have tumor-specific phenotypes and are suitable for chemical and
genetic screens. Cell Stem Cell 4: 568–580.
Rawlins EL. (2008). Lung epithelial progenitor cells: lessons from
development. Proc Am Thorac Soc 5: 675–681.
Rawlins EL, Okubo T, Xue Y, Brass DM, Auten RL, Hasegawa H
et al. (2009). The role of Scgb1a1+ Clara cells in the long-term
maintenance and repair of lung airway, but not alveolar, epithelium.
Cell Stem Cell 4: 525–534.
Reya T, Morrison SJ, Clarke MF, Weissman IL. (2001). Stem cells,
cancer, and cancer stem cells. Nature 414: 105–111.
Reynolds BA, Weiss S. (1992). Generation of neurons and astrocytes
from isolated cells of the adult mammalian central nervous system.
Science 255: 1707–1710.
Reynolds SD, Giangreco A, Power JH, Stripp BR. (2000). Neuroepithelial
bodies of pulmonary airways serve as a reservoir of
progenitor cells capable of epithelial regeneration. Am J Pathol 156:
269–278.
Ricci-Vitiani L, Lombardi DG, Pilozzi E, Biffoni M, Todaro M,
Peschle C et al. (2007). Identification and expansion of human
colon-cancer-initiating cells. Nature 445: 111–115.
Riely GJ, Marks J, Pao W. (2009). KRAS mutations in non-small cell
lung cancer. Proc Am Thorac Soc 6: 201–205.
Ross JS, Slodkowska EA. (2009). Circulating and disseminated tumor
cells in the management of breast cancer. Am J Clin Pathol 132:
237–245.
Scagliotti G, Hanna N, Fossella F, Sugarman K, Blatter J, Peterson P
et al. (2009). The differential efficacy of pemetrexed according to
NSCLC histology: a review of two phase III studies. Oncologist 14:
253–263.
Schatton T, Frank NY, Frank MH. (2009). Identification and
targeting of cancer stem cells. Bioessays 31: 1038–1049.
Sekido Y, Fong KM, Minna JD. (2003). Molecular genetics of lung
cancer. Annu Rev Med 54: 73–87.
Sholl LM, Long KB, Hornick JL. (2009). Sox2 expression in
pulmonary non-small cell and neuroendocrine carcinomas. Appl
Immunohistochem Mol Morphol 18: 55–61.
Shultz LD, Lyons BL, Burzenski LM, Gott B, Chen X, Chaleff S et al.
(2005). Human lymphoid and myeloid cell development in NOD/
LtSz-scid IL2R gamma null mice engrafted with mobilized human
hemopoietic stem cells. J Immunol 174: 6477–6489.
Singh SK, Clarke ID, Terasaki M, Bonn VE, Hawkins C, Squire J
et al. (2003). Identification of a cancer stem cell in human brain
tumors. Cancer Res 63: 5821–5828.
Singh SK, Hawkins C, Clarke ID, Squire JA, Bayani J, Hide T et al.
(2004). Identification of human brain tumour initiating cells. Nature
432: 396–401.
Song W, Li H, Tao K, Li R, Song Z, Zhao Q et al. (2008). Expression
and clinical significance of the stem cell marker CD133 in
hepatocellular carcinoma. Int J Clin Pract 62: 1212–1218.
Storms RW, Trujillo AP, Springer JB, Shah L, Colvin OM, Ludeman
SM et al. (1999). Isolation of primitive human hematopoietic
progenitors on the basis of aldehyde dehydrogenase activity.
Proc Natl Acad Sci USA 96: 9118–9123.
Stripp BR, Maxson K, Mera R, Singh G. (1995). Plasticity of airway
cell proliferation and gene expression after acute naphthalene
injury. Am J Physiol 269: L791–L799.
Stripp BR, Reynolds SD. (2008). Maintenance and repair of the
bronchiolar epithelium. Proc Am Thorac Soc 5: 328–333.
Theodoropoulos PA, Polioudaki H, Agelaki S, Kallergi G, Saridaki Z,
Mavroudis D et al. (2009). Circulating tumor cells with a putative
stem cell phenotype in peripheral blood of patients with breast
cancer. Cancer Lett. 288: 99–106.
Tiseo M, Bartolotti M, Gelsomino F, Ardizzoni A. (2009). First-line
treatment in advanced non-small-cell lung cancer: the emerging
role of the histologic subtype. Expert Rev Anticancer Ther 9:
425–435.
Todaro M, Alea MP, Di Stefano AB, Cammareri P, Vermeulen L,
Iovino F et al. (2007). Colon cancer stem cells dictate tumor growth
and resist cell death by production of interleukin-4. Cell Stem Cell 1:
389–402.
Todaro M, Perez Alea M, Scopelliti A, Medema JP, Stassi G. (2008).
IL-4-mediated drug resistance in colon cancer stem cells. Cell Cycle
7: 309–313.
Van Winkle LS, Buckpitt AR, Nishio SJ, Isaac JM, Plopper CG.
(1995). Cellular response in naphthalene-induced Clara cell
injury and bronchiolar epithelial repair in mice. Am J Physiol 269:
L800–L818.
Ventura JJ, Tenbaum S, Perdiguero E, Huth M, Guerra C,
Barbacid M et al. (2007). p38alpha MAP kinase is essential in lung
stem and progenitor cell proliferation and differentiation. Nat Genet
39: 750–758.
Visvader JE, Lindeman GJ. (2008). Cancer stem cells in solid tumours:
accumulating evidence and unresolved questions. Nat Rev Cancer 8:
755–768.
Wang JC, Dick JE. (2005). Cancer stem cells: lessons from leukemia.
Trends Cell Biol 15: 494–501.
Watkins DN, Berman DM, Burkholder SG, Wang B, Beachy PA,
Baylin SB. (2003). Hedgehog signalling within airway
epithelial progenitors and in small-cell lung cancer. Nature 422:
313–317.
Yanagi S, Kishimoto H, Kawahara K, Sasaki T, Sasaki M, Nishio M
et al. (2007). Pten controls lung morphogenesis, bronchioalveolar
stem cells, and onset of lung adenocarcinomas in mice. J Clin Invest
117: 2929–2940.
Yang ZJ, Ellis T, Markant SL, Read TA, Kessler JD, Bourboulas M
et al. (2008). Medulloblastoma can be initiated by deletion of
patched in lineage-restricted progenitors or stem cells. Cancer Cell
14: 135–145.
Yu F, Yao H, Zhu P, Zhang X, Pan Q, Gong C et al. (2007). Let-7
regulates self renewal and tumorigenicity of breast cancer cells.
Cell 131: 1109–1123.
Zeng Z, Shi YX, Samudio IJ, Wang RY, Ling X, Frolova O et al.
(2009). Targeting the leukemia microenvironment by CXCR4
inhibition overcomes resistance to kinase inhibitors and chemotherapy
in AML. Blood 113: 6215–6224.
Zeppernick F, Ahmadi R, Campos B, Dictus C, Helmke BM,
Becker N et al. (2008). Stem cell marker CD133 affects clinical
outcome in glioma patients. Clin Cancer Res 14: 123–129.
Zhou BB, Zhang H, Damelin M, Geles KG, Grindley JC,
Dirks PB. (2009). Tumour-initiating cells: challenges and opportunities
for anticancer drug discovery. Nat Rev Drug Discov 8:
806–823.
Zhu L, Gibson P, Currle DS, Tong Y, Richardson RJ, Bayazitov IT
et al. (2009). Prominin 1 marks intestinal stem cells that are
susceptible to neoplastic transformation. Nature 457: 603–607.
