Role of stem cells in cancer therapy and cancer stem cells: a review.

Abstract

For over 30 years, stem cells have been used in the replenishment of blood and immune systems damaged by the cancer cells or during treatment of cancer by chemotherapy or radiotherapy. Apart from their use in the immuno-reconstitution, the stem cells have been reported to contribute in the tissue regeneration and as delivery vehicles in the cancer treatments. The recent concept of 'cancer stem cells' has directed scientific communities towards a different wide new area of research field and possible potential future treatment modalities for the cancer. Aim of this review is primarily focus on the recent developments in the use of the stem cells in the cancer treatments, then to discuss the cancer stem cells, now considered as backbone in the development of the cancer; and their role in carcinogenesis and their implications in the development of possible new cancer treatment options in future.

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BioMed Central
C
INTERNA
TIONAL
CANCER CELL
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Cancer Cell International
Open Access
Review
Role of stem cells in cancer therapy and cancer stem cells: a review
Jayesh Sagar*1,2, Boussad Chaib2, Kevin Sales2, Marc Winslet1,2 and
Alexander Seifalian1,2
Address: 1Academic Department of Surgery, Royal Free Hospital, London, UK and 2University College of London, London, UK
Email: Jayesh Sagar* - jsagar_2001@yahoo.com; Boussad Chaib - zchay3e@ucl.ac.uk; Kevin Sales - k.sales@medsch.ucl.ac.uk;
Marc Winslet - m.winslet@medsch.ucl.ac.uk; Alexander Seifalian - a.seifalian@medsch.ucl.ac.uk
* Corresponding author
Abstract
For over 30 years, stem cells have been used in the replenishment of blood and immune systems
damaged by the cancer cells or during treatment of cancer by chemotherapy or radiotherapy. Apart
from their use in the immuno-reconstitution, the stem cells have been reported to contribute in
the tissue regeneration and as delivery vehicles in the cancer treatments. The recent concept of
'cancer stem cells' has directed scientific communities towards a different wide new area of
research field and possible potential future treatment modalities for the cancer. Aim of this review
is primarily focus on the recent developments in the use of the stem cells in the cancer treatments,
then to discuss the cancer stem cells, now considered as backbone in the development of the
cancer; and their role in carcinogenesis and their implications in the development of possible new
cancer treatment options in future.
Background
Cancer is the most common cause of mortality and mor-
bidity in U.K. Despite recent advances in the treatments of
cancer, the clinical outcome is yet far away from expecta-
tion. Use of stem cells in immuno-modulation or recon-
stitution is one of the methods used for decades in cancer
therapy. Stem cells have self-renewal capacity with highly
replicative potential in multilineage differentiation capac-
ity [1].
Stem cells can be divided into main three categories:
embryonic, germinal, and somatic. Embryonic stem cells
(ESCs) originate from the inner cell mass of the blastocyst.
ESCs are omnipotent and have indefinite replicative life
span, which is attributable to their telomerase expres-
sion[2]. Germinal stem cells are derived from primary ger-
minal layers of embryo. They differentiate into progenitor
cells to produce specific organ cells. Somatic/adult stem
cells are progenitor cells as they are less totipotent i.e. less
replicative life span than ESCs. They exist in mature tissues
such as haematopoietic, neural, gastrointestinal and mes-
enchymal tissues. The most commonly used adult stem
cells (ASCs) derived from bone marrow are haemopoietic
stem cells (HSCs) and other primitive progenitor cells
including mesenchymal stem cells (MSCs) and multipo-
tent adult progenitor cells (MAPCs)[3,4] The microRNAs
expression has been reported as a requisite to bypass G1/
S checkpoint, thus for the self-renewal characteristic of
stem cells[5]. Figure 1 shows hierarchy of stem cells with
cell determination and differentiation. In this review, we
highlight the potential of the adult stem cells in the cancer
treatment and also focus on the new concept of the cancer
stem cell.
Published: 4 June 2007
Cancer Cell International 2007, 7:9doi:10.1186/1475-2867-7-9
Received: 18 October 2006
Accepted: 4 June 2007
This article is available from: http://www.cancerci.com/content/7/1/9
© 2007 Sagar et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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The choice of source of stem cells for cancer
therapy
Ideally, ESCs would be the source of stem cells for thera-
peutic purposes due to higher totipotency and indefinite
life span compared to ASCs with lower totipotency and
restricted life span. However, use of ESCs have ethical con-
straints (Department of Health, UK, National Institutes of
Health and International Society for Stem Cell Research)
and their use for research and therapeutic purposes are
restricted[6] and prohibited in many countries through-
out the world. In addition, the stem cells with higher
totipotency have been shown to be more tumorogenic in
mice [7]. Thus, for ease of availability and lesser con-
strained on ethical issue, ASCs are the stem cells most
commonly used for research and therapeutic purposes.
The other reason for the use of ASCs is their easy accessi-
bility compared to ESCs. According to literature, ASCs
from bone marrow (HSCs & MSCs) are the most com-
monly studied[8] stem cells. MSCs support HSCs in the
bone marrow and have the ability to differentiate both in
vivo and in vitro into the different mesenchymal cells such
as bone, cartilage, fat, muscle, tendon and marrow
stroma[9].
Stem cell sources
ESCs are derived from a 5-day old pre-implantation
human embryos, however it posses potential risk of
destroying the embryo. ASCs can be obtained from many
tissues including bone, synovium, deciduous teeth, adi-
pose tissue, brain, blood vessels, blood and umbilical
cord blood[10-13]. Due to legal and ethical reasons, use
of ESCs is restricted in research and clinical fields and
ASCs remain the main supplement for the stem cells.
Although ASCs can be obtained from the various sites, the
ideal source of ASCs is yet to be found. Most commonly,
ASCs are acquired from the bone marrow and peripheral
blood. The bone marrow (BM) aspiration is one of the
common procedures performed to obtain ASCs, but it is
associated with morbidity in the form of wound infection
and sepsis complications[14]. ASCs can also be obtained
from adipose tissues such as abdominal fat and infra-
patellar fat[15,16] which is less invasive and less morbid
procedure than the bone marrow aspiration. It has been
shown that there is no significant difference in the cell
growth kinetics, cell senescence, gene transduction of
adherent stromal cells and yield from stem cells obtained
from bone marrow or adipose tissues[17]. The peripheral
blood also provides a safe and easily accessible route for
isolating ASCs with minimal morbidity. Use of ASCs
through peripheral blood has shown to induce more T
and NK (Natural Killer) cells compared to bone marrow
ASCs[18]. Recently, the stem cells have been claimed to be
obtained from the amniotic fluid without any harm to
mother and embryo (posted on cnn.com on 08/01/
2007).
Stem cells in immuno-reconstitution
The stem cells have been used since many years in
immuno-reconstitution following cancer development or
following cancer treatments. The high dose chemotherapy
have the adverse effects on the bone marrow causing mye-
losupression[19]. Usually this is followed by the blood
cell recovery through the haematopoietic progenitor cells
residing in the bone marrow by the complex interactions
between the progenitor cells and the marrow microenvi-
ronment under the influence of various stimulatory and
inhibitory factors [20-22]. However, time for haemat-
opoietic recovery is proportional to the doses and number
of cycles of chemotherapy [23] It has been shown that
chemotherapy can induce inhibitory factors such as
Tumour Growth Factor (TGF)-β, Interferon(IFN)-γ – IFN-
α, Tumour Necrosis Factor(TNF)-α and Interleukin(IL)-4
with cytokines that causes myelosupression[24]. HSCs are
the most commonly used and they are the stem cells of
choice for the haematopoietic cell transplantation follow-
ing high dose chemotherapy to restore bone marrow and
immune system to pre-chemotherapy levels [25]. Ran-
domised clinical trials regarding the use of HSCs for hae-
matopoietic cell transplantation have been published
with controversial results, however most of the trials sug-
gested improved disease free survival rates, shorter hospi-
tal stay, overall survival rates and event free survival
rates[26-31], while fewer of the studies have reported no
Hierarchy of stem cells with cell determination, differentia-tion and maturation
Figure 1
Hierarchy of stem cells with cell determination, differentia-
tion and maturation. It also shows potential areas of A.
Trans-germal plasticity – differentiation from one stem to
other stem cell type; B. De-differentiation – regression of a
fixed lineage cell type to a more primitive cell type; C. Trans-
determination – differentiation from one progenitor cells to
another; and D. Trans-differentiation – hypothetical differen-
tiation of one cell type to another without dedifferentiation.

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statistically significant differences when assessed those
parameters[32-35]. The adequate number of the stem
cells therapy is also reported a crucial factors for speedy
recovery[36]. Some of the chemotherapeutic agents, espe-
cially alkaylating agents, should be avoided as they are
reported to adversely affect stem cell yield and haemo-
topoietic recovery[36,37]. The post-transplant period
thrombocytopenia and neutropenia may be reduced by
re-infusion of ex vivo expanded megacaryocyte progeni-
tors[38] and re-infusion of ex vivo expanded peripheral
blood stem cells (PBSC)[39] respectively.
Choice of type of stem cell : bone marrow or
peripheral blood
The source of stem cells is most commonly either from the
bone marrow or the peripheral blood. The procedure of
the bone marrow aspiration is invasive and is associated
with the potential possible complications including frac-
ture, wound infection and sepsis while the procedure for
PBSCs isolation is much less invasive and less morbid.
PBSCs have also been shown to induce higher number of
CD4 T and NK cells compared to stem cells obtained from
the bone marrow [18]. Thus, the stem cells from periph-
eral blood are considered the preferred source of stem
cells however various clinical trials have publicized con-
troversial conclusions comparing PBSCs and BM stem
cells. It is also noticed that the occurrence of graft versus
host reaction varies with PBSCs compared to BM stem
cells [40,41]. Table 1 shows published human clinical tri-
als comparing outcomes following transplantation of
stem cells from BM and peripheral blood. These clinical
trials provide different outcomes such as Storek et al sug-
gested that PBSC yields higher lymphocyte subset
counts[42] while Hernandez et al noticed no difference in
the number of lymphocyte counts but noted faster recon-
stitution of cytotoxic subsets [43]. Similarly, these trials
present controversial results including graft versus host
disease, overall survival, disease free survival and immune
recovery. Double stem cell transplantation has been doc-
umented to improve overall survival compared to single
stem cell transplantation[44]. Granulocyte-colony stimu-
lating factor (G-CSF) helps in proliferation and differenti-
ation of haematopoietic progenitor cells [45]. G-CSF has
also been reported to mobilise autologous peripheral
blood stem cells and to preserve and increase the length of
telomerase [45]. There are various different agents which
are shown to enhance the G-CSG activity in mobilising
stem cell. These are paclitaxel and docetaxel[46], recom-
binant human thrombopoietin[47], lithium[48] and
recombinant methionyl human stem cell factor (r-metH-
uSCF)[49].
Role of purging in the isolation of stem cells
The isolation of stem cells from the allogeneic donor is
the most preferable method, however only 30% of candi-
dates are eligible due to the lack of donors and age restric-
tions[50]. Stem cells from autologous source are easily
available but they carry the risk of coexistence of normal
haematopoietic progenitors with malignant counterparts
and may lead to the relapse of cancer. In population of
patients with breast cancer, PBSC transplantation has
been related to a rapid and sustained haematopoietic
Table 1: Published human clinical trials comparing outcomes following isolation of stem cells from bone marrow vs peripheral blood.
AuthorsType of cancer No. of pats RCT/CTConclusion
Storek et al [42] Haematological malignancies140RCTPBSC yields higher lymphocyte subset counts and is associated
with fewer infections
No significant difference in T, B and NK lymphoid cells
reconstitution but PBSC influence faster reconstitution of
cytotoxic subsets (CD8+/HLADR+ and NK lymphoid cells)
The CD4:CD8 and CD45RA:CD45RO ratios were higher in the
PBSC group. Accelerated reconstitution of NK cell activity
following PBSC compared to BM.
No statistically significant difference in acute or chronic GVHD,
OS and disease free survival.
Statistically significant enhanced graft versus leukemia effect in
allo PBSC group
Faster haematological recovery and improved survival in PBSC
but no difference in GVHD
Shorter duration of neutropenia in PBSC group but higher
incidence of extensive chronic GVHD
Faster haematopoietic and immune recovery in PBSC and no
difference in GVHDand OS
Faster haematopoietic reconstitution in PBSC group with no
difference in GVHD
Hernandez et al [43] Haematological malignancies12 RCT
Talmadge et al [18]Intermediate and high grade
non-Hodgkin's lymphoma
116RCT
Oehler et al [115]Chronic myeloid leukaemia 72CT
Heldal et al [116] Haematological malignancies61 CT
Couban et al [41] Haematological malignancies228RCT
Nucci et al [117] Haematological malignancies56 RCT
Powles et al [118] Haematological malignancies39 RCT
Mahmoud et al [119]Haematological malignancies 30 RCT
Keys: RCT – Randomised Control Trial, CT – Controlled Trial, PBSC – Peripheral Blood Stem Cells, NK – Natural Killer, CD8, CD4, CD45RA &
CD45RO – Different types of T Cells, HLA – Human Leucocyte Antigen, GVHD – Graft Vs Host Disease, OS – Overall Survival, Pats – Patients, No
– Number.

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engraftment and has shown to be less contaminated than
bone marrow stem cells[51]. There was however no over-
all improvement in survival outcome[52].
The contamination of the retrieval of stem cells with
tumour cells have been major problem which reported by
many studies [51,53,54], however the effect on clinical
cell therapy has been less problematic[55,56].
Purging procedures are used in an attempt to remove
these contaminant cancer cells from stem cells. Table 2
shows published clinical trials with various in vitro and in
vivo techniques to purge the stem cells such as use of
monoclonal antibodies, continuous flow immunoad-
sorption technique, dielectroforetic-field-flow-fractiona-
tion, use of rituximab, pulsed electric field, and
hyperthermia. Amifostine has been shown to protect nor-
mal haematopoietic progenitor cells from damage by
alkylating agents used for purging of stem cells[57,58].
The double procedure using 'positive CD34' and 'negative
CD19' double selection method for purging is reported to
be better than single procedure in the poor prognosis lym-
phoproliferative disorders, but it is associated with
increased risk of life-threatening infections [59].
Stem cells in tissue regeneration and as delivery
vehicles
Apart from long lasting replicative property of stem cells,
stem cells from haemopoietic tissues seem to have
'extraordinary' abilities to generate or switch between hae-
matopoietic and non-haematopoietic lineages, exhibiting
an unexpected degree of developmental or differentiation
potential. On theoretical grounds, this allows HSC to be
used to regenerate any non-haematopoietic tissue [60].
This technique has particular implications in bone
tumours as reconstruction of bone following chemother-
apy and surgery is always a major problem. The stromal
stem cells derived from bone marrow have been used in
the cell-based bone reconstruction following chemother-
apy and surgery in osteosarcoma and Ewing sarcoma[61].
Jager et al have shown the regeneration of osteoblasts
from the survived mesenchymal progenitor cells follow-
ing COSS-96 (the cooperative osteosarcoma study) poly-
chemotherapy in vitro and its potential in vivo use [62].
There are clinical trials showing role of the stem cells in
the regeneration of myocardial tissue following myocar-
dial infarction [63-65].
Systematic delivery of drug or gene therapy has promising
future but is currently limited by various factors such as
immune detection, non-specific accumulation in normal
tissues and poor permeation. The effects of many antican-
cer agents are limited due to either their toxicities or their
short half lives such as interferon β, which shows anti-pro-
liferative and pro-apoptotic activities in vitro, but has
shown restricted effects on human malignancies in vivo
[66-68]. One proposed solution for these would be the
cell-based carriers that may target the desired site.
The recent concept of use of stem cells as delivery vehicles
came from the fact that the tumours, similar to the
wounds, send out chemo-attractants such as the vascular
endothelial growth factor (VEGF) to recruit MSC to form
the supporting stroma of the tumour, and pericytes for
angiogenesis. MSC transduced with an adenoviral expres-
sion vector carrying interferon-β gene has been demon-
Table 2: Published clinical trials with various in vitro and in vivo stem cell purging techniques.
AuthorsIn Vivo/In VitroType of cancer(cells) Purging technique Conclusion
Barbui et al [120]IV & IT Multiple myelomaTwo step negative selection
procedure with combination of
monoclonal antibodies
CEPRATE SC System – continuous
flow immunoadsorption technique
CEPRATE SC System – continuous
flow immunoadsorption technique
Safe procedure of purging stem cells.
Higher event free survival rate
Stewart et al [121]ITMultiple myeloma No advantage of purging of stem
cells
Significantly reduce tumour cell
contamination and provides safe and
rapid haematological recovery
Reduced time to engraftment
Vescio et al. [122] ITMultiple myeloma
Shpall et al [123]IVBreast cancer WR-2721 (amifostine) to 4-
hydroperoxycyclophosphamide (4-
HC)
Dielectrophoretic field-flow-
fractionation (DEP-FFF)
Rituximab
Huang et al [124] ITBreast cancerEfficient separation was observed in
12 minutes with purity of > 99.2%
Rituximab can be used in stem cell
purging
Promising technology for rapid stem
cell purging
Promising method for stem cell
purging
Borbolla-Escoboza
et al [125]
Craiu et al [126].
IT B cell lymphoma
ITMultiple myelomaPulsed electric fields
Wierenga et al [127]IT Acute myeloid leukaemiaHyperthermia
Keys: IT – in vitro, IV – in vivo.

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strated to increase the production of interferon-β at the
local site[69]. However this in vivo function of MSC
depends partly on signals from the target tissue microen-
vironment, for example, the tissues such as skin would
have high cell turn over where there would be more sig-
nals for MSC compared to connective tissues where the
high cell turn over is apparent only during healing proc-
ess[70]. Likewise, MSC engineered to release interferon-β
has been reported to create high local interferon-β levels
in the mice glioma[71]. The neural stem cells have been
reported as the delivery vehicles for the gene therapy for
CNS disorders[72]. Similarly, interest has been shown in
the use of the endothelial progenitor cells as the delivery
vehicles for gene therapy because of their attraction
towards the site of angiogenesis rather than the quiescent
vasculature[73]. It may be possible to deliver immune-
activating cytokines and other secreted proteins to brain
and breast tumours though the stem cells.
Life span of ASC
The major limiting factor in the use of stem cells in clinical
area is the life span of the stem cells. Theoretically, the
embryonic stem cells are best from this perspective due to
their indefinitive replicative life span attributed to their
telomerase expression[2]. However, practically, use of the
embryonic stem cells in clinical area is very much
restricted. Most of ASC do not possess sufficient telomer-
ase activity and thus cannot prevent loss of telomerase. At
each division, the telomerase shortens and the replication
slows down (aging) and at the end, cells cease to divide
(crisis phase) [74]. Thus we may not be able to obtain
enough adult stem cells to perform our clinical task. One
proposed solution is the use of genetic manipulation to
extend the replicative span of the stem cells through the
introduction of genes involved in controlling the replica-
tive lifespan. In humans, this can be achieved by overcom-
ing the replicative senescence by using the ectopic
expression of telomerase hTERT gene [75]. In recent years,
many studies suggested that hTERT-expressing stem cells
continue to proliferate longer and maintain their ability
to differentiate[76,77,7,78,9] Similarly, hMSCs have been
immortalized by transduction with HPV16 E6/E7 in vitro
without any neoplastic changes[79]. If it becomes success-
ful to imply this principle in clinical practice, quantitative
amount of the stem cells may not be one of the prognostic
factors in the outcome in future.
Cancer stem cells
Why a tumour does not respond to treatment? Why
tumours recur? Why cancer cells develop resistance to
treatment? These and many other raised questions may be
answered by the new concept of "Cancer Stem Cells"[80].
Cancer stem cells can be defined as cells in the tumour
growth with a tumour initiating potential. Normal stem
cells are characterised by three properties: 1 Capability of
self-renewal; 2 Strict control on stem cell numbers; 3 Abil-
ity to divide and differentiate to generate all functional
elements of that particular tissue [81]. Compared to nor-
mal stem cells, the cancer stem cells are believed to have
no control on the cell numbers. Cancer stem cells form
very small numbers in whole tumour growth and they are
said to be responsible for the growth of the tumour cells.
It has been well-known that in order to induce a tumour
in an animal model, hundreds of thousands of cancer
cells need to be injected [82]. This has been explained to
be due to limitations in the assay to support tumour
growth, or due to tumour formation deficiency [1]. With
the recent concept of the cancer stem cells, it may be
explained that higher numbers of cancer cells are needed
to maximize the probability of injecting cancer stem cells
in animal model. At present, the shrinkage in the size of a
tumour is considered as a response to the treatment. How-
ever, tumour often shrinks in response to the treatment
only to recur again. This may be explained by cancer stem
cells that the treatment targeting the cancer cells may not
be able to target the cancer stem cells
A fundamental problem in the cancer is the identification
of the cell type capable of sustaining the neoplastic
growth. There is evidence that the majority of the cancers
are clones and that the cancer cells represent the progeny
of one cell, however it is not clear which cells possess the
tumour-initiating cell (TIC) function (cancer stem cells)
and how to recognise them [83]. Though the idea of can-
cer stem cells is considered as a new concept in science, it
was thought almost 35 years back in 1971 when they were
called as leukaemic stem cells [84]. A small subset of can-
cer cells capable of extensive proliferation in leukaemia
and multiple myeloma were found and named as leukae-
mic stem cells (LSC) [84]. Two possibilities were pro-
posed: either all leukaemia cells had a low probability of
proliferation and therefore all leukaemia cells behave as
LSC, or only a small subset was clonogenic. The later the-
ory was favoured by Dick and colleagues who were able to
separate the LSC as CD34+CD38- from patients' samples
[85]. Despite being small in numbers (0.2%), these were
the only cells capable to transfer Acute Myeloid Leukae-
mia from patients to NOD-SCID (non-obese diabetic-
severe combined immunodeficiency) mice.
Recently, the cancer stem cells were also shown in the
solid tumours such as breast cancer and brain tumours
[86,87]. The cancer stem cells have been shown to have
not only self-renewal capability but also generating wide
spectrum of progeny, like normal stem cells [88]. In pae-
diatric brain tumours, including medulloblastomas and
gliomas, a subset of cells, called neurospheres, have been
shown to have self-renewal capability. In conditions to