Cancer Stem Cells Hematopoietic

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INTRODUCTION

The association between cancer and inf lammation hasbeen recognized for over 2 000 years [1,2]. Virchow rec-ognized that many tumors arise in a setting of chronicin ammation, and later, Dvorak aptly described cancer as“wounds that do not heal” [3]. It is estimated that up to 15%of cancers worldwide are associated with chronic infec-tion[4] and there are many more examples throughout thebody of cancer associated with in ammation of unclearetiology. Within the GI tract, examples include esophagealadenocarcinoma arising in the setting of Barrett’s meta-plasia, gastric cancer occurring secondary to Helicobacter infection, colorectal cancer occurring in longstanding in-flammatory bowel disease and hepatocellular carcinomasecondary to viral hepatitis (Table 1). There are severalmechanisms by which in ammation and a cycle of chronicinjury/repair has been postulated to lead to cancer, includ-ing the push for continued proliferation, an abnormalinflamed-stromal environment, and more recently werecognize a role for engraftment of circulating marrow-derived stem cells, which may contribute to the stromalcomponents of the tumor as well as the epithelial compo-nent of the tumor mass itself.

INFLAMMATION: THE RELATIONSHIP

BETWEEN TISSUE INJURY, REPAIR AND

CANCER

Proliferation of cells alone is not suf cient to cause cancer,

rather proliferation in the setting of altered growth signalsfrom inflammatory cell infiltrates and DNA damagingagents released from infiltrating leukocytes promotemalignant degeneration. Wound healing, which requiressubversion of usual growth programs, mobilization andmigration of cells and a heightened resistance to apoptosis,embodies all the properties of malignant cells with oneexception, that is, healing is self limited. Successful

wound healing results in restoration of tissue integrityand resolution of inflammation, reinstating homeostaticgrowth control. The tissue environments of longstandingunrelenting chronic infection or idiopathic chronicinflammation are persistent states of inadequate woundhealing. Within this setting, inflammatory cells producehighly reactive oxygen and nitrogen species, which interact

with cellular DNA inducing point mutations, deletions orrearrangements. Usually DNA damage such as this triggers

REVIEW

Stem cells and cancer: Evidence for bone marrow stem cells

in epithelial cancers

Han-Chen Li, Calin Stoicov, Arlin B Rogers, JeanMarie Houghton

Han-Chen Li, Calin Stoicov, JeanMarie Houghton, Departmentof Medicine, Division of Gastroenterology; Department of CancerBiology, University of Massachusetts Medical School, Worcester,MA 01605, United States

Arlin B Rogers, Division of Comparative Medicine, Massachu-setts Institute of Technology, Cambridge, MA 02139,United States

Correspondence to: JeanMarie Houghton, University ofMassachusetts Medical School, LRB-Second Floor, Room 209,364 Plantation Street, Worcester, MA 01605-2324,United States. [email protected]: +1-508-856-6441Received: 2005-06-10 Accepted: 2005-07-28

Abstract

Cancer commonly arises at the sites of chronic in am-mation and infection. Although this association has long

been recognized, the reason has remained unclear. With-in the gastrointestinal tract, there are many examplesof in ammatory conditions associated with cancer, andthese include re ux disease and Barrett’s adenocarcino-ma of the esophagus, Helicobacter infection and gastriccancer, in ammatory bowel disease and colorectal cancerand viral hepatitis leading to hepatocellular carcinoma.There are several mechanisms by which chronic in am-mation has been postulated to lead to cancer whichincludes enhanced proliferation in an endless attempt toheal damage, the presence of a persistent in ammatoryenvironment creating a pro-carcinogenic environmentand more recently a role for engraftment of circulating

marrow-derived stem cells which may contribute to thestromal components of the tumor as well as the tumormass itself. Here we review the recent advances in ourunderstanding of the contributions of circulating bonemarrow-derived stem cells to the formation of tumors inanimal models as well as in human beings.

© 2006 The WJG Press. All rights reserved.

Key words: Epithelial cancer; Stem cells

Li HC, Stoicov C, Rogers AB, Houghton J. Stem cells andcancer: Evidence for bone marrow stem cells in epithelialcancers. World J Gastroenterol 2006; 12(3): 363-371

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PO Box 2345, Beijing 100023, China World J Gastroenterol 2006 January 21; 12(3): 363-371www.wjgnet.com World Journal of Gastroenterology ISSN [email protected] © 2006 The WJG Press. All rights reserved.

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apoptosis. However, in areas of chronic inflammationand repair, growth programs are corrupted and theenvironment is poised to allow replication and survivalof cells, which under normal situations would either bequiescent or lost to apoptosis. Allowing cells to continuallydivide in an environment conducive to DNA damagemay result in the accumulation of genetic defects and theemergence of malignant cells.

Which cell is the target of malignant transformationhas been the area of much debate. Original thought wasthat a terminally differentiated cell would acquire enoughgenetic damage to replicate endlessly. This would require

multiple genetic alterations in key cell signaling cascadesto allow autonomous growth. A more likely scenario,however, is that these cells would undergo apoptosis orbe sloughed off as a normal part of organ turnover priorto “backing up” the differentiation ladder sufficiently toacquire independent growth. More recently, focus hasbeen on tissue-derived stem cells as the source of cancer.

Tissue stem cells possess several important features mak-ing them attractive candidates for malignant degeneration.

These include long life span, relative apoptosis resistanceand ability to replicate for extended periods of time. In thesetting of chronic in ammation, progenitor or stem cells

within the peripheral tissue are forced to undergo multiple

rounds of cell division predisposing to the accumulationof mutations. While restricted progenitors or even differ-entiated cells can still become transformed, in most cases,it has been believed that early stem cells are the targets oftransformation.

If one looks at areas of high proliferative capacity (highBrdU or PCNA staining in tissue) as the stem cell zone,then this theory of peripheral stem cells as the cells oforigin of cancer must be reconciled with the fact that thissupposed stem cell compartment is often the most dam-aged and depleted by agents thought to be carcinogens [5].

With chronic inflammation leading to atrophic changesattributed to peripheral stem cell exhaustion, the very cellthought to be transformed is lost - leading us to investigateif an alternate stem cell compartment is responsible forperipheral cancers arising in the setting of in ammation.In order to understand this concept, and why the bonemarrow-derived stem cell (BMDC) is a “logical choice”

for a cancer stem cell, we need to understand a few pointsabout cancer stem cells, and BMDCs in general.

Stem cells in cancer - cancer as an abnormal organ A tumor mass can be compared to an abnormal organ;in that it is composed of a heterogeneous mixture of cell

types and of cells possessing different proliferative capaci-ties and different levels of differentiation. Tumor cells areadmixed with broblasts, endothelial cells and in amma-tory cells comprising the tumor stroma. This tumor stromais being increasingly recognized as a critical contributor tomalignant growth and survival of the tumor mass; how-ever, stromal cells themselves are usually not malignant.Likewise, not all cells within a tumor are malignant - mean-ing not all cells can form tumors if transplanted at anothersite, or into a secondary host. In fact, the majority of cells

within a tumor are incapable of independent growth andare readily susceptible to apoptosis. Only a small frac-tion of cells within a tumor are capable of independentgrowth, and ful ll the criteria described for cancer stemcells[6-8]. These cells have metastatic potential, form tumorsin secondary hosts and are believed to be responsible forcontinual renewal of cells within the tumor mass. Thesecells are likely to proliferate slowly and asymmetrically,

Cause Site

Chewing tobacco/oral irritation Oral cancersSmoking/chronic bronchitis LungAsbestos MesotheliomaRe ux disease Barretts’ adenocarcinoma of

the esophagusChronic Helicobacter infection Gastric adenocarcinoma andlymphoma

Chronic pancreatitis Pancreatic cancerOpisthorchis sinensis infection (liver uke) CholangiocarcinomaViral hepatitis Hepatocellular carcinomaUlcerative colitis and Crohn’s disease Colorectal carcinomaHuman papilloma virus Anogenital carcinomasSchistosomiasis Bladder cancerPelvic in ammatory disease Ovarian cancerChronic osteomyelitis OsteosarcomaChronic scar tissue “Scar” cancer arising in the

lung, skin and other areas ofscarring

Table 1 Infection and in ammation associated with cancers

High proliferation rate

Quiescent

→

→

→

→

→

→

→→

→

→

→→

→→

→→

Figure 1 A proposed model for the cancer stem cell. The cancer stem cell repli-cates itself asymmetrically, thus maintaining one daughter stem cell identical toitself. This remains in a relatively quiescent state. The asymmetric division alsoproduces another daughter cell with a high proliferative rate which rapidly dividesto sustain the tumor mass.

Rapidly growing cells killedStem cell remains

Re-growth of tumor

Tumor growthand metastasis

ConventionalChemotherapy

→ →

Targetedchemotherapy

Stem cell killed

Residual cells can notSustain tumor mass

Apoptosis ordifferentiation→ →

Figure 2 Conventional versus stem cell-targeted chemotherapy. Conventional che-motherapy and radiotherapy targets rapidly dividing cells, and may shrink tumormass substantially. However, the stem cell (gray), which is relatively quiescent, isnot affected. Regrowth of tumor from surviving stem cell leads to regrowth of tumorand treatment failure. Chemotherapy targeted at the stem cell would remove thesource of new cell growth, and allow residual cells within the tumor to be targetedwith chemotherapy, differentiating agents or therapy aimed at inducing apoptosis,thus successfully eliminating the tumor.

364 ISSN 1007-9327 CN 14-1219/ R World J Gastroenterol January 21, 2006 Volume 12 Number 3

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self renewing the stem cell population and giving rise todaughter cells, which proliferate to sustain tumor growth(Figure 1). Conventional anti-cancer chemotherapy andradiotherapy target rapidly dividing daughter cells, affect-ing the bulk of the tumor mass, but leave the cancer stemcell intact, explaining the often rapid recurrence of tumor

bulk, once therapy is stopped (Figure 2). At present, themost pressing issue for cancer research is to identify thecancer stem cell and exploit its unique characteristics withtargeted therapies.

Bone marrow stem cellsBMDCs are a heterogenous group of cells isolated fromthe bone marrow which are capable of repopulating thehematopoietic system of a lethally irradiated immunologi-cally compatible secondary host. These cells have been di-

vided into at least two main categories; the hematopoieticand mesenchymal stem cells (MSC). Hematopoietic stemcells are traditionally regarded as the cells which give riseto the formed elements of the blood and have been usedextensively in human bone marrow transplantation. Thus,hematopoietic stem cells have been extensively studied andde ned with regard to surface markers, growth character-istics and repopulation potential [9]. Less well defined arethe MSC. This term MSC, as de ned in the literature, isthe heterogenous population of cells isolated as the adher-ent population, when total marrow is placed in culture [10].

These cells give rise to adipocyte, chondrocyte, cells ofosteocyte lineages and the marrow mesenchyma, which is

vital for optimal hematopoiesis[11-13]. Work from multiple laboratories demonstrates sur-

prising roles for marrow-derived stem cells in addition tohematopoiesis, stressing that the potential for differentia-tion may be much greater than originally believed. Markersdefining cell subpopulations within the marrow are notstandardized in these studies, making direct comparison ofdata between laboratories challenging; however, one thingremains consistent - cells within the bone marrow have amarkedly greater differentiation potential than originallybelieved. For the purposes of this discussion, the termBMDCs will be broadly used to refer to cells derived fromthe marrow, and will encompass hematopoietic stem cells,MSC multipotent progenitor cells and whole marrow.

In vitro and in vivo studies-plasticity of BMDCsMultiple and elegant studies from independent groupshave shown quite clearly that bone marrow stem cells candifferentiate along multiple diverse lineage pathways[14-17].

These ndings challenge the conventional view that bonemarrow stem cells give rise only to the marrow mesen-chyma or formed elements in the peripheral blood. In vitro,BMDCs have been shown to differentiate at the single celllevel and acquire characteristics of mesoderm, neuroecto-derm and endoderm [15,16]. These cells appear to use cultureenvironmental factors to guide lineage decisions. Strik-ingly, in vivo studies in the mouse model have con rmedthis plasticity. Elegant studies utilizing transplanted singlecells demonstrate differentiation along multiple lineages,supporting a central role for the local tissue environmentin dictating differentiation of stem cells, con rming thata single cell is multipotent, and supporting the assertion

that experimental ndings demonstrating multiple cell lin-eage differentiation is not due to circulating tissue speci cprogenitor cells, but rather to a single multipotent cell. Inthese studies, multiple types of epithelial cells have beenshown to be derived from BMDCs including epitheliumof the lung, gastrointestinal tract and skin after transplan-

tation of a single bone marrow-derived stem cell[14]

. This isnot a transient event, as cells can be recovered nearly a yearafter transplantation. In the gastrointestinal tract, engraftedcells are seen as isolated epithelial cells in the gastric pitsof the stomach, the small intestinal villi, the colonic crypt,and rarely in the esophagus. Under these experimentalconditions, cells were recovered as single differentiatedepithelial cells, and did not appear to engraft into the stemcell niche as clonal expansion was not seen. Infusion oflabeled BMDCs into a non-irradiated host, also led to theengraftment ( albeit to a lesser degree) and differentiation asepithelial cells of the liver, lung and gut in a similar patternto that seen with marrow ablation and transplantation [15],demonstrating that engraftment and differentiation aretrue physiological events and not merely artifacts of ir-radiation and experimental manipulation. While epithelialcell damage is not necessary for engraftment, studies sup-port the notion that damage to the epithelium increasesengraftment.

The mechanism by which the marrow-derived cellsacquire the appropriate phenotype of epithelial cells is notknown, with evidence supporting both direct differentia-tion or fusion with a peripheral cell[18-21]. The method ofengraftment and differentiation may be specific to theindividual tissues and/or may depend on the mechanism

of injury inducing engraftment. Irrespective of the mecha-nism involved, BMDCs have been shown to engraft andtake on the function of cells within the peripheral tis-sues[18-24].

HUMAN STUDIES - EVIDENCE FOR

PLASTICITY OF BMDCS IN PATIENTS

T R A N S P L A N T E D W I T H G E N D E R

MISMATCHED BONE MARROW

Human studies have con rmed that plasticity of BMDCs

is not restricted to mice, and may be a physiologicallyrelevant phenomenon in man as well. Studies, examin-ing peripheral tissue of female patients transplanted withbone marrow derived from male donors, have shown thatBMDCs from the donor can differentiate into skin, gutepithelium and mature hepatocytes [25,26]. Identi cation ofthe Y chromosome in cells of these tissues con rms thatBMDCs can substantially repopulate the GI tract epithe-lium[26], and this repopulation does not appear to be a rareevent. Patients in these studies have some level of graft-

versus-host disease, and the level of in ammation in thetissue correlates with the level of donor cell engraftment.

These ndings are consistent with the data derived frommurine studies and suggest the in ammatory environmentis crucial for optimal engraftment and differentiation ofBMDCs. The fact that BMDCs have the capacity to dif-ferentiate along organ-speci c lineages appropriate for theorgan of engraftment, and are found in increasing num-

Li HCet al. Stem cells in epithelial cancers 365

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bers during chronic in ammation (a condition associated with cancer), places these cells “in the right place, at theright time” to be candidates for the cancer stem cell.

Similarities between BMDCs and cancer cellsIn addition to what appears to be of immense plasticityof cells within the bone marrow, BMDCs have other traits

which make them attractive candidates for cancer stemcells. BMDCs have the capacity for self renewal, are longlived, are chemoresistant, and may be inherently muta-genic[27-30]. Intriguing is the fact that similar growth regula-tors and control mechanisms are involved in both cancerand stem cell maintenance. For example, proteins from thepolycomb group, the epigenetic chromatin modi ers, areinvolved in both cancer development and maintenance ofembryonic and adult stem cells[31]. Also, pathways used bybone marrow stem cells for trafficking appear to be ex-ploited by tumor cells for metastasis[32]. For instance, che-mokines and cytokines produced during chronic inflam-mation (such as SDF-1) in uence the behavior and migra-tion of cancer cells. These are the same chemokines andcytokines responsible for physiological stem cells homingback to the marrow cavity [33-37]. Identification of bone

marrow stromal cell-derived growth inhibitor as a potentinhibitor of breast cancer cell migration, and the capabilityof this protein to induce cell cycle arrest and apoptosis inbreast cancer stem cells further supports the use of simi-lar growth mechanisms between stem and cancer cells[38].In ammation of the GI tract is associated with IL-6 andIL-8 production which initiate neutrophil infiltration [40].Interestingly, IL-6 is also chemotactic for MSC[34]. Othercytokines and chemokines prominent in the setting ofmucosal in ammation such as VEGF and MIP-1 α are alsochemoattractants for MSC [34,35]. Receptors such as CXCR2 and 4 are found on both cancer cells and stem cells, andin uence the homing of stem cells, or invasion/metastasesof cancer cells, suggesting a link between the two popula-tions of cells. One might suppose that a mechanism simi-lar to that used to regulate BMDC circulation and homingback to areas of bone may also facilitate migration andengraftment of BMDCs into peripheral tissues as a result

of chronic in ammation, if the peripheral tissue secretesthe appropriate homing signals.

Additionally, immune escape has long been a perplex-ing property of cancer cells; MSCs have unique immuno-logical properties in that they are not immunogenic, theydo not stimulate alloreactivity, and they escape lysis by cy-

totoxic T cells and natural killer cells[40]

. This inherent abil-ity to evade immune recognition may explain why manycancer cells evade the host immune response.

BMDCs as the origin of epithelial cancer: helicobacterinduced gastric cancer as a model system

We reasoned that BMDCs, as the ultimate uncommittedadult stem cell, might represent the ideal candidate fortransformation, if placed in a favorable environment. Weused the well-described H. felis /C57BL/6 mouse model ofgastric cancer to test this theory [41]. This model is optimalfor studying the role of stem cells in in ammatory-mediat-ed cancers because C57BL/6 mice do not develop gastriccancer under controlled conditions. With Helicobacterinfection, however, the gastric mucosa progresses througha series of changes including metaplasia and dysplasia,culminating in gastrointestinal intraepithelial neoplasia(GIN) [42] by 12-15 mo of infection, thus reiterating humandisease, where gastric cancer in the absence of Helico-bacter infection is unusual, while longstanding infectioncarries a signi cant (up to 1-3%) risk of gastric cancer[43-48].In order to test the role for BMDCs in gastric cancer (Figure3), C57BL/6J mice were myeloablated and transplanted

with gender-mismatched bone marrow from mice thatexpressed a non-mammalian beta-galactosidase enzyme[C57BL/ 6JGtrosa26 (ROSA 26)], mice that expressed green uorescent protein [C57BL/6J- beta-actin-EGFP (GFP)], orcontrol C57BL/6J liter mates. Engraftment of ROSA26BMDCs into the gastric mucosa was con rmed by severalindependent methods including detecting enzyme activity,speci c B-galactosidase immunohistochemistry (IHC, twocytoplasmic markers) (Figures 4 and 5), and detection ofLacZNeo fusion gene sequence (nuclear marker) by PCR

within beta-galactosidase positive gastric glands isolated bylaser capture microscopy. In those mice transplanted withGFP marrow, GFP was detected by uorescence activatedcell sorting of cytokeratin positive-single cell preparations,

and GFP immunohistochemistry of tissue sections. X and Y chromosome fluorescent in situ hybridization (X and Y-FISH) was used as an additional means to detect BM-DCs in gender mismatched transplants [41].

As expected, acute Helicobacter infection was associ-ated with an in ux of bone marrow-derived in ammatorycells (Figure 4A - blue staining) into the tissue. At earlytime points, we did not detect any engraftment or differen-tiation of BMDCs to an epithelial cell phenotype. At 20 wkof infection, rare glands entirely replaced by BMDCs wereisolated, suggesting that engraftment into the stem cellniche had occurred. These ndings were more pronouncedat 30 wk, where antralized glands and metaplastic cells atthe squamocolumnar junction were entirely replaced bymarrow-derived cells (Figures 4D and E). The severity ofintraepithelial dysplasia increased over time, and by oneyear of infection, most mice developed invasive neoplasticglands. All of the intraepithelial neoplasia in mice infected

Figure 3 An experimental mouse model for bone marrow transplantation andH. felis -induced gastric carcinoma.

↓

↓

↓

↓

900 radsC57BL/6

Transplant withGFPROSA26 orwt marrow

H. felis infection

Evaluate forGFP,Beta-gal activity and

Y-FISH


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for 12-16 mo rose from donor marrow cells, strongly sug-gesting an inherent vulnerability of this population of cellsto malignant progression. Progressive parietal and chiefcell loss is a hallmark of chronic Helicobacter infection.Of the few parietal or chief cells which we isolated from

the infected mice, none were derived from the bone mar-row, strongly suggesting that marrow cells do not differen-tiate toward the parietal or chief cell phenotype under theexperimental conditions that were used [41].

Normal healing of the gastric mucosa after iatrogeniculceration likewise did not require BMDCs[41], nor didloss of speci c cell lineages, such as targeted ablation ofparietal cells, lead to marrow engraftment[41]. Rather, itseems that long standing in ammation and in ammatorymediated damage to the epithelium is required - an envi-ronment strongly linked to the development of cancer inmany settings. In our Helicobacter-gastric cancer model,infection and inflammation reached a plateau at 8 wk;however, engraftment was not apparent until 20 wk, sug-gesting that events other than increased in ammation areresponsible for engraftment. Between 8 and 20 wk, thereis loss of the oxyntic glands, and a restructuring of thegastric architecture to include metaplastic cell lineages, re-

ecting the effects of an abnormal tissue milieu on rapidlyproliferating cells[49]. Once engraftment began, however,the number of bone marrow-derived glands increased dra-matically, suggesting that a threshold for recruitment hadbeen reached[41].

In addition to epithelial cells within the tumor, BMDCsalso comprise a subset of cells within the tumor stromaand within seemingly uninvolved epithelium and subepi-thelial spaces adjacent to the tumors. We have recoveredadipocytes (Figure 5C), fibroblast, endothelial cells andmyofibroblasts (Figures 4 B and C) derived from bonemarrow precursors in areas adjacent to dysplasia and neo-plasia.

Based on these experiments, we have proposed a newparadigm for epithelial cancer (Figure 6). Chronic tissuein ammation leads to tissue injury and with time, to tis-sue stem cell failure. Peripheral stem cell failure leads torecruitment and permanent engraftment of BMDCs intothe tissue stem cell niche, where the BMDCs essentiallytake over the function of the tissue stem cell. In the settingof in ammation, speci cally with Th1 type cytokines andan abnormal tissue environment (for example, one lackingchief and parietal cells), the BMDCs initiate differentia-

A D

B E

C

Figure 4 Engraftment of donor-derived ROSA-26 marrow by x-gal staining.A: Mice transplanted with ROSA 26 marrow and infected withH. felis for 4 wk had donor-derivedleukocytes (blue) inltrating the gastric mucosa, and no engraftment into gland structures.B and C: A higher power view reveals myocytes and myobroblasts in the sub-mucosal tissue adjacent to vascular structures (arrows).D: After 30 wk of infection, marked architectural distortion is seen with antralization and appearance of metaplasticglands. Entire gland structures are derived from donor marrow (blue staining). Gland shown in panel D (arrow) is shown at higher power inE.

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tion, but fail to regulate growth programs appropriatelyand progresses through stages of metaplasia and dysplasia.

We speculate that the inappropriate retention of primitivegrowth programs in a stem cell forced to replicate maypermit survival despite otherwise lethal mutations, thusallowing transformation. This new model brings togetherpreviously unexplained observations regarding the behav-ior of cancer, and presupposes that properties inherent tocancer such as their resistance to apoptosis, their unlimitedgrowth potential and their ability for local spread and dis-tant metastasis are fundamental to the origin of the cell,rather than traits acquired. The concept of cancer initia-tion and promotion can also be viewed within the contextof this model. Initiation may represent BMDCs traf ckinginto the stem cell niche as a result of tissue stem cell dam-age. In the absence of continued in ammation and injury,these engrafted cells may behave in a way indistinguishablefrom endogenous tissue stem cells. Promotion may rep-resent an additional stimulus received at a later time thatallows sustained proliferation of BMDCs and transforma-tion.

In vitro experiments and animal models supporting theBMDC-epithelial cancer model In addition to the Helicobacter-gastric cancer model, otherstudies have begun to address the role of BMDCs in can-cer using various in vitro and in vivo models. For example,BMDCs have been shown to localize to a known stem cell

niche within the epidermis known as the CD34 positivebulge region of the hair follicle, and clonally expand torepopulate portions of the epidermis, functioning as anepidermal stem cell[50]. Similar to our ndings, engraftmentof BMDCs to the stem cell niche is dramatically increased

with injury severe enough to deplete peripheral stem cellsin the region. However, these are short-term studies. Lon-ger term studies utilizing carcinogen exposure will deter-mine the eventual fate of these BMDCs, and determineif BMDCs in the stem cell niche behave differently fromperipheral stem cells occupying the same niche. It is in-triguing, however, to speculate the ultimate fate of thesestem cells given the prevalence of BMDC-skin carcinoma

in solid organ recipients (see below).In addition to residing in the epithelial stem cell niche,

bone marrow-derived myo broblasts have been recovered within the colonic subepithelial compartments in bothmice and human beings [51,52]. Interestingly, Direkze et al. ob-served that in the IL-10 knockout mouse model of colitis,up to 45% of subepithelial myo broblasts were marrowderived[52], suggesting that in the setting of chronic in am-mation, damaged tissue is replaced by BMDCs. When thesame group looked at tumor-associated myo broblasts and broblasts, they also found a signi cant portion of thesecells derived from bone marrow cells[51]. It is not clear

from these data, if tumors recruit bone marrow cells intothe stromal compartment or if resident myofibroblastsand broblasts derived from marrow contribute to tumorformation because of abnormal signaling behavior.

Adenocarcinoma of the distal esophagus (Barretts’ ad-enocarcinoma) results from re ux-induced mucosal dam-age followed by healing with a metaplastic intestinal celllineage. This intestinal metaplasia is prone to malignantdegeneration and is another ideal model to test the roleof BMDCs in in ammatory-mediated cancers. Using a ratmodel of Barrett’s metaplasia, a signi cant contribution ofBMDCs to the stroma and the metaplastic epithelium hasbeen demonstrated, supporting a role for BMDCs in thesepre-neoplastic lesions[53]. Though these ndings have onlybeen reported in an abstract form so far, this informationis especially exciting because it provides evidence of directBMDC involvement in carcinogenesis from both an ad-ditional species (rat) and tissue type (esophagus), providing

A B C

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Figure 5 Immunohistochemistry for bacterial beta-galactosidase conrms uniform signal in gastrointestinal neoplasia. Mice developed severe dysplasia and intraepithelialneoplasia derived from donor marrow, 12-15 mo after infection withH. felis (A) and (B). Immunohistochemistry for bacterial beta-galactosidase demonstrates cytoplasmicstaining in dysplastic glands. A population of adipocytes in the submucosa are also stained for beta-galactosidase (arrow).

Inammation Stem cell zoneapoptosis-tissue

stem cell failure

BMD-stemcell recruitment

Stem cellengraftment

Metaplasiadysplasia

cancerformation

IFN-γ

Th1cytokines

Proliferation anddifferentiation of stem cells

Entire gland unitsrepopulated

↑↑ ↑↑ ↑

Figure 6 A new paradigm proposed for epithelial cancer.


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further support for our BMDC-epithelial cancer model.

Human data supporting the BMDC-epithelial cancer model In human beings, the incidence of solid tumors is signi -cantly increased following bone marrow transplantation [54] and may be related to persistent chronic in ammation of

graft vs. host disease. The data on BMDCs in human can-cers, however, have been con icting. First, it is dif cult toexamine the contribution of donor marrow to tumor for-mation in human beings because of a paucity of cell mark-ers to consistently identify autologous BMDCs or donorcells after BM transplantation. The most reliable marker

we have to date is identi cation of the sex chromosomesin sex mismatched transplants. However, there are inherentdif culties with using Y-chromosome identi cation. X/Y uorescent in situ hybridization (FISH) analysis of archivedtumors is estimated to miss more than 50% of Y-positivecells due to sectioning bias, where only a portion of thenucleus and thus only a portion of the chromosomes areincluded in the tissue section. Additionally, females witha history of carrying a male fetus may show peripheralblood chimerism confounding interpretation of data, andeliminating this population from the study. Also, tumorsidenti ed within a short time after transplant may re ectthe effects of immunosuppression on previously unde-tected early malignancy and not newly formed tumors, andmay explain why some studies conclude tumors in thesepatients are host derived [55], while other studies demon-strate a de nite contribution of donor’s-BMDCs [56]. Stud-ies utilizing larger numbers of patients followed for longerperiods of time will better address this new and controver-sial area, and determine if the BMDCs are con ned to thestroma, involved in angiogenesis or constitute the epithelialcomponent of the tumor mass in human beings.

In addition to patients receiving bone marrow trans-plants, recipients of solid organ transplants also have ahigher incidence of secondary malignancy. Interestingly,in solid organ transplant recipients, hematopoietic cells ofdonor origin are often found in the circulation, indicat-ing that hematopoietic stem cells are transferred with thetransplanted organ [57,58]. These transferred stem cells havebeen shown to give rise to Kaposi sarcoma (KS), a vasculartumor [59], and skin carcinoma[60]. The detected KS lesions

occurred distal to the graft site, and formed presumably via mobilization of donor progenitor cells with subsequenttransformation at a distant site. Donor-derived stem cellscontribute to skin carcinomas, and have been recoveredas components of squamous cell carcinoma, basal cellcarcinoma, actinic keratosis, keratoacanthomas and benigncutaneous lesions[60], attesting to the great potential for ab-normal differentiation of these cells. BMDCs as terminallydifferentiated cells in other organs including hepatic endo-thelial cells, hepatocytes and biliary epithelial cells[61], sug-gesting that these cells may play a role in transformation

within these organs as well, if subjected to the appropriateenvironmental conditions.

CONCLUSION

One of the greatest and most elusive challenges in cancerbiology has been to identify the cellular origin of cancer.

We have identi ed the bone marrow stem cell as the cell oforigin of Helicobacter-induced gastric cancer in a mousemodel, radically altering our current view of gastric cancerformation in particular, and of inflammation-mediatedcancers in general. The concept of BMDC plasticity is be-ing increasingly recognized and validated by independent

groups. Our recent observation that BMDCs are the originof Helicobacter-induced gastric cancer[41] combined withsupporting observations of BMDCs in other tumors suchas benign and malignant tumors of the skin [60], Kaposissarcoma[59] Barretts’ adenocarcinoma of the esophagus [53] as well as demonstration of BMDCs as constituents oftumor stroma and tumor vascular structures [52-56] suggestsexciting approaches for cancer therapy. If the propen-sity for BMDCs to transform is based on inappropriateregulation of immature growth programs, with growthprograms left “turned on” rather than the previously heldconcept of mutation driven-reactivation of programs, can

we target these pathways? Undoubtedly, genetic mutationshave occurred which are irreversible; but if we can targetand switch off inappropriately activated growth cascades,perhaps we can push these damaged cells into apoptosisor enhance the sensitivity to conventional chemo- and ra-diotherapy. These approaches may lead to novel and moreef cacious cancer therapy.

Presently, our laboratory is involved in identifyingthe cell population within the bone marrow capable ofcancer formation as well as defining the homing anddifferentiation signals which allow these cells to accessto gastric mucosa, and to differentiate as metaplastic anddysplastic cells. Studies designed to determine if fusionis a means of bone marrow cell integration into gastricmucosa and gastric cancer are underway. The applicabilityof these ndings to other epithelial cancers will be testedas well as our ability to control the growth of these cellsby manipulations of the local tissue environment. Theseefforts are aimed at identifying cell-specific targets forchemotherapy. Findings from these studies will radicallyalter our approach to the treatment of gastric cancer as

well as other solid tumors, and offer hope for improvedsurvival and potential cure.

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