Lung Cancer and Lung Stem Cells - ATS Journals

3 downloads 92 Views 1MB Size Report
Jul 19, 2006 - field carcinogenesis theory, which suggests that multiple primary cancers ..... A genetic explanation of Slaughter's concept of field cancerization:.
Pulmonary Perspective Lung Cancer and Lung Stem Cells Strange Bedfellows? Adam Giangreco1, Karen R. Groot2, and Sam M. Janes2 1

Keratinocyte Laboratory, Cancer Research UK, London, United Kingdom; and 2Centre of Respiratory Research, University College London, London, United Kingdom

Lung cancer is a significant disease with survival rates remaining poor despite numerous therapeutic advances during the last 30 years. Understanding lung cancer pathogenesis through murine modeling may improve future human therapies, and new data indicate that mutations within different endogenous stem cells situated throughout airways can drive cancer formation. Airway stem cells maintain prototumorigenic characteristics, including high proliferative capacity, multipotent differentiation, and a long lifespan relative to other cells. These cells localize to proximal airway submucosal glands/intercartilagenous rings, neuroepithelial bodies, and terminal bronchioles/bronchoalveolar duct junctions. Recent studies suggest that endogenous stem cell signaling and differentiation pathways are maintained within distinct cancer types, and that destabilization of this signaling machinery may initiate regionspecific lung cancers. A better understanding of this relationship among stem cell regulation, cellular mutation, and lung cancer oncogenesis is critical for developing the next wave of lung cancer therapies. Keywords: lung cancer; stem cells; pathogenesis; signaling; progenitors

Lung cancer kills more people than any other cancer. The majority of patients present too late for curative surgery and the 5-year survival rate remains relatively poor despite active medical therapy and significant therapeutic advances. It is estimated that more people in the West die of lung cancer than prostate, breast, colon, and cervical cancer combined (1). Sadly, this devastating disease is also the most preventable cancer, with cigarette smoking causing 90% of lung cancers worldwide (2). With the increased social vilification and awareness of dangers of smoking in Europe and the United States, it was hoped that the worldwide incidence of lung cancer would decline. Unfortunately, this has not yet occurred, and an increase in lung cancer incidence in several developing countries has recently been fuelled by a new, predominately young, smoking population. Indeed, China expects one-third of its male population to die of tobacco-related disease (3). Although cancer prevention through public awareness and lifestyle choice remains a primary strategy in the fight against lung cancer, clinicians and scientists together must develop a better appreciation of the underlying cellular and molecular events driving this disease to design more effective therapies. In contrast to the slow improvement in lung cancer prevention and treatment, pulmonary stem cell biology (driven by mouse

(Received in original form July 19, 2006; accepted in final form December 7, 2006 ) Correspondence and requests for reprints should be addressed to Sam M. Janes, M.D., Ph.D., Centre of Respiratory Research, Rayne Building, University College London, 5 University Street, London, WC1E 6JJ UK. E-mail: [email protected] Am J Respir Crit Care Med Vol 175. pp 547–553, 2007 Originally Published in Press as DOI: 10.1164/rccm.200607-984PP on December 7, 2006 Internet address: www.atsjournals.org

models) is rapidly revealing progenitor cell populations throughout lungs (4, 5). Multipotent, long-lived cells (stem cells) have been identified throughout airways and give rise to both transiently amplifying (TA) and terminally differentiated (TD) daughters (see Table 1). These cells, like stem cells in other organs, are critically important for local tissue maintenance and repair after injury (6, 7). Despite an established tissue-maintenance role, recent mouse data also support a stem cell–mediated origin for leukemia (8–13). It appears leukemias arise either from transformation of hematopoietic stem cells or via mutation in partially committed cells, resulting in selective expression of genes and enhancing their self-renewal potential (10). Hence, stem cells might accurately be considered prototumorigenic, lacking only those genetic mutations that may induce aberrant, cancerous growth and tissue invasiveness. Thus, stem cells and solid tumors may not be such strange bedfellows after all. This Pulmonary Perspective highlights evidence that supports roles for lung stem cells as likely originating cells for specific lung cancers. Recently, several studies have also demonstrated that some tumors contain their own subpopulations of drugresistant, cancerous stem cells (see Table 1) (11–17). Despite being distinct entities, normal tissue and cancerous stem cells appear to share common properties, including enhanced pollutant/drug resistance, robust differentiation, and increased mitotic capacity relative to neighboring cells (14). Whether these similarities are indicative of a common originating population remains unexplored. In the interest of space, we will not discuss the phenomenon of cancer stem cells within lung tumors. Rather, the current Perspective will highlight links between normal lung stem cell function and these stems cells’ involvement during lung cancer oncogenesis.

PROTOTUMORIGENIC STEM CELLS Characteristics that define stem cells include their capacity for self-renewal, production of daughter cells, and extensive proliferative capacity (18, 19). In this sense, stem cells could be considered ideal tumor initiation candidates, because dysregulation of this robust proliferative capacity through mutation may rapidly cause dysplastic, tumorlike growth. Drug-dependent, regulable transgenic models that reversibly mimic single oncogenic mutations (resulting in suppression of tumor suppressor genes or activation of oncogenes) provide crucial evidence that continued signaling is required to maintain tumor phenotypes (20–22). However, it appears that simply expanding stem cells without additional genetic mutation may not produce fully invasive tumors (20). Thus, stem cells appear to be prototumorigenic, and must first receive at least one permanent genetic mutation to destabilize their growth prior to cancer initiation (23). In rapidly dividing tissues such as the blood, gut, and skin, stem cells persist throughout an individual’s life and can easily acquire numerous oncogenic mutations. In contrast, committed

548

AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL 175 2007 TABLE 1. DEFINITION OF TERMS Name Progenitor cell Stem cell

TA cell TD cell Cancer cell Cancer stem cell

Definition

Examples

Any proliferation-competent cell capable of giving rise to at least one offspring Long-lived, self-renewing progenitor that exhibits extensive proliferative capacity and typically multipotent differentiation Short-lived progenitor cell with limited proliferative capacity and restricted differentiation potential Fully differentiated cell with no identified proliferative capacity Mutated, tumor-derived cell with dysregulated proliferation and differentiation Cancer cell exhibiting enhanced tumor-regenerative potential and/or pollutant resistance

PNEC BASC, variant CCSP-expressing cell (vCE)

Clara cell, type II pneumocyte Type I pneumocyte, ciliated cell

Definition of abbreviations: BASC ⫽ bronchioalveolar stem cell; CCSP ⫽ Clara cell secretory protein; PNEC ⫽ pulmonary neuroendocrine cell; TA ⫽ transiently amplifying; TD ⫽ terminally differentiated.

daughter progenitor cells (TA cells) and fully differentiated cells appear to have too limited a lifespan to accumulate this damage. Indeed, targeted oncogenic mutagenesis within these committed daughters has repeatedly failed to yield appropriate tumor models (24–26). It is therefore unsurprising that, within rapidly dividing tissues, most evidence supports a stem cell–mediated origin for cancers (10, 27). Alternatively, in tissues where cell turnover is slow (e.g., lung) daughter cells, including ciliated and Clara cells, exhibit significantly longer tissue transit times. In these organs, stem cells could theoretically accumulate only some oncogenic mutations, with their long-lived daughter cells receiving additional transforming mutations (10). Whether this actually occurs in lung and other slow-turnover organs remains largely unstudied. An additional stem cell characteristic that allows for their prototumorigenic classification is an inherent resistance to potentially toxic compounds (28). In murine lung, several distinct properties confer pollutant resistance among airway progenitors. Bronchiolar progenitor cells localized within neuroepithelial body (NEB) microenvironments express reduced levels of cytochrome p450 xenobiotic metabolizing enzymes (29). This reduces intracellular bioactivation of lipophilic compounds to toxic metabolites, allowing stem cells to survive several types of airway injury. These same cells also exhibit efficient drug efflux or a “side population” phenotype via the overexpression of ABCGtype transporters (30, 31). ABCG2 (breast cancer resistance protein 1) is an ATP binding cassette (ABC) transporter with affinity for numerous cytotoxic molecules, including mitoxantrone (32). ABC transporter–dependent “side” population (SP) cells have also been observed within other murine lung preparations, and most likely represent subsets of lung endothelial or fibroblast-like cells (31, 33). However, it remains untested in vivo whether any of these isolated SP cells truly exhibit bona fide lung stem cell function (33). ABCG-dependent drug efflux has also been observed within lung and other tissue tumor cell lines in vitro (12, 14, 34). SP cells within these tumor tissues appear to have the same pollution-resistance phenotype as endogenous stem cells and have cancer stem cell characteristics, including enhanced clonogenicity and secondary tumor formation capacity. The ABCG transporter proteins that define SP cells also pump various chemotherapy agents from these cells, protecting them from many current therapeutic drug strategies (14).

LOCAL PULMONARY ENVIRONMENTS INFLUENCE TUMOR FORMATION Lung cancer is not simply a single disease but a collection of several phenotypically diverse, regionally distinct neoplasias.

These encompass several major tumor subclasses and, in murine models, roughly follow a proximal-to-distal distribution pattern: moving distally from the trachea, major tumor types include squamous cell carcinomas (SCCs), small cell lung carcinomas (SCLCs), and adenocarcinomas/bronchoalveolar carcinomas. The regional segregation among tumor types using mouse models suggests that only finite numbers of distinct cells and/or pulmonary environments are capable of supporting tumor growth. Although humans also present with these tumor types in similar lung locations, determining the exact location in which human tumors originate remains difficult. Observed phenotypic heterogeneity between distinct tumor types suggests that the tumor’s local pulmonary environment profoundly impacts a cancer cell’s fate (35). Studies by Franklin and colleagues have assessed whether a case of multiple premalignant lesions localized within discrete lung microenvironments could have been derived from a common progenitor (36). They discovered that identical p53 mutations were present within discontinuous lesions throughout similar caliber airways, supporting their hypothesis that multiple simultaneous lung tumors may be clonally derived. These observations support Slaughter’s 1953 field carcinogenesis theory, which suggests that multiple primary cancers are derived from common, clonally derived precursor cells (37). These data support the role of individual stem cells in generating phenotypically similar lung tumors. To overcome the inherent problems related to human studies, scientists have increasingly favored genetically modified mouse cancer models. Mouse models for lung cancer have traditionally involved either global knockout strategies to remove putative tumor suppressors (e.g., p53) or transgenic expression of oncogenes/proto-oncogenes under the regulation of widely expressed, pulmonary-specific promoters (including Clara cell secretory protein [CCSP] or surfactant protein C [SPC]) (35, 38). These modifications generate identical mutations throughout large portions of lungs in a pattern similar to what would be expected after mutation and subsequent expansion of stem or progenitor cells. Within the context of field carcinogenesis, these global changes should therefore be capable of generating cancers throughout the entire lung. This has not turned out to be the case. Rather, it appears that very particular airway regions exhibit tumorigenic properties, and only when associated with specific cellular mutations. Of particular interest, several independent mouse models have recently been developed that introduce Kras mutations, which are common in human cancers throughout the lungs (21, 39–42). Amazingly, in all models tested, mice only produced

Pulmonary Perspective

adenomatous hyperplastic lesions localized to the bronchoalveolar zone despite identical mutations in virtually all airway cells. This strongly suggests that different lung cancers originate not only because of permissive oncogenic mutations but also because an individual cell’s local environment (niche) supports growth under these conditions. In other words, field carcinogenesis theory may be broadly applicable only within airway regions harboring equivalently functioning progenitor cells and may not be valid when comparing diverse pulmonary environments (e.g., tracheal vs. alveolar epithelia). Intriguingly, most originating sites identified using murine models of SCC, SCLC, and adeno-/ bronchoalveolar carcinomas appear to coincide with recently identified airway stem cell niches (4).

LUNG CANCERS AND ASSOCIATED PROTO-ONCOGENIC STEM CELL NICHES SCC—Major Airway Basal Cells

SCCs are the most common lung cancer in the United Kingdom and are highly associated with tobacco smoking (2, 43, 44). However, there are relatively few mouse models of human SCC compared with other cancers. Murine SCCs generally occur in the proximal airways down to the second or third bifurcation and are rarely observed distally. Existing SCC models have invariably used chemically mediated carcinogenesis, and to date no genetically modified mouse models of human SCC exist. Despite the obvious disadvantages related to chemical carcinogenesis, including uncontrolled cellular targeting and unknown mutations, these experiments do yield hyperplastic lesions and SCCs that resemble the human disease (45–48). Histologically, carcinogentreated airways undergo stepwise changes, eventually developing a full SCC phenotype. These are staged as a primary, generalized basal cell hyperplasia, followed by squamous metaplasia, dysplasia, carcinoma in situ, and, finally, invasive SCC (Figure 1) (49). It is important to note that, in SCC chemical carcinogenesis studies, most tumors lacked mutations common among other lung cancer types despite numerous carcinogen treatments. This suggests that very particular mutations occurring in specific cell populations are necessary to achieve SCC formation. Recent chemical carcinogenesis studies have identified unique genetic quantitative trait loci (QTL) associated with SCC progression and/or susceptibility that are not found in other lung cancers (48). Mutant mouse models of these QTL-associated genes are now needed to determine their relevance toward human SCC. Although mouse models have not identified the specific genes or cells involved in SCC formation, the morphology and gene expression patterns of mouse SCC-like hyperplastic lesions frequently resemble tracheal basal cell progenitors. These proximal airway progenitors are keratin 5/14 positive and are located at submucosal gland duct junctions or intracartilaginous boundaries (50–52) (Figure 1). Keratin 14 (K14)–positive basal cells decrease in frequency distally toward the carina and, like SCCs, are not normally observed beyond the main bronchi. K14 expression may be a hallmark of basal cells with enhanced regenerative potential, and reactivity is expanded after chemically mediated proximal airway injury and repair. Lineage tagging experiments confirm that these cells contain significant regenerative and differentiation potentials both in vitro and in vivo (51, 53). Similar K14 cell expansion occurs in hyperproliferative, preneoplastic lesions in SCC (54). To confirm the direct linkage between these cells and SCC formation, more rigorous approaches, such as lineage tagging plus chemical carcinogenesis, will be needed. Nonetheless, current data do support a direct relationship between proximal airway basal progenitors and cells associated with carcinogenesis in murine models for human SCC.

549

SCLC—NEBs

SCLC is a form of lung cancer with a particularly poor prognosis because of a high rate of metastatic dissemination. Human SCLCs predominately localize to midlevel bronchioles and typically express a range of neuroendocrine cell markers, including calcitonin gene–related peptide (CGRP) and other neuropeptides normally expressed within pulmonary neuroendocrine cells (PNECs). On the basis of these observations, PNECs have been proposed as likely originating cells for SCLC (55). Retinoblastoma (Rb) and TP53 gene mutations are frequently associated with human SCLC. The combination of Rb depletion and knockout of the TP53 gene in mice resulted in the formation of multiple, distinct, SCLC-like hyperplastic foci (56). These foci corresponded to PNEC-containing NEB microenvironments (Figure 1). This model, however, failed to generate fully metastatic lung tumors and mimicked only the early stages of SCLC-like disease. More recently, a lung-specific conditional Rb inactivation model alone yielded strikingly similar results, with exclusive PNEC hyperplasia despite Rb gene deletion throughout the airways (57). Finally, pulmonary models deleting both Rb and TP53 in adult mice resulted in progressive epithelial hyperplasia that was restricted to the NEB microenvironment (58, 59). Importantly, these early lesions went on to form metastatic tumors resembling human SCLC. The observation that only PNECs respond, despite this method’s ability to cause widespread mutation throughout lungs, strongly implicated these cells as obligate targets for SCLC tumorigenesis. NEB microenvironments maintain putative stem cell populations in the bronchiole mucosa. They are defined as widely dispersed clusters of both PNECs and variant CCSP-expressing (vCE) cells and are the first regenerative airway sites after Clara cell injury (60). Suicide gene ablation studies have demonstrated that NEB-associated vCE cells are obligatory progenitors after Clara cell depletion (6), and that they divide infrequently in the steady state and are capable of multipotent differentiation (29). Interestingly, a subset of CCSP-expressing cells exhibit antigens specific to both PNECs and CE cells (60). Expansion of this dual-positive population may relate to the existence of mixed small cell and non–small cell lung carcinoma (NSCLC)–like tumors and/or nonsquamous NSCLC observed throughout airways in some transgenic mouse models of human lung cancer (61–63). Whether a comparable population of Clara or vCE cells could account for the development of human nonsquamous NSCLCs originating from pseudostratified airways remains unexplored. NEB-associated PNECs exhibit properties of unipotent progenitor cells and maintain a high proliferative capacity. In this sense, they are not true stem cells, but nonetheless, data support their contributory role within some lung cancer models. During repair-mediated NEB hyperplasia, PNEC-derived growth factors are released, which probably facilitates the rapid expansion of both PNEC and vCE populations (64). Chronic injury models reveal extensive PNEC hyperplasia similar to early hyperplastic lesions observed after lung Rb/TP53 mutations (60). If either vCE cells or PNECs develop mutations that enhance their growth independent of injury, this could in turn rapidly produce hyperplastic lesions similar to those observed in mouse models for human SCLC. Evidence that NEB-associated PNECs and SCLCs use identical signaling pathways further strengthens the possible common origins between these two populations. Sonic hedgehog (Shh) reactivity is elevated in NEBs during lung development and after airway Clara cell depletion, whereas the Hedgehog receptor Patched (Ptc) also shows increased expression within PNECs during repair-associated NEB hyperplasia (65, 66). Similarly, SCLC tumors frequently overexpress both Shh receptor and

550

AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL 175 2007

Figure 1. Airway stem cell microenvironments and associated human carcinomas. Fluorescent whole mounts of mouse lungs and schematic drawings reveal identified airway stem cell microenvironments. Upper airways likely contain submucosal gland and intercartilagenous stem cell niches (highlighted in red) defined by keratin 14 (K14)–expressing basal cells (red K14 staining) adjacent to ciliated cells expressing acetylated tubulin (green stained) as shown in A and inset, and drawn in C (highlighted in red). In humans, basal cells may generate squamous cell carcinomas via transitional stages, including metaplasia (D ), moderate dysplasia (E ), carcinoma in situ (F ), and, finally, invasive carcinoma (G ). Highlighted in yellow are midlevel, bronchiolar airway–mediated stem cell populations localized within calcitonin gene–related peptide (CGRP)–expressing neuroepithelial bodies (NEBs) adjacent to pulmonary neuroendocrine cells (PNECs). These are shown in B stained with Clara cell secretory protein (CCSP) (red) and CGRP (green), and drawn schematically in C. Within the NEBs of humans, neuroendocrine cells themselves likely generate small cell carcinomas (H ), whereas variant airway CCSP-expressing cells (vCE) may contribute to bronchiolar adenomas (not shown). CCSP-expressing bronchioalveolar stem cells (BASCs) within green-highlighted terminal airways (CCSP-positive cells in red (B ) and BASCs drawn in C function as distal airway stem cells and, in humans, may generate atypical adenomatous hyperplasia (I ), adenocarcinomas (J ), and bronchoalveolar cell carcinomas (K ). Abnormal human lung tissues are indicated by asterisks (D–K ).

ligand and may require continued Shh signaling for continuous growth. These observations led to the hypothesis that PNECderived human SCLC tumors may undergo specific mutations permitting autonomous Shh signaling, thereby circumventing existing control mechanisms that regulate normal NEB-associated proliferation (66). Similar studies regarding the Notch-delta pathway also indicate roles for this signaling pathway in NEB growth and human SCLC progression (67–69). Central Bronchiolar Adenocarcinoma/Bronchoalveolar Cell Carcinoma—Bronchioalveolar Stem Cells

Central bronchiolar adenocarcinomas and peripheral bronchoalveolar cell carcinomas are among the most common lung cancer types in the United States. Although they occur frequently in smokers, they are also the most common lung cancer type in nonsmokers, with a high incidence in Asian women (2). Central bronchiolar adenocarcinomas classically show acinar–glandular differentiation, and murine models suggest that these adenocarcinomas arise from the junction between the terminal bronchiole and the alveolus termed the “bronchoalveolar duct junction” (BADJ) (70) (Figure 1). These adenocarcinomas frequently exhibit mixed airway and alveolar characteristics, including CCSP

and SPC coexpression (70). Although human cancers largely exhibit either airway or alveolar differentiation, mouse models and some human cancers do express both airway and alveolar differentiation characteristics. This has led to hypotheses that either Clara or alveolar type 2 (AT2) cells function as the originating cells for adenocarcinomas (21, 70). Murine adenocarcinoma models represent a disproportionately high number of the existing models for human lung cancer. One reason for this bias is the extensive use of either airway CCSP or alveolar SPC promoters to express mutated proteins in both lung compartments (35). In humans, CCSP and SPC expression is retained within many adenocarcinomas, indicating their possible Clara or AT2 origins. CCSP and SP-C promoter– driven adenocarcinoma models include mutated epidermal growth factor (EGF) receptor (71, 72), active K-ras (Kras G12D) (40, 41), dominant negative transforming growth factor-␤ (73), and large T antigen (74, 75). Several of these and other transgenic mouse models develop mixed-phenotype tumors adjacent to terminal bronchioles most closely resembling human bronchoalveolar carcinoma. These results strongly suggest that originating cells for adeno- and bronchoalveolar carcinoma localize near BADJs.

Pulmonary Perspective

Recently, several improved mouse models for human adenocarcinoma have been developed that use a conditionally activated mutant Kras G12D (39, 40, 42, 76). These include a spontaneously mutating “hit and run” model, a doxicycline-inducible transgenic system, and a transgenic promoter-independent adenovirus-dependent recombination system in which intratracheal and intranasal adenovirus expressing Cre recombinase (AdenoCre) triggers Kras G12D expression throughout infected airways (21, 39–42). Each model produces initial lesions resembling injury-dependent terminal bronchiolar hyperplasia plus alveolar bronchiolization, or induces the transformation of alveolar septae into regions exhibiting a strong airway-like morphology. The AdenoCre model is particularly informative because it provides widespread lung-specific genetic recombination and eliminates confounding developmental effects. Strikingly, each improved model exhibits very similar results typified by mixed airway– alveolar lineage, BADJ-associated adenocarcinomas. Together, results of these newer models indicate that, as well as being progenitor cell specific, certain mutations (here, Kras G12D) are additionally favored during adenocarcinoma initiation relative to other lung cancers. Early observations identified multiple foci of epithelial regeneration localized to both airway branch points and terminal bronchioles after naphthalene-mediated Clara cell depletion (77). Although the former turned out to be NEB-associated vCE progenitors as described above (29), terminal bronchiolar regeneration was due to NEB-independent, BADJ-restricted CE stem cells (78). Significantly, these BADJ stem cells (defined as bronchioalveolar stem cells [BASC]) also demonstrate a steady-state mixed airway–alveolar phenotype and are uniquely capable of in vitro expansion (70). Using the AdenoCre adenocarcinoma model described above, Kim and colleagues have now demonstrated selective, dose-dependent BASC expansion in vitro and in vivo after Kras G12D mutation (70). In addition, in vivo BASC activation through injury followed by Kras mutation resulted in much greater increases to both tumor number and area versus Kras mutation alone. These data provide the most compelling evidence to date that normal airway stem cells can directly act as originating cells for lung cancers.

SUMMARY AND FUTURE DIRECTIONS The relationship between stem cells and cancer formation is becoming increasingly scrutinized with respect to lung cancers. Stem cell populations within proximal airways, NEB microenvironments, and BASCs are ideally situated to serve a role as lung cancer progenitors. Several properties unique to stem cells are similar to those found within different lung cancer types. In addition, there is emerging evidence that signaling pathways governing airway stem cell fate are exploited by lung cancers after mutation. Despite these discoveries and recent studies demonstrating a direct relationship between BASCs and bronchoalveolar carcinomas, the relationship between lung stem cells and lung cancer remains underexplored. Additional studies using cellular lineage tagging of putative stem/cancer progenitors and tightly regulated transgenic mouse models for all human lung cancer types are needed. Only once these cells and lineage relationships are more fully understood can we hope to develop more effective, targeted therapies to treat this devastating disease. Conflict of Interest Statement : None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript. Acknowledgment : The authors gratefully acknowledge support from the histopathology unit at University College London, specifically Vinu Sheshappanavar, Andrew Nicholson, and Mary Falzon for their assistance in providing human lung carcinoma tissue samples and images. They also thank Mark Griffiths, Soline Estrach, Robert Buttery, and Dawn J. Mazzatti for critical reading of the manuscript.

551 References 1. Jemal A, Thomas A, Murray T, Thun M. Cancer statistics, 2002. CA Cancer J Clin 2002;52:23–47. 2. Minna JD, Roth JA, Gazdar AF. Focus on lung cancer. Cancer Cell 2002;1:49–52. 3. Liu BQ, Peto R, Chen ZM, Boreham J, Wu YP, Li JY, Campbell TC, Chen JS. Emerging tobacco hazards in China: 1. Retrospective proportional mortality study of one million deaths. BMJ 1998;317:1411–1422. 4. Rawlins EL, Hogan BL. Epithelial stem cells of the lung: privileged few or opportunities for many? Development 2006;133:2455–2465. 5. Berns A. Stem cells for lung cancer? Cell 2005;121:811–813. 6. Hong KU, Reynolds SD, Giangreco A, Hurley CM, Stripp BR. Clara cell secretory protein-expressing cells of the airway neuroepithelial body microenvironment include a label-retaining subset and are critical for epithelial renewal after progenitor cell depletion. Am J Respir Cell Mol Biol 2001;24:671–681. 7. Benitah SA, Frye M, Glogauer M, Watt FM. Stem cell depletion through epidermal deletion of Rac1. Science 2005;309:933–935. 8. Blair A, Hogge DE, Ailles LE, Lansdorp PM, Sutherland HJ. Lack of expression of Thy-1 (CD90) on acute myeloid leukemia cells with longterm proliferative ability in vitro and in vivo. Blood 1997;89:3104–3112. 9. Bonnet D, Dick JE. Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nat Med 1997;3:730–737. 10. Passegue E, Jamieson CH, Ailles LE, Weissman IL. Normal and leukemic hematopoiesis: are leukemias a stem cell disorder or a reacquisition of stem cell characteristics? Proc Natl Acad Sci USA 2003;100:11842– 11849. 11. Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF. Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci USA 2003;100:3983–3988. 12. Kondo T, Setoguchi T, Taga T. Persistence of a small subpopulation of cancer stem-like cells in the C6 glioma cell line. Proc Natl Acad Sci USA 2004;101:781–786. 13. Patrawala L, Calhoun T, Schneider-Broussard R, Zhou J, Claypool K, Tang DG. Side population is enriched in tumorigenic, stem-like cancer cells, whereas ABCG2⫹ and ABCG2- cancer cells are similarly tumorigenic. Cancer Res 2005;65:6207–6219. 14. Hirschmann-Jax C, Foster AE, Wulf GG, Nuchtern JG, Jax TW, Gobel U, Goodell MA, Brenner MK. A distinct “side population” of cells with high drug efflux capacity in human tumor cells. Proc Natl Acad Sci USA 2004;101:14228–14233. 15. Galli R, Binda E, Orfanelli U, Cipelletti B, Gritti A, De Vitis S, Fiocco R, Foroni C, Dimeco F, Vescovi A. Isolation and characterization of tumorigenic, stem-like neural precursors from human glioblastoma. Cancer Res 2004;64:7011–7021. 16. Hemmati HD, Nakano I, Lazareff JA, Masterman-Smith M, Geschwind DH, Bronner-Fraser M, Kornblum HI. Cancerous stem cells can arise from pediatric brain tumors. Proc Natl Acad Sci USA 2003;100:15178– 15183. 17. Singh SK, Clarke ID, Terasaki M, Bonn VE, Hawkins C, Squire J, Dirks PB. Identification of a cancer stem cell in human brain tumors. Cancer Res 2003;63:5821–5828. 18. Smith A. A glossary for stem-cell biology. Nature 2006;441:1060. 19. Weissman IL, Anderson DJ, Gage F. Stem and progenitor cells: origins, phenotypes, lineage commitments, and transdifferentiations. Annu Rev Cell Dev Biol 2001;17:387–403. 20. Hochedlinger K, Yamada Y, Beard C, Jaenisch R. Ectopic expression of Oct-4 blocks progenitor-cell differentiation and causes dysplasia in epithelial tissues. Cell 2005;121:465–477. 21. Fisher GH, Wellen SL, Klimstra D, Lenczowski JM, Tichelaar JW, Lizak MJ, Whitsett JA, Koretsky A, Varmus HE. Induction and apoptotic regression of lung adenocarcinomas by regulation of a K-Ras transgene in the presence and absence of tumor suppressor genes. Genes Dev 2001;15:3249–3262. 22. Lo Celso C, Prowse DM, Watt FM. Transient activation of beta-catenin signalling in adult mouse epidermis is sufficient to induce new hair follicles but continuous activation is required to maintain hair follicle tumours. Development 2004;131:1787–1799. 23. Knudson AG Jr, Strong LC, Anderson DE. Heredity and cancer in man. Prog Med Genet 1973;9:113–158. 24. Barrandon Y, Morgan JR, Mulligan RC, Green H. Restoration of growth potential in paraclones of human keratinocytes by a viral oncogene. Proc Natl Acad Sci USA 1989;86:4102–4106.

552

AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL 175 2007

25. Pelengaris S, Littlewood T, Khan M, Elia G, Evan G. Reversible activation of c-Myc in skin: induction of a complex neoplastic phenotype by a single oncogenic lesion. Mol Cell 1999;3:565–577. 26. Owens DM, Watt FM. Contribution of stem cells and differentiated cells to epidermal tumours. Nat Rev Cancer 2003;3:444–451. 27. Roskams T. Liver stem cells and their implication in hepatocellular and cholangiocarcinoma. Oncogene 2006;25:3818–3822. 28. Reya T, Morrison SJ, Clarke MF, Weissman IL. Stem cells, cancer, and cancer stem cells. Nature 2001;414:105–111. 29. Reynolds SD, Giangreco A, Power JH, Stripp BR. Neuroepithelial bodies of pulmonary airways serve as a reservoir of progenitor cells capable of epithelial regeneration. Am J Pathol 2000;156:269–278. 30. Giangreco A, Shen H, Reynolds SD, Stripp BR. Molecular phenotype of airway side population cells. Am J Physiol Lung Cell Mol Physiol 2004;286:L624–L630. 31. Summer R, Kotton DN, Sun X, Ma B, Fitzsimmons K, Fine A. Side population cells and Bcrp1 expression in lung. Am J Physiol Lung Cell Mol Physiol 2003;285:L97–104. 32. Zhou S, Morris JJ, Barnes Y, Lan L, Schuetz JD, Sorrentino BP. Bcrp1 gene expression is required for normal numbers of side population stem cells in mice, and confers relative protection to mitoxantrone in hematopoietic cells in vivo. Proc Natl Acad Sci USA 2002;99:12339– 12344. 33. Summer R, Kotton DN, Sun X, Fitzsimmons K, Fine A. Translational physiology: origin and phenotype of lung side population cells. Am J Physiol Lung Cell Mol Physiol 2004;287:L477–L483. 34. Scharenberg CW, Harkey MA, Torok-Storb B. The ABCG2 transporter is an efficient Hoechst 33342 efflux pump and is preferentially expressed by immature human hematopoietic progenitors. Blood 2002; 99:507–512. 35. Meuwissen R, Berns A. Mouse models for human lung cancer. Genes Dev 2005;19:643–664. 36. Franklin WA, Gazdar AF, Haney J, Wistuba II, La Rosa FG, Kennedy T, Ritchey DM, Miller YE. Widely dispersed p53 mutation in respiratory epithelium: a novel mechanism for field carcinogenesis. J Clin Invest 1997;100:2133–2137. 37. Braakhuis BJ, Tabor MP, Kummer JA, Leemans CR, Brakenhoff RH. A genetic explanation of Slaughter’s concept of field cancerization: evidence and clinical implications. Cancer Res 2003;63:1727–1730. 38. Kwak I, Tsai SY, DeMayo FJ. Genetically engineered mouse models for lung cancer. Annu Rev Physiol 2004;66:647–663. 39. Johnson L, Mercer K, Greenbaum D, Bronson RT, Crowley D, Tuveson DA, Jacks T. Somatic activation of the K-ras oncogene causes early onset lung cancer in mice. Nature 2001;410:1111–1116. 40. Meuwissen R, Linn SC, van der Valk M, Mooi WJ, Berns A. Mouse model for lung tumorigenesis through Cre/lox controlled sporadic activation of the K-Ras oncogene. Oncogene 2001;20:6551–6558. 41. Jackson EL, Willis N, Mercer K, Bronson RT, Crowley D, Montoya R, Jacks T, Tuveson DA. Analysis of lung tumor initiation and progression using conditional expression of oncogenic K-ras. Genes Dev 2001;15:3243–3248. 42. Guerra C, Mijimolle N, Dhawahir A, Dubus P, Barradas M, Serrano M, Campuzano V, Barbacid M. Tumor induction by an endogenous K-ras oncogene is highly dependent on cellular context. Cancer Cell 2003;4:111–120. 43. Hamilton W, Peters TJ, Round A, Sharp D. What are the clinical features of lung cancer before the diagnosis is made? A population based casecontrol study. Thorax 2005;60:1059–1065. 44. Papi A, Casoni G, Caramori G, Guzzinati I, Boschetto P, Ravenna F, Calia N, Petruzzelli S, Corbetta L, Cavallesco G, et al. COPD increases the risk of squamous histological subtype in smokers who develop nonsmall cell lung carcinoma. Thorax 2004;59:679–681. 45. Yoshimoto T, Inoue T, Iizuka H, Nishikawa H, Sakatani M, Ogura T, Hirao F, Yamamura Y. Differential induction of squamous cell carcinomas and adenocarcinomas in mouse lung by intratracheal instillation of benzo(a)pyrene and charcoal powder. Cancer Res 1980;40: 4301–4307. 46. Yoshimoto T, Hirao F, Sakatani M, Nishikawa H, Ogura T. Induction of squamous cell carcinoma in the lung of C57BL/6 mice by intratracheal instillation of benzo[a]pyrene with charcoal powder. Gann 1977;68: 343–352. 47. Henry CJ, Billups LH, Avery MD, Rude TH, Dansie DR, Lopez A, Sass B, Whitmire CE, Kouri RE. Lung cancer model system using 3-methylcholanthrene in inbred strains of mice. Cancer Res 1981;41: 5027–5032. 48. Wang Y, Zhang Z, Yan Y, Lemon WJ, LaRegina M, Morrison C, Lubet R, You M. A chemically induced model for squamous cell carcinoma

of the lung in mice: histopathology and strain susceptibility. Cancer Res 2004;64:1647–1654. 49. George PJ, Banerjee A, Read CA, O’Sullivan C, Falzon M, Pezzella F, Nicholson A, Shaw P, Laurent G, Rabbitts P. Surveillance for the detection of early lung cancer in patients with bronchial dysplasia. Thorax 2007;62:43–50. 50. Hong KU, Reynolds SD, Watkins S, Fuchs E, Stripp BR. Basal cells are a multipotent progenitor capable of renewing the bronchial epithelium. Am J Pathol 2004;164:577–588. 51. Hong KU, Reynolds SD, Watkins S, Fuchs E, Stripp BR. In vivo differentiation potential of tracheal basal cells: evidence for multipotent and unipotent subpopulations. Am J Physiol Lung Cell Mol Physiol 2004; 286:L643–L649. 52. Borthwick DW, Shahbazian M, Krantz QT, Dorin JR, Randell SH. Evidence for stem-cell niches in the tracheal epithelium. Am J Respir Cell Mol Biol 2001;24:662–670. 53. Schoch KG, Lori A, Burns KA, Eldred T, Olsen JC, Randell SH. A subset of mouse tracheal epithelial basal cells generates large colonies in vitro. Am J Physiol Lung Cell Mol Physiol 2004;286:L631–L642. 54. Barth PJ, Koch S, Muller B, Unterstab F, von Wichert P, Moll R. Proliferation and number of Clara cell 10-kDa protein (CC10)-reactive epithelial cells and basal cells in normal, hyperplastic and metaplastic bronchial mucosa. Virchows Arch 2000;437:648–655. 55. Murray JF, Nadel JA. Textbook of respiratory medicine, 2nd ed. Philadelphia: W.B. Saunders; 1994. 56. Williams BO, Remington L, Albert DM, Mukai S, Bronson RT, Jacks T. Cooperative tumorigenic effects of germline mutations in Rb and p53. Nat Genet 1994;7:480–484. 57. Wikenheiser-Brokamp KA. Rb family proteins differentially regulate distinct cell lineages during epithelial development. Development 2004;131:4299–4310. 58. Meuwissen R, Linn SC, Linnoila RI, Zevenhoven J, Mooi WJ, Berns A. Induction of small cell lung cancer by somatic inactivation of both Trp53 and Rb1 in a conditional mouse model. Cancer Cell 2003;4:181–189. 59. Minna JD, Kurie JM, Jacks T. A big step in the study of small cell lung cancer. Cancer Cell 2003;4:163–166. 60. Reynolds SD, Giangreco A, Power JH, Stripp BR. Neuroepithelial bodies of pulmonary airways serve as a reservoir of progenitor cells capable of epithelial regeneration. Am J Pathol 2000;156:269–278. 61. Linnoila RI, Naizhen X, Meuwissen R, Berns A, DeMayo FJ. Mouse lung neuroendocrine carcinomas: distinct morphologies, same transcription factors. Exp Lung Res 2005;31:37–55. 62. Linnoila RI, Sahu A, Miki M, Ball DW, DeMayo FJ. Morphometric analysis of CC10-hASH1 transgenic mouse lung: a model for bronchiolization of alveoli and neuroendocrine carcinoma. Exp Lung Res 2000; 26:595–615. 63. Linnoila RI, Zhao B, DeMayo JL, Nelkin BD, Baylin SB, DeMayo FJ, Ball DW. Constitutive achaete-scute homologue-1 promotes airway dysplasia and lung neuroendocrine tumors in transgenic mice. Cancer Res 2000;60:4005–4009. 64. Van Lommel A, Bolle T, Fannes W, Lauweryns JM. The pulmonary neuroendocrine system: the past decade. Arch Histol Cytol 1999;62: 1–16. 65. Miller LA, Wert SE, Whitsett JA. Immunolocalization of sonic hedgehog (Shh) in developing mouse lung. J Histochem Cytochem 2001;49:1593– 1604. 66. Watkins DN, Berman DM, Burkholder SG, Wang B, Beachy PA, Baylin SB. Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer. Nature 2003;422:313–317. 67. Collins BJ, Kleeberger W, Ball DW. Notch in lung development and lung cancer. Semin Cancer Biol 2004;14:357–364. 68. Ball DW. Achaete-scute homolog-1 and Notch in lung neuroendocrine development and cancer. Cancer Lett 2004;204:159–169. 69. Ball DW, Azzoli CG, Baylin SB, Chi D, Dou S, Donis-Keller H, Cumaraswamy A, Borges M, Nelkin BD. Identification of a human achaetescute homolog highly expressed in neuroendocrine tumors. Proc Natl Acad Sci USA 1993;90:5648–5652. 70. Kim CF, Jackson EL, Woolfenden AE, Lawrence S, Babar I, Vogel S, Crowley D, Bronson RT, Jacks T. Identification of bronchioalveolar stem cells in normal lung and lung cancer. Cell 2005;121:823–835. 71. Politi K, Zakowski MF, Fan PD, Schonfeld EA, Pao W, Varmus HE. Lung adenocarcinomas induced in mice by mutant EGF receptors found in human lung cancers respond to a tyrosine kinase inhibitor or to down-regulation of the receptors. Genes Dev 2006;20:1496–1510. 72. Ji H, Li D, Chen L, Shimamura T, Kobayashi S, McNamara K, Mahmood U, Mitchell A, Sun Y, Al-Hashem R, et al. The impact of human EGFR

Pulmonary Perspective kinase domain mutations on lung tumorigenesis and in vivo sensitivity to EGFR-targeted therapies. Cancer Cell 2006;9:485–495. 73. Bottinger EP, Jakubczak JL, Haines DC, Bagnall K, Wakefield LM. Transgenic mice overexpressing a dominant-negative mutant type II transforming growth factor beta receptor show enhanced tumorigenesis in the mammary gland and lung in response to the carcinogen 7,12dimethylbenz-[a]-anthracene. Cancer Res 1997;57:5564–5570. 74. Wikenheiser KA, Clark JC, Linnoila RI, Stahlman MT, Whitsett JA. Simian virus 40 large T antigen directed by transcriptional elements of the human surfactant protein C gene produces pulmonary adenocarcinomas in transgenic mice. Cancer Res 1992;52:5342–5352.

553 75. DeMayo FJ, Finegold MJ, Hansen TN, Stanley LA, Smith B, Bullock DW. Expression of SV40 T antigen under control of rabbit uteroglobin promoter in transgenic mice. Am J Physiol 1991;261:L70–L76. 76. Johnson L, Mercer K, Greenbaum D, Bronson RT, Crowley D, Tuveson DA, Jacks T. Somatic activation of the K-ras oncogene causes early onset lung cancer in mice. Nature 2001;410:1111–1116. 77. Stevens TP, McBride JT, Peake JL, Pinkerton KE, Stripp BR. Cell proliferation contributes to PNEC hyperplasia after acute airway injury. Am J Physiol 1997;272:L486–L493. 78. Giangreco A, Reynolds SD, Stripp BR. Terminal bronchioles harbor a unique airway stem cell population that localizes to the bronchoalveolar duct junction. Am J Pathol 2002;161:173–182.