Stem cells are vital to all stages of life, yet the specific roles that stem cells play during different stages of ontogeny change considerably. During early embryogenesis, pluripotent stem cells differentiate to give rise to the three germ layers that establish the basic vertebrate body plan.18
As development proceeds, distinct subsets of stem cells emerge to orchestrate the construction of tissues and organs. These processes are often incomplete at birth and carry over into postnatal life. Once tissues are fully established, stem cells undergo a fundamental change as their role turns from one of tissue building to one of tissue maintenance and repair, which persists throughout adult life. Although best exemplified in the hematopoietic system,19
the existence of hierarchical relationships between stem and progenitor cells is emerging as a common feature of other tissue-specific stem cell compartments.
Cells with long life spans are intuitively more likely to accrue the requisite number and variety of genetic alterations necessary to acquire full tumorigenic capacity.20
The cancer stem cell (CSC) hypothesis has arisen from observations that neoplastic clones are maintained exclusively by rare fractions of cells with stem cell properties. Not only is the existence of CSCs in human leukemia well established,21
the identification of CD133+
human colon CSCs22
and brain tumor CSCs23
provide further evidence for the potential existence of CSCs in solid tumors. A tumor can be viewed as an aberrant organ initiated by a CSC that acquired the capacity for indefinite proliferation through accumulated mutations.21
Both normal stem cells and tumorigenic cells have extensive proliferative potential and the ability to give rise to new (normal or abnormal) tissues. Both tumors and normal tissues are composed of interacting heterogeneous combinations of cells, with different phenotypic characteristics and different proliferative potentials. This suggests that CSC undergo processes that are analogous to the self-renewal and differentiation of normal stem cells. Thus, the principles of normal stem cell biology may be applicable to understand better how tumors develop.24
Recent observations by Kim et al. have ignited interest in the potential role of adult stem cells in lung development, injury and repair, and as a potential cell of origin for tumor initiation.25
A rare population of bronchioalveolar stem cells (BASCs) was described at the bronchoalveolar duct junction in adult mice. BASCs harbor both the alveolar epithelial type II (AT2) cell marker, surfactant protein C (SP-C), and the Clara cell marker, Clara cell secretary protein (CCSP or CC10). They were identified and purified by FACS based on being positive for Sca-1 and CD34, and negative for CD45 and CD31. This population retained the double positive staining characteristics for SP-C and CC10 with serial passage in culture. BASCs demonstrated resistance to naphthalene-induced lung injury and increased in number during a period of bronchiolar epithelial repair. Interestingly, BASC populations demonstrated early expansion in mice harboring an oncogenic K-ras mutation. Viable BASCs were maintained through multiple passages in vitro under distinct culture conditions, and retained the ability to differentiate into Clara cell and AT2 lineages with culture on Matrigel.25
The observation that almost all cells in the epithelium of the smaller distal airway during early embryonic lung development (E13–E15) express markers of AT2, Clara and neuroendocrine (NE) cells26
supports the potential existence of BASCs that are maintained into adulthood once development is complete.
Identification of BASCs makes it possible to begin to map out the pathways that are required for stem cell function in lung morphogenesis and lung tumorigenesis. Using p38a conditional lung knockout mice, Ventura et al.27
found that inactivation of p38a leads to an immature and hyperproliferative lung epithelium that is highly sensitized to K-RasG12V-induced tumorigenesis. Coincident expansion of the BASC population was observed. This suggests that p38a has a key role in the regulation of lung cell renewal and tumorigenesis by coordinating proliferation and differentiation signals in lung stem and progenitor cells. Dovey et al.28
demonstrated that loss of Bmi1 decreases the number and progression of lung tumors at a very early point in a K-ras-initiated mouse model of lung cancer. This correlates with a defect in the ability of Bmi1-deficient BASCs to proliferate in response to the oncogenic stimulus. Yanagi et al.29
generated PTEN knockout mice under control of the SP-C promoter where 90% of mice that received doxycycline in utero died of hypoxia soon after birth. Postnatal deletion of PTEN resulted in spontaneous lung adenocarcinomas with increased BASC numbers. Expression of the oncogenes Spry2, c-Myc and Shh were elevated in the lungs of mutant mice. Zhang et al.30
show that deletion of Gata6 resulted in the expansion of the BASC population and loss of epithelial differentiation with pronounced activation of the canonical Wnt signaling pathway. These molecular mechanisms regulating the balance between BASC expansion and epithelial differentiation and regeneration strongly imply that the process of lung tumorigenesis may share critical common pathways with lung organogenesis.
Early descriptions of putative CD133+
cancer stem cell populations from NSCLC point to further onco/devo links in the lung. Recently, Eramo et al.31
and Chen et al.32
have reported isolation of CD133+
cells from tumor samples of human NSCLC. These CD133+
cells displayed higher Oct-4 expression with the ability to self-renew and may represent a reservoir with proliferative potential for generating further lung cancer cells during tumor propagation and metastasis. Oct-4, a member of the family of POU-domain transcription factors, is expressed in pluripotent embryonic stem and germ cells. Oct-4 was identified as a stem cell biomarker and as one of four essential genes for reprogramming fibroblasts into a pluripotent embryonic stem cell-like state.33
The injection of CD133+
cells either subcutaneously or through the tail vein into immunocompromised mice generated tumor xenografts phenotypically identical to the original tumor. Knock-down of Oct-4 expression in CD133+
cells significantly inhibited tumor invasion and colony formation.
Ling et al. have reported a serum-free culture system for primary neonatal pulmonary cells that can support the growth of Oct-4+
epithelial colonies. In addition to Oct-4, these cells also express other stem cell markers such as stage-specific embryonic antigen 1 (SSEA-1), stem cell antigen 1 (Sca-1), and CCSP. These cells can be maintained for weeks in primary cultures and undergo terminal differentiation to AT1 and AT2-like pneumocytes. They demonstrated these Oct-4+
cells were located at the bronchoalveolar junction of neonatal lung.34
These findings overall provide further supportive evidence to the hypothesis that stem cell biology is a key interface between lung development and lung cancer.