Cancer Stem Cells in Lung Tumorigenesis

The proximal airways, composed of the trachea and main bronchi, are lined by pseudostratified epithelium [17]. Keratin (K)-expressing basal cells have been shown to have capacities for self-renewal, proliferation, and multipotency, making them putative major airway stem cells [18, 19, 21, 22]. Borthwick and colleagues [21] used an epithelial injury mouse model to demonstrate the presence of tracheal K5-expressing basal stem cells. Mouse airways were injured by intratracheal detergent or sulfur dioxide inhalation, and the proliferation and localization of bromodeoxyuridine label-retaining cells was observed in the trachea during and after injury. These label-retaining cells were found in the same niches as cells with high K5 promoter activity [21]. Transgenic mice expressing K5-promoter-driven enhanced green fluorescent protein demonstrated enhanced green fluorescent protein expression in these putative stem cell niches, confirming K5 expression in these self-renewing tracheal stem cells [22]. K5-expressing cells also demonstrated a 4.5-fold greater capacity to form colonies and a 12-fold greater capacity to form large colonies in vitro, demonstrating the proliferative potential of these self-renewing tracheal stem cells [22].

Rawlins and colleagues [23] confirmed these observations by using elegant lineage tracing experiments that support the pluripotency of tracheal K5-expressing basal cells. Hong and colleagues [18, 19] used a naphthalene airway injury mouse model to demonstrate the multipotency of tracheal keratin-expressing basal cells [18, 19]. In their model, injury resulted in the rapid induction of K14-expressing basal cells. Transgenic mice harboring a K14 promoter linked to a lox-cre reporter were used to show that these rapidly proliferating K14-expressing basal cells gave rise to phenotypically heterogenous progeny, including clonal species of the original K14-expressing basal cells, demonstrating self-renewal, proliferation, and multipotency of these tracheal stem cells [18, 19].

The bronchioles of middle airways are lined by simplified columnar epithelium [17]. Two types of bronchiolar progenitor cells that proliferate in response to injury have been described. Rapidly proliferating bronchiolar progenitor cells were first reported in classic studies by Evans and colleagues [31, 32] in a rat model of nitrogen oxide/ozone inhalation injury. They demonstrated that mature Clara cells could transiently dedifferentiate and give rise to phenotypically different bronchiolar epithelial cells after inhalation injury [31, 32]. These Clara cells are functionally equivalent to mature differentiated epithelial cells in the quiescent steady state but have the potential to become transit-amplifying cells (Clara type A cells) that give rise to diverse progeny in response to injury [29, 33]. Although originally described as “lung stem cells,” these Clara cells have recently been redesignated as “facultative progenitor cells” [29, 33].

More recent studies have identified other populations of bronchiolar progenitor cells resistant to naphthalene-induced injury that likely represent true bronchial airway stem cells [34]. Because Clara cells are selectively damaged by naphthalene [35], these models allow for the study of progenitor cells in a Clara-cell depleted environment. These models have shown that neuroepithelial bodies contain two separate populations of cells that proliferate in response to naphthalene-induced injury [24]. The first population consists of calcitonin gene-related peptide-positive pulmonary neuroendocrine cells that proliferate and form hyperplastic lesions after naphthalene injury [25]. The second population consists of Clara-cell secretory protein (CCSP)-positive cells that proliferate and repopulate bronchiole airways with phenotypically diverse progeny after naphthalene exposure [24–26]. The inability of calcitonin gene-related peptide-positive pulmonary neuroendocrine cells to independently repopulate naphthalene-injured airways implies that neuroepithelial bodies CCSP⁺ cells represent true bronchial airway stem cells [26] (Table 1).

The distal airways are composed of respiratory bronchioles and alveoli that are lined by cuboidal epithelium [17]. The bronchioalveolar duct junction (BADJ) has been identified in recent studies as a microenvironment harboring airway stem cells. In 2002 Giangreco and colleagues [36] identified a neuroepithelial bodies-independent CCSP-expressing population in the BADJ that was capable of proliferation and self-renewal after naphthalene-induced injury. These CCSP⁺ BADJ cells gave rise to populations of label-retaining cells with different mitotic rates, suggestive of multipotent potential [36]. In 2005 Kim and colleagues [38] reported that these CCSP⁺ BADJ cells coexpressed surfactant protein C (SP-C). Further characterization demonstrated that these cells displayed a Sca-1⁺/CD45⁻/platelet-endothelial cell adhesion molecule (PECAM) ⁻/CD34⁺ cell-surface marker phenotype [28]. This Sca-1⁺/CD45⁻/PECAM⁻/CD34⁺ population was capable of proliferation, self-renewal, and multilineage differentiation in culture, suggesting they represented true distal airway stem cells [28].