Background
The cancer stem cell theory hypothesizes that cancers are perpetuated by cancer stem cells (CSC) or tumor initiating cells (TIC) possessing self-renewal and other stem cell-like properties while differentiated non-stem/initiating cells have a finite life span. To investigate whether the hypothesis is applicable to lung cancer, identification of lung CSC and demonstration of these capacities is essential. Methodology/principal finding: The expression profiles of five stem cell markers (CD34, CD44, CD133, BMI1 and OCT4) were screened by flow cytometry in 10 lung cancer cell lines. CD44 was further investigated by testing for in vitro and in vivo tumorigenecity. Formation of spheroid bodies and in vivo tumor initiation ability were demonstrated in CD44(+) cells of 4 cell lines. Serial in vivo tumor transplantability in nude mice was demonstrated using H1299 cell line. The primary xenografts initiated from CD44(+) cells consisted of mixed CD44(+) and CD44(-) cells in similar ratio as the parental H1299 cell line, supporting in vivo differentiation. Semi-quantitative Real-Time PCR (RT-PCR) showed that both freshly sorted CD44(+) and CD44(+) cells derived from CD44(+)-initiated tumors expressed the pluripotency genes OCT4/POU5F1, NANOG, SOX2. These stemness markers were not expressed by CD44(-) cells. Furthermore, freshly sorted CD44(+) cells were more resistant to cisplatin treatment with lower apoptosis levels than CD44(-) cells. Immunohistochemical analysis of 141 resected non-small cell lung cancers showed tumor cell expression of CD44 in 50.4% of tumors while no CD34, and CD133 expression was observed in tumor cells. CD44 expression was associated with squamous cell carcinoma but unexpectedly, a longer survival was observed in CD44-expressing adenocarcinomas.
Significance
Overall, our results demonstrated that stem cell-like properties are enriched in CD44-expressing subpopulations of some lung cancer cell lines. Further investigation is required to clarify the role of CD44 in tumor cell renewal and cancer propagation in the in vivo environment.