POTENTIAL CAVEATS IN THE STUDY OF LUNG CANCER STEM CELLS

Several aspects of CSC biology may limit the ability to
propagate human lung cancer cells in mice or in culture,
and thus make it difficult to test the CSC hypothesis in
lung cancer. First, human lung CSCs may require human
stromal cells or other factors from the human lung microenvironment
for growth and tumor propagation. Instead of
sorted cells, it is possible to implant intact pieces of tumor
subcutaneously for a first round of tumor growth in mice.
This technique has been performed in prior studies used to
identify breast CSCs. 2 In addition, lung tumor samples that
can be obtained from patients will be early stage tumors, as
these are typically the only stage of lung cancer that is treatable
with surgery. It is possible that these early stage tumors
will not have an adequate number of CSCs for propagation.
It may be necessary to use more advanced cancer samples to
identify lung CSCs.
It remains possible that no unique population of cells
has the ability to preferentially propagate human lung adenocarcinoma
tumors, which would argue more in favor of the
stochastic theory of tumor cell clonogenicity (above) in lung
cancer rather than the CSC hypothesis. For example, different
subsets of cancer cells may acquire CSC activity through mutation
during tumor growth, and there may not be cell-intrinsic
properties that separate cancer-propagating and nonpropagating
populations within lung tumors beyond their genetic component.
CSCs from brain tumors do not have a significantly
different pattern of genetic alterations compared with the non-
CSC from the same tumors, 3 indicating that genomic changes
alone do not account for all observed CSC activity; yet, it
will be important to compare the genomic status of cancer
cell subpopulations. Second, the inherent difficulty with the
CSC hypothesis is that negative results may not completely
rule out the hypothesis. It may not be possible to formally
exclude that the correct marker or strategy was not applied
to identify the elusive lung CSC. Given the potential promise
for future cancer therapy that has been associated with the
CSC concept, it is imperative to perform studies such as those
discussed here to begin determining if the CSC hypothesis is
a valid model for understanding lung cancer biology despite
these challenges.

At least three distinct, yet complementary, aspects of the CSC
hypothesis remain to be addressed for human lung adenocarcinomas
and all forms of lung cancer: to determine if a
CSC population exists in human lung cancers, to determine
how tumor-propagating frequency and activation of stem cell
pathways are changed in chemoresistant lung cancer cells, and
to identify molecular markers that may connect lung CSCs
and metastatic cells. If CSCs are identified in human lung
cancers, many important future directions will be possible.
Gene-expression profiling of human lung CSCs may lead to
the identification of new lung CSC markers or pathways that
regulate CSCs. Furthermore, molecules identified as differentially
expressed in lung CSCs, chemoresistant cancer cells,
or adenocarcinoma metastases could be investigated for their
causal role in these aspects of tumorigenesis and their usefulness
as biomarkers for screening patients for lung cancer.
Strategies such as those described here will make important
inroads to future studies to use the biology of stem cells to
improve the outcome of lung cancer patients.