Characterization of side population cells from spontaneously transformed mouse lung carcinoma cell lines

Background The side population (SP) phenotype, a subset of cells that extrude the nucleic acid dye Hoechst 33342, has been reported to be enriched for stem cells in several human normal tissues, cancers and cell lines, and thus may be useful for the identification and isolation of cancer stem cells. Methods We demonstrated the presence of SP fractions in all analyzed tumor cell lines ranging between 7- 20% of cells. To identify gene expression patterns that contribute to SP phenotype, microarray analysis of SP and non-SP cells was performed. We additionally confirmed regulation of some genes by qRT-PCR. Results Surprisingly, only a subset of few genes in SP cells showed altered gene expression. A total of 11 genes in A2C12, 103 genes in cRAF_cMYC and 101 genes in beta5 SP cells were regulated. Most regulated genes are involved in transcription / transcriptional regulation and transport. In addition, we found no enrichment of previously described stem cell marker like CD24a, CD90 or CD133 and also the ABC transporter ABCG2 was only slightly increased in side population fraction of two cell lines. But despite the few differences between SP and non-SP cells, the beta5 tumor cells were highly tumorigenic due to their capacity to form an original murine tumor when injected in NOD/SCID mice. Conclusion These findings stand in contrast to other observations but indicate that there are further factors responsible for the SP phenotype and that SP cells alone are not suitable as a universal stem cell marker.

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