Differences in stemness properties associated with the heterogeneity of luminal-type breast cancer

Background: Luminal-type breast cancers are the most abundant subtype. Endocrine therapies targeting estrogen receptor (ER) or estradiol (E2) synthesis have achieved marked improvement in disease-free and overall survival of ER-positive cancers. However, approximately one-third of these cancers are poorly responsive to endocrine therapies, suggesting nonuniform tumor cell characteristics of this subtype. Recently, the tumorigenesis theory which states that CSCs are capable of self-renewal, tumorigenicity, and therapeutic resistance, became widely accepted. We investigated the relationship between the heterogeneity of luminal-type breast cancer and stemness properties.

Materials and methods: CSC surface markers and expression of stemness-related genes, including Octamer-binding transcription factor 4 (OCT4), Nanog homeobox (NANOG), and Kruppel-like factor 4 (KLF4), were analyzed in clinical samples. ER activities were analyzed using the adenovirus vector carrying the ERE-green fluorescent protein (GFP). We separated the luminal-type breast cancers into 2 groups according to stemness-related gene expression patterns in mammospheres.

Results: The group that predominantly expressed NANOG mRNA showed a high percentage of the cells that were positive for CD44 and negative for CD24 and Hoechst (possessing high-stemness properties), younger patient age, higher p53 expression, and tended to show higher histological grade and higher topoisomerase IIα expression. The ERE-GFP assay revealed that the luminal-type breast cancer mammospheres were heterogeneous. Mammospheres from several specimens lacked ER activity and responsiveness to E2 but some retained ER activities.

Conclusion: ERE activity differences might be associated with endocrine therapy effectiveness. Mammosphere stemness properties could be a useful and novel criterion for subclassification of luminal-type breast cancers.