Exploring the Role of Hypoxia-Inducible Carbonic Anhydrase IX (CAIX) in Circulating Tumor Cells (CTCs) of Breast Cancer

Circulating tumor cells (CTCs) in the peripheral blood are believed to be the source of metastasis and can be used as a liquid biopsy to monitor cancer progression and therapeutic response. However, it has been challenging to accurately detect CTCs because of their low frequency and the heterogeneity of the population. In this study, we have developed an in vitro model of CTCs by using non-adherent suspension culture. We used this model to study a group of breast cancer cell lines with distinct molecular subtypes (TNBC, HER2(+), and ER+/PR+). We found that, when these breast cancer cell lines lost their attachment to the extracellular matrix, they accumulated a subtype of cancer stem cells (CSC) that expressed the surface markers of stem cells (e.g., CD44(+)CD24(-)). These stem-like CTCs also showed high expressions of hypoxia-inducible gene products, particularly the hypoxia-inducible carbonic anhydrase IX (CAIX). Inhibition of CAIX activity was found to reduce CAIX expression and stem cell phenotypes in the targeted CTCs. Further studies are needed, using CTC samples from breast cancer patients, to determine the role of CAIX in CTC survival, CSC transition, and metastasis. CAIX may be a useful surface marker for the detection of CSCs in the blood, and a potential target for treating metastatic breast cancers.

Automated summary

Circulating tumor cells (CTCs) are the source of metastasis and can be used as a liquid biopsy for monitoring cancer progression and treatment response. However, accurately detecting CTCs has been challenging due to their low frequency and heterogeneity. In this study, an in vitro model of CTCs was developed using non-adherent suspension culture to study breast cancer cell lines of different molecular subtypes. It was found that detached breast cancer cells accumulated cancer stem cells (CSCs) expressing stem cell markers and exhibiting high expressions of hypoxia-inducible gene products, especially CAIX. Inhibiting CAIX activity reduced CAIX expression and stem cell phenotypes in the targeted CTCs.