Long-term exposure to carcinoma-associated fibroblasts makes breast cancer cells addictive to integrin β1

Introduction:

Short-term exposure to carcinoma-associated fibroblasts (CAFs) or mesenchymal stem cells have shown to decrease the sensitivity of breast cancer cells to fulvestrant, an event based mainly on upregulation of the transcription factor Bcl-3 (Oncotarget 2015, 6: 39307 – 28), a modulator of NFκB-dependent transcription. However, the stroma-induced increase in Bcl-3 is transient prompting us to analyze the long-term effect of CAFs on fulvestrant resistance.

Methods:

Fulvestrant-tolerating dormant (D) cells and fulvestrant-resistant (FR) sublines were analyzed for their responses to fulvestrant in the presence or absence of conditioned medium (CM) from CAFs. Responses in terms of fulvestrant sensitivity, expression of selected proteins, cell growth, migration and spheroid formation were measured. RNA interference was used to determine the importance of proteins upregulated in response to CAFs.

Results:

Sublines established from CAF-CM-treated FR-cells and CAF-CM-treated D-cells show permanently high expression of integrin β1 as well as Bcl-3 and P-STAT3 (FR) or IGF1R (D). Yet, cells fail to withstand fulvestrant better and do not migrate or grow faster than control cells. Instead, CAF-CM-treated D cells rely on stromal factors to perform as well as control cells and CAF-CM-treated FR cells adapted to integrin β1 for growth in 3D cultures.

Conclusions:

Our data suggest that long-term exposure to stromal factors in the presence of fulvestrant leads to addiction rather than better performance in cellular activities in vitro. Nevertheless, as integrin β1 has been linked to metastasis, the elevated integrin β1 levels as established in the presence of CAF-CM may bestow higher metastatic activity to breast cancer cells.