Tumor microenvironment and stem cells as Players in endocrine resistant breast cancer

SAMPAYO ROCÍO; RAFFO, D; PONTIGGIA, O; TOSCANI, M; COLLUCIO LESKOW, F; BAL DE KIER JOFFÉ, E; SIMIAN, M

Mar del Plata

Congreso; Reunion anual numero 57 de la SAIC; 2012

SAIC

Endocrine resistance in breast cancer is today a major clinical problem. Seventy five percent of diagnosed breast cancers are positive for estrogen and progesterone receptors. Patients with this type of breast cancer receive adjuvant endocrine therapy for 5 years mostly in the form of tamoxifen, a selective estrogen receptor (ER) modulator. However, within 15 years approximately 35% of these patients will develop a recurrence. Most research on tamoxifen resistance has focused on autocrine or paracrine mechanisms between identical cells. Deregulation of the HER-2/Neu pathway has been shown to contribute to tumor progression and tamoxifen resistance, leading to the development of drugs such as Trastuzamab2, 3. However, clinical data show that only a relatively small group (15-20%) of ER+ patients who acquire tamoxifen resistance actually show changes in HER-2 status, implicating the involvement of other mechanisms4, 5. Tumors are complex organs composed not only of neoplastic cells, but also contain fibroblasts, blood vessels, immune cells and extracellular matrix. Evidence suggests that both tumor progression and response to therapy are modulated by the tumor microenvironment6 indeed, several papers have implicated stromal signatures as predictors of response to therapy in breast cancer. On the other hand cancer stem cells have been associated in the last years to resistance to chemo and radio therapy, but whether they are involved in endocrine resistance is still controversial. Using the ER+ M05 mouse mammary tumor model and the cells lines derived from it together with the human MCF-7 cell line we have established that fibronectin and laminin, through b1 and a6 integrins respectively, confer tamoxifen resistance to otherwise sensitive cells. Fibronectin leads to the phosphorylation of the estrogen receptor but does not induce its transcriptional activity. To further understand the mechanism involved in fibronectin induced tamoxifen resistance we began to investigate whether a direct interaction between ER-a and b1 integrin was possible. To our surprise we found that ER-a and b1 integrin interact at the cell membrane. on the other hand to further understand the implications of fibronectin signaling on tamoxifen resistance we investigated whether this extracellular matrix component was able to affect the abundance of stem/progenitor cells in culture. We found a reduction in stem/progenitor cells upon treatment with fibronectin, suggesting that the effect on endocrine resistance was not mediated through the enrichment of this cell population.We next investigated whether tamoxifen leads to an increase in cells with self-renewal capacity. This hypothesis was confirmed in vitro for MCF-7 cells and in vivo for the M05 mouse mammary tumor. Mammosphere assays using cells from tumors derived from tamoxifen treated mice showed a greater ability to form mammospheres compared to those derived from vehicle Treated mice. Interestingly, when these tumors were passaged to untreated mice and allowed to grow, the difference in mammosphere forming capacity was maintained suggesting that interruption of the treatment does not revert the increase of cells with self-renewing capacity. Our results suggest that microenvironmental factors such as fibronectin modulate tamoxifen resistance in breast cancer cells through b1 integrin and that the stem/progenitor cell population is susceptible to both the microenvironment and endocrine treatment. Moreover, a direct cross talk between b1 integrin and ER-a could play a role in modulating these effects.