| Doxorubicin is a common chemotherapeutic treatment for ER+ breast tumors despite clinical reports demonstrating that ER+ patients respond less well to chemotherapy than ER- patients. Studies using ER+ breast cancer cell lines have shown that estrogen treatment counters chemotherapeutic effects, suggesting estrogen-induced signaling may confer chemotherapeutic resistance. However, little is known about whether and how chemotherapy affects estrogen/ER signaling, the effectors mediating chemotherapy's effects, and whether these signaling molecules promote chemotherapeutic resistance. Mimicking the prolonged estrogen and doxorubicin exposure seen clinically, this study used an ER+ breast cancer cell model to identify estrogen effectors that could serve as therapeutic targets to enhance doxorubicin's efficacy. Alterations in protein and phosphorylation levels of known estrogen signaling proteins (ER-alpha, c-Src, ERK, PI3K, AR, and EGFR) induced by doxorubicin or estrogen treatment individually were compared to untreated controls or combined treatment. Prolonged estrogen treatment alone modulated levels and phosphorylation of specific signaling molecules much like that seen previously with shorter estrogen treatment. Doxorubicin further upregulated some hormone and growth factor signaling molecules that were downregulated by estrogen alone, suggesting a mechanism by which doxorubicin could molecularly counteract the estrogen's biological effects. Co-treatment with estrogen and doxorubicin modestly enhanced changes induced by estrogen alone and markedly enhanced pro-growth alterations when compared to doxorubicin alone. Thus, the net effect of doxorubicin/estrogen co-treatment is pro-proliferative and pro-survival, suggesting a molecular mechanism for the poorer chemotherapeutic responses of ER+ breast tumors. We show that inhibitors of MEK and metalloprotienases cooperated with doxorubicin to reduce cell proliferation, while inhibitors of the ER, SFKs, EGFR, and PI3K functioned independently of doxorubicin. Results indicate that both classes of inhibitors are potential candidates for overcoming doxorubicin resistance in ER+ breast cancers.;Additionally, the AR upregulation seen with estrogen and/or doxorubicin treatment suggested that AR may mediate the biological responses of ER+ breast cancer cells to these agents. Studies in the aforementioned conditions tested the contribution of AR to proliferation, survival, migration, and invasion. Preliminary results suggest that the AR contributes little to proliferation or survival, but might regulate migration and invasion in the absence or presence of estrogen and/or doxorubicin. |
