The estrogen receptor (ER) plays a cardinal role in estrogen-responsive breast carcinogenesis. It is, however, unclear as to how estrogen-ER interaction potentiates breast cancer progression. Compelling evidence supports estrogen-induced redox alterations, such as augmented reactive oxygen species (ROS) levels, as having a crucial role in breast carcinogenesis. Despite ER being a biological mediator of the majority of estrogen-induced cellular responses; its role in estrogen-induced tissue-specific ROS generation remains largely debatable. We examined a panel of human breast cancer specimens and found that ER-positive breast cancer specimens exhibited a higher incidence of augmented O(2)(•-) levels compared to matched normal tissue. ROS are known to function as signal transducers and ROS-mediated signaling remains a key complementary mechanism that drives carcinogenesis by activating redox-sensitive oncogenic pathways. Additional studies revealed that augmented O(2)(•-) levels in breast cancer specimens coincided with mammalian target of rapamycin complex 2 (mTORC2) hyperactivation. Detailed investigations using in vitro experiments established that 17β-estradiol (E2)-stimulated breast cancer cells exhibited transiently upregulated O(2)(•-) levels, with the presence of ER being a crucial determinant for the phenomenon to take place. Gene expression, ER transactivation, and confocal studies revealed that the E2-induced transient O(2)(•-) upregulation was effected by ER through a nongenomic pathway possibly involving mitochondria. Furthermore, E2 treatment activated mTORC2 in breast cancer cells in a characteristically ER-dependent manner. Interestingly, altering O(2)(•-) anion levels through chemical/genetic methods caused significant modulation of the mTORC2 signaling cascade. Taken together, our findings unravel a novel nongenomic pathway unique to estrogen-responsive breast cancer cells wherein, upon stimulation by E2, ER may regulate mTORC2 activity in a redox-dependent manner by transiently modulating O(2)(•-) levels particularly within mitochondria. The findings suggest that therapies aimed at counteracting these redox alterations and/or resultant signaling cascades may complement conventional treatments for estrogen-responsive breast cancer.
Copyright © 2012 Elsevier Inc. All rights reserved.