Non-tumorigenic epithelial cells secrete MCP-1 and other cytokines that promote cell division in breast cancer cells by activating ERα via PI3K/Akt/mTOR signaling

Efforts in understanding the role of the microenvironment in the development of breast cancer have focused on tumor-stroma cross-talk, but the possibility that normal epithelial cells might also play a role in tumor progression has received little attention. Here, we show that non-tumorigenic human mammary epithelial cells (MCF10A and HMEC) secrete factors able to enhance the proliferation of estrogen receptor alpha (ER alpha) positive breast cancer cells (MCF7 and T47D) and suppress their ability to undergo apoptosis. Conditioned medium (CM) derived from MCF10A and HMEC cells was capable of activating ER alpha in a hormone-independent way, by phosphorylating ER on Ser167. Co-exposure with PI3K and mTORC1 inhibitors significantly reduced the ER Ser167 phosphorylation and suppressed the proliferation-enhancing effects of both 10A-CM and HMEC-CM on MCF7 cells. We show that MCF10A and HMEC secrete numerous cytokines, among them MCP-1, which was one of the most prevalent. MCP-1 was shown to have a role in the effects elicited by the 10A-CM. It activated the ER by phosphorylating Ser167 via the PI3K/Akt/mTORC1 signaling pathway, an effect which was further confirmed by silencing the MCP-1 receptors, CCR2 and CCR4. To our knowledge, this is the first time MCP-1 has been shown to contribute to ER signaling activation. These data suggest that normal mammary cells could have the capability of supporting the proliferation of breast cancer cells via paracrine interactions. A better understanding of the role of these cells may be useful for designing strategies for the prevention of tumor progression at early stages. (C) 2014 Elsevier Ltd. All rights reserved.