RISING STARS: Hormonal regulation of the breast cancer microenvironment.

The tumor microenvironment is a dynamic ecosystem of stromal and immune cells that, under the influence of cancer cells, govern biochemical signaling, mechanical signaling via production and remodeling of the extracellular matrix (ECM), formation of vascular networks, and ultimately promotion of tumor growth. In breast cancer, hormone receptor-mediated signaling is a key coordinator of cancer cell proliferation and invasiveness not only through cell-autonomous means but also via cancer cell-stroma cross-talk. In the absence of hormone receptors, a different microenvironment landscape emerges, which comes with its own challenges for therapy. This review summarizes the current knowledge regarding the associations of hormone receptor profiles with composition of the microenvironment, how hormones directly influence stromal cells, immune cells and cells associated with the vasculature, and the paracrine mechanisms that lead to the formation of a tumor-promoting ECM.