PTGS2 and breast cancer: PGE2 in the tumor micro-environment resulting from elevated COX-2 expression by cancer or host cells promotes breast cancer progression by multiple mechanisms: inactivation of host anti-tumor immune cells [20, 21], stimulation of tumor cell migration [22, 23], invasiveness [23, 24], induction of stem-like cells (SLC) [25], tumor-associated angiogenesis [22] and lymphangiogenesis [26].