CXCL1 and breast cancer: CXCL1 expression in breast cancer cells is also increased by secretory factors such as prostaglandin E2 (PGE2) [58], interleukin-17 (IL-17) derived from T helper type 17 (Th17) cells [59,60], loss of response to transforming growth factor-β (TGF-β) [61,62], tumor necrosis factor-α (TNF-α) [42], leukemia inhibitory factor (LIF) [63], plasminogen activator inhibitor-1 (PAI-1) [64], and Notch signaling [65].