DDR2 and breast cancer: Investigating DDR2 signaling pathways, the collagen-dependent protease pappalysin-1 (PAPP-1) plays a critical role in postpartum breast cancer, with increased IGF signaling resulting from PAPP-1-mediated degradation of IGFBP-4 and IGFBP-5, promoting DDR2 signaling (Slocum and Germain, 2019).