DKK1 and breast cancer: Another parallel between OP and BC is provided by the dysregulation of the Wnt pathway, which may be down-regulated by BC cells through synthesis and secretion of the Wnt antagonist DKK-1 [127], thereby inhibiting intrinsic bone regeneration and indirectly enhancing osteoclast-activating pathways such as NF-κB and RANK/RANKL, contributing to osteolysis in BC patients [128] (Figure 5).