Prolactin-Induced Prolactin (PIP) is widely expressed in breast cancer and has key cellular functions in this disease that include promoting invasion and cell cycle progression. Notably, we have recently identified a strong association between PIP-binding partners and a number of cell functions that are involved in cell adhesion. Therefore in this study, we investigated the effect of PIP on the regulation of cell adhesion using PIP-silencing in breast cancer cell lines T-47D, BT-474, and MFM-223. Our findings suggest that PIP expression is necessary for cell adhesion in a process that shows variation in the pattern of PIP regulation of cell-matrix and cell cell adhesions based on the types of adhesion surface and breast cancer cell line. In this respect, we observed that PIP-silencing markedly reduced cell adhesion to uncoated plates in all three cell lines. In addition, in T-47D and MFM-223 cells fibronectin matrix induced baseline adhesion and reversed the PIP-silencing mediated reduction of cell adhesion. However, in BT-474 cells we did not observe an induction of baseline adhesion by fibronectin and PIP-silencing led to a marked reduction in cell adhesion to both uncoated and fibronectin-coated plates. Furthermore, we observed a significant reduction in cell cell adhesion of BT-474 cell line following PIP-silencing. To explain an underlying mechanism for PIP regulation of cell adhesion, we found that PIP expression is necessary for the formation of alpha-actinin/actin-rich podosomes at the adhesion-sites of breast cancer cells. In summary, this study suggests that PIP expression regulates the process of cell adhesion in breast cancer. (C) 2015 Elsevier Inc. All rights reserved.