The protein kinase Pak4 disrupts mammary acinar architecture and promotes mammary tumorigenesis.

The Pak4 serine/threonine kinase is highly expressed in many cancer cell lines and human tumors. Although several studies have addressed the role for Pak4 in transformation of fibroblasts, most human cancers are epithelial in origin. Epithelial cancers are associated not only with changes in cell growth but also with changes in the cellular organization within the three-dimensional (3D) architecture of the affected tissues. In this study we used immortalized mouse mammary epithelial cells (iMMECs) as a model system to study the role for Pak4 in mammary tumorigenesis. iMMECs are an excellent model system for studying breast cancer, as they can grow in 3D-epithelial cell culture, in which they form acinar structures that recapitulate in vivo mammary morphogenesis. Although Pak4 is expressed at low levels in wild-type iMMECs, it is overexpressed in response to oncogenes, such as oncogenic Ras and Her2/neu. In this study we found that overexpression of Pak4 in iMMECs leads to changes in 3D acinar architecture that are consistent with oncogenic transformation. These include decreased central acinar cell death, abrogation of lumen formation, cell polarity alterations and deregulation of acinar size and cell number. Furthermore, iMMECs overexpressing Pak4 form tumors when implanted into the fat pads of athymic mice. Our results suggest that overexpression of Pak4 triggers events that are important for the transformation of mammary epithelial cells. This is likely to be owing to the ability of Pak4 to inhibit apoptosis and promote cell survival and thus subsequent uncontrolled proliferation, and to its ability to deregulate cell shape and polarity.