Podocalyxin is an anti-adhesive transmembrane sialomucin that has recently been implicated in the development of more aggressive cancer. The mechanism through which podocalyxin increases cancer aggressiveness is poorly understood but may involve the interaction of podocalyxin with ezrin, a protein that is an established mediator of cancer metastasis. Here, we demonstrate for the first time that over-expression of podocalyxin in the human breast cancer cell-line MCF7 increased the aggressiveness of these cells in vitro as demonstrated by enhanced invasion and migration along with increased expression of the matrix metalloproteases MMP1 and MMP9. These effects were dependent upon podocalyxin-induced increases in MAPK and PI3K pathway activity. Importantly, the ability of podocalyxin to increase MAPK and PI3K activity as well as increase expression of MMPs was dependent on ezrin. Utilizing co-immunoprecipitation and co-localization we demonstrated that podocalyxin forms a complex with ezrin in MCF7 cells. Furthermore, expression of podocalyxin correlated with a change in the subcellular localization of ezrin and increased phosphorylation of ezrin on Tyr353, a residue that mediates interaction between ezrin and PI3K. Transient knock-down of ezrin protein levels abrogated MAPK and PI3K signaling as well as MMP expression in MCF7 cells over-expressing podocalyxin. These findings suggest that the ability of podocalyxin to form a complex with ezrin and influence ezrin regulated signaling events may be a key mechanism through which podocalyxin increases the aggressiveness of cancer cells.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA