The cell surface mucin podocalyxin regulates collective breast tumor budding.

BACKGROUND: Overexpression of the transmembrane sialomucin podocalyxin, which is known to play a role in lumen formation during polarized epithelial morphogenesis, is an independent indicator of poor prognosis in a number of epithelial cancers, including those that arise in the breast. Therefore, we set out to determine if podocalyxin plays a functional role in breast tumor progression. METHODS: MCF-7 breast cancer cells, which express little endogenous podocalyxin, were stably transfected with wild type podocalyxin for forced overexpression. 4T1 mammary tumor cells, which express considerable endogenous podocalyxin, were retrovirally transduced with a short hairpin ribonucleic acid (shRNA) targeting podocalyxin for stable knockdown. In vitro, the effects of podocalyxin on collective cellular migration and invasion were assessed in two-dimensional monolayer and three-dimensional basement membrane/collagen gel culture, respectively. In vivo, local invasion was assessed after orthotopic transplantation in immunocompromised mice. RESULTS: Forced overexpression of podocalyxin caused cohesive clusters of epithelial MCF-7 breast tumor cells to bud off from the primary tumor and collectively invade the stroma of the mouse mammary gland in vivo. This budding was not associated with any obvious changes in histoarchitecture, matrix deposition or proliferation in the primary tumour. In vitro, podocalyxin overexpression induced a collective migration of MCF-7 tumor cells in two-dimensional (2-D) monolayer culture that was dependent on the activity of the actin scaffolding protein ezrin, a cytoplasmic binding partner of podocalyxin. In three-dimensional (3-D) culture, podocalyxin overexpression induced a collective budding and invasion that was dependent on actomyosin contractility. Interestingly, the collectively invasive cell aggregates often contained expanded microlumens that were also observed in vivo. Conversely, when endogenous podocalyxin was removed from highly metastatic, but cohesive, 4T1 mammary tumor cells there was a decrease in collective invasion in three-dimensional culture. CONCLUSIONS: Podocalyxin is a tumor cell-intrinsic regulator of experimental collective tumor cell invasion and tumor budding.

The anti-adhesive mucin podocalyxin may help initiate the transperitoneal metastasis of high grade serous ovarian carcinoma.

High grade serous ovarian tumors often metastasize transperitoneally, a process that begins when small tumor nodules de-adhere and are released into the fluid of the abdominal cavity where they float freely to reach new sites on the peritoneal wall. Podocalyxin, a small anti-adhesive sialomucin, has been shown to contribute to non-adhesive membrane domain formation in some epithelia and is overexpressed in a variety of cancers. We therefore assessed podocalyxin expression on a previously characterized tissue microarray and found that 87% (169/194) of high grade serous epithelial ovarian carcinomas were positive for podocalyxin. In addition, cell surface localization of podocalyxin was associated with a significant decrease in disease-free survival in these tumors. When podocalyxin was force-expressed in serous ovarian carcinoma-derived OVCAR-3 cells it was targeted to the cell surface and it decreased the adhesion of these cells to mesothelial monolayers, fibronectin and immobilized ß1 integrin-binding antibodies. This decrease in adhesion was associated with a modest decrease in cell surface ß1 integrin. In monolayer culture, podocalyxin was targeted to the free, apical domains of OVCAR-3 cells and it appeared to decrease ß1 integrin levels on the attached basolateral domains of the same cells. Furthermore, in 3-dimensional basement membrane gel culture, the cells formed small, cohesive nodules and podocalyxin localized to membrane domains at the cell-basement membrane interface. Therefore, podocalyxin's ability to facilitate the formation of non-adhesive membrane domains may contribute to the formation of free-floating high grade serous tumor nodules during the initial steps of transperitoneal metastasis.