RSPO3 EXPRESSION BY MAMMARY TUMOR CELLS INDUCES CANONICAL WNT-PATHWAY ACTIVATION AND PROMOTES TUMORIGENIC BEHAVIOR IN VIVO�

TOCCI J. M.; FELCHER C; ABBA MC; MEISS RP; KORDON E.

Congreso; Gordon Research Conference on Mammary Gland Biology; 2017

R-spondins (RSPOs), which are usually co-expressed with WNT proteins, are described as positive modulators of both the canonical and non-canonical WNT signaling pathways. They have been implicated in significant processes like embryonic development, tissue differentiation and human diseases, and have been postulated as potent stem-cell growth factors. RSPOs, particularly RSPO3, are frequently mutated and over-expressed in Mouse Mammary Tumor Virus (MMTV)- induced tumors. We have analyzed RSPO3 expression in human and mouse mammary cancer cells (not infected by MMTV) and we found high levels of Rspo3 in basal tumor cell lines. In addition, we found that Rspo3 mRNA is expressed by the basal compartment of normal mammary epithelium. To determine the relevance of Rspo3 expression in the tumorigenic behavior of basal mammary cancer cells, stable down-regulation of Rspo3 was induced in the mouse mammary SCg6 cell line. Our results show that in these cells, specific shRNA transfection caused repression of the canonical WNT signaling pathway and when transplanted into mice, shRSPO3 cells generated less tumors, which grew significantly later than control cells. Based on these results we propose that in the normal gland RSPO3 is secreted by the basal epithelial compartment, where it remains due to its high affinity to the extracellular matrix, participating in the maintenance of stem/basal cell features. On the other hand,RSPO3 over-expression and/or expression induction in the luminal compartment may lead to tumorigenic behavior of human and mouse mammary cells.