Signaling from a Novel Receptor Tyrosine Kinase and the Control of Multipotent Mammary Progenitor Cells

Abstract

Abstract : Our objective was to determine if signaling through the ROR1 receptor tyrosine kinase controls the proliferation, self-renewal and/or differentiation of mammary epithelial cells. We silenced ROR1 by stable expression of siRNA ROR1 sequences using a retroviral expression system in human cell lines with progenitor properties. We also overexpressed the ROR1 cDNA in immortalized human mammary epithelial cell lines (184A1, 184B5). The effects of depleted and overexpressed ROR1 were assayed using in vitro matrigel TDLU formation assays and in vivo xenograft tumor formation assays. We characterized the CAL51 breast cancer line extensively because of its high level of ROR1 expression and its interesting multi-potent properties. When we silenced ROR1 in this cell line we observed compelling differences in the formation of in vitro 3D matrigel structures and also in the histology and nuclear morphology of the resulting xenograft tumors. Expression profiling experiments comparing CAL51 tumor xenografts with and without ROR1 depletion uncovered a potential role for ROR1 in controlling the developmental fate of breast cells. These experiments led us to hypothesize that ROR1 antagonizes the Hedgehog pathway that is thought to play a critical role in the maintenance of adult stem cells.