Receptor-tyrosine-kinase-like orphan receptor 1 (ROR1) is expressed during embryogenesis and by certain leukemias, but not by normal adult tissues. Here we show that the neoplastic cells of many human breast cancers express the ROR1 protein and high-level expression of ROR1 in breast adenocarcinoma was associated with aggressive disease. Silencing expression of ROR1 in human breast cancer cell lines found to express this protein impaired their growth in vitro and also in immune-deficient mice. We found that ROR1 could interact with casein kinase 1 epsilon (CK1ε) to activate phosphoinositide 3-kinase-mediated AKT phosphorylation and cAMP-response-element-binding protein (CREB), which was associated with enhanced tumor-cell growth. Wnt5a, a ligand of ROR1, could induce ROR1-dependent signaling and enhance cell growth. This study demonstrates that ROR1 is expressed in human breast cancers and has biological and clinical significance, indicating that it may be a potential target for breast cancer therapy.

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ROR1 is an orphan-receptor tyrosine-kinase-like surface antigen that is expressed by many tissues during embryogenesis, some B-cell malignancies, and various cancer cell lines but not by virtually all normal adult tissues. Here, we report that large proportions of many different human cancers also express ROR1, particularly those cancers that have high-grade histology. Primary cancers that expressed ROR1 more commonly expressed high levels of phosphorylated AKT (p-AKT) and phosphorylated cAMP response element binding-factor (p-CREB) than similar cancers that lacked expression of ROR1. Induced expression of ROR1 could enhance basal p-AKT and p-CREB levels and could promote the growth of a cancer cell line, MEC1. Conversely, silencing ROR1 resulted in lower levels of p-AKT and p-CREB, which was associated with impaired tumor cell growth. In summary, this study found that many different human cancers express ROR1 and that ROR1 may play a functional role in promoting tumor cell growth, suggesting that this orphan-receptor tyrosine-kinase-like protein may be a potential target for therapy directed against a variety of human cancers.

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Metastasis is responsible for 90% of cancer-related deaths. Strategies are needed that can inhibit the capacity of cancer cells to migrate across anatomic barriers and colonize distant organs. Here we show an association between metastasis and expression of a type I receptor-tyrosine-kinase-like orphan receptor, ROR1, which is expressed during embryogenesis and by various cancers, but not by normal post-partum tissues. We found that expression of ROR1 associates with the epithelial-mesenchymal transition (EMT), which occurs during embryogenesis and cancer metastasis. Breast adenocarcinomas expressing high-levels ROR1 were more likely to have gene-expression signatures associated with EMT and had higher rates of relapse and metastasis than breast adenocarcinomas expressing low-levels of ROR1. Suppressing expression of ROR1 in metastasis-prone breast-cancer cell-lines, MDA-MB-231, HS-578T, or BT549, attenuated expression of proteins associated with EMT (e.g. vimentin, SNAIL-1/2, and ZEB1), enhanced expression of E-cadherin, epithelial cytokeratins (e.g. CK-19), and tight-junction proteins (e.g. ZO-1), and impaired their migration/invasion capacity in vitro and the metastatic potential of MDA-MB-231 cells in immune-deficient mice. Conversely, transfection of MCF-7 cells to express ROR1 reduced expression of E-cadherin and CK-19, but enhanced expression of SNAIL-1/2 and vimentin. Treatment of MDA-MB-231 with a mAb specific for ROR1 induced down-modulation of vimentin, and inhibited cancer-cell migration and invasion in vitro and tumor metastasis in vivo. Collectively, this study indicates that ROR1 may regulate EMT and metastasis, and that antibodies targeting ROR1 can inhibit cancer progression and metastasis.