Although the role of miR-200s in regulating E-cadherin expression and epithelial-mesenchymal transition is well established, their influence on metastatic colonization remains controversial. Here, we use clinical and experimental models of breast cancer metastasis to discover a pro-metastatic role of miR-200s that goes beyond their regulation of E-cadherin and epithelial phenotype. Overexpression of miR-200s is associated with increased risk of metastasis in breast cancer and promotes metastatic colonization in mouse models, phenotypes that cannot be recapitulated by E-cadherin expression alone. Genomic and proteomic analyses revealed global shifts in gene expression upon miR-200 overexpression toward that of highly metastatic cells. MiR-200s promote metastatic colonization partly through direct targeting of Sec23a, which mediates secretion of metastasis suppressive proteins, including Igfbp4 and Tinagl1, as validated by functional and clinical correlation studies. Overall, these findings suggest a pleiotropic role of miR-200s in promoting metastatic colonization by influencing E-cadherin-dependent epithelial traits and Sec23a-mediated tumor cell secretome.

SUMMARY Breast cancer patients often develop locoregional or distant recurrence years after mastectomy. Understanding the mechanism of metastatic recurrence after dormancy is crucial for improving the cure rate for breast cancer. Here, we characterized a bone metastasis dormancy model to show that aberrant expression of vascular cell adhesion molecule 1 (VCAM-1), in part dependent on the activity of the NFκB pathway, promotes the transition from indolent micrometastasis to overt metastasis. By interacting with the cognate receptor integrin α4β1, VCAM-1 recruits monocytic osteoclast progenitors and elevates local osteoclast activity. Antibodies against VCAM-1 and integrin α4 effectively inhibit bone metastasis progression and preserve bone structure. These findings establish VCAM-1 as a promising target for the prevention and inhibition of metastatic recurrence in bone.

SUMMARY The Metadherin gene (MTDH) is prevalently amplified in breast cancer and associated with poor prognosis but its functional contribution to tumorigenesis is poorly understood. Using mouse models representing different subtypes of breast cancer, we demonstrated that MTDH plays a critical role in mammary tumorigenesis by regulating oncogene-induced expansion and activities of tumor-initiating cells (TICs), whereas it is largely dispensable for normal development. Mechanistically, MTDH supports the survival of mammary epithelial cells (MECs) under oncogenic/stress conditions by interacting with and stabilizing Staphylococcal nuclease domain-containing 1 (SND1). Silencing MTDH or SND1 individually or disrupting their interaction compromises tumorigenenic potential of TICs in vivo. Finally, this functional significance of MTDH-SND1 interaction is supported by clinical analysis of human breast cancer samples.

Metastasis is the deadliest and most poorly understood feature of malignant diseases. Recent work has shown that Metadherin (MTDH) is overexpressed in over 40% of breast cancer patients and promotes metastasis and chemoresistance in experimental models of breast cancer progression. Here we applied mass spectrometry-based screen to identify staphylococcal nuclease domain-containing 1 (SND1) as a candidate MTDH-interacting protein. After confirming the interaction between SND1 and MTDH, we tested the role of SND1 in breast cancer and found that it strongly promotes lung metastasis. SND1 was further shown to promote resistance to apoptosis and to regulate the expression of genes associated with metastasis and chemoresistance. Analyses of breast cancer clinical microarray data indicated that high expression of SND1 in primary tumors is strongly associated with reduced metastasis-free survival in multiple large scale data sets. Thus, we have uncovered SND1 as a novel MTDH-interacting protein and shown that it is a functionally and clinically significant mediator of metastasis.Metastasis is responsible for the majority of cancer-related deaths (1). An increasing number of genes have been implicated in mediating different steps of metastasis, but relatively few are mechanistically well characterized (2). One recently verified mediator of distant metastasis is Metadherin (MTDH 3 ; also called Lyric and AEG1 (3-5)). MTDH has been shown to be a key functional target of the 8q22 genomic gain that is frequently observed in poor prognosis breast cancer patients (6). Beyond promoting experimental lung metastasis (3, 6), multiple studies have implicated MTDH as a mediator of several cancer-related processes, such as oncogenesis and angiogenesis (7,8), invasion (9), chemoresistance (6, 10 -12), apoptosis resistance (13), and autophagy (14). Despite the abundance of MTDH phenotypes, a consensus understanding of its underlying molecular mechanisms has not yet been reached. MTDH has been shown to influence several oncogenic signaling pathways and transcription factors, such as Ha-Ras (15), PI3K/AKT (13), ERK, Wnt/␤-catenin (8), NF-B (16), c-Myc (15), and FOXO1/FOXO3a (17, 18). However, MTDH regulation of signaling pathways appears to be context-dependent with affected pathways varying by tumor type and cell line (19,20). Furthermore, prior work on MTDH molecular mechanisms has largely focused on hypothesis-driven investigations into the ability of MTDH to influence classical oncogenic pathways with little exploration to date on protein-level interactions (16,21).In this study, we aimed to expand the knowledge of MTDH molecular functionality and also uncover novel genes involved in promoting distant metastasis. Our approach utilized an unbiased, mass spectrometry-based screen for MTDH-interacting partners. We identified and characterized the interaction between MTDH and one such protein, SND1. SND1 is a multifunctional protein with reported roles in transcriptional activation (22), RNA editing (23, 24), formation of the RNAinduced ...