Transcriptional networks controlling breast cancer metastasis : molecular mechanisms shaping the SOX4 response

Breast cancer is the most commonly diagnosed cancer in women. Despite great improvements in diagnosis and treatment of this disease, mortality remains high due to the development of metastatic disease resulting in clinical relapse. The majority of current treatment options primarily target the primary tumor and fail to effectively stop the development of metastases. It is imperative to further our understanding of the molecular mechanisms driving breast cancer metastasis to identify novel therapeutic targets. Where primary tumor formation is fueled by the acquisition of specific mutations enabling unrestrained proliferation whilst preventing cell death, the process of metastasis is associated the aberrant activation of developmental processes. Activation of processes such as the epithelial to mensenchymal transition (EMT), endow cancer cells with novel traits resulting in the ability to overcome natural barriers and ultimately initiate the formation of a secondary tumors. These developmental pathways are fundamentally driven by the aberrant activation of selected set of transcription factors, which activate complex transcriptional networks. The transcription factor SOX4 is frequently upregulated in human cancers compared to the matching benign tissue and is part of a general cancer signature. Increased expression of SOX4 is generally associated with poor-disease outcome. Despite this correlation, in depth analysis of the SOX4 induced pro-oncogenic effects is lacking. Here, we have utilized a unbiased genome-wide approach using state-of-the-art techniques such as chromatin-immunoprecipitation sequencing and RNA-sequencing, to characterize the SOX4-induced transcriptional response in breast cancer. We demonstrate that the core SOX4 target gene signature in breast cancer is strongly associated with decreased survival in patients and is enriched for pro-metastatic processes including EMT and angiogenesis. We show that SOX4 regulates EMT and angiogenesis in breast cancer both in vitro and in vivo, thus uncovering novel pro-metastatic functions for this cancer-associated transcription factor. Consistent with these observations, we demonstrate that depletion of SOX4 reduces the formation of metastases in vivo in a breast cancer xenograft model, and that in breast cancer patients SOX4 expression correlates with poor-disease prognosis, increased metastasis, therapy resistance and elevated blood vessel density. Interestingly, we also find that SOX4 is an integral component of the TGF-β signaling pathway, acting as a transcriptional target and a SMAD3 interaction partner. In this way, SOX4 modulates pro-metastatic responses in breast cancer induced by TGF-β including EMT. Finally, we provide the first evidence of post-translational regulation of SOX4 by the adaptor protein Syntenin, which stabilizes SOX4 by protecting it from proteasomal degradation. Taken together, these findings provide novel insights into molecular mechanisms that shape the SOX4 response in breast cancer and identify SOX4-regulated pathways that could be targeted therapeutically to halt the development of metastatic disease