A novel TGF-β-responsive enhancer regulates SRC expression and epithelial-mesenchymal transition-associated cell migration

The non-receptor tyrosine kinase SRC is overexpressed and/or hyperactivated in various human cancers, and facilitates cancer progression by promoting invasion and metastasis. However, the mechanisms underlying SRC upregulation are poorly understood. In this study, we demonstrated that transforming growth factor-β (TGF-β) induces SRC expression at the transcriptional level by activating a novel intragenic SRC enhancer. In the human breast epithelial cell line MCF10A and triple-negative breast cancer cell line BT-549, TGF-β1 stimulation upregulated the SRC1A promoter, resulting in increased SRC mRNA and protein levels. Chromatin immunoprecipitation (ChIP)-sequencing analysis revealed that SMAD2 binds to three enhancer regions ~15 kb upstream and downstream of the SRC promoter, and one of them is capable of activating the SRC promoter in response to TGF-β. In addition, JUN, which is a member of the activator protein (AP)-1 family, also localizes to the enhancer and regulates TGF-β-induced SRC expression. Moreover, the total amount of active SRC increased, coinciding with the TGF-β-induced SRC expression. In TGF-β-induced epithelial-mesenchymal transition (EMT), TGF-β-induced SRC upregulation plays a crucial role in EMT-associated cell migration by activating the SRC/focal adhesion kinase (FAK) circuit. Overall, these results suggest that TGF-β-induced SRC upregulation promotes cancer cell invasion and metastasis in a subset of human malignancies.