TGF-b1 stimulates lymphatic endothelial cells to produce IL-7 and IL-15, which act as chemotactic factors for breast cancer cells with EMT properties

The lymphatic system is a major gateway for tumor cell dissemination but the mechanisms of how tumor cells gain access to lymphatic vessels are not completely understood. Breast cancer cells undergoing epithelial-mesenchymal transition (EMT) gain invasive and migratory properties. Overexpression of the cytokine transforming growth factor β1 (TGF-b1), a potent inducer of EMT, is frequently detected in the tumor microenvironment and correlates with invasion and lymph metastasis. Recently, we reported that TGF-b1 stimulated breast cancer cells with EMT properties migrate in a targeted fashion towards the lymphatic system via CCR7/CCL21-mediated chemotaxis, similar to dendritic cells during inflammation. Here, we aimed to identify additional chemotactic factors and receptors that could be involved in this. Through a combination of RNA sequencing analysis, database screening and invasion assays we identified IL-7/IL7R and IL-15/IL15R as pairs of chemokines and chemokine receptors with potential roles in promoting chemotactic migration of breast cancer cells with EMT properties towards the lymphatics. The results warrant studies to further explore their possible roles in lymph metastasis in breast cancer. Moreover, they demonstrate the capacity of TGF-b1 to orchestrate crosstalk between tumor cells and lymphatic endothelial cells. Total RNA was extracted from quadruplicate samples of svLEC cells that were either left untreated or treated for 72 h with 10 ng/ml TGFB1 (R&D Systems, Abingdon, UK), using RNeasy Mini Kit (Qiagen, Valencia, CA) according to the manufacturer´s protocol. Libraries were prepared using TruSeq Stranded mRNA kit (Illumina) and sequenced on the Illumina Nextseq 550 platform, at single end mode and generating 75 bp reads that were mapped to the mouse reference genome (GRCm38/mm10) and Differential Expression Analysis was performed using EdgeR [20].