ST6GAL1-Mediated Sialylation of PECAM-1 Promotes a Transcellular Diapedesis–Like Process That Directs Lung Tropism of Metastatic Breast Cancer

Metastasis is the leading cause of mortality in breast cancer, with lung metastasis being particularly detrimental. Identification of the processes determining metastatic organotropism could enable the development of approaches to prevent and treat breast cancer metastasis. In this study, we found that lung-tropic and non-lung-tropic breast cancer cells differ in their response to sialic acids, affecting the sialylation of surface proteins. Lung-tropic cells showed higher levels of ST6GAL1, whereas non-lung-tropic cells had more ST3GAL1. ST6GAL1-mediated α-2,6-sialylation, unlike ST3GAL1-mediated α-2,3-sialylation, increased lung metastasis by promoting cancer cell migration through pulmonary endothelial layers and reducing junction protein levels. α-2,6-Sialylated platelet/endothelial cell adhesion molecule 1 (PECAM-1) on breast cancer cells facilitated extravasation through the pulmonary endothelium, a critical step in lung metastasis. Knockdown of ST6GAL1 or PECAM-1 significantly reduced lung metastasis. The human pulmonary endothelium displayed high PECAM-1 levels. Through transhomophilic interaction with pulmonary PECAM-1, α-2,6-sialylated PECAM-1 on ST6GAL1-positive cancer cells increased pulmonary extravasation in a diapedesis-like, cell-autonomous manner. Additionally, lung-tropic cells and their exosomes increased the permeability of pulmonary endothelial cells, promoting metastasis in a non-cell-autonomous manner. Analysis of human breast cancer samples showed a correlation between elevated ST6GAL1/PECAM-1 expression and lung metastasis. These results suggest that targeting ST6GAL1-mediated α-2,6-sialylation could be a potential therapeutic strategy to prevent lung metastasis in patients with breast cancer. Significance: ST6GAL1-mediated α-2,6-sialylation of PECAM-1 dictates lung-tropic metastasis of breast cancer, revealing that the pattern of sialylation of breast cancer cells is a determinant of metastatic organ tropism and a potential therapeutic target.