Galectin-9 signaling drives breast cancer invasion through matrix

Aberration in expression and function of glycans and their binding proteins (lectins) in transformed cells constitutes one of the earliest discovered hallmarks of cancer. Galectins are a conserved family of lectins that can bind to {beta}-galactosides. Among them, the role of Galectin-9, a galectin with two carbohydrate binding domains in immune-tumor cell interactions has been well-established, although its effect on cancer cell behavior remains as yet unclear. In this study, we used a spectrum of cell lines from homeostatic breast cells to transformed non-invasive and invasive cell lines cultured in microenvironment-diverse conditions to show that Galectin-9 expression shows an elevation in association with invasiveness of breast cancer epithelia. Our observations were supported by immunohistochemical studies of breast tumors and adjacent normal-tissues from patients. Genetic perturbation of Galectin-9 as well as the pharmacological inhibition of activity using cognate inhibitors confirmed a positive correlation between Galectin-9 levels and the adhesion of the aggressive triple negative breast cancer cells MDA-MB-231 to- and their invasion through-extracellular matrices (ECM). Within a constituted organomimetic multiECM microenvironment, Galectin-9 enhanced both the solitary and the collective invasion of cancer cells. Quantitative proteomics led us to uncover the inductive role of Galectin-9 in the expression of the proinvasive protein S100A4. In addition, Galectin-9 expression correlated with FAK signaling, the inhibition of which decreased S100A4 mRNA levels. Our results provide crucial signaling insights into how the elevation in Galectin-9 expression in breast cancer cells potentiates their invasiveness through ECM during early steps of metastasis.