Cell aggregation activates small GTPase Rac1 and induces CD44 cleavage by maintaining lipid rafts integrity

Lipid rafts are highly ordered membrane domains that are enriched in Cholesterol and glycosphingolipids, and serve as major platforms for signal transduction. Cell detachment from the extracellular matrix (ECM) triggers lipid rafts disruption and anoikis, which is a barrier for Cancer cells to metastasize. Compared to single circulating tumor cells (CTCs), our recent studies have demonstrated that CD44-mediatd cell aggregation enhances the stemness, survival and metastatic ability of aggregated cells. Here, we investigated whether and how lipid rafts are involved in CD44-mediated cell aggregation. We found that cell detachment, which mimics the condition when tumor cells detach from the ECM to metastasize, induced lipid raft disruption in single cells, but lipid rafts integrity was maintained in aggregated cells. We further found that lipid rafts integrity in aggregated cells was required for Rac1 activation to prevent anoikis. In addition, CD44 and γ-secretase coexisted at lipid rafts in aggregated cells, which promoted CD44 cleavage and generated CD44 intracellular domain (CD44 ICD) to enhance stemness of aggregated cells. Consequently, lipid rafts disruption inhibited Rac1 activation, CD44 ICD generation, and metastasis. Our findings reveal two new pathways regulated by CD44-mediated cell aggregation via maintaining lipid rafts integrity. These findings also suggest that targeting cell aggregation-mediated pathways could be a novel therapeutic strategy to prevent CTC cluster-initiated metastasis.

CD44 ICD; Cell aggregation; Rac1; anoikis; breast cancer; circulating tumor cells; lipid raft; metastasis.