The GPI-Anchored Receptor CD24 Increases Cancer Cell Invasion through Enhanced Contractile Forces

Abstract

The malignancy of a tumor depends on the capability of cancer cells to metastasize. The process of metastasis involves cell invasion through connective tissue and transmigration through endothelial monolayers. The GPI-anchored receptor CD24 expression is increased in several tumor types and is consistently associated with increased metastasis formation in patients. Furthermore, the localization of β1 integrins in lipid rafts depends on CD24. Cell invasion is a fundamental biomechanical process and usually requires cell adhesion to the extracellular matrix (ECM) through mainly β1 heterodimeric integrin receptors. Here, we studied the invasion of human lung A125 cancer cells with different CD24 expression levels in 3D ECMs. We hypothesized that CD24 expression increases the invasiveness of cancer cells by enhanced contractile force transmission or generation.To analyze this, CD24neg cells (CD24-negative) were stably transfected with CD24 and sorted for high and low CD24 expression. The invasiveness of the CD24high and CD24low transfectants were determined in 3D-ECMs. The percentage of invasive cells and their invasion depth was increased in CD24high cells compared to CD24low cells.Knock-down of CD24 and of the β1 integrin subunit in CD24high cells decreased their invasiveness, indicating that the increased invasiveness is CD24 and β1 integrin subunit dependent.Fourier transform traction microscopy revealed that the CD24high cells generated 5-fold higher contractile forces compared to CD24low cells. To test whether contractile forces were functional related to cell invasion. Cell invasiveness was reduced after addition of myosin light chain kinase inhibitor ML-7 as well as Rho kinase inhibitor Y27632 in CD24high cells, but not in CD24neg cells, whereas an increase in prestress in CD24neg cells after addition of LPA increased cell invasiveness. Taken together, these results suggest that CD24 enhances cell invasion through increased transmission or generation of contractile forces.