CD98 activation increases surface expression and clustering of β1 integrins in MCF-7 cells through FAK/Src- and cytoskeleton-independent mechanisms

Abstract

CD98, a disulfide-linked 125-kDa heterodimeric type II transmembrane glycoprotein, regulates β1 integrin-mediated cell adhesion. However, the molecular mechanisms underlying CD98-mediated activation of β1 integrin are presently unclear. In this study, the effects of CD98 signaling on the expression and clustering of β1 integrin were investigated. Activation of CD98 augmented surface expression of β1 integrin on MCF-7 cells. Cross-linking CD98 induced clustering of β1 integrins. Inhibition of phosphorylation of focal adhesion kimase (FAK) by PP2, an inhibitor of Src family kinase, reduced cell-extracellular matrix adhesion, but not surface expression and clustering of β1 integrin on MCF-7 cells. This result was confirmed by over-expression of dominant negative forms of FAK. In addition, phalloidin or cytochalasin D inhibited CD98-mediated induction of cell-ECM adhesion, but not surface expression and clustering of β1 integrins. The inhibitory effects of PP2, cytochalasin D or phalloidin on CD98-stimulated cell adhesion were diminished by pretreatment of cells with Mn2+, which is shown to induce conformational change of integrins. These results provide the first evidence that CD98 activation increases not only β1 integrin affinity but also its surface expression and clustering and the latter is independent of FAK/Src and cytoskeleton.