p70 S6 kinase regulates the actin cytoskeleton in ovarian cancer cell motility by binding and cross-linking actin filaments

Abstract

p70 S6 kinase (p70S6K) is a downstream effector of phosphatidylinositol 3-kinase and is frequently activated in human ovarian cancer. Here we show for the first time that p70S6K functions in actin cytoskeleton reorganization responsible for the acquisition of invasiveness during tumor progression. Ectopic expression of active p70S6K in ovarian cancer cells induced dramatic reorganization of the actin cytoskeleton and promoted cell migration. Inhibition of p70S6K by siRNA or small molecule inhibitor resulted in diminished cell migration and actin cytoskeleton reorganization, confirming that these effects were p70S6K specific. By using cosedimentation and differential sedimentation assays, p70S6K was found to bind actin filaments, and was more effective in the presence than in the absence of phosphorylation. Importantly, purified p70S6K could be cosedimented with purified filamentous actin, indicating a direct association between p70S6K and actin. The response to p70S6K could be blocked with cytochalasin D, indicating that the binding of p70S6K to the cytoskeleton is mediated by actin. In addition, our results suggest that this is a functional interaction, because p70S6K was capable of generating specialized, actin-based cell morphologies via direct cross-linking activity. This bundling activity was confirmed by light scattering and electron microscopy. These results, in conjunction with our prior evidence that p70S6K regulates Rac and Cdc42 GTPases, suggest that p70S6K serves as an important regulator of the actin cytoskeleton, highlighting a role for p70S6K in the regulation of tumor progression and metastasis in ovarian cancer.