p70 S6 kinase in the control of actin cytoskeleton dynamics and directed migration of ovarian cancer cells

Abstract

Ovarian cancer is highly metastatic with a poor prognosis. The serine/threonine kinase, p70 S6 kinase (p70 S6K ), which is a downstream effector of phosphatidylinositol 3-kinase/Akt pathway, is frequently activated in ovarian cancer. Here, we show that p70 S6K is a critical regulator of the actin cytoskeleton in the acquisition of the metastatic phenotype. This regulation is through two important activities: p70 S6K acts as an actin filament cross-linking protein and as a Rho family GTPase-activating protein.Ectopic expression of constitutively active p70 S6K in ovarian cancer cells induced a marked reorganization of the actin cytoskeleton and promoted directional cell migration. Using cosedimentation and differential sedimentation assays, p70 S6K was found to directly bind to and cross-link actin filaments. Immunofluorescence studies showed p70S6K colocalized with cytochalasin D-sensitive actin at the leading edge of motile cells. The p70 S6K did not affect the kinetics of spontaneous actin polymerization, but could stabilize actin filaments by the inhibition of cofilin-induced actin depolymerization. In addition, we showed that p70 S6K stimulated the rapid activation of both Rac1 and Cdc42, and their downstream effector p21-activated kinase (PAK1), but not RhoA. Depletion of p70 S6K expression or inhibition of its activity resulted in significant inhibition of actin cytoskeleton reorganization and reduced migration, with a concomitant reduction in Rac1, Cdc42 and PAK1 activation, confirming that the effect was p70 S6K specific. Similarly, the actin cytoskeleton reorganization/migratory phenotype could be reversed by expression of dominant negative Rac1 and Cdc42, or inhibition of PAK1. These results reveal a new direction for understanding the oncogenic roles of p70 S6K in tumor progression.

Neuritic complexity of hippocampal neurons depends on WIP‐mediated mTORC1 and Abl family kinases activities

“…We therefore tested whether this reduction was greater in actin‐rich areas, where S6K‐pThr 389 concentrates in tumor cells (Ip et al. 2011). We focused on growth cones and in neuritic actin patches, the main actin‐rich structures in developing neurons.…”

“…2004; Ip et al. 2011), therefore we analyzed the distribution of this kinase at the actin‐rich leading edges of the neurites, the growth cones. The signal intensities and areas per growth cone of S6K‐pThr 389 , S6K and F‐actin were comparable in WT and WIP −/− neurons (Fig.…”

“…In ovarian cancer cells, phosphorylation of S6K‐pThr 389 promotes S6K binding to F‐actin (Ip et al. 2011). As the levels of active S6K and F‐actin are equivalent in the growth cones of both genotypes, WIP might play a potential role as a direct or indirect scaffold protein tethering S6K‐pThr 389 to MF in actin‐rich areas.…”

“…2003) and its ability to bind and stabilize F‐actin (Ip et al. 2011). As we described here, WIP −/− neurons present reduced levels of S6K‐pThr 389 over the neurites and in neuritic F‐actin patches compared to controls.…”

“…2005; Ip et al. 2011). The mTORC1‐S6K pathway is involved in actin cytoskeleton reorganization, stimulated by growth factors that induce phosphorylation of focal adhesion proteins (Liu et al.…”