p70 S6 kinase in the control of actin cytoskeleton dynamics and directed migration of ovarian cancer cells.
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 p70(S6K) 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.
School of Biological Sciences, University of Hong Kong, Hong Kong.
C K M
CK
Ip
A N Y
AN
Cheung
H Y S
HY
Ngan
A S T
AS
Wong
D000199:Actins / Q000378:metabolism* / Q000648:ultrastructure; D015153:Blotting, Western; D045744:Cell Line, Tumor; D002465:Cell Movement; D003599:Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D056945:Hep G2 Cells; D017228:Hepatocyte Growth Factor / Q000494:pharmacology; D006801:Humans; D008854:Microscopy, Electron; D008856:Microscopy, Fluorescence; D011485:Protein Binding; D034622:RNA Interference; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000235:genetics / Q000378:metabolism*; D014162:Transfection; D014407:Tumor Cells, Cultured; D020764:cdc42 GTP-Binding Protein / Q000235:genetics / Q000378:metabolism; D054462:p21-Activated Kinases / Q000235:genetics / Q000378:metabolism; D020830:rac1 GTP-Binding Protein / Q000235:genetics / Q000378:metabolism
D000199:Actins; D049452:Green Fluorescent Proteins; D017228:Hepatocyte Growth Factor; C515903:PAK1 protein, human; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D054462:p21-Activated Kinases; D020764:cdc42 GTP-Binding Protein; D020830:rac1 GTP-Binding Protein