Disruption of the Actin Cytoskeleton Leads to Inhibition of Mitogen-Induced Cyclin E Expression, Cdk2 Phosphorylation, and Nuclear Accumulation of the Retinoblastoma Protein-Related p107 Protein

Abstract

The actin cytoskeleton has been found to be required for mitogen-stimulated cells to passage through the cell cycle checkpoint. Here we show that selective disruption of the actin cytoskeleton by dihydrocytochalasin B (H2CB) blocked the mitogenic effect in normal Swiss 3T3 cells, leading to cell cycle arrest at mid to late G1 phase. Cells treated with H2CB remain tightly attached to the substratum and respond to mitogen-induced MAP kinase activation. Upon cytoskeleton disruption, however, growth factors fail to induce hyperphosphorylation of the retinoblastoma protein (pRb) and the pRb-related p107. While cyclin D1 induction and cdk4-associated kinase activity are not affected, induction of cyclin E expression and activation of cyclin E–cdk2 complexes are greatly inhibited in growth-stimulated cells treated with H2CB. The inhibition of cyclin E expression appears to be mediated at least in part at the RNA level and the inhibition of cdk2 kinase activity is also attributed to the decrease in cdk2 phosphorylation and proper subcellular localization. The expression patterns of cdk inhibitors p21 and p27 are similar in both untreated and H2CB-treated cells upon serum stimulation. In addition, the changes in subcellular localization of pRb and p107 appear to be linked to their phosphorylation states and disruption of normal actin structure affects nuclear migration of p107 during G1-to-S progression. Taken together, our results suggest that the actin cytoskeleton-dependent G1 arrest is linked to the cyclin–cdk pathway. We hypothesize that normal actin structure may be important for proper localization of certain G1 regulators, consequently modulating specific cyclin and kinase expression.