Cell division and differentiation depend on massive and rapid organelle remodeling. The mitotic oscillator, centered on the cyclin-dependent kinase 1-anaphase-promoting complex/cyclosome (CDK1-APC/C) axis, spatiotemporally coordinates this reorganization in dividing cells. Here we discovered that nondividing cells could also implement this mitotic clocklike regulatory circuit to orchestrate subcellular reorganization associated with differentiation. We probed centriole amplification in differentiating mouse-brain multiciliated cells. These postmitotic progenitors fine-tuned mitotic oscillator activity to drive the orderly progression of centriole production, maturation, and motile ciliation while avoiding the mitosis commitment threshold. Insufficient CDK1 activity hindered differentiation, whereas excessive activity accelerated differentiation yet drove postmitotic progenitors into mitosis. Thus, postmitotic cells can redeploy and calibrate the mitotic oscillator to uncouple cytoplasmic from nuclear dynamics for organelle remodeling associated with differentiation.

Cyclin-dependent kinases (Cdks) are key regulators of the eukaryotic cell division cycle. Cdk1 (Cdc2) and Cdk2 should be bound to regulatory subunits named cyclins as well as phosphorylated on a conserved Thr located in the T-loop for full enzymatic activity. Cdc2-and Cdk2-cyclin complexes can be inactivated by phosphorylation on the catalytic cleft-located Thr-14 and Tyr-15 residues or by association with inhibitory subunits such as p21 Cip1 . We have recently identified a novel Cdc2 regulator named RINGO that plays an important role in the meiotic cell cycle of Xenopus oocytes. RINGO can bind and activate Cdc2 but has no sequence homology to cyclins. Here we report that, in contrast with Cdc2-cyclin complexes, the phosphorylation of Thr-161 is not required for full activation of Cdc2 by RINGO. We also show that RINGO can directly stimulate the kinase activity of Cdk2 independently of Thr-160 phosphorylation. Moreover, RINGO-bound Cdc2 and Cdk2 are both less susceptible to inhibition by p21 Cip1 , whereas the Thr-14/Tyr-15 kinase Myt1 can negatively regulate the activity of Cdc2-RINGO with reduced efficiency. Our results indicate that Cdk-RINGO complexes may be active under conditions in which cyclin-bound Cdks are inhibited and can therefore play different regulatory roles.

Activating epidermal growth factor receptor (EGFR) mutations have been linked with sensitivity to gefitinib and erlotinib; however, there are no established predictive markers for response to the combination of EGFR inhibitors with standard chemotherapy in non -small cell lung cancer (NSCLC) patients. In this study, we characterized a panel of human EGFR wild-type and mutant NSCLC cells for their sensitivity to gefitinib alone and in combination with cisplatin or Taxol. Cell viability was assessed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and crystal violet cell viability assays. Cell cycle distribution was measured by flow cytometry. EGFR expression was measured by flow cytometry, real-time PCR, and Western blotting. EGFR/ Her2/Akt and extracellular signal-regulated kinase 1/2 (Erk1/2) phosphorylation were measured by Western blotting. Two of nine EGFR wild type and one of two EGFR mutant NSCLC cells were sensitive to gefitinib, and this was associated with a decrease in phospho (p) -Akt and pErk1/2 following gefitinib exposure. There was no correlation between constitutive EGFR expression or activity and sensitivity to gefitinib nor was there a correlation between Her2/Akt and Erk1/2 activity and gefitinib sensitivity. However, in cells displaying a synergistic interaction between gefitinib and chemotherapy (cisplatin or Taxol), a dose-dependent increase in pEGFR was observed following chemotherapy exposure. In contrast, in cells where no change or a decrease in pEGFR following drug treatment was observed, we found an antagonistic or (at best) an additive interaction between the two compounds. Furthermore, the nature of this interaction was not dependent on the presence of a mutant EGFR. These novel findings suggest that modulation of EGFR activity following drug treatment determines response to gefitinib in combination with chemotherapy in NSCLC cells.