Overexpression of wild type p70 S6 kinase interferes with cytokinesis

p70 S6 kinase (p70s6k) is a serine/threonine kinase which is activated through an unidentified pathway by mitogenic stimuli. Here we demonstrate that stable- and transient-overexpression of wild type (WT)-p70s6k results in perturbation of cytokinesis in NIH3T3 cells. Cells overexpressing WT-p70s6k demonstrated a slow growth rate and lower viability, whereas cells overexpressing a mutant T229A-p70s6k had a similar growth rate and viability as those of parental NIH3T3 cells. Moreover, WT-p70s6k cells demonstrated several abnormalities in cellular morphology; both the cytoplasm and the nuclei were large and a proportion of the cells was multinucleated. Flow cytometric analysis of propidium iodide-stained cells demonstrated that about 50% of the cells had 4N or greater content of DNA. When cell division in WT-p70s6k cells was monitored by time-lapse video microscopy, cytokines was often incomplete after mitosis, producing binucleated cells. Addition of rapamycin to WT-p70s6k cells, which inactivates endogenous and transfected p70s6k, failed to reverse any of the morphological abnormalities. These indicate that overexpression of p70s6k but not increases in activity of p70s6k, is responsible for the phenotype. A molecule which interacts with p70s6k may be involed in this regulation of cytokinesis.