Serine/threonine protein phosphatase type 1γ1 is required for the completion of cytokinesis in human A549 lung carcinoma cells

In lower eukaryotic organisms, the loss of serine/threonine protein phosphatase type 1 (PP1) results in growth arrest after the onset of mitosis. In humans, four highly homologous isoforms of PP1 (PP1 alpha, PP1 delta, PP1 gamma 1, and PP1 gamma 2) have been identified. Determining the roles of these phosphatases, however, has proven difficult due to the lack of subtype-specific inhibitors. In this study, we developed chimeric antisense 2'-O-(2-methoxy)ethylphosphothioate oligonucleotides targeting human PP1 gamma 1 that specifically inhibit PP1 gamma 1 gene expression. Two potent antisense oligonucleotides (ISIS 14435 and 14439; IC50 similar to 50 nM) were then employed to elucidate the cellular functions of PP1 gamma 1 during cell cycle progression. In A549 cells, the inhibition of PP1 gamma 1 expression resulted in a dose-dependent inhibition of cellular proliferation, with growth arrest occurring after similar to 36-48 h, when PP1 gamma 1 mRNA expression was inhibited by >85%. Fluorescence-activated cell sorter analysis revealed that ISIS 14435/14439-induced growth arrest was associated with an increase in the number of cells containing 4N DNA. Immunostaining of treated cells revealed that the inhibition of PP1 gamma 1 expression had no apparent effect on the formation of mitotic spindles. However, decreased expression was associated with the failure of cell division in a late stage of cytokinesis and the formation of dikaryons.