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

In lower eukaryotic organisms, the loss of serine/threonine protein phospha tase type 1 (PP1) results in growth arrest after the onset of mitosis. In h umans, four highly homologous isoforms of PP1 (PP1 alpha, PP1 delta, PP1 ga mma 1, and PP1 gamma 2) have been identified. Determining the roles of thes e phosphatases, however, has proven difficult due to the lack of subtype-sp ecific inhibitors. In this study, we developed chimeric antisense 2'-O-(2-m ethoxy)ethylphosphothioate oligonucleotides targeting human PP1 gamma 1 tha t specifically inhibit PP1 gamma 1 gene expression. Two potent antisense ol igonucleotides (ISIS 14435 and 14439; IC50 similar to 50 nM) were then empl oyed to elucidate the cellular functions of PP1 gamma 1 during cell cycle p rogression. In A549 cells, the inhibition of PP1 gamma 1 expression resulte d in a dose-dependent inhibition of cellular proliferation, with growth arr est occurring after similar to 36-48 h, when PP1 gamma 1 mRNA expression wa s inhibited by >85%. Fluorescence-activated cell sorter analysis revealed t hat ISIS 14435/14439-induced growth arrest was associated with an increase in the number of cells containing 4N DNA. Immunostaining of treated cells r evealed that the inhibition of PP1 gamma 1 expression had no apparent effec t on the formation of mitotic spindles. However, decreased expression was a ssociated with the failure of cell division in a late stage of cytokinesis and the formation of dikaryons.