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

Abstract

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 (PP1α, PP1δ, PP1γ1, and PP1γ2) 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 PP1γ1 that specifically inhibit PP1γ1 gene expression. Two potent antisense oligonueleotides (ISIS 14435 and 14439; IC50 ~ 80 nM) were then employed to elucidate the cellular functions of PP1γ1 during cell cycle progression. In A549 cells, the inhibition of PP1γ1 expression resulted in a dose- dependent inhibition of cellular proliferation, with growth arrest occurring after ~36-48 h, when PP1~1 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 PP1γ1 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.