Decrease of nuclear protein phosphatase 1 activity and induction of mitotic arrest and apoptosis by a marine microalgal polysaccharide in human myeloid leukemia U937 cells

An extracellular polysaccharide, which we designate GA3P, produced from a marine microalga dinoflagellate Gymnodinium sp. A₃, has been previously reported to induce apoptosis in lymphoid and myeloid cell lines. We found that the GA3P accumulates cells into the mitotic phase of the cell cycle and decreases nuclear protein phosphatase 1 (PP1) activity in a dose-dependent manner in myeloid leukemia U937 cells. Dose-dependent patterns in the decrease of nuclear PP1 activity and in the accumulation of cells into mitotic phase or apoptotic status by the GA3P were concordant with each other, indicating that the decrease of nuclear PP1 activity at least mediates some of the etiological steps in development of mitotic arrest and apoptosis induced by the GA3P. In addition, the GA3P repressed the expression of protein levels of the PP1 catalytic subunit isoform PP1γ1. We thus suggest that the decrease of nuclear PP1 activity is due to down-regulation of the protein levels of the PP 1γ1.