Effects of okadaic acid on mitotic HeLa cells

Mitotic HeLa cells were treated with different concentrations of okadaic acid (OA), known to inhibit phosphatase 1 and 2A activities. The cytological effects on the course of mitosis were studied at the light microscopic, immunoflourescence and electron microscopic levels. At the lowest concentration used (1 nM), OA did not show any effect on mitosis, but at higher concentrations it showed pronounced effects. The mitotic chromosomes became scattered, the mitotic spindle became deranged and the cells failed to enter anaphase. At the electron microscopic level formation of isolated microtubules and regular trilaminar kinetochores were observed. An extensive growth of the endoplasmic reticulum could be noted in these cells. Decondensation of chromatin and nuclear envelope re-formation could be seen only after withdrawal of OA. A high frequency of multinucleate cells could be found after 24 h of recovery. Cells treated with 100 nM OA for 3 hours showed diplochromosomes in over 50% of mitotic cells after 24 h recovery. These were presumably formed due to the failure of sister chromatid separation in the earlier mitosis in the presence of OA. At the electron microscopic level the diplochromosomes showed a quadruplet structure. The role of phosphatase 1 in controlling some late mitotic events, i.e. sister chromatid separation, MPF-inactivation and nuclear envelope re-formation etc., is discussed.