Induction of actin cytoskeleton rearrangement by methyl okadaate--comparison with okadaic acid

Methyl okadaate is a derivative of the lipophilic polyether okadaic acid (OA), a well-known inducer of apoptosis. OA inhibits Ser/Thr protein phosphatases (PPs), among them types 1 and 2A (PP1 and PP2A), whereas methyl okadaate lacks PP1/PP2A inhibitory activity in vitro. As progressive loss of neuronal cytoarchitecture is a major event that precedes neuronal death, in this work we studied comparatively the effects of both toxins on actin cytoskeleton organization in human neuroblastoma cells by filamentous actin (F-actin) labeling with the specific dye Oregon Green 514 Phalloidin. Neither methyl okadaate nor OA modified the amount of F-actin per cell. However, confocal microscopy imaging showed that methyl okadaate induced reorganization of actin cytoskeleton, loss of the typical flattened morphology and adoption of a round shape, and a reduction in the number of neurites, with a consequent loss of cell attachment. These effects were identical to those induced by OA, although methyl okadaate potency was approximately 10-fold lower. In order to investigate the role of membrane potential and cytosolic Ca2+ concentration in morphological changes induced by these toxins, the cells were stained with bis-(1,3-dibutylbarbituric acid)-trimethine oxonol and fura-2. No toxin effect was detected on membrane potential or calcium influx, indicating that these two signals are not responsible for cytoskeletal/morphological change induction. Methyl okadaate induced an increase of Ser/Thr phosphorylation of cellular proteins detected by western blot, showing similar phosphorylation profiles to OA. Our data suggest that methyl okadaate is an active compound that shares a pharmacological target with OA that may be a Ser/Thr phosphatase, probably different from PP1 and PP2A.