Phorbol ester-induced change in astrocyte morphology: correlation with protein kinase C activation and protein phosphorylation.

Treatment with 300 nM phorbol 12-myristate 13-acetate (PMA) transforms polygonal-shaped cultured astrocytes into process-bearing cells and produces a shift in protein kinase C (PK-C) from the cytosol to the membrane. Exposure to PMA also produces increases in the phosphorylation of several proteins including vimentin, glial fibrillary acidic protein (GFAP), an acidic 80,000 molecular weight protein, and two 30,000 molecular weight proteins (pI 5.5 and 5.7). The effects of PMA on the translocation of PK-C and on protein phosphorylation precede the PMA-induced changes in astrocyte morphology, and a close correlation exists between the concentration of PMA necessary to elicit half-maximal and maximal effects on the shift of PK-C to the membrane and on protein phosphorylation. In addition, the PMA-induced alterations in cell morphology are not permanent, and within 24 hr after PMA treatment the cells have reverted almost to their original morphology. A second exposure to PMA at this time fails to elicit further change in cell shape and is also incapable of producing increases in the phosphorylation of proteins. It was determined that there is little, if any, PK-C present in these PMA-pretreated cells. The morphological responsiveness to PMA gradually returns in 5 to 8 days after the initial treatment with PMA, and this is accompanied by the recovery of PK-C activity and the phosphorylation response. Therefore, these studies suggest that the effect of PMA on astrocyte morphology is mediated by the activation of PK-C and subsequent protein phosphorylation.