PROTEIN PHOSPHATASE INHIBITOR CALYCULIN-A INDUCES HYPERPHOSPHORYLATION OF CYTOKERATINS AND INHIBITS AMYLASE EXOCYTOSIS IN THE RAT PAROTID ACINI

Calyculin A, a protein phosphatase inhibitor with a chemical structure completely different from that of okadaic acid, reproduced the inhibitory effect of okadaic acid on cyclic AMP-mediated amylase release from rat parotid acinar cells. Calyculin A markedly enhanced phosphorylation of cytokeratins in the cytoskeletal fraction of the cells, whereas cAMP had apparently no effect on the phosphorylation. Microscopic observations showed that parotid acini incubated with 100 nM calyculin A for 15 min had large vacuoles in the cytoplasm and conspicuous blebs on the basal plasma membrane. K252a, a nonselective protein kinase inhibitor, clearly reduced calyclin A-induced phosphorylation of cytokeratins, and it markedly blocked the inhibition of amylase release and morphological changes evoked by calyculin A. These results suggest that hyperphosphorylation of cytokeratins profoundly affects the morphology and secretory activity of parotid acinar cells.