Hepatocyte DNA replication is abolished by inhibitors selecting protein phosphatase 2A rather than phosphatase 1.

Primary rat hepatocytes exposed to the phosphoprotein phosphatase (PP) inhibitors microcystin-LR and okadaic acid showed extensive surface protrusions and release of cell fragments, like cells in apoptosis. Microinjected microcystin fully reproduced these effects; the calculated intracellular concentration required for 50% effect being about 1 microM. The effects were counteracted by antagonists of calmodulin or of the multifunctional calmodulin-activated protein kinase II. The DNA replication of the epidermal growth factor-stimulated hepatocytes was nearly completely inhibited by okadaic acid at concentrations below those giving overt morphological effects. However, microcystin did not inhibit the DNA replication. Calmodulin antagonists counteracted the effect of okadaic acid on DNA replication. Microinjection of inhibitor-1 and inhibitor-2 (both directed against PP1) had no effect on DNA replication. Based on the known selectivity of okadaic acid for PP type 2A versus that of type 1, and the lack of such selectivity for microcystin, it is concluded that DNA replication is abolished by moderate inhibition of PP2A. Inhibition of PP1 did not impede DNA replication, suggesting that the two major liver phosphatases may have opposite roles in the regulation of hepatocyte DNA replication.