OKADAIC ACID, CAMP, AND SELECTED NUTRIENTS INHIBIT HEPATOCYTE PROLIFERATION AT DIFFERENT STAGES IN G(1) - MODULATION OF THE CAMP EFFECT BY PHOSPHATASE INHIBITORS AND NUTRIENTS

The protein phosphatase inhibitor okadaic acid (>100 nM) caused an abr upt and complete cessation of primary rat hepatocyte cell cycle progre ssion at the restriction point in late G(1). A decline in the G(1)/S t ransition rate was observed in response to elevated cAMP, excess selec ted nutrients, and okadaic acid (<100 nM). Excess nutrients (40 mM glu cose +/- 5 mM dihydroxyacetone) acted by imposing an incomplete block in early G(1). The cAMP action was potentiated by the phosphatase inhi bitor microcystin, which in itself did not affect DNA replication. Thi s suggests that cAMP acted by phosphorylating substrate(s) that is dep hosphorylated by a microcystin-sensitive phosphatase. The additive eff ects of submaximal concentrations of okadaic acid and cAMP analogs ind icated that okadaic acid and cAMP acted via different pathways. In con clusion, okadaic acid, cAMP, and excess nutrients, acting through dist inct pathways, inhibited hepatocytes in different parts of the G(1) ph ase. (C) 1995 Wiley-Liss, Inc.