EFFECTS OF OKADAIC ACID ON EXPRESSION OF PHOSPHOENOLPYRUVATE CARBOXYKINASE IN CULTURED RAT HEPATOCYTES

In normal rat hepatocytes in primary culture the level of mRNA encodin g the key gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPC K) is increased by the cyclic AMP analogue, chlorophenylthio cyclic AM P (cpt cAMP), and this response is reversed by insulin. The protein-ph osphatase inhibitor okadaic acid diminished the stimulatory effects of cpt cAMP on PEPCK mRNA. Protein kinase A remained fully active in the presence of okadaic acid, therefore, the insulin-mimetic actions of o kadaic acid were localised to a site subsequent to initial protein kin ase A activation. Insulin produced a decrease in PEPCK mRNA expression which was similar to that of okadaic acid both in extent and mechanis m (i.e., lack of change in protein kinase A activation). The effects o f okadaic acid on PEPCK mRNA amount were not additive with those of in sulin and the effects of insulin were not abolished by okadaic acid. T hese data suggest that okadaic acid and insulin may interact with the cAMP regulation of the PEPCK gene expression at a common site. The mec hanisms by which this may be attained are discussed in relation to wha t is known about the control of specific protein kinases and protein p hosphatases by insulin and okadaic acid and of the importance of prote in phosphorylation state to regulation of gene-transcriptional process es.