INVOLVEMENT OF CA2+ CALMODULIN-DEPENDENT PROTEIN-KINASE-II IN THE ACTIVATION OF CARNITINE PALMITOYLTRANSFERASE-I BY OKADAIC ACID IN RAT HEPATOCYTES/

The present work was undertaken to study the mechanism by which okadai c acid (OA), an inhibitor of protein phosphatases 1 and 2A, stimulates carnitine palmitoyltransferase I (CPT-I) in isolated rat hepatocytes [Guzman, Kolodziej, Caldwell, Costorphine and Zammit (1994) Biochem. J . 300, 693-699]. The OA-induced stimulation of CPT-I was abolished by the general protein kinase inhibitor K-252a as well as by KN-62, a spe cific inhibitor of Ca2+/calmodulin-dependent protein kinase II (Ca2+/C M-PKII). However. neither the protein kinase C-specific inhibitor bisi ndolylmaleimide nor the protein kinase A/protein kinase C inhibitor H- 7 was able to prevent the OA-induced stimulation of CPT-I. Hepatocyte- shrinkage-induced stimulation of CPT-I as well as OA-induced hepatocyt e shrinkage was prevented by KN-62. KN-62 also antagonized the OA-enha nced release of lactate dehydrogenase from digitonin-permeabilized hep atocytes. Exposure of P-32-labelled hepatocytes to OA increased the de gree of phosphorylation of Ca2+/CM-PKII, as immunoprecipitated by a mo noclonal antibody raised against the alpha-subunit of rat brain kinase , This effect of OA was also antagonized by KN-62. The results thus in dicate that the OA-dependent stimulation of CPT-I may be mediated (at least in part) by increased phosphorylation and subsequent activation of Ca2+/CM-PKII.