Stimulatory Release of Hepatic Lipase Activity from Cultured Rat Hepatocytes by Sodium Orthovanadate : Rapid Increase in Cyclic Adenosine Monophosphate Content

Abstract

The release of hepatic triacylglyceride lipase [EC 3.1.1.3] has been examined in isolated hepatocytes in primary culture. The stimulatory release of activity from the hepatocytes into the medium by sodium orthovanadate (vanadate) was observed in a time-and dose-dependent manner. However, insulin failed to have this stimulatory action. Moreover, vanadate rapidly increased the cyclic adenosine monophosphate (cyclic AMP) content in hepatocytes in a time-and dose-dependent manner. The treatment of hepatocytes with H-89, which is a potent cyclic AMP-dependent protein kinase inhibitor, decreased the stimulatory release of hepatic lipase activity by vanadate. The vanadate-stimulated release of the enzyme activity was suppressed by uncouplers. In addition, the incorporation of [³H] leucine into protein was increased in the presence of vanadate. Under the marked inhibition of protein synthesis by cycloheximide, vanadate still showed a full effect on the release of the enzyme activity. These results suggest that the vanadatestimulated release of hepatic lipase activity from the cultured hepatocytes is associated with a rapid increase in intracellular cyclic AMP content, probably due to an activation of cyclic AMP-dependent protein kinase which requires a metabolic energy process rather than an elevation in enzyme molecule synthesis.