Hepatic lipase (HTGL; hepatic triacylglyceride lipase, EC 3.1.1.3), is synthesized primarily by hepatocytes, hydrolyzes triacylglyceride-rich lipoproteins, such as high and intermediate density lipoprotein, while on the luminal endothelial cell surface. [1][2][3] The enzyme, which is found in plasma membranes, microsomes, and cytosol in hepatocytes, is thought to play important roles in the lipid metabolism including catabolic action for the clearance of circulatory triacylglycerides and the production of free fatty acids that are available for local uptake.1,4-6) It is suggested that HTGL is found in the endothelium by electrostatic interactions with cell surface proteoglycans. It has been shown that the intravenous injection of heparin leads to the appearance of large amounts of HTGL activity in the blood-stream.1,2) Inclusion of heparin in the medium decreases HTGL binding to the surface of cultured cells, and causes release of HTGL into the medium. 1,3,4,6) Heparin releases HTGL by direct interaction with the enzyme or by competing for the binding sites on the cell surface. The mechanism of this action is thought to be due to an interaction with the negative charge of each molecule. 4)Previously, we showed that the release of lipoprotein lipase (LPL) activity from tumor cells is stimulated via a process involving the activation of various protein kinases by dextran sulfate, which is a type of heparinoid. 7,8) However, the involvement of the protein kinases in the heparin-stimulated release of HTGL is still unknown.In this report, we show that the release of HTGL produced by heparin from cultured rat hepatocytes is, in part, associated with an increase in the activities of tyrosine kinase (TK) and Ca 2+ /calmodulin-dependent protein kinase II (CaMK-II). Quin2/AM and collagenase were purchased from Wako Pure Chemical Industries (Osaka, Japan). KN-93, KN-92, W-7 and W-5 were from Seikagaku Industries (Tokyo, Japan). Biochanin A and poly (glutamate : tyrosine, 4 : 1) were purchased from Sigma (MO, U.S.A.). ST-638 was provided by Dr. Tadayoshi Shiraishi (Kanegafuchi Chemical Industry, Osaka). Williams' medium E was from Gibco (N.Y., U.S.A.). All other chemicals used were of analytical grade. MATERIALS AND METHODS MaterialsPreparation and Incubation of Hepatocytes Male Wistar rats, weighing 200-250 g, were fed a commercial laboratory chow ad libitum and fasted for 24 h before the experiments. Hepatocytes were isolated by in vitro collagenase perfusion and low speed centrifugation with modifications.9-11) Cell viability was determined by trypan blue exclusion and ranged from 85 to 95%. The hepatocytes were cultured for 24 h in monolayers in a plastic dish (1ϫ10 5 cells/cm 2 ) in Williams' medium E containing 10% fetal calf serum, 10 nM insulin, 10 nM dexamethasone and 5 KIU/ml aprotinin under 5% CO 2 atmosphere. After removal of the medium by aspiration, monolayers of hepatocytes in the dish were further incubated for 0-90 min in Williams' medium E containing 2% bovine serum albumin with or without the addition of ...