Rat liver contains a limited number of binding sites for hepatic lipase

Abstract

The binding of hepatic lipase to rat liver was studied in an ex vivo perfusion model. The livers were perfused with media containing partially purified rat hepatic lipase or bovine milk lipoprotein lipase. The activity of the enzymes was determined in the perfusion media before and after passage through the liver. During perfusion with a hepatic-lipase-containing medium the lipase activity in the medium did not change, indicating that there was no net binding of lipase by the liver. In contrast, more than 80% of the lipoprotein .lipase was removed from the medium. This lipoprotein lipase activity could be recovered into the perfusion medium completely by heparin perfusion of the liver. If livers, first depleted of hepatic lipase by heparin, were subsequent perfused with a hepatic-lipase-containing medium, 90 + 24 m-units of the lipase activity was bound per g of liver (up to 1000 m-units/total liver). However, heparin treatment of the liver decreases the ability of the liver to re-bind hepatic lipase by 80 %. Perfusion of rat livers with 0.3 M NaCl released 60 % of the lipase activity into the medium. Upon subsequent perfusion of these livers with hepatic-lipase-containing media, 541 + 164 munits of hepatic lipase could be bound per g of liver (up to 5000 m-units/total liver). The binding of hepatic lipase was also studied in livers of corticotropin (ACTH)-pre-treated rats. In these rats also, hepatic lipase bound only to livers which had been pre-perfused with heparin or 0.3 M NaCl. After heparin pre-perfusion, 88 + 12 m-units of hepatic lipase could be bound per g of liver, similar to that with livers of control rats not treated with ACTH. After prior salt perfusion, however, the capacity of the livers of ACTH-pre-treated rats to bind hepatic lipase was 212 + 60 m-units/g of liver. This is less than in livers of control rats (541 + 164 m-units/g of liver). These results indicate that in rat liver the binding ofhepatic lipase is heterogeneous in character and consists of heparin-resistant and heparin-sensitive components. The hepatic-lipase binding capacity of the liver is saturable and fully utilized under various conditions. The heparin-sensitive binding capacity is lowered in ACTH-treated rats, whereas the heparin-resistant binding is unaffected. We postulate that the functional hepatic lipase activity can be regulated by changes in the binding capacity of the liver.

Endocytosis of Hepatic Lipase and Lipoprotein Lipase into Rat Liver Hepatocytes in Vivo Is Mediated by the Low Density Lipoprotein Receptor-related Protein

“…The extent of enrichment of HL in multivesicular bodies over liver homogenate (20 -30-fold) was lower than that observed for ligands such as LDL (ϳ200-fold). This presumably reflects the relatively large pool of HL on liver cell surfaces (3,33). These results are consistent with the hypothesis that HL may leave this pool by binding to receptors that enter hepatocytes via coated pits.…”

“…However, the liver still contained 99% of the total mass of HL in liver ϩ plasma in rats given GST-RAP, as it did in those given GST (Table II). The increasing concentration of plasma HL in the GST-RAP group is consistent with reduced internalization of hepatic HL, which is mainly extracellular (3,33). Indeed, as shown in Fig.…”

“…Little HL circulates in the blood of rats, and most is present in the extracellular space of the liver (space of Disse), presumably bound to proteoglycans (3,33). We found, however, that this protein, evaluated by its activity against triglyceride substrate and mass (estimated immunochemically), was enriched in three endosome fractions from estradiol-treated rats in a distinctive pattern: highest in multivesicular bodies (late endosomes), intermediate in early endosomes, and lowest in receptor-recycling endosomes (Table I).…”

Hepatic Lipase, High Density Lipoproteins, and Hypertriglyceridemia

“…25 In rodents, HL is also synthesized in the liver, but predominantly is found circulating in the bloodstream. 26,27 Murine HL appears to be more readily displaced from cell-surface HSPG, because of differences in the C-terminal amino acid composition of the enzyme. 27 Human HL can be released or liberated from cell-surface HSPG by both heparin and HDL.…”

The amino acid sequences of the carboxyl termini of human and mouse hepatic lipase influence cell surface association

“…Infusing heparin in vivo increases HL activity in the serum by 1,000-fold, suggesting an interaction of hHL with heparan sulfate proteoglycans (HSPGs) (20). In contrast to that of hHL (21), the major proportion of mouse HL (mHL) mass and activity (60-70% of total) circulates in the plasma (20). The low affinity of mHL to the cell surface could be attributable to the lack of glycosaminoglycans specific for HL binding that are present in humans but not in mice.…”