Hepatic lipase-mediated hydrolysis versus liver uptake of HDL phospholipids and triacylglycerols by the perfused rat liver

Endoplasmic reticulum quality control in lipoprotein metabolism

Lipids play a critical role in energy metabolism, and a suite of proteins is required to deliver lipids to tissues. Several of these proteins require an intricate endoplasmic reticulum (ER) quality control (QC) system and unique secondary chaperones for folding. Key examples include apolipoprotein B (apoB), which is the primary scaffold for many lipoproteins, dimeric lipases, which hydrolyze triglycerides from circulating lipoproteins, and the low-density lipoprotein receptor (LDLR), which clears cholesterol-rich lipoproteins from the circulation. ApoB requires specialized proteins for lipidation, dimeric lipases lipoprotein lipase (LPL) and hepatic lipase (HL) require a transmembrane maturation factor for secretion, and the LDLR requires several specialized, domain-specific chaperones. Deleterious mutations in these proteins or their chaperones may result in dyslipidemias, which are detrimental to human health. Here, we review the ER quality control systems that ensure secretion of apoB, LPL, HL, and LDLR with a focus on the specialized chaperones required by each protein.

Biliary lipid secretion, bile acid metabolism, and gallstone formation are not impaired in hepatic lipase-deficient mice

Whereas hepatic lipase (HL) has been implicated in lipoprotein metabolism and atherosclerosis, its role in controlling biliary lipid physiology has not been reported. This work characterizes plasma lipoprotein cholesterol, hepatic cholesterol content, bile acid metabolism, biliary cholesterol secretion, and gallstone formation in HL-deficient mice and C57BL/6 controls fed standard chow, a cholesterol-supplemented diet, or a lithogenic diet. Compared with C57BL/6 controls, HL knockout mice exhibited increased basal plasma high-density lipoprotein (HDL) cholesterol as well as reduced cholesterol levels transported in large lipoproteins in response to cholesterol-enriched diets. Hepatic cholesterol content and biliary cholesterol secretion of chow-fed HL knockout and wild-type mice were not different and increased similarly in both strains after feeding dietary cholesterol or a lithogenic diet. There were no differences in biliary bile acid secretion, bile acid pool size and composition, or fecal bile acid excretion between HL-deficient and control mice. HL knockout mice had a similar prevalence of gallstone formation as compared with control mice when both strains were fed with a lithogenic diet. In conclusion, the deficiency of HL has no major impact on the availability of lipoprotein-derived hepatic cholesterol for biliary secretion; HL expression is not essential for diet-induced gallstone formation in mice.

Transformation of high density lipoprotein 2 particles by hepatic lipase and phospholipid transfer protein

High density lipoprotein 2 (HDL2) was incubated with phospholipid transfer protein (PLTP) or with hepatic lipase (H-TGL), and the incubation products were separated into a d < 1.22 g/ml and a d > 1.22 g/ml fractions. The d < 1.22 g/ml fraction produced by PLTP was larger, had lower apolipoprotein A-I and higher lipid and apolipoprotein A-II content than native HDL2. The d > 1.22 g/ml fraction represented 30% of the initial HDL2 protein and consisted of small, apolipoprotein A-I and phospholipid-rich particles, with a high sphingomyelin:phosphatidylcholine ratio. Incubation with H-TGL led to a d < 1.22 g/ml fraction which was comparable to native HDL2 regarding size and chemical composition. The d > 1.22 g/ml particles represented only 5% of the initial HDL2 protein and had slightly higher diameter and sphingomyelin:phosphatidylcholine ratio than those produced by PLTP. Enrichment of HDL2 with triglyceride prior to incubation increased the amount of protein released into the d > 1.22 g/ml fraction (20%) but had no effect on size and chemical composition of the particles. We conclude that PLTP and H-TGL promote the formation of small, pre-beta-like HDL particles from HDL2.

Transfer of cholesterol between high density lipoproteins and cultured rat Sertoli cells

In the testes, the Sertoli cells are separated from the blood capillaries by the basement membrane, thereby excluding the passage of low density lipoproteins (LDLs) but allowing the passage of high density lipoproteins (HDLs). The present study examines first the capacity of Sertoli cells to uptake cholesterol from HDL and secondly the role of apolipoproteins (apo) A-I and E in cholesterol flux between HDL and cultured rat Sertoli cells. In the presence of HDL in cultured medium, rat Sertoli cells accumulated few amounts of esterified cholesterol. Incubation of [14C] cholesterol-labelled Sertoli cells with [3H]cholesterol-labelled HDL showed that the amount of cholesterol influx slightly exceeded its efflux, thus resulting in a net uptake of cholesterol from HDL to rat Sertoli cells. The amount of HDL-cholesterol converted to steroids by Sertoli cells was about 32% of influx. Uptake of cholesterol by Sertoli cells was three times higher with phospholipid-apo A-I vesicles and seven times higher with phospholipid- apo E vesicles than that with phospholipid vesicles without apolipoprotein. Phospholipid- apo A-I vesicles promoted cholesterol efflux at the same rate as native HDL and twice as efficiently as phospholipid- apo E vesicles. Thus, this study shows that rat Sertoli cells have the capacity to take up HDL-cholesterol for membrane renewal and steroid production mainly by apo E dependent pathways.