Endotoxin promotes preferential periportal upregulation of VLDL secretion in the rat liver

Abstract

Zonation affects liver parenchymal cell function and metabolism as well as nonparenchymal cell activation, but whether VLDL production is zonated has yet to be elucidated. Infection induces enhanced VLDL secretion by the liver. Ex vivo studies were undertaken to examine the liver heterogeneity for VLDL formation and secretion and their in vivo response to endotoxin. Highly pure periportal (PP) and perivenous (PV) hepatocytes were isolated from fasted lipopolysaccharide-treated, fasted, and fed rats. They were used to assess their capacity to release VLDL-apolipoprotein B (apoB) and lipid classes in relation to de novo lipid synthesis and the expression of genes crucial to VLDL production. Despite the common superior ability of PP hepatocytes for lipid release and zonal differences in lipid synthesis, zonated secretion of VLDL particles was observed in septic but not in normal fed or fasted livers. The endotoxininduced apoB secretion was more accentuated in PP hepatocytes; this was accompanied by a preferential PP increase in apoB and microsomal triglyceride transfer protein mRNA levels, whereas lipogenesis indicators were, if anything, similarly modified in hepatocytes of either acinar origin. We conclude that PP and PV hepatocytes exhibited similar capabilities for VLDL formation/secretion in normal conditions; however, the endotoxic pressure did zonate periportally. Septic hyperlipidemia and hyperlipoproteinemia have been postulated to be components of the innate defense against infection in humans and experimental models (1). The insertion of the lipid A component of Gram-negative bacterial lipopolysaccharide (LPS; or endotoxin) in the phospholipid layer of lipoproteins diminishes toxicity (2), as it enables endotoxin clearance and excretion by hepatic parenchymal cells (3). Septic hypertriglyceridemia is mainly caused by the accumulation of VLDL particles in the plasma (4) attributable to many changes in their metabolism (5, 6; see recent review in 7), including increased secretion by the liver (8).VLDL particles are assembled at the endoplasmic reticulum (ER) of hepatocytes in a process that depends absolutely upon the cellular availability of lipids, such as triglycerides (TGs), phospholipids, cholesterol, and cholesteryl esters, to correctly translate and translocate apolipoprotein B (apoB) to the lumen. ApoB-100 and apoB-48 are the essential apolipoproteins for VLDL assembly in the rat (9). They are expressed in excess, and the protein that is not lipidated to a sufficient extent is targeted to and eventually degraded by proteasomes (10). Correct apoB lipidation and translocation are controlled by the microsomal triglyceride transfer protein (MTP), whose binding and lipid transfer activities are probably the major determinant in VLDL secretion (11-13). The assembly of VLDL is a complex process that includes two major lipidation steps. A relatively small, dense, TG-poor lipoprotein particle is formed in the ER by cotranslational loading of apoB chains with lipid. Then, bulk lipidation and final m...

High insulin levels are required for FAT/CD36 plasma membrane translocation and enhanced fatty acid uptake in obese Zucker rat hepatocytes

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A subset of dysregulated metabolic and survival genes is associated with severity of hepatic steatosis in obese Zucker rats

“…After 2 h, blood was collected and VLDLs (d < 1.006 g/ml) were isolated and characterized ( 23 ). The VLDL-TG and VLDL-apoB post-Triton accumulation in the serum serves as a measure of VLDL production rate.…”

“…Rat hepatocytes were isolated as previously described ( 23 ) and used immediately for transcriptomics or functional studies. The hepatocyte suspensions had >80% viability and were >99% nonparenchymal cell free.…”

Hepatic denervation impairs the assembly and secretion of VLDL‐TAG

“…The distribution of these cells may contribute to zonation of VLDL assembly, as in endotoxin-treated rats the increase in VLDL secretion occurs in periportal hepatocytes and is accompanied by higher apoB and MTP levels. 58 During fasting, the secretion of apoB was found to be decreased 58 and this was more pronounced in periportal hepatocytes, but the synthesis of lipids dominated perivenously. Another study 59 also showed that the zonation of oxidation of fatty acids is altered depending on the physiological status of the animal and that AMP activated protein kinase distribution, which is greater in the perivenous region, may also contribute 60 to the metabolic heterogeneity observed.…”