Familial combined hyperlipidemia plasma stimulates protein secretion by HepG2 cells

Abstract

The aim of this study was to evaluate whether soluble factors in plasma of familial combined hyperlipidemia (FCHL) patients affect hepatic protein secretion. Cultured human hepatocytes, i.e., HepG2 cells, were incubated with fasting plasma (20%, v/v, in DMEM) from untreated FCHL patients or normolipidemic controls. Overall protein secretion was 10-15% higher after incubation with FCHL plasma. This was specifically caused by an increase in four secreted proteins, with estimated sizes of 240, 180, 120, and Ͻ 40 kD ( P Ͻ 0.001, P Ͻ 0.006, P Ͻ 0.002, P Ͻ 0.02, respectively). The 240 kD protein in the secretion proteome was identified as fibronectin by mass spectrometry. Plasma fibronectin concentrations were elevated in FCHL patients, confirming biological relevance of these data. Overall protein secretion by HepG2 cells correlated with concentrations of triglycerides ( r ‫؍‬ 0.61, P Ͻ 0.001) in the applied plasma samples. VLDL ؉ IDL isolated from FCHL patients, induced a higher protein secretion than lipoproteins isolated from controls ( P Ͻ 0.001). Remarkably, secretion of apoB, the structural protein of VLDL, was stimulated to a similar extent by FCHL and control plasma. Familial combined hyperlipidemia (FCHL) is a common dyslipidemia with a strong genetic component. The prevalence of FCHL in the population is 1-2% and FCHL is estimated to cause 10-20% of premature coronary heart disease (CHD) (1). Metabolic disturbances in FCHL include overproduction of atherogenic apolipoprotein B (apoB)-containing lipoproteins (i.e., VLDL), delayed clearance of lipoproteins, increased plasma apoB concentrations, increased free fatty acid (FFA) fluxes, and insulin resistance. The molecular mechanism(s) that underlie these metabolic abnormalities are not yet known. It has been suggested that increased plasma FFA fluxes, potentially induced by insulin resistance, may increase hepatic secretion of apoB containing lipoproteins. However, available data on the direct effects of increased FFA fluxes on hepatic apoB production in normal human subjects are limited and apparently conflicting (2, 3). These data do not yet allow defenitive conclusions on the hepatic abnormality in FCHL.The liver plays a central role in the development of atherosclerosis (4). It is an important source of vaso-active compounds as well as atherogenic, apoB containing lipoproteins. Current knowledge on intrahepatic assembly of apoB containing lipoproteins mainly derives from studies in laboratory animals and cells in culture (5). Following intracellular association of lipid with apoB, the precursor lipoproteins fuse with lumenal triglyceride droplets to form mature VLDL that is secreted from the cells (6). The scarce availability of hepatocytes from human FCHL patients renders it not feasible to directly study the role of the liver-specific pathways in FCHL in vitro. It is presently unresolved whether the hepatic hypersecretion of lipoproteins that is observed in FCHL is a process inherent to metabolic abnormalities in the liver, or may be driven by s...

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