Liver endothelial cells promote LDL-R expression and the uptake of HCV-like particles in primary rat and human hepatocytes

Abstract

Low-density lipoprotein (LDL) is an important carrier of plasma cholesterol and triglycerides whose concentration is regulated by the liver parenchymal cells. Abnormal LDL regulation is thought to cause atherosclerosis, while viral binding to LDL has been suggested to facilitate hepatitis C infection. Primary hepatocytes quickly lose the ability to clear LDL during in vitro culture. Here we show that the coculture of hepatocytes with liver sinusoidal endothelial cells (LSEC) significantly increases the ability of hepatocytes to uptake LDL in vitro. LDL uptake does not increase when hepatocytes are cocultured with other cell types such as fibroblasts or umbilical vein endothelial cells. We find that LSECs induce the hepatic expression of the LDL receptor and the epidermal growth factor receptor. In addition, while hepatocytes in single culture did not take up hepatitis C virus (HCV)-like particles, the hepatocytes cocultured with LSECs showed a high level of HCV-like particle uptake. We suggest that coculture with LSECs induces the emergence of a sinusoidal surface in primary hepatocytes conducive to the uptake of HCV-like particles. In conclusion, our findings describe a novel model of polarized hepatocytes in vitro that can be used for the study of LDL metabolism and hepatitis C infection. L ow-density lipoprotein (LDL) is a plasma-carried particle whose lipid component includes cholesterol and triglycerides. 1 LDL originates from very low-density lipoprotein (vLDL) synthesized by the liver with apoprotein B-100. The vLDL is converted to LDL by an endothelial lipase, which releases free fatty acids, increasing the density of the particle to form LDL. Excess LDL is then taken up by hepatocytes through LDL receptor (LDL-R)-mediated endocytosis. 1 Improper hepatic clearance of LDL results in elevated plasma levels of LDL which is a serious risk factor for the development of atherosclerosis. 2 One of the first steps in atherosclerosis is the passage of LDL into the vascular wall. Once trapped in the vessel wall, LDL can undergo oxidation. 3 Oxidized LDL is no longer a ligand for the LDL receptor. 4 Accumulation of oxidized LDL and LDL aggregates in the vessel wall stimulates an inflammatory response causing endothelial injury, macrophage recruitment, foam cell formation, and smooth muscle cell proliferation. 3 Current therapeutic intervention includes administration of statins, which block cholesterol production and increase expression of the LDL-R by the liver parenchymal cells, 2 removing LDL from circulation. 5 In addition, liver sinusoidal endothelial cells (LSECs) remove oxidized LDL from the circulation via the scavenger cell receptor. 5 LDL-and LDL-R-mediated pathways have also been shown to play a role in hepatitis C virus (HCV) infection. 6 The HCV surface receptors, glycoproteins E1 and E2, have been shown to associate with LDL and vLDL. 7 The virus particle itself was shown to associate with the

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