Uptake of HDL Subfractions by Lymphocytes through the LDL Receptor

Abstract

When endogenous synthesis, of cholesterol is blocked by an HMG-CoA reductase inhibitor, proliferation of Phytohemagglutinin (PHA)-stimulated human lymphocytes is markedly inhibited, provided an exogenous source of cholesterol is not supplied. Based on this principle, Lipsky et al. developed a simplified method to assay functional LDL receptors. However, it is still a controversial point whether HDL can be taken up by the cell via an LDL receptor. We modified Lipsky's method by using pravastatin (HMG-CoA reductase inhibitor) and lipoprotein-depleted fetal calf serum, and examined the capacity of LDL and HDL subfractions to supply cholesterol to lymphocytes. Peripheral blood lymphocytes were isolated from venous blood of 10 healthy adults and from l patient with FH homozygote. Human LDL and HDL were isolated from the serum of normal fasting adults by ultracentrifugation, and HDL_2b and HDL_2a fractions which contained Apo E but not Apo B were prepared using a heparin-Sepharose column. The ability of LDL cholesterol to reverse pravastatin-mediated inhibition of normal lymphocyte responses was found to be concentration dependent, and the suppression ability of pravastatin was reversed by the HDL_2b and HDL_2a fractions. However, the effect of these fractions was weaker than that of LDL. HDL₃ showed no capacity to recover the cells from suppression. On the other hand, lymphocytes lacking normal LDL receptors from FH homozygote could not obtain cholesterol from LDL or any subfractions of HDL. Therefore, our result suggests that both HDL_2b and HDL_2a can be taken up by lymphocytes via LDL receptors.