METHYLAMINE-TREATED LOW-DENSITY LIPOPROTEINS ELICIT DIFFERENT RESPONSES IN HEPG2 CELLS AND MACROPHAGES

Recent results from this laboratory have demonstrated the existence of labile thiolester bonds in apolipoprotein B (ApoB). Thiolester bonds can be cleaved with nucleophiles such as methylamine, resulting in con formational change. The purpose of this study was to explore whether t he cellular interactions would be altered after methylamine treatment of low density lipoproteins (LDL). Human hepatoma cells, HepG2, and hu man monocyte derived macrophages were used for these studies. Fresh LD L were incubated with methylamine under mild alkaline conditions under N2 and with preservatives for 24 h. The methylamine-treated LDL showe d particle size and net charge identical to fresh native LDL. In addit ion, no oxidative modification of LDL occurred under the experimental conditions. The methylamine-treated LDL were indistinguishable from na tive LDL in HepG2 cells as judged by binding, degradation, cholesterol accumulation and de novo sterol synthesis. However, methylamine-treat ed LDL caused an increased accumulation of cholesteryl esters in macro phages which was comparable to the accumulation caused by acetylated L DL. Dual color digital imaging fluorescence microscopy revealed no com petition between acetylated and methylamine-treated LDL, suggesting th at the excessive uptake of methylamine-treated LDL was not mediated by the 'scavenger' receptor. The increased accumulation of cholesteryl e ster in macrophages also did not appear to stem from the classical LDL receptor. These results suggest that a new receptor binding domain is exposed due to the conformational change upon treatment of LDL with m ethylamine.