Regulation of apolipoprotein E secretion by high density lipoprotein₃ in mouse macrophages

Recent reports from this laboratory indicate that exposure of cholesterol-loaded macrophages to high density lipoprotein₃ (HDL₃) stimulates not only cholesterol efflux, but also results in a two- to threefold increase in apoE accumulation in the media (Dory, L., 1989. J. Lipid Res. 30: 809-816). The present experiments demonstrate that the effect of HDL₃, and to a lesser extent HDL₂, on apoE secretion is specific, concentration-dependent, and may required interaction with the HDL receptor. Very low density lipoproteins (VLDL) and low-density lipoproteins (LDL) fail to specifically stimulate apoE secretion by cholesterol-loaded macrophages. The effect of HDL₃ is maximal at 25-50 μg/ml (0.26-0.52 μM) and can be totally abolished by mild nitrosylation (with 3 mM tetranitromethane (TNM)). Data are also presented to indicate that the increased rate of apoE secretion in the presence of HDL₃ is not due to a 'protective' effect of this lipoprotein on possible proteolytic degradation or cellular reuptake of apoE secreted into the media. The stimulatory effect of HDL on apoE secretion can be clearly dissociated from cholesterol efflux; HDL stimulates apoE secretion from oxysterol-treated cells in the absence of measurable cholesterol efflux, while TNM-HDL promotes substantial cholesterol efflux from cholesterol-loaded cells but has no effect on apoE secretion. The kinetics of apoE synthesis and secretion, determined in short-term labeling studies, demonstrate that under all experimental conditions examined a substantial portion of cellular apoE is not secreted. Furthermore, in cholesterol-loaded cells HDL₃ increases apoE secretion essentially by diversion of a greater portion of cellular apoE pool for secretion. While HDL₃ has no effect on the rate of apoE synthesis, cellular apoE turns over two-fold faster in cells incubated in the presence of HDL₃ than in its absence of (t( 1/2 ) = 11 ± 2 and 22 ± 4 min, respectively), an observation corresponding well with the changes in the rates of apoE secretion under similar conditions. The HDL₃-mediated increase in apoE secretion by cholesterol-loaded macrophages suggests another mechanism by which HDL exerts a protective effect in the development of atherosclerosis; increased contribution to the metabolic pool of apoE by peripheral tissues may lead to a more effective clearance of peripheral cholesterol by the liver (reverse cholesterol transport).