Regulation of apolipoprotein E synthesis and mRNA by diet and hormones

Rats were fasted or fasted and refed simple purified diets so the effects of individual carbohydrates or fats could be studied. Freshly isolated hepatocytes from these animals were used to measure both apoE synthesis and mRNA levels so any changes in apoE synthesis that might occur without changes in its mRNA could be detected. Some of these experiments were done with both sexes. Both fasting and fasting and refeeding a 60% glucose fat-free diet significantly increased spoE synthesis. However, cyclic AMP is not likely to rapidly mediate the effect of fasting since dibutyryl cAMP slightly lowered (rather than increased) apoE synthesis and mRNA when injected into rats for 4.5 h. Dietary fat had no effect either in the absence of carbohydrate or when consumption of carbohydrate was constant in pair-fed rats. ApoE mRNA levels remained normal for 4 days in primary hepatocytes cultured in medium that had only amino acids as an energy source. Added hormones or fructose had no significant effect. Thus, only fasting and fasting and refeeding glucose were able to significantly change apoE synthesis or mRNA levels. Synthesis of apoE may be regulated to increase when apoE is secreted with very low density lipoprotein or when apoE in secreted high density lipoprotein is needed to acquire cholesteryl esters for the synthesis of bile salts and acids by liver.     

Regulation of reconstituted high density lipoprotein structure and remodeling by apolipoprotein E

Apolipoprotein E (apoE) enters the plasma as a component of discoidal HDL and is subsequently incorporated into spherical HDL, most of which contain apoE as the sole apolipoprotein. This study investigates the regulation, origins, and structure of spherical, apoE-containing HDLs and their remodeling by cholesteryl ester transfer protein (CETP). When the ability of discoidal reconstituted high density lipoprotein (rHDL) containing apoE2 [(E2)rHDL], apoE3 [(E3)rHDL], or apoE4 [(E4)rHDL] as the sole apolipoprotein to act as substrates for LCAT were compared with that of discoidal rHDL containing apoA-I [(A-I)rHDL], the rate of cholesterol esterification was (A-I)rHDL > (E2)rHDL approximately (E3)rHDL > (E4)rHDL. LCAT also had a higher affinity for discoidal (A-I)rHDL than for the apoE-containing rHDL. When the discoidal rHDLs were incubated with LCAT and LDL, the resulting spherical (E2)rHDL, (E3)rHDL, and (E4)rHDL were larger than, and structurally distinct from, spherical (A-I)rHDL. Incubation of the apoE-containing spherical rHDL with CETP and Intralipid(R) generated large fusion products without the dissociation of apoE, whereas the spherical (A-I)rHDLs were remodeled into small particles with the formation of lipid-poor apoA-I. In conclusion, i) apoE activates LCAT less efficiently than apoA-I; ii) apoE-containing spherical rHDLs are structurally distinct from spherical (A-I)rHDL; and iii) the CETP-mediated remodeling of apoE-containing spherical rHDL differs from that of spherical (A-I)rHDL.     

Rat granulosa cell apolipoprotein E secretion. Regulation by cell cholesterol

We have demonstrated previously that cultured rat ovarian granulosa cells synthesize and secrete apoE, and this production of apoE is increased by agents that stimulate protein kinase A (cyclic AMP-dependent enzyme) (for example, cholera toxin) and protein kinase C (Ca2+/phospholipid-dependent enzyme) (for example, 12-O-tetradecanoylphorbol-13-acetate, a phorbol ester). In the studies presented in this report, we have examined the effect of changes in cell cholesterol synthesis on the production of apoE by rat ovarian granulosa cells. Mevinolin, an inhibitor of hydroxymethylglutaryl (HMG)-CoA reductase (the rate-limiting enzyme in cholesterol synthesis), and 4,4,10 beta-trimethyl-trans-decal-3 beta-ol, an inhibitor of squalene cyclization, both attenuate the cholera toxin or 12-O-tetradecanoylphorbol-13-acetate stimulation of granulosa cell apoE secretion and apoE mRNA content in a dose-responsive manner. The inhibitory effect of mevinolin is reversed by the concomitant administration of mevalolactone, which provides the cells with the product of the reaction catalyzed by HMG-CoA reductase. Steroidogenesis per se has no effect on apoE production. Aminoglutethimide, which blocks the rate-limiting step in steroidogenesis, has no effect on apoE or apoE mRNA. The data indicate that products of HMG-CoA reductase (isoprenes, cholesterol and/or cholesterol metabolites) are required along with stimulators of protein kinases A and C, to regulate ovarian granulosa cell apoE production.