Rabbit and rat liver nuclei both contain proteins which bind to the regions controlling apolipoprotein A-I gene expression

The full induction of human apoprotein A-I gene expression by the experimental nephrotic syndrome in transgenic mice depends on cis-acting elements in the proximal 256 base-pair promoter region and the trans-acting factor early growth response factor 1

To identify molecular factors regulating apo A-I production in vivo, we induced in transgenic mice the experimental nephrotic syndrome, which results in elevated levels of HDL cholesterol (HDL-C), plasma apo A-I, and hepatic apo A-I mRNA. Human (h) apo A-I transgenic mice with different length 5' flanking sequences (5.5 or 0.256 kb, the core promoter for hepatic-specific basal expression) were injected with nephrotoxic (NTS) or control serum. With nephrosis, there were comparable (greater than twofold) increases in both lines of HDL-C, h-apo A-I, and hepatic h-apo A-I mRNA, suggesting that cis-acting elements regulating induced apo A-I gene expression were within its core promoter. Hepatic nuclear extracts from control and nephrotic mice footprinted the core promoter similarly, implying that the same elements regulated basal and induced expression. Hepatic mRNA levels for hepatocyte nuclear factor (HNF) 4 and early growth response factor (EGR) 1, trans-acting factors that bind to the core promoter, were measured: HNF4 mRNA was not affected, but that of EGR-1 was elevated approximately fivefold in the nephrotic group. EGR-1 knockout (EGR1-KO) mice or mice expressing EGR-1 were injected with either NTS or control serum. Levels of HDL-C, apo A-I, and hepatic apo A-I mRNA were lowest in nonnephrotic EGR1-KO mice and highest in nephrotic mice expressing EGR-1. Although in EGR1-KO mice HDL-C, apo A-I, and apo A-I mRNA levels also increased after NTS injection, they were approximately half of those in the nephrotic EGR-1-expressing mice. We conclude that in this model, basal and induced apo A-I gene expression in vivo are regulated by the trans-acting factor EGR-1 and require the same cis-acting elements in the core promoter.

Transgenic rabbits expressing human apolipoprotein A-I in the liver

Human apolipoprotein A-I (apo A-I) transgenic rabbits were created by use of an 11-kb genomic human apo A-I construct containing a liver-specific promoter. Five independent transgenic lines were obtained in which human apo A-I gene had integrated and was expressed. Plasma levels of human apo A-I ranged from 8 to 100 mg/dL for the founder and up to 175 mg/dL for the progeny. Rabbit apo A-I levels were substantially decreased in the transgenic rabbits. HDL cholesterol (HDL-C) levels were higher in two of the five transgenic rabbit lines than in controls (line 20 versus nontransgenic littermate, HDL-C = 80 +/- 7 versus 37 +/- 6 mg/dL; line 8 versus nontransgenic littermate, HDL-C = 54 +/- 16 versus 35 +/- 6 mg/dL). This resulted in less atherogenic lipoprotein profiles, with very low (VLDL + LDL-C)/HDL-C ratios. HDL size and protein and lipid compositions were similar between transgenic and littermate nontransgenic rabbits. However, a large amount of pre-beta apo A-I-containing lipoproteins was observed in the plasma of the highest human apo A-I expressor. Cell cholesterol efflux was evaluated with the incubation of whole serum from transgenic and control rabbits. Cell cholesterol efflux was highly correlated with HDL cholesterol, with apo A-I, and with the presence of pre-beta apo A-I-containing lipoproteins. These rabbits will be an extremely useful model for the evaluation of the effect of increased hepatic apo A-I expression on atherosclerosis.