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, w e induced in transgenic mice the experimental nephrotic syndrome, whic h 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 di fferent length 5' flanking sequences (5.5 or 0.256 kb, the core promot er for hepatic-specific basal expression) were injected with nephrotox ic (NTS) or control serum. With nephrosis, there were comparable (grea ter than twofold) increases in both lines of HDL-C, h-apo A-I, and hep atic h-apo A-I mRNA, suggesting that cis-acting elements regulating in duced apo A-I gene expression were within its core promoter. Hepatic n uclear extracts from control and nephrotic mice footprinted the core p romoter 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 factor s that bind to the core promoter, were measured: HNF4 mRNA was not aff ected, but that of EGR-1 was elevated approximately fivefold in the ne phrotic 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 non-nephrotic EGR1-KO mice and highest in nephrotic mice expressing EGR-1, Although in EGR1-KO m ice HDL-C, apo A-I, and apo A-I mRNA levels also increased after NTS i njection, they were approximately half of those in the nephrotic EGR-1 -expressing mice. We conclude that in this model, basal and induced ap o 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.