TISSUE-SPECIFIC INHIBITION OF APOLIPOPROTEIN-B MESSENGER-RNA EDITING IN THE LIVER BY ADENOVIRUS-MEDIATED TRANSFER OF A DOMINANT-NEGATIVE MUTANT APOBEC-1 LEADS TO INCREASED LOW-DENSITY-LIPOPROTEIN IN MICE

APOBEC-1 is a catalytic subunit of an apolipoprotein B (apoB) mRNA edi ting enzyme complex. In humans it is expressed only in the intestine, whereas in mice it is expressed in both the Liver and intestine. APOBE C-1 exists as a spontaneous homodimer (Lau, P. P., Zhu, H. -J., Baldin i, A., Charnsangavej, C., and Chan, L. (1994) Proc. Natl. Acad. Sci. U .S.A. 91, 8522-8526). We tested the editing activity and dimerization potential of three different mouse APOBEC-1 mutants using in vitro edi ting activity assay and immunoprecipitation in the presence of epitope -tagged APOBEC-1. One catalytically inactive mutant, mu1 (H61K/C93S/C9 6S), that retains its capacity to dimerize with wild-type APOBEC-1 was found to inhibit the editing activity of the latter and was thus a do minant negative mutant. Two other inactive mutants that dimerized poor ly with APOBEC-1 failed to inhibit its activity. Intravenous injection of a mu1 adenovirus, Admu1, in C57BL/6J mice in vivo resulted in live r-specific expression of mu1 mRNA. On days 4 and 9 after virus injecti on, endogenous hepatic apoB mRNA editing was 23.3 +/- 5.0 and 36.8 +/- 5.7%, respectively, compared with 65.3 +/- 11 and 71.3 +/- 5.2%, resp ectively, for luciferase adenovirus-treated animals. Plasma apoB-100 a ccounted for 95 and 93% of total plasma apoB in Admu1 animals on days 4 and 9, respectively, compared with 78 and 72% in luciferase adenovir us animals. Plasma cholesterol on day 9 was 98 +/- 17 mg/dl in the mu1 -treated animals, substantially higher than phosphate-buffered saline- treated (57 +/- 9 mg/dl) or luciferase-treated (71 +/- 12 mg/dl) contr ols. Fast protein liquid chromatography analysis of mouse plasma showe d that the intermediate density/low density lipoprotein fractions in t he animals treated with the dominant negative mutant adenovirus were m uch higher than those in controls. We conclude that active APOBEC-1 fu nctions as a dimer and its activity is inhibited by a dominant negativ e mutant. Furthermore, apoB mRNA editing determines the availability o f apoB-100, which in turn limits the amount of intermediate density/lo w density lipoprotein that can be formed in mice. Liver-specific inhib ition of apoB mRNA editing is an important component of any strategy t o enhance the value of mice as a model for human lipoprotein metabolis m.