Tissue-specific inhibition of apolipoprotein B mRNA 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 editing enzyme complex. In humans it is expressed only in the intestine, whereas in mice it is expressed in both the liver and intestine. APOBEC-1 exists as a spontaneous homodimer (Lau, P. P., Zhu, H.-J., Baldini, 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 editing activity assay and immunoprecipitation in the presence of epitope-tagged APOBEC-1. One catalytically inactive mutant, mu1 (H61K/C93S/C96S), 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 dominant negative mutant. Two other inactive mutants that dimerized poorly with APOBEC-1 failed to inhibit its activity. Intravenous injection of a mu1 adenovirus, Admu1, in C57BL/6J mice in vivo resulted in liver-specific expression of mu1 mRNA. On days 4 and 9 after virus injection, 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%, respectively, for luciferase adenovirus-treated animals. Plasma apoB-100 accounted for 95 and 93% of total plasma apoB in Admu1 animals on days 4 and 9, respectively, compared with 78 and 72% in luciferase adenovirus 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) controls. Fast protein liquid chromatography analysis of mouse plasma showed that the intermediate density/low density lipoprotein fractions in the animals treated with the dominant negative mutant adenovirus were much higher than those in controls. We conclude that active APOBEC-1 functions as a dimer and its activity is inhibited by a dominant negative mutant. Furthermore, apoB mRNA editing determines the availability of apoB-100, which in turn limits the amount of intermediate density/low density lipoprotein that can be formed in mice. Liver-specific inhibition of apoB mRNA editing is an important component of any strategy to enhance the value of mice as a model for human lipoprotein metabolism.
Oka
K
K
Department of Cell Biology, Baylor College of Medicine, Houston, Texas 77030-3498, USA.
Kobayashi
K
K
Sullivan
M
M
Martinez
J
J
Teng
B B
BB
Ishimura-Oka
K
K
Chan
L
L
D000071479:APOBEC-1 Deaminase; D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D001055:Apolipoproteins B / Q000235:genetics*; D003564:Cytidine Deaminase / Q000235:genetics*; D018014:Gene Transfer Techniques; D006801:Humans; D008077:Lipoproteins, LDL / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D051379:Mice; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D017393:RNA Editing / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics*; D000098830:AICDA (Activation-Induced Cytidine Deaminase)
D001055:Apolipoproteins B; D008077:Lipoproteins, LDL; D012333:RNA, Messenger; D000098830:AICDA (Activation-Induced Cytidine Deaminase); D000071479:APOBEC-1 Deaminase; C000605089:APOBEC1 protein, human; C000605090:Apobec1 protein, mouse; D003564:Cytidine Deaminase