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Complementation of apolipoprotein B mRNA editing by human liver accompanied by secretion of apolipoprotein B48
F Giannoni1, D K Bonen, T Funahashi
1Department of Medicine, University of Chicago, Illinois 60637.
Abstract:
Mammalian small intestine secretes a truncated apolipoprotein B (apoB48) species as a result of tissue-specific post-transcriptional RNA editing. The human liver, by contrast, contains only unedited apoB mRNA and secretes only apoB100. We have recently isolated a cDNA clone from rat small intestine which encodes an apoB mRNA editing protein, REPR (Teng, B., Burant, C.F., and Davidson, N.O. (1993) Science 260, 1816-1819). The current study demonstrates that homogenates of Xenopus oocytes expressing REPR confer editing ability upon S100 extracts prepared from human liver when tested on a synthetic apoB RNA template in vitro. Transfection of REPR into HepG2 cells resulted in editing of endogenous apoB mRNA and the appearance of an apoB48-like protein in the media. Extracts prepared from these transfected cells edit mammalian apoB RNA templates when incubated alone and with enhanced efficiency in the presence of chicken intestinal S100 extracts. The results suggest that human liver expresses factor(s) which are critical to apoB mRNA editing and which allow functional complementation of REPR in vivo.
Insights
Researchers identified a key protein, REPR, essential for apolipoprotein B (apoB) mRNA editing in the small intestine. This protein, when expressed in liver cells, enables the production of apoB48, a truncated form of apoB.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- Mammalian small intestine uniquely produces truncated apolipoprotein B (apoB48) via tissue-specific RNA editing.
- Human liver synthesizes only unedited apoB mRNA, resulting in apoB100 secretion.
- A rat small intestine cDNA clone encoding an apoB mRNA editing protein, REPR, was previously identified.
Purpose of the Study:
- To investigate the role of REPR in apoB mRNA editing.
- To determine if REPR can confer editing ability to human liver extracts.
- To examine the effects of REPR expression on endogenous apoB mRNA editing in human liver cells.
Main Methods:
- Expressed REPR in Xenopus oocytes and tested its effect on human liver S100 extracts using a synthetic apoB RNA template in vitro.
- Transfected HepG2 cells (human liver cell line) with REPR.
- Analyzed apoB mRNA editing and protein products in the media of transfected cells.
Main Results:
- Xenopus oocyte homogenates expressing REPR conferred editing ability to human liver S100 extracts.
- REPR transfection into HepG2 cells led to endogenous apoB mRNA editing and secretion of an apoB48-like protein.
- Extracts from REPR-transfected cells demonstrated editing activity on mammalian apoB RNA templates, enhanced by chicken intestinal extracts.
Conclusions:
- Human liver possesses factors crucial for apoB mRNA editing.
- REPR can functionally complement these liver factors in vivo.
- These findings suggest a conserved mechanism for apoB mRNA editing involving REPR and liver-specific factors.