Complementation of apolipoprotein B mRNA editing by human liver accompanied by secretion of apolipoprotein B48

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.