Tissue-specific, developmental and nutritional regulation of the gene encoding the catalytic subunit of the rat apolipoprotein B mRNA editing enzyme: functional role in the modulation of apoB mRNA editing.

Apolipoprotein B (apoB) mRNA editing, a posttranscriptional site-specific cytidine deamination reaction, is mediated by a protein complex of which the catalytic component (REPR) has recently been cloned. REPR mRNA was demonstrated by RNase protection at highest abundance in small intestine and colon but the transcript was detectable in all tissues examined including kidney, spleen, lung, liver, and ovary. ApoB mRNA was found predominantly in the liver and small intestine but low levels were detected in all adult tissues examined and found to be variably (29-86% TAA) edited. In addition, S100 extracts prepared from spleen and kidney were competent to edit an apoB RNA template in vitro, suggesting that the entire apoB mRNA editing complex is present and functionally active in these tissues. In situ hybridization demonstrated REPR mRNA to be distributed along the entire villus-crypt axis, while apoB mRNA distribution did not extend into the crypts. In the liver, both apoB and REPR mRNA were detected in all cells of the hepatic lobule without an apparent gradient of expression. REPR mRNA was found in the red pulp of the spleen and in the superficial crypt cells of the colon. This distribution of REPR mRNA was recapitulated by immunocytochemical localization of the protein within these tissues. Finally, the developmental and nutritional modulation of REPR was examined in relation to endogenous apoB mRNA editing. Small intestinal apoB mRNA editing was found to undergo a developmentally regulated increase beginning at gestational day 20, preceding a developmental increase in REPR mRNA abundance. Additionally, hepatic and kidney apoB mRNA editing both revealed a temporal dissociation from alterations in REPR mRNA abundance. By contrast, adult rats subjected to fasting and refeeding a high carbohydrate diet, demonstrated concordant modulation of endogenous apoB mRNA editing and REPR mRNA abundance (r = 0.92, P < 0.001). Taken together, the data demonstrate that REPR and other components of the rat apoB mRNA editing complex are widely distributed and undergo distinct developmental and metabolic regulation that interact to regulate apoB mRNA editing in a tissue-specific manner.