Tissue-specific expression of genes encoding apolipoprotein E and apolipoprotein A-I in rabbits.

We determined the site of synthesis of apolipoprotein (apo) E and apo-A-I in rabbit by measuring in vitro translational activity of their mRNAs from the liver and from the intestine. Poly(A+) RNA isolated from liver and intestinal epithelium of rabbits fed either a chow diet or a cholesterol-rich diet was translated in vitro in the rabbit reticulocyte lysate system using [35S] methionine as the labeled precursor. Newly synthesized apolipoproteins were immunoprecipitated with specific antisera and quantitated after electrophoresed on 10% polyacrylamide slab gels in the presence of 0.2% sodium dodecyl sulfate. The levels of liver apo-E and apo-A-I mRNAs from chow-fed rabbits are 0.41 and 0.002% of total translatable mRNA, respectively. The level of liver apo-A-I mRNA in the rabbit is approximately 500-fold lower than the reported level of apo-A-I mRNA in rat and human livers. Rabbit intestinal apo-E and apo-A-I mRNAs levels are 0.0036 and 0.67%, respectively. Our results indicate that in rabbits apo-E is synthesized primarily in the liver and that apo-A-I is synthesized primarily in the intestine. When rabbits are fed a cholesterol-rich diet, liver and intestinal apo-E in mRNA levels and intestinal apo-A-I mRNA levels are not changed. In contrast, the liver apo-A-I mRNA level increases 5-fold in response to the cholesterol-rich diet. However, because the intestinal liver apo-A-I mRNA level is so low, the 5-fold induction only increases liver mRNA levels to 2.7% of the corresponding intestinal apo-A-I mRNA level.