Transcriptional regulation of the apolipoprotein A-I gene. Species-specific expression correlates with rates of gene transcription.

Previous studies have shown that the abundance of apoA-I mRNA in liver and intestine correlates with a 2-3-fold species-specific difference in the plasma concentration of high density lipoprotein and apoA-I. In order to determine the role of gene transcription in regulating the tissue, steady state apoA-I mRNA abundance nuclear run-on assays were conducted using nuclei from two species of nonhuman primates. The transcriptional activity expressed as the ratio of apoA-I signal intensity to that for the constitutively expressed glyceraldehyde-3-phosphate dehydrogenase was calculated for seven individual animals in each of the two nonhuman primate species. The African green monkey showed a ratio of 4.18 +/- 0.35, and the cynomolgus monkey showed 2.03 +/- 0.13 (n = 7, p less than 0.004). To identify possible cis-acting elements that may be involved in transcriptional regulation of the apoA-I gene, a portion of the apoA-I regulatory region, corresponding to nucleotides -231 to +263 (where +1 is the start site of transcription), was isolated from both species using the polymerase chain reaction. The nucleotide sequence of this region was compared between monkey species, as well as with the same region from the apoA-I gene isolated from human genomic DNA. In this region, the African green monkey apoA-I gene showed 95% similarity, whereas the cynomolgus monkey showed 94% similarity to the human sequence. Although a high degree of sequence similarity was observed among all species, numerous sequence specific differences were noted in the first intron between the two primate species and between nonhuman and human primate sequences. Results from studies measuring relative promoter strength indicated that the African green monkey 5'-regulatory region had a consistently higher level of activity (1.4-3.0-fold) than the same region from the cynomolgus monkey. Interestingly, the African green monkey promoter also showed a significantly higher transcriptional activity than the human or rabbit promoter, suggesting the presence of a nonhuman primate specific cis-acting element(s) regulating apoA-I gene expression. These results demonstrate that a portion of the species-specific difference in apoA-I gene expression may be explained by sequence divergence in the 5'-regulatory region including exon/intron 1 of the apoA-I gene.