Journal of Lipid Research, Vol 37, 2324-2331, Copyright © 1996 by Lipid Research, Inc.
Identification of promoter sequences in the 5' untranslated region of the baboon apolipoprotein[a] gene
JE Hixson, C Jett and S Birnbaum
Department of Genetics, Southwest Foundation for Biomedical Research, San Antonio, TX 78228-0147, USA.
Like humans, baboons possess apolipoprotein[a] (apo[a]), a unique protein
component of the atherogenic lipoprotein [a] (Lp[a]) particle. Baboon
apo[a] also exhibits extensive variation with respect to size and serum
levels. In this report, we have cloned the 5' flanking region of the baboon
apo[a] gene (I isoform) and performed promoter mapping studies to identify
sequences that control apo[a] transcription. The sequence of the baboon
apo[a] 5' flanking region is similar to the human gene, and contains two
Alu repeats that distinguish the apo[a] gene from plasminogen and other
apo[a]-like genes. The transcription start site for the baboon apo[a]gene
is located 85 bp upstream from the major start site for the human apo[a]
gene. For promoter mapping studies, we constructed two sets of deletion
clones (5' to 3' and 3' to 5') in luciferase reporter plasmids for
transfection of hepatic cell lines (HepG2 and HUh7). These experiments
showed that the 5' untranslated region (5' UTR) contains a positive
promoter element with 85% identity to the consensus binding site for
hepatocyte nuclear factor 1 alpha (HNF-1 alpha), and a negative element
that is functional in HepG2 cells, but not Huh7 cells. Transfection assays
with HeLa cells showed that the positive promoter element acts in an
hepatocyte- specific manner. We also cloned the 5' flanking region from a
baboon carrying a null allele that produced no detectable hepatic
transcripts or serum isoforms in vivo. Surprisingly, the 5' flanking
regions of the null allele possessed a promoter that was functional in
transfection assays. We conclude that the baboon apo[a] gene 5'UTR contains
hepatocyte-specific promoter elements, but that other unknown sequences
must influence apo[a] expression in vivo.