CHARACTERIZATION OF MULTIPLE ENHANCER REGIONS UPSTREAM OF THE APOLIPOPROTEIN(A) GENE

Plasma concentrations of the atherogenic lipoprotein(a) (Lp(a)) are pr edominantly determined by inherited sequences within or closely linked to the apolipoprotein(a) gene locus, Much of the interindividual vari ability in Lp(a) levels is likely to originate at the level of apo(a) gene transcription, However, the liver-specific apo(a) basal promoter is extremely weak and does not exhibit common functional variations th at affect plasma Lp(a) concentrations, In a search for additional apo( a) gene control elements, we have identified two fragments with enhanc er activity within the 40-kilobase pair apo(a)-plasminogen intergenic region that coincide with DNase I-hypersensitive sites (DHII and DHIII ) observed in liver chromatin of mice expressing a human apo(a) transg ene. Neither enhancer exhibits tis sue specificity. DHIII activity was mapped to a BOG-base pair fragment containing nine DNase I-protected elements (footprints) that stimulates luciferase expression from the a po(a) promoter 10-15-fold in HepG2 cells, Binding of the ubiquitous tr anscription factor Spl plays a major role in the function of this enha ncer, but no single site was indispensable for activity, DHIII com pri ses part of the regulatory region of an inactive long interspersed nuc leotide element 1 retrotransposon, raising the possibility that retrot ransposon insertion can influence the regulation of adjacent genes. DH II enhancer activity was localized to a 180-base pair fragment that st imulates transcription from the aps(a) promoter 4-8-fold in HepG2 cell s. Mutations within an Spl site or either of two elements composed of direct repeats of the nuclear hormone receptor half-site AGGTCA in thi s sequence completely abolished enhancer function. Both nuclear hormon e receptor elements were shown to bind peroxisome proliferator-activat ed receptors and other members of the nuclear receptor family, suggest ing that this enhancer may mediate drug and hormone responsiveness.