A BASE SUBSTITUTION IN THE PROMOTER ASSOCIATED WITH THE HUMAN HAPTOGLOBIN-2-1 MODIFIED PHENOTYPE DECREASES TRANSCRIPTIONAL ACTIVITY AND RESPONSIVENESS TO INTERLEUKIN-6 IN HUMAN HEPATOMA-CELLS

An A-to-C base substitution at nucleotide position -61 in the promoter region of the human haptoglobin gene (Hp) has been shown to be strong ly associated with the haptoglobin 2-1 modified (Hp2-1 mod) phenotype. In order to investigate whether this base substitution is the cause o f reduced expression of the Hp2 allele relative to the Hp1 allele in i ndividuals with the Hp2-1 mod phenotype, we used the chloramphenicol a cetyl transferase (CAT) expression system to evaluate promoter functio n. In HepG2 cells, which normally express their endogenous haptoglobin genes, CAT plasmid constructs with the -61C base change in the promot er had about 10-fold-lower transcriptional activity after transfection than did the Hp control construct. The -61C substitution also rendere d the construct unresponsive to treatment by interleukin-6 after trans fection into Hep3B2 cells, which normally do not express haptoglobin b ut do so in response to stimulation by acute-phase reactants. In addit ion, two base substitutions, T to A and A to G, at positions -104 and -55G, respectively, in the promoter region of the Hp1 allele, are also associated with the Hp2-1 mod phenotype. CAT constructs with both sub stitutions (-104A-55G) and with one substitution (-55G) showed activit y similar to that in the Hp control when transfected into both HepG2 a nd Hep3B2 cells, although interleukin-6 induction was less than with t he Hp control construct. These results further support the hypothesis that the Hp2-1 mod phenotype results, in part, from the -61C mutation in the promoter region of the Hp2 gene.