Allylisopropylacetamide restricts expression of β minor globin gene in Friend cells

Abstract

Two structural genes, β major and β minor, are present at the β-globin locus (Hbb) of a variety of mice phenotypically characterised by adult haemoglobin of the diffuse type¹. These genes may be the result of duplication of an ancestral gene², but there is increasing evidence that their expression is independently regulated. For example, in erythrocytes of the adult DBA/2 mouse, the ratio of the two nonallelic variant gene products is 4:1 in favour of the β-major chain whereas in Friend erythroleukaemia cells, clone 745, derived from the same mouse strain, the ratio varies from 3 to 0.6, depending on the inducer of erythroid differentiation used^3–6. We reported previously that treatment of clone 745 cells for 6 d with 1×10⁻⁴ M haemin, an inducer of haemoglobin synthesis⁷, induces synthesis of the β minor, but not the β major globin chain⁵. This observation suggested the possibility that intracellular levels of haem (the reduced form of haemin) might act physiologically to selectively control the expression of one of the two β globin genes. To test this hypothesis, we determined the effect of allylisopropylacetamide (AIA) on the relative amounts of β globin chains present in induced erythroleukaemia cells. AIA is a strong inducer of δ-aminolevulinic acid synthetase in rat hepatocytes⁸ and in mouse erythroleukaemia cells⁹. Induction of the enzyme is due to breakdown by the drug of the haem moiety of the haemoproteins¹⁰. We report here that AIA treatment of Friend cells induced to differentiate by dimethylsulphoxide (DMSO) causes a reduction in the intracellular haem concentration and at the same time a marked decrease in the amount of the β minor globin chain. The ratio of the two β chains thus becomes very close to the ratio observed in normal erythrocytes of adult DBA/2 mouse.