Differential regulation of gene activity and chromatin structure within the human serpin gene cluster at 14q32.1 in macrophage microcell hybrids

The human gene encoding α1-antitrypsin (α1AT, gene symbol PI) is highly expressed in the liver and in cultured hepatoma cells and, to a lesser extent, in macrophages, where transcription originates from a separate upstream promoter. α1AT maps to a region of human chromosome 14q32.1 that includes a related serine protease inhibitor (serpin) gene that encodes corticosteroid-binding globulin (CBG). We recently reported the chromatin organization of this ~130 kb region, as defined by DNase I hypersensitive sites (DHSs) and matrix-attachment regions, in expressing and non-expressing cells. Furthermore, we demonstrated that transfer of human chromosome 14 from non-expressing fibroblasts to rat hepatoma cells resulted in activation of both α1AT and CBG transcription and gene activation was accompanied by long range chromatin reorganization of the entire region. In this study, we transferred human chromosome 14 from fibroblasts to mouse macrophages and documented activation of α1AT but not CBG gene expression. RT–PCR experiments indicated that transcription of the human α1AT gene in the microcell hybrids initiated at the macrophage promoter. Furthermore, DHS mapping experiments revealed a distinctive chromatin configuration of the locus that resembled the structure found in human macrophage-like cell lines, with many DHSs around α1AT but few in CBG. Thus, mouse macrophage cell lines will provide a useful cell type to study the effects of targeted modifications of the human α1AT–CBG locus on the regulation of cell-specific gene activity and chromatin structure.