Extinction of α1-antitrypsin expression in cell hybrids is independent of HNF1α and HNF4 and involves both promoter and internal DNA sequences

Abstract

In rat hepatoma x fibroblast somatic cell hybrids, extinction of rat α1-antitrypsin (α1AT) gene expression is accompanied by the loss of liver-enriched transcription factors hepatocyte nuclear factor 1 (HNF1α) and hepatocyte nuclear factor 4 (HNF4). Previous analysis showed that forced expression of functional HNF1α failed to prevent extinction of the rat α1AT locus in cell hybrids. Here I show that ectopic co-expression of HNF1α plus HNF4 fails to prevent extinction of either rat or human α1AT genes in cell hybrids. A 40 kb human α1AT minilocus integrated into the rat genome is fully silenced in cell hybrids in the presence of transacting factors. The integrated α1AT promoter, but not a viral or ubiquitously active promoter, is repressed 35-fold in the cell hybrids. In addition, position effects also contributed to extinction of many integrated transgenes in a cell type-dependent manner. Finally, internal DNA sequences within the human α1AT gene contributed dramatically to the extinction phenotype, resulting in a further 10- to 30-fold reduction in α1AT gene expression in cell hybrids. Thus, multiple mechanisms contribute to silencing of tissue-specific gene expression of the α1AT gene in cell hybrids.