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

In rat hepatoma x fibroblast somatic cell hybrids, extinction of rat alpha 1-antitrypsin (alpha 1AT) gene expression is accompanied by the loss of liver-enriched transcription factors hepatocyte nuclear factor 1 (HNF1 alpha) and hepatocyte nuclear factor 4 (HNF4), Previous analysis showed that forced expression of functional HNF1 alpha failed to prevent extinction of the rat alpha 1AT locus in cell hybrids. Here I show that ectopic co-expression of HNF1 alpha plus HNF4 fails to prevent extinction of either rat or human alpha 1AT genes in cell hybrids. A 40 kb human alpha 1AT minilocus integrated into the rat genome is fully silenced in cell hybrids in the presence of transacting factors, The integrated alpha 1AT 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 alpha 1AT gene contributed dramatically to the extinction phenotype, resulting in a further 10- to 30-fold reduction in alpha 1AT gene expression in cell hybrids, Thus, multiple mechanisms contribute to silencing of tissue-specific gene expression of the alpha 1AT gene in cell hybrids.