PHENOTYPIC EFFECTS OF THE FORCED EXPRESSION OF HNF4 AND HNF1-ALPHA ARE CONDITIONED BY PROPERTIES OF THE RECIPIENT CELL

Tagged versions of HNF4 or HNF1 alpha cDNAs in expression vectors have been introduced by transient and stable transfection into three cell lines of hepatic origin that all fail to express these two liver-enric hed transcription factors and hepatic functions. C2 and H5 cells are d edifferentiated rat hepatoma variants and WIF12-E cells are human fibr oblast-rat hepatoma hybrids with a reduced complement of human chromos omes. Transfectants were analyzed for the expression state of the endo genous genes coding for these transcription factors and for hepatic fu nctions. Each cell line showed a different response to the forced expr ession of the transcription factors, In C2 cells, no measurable effect was observed, either upon transitory or stable expression. H5 cells r eexpressed the endogenous HNF4 gene only upon transient HNF1 alpha tra nsfection, and the endogenous HNF1 alpha gene only in stable HNF4 tran sfectants, WIF12-E cells responded to the forced transient or stable e xpression of either HNF1 alpha or HNF4 by cross-activation of the corr esponding endogenous gene. In addition, the stable transfectants reexp ress HNF3 alpha and C/EBP alpha, as well as all of the hepatic functio ns examined. Hybrid cells similar to WIF12-E lad previously been obser ved to show pleiotropic reexpression of the hepatic phenotype in paral lel with loss of human chromosome 2, For the stable WIF12-E transfecta nts, it was verified that reexpression of the hepatic phenotype was no t due to loss of human chromosome 2. The demonstration of reciprocal c ross-regulation between HNF4 and HNF1 alpha in transient as well as st able transfectants implies that direct effects are involved.