OVERPRODUCTION OF A TRUNCATED HEPATOCYTE NUCLEAR FACTOR-3 PROTEIN INHIBITS EXPRESSION OF LIVER-SPECIFIC GENES IN HEPATOMA-CELLS

Transcription of hepatocyte-specific genes requires the interaction of their regulatory regions with several nuclear factors, Among them is the hepatocyte nuclear factor 3 (HNF3) family, composed of the HNF3 al pha, HNF3 beta, and HNF3 gamma proteins, which are expressed in the li ver and have very similar fork head DNA binding domains. The regulator y regions of numerous hepatocyte-specific genes contain HNF3 binding s ites, We examined the role of HNF3 proteins in the liver-specific phen otype by turning off the HNF3 activity in well-differentiated mhAT3F h epatoma cells, Cells were stably transfected with a vector allowing th e synthesis of an HNF3 beta fragment consisting of the fork head DNA b inding domain without the transactivating amino- and carboxy-terminal domains, The truncated protein was located in the nuclei of cultured h epatoma cells and competed with endogenous HNF3 proteins for binding t o cognate DNA sites, Overproduction of this truncated protein, lacking any transactivating activity, induced a dramatic decrease in the expr ession of liver-specific genes, including those for albumin, transthyr etin, transferrin, phosphoenolpyruvate carboxykinase, and aldolase B, whereas the expression of the L-type pyruvate kinase gene, containing no HNF3 binding sites, was unaltered, Neither were the concentrations of various liver-specific transcription factors (HNF3, HNF1, HNF4, and C/EBP alpha) affected. In partial revertants, with a lower ratio of t runcated to full-length endogenous HNF3 proteins, previously extinguis hed genes were re-expressed, Thus, the transactivating domains of HNF3 proteins are needed for the proper expression of a set of liver-speci fic genes but not for expression of the genes encoding transcription f actors found in differentiated hepatocytes.