Studies of Gene Transcription and Translation in Regenerating Rat Liver

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Abstract
Specific transcriptional and translational products associated with regenerating liver were analyzed by differential hybridization to a cDNA library and by two–dimensional electrophoresis of hepatic proteins, respectively. Comparisons of approximately 800 soluble and 800 particulate liver proteins from normal and 70% partially hepatectomized Fischer rats resulted in the identification of only three apparently unique polypeptides in 70% partially hepatectomized livers, although many quantitative changes were observed. A subset of these quantitative changes were also observed after sham operation. A cDNA library was generated from polyadenylated RNA isolated 18 hr post–70% partial hepatectomy. Comparative analysis of 6,000 transformantswith single–stranded cDNA probes prepared from 18 hr post–70% partial hepatectomy and sham–operated animals identified three clones whose sequences were preferentially expressed 4– to 6–fold 18 hr post–70%partial hepatectomy. Southern blot analysis of one clone, REG–A, showed no homology to albumin, α–fetoprotein, three different forms of cytochrome P–450, ornithine decarboxylase, globin, or to a putative tumor promotion associated gene called PRO–2. A single, REG–A specific 2.5 kb band was identified by Northern blot analysis of liver samples. REG–A expression was increased 2–fold 18 hr postsham operation; 4–fold 18 hr post–70% partialhepatectomy and following chronic 2,3,7,8–tetrachlorodibenzo–p–dioxin or phenobarbital treatment. REG–A expression returned to control levels 1 week after 70% partial hepatectomy. Furthermore, expression of REG–Awas reduced in chemically induced preneoplastic nodules and in primary and transplantable hepatomas. Hybrid selection studies indicated that the REG–A sequence selected a mRNA(s) species, that in an in vitro translation assay, produced two major polypeptides of 21,000 and 25,000 molecular weight with a pI of 6.9. Thus, these data support the hypothesis that liver regeneration is characterized byquantitative changes in genes normally expressed at low levels in the Go hepatocyte andis not the result of major qualitative changes in gene expression.