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The goal of this study is to explain the molecular basis of the marked deinduction of Xenopus albumin synthesis and secretion accompanying the activation of vitellogenin genes by estrogen. We have characterized by restriction analysis, DNA sequencing and hybrid-selected translation of mRNA, a cloned cDNA specifying the two 74-kDa albumins which constitute the predominant circulating form of albumin in Xenopus laevis. Using this recombinant DNA plasmid as a hybridization probe, we have determined the steady-state levels of albumin mRNA, the rate of transcription of the two 74-kDa albumin genes and the stability of the mRNA in male and female Xenopus hepatocytes in vivo and in primary cell cultures following estrogen treatment. In both whole liver and cultured hepatocytes estradiol caused a rapid drop in the steady-state levels of 74-kDa albumin mRNAs, which was reversed spontaneously in the continued presence of the hormone. The concentration of albumin mRNA was substantially higher in male than in female hepatocytes, the hormonal effect being more marked in male than in female hepatocytes. The decrease in steady-state levels of mRNA was anticipated in male hepatocytes by a 70% inhibition of rate of transcription of albumin genes within 2 h of exposure to estradiol, as measured by run-off transcription in liver nuclei isolated from animals treated in vivo or by determining the absolute transcription rate in cell cultures. In the latter the diminished transcription rate returned to normal within 12 h in the continued presence of the hormone. Estradiol caused a threefold destabilization of albumin mRNA in both male and female hepatocyte cultures to t 1/2 = 3 h and 2 h respectively. The combined effects on rate of or transcription and mRNA stability largely explain the changes in the steady-state levels of mRNA caused by hormone administration. Comparison of the kinetics of transcription rates of vitellogenin and albumin genes in vivo and in vitro reveals a striking reciprocity in the selective activation of the inducible genes and deinduction of the constitutively expressed genes at the early stages of response of Xenopus hepatocytes to estrogen.
Wolffe et al. (Fri,) studied this question.