Regulation by estrogen receptor of vitellogenin gene transcription in Xenopus hepatocyte cultures.

We have used primary cell cultures of hepatocytes from male or female Xenopus laevis to study the mechanisms by which estrogen induces vitellogenin gene transcription and how primary exposure to estrogen renders cells more responsive to secondary stimulation. We have characterized the estrogen receptor in hormonally naïve cells and in hepatocytes treated with estrogen under a variety of conditions. Under all conditions the receptor has a Kd congruent to 4 X 10(-10) M. Hormonally naïve male cells contain 300 binding sites whereas female cells or male cells previously exposed to estradiol exhibit 6-7-fold higher levels. In parallel cultures, the absolute rate of vitellogenin gene transcription was determined by hybridization of newly synthesized RNA pulse-labelled with [3H]uridine to cloned Xenopus vitellogenin cDNA. Naïve male cells on primary stimulation with estradiol synthesized vitellogenin mRNA at an average rate of approximately 150 moles/cell/h compared to 1200 moles/cell/h for cells previously exposed to estrogen, thus bearing a close correlation with receptor number. Furthermore, we show that the kinetics of the induced up-regulation of receptor exactly parallel those of the increase in the rate of vitellogenin gene transcription upon secondary hormonal stimulation following various periods of primary exposure to estrogen. Addition of cycloheximide to cell cultures during primary estrogen treatment abolishes both receptor up-regulation and increased rate of vitellogenin gene transcription on secondary stimulation. In addition, primary treatment with the antiestrogen tamoxifen prevents both receptor up-regulation and an enhanced rate of transcription or accumulation of vitellogenin mRNA on secondary hormonal exposure. These results demonstrate that estrogen treatment of male Xenopus hepatocytes results in the rapid up-regulation of its own receptor to female levels via new receptor synthesis, and that receptor number is rate-limiting in vitellogenin gene transcription.