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Estrogen activates transcription of the vitellogenin genes in livers of male Xenopus laevis. We have examined the conformation of the vitellogenin genes in chromatin and the methylation state of one vitellogenin gene during the process of estrogen stimulation and withdrawal. Sensitivity of the vitellogenin genes to DNase I digestion parallels transcription. The vitellogenin genes are insensitive to DNase I digestion in unstimulated liver cells, become more sensitive to DNase I digestion following estrogen activation of vitellogenin gene transcription, and are insensitive to DNase I digestion in liver cells withdrawn from estrogen. In contrast, the methylation state of nine potential methylation sites within the vitellogenin A1 gene is identical in red blood cells, unstimulated and withdrawn liver, estrogen-stimulated liver, and hepatocytes purified from estrogen-stimulated liver. Rapid transcription of the vitellogenin genes in estrogen-stimulated liver cells occurs with six of the nine methylation sites examined fully methylated.

The levels of cytoplasmic and nuclear estrogen receptor have been determined in livers of male Xenopus laevis stimulated by estradiol-17 beta to synthesize vitellogenin mRNA. Estrogen receptor levels were also determined in unstimulated liver and following long term withdrawal of estrogen. In unstimulated liver cells, which do not contain detectable vitellogenin mRNA, more than 80% of the estrogen receptor is located in the nucleus (550 high affinity estrogen binding sites/nucleus), while the cytoplasm contains only 100 high affinity estrogen binding sites/cell. Administration of estradiol-17 beta, which induces massive synthesis and accumulation of vitellogenin mRNA, induces the estrogen receptor as well. The nuclear receptor level rises to approximately 2,000 estrogen binding sites/cell, while the cytosol receptor increases to only 150 sites/cel. Liver cells of male X. laevis which have been withdrawn from estrogen for 70 days exhibit a striking change in receptor levels. The nuclear receptor returns to the level prevailing in unstimulated cells (approximately 500 sites/cell) while the cytosol receptor level rises to more than 1,200 sites/cell (equivalent to 260 fmol/g of tissue). The existence of a pool of cytosol receptor, which is rapidly available for induction of vitellogenin mRNA, may in part explain the shorter lag period and more rapid induction of vitellogenin mRNA observed during secondary estrogen stimulation of withdrawn Xenopus liver cells.