Estrogen-dependent modification of ribosomal proteins. Effects of estrogen withdrawal on the distribution of constitutive and hormonally regulated mRNAs.

Estrogen-dependent modification of ribosomal proteins during induction of egg-yolk protein synthesis in avian liver was examined in vivo and in cultured hepatocytes. Modification of two proteins of the 40S ribosomal subunit was detected in vivo, within 40 min of injection of hormone. One of the proteins was identified as S6 and the other, tentatively, as S3a. Estrogen treatment resulted in the appearance of multiple, phosphorylated forms of S6 and a shift in electrophoretic mobility of the other protein that was consistent with its dephosphorylation. The steady state achieved within 2 h of injection could be maintained for up to 2 weeks when the hormone was administered from silastic implants. Removal of the implants resulted in a return to the preinduction state within 20-40 min. Similar modifications were induced in hepatocytes maintained in defined medium, with 17 beta-estradiol as the only hormonal supplement. In order to check on the possibility that the modifications observed could selectively influence mRNA utilization, the cytoplasmic distributions of two abundant mRNAs were monitored during the first few hours following withdrawal. One of these was serum albumin mRNA, the levels of which are unaffected by estrogen. The other was very-low-density apolipoprotein II mRNA which specifies a major egg-yolk protein. The synthesis of this mRNA is absolutely dependent on estrogen and its half-life is also markedly affected by the hormone.