No abstract

Oestrogen (E2) regulates the expression of its target genes at transcriptional and post-transcriptional levels. To clarify the mechanism of E2-induced post-transcriptional regulation, with attention to the involvement of the oestrogen receptor (ER), we studied the effect of tamoxifen (TAM), a synthetic E2 antagonist that inhibits ER-mediated transcription, on E2-induced transcriptional and post-transcriptional regulation of the chicken ovalbumin (OVA) gene in chick oviducts. Run-on analysis with oviduct nuclei isolated from E2-treated chicks showed that TAM treatment completely blocked E2-induced transcription of the OVA gene within 24 h without affecting ER gene expression. Likewise, the rate of transcription fell to below the limit of detection after E2 withdrawal from the chicks. Reflecting the transcription rate, OVA mRNA accumulated linearly in E2-treated chicks, and E2 withdrawal caused a rapid loss of OVA mRNA. However, in the chicks treated with TAM and E2, OVA mRNA was degraded slowly over 48 h with a half-life of 24 h, suggesting that TAM does not inhibit E2-induced mRNA stabilization. Moreover, E2-induced mRNA stabilization was observed even when transcription of the OVA gene was blocked by a transcription inhibitor. Western-blot analysis showed that the remaining OVA mRNA was translatable. Thus the present study indicates that E2 regulates expression of the OVA gene via distinct pathways at transcriptional and post-transcriptional levels.

The stabilization of chicken ovalbumin (OVA) mRNA by different classes of steroid hormones (estrogen, progesterone, glucocorticoid, and androgen) was studied in the oviducts of chicks treated with combinations of four steroids. The combination of estrogen with progesterone, glucocorticoid, or androgen enhanced the induction of the OVA gene more than did estrogen alone. Run-on analysis of the isolated oviduct nuclei to measure the transcription rate of the OVA gene showed that the enhanced induction of the OVA gene by the combined hormone treatments was partly caused by an increased rate of transcription. The half-life of OVA mRNA as determined using a transcription inhibitor (actinomycin D) was estimated to be about 24 h irrespective of the hormone treatment, though the half-life was about 6 h in the absence of hormones. These results suggested that the prolongation of the half-life of OVA mRNA by steroid hormones is constant irrespective of differential transcription rates of the OVA gene.