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Oncogene mediated repression of glucocorticoid hormone response elements and glucocorticoid receptor levels
1Ludwig Institute for Cancer Research, Bern Branch, Inselspital, Switzerland.
Abstract:
We have previously described the inhibition of glucocorticoid-dependent transcription from the mouse mammary tumor virus long terminal repeat promoter by products of the H-ras and v-mos oncogenes. We have studied the effects of conditional oncogenes on expression of glucocorticoid-dependent indicator genes. Expression of the glucocorticoid-dependent transcription of the tyrosine aminotransferase gene was monitored in FTO-2B rat hepatoma cells during Mr 21,000 protein (p21) H-ras induction. A strong transcriptional repression of the tyrosine aminotransferase gene followed p21 H-ras expression. The sequences in a glucocorticoid-dependent promoter which are responsible for the oncogene-mediated repression could be localized to the glucocorticoid response element; a construct in which a 15-base pair glucocorticoid response element was inserted 5' of the thymidine kinase promoter exhibited the oncogene-mediated repression of transcription. We observed a strong repression of glucocorticoid-dependent promoters and promoter constructs not only in the presence of p21 H-ras and p37 v-mos but also with p60 v-src. p57 v-myc, however, had no effect. Oncogene expression is not a sufficient prerequisite for an initial repression of glucocorticoid hormone-dependent gene transcription, since even in the presence of constitutively high levels of oncogene product a transient stimulation of glucocorticoid-dependent gene expression was found. Protein synthesis inhibition experiments revealed that no hormonally induced cellular protein is needed for the oncogene-mediated repression. It seemed reasonable that this phenomenon might reflect oncogene effects on the glucocorticoid receptor. We, therefore, made measurements of the glucocorticoid receptor protein. In the presence of glucocorticoid hormone the receptor translocated rapidly from the cytoplasm to the nucleus. In normal NIH 3T3 cells, after 24-h treatments the nuclear receptor levels had declined to about 50% of those determined at 2 h and in the presence of p21 H-ras they declined to 15%. The levels of cytoplasmic receptor were not affected by p21 H-ras expression.
Insights
Oncogenes like H-ras repress glucocorticoid-dependent gene transcription by affecting the glucocorticoid receptor. This repression occurs at the glucocorticoid response element and does not require new protein synthesis.
Area of Science:
- Molecular Biology
- Oncology
- Gene Regulation
Background:
- Oncogenes can interfere with normal cellular processes.
- Glucocorticoids regulate gene expression via specific promoter elements.
- Previous work showed H-ras and v-mos oncogenes inhibit MMTV-LTR transcription.
Purpose of the Study:
- To investigate the effect of conditional oncogenes on glucocorticoid-dependent gene expression.
- To identify the specific promoter regions involved in oncogene-mediated repression.
- To explore the mechanism underlying oncogene-induced transcriptional repression.
Main Methods:
- Monitoring tyrosine aminotransferase gene expression in FTO-2B cells upon H-ras induction.
- Analyzing promoter constructs with inserted glucocorticoid response elements.
- Assessing the impact of various oncogenes (H-ras, v-mos, v-src, v-myc) on gene transcription.
- Measuring glucocorticoid receptor protein levels and localization.
Main Results:
- p21 H-ras induction strongly repressed tyrosine aminotransferase gene transcription.
- Repression localized to the glucocorticoid response element.
- p21 H-ras, p37 v-mos, and p60 v-src repressed transcription; p57 v-myc did not.
- Repression occurred without requiring new protein synthesis.
- p21 H-ras expression led to a significant decline in nuclear glucocorticoid receptor levels.
Conclusions:
- Oncogenes, particularly H-ras, can repress glucocorticoid-dependent transcription.
- The glucocorticoid response element is a key target for oncogene-mediated repression.
- The mechanism involves a reduction in nuclear glucocorticoid receptor levels.
- This suggests a direct interaction or interference of oncogene products with the glucocorticoid receptor pathway.