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Regulation of retinoblastoma gene expression in hormone-dependent breast cancer
M M Gottardis1, M Saceda, P Garcia-Morales
1Vincent T. Lombardi Cancer Center, Georgetown University, Washington, D.C. 20007, USA.
Abstract:
Studies have shown an increased risk for breast cancer in the mothers of children suffering from retinoblastoma and osteosarcoma, suggesting a role for the retinoblastoma susceptibility (Rb) gene product in breast cancer. We now show that estradiol decreases the expression of Rb at the level of protein and messenger RNA (mRNA) in estrogen-dependent breast cancer cell lines. Treatment of MCF-7 cells with 10(-9) M estradiol for 48 h resulted in a 70% decrease in the level of Rb protein. Ribonuclease protection assays showed a 50% decrease in the steady state levels of Rb mRNA by 12 h and a 70% decrease in Rb mRNA by 24 h. Treatment with estradiol had no effect on the rate of Rb gene transcription or on Rb mRNA stability, but resulted in an increase in the steady state level of Rb mRNA in the nucleus. The effect of estradiol was inhibited by 10(-7) M 4-hydroxytamoxifen. In the absence of estradiol, the antiestrogens 4-hydroxytamoxifen and ICI 164,384 increased Rb mRNA by 50% over that in estrogen-depleted conditions. Estradiol regulation of Rb mRNA also occurred in other estrogen-dependent breast cancer cell lines. Insulin-like growth factor I, insulin, progestins, and epidermal growth factor had no effect on Rb expression. In summary, these results show that estradiol specifically regulates the expression of the Rb susceptibility gene product in hormone-dependent breast cancer by a posttranscriptional mechanism that occurs in the nucleus. The results from this study suggest that the negative regulation of Rb expression by estradiol, rather than Rb loss or mutation, may play an important role in breast carcinogenesis.
Insights
Estradiol, a key hormone, reduces retinoblastoma (Rb) gene expression in breast cancer cells. This hormone-driven regulation, rather than gene mutation, may drive breast cancer development.
Area of Science:
- Molecular Biology
- Oncology
- Genetics
Background:
- Increased breast cancer risk observed in mothers of children with retinoblastoma suggests a link to the retinoblastoma (Rb) gene.
- The retinoblastoma susceptibility gene product's role in breast cancer is implicated but not fully understood.
Purpose of the Study:
- To investigate the effect of estradiol on retinoblastoma (Rb) gene expression in estrogen-dependent breast cancer cells.
- To elucidate the mechanism by which estradiol influences Rb expression and its potential role in breast carcinogenesis.
Main Methods:
- Utilized MCF-7 and other estrogen-dependent breast cancer cell lines.
- Administered estradiol and antiestrogens (4-hydroxytamoxifen, ICI 164,384) to cell cultures.
- Quantified Rb protein and mRNA levels using Western blotting and ribonuclease protection assays.
- Assessed Rb gene transcription rates and mRNA stability.
- Analyzed nuclear and cytoplasmic mRNA fractions.
Main Results:
- Estradiol significantly decreased Rb protein and mRNA levels in a time- and dose-dependent manner.
- Estradiol's effect was posttranscriptional, impacting nuclear mRNA levels without altering transcription or stability.
- Antiestrogens demonstrated an inverse effect, increasing Rb mRNA in the absence of estradiol.
- Estradiol regulation of Rb mRNA was specific and not mimicked by other growth factors or hormones.
Conclusions:
- Estradiol specifically downregulates retinoblastoma (Rb) gene expression in hormone-dependent breast cancer cells via a nuclear, posttranscriptional mechanism.
- Negative regulation of Rb expression by estradiol, rather than Rb gene mutation, may be a critical factor in breast cancer development.
- Findings suggest novel therapeutic targets for hormone-dependent breast cancers by modulating Rb expression.