Roles of ER, SRC-1, and CBP Phosphorylation in Estrogen Receptor- Regulated Gene Expression

摘要

Breast cancer patients who possess cancers that are estrogen- dependent usually respond well initially to the antiestrogen, tamoxifen. However, the cancer subsequently becomes resistant to tamoxifen, possibly through increases in cAMP and protein kinase A or through stimulation of the MAP kinase pathway, which have been associated with the conversion of the tamoxifen metabolite, 4-hydroxytamoxifen (4HT), into an estrogen receptor-alpha (ER) agonist. Tamoxifen resistance may also occur through cellular alterations in the balance of coactivators and corepressors whose protein levels may be regulated by the ubiquitin-proteasome protein degradation pathway. The relationship between phosphorylation of ER, its response to 4HT and its proteasome-mediated degradation was examined in transiently transfected immortialized cell lines. An inhibitor of the proteasome, MGl32, interferes with ER-mediated transcription and blocks the agonist activity of 4HT. MG132 also stabilizes ER and prevents its ligand-mediated down-regulation. Phosphorylan-defective ER mutants were found to be more stable than the wild-type receptor suggesting that receptor phosphorylation plays a role in regulating its stability which has been observed for other transcription factors. Mutation of coactivator-binding surfaces in the AF-2 of the receptor also influenced the stability of the receptor, suggesting that a coactivator(s) promotes ER degradation through their association with the liganded receptor. These results provide evidence that the ubiquitin-proteasome protein degradation pathway plays an important role in ER function and its response to tamoxifen.