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The nuclear receptor corepressor N-CoR plays a crucial role in the repressive activity of diverse transcription factors, yet little is known about what regulates its cellular level. We have found that estrogen markedly down-regulates N-CoR protein levels in estrogen receptor (ER)-positive breast cancer cells without affecting N-CoR mRNA levels, whereas levels of the related corepressor SMRT are unaffected. This effect is attributable to estrogen up-regulation of the ubiquitin ligase Siah2, which is a rapid and primary transcriptional response mediated by the ER, and precedes the loss of N-CoR. Treatment with proteasomal inhibitor or with small interfering RNA against Siah2 prevented the down-regulation of N-CoR by estrogen. Furthermore, the expression of 24-hydroxylase, a gene repressed by unliganded vitamin D receptor through its interaction with N-CoR, was up-regulated by estrogen and required Siah2. Our results illustrate a mechanism by which the estrogen-ER complex markedly reduces the level of N-CoR through a process involving the up-regulation of Siah2 and the subsequent targeting of N-CoR for proteasomal degradation. These findings reveal that, although estrogen directly regulates the transcription of many genes, by regulating a gene such as Siah2 it can exert profound "secondary" effects on cellular activity through mechanisms such as targeting regulatory proteins for degradation. This estrogen-evoked down-regulation of N-CoR could have a global derepressive effect on genes whose repression depends on N-CoR and thereby have broad impact on the activity of transcription factors and nuclear receptors whose actions involve N-CoR.

Estrogen down-regulation of the corepressor N-CoR: Mechanism and implications for estrogen derepression of N-CoR-regulated genes