A conformational switch in nuclear hormone receptors is involved in coupling hormone binding to corepressor release.

Nuclear hormone receptors are ligand-regulated transcription factors that modulate gene expression in response to small, hydrophobic hormones, such as retinoic acid and thyroid hormone. The thyroid hormone and retinoic acid receptors typically repress transcription in the absence of hormone and activate it in the presence of hormone. Transcriptional repression is mediated, in part, through the ability of these receptors to physically associate with ancillary polypeptides called corepressors. We wished to understand the mechanism by which corepressors are recruited to unliganded nuclear hormone receptors and are released on the binding of hormone. We report here that an alpha-helical domain located at the thyroid hormone receptor C terminus appears to undergo a hormone-induced conformational change required for release of corepressor and that amino acid substitutions that abrogate this conformational change can impair or prevent corepressor release. In contrast, retinoid X receptors appear neither to undergo an equivalent conformational alteration in their C termini nor to release corepressor in response to cognate hormone, consistent with the distinct transcriptional regulatory properties displayed by this class of receptors.

TCDD suppression of tissue transglutaminase stimulation by retinoids in malignant human keratinocytes.

The human keratinocyte line SCC-4 is a model system in which to explore the mechanism by which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) interferes with the action of hormones in the steroid receptor superfamily. In present work, retinoid induction of tissue transglutaminase mRNA was suppressed 60-70% by 10 nM TCDD in the human squamous carcinoma cell line SCC-4. This effect occurred without enhanced degradation of the mRNA and thus appeared to result from altered transcription. The actions of all-trans-retinoic acid and the synthetic retinoid TTNPB ((E)4-[2-(5,6,7,8-tetrahydro-5,5,8, 8-tetramethyl-2-naphthylenyl)-1propenyl] benzoic acid), which resists metabolic degradation, were suppressed to the same extent without obvious changes in their EC(50)s. In addition, TCDD suppression of reporter transcription, driven by a retinoic acid response element, was not evident in transient or stable transfections of SCC-4 cells. Sodium butyrate (3 mM) alone induced tissue transglutaminase and augmented retinoid induction. In the presence of butyrate, TCDD acted as an inducer and did not reduce retinoid stimulation. Retinoic acid induction of tissue transglutaminase displayed a lag phase of >24 h, indicating that the induction has an indirect component. Rather than depleting active retinoid in the culture medium or generally inactivating retinoid receptor function, TCDD may suppress retinoid action in this case by interfering with the late phase of induction.

ZNF217, a candidate breast cancer oncogene amplified at 20q13, regulates expression of the ErbB3 receptor tyrosine kinase in breast cancer cells.

Understanding the mechanisms underlying ErbB3 overexpression in breast cancer will facilitate the rational design of therapies to disrupt ErbB2-ErbB3 oncogenic function. Although ErbB3 overexpression is frequently observed in breast cancer, the factors mediating its aberrant expression are poorly understood. In particular, the ErbB3 gene is not significantly amplified, raising the question as to how ErbB3 overexpression is achieved. In this study we showed that the ZNF217 transcription factor, amplified at 20q13 in ∼20% of breast tumors, regulates ErbB3 expression. Analysis of a panel of human breast cancer cell lines (n = 50) and primary human breast tumors (n = 15) showed a strong positive correlation between ZNF217 and ErbB3 expression. Ectopic expression of ZNF217 in human mammary epithelial cells induced ErbB3 expression, whereas ZNF217 silencing in breast cancer cells resulted in decreased ErbB3 expression. Although ZNF217 has previously been linked with transcriptional repression because of its close association with C-terminal-binding protein (CtBP)1/2 repressor complexes, our results show that ZNF217 also activates gene expression. We showed that ZNF217 recruitment to the ErbB3 promoter is CtBP1/2-independent and that ZNF217 and CtBP1/2 have opposite roles in regulating ErbB3 expression. In addition, we identify ErbB3 as one of the mechanisms by which ZNF217 augments PI-3K/Akt signaling.