Nuclear receptor corepressor complexes in cancer: mechanism, function and regulation.

Nuclear receptor corepressor (NCoR) and silencing mediator for retinoid and thyroid hormone receptors (SMRT) function as corepressors for diverse transcription factors including nuclear receptors such as estrogen receptors and androgen receptors. Deregulated functions of NCoR and SMRT have been observed in many types of cancers and leukemias. NCoR and SMRT directly bind to transcription factors and nucleate the formation of stable complexes that include histone deacetylase 3, transducin b-like protein 1/TBL1-related protein 1, and G-protein pathway suppressor 2. These NCoR/SMRT-interacting proteins also show deregulated functions in cancers. In this review, we summarize the literature on the mechanism, regulation, and function of the core components of NCoR/SMRT complexes in the context of their involvement in cancers and leukemias. While the current studies support the view that the corepressors are promising targets for cancer treatment, elucidation of the mechanisms of corepressors involved in individual types of cancers is likely required for effective therapy.

Promoter-bound p300 complexes facilitate post-mitotic transmission of transcriptional memory.

A central hallmark of epigenetic inheritance is the parental transmission of changes in patterns of gene expression to progeny without modification of DNA sequence. Although, the trans-generational conveyance of this molecular memory has been traditionally linked to covalent modification of histone and/or DNA, recent studies suggest a role for proteins that persist or remain bound within chromatin to "bookmark" specific loci for enhanced or potentiated responses in daughter cells immediately following cell division. In this report we describe a role for p300 in enabling gene bookmarking by pre-initiation complexes (PICs) containing RNA polymerase II (pol II), Mediator and TBP. Once formed these complexes require p300 to enable reacquisition of protein complex assemblies, chromatin modifications and long range chromatin interactions that facilitate post-mitotic transmission of transcriptional memory of prior environmental stimuli.

Genome-wide profiles of CtBP link metabolism with genome stability and epithelial reprogramming in breast cancer.

The C-terminal binding protein (CtBP) is a NADH-dependent transcriptional repressor that links carbohydrate metabolism to epigenetic regulation by recruiting diverse histone-modifying complexes to chromatin. Here global profiling of CtBP in breast cancer cells reveals that it drives epithelial-to-mesenchymal transition, stem cell pathways and genome instability. CtBP expression induces mesenchymal and stem cell-like features, whereas CtBP depletion or caloric restriction reverses gene repression and increases DNA repair. Multiple members of the CtBP-targeted gene network are selectively downregulated in aggressive breast cancer subtypes. Differential expression of CtBP-targeted genes predicts poor clinical outcome in breast cancer patients, and elevated levels of CtBP in patient tumours predict shorter median survival. Finally, both CtBP promoter targeting and gene repression can be reversed by small molecule inhibition. These findings define broad roles for CtBP in breast cancer biology and suggest novel chromatin-based strategies for pharmacologic and metabolic intervention in cancer.

Loss of H2A.Z Is Not Sufficient to Determine Transcriptional Activity of Snf2-Related CBP Activator Protein or p400 Complexes.

The p400 and SRCAP (Snf2-related CBP activator protein) complexes remodel chromatin by catalyzing deposition of histone H2A.Z into nucleosomes. This remodeling activity has been proposed as a basis for regulation of transcription by these complexes. Transcript levels of p21 or Sp1 mRNAs after knockdown of p400 or SRCAP reveals that each regulates transcription of these promoters differently. In this study, we asked whether deposition of H2A.Z within specific nucleosomes by p400 or SRCAP dictates transcriptional activity. Our data indicates that nucleosome density at specific p21 or Sp1 promoter positions is not altered by the loss of either remodeling complex. However, knockdown of SRCAP or p400 reduces deposition of H2A.Z∼50% into all p21 and Sp1 promoter nucleosomes. Thus, H2A.Z deposition is not targeted to specific nucleosomes. These results indicate that the deposition of H2A.Z by the p400 or SRCAP complexes is not sufficient to determine how each regulates transcription. This conclusion is further supported by studies that demonstrate a SRCAP(ΔATP ) mutant unable to deposit H2A.Z has similar transcriptional activity as wild-type SRCAP.

Dynamic bookmarking of primary response genes by p300 and RNA polymerase II complexes.

Profiling the dynamic interaction of p300 with proximal promoters of human T cells identified a class of genes that rapidly coassemble p300 and RNA polymerase II (pol II) following mitogen stimulation. Several of these p300 targets are immediate early genes, including FOS, implicating a prominent role for p300 in the control of primary genetic responses. The recruitment of p300 and pol II rapidly transitions to the assembly of several elongation factors, including the positive transcriptional elongation factor (P-TEFb), the bromodomain-containing protein (BRD4), and the elongin-like eleven nineteen lysine-rich leukemia protein (ELL). However, transcription at many of these rapidly induced genes is transient, wherein swift departure of P-TEFb, BRD4, and ELL coincides with termination of transcriptional elongation. Unexpectedly, both p300 and pol II remain accumulated or "bookmarked" at the proximal promoter long after transcription has terminated, demarking a clear mechanistic separation between the recruitment and elongation phases of transcription in vivo. The bookmarked pol II is depleted of both serine-2 and serine-5 phosphorylation of its C-terminal domain and remains proximally positioned at the promoter for hours. Surprisingly, these p300/pol II bookmarked genes can be readily reactivated, and elongation factors can be reassembled by subsequent addition of nonmitogenic agents that, alone, have minimal effects on transcription in the absence of prior preconditioning by mitogen stimulation. These findings suggest that p300 is likely to play an important role in biological processes in which transcriptional bookmarking or preconditioning influences cellular growth and development through the dynamic priming of genes for response to rechallenge by secondary stimuli.

The chromatin remodeling protein, SRCAP, is critical for deposition of the histone variant H2A.Z at promoters.

Studies in Saccharomyces cerevisiae indicate the histone variant H2A.Z is deposited at promoters by the chromatin remodeling protein Swr1 and plays a critical role in the regulation of transcription. In higher eukaryotes, however, little is known about the distribution, method of deposition, and function of H2A.Z at promoters. Using biochemical studies, we demonstrated previously that SRCAP (SNF-2-related CREB-binding protein activator protein), the human ortholog of Swr1, could catalyze deposition of H2A.Z into nucleosomes. To address whether SRCAP directs H2A.Z deposition in vivo, promoters targeted by SRCAP were identified by a chromatin immunoprecipitation (ChIP)-on-chip assay. ChIP assays on a subset of these promoters confirmed the presence of SRCAP on inactive and active promoters. The highest levels of SRCAP were observed on the active SP-1, G3BP, and FAD synthetase promoters. Detailed analyses of these promoters indicate sites of SRCAP binding overlap or occur adjacent to the sites of H2A.Z deposition. Knockdown of SRCAP levels using siRNA resulted in loss of SRCAP at these promoters, decreased deposition of H2A.Z and acetylated H2A.Z, and a decrease in levels of SP-1, G3BP, and FAD synthetase mRNA. Thus, these studies provide the first evidence that SRCAP is recruited to promoters and is critical for the deposition of H2A.Z.

Epstein-Barr virus serology in the diagnosis of nasopharyngeal carcinoma.

The antibody levels to viral capsid antigen (VCA) and early antigen (EA) of Epstein-Barr virus (EBV) in 164 nasopharyngeal carcinoma (NPC) patients from Sarawak, East Malaysia were significantly higher than those in 147 sex, age and ethnically matched healthy controls. As diagnostic markers of NPC, IgG/VCA at reciprocal titers > or =160 was the most sensitive (89%, with 98% specificity), while IgA/EA at > or =5 was the most specific (100%) but the least sensitive (75%). The sensitivity and specificity of IgA/VCA at reciprocal titers > or =10 were 84% and 97%. IgA/VCA has an advantage over IgG/VCA despite the slightly lower sensitivity due to its consistently more distinct fluorescence reaction. The sensitivity and specificity can be marginally improved by a combination of two tests.