Targeting of CTCF to the nucleolus inhibits nucleolar transcription through a poly(ADP-ribosyl)ation-dependent mechanism

Abstract

Multiple functions have been reported for the transcription factor and candidate tumour suppressor, CTCF. Among others, they include regulation of cell growth, differentiation and apoptosis, enhancer-blocking activity and control of imprinted genes. CTCF is usually localized in the nucleus and its subcellular distribution during the cell cycle is dynamic; CTCF was found associated with mitotic chromosomes and the midbody, suggesting different roles for CTCF at different stages of the cell cycle. Here we report the nucleolar localization of CTCF in several experimental model systems. Translocation of CTCF from nucleoplasm to the nucleolus was observed after differentiation of K562 myeloid cells and induction of apoptosis in MCF7 breast cancer cells. CTCF was also found in the nucleoli in terminally differentiated rat trigeminal ganglion neurons. Thus our data show that nucleolar localization of CTCF is associated with growth arrest. Interestingly, the 180 kDa poly(ADP-ribosyl)ated isoform of CTCF was predominantly found in the nucleoli fractions. By transfecting different CTCF deletion constructs into cell lines of different origin we demonstrate that the central zinc-finger domain of CTCF is the region responsible for nucleolar targeting. Analysis of subnucleolar localization of CTCF revealed that it is distributed homogeneously in both dense fibrillar and granular components of the nucleolus, but is not associated with fibrillar centres. RNA polymerase I transcription and protein synthesis were required to sustain nucleolar localization of CTCF. Notably, the labelling of active transcription sites by in situ run-on assays demonstrated that CTCF inhibits nucleolar transcription through a poly(ADP-ribosyl)ation-dependent mechanism.

Ribosomal RNA Gene Transcription Mediated by the Master Genome Regulator Protein CCCTC-binding Factor (CTCF) Is Negatively Regulated by the Condensin Complex

“…CTCF has been shown to localize to the nucleolus and repress nucleolar transcription in UR61 cells (33). However, in this study, we found that CTCF positively regulates rRNA gene transcription in HeLa-S3 cells.…”

“…It has been reported that translocation of CTCF from nucleoplasm to the nucleolus was observed after differentiation of K562 myeloid cells or induction of apoptosis in MCF7 breast cancer cells (33). Nucleolar protein nucleophosmin/B23 has been shown to interact with CTCF and is present in vivo at insulator sites in the chicken ␤-globin locus (31).…”

Functional role of CCCTC binding factor (CTCF) in stress-induced apoptosis

“…In lymphocyte B cells, increased expression of CTCF is associated with downregulation of c-Myc, resulting in cell growth arrest and cell death [15]. Accumulation of CTCF in the nucleoli is related to growth arrest and apoptosis during differentiation in human K562 myeloid cells and in human breast carcinoma cells, respectively [16,17]. These results suggest that CTCF may be a determinant factor to death signaling pathways in these cells.…”

CTCF Interacts with and Recruits the Largest Subunit of RNA Polymerase II to CTCF Target Sites Genome-Wide

“…Thus, specific phosphorylation of CTCF by the protein kinase CK2 (former casein kinase II) affects CTCF function in transcriptional regulation (15,29). Poly(ADP-ribosyl)ation is another recently discovered modification of CTCF that is important for insulator function (27,67) and nucleolar transcription (60). Posttranslational modifications of CTCF have also been implicated in human myeloid cell differentiation (14).…”