Nuclear organization and chromatin dynamics-Sp1, Sp3 and histone deacetylases

The TFF1 gene provides an example in which there are several routes involving different transcription factors, chromatin remodeling complexes and histone modifications in response to different agents to induce TFF1 gene expression in estrogen receptor α positive breast cancer cells (e.g. MCF-7 cells). A TFF1 promoter binds either Sp1 or Sp3 but not both factors. In MCF-7 cells Sp1 and Sp3 are located in non-overlapping foci which are associated with the nuclear matrix. In response to estrogens, Sp3 is preferred over Sp1 to drive ligand bound ERα mediated TFF1 gene expression. Sp3 or Sp1 recruits the Sin3 HDAC complex containing phosphorylated HDAC2 and HDAC1 to the TFF1 promoter. HDAC2 is phosphorylated by protein kinase CK2. This phosphorylation event is important for the binding of HDAC2 to RbAp48, which is a core component of the Sin3 and NuRD HDAC complexes. In the presence or absence of estrogens, the TFF1 promoter is engaged in dynamic acetylation. In the absence of estrogens, the steady state level of histone acetylation at the TFF1 promoter is low relative to a higher steady state level of acetylation when ligand bound ERα and associated coactivators with HAT activity are recruited to the promoter. Our studies on phosphorylated HDAC2 and MCF-7 karyotype illustrate the limitations of the X-ChIP assay in which chromatin associated proteins are cross-linked to DNA with formaldehyde. Formaldehyde preferentially cross-links the phosphorylated form of HDAC2 to chromatin while the more abundant unmodified form of HDAC2 cross-links poorly. Analyses of the MCF-7 karyotype show that these cells are aneuploid with each cell having a different chromosome composition. Hence studies using the X-ChIP assay to analyze proteins associated with a specific chromosome region in breast cancer cells are being done in a background of genomic instability. © 2007 Elsevier Ltd. All rights reserved.