The chromatin of eukaryotic cells is organized in nucleosomes. This organization allows the efficient packaging of chromosomal DNA into the nucleus but limits the access of highmolecular-weight protein complexes of the transcription machinery. At least two different mechanisms enable the eukaryotic cell to relieve nucleosomal repression: the chromatinremodeling complexes (reviewed in references 55 and 57) and reversible histone acetylation. Two recent reports indicate a direct link between these two activities (60, 67). Posttranslational acetylation on conserved lysine residues within the Nterminal regions of nucleosomal histones is assumed to lead to a reduced attraction between chromosomal DNA and histone tails and changed interactions with neighboring nucleosomes or other nonhistone proteins. The resulting local chromatin decondensation increases the accessibility of particular DNA regions for RNA polymerase complexes. Consistent with this idea, transcriptionally active chromatin correlates with histone hyperacetylation (reviewed in references 18, 30, 47, 49, 61, and 62). This model predicts that histone acetyltransferases would promote transcription, while histone deacetylases (HDACs) should act as repressors. In accordance with this model, several transcriptional adapters and coactivators, such as GCN5 (8, 31), p300/CBP (4, 46), TAFII250 (40), SRC-1 (54), and ACTR (10), have been classified as histone acetyltransferases. Five HDACs have been identified in mammalian cells (12,14,56,58,63,64). Three of them, HDAC1, HDAC2, and HDAC3, have significant homology to yeast Rpd3 (44,50,59). HDAC4 and HDAC5 belong to the histone deacetylase A (HDA) family (9, 58). HDAC1 and HDAC2 are found in high-molecularweight complexes associated with adapter proteins like SIN3, SAP18, and SAP30 and nuclear corepressors like N-CoR, SMRT, and 24,32,42,65,66). Recently it was demonstrated that several mammalian transcription factors, such as Mad (21, 24, 32, 52), YY1 (64), hormone-dependent nuclear receptors (24, 42), MeCP2 (26, 43), CBF (27), retinoblastoma protein (Rb) (7, 38, 39), and related pocket proteins (16), can repress transcription by recruiting HDACs to specific promoters. In addition, the aberrant recruitment of HDACs by PLZF, PML, and ETO fusion proteins can interfere with the differentiation of hematopoietic precursor cells in acute promyelocytic leukemia (13,17,19,35).In this study we investigated the potential function of HDACs as transcriptional repressors during the growth arrest of mammalian cells. Using the S-phase-specific mouse thymidine kinase (TK) promoter as a model system, we show that HDAC1 can mediate transcriptional repression via the Sp1 binding site. HDAC1 is associated with Sp1 and binds directly to the C-terminal part of Sp1 that was previously identified as interacting domain for E2F1 (28). Sp1 and E2F1 cooperate in the activation of S-phase-specific promoters (28, 36). Here we show that E2F1 but not E2F4 can compete with HDAC1 binding to Sp1, thereby relieving HDAC1-mediated repression of the TK...

The INK4a and ARF genes found at the CDKN2A locus are key effectors of cellular senescence that is believed to act as a powerful anticancer mechanism. Accordingly, mutations in these genes are present in a wide variety of spontaneous human cancers and CDKN2A germ line mutations are found in familial melanoma. The TBX2 gene encoding a key developmental transcription factor is amplified in pancreatic cancer cell lines and preferentially amplified and overexpressed in BRCA1 and BRCA2 mutated breast tumors. Overexpression of Tbx2 and the related factor Tbx3, which is also overexpressed in breast cancer and melanomas, can suppress senescence in defined experimental systems through repression of ARF expression. However, it is not known how Tbx2 mediates its repressive effect nor whether endogenous Tbx2 or Tbx3 perform a similar antisenescence function in transformed cells. This is a particularly important question because the loss of CDKN2A in many human cancers would, in principle, bypass the requirement for Tbx2/3-mediated repression of ARF in suppressing senescence. We show here that Tbx2 is overexpressed in melanoma cell lines and that Tbx2 targets histone deacetylase 1 to the p21 Cip1 (CDKN1A) initiator. Strikingly, expression of an inducible dominant-negative Tbx2 (dnTbx2) leads to displacement of histone deacetylase 1, up-regulation of p21 Cip1 expression, and the induction of replicative senescence in CDKN2A-null B16 melanoma cells. In human melanoma cells, expression of dnTbx2 leads to severely reduced growth and induction of senescence-associated heterochromatin foci. The results suggest that the activity of endogenous Tbx2 is critically required to maintain proliferation and suppress senescence in melanomas. (Cancer Res 2005; 65(6): 2260-8)

The steady state of histone acetylation is established and maintained by multiple histone acetyltransferases and deacetylases, and this steady state affects chromatin structure and function. The identification of a maize complementary DNA encoding the chromatin-bound deacetylase HD2 is reported. This protein was not homologous to the yeast RPD3 transcriptional regulator. It was expressed throughout embryo germination in correlation with the proliferative activity of cells. Antibodies against recombinant HD2-p39 immunoprecipitated the native enzyme complex, which was composed of phosphorylated p39 subunits. Immunofluorescence microscopy and sequence homologies suggested nucleolar localization. HD2 is an acidic nucleolar phosphoprotein that might regulate ribosomal chromatin structure and function.