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The Rb/Chromatin Connection and Epigenetic Control: Opinion

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The Rb/Chromatin Connection and Epigenetic Control: Opinion Oncogene (2001) 20, 3128 ± 3133 ã 2001 Nature Publishing Group All rights reserved 0950 ± 9232/01 $15.00 www.nature.com/onc The Rb/chromatin connection and epigenetic control: opinion Roger Ferreira1, Irina Naguibneva1, Linda L Pritchard1, Slimane Ait-Si-Ali1 and Annick Harel-Bellan*,1 1Laboratoire `OncogeneÁse, DieÂrenciation et Transduction du Signal', CNRS UPR 9079, Institut Andre Lwo, 7 rue Guy Moquet, Villejuif, France The balance between cell dierentiation and proliferation proteins' family that also includes p130 and p107 (for is regulated at the transcriptional level. In the cell cycle, reviews see Grana et al., 1998; Mulligan and Jacks, the transition from G1 to S phase (G1/S transition) is of 1998). Rb exerts its anti-proliferative activity, at least paramount importance in this regard. Indeed, it is only in part, by inhibiting the E2F transcription factor. Rb before this point that cells can be oriented toward the is regulated by phosphorylation: in non-cycling cells, or dierentiation pathway: beyond, cells progress into the in early G1, Rb is hypophosphorylated and inhibits cycle in an autonomous manner. The G1/S transition is E2F activity; during G1, Rb is progressively phos- orchestrated by the transcription factor E2F. E2F controls phorylated by cyclin-CDK complexes (for review see the expression of a group of checkpoint genes whose Harbour and Dean, 2000) and, as a consequence, loses products are required either for the G1-to-S transition its anity for E2F. The release of Rb triggers the itself or for DNA replication (e.g. DNA polymerase a). activation of E2F target genes, which allows the cells E2F activity is repressed in growth-arrested cells and in to proceed through the G1/S transition. early G1, and is activated at mid-to-late G1. E2F is Rb physically interacts with E2F's transactivation controlled by the retinoblastoma tumor suppressor protein domain (Flemington et al., 1993; Helin et al., 1993; Rb. Rb represses E2F mainly by recruiting chromatin Ross et al., 1999). It is thought that the masking of this remodeling factors (histone deacetylases and SWI/SNF domain participates in E2F inhibition. In addition, a complexes), the DNA methyltransferase DNMT1, and a second mechanism is based on active repression: Rb histone methyltransferase. This review will focus on the recruits `chromatin remodeling factors', including molecular mechanisms of E2F repression by Rb during the Histone Deacetylases (HDACs) (Brehm et al., 1998; cell cycle and during cell-cycle exit by dierentiating cells. Luo et al., 1998; Magnaghi-Jaulin et al., 1998), A model in which Rb irreversibly represses E2F-regulated members of the ATP-dependent chromatin remodeling genes in dierentiated cells by an epigenetic mechanism complex SWI/SNF (Dunaief et al., 1994; Trouche et linked to heterochromatin, and involving histone H3 and al., 1997), DNA Methyltransferase 1 (DNMT1) promoter DNA methylation, is discussed. Oncogene (Robertson et al., 2000), and a Histone H3 Methyl- (2001) 20, 3128 ± 3133. transferase (most likely Suv39H1) (Didier Trouche, personal communication). Keywords: Rb; cell cycle; dierentiation; histone dea- cetylase; heterochromatin; methylation Chromatin remodeling factors Introduction In eukaryotes, genomic DNA is packaged by histones (H1, H2A, H2B, H3, and H4) and non-histone proteins E2F controls the G1/S transition, a critical moment in into chromatin, the basic unit of which is the the cell cycle, by regulating the transcription of families nucleosome. Linker histones, non-histone proteins, of genes whose products are either required for DNA and potential interactions of core histone N-terminal synthesis or involved in the regulation of S phase entry tails with adjacent nucleosomes contribute to compac- (Johnson et al., 1993). E2F is under the control of the tion/folding of nucleosomal DNA and formation of retinoblastoma protein Rb (Flemington et al., 1993; higher order structures. These structures restrict the Helin et al., 1993) (for review see Grana et al., 1998) access of proteins in general, and in particular that of and of CREB Binding Protein, CBP (Trouche et al., proteins involved in transcription (reviewed in Felsen- 1996; Ait-Si-Ali, 2000). Rb, whose inactivation is feld, 1992; Kingston and Narlikar, 1999). associated with the development of retinoblastoma Dynamic changes in chromatin structure facilitate or and other types of human cancers (for reviews see prevent the access of transcription factors to nucleo- Weinberg, 1992, 1995), is a member of the `pocket somal DNA. At least two mechanisms can be used to alter or remodel chromatin structure. One mechanism involves multi-subunit protein complexes that use ATP as a source of energy to remodel nucleosomes (e.g. *Correspondence: A Harel-Bellan, CNRS UPR 9079, Institut Andre Lwo, 7 rue Guy Moquet, 94801 Villejuif, France; SWI/SNF, NURF, RSC, CHRAC, and ACF) (re- E-mail: ahbellan@vjf.cnrs.fr viewed in Kingston and Narlikar, 1999; Muchardt and Rb/chromatin connection R Ferreira et al 3129 Yaniv, 1999; Tyler and Kadonaga, 1999). These and Corces, 2000; Wei et al., 1998), and by methylation complexes alter chromatin structure by changing the of distinct speci®c lysine residues (Rea et al., 2000; location and/or conformation of the nucleosomes Strahl et al., 1999). These modi®cations seem to play a (Hamiche et al., 1999). A second mechanism involves role in chromatin remodeling and might be important covalent modi®cations either of DNA or of histone N- for the epigenetic regulation of gene expression. They terminal tails that protrude from the core nucleosome. seem to act in concert with ATP-dependent chromatin DNA can be methylated at position 5 of cytosine remodeling complexes to achieve chromatin remodeling within CpG dinucleotides (reviewed in Bird and and regulation of gene activity. Wole, 1999; Ng and Bird, 1999; Singal and Ginder, One of the questions that remains without a clear 1999), a modi®cation that is associated with transcrip- answer to date is the order of intervention of each of tional repression; this methylation in¯uences chromatin these partners: HATs or HDACs, remodeling com- structure as well as gene activity (reviewed in Ng and plexes, DNA (and other) methylases (such as histone Bird, 1999; Razin, 1998). DNA is methylated by methyl-transferases). ATP-dependent chromatin remo- Methyltransferases (DNMTs). Sequences that include deling complexes are involved in either activation or methylcytosines are speci®cally recognized by Methyl- inhibition of transcription (Holstege et al., 1998). A cytosine Binding Proteins (MBDs) such as MeCP2 and number of experimental observations suggest that, for MBDs 1/2/3. MBDs have been shown to be transcrip- transcriptional activation, the action of the ATP- tional repressors (for reviews see Robertson and dependent complexes precedes the action of HATs Wole, 2000; Singal and Ginder, 1999). Histone tails (Cosma et al., 1999; for review see Kingston and can be modi®ed in several ways, including phosphor- Narlikar, 1999). The mechanism responsible for their ylation (Nowak and Corces, 2000; Wei et al., 1998), recruitment to speci®c target genes is a matter of methylation (Rea et al., 2000; Strahl et al., 1999), and speculation. In some instances, chromatin remodeling acetylation (Roth and Allis, 1996). The acetylation complexes have been shown to be recruited through status of lysine residues in the core histone tails is their interactions with DNA-bound transcriptional under the control of nuclear Histone Acetyl-Trans- regulators (Workman and Kingston, 1998). This ferases (HATs) and of Histone Deacetylases (HDACs). requires, however, that the transcription factors have Several mammalian HATs have been identi®ed. These access to the DNA before the action of remodeling include the transcriptional coactivators CBP/p300, complexes, and thus that the initial structure have PCAF, GCN5L, SRC1, ACTR and TAFII250 (for some level of accessibility. In this regard, sub-nuclear reviews see Berger, 1999; Roth and Allis, 1996; Wade localization could provide a clue as to how this general and Wole, 1997). On the other hand, eight HDACs level of accessibility is achieved. have been characterized to date (HDACs 1 to 8). A general correlation between histone acetylation and gene activity in vivo is now well established, although Spatio-temporal chromatin organization and regulation there are some exceptions. Furthermore, a number of of gene expression transcriptional coactivators display a histone acetyl- transferase activity; and several transcriptional core- The chromatin structure and its level of condensation pressors have, or recruit, a histone deacetylase activity. are not homogeneous in cell nuclei. In the interphase Several models have been proposed for the involve- nucleus, individual chromosomes occupy speci®c spaces ment of HATs and HDACs in transcription (reviewed referred to as chromosome territories (Schardin et al., in Kingston and Narlikar, 1999). However, how core 1985). The active genes are preferentially localized histone acetylation or deacetylation modulates tran- either to the periphery of these chromosome territories scription remains to be fully elucidated. HATs and or in the central nucleoplasm (Kurz et al., 1996; HDACs are generally contained in multimolecular Wansink et al., 1996; for review see Lamond and complexes. Some of these complexes, such as the Earnshaw, 1998). During interphase, there are two Mi2/NURD complex, also include remodeling factors structural forms of chromatin: euchromatin, which
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