SMRT corepressor interacts with PLZF and with the PML-retinoic acid receptor α (RARα) and PLZF-RARα oncoproteins associated with acute promyelocytic leukemia

Abstract

Retinoic acid receptors (RARs) are hormone-regulated transcription factors that control key aspects of normal differentiation. Aberrant RAR activity may be a causal factor in neoplasia. Human acute promyelocytic leukemia, for example, is tightly linked to chromosomal translocations that fuse novel amino acid sequences (denoted PML, PLZF, and NPM) to the DNA-binding and hormone-binding domains of RAR␣. The resulting chimeric receptors have unique transcriptional properties that may contribute to leukemogenesis. Normal RARs repress gene transcription by associating with ancillary factors denoted corepressors (also referred to as SMRT, N-CoR, TRAC, or RIP13). We report here that the PML-RAR␣ and PLZF-RAR␣ oncoproteins retain the ability of RAR␣ to associate with corepressors, and that this corepressor association correlates with certain aspects of the leukemic phenotype. Unexpectedly, the PLZF moiety itself can interact with SMRT corepressor. This interaction with corepressor is mediated, in part, by a POZ motif within PLZF. Given the presence of POZ motifs in a number of known transcriptional repressors, similar interactions with SMRT may play a role in transcriptional silencing by a variety of both receptor and nonreceptor transcription factors.Retinoids regulate many aspects of vertebrate cell proliferation and differentiation through receptors that function as hormone-regulated transcription factors (1-6). Two major classes of retinoid receptors have been identified: retinoic acid receptors (RARs) and retinoid X receptors (RXRs) (3-6). Both RARs and RXRs bind to specific sites on the DNA, and they can activate or repress expression of adjacent target genes (1-6). Aberrant RARs appear to play a crucial role in human acute promyelocytic leukemia (APL) (7-15). In over 95% of all APL patients, specific chromosomal translocations create abnormal RARs by replacing the N terminus of RAR␣ with novel ORFs. The breakpoint in RAR␣ is identical in all cases, whereas the nature of the novel N-terminal sequences can vary. In the common t(15;17) translocation, the RAR␣ N terminus is replaced with an ORF denoted PML (for promyelocytic leukemia) (7-12). Alternatively, t(11;17) and t(5;17) translocations result in chimeric proteins denoted PLZF (promyelocytic leukemia zinc finger)-RAR␣ and NPM (nucleophosmin)-RAR␣, respectively (13-15). Although possessing a number of recognizable structural motifs, including several prevalent in transcription factors, the PML, PLZF, and NPMderived sequences exhibit little structural interrelatedness, and the functions of these proteins in the normal cell are not fully understood (reviewed in refs. 16-18).The near-invariant association of acute promyelocytic leukemia with the expression of chimeric RAR␣ proteins, and the ability of retinoids to drive leukemias bearing the PML-RAR␣ translocation into differentiation and clinical remission (16-18), suggest an active role for these chimeras in conferring the leukemogenic phenotype. Furthermore, ectopic expression of PML-RAR␣ in murine or av...

Human endogenous retrovirus protein cORF supports cell transformation and associates with the promyelocytic leukemia zinc finger protein

“…The promyelocytic zinc ®nger protein PLZF is a transcriptional repressor that was originally identi®ed as part of a fusion with the retinoic acid receptor alpha (RARa) implicated in acute promyelocytic leukemia (APL; Chen et al, 1993a,b). In attempts to identify protein binding partners, several interacting proteins such as the co-repressors N-CoR, SMRT, Sin3A and HDAC1 were found to associate with the N-terminal POZ domain of PLFZ (Hong et al, 1997;He et al, 1998;Lin et al, 1998;Grignani et al, 1998;David et al, 1998). cORF is the third PLZF binding partner aside from the promyelocytic leukemia protein (PML; Melnick and and the epsin 1 protein that has been shown to target the boundary region of the proline-rich domain and the proximal of the nine zinc®ngers.…”

“…It recognizes speci®c DNA sequences via its C-terminal zinc ®nger domain consisting of nine KruÈ ppel-like C2H2 elements (Li et al, 1997). However, PLZF does not function as a transactivator; rather, target genes are suppressed upon DNA binding by the interaction of the N-terminal POZ domain with the co-repressors N-CoR, SMRT, Sin3A and HDAC1 (histone deacetylase 1; Hong et al, 1997;He et al, 1998;Lin et al, 1998;Grignani et al, 1998;David et al, 1998). Other factors bind to PLZF and may modulate its function as a transcriptional repressor (Melnick and Licht, 1999).…”

Histone deacetylase associated with mSin3A mediates repression by the acute promyelocytic leukemia-associated PLZF protein

“…Although the PLZF natural target genes have not yet been identi®ed, a high a nity PLZF-binding site was recently isolated in the lexA operator (Sitterlin et al, 1997). When fused to a GAL4 DNA-binding domain, PLZF strongly represses transcription from a GAL4-dependent reporter (Hong et al, 1997). In addition, the native PLZF protein has been shown to repress cyclin A expression, possibly through binding a speci®c TA-rich motif in the cyclin A promoter (Yeyati et al, 1997).…”

Overexpressed BCL6 (LAZ3) oncoprotein triggers apoptosis, delays S phase progression and associates with replication foci

“…Five mg of total RNA were harvested after 48 h, loaded and hybridized with a probe Growth suppression and apoptosis by BCL6 (LAZ3) O Albagli et al tion at least in part by locally inducing a repressive (hypoacetylated) chromatin structure. This mechanism appears to be shared by other POZ/zinc ®nger (POK) transcriptional repressors since PLZF, a relative of BCL6 involved in certain cases of acute promyelocytic leukemia, recruits the same SIN3/ SMRT(NcoR)/HDAC complex and also depends upon HDAC activity to repress transcription (Dhordain et al, 1997;Hong et al, 1997;David et al, 1998;Dhordain et al, 1998;Grignani et al, 1998;Guidez et al, 1998;Lin et al, 1998;Wong and Privalsky, 1998;Huynh and Bardwell, 1998). BCL6 is expressed in most tissues and in many cell types in vitro (Kerckaert et al, 1993;Fukuda et al, 1995;Allman et al, 1996;Albagli et al, 1998).…”