The Viral Oncogene Human Papillomavirus E7 Deregulates Transcriptional Silencing by Brm-related Gene 1 via Molecular Interactions

Abstract

BRG-1, a component of the human SWI/SNF complex, either activates or represses cellular promoters by modulating chromatin structure via the formation of a multiple polypeptide complex. Human papillomavirus E7 binds and destabilizes pRb, resulting in the blockage of G 1 arrest in the cell cycle. We show here that the highrisk human papillomavirus E7 protein group binds BRG-1 and modulates repression of the c-fos promoter mediated by this protein. In addition, both wild-type and Rb binding-defective E7 proteins abolish flat cell formation by BRG-1 in SW13 cells, whereas E7 COOHterminal mutants do not affect this process. BRG-1-triggered repression of the c-fos promoter is sensitive to trichostatin A. We further establish that BRG-1 contains an activation domain and a trichostatin A-sensitive repression domain. These results collectively suggest that the viral oncogene E7 targets both pRb and BRG-1 via protein-protein interactions, resulting in the deregulation of host cell cycle control.Oncogenic transformation is often caused by the aberrant expression of key genes that regulate growth properties of the cell. Many transcription factors possess DNA binding properties to modulate cellular gene expression. These DNA-binding proteins frequently communicate with other cellular co-factors, such as co-activators and co-repressors, and regulate target gene expression through modification of the chromatin structure (1).Brm-related gene 1 (BRG-1), 1 a component of the human SWI/SNF complex, is a homolog of Drosophila melanogaster brahma and Saccharomyces cerevisiae SNF2 (2). BRG-1 is crucial to the function of the SWI/SNF nucleosome remodeling complex and interacts with various cellular factors, such as BRCA1, pRb, and several transcription factors (3-8). BRG-1 also binds to cyclin E and alters the ability of this protein to induce growth arrest (9). In SW13 cells that do not express either BRG-1 or hBrm but contain wild-type pRb, overexpression of BRG-1 leads to flat cell formation (5). A recent study by de la Serna et al. (10) demonstrated that the dominant negative form of BRG-1 blocks MyoD-mediated muscle differentiation. Interestingly, a BRG-1 null mouse displays an embryonic lethal phenotype and its heterozygotes are predisposed to exencephaly and tumors, indicating that this protein plays a role in the suppression of tumor formation (11). The nuclear receptor co-repressor and mSin3A co-purify with the SWI/SNF complex, suggesting that these distinct multiprotein complexes are involved in transcriptional repression (12, 13). Although the detailed molecular mechanism of interaction between SWI/SNF and the co-repressor complex remains to be elucidated, we speculate that the two complexes act in concert to repress target genes, using histone deacetylase (HDAC) and chromatin remodeling activity.Human papillomaviruses (HPVs) of the high-risk group (e.g. HPV-16) cause cancers in humans, whereas papillomaviruses of the low-risk group (e.g. HPV-11) cause benign epithelial hyperproliferation (14). Papillomavirus E7 associates...

Human Papillomavirus Oncoprotein E7 Targets the Promyelocytic Leukemia Protein and Circumvents Cellular Senescence via the Rb and p53 Tumor Suppressor Pathways

“…One mechanism is inactivation of the Rb tumor suppressor through the LXCXE motif-mediated interaction. The results presented here and elsewhere, however, indicate that E7 also targets molecules unrelated to Rb, including PML, p21/p27, and chromatin remodeling complexes, through its C-terminal Zn finger (2,7,24,33,34,62). In sum, this enables E7 to alter expression of cellular genes that would otherwise be unfavorable for viral propagation.…”

“…Retroviral vectors pBABE-puro and pLXSN-neo were as reported previously (5). pLXSN-E6, -E7, and -E6E7 were a gift of D. Galloway; C-terminal E7-FLAG, CR2, and CR3 mutant E7 constructs were kindly provided by T. Kouzarides and J. Choe (7,34); pWZLHy-Cdk 4 R24C was a gift of S. Lowe (50); and small interfering RNA (siRNA)-p16 vector retro-pRSHyg-16 was a gift of R. Agami (56). Wild-type cDNAs for PML III and IV (new nomenclature) were as described previously (31); pBABE-RasV12, pBABE-PML IV, and pBABE-PML III were as reported previously (5).…”

Retinoic Acid-Induced Chromatin Remodeling of Mouse κ Opioid Receptor Gene

“…As such, BRG-1 was proposed to form a repressive complex with Mad-1 (instead of c-Myc) or other factors, such as HDACs and corepressors, to assemble an inaccessible chromatin structure. BRG-1-triggered repression of c-fos promoter was found to synergize with recruitment of HDAC components such as SMRT and HDAC1 (Lee et al, 2002). It has also been proposed that the Max/Mad1 complex could remodel chromatin of their target genes by recruiting chromatin remodeling proteins (Luscher, 2001).…”

“…Although most chromatin remodeling activated transcription, Isw2p was recruited to promoters of mitotic genes by the transcriptional repressor UME6 and led to the formation of a repressive chromatin structure (Goldmark et al, 2000;Fazzio and Tsukiyama, 2003). Ets-2 and human papillomavirus E7 were also reported to form a repressor complex with components of SWI/SNF such as BRG-1 to silence gene transcription (Lee et al, 2002;Baker et al, 2003). As such, BRG-1 was proposed to form a repressive complex with Mad-1 (instead of c-Myc) or other factors, such as HDACs and corepressors, to assemble an inaccessible chromatin structure.…”

Interaction of Papillomavirus E2 Protein with the Brm Chromatin Remodeling Complex Leads to Enhanced Transcriptional Activation

“…Viral proteins are known to interact with chromatin-modifying factors or remodeling complexes (45). HPV E7 interacts with the chromatin remodeling factor Brg1 and deregulates the transcriptional properties of Brg1 (40). Likewise, several viral proteins, such as the human immunodeficiency virus type 1 integrase, Tat, Tax, EBNA2, E1A, and SV40 large T protein, interact with chromatin-modifying factors in modulating their transcriptional properties (21-23, 50, 86, 90).…”