When viral oncoprotein meets tumor suppressor: a structural view

Understanding Retinoblastoma Post-Translational Regulation for the Design of Targeted Cancer Therapies

Simple Summary

Rb1 is a regulator of cell cycle progression and genomic stability. This review focuses on post-translational modifications, their effect on Rb1 interactors, and their role in intracellular signaling in the context of cancer development. Finally, we highlight potential approaches to harness these post-translational modifications to design novel effective anticancer therapies.

Abstract

The retinoblastoma protein (Rb1) is a prototypical tumor suppressor protein whose role was described more than 40 years ago. Together with p107 (also known as RBL1) and p130 (also known as RBL2), the Rb1 belongs to a family of structurally and functionally similar proteins that inhibits cell cycle progression. Given the central role of Rb1 in regulating proliferation, its expression or function is altered in most types of cancer. One of the mechanisms underlying Rb-mediated cell cycle inhibition is the binding and repression of E2F transcription factors, and these processes are dependent on Rb1 phosphorylation status. However, recent work shows that Rb1 is a convergent point of many pathways and thus the regulation of its function through post-translational modifications is more complex than initially expected. Moreover, depending on the context, downstream signaling can be both E2F-dependent and -independent. This review seeks to summarize the most recent research on Rb1 function and regulation and discuss potential avenues for the design of novel cancer therapies.

An Integrated View of Virus-Triggered Cellular Plasticity Using Boolean Networks

Virus-related mortality and morbidity are due to cell/tissue damage caused by replicative pressure and resource exhaustion, e.g., HBV or HIV; exaggerated immune responses, e.g., SARS-CoV-2; and cancer, e.g., EBV or HPV. In this context, oncogenic and other types of viruses drive genetic and epigenetic changes that expand the tumorigenic program, including modifications to the ability of cancer cells to migrate. The best-characterized group of changes is collectively known as the epithelial–mesenchymal transition, or EMT. This is a complex phenomenon classically described using biochemistry, cell biology and genetics. However, these methods require enormous, often slow, efforts to identify and validate novel therapeutic targets. Systems biology can complement and accelerate discoveries in this field. One example of such an approach is Boolean networks, which make complex biological problems tractable by modeling data (“nodes”) connected by logical operators. Here, we focus on virus-induced cellular plasticity and cell reprogramming in mammals, and how Boolean networks could provide novel insights into the ability of some viruses to trigger uncontrolled cell proliferation and EMT, two key hallmarks of cancer.

Viral Infections: Negative Regulators of Apoptosis and Oncogenic Factors

The disruption of apoptotic cell death process is closely associated with the etiology of various diseases, including cancer. Permanent viral infections can cause different types of cancers. Oncogenic viruses manipulate both external and internal apoptosis pathways, and inhibit the activity of proapoptotic proteins and signaling pathways, which facilitates carcinogenesis. Ineffective immune surveillance or immune response suppression can induce uncontrolled virus propagation and host cell proliferation. In this review, we discuss current data that provide insights into mechanisms of apoptotic death suppression by viruses and their role in oncogenesis.

Double-edged role of PML nuclear bodies during human adenovirus infection

PML nuclear bodies are matrix-bound nuclear structures with a variety of functions in human cells. These nuclear domains are interferon regulated and play an essential role during virus infections involving accumulation of SUMO-dependent host and viral factors. PML-NBs are targeted and subsequently manipulated by adenoviral regulatory proteins, illustrating their crucial role during productive infection and virus-mediated oncogenic transformation. PML-NBs have a longstanding antiviral reputation; however, the genomes of Human Adenoviruses and initial sites of viral transcription/replication are found juxtaposed to these domains, resulting in a double-edged capacity of these nuclear multiprotein/multifunctional complexes. This enigma provides evidence that Human Adenoviruses selectively counteract antiviral responses, and simultaneously benefit from or even depend on proviral PML-NB associated components by active recruitment to PML track-like structures, that are induced during infection. Thereby, a positive microenvironment for adenoviral transcription and replication is created at these nuclear subdomains. Based on the available data, this review aims to provide a detailed overview of the current knowledge of Human Adenovirus crosstalk with nuclear PML body compartments as sites of SUMOylation processes in the host cells, evaluating the currently known principles and molecular mechanisms.

p53 and the Viral Connection: Back into the Future ‡

The discovery of the tumor suppressor p53, through its interactions with proteins of tumor-promoting viruses, paved the way to the understanding of p53 roles in tumor virology. Over the years, accumulating data suggest that WTp53 is involved in the viral life cycle of non-tumor-promoting viruses as well. These include the influenza virus, smallpox and vaccinia viruses, the Zika virus, West Nile virus, Japanese encephalitis virus, Human Immunodeficiency Virus Type 1, Human herpes simplex virus-1, and more. Viruses have learned to manipulate WTp53 through different strategies to improve their replication and spreading in a stage-specific, bidirectional way. While some viruses require active WTp53 for efficient viral replication, others require reduction/inhibition of WTp53 activity. A better understanding of WTp53 functionality in viral life may offer new future clinical approaches, based on WTp53 manipulation, for viral infections.

Mapping the interactions of adenoviral E1A proteins with the p160 nuclear receptor coactivator binding domain of CBP

Many viruses deregulate the cell and force transcription of viral genes by competing with cellular proteins for binding to the transcriptional co-activators CREB-binding protein (CBP) and p300. Through its interactions with CBP/p300 and the retinoblastoma protein, the adenovirus (AdV) early region 1A (E1A) oncoprotein hijacks the cell cycle and, in rodents, transforms the cell; the mechanistic and structural basis for these effects remain unclear. In this study we compare the affinity of protein constructs from the E1A proteins from two adenovirus serotypes, non-oncogenic AdV5 and highly oncogenic AdV12, for binding to the nuclear receptor coactivator binding domain (NCBD) of CBP. NMR spectra show that the E1A constructs from both serotypes are intrinsically disordered in the free state and that each contains three homologous binding sites for the NCBD, one in the N-terminal region and two within conserved region 1 (CR1) of E1A. The binding sites in CR1 correspond to the motifs that bind the retinoblastoma protein and the TAZ2 domain of CBP/p300. The E1A and NCBD peptides fold synergistically upon complex formation. Binding affinities determined from NMR titrations show that, although the overall affinities for AdV5 and AdV12 E1A are comparable, there are significant differences between the two E1A serotypes in the relative strength with which their constituent interaction motifs bind to the NCBD. The individual E1A interaction motifs were unable to compete effectively with p53 for binding to the NCBD and both the N-terminal region and CR1 region of E1A are required for efficient competition with p53.

The transcription factor FOXM1 (Forkhead box M1): proliferation-specific expression, transcription factor function, target genes, mouse models, and normal biological roles

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor, which stimulates cell proliferation and exhibits a proliferation-specific expression pattern. Accordingly, both the expression and the transcriptional activity of FOXM1 are increased by proliferation signals, but decreased by antiproliferation signals, including the positive and negative regulation by protooncoproteins or tumor suppressors, respectively. FOXM1 stimulates cell cycle progression by promoting the entry into S-phase and M-phase. Moreover, FOXM1 is required for proper execution of mitosis. Accordingly, FOXM1 regulates the expression of genes, whose products control G1/S-transition, S-phase progression, G2/M-transition, and M-phase progression. Additionally, FOXM1 target genes encode proteins with functions in the execution of DNA replication and mitosis. FOXM1 is a transcriptional activator with a forkhead domain as DNA binding domain and with a very strong acidic transactivation domain. However, wild-type FOXM1 is (almost) inactive because the transactivation domain is repressed by three inhibitory domains. Inactive FOXM1 can be converted into a very potent transactivator by activating signals, which release the transactivation domain from its inhibition by the inhibitory domains. FOXM1 is essential for embryonic development and the foxm1 knockout is embryonically lethal. In adults, FOXM1 is important for tissue repair after injury. FOXM1 prevents premature senescence and interferes with contact inhibition. FOXM1 plays a role for maintenance of stem cell pluripotency and for self-renewal capacity of stem cells. The functions of FOXM1 in prevention of polyploidy and aneuploidy and in homologous recombination repair of DNA-double-strand breaks suggest an importance of FOXM1 for the maintenance of genomic stability and chromosomal integrity.

Understanding the targeting of the RB family proteins by viral oncoproteins to defeat their oncogenic machinery

The retinoblastoma (RB) family consists of three genes, RB1, RBL1, and RBL2, that code for the pRb, p107, and pRb2/p130 proteins, respectively. All these factors have pivotal roles in controlling fundamental cellular mechanisms such as cell cycle, differentiation and apoptosis. The founder and the most investigated RB family protein is pRb, which is considered to be the paradigm of tumor suppressors. However, p107 and pRb2/p130 clearly display a high degree of structural and functional homology with pRb. Interestingly, these factors were first identified as physical targets of the Adenovirus E1A oncoprotein. Indeed, RB family proteins are the most important and widely investigated targets of small DNA virus oncoproteins, such as Adenovirus E1A, human papillomavirus E7 and Simian virus 40 large T antigen. By interacting with pRb and with other RB family members, these oncoproteins neutralize their growth suppressive properties, thus stimulating proliferation of the infected cells, de-differentiation, and resistance to apoptosis. All these acquired features strongly favor the rise and selection of immortalized and mutation-prone cells, leading to a higher propensity in undergoing transformation. Our present work aims to illustrate and delve into these protein-protein interactions. Considering that these viral oncoproteins are dispensable for normal cellular functions, they can create "oncogene addiction" in the infected/transformed cells. This makes the possibility to dismantle these interactions extremely attractive, thus promoting the development of highly specific smart molecules capable of targeting only the infected/transformed cells that express these viral factors.

FOXM1 (Forkhead box M1) in tumorigenesis: overexpression in human cancer, implication in tumorigenesis, oncogenic functions, tumor-suppressive properties, and target of anticancer therapy

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor and is also intimately involved in tumorigenesis. FOXM1 stimulates cell proliferation and cell cycle progression by promoting the entry into S-phase and M-phase. Additionally, FOXM1 is required for proper execution of mitosis. In accordance with its role in stimulation of cell proliferation, FOXM1 exhibits a proliferation-specific expression pattern and its expression is regulated by proliferation and anti-proliferation signals as well as by proto-oncoproteins and tumor suppressors. Since these factors are often mutated, overexpressed, or lost in human cancer, the normal control of the foxm1 expression by them provides the basis for deregulated FOXM1 expression in tumors. Accordingly, FOXM1 is overexpressed in many types of human cancer. FOXM1 is intimately involved in tumorigenesis, because it contributes to oncogenic transformation and participates in tumor initiation, growth, and progression, including positive effects on angiogenesis, migration, invasion, epithelial-mesenchymal transition, metastasis, recruitment of tumor-associated macrophages, tumor-associated lung inflammation, self-renewal capacity of cancer cells, prevention of premature cellular senescence, and chemotherapeutic drug resistance. However, in the context of urethane-induced lung tumorigenesis, FOXM1 has an unexpected tumor suppressor role in endothelial cells because it limits pulmonary inflammation and canonical Wnt signaling in epithelial lung cells, thereby restricting carcinogenesis. Accordingly, FOXM1 plays a role in homologous recombination repair of DNA double-strand breaks and maintenance of genomic stability, that is, prevention of polyploidy and aneuploidy. The implication of FOXM1 in tumorigenesis makes it an attractive target for anticancer therapy, and several antitumor drugs have been reported to decrease FOXM1 expression.

Identification and characterization of small molecule antagonists of pRb inactivation by viral oncoproteins

The retinoblastoma protein pRb is essential for regulating many cellular activities through its binding and inhibition of E2F transcription activators, and pRb inactivation leads to many cancers. pRb activity can be perturbed by viral oncoproteins including human papillomavirus (HPV) that share an LxCxE motif. Because there are no treatments for existing HPV infection leading to nearly all cervical cancers and other cancers to a lesser extent, we screened for compounds that inhibit the ability of HPV-E7 to disrupt pRb/E2F complexes. This lead to the identification of thiadiazolidinedione compounds that bind to pRb with mid-high nanomolar dissociation constants, are competitive with the binding of viral oncoproteins containing an LxCxE motif, and are selectively cytotoxic in HPV-positive cells alone and in mice. These inhibitors provide a promising scaffold for the development of therapies to treat HPV-mediated pathologies.

Understanding pRb: toward the necessary development of targeted treatments for retinoblastoma

Retinoblastoma is a pediatric retinal tumor initiated by biallelic inactivation of the retinoblastoma gene (RB1). RB1 was the first identified tumor suppressor gene and has defined roles in the regulation of cell cycle progression, DNA replication, and terminal differentiation. However, despite the abundance of work demonstrating the molecular function and identifying binding partners of pRb, the challenge facing molecular biologists and clinical oncologists is how to integrate this vast body of molecular knowledge into the development of targeted therapies for treatment of retinoblastoma. We propose that a more thorough genetic understanding of retinoblastoma would inform targeted treatment decisions and could improve outcomes and quality of life in children affected by this disease.

HPV E7 viral oncoprotein disrupts transcriptional regulation of L1Md retrotransposon

Murine L1Md-A5 retrotransposon is a redox-inducible element regulated by Nrf-2/JunD and E2F/Rb-binding sites within its promoter (5'-UTR). Because the human papillomavirus (HPV) oncoprotein E7 interacts with retinoblastoma (pRb) and members of the AP1 family, studies were conducted to examine functional interactions between HPV E7, pRb, and histone deacetylase 2 (HDAC2) in the regulation of L1Md-A5. Using a transient heterologous transcription system we found that HPV E7 alone, or in combination with HDAC2, disrupted pRb-mediated L1MdA-5 transactivation. HPV E7 also ablated the transcriptional response of L1Md-A5 to genotoxic stress, but did not interfere with basal activity. We conclude that HPV E7 associates with proteins involved in the assembly of macromolecular complexes that regulate antioxidant and E2F/Rb sites within L1MdA-5 to regulate biological activity.

Structural basis for subversion of cellular control mechanisms by the adenoviral E1A oncoprotein

The adenovirus early region 1A (E1A) oncoprotein mediates cell transformation by deregulating host cellular processes and activating viral gene expression by recruitment of cellular proteins that include cyclic-AMP response element binding (CREB) binding protein (CBP)/p300 and the retinoblastoma protein (pRb). While E1A is capable of independent interaction with CBP/p300 or pRb, simultaneous binding of both proteins is required for maximal biological activity. To obtain insights into the mechanism by which E1A hijacks the cellular transcription machinery by competing with essential transcription factors for binding to CBP/p300, we have determined the structure of the complex between the transcriptional adaptor zinc finger-2 (TAZ2) domain of CBP and the conserved region-1 (CR1) domain of E1A. The E1A CR1 domain is unstructured in the free state and upon binding folds into a local helical structure mediated by an extensive network of intermolecular hydrophobic contacts. By NMR titrations, we show that E1A efficiently competes with the N-terminal transactivation domain of p53 for binding to TAZ2 and that pRb interacts with E1A at 2 independent sites located in CR1 and CR2. We show that pRb and the CBP TAZ2 domain can bind simultaneously to the CR1 site of E1A to form a ternary complex and propose a structural model for the pRb:E1A:CBP complex on the basis of published x-ray data for homologous binary complexes. These observations reveal the molecular basis by which E1A inhibits p53-mediated transcriptional activation and provide a rationale for the efficiency of cellular transformation by the adenoviral E1A oncoprotein.

Evidence for a structural relationship between BRCT domains and the helicase domains of the replication initiators encoded by the Polyomaviridae and Papillomaviridae families of DNA tumor viruses

Studies of DNA tumor viruses have provided important insights into fundamental cellular processes and oncogenic transformation. They have revealed, for example, that upon expression of virally encoded proteins, cellular pathways involved in DNA repair and cell cycle control are disrupted. Herein, evidence is presented that BRCT-related regions are present in the helicase domains of the viral initiators encoded by the Polyomaviridae and Papillomaviridae viral families. Of interest, BRCT domains in cellular proteins recruit factors involved in diverse pathways, including DNA repair and the regulation of cell cycle progression. Therefore, the viral BRCT-related regions may compete with host BRCT domains for particular cellular ligands, a process that would help to explain the pleiotropic effects associated with infections with many DNA tumor viruses.

Structure of the retinoblastoma protein bound to adenovirus E1A reveals the molecular basis for viral oncoprotein inactivation of a tumor suppressor

The adenovirus (Ad) E1A (Ad-E1A) oncoprotein mediates cell transformation, in part, by displacing E2F transcription factors from the retinoblastoma protein (pRb) tumor suppressor. In this study we determined the crystal structure of the pRb pocket domain in complex with conserved region 1 (CR1) of Ad5-E1A. The structure and accompanying biochemical studies reveal that E1A-CR1 binds at the interface of the A and B cyclin folds of the pRb pocket domain, and that both E1A-CR1 and the E2F transactivation domain use similar conserved nonpolar residues to engage overlapping sites on pRb, implicating a novel molecular mechanism for pRb inactivation by a viral oncoprotein.

BK virus as a cofactor in the etiology of prostate cancer in its early stages

Prostate cancer has been projected to cause almost 10% of all male cancer deaths in the United States in 2007. The incidence of mutations in the tumor suppressor genes Rb1 and p53, especially in the early stages of the disease, is low compared to those for other cancers. This has led to the hypothesis that a human virus such as BK virus (BKV), which establishes a persistent subclinical infection in the urinary tract and encodes oncoproteins that interfere with these tumor suppressor pathways, is involved. Previously, we detected BKV DNA in the epithelial cells of benign and proliferative inflammatory atrophy ducts of cancerous prostate specimens. In the present report, we demonstrate that BKV is present at a much lower frequency in noncancerous prostates. Additionally, in normal prostates, T-antigen (TAg) expression is observed only in specimens harboring proliferative inflammatory atrophy and prostatic intraepithelial neoplasia. We further demonstrate that the p53 gene from atrophic cells expressing TAg is wild type, whereas tumor cells expressing detectable nuclear p53 contain a mix of wild-type and mutant p53 genes, suggesting that TAg may inactivate p53 in the atrophic cells. Our results point toward a role for BKV in early prostate cancer progression.