Mutagenesis of the pRB pocket reveals that cell cycle arrest functions are separable from binding to viral oncoproteins

A Promising New Model: Establishment of Patient-Derived Organoid Models Covering HPV-Related Cervical Pre-Cancerous Lesions and Their Cancers

The lack of human-derived in vitro models that recapitulate cervical pre-cancerous lesions has been the bottleneck in researching human papillomavirus (HPV) infection-associated pre-cancerous lesions and cancers for a long time. Here, a long-term 3D organoid culture protocol for high-grade squamous intraepithelial lesions and cervical squamous cell carcinoma that stably recapitulates the two tissues of origin is described. Originating from human-derived samples, a small biobank of cervical pre-tumoroids and tumoroids that faithfully retains genomic and transcriptomic characteristics as well as the causative HPV genome is established. Cervical pre-tumoroids and tumoroids show differential responses to common chemotherapeutic agents and grow differently as xenografts in mice. By coculture organoid models with peripheral blood immune cells (PBMCs) stimulated by HPV antigenic peptides, it is illustrated that both organoid models respond differently to immunized PBMCs, supporting organoids as reliable and powerful tools for studying virus-specific T-cell responses and screening therapeutic HPV vaccines. In this study, a model of cervical pre-cancerous lesions containing HPV is established for the first time, overcoming the bottleneck of the current model of human cervical pre-cancerous lesions. This study establishes an experimental platform and biobanks for in vitro mechanistic research, therapeutic vaccine screening, and personalized treatment for HPV-related cervical diseases.

Current In Vitro and In Vivo Models to Study MCPyV-Associated MCC

Merkel cell polyomavirus (MCPyV) is the only human polyomavirus currently known to cause human cancer. MCPyV is believed to be an etiological factor in at least 80% of cases of the rare but aggressive skin malignancy Merkel cell carcinoma (MCC). In these MCPyV+ MCC tumors, clonal integration of the viral genome results in the continued expression of two viral proteins: the viral small T antigen (ST) and a truncated form of the viral large T antigen. The oncogenic potential of MCPyV and the functional properties of the viral T antigens that contribute to neoplasia are becoming increasingly well-characterized with the recent development of model systems that recapitulate the biology of MCPyV+ MCC. In this review, we summarize our understanding of MCPyV and its role in MCC, followed by the current state of both in vitro and in vivo model systems used to study MCPyV and its contribution to carcinogenesis. We also highlight the remaining challenges within the field and the major considerations related to the ongoing development of in vitro and in vivo models of MCPyV+ MCC.

Merkel cell polyomavirus large T antigen binding to pRb promotes skin hyperplasia and tumor development

Clear evidence supports a causal link between Merkel cell polyomavirus (MCPyV) and the highly aggressive human skin cancer called Merkel cell carcinoma (MCC). Integration of viral DNA into the human genome facilitates continued expression of the MCPyV small tumor (ST) and large tumor (LT) antigens in virus-positive MCCs. In MCC tumors, MCPyV LT is truncated in a manner that renders the virus unable to replicate yet preserves the LXCXE motif that facilitates its binding to and inactivation of the retinoblastoma tumor suppressor protein (pRb). We previously developed a MCPyV transgenic mouse model in which MCC tumor-derived ST and truncated LT expression were targeted to the stratified epithelium of the skin, causing epithelial hyperplasia, increased proliferation, and spontaneous tumorigenesis. We sought to determine if any of these phenotypes required the association between the truncated MCPyV LT and pRb. Mice were generated in which K14-driven MCPyV ST/LT were expressed in the context of a homozygous RbΔLXCXE knock-in allele that attenuates LT-pRb interactions through LT's LXCXE motif. We found that many of the phenotypes including tumorigenesis that develop in the K14-driven MCPyV transgenic mice were dependent upon LT's LXCXE-dependent interaction with pRb. These findings highlight the importance of the MCPyV LT-pRb interaction in an in vivo model for MCPyV-induced tumorigenesis.

The Mus musculus Papillomavirus Type 1 E7 Protein Binds to the Retinoblastoma Tumor Suppressor: Implications for Viral Pathogenesis

The species specificity of papillomaviruses has been a significant roadblock for performing in vivo pathogenesis studies in common model organisms. The Mus musculus papillomavirus type 1 (MmuPV1) causes cutaneous papillomas that can progress to squamous cell carcinomas in laboratory mice. The papillomavirus E6 and E7 genes encode proteins that establish and maintain a cellular milieu that allows for viral genome synthesis and viral progeny synthesis in growth-arrested, terminally differentiated keratinocytes. The E6 and E7 proteins provide this activity by binding to and functionally reprogramming key cellular regulatory proteins. The MmuPV1 E7 protein lacks the canonical LXCXE motif that mediates the binding of multiple viral oncoproteins to the cellular retinoblastoma tumor suppressor protein, RB1. Our proteomic experiments, however, revealed that MmuPV1 E7 still interacts with RB1. We show that MmuPV1 E7 interacts through its C terminus with the C-terminal domain of RB1. Binding of MmuPV1 E7 to RB1 did not cause significant activation of E2F-regulated cellular genes. MmuPV1 E7 expression was shown to be essential for papilloma formation. Experimental infection of mice with MmuPV1 expressing an E7 mutant that is defective for binding to RB1 caused delayed onset, lower incidence, and smaller sizes of papillomas. Our results demonstrate that the MmuPV1 E7 gene is essential and that targeting noncanonical activities of RB1, which are independent of RB1's ability to modulate the expression of E2F-regulated genes, contribute to papillomavirus-mediated pathogenesis. IMPORTANCE Papillomavirus infections cause a variety of epithelial hyperplastic lesions, or warts. While most warts are benign, some papillomaviruses cause lesions that can progress to squamous cell carcinomas, and approximately 5% of all human cancers are caused by human papillomavirus (HPV) infections. The papillomavirus E6 and E7 proteins are thought to function to reprogram host epithelial cells to enable viral genome replication in terminally differentiated, normally growth-arrested cells. E6 and E7 lack enzymatic activities and function by interacting and functionally altering host cell regulatory proteins. Many cellular proteins that can interact with E6 and E7 have been identified, but the biological relevance of these interactions for viral pathogenesis has not been determined. This is because papillomaviruses are species specific and do not infect heterologous hosts. Here, we use a recently established mouse papillomavirus (MmuPV1) model to investigate the role of the E7 protein in viral pathogenesis. We show that MmuPV1 E7 is necessary for papilloma formation. The retinoblastoma tumor suppressor protein (RB1) is targeted by many papillomaviral E7 proteins, including cancer-associated HPVs. We show that MmuPV1 E7 can bind RB1 and that infection with a mutant MmuPV1 virus that expresses an RB1 binding-defective E7 mutant caused smaller and fewer papillomas that arise with delayed kinetics.

Molecular aspects of the teratogenesis of rubella virus

Rubella or German measles is an infection caused by rubella virus (RV). Infection of children and adults is usually characterized by a mild exanthematous febrile illness. However, RV is a major cause of birth defects and fetal death following infection in pregnant women. RV is a teratogen and is a major cause of public health concern as there are more than 100,000 cases of congenital rubella syndrome (CRS) estimated to occur every year. Several lines of evidence in the field of molecular biology of RV have provided deeper insights into the teratogenesis process. The damage to the growing fetus in infected mothers is multifactorial, arising from a combination of cellular damage, as well as its effect on the dividing cells. This review focuses on the findings in the molecular biology of RV, with special emphasis on the mitochondrial, cytoskeleton and the gene expression changes. Further, the review addresses in detail, the role of apoptosis in the teratogenesis process.

Cyclin D-Cdk4,6 Drives Cell-Cycle Progression via the Retinoblastoma Protein's C-Terminal Helix

The cyclin-dependent kinases Cdk4 and Cdk6 form complexes with D-type cyclins to drive cell proliferation. A well-known target of cyclin D-Cdk4,6 is the retinoblastoma protein Rb, which inhibits cell-cycle progression until its inactivation by phosphorylation. However, the role of Rb phosphorylation by cyclin D-Cdk4,6 in cell-cycle progression is unclear because Rb can be phosphorylated by other cyclin-Cdks, and cyclin D-Cdk4,6 has other targets involved in cell division. Here, we show that cyclin D-Cdk4,6 docks one side of an alpha-helix in the Rb C terminus, which is not recognized by cyclins E, A, and B. This helix-based docking mechanism is shared by the p107 and p130 Rb-family members across metazoans. Mutation of the Rb C-terminal helix prevents its phosphorylation, promotes G1 arrest, and enhances Rb's tumor suppressive function. Our work conclusively demonstrates that the cyclin D-Rb interaction drives cell division and expands the diversity of known cyclin-based protein docking mechanisms.

A Code of Mono-phosphorylation Modulates the Function of RB

Hyper-phosphorylation of RB controls its interaction with E2F and inhibits its tumor suppressor properties. However, during G1 active RB can be mono-phosphorylated on any one of 14 CDK phosphorylation sites. Here, we used quantitative proteomics to profile protein complexes formed by each mono-phosphorylated RB isoform (mP-RB) and identified the associated transcriptional outputs. The results show that the 14 sites of mono-phosphorylation co-ordinate RB's interactions and confer functional specificity. All 14 mP-RBs interact with E2F/DP proteins, but they provide different shades of E2F regulation. RB mono-phosphorylation at S811, for example, alters RB transcriptional activity by promoting its association with NuRD complexes. The greatest functional differences between mP-RBs are evident beyond the cell cycle machinery. RB mono-phosphorylation at S811 or T826 stimulates the expression of oxidative phosphorylation genes, increasing cellular oxygen consumption. These results indicate that RB activation signals are integrated in a phosphorylation code that determines the diversity of RB activity.

Recruitment of Antigen Presenting Cells to Skin Draining Lymph Node From HPV16E7-Expressing Skin Requires E7-Rb Interaction

“High-risk” human papillomaviruses (HPV) infect keratinocytes of squamous epithelia. The HPV16E7 protein induces epithelial hyperplasia by binding Rb family proteins and disrupting cell cycle termination. Murine skin expressing HPV16E7 as a transgene from a keratin 14 promoter (K14.E7) demonstrates epithelial hyperplasia, dysfunctional antigen presenting cells, ineffective antigen presentation by keratinocytes, and production of immunoregulatory cytokines. Furthermore, grafted K14.E7 skin is not rejected from immunocompetent non-transgenic recipient animals. To establish the contributions of E7, of E7-Rb interaction and of epithelial hyperplasia to altered local skin immunity, K14.E7 skin was compared with skin from K14.E7 mice heterozygous for a mutant Rb unable to bind E7 (K14.E7xRb^ΔL/ΔL mice), that have normoplastic epithelium. Previously, we demonstrated that E7-speicfic T cells do not accumulate in K14.E7xRb^ΔL/ΔL skin grafts. Here, we further show that K14.E7xRb^ΔL/ΔL skin, like K14.E7 skin, is not rejected by immunocompetent non-transgenic animals. There were fewer CD11b⁺ antigen presenting cells in skin draining lymph nodes from animals recipient of K14.E7xRb^ΔL/ΔL grafts, when compared with animals receiving K14.E7 grafts or K5mOVA grafts. Maturation of migratory DCs derived from K14.E7xRb^ΔL/ΔL grafts found in the draining lymph nodes is significantly lower than that of K14.E7 grafts. Surprisingly, K14.E7xRb^ΔL/ΔL keratinocytes, unlike K14.E7 keratinocytes, are susceptible to E7 directed CTL-mediated lysis in vitro. We conclude that E7-Rb interaction and its associated epithelial hyperplasia partially contribute to the suppressive local immune responses in area affected by HPV16E7 expression.

HPV16E7-Induced Hyperplasia Promotes CXCL9/10 Expression and Induces CXCR3+ T-Cell Migration to Skin

Chemokines regulate tissue immunity by recruiting specific subsets of immune cells. Mice expressing the E7 protein of human papilloma virus 16 as a transgene from a keratin 14 promoter (K14.E7) show increased epidermal and dermal lymphocytic infiltrates, epidermal hyperplasia, and suppressed local immunity. Here, we show that CXCL9 and CXCL10 are overexpressed in non-hematopoietic cells in skin of K14.E7 mice when compared with non-transgenic animals, and recruit CXCR3⁺ lymphocytes to the hyperplastic skin. Overexpression of CXCL9 and CXCL10 is not observed in E7 transgenic mice with mutated Rb gene whose protein product cannot interact with E7 (K14.E7xRb^ΔL/ΔL) and in consequence lack hyperplastic epithelium. CXCR3⁺ T cells are preferentially recruited by CXCL9 and CXCL10 in supernatants of K14.E7 but not K14.E7xRb^ΔL/ΔL skin cultures in vitro. CXCR3 signalling promotes infiltration of a subset of effector T lymphocytes that enables donor lymphocyte deficient, E7-expressing skin graft rejection. Taken together, this suggests that recruitment of CXCR3⁺ T cells can be an important factor in the rejection of precancerous skin epithelium providing they can overcome local immunosuppressive mechanisms driven by skin-resident lymphocytes.

HPV/E7 induces chemotherapy-mediated tumor suppression by ceramide-dependent mitophagy

Human papillomavirus (HPV) infection is linked to improved survival in response to chemo-radiotherapy for patients with oropharynx head and neck squamous cell carcinoma (HNSCC). However, mechanisms involved in increased HNSCC cell death by HPV signaling in response to therapy are largely unknown. Here, using molecular, pharmacologic and genetic tools, we show that HPV early protein 7 (E7) enhances ceramide-mediated lethal mitophagy in response to chemotherapy-induced cellular stress in HPV-positive HNSCC cells by selectively targeting retinoblastoma protein (RB). Inhibition of RB by HPV-E7 relieves E2F5, which then associates with DRP1, providing a scaffolding platform for Drp1 activation and mitochondrial translocation, leading to mitochondrial fission and increased lethal mitophagy. Ectopic expression of a constitutively active mutant RB, which is not inhibited by HPV-E7, attenuated ceramide-dependent mitophagy and cell death in HPV(+) HNSCC cells. Moreover, mutation of E2F5 to prevent Drp1 activation inhibited mitophagy in HPV(+) cells. Activation of Drp1 with E2F5-mimetic peptide for inducing Drp1 mitochondrial localization enhanced ceramide-mediated mitophagy and led to tumor suppression in HPV-negative HNSCC-derived xenograft tumors in response to cisplatin in SCID mice.

Multiple molecular interactions redundantly contribute to RB-mediated cell cycle control

Background

The G1-S phase transition is critical to maintaining proliferative control and preventing carcinogenesis. The retinoblastoma tumor suppressor is a key regulator of this step in the cell cycle.

Results

Here we use a structure–function approach to evaluate the contributions of multiple protein interaction surfaces on pRB towards cell cycle regulation. SAOS2 cell cycle arrest assays showed that disruption of three separate binding surfaces were necessary to inhibit pRB-mediated cell cycle control. Surprisingly, mutation of some interaction surfaces had no effect on their own. Rather, they only contributed to cell cycle arrest in the absence of other pRB dependent arrest functions. Specifically, our data shows that pRB–E2F interactions are competitive with pRB–CDH1 interactions, implying that interchangeable growth arrest functions underlie pRB’s ability to block proliferation. Additionally, disruption of similar cell cycle control mechanisms in genetically modified mutant mice results in ectopic DNA synthesis in the liver.

Conclusions

Our work demonstrates that pRB utilizes a network of mechanisms to prevent cell cycle entry. This has important implications for the use of new CDK4/6 inhibitors that aim to activate this proliferative control network.

Cell-specific Kaiso (ZBTB33) Regulation of Cell Cycle through Cyclin D1 and Cyclin E1

The correlation between aberrant DNA methylation with cancer promotion and progression has prompted an interest in discerning the associated regulatory mechanisms. Kaiso (ZBTB33) is a specialized transcription factor that selectively recognizes methylated CpG-containing sites as well as a sequence-specific DNA target. Increasing reports link ZBTB33 overexpression and transcriptional activities with metastatic potential and poor prognosis in cancer, although there is little mechanistic insight into how cells harness ZBTB33 transcriptional capabilities to promote and progress disease. Here we report mechanistic details for how ZBTB33 mediates cell-specific cell cycle regulation. By utilizing ZBTB33 depletion and overexpression studies, it was determined that in HeLa cells ZBTB33 directly occupies the promoters of cyclin D1 and cyclin E1, inducing proliferation by promoting retinoblastoma phosphorylation and allowing for E2F transcriptional activity that accelerates G₁- to S-phase transition. Conversely, in HEK293 cells ZBTB33 indirectly regulates cyclin E abundance resulting in reduced retinoblastoma phosphorylation, decreased E2F activity, and decelerated G₁ transition. Thus, we identified a novel mechanism by which ZBTB33 mediates the cyclin D1/cyclin E1/RB1/E2F pathway, controlling passage through the G₁ restriction point and accelerating cancer cell proliferation.

APC/C and retinoblastoma interaction: cross-talk of retinoblastoma protein with the ubiquitin proteasome pathway

The ubiquitin (Ub) ligase anaphase promoting complex/cyclosome (APC/C) and the tumour suppressor retinoblastoma protein (pRB) play key roles in cell cycle regulation. APC/C is a critical regulator of mitosis and G₁-phase of the cell cycle whereas pRB keeps a check on proliferation by inhibiting transition to the S-phase. APC/C and pRB interact with each other via the co-activator of APC/C, FZR1, providing an alternative pathway of regulation of G₁ to S transition by pRB using a post-translational mechanism. Both pRB and FZR1 have complex roles and are implicated not only in regulation of cell proliferation but also in differentiation, quiescence, apoptosis, maintenance of chromosomal integrity and metabolism. Both are also targeted by transforming viruses. We discuss recent advances in our understanding of the involvement of APC/C and pRB in cell cycle based decisions and how these insights will be useful for development of anti-cancer and anti-viral drugs.

Structural Conservation and E2F Binding Specificity within the Retinoblastoma Pocket Protein Family

The human pocket proteins retinoblastoma (Rb), p107, and p130 are critical negative regulators of the cell cycle and contribute to tumor suppression. While strong structural conservation within the pocket protein family provides for some functional redundancy, important differences have been observed and may underlie the reason that Rb is a uniquely potent tumor suppressor. It has been proposed that distinct pocket protein activities are mediated by their different E2F transcription factor binding partners. In humans, Rb binds E2F1-E2F5, whereas p107 and p130 almost exclusively associate with E2F4 and E2F5. To identify the molecular determinants of this specificity, we compared the crystal structures of Rb and p107 pocket domains and identified several key residues that contribute to E2F selectivity in the pocket family. Mutation of these residues in p107 to match the analogous residue in Rb results in an increase in affinity for E2F1 and E2F2 and an increase in the ability of p107 to inhibit E2F2 transactivation. Additionally, we investigated how phosphorylation by Cyclin-dependent kinase on distinct residues regulates p107 affinity for the E2F4 transactivation domain. We found that phosphorylation of residues S650 and S975 weakens the E2F4 transactivation domain binding. Our data reveal molecular features of pocket proteins that are responsible for their similarities and differences in function and regulation.

A Novel Retinoblastoma Protein (RB) E3 Ubiquitin Ligase (NRBE3) Promotes RB Degradation and Is Transcriptionally Regulated by E2F1 Transcription Factor

Retinoblastoma protein (RB) plays critical roles in tumor suppression and is degraded through the proteasomal pathway. However, E3 ubiquitin ligases responsible for proteasome-mediated degradation of RB are largely unknown. Here we characterize a novel RB E3 ubiquitin ligase (NRBE3) that binds RB and promotes RB degradation. NRBE3 contains an LXCXE motif and bound RB in vitro. NRBE3 interacted with RB in cells when proteasome activity was inhibited. NRBE3 promoted RB ubiquitination and degradation via the ubiquitin-proteasome pathway. Importantly, purified NRBE3 ubiquitinated recombinant RB in vitro, and a U-box was identified as essential for its E3 activity. Surprisingly, NRBE3 was transcriptionally activated by E2F1/DP1. Consequently, NRBE3 affected the cell cycle by promoting G1/S transition. Moreover, NRBE3 was up-regulated in breast cancer tissues. Taken together, we identified NRBE3 as a novel ubiquitin E3 ligase for RB that might play a role as a potential oncoprotein in human cancers.

CDK4/6 and IGF1 receptor inhibitors synergize to suppress the growth of p16INK4A-deficient pancreatic cancers

Loss-of-function mutations in p16(INK4A) (CDKN2A) occur in approximately 80% of sporadic pancreatic ductal adenocarcinoma (PDAC), contributing to its early progression. Although this loss activates the cell-cycle-dependent kinases CDK4/6, which have been considered as drug targets for many years, p16(INK4A)-deficient PDAC cells are inherently resistant to CDK4/6 inhibitors. This study searched for targeted therapies that might synergize with CDK4/6 inhibition in this setting. We report that the IGF1R/IR inhibitor BMS-754807 cooperated with the CDK4/6 inhibitor PD-0332991 to strongly block proliferation of p16(INK4A)-deficient PDAC cells in vitro and in vivo. Sensitivity to this drug combination correlated with reduced activity of the master cell growth regulator mTORC1. Accordingly, replacing the IGF1R/IR inhibitor with the rapalog inhibitor temsirolimus broadened the sensitivity of PDAC cells to CDK4/6 inhibition. Our results establish targeted therapy combinations with robust cytostatic activity in p16(INK4A)-deficient PDAC cells and possible implications for improving treatment of a broad spectrum of human cancers characterized by p16(INK4A) loss.

Loss of the mammalian DREAM complex deregulates chondrocyte proliferation

Mammalian DREAM is a conserved protein complex that functions in cellular quiescence. DREAM contains an E2F, a retinoblastoma (RB)-family protein, and the MuvB core (LIN9, LIN37, LIN52, LIN54, and RBBP4). In mammals, MuvB can alternatively bind to BMYB to form a complex that promotes mitotic gene expression. Because BMYB-MuvB is essential for proliferation, loss-of-function approaches to study MuvB have generated limited insight into DREAM function. Here, we report a gene-targeted mouse model that is uniquely deficient for DREAM complex assembly. We have targeted p107 (Rbl1) to prevent MuvB binding and combined it with deficiency for p130 (Rbl2). Our data demonstrate that cells from these mice preferentially assemble BMYB-MuvB complexes and fail to repress transcription. DREAM-deficient mice show defects in endochondral bone formation and die shortly after birth. Micro-computed tomography and histology demonstrate that in the absence of DREAM, chondrocytes fail to arrest proliferation. Since DREAM requires DYRK1A (dual-specificity tyrosine phosphorylation-regulated protein kinase 1A) phosphorylation of LIN52 for assembly, we utilized an embryonic bone culture system and pharmacologic inhibition of (DYRK) kinase to demonstrate a similar defect in endochondral bone growth. This reveals that assembly of mammalian DREAM is required to induce cell cycle exit in chondrocytes.

A retinoblastoma allele that is mutated at its common E2F interaction site inhibits cell proliferation in gene-targeted mice

The retinoblastoma protein (pRB) is best known for regulating cell proliferation through E2F transcription factors. In this report, we investigate the properties of a targeted mutation that disrupts pRB interactions with the transactivation domain of E2Fs. Mice that carry this mutation endogenously (Rb1(ΔG)) are defective for pRB-dependent repression of E2F target genes. Except for an accelerated entry into S phase in response to serum stimulation, cell cycle regulation in Rb1(ΔG/ΔG) mouse embryonic fibroblasts (MEFs) strongly resembles that of the wild type. In a serum deprivation-induced cell cycle exit, Rb1(ΔG/ΔG) MEFs display a magnitude of E2F target gene derepression similar to that of Rb1(-/-) cells, even though Rb1(ΔG/ΔG) cells exit the cell cycle normally. Interestingly, cell cycle arrest in Rb1(ΔG/ΔG) MEFs is responsive to p16 expression and gamma irradiation, indicating that alternate mechanisms can be activated in G1 to arrest proliferation. Some Rb1(ΔG/ΔG) mice die neonatally with a muscle degeneration phenotype, while the others live a normal life span with no evidence of spontaneous tumor formation. Most tissues appear histologically normal while being accompanied by derepression of pRB-regulated E2F targets. This suggests that non-E2F-, pRB-dependent pathways may have a more relevant role in proliferative control than previously identified.

Conserved region 3 of human papillomavirus 16 E7 contributes to deregulation of the retinoblastoma tumor suppressor

The human papillomavirus (HPV) E7 oncoprotein binds cellular factors, preventing or retargeting their function and thereby making the infected cell conducive for viral replication. A key target of E7 is the product of the retinoblastoma susceptibility locus (pRb). This interaction results in the release of E2F transcription factors and drives the host cell into the S phase of the cell cycle. E7 binds pRb via a high-affinity binding site in conserved region 2 (CR2) and also targets a portion of cellular pRb for degradation via the proteasome. Evidence suggests that a secondary binding site exists in CR3, and that this interaction influences pRb deregulation. Additionally, evidence suggests that CR3 also participates in the degradation of pRb. We have systematically analyzed the molecular mechanisms by which CR3 contributes to deregulation of the pRb pathway by utilizing a comprehensive series of mutations in residues predicted to be exposed on the surface of HPV16 E7 CR3. Despite differences in the ability to interact with cullin 2, all CR3 mutants degrade pRb comparably to wild-type E7. We identified two specific patches of residues on the surface of CR3 that contribute to pRb binding independently of the high-affinity CR2 binding site. Mutants within CR3 that affect pRb binding are less effective than the wild-type E7 in overcoming pRb-induced cell cycle arrest. This demonstrates that the interaction between HPV16 E7 CR3 and pRb is functionally important for alteration of the cell cycle.

Regulation of transcription and chromatin structure by pRB: here, there and everywhere

Commitment to divide is one of the most crucial steps in the mammalian cell division cycle. It is critical for tissue and organismal homeostasis, and consequently is highly regulated. The vast majority of cancers evade proliferative control, further emphasizing the importance of the commitment step in cell cycle regulation. The Retinoblastoma (RB) tumor suppressor pathway regulates this decision-making step. Since being the subject of Knudson's 'two hit hypothesis', there has been considerable interest in understanding pRB's role in cancer. It is best known for repressing E2F dependent transcription of cell cycle genes. However, pRB's role in controlling chromatin structure is expanding and bringing it into new regulatory paradigms. In this review we discuss pRB function through protein-protein interactions, at the level of transcriptional regulation of individual promoters and in organizing higher order chromatin domains.

DNA damage signals through differentially modified E2F1 molecules to induce apoptosis

E2F transcription can lead to cell proliferation or apoptosis, indicating that E2Fs control opposing functions. In a similar manner, DNA double-strand breaks can signal to induce cell cycle arrest or apoptosis. Specifically, pRB is activated following DNA damage, allowing it to bind to E2Fs and block transcription at cell cycle promoters; however, E2F1 is simultaneously activated, leading to transcription at proapoptotic promoters. We examined this paradoxical control of E2F transcription by studying how E2F1's interaction with pRB is regulated following DNA damage. Our work reveals that DNA damage signals create multiple forms of E2F1 that contain mutually exclusive posttranslational modifications. Specifically, E2F1 phospho-serine 364 is found only in complex with pRB, while E2F1 phosphorylation at serine 31 and acetylation function to create a pRB-free form of E2F1. Both pRB-bound and pRB-free modifications on E2F1 are essential for the activation of TA-p73 and the maximal induction of apoptosis. Chromatin immunoprecipitation demonstrated that E2F1 phosphorylated on serine 364 is also present at proapoptotic gene promoters during the induction of apoptosis. This indicates that distinct populations of E2F1 are organized in response to DNA damage signaling. Surprisingly, these complexes act in parallel to activate transcription of proapoptotic genes. Our data suggest that DNA damage signals alter pRB and E2F1 to engage them in functions leading to apoptotic induction that are distinct from pRB-E2F regulation in cell cycle control.

Analysis of cell cycle position in mammalian cells

The regulation of cell proliferation is central to tissue morphogenesis during the development of multicellular organisms. Furthermore, loss of control of cell proliferation underlies the pathology of diseases like cancer. As such there is great need to be able to investigate cell proliferation and quantitate the proportion of cells in each phase of the cell cycle. It is also of vital importance to indistinguishably identify cells that are replicating their DNA within a larger population. Since a cell′s decision to proliferate is made in the G1 phase immediately before initiating DNA synthesis and progressing through the rest of the cell cycle, detection of DNA synthesis at this stage allows for an unambiguous determination of the status of growth regulation in cell culture experiments.

DNA content in cells can be readily quantitated by flow cytometry of cells stained with propidium iodide, a fluorescent DNA intercalating dye. Similarly, active DNA synthesis can be quantitated by culturing cells in the presence of radioactive thymidine, harvesting the cells, and measuring the incorporation of radioactivity into an acid insoluble fraction. We have considerable expertise with cell cycle analysis and recommend a different approach. We Investigate cell proliferation using bromodeoxyuridine/fluorodeoxyuridine (abbreviated simply as BrdU) staining that detects the incorporation of these thymine analogs into recently synthesized DNA. Labeling and staining cells with BrdU, combined with total DNA staining by propidium iodide and analysis by flow cytometry¹ offers the most accurate measure of cells in the various stages of the cell cycle. It is our preferred method because it combines the detection of active DNA synthesis, through antibody based staining of BrdU, with total DNA content from propidium iodide. This allows for the clear separation of cells in G1 from early S phase, or late S phase from G2/M. Furthermore, this approach can be utilized to investigate the effects of many different cell stimuli and pharmacologic agents on the regulation of progression through these different cell cycle phases.

In this report we describe methods for labeling and staining cultured cells, as well as their analysis by flow cytometry. We also include experimental examples of how this method can be used to measure the effects of growth inhibiting signals from cytokines such as TGF-β1, and proliferative inhibitors such as the cyclin dependent kinase inhibitor, p27KIP1. We also include an alternate protocol that allows for the analysis of cell cycle position in a sub-population of cells within a larger culture⁵. In this case, we demonstrate how to detect a cell cycle arrest in cells transfected with the retinoblastoma gene even when greatly outnumbered by untransfected cells in the same culture. These examples illustrate the many ways that DNA staining and flow cytometry can be utilized and adapted to investigate fundamental questions of mammalian cell cycle control.

RB restricts DNA damage-initiated tumorigenesis through an LXCXE-dependent mechanism of transcriptional control

The LXCXE peptide motif facilitates interaction between the RB tumor suppressor and a large number of cellular proteins that are expected to impinge on diverse biological processes. In vitro and in vivo analyses demonstrated that LXCXE binding function is dispensable for RB promoter association and control of basal gene expression. Dependence on this function of RB is unmasked after DNA damage, wherein LXCXE binding is essential for exerting control over E2F3 and suppressing cell-cycle progression in the presence of genotoxic stress. Gene expression profiling revealed that the transcriptional program coordinated by this specific aspect of RB is associated with progression of human hepatocellular carcinoma and poor disease outcome. Consistent with these findings, biological challenge revealed a requirement for LXCXE binding in suppression of genotoxin-initiated hepatocellular carcinoma in vivo. Together, these studies establish an essential role of the LXCXE binding motif for RB-mediated transcriptional control, response to genotoxic insult, and tumor suppression.

Retinoblastoma family of proteins and chromatin epigenetics: a repetitive story in a few LINEs

The retinoblastoma (RB) protein family in mammals is composed of three members: pRB (or RB1), p107, and p130. Although these proteins do not directly bind DNA, they associate with the E2F family of transcription factors which function as DNA sequence-specific transcription factors. RB proteins alter gene transcription via direct interference with E2F functions, as well as recruitment of transcriptional repressors and corepressors that silence gene expression through DNA and histone modifications. E2F/RB complexes shape the chromatin landscape through recruitment to CpG-rich regions in the genome, thus making E2F/RB complexes function as local and global regulators of gene expression and chromatin dynamics. Recruitment of E2F/pRB to the long interspersed nuclear element (LINE1) promoter enhances the role that RB proteins play in genome-wide regulation of heterochromatin. LINE1 elements are dispersed throughout the genome and therefore recruitment of RB to the LINE1 promoter suggests that LINE1 could serve as the scaffold on which RB builds up heterochromatic regions that silence and shape large stretches of chromatin. We suggest that mutations in RB function might lead to global rearrangement of heterochromatic domains with concomitant retrotransposon reactivation and increased genomic instability. These novel roles for RB proteins open the epigenetic-based way for new pharmacological treatments of RB-associated diseases, namely inhibitors of histone and DNA methylation, as well as histone deacetylase inhibitors.

The biochemical basis of CDK phosphorylation-independent regulation of E2F1 by the retinoblastoma protein

The pRB (retinoblastoma protein) has a central role in the control of the G(1)-S phase transition of the cell cycle that is mediated in part through the regulation of E2F transcription factors. Upon S-phase entry pRB is phosphorylated extensively, which in turn releases bound E2Fs to drive the expression of the genes required for S-phase progression. In the present study, we demonstrate that E2F1-maintains the ability to interact with ppRB (hyperphosphorylated pRB). This interaction is dependent upon the 'specific' E2F1-binding site located in the C-terminal domain of pRB. A unique region of the marked box domain of E2F1 contacts the 'specific' site to mediate the interaction with ppRB. The mechanistic basis of the interaction between E2F1 and ppRB is subtle. A single substitution between valine and proline residues in the marked box distinguishes E2F1's ability to interact with ppRB from the inability of E2F3 to bind to the 'specific' site in ppRB. The E2F1-pRB interaction at the 'specific' site also maintains the ability to regulate the transcriptional activation of E2F1 target genes. These data reveal a mechanism by which E2F1 regulation by pRB can persist, when pRB is hyperphosphorylated and presumed to be inactive.

Mitotic chromosome condensation mediated by the retinoblastoma protein is tumor-suppressive

Condensation and segregation of mitotic chromosomes is a critical process for cellular propagation, and, in mammals, mitotic errors can contribute to the pathogenesis of cancer. In this report, we demonstrate that the retinoblastoma protein (pRB), a well-known regulator of progression through the G1 phase of the cell cycle, plays a critical role in mitotic chromosome condensation that is independent of G1-to-S-phase regulation. Using gene targeted mutant mice, we studied this aspect of pRB function in isolation, and demonstrate that it is an essential part of pRB-mediated tumor suppression. Cancer-prone Trp53(-/-) mice succumb to more aggressive forms of cancer when pRB's ability to condense chromosomes is compromised. Furthermore, we demonstrate that defective mitotic chromosome structure caused by mutant pRB accelerates loss of heterozygosity, leading to earlier tumor formation in Trp53(+/-) mice. These data reveal a new mechanism of tumor suppression, facilitated by pRB, in which genome stability is maintained by proper condensation of mitotic chromosomes.

A cancer derived mutation in the retinoblastoma gene with a distinct defect for LXCXE dependent interactions

BACKGROUND

The interaction between viral oncoproteins such as Simian virus 40 TAg, adenovirus E1A, and human papilloma virus E7, and the retinoblastoma protein (pRB) occurs through a well characterized peptide sequence, LXCXE, on the viral protein and a well conserved groove in the pocket domain of pRB. Cellular proteins, such as histone deacetylases, also use this mechanism to interact with the retinoblastoma protein to repress transcription at cell cycle regulated genes. For these reasons this region of the pRB pocket domain is thought to play a critical role in growth suppression.

RESULTS

In this study, we identify and characterize a tumor derived allele of the retinoblastoma gene (RB1) that possesses a discrete defect in its ability to interact with LXCXE motif containing proteins that compromises proliferative control. To assess the frequency of similar mutations in the RB1 gene in human cancer, we screened blood and tumor samples for similar alleles. We screened almost 700 samples and did not detect additional mutations, indicating that this class of mutation is rare.

CONCLUSIONS

Our work provides proof of principal that alleles encoding distinct, partial loss of function mutations in the retinoblastoma gene that specifically lose LXCXE dependent interactions, are found in human cancer.

Targeting mechanism of the retinoblastoma tumor suppressor by a prototypical viral oncoprotein. Structural modularity, intrinsic disorder and phosphorylation of human papillomavirus E7

DNA tumor viruses ensure genome amplification by hijacking the cellular replication machinery and forcing infected cells to enter the S phase. The retinoblastoma (Rb) protein controls the G1/S checkpoint, and is targeted by several viral oncoproteins, among these the E7 protein from human papillomaviruses (HPVs). A quantitative investigation of the interaction mechanism between the HPV16 E7 protein and the RbAB domain in solution revealed that 90% of the binding energy is determined by the LxCxE motif, with an additional binding determinant (1.0 kcal.mol(-1)) located in the C-terminal domain of E7, establishing a dual-contact mode. The stoichiometry and subnanomolar affinity of E7 indicated that it can bind RbAB as a monomer. The low-risk HPV11 E7 protein bound 2.0 kcal.mol(-1) more weakly than the high-risk HPV16 and HPV18 type counterparts, but the modularity and binding mode were conserved. Phosphorylation at a conserved casein kinase II site in the natively unfolded N-terminal domain of E7 affected the local conformation by increasing the polyproline II content and stabilizing an extended conformation, which allowed for a tighter interaction with the Rb protein. Thus, the E7-RbAB interaction involves multiple motifs within the N-terminal domain of E7 and at least two conserved interaction surfaces in RbAB. We discussed a mechanistic model of the interaction of the Rb protein with a viral target in solution, integrated with structural data and the analysis of other cellular and viral proteins, which provided information about the balance of interactions involving the Rb protein and how these determine the progression into either the normal cell cycle or transformation.

Two new miR-16 targets: caprin-1 and HMGA1, proteins implicated in cell proliferation

BACKGROUND INFORMATION

miRNAs (microRNAs) are a class of non-coding RNAs that inhibit gene expression by binding to recognition elements, mainly in the 3' UTR (untranslated region) of mRNA. A single miRNA can target several hundred mRNAs, leading to a complex metabolic network. miR-16 (miRNA-16), located on chromosome 13q14, is involved in cell proliferation and apoptosis regulation; it may interfere with either oncogenic or tumour suppressor pathways, and is implicated in leukaemogenesis. These data prompted us to search for and validate novel targets of miR-16.

RESULTS

In the present study, by using a combined bioinformatics and molecular approach, we identified two novel putative targets of miR-16, caprin-1 (cytoplasmic activation/proliferation-associated protein-1) and HMGA1 (high-mobility group A1), and we also studied cyclin E which had been previously recognized as an miR-16 target by bioinformatics database. Using luciferase activity assays, we demonstrated that miR-16 interacts with the 3' UTR of the three target mRNAs. We showed that miR-16, in MCF-7 and HeLa cell lines, down-regulates the expression of caprin-1, HMGA1a, HMGA1b and cyclin E at the protein level, and of cyclin E, HMGA1a and HMGA1b at the mRNA levels.

CONCLUSIONS

Taken together, our data demonstrated that miR-16 can negatively regulate two new targets, HMGA1 and caprin-1, which are involved in cell proliferation. In addition, we also showed that the inhibition of cyclin E expression was due, at least in part, to a decrease in its mRNA stability.

A functional connection between pRB and transforming growth factor beta in growth inhibition and mammary gland development

Transforming growth factor beta (TGF-beta) is a crucial mediator of breast development, and loss of TGF-beta-induced growth arrest is a hallmark of breast cancer. TGF-beta has been shown to inhibit cyclin-dependent kinase (CDK) activity, which leads to the accumulation of hypophosphorylated pRB. However, unlike other components of TGF-beta cytostatic signaling, pRB is thought to be dispensable for mammary development. Using gene-targeted mice carrying subtle missense changes in pRB (Rb1(DeltaL) and Rb1(NF)), we have discovered that pRB plays a critical role in mammary gland development. In particular, Rb1 mutant female mice have hyperplastic mammary epithelium and defects in nursing due to insensitivity to TGF-beta growth inhibition. In contrast with previous studies that highlighted the inhibition of cyclin/CDK activity by TGF-beta signaling, our experiments revealed that active transcriptional repression of E2F target genes by pRB downstream of CDKs is also a key component of TGF-beta cytostatic signaling. Taken together, our work demonstrates a unique functional connection between pRB and TGF-beta in growth control and mammary gland development.

Epstein-Barr virus nuclear protein EBNA3C residues critical for maintaining lymphoblastoid cell growth

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is essential for efficient conversion of primary human B lymphocytes to lymphoblastoid cell lines (LCLs) and for continued LCL growth. We used a transcomplementation assay in the context of LCLs transformed by an EBV with a conditional EBNA3C to identify the EBNA3C amino acids (aa) necessary for maintaining LCL growth. Surprisingly, we found that most EBNA3C aa were essential for continued LCL growth. Only EBNA3C mutants deleted for residues within aa 507-515, 516-620, 637-675, or 676-727 maintained full LCL growth, and EBNA3C mutants deleted for residues within aa 728-732 or 910-992 maintained slow LCL growth. In contrast, EBNA3C lacking aa 180-231, which mediate RBP-Jkappa association and are necessary for EBNA3C abrogation of EBNA2-induced transcription through RBP-Jkappa, could not support LCL growth. Furthermore, 2 EBNA3C alanine substitution mutants within aa 180-231, which were wild-type (wt) in abrogating EBNA2-mediated transcription through RBP-Jkappa, maintained LCL growth, and 2 alanine substitution mutants within aa 180-231, which were null in abrogating EBNA2-mediated transcription through RBP-Jkappa, did not maintain LCL growth. This indicates that EBNA3C regulation of transcription through RBP-Jkappa is critical to maintaining LCL growth. Several other EBNA3C functions also are critical for LCL growth, because EBNA3C mutants deleted for residues within aa 130-159, 251-506, or 733-909 were wt in abrogating transcription through RBP-Jkappa and expression level, but did not maintain LCL growth.

E2F1 represses beta-catenin transcription and is antagonized by both pRB and CDK8

The E2F1 transcription factor can promote proliferation or apoptosis when activated, and is a key downstream target of the retinoblastoma tumour suppressor protein (pRB). Here we show that E2F1 is a potent and specific inhibitor of beta-catenin/T-cell factor (TCF)-dependent transcription, and that this function contributes to E2F1-induced apoptosis. E2F1 deregulation suppresses beta-catenin activity in an adenomatous polyposis coli (APC)/glycogen synthase kinase-3 (GSK3)-independent manner, reducing the expression of key beta-catenin targets including c-MYC. This interaction explains why colorectal tumours, which depend on beta-catenin transcription for their abnormal proliferation, keep RB1 intact. Remarkably, E2F1 activity is also repressed by cyclin-dependent kinase-8 (CDK8), a colorectal oncoprotein. Elevated levels of CDK8 protect beta-catenin/TCF-dependent transcription from inhibition by E2F1. Thus, by retaining RB1 and amplifying CDK8, colorectal tumour cells select conditions that collectively suppress E2F1 and enhance the activity of beta-catenin.

Hepatocellular carcinoma and the ubiquitin-proteasome system

Hepatocellular carcinoma is one of the largest causes of cancer-related deaths worldwide for which there are very limited treatment options that are currently effective. The ubiquitin-proteasome system has rapidly become acknowledged as both critical for normal cellular function and a frequent target of de-regulation leading to disease. This review appraises the evidence linking the ubiquitin-proteasome system with this devastatingly intractable cancer and asks whether it may prove to be fertile ground for the development of novel therapeutic interventions against hepatocellular carcinoma.

pRB-E2F1 complexes are resistant to adenovirus E1A-mediated disruption

Disruption of pRB-E2F interactions by E1A is a key event in the adenoviral life cycle that drives expression of early viral transcription and induces cell cycle progression. This function of E1A is complicated by E2F1, an E2F family member that controls multiple processes besides proliferation, including apoptosis and DNA repair. Recently, a second interaction site in pRB that only contacts E2F1 has been discovered, allowing pRB to control proliferation separately from other E2F1-dependent activities. Based on this new insight into pRB-E2F1 regulation, we investigated how E1A affects control of E2F1 by pRB. Our data reveal that pRB-E2F1 interactions are resistant to E1A-mediated disruption. Using mutant forms of pRB that selectively force E2F1 to bind through only one of the two binding sites on pRB, we determined that E1A is unable to disrupt E2F1's unique interaction with pRB. Furthermore, analysis of pRB-E2F complexes during adenoviral infection reveals the selective maintenance of pRB-E2F1 interactions despite the presence of E1A. Our experiments also demonstrate that E2F1 functions to maintain cell viability in response to E1A expression. This suggests that adenovirus E1A's seemingly complex mechanism of disrupting pRB-E2F interactions provides selectivity in promoting viral transcription and cell cycle advancement, while maintaining cell viability.

Crystal structure of the retinoblastoma protein N domain provides insight into tumor suppression, ligand interaction, and holoprotein architecture

The retinoblastoma susceptibility protein, Rb, has a key role in regulating cell-cycle progression via interactions involving the central "pocket" and C-terminal regions. While the N-terminal domain of Rb is dispensable for this function, it is nonetheless strongly conserved and harbors missense mutations found in hereditary retinoblastoma, indicating that disruption of its function is oncogenic. The crystal structure of the Rb N-terminal domain (RbN), reveals a globular entity formed by two rigidly connected cyclin-like folds. The similarity of RbN to the A and B boxes of the Rb pocket domain suggests that Rb evolved through domain duplication. Structural and functional analysis provides insight into oncogenicity of mutations in RbN and identifies a unique phosphorylation-regulated site of protein interaction. Additionally, this analysis suggests a coherent conformation for the Rb holoprotein in which RbN and pocket domains directly interact, and which can be modulated through ligand binding and possibly Rb phosphorylation.

Critical roles for non-pRb targets of human papillomavirus type 16 E7 in cervical carcinogenesis

High-risk human papillomaviruses (HPV) encode two oncogenes, E6 and E7, expressed in nearly all cervical cancers. In vivo, HPV-16 E7 has been shown to induce multiple phenotypes in the context of transgenic mice, including cervical cancer. E7 is a multifunctional protein known best for its ability to inactivate the tumor suppressor pRb. To determine the importance of pRb inactivation by E7 in cervical cancer, we pursued studies with genetically engineered mice. E7 expression in estrogen-treated murine cervix induced dysplasia and invasive cancers as reported previously, but targeted Rb inactivation in cervical epithelium was not sufficient to induce any cervical dysplasia or neoplasia. Furthermore, E7 induced cervical cancer formation even when the E7-pRb interaction was disrupted by the use of a knock-in mouse carrying an E7-resistant mutant Rb allele. pRb inactivation was necessary but not sufficient for E7 to overcome differentiation-induced or DNA damage-induced cell cycle arrest, and expression patterns of the E2F-responsive genes Mcm7 and cyclin E indicate that other E2F regulators besides pRb are important targets of E7. Together, these data indicate that non-pRb targets of E7 play critical roles in cervical carcinogenesis.

The LxCxE pRb interaction domain of cyclin D1 is dispensable for murine development

Cyclin D1 is a multifunctional, tumor-associated protein that interacts with pRb via a conserved LxCxE motif, activates a kinase partner, directs the phosphorylation of pRb, activates cyclin E-cyclin-dependent kinase 2 (cdk2) by titrating Cip/Kip cdk inhibitors, and modulates the activity of a variety of transcription factors. It is thought that some of the proproliferative function of cyclin D1 is exerted by LxCxE-dependent binding to the pRb pocket domain, which might interfere with the ability of pRb to repress transcription by recruiting cellular chromatin remodeling proteins to E2F-dependent promoters. To test the importance of the LxCxE domain in vivo, we have generated a "knock-in" mouse by replacing the wild-type cyclin D1 gene with a mutant allele precisely lacking the nucleotides encoding the LxCxE domain. Analysis of this mouse has shown that the LxCxE protein is biochemically similar to wild-type cyclin D1 in all tested respects. Moreover, we were unable to detect abnormalities in growth, retinal development, mammary gland development, or tumorigenesis, all of which are affected by deleting cyclin D1. Although we cannot exclude the presence of subtle defects, these results suggest that the LxCxE domain of cyclin D1 is not necessary for function despite the absolute conservation of this motif in the D-type cyclins from plants and vertebrates.

Human herpesvirus 6A (HHV-6A) and HHV-6B alter E2F1/Rb pathways and E2F1 localization and cause cell cycle arrest in infected T cells

E2F transcription factors play pivotal roles in controlling the expression of genes involved in cell viability as well as genes involved in cell death. E2F1 is an important constituent of this protein family, which thus far contains eight members. The interaction of E2F1 with its major regulator, retinoblastoma protein (Rb), has been studied extensively in the past two decades, concentrating on the role of E2F1 in transcriptional regulation and the role of Rb in cell replication and cancer formation. Additionally, the effect of viral infections on E2F1/Rb interactions has been analyzed for different viruses, concentrating on cell division, which is essential for viral replication. In the present study, we monitored E2F1-Rb interactions during human herpesvirus 6A (HHV-6A) and HHV-6B infections of SupT1 T cells. The results have shown the following dramatic alterations in E2F1-Rb pathways compared to the pathways of parallel mock-infected control cultures. (i) The E2F1 levels were elevated during viral infections. (ii) The cellular localization of E2F1 was dramatically altered, and it was found to accumulate both in the cytoplasmic and nuclear fractions, as opposed to the strict nuclear localization seen in the mock-infected cells. (iii) Although E2F1 expression was elevated, two exemplary target genes, cyclin E and MCM5, were not upregulated. (iv) The Rb protein was dephosphorylated early postinfection, a trait that also occurred with UV-inactivated virus. (v) Infection was associated with significant reduction of E2F1/Rb complexing. (vi) HHV-6 infections were accompanied by cell cycle arrest. The altered E2F1-Rb interactions and functions might contribute to the observed cell cycle arrest.

Characterization of an E2F1-specific binding domain in pRB and its implications for apoptotic regulation

The retinoblastoma protein (pRB) has the dual capability to negatively regulate both E2F-induced cell cycle entry and E2F1-induced apoptosis. In this report, we characterize a unique pRB-E2F1 interaction. Using mutagenesis to disrupt E2F1 binding, we find that the ability of pRB to regulate E2F1-induced apoptosis is diminished when this interaction is lost. Strikingly, this mutant form of pRB retains the ability to control E2F responsive cell cycle genes and blocks cell proliferation. These functional properties are the reciprocal of a previously described E2F binding mutant of pRB that interacts with E2F1, but lacks the ability to interact with other E2Fs. Our work shows that these distinct interactions allow pRB to separately regulate E2F-induced cell proliferation and apoptosis. This suggests a novel form of regulation whereby separate types of binding contacts between the same types of molecules can confer distinct functional outcomes.

Structure-function analysis of the retinoblastoma tumor suppressor protein - is the whole a sum of its parts?

Biochemical analysis of the retinoblastoma protein's function has received considerable attention since it was cloned just over 20 years ago. During this time pRB has emerged as a key regulator of the cell division cycle and its ability to block proliferation is disrupted in the vast majority of human cancers. Much has been learned about the regulation of E2F transcription factors by pRB in the cell cycle. However, many questions remain unresolved and researchers continue to explore this multifunctional protein. In particular, understanding how its biochemical functions contribute to its role as a tumor suppressor remains to be determined. Since pRB has been shown to function as an adaptor molecule that links different proteins together, or to particular promoters, analyzing pRB by disrupting individual protein interactions holds tremendous promise in unraveling the intricacies of its function. Recently, crystal structures have reported how pRB interacts with some of its molecular partners. This information has created the possibility of rationally separating pRB functions by studying mutants that disrupt individual binding sites. This review will focus on literature that investigates pRB by isolating functions based on binding sites within the pocket domain. This article will also discuss the prospects for using this approach to further explore the unknown functions of pRB.

Structure of the Oncoprotein Gankyrin in Complex with S6 ATPase of the 26S Proteasome

Gankyrin is an oncoprotein commonly overexpressed in most hepatocellular carcinomas. Gankyrin interacts with S6 ATPase of the 19S regulatory particle of the 26S proteasome and enhances the degradation of the tumor suppressors pRb and p53. Here, we report the structure of gankyrin in complex with the C-terminal domain of S6 ATPase. Almost all of the seven ankyrin repeats of gankyrin interact, through its concave region, with the C-terminal domain of S6 ATPase. The intermolecular interactions occur through the complementary charged residues between gankyrin and S6 ATPase. Biochemical studies based on the structure of the complex revealed that gankyrin interacts with pRb in both the presence and absence of S6 ATPase; however, the E182 residue in gankyrin is essential for the pRb interaction. These results provide a structural basis for the involvement of gankyrin in the pRb degradation pathway, through its association with S6 ATPase of the 26S proteasome.

Dimerization of CtIP may stabilize in vivo interactions with the Retinoblastoma-pocket domain

CtIP is a tumor suppressor that interacts with Retinoblastoma protein (Rb) to regulate the G1/S-phase transition of the cell cycle. Despite its large size (897 residues) CtIP has few known structured regions. Rather it contains several linear motifs that interact with known binding partners, including an LXCXE motif that binds the pocket domain of Rb-family proteins. This LXCXE motif lies at the C-terminus of the only known structured domain, an N-terminal coiled-coil dimerization domain (DD; residues 45-160). Yeast two-hybrid (Y2H) and GST-pulldown analyses showed that CtIP requires the LXCXE motif to bind the Rb-pocket. Although isothermal titration calorimetry data indicates that the LXCXE motif is the sole determinant of binding affinity for the Rb-pocket domain (K(A) approximately 10(6)M(-1)), Y2H data indicates that the DD is required to stabilize the interaction in vivo. Thus dimerization may increase the apparent stability of the proteins and/or the lifetime of the complexes.

Retinoblastoma protein and anaphase-promoting complex physically interact and functionally cooperate during cell-cycle exit

The retinoblastoma protein (pRB) negatively regulates the progression from G1 to S phase of the cell cycle, in part, by repressing E2F-dependent transcription. pRB also possesses E2F-independent functions that contribute to cell-cycle control--for example, during pRB-mediated cell-cycle arrest pRB associates with Skp2, the F-box protein of the Skp1-Cullin-F-box protein (SCF) E3 ubiquitin ligase complex, and promotes the stability of the cyclin-dependent kinase-inhibitor p27(Kip1) through an unknown mechanism. Degradation of p27(Kip1) is mediated by ubiquitin-dependent targeting of p27(Kip1) by SCF -Skp2 (ref. 4). Here, we report a novel interaction between pRB and the anaphase-promoting complex/cyclosome (APC/C) that controls p27(Kip1) stability by targeting Skp2 for ubiquitin-mediated degradation. Cdh1, an activator of APC/C, not only interacts with pRB but is also required for a pRB-induced cell-cycle arrest. The results reveal an unexpected physical convergence between the pRB tumour-suppressor protein and E3 ligase complexes, and raise the possibility that pRB may direct APC/C to additional targets during pRB-mediated cell-cycle exit.

Functional identification of Api5 as a suppressor of E2F-dependent apoptosis in vivo

Retinoblastoma protein and E2-promoter binding factor (E2F) family members are important regulators of G1-S phase progression. Deregulated E2F also sensitizes cells to apoptosis, but this aspect of E2F function is poorly understood. Studies of E2F-induced apoptosis have mostly been carried out in tissue culture cells, and the analysis of the factors that are important for this process has been restricted to the testing of a few candidate genes. Using Drosophila as a model system, we have generated tools that allow genetic modifiers of E2F-dependent apoptosis to be identified in vivo and developed assays that allow effects on E2F-induced apoptosis to be studied in cultured cells. Genetic interactions show that dE2F1-dependent apoptosis in vivo involves dArk/Apaf1 apoptosome-dependent activation of both initiator and effector caspases and is sensitive to levels of Drosophila inhibitor of apoptosis-1 (dIAP1). Using these approaches, we report the surprising finding that apoptosis inhibitor-5/antiapoptosis clone-11 (Api5/Aac11) is a critical determinant of dE2F1-induced apoptosis in vivo and in vitro. This functional interaction occurs in multiple tissues, is specific to E2F-induced apoptosis, and is conserved from flies to humans. Interestingly, Api5/Aac11 acts downstream of E2F and suppresses E2F-dependent apoptosis without generally blocking E2F-dependent transcription. Api5/Aac11 expression is often upregulated in tumor cells, particularly in metastatic cells. We find that depletion of Api5 is tumor cell lethal. The strong genetic interaction between E2F and Api5/Aac11 suggests that elevated levels of Api5 may be selected during tumorigenesis to allow cells with deregulated E2F activity to survive under suboptimal conditions. Therefore, inhibition of Api5 function might offer a possible mechanism for antitumor exploitation.

The retinoblastoma protein (pRB) was the first human tumor suppressor to be described, and it works by limiting the activity of the E2F transcription factor. The pRB pathway is inactivated in most forms of cancer, and, accordingly, most tumor cells have deregulated E2F. Uncontrolled E2F drives cell proliferation, but it also sensitizes cells to die (apoptosis). E2F-induced apoptosis is not well understood, but it affects the development of cancer and, potentially, could be exploited for cancer treatment. To date, however, there have been very few studies of E2F-induced apoptosis in animal models. The authors describe a series of genetic tools that allow systematic studies of E2F-induced apoptosis in Drosophila. As validation, this approach identified some known regulators of E2F-dependent apoptosis and also identified Api5, a little-studied gene that had not previously been linked to E2F, as a potent suppressor of E2F-induced cell death. The effects of Api5 on E2F occur in several different tissues and are conserved from flies to humans. This last point is significant since Api5 is upregulated in cancer cells. The discovery of the E2F–Api5 interaction demonstrates that important modulators of E2F-induced apoptosis are waiting to be discovered and that they can be found using Drosophila.

Separation of anti-proliferation and anti-apoptotic functions of retinoblastoma protein through targeted mutations of its A/B domain

Background

The human retinoblastoma susceptibility gene encodes a nuclear phosphoprotein RB, which is a negative regulator of cell proliferation. The growth suppression function of RB requires an evolutionarily conserved A/B domain that contains two distinct peptide-binding pockets. At the A/B interface is a binding site for the C-terminal trans-activation domain of E2F. Within the B-domain is a binding site for proteins containing the LxCxE peptide motif.

Methodology/Principle Findings

Based on the crystal structure of the A/B domain, we have constructed an RB-K530A/N757F (KN) mutant to disrupt the E2F- and LxCxE-binding pockets. The RB-K530A (K) mutant is sufficient to inactivate the E2F-binding pocket, whereas the RB-N757F (N) mutant is sufficient to inactivate the LxCxE-binding pocket. Each single mutant inhibits cell proliferation, but the RB-KN double mutant is defective in growth suppression. Nevertheless, the RB-KN mutant is capable of reducing etoposide-induced apoptosis.

Conclusion/Significance

Previous studies have established that RB-dependent G1-arrest can confer resistance to DNA damage-induced apoptosis. Results from this study demonstrate that RB can also inhibit apoptosis independent of growth suppression.

Isolation and characterization of the first American bottlenose dolphin papillomavirus: Tursiops truncatus papillomavirus type 2

A novel papillomavirus (PV) was isolated from a genital condyloma of a free-ranging bottlenose dolphin inhabiting the coastal waters of Charleston Harbor, SC, USA: Tursiops truncatus papillomavirus type 2 (TtPV2). This novel virus represents the first isolated North American cetacean PV and the first American bottlenose dolphin PV. After the viral genome was cloned, sequenced and characterized genetically, phylogenetic analyses revealed that TtPV2 is most similar to the only published cetacean PV isolated and characterized thus far, Phocoena spinipinnis PV type 1 (PsPV1). A striking feature of the genome of TtPV2, as well as that of PsPV1, is the lack of an E7 open reading frame, which typically encodes one of the oncogenic proteins believed to be responsible for malignant transformation in the high-risk mucosotropic human papillomaviruses (HPVs). TtPV2 E6 contains a PDZ-binding motif that has been shown to be involved in transformation in the case of high-risk genital HPVs.